Advances in Agronomy, Volume 1

ADVANCES IN AGRONOMY VOLUME I

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ADVANCES IN

AGRONOMY Prepared under the Auspices of the

AMERICAN SOCIETY OF AGRONOMY

VOLUME I Edited by A. G. NORMAN Camp Detrick, Prederick, Maryland

ADVISORY BOARD H. BRADFIELD H. H. LAUDE N. P. NEAL

L. A. RICHARDS V. G. SPRAGUE E. WINTERS

1949

ACADEMIC PRESS INC., PUBLISHERS

NEW YORK

Copyright 1949, by ACADEMIC PRESS INC. 125 EAST ,23m STREET NEW YORK

10,

N. Y.

All Rights Reserved

N o part of this book may be reproduced in any form, b y photostat, microfilm, or any other means, without written permission from the publishers.

PRINTED I N THE UNITED STATES OF AMERICA

CONTRIBUTORS TO VOLUME I K. C. BERGER, Associate Professor of Soils, University of Wisconsin, Madison, Wisconsin. FRANCIS E. CLARK, Bacteriologist, U . S. Department of Agriculture, Department of Agronomy, Ames, Iowa. A. S. CRAFTS, Professor of Botany, University of California, Davis, California.

L. A. DEAN, Senior Soil Scientist, Division of Soil Management and Irrigation, Bureau of Plant Industry, Beltsville, Maryland. J. E. GIESEKING, Professor of Soil Physics, University of Illinois, Urbarn, Illinois. W. A. HARVEY, Associate in Botany, University of California, Davis, California. H .

E. HAYWARD, Director, U . S. Regional Salinity and Rubidoux Laboratories, Riverside, California.

RANDALL J. JONES,Chief, Soils and Fertilizer Research Section, Division of Agricultural Relations, Tennessee Valley Authority, Knoxville, Tennessee. HovAm T. ROGERS, Agronomist, Soils and Fertilizer Research Sectiom, Division of Agricultural Relations, Tennessee Valley Authority, Knoxville, Tennessee. ORA SMITH,Professor of Vegetable Crops, Cornell University, Ithaca, New York.

C. H. WADLEIGH, Principal Plant Physiologist, U . S. Regional Salinity and Rubidoux Laboratories, Riverside, California.

MARTING. WEISS,Professor of Farm Crops, Iowa State College, Ames, Iowa. WILLIAM J. WHITE,Oficer-in-Charge, Dominion Forage Crops Laboratory, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

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Preface Many sciences and skills contribute to the subject of agronomy; many persons with widely different duties can properly call themselves agronomists. Not all of these agronomists would agree as to the precise definition of the word agronomy, yet all, in some way or another, have interests that relate directly or indirectly to the growth of plants in soil. The kind of professional training required of those studying the genesis and classification of soils has few points in common with that required of men engaged in genetical studies of a particular crop. Yet their fields of activity are linked by their colleagues who must develop the proper fertilizer recommendation for profitable production of adapted varieties of that crop on various soil types. The great body of knowledge about plants-their nutritive requirements and growth, their management and improvement, their adaptation and utilization-is continually expanding. It is becoming increasingly difficult for many of those involved in one way or another in the theory or practice of soil management and crop production to keep themselves even reasonably well informed of the newer developments in all but their immediate fields of activity. Progress is to a degree centrifugal in it.s effects and through specialization tends to throw us apart. This volume, Advances in Agronomy, has as its objective the survey and review of progress in agronomic research and practice. The articles are written by specialists. They are critical and reasonably comprehensive in treatment. They are written primarily for fellow agronomists across the hall and across the continents who also find it difficult to keep well informed in all phases of agronomy. The authors of this volume all live on the North American continent, and it is primarily North American agronomy that is reviewed. It is not intended that this shall always be the case, and contributions to latcr volumes will be sought from workers in other countries overseas. I n the selection of topics for these volumes an attempt will be made to include material that will be helpful to workers with diverse subject matter and regional interests. The edit,ors’ definition of what constitutes agronomy is catholic; they will be guided in their choice more by what information may be of use to agronomists than by what constitutes agronomy. The central theme must be soil-crop relationships, for soils without crops are barren and field crops cannot be considered without vii

viii

PREFACE

reference to the soil on which they are produced. From time to time articles may be included that deal with related fields of horticulture and forestry. The editors will take cognizance of other publishing plans, in so far as they are known to them, in order to avoid duplication of treatment. For example, such considerat,ions led them to omit from this volume the subject of pastures, which was comprehensively surveyed in the 1948 Yearbook of Agriculture, and the subject of soil classification which was reviewed in a recent issue of Soil Science. The editors wish to acknowledge the co-operation of the several contributors to this volume, whose articles have been prepared as a service to the profession of agronomy.

A. G. NORMAN

Frederick, Md. October, 1949.

CONTENTS Page

Contributors t.o Volume I Preface., . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

v vii

Plant Growth on Saline and Alkali Soils

BY H . E . HAYWARD AND C . H . WADLEIGH. U . S. Regional Salinity and Rubidoux Laboratories. Riverside. California I. Introduction . . . . . . . . . . . . I1. Characteristics of Saline and Alkali Soils I11. Physiological Basis of Salt Tolerance . .

. . . . . .. . . . .

. . . . . . . . . . . . . . IV . Physiological Basis of Alkali Tolerance . . . . . . . . V. How Saline and Alkali Soils Affect Plant Growth . . . .

. . . . VI . Salt Tolerance as Related to the Life Cycle of the Plant . . VII . Specificity in Salt Tolerance . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

1 2 5 9 10 20 29

35

New Fertilizers and Fertilizer Practices BY RANDALL J . JONES

AND

HOWARD T . ROGERS. Tennessee Valley Authority. Knoxville. Tennessee

I. Introduction . . . . . . . . . . . . . I1. New and Improved Fertilizer Materials I11. Recent Developments in Fertilizer Use . References . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

39 41 53 72

Soybeans BY

MARTIN G. WEISS. Iowa State College. Ames. Iowa

. .

I Introduction . . . . . . . . . . . . . . . . . . . . . . . 78 I1 Production and Distribution . . . . . . . . . . . . . . . . 80 I11. Disposition and Utilization . . . . . . . . . . . . . . . . . 83 I V . Physiology of the Soybean Plant . . . . . . . . . . . . . . . 85 V. Effect of Climate and Location . . . . . . . . . . . . . . . 97 VI . Effect of Cultural Practices . . . . . . . . . . . . . . . . . 101 VII . Genetics and Cytology . . . . . . . . . . . . . . . . . . . 115 VIII . Variety Improvement . . . . . . . . . . . . . . . . . . . 123 I X . Effect on Soils . . . . . . . . . . . . . . . . . . . . . . 136 X . Disease and Insect Pests . . . . . . . . . . . . . . . . . . 143 XI . The Regional Approach to Soybean Research . . . . . . . . . . 150 References . . . . . . . . . . . . . . . . . . . . . . 152 I

ix

X

CONTENTS

The Clay Minerals in Soils

HY J . E . GIESEKING.Universil// n j Illinois. lirbnrra. Il1iiini.s Pagc I . Introduction . . . . . . . . . . . . . . . . . . . . . . . 159 I1. Historical Developiiicnt ol Clay Mineralogy . . . . . . . . . . 160 I11. Crystal Structure of the Clay Minerals in Soil Clays . . . . . . . 162 IV Qualitative Identification and Quantitative Estimation of the Clay Minerals . . . . . . . . . . . . . . . . . . . . . . . 171 V Distribution of the Clay Minerals in Soils . . . . . . . . . . . 177 VI . The Configuration of the Clay Mineral Crystals as Related to their Properties . . . . . . . . . . . . . . . . . . . . . . 180 VII. The Physicochemical Reactions of the Clay Minerals . . . . . . . 184 VIII . Functions of the Clay Minerals . . . . . . . . . . . . . . . 196 IX. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 199 References . . . . . . . . . . . . . . . . . . . . . . . 200

. .

Alfalfa Improvement

BY WILLIAMJ . WHITE.Dominion Forage Crops Laboratory. Univeraily of Saskatchewan. Saskatoon. ,S%skalchewan. Canada

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . 205 I1. Seed Setting and Production . . . . . . . . . . . . . . . . 206 I11. Progress in Methods of Breeding . . . . . . . . . . . . . . 225 IV. Conquering Some Diseases . . . . . . . . . . . . . . . . . 232 V Summary and Conclusions . . . . . . . . . . . . . . . . . 237 References . . . . . . . . . . . . . . . . . . . . . . . 238

.

Soil Microorganisms and Plant Roots

BY FRANCIS E . CLARK.U . S. Department of Agriculture and Iowa Agricultural Experiment Station. Ames. Zowa I. Introduction . . . . . . . . . . . . . . . . . . . . . . . I1. Types of Relationships between Microorganisms and Plant Roots . . I11. The Rhiaosphere Microflora in Relation to the Growth of Higher Plants IV . The Numbers of Microorganisms Associated with Plant Roots . . . V . The Kinds of Microorganisms Found on Plant Roots . . . . . . VI . Modification of the Root Surface Microflora . . . . . . . . . . VII . Influences of the Rhizosphere Flora on Succeeding or Associated Plants . . . . . . . . . . . . . . . . . . . . . . . References

242 247 249 264 270 274 278 282

Weed Uontrol

.

BY A S. CRAFTSA N D W . A . HARVEY. University of California. Davis. California I. Introduction . . . . . . . . . . . . . . . . . . . . . . . I1 Tillage. Cropping. and Competition in the Control of Weeds . . I11 Chemical Weed Control . . . . . . . . . . . . . . . . . IV Principles of Chemical Weed Control . . . . . . . . . . . .

. . .

289

. 290 . 293 .

293

xi

CONTENTS

I’agc V. Herbicidal Action . . . . . . . . . . . . . . . . . . . . . 295 VI . Molecular Properties of Herbicides . . . . . . . . . . . . . . 296 VII . Emulsions and Emulsion Stabilizers . . . . . . . . . . . . . 298 VIII . Selectivity of Herbicides . . . . . . . . . . . . . . . . . . 299 I X . The 2,4-D Herbicides . . . . . . . . . . . . . . . . . . . . 300 X . Uses of 2.4-D . . . . . . . . . . . . . . . . . . . . . . 303 XI . Nitro- and Chloro-substituted Phenols . . . . . . . . . . . . . 307 XII.0ils . . . . . . . . . . . . . . . . . . . . . . . . . . 308 XI11. Other Organic and Inorganic Chemicals . . . . . . . . . . . . 310 XIV . Water Weed Control . . . . . . . . . . . . . . . . . . 312 XV . Herbicide Application Equipment . . . . . . . . . . . . . . 312 XVI . Drift, Volatilization, Blowing of Herbicides . Secondary and Residual 312 Effects . . . . . . . . . . . . . . . . . . . . . . . S V I I. Flame Cultivation . . . . . . . . . . . . . . . . . . . . 314 S V I I I. Thc New Agronomy . . . . . . . . . . . . . . . . . . . 314 References . . . . . . . . . . . . . . . . . . . . . . . 315

Boron in Soils and Crops BY K . C . BERGER.University of Wisconsin. Madison. Wisconsin

. . .

I Introduction . . . . . . . . . . . . I1 Boron Determination . . . . . . . 111. Boron Availability in Soils . . . . . IV Boron Requirement of Plants . . . . V . Summayy . . . . . . . . . . . . . References . . . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . 321 . . . . . 323 . . . . . 327 . . . . . 336 . . . . 347 . . . . 348

Potato Production

BY OM SMITH.Cornell University. Illmca. New Yolk

. . . . . . . . . . . . . . . . . . . . . . .

I. Introduction 363 I1. Breeding and Improving Potato Varieties . . . . . . . . . . 355 111. Chemical Weed Control . . . . . . . . . . . . . . . . . 357 IV . Fertilizer Practices . . . . . . . . . . . . . . . . . . . . 360 V . Rotations and Green Manures . . . . . . . . . . . . . . . . 363 VI . Response to Nitrogen Fertilization . . . . . . . . . . . . . . 365 VII Response to Phosphorus Fertilization . . . . . . . . . . . . . 366 VIII . Response to Potassium Fertilization . . . . . . . . . . . . . . 367 IX. Effects of Magnesium. Liming. and Soil Reaction . . . . . . . . . 369 X . Minor Elements . . . . . . . . . . . . . . . . . . . . . 371 X I Time and Method of Application of Fertilizers . . . . . . . . . 372 XI1 Relation of Yield and Tuber Composition to Plant and Soil Analyses 374 XIII. Killing Potato Vines . . . . . . . . . . . . . . . . . . . 377 XIV. Recent Developments in Insect Control . . . . . . . . . . . . 381 XV Recent Developments in Disease Control . . . . . . . . . . . 385 References . . . . . . . . . . . . . . . . . . . . . . . 386

.

. . .

xii

CONTENTS

Fixation of Soil Phosphorus

BY L . A . DEAN.U . S. Department

of Agriculture. Beltsville. Maryland

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . I1. Accumulation of Phosphorus in Soils . . . . . . . . . . . . I11. Phosphorus Fixation by Soils. Clay Minerals. and Hydrous Oxides . IV . Chemically Precipitated Phosphorus . . . . . . . . . . . . V Fixation of Phosphorus by Surface Reactions . . . . . . . . . VI . Biological Fixation of Phosphorus in Soils . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .

.

Author Index Subject Index

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Page 391 . 392 . 393 . 397 . 400 ! 406 409

413 436

Plant Growth on Saline and Alkali Soils* H . E . HAYWARD AND C . H. WADLEIGH U S. Regionnl Salinity and Rubidoux Laboratories, Riverside. California C0N TEN TS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . I1. Characteristics of Saline and Alkali Soils . . . . . . . . . . . . I11. Physiological Basis of Salt Tolerance . . . . . . . . . . . . . IV . Physiological Basis of Alkali Tolerance . . . . . . . . . . . . . V . How Saline and Alkali Soils Affect Plant Growth . . . . . . . . . 1. Saline Soils . . . . . . . . . . . . . . . . . . . . . . a . Sodium . . . . . . . . . . . . . . . . . . . . . . b.Calcium . . . . . . . . . . . . . . . . . . . . . . c. Magnesium . . . . . . . . . . . . . . . . . . . . . d . Potassium . . . . . . . . . . . . . . . . . . . . . e . Chloride . . . . . . . . . . . . . . . . . . . . . . f . Sulfate . . . . . . . . . . . . . . . . . . . . . . . g. Bicarbonate . . . . . . . . . . . . . . . . . . . . . h.Nitrate . . . . . . . . . . . . . . . . . . . . . . 2. Alkali Soils . . . . . . . . . . . . . . . . . . . . . . . VI . Salt Tolerance as Related to the Life Cycle of the Plant . . . . . . 1. Germination . . . . . . . . . . . . . . . . . . . . . . 2. Vegetative Growth and Maturation . . . . . . . . . . . . VII . Specificity in Salt Tolerance . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

Page 1

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. .

2 5 9 10 11

15 15 16 16 16

17

.

. .

18 19 19 20 20 25 29 35

I . INTRODUCTION The yield of a given crop is the net resultant of the effects of the prevailing weather conditions, the ravages of pathogens, and the existing status of the soil, within the genetic limitations of the plant . Under normal conditions, soils affect yield through three primary factors: (a) moisture availability, (b) nutrient availability, and (c) physical condition . A fourth factor. excess salt.. may be present due to the accumulation of chemical components in the soil that are inhibitive to plant growth . I n the irrigated soils of arid or semi-arid regions. this factor

* Contribution from the U.S. Regional Salinity and Rubidoux Laboratories. Bureau of Plant Industry. Soils and Agricultural Engineering. Agricultural Research Administration. U S . Dept. of Agriculture. Riverside. Calif., in coorperation with the eleven Western States and the Territory of Hawaii . 1

2

H. E. HAYWARD AND C. H. WADLEIGH

may be a principal consideration on those soils t.hat contain accumulations of salts or alkali. The problems of plant growth on saline and alkali soils are related primarily to the irrigated areas west of the Mississippi River. According to the Bureau of the Census, there were 20,258,191 acres of irrigated land in this region in 1944. This represents a substantial increase since 1939 of 2,435,228 acres, or 13.7 per cent; and additional areas are coming under irrigation as a result of new irrigation projects. For example, about 1,000,000 acres are proposed for development in the Columbia Basin in the Northwest, approximately 500,000 acres of new land are being developed in the Lower Colorado Basin, and the proposed development of the Missouri Basin may involve as much as 4,500,000 acres. Although the soils of some irrigated areas are nonsaline, the accumulation of salt is a continuing threat to crop production on much of the irrigated land. The trend in irrigation agriculture is in the direction of using all the available water including the drainage water and return flow from older irrigated lands. The increased salt content of such water may be expected to increase rather than diminish the salt problem. Owing to the importance of irrigation agriculture in the Western States and the fact that salt accumulation is a major problem in many of the irrigated soils of this region, this review is designed to consider some aspects of plant growth on saline and alkali soils. The classification and composition of saline and alkali soils and their chemical nature have been reviewed by Magistad (1945). Therefore, t.he consideration of these topics will be limited to a brief statement of the characteristics of saline and alkali soils and to definitions of soil terminology as used by the authors. Four major segments of the plant aspects of the problem will be reviewed: (a) the physiological basis of salt and alkali tolerance, (b) how saline and alkali soils affect plant growth, (c) salt tolerance as related to the life cycle of the plant, and (d) specificity in salt tolerance.

11. CHARACTERISTICS OF SALINEAND ALEALI SOILS Saline and alkali soils occur for the most part in regions of arid or semi-arid climate and the process of salinization is frequently accelerated by injudicious irrigation and poor drainage. I n arid regions, leaching and transport of soluble salts to the ocean is not as effective or complete as in humid regions. Leaching is usually local and the soluble salts may not be transported far, owing to low rainfall and the high rates of evaporation characteristic of arid climates. On the other hand, water is plentiful during the early development of an irrigation system and there is a tendency to use it in excess. This may accelerate the rise of the water table unless provision is made for adequate drainage, and under such

TABLE I Chemical Composition of Some River Waters Used for Irrigation in Western United States a

River Gila Cobrado Sacramento Arkansas Boise Rio Grande Pecos Sevier Columbia Big Horn

Sampling location

Date sampled

P.p.m.

Eel06b

Ashurst, Ariz. Yuma, Ariz. Tisdale, Calif. Ldunta, Colo. Boise, Idaho Eleph. B., N. Mes. Comstock, Tex. Delta, Utah Wenatchee, Wn. Thermopolis, Wyo.

4-10-32 3-21-43 2-15-47 7-21-44 11-21-38 6- -46 5- -46 10-17-45 11-29-35 7-29-35

1089 755 73 1000 99 494 2292 1634 116 428

1720 1060 94 1210 133 694 3700 2650 151 612

Ca

Mg

3.59 4.79 0.47 7.18 0.81 28 4 7.63 3 30 0.90

1.99 2.11 03 2 3.49 0.34 1.05 6.78 7.50 0.39 1.19

--

3.08

-

Milliequivalents per liter Na €COI

1127 4.06 022 3.47 025 3.00 23.02 1520 0.19 1.96

3.68 2.64 0.73 3.95 0.91 2.67 1.70 4.10 126 2.18

-

“These analyses were made by the U. S. Regional Salinity and Rubidoux Laboratories, Riverside, California. ECxl06 = conductivity expressed in micromhos per centimeter. T =trace.

G:

Na,

c1 9.95 2.05 0.09 0.62 0.05 1.10 2333 14.00 0.07 0.76 -

326 639 0.15 9.80 0.32 320 12.44 8.30 021 3.17

-

%

67.0 37.0 21.0

z

242 17.9 43.5 615 578 12.7 31.4

--

E

%

m

&-

3

m

8

E:

w

i

4

H. E. HAYWARD AND C. H. WADLEIGH

conditions ground water may contribute to the salinization of the soil. This is particularly true if the water applied carries appreciable amounts of dissolved salts as is frequently the case in irrigated areas. Furthermore, loss of drainage water from irrigated areas upstream and the pick-up of saline ground water result in more salt downstream. The range of quality in irrigation waters is shown in Table I which gives t,he parts per million, electrical conductivity, chemical composition and sodium percentage for a number of river waters used for irrigation in western United States. Although many salt problems are man-made, it should be recognized that the occurrence of saline and alkali areas is related fundamentally to changes in climatic conditions, the chemical composition of soil-forming materials in the primary rocks, and to geologic changes that have taken place with time due to deposition, erosion, weathering and other processes (Harris, 1920; Hilgard, 1906; de Sigmond, 1938). There are numerous publications dealing with various aspects of saline and alkali soils, some of which go back before the turn of the century (Burgess, 1928; Gardner, 1945; Goss and Griffin, 1897; Hibbard, 1937; Hilgard, 1886, 1895-1898; Kelley, 1937; Powers, 1946; Tinsley, 1902). Magistad (1945) has reviewed a number of the schemes of classification for saline and alkali soils and has reported the terminology proposed for them. I n view of the differences in the meanings of terms as used in the literature, the U S . Salinity Laboratory (1947) has published a terminology and description of saline and alkali soils. The terms as defined in that publication will be followed in this review and are given below:

Alkali Soil-A soil that contains sufficient exchangeable sodium to int.erfere with the growth of most crop plants, either with or without appreciable quantities of soluble salts. (See Saline-Alkali and NomalineAlkali Soil). Nonsaline-Alkali S o i G A soil which contains sufficient exchangeable sodium to interfere with the growth of most crop plants and does not contain appreciable quantities of soluble salts. The exchangeablesodium-percentage is greater than 15, the conductivity of the saturation extract is less than 4 millimhos per centimeter (at 25°C.) and the pH of the saturated soil usually ranges between 8.5 and 10. Saline-Alkali Soil-A soil containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. The exchangeable-sodium-percentage is greater than 15 and the conductivity of the saturation extract is greater than 4 millimhos per centimeter (at 25°C.). The pH of the saturated soil is usually less than 8.5.

5

PLANT GROWTH ON SALINE AND ALKALI SOILS

Saline Soil-A nonalkali soil containing soluble salts in such quantities that they interefere with the growth of most crop plants. The conductivity of the saturation extract is greater than 4 millimhos per centimeter (at 25"C.), the exchangeable-sodium-percentage is less than 15, and the pH of the saturated soil is usually less than 8.5. Alkalization--A process whereby the exchangeable sodium content of the soil is increased. Salinization-The process of accumulation of salts in the soil. Exchangeable-sodium-percentage-This term indicates the degree of saturation of the soil exchange complex with sodium and is defined as follows: Exchangeable sodium (m.e. per 100 g. soil) x 100 ESP = Cation exchange capacity (m.e. per 100 g. soil) Soluble-sodium-percentage-The proportion of sodium ions in solution in relation to the total cation concentration, defined as follows:

SSP =

Soluble sodium concent.ration (m.e. per liter) Total salt concentration (m.e. per liter)

x 100

This term is used in connection with irrigation waters and soil extracts.

111. PHYSIOLOGICAL BASISOF SALTTOLERANCE Successful agriculture on saline and alkali soils requires the use of crops capable of producing a sat.isfactory yield under moderate intensities of salt or alkali accumulation. The question arises immediately as to what constitutes the physiological capacity of a plant to tolerate salt or alkali. That is, what is salt tolerance and how may it be defined? The salt tolerance of a variety or a species may be evaluated in three ways. Firstly, salt tolerance may be looked upon as the capacity to persist in the presence of increasing degrees of salinity. A given species may make little or no growth a t the higher levels of salt accumulation, but i t does survive. That is, power of survival in increasingly saline soils regardless of growth would be the measure of salt tolerance. This is largely the criterion of the ecologist in evaluating halophytic environments, since the species most capable of persisting in a saline area becomes the climax vegetation of that area. Secondly, salt tolerance may be regarded from the standpoint of productive capacity a t a given level of salinity. For example, a number of varieties of a given crop may be tested in a soil having a certain degree of salinization and the highest yielding variety may be designated as the most salt tolerant. This method of interpretation may give a differen& evaluation of salt tolerance from the previous one, since experience has

6

H. E. HAYWARD AND C. H. WADLEIGH

shown that the capacity to produce well a t moderate levels of salinity does not necessarily imply the ability to persist a t higher levels of salt accumulation. This second criterion is especially useful to the agronomist in comparing the performance of strains and varieties of a given crop. Thirdly, the relative performance of a crop a t a given level of soil salinity as compared to its performance on a comparable nonsaline soil may be used as a criterion of salt tolerance. This method has certain advantages over the previously mentioned concepts in that comparisons between species are more readily evaluated. For example, although preference as to salt tolerance should be given to that variety of alfalfa having the highest production on saline soil regardless of performance in the absence of salinity, one could hardly compare salt tolerance in alfalfa with that in cotton without taking into account the yielding power of these respective crops when growing on comparable nonsaline soils. Evaluating salt tolerance on the basis of relative yield will not necessarily result in the same order of classification as power of survival a t high levels of salinity, but it will provide a more useful basis of appraising agronomic crops to be grown on moderately saline soil. I n variety and strain testing, tshe data on relative yield should be supplemented by data on absolute yield; ie., a strain may have a comparably poor relative yield because of unusual vigor of growth on the nonsaline soil, and yet yield the best of any of the strains a t the given level of salinity. Everything considered, defining salt tolerance on the basis of relative yield to that of the nonsaline condition is to be preferred for general agronomic use. I n discussing the physiological basis for the various degrees of salt tolerance which prevail among crop plants, it may be helpful to consider the characteristics of the natural halophytes. I n a review of this group of plants, Uphof (1941) discusses the physiological characteristics of halophytes, but it is apparent that the specific physiology of these plants is not well known. The early investigators concluded that halophytism was essentially xerophytism, since both halophytes and xerophytes are adapted physiologically or anatomically to a scarcity of water. Anatomical studies, such as those of Chermezon (1910), later revealed that the two groups of plants must be regarded as distinct physiologically. Halophytes tend to have relatively high values for the osmotic pressure of the tissue fluids. Fitting (1911) used an indirect method to measure the osmotic pressure of the cell contents of various species of plants on the North African Desert. The highest osmotic pressures, 100 atmospheres or above, were found in plants growing on dry or highly saline soils. Those growing on moist nonsaline soils had osmotic pressures of 10-20 atm. The osmotic pressure of the various species tended to vary

PLANT GROWTH ON SALINE AND ALKALI SOILS

7

with the physiological scarcity of water in the environment in which the plants were growing. This generalization has been verified by Harris et al. (1916, 1924), Keller (1920) and others. There may be a wide variation in the osmotic pressure of the tissue fluids depending on the environmental stress under which i t is growing. Harris et al. (1924) found variations in the osmotic pressure of the tissue fluids of leaves of Atriplex confertifolia from 31.2 to 153 atm. ; in Allenrolfeu occidentalis from 22.5 to 61.8 atm.; in Sarcobatus vermiculatus from 22.7 to 39.8 atm.; and in Salicornia utahensis from 36.8 to 51.9 atm. 'Much of the variation in osmotic pressure of the tissue fluids was found to be associated with variations in chloride content, but not all of it. Keller (1925) observed that some halophytes may regulate the salt content of their tissue fluids somewhat independently of the salinity of the environment-. Salicornia may contain a lower concentration of sodium chloride than exists in the soil, or i t may accumulate NaCl far above the concentration of the soil, depending on the degree of soil salinity. Iljin (1922, 1932) states that only those plants should be considered halophytes whose protoplasm is resistant to relatively high accumulations of sodium ions in the cell sap. Thus, halophytes may be described as having a t least three attributes which are important to their survival on saline soil; (a) the capacity to develop rather high osmotic pressures of the tissue fluids in counteraction to the increased osmotic pressure of the substrate; (b) the capacity to accumulate considerable quantities of salts in the tissue fluids and to regulate that accumulation; and (c) a protoplasm which is characteristically resistant to the deleterious effects of accumulations of sodium salts in the cell sap. Application of the above criteria to an evaluation of the relative salt tolerance of economic crops is not sharply defined, and the varying physiological responses of different crop plants to saline soils prevent any generalization. Brown and Cooil a t the U.S. Regional Salinity Laboratory found in 1947 that the osmotic pressures of the tissue fluids of alfalfa tops were 12.3, 14.5, 17.9, and 19.9 atm. when grown on artificially salinized soils in which the average osmotic pressures of the soil solutions were 0.9, 4.2, 6.6, and 8.2 atm. respectively. Thus, even though there was but little variation in the net osmotic gradient between soil and plant tops, there were marked reductions in yield. If the yield on the control plot that had 0.9 atm. osmotic pressure in the soil solution be taken as 100 per cent, the yields on the other plots were 62.5, 32.4, and 21.5 per cent respectively. That is, the marked reduction in yield did. not reflect the relative constancy in osmotic gradient. The increase in osmotic pressure of the tissue fluids of the tops of these alfalfa plants could be largely accounted for by the increase in chloride salts in the

8

H. E. HAYWARD AND C. H. WADLEIGH

tissue fluids. Alfalfa is regarded as one of thc more salt tolerant crops, and the theory could he advanced that its salt tolerance is related to thc intake of salt and the resiiltant increase in osmotic pressure of the tissue fluids as the salinity of the soil is increased. Such a theory could not be applied to certain other forage crops. Ayers and Kolisch * determined the osmotic pressure of the expressed sap of seven different leguminous forage plants grown on soil irrigated with water containing 0, 2500, 5000, and 7500 p.p.m. of added salts. Observations on red clover, Trifolium pratense, harvested in July showed osmotic pressures of the expressed sap of 11.5, 20.6, and 23.7 atm. respectively, for the first t.hree treatments. The most saline irrigation water, 7500 p.p.m., killed the plants. By August, the plants irrigated with water containing 5000 p.p.m. of salt were killed, and by September only one or two plants survived that were irrigated with water containing 2500 p.p.m. added salts. All control plants survived but they did not thrive during the hottest part of the summer. Thus, red clover showed very poor salt tolerance, yet the increase in the osmotic pressure of the tissue fluids for a given increase in salinity of the substrate was greater than that observed for alfalfa. This suggests that capacity to adjust internal osmotic pressure with respect to the substrate may be a poor criterion of salt tolerance. It is pertinent to note that for comparable levels of salinization, the expressed sap of red clover contained nearly three times as much chloride as that of alfalfa. It,appears that red clover plants were capable of effecting internal osmotic adjustments to compensate for the external increase in salinity, but the protoplasm of these plants was not sufficiently resistant to the deleterious effects of the ions so accumulated. I n this connection, the observations of Ayers and Kolisch * on two species of trefoil are of interest. The osmotic pressure of the expressed sap of the herbage of birdsfoot trefoil, Lotus corniculatus var. TENNUIFOLIUS, which is a very salt tolerant legume (Ayers, 1948) was 12.0, 16.6, 17.3, and 19.1 atm. respectively for the same qualities of irrigation water used on red clover. Comparable values for big trefoil, Lotus uliginosus, were 10.6, 16.9, 18.4, and 21.9 atm. osmotic pressure. There was a greater internal adjustment in osmotic pressure over a range of soil salinization in big trefoil than in birdsfoot t.refoil, yet the big trefoil showed relatively poor salt tolerance. At a given level of salinity, however, the expressed sap of the herbage of big trefoil contained nearly twice as much chloride as did the birdsfoot trefoil. *This, and subsequent references in which the author’s name is followed by an asterisk, relate to unpublished data obtained at the US. Regional Salinity Laboratory.

PLANT GROWTH ON SALINE AND ALKALI SOlLS

9

Additional evidence available on other economic crops (see below) indicates t.hat the salt tolerance of a given species depends upon three attributes: ( a ) the capacity to increase the osmotic pressure of the tissue fluids to compensate for increases in osmotic pressure of the substrate; (b) the capacit,y to regulate the intake of ions so as to bring about the increase in osmotic pressure and yet avoid an excess accumulation of ions, and (c) the inherent ability of the protoplasm to resist deleterious effects of accumulated ions. These are the same three attributes that were stipulated as essential for halophytism. It is apparent that the main deficiencies of economic crops which lack salt tolerance are the inabi1it.y to regulate adequately the intake of salt and the specific sensitivity of their protoplasm to accumulations of salt within the tissues.

IV. PHYSIOLOGICAL BASISOF ALKALITOLERANCE Very little is known concerning the physiological basis for the tolerance of plants to alkali soils. There appears to be considerable variat*ion among halophytes as to their tolerance to alkali as contrasted with salinity. Hilgard (1906) points out that Allenrolfea occidentalis and Salicornia subterminalis are two of the most salt tolerant halophtes, but their tolerance to “black alkali” (alkali) is relatively poor. On the other hand, Sarcobatus vermiculatus and Sporobolus airoides are also highly salt tolerant, and have a remarkably high tolerance of “black alkali.” I n evaluating tolerance of plants to alkali soils distinction must be made as to whether the soil is (a) high in exchangeable sodium but having a moderate pH, (b) high in exchangeable sodium, but with a pH of 8.5 or above, and (c) high in exchangeable sodium but with a considerable accumulation of titrat,able carbonate. The latter condition represents the status in “black alkali” soils as described by Hilgard (1906). Although concrete evidence is very meager, it may be inferred that tolerance of a species to high percentages of adsorbed sodium is modified by the pH of the soil and the accumulation of soluble carbonate. Breazeale (1927) concluded from his studies, however, that sodium carbonate occurs in “black alkali” soils in insufficient concentration to be toxic. Thus, the infertility of most of these soils must be sought in their poor permeability to water and to other nutritional disturbances. Ratner (1935, 1944) presents evidence that plant growth is inhibited on high-sodium soils owing to availability of calcium. Hence, tolerance to soil alkali may involve the capacity by the plant to secure an adequate supply of calcium under conditions of relatively low availability. Bower and Wadleigh (1948) studied the influence of various levels of exchangeable sodium upon growt,h and cationic accumulation by dwarf red kidney beans, garden beets and Rhodes and Dallis grasses under controlled cul-

10

H. E. HAYWARD AND C. 13. WADLEIGH

tural conditions in the greenhouse. The culture media consisted of a mixture of sand and synthetic cation- and anion-exchange resins (“Amberlites”) containing the desired amounts of various cations and anions in adsorbed form. Adsorbed K, H2P04,NOs and SOr were supplied in constant amounts to all cultures, the potassium making up 10 per cent of the cation exchange capacity. Six levels of exchangeable sodium, wiz., 0, 15, 30, 45, 60, and 75 per cent of the cation exchange capacity, constituted the treatments. The remainder of the cation exchange capacity was satisfied by calcium and magnesium, the Ca:Mg ratio being 3 : l . The p H value of all cultures was approximately 6.5. The tolerance of the different species to the presence of exchangeable sodium in the substrate varied greatly. Beans were found to be especially sodium-sensitive. Growth of this species was markedly decreased a t exchangeable-sodium-percentages as low as 15 and almost completely inhibited a t the three highest levels of sodium employed. I n sharp contrast with the data for beans, Rhodes grass and garden beets were found to be very sodium-tolerant. Significant reductions in the growth of these species occurred only a t the highest level of sodium. The growth of Dallis grass was not significantly lowered a t exchangeable-sodium-percentages of 30 or less but a t the higher sodium levels practically no growth was obtained. The Ca, Mg, K, and Na contents of the roots and tops of each species were determined after harvest. Accumulation of Ca, Mg, and K by the plants as a whole tended to decrease and that of sodium to increase progressively as higher proportions of exchangeable sodium were supplied. The magnitude of the decreases in Ca, Mg, and K accumulation and the extent of sodium accumulations varied greatly among the species studied and between the roots and top parts of the plant. These observations suggest the possibility that the species that are more tolerant to high levels of exchangeable sodium are the ones which normally take in considerable amounts of sodium, whereas the more sensitive species are the ones which normally tend to exclude sodium.

V. How SALINEAND ALKALISOILSAFFECTPLANT GROWTH Saline soils may affect plant. growth in two distinct ways: (a) the increased osmotic pressure of the soil solution effects an accompanying decrease in the physiological availability of water to the plant; and (b) t.he concentrated soil solution may be conducive to the accumulation of toxic quantities of various ions within the plant. Alkali soils may possess three attributes, any one of which may seriously inhibit or entirely prevent plant growth: (a) the relatively high percentage of adsorbed alkali cations on the exchange complex of these soils may effectively depress

PLANT GROWTH ON SALINE AND ALKALI SOILS

11

the availability of calcium and magnesium; (b) the activity of thc hydroxyl ion may be sufficiently high to be toxic per se to the plant; and (c) an accumulation of adsorbed Na on the exchange complex may have a dispersive effect on the soil, and thereby bring about a “puddled” condition which may seriously curtail permeability to water and air. 1. Saline Soils

Most evidence indicates that accumulations of neutral salts in the substrate inhibit plant growth primarily as a consequence of the increase in osmotic pressure of the soil solution and the accompanying decrease in the physiological availability of water. Magistad et al. (1943) studied the growth response of numerous crops in sand cultures in which relatively large quantities of chloride and sulfate salts were added to a control nutrient solution. Growth inhibition accompanying increasing concentrations of added salts was virtually linear with increase in osmotic pressure, and was largely independent of whether the added salts were chlorides or sulfates. The slope of the negative regressions of yield on osmotic pressure of the substrate varied with the salt tolerance of a given crop. The experiment was carried on under three different climatic conditions, and it was found that the slope of the regressions of yield on osmotic pressure for a given crop varied with climate. Gauch and Wadleigh (1944) studied the growth response of beans to increasing concentrations of NaC1, CaC12, Na2S04,MgC12, and MgS04 added to a control nutrient solution. Growth depression was linear with respect to the osmotic pressure of the substrate and independent of whether a given level of osmotic pressure was developed by NaCl, CaC12, or Na2S04. Magnesium salts had a toxic effect in addition to that which might be attributed to osmotic pressure. Hayward and Spurr (1943) attached potometers to corn roots and measured the rate of entry of water into the roots as conditioned by the osmotic pressure of the substrate. They found that for a given location on the root, the rate of entry was inversely proportional to the osmotic pressure of the substrate and virtually independent of whether the increased osmotic pressure was developed by NaCl, CaC12,Na2S04,sucrose, or mannitol. Entry of water ceased when the osmotic pressure of the substrate was maintained a t 6.8 atm.; in fact, a small outward movement of water was recorded. Significantly, an osmotic pressure of the tissue fluids of 5.7 atm. was recorded for roots comparable to the ones studied potometrically. Roots which were permitted to become at least partially adjusted to a given saline substrate had a higher rate of entry of water than comparable roots which were not subjected to a preconditioning treatment prior to the observation period (Hayward and Spurr, 1943).

12

H. E. HAYWARD AND C. H. WADLEIGH

Eaton (1941) and Long (1943) using divided root systems have also shown that the rate of entry of water into roots is inversely proportional to the physiological availability of the water as mensiired by the osmotic pressure of the nutrient solution. The evaluation of plant response to salinized sand or water cultures behavior on is relatively simple as compared to the appraisal of growth saliniaed soils. The osmotic pressure of the artificial substrate may be controlled r a t h e r precisely, but such control is not possible in a salinized soil. The osmotic pressure of the soil solution at a given salt content of the soil will vary inversely with changes in the moisture c o n t e n t of the soil. That is, the normal fluctuation in soil moisture content between rains or irrigations is a c c o m p a n i e d by inverse fluctuations in osmotic pressure of the soil solution. Also, water cannot move into or through a soil without carrying solute PERCENT SOIL MOISTURE (DRY BASIS) with it. Consequently, Fig. 1. Relationship between soil moisture stress and marked variations in moisture percentage with the salt content at different Q the salt contentof the values in a sample of Panoche loam. soil may occur within the root zone as a result of water movement (Wadleigh and Fireman, 1948). Furt,her, the withholding of water from the plant through surface force action by the soil varies with the moisture content of the soil, and the effect of this retentive force is theoretically additive to that of physiological unavailability of water induced by the osmotic pressure of the soil solution. Wadleigh (1946) has discussed the complexities cont.ributed by these variables in determining the relationship between salt content of soil and plant response.

PLANT GROWTH ON SALINE AND ALKALI SOILS

13

This problem may be illustrated by reference to Fig. 1 showing the moisture tension curve of a sample of Panoche loam together with the effect of increasing degrees of salinization upon the total soil moisture stress as conditioned by t.he moisture content of the soil (Wadleigh, 1946). The level of salinity in this instance is measured by an arbitrarily chosen “Q value” which specifies the osmotic pressure of the soil solution. The “total soil moisture stress” is defined as the summation of the osmotic pressure of t,he soil solution and the soil moisture tension expressed in atmospheres. The most useful concept of soil moisture from the agronomic standpoint, is the “available range” as delimited by “field capacity” and “permanent wilting percentage” (Veihmeyer and Hendrickson, 1927). The soil moisture tension a t field capacity is evident.ly somewhere in the neighborhood of 1/10 to 1/3 atm. The moisture retained by a soil in equilibrium with a displacing force of 15 atm. has been found to approximate the permanent wilting percentage for many soils (Richards and Weaver, 1943). Thus, the curve on the left. in Fig. 1 shows the change in moisture tension between the field capacity and the permanent wilting percentage of this sample of Panoche loam. The hyperbolic nature of this curve is a prime consideration in the evaluation of plant responses to variations in soil moisture in terms of the energy status of the moisture. At the higher levels of soil moisture within the “available range,” there is little change in the energy Bf retention over a considerable range in moist.ure content, whereas a t moisture levels just above the wilting percentage, there is a marked change in surface force action with little change in moisture content. This hyperbolic relationship is common to most soils and part.ially explains the observation that for all practical purposes under field conditions in the nonsaline soil, the soil moisture between field capacity and permanent wilting percentage is “equally available” to the plant (Conrad and Veihmeyer, 1929; Hendrickson and Veihmeyer, 1929, 1942; and Veihmeyer, 1927). Certainly, moisture withheld from the plant b y a force of 15 atm. is not. as readily available as that retained by a forcc of only 1/3 atm.; but, the hyperbolic relationship found for most soils indicates that most of the available water is absorbed from the soil before the moisture tension reaches 2 or 3 atm. The remaining curves in Fig. 1 show how increasing concentrations of salt in the soil affect the relationship between soil moisture stress and moisture content. That is, the soil moisture stress may approach or even exceecl a value of 15 atm. a t tlie moisture content of field capacity. Richards and Weaver (1944) indicated that growth of most plants ceases

14

H. E. HAYWARD AND C. H. WADLEIGH

when the moisture tension reaches about 15 atm., and Wadleigh and Gauch (1948) found that leaf elongation of cotton stopped when the total soil moisture stress in a saline soil reached about 15 atm. There is evidence, however, that different species of plants vary considerably as to the level of soil moisture stsress a t which symptoms of marked water deficit will be in evidence, Wadleigh et al. (1947) grew bean, corn, alfalfa, and cotton plants in containers of soil, 1 foot square and 36 inches deep, varying in added salt content from none in the surface 6 inches to 0.25 per cent a t the bottom. Observations on these soil columns when the plants of each species were showing marked moisture stress revealed that as the salt content of the soil strata increased, the roots of the various species showed a corresponding decrease in their ability to remove water. Comparable cultures of nonsaline soil showed that roots of all species were normally capable of penetrating the deepest layer in the culture and removing all available water. I n the salinized cultures, water was removed from each layer to such n degree that final osmotic pressures of all layers in the soil column were nearly uniform. These critical osmotic pressures of the soil solution were found to be 7 to 8 atm. for beans; 10.5 to 11.5 atm. for corn; 12 to 13 atm. for alfalfa; and 16 to 17 atm. for cotton. Wadleigh and Fireman (1948) found a comparable inverse relationship in the patterns of salt distribution and water removal in the root zone of furrow-irrigated cotton. On the basis of the preceding st.atements, it is evident that plant growth on saline soil, as conditioned by water relations, involves an integration of the following variables affecting moisture availability in the root zone: (a) variation in salt distribution within the soil mass and its consequent effect on the variation in the osmotic pressure of the soil solution a t a given moisture content; (b) variation in osmotic pressure in relation to change in moisture content; (c) variation in moisture tension in relation to moisture content; (d) variation in moisture content within the soil mass a t a given time; and (e) variation in total water content of the soil in the root zone with time. A mathematical method (Wadleigh, 1946) has been developed to integrate these variables and permit. the derivation of the average moisture stress affecting the plant over an extended period of time. It has been found that vegetative growth of beans (Wadleigh and Ayers, 1945) and guayule (Wadleigh et al., 1946) is rather closely related to the average moisture stress if other factors are not limiting t o growth. The daily rate a t which cotton leaves enlarge has also been found to be correlated with the intensity of the soil moisture stress (Wadleigh and Gaucli, 1948). I n summary, one of the main effects of moderate levels of soil salinity

PLANT GROWTH ON SALINE AND ALKALI SOILS

15

is that of limiting water supply to the plant and thereby inducing those modifications in plant behavior normally associated with water deficits in the tissues. Obviously, a soil may become sufficiently saline to prevent even the growth of halophytes, just as it may become too dry to support growth of xerophytes. Depending on the species, each of the various components that may be present in saline solutions may have some specific toxic effect on the plant over and above that which may be accounted for on the basis of the osmotic pressure of the soil solution. The ions which may accumulate in saline soils are: N a + , C a + + , Mg++, K+, C1-, SOa=, HC03-, and NOs-. a. Sodium. There is relatively little evidence that indicates positively the specific toxicity of the sodium ion to plants growing in saline soils. Many species tend to exclude sodium (Collander, 1941; Gauch and Wadleigh, 1945; Hayward e t al., 1946; Wallace et al., 1948); and specific toxic effects may arise from such exclusion of sodium along with accumulation of accompanying anions from the substrate (Hayward, 1946). Such instances should not be classed as sodium toxicity. Lilleland e t al. (1945) found that a tip-burn condition on almond leaves in California was directly related to the sodium content of the leaf. Neither the salinity of the soil nor the sodium content of the soil solution was high, and the condition may have been more indicative of an alkali soil condition rather than salinity. It is possible that accumulation of sodium within the plant may be associated with a depression in the accumulation of the other cations to the extent that their content may be below adequate levels, or an unfavorable cationic balance may be induced. Whether or not such a condition should be designated as sodium toxicity is merely a question of definition. At present, there exists little clearcut evidence that strictly saline soils may induce sodium toxicity per se. b. Calcium. The calcium ion may accumulate to high concentrations in saline soil solutions, and this concentration may be specifically toxic. The specific effect of high concentration of calcium varies with the species. For example, guayule was found to be relatively more tolerant of a saline substrate induced by CaClz than to those induced by other neutral salts (Wadleigh and Gauch, 1944). Masaewa (1936) found that applications of CaClz to soil cultures of flax were more highly toxic than applications of NaC1. The chloride ion accumulated to high levels in the plants on CaClz cultures, so she ascribed the difficulty to chloride toxicity and to an unfavorable Ca/K ratio since Ca was also found to accumulate to rather high levels. Wadleigh and Gauch * observed the same effect on orchard grass, but they also' noted that salinization of the

16

H. E. HAYYWARD AND C. H.

WADLnIGH

soil with Ca(N03)a had the samc effect as CaC12. There was a high accumulation of Ca in orchard grass on the Ca(N03)2 treatments but only a small amount of chloride, hence chloride toxicity was not involved. Ayers * has secured comparable data for tall fescue grass. Lehr (1942) attributes the stimulative effect of sodium on sugar beets to the fact that the sodium effectively counteracts absorption of calcium, thereby preventing the development of what hc calls a “calcium-type-plant.” Such a plant has a bluish-green east and appears to be stunted in growth. c. Magnesium. High accumulations of magnesium in the substrate have been found to be especially toxic to plants over and above any inhibition in growth that might be associated wit.h osmotic pressure (de Sigmond, 1938; Trelease and Trelease, 1931 ; Wadleigh and Gauch, 1944). Magnesium injury may be associated with an inadequate supply of calcium within the tissue (Gauch, 1940). Both Ayers * and Wadleigh and Gauch * have obtained evidence that plants may not show specific symptoms of magnesium toxicity under conditions of accumulated magnesium in the soil solution when calcium ions are also present, a t a relatively high level. d. Potassium. Accumulations of potassium in the soil solut.ian are rather rare, but they may occur. If such an accumulation is partially balanced by calcium no specific inhibitive effects of potassium on plant response are noticeable (Ayers *; Wadleigh and Gauch *) . Cases have been reported in which relatively high levels of potassium have induced characteristic symptoms of iron chlorosis (Walsh and Clarke, 1942) and magnesium deficiency (Boynton and Burrell, 1944). Cations differ markedly in their effect upon the physical properties of the colloidal constituents of protoplasm; and there are pronounced antagonistic effects exhibited between various cation pairs in counteracting the adverse effect of one or both cations upon protoplasmic activity (Chambers et al., 1937; Heilbrunn and Daugherty, 1932; Moyer and Bull, 1935). As Lundegbrdh (1940) points out, the monovalent cations have such a dispersive effect upon protoplasmic colloids that they may induce complete disorganization and deat.h unless balanced by a divalent cation, especially calcium. On the other hand, the divalent ions tend to have a coagulative eff eet and may seriously inhibit permeability of the membranes. There are instances, however, in which the effect of Mg++ on protoplasm is more nearly comparable to K+ and Na+ than to C a + + (Heilbrunn and Daugherty, 1932). Hence the frequently noted mutual antagonism between Ca++ and Mg++. It is apparent that, species differ widely as to the extent to which they may be susceptible to the adverse effect of abnormally wide ratios between various cations. e. Chloride. There has been some tendency to regard chloride toxicity and the adverse effect of soil salinity as synonymous. For many species of

PLANT GROWTH ON SALINE AND ALKALI SOILS

17

plants, chloride salts are no more inhibitive to growth than isosmotic concentrations of sulfate salts (Eaton, 1942; Hayward and Long, 1941; Magistad, et al. 1943). Eaton (1942) found that lemon cuttings, navy beans, and dwarf milo were more sensitive to chloride than to isosmotic concentrations of sulfate when grown in sand cultures. Hayward et al. (1946) noted that peach trees were especially sensitive t o chloride salts, and Harper (1946) that pecans were quite sensitive to chloride. The work of Garner et al. (1930) on tobacco provides a good insight into the mechanism of chloride toxicity. They found that the high level of chloride accumulation in tobacco leaves resulting from heavy fertilization with potassium chloride was associated with a pronounced dissipation of the malic acid content of t.he leaves. Since the organic acids are the major components of the buffer mechanism of plant cells, conditions effecting dissipation of organic acids could have a significant effect on the pH control within the cell and the associated activity of the protoplasm. These investigators also noted that if the tobacco plant receives an excess of chloride the normal amylolytic activity is disturbed and the leaves become gorged with starch. Baslavskaja (1936) observed that accumulation of the chloride ion in potato leaves interferes with the photosynthetic mechanism, i.e., causes a reduction in chlorophyll content; and, consequently, a reduction in total carbohydrate content, even though there was a definite increase in the starch/sugar ratio. Schuphan (1940) concluded from his data that it is not possible to make sweeping conclusions as to the effect of chloride on carbohydrate metabolism since species vary so greatly in their response to the chloride ion. Beneficial effects from added chloride salts have been noted for table beets (Raleigh, 1948), sugar beets (Eaton, 1942; Tottingham, 1919) tomato (Eaton, 1942), and spinach (Schuphan, 1940). f. Sulfate. There are numerous observations on several species of crop plants indicating specific toxicity of high concentrations of the sulfate ion. This has been reported for flax (Hayward and Spurr, 1944), tomato (Eaton, 1942), cotton and orchard grass (Wadleigh and Gauch"), and leek (Schuphan, 1940). But there is a dearth of information to explain why the sulfate ion has an inhibitive effect on the growth of certain species. Harris et al. (1925) found that Egyptian cotton varieties tend to accumulate considerably more chloride in their tissues than do American upland varieties, and that the converse tendency was expressed with respect to the sulfate ion. Cotton variety tests conducted in large outdoor sand cultures by Wadleigh and Gauch * failed to show any clearcut distinction between these two types of cotton in their respective tolerance of high accumulations of sulfate. It is obvious, however, that high concentrations of sulfate in the substrate definitely limit the activity

18

H. E. HAYWARD AND C. H. WADLEIGH

of the calcium ion and thereby condition cationic intake by plants. Analyses of leaves of beans (Gauch and Wadleigh, 1945), peach trees (Hayward et al., 1946), orchard grass and cotton (Wadleigh and Gauch ") showed that the tissues contained an appreciably lower content of calcium and higher contents of sodium and potassium when sulfate was the predominant anion in the substrate as compared to similar cultural conditions in which chloride was the predominant anion. It may be presumed that specific adverse effects of sulfate are related to a disturbance in the optimum cationic balance within the plant, but the evidence is too limited t o warrant a broad generalization. g. Bicarbonate. As pointed out by Heller et al. (1940), the bicarbonate ion is quite toxic to plants. They noted that the presence of accumulations of bicarbonate in the substrate markedly inhibited the intake of calcium by plants. Harley and Lindner (1945) reported that apple orchards in Washington irrigated with water relatively high in bicarbona.te tended to become chlorotic, and that the condition could be partially alleviated by subsequent irrigation with low bicarbonate water. They found also a heavy incrustation of calcium and magnesium carbonates upon the roots of the apple trees in orchards which had been irrigated for some time with water relatively high in calcium and magnesium bicarbonates. They suggested that such a condition could seriously affect the mineral nutrition of the tree, and as a consequence induce the symptoms of chlorosis so frequently observed. Gauch" found evidence of marked specificity in tolerance of the bicarbonate ion. His data showed that the addition of 12 m.e./l. of bicarbonate to a nutrient solution had virtually no effect on the growth of Rhodes grass, whereas the same concentration caused Dallis grass to become seriously chlorotic or to bc killed. Increasing concentrations of bicarbonate have caused pronounced chlorosis and inhibition of growth in beans (Wadleigh and Brown") whereas the same treatments effected a comparably small decrease in growth and little visual evidence of chlorosis on garden beets (Brown and Wadleigh ") Steward and Preston (1941) studied the effect of bicarbonate on ionic absorption and metabolism by potato disks. At a constant pH, increasing the external concentration of potassium bicarbonate depressed botli protein synthesis and bromide accumulation. Indirect evidence indicated that KHC03 also depressed respiration and carbohydrate metabolism. The study with bean plants (Wadleigh and Brown") showed tshat increasing concentrations of bicarbonate induced a pronounced depression in the intake of calcium, and an increase in intake of potassium. I n beets, with their normally low calcium content, intake of calcium was not affected, that of magnesium was depressed markedly, that of potas-

.

PLANT GROWTH ON SALINE AND ALKALI SOILS

19

sium depressed slightly, and intake of sodium increased. These data serve to illustrate that the adverse effect of bicarbonate upon plant response is intimately associated with the specificity of the plant with respect to ionic intake and metabolism. h. Nitrate. The nitrate ion may accumulate to rather high levels in certain naturally saline soils, the condition being characterized as the development of “niter spots” (Stewart and Peterson, 1915). There are several instances known in which high levels of nitrate supply inhibited growth (Chapman and Liebig, 1940; Eaton and Rigler, 1945; Leonard et al., 1948), but it is usually difficult to draw a olearcut distinction between any specific effect of the nitrate ion and concomitant effects induced by the higher osmotic pressure of the substrate or the effect of the complementary cations. Howcver, Headden’s early observation (1912) that nitrate accumulations in the soil contribute to the production of inferior quality sugar beets because of low sugar content has been verified many times. 6. Alkali Soils Kelley (1928) suggested 20 years ago that the presence of a relatively high proportion of sodium on the exchange complex of soils may prevent the plant roots from obtaining an adequate supply of calcium because of “the pronounced avidity of the sodium-exchange complex for calcium.” This effect of adsorbed sodium on the availability of calcium has been observed many times (Gedroix, 1931; Ratner, 1935, 1944; Thorne, 1944; Van Itallie, 1938). For most crop plants, the calcium becomes unavailable when the exchangeable-sodium-percentage approaches 50. Bower and Turk (1946) found that high percentages of exchangeable potassium were just as effective as those of sodium in preventing calcium and magnesium availability to plants. Although alkali soils may actually have an acid reaction, many are found that have a p H of 9 or even 10. Few if any crop plants can thrive under such alkalinity. Arnon and Johnson (1942) grew tomatoes, lettuce and Bermuda grass in nutrient solution in which a range from p H 3 to pH 9 was maintained. Although all three species were tolerant of a wide range in p H value of the subshate, a marked decline in growth was observed a t pH 9. Breazeale and McGeorge (1932) found that the carbon dioxide content of alkaline-calcareous soils was extremely low. They concluded that the low level of COz in such soils was a major factor in the lack of availability of phosphate which also was observed for these soils. That is, their observations indicated that phosphorus unavailability was the major limiting factor in these alkaline-calcareous soils, and

20

13. E. HAYWARD AND C. H. WADLEIGH

that this condition was brought about by thc low level of CO, occurring in a substrate high in alkalinity. Sodium soils readily become dispersed and a dispersed soil is not conducive to vigorous growth of plants. McGeorge and Breazeale (1938) studied the effect of puddled soils on plant growth, and found that plants growing under normal conditions will wilt after the soil is puddled, even though an abundant supply of moisture may be present. The puddled soil has a lowered capacity for gaseous interchange which may result, in oxygen deficiency a t the absorbing surfaces of the roots, and most plants are unable to take in adequate quantities of water a t low oxygen tensions. These investigators also presented evidence that nutrient availability as well as water availability is lowered in a puddled soil. Fireman and Reeve (1948) made a study of alkali soils in Gem County, Idaho. I n this area, barren islands of alkali soil are interspersed among soil areas supporting fairly good crop growth. They made a study of various soil attributes which might show a wide differential between soil supporting good growth and adjacent barren areas. They found that rate of infiltration was the most consistent criterion, the poor infiltration on the barren areas causing the soils to be deficient in moisture most of the time. That is, the very poor structural status of the alkali soil in the barren areas prevented a replenishment of the soil moisture reservoir that is essential for plant growth.

VI. SALTTOLERANCE AS RELATED TO

THE

LIFE CYCLEOF

THE

PLANT

When the life cycle of the plant is considered in relation to salt tolerance, it is desirable to recognize three phases of growth and development since the effect of salt may be different with respect to germination and seedling growth, vegetative growth, and maturation and fruition. 1. Germination

Under saline soil conditions, the first phase, germination and seedling growth, is critical, since the ability of a given variety to germinate and est*ablishthe seedling is frequently the limiting factor in crop producton. There are two ways in which saline soils may effect germination: (a) there may be enough soluble salt in the seed bed to build up the osmot.ic pressure of the soil solution to a point which will retard or prevent intake of necessary water, and (b) certain constituent salts or ions may be toxic to the embryo and seedling. The effect of high osmotic pressure of the soil solution was investigated in early work by Buffum (1896, 1899) who concluded that “the retarding effect of a salt solution on the germination of seeds is in direct proportion to its osmotic pressure when the solutions are strong.’’ Similar

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21

conclusions were reached by Slosson and Buffum (1898) and Stewart (1898). They found that if the osmotic pressure was high enough, no germination occurred ; but it was noted that at a given salt concentration various species of agricultural plants exhibited differential salt tolerance with respect to germination. Stewart (1898) found that the cereals as a group were more tolerant of salt than the legumes, and listed their relative salt tolerance in the following descending order, barley, rye, wheat, oats. His order of tolerance for legumes was peas, red clover, alfalfa, and white clover. Early investigators tested a large number of agricultural crops to determine the limits within which seeds would germinate; but, in many instances, the methods used were not standardized and comparison of data is impossible. Harris (1915) has reviewed the early literature on seed germination which was done chiefly in solution cultures, in many cases using single salts. He points out that. conclusions drawn from such studies “should not be too definitely applied to the action of alkali as it is found in the soil,” citing as an example that “the salts of magnesium when present alone are very toxic, while if added to a normal soil they are no more toxic than a number of other salts.” I n his first germination tests, Harris (1915) used glass tumblers which held about 200 g. of soil. The salt levels ranged from no salt to 10,000 p.p.m., or 1 per cent on a dry weight basis; various single salts and combinations of salts were used and over 18,000 determinations were reported. Like earlier workers, he found that crops varied greatly in their relative resistance to alkali salts and listed crops tested in the following descending order of tolerance, barley, oats, wheat, alfalfa, sugar beets, corn, Canada field peas. Shive (1916) using a sand culture technic and single salts tested the germination of beans and corn at osmotic pressures ranging from 0.5 to 8.0 atm. His data indicate that “retarded germination is directly related to the amount of water absorbed by the seeds, which in turn is dependent upon the concentration of the soil solut.ions.” Rudolfs (1925) tested seeds of white lupine, watermelon, Canada field peas, buckwheat, soybeans, wheat, corn, beans, alfalfa, and dwarf rape. He used presoaked seeds and subsequent germination on beds of filter paper with single salts, NaNOs, Ca NaC1, K2C03, KCl and MgS04, a t osmotic pressures up to 7 atm. Except for some of the weaker solutions, absorption, germination, and root-growth decreased with increase in concentration of the salts. Peas, alfalfa, lupine, buckwheat and watermelon were far less salt tolerant than corn and wheat. It is difficult to evaluate the level of salinity conditioning the germination of seeds under field conditions since the amount of soil moisture

22

H. E.

HAYWARD AND C. H. WADLEIGH

and the salt conccntration adjacent to the seed are continually changing, owing to evaporation, capillary transmission, and rainfall or irrigation. Ayers and Hayward (1948) have reported a method for measuring the effects of. soil salinity on germination which involves moistening and salinizing nonsaline soil so that a specified soil moisture percentage and salinity level are obtained. The moisture content of the soil and the salt content of the extract from the saturated soil are determined on subsamples and these data permit a calculation of the osmotic pressure of the soil solution in the germination culture. Weighed amounts of the preconditioned soil are placed in large culture dishes and planted with a definite number of seeds. The covered cultures are maintained in a constant temperature room (70°F.)to eliminate temperature as a variable and to prevent moisture distillation in the germinators which occurs under fluctuating temperahres. Several salinity levels were set up, ranging from 0.05 to 0.4 per cent sodium chloride on a dry soil basis. The osmotic pressures of the soil solutions, calculated from the electrical conductivity of the saturation extract and the soil moisture content at time of planting, ranged from 0.7 to 25.3 atm. Alfalfa, sugar beets, two varieties of barley, Mexican June corn and red kidney beans were tested. No seeds germinated a t the 0.4 per cent level, but barley, (California Marriout), gave 80 per cent germination a t the 0.3 per cent salt level (20 atm. osmotic pressure). Although alfalfa and sugar beets are regarded as salt tolerant crops, the data indicate that they are relatively sensitive during germination. Alfalfa gave 80 per cent germination with 0.1 per cent added salt (7.3atm. osmotic pressure) and the germination of sugar beets was reduced to 50 per cent a t 5.8 atm. osmotic pressure. Corn, which is less tolerant than sugar beets or alfalfa during later stages of growth, gave satisfactory germination (93 per cent) a t approximately 10 atm. osmotic pressure and red kidney beans, which are very sensitive to salt, germinated slightly better than sugar beets. These data indicate that there is not always a positive correlation between salt tolerance a t germination and during later phases of growth. The differential toxic effects of salts 01: ions in the substrate on germination and the development of the embryo and seedling have been studied by a number of inxestigators. Harris (1915) found the relative toxicity of soluble salts to be in the following descending order: NaC1, CaC12, KCl, MgCL, KN03, Mg(NOd2, NazC03, NazSO, and MgSOr. With respect to antagonism, he concluded that the effect of combined salts was not so great in soils as in solution cultures. Harris and Pittman (1918) in a continuation of the above study compared the relative toxicity of NaCl a t concentrations of 0 to 4,000 p.p.m. and of NazCOs and NazSOl

PLANT GROWTH ON SALINE AND ALKALI SOILS

23

at concentrations up to 10,000 p.p.m. a t moisture levels ranging from 20 to 32 per cent. Up to 1,000 p.p.m., all salts were beneficial, but above 1,500 p.p.m. all salts were increasingly toxic, chloride being most so, sulfate the least, and carbonate halfway between. The alkali carbonates are usually found to be the most toxic salts. Stewart (1898) found Na2S04 less injurious than NaCl and Na2C03 most toxic. Kearney and Harter (1907) tested seedlings of maize, sorghum, oats, cotton and sugar beets in water cultures, using NaC1, MgC12 and MgS04 as single salts, to determine the critical concentrations a t which half of the root tips of seedlings exposed to these concentrations for 24 hours failed to survive when subsequently transferred to water. They found great differences in resistance to magnesium and sodium salts in solution among the eight species tested, maize being most resistant and cotton the least. The presence of CaS04 in excess greatly diminished the toxicity of magnesium and sodium salts, the neutralizing effect being greatest when added to MgS04 cultures and least in combination with Na2C03. Rudolfs (1925) found that presoaking in distilled water retarded germination of all seeds, and noted differential responses to various single salts. All seeds were injured in K2C03 solutions and abnormalities occurred when this salt or MgS04 was used. C a ( N O d 2 had a detrimental effect on germination and root, growth with nearly all varieties of seeds except corn. Uhvits (1946) studied the effect of osmotic pressure on water absorption and germination of alfalfa seeds using concentrations of sodium chloride and mannitol ranging from 1 to 15 atm. osmotic pressure. These tests were made on filter paper in Petri dishes maintained at a constant temperature of 71°F. 2". Other tests were made in sand cultures under greenhouse conditions using sodium chloride a t concentrations of 1 to 12 atm. osmot,ic pressure. She found that germination was virtually inhibited when N a C l solutions of 12 to 15 atm. osmotic pressure were used, and that reduction and retardation of germination were greater on N a C l than on mannitol substrates. The difference in response on the tewo substrates a t isomotic concentrations suggests a toxic effect of N a C l and this assumption is supported 'by data showing the accumulation of chloride in alfalfa seeds after 4 days of treatment. For example, on a dry weight basis the per cent chloride in the seeds increased from 0.04 per cent in t a p water to 1.18 and 1.79 respectively on the 3 and 15 atm. substrates. The data indicate that a t high concentrations, total absorpt.ion values were greater with mannitol than with sodium chloride; consequently if given enough time, relatively high germination rates were obtained with mannitol a t 12 and 15 atm. (71 and 57 per cent, respectively). That high concentrations of sodium chloride are toxic is

*

24

H. E. HAYWARD AND C. H. WADLBIGH

supported further by studies which showed that recovery of seeds transferred from a 12 atm. substrate of sodium chloride to tap water was considerably greater than the recovery of seeds treated for the same length of time on a 15 atm. substrate of NaCl. The percentage of deformed seeds on the sodium chloride substrate a t 15 atm. osmotic pressure was greater than a t 12 atm., and the number of deformed seedlings in all concentrations of sodium chloride was much greater than in the corresponding concentrations of mannitol. The influence of tcmperature as related to the effect of salt on germination should be mentioned. Ulivits (1946) found that. an increase in the mean greenhouse temperature of 5°F. reduced the per cent germination a t all levels of salt treatment, the differences being more pronounced a t the higher salt levels. Ahi and Powers (1938) studied the effect of temperature and other factors affecting salt tolerance using salt grass, alfalfa, sweet clover, strawberry clover and Astmgulus rubyii as test plants. The plants were grown in sand and water cultures; and sea water, fortified with n nutrient solution and adjusted to salt concentrations ranging from 306 to 11,200 p.p.m., was used. I n one study wit.h strawberry clover and alfalfa, temperatures were controlled a t 55", 70" and 90°F. There was a definite decrease in the per cent germination with increase of temperature or salt concentrat,ion. At 90°F. there was practically no germination regardless of salt level; but a t 55"F., 47.7 per cent of the strawberry clover and 38 per cent of the alfalfa seeds germinated. The work of Ogasa (1939) on the effect of sodium chloride solut,ions on soybeans a t high and low temperatures confirms the above findings. He found the limit of concentration of N a C l solutions a t which germination occurred to be 200 m.e./l for high temperature (30°C.) and 300 m.e./l for low temperature (15°C.). To summarize, it is evident that germination is retarded or inhibited by the presence of soluble salts in the soil and that this effect is related primarily to the osmotic pressure of the soil solution. As osmotic pressure increases, rate and per cent germination decrease. There is evidence that certain salts or ions may be toxic to the embryo or seedling if occurring in sufficiently high concentrations. This toxicity may be reflected in reduced germination and is frequently accompanied by abnormalities in the growth and development of the seedling. High temperature is an important consideration ; and, a t isosmotic concentrations of salt, per cent, germination decreases with increase of temperature above optimum levels. The studies reported indicate that species and varieties of plants exhibit varying degrees of salt tolerance with respect to germination and seedling growth, and they emphasize the importance of crop selection on the basis of salt tolerance in areas where salinity is a problem.

PLANT GROWTH ON SALINE AND ALKALI SOILS

25

2. Vegetative Growth and Maturation

Vegetative growth is retarded as the osmotic pressure of the substrate is increased. Buffum (1896) noted that growth is in proportion t o the amount of salts present in the substrate and similar conclusions were reached by Harris (1915), Harris and Pitt.man (1918), Hayward and Spurr (1944), and others. Eaton (1942) in studies on the toxicity of chloride and sulfate salts has pointed out that the growth depression curves showed no evidence of an abrupt point a t which the effect of increasing osmotic pressure became pronounced. Magistad et al. (1943) reported that growth reduction was in most cases linear with increasing osmotic concentration of the substrate, and Gauch and Magistad (1943) in a study of the effect of salt on legumes, found no evidence that there is a given concentration of solution which may be regarded as critical, but, rather there tended to be a linear relationship between growth reduction and increase in salt concentration of the solutions as expressed in atmospheres.” The first effect of increasing concentration of salt on vegetative development is usually a reduction in rate of growth which may not be accompanied by any visible symptoms of injury. As Eaton (1942) has pointed out, this absence of leaf symptoms of diagnostic significance or other pronounced outward abnormalities suggests “that a substantial proportion of the curtailed production of crops in irrigated areas that was attributed to nutritional deficiencies or unfavorable water relations was in fact due to saline conditions customarily regarded as insufficiently high to be a cause of reduced yields.” Under marginal conditions of salinity, and in t.he absence of detectable symptoms of salt injury, it is difficult to recognize salt effects under field conditions. Controlled studies, however, have shown that there may be morphological changes before other symptoms are evident. I n general, the first physiological reaction to increased salt concentration is reduced entry of water into the roots (Hayward and Spurr, 1944; Long, 1943; Rosene, 1941; and Tagawa, 1934). This tends to inhibit meristemat,ic activity and elongation of the root (Hayward and Spurr, 1943). Hayward and Long (1941) have shown that the growth of tomato stems as measured by height, diameter and dry weight was less a t high salt concentrations than at control levels. The smaller diameter of stems was correlated with significant differential reductions in the tissue systems. I n general, the reduction in thc vascular system on the basis of percentage of total area was greater than that of the parenchymatous tissues of the cortex and pith. Cambial act.ivity was inhibited and secondary xylem vessels and fibers were smaller in diameter and propor-

26

H. E. HAYWARD AND C. H. WADLEIGH

tionately thicker walled. Somewhat similar results were observed for flax (Hayward and Spurr, 1944) grown under high concentrations of salt. The cambium was less active, the cells of the secondary xylem were smaller, and the number and diameter of the phloem fibers was less than in the control plants. Various effects of increased salt concentrations on the growth and structure of leaves have been reported, Harter (1908) working with wheat, oats, and barley, found that increasing the salinity of a nonsaline soil to 0.5 per cent soluble salts on a dry weight basis caused significant modifications in leaf structure. The leaves developed a pronounced waxy bloom, a thickened cuticle, and the size of the epidermal cells was decreased. Uphof (1941) in his review on halophytes, points out that such plants show a tendency towards succulence by having thicker leaves and stems, more pronounced palisade parenchyma and smaller intercellular spaces. Lesage (1890) working with three nonhalophytes Pisum sativum, Linum grandiflorum and Lepidium sativum, found that sodium chloride produced thicker leaves, st.rengthened the palisade parenchyma, and reduced the intercellular spaces. Hayward and Long (1941), using osmotic concentrations ranging from 0.5 to 6.0 atm., noted increases in the thickness of tomato leaves of from 9 to 30 per cent a t the 4.5 and 6.0 atm. levels. The increased succulence of leaves was in agreement with results reported by Wuhrmann (1935) who found that the thickness and degree of succulence of leaves of Lepidium sativum and Nicotiana could be modified by the addition of sodium chloride to nutrient solutions. Eaton (1942), on the other hand, found no increase in the succulence of leaves of milo, cotton, tomato, and sugar beets, or in alfalfa plants when the osmotic pressure of the substrate was increased. I n barley, he found succulence decreased with the additions of salt. Recent studies by Bernstein and Ayers * have provided additional information which indicates that with increasing levels of salinity succulence of leaf tissues may be either decreased or increased. Decrease in succulence has been obtained with some cucurbits and with alfalfa and grasses. With some crops, however, succulence increases with salinity. Bean leaves have shown this response to salinity in both field plot and water culture studies. I n other cases, there is little effect of salinity on succulence. Tomato leaves in a field plot experiment showed increased succulence a t low and medium salt levels, but a t high salt levels there was no change in succulence as compared with leaves of the control plants grown in nonsaline plots. Tomatoes have been used in several studies to illustrate the effect of salt on vegetative growth and yield. Eaton (1942) tested the growth and yield of Stone tomatoes on substrates adjusted to .72 (control), 2.5 and

PLANT GROWTH ON SALINE AND ALKALI SOILS

27

6.0 atm. with sodium chloride as the added salt. The relative dry weights of t,he vines excluding fruit were 100, 77 and 27 per cent respectively and those of the fruits were 100, 81, and 4 per cent. Hayward and Long (1943) obtained comparable results with Marglobe tomatoes using NaCl and NazSOl at osmotic pressures ranging from 1.6 to 7.7 atm. Their work indicated that the osmotic pressure of t.he substrate was more significant than the specific effect of the C1- and SO4= ions in relation to vegetative responses and production of fruit. Other crops where tshis generalization appears to hold are flax (Hayward and Spurr, 1944), beans (Ayers et al., 1943) and peaches (Hayward et al., 1946). Visible symptoms of salt injury may occur if the salt concentration of t,he substrate is high. When chlorides are present, characteristic symptoms are incipient chlorosis accompanied by a drying and browning of the apex of the leaf blade. The initial tip burn is usually followed by progressive involvement of additional tissue extending along the margins of the blade until one-half to two-thirds, or in some cases the entire surface, becomes brown and necrotic. In severe cases, abscission of the leaves occurs, dieback of the terminal axis or small branches is evident, and death may ensue. These symptoms have been described by Hayward et al. (1946) for peaches, and Harper (1946) reports chloride injury for a number of trees including pecan, elm, and ash, the tip burn and marginal browning being most pronounced on the former. With one exception, scorched leaves contained in excess of .88 per cent chloride in the ash content-. Hayward and Blair (1942) observed moderate to severe chlorosis and tip burn on leaves of Valencia orange seedlings on a substrate containing 50 m.e./l. chloride and very severe symptoms when 100 m.e./l. of mixed chlorides were added. Hayward, Cooil and Brown * studied the effects of NaC1, CaCla and mixed chlorides on Marsh grapefruit grown in sand cultures, with solutions adjusted to 0.5 (control), 2.5 and 3.5 atm. osmotic pressure. Incipient chlorosis and marginal and tip burn were evident after two months, abscission of leaves was severe at 3.5 atm. osmotic pressure, and at the end of 10 months the trees had lost approximately one-third of their leaves. A t the highest level of salt concentration, vegetative growth was reduced t o 45 per cent of the controls with mixed chlorides, 34 per cent with N a C l and to 22 per cent with CaCl2. Kelley and Thomas (1920) studied the effects of excessive concentrations of salt in irrigation water on citrus trees grown under orchard conditions. They report that an excess of chlorides causes yellowing of the margins and tip burn followed by heavy shedding of leaves on lemon trees. With orange trees, mottle leaf was one of the first symptoms, sometimes accompanied by browning and curling of leaves and dieback of young, tender shoots.

28

H. E. HAYWARD AND C. H. WADLEIGH

Eaton (1942) observed somc yellowing in lemon leaves, with occasional tip burn and subsequent abscission a t 50 m.e./l. chloride and noted marked bronzing of leaves in an orchard where high chloride water was used for irrigateion. Barley, milo and navy bean leaves wcre burncd by chloride and sulfate salts, but no injury was observed on alfalfa, cotton, tomato, and beet plants (Eaton, 1942). Gauch and Wadleigh (1944) report darker green color in the younger trifoliate leaves of bean plants tested on N a C l and Na2S04 substrates and a pronounced marginal and tip burn a t high salt concentrations (3.5 atm.) . Retzer and Mogen (1946) found that guayule usually was killed when salt concentrations were 0.6 per cent in either the first or second foot of soil and observed considerable amount of tip burn in some fields, especially a t Coalinga, California. Tip burn is not always associated with accumulations of the chloride ion. Lilleland et al. (1945) have shown that sodium may cause tip burn in almond trees and they describe symptoms which are very similar to those noted above for chloride injury. They found moderate tip burn where sodium in the leaf (moisture-free basis) ranged from 1.30 to 2.10 per cent. The scorch became worse as the season advanced and was correlated with increasing accumulation of sodium. Although high osmotic pressure of the substrate or soil solution usually results in depression of both vegetative growth and yield, exceptions have been noted. Eaton (1942) found that the vegetative growth of cotton plants was reduced relatively more by increased concentrations of chloride and sulfate salts than was yield of seed cotton, but the differences were not great with chloride salts. Recent studies by Fireman and Wadleigh * indicate that there may be differences in the vegetative and fruiting responses of cotton t o increased levels of salinity. These differences may be related to variations in water regime, t o climatic factors, or to the variety of cotton. Acala cotton was grown in salinized plots adjusted to 0.1 and 0.2 per cent salt on a dry weight basis. The “dry” plots received 20 surface inches of water in five irrigations, while thc “wet” plots received 32 inches in sixteen irrigations. Vegetative growth was better under the “wet” regime than on the “dry” plots a t all salt levels, but in both series vegetative growth was reduced with increase in salt concentration. Yield of seed cot,ton was inversely related to the salt added in all cases except the “wet plot” with 0.1 per cent added salt. I n a second study with Acala cotton this exception did not occur. Ayers and Wadleigh * tested eight varieties of western barley in salinized plots irrigated with water to which 3,000, 6,000, and 9,000 p.p.m. of salt were added. The salts were supplied as a 50-50 mixture of sodium and calcium chlorides. On the average, the levels of salinization had no effect

PLANT GROWTH ON SALINE AND ALKALI SOILS

29

on the yield of grain, but thc lowest yields of straw and the highest grain-draw ratios were found in the highest, salt t,reatment. The ratio of grain to straw for eight varieties tested was .50 in the control.plots and .72 under the 9,000 p.p.m. salt treatment.

VII. SPECIFICITY IN SALTTOLERANCE Early investigations indicated that various species and varieties of crop plants exhibit differential salt tolerances when tested under uniform conditions of salinity. Before the turn of the century, Buffum (1896) pointed out that “the amount of alkali in the soil that is injurious to crops depends upon its composition, the character of the soil, whether the salts are upon the surface and the kind of crop grown,” Loughridge (1901) .investigated the salt tolerance of fruit trees, truck, cereal and forage crops grown under field conditions and reported his findings in terms of the highest amounts of alkali in which the plants were unaffected The relative tolerance was expressed as total alkali in lbs. per acre in 4 feet depth. H e also presented values showing the maximum tolerance for each of the three salts commonly found, Na2S04, Na2COs and NaCI. Hilgard (1906) in commenting on this work, points out that “it is certain that the tolerance-figures will be quite different in presence of other salts, from those that would be obtained for each salt separately; or for the calculated mean of such separate determinations, proportionately prorated.” Harris (1920) recognized the difficulty of evaluating the relative salt tolerance on the single basis of the quantity of salt that various crops have been found to endure safely and pointed out that “soil, moisture, climate, and perhaps other things will often change the relative tolerance of the different crops to some extent so that slight differences in tolerance mean little or nothing.” Kearney and Scofield (1936) have reported on the choice of crops for saline land using as a basis the percentage of soluble salts by weight to the tot.al dry weight of the depth of soil reached by the roots. They set up the following classes with respect t o degree of salinity: excessive, more than 1.5 per cent; very strong, 1.0 to 1.5; strong, 0.8 to 1.0; medium strong, 0.6 to 0.8; medium 0.4 to 0.6; weak, 0.1 to 0.4; and negligible, less than 0.1. They recognized that a classification on a dry soil basis ignores the soil-moisture relation by pointing out that “it is the concentration of the soil solution and not the total quantity of salts present in the soil which determines the effect on plant growth,” and assumed that “the soil contains a degree of moisture favorable for the growth of the crop in question.” Their studies dealt with the relative salt tolerance of crops under the following classes: forage plants, root crops, cereals, fiber plants, garden vegetables and truck crops, and trees and shrubs including fruit

30

H. E. HAYWARD AND C. H. WADLEIGH

trees and ornamental and shade trees. They report a wide variation in salt tolerance among various members of these groups of crops. The U. S. Regional Salinity Laboratory has included the problem of salt tolerance as a major segment of its research program and a number of lists of salt tolerant plants have been published (Hayward and Magistad, 1946; Magistad and Christiansen, 1944; U. S. Regional Salinity Laboratory, 1947). I n the most recent publication (U.S. Regional Salinity Laboratory, 1947), fruit crops, field and truck crops, and forage crops are classed on the basis of good, moderate, and poor salt tolerance. The electrical conductivity of the extract of saturated soil is regarded as the most suitable measurement for appraising soil salinity and its relation to crop condition and plant growth. On this basis, it would be expected that an electrical conductivity of the saturation extract equal to 4 millimhos/cm. (0.1 per cent salt in a medium-textured soil) may cause significant reduction in growth for plants listed as having poor salt tolerance. Moderately tolerant crops may do well where the conductivity does not exceed 8 millimhos/cm., crop growth is restricted if the conductivity is between 8 and 15 millimhos/cm., and no crops and few species of native halophytes can do well a t conductivities in excess of that value. Forage plants, grasses and legumes, as a rule exhibit the highest degree of salt tolerance on saline lands (Harris, 1920; Kearney and Scofield, 1936; Magistad and Christiansen, 1944), but there are marked specific differences in this regard. The grasses are more salt resistant than the legumes, outstanding species being alkali sacaton (Sporobolus uiroides) , salt grass (Distichlissp&xzta),Nut,tall alkali grass (Puccinellia Nuttalliana) ,Bermuda grass (CynodonDactylon),Rhodes grass (Chloris gayuna), and western wheatgrass (Agropyron Smithii) . A number of other grasses have been reported as having a moderate to high degree of salt tolerance depending upon other factors. Studies a t Riverside, California, have indicated that the salt tolerance of some grasses is seriously affected by high soil temperatures, i.e., reed canary grass, perennial ryegrass, meadow fescue and orchard grass (Wadleigh and Gauch *). Among the leguminous forage plants, alfalfa, white and yellow sweet clovers, birdsfoot trefoil, strawberry clover, and hubam clover are moderately salt tolerant (Harris, 1920; Kearney and Scofield, 1936; U. s. Regional Salinity Laboratory, 1947). Ayers (1948) has found that birdsfoot trefoil (Lotus cmiculatus var. TENNUIFOLIUS) has a high salt tolerance and can withstand high summer temperatures. I n salinized plots irrigated with water containing 5,000 p.p.m. added salts, the relative yields, expressed as per cent of the yield on the nonsaline control plots, were: birdsfoot trefoil, 43.6; California Common alfalfa, 40.4; and buf-

PLANT GROWTH ON SALINE AND ALKALI SOILS

31

falo alfalfa, 32.0. Birdsfoot trefoil gave better yields a t medium and high salt levels (5,000 and 7,500 p.p.m .added salts) than big trefoil, alsike, red, ladino, and strawberry clovers, in that order. The data regarding the salt tolerance of strawberry clover appear to be conflicting. Kearney and Scofield (1936) regarded it as outstanding and reported satisfactory growth where the salt content of the surface soil was more than 2 per cent, and Ahi and Powers (1938) rated it as the most promising resistant legume followed by sweet clover and alfalfa. On the other hand, Gauch and Magistad (1943) reported that on an actual yield basis, alfalfa and ladino clover produced 1.3 and 1.9 times as much forage, respectively, as strawberry clover, and Ayers (1948) found that it failed to make an appreciable growth a t a high level of salinity (irrigated with water containing 7,500 p.p.m. added salt). It seems probable that climatic and soil moisture may account for t.he differences noted above. Strawberry clover appears to be well adapted to wet, saline pastures and can tolerate high water tables and the cooler Summer temperatures in the Northwest. The study of varieties of strawberry clover by Gauch and Magistad (1943) illustrates the possibility of differences in salt tolerance within a species. Five strains were tested at osmotic pressure of 0.5, 2.5, 3.5, and 4.5 atm. On the basis of actual yields, significant differences in salt tolerance were observed with respect to the strains from various sections of western United States. The salt tolerance of alfalfa, one of the leading forage crops in the Western States, has been studied extensively (Ahi and Powers, 1938; Eaton, 1942; Gauch et al., 1943; Harris, 1920; Kearney, 1911; Kearney and Cameron, 1902; Kearney and Scofield, 1936; and Magistad e t al., 1943). It has been noted that alfalfa exhibits differences in salt tolerance during its life cycle being more tolerant with age (Harris, 1920; Kearney and Scofield, 1936), but there are few data on the relative tolerance of varieties. Cooil and Brown * a t the U. S. Salinity Laboratory have tested several varieties using sand culture and soil plot technics. Six varieties, California Common. Arizona Chilean, Ranger, Hegazi, Demnat and Tunisia, were tested in large sand cult.ure tanks at salt concentrations from 0.5 to 6.5 atmowpheres. Based on relative yields, it was evident that there were significant varietal differences in salt tolerance. At the highest salt concentration (6.5 atm.), California Common yielded 77 per cent of t.he control and Arizona Chilean 72 per cent, whereas Ranger produced only 38 and Demnat 36 per cent. On an absolute yield basis, California Common, Hegazi and Arizona Chilean were superior to other varieties tested. Tests with California Common and Hegazi in salinized soil plots gave

32

11. E. HAYWARD AND C. H. WADLEIGH

consistently higher yields for the latter although relative yields on the saliniaed plots were higher for California Common. The cereals are moderately tolerant to salt, and some reports indicate that they are more tolerant as forage than as grain crops, since the grain may be inferior when grown under conditions of high salinity (Kearney and Scofield, 1936; Magistad and Christiansen, 1944). Although reports differ in regard to the relative tolerance of cereals; barley, rye, wheat and oats for hay, and barley, rye, oats, rice, wheat and corn for grain have been listed in that order of tolerance (U. S. Regional Salinity Laboratory, 1947). There are few data on the salt tolerance of variet,ies of cereals. Harris and Pittman (1919) tested the salt tolerance of a number of varieties of oats, wheat, barley and corn, but their experiments were primarily to determine relative germination and were terminated a t the end of a 3-weeks growing period. Loughridge (1901) compared Russian wheats and gluten wheat in field plots a t Tulare, California, and found that the latter made good growth in soil containing 0.15 per cent salt.. Hayward and Uhvits * found White Federation 38 wheat, to be moderately salt tolerant and fair yields were obtained in sand culture studies on a saline substrate containing 4 atm. (5,612 p.p.m.) of sodium chloride. At this concentration, however, t.liere was a reduction of approximately 25 per cent in growth and yield of grain and there was some evidence that the grain did not fill well when grown on the saline substrates. Wasatch wheat was used by Reeve e t al. (1948) in leaching studies at Delta, Utah. They found that yields varied inversely with the residual salinity of the soil, and that a t low salt levels slight reductions in salt, content resulted in large increases in yield. For example, a t one site, (A), reduction in the conductivity of the saturation extract from 40 to 6 millimhos per cm. resulted in an increase in yield from 0.7 to 42.6 bushels per acre. Recent studies by Ayers and Wadleigh * with eight varieties of barley indicate a high degree of salt tolerance, and relative yields were not reduced when the plots were irrigated with water containing 9,000 p.p.m. added salt. The average conductivity of the saturated soil extracts for the 0-16 inch depth a t this salinity level was 8.4 millimhos per cm. as compared with 2.9 millimhos per cm. in the control plots. Corn is t,he most salt-sensitive cereal and may not produce a satisfactory crop even on slightly saline soils (Harris, 1920; Kearney and Scofield, 1936). Wadleigh et al. (1947) found Mexican June corn to be less salt tolerant than alfalfa and more so than beans. Only a few roots penetrated a soil layer containing 0.2 per cent salt and none were found in the layer where 0.25 per cent salt was added.

PLANT GROWTH ON SALINE AND ALKALI SOILS

33

Root and vegetable crops have a wide variation in salt tolerance (Harris, 1920; U. S. Regional Salinity Laboratory, 1947). Sugar beets, table beets, tomato, and asparagus, have shown good to moderate salt tolerance, but most vegetable crops tested do not appear to be able to withstand conditions of high salinity (Magistad and Christiansen, 1944). Beans are very sensitive to salt; and carrots ,onions, lettuce and many cucurbits have poor to moderate tolerance. Bernstein and Ayers * have found that lettuce is least affected by given levels of salinity, cantaloupes are intermediate and beans the most sensitive of these 3 crops. I n each instance, several varieties were tested and varietal differences were noted in some groups. With lettuce the relative salt tolerance of 6 variet
34

H. 1. HAYWARD AND C. H. WADLEIGH

technic and single salts (NaC1, Na2S04 and CaCI2) adjusted to 4.5 atm. osmotic pressure. All varieties were also grown in well-irrigated and well-fertilized soil. A summary of yields for 12 varieties over a 3-year period indicates a marked variation in relative salt tolerance. AmericanEgyptian varieties (SXP, Amsak and Sakel) , Acala 1517 and Acala P-18 have consistently shown a good salt tolerance, Stoneville strains made very good relative yields on saline cultures but always showed marked symptoms of salt toxicity. Other strains, Coker 100-6, Deltapine 14 and Delfos 9252 have not produced well under control conditions a t Riverside, California, and do not. exhibit any marked degree of salt tolerance. Flax is moderately salt tolerant (Hayward and Spurr, 1944; Kearney, 1911). Kearney and Scofield (1936) report good crops where the salinity does not exceed 0.4 per cent. Hayward and Spurr (1944) in tests of Punjab flax in sand cultures under greenhouse conditions found it to be moderately salt tolerant. The tests were made a t osmotic concentrations of 1.5 to 4.5 atm. using NaCl, CaClz and Na2S04 as single salts added to a nutrient substrate. At high concentrations of salt (3.5 and 4.5 atm.) relative yields of seed were reduced 25 to 62 per cent and no mature seeds were produced at the highest concentration of sodium sulfate. Information on the specific salt tolerance of trees and shrubs is meager. The date palm is perhaps one of the most salt tolerant of all cultivated plants (Harris, 1920). Magistad and Reitemeier (1943) report fair growth of dates in the Imperial Valley where the soil solution a t a moisture content near the wilting percentage contained 15,000 p.p.m. of salts or an osmotic concentration of 7 atm. Figs, grapes and olives are moderately salt tolerant, while citrus fruits, apples and pears, and drupaceous fruits are generally regarded as low in salt tolerance (Magistad and Christiansen, 1944; U. S. Regional Salinity Laboratory, 1947). Thrifty growth of Persian varieties of grapes has been observed in soils containing as much as 0.28 per cent salt (Loughridge, 1901). Allison and Christiansen i t reported good growth of Thompson seedless grapes in the Coachella Valley, California, when the conductivity of the saturation extract was as high as 3.5 millimhos/cm. in the second foot of soil. Hayward et al. (1946) report that the Elberta peach is sensitive to moderate concentrations of salt and state that over a period of years yields can be expected to decline if t,he salt concentration exceeds two atmospheres. The Love11 rootstock appeared to be more suitable to moderately saline conditions than the Shalil. Lilleland et al. (1945) have noted differential toxicity to sodium in almonds. The Texas variety appears to be more susceptible than Nonpareil, Ne Plus Ultra and others. Species of citrus are quite sensitive to salt, especially to sodium chloride (Kelley and

PLANT GROWTH ON SALINE AND ALKALI SOILS

35

Thomas, 1920; Loughridge, 1901). Lemons are the most sensitive and oranges occupy a position between lemons and grapefruit. As Eaton (1942) has pointed out, it is difficult and frequently misleading to evaluate the salt tolerance of a species on the basis of appearance. This is especially true of trees and shrubs. There is good evidence that the effect of saline substrates may be cumulative and that over a period of years even low concentrations of salt may result in a slow but progressive decline of a tree crop (Hayward e t al., 1946) I n summary, two considerations may be stressed with respect to specificity in salt tolerance. First, it is evident that there are marked differences in relative salt tolerance among various genera, species and varieties of agricultural crops. This fact emphasizes the importance of proper selection of crops for use on lands that are marginal because of salinity. It suggests also the necessity of furtsher testing and srreening for salt tolerance on a genetic basis. The second point is that the various criteria used for appraising salt tolerance may be inadequate if soil moisture status and other factors are not taken into account.

REFERENCE^ Ahi, 5. M., and Powers, W. L. 1938. Plant Physiol. 13, 767-789. Arnon, D. I., and Johnson, C. M. 1942. Plant Physiol. 17, 525-539. Ayers, A. D. 1948. J . Am. SOC.Agron. 40, 331-334. Ayers, A. D., and Hayward, H. E. 1948. Soil Sci. SOC.Am., Proc. 13, in press. Ayers, A. D., Wadleigh, C. H., and Magistad, 0. C. 1943. J . Am. SOC.Agron. 35, 796-810.

Baslavskaja, S. S. 1936. Plant Physiol. 11, 863-872. Bower, C. A., and Turk, L. M. 1946. J . Am. SOC.Agron. 38, 123-727. Bower, C. A., and Wadleigh, C. H. 1948. Soil Sci. SOC.Am., Proc. 13, in press. Boynton, D., and Burrell, A. B. 1944. Soil. Sci. 58, 441-454. Breazeale, J. F. 1927. Ariz. Agr. Expt. Sta. Tech. Bull. 14, Breazeale, J. F., and McGeorge, W. T. 1932. Ariz. Agr. Expt. Sta. Tech. Bull. 41. Buffum, B. C. 1896. Wyo. Agr. Expt. Sta .Bull. 29. Buffum, B. C. 1899. Wyo. Agr. Expt. Sta. 9th Ann. Rept. Burgess, P. S. 1928. Ariz. Agr. Expt. Sta. Bull. 123. Chambers, R., Kopac, M. J., and Grand, C. G. 1937. Znd. Eng. Chem., Anal. Ed. 9, 143-145.

Chspman, H. D., and Liebig, G. F., Jr. 1940. Hilgardia 13, 141-172. Chermezon, H. 1910. Ann. &. Nut. Botan. ZX 12, 117-125. Collander, R. 1941. Plant Physiol. 16, 691-720. Conrad, J. P., and Veihmeyer, F. J. 1929. Hilgardia 4, 113-134. Eaton, F. M. 1941. Plant Physiol. 16, 545-564. Eaton, F. M. 1942. J . Agr. Research 64, 357-399. Eaton, F. M., and Rigler, N. E. 1945. Plant Physiol. 20, 380-411. Fireman, M., and Reeve, R. 1948. Soil Sci. SOC.Am., Proc. 13, in press. Fitting, H. 1911. Z. Botan. 3, 209-275. Gardner, R. 1945. U.S. Dept. Agr. Tech. Bull. 902.

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Garner, W. W., McMurtrey, J. E., Jr., Bowling, J. D., Jr., and Moss, E. G. 1930. J. Agr. Research 40, 627-648. Gauch, H. G. 1940. Plant Physiol. 15, 1-21. Gauch, H. G., and Magistad, 0. C. 1943. J . Am. SOC.Agron. 35, 871-879. Gauch, H. G., and Wadleigh, C. H. 1944. Botan. Gaz. 105, 379-387. Gauch, H. G., and Wadleigh, C. H. 1945. Soil Sci. 59, 139-153. Gedroix, K. K. 1931. Soil Sci. 32, 51-63. Goss, A., and Griffi, H. H. 1897. N . Mez. Agr. Ezpt. Sta. Bull. 22 Harley, C. P., and Lindner, R. C. 1945. Proc. Am. SOC.Hort. Sci. 46, 35-44. Harper, H. J. 1946. Okla. Agr. Expt. Sta. Bull. T-23. Harris, F. S. 1915. 'J.Ayr. Research 5, 1-53. Harris, F.S. 1920. Soil Alkali. Wiley, New York. Harris, F. S., and Pittman, D. W. 1918. J. Agr. Research 15, 287-319. Harris, F.S., and Pittman, D. W. 1919. Utah Agr. Ezpt. Sta. Bull. 168. Harris, J. A., Gortner, R. A., Hoffman, W. F., Lawrence, J. V., and Valentine, A. T. 1924. J. Agr. Research 27, 893-924. Harris, J. A.,Hoffman, W. F., and Lawrence, J. V. 1925. Proc. SOC.Ezp. Biol. Med. 22, 350-352. Harris, J. A., Lawrence, J. V., and Gortner, R. A. 1916. Physiol. Researches 2, 1-49. Harter, L.L. 1908. U.S. Dept. Agr. Bur. Plant Znd. Bull. 134. Hayward, H. E.,and Blair, W. M. 1942. Am. J. Botan. 29, 148-155. Hayward, H. E.,and Long, E. M. 1941. Botan. Gaz. 102, 437-462. Hayward, H. E.,and Long, E. M. 1943. Plant Physiol. 18, 556-569. Hayward, H. E.,Long, E. M., and Uhvits, R. 1946. U . S.Dept. Agr. Tech. Bull. 922. Hayward, H. E.,and Magistad, 0. C. 1946. U.S.Dept. Agr. Misc. Pub. 607. Hayward, H. E.,and Spurr, W. B. 1943. Botan. Gaz. 105, 152-164. Hayward, H. E.,and Spurr, W. B. 1944. J. Am. SOC.Agron. 36, 287-300. Hayward, H. E.,and Spurr, W. B. 1944. Botan. Gaz. 106, 131-139. Headden, W.P. 1912. Colo. Agr. Expt. Sta. Bull. 183. Heilbrunn, L. V., and Daugherty, K. 1932. Physiol. Zool. 5, 254-274. Heller, V. G.,Hageman, R. H., and Hartman, E. L. 1940. Plant Physiol. 15, 727734. Hendrickson, A. H., and Veihmeyer, F. J. 1929. Proc. Am. &c. Hort. Sci. 26, 105108. Hendrickson, A. H., and Veihmeyer, F. J. 1942. Calif. Agr. Ezpt. Sta. Bull. 667. Hibbard, P. L. 1937. Calif. Agr. Expt. Sta. Circ. 292 (revised). Hilgard, E.W. 1886. Univ. Calif. Coll. Agr. Rept. App. VZZ. Hilgard, E. W. 1895-1898. Rep. of Expt. Sta., Univ. Calif. Hilgard, E. W. 1906. Soils. Macmillan, New York. Iljin, W. S. 1922. Biochem. 2. 132, 526-540. Iljin, W. 5. 1932. Planta 16, 352-366. Kearney, T.H. 1911. US.Dept. Agr. Farmers Bull. 446. Kearney, T. H., and Cameron, F. K. 1902. U.S. Dept. Agr. Repl. 71. Kearney, T.H., and Harter, L. L. 1907. US.Dept. Agr., Bur. Plant Znd. Bull. 113 Kearney, T. H., and Scofield, C. S. 1936. U.S. Dept. Agr. Circ. 404. Keller, B. 1920. Proc. Russ. Botan. SOC.5, 84-92. Keller, B. 1925. J. Ecol. 13, 224-261. Kelley, W.P. 1928. PTOC. First Intern. Cong. Soil Sci. 4, 483-489. Kelley, W.P. 1937. Calif. Agr. Expt. Sta. Bull. 617. Kelley, W. P., and Thomas, E. E. 1920. Calif.Agr. Expt. Sta. Bull. 318.

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Lehr, J. J. 1942. Soil Sci. 55, 399-411. Leonard, 0. A,, Anderson, W. S., and Gieger, M. 1948. Plant Physiol. 23, 223-237. Lesage, P. 1890. Rev. Gen. Bolnn. 2, 55-65, 106-109, 163-170. Lilleland, O., Brown, J. G., and Swmson, C. 1945. Almond Facts 9, 1, 5 . Long, E. M. 1943. A m . J . Bo[an. 30, 594-601. Loughridge, R. H. 1901. Calif. Ayr. Expt. P l o . Bull. 133. Lundeg&rdh,H. 1940. Lantbrh6gsk. Ann. 8, 234-404. McGeorge, W. T., and Breaaeale, J. F. 1938. Ariz. Agr. Expt. Sta. Tech. Bull. 72. Magistad, 0. C. 1945. Botan. Rev. 11, 181-230. Magistad, 0.C., Ayers, A. D., Wadleigh, C. H., and Gauch, H. G. 1943. Plant Physiol. 18, 151-166. Magistad, 0. C., and Christiansen, J. E. 1944. U.S. Dept. Agr. Circ. 707. Magistad, 0. C., and Reitemeier, R. F. 1943. Soil Sci. 55, 351-360. Masaewa, M. 1936. Bodenk. PfEernhhr. 1, 39-57. Moyer, L. S., and Bull, H. B. 1935. J . Gen. Physiol. 19, 239-247. Ogasa, T. 1939. Rept. Inst. Sci. Research, Manchoukuo 3, 303-315, (Chem. Abstracts 36, 2588. 1942). Powers, W. L. 1946. Ore. Agr. Expt. Sta. Tech. Bull. 10. Raleigh, G. J. 1948. Proc. A m . SOC.Hort. Sci. 51, 433-436. Ratner, E.I. 1935. Soil Sci. 40, 459-471. Ratner, E. I. 1944. Pedology 4-5, 205-227. Reeve, R. C., Allison, I,. E., and Peterson, D. F., Jr. 1948. Utah Agr. Expt. Sta. Bull. 335. Retzer, J. L., and Mogen, C. A. 1946. J . A m . SOC.Agron. 38, 728-742. Richards, L. A,, and Weaver, L. R. 1943. Sozl Sci. 56, 331-339. Richards, L. A., and Weaver, 1,. R . 1944. J . Agr. Research 69, 215-235. Rosene, H. F. 1941. Am. J . Botnn. 28, 402-410. Rudolfs, W. 1925. Soil Sci. 20, 15-37. Schuphan, W. 1940. Bodenk. PfEernhhr. 19, 265-315. Shaw, G.W. 1905. Calif. Agr. Expt. Sta. Bdl. 169. Shive, J. W. 1916. N.J. A9r. Expt. Sta. S7th Ann. Rept. 455-457. de Sigmond, A. A. J. 1938. The Principles of Soil Science. Thomas Murby & Co., London. Slosson, E. E., and Buffum, B. C. 1898. Wyo. Agr. Expt. Sta. Bull. 39. Steward, F. C., and Preston, C. 1941. Plant Physiol. 16, 481-519. Stewart, John. 1898. Utah Agr. Expt. Sta. 9th Ann. Rept. 26-35. Stewart, R., and Peterson, W. 1915. J . A m . SOC.Agron. 6, 241-248. Tagawa, T. 1934. Japan. J . Botany 7, 33-60. Thorne, D. W. 1944. Soil Sci. SOC.Am., Proc. 9, 185-189. Tinsley, J. D. 1902. N . Mex. Agr. h’xpl. Sin. Bull. 42. Tottingham, W. E. 1919. J . Am. BOG.Aqrou. 11, 1-32. Trelease, S. F., and Trelease, H. M . 1931. Bull. ?‘ot/ey Bolati. Club. 58, 127-149. Uhvits, R. 1946. Am. J . Botan. 33, 278-285. U.S. Regional Salinity Laboratory. 1947. Laboratory Staff, L. A. Richards, Editor. Multilithed. Uphof, J. C. Th. 1941. Botan. Rev. 7, 1-58. Van Itallie, Th. B. 1938. Soil Sci. 46, 175-186. Veihmeyer, F.J. 1927. Hilgardia 2, 125-291. Veihmeyer, F. J., and Hendrickson, A. H. 1927. Plnril Physiol. 2, 71-82. Wadleigh, C. H. 1946. Soil Sci. 61, 225-238.

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Wadleigh, C. H., and Ayers, A. D. 1945. Plant Physiot. 20, 106-132. Wadleigh, C. € and I.Fireman, , M. 1948. Soil Sci. Soc. Am., Proc. 13. In press. Wadleigh, C. H., and Gauch, H. G. 1944. Soil Sci. 58, 399-403. Wadleigh, C. H., and Gauch, H. G. 1948. Plant Physiol. 23, 485-495. Wadleigh, C. H., and Gauch, H. G., and Magistad, 0. C. 1948. U.S. Dept. Agr. Tech. Bull. 925. Wadleigh, C. H., Gauch, H. G., and Strong, D. C. 1947. Soil Sci. 63, 341-349. Wallace, A., Toth, S. J., and Bear, F. E. 1948. sbil Sci. 65, 249-258. Walsh, T., and Clarke, E. J. 1942. Eire Dept. Agr., J. 39, 316-325. Wuhrmann, K. 1935. Eidgenoss Tech. Hochschule, Ziirich.

New Fertilizers and Fertilizer Practices

.

RANDALL J JONES A N D HOWARD T . ROGERS Tennessee Valley Authority. Division of Agricultural Relations. Knoxville. Tennessee CONTENTS

Page 39

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . I1. New and Improved Fertilizer Materials . . . . . . . . . . . . . . 1. Phosphate Fertilizers . . . . . . . . . . . . . . . . . . . a . Concentrated Superphosphate (40-50 Per cent PnOd . . . . . b . Defluorinated Phosphates . . . . . . . . . . . . . . . c. Phosphate Rock-Magnesium Silicate Glass . . . . . . . . . d . Metaphosphates . . . . . . . . . . . . . . . . . . . (1) Calcium Metaphosphate . . . . . . . . . . . . . (2) Potassium Metaphosphate . . . . . . . . . . . . . 2. Phosphorus-Nitrogen Fertilizers . . . . . . . . . . . . . . . a . Ammonium Phosphates . . . . . . . . . . . . . . . . b . Dicalcium Nitraphosphate Products . . . . . . . . . . . c. Ammoniated Superphosphate . . . . . . . . . . . . . . 3 . Nitrogen Fertilizers . . . . . . . . . . . . . . . . . . . a . Ammonium Nitrate . . . . . . . . . . . . . . . . . . b . Urea-Form . . . . . . . . . . . . . . . . . . . . . c. Anhydrous Ammonia . . . . . . . . . . . . . . . . . 4 . Potash Fertilizers . . . . . . . . . . . . . . . . . . . . 5 . Mixed Fertilizers . . . . . . . . . . . . . . . . . . . . I11. Recent Developments in Fertilizer Use . . . . . . . . . . . . 1 High Rates of Fertilizer for Corn . . . . . . . . . . . . . 2. Use of Anhydrous Ammonia as a Fertilizer . . . . . . . . . . 3. Methods of Application . . . . . . . . . . . . . . . . . . a . Furrow-Bottom or “Plow-Sole” Placement of Fertilizers . . . . b . Subsurface Placement of Fertilizers for Sod Crops . . . . . . c. Application of Fertilizers in Irrigation Water . . . . . . . . d . Direct Application of Liquid Fertilizers . . . . . . . . . . e . Plant-Nutrient Sprays . . . . . . . . . . . . . . . . . 4 . Fertilizing For Winter Grazing in the Southeastern States . . . . . 5 . Miscellaneous Practices . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .

.

41

41 42 43 45 46 46

48 49 49 50 51 51

51 51 52 52 53 53 53 56 59 59 61

63 65 67 69 71 72

I . INTRODUCTION The estimated fertilizer bill of the American farmer in 1947 was 615 million dollars Fertilizer consumption that year reached a figure of 16.000.000 tons. which was more than double that applied to soils in the

.

39

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RANDALL J . JONES A N D HOWARD T. ROGERS

United States in 1939. It is believed that the 1948 figures will be somewhat in excess of those for 1947. Although fertilizer production has increased phenomenally during recent years, the current demand for many materials still exceeds the supply. A few years ago the north central region, an area of highly fertile soils, was not considered to be of great importance from the standpoint of fertilizer use. Now this is the most rapidly expanding fertilizerconsuming area in the United States, as reported by Mehring (1948). I n the north central states plant-nutrient consumption increased from

Fig, 1. Increase in

U.S.fertilizer consumption, 1935-1947,by regions,

393,398 tons as an annual average for the 1935-39 period to 779,986 tons in 1947, or an increase of 303 per cent. Significant increases in fertilizer use have also occurred in the western states and in the traditionally highfertilizer-consuming Southeast, as illustrated in Fig. 1. With this upward trend in consumption there has also developed, especially in the Midwest, a demand for high-analysis materials. Considerable progress has been made by the chemical engineer and fertilizer chemist during recent years in the development of new fertilizers and improvement of processes for fertilizer production. Likewise, the agronomist, with assistance from specialists in other fields, has found new ways of using fertilizer which add to its efficiency in crop production and value for soil improvement,. These recent advancements have a farreaching significance for agricultural development in this country.

NEW FERTILIZERS AND FERTILIZER PRACTICES

41

The review covered in this paper is an attempt to indicate the more important developments, both in new and improved fertilizer products and in recent trends in fert,ilizer use. Some of the discussion is devoted to materials and practices which are not strictly new, but i t is included to provide background information.

11. NEWAND IMPROVED FERTILIZER MATERIALS 1. Phosphate Fertilizers

Within the past 15 years a great deal of attention has been given to the development of new phosphate fertilizers and the improvement of processing methods. Contributions in this field have been significant. Some of the processes have been expanded to a commercial scale, and t*hereis reason to believe that others will find rather extensive use in the future. Ordinary superphosphate, 16-20 per cent P205, made by acidulating phosphate rock with sulfuric acid, has served as the major source of phosphate produced by the fertilizer industry since it was first developed over 100 years ago. Superphosphate is a good source of phosphorus for crop production, and its manufacture is a relatively simple, low-cost process. During recent years there has been considerable improvement in the physical condition of ordinary superphosphate by means of granulation. Also, improvements in the manufacturing processes have been achieved, especially through greater use of mechanical den systems and the development of continuous processes. It has been well suited to the type of agriculture commonly practiced in the high-fertilizer-consuming area of the South. However, ordinary superphosphate has the disadvantage of being a product of relatively low phosphorus content which is an important factor, especially in areas that are located a long distance from the source of supply. High-analysis phosphates have a decided advantage over 20-per cent material from the standpoint of lower shipping and handling costs per unit of plant nutrients when long hauls are involved. Also, concentrated materials are preferable for use on steep land where fertilizer distribution is difficult and is frequently done by hand. Obviously, the cost of P205 applied to the land will be an important factor determining the use of any particular form of phosphate. Two factors focus attention upon the need for high-analysis phosphates: (a) the rapid increase in fertilizer consumption in the Midwest and (b) the importance of developing the extensive western phosphate reserves. Research in developing new phosphate fertilizers must take into account the geographical location of the major phosphate rock deposits. Continental United States contains about one-half of the known

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RANDALL J. JONES AND HOWARD T. ROGERS

world reserves of phosphate which, according to calculations based on estimates made by Mansfield (1942), are distributed in three major areas as follows: Tennessee, 1 per cent; Florida, 24 per cent; and western intermountain states, 75 per cent. Prior to 1946, Florida and Tennessee produced approximately 95 per cent of the phosphate rock of the United States, I n 1946, however, the western states produced more than 7 per cent of the US. production; and in 1947, more than 13 per cent (Editorial, 1948). Bell and Griffith (1947) have studied transportation costs in relation to development of new phosphorus industries in the West. They have shown that, on the basis of calculated production costs, triple superphosphate manufactured in the western phosphate fields would have a trade area of a t least 17 western and central states. This area would be extended to several other states if more concentrated phosphates were produced. a. Concentrated Superphosphate (40-50 Per cent P,05). Concentrated superphosphate, 40-50 per cent P206, has been variously designated as “double,” “triple,” and “treble” superphosphate. Although this product has been manufactured for a long period on a commercial scale, only in recent years has production expanded appreciably. According to Jacob (1948), the amount of P20aproduced in this form increased from about 43,000 tons in 1930 to over 170,000 tons in 1947; and a considerable expansion of the present production capacity is anticipated. The current demand is considerably greater than the supply, Concentrated superphosphate is produced by t.reating phosphate rock with phosphoric acid which is made by either the wet-process method or by the electric-furnace process. The wet-process method is used most extensively for current production. It involves treating phosphate rock with sulfuric acid to obtain the phosphoric acid necessary for acidulating additional phosphate rock. In the electric-furnace process, elemental phosphorus is produced and then burned to form P2OS.The P2OS gas is absorbed in water to produce phosphoric acid which is used in the same manner as indicated for the wet process. Electric-furnace acid is of higher purity than the wet-process acid; consequently, it has been used extensively by the chemical industry. Extensive tests have been conducted with concentrated superphosphate, using a wide variety of crops and soils. As a source of phosphate it is fully as effective as ordinary 18-20 per cent superphosphate. This is borne out by results obtained in all the major agricultural regions in the United St,ates. It should be pointed out that concentrated superphosphate made by the wet process carries only a small quantity of sulfur (about 3 per cent Sod, whereas, ordinary superphosphate contains

NEW FERTILIZERS AND FERTILIZER PRACTICES

43

about 50 per cent by weight of gypsum. The amount of sulfur in superphosphate made from electric-furnace acid is negligible. This may be an important factor in areas of sulfur-deficient soils. An average of results obtained from 1060 cotton tests in the four states of Alabama, Georgia, Mississippi, and Tennessee showed a 5-per cent increase in yield from adding sulfur in the form of either gypsum or ammonium sulfate to triple superphosphate, according to a recent summary by the Tennessee Valley Authority (1946). Similar tests conducted wit.h wheat and corn on many of the same soils gave no indication of sulfur response. Significant response to sulfur applications for clover has been observed in Florida (Bledsoe and Blaser, 1947). Sulfur deficiency is probably most acute in the western states, particularly in Oregon, California, and Washington. This lack of sulfur in most of the recently developed high-analysis phosphate fertilizers should not be overlooked. b. Defluorinated Phosphates. Some of the most intensive research during recent years directed toward the development of new fertilizer products of commercial value has been centered on various phosphate fusion or sintering processes. The principle of defluorination of rock phosphate has been extensively developed by the U.S.Department of Agriculture and the Tennessee Valley Authority. On the basis of this research, commercial production of defluorinated phosphate is now under way by the Curonet Phosphate Company. Also, the TVA is continuing developmental work with defluorinated phosphate on experimental-plant scale. Although the processes are different in certain respects, in both cases the principal product formed is alpha tricalcium phosphate. In the Coronet process, rock phosphate is defluorinated by adding a high percentage of Si02 (about 45 per cent) and sintering, but not fusing, the material. This process has been described by Whitney and Hollingsworth (1949). The final product contains about 20 per cent total P205 and 0.05 to 0.15 per cent fluorine. Because of the low fluorine content it has been used chiefly as a mineral supplement for livestock during the last few years, since the product commands a higher price for this use than as a fertilizer. The effectiveness of this product as a fertilizer material has been investigated to only a limited extent in greenhouse pot experiments. Results from these studies show its availability as a source of phosphorus for plants to be about equal to that of superphosphate, This material has the disadvantage of being low in P205 content. The TVA product, fused tricalcium phosphate, containing about 27 per cent P205, is produced by the following process: Phosphate rock is defluorinated by heating in the presence of silica and water vapor until the charge becomes fluid and the fluorine content is reduced to about,

44

RANDALL J. JONES AND HOWARD T. ROGERS

0.4 per cent. A unique feature of the process is the quenching of the molten material as it comes from the furnace, which gives a product approximately 90 per cent finer than 10 mesh and about 50 per cent of which passes a 40-mesh screen. This product, like the Coronet material, does not absorb moisture, is free-flowing, and remains in excellent physical condition. This process was described by Hignett and Hubbuch (1946) and is illustrated in Fig. 2.

Pig. 2. Manufacture of fused tricalcium phosphate.

On the basis of greenhouse and field tests, it appears that satisfactory crop response is obtained when the fluorine content is reduced to about 0.4 per cent (MacIntire et al., 1944; Tennessee Valley Authority, 1945; and Terman, 1944). Rather extensive field tests have been conducted with fused tricalcium phosphate, especially in the Southeast. The effect of particle size on availability to crops has been of particular interest. This product has compared favorably with concentrated superphosphate in most tests on acid soil (Karraker et al., 1941; O’Brien, 1944; and Roberts et al., 1942). Row crops such as corn, tobacco, and cotton, however, do iiot appear to respond quite so well as vetch, alfalfa, and permanent pastures. I n some experiments response has been greater when the product was ground to pass a 40-mesh screen as compared with the unground material screened a t 6 or 10 mesh (Tennessee Valley Authority, 1915; Terman, 1944). Grinding, of course, increases cost of production; and on the basis of present information, it does not appear justifiable if approximately 50 per cent of the quenched unground material passes a 40-mesh screen.

NEW FERTILIZERS AND FERTILIZER PRACTICES

45

The level of phosphorus in the soil apparently affects crop response to fused tricalcium phosphate, and crops on soils of extremely low phosphorus content give lower yields when fertilized a t low or moderate rates with this source of phosphorus than with superphosphate. Tests in the western states indicate that this product is not a promising source of phosphorus on alkaline and calcareous soils (Hinkle, 1942; Jones, 1947 ”) . Investigations in this region have not been adequate for final conclusions, but expanded research now under way should make possible a better evaluation of this material in the near future. Extensive solubility studies have been conducted by Jacob et al. ( 1947) with alpha phosphates, using the standard neutral ammonium citrate and 2 per cent citric acid procedures. They use the term “alpha phosphate” to represent a group of defluorinated phosphates that are composed largely of alpha tricalcium phosphate and which iAcludes both the Coronet and the TVA products. These investigators reported that the solubility of defluorinated phosphates containing less than 0.5 per cent fluorine ranged from approximately 65 to over 90 per cent. The solubility was dependent, on particle size, amount of glassy material, the fluorine content, and whether the product was fused, calcined, or sintered. Slightly higher values were usually obtained from citric acid extraction than from ammonium citrate. Reynolds et al. (1934) showed that as the fluorine content of defluorinated phosphates decreased, the solubility of the phosphorus in neutral ammonium citrate increased. MacIntire et al. (1944) reported that the fluorine remaining in fused tricalcium phosphate exists as apatite and is combined with about 15 per cent of the phosphorus. The somewhat lower rate of solubility of the defluorinated phosphates, as compared with superphosphate, may result in increased residual effects on crop yields. There is some indication that this is true, but additional long-term field tests are needed to establish this point. Production of phosphate fertilizer by defluorination of rock phosphate appears to be a promising process economically. Such products are not as concentrated as would be desired, and they must also be considered for use primarily as straight materials since they are not suitable for use in mixtures containing ammonium salts. c. Phosphate Roclc-Magnesium Silicate Glass. A process developed by Walthall and Bridger (1943) has led to commercial production on the West Coast of a fertilizer produced by the fusion of phosphate rock with magnesia and silica. The original process involved fusing a mixture of rock phosphate and olivine in an electric furnace. Defluorination is not

* Designates references to work as yet unpublished.

46

RANDALL J. JONES AND HOWARD T. ROGERS

required in this process, although part of the fluorine is volatilized during fusion. The product obtained was a glass containing about 22 per cent P206which had a high solubility in ammonium citrate. Limited greenhouse tests on two Tennessee soils (pH 6.0 and 6.3) indicated that this phosphate was virtually as effective as superphosphate. A modification of this process in which serpentine is substituted for olivine has been adopted by the Permanente Metals Corporation in California. . The product has been marketed under the name “ThermoPhos.” Greenhouse tests with this and similar products conducted by Hill et al. (1948) showed that when these materials are ground finer than 100-mesh, crop response compares favorably with superphosphate on acid soils. Their results on calcareous soils were not consistently so favorable. Quenched material that only passed a 6-mesh screen gave consistently lower plant growth response and particularly so on calcareous soil. It was indicated that R fineness exceeding 60 mesh was required for satisfactory plant response to this material. Field tests in California and Washington indicate that the coarse material (passing a 6-mesh screen) is inferior to superphosphate; whereas, the finely ground product appears to give satisfactory results (Lorens, 1948; Wheeting, 1948) .” The Manganese Products, Inc., of Seattle, Washington, is producing a fertilizer material containing about 20 per cent P205by fusing olivine and rock phosphate, as reported by Moulton (1947) and Granberg (1048). This is essentially the same process as described by Walthall and Bridger (1943) above. Although no experimental data are available for evaluating this product, it is assumed that crop response would be about the same as for the products described above. A considerable expansion of field experimenk is needed to evaluate these products adequately. The process of fusing rock phosphate with magnesia and silica appears to have possibilities for the production of a low-cost phosphate fertilizer. d. Metaphosphates. (1) Calcium Metaphosphate. The high phosphorus content of calcium metaphosphate (60-63 per cent P206) has made this product of particular interest in certain areas since it was first produced by the TVA in 1935, as reported by Curtis et al. (1938). Since that time several thousand tons have been produced in pilot plants and in a full-scale unit for use in an experimental testing program on field plots and on test-demonstration farms in cooperation with the land-grant colleges in a large number of states. The process for producing calcium metaphosphate has gone through several stages of development. Originally the method was essentially one of burning elemental phosphorus with air and reacting the hot

NEW FERTILIZERS AND FERTILIZEB PRACTICES

47

products of combustion with lump rock phosphate in a vertical shaft. The molten material collected in the bottom of the reacting chamber and was tapped from the furnace periodically. The resulting glassy product was then ground. Improvements have been made in the process which involve blowing fine phosphate sands into a combustion chamber in which the phosphate fines react with hot Pz05. The PzOsthat does not react in the combustion chamber passes into an absorption tower packed with lump phosphate rock. A flow diagram giving the essential steps in the process is shown in Fig. 3. A new experimental-scale plant embody-

TUNER

ORUM

w L % 2 k % w T E PULVERIZER

Fig. 3. Calcium metaphosphate process.

ing the most recent process improvements is now under construction by the TVA. Tests to determine the efficiency of calcium metaphosphate as a source of phosphorus have been conducted throughout the United States. Crop response data are rather conclusive in showing that this product is equal to superphosphate as a phosphorus source on acid soils of the humid region. Thus, O’Brien (1944) reported calcium metaphosphate to be an effective fertilizer for a wide variety of crops grown in rotation a t eight different locations in Virginia on major soil types. I n a series of greenhouse tests with three important soil types in Alabama, Volk (1944) concluded that calcium metaphosphate compared favorably with superphosphate. Results from field-plot experiments in New York with legume and grass hay, corn, and wheat showed this

48

RANDALL J. J O N E S AND HOWARD T.

product to be quite satisfactory as a source of phosphorus (Chandler and Musgrave, 1944). A summary of several hundred field experiments with calcium metaphosphate conducted in the states of Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee, and Virginia showed an average relative crop yield of 99 as compared with a value of 100 for superphosphate (Tennessee Valley Authority, 1946). These tests were conducted with cotton, corn, wheat, and legume hay. Results from field experiments on alkaline soils in the western states are conflicting. Variable results were reported by Toevs and Baker (1939) in Idaho from two alfalfa experiments in which one test showed no increase in yield from calcium metaphosphate ; whereas, met.aphosphate was about 70 per cent as effective as superphosphate in the other test. On the other hand, Hinkle (1942) reported calcium metaphosphate to be only slightly less effective than superphosphate for alfalfa in New Mexico experiments. Alway and Nesom (1944) found in alfalfa experiments in Minnesota that calcium metaphosphate was as effective as superphosphate when incorporated with the soil prior to seeding the crop, except on calcareous soils. Placement of fertilizer, soil moisture, and rate of hydrolysis of metaphosphate to orthophosphate may be factors which affect the efficiency of calcium metjaphosphate as a source of phosphorus. A careful study of these and other factors should be made along with additional experiments to determine crop yield response. The calcium metaphosphate process looks promising as an economical method for producing phosphate. Because of its high concentration, this material would seem particularly well suited for areas, such as the Midwest, distant from the phosphate deposits. (2) Potassium Metaphosphate. Potassium metaphosphate, like calcium metaphosphate, has the advantage of being a fertilizer material of high analysis. Potassium metaphosphate from pilot-plant production contains approximately 55 per cent PzOs and 35 per cent KzO. Considerable work was done by the U.S. Dept. Agr. on the development of this product on a laboratory scale (Madorsky and Clark, 1940). The Tennessee Valley Authority produced potassium metaphosphate on a pilot-plant scale by blowing powdered muriate of potash into a phosphorus combustion chamber where the temperature was maintained at 800-900°C.(Copson e t al., 1942). The molten material was then tapped from the furnace and cooled to form a crystalline product which was ground for fertilizer use. Hydrochloric acid is formed as 8 byproduct from the muriate of potash. Thus far the economics of production of potassium metaphosphate do not appear to be promising, since the

NEW FERTILIZERS AND FERTILIZER PRACTICES

49

cost of production exceeds to a considerable extent the cost of equivalent quantities of PeOa and K 2 0 contained in superphosphate and muriate of potash. Potassium metaphosphate, like calcium metaphosphate, is only slightly soluble in water, but. it hydrolyzes in the soil to form orthophosphate which is a more soluble product. Chandler and Musgrave (1944), in New York, reported that potassium metaphosphate was fully as effective as calcium metaphosphate and superphosphate in field experiments with wheat, alfalfa, and legume-grass hay. Houghland et aZ. (1942) used potassium metaphosphate, along with several other phosphate sources, in potato experiments. From the results obtained on Caribou loam in Maine, yields from potassium metaphosphate either equaled or exceeded the yields obtained from superphosphate. In greenhouse experiments conducted by Brown and Clark (1943), using millet, oats, and wheat as indicator crops on four different soils, potassium metaphosphate gave higher yields than superphosphate in six out of twelve tests. A summary of unpublished data from the states of North Carolina, Georgia, Virginia, Kentucky, Alabama, and Mississippi shows that out of a total of 71 tests, 34 gave yields for potassium metaphosphate higher than those for superphosphate. On the other hand, 215 tests out of a total of 233 test3sin Tennessee gave yields which were somewhat below those from superphosphate. The explanation as to why the results in Tennessee should be consistently low for pot.assium metaphosphate is not known. Unless process improvements are made which would lower the cost of production, it would not be feasible to manufacture this material on a commercial basis. If developments lead to more economical production, this material should be more thoroughly tested, particularly in the midwestern and northeastern states where highly concentrated fertilizers are in demand. 2. Phosphorus-Nitrogen Fertilizers

There has been increased interest recently in the possibility of lowering the cost of nitrogen and phosphate fertilizers by the use of processes in which both nutrients are combined either as single compounds or in mixtures. Some of these products have been used as fertilizer for many years, while others are of comparatively recent development. a. Ammonium Phosphates. Monoammonium phosphate has been in commercial production a number of years, but it is mentioned here because the total production and area of distribution appear t o be expanding to some extent. The product is manufactured either in the form of

50

RANDALL J . JONES AND HOWARD T. ROGERS

a straight material analyzing 11 per cent nitrogen and 48 per cent PzOb or in combination with ammonium sulfate which results in 8 16-20-0 fertilizer. I n Nort-h America, the 11-48-0grade is now produced only a t Trail, British Columbia, while the 16-20-0grade is made a t Trail and a t Pasadena, Texas. The use of these products as fertilizer materials is generally accepted. As pointed out by Volk et a!. (1945),however, the acidity resulting from ammonium phosphate must be corrected by liming; and in some cases, continued use of this form of nitrogen and phosphorus results in a sulfur deficiency. Although only negligible quantities of diammonium phosphate for use as fertilizer have been produced in the United States, the fertilizer grade of this compound has been used for some years in Europe, both as an individual material and as a constituent of some of the Nitrophoska types of mixed fertilizers. I n these forms it was imported into the United States in certain years before World War 11. More recently a process has been developed by the TVA for producing diammonium phosphate which gives a product of superior physical characteristics for use as a fertilizer material. It analyzes approximately 21 per cent nitrogen and 54 per cent P20B, giving a high-analysis fertilizer. The process, developed on a pilot-plant scale, consists of reacting anhydrous ammonia and electric-furnace phosphoric acid in a saturator to form aggregates of thin tabular crystals which are primarily diammonium phosphate with small amounts of monoammonium phosphate. The material is not hygroscopic, and it handles satisfactorily in ordinary fertilizer distributors. The economics of this process for the manufacture of ammonium phosphates appear attractive. The ammonium phosphates are particularly suitable for use in highanalysis mixed fertilizers and may be used for direct application where only nitrogen and phosphorus are required, as is the case in many of the western states. b. Dicalcium Nitraphosphate Products. Within the last year renewed attention has been given in the United States to processes which involve treating raw rock phosphate with a mixture of nitric and phosphoric acids. Many such processes have been investigated, and some of them have been used in Europe. It is understood that large-scale production of a dicalcium phosphate-ammonium nitrate mixture (20-20-0)is in operation in Holland a t present. A process has been developed on a pilot-plant scale by the TVA. One of the materials that is produced contains dicalcium phosphate and ammonium nitrate and analyzes about 17 per cent nitrogen and 22 per cent P20e.The phosphorus is 98 per cent citrate-soluble and the nitrogen

NEW FERTILIZERS AND FERTILIZER PRACTICES

51

is water-solublc. This process, though in its early stages of development, promises to be an economic source of nitrogen and phosphate fertilizer, since the nitric acid which is normally used for making ammonium nitrate can be put to double use by acidulation of rock phosphate prior to ammoniation. The physical condition of this product appears to be satisfactory since it is free-flowing. Preliminary greenhouse tests indicate that both the nitrogen and phosphorus are available for plant growth and are approximately as efficient as superphosphate and ammonium nitrate on acid soils. c. Ammoniated Superphosphate. Ammonia and ammonia solutions have been used to ammoniate superphosphate since about 1928. This has afforded a convenient way of utilizing a form of nitrogen which is relatively low in cost. The practice of ammoniating superphosphate has continued to expand until more than 250,000 tons of nitrogen are now used annually for this purpose. 3. Nitrogen Fertilizers

a. Ammonium Nitrate. Since 1943 ammonium nitrate for direct application has been used in increasing quantities until in the year ended June 30, 1947,there were 367,093 tons used in the United States (Scholl and Wallace, 1948). Interest in ammonium nitrate as a fertiliaer material was greatly stimulated during World War I1 due to the expansion in plant capacity for producing ammonium nitrate for munitions. The major problem that originally restricted the use of -ammonium nitrate was its tendency to absorb moisture. Ross et al. (1946)described in detail methods which have been used for treating ammonium nitrate to make it a suitable fertilizer product. Ammonium nitrate is treated with suitable conditioning agents to give it satisfactory physical properties. The final product contains 32 to 34 per cent nitrogen, As summarized by Whittaker et al. (1948) field tests with a number of common crops show conclusively that ammonium nitrate is a satisfactory source of nitrogen for crop production. The indications are that the use of this product will continue to expand in this country because of economy of production, high analysis, and satisfactory crop response. b. Urea-Form. A reaction product of urea and formaldehyde designated as “urea-form,” which is slightly soluble in water and nitrifies slowly in the soil, has been developed recently by the U.S.Dept. Agr. Armiger et al. (1948)of the U.S. Dept. Agr. have studied the properties of this product and have tested it both in greenhouse and field experiments. It is reported by Fuller and Clark (1947)that the most promising products have urea-formaldehyde mole ratios of 1.2 to 1.4 and

52

RANDALL J. JONES AND HOWARD T. ROGERS

nitrogen contents of 36 to 38 per cent. These products are not yet in commercial production. Nitrification tests reported by Yes and Love (1946) indicate that the rate of nitrate formation in the soil is sufficient to meet crop needs. Ureaform appears to be particularly well suited for grasses and other crops which require an available source of nit,rogen over a long period. Extensive cooperative field tests now under way between the U.S. Dept. Agr. and several state agricultural experiment stations should give fairly conclusive data as to the suitability of this product,. Since urea-form can absorb a high percentage of moisture without any change in its physical condition, it appears to be well suited for use as a conditioning agent in mixed fertilizers and, as suggested by Jacob and Mehring (1947)’ could replace to advantage some of the inert or low-analysis materials now used for such purposes. c. Anhydrous Ammonia. The use of anhydrous ammonia as a nitrogen fertilizer was initiated by the Shell Chemical Company in California in the 1930’s. This practice has increased to a great extent in that area as well as in other western states. A method for direct fertilization of the soil with anhydrous ammonia was patented by Leavitt (1942). Recently there has been considerable interest in other parts of the United States in the use of anhydrous ammonia for direct application. I n 1943, research on the use of anhydrous ammonia was initiated by the Mississippi Agricu!tural Experiment Station in cooperation with TVA, and the practice has spread to several other southern states. Since the cost of anhydrous ammonia per unit of nitrogen is much lower than that of any other presently available nitrogen source, it appears to be a potentially important source of fertilizer nitrogen for certain areas. A discussion of the use of this material and crop response data will be given under Section 111-2.

4. Potash Fertilizers The outstanding contributions made in the manufacture of potash fertilizers in recent years have been in improved technology for producing high-analysis nuriate of potash. According to Jacob and Mehring (1947), approximately 80 per cent of the potash now consumed as fertilizer in the United States is in the form of potassium chloride containing 60 per cent KzO. Large quantities of concentrated muriate of potash are used in the manufacture of high-analysis mixed fertilizers. Turrentine (1943) has given an excellent discussion of recent advances made in the technology of potash production in the United St.ates. Potassium metaphosphate was discussed in the section dealing with phosphates, but it should be mentioned here also since it is a new type

53

NEW FERTILIZERS AND FERTILIZER PRACTICES

of potash carrier. Chandler and Musgrave (1944) were interested in potassium metaphosphate as a source of potassium of lower solubility than muriate of potash. I n a greenhouse experiment with Ladino clover, they showed a considerably lower rate of potassium uptake from potassium metaphosphate than from muriate of potash. It was suggested that the use of potassium metaphosphate might serve to prevent the excessive absorption of potash by such crops as alfalfa during the early part of the growing season, which frequently leads t o potash deficiency later in the year. 5 . Mixed Fertilizers The discussion thus far has been rest,ricted to single fertilizer materials. It is recognized, however, that important developments have occurred in the manufacture of mixed fertilizers. It should be mentioned that ( a ) about 75 per cent of the total consumption of available plant nutrients in the United States is in the form of mixed fertilizers; (b) the total plant nut,rient content of mixed fertilizers has increased progressively from 19.9 per cent in 1940 t o about 21.5 per cent in 1947; (c) progress has been made in solving the problems of physical condition and chemical reactions in mixtures; and (d) attention has been given to the development of granular mixtures. 111. RECENTDEVELOPMENTS IN FERTILIZER USE 1. High Rates of Fertilizer for Corn

One of the most significant recent developments in the field of fertilizer use in the heavy fertilizer-consuming area of the southeastern United States has been the recognition of a great pot.entia1 for increasing per acre yields of corn by combining adequate fertilization with close spacing of adapted hybrids. The possibility of increasing corn yields in the 7 southeastern states of Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee, and Virginia is suggested by the following statistics on yields of corn, cotton, and tobacco since 1909 (Tenn. Dept. Agr., 1948; US. Dept. Agr., 1909-1935, 1936-1946, 1948). 1909-13 1914-20 1921-25 1926-30 1931-35 1936-40 1941-45 1946-48 21.2 19.5 18.6 18.2 18.4 20.6 21.4 26.5 104 129 103 95 90 88 100 89

Corn

Bu./acre Relative"

Cotton

Lb./acre Relative"

207 100

193 93

183 88

201 97

234 113

279 135

354 171

348 168

Lb./acrc: Relative"

751

769 102

700 93

725 97

775 103

881 117

1010 134

1176 157

100

Based on yield 1909-13 = 100.

54

RANDALL J . J O N E S A N D HOWARD T. ROGERS

It is apparent that no substantial progress was made in increasing corn yields in this area from 1909 to 1945. Cotton and tobacco yields, however, began an upward trend during the thirties and for the 1941-1945 period were 171 and 134 per cent, respectively, of the 1909-1913 yields. Increased per acre applications of fertilizer to cotton and tobacco has been an important factor in increasing the yields of these crops. It would be expected that in this region of favorable climatic conditions for corn production higher rates of fertilization should also result in increased corn yields. The importance of increased corn yields in the Southeast is emphasized.by the fact that approximately one-third of the crop acreage is devoted to corn, which is not greatly different from the proportion of land planted to this crop in the Corn Belt. Volk (1942) estimated that 65 per cent of the land in Alabama planted to corn would produce about 11 bushels per acre without applying nitrogen, and that only 12 per cent of the acreage would produce 30 bushels or more without adding this plant nutrient. Jones (1942), summarizing 12 years’ results from seven experiment fields in Alabama, showed that the increase in yield due to increments of nitrogen was nearly a linear response up to 36 lbs. of nitrogen per acre. At that time, very few experiments in which high rates of nitrogen were used had been conducted, and most of the tests employed open-pollinated varieties of corn with relatively wide spacing of plants. Generally, the genetic 1imitat.ion of open-pollinated varieties combined with a low number of plants per acre had concealed the possibilities of fertilization in the few cases where high rates had been used. Volk (1944), however, reported results from 15 tests in Alabama conducted cooperatively with farmers, which showed substantial increases in corn yields for each 15-lb. increment of nitrogen from 0 to 75 lbs. per acre. The potential for high yields was pointed up when Krantz (1945) reported an increase from 19 bushels per acre without nitrogen to 107 bushels of corn with 120 lbs. of nitrogen added to Norfolk sandy loam. Cummings (1947), summarizing 3 years’ results of 38 fertilizer tests in North Carolina with corn, reported average yields of 28, 50, 68, and 78 bushels per acre from plots receiving 0 , 40, 80, and 120 lbs. of nitrogen, respectively. All plots received adequate phosphate and potash and were planted to adapted hybrids with 9,000 to 10,000 plants per acre. Investigators in Georgia, Mississippi, and North Carolina (Brooks, 1948; Jordan, 1947; Krantz, 1947) demonstrated the need for an adequate number of plants per acre with high levels of fertilization for maximum yields. Krantz (1947), using spacings of 4,000, 9,000, and 12,000 plants per acre in one experiment, obtained per acre yields of 53, 82, and 93 bushels, respectively. Brooks (1948) found that there was no yield

NEW FERTILIZERS AND FERTILIZER PRACTICES

55

increase for spacings above about 10,000 plants per acre, even when fertilized with adequate phosphate and potash and rates of nitrogen up to 150 lbs. per acre. In the Georgia tests, maximum yields ranged from 78 to 117 bushels per acre at different locations, and increases were obtained for rates of nitrogen up to 90 lbs. per acre. I n the North Carolina tests, nearly linear response was obtained to rates of nitrogen up to 120 lbs. per acre when fair to good moisture condiTions prevailed. Similarly, Jordan (1947) showed increases in yields of corn from rates of nitrogen up to 120 pounds per acre a t several locations in Mississippi. In most of the experiment,al work on high rates of fertilization for corn, nitrogen has been the nutrient producing most of the spectacular increases. The results from Georgia, however, show that either phosphate or potash may be as limiting as nitrogen on some soils. At one location, a 40-bushel increase was obtained for the application of 60 Ibs. of PzOs per acre. Likewise, a t one location, a 32-bushel increase was attributed to the application of 60 lbs. of K 2 0 per acre. While only one experiment out of seven, in 1947, gave less than 30 bushels per acre increase for 90 Ibs. of nitrogen, in most cases the increase duc to either phosphate or potash was less than 10 bushels per acre. Krantz (1947) pointed out the need for nutrient balance, although in most tests nitrogen was the key to high yields. For example, Dunbar sandy loam produced a 24-bushel increase for potash when high rates of nitrogen wc3re applied, but no response to potash without nitrogen. Conversely, a striking response to nitrogen was obtained when potash was supplied, but no nitrogen response occurred without potash. Striking crop response to high rates of fertilization is not restricted to the southeastern states. Scarseth et al. (1943) showed that during 1939 yields of corn on Clermont silt loam in Indiana were increased from 11 bushels per acre without nitrogen to 71 bushels where 120 lbs. of nitrogen per acre were applied. Likewise, yields on Vigo silt loam were increased from 26 bushels without nitrogen to 91 bushels wit.h 120 Ibs. of nitrogen. This is indicative of the response that may be expected under some conditions in the Midwest. Results obtained by various investigators (Jones, 1942; Krantz, 1945; Ohlrogge et al., 1944) show that, within the range in which nitrogen is a limiting factor, about one bushel of corn is produced for every 2 Ibs. of commercial nitrogen applied. Widespread interest in these experimental results and in corn fertilizer demonstrations has been evident, but, the supply of nitrogen fertilizer has been inadequate to meet the demand in recent years. Statistics are not available to show to what extent the recent increased consumption of fertilizers in the southeastern states is due to heavier fertilization of

56

RANDALL J . J O N E S A N D H O V A R D T. ROGERS

the corn crop. From 1946 to 1948, however, the average corn yield for the 7 southeastern stat,es previously mentioned was 129 per cent of the yield during the 1909-1913 base period. This was the first significant increase in corn yields in this area for over 30 years. 6. Use of Anhydrous Ammonia as a Fertilizer Some of the advantages that have been suggested for using anhydrous ammonia as a fertilizer, as compared with solid nitrogen materials, include (a) a cost approximately 50 per cent less than solid forms of nitrogen, (b) immediate absorption by the soil, even at low moisture content, (c) more uniform distribution, and (d) less expense in application under certain conditions. Anhydrous ammonia is a gas at ordinary temperatures and pressure and contains 82 per cent nitrogen. Thirty per cent aqueous ammonia has also been used to some extent. At 100°F. the pressure over anhydrous ammonia in a closed vessel is about 200 lbs. per square inch. It is in liquid form under high pressures and changes to a gas as it is released from the container. Waynick (1934) claim the first attempt to apply anhydrous ammonia as a fertilizer material in surface irrigation water. The use of anhydrous ammonia in irrigation water for citrus began in California about 1934 and appears to have steadily increased until it is now a widespread practice and is being used on a variety of crops in that state. Chapman (1944) stated that anhydrous ammonia was being used in California on practically every type of irrigated crop which requires nitrogen fertilizat,ion. The field crops thus treated include cotton, corn, barley, oats, wheat, flax, sugarcane, sugar beets, hops, and rice, as reported by various investigators. Among the fruit crops are citrus, nearly all varieties of deciduous tree crops, and cane berries. Anhydrous ammonia has also been used on melons, carrots, onions, artichokes, tomatoes, beets, and other vegetables. Merrill (1948) estimated that 18,000 to 20,000 tons of anhydrous ammonia were used in California during the 1947-1948 crop year. Anhydrous ammonia constituted about one-third of the total sales of nitrogen for fertilizer in Arizona during t.he past crop season. Since about 1944, the practice of direct application of anhydrous ammonia to the soil has developed rapidly in California and in the Mississippi Delta area. I n Mississippi, Louisiana, and Arkansas approximately 15,500 tons of anhydrous ammonia were used during the 1947-1948 crop year. Moat of this nitrogen was applied to cotton on more than 600,000 acres of land a t an average rate of about 40 lbs. per acre (Andrews et al., 1948; Garman, 1948,* Louisiana Dept. Agr. and

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Immigration, 1948"). The indications are that a limited supply of ammonia was hhe major factor in restrirting the wider use of this material in 1948. Waynick (1934) studied the behavior of anhydrous ammonia in alkaline soils and found that the rate of nitrification was more rapid than t.hat of ammonium sulfate. The Arizona Agricultural Experiment Station (1945) reported that the applicatior, of anhydrous ammonia to some soils raised the pH t o as high as 9.5 immediately after treatment. I n these desert soils the pH soon dropped to about 7.7, where it remained for a period, and nitrification proceeded a t a satisfactory rate. Anhydrous ammonia was nitrified as readily as urea or ammonium sulfate in highly buffered soils. Ammonia applied in irrigation water was retained in the surface inch of soil where it was readily nitrified. Jackson and Chang (1947) studied the factors affecting absorption of NH3 by soil by releasing ammonia gas into a beaker of soil which was placed in a vacuum desiccator with suitable connections for aspiration, These investigators concluded that (a) soil of intermediate texture, moisture content, and p H value will absorb 60 lbs. of nitrogen per acre from NH, released a t a depth of only 1 to 2 inches, (b) a soil containing only 6 per cent clay provided adequate sorption capacity for NH3, (c) soils of high pH value with free calcium carbonate will retain 600 lbs. of nitrogen per acre from NH3 released 2 to 4 inches below the surface, and (d) air-dry soil absorbed instantly almost three times its own bulk volume of NH3. Chapman (1944) concluded that the major factors affecting the evaporational loss of ammonia from irrigation water were soil permeability, temperature, and degree of agitation of the water. His results indicated that under most conditions losses would be under 10 per cent by furrow irrigation. With high water temperahre and low soil permeability, however, losses may be of the order of 25 per cent. Dyke (1948) pointed out that rather high losses of ammonia may occur by floodirrigation methods such as those used on rice fields. Kennedy (1944)" studied the effects of concentrat,ion and drop size on losses of ammonia applied in sprinkler irrigation water. He reported losses of the order of 20 per cent. Although few data are available on the efficiency of anhydrous ammonia in comparison with other sources of nitrogen in California and adjoining states, there have been rather sat,isfactory yield increases from this material. Rhoades (1948) " reported increases from the application of anhydrous ammonia to wheat equivalent to those obtained from the use of ammonium nitrate. When 30 lbs. of nitrogen were applied, the

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RANDALL J . J O N E S A N D HOWARD T. ROGERS

average yield increases a t two locations were 9.9 bushels per acre for ammonium nitrate and 13.4 bushels for anhydrous ammonia. According to Garman (1948)*, a rice experiment in Arkansas showed a yield of 76.9 bushels from use of anhydrous ammonia and 72.7 bushels from ammonium nitrate when each was applied a t t,he rate of 50 lbs. of nitrogen per acre. Rather extensive field studies have been made by Andrews et al. (1948) in Mississippi. These investigators reported that when 32 lbs. of nitrogen were applied 4 inches deep as a side dressing anhydrous ammonia gave an average yield of 44.3 bushels per acre and ammonium nitrate produced 42.8 bushels in 13 tests on corn. In similar tests with cotton a t 18 different locations, anhydrous ammonia gave an average increase in yiela of 296 lbs. of seed cotton per acre, as compared with 313 Ibs. for ammonium nitrate. Anhydrous ammonia was used successfully for preplanting application on oats, but some difficulty was experienced in making top dressings in the spring due to excessive soil moisture. Leavitt (1948) * described the development of equipment for the introduction of anhydrous ammonia into irrigation water and for direct application to the soil in California. Approximately 15,000 steel cylinders of 150-lb. capacity are being used in that area to transport ammonia from filling depots to the farm where the ammonia is metered into the irrigation water. A machine built especially for injection of anhydrous ammonia directly into the soil was adapted from a Killifer cultivator. This machine has self-sealing injection shanks. I n the latest model, a trailer with a 4,500-lb. capacity tank is attached behind the applicator. In California the rate of application ranges from about 60 to 120 lbs. of nitrogen per acre on the various crops. Andrews et al. (1948) listed the specifications of equipment for storage of ammonia, transportation to farms, and application to the soil in the Mississippi Delta area. Anhydrous ammonia is transported from the railhead or from storage tanks in 1,000-gallon field transport trucks from which it is transferred to smaller tractor tanks of 80- to 110-gallon capacity. These invest.igators point out that anhydrous ammonia has been satisfactorily applied in the Delta area under the following conditions: (a) to prepared level land before planting, (b) to bedded land before planting, (c) during the process of bedding before planting, (d) during the planting operation, and (e) as a side dressing. Machines have been designed to apply the ammonia in the soil 4 to 6 inches deep. Applicators are specially designed knife-type openers with flat suctiontype points equipped with disc hillers or other apparatus for sealing in the ammonia vapor. The use of ammonia as a fertilizer will probably continue to expand

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since it is a low cost source of nitrogen, and there appear to be no serious mechanical difficulties in the application of either the anhydrous or aqueous form to the soil. 3. Methods of Application

a. Furrow-Bottowh or “Plow-Sole” Placement of Fertilizers. Generally accepted principles which might be considered the basis for determining best methods of fertilizer placement have been described by the National Joint Committee on Fertilizer Application (1948). Some of the important factors affecting placement are (1) nutrient balance within the root zone, (2) early stimulation of seedlings, (3) fixat,ion of added nutrients by the soil, (4) suitable crop rotations to utilize available plant nutrients to a maximum, (5) adaptation of methods to fit soil and plant requirements, and (6) avoidance of high salt concentration in contact with seed or roots. Various advantages have been suggested for deep placement of fertilizer. It was t,heorized that (a) the fertilizer would be kept in a moist zone of soil throughout the growing season, (b) band placement on the furrow-bottom should reduce fixation of phosphate by the soil, (c) reduced nitrification of ammonia nitrogen by deep placement would decrease leaching losses during wet years and prevent upward movement of the nitrogen during dry seasons, (d) possible injury by high salt concentration from larger amounts of fertilizer would be avoided, and (e) the application of fertilizer before the rush season would give better labor distribution. Scarseth et al. (1943) proposed plow-sole application of fertilizer in Indiana as a possible means of insuring ample plant nutrient supply, particularly nitrogen, during dry seasons. They reported that up until about 1939 response to small amounts of nitrogen applied in the row or as side dressing to corn was uncertain. These investigators used higher rates of fertilizer than were generally used in the fertilization of corn in the Corn Belt a t that time and showed significant increases in corn yields by supplementing row applications with furrow-bottom placement of additional nitrogen. These tests were not designed to compare equal amounts of plant nutrients supplied by different methods. Essentially, these experiments showed that high rates of fertilizer would greatly increase the yields of corn, particularly on the less fertile soils of the Corn Belt. Furthermore, they showed t.hat good response to corn fertilization was possible during years of low rainfall. These investigation3 stimulated interest in deep placement of fertilizers throughout the Corn Belt. Yoder (1945), reporting results on Wooster-Canfield silt loam in

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JONES AND HOWARD T. ROGERS

Ohio, compared plow-sole application with row application and concluded that plow-down fertilization was no more effective for corn than other methods, even under extreme drouth conditions. I n these tests equal amounts of plant nutrients were used by several methods of placement, including combinations of row and plow-sole fertilization. Millar (1944) reported ll-year averages of corn yields on Hillsdale sandy loam, showing that equivalent amounts of R complete fertilizer were more effective when applied in the row at planting than when placed on the furrow bottom or broadcast and plowed under. Caldwell et al. (1946) concluded from 3 years of tests on deep placement that this method was not effective in Minnesota even during dry periods. Rich and Odland (1947) concluded after one dry season and two normal years that the usual band application was fully as effective for silage corn in Rhode Island as plow-sole or other deep placement of all or part of the fertilizer. These investigators pointed out that the rapid early growth obtained by row placement lessened weed competition. In placement tests in Nebraska with nitrogen on corn, Fitts et al. (1946) showed that nitrogen applied a t planting time or last cultivation was equally as good as furrow-bottom placement. Volk (1946) pointed out that deep placement of fertilizer has been long practiced in the southeastern United States, since farmers in the Cotton Belt placed fertilizer in the “middle burster” bottom by hand and bedded on it, many years before mechanical distributors were developed. Tests in the southern states, however, have failed to show that deep placement of fertilizers for corn has any marked advantage over row placement. Bartholomew (1948) reported that there was no consistent benefit from plow-sole application in 37 tests a t various locations in Arkansas. Krantz (1948) found no difference between side-dressed and plow-sole applications of nitrogen on corn in experiments in North Carolina. A limited number of tests has been conducted with small grain and other crops. Smith (1947) failed to obtain response to nitrogen and phosphate on winter wheat in Kansas from furrow-bottom placement, although significant increases in yields were obtained when these fertilizers were placed with the seed or when the nitrate was top-dressed in the spring. Yoder (1945) concluded that all of the fertilizers for small grain should be applied with the drill a t time of seeding, and Weidemann (1943) reported that placing the fertilizer deep in the soil by plow-under methods or deep drilling was not as effective on wheat yields as broadcasting and discing the fertilizer materials into the surface soil. Experiments with soybeans on Miami loam in Michigan failed t o give favorable increases for plowed-under applications of fertilizer, according

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to Millar (1944). Karraker and Freeman (1944) failed to obtain any benefit to yield or quality of burley tobacco from placing part of the fertilizer on the furrow hottom as compared with row-side band placement. Merrill (1948) pointed out that the best method of application of fertilizers has been such a controversial matter that it has been very difficult for the equipment manufacturer to develop new machines for distributing fertilizers. Widespread interest in plow-sole placement encouraged farm equipment manufacturers to develop special fertilizer distributors. It is estimated that a total of approximately 30,000 attachments for plow-sole application has been sold, principally in the Corn Belt. The practice was most widely used in Wisconsin, Illinois, and Indiana, but recent reports from the Corn Belt states reveal that many of the fertilizer distributors developed for plow-sole applications. have been discarded by the farmers. Interest in this method of placement is apparently decreasing, as evidenced by the following record of sales of one manufacturer of plow-sole fertilizer distributors: Date 1945 1946 1947 1948

Number of distributors sold 9,W 2,000 600

None

Some of the disadvantages of furrow-bottom placement which have been mentioned in various reports include (a) fertilizer is placed too far below roots of small plants, (b) in cool, wet seasons conversion of ammonia nitrogen to nitrate is too slow, (c) plowing operations are interfered with, (d) distributors on the market have insufficient hopper capacity and are adapted only to two-bottom plows, and (e) restricted aeration in the fertilizer zone in some soils results in poor response to plow-sole application. Experimental findings to date show little or no advantage of deep placement over conventional methods for most crops on which it has been tested, when equivalent amounts of fertilizer are used. b. Subsurface Placement of Fertilizers for Sod Crops. Agronomists have been interested for a t least 20 years in the idea of subsurface placement of fertilizers and liming materials for sod crops. Rogers (1942) found that appreciable losses of surface-applied fertilizer may occur through runoff from pasture lands under certain conditions. It has been commonly observed that poor growth is obtained from permanent pasture sods during dry seasons. Furthermore, experimental data show that

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RANDALL J . J O N E S A N D HOWARD T. ROGERS

phosphate fertilizer moves down in the soil very slowly. These observations suggest that there might be some benefit from subsurface placement of this material as contrasted with the conventional method of surface application. Before the development of suitable machines for subsurface placement of fertilizers in grasslands, Midgley (1931) placed superphosphate in knife grooves 4 inches apart and 6 inches deep in small plots of bluegrass sod. He reported a 57 per cent increase in growth of the bluegrass over surface application. An experimental fertilizer placement machine which will place the fertilizer in sod crops in bands a t any desired depth from about 2 to 9 inches and a t any spacing from about 6 to 32 inches was described by Schroeder (1947). Placement tests were conducted a t two locations by the Kentucky Agricultural Experiment Station (1947)" in which phosphate from four different sources was placed in bands 12 inches apart and 4 inches below the surface. There were no beneficial effects the first year from placing all of the phosphate below the surface. These tests did not include a combination of surface and subsurface placement. More recently tests have been initiated in Virginia, Kentucky, and Georgia which have included a split spplication with part of the phosphate on the surface and part below the surface a t various depths and spacings, but results are not yet available. Studies are under way in New York state using the tracer technique with radioactive phosphorus to compare subsurface placement of phosphate fertilizer with surface applications on permanent pasture sods. Drake (1948)" failed to find any benefit on alfalfa from subsoil placement of part of the phosphate in bands 12 inches apart and 8 inches deep over 'standard placement in Cecil clay loam during a dry season. Caldwell et al. (1946) reported that broadcast surface applications of fertilizer for alfelfa were as effective as plow-sole placement on Clarion soil in tests in Minnesota. Reports from the North Carolina Agricultural Experiment Station (1947)" did not show any benefit to alfalfa or to a lespedeza-Dallis grass mixture from subsurface localized applications of phosphate or potash fertilizers over mixing in the upper 4 inches of soil or broadcasting on the surface. I n this test the subsurface treatment was the application of 90 per cent of the fertilizer a t a depth of 4 inches, with the remaining 10 per cent applied to the surface. Parberry (1946), however, reported that a 400-lb. per acre application of superphosphate placed 1 inch below the surface of a brown iateritic soil in Australia was definitely superior to surface applications in stimulating ryegrass yields. Placement a t the l-inch depth was as good as 2- or 3-inch placements. Brown and Munsell (1938) found that limestone applied on the sur-

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face a t the rate of 2 tons per acre to a fine sandy loam had penetrated to a depth of 6 inches in a pasture sod after 10 years. They concluded, however, that the rate of penetration was sufficient to make surface application to grassland an effective and efficient method of liming. Pohlman (1946), using glazed tile cylinders, showed that liming the 16-24 inch layer of Gilpin silt loam tripled the yield of alfalfa when the surface 0-8 inches had a pH of 5.6. Maximum yields were obtained when the entire 0-16 inch layer was limed to neutrality. A 50-per cent increase in root growth in the 16-24 inch zone was obtained by liming this layer. I n tests conducted by the North Carolina Agricultural Experiment Station (1947) ,+'either eubsurface placement of limestone or mixing the liming material with the surface 4 inches of soil was superior to surface placement on alfalfa. Experimental findings thus far on subsurface placement of fertilizers for sod crops do not consistently show an advantage for this method. Possibly, Volk's (1946) observation that plants appear t o be able to take plant nutrients from a soil zone the moisture content of which is below wilting point, if some of the plant roots are in a moist medium, may be related to this problem. The need for more fundamental studies on root distribution and plant nutrient feeding a t various moisture levels is apparent. c. Application of Fertilizers in Irrigation Water. Various liquid fertilizer materials have been applied through irrigation waters, including both anhydrous and aqueous ammonia, phosphoric acid, sulfuric acid, sulfur dioxide, ond water solutions of various carriers of nitrogen and phosphate, as well as mixed fertilizers. The application of fertilizer in surface irrigation wat,er, according to Proebsting (1948),* started in California about 30 years ago; and McGeorge (1948)* reported that it began in Arizona about 1933. The practice, however, has become reasonably widespread only within the last 5 years, as indicated by statistics on liquid fertilizers compiled by the California State Department of Agriculture (1947). The combined amount of liquid mixed fertilizer and phosphoric acid used in California increased from about 2,000 tons in 1943 to over 13,000 tons in 1947. I n addition, a large tonnage of anhydrous ammonia was used in that state during the 1947-1948 crop year, most of which was applied in irrigation water. During the first quarter of 1948, approximately $750,000 was spent in California for liquid mixed fertilizer, although plant nutrients in this form cost approximately 4 times as much per unit as in solid materials. This represented about one-sixth of the total expenditure for

64

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JONES AND HOWARD T. ROGERS

mixed fertilizer during these 3 months, according to the California State Department of Agriculture (1948). McCollam and Fullmer (1948) reviewed the history of the use of fertilizer solutions in California from 1923, when the first liquid fertilizer plant was built, to 1948 when 30 companies were distributing fertilizers in “liquid” form. They pointed out the following developments in the compounding of liquid mixed fertilizers: (a) nitrogen is generally added in the forms of ammonium nitrate, nitrogen liquors, urea, and potassium nitrate; (b) phosphorus is added almost exclusively as phosphoric acid, with some ammonium phosphate being used; and (c) muriate of potash and potassium nitrate are used as sources of potash. One manufacturer sold for agricultural use 5,500 tons of 53-per cent phosphoric acid during 1947 and approximately 6,000 tons in 1948. The largest tonnage was applied in California and most, of the remainder in Arizona, Utah, and Colorado. Records of the Arizona Fertilizer Control Office (1948) show that 11 registrants listed 36 products for sale as liquid fertilizer in that state during 1947. -4side-dressing service by which any desired fertilizer mixture can be applied t,hrough irrigation waters is the latest development in Arizona (McGeorge, 1948)*. These mixtures are usually made from “simples” which are mixed a t the time of application in the water a t the irrigation ditch. The Washington State Department of Agriculture (1948)* reported statistics showing that 23 per cent of the nitrogen sold as straight nitrogen materials in that state during the 1946-1947 crop year was in the form of liquid ammonia which is assumed to have been used in irrigation water. Although Jones and Green (1946) did not report supporting data, they stated that, phosphorus added as phosphoric acid in irrigation water penetrated the root zone of such crops as citrus, sugar beets, and alfalfa and tended to convert native soil phosphates to more available forms. Chapman et al. (1945) concluded that experimental findings did not show a sound basis for the use of sulfur, sulfur dioxide, sulfuric acid, and other acidifying agents in California citrus groves. Surveys by Smith (1946) and Turnell (1948), however, indicated widespread interest in this practice and rather extensive use of sulfur for acidifying soils in some of the western states. Fertilizers are applied through sprinkler irrigation in the Willamette Valley, according to Powers (1947). He has stated also that the fertilization of mint in Oregon through irrigation water is a general practice. The addition of fertilizer to pastures through irrigation in the coastal area of Oregon and Washington is becoming increasingly important. King et al. (1943) in Oregon, Nissley (1946) in New Jersey, and Davis

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(1948) in Georgia, described the development of simple equipment for handling and introducing fertilizers into sprinkler irrigation systems. Generally, the solutions are introduced into the system through the suction pipe ahead of the pump. The advantages and disadvantages of applying nitrogen, phosphate, and potash fertilizers through irrigation water are pointed out by Peikert and Cook (1948). They concluded that (a) since nitrogen carriers are readily soluble, and reactions in the soil 'do not prevent broadcast applications, nitrogen may well be applied through irrigation; (b) phosphate fertilizers are not generally suitable for irrigation application because of low solubility and reduced efficiency from broadcasting; and (c) potash may be applied in irrigation water but it is probably desirable to use this method only as a supplement to direct soil application a t planting. There is increasing interest in irrigation for a wide variety of crops, even in regions with 30 to 50 inches of rainfall. It appears that the practice of applying fertilizers through irrigation water may become more widespread, particularly for supplemental applications on vegetables and grazing crops and in special cases where crop growth does not permit the use of conventional fertilizer distributors. d. Direct Application of Liquid Fertilizers. Some of the advantages claimed by Tiedjens and Schermerhorn (1942a) for the use of liquid fertilizers were (a) less fertilizer required; (b) avoidance of injury to plant roots from heavy applications of dry fertilizer; (c) more uniform distribution of small quantities; (d) fertilizers in poor physical condition can be utilized; (e) plant response in dry weather; and ( f ) light applications may be applied as needed by the plant. The use of starter solutions is one of the most important practices in which liquid fertilizers are applied directly to the soil. According to Hester (1939), the practice of applying plant nutrients in transplanting water for tomatoes was proposed for New Jersey about 1937. Nine years later, Hester (1948)" stated that the use of starter solutions was a generally accepted practice in transplanting tomatoes and some other vegetable crops. Experiments reported by Hester (1939) showed that starter solutions increased the earliness of tomatoes about one week and increased the yields from 1 to 2 tons per acre, as compared with transplanting with water alone, As would be expected, t.he results obtained from starter solutions were governed by method and rate of fertilization. Tiedjens and Schermerhorn (1942a) reported favorable results from starter solutions on sweet potatoes, tomatoes, lima beans, snap beans, sweet corn, celery, and peppers. Some of the best results from the use of transplanting solutions were obtained with sweet potatoes. These

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RANDALL J . JONES AND HOWARD T. ROGERS

investigators (1942b) reported the following yields of sweet potatoes which were obtained from starter solutions used in conjunction wit,h 800 lbs. of 3-12-15 fertilizer placed in bands to side of row: N-P20n-K20 Water 0-31-0 17-0-16 13-26-13

Lb./50 gal. water

5 .O

Bu./acre 156 160 226

3 .O

298

6.0

Sayre (1943) compared 23 different starter solution treatments on tomatoes and reported t,hat the best solutions increased early yields about lv2 tons per acre. The higher analysis mixtures left less insoluble residue, but nearly all mixtures which were not too acid in reaction made effective starter solut,ions. Satisfactory solutions were made by dissolving 3 to 4 lbs. of 13-26-13, 11-32-14, or 16-48-18 fertilizer mixtures in 50 gallons of water. About 90 gallons of this solution were used in transplanting one acre of tomato plants. Raleigh (1942) recommended an 11-32-14 mixture composed of 2 parts of Ammo-Phos and 1 of potassium nitrate. Prince and Tiedjens (1942) studied the solubility and reaction of 10 grades of mixed fertilizers and reported that practically all of the nitrogen and potash in medium or high analysis commercial fertilizer mixtures was water-soluble, while one-half to two-thirds of the phosphate was water-soluble. The pH of water solutions of these commercial grades of fert,ilizer ranged from 3.3 to 6.3. These investigators concluded that fertilizer solutions with a pH value below 5.0 are not considered suitable for starter solutions. The reaction of liquid fertilizers sold in California (Calif. State Dept. Agr., (1946)), which are used primarily in irrigation water, varied from pH 2.0 to 10.0, with most solutions definitely acid. According to Heater (1948)* the trend is toward all soluble mixtures such as 11-52-17, which may be made from diammonium phosphate and monopotassium phosphate. These water-soluble mixtures are offered at considerably higher prices per unit of plant nutrients than grades of mixed fertilizer such as 8-28-8 or 10-20-10, but growers object to the sludge left behind by these ordinary commercial fertilizer mixtures. The direct application of liquid mixed fertilizers to the soil, other than in the role of sta.rter solutions, is in the experimental stage. Prince and Tiedj ens (1942) reported favorable results from side-dressing sweet. potatoes and other horticultural crops with mixed fertilizer solutions and pointed out that the best response to side dressing with liquid fertilizers appeared on sandy soils. Except for starter solutions, it would appear doubtful that the prac-

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tice of using mixed fertilizer solutions by direct application to the soil will become widespread, because of the labor requirements and other costs of transportsing and handling in the field large volumes of lowanalysis material. e. Plant-Nutrient Spraps. Miscellaneous methods of fertilizer application, including soil and plant injections and application in solution, were covered in a recent review (Editorial, 1947c), which emphasized European developments in this field. This discussion will be restricted largely to the application of plant nutrients through foliage or dormant sprays and will indicate the spread of this practice in the United States. According to Woodhams (1947), the use of minor element sprays on citrus began about 1933 in San Bernardino County, California. Parker and Batchelor (1942) concluded from experiments on oranges that zinc was most satisfactorily applied to citrus through foliage sprays. As might be anticipated, this practice has been found to have a distinct advantage on calcareous soils where difficulty is experienced in maintaining in the soil solution a satisfactory concentration of several of the essential elements. It was estimated by Woodhams (1947) that 138,000 Ibs. of chemicals containing zinc combinations were used in San Bernardino County in 1945 in sprays on 402,000 trees. The same year about 35,000 trees were sprayed with nutrient sprays of copper and about 10,000 trees with manganese. Chapman et al. (1945), discussing the status of nutrient deficiencies of citrus in California, concluded that (a) iron sprays, injections into trunks, or attempts to acidify the soil to overcome iron chlorosis, have not been very successful; (b) manganese deficiency associated with alkaline soil reactions can be partially corrected with sprays containing manganese sulfate; and (c) zinc and copper deficiencies may be controlled by foliage sprays with the sulfate salts of these metals, but their use should be timed with reference to fumigation. Serious injury is reported to result from HCN-fumigation immediately after spraying with copper compounds. Although the sulfates of copper, zinc, and manganese are compatible in a single spray, their general use is not recommended because certain spray residues encourage spider and scale infestations, increase injury from fumigation, and leave undesirable deposits on fruit. However, where zinc deficiency is perennial and manganese and copper deficiencies are recurrent problems, a combination of 5 lbs. of zinc sulfate, 1 Ib. each of manganese and copper sulfates, plus 4 Ibs. of soda ash or hydrated lime in 100 gallons water is recommended as a nutrient spray. Serious fruit drop in late summer or splitting of navel oranges in the fall may be associated with minor element nutrition.

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RANDALL J. JONES AND HOWARD T. ROGERS

Zinc sprays are more effective if applied just prior to growth flushes in the spring or fsll. Powers and Wood (1947) reported that in Oregon the use of zinc tacks or injections had been replaced by foliar or dormant sprays of this nutrient. Dormant sprays containing zinc were reported effective on cane and deciduous tree fruits. One application of ninc sulfate as a spray was reported by Malan (1946) as sufficient to correct zinc deficiency of citrus in South Africa for 2 years. Applications of zinc sulfate to the soil were described as uneconomical. Camp (1948) ’reported that copper, zinc, and manganese sprays were widely used in Florida on citrus. Ruehle (1947a, 1947b) recommended nutritional sprays containing copper and zinc for mangos and guavas in Florida and pointed out that heavy nitrogen fertilization may intensify zinc deficiency in bearing mango trees. Widespread use of nutritional sprays to correct zinc deficiency in avocados is indicated by Ruehle (1948)’ who estimated that 85 per cent of the acreage planted to this crop in the Homestead, Florida, area received at least one zinc-copper spray each year. I n the same area, potato and tomato growers prefer Dithane-zinc su!fate as a fungicide in order to combine disease control with applications of zinc as a nutrient. The use of copper-lime spray mixture to correct copper deficiency of tung trees in t,he northern peninsula of Florida is reported by Dickey e t al. (1948). Although Jardine (1946) stated that soil treatment was preferred to sprays, he found that spraying grapes with a solution of borax 3 weeks before blossoming, combined witsh swabbing freshly cut surfaces, produced large, evenly developed grapes instead of “hen and chicken” bunches, a fruit,ing habit attributed to boron deficiency. According to MacLachlan and Strong (1948), spraying or dusting with borax to correct “water-core” or “brown-heart” of rutabagas is becoming widespread on the high-lime soils of the Province of Ontario in Canada, where soil applications at time of planting are not satisfactory. Curative sprays of manganese sulfate applied to wheat, peas, and potatoes growing on the marshlands of Kent produced significant increases in yields of these crops, according to Roach (1944). Results of experiments in New York, Massachusetts, and Maine are reported by Boynton (1947) to indicate that sprays containing epsom salts may be used to control magnesium deficiency of apples. The use of acid-forming fertilizers, sulfur dusting, and heavy potassium fertilization are factors contributing to magnesium deficiency of soils in New York orchards. Some of these soils are as acid as pH 4.0. Due to slow-

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ness of response to soil treatments, magnesium sprays may be used to control “blotch” until applications to the soil take effect. Fisher et al. (1948) found that nitrogen sprays may be used under some conditions to influence color and set of fruit in apple orchards. By proper timing and dosage, better control of these factors may be possible through sprays than by soil application. The commercial possibilities of nitrogen sprays, however, have not yet been determined. Sayre (1943) tried foliage sprays of nitrogen and phosphate on tomatoes but found the method less satisfactory than starter solutions. He reported that some nitrogen was absorbed through the leaves, but phosphate was not effective when applied in this manner.

4. Fertilizing f o r Winter Grazing in the Southeastern States The fertilization of grasslands is not a new practice, but the tonnage of fertilizer used in the United States for this purpose has been small until recent years. One of the most significant developments in the field of pasture fertilization has been the application of commercial fertilizers to fall and winter grazing crops to make possible year-round grazing in the southeastern states. The development of this practice, which has become widely accepted, resulted primarily from the discovery of the importance of adequate fertilization and early planting of certain forage crops which make good growth during the fall and early spring. The dairymen who have succeeded in growing fall and winter pastures report stimulated milk flow which is frequently greater than might be expected from the total amount of forage provided. Based on experiments in Alabama, Koehn and Salmon (1948) reported large increases in the vitamin A value of milk and butter as a result of small amounts of green pasturage in the winter months. Lowery (1948) * attributes the development of fall and winter grazing in Alabama primarily to the cooperative test-demonstration program of the extension service and the TVA in which the role of fertilizers in increasing feed production has been emphasized. Based on reports from Lowery (1948) * in Alabama, Alexander (1948) * in Georgia, Dobson (1948)* in North Carolina, Walrath (1948) * in Tennessee, and Woodle (1948)* in South Carolina, the total area planted expressly for fall and winter grazing in 1947-1948 in these five states approached 1% million acres. The major crops used were small grains, ryegrass, and crimson clover. These crops were planted alone or in mixtures. Where farm records have been kept in Alabama and South Carolina, the average number of days during which the winter crops were actually grazed varied from about 100 to 125 during any one grazing season. The

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RANDALL J . JONES AND HOWARD T. ROGERS

potential grazing season for the crops mentioned is as much as 200 days in certain areas. A conservative estimate of the winter-grazing potential in the seven southeastern states of Alabama, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee, based on 1947-1948 plantings, would be about 3,000,000 acres of small grain and ryegrass or mixtures of these crops with winter legumes. The average rate of fertilizer application for these crops in states which kept records appears to be about 20-40-25 lbs. per acre of N-P20s-K20 at planting plus a fall top dressing of 32 lbs. of nitrogen. Thus, the acreage of such crops planted in 1947-1948 would require about 30,000 tons of N, 60,000 tons of P ~ O S , and 37,500 tons of KzO a t planting, plus about 48,000 tons of additional nitrogen for fall top dressing. All of the stsates surveyed indicated a heavy, unsatisfied demand in the fall of 1948 for nitrogen fertilizer for fall and winter grazing crops. Jon'es (1944) in Alabama, and Woodhouse et al. (1946) in North Carolina, showed that the growth of such crops as oats, barley, and rye during the winter and spring months was almost direct.ly proportional to the amount of nitrogen added. For example, Jones (1944) reported the following dry weight yields of oats from clippings taken March 20 a t Auburn, Alabama : N applied, Ib./acre None 40 80 120 160

Dry matter yields, lb./acre 361 1,852 3,113 3,802 4,621

All plots received lime plus 800 lbs. of 0-14-10 fertilizer, and top dressing began 2 weeks after planting, with 40-pound increments of nitrogen made periodically. Grazing small grains in the late fall and winter has been a common practice in the South for years. Generally, livestock were removed in late winter, and a top dressing of nitrogen was made to the small grain. Oats and barley have been the principal crops used in this manner, but the amount of grazing furnished was quite small without liberal application of fertilizer in the fall, particularly nitrogen. Jones (1945) reported data which showed the possibilities of combined grazing and grain production from oats on Decatur clay loam in north Alabama. Returns from winter grazing systems in terms of animal products have been reported by Hazlewood (1936) and Means and Bennet.4 (1947). The latter investigators reported average gains ranging from 209 to 300

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lbs. of beef per acre produced on oats in the Black Belt of Mississippi during the winter and spring season of 1947-1948. Six lots of 11 calves each were grazed on 9-acre paddocks of oats and oat-legume mixtures from December to June, except for a 15-day period in April. The calves gained on the average 250 lbs. in weight during this period, with an average daily gain ranging from 1.1 to 1.6 lbs. The cost per acre to furnish the winter grazing was estimated a t $20, with a net return of $30 to $47 per acre. Recent grazing demonstrations throughout the area point up the possibilities of all-year pasture systems which are made possible through adequate fertilization, selection of adapted crops, proper seedbed preparation, and timely planting of crops. 5. Miscellaneous Practices

The discovery of the effects o i fertilizers on marine and fresh-water populations opened up a new field for the use of fertilizers, which has attained commercial importance in the form of farm pond fertilization. After extensive investigations on the use of fertilizers for fish production, Swingle and Smith (1947) reported that unfertilized ponds in Alabama produced from 40 to 200 lbs. of fish per acre, as contrasted with 400 to 600 Ibs. from properly fertilized ponds. These investigators recommended for farm ponds in Alabama the application of 100 lbs. of 6-8-4 fertilizer plus 10 lbs. of nitrate of soda, beginning in March, and indicated that this treatment should be repeated 8 to 14 times during the year. Ponds which stay muddy or those with excessive flowing watet cannot be satisfactorily fertilized. Experiments were designed to measure the effects of fertilizing sea waters in the lochs along the west coast of Scotland (Editorial, 1947b). These tests showed that (a) the spring surge of plankton could be maintained in the sea water through the summer by the application of about 225 lbs. of sulfate of ammonia or its equivalent as nitrate of soda plus 100 lbs. of superphosphate per acre; (b) there was a remarkably rapid uptake of applied nitrogen and phosphate by plankton; (c) threefold increases in growth of minute forms of sea life resulted from fertilization; and (d) flounder and plaice grew to marketable size in one-half to twothirds of the time required in unfertilized lochs. The use of fertilizers as plant defoliants and for weed control has attracted considerable attention in recent years. As an example, Cox (1948)' stated that about 20,000 tons of calcium cyanamid were used in 1948 as a defoliant on cotton and that another 16,000 tons were used on tobacco plant beds for weed control. Measurable response by succeeding crops would not be expected from a single application of 25 to

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30 lbs. of cyanamid per acre, 5 to 6 lbs. of nitrogen, which is the average rate used on cotton. The rates of application to plant beds, however, are usually sufficient to be reflected in plant growth. Another practice which appears to be gaining in popularity is the use of large spreader trucks for the application of bulk fertilizers. While statistics on the practice of broadcasting fertilizers with spreader attachments on transport trucks are not available, several reports (Editorial, 1947a; Lang, 1948; McLemore, 1948; Wolf, 1948) indicate that considerable quantities of fertilizer materials are applied by this method. Spreader trucks of 5- to 8-ton capacity were used to apply about onehalf of the fertilizer for 14,000 acres of truck crops grown on Seabrook Farms in New Jersey (Wolf, 1948). It was pointed out that “green” fertilizer direct from the mixers may be applied with these trucks if spread immediately. This bulk handling of fertilizers eliminates the costs of bags, bagging, conditioners, storage while curing, and rehandling. At least one mixing plant in northern Virginia mixes fertilizer of the desired analysis and delivers it in bulk directsly to spreader trucks (Editorial, 1947a). McLemore (1948) described the use of spreader trucks for applying fertilizer to a considerable acreage of cotton in Alabama. As pointed out by Lang (1948) these trucks are used in Illinois for broadcasting large amounts of rock phosphate. Long-term experiments are showing marked residual effects from fertilizers, particularly phosphates, which will tend to offset any temporary lowering of efficiency through broadcasting. I n view of this fact and because‘of the economy of bulk handling of fertilizers, it would appear that this practice of broadcasting fertilizers with spreader trucks may be a promising method under some conditions, particularly where high rates per acre are used. Other developments in the field of fertilizer use which can be only mentioned here are (a) the increasingly widespread use of a number of secondary and minor plant nutrient elements both in mixed fertilizers and as straight materials; (b) the improvement of diagnostic soil and plant tests, including visual deficiency symptoms, tissue tests, leaf injections and paints, and soil analysis; and (c) the evaluation of the residual effects of fertilizer applications.

REFERENCES AlexaL-er, E. D. 1948. Personal communication. Alway, F. J., and Nesom, G . H. 1944. J. Am. SOC.Agron. 36, 73-88. Andrews, W. B., Edwards, F. E., and Hammons, J. G . 1948. Miss. Agr. Expt. Sta. B d l . 451. Aria. Agr. Expt. Sta. 1945. 66th Ann. Rept. pp. 9-10.

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Arizona Fertilizer Control Office. 1948. Ariz. Agr. Expt. Sta. Spec. Bull. Armiger, W. H., Forbes, I., Jr., Wagner, R. E., and Lundstrom, F. 0. 1948. J . Am. SOC.Agron. 40, 342-356. Bartholomew, R.P. 1948. Ark. Agr. Expt. Sta. Bull. 473. Bell, R. E., and Griffith, D. T. 1947. Bonneville Power Administration, US.Dept. Interior, Mimeo. Rept. Bledsoe, R. W., and Blaser, R. E. 1947. J. Am. SOC.Agron. 39, 146-152. Boynton, D. 1947. Soil Sci. 63, 53-58. Brooks, 0.L. 1948. Ga. Agr. Expt. Sta. Mimeo. Rept. Brown, B. A., and Munsell, R. I. 1938. Soil Sci. SOC.Am. Proc. 3, 217-221. Brown, B. E., and Clark, K. G. 1943. Am. Fertilizer 98(5), 10-11,24, 26. Caldwell, A. C., MacGregor, J. M., and Rost, C. 0. 1946. Minn. Farm Home Sci. III(Z), 6-7. Calif. State Dept. Agr., Bur. Chem. 1946. 27th Ann. Rept. XXXV, 264-282. Calif. State Dept. Agr., Bur. Chem. 1947. 28th Ann. Rept. XXXVI, 243-258. Calif. State Dept. Agr., Bur. Chem. 1948. Announcement FM-164. Camp, A. F. 1948. Personal communication. Chandler, R. F., Jr., and Musgrave, R. B. 1944. Soil Sci. SOC.Am. Proc. 9, 151-153. Chapman, H. D. 1944. Proc. Natl. Joint Conam. Fertilizer Application pp. 18-23. Chapman, H. D.,Brown, S. M., and Rayner, D. S. 1945. Citrus Leaves 25, 17281739. Copson, R. L., Pole, G. R., and Baskervill, W. H. 1942. Znd. Eng. Chem. 34, 26-32. Cox, T.R. 1948. Personal communication. Cummings, R. W. 1947. Plant Food J . 1(1), 4-7. Curtis, H. A., Copson, R. L., Adams, A. J., and Junkins, J. N. 1938. Chem. Met. Eng. 45, 318-322. Davis, E. H. 1948. Better Crops with Plant Food XXXII(6), 17-23,45-46. Dickey, R.D., Drosdoff, M., and Hamilton, J. 1948. Flu. Agr. Expt. Sta. BulE. 447. Dobson, S. H. 1948. Personal communication. Drake, M. 1948. Personal communication. Dyke, K. 1948. Chronica Naturae 104, 150-154. Editorial, 1947a. Am. Fertilizer l07(10), 14. Editorial. 1947b. The Chem. Age, London LVII,531-533. Editorial. 1947c. Soils and Fertilizers X, 117-122. Editorial. 1948. Am. Fertilizer 109(5), 11. Fisher, E.,Boynton, D., and Skodvin, K. 1948. Proc. Am. SOC.Hort. Sci. 51, 23-32. Fitts, J. W., McHenry, J. R., and Allaway, W. H. 1946. Nebr. Agr. Ezpt. Sta. Bull. 382. 12, 198-202. Fuller, W. H., and Clark, K. G. 1947. Soil Sci. SOC.Am. PTOC. Garman, W. H. 1948. Personal communication. Granberg, W. J. 1948. Rock Products 51(10), 108, 110. Harlewood, B. P. 1936. Tenn. Agr. Expt. Sta. Circ. 58. Hester, J . B. 1939. Report of Ten-Ton Tomato Club of N e w Jersey. Hester, J . B. 1948. Personal communication. Hignett, T. P., and Hubbuch, T. N. 1946. Ind. Eng. Cheni. 38, 1208-1216. Hill, W. L., Ward, F. N., Armiger, W. H., and Jacob, K. D. 1948. J . Assoc. Of. Agr. Chem. 31, 381-397. Hinkle, D. A. 1942. N . Mex. Agr. Expt. Sta. Bull. 289. Houghland, G. V. C., Clark, K. G., Hawkins, A., and Campbell, J. C. 1942. Am. Fertilizer 97(7), 6-8,24, 26.

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Jackson, M. L., and Chang, S. C. 1947. J. Am. SOC.Agron. 39, 623-633. Jacob, K. D. 1948. Fertilizer Rev. X X I I I ( l ) , 1-9, 19-20. Jacob, K. D., and Mehring, A. L. 1947. Agr. Chem. II(12), 21-24, 61-67. Jacob, K. D., Ward, F. N., Hill, W. L., and Pinkerton, C. 1947. J . Assoc. Of. Agr. Chem. 3 4 529-548. Jardine, F. A. L. 1946. Queen.sland Agr. J . 62(2), 74-78. Jones, R. A., and Green, J. 1946. Proc. Am. SOC.Sugar Beet Technol. pp. 36-39. Jones, R. J. 1942. Proc. Natl. Joint Comm. Nitrogen Utilization pp. 16-17. Jones, R. J . 1944. Proc. Natl. Joint Comm. Nitrogen Utilization pp. 12-16. Jones, R. J. 1945. Proc. Natl. Joint Comm. Nitrogen Utilization pp. 131-132. Jones, R. J. 1947. Paper presented at meeting of Western Society of Soil Science, San Diego, California. Unpublished. Jordan, H. V. 1947. Miss. Farm Research 10(2), 1,3, 7. Karraker, P. E., and Freeman, J. F. 1944. Proc. Natl. Joint Comm. Fertilizer Application pp. 141-143. Karraker, P. E., Miller, H. F., Bortner, C. E., and Todd, J. R. 1941. B y . Agr. Expl. Sta. Bull. 413. Kennedy, A. L. 1944. Unpublished data. Kentucky Agr. Expt. Sta. 1947. Ann. Rept. to T V A of Coop. Fertilizer Invest. Unpublished. King, A. S., Newcomb, G . T., and Chenoweth, 0. V. 1943. Oreg. State Coll. Ext. Bull. 626. Koehn, C. J., and Salmon, W. D. 1948. Ala. Agr. Expt. Sta. Bull. 266. Krantz, B. A. 1945. Better Crops with Plant Food XX IX(3), 19-22, 48-49. Kranta, B. A. 1947. Better Crops with Plant Food XXXI(Z), 6-10, 45-47. Krantz, B. A. 1948. N.C. Agr. Expt. Sta. Agron. Inf. Circ. 142. Lang, A. L. 1948. Fertilizer R e v . XXIII(6), 9-12. Leavitt, F. H. 1942. US. Patent 2,285,932. Leavitt, F. H. 1948. Paper presented at 22nd Fall Meeting, The Natl. Fertilizer Assoc., Atlanta, Ga. Unpublished. Lorenz, 0. A. 1948. Personal communication. Louisiana Dept. Agr. and Immigration. 1948. Personal communication. Lowery, J. C. 1948. Personal communication. McCollam, M. E., and Fullmer, F. S. 1948. Belter Crops with Plant Food XXXII(6), 6-8, 46-47. McGeorge, W. T. 1948. Personal communication. MacIntire, W. H., Winterberg, S. H., Hatcher, B. W., and Palmer, G. 1944. Soil Sci. 57, 425-442. MacLachlan, J. D., and Strong, W. F. 1948. Sci. Agr. 28, 61-65. McLemore, P. 1948. Progressive Farmer 63(3), 22. Madorsky, S. L., and Clark, K. G. 1940. Znd. Eng. Chern. 32, 244-248. Malan, P. F. 1946. Farming in S. Africa 21, 8, 18. Mansfield, G. R. 1942. Ind. Eng. Chem. 34, 9-12. Means, R. H., and Bennett, H. W. 1947. Miss. Agr. Ezpt. Sta. Circ. 133. Mehring, .A. L. 1948. Better Crops with Plant Food XXXII(S), 6-14, 40-47. Merrill, R. M. 1948. Proc. Natl. Joint Comm. Fertilizer Application, pp. 32-35. Midgley, A. R. 1931. J . Am. SOC.Agron. 23, 788-799. Millar, C. E. 1944. Mich. Agr. Expt. Sta. Quart. Bull. 26, 173-176. Moulton, R. W. 1947, Chem. Eng. Progress 43(4), 163-164.

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N. Carolina Agr. Expt. Sta. 1947. Ann. Rept. to T V A of Coop. Fertilizer Invesi. Unpublished. Natl. Joint Comm. Fertilizer Application. 1948. Special Bull. Nissley, C. H. 1946. N.J. State Hort. SOC.News 27, 1884-1885. O’Brien, R. E. 1944. V a . Agr. Expt. Sta. Bull. 364. Ohlrogge, A. J., Krantz, B. A., and Rcarseth, G. D. 1944. Soil Sci. Soc. Am. Proc. 8, 196-200.

Parberry, N. H. 1946. Agr. Gaz. N . S. Wales 57(8), 405409. Parker, E. R., and Batchelor, L. D. 1942. Colo. Agr. Expt. Sta. Bull. 673. Peikert, F . W., and Cook, R. L. 1948. Mich. Agr. Expt. Sda. Quart. Bull. 3 4 437444.

Pohlman, G. G. 1946. Soil Sci. 62, 255-266. Powers, W. L. 1947. Better Crops with Plant Food XXXI(l2), 15-20, 46. Powers, W. L., and Wood, L. K. 1947. Oreg. Agr. Expt. Sta. Circ. of Inf.418. Prince, A. L., and Tiedjens, V. A. 1942. N.J. Agr. Expt. Sta. Circ. 449. Proebsting, E. L. 1948. Personal communication. Raleigh, G. J. 1942. Cornell Ext. Bull. 377. Reynolds, D. S., Jacob, K. D., and Rader, L. F., Jr. 1934. Ind. Eng. Chem. 26, 406-412.

Rhoades, H. F. 1948. Personal communication. Rich, A. E., and Odland, T. E. 1947. Soil Sci. SOC.Am. Proc. 12, 253-254. Roach, W. A. 1944. East Mulling Research Sta. Ann. Rept. pp. 43-60. Roberts, G., Freeman, J. F., and Miller, H. 1942. Icy. Agr. Expt. Sta. Bull. 420. Rogers, H . T. 1942. Soil Sci. SOC.Am. Proc. 7, 69-76. Ross, W.H., Adams, J. R., Yee, J. Y., and Love, K. S. 1946. US.Dept. Agr. Tech. Bull. 912. Ruehle, G. D. 1947a. Flu. Sub-Tropical Expt. Sta. Mimeo Rept. 10. Ruehle, G. D. 1947b. Flu. Sub-Tropical Expt. Sta. Mimeo. Rept. 12. Ruehle, G. D. 1848. Personal communication. Sayre, C. B. 1943. N.Y. State Agr. Expt. Sta. Bull. 706. Scarseth, G. D., Cook, H. L., Kranta, B. A., and Ohlrogge, A. J. 19-13. Purdue Agr. Expt. Sta. Bull. 482. Scholl, W., and Wallace, H. M. 1948. Agr. Chem. III(6), 24-29, 59. Schroeder, E. W. 1947. Agr. Eng. Div., Commerce Dept., TVA, Special Rept. 33. Smith, A. M. 1946. Am. Fertilizer lOS(lO), 9-11, 26-28. Smith, F. W. 1947. Soil Sci. SOC.Am. Proc. 12, 262-264. Swingle, H. S., and Smith, E. V. 1947. Ala. Agr. Expt. Sta. Bull. 254. Tenn. Dept. Agr., Dept. Agr. Statistical Service. 1948. U S . Dept. Agr., Bur. Agr. Econ., Release 860. Tennessee Valley Authority, Agricultural Relations Staff. 1946. Soil Sci. SOC.Am. Proc. 11, 369-373. Tennessee Valley Authority, Dept. Agr. Relations. 1945. Mimeo. Rept. 3. Terman, G. L. 1944. Soil Sei. SOC.Am. Proc. 9, 154-158. Tiedjens, V. A., and Schermerhorn, L. G. 1942a. N.J. Agr. Expt. Sta. Bull. 694. Tiedjens, V. A., and Schermerhorn, L. G. 1942b. N.J. Agr. Expt. Sta. Bull. 697. Toevs, J. L., and Baker, G. 0. 1939. Idaho Agr. Expt. Stn. Bull. 230. Turnell, K. 1948. Comm. Fertilizer 78(3), 34. Turrentine, J. W. 1943. Potash in North America. Am. Chem. SOC.,Monograph Series 91. U.S. Dept. Agr. 1909-1935. Yearbooks.

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U.S. Dept. Agr. 1936-1946. Agr. Rtatistics. 1948. Crop Production, Octoher i,wiie. Volk, G. M. 1946. Fla. Agr. Expt. Sta., Ann. Rept. pp. 92-93. Volk, G. W. 1944. J. A m , SOC.Agron. 36, 46-56. Volk, N. J. 1942. Comm. Fertilizer “Yearbook” p p . 23-28. Volk, N. J. 1944. Proc. Natl. Joint Comm. Nitrogen Utilization p . 69. Volk, N . J. 1946. Hoard‘s Dairyman 91, 241-259. Volk, N. J., Tidmore, J. W., and Meadows, D. T. 1945. Soil Sci. 60, 427-435. Walrath, F. J. 1948. Personal communication. Walthall, J. H., and Bridger, G. L. 1943. Ind. Eng. Chem. 35, 774-777. Wash. State Dept. Agr. 1948. Personal communication. Waynick, D. D. 1934. Calif. Citrograph 19, 295, 310-311. Weidemann, A. G. 1943. J. Am. Soe. Agron. 35, 747-767. Wheeting, L. C. 1948. Personal communication. Whitney, W. T., and Hollingsworth, C. A. 1949. Znd. Eng. Chem.41, 1325-1327. Whittaker, C. W., Brown, B. E., and Adams, J. R. 1948. U S . Dept. Agr. Circ. 771. Wolf, B. 1948. Agr. Chem. III(11), 37, 39, 41, 73. Woodhams, G. E. 1947. Calif. Citrograph 32, 112-113. Woodhouse, W. W., Jr., Lovvorn, R. L., and Chamblee, D. S. 1946. N.C. Agr. Erpt. Sta., Agron. In!. Circ, 141. Woodle, H. A. 1948. Personal communication. Yee, J. Y., and Love, K. S. 1946. Soil Sci. SOC.Am. Proc. 11, 389-392. Yoder, R. E. 1945. Ohio Agr. Expt. Sta. Bull. 659, pp. 22-23.

U.S.Dept. Agr., Bur. Agr. Econ.

Soybeans * MARTIN G . WEISS

lozoa State College. Antes. Iowa CONTENTS

Page I. Introduction . . . . . . . . . . . . . . . . . . . . . . 78 I1. Production and Distribution . . . . . . . . . . . . . . . . . 80 1. In the World . . . . . . . . . . . . . . . . . . . . . 80 2. I n the United States . . . . . . . . . . . . . . . . . 81 111. Disposition and Utilization . . . . . . . . . . . . . . . . . . 83 IV. Physiology of the Soybean Plant . . . . . . . . . . . . . . . 85 1 . Floral Initiation . . . . . . . . . . . . . . . . . . . . 85 a . Varietal Differences . . . . . . . . . . . . . . . . . 86 b . Duration of Dark and Photoperiods . . . . . . . . . . . 87 c. Light Intensity and Photosynthesis . . . . . . . . . . . 88 d . Age and Position of Induced Tissue . . . . . . . . . . .I 89 e . Temperature Effects . . . . . . . . . . . . . . . . . 89 2 . Nutrition . . . . . . . . . . . . . . . . . . . . . . . 90 90 a . Nitrogen . . . . . . . . . . . . . . . . . . . . . 92 b . Phosphorus . . . . . . . . . . . . . . . . . . . . c . Major Cations . . . . . . . . . . . . . . . . . . 93 d . Micronutrients . . . . . . . . . . . . . . . . . . . 95 3. Root Temperatures . . . . . . . . . . . . . . . . . . 97 V . Effect of Climate and Location . . . . . . . . . . . . . . . . 97 1 . Location and Season Effects . . . . . . . . . . . . . . . . 97 2 . Simulated Hail Damage . . . . . . . . . . . . . . . . . 99 VI . Effect of Cultural Practices . . . . . . . . . . . . . . . . . . 101 1 . Rotations . . . . . . . . . . . . . . . . . . . . . . . 101 2. Fertilizers and Soil Management . . . . . . . . . . . . . . 102 a . Response . . . . . . . . . . . . . . . . . . . . 102 b . Placement . . . . . . . . . . . . . . . . . . . . 104 3. Seed Inoculation . . . . . . . . . . . . . . . . . . . 105 4. Seed Germinability . . . . . . . . . . . . . . . . . . . 107 a . Viability . . . . . . . . . . . . . . . . . . . . . 107 b . Disinfectants and Protectants . . . . . . . . . . . . . 108 c. Hormones . . . . . . . . . . . . . . . . . . . . . 109 5. Time of Planting . . . . . . . . . . . . . . . . . . . . 110 6. Method and Rate of Planting . . . . . . . . . . . . . . 111 7. Weed Control . . . . . . . . . . . . . . . . . . . . . 114 8. Harvesting . . . . . . . . . . . . . . . . . . . . . 114

* Paper

No . 5-1620 from the Iowa Agricultural Experiment Station, Project 719 . 77

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VII. Genetics and Cytology . . . . . . . . . . . . . . . . . . . 115 1. Genetic Analysis of Simply Inherited Characters . . . . . . . . 115 2. Linkage . . . . . . . . . . . . . . . . . . . . . . . 117 3 . Analysis of Yield Components . . . . . . . . . . . . . . . 118 4 . Inheritance of Oil and Protein Content . . . . . . . . . . . . 119 5. Polyploidy . . . . . . . . . . . . . . . . . . . . . . 121 6. Interspecific Crosses . . . . . . . . . . . . . . . . . . . 121 123 VIII. Variety Improvement . . . . . . . . . . . . . . . . . . . . 1. Introduction and Selection . . . . . . . . . . . . . . . . 123 2. Hybridization . . . . . . . . . . . . . . . . . . . . . 123 a . Making the Cross . . . . . . . . . . . . . . . . . . 123 b . Breeding Methods . . . . . . . . . . . . . . . . . . 125 c. Early Generation Testing . . . . . . . . . . . . . . . 126 131 3. New Varieties . . . . . . . . . . . . . . . . . . . . . 4 . Experimental Techniques . . . . . . . . . . . . . . . . . 131 I X . Effect on Soils . . . . . . . . . . . . . . . . . . . . . . 136 1. Erosion . . . . . . . . . . . . . . . . . . . . . . . 136 140 2. Soil Productivity . . . . . . . . . . . . . . . . . . . . X . Disease and Insect Pests . . . . . . . . . . . . . . . . . . . 143 143 1. Diseases . . . . . . . . . . . . . . . . . . . . . . . a . Stem Diseases . . . . . . . . . . . . . . . . . . . 144 b. Root and Crown Diseases . . . . . . . . . . . . . . . 146 c. Foliage Diseases . . . . . . . . . . . . . . . . . . 146 d . Control . . . . . . . . . . . . . . . . . . . . . . 148 2. Insects . . . . . . . . . . . . . . . . . . . . . . . . 149 XI. Regional Approach to Soybean Research . . . . . . . . . . . 150 References . . . . . . . . . . . . . . . . . . . . . . . . 152

I . INTRODUCTION Although the soybean. Glycine ma5 (L.) Merrill. is recognized as one of the oldest species cultivated by man. it failed to become prominent in the agricultural hist.ory of the United States until the twentieth century . The first recorded evidence of its existence is thought to be in Chinese literature in 2838 B.c., but it is considered to have been extensively cultivated in China centuries prior to this date . The botanical classification of the soybean has been a controversial matter . It has alternately been known as Glycine hispida. Clycine soja. Soja .max. and Glycine max. The newest edition of Standardized Plant Names prepared by Kelsey and Dayton (1942) lists Glycine soja as t.he approved botanical name . However. Ricker and Morse (1948) contend that under the International Rules the correct botanical name for the soybean is Glycine max (L.) Merrill. a viewpoint shared by most taxonomists. The cultivated soybean is an upright. branching. herbaceous. summer annual legume. Plant height may range from 1 to 6 feet . Early varieties

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which cannot utilize the maximum growing season, do not attain as great height as later varieties when grown a t the same locat,ion. Considerable variability exists in the species relative to size and shape of trifoliate leaves. Most varieties grown commercially in the United States have dense, upright, tawny or gray colored pubescence on stems, pods, and leaves. The small flowers, purple or white in color, are borne in clusters in the axils of the leaves. Flowers are perfect and normally selfpollinated. Although the number of seeds per pod may vary from one to five, most commercial varieties have two or three seeds per pod. Seed size of the cultivated types varies frum 5 to 35 g. per 100 seeds, t.he larger seeded types being used largely as “vegetable” or “edible” varieties. Seed coat colors found in the species include yellow, green, brown, black and buff. Many varieties with predominantly yellow or green seed coats have black, brown or buff pigmentation to a limited degree. In some genotypes the darker pigments are restricted to the hilum whereas in others they extend beyond the hilum to either side of the bean in a saddle-like pattern. Under certain environmental conditions extension of the dark pigments beyond the hilum occurs in irregular patterns resulting in a mottled condition of the seed coat. The cotyledon color, either green or yellow, is partially visible through yellow or green seed coats and this may somewhat modify the seed color. A close relative of the cultivated soybean grows wild throughout much of eastern Asia. This species, first described as Glycine soja and more recently known as Glycine ussuriensis Regal and Maack, is procumbent, has fine, twiny stems, and small, narrow leaflets. The seeds are sootyblack in color and range from 1 to 2 g. per 100 seeds. Forms intermediate to the cultivated and wild soybeans also occur and this intermediate group has been named Glycine grasilis Skvortzov. Several hundred varieties of this species have been found in Manchuria. All of the forms have been found normally to contain 40 chromosomes, to be self-fertilized and cross-compatible, and the hybrids have fairly high fertility. Due to the occurrence of intermediate forms, Piper and Morse (1910) contend that the wild soybean is the progenitor of the cultivated form. These authors also believe that, as overlapping of types occurs, all of the forms could well be considered to be one botanical species. Most agronomists concur, however, in recognizing three species: the wild soybean, G. ussuriensis; the intermediate type, G. grasilis; and the cultivated form, G. max. A tropical group of species closely related to the above temperate groups is worthy of mention. To this group belong the wild species Soja javanica, S. tomentosa, and S. pentaphylla, which are indigenous to Java, the Philippine Islands, and India.

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11. PRODUCTION AND DISTRIBUTION I . In the World Prior to World War I1 t,he countries which led in soybean production were China and Manchuria. The war-stimulated increase in production in the United States and the concurrent decrease in production in China and Manchuria resulted in the United States becoming the leading country in soybean production by producing 38 per cent of the world total (excluding U.S.S.R.) during the period 1944-46. Recent soybean production trends in the principal producing countries are shown in Table I. Soybeans may become a major crop in a number of additional countries. I n Canada, primarily in southwestern Ontario, over one million bushels were produced in 1946. The development of new early maturing varieties is expected to increase production in more northern regions. In Bulgaria, Hungary, Rumania, and Yugoslavia production also is increasing. Although production records of the Union of Soviet Socialist Republics are not available, according to Ryahikov (1947), soybean production in the Soviet Far East and in Khabarovsk territory has become extensive. Although, as a consequence of World War 11, serious TABLE I Production of Soybeans for the Periods 1930-1934 and 1944-1946 in the Principal Producing Counties of the World Soybean production (1,000 bushels) 1930-1934

United States China Proper (22 provinces) Manchuria and Jehol Japan Proper Korea Netherlands Indies World Total (excluding U.S.S.R.)

16,603 231,327 167,571 12,231 20,286 5,602 455,000

1944-1946 193,586 159,954 126,938 9,939 7,265 3,540 506,800

' Source of data: U.S. Department of Agriculture, Agricultural Statistics, 1947

reduction in production occurred in the Ukraine, North Caucasus, and Moldavia, special efforts were made to increase acreage in an attempt to attain pre-war production in 1948. World soybean production has been reviewed recently by Morse (1948). Minimum climatic requirements for soybeans have been summarized

SOYBEIANS

81

by Riede (1938) as being a 5-month growing period during which a total warmth of 2400°C-days and a total precipitation of 300 mm. are attained. 2.

In the United

States

The increase in soybean production in the United States has been remarkable. I n the 40-year period, 1907-1947, plantings increased from 50,000 t o over 13,000,000 acres. Production of beans increased from 3,000,000 bushels in 1920 to 200,000,000 bushels in 1946. The increase in production of seed, largely due t o increased acreage, also was aided by increased acre yields which rose from 11 to 20 bushels per acre during the period 1924 to 1938. Soybean acreages in the United Stat,es for the period 1924-1948 appear in Fig. 1. Since 1934 the increase in acreage has been largely for production of seed. The demand for oil seeds during World War I1 and postwar years

YEAR

Fig. 1. Soybean acreage harvesled for beans, hay, and grazed, plowed under, or abandoned in the United States for the year8 1924-1948.*

* Source of data: U.8. Department of Agriculture, Agricultural Statistics, and publications of the Bureau of Agricultural Economics, U.S. Department of Agriculture. Data for 1947 and 1948 are preliminary.

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resulted in lower acreages being harvested for hay whereas acreages harvested for beans increased substantially. It is of interest to note that prior to 1941 the acreage harvested for hay exceeded that harvested for beans, whereas the estimates for 1947 indicate t.hat 10 per cent of the total soybean acreage was harvested for hay and over 81 per cent was harvested for beans. As an appreciable increase in soybean production in the United States was stimulated by the demand for oil seeds during World War 11, a decrease in acreage in later years is anticipated by many authorities. However, such decrease is not expected to be of such magnitude to remove soybeans from their role as a major crop. Hansen (1946) estimates soybean production in the United States will be approximately 140 million bushels in 1955. The heaviest area of soybean production in the Unit.ed States occurs in the North Central region. Here, as in the eastern portion of the Great Central Plain of Manchuria, a large area has been found to be particularly well adapted to soybean production. Two additional areas of production occur, one in the Mississippi Delta region and the other in the Atlantic Coast region. A distribtion map of soybean production in the United States appears as Fig. 2.

Fig. 2. Soybean acreage harvested for beans, 1947.*

* Prepared from data published by the Bureau of Agricultural Economics, U.S. Department of Agriculture,

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83

The leading states ranked in extent of soybean production for the period 1943-1947 were as follows : Illinois, Iowa, Indiana, Ohio, Missouri, and Minnesota. During this period these six states produced over 90 per cent of all soybeans in the United States and accounted for 73 per cent of the total acreage. The shift, in the heavy production areas may be demonstrated by noting that North Carolina, Virginia, Alabama, Missouri, and Kentucky were the leading states in soybean production in 1920. Production in the latter states, with the exception of Missouri, did not increase appreciably in the succeeding 10-year period, whereas substantial increase in production occurred in the North Central states, an increase which was further magnified by t*heWorld War I1 stimulus. The North Central production area was expanded to the north and west during the period 1935-1948, particularly in the states of Minnesota, South Dakota, and Nebraska. During this period production did not change appreciably in the Atlantic Coast region and increased in the Delta region where leadership in production shifted to Arkansas. Trends of soybean production in the United States and factors affecting these trends have been thoroughly analyzed recently by Strand (1948).

111. DISPOSITION AND UTILIZATION Prior to 1920 the soybean was used principally as a forage crop in the United States, for hay, silage, soilage, or pasture. Development of processing facilities and industrial uses of the beans has stimulated the harvesting of progressively larger acreage as beans. During the period 1946-1947, 79 per cent of the total soybean acreage was grown for beans, 12 per cent for hay, and less than 9 per cent plowed under, grazed, or abandoned. I n the heavy producing north-central area, approximately 93 per cent of the crop is now harvested for beans, 5 per cent for hay, and less than 3 per cent for other uses. Throughout the southern United States approximately one-third of the acreage is harvested for hay, the extensive use of soybeans as forage being well illustrated by the fact that in Louisiana, Tennessee, and South Carolina greater acreages of soybeans are presently interplanted with other crops than are grown alone. With the development of high oil varieties suitable for industrial utilization, a trend toward greater bean production in the South is anticipated. Current disposition and utilization of soybeans in the United States is shown in Fig. 3. Nearly 90 per cent of the soybeans produced during the period 19451947 was sold from farms, approximately 8 per cent was used for seed, and less than 2 per cent was fed on farms to livestock. Limited use of

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MARTIN G. WEISS

the whole bean as livestock feed is largely due to its unfavorable effects in causing soft pork from hogs, soft butter from dairy cattle, and scours in cattle when fed in too large proportions. Over 93 per cent of all soybeans sold from farms was crushed for oil extraction in t.he immediate postwar years. Less than 2 per cent was

0-

SOLD FROM FARMS

(-/)UAL

\ 1 3 %NET EXPORTS

/

CRUSHED FOR oiL

\

91.5% FEED 3.2 % OTHER USES 5.3 % FLOUR (Low fat)

DOMESTIC DISAPPEARANCE OF CRUSHINGS

-

EDIBLE (93.3%) 63.8% SHORTENING 143% OLEOMARGARINE 10.2% OTHER EDIBLE NON-EDIBLE (6.7%) 4.2% OTHER USES \2.5% DRYING OIL

DOMESTIC DISAPPEARANCE OF OIL

Fig. 3. Disposition and utilization of soybeans grown in the United States, 1945-1947.*

exported, approximately 1 per cent was converted into full fat flour, and 4 per cent was used as whole soybeans for human food, was fed to livestock on farms other than where produced, or found miscellaneous uses. Several types of processing plants are used to extract oil from soy-

* Calculated from data in publications of the Bureau of Agricultural Economics, US. Department of Agriculture. Averages for the period 1945-1947 were used whenever available, otherwise for the period 1945-1946.

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SOYBBANS

beans, Mills specializing in soybean processing employ either the expeller (screw processing) or solvent extraction methods. I n certain mills which also crush other oilseeds, however, the hydraulic press method is used. Products derived from processing one bushel of soybeans for the production of meal when mechanical pressing and solvent extraction methods are used were given by Shollenberger and Goss (1945) as follows: Products from 60 Ibs. (1 bu.) of soybeans

Crude soybean oil Soybean oil meal Loss, approximately

Mechanical pressing lbs.

Solvent extraction lbs.

48 (41 per cent protein)

46 (44 per cent protein)

9

3

11

3

The oil meal derived from mechanical pressing (old process) contains from 4.0 to 5.5 per cent oil, whereas meal from solvent extraction (new process) contains 1 per cent or less oil. Medium and low fat flours also are produced by mechanical pressing and solvent extraction methods, respectively. Over 91 per cent of the oil meal produced is used as a protein supplement feed for livestock. Approximately 5 per cent of all processed beans are used for flour, and 3 per cent for mixed fertilizers and industrial products such as plywood glue, plastics, paper coating and sizing, and molding and core-bind compounds. Soybean oil, classified as semidrying, has qualities that make it adaptable to a wide variety of uses. Currently over 93 per cent of the refined soybean oil in the United States is used for edible compounds such as shortening, oleomargarine, mayonnaise, salad dressing, etc. During the period 1944-1946 soybean oil was used for 51 per cent of all shortening, 43 per cent of all oleomargarine and 21 per cent of all other edible oil products consumed in the United States. Use as drying oils in paints, varnishes, linoleum, oil cloth, and printing inks accounted for less than 3 per cent of all soybean oil and supplied less than 5 per cent of the total oils required in the United States for these products. Approximately 4 per cent was used for miscellaneous pr0duct.s such as soap, rubber substitutes, cutting fluid, eto.

IV. PHYSIOLOGY OF THE SOYBEAN PLANT 1. Floral Initiation

Initiation of flowering in plants is critically associated with the duration of light and dark periods. Exposure to long photoperiods was found by Garner and Allard (1920) to stimulate flowering in some species while

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others failed to flower unless exposed to relatively short days. Relatively late maturing varieties of soybeans are particularly sensitive to the photoperiod and have been widely used in studies of “Fjhort day” plants. Review of all photoperiod studies involving soybeans is not considered pertinent here, and consequently, hrief consideration will be given only to those aspects of photoperiodism which relate to soybeans as a crop and which may serve to explain the response of soybean varieties when grown in latitudes to which they are not well adapted. A more detailed summary of the relation of day length and flowering in plants has recently been made by Murneek et al. (1948) and by Borthwick (1947). a. Varietal Differences. Varieties of soybeans were found by Garner and Allard to differ strikingly in their response to photoperiod. When grown under summer daylight conditions in Washington, D.C., four varieties ranged from 27 to 105 days from germination to blossoming. When the day length was reduced to 12 hours, however, the varieties all became early maturing varieties and blossomed 21 to 28 days after germination, Reduction of the photoperiod altered the blossoming time of the earliest variety, Mandarin, only slightly, whereas that of the latest variety, Biloxi, was radically reduced. Biloxi was considered a “short day” plant and Mandarin was thought to be indeterminate or day neutral. Flower primordia were found by Borthwick and Parker (1939) to be initiated in very early varieties a t all photoperiods, including continuous illumination, while relatively late varieties differentiated reproductive primordia only when subjected to periods of illumination not in excess of 14 hours. Nevertheless, several observations indicated that the fundamental character of the reactions in early and late varieties was similar. Although flower primordia in early varieties were initiated with exceptionally long light exposures, with photoperiods of greater than 18 hours the primordia failed to develop into flowers and pods. Similarly, in late varieties flowers failed to develop a t maximum photoperiods resulting in primordia initiation. I n later confirmation of these findings, histological investigations by Neilsen (1942) indicated that the frequency of disintegration of the young sporocytes was associated with the magnitude of deviation from optimum photo-inductive conditions. Initiation of primordia in early varieties under continuous light was further found by Borthwick and Parker to occur much less promptly than when illumination was interspersed with dark periods. During late stages of growth, late varieties flowered with longer photoperiods than during early stages. It was, therefore, concluded that the fundamental response to photoperiod is similar in early and late soybean varieties although they may differ greatly as to relative photoperiod when primordia initiation occurs.

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87

Several soybean varieties have been noted to respond differentially in seed and forage yields when grown on soils of different productivity levels. I n Missouri, the variety Virginia commonly outyields Morse on soil with low productivity while on soils of high productivity Morse gives higher yields. Lincoln has been reported to respond relatively more to fertilizer applications than T 48. T o determine whether the initiation of flower primordia was associated with the above differential yields, the varieties, Lincoln and T 48, were grown by Scully et a2. (1945) a t low, intermediate, and high nitrogen levels with long and short photoperiods. With long photoperiods total nodes a t which flower primordia were initiated increased progressively with nitrogen concentration in Lincoln and decreased substantially in T 48. The increase of total nodes per plant a t higher nitrogen levels was more pronounced in Lincoln. A delay in flowering was stimulated by higher nitrogen but this delay was more pronounced in T 48. With short photoperiods neither time of flower initiation nor total nodes per plant differed with nitrogen levels. I n a similar experiment with Morse and Virginia, conducted only with moderately short photoperiods, varietal differentiation was not great. Differences as conditioned by nitrogen level seem to occur only a t threshold durations of the photoperiod when production of flower-producing substances is relatively low. Although these results are insufficient to explain differential yield response of varieties a t various soil productivity levels, the differential response of these varieties at various nitrogen levels with respect t o time and amount of flower initiation is significant. b. Duration of Dark and Photoperiods. The effect of duration of the dark and photoperiods on flower initiation in late soybean varieties has been the subject of several recent investigations (Allard and Garner, 1941; Borthwick and Parker, 1938a; Hamner, 1940; Long, 1939; Snyder, 1940). Certain trends have been established which clarify the nature of photoperiodic induction. I n late maturing varieties, such as Biloxi, alternate periods of darkness and light are required for flower initiation. No initiation will result from exposure either to continuous light or darkness. Although exposure to as few as two photo-inductive cycles will result in flower initiation, an increase in the number of cycles up to 10 will result in progressively more rapid and more abundant flowering. No evidence of residual effect is apparent. Exposure to repeated, single, short photoperiods, when separated by long photoperiods, resulted in no flowering. Within the range of photoperiods under which Biloxi flower primordia are stimulated in the 24-hour cycles, photoperiods intermediate in length (6, 8, 10 and 12 hours) are more conducive to flowering t,han either extreme (2, 4, or 14 hours). Therefore, periods of darkness 12 to 18 hours in length are most favorable for flower initiation with the 24-

88

MARTIN G. WEISS

hour cycles. Use of cycles other than 24 hours alters the length of the optimum periods somewhat. With long (16-hour) dark periods, flower primordia are produced more abundantly with 10- to 14-hour photoperiods, and no flowers are initiated with photoperiods in excess of 18 hours. It is apparent that dark periods of sufficient length to stimulate flowering with the normal 24-hour cycle are ineffective when interspersed with long photoperiods. With long (16-hour) photoperiods, flowers are initiated when the dark period is in excess of 10 hours, and most abundant flowering occurs with 12- to 16-hour dark periods. Even with very short (4-hour) photoperiods, no flower initiation occurs unless the durat4ionof the dark periods exceeds 10 hours. It may be concluded that with a long photoperiod, which results in no flowering in the 24-hour cycle, flowers are differentiated provided the dark period is of adequate length. Furthermore, regardless of length of the photoperiod, the period of darkness must exceed 10 hours for flowering to occur in this variety. The most critical prerequisite of flower initiation apparently is dark periods which must exceed minimum durations. Although interspersed periods of light are necessary, their duration does not seem to be as critical in fulfilling the requirements for flower differentiation. c. Light Intensity and Photosynthesis. The intensity of light during the photoperiod has been found to affect materially flower initiation in Biloxi soybeans (Borthwick and Parker, 1938c; Hamner, 1940; Parker and Borthwick, 1940). Intensities of 100 foot-candles for 10 hours or 150 foot-candles for 5 hours are the minimum to stimulate flower initiation. The quantity of flowers produced progressively increases with intensity of light up to full daylight intensities. With 8 hours of natural daylight, however, the minimum intensity of an 8-hour period of supplementary light necessary to prevent flower induction is 0.6 foot-candles. The effective length of the photoperiod apparently consists of the period during which light intensities are in excess of 0.6 foot-candles. However, high light intensities during a portion of the short photoperiod are a requisite for flower initiation. Lack of floral initiation a t low light energy levels is thought to be associated with low photosynthetic activity within the leaf. Substantiation of the latter theory has been obtained by retarding photosynthesis by other means. When photosynthesis is materially reduced by exclusion of COz from the air in which the plants are grown, no flower initiation results. Adequate photosynthesis per unit area of leaf rather than total photosynthesis per plant is considered the limiting requirement. The relative effectiveness of various wave lengths of visible light to prevent flower initiation also has been studied (Katunsky, 1937; Parker et al., 1945, 1946). Regions of the spectrum exhibiting maximum effec-

SOYBEANS

89

tiveness were observed to contain the wave bands absorbed by chlorophyll to the greatest extent. It has been postulated that a flower-forming substance, thought to be normally produced in a long dark period, may be destroyed as a consequence of some reaction stimulated by energy absorbed by the chlorophyll. d. Age and Position of Induced Tissue. Sensitivity of various soybean tissues to photoperiodic induction has been determined (Borthwick and Parker, 1938b, c, 1940; Heinze et al., 1942). Photoperiodic induction processes have been found to be initiated within the leaves and not in the stems or growing points of the plant. Based on abundance of flower primordia produced, the most effective leaf of the plant is that compound leaf which has attained its full size most recently. I n fact, subjecting this leaf to short photoperiods results in as great floral stimulation as when all the leaves of the plant are exposed, and exposure of as little as one quarter of this leaf results in flower stimulation. The capacity of leaves to stimulate floral initiation seems associated with their state of maturity rather than relative position or proximity to growing points. The flower-forming substance seems to move readily up or down the stem, into a second branch that has been defoliated, or through grafts of induced leaves on noninduced plants. It is of interest to note that Biloxi plants will flower with long photoperiods when leaves of an early variety, which will blossom under continuous illumination, are grafted on to them. Defoliation of the stock hastens this process. Differentiation of flower primordia occurs very early in the life of the soybean plant when subjected to adequately short photoperiods. Limited floral initiation occurs when Biloxi plants are exposed to short photoperiods two weeks after planting. Flowering is, however, more abundant when exposure occurs later in the life of the plant. Flower primordia are not stimulated a t the lower nodes if the plant has attained relatively large size prior to photo induction. e. Temperature Effects. Flower initiation has been found to be influenced by temperature differences (Borthwick and Heinze, 1941; Parker and Borthwick, 1939, 1943). Maximum flower initiation occurs with day temperatures of 75" and 85" and night temperatures of 65" and 75°F. Complete inhibition occurs with temperatures as low as 55°F. Exposure of localized tissues to various temperatures indicates that floral initiation is influenced much more by differences in temperature throughout the dark period than by differences throughout the photoperiod. Furthermore, low temperatures in the leaves are more effective in limiting flower initiation than low temperatures in the petioles or growing points. Inhibition of flower initiation caused by low temperatures, therefore, seems

90

MARTIN G. WEISS

atttibutable to the effect on the photoperiodic reactions occurring within the leaves during the dark period. b. Nutrition

Nutritional requirements of the soybean plant are high in comparison with that of grain crops. It has been estimated by Sears (1939) that a crop of soybeans removes somewhat more phosphorus and magnesium and appreciably more potassium and calcium from the growing medium than crops of corn, oats, or wheat with comparable yields. Norman (1944b) states that per unit area under Iowa conditions the nitrogen needed by soybeans is somewhat in excess of that needed by comparable yields of corn. With proper inoculation, of course, a considerable portion of the required nitrogen is obtained from the air. The appearance in soybeans of numerous deficiencies of minor elements required for plant growth emphasizes the heavy drain of t+hiscrop upon certain of the micronutrients as well. Although much basic research on plant nutrition was done prior to the past decade, only recent references are cited herein. I n many cases the researches cited constitute confirmation of earlier findings or theories. a. Nitrogen. Although the soybean plant is capable of utilising soil nitrogen, as are grain crops, it also has supplementary means of obtaining nitrogen. Like most other legumes it has the ability to enter into a symbiotic relationship with one of the species of Rhizobium, thereby making possible the utilization of nitrogen from the air. The comparative efficiency of free and combined nitrogen for the nutrition of the soybean plant has been the subject of numerous investigations. The effectiveness of free or combined nitrogen is thought by Umbreit and Fred (1936) to be a function of the carbohydrate-nitrogen relation in the plant. When the carbohydrate-nitrogen relation is balanced, the soybean plant primarily utilizes free nitrogen. Under conditions which result in an unbalanced carbohydrate-nitrogen relation, however, such as low light intensity or unfavorable pH, fixed nitrogen is required in that it enables plants to survive the unfavorable environment. Under field conditions these workers believe unfavorable conditions are the exception and maximum yields should be obtained with nodulated plants rather than plants which are dependent on fixed forms of nitrogen. This viewpoint is not entirely supported by other findings. When Mukden soybeans were grown to maturity by Norman (1944a) in field experiments on a loess soil low in nitrogen, yield and nitrogen content of beans from nodulated plots slightly exceeded those from nonnodulated plots receiving 94 Ibs. nitrogen per acre, but were significantly less than obtained from plots to which 158 lbs, of nitrogen were applied

SOYBEANS

91

per acre. The recovery of nitrogen in plant tops was 24 and 42 per cent, respectively, for the 94- and 158-lb. rates of application. It was estimated that inoculation resulted in the fixation of approximately 26 lbs. of nitrogen per acre in the plant tops, which is less than 30 per cent of the total nitrogen in the tops of nodulated plants. Luxury consumption was apparent a t the highest rate of application of nitrogen, part of which was applied at midseason. The oil content- decreased proportionately with added increments of nitrogen from either combined or atmospheric nitrogen. With higher bean yields, however, the oil yield per acre increased with additions of nitrogen. It would appear that under these conditions the inoculated plants did not receive adequate nitrogen through the fixation process to permit maximum growth and production. The effect of combined nitrogen on the fixation process has been the subject of controversial thought by various workers. Early workers generally agreed that addition of nitrogen depressed nodulation, and it was speculated by some that nitrogen fixation could be entirely suppressed by the addition of adequate quantities of nitrogen. Considerably more information has been made available during the past decade. The response of inoculated and uninoculated soybeans grown on acid soils in the field to lime and various sources of nitrogen was studied by Andrews (1937, 1938). Based on results obtained when the soybeans were harvested in the small bean stage it was found that addition of ammonium sulfate in large amounts to nodulated soybeans increased total yields substantially but had little influence on the nitrogen content. The total nitrogen increase due to inoculation was as great with a 600-lb. application of ammonium sulfate as with a 75-lb. application, leading the author to conclude that under field conditions application of ammonium sulfate at rates as high as 600 Ibs. per acre failed to inhibit nitrogen fixation. The sources of combined nitrogen, ammonium sulfate, ammonium nitrate, urea, and cyanamid, varied in effectiveness in stimulating yield. Furthermore, the effectiveness of these forms was influenced differentially by the addition of lime, and by nodulation of the plants. With heavy applications of combined nitrogen in any form, the nitrogen content of the plants was not altered by nodulation. The effect of combined nitrogen on nodulation of soybeans was studied by Doolas (1938). Inoculated plants were grown in small paraffinedscreen pots placed within much larger pots. As the roots of the plants penetrated into the outer compartment, a study of the effect of combined nitrogen on nodulation of the inner and outer zones was made possible. Addition of nitrogen as calcium nitrate to either the inner or outer zone reduced the size of nodules materially in that zone but the number of nodules was reduced only slightly. When nitrogen was added t o the

92

MARTIN Q. WEISS

outer zone, the inhibitory effects on nodule size were transferred to the inner zone, whereas only slight effects were transferred to the outer zone when the inner zone was enriched with nitrogen. Reduction in size rather than number of nodules by nitrogen application caused the author to deduce that the effect of added nitrate occurs after absorption by the plant root, and that entrance of the organism into the root is not affected. During the seedling stage soybeans, even though inoculated, frequently undergo a nitrogen hunger period (Fred et al., 1938). This period of nitrogen deficiency occurs after cotyledonary nitrogen is exhausted and before the nodules supply adequate nitrogen. The nitrogen hunger period was observed to be prolonged in Manchu soybeans when grown under bright sunlight. Inhibition of nitrogen fixation was attributed by the authors to an excessive carbohydrate-nitrogen balance within the plant stimulated by the conditions of bright sunlight. A reduction in this ratio, accomplished either by shading the plants or by addition of combined nitrogen, terminated the period of inhibition and the nitrogen fixation process was initiated. Studies on effect of combined nitrogen on the fixation process have been facilitated by the availability of the stable isotope of nitrogen, N15. Addition of a certain percentage of this fractionally heavier form of nitrogen to the ordinary nitrogen supplied in the growing medium permits determination of the proportion of nitrogen in the plant derived from the Foil. With this isotope of nitrogen, Norman and Krampitz (1946) confirmed previous findings that as combined nitrogen was in greater abundance, the amount of nitrogen fixation decreased even though total nitrogen per plant increased. The percentage of nitrogen derived from the atmosphere varied from 100 per cent, when no combined nitrogen was added, to 30 per cent when large quantities of nitrogen were applied to the soil. Thornton (1947) similarly using isotopic nitrogen also found that the amount of nitrogen fixation was inversely proportional to the amount of nitrogen added. Some degree of nitrogen fixation occurred, however, even when a near adequate supply of combined nitrogen was available. Since nitrogen nutrition is closely associated with other nutrient elements, further aspects of nitrogen nutrition will be discussed in succeeding sections. b. Phosphorus. During the early stages of development of the soybean plant phosphorus was found by Hutchings (1936) to be most efficient in promoting growth on colloidal clay substrate when adequate levels of calcium are available. With adequate calcium, application of phosphorus resulted in higher concentrations of the latter element in the plant tissues. Under similar conditions phosphorus applications also in-

SOYBEANS

93

creased potassium concentrations, although with low calcium concentrations in the substrate the opposite relation existed. Phosphorus was not found to be a significant factor in controlling nodulation during the early growth stages of tshe plant. However, applications of phosphorus to the substrate materially increased the nitrogen concentration of the tissue. c. Major cations. Calcium has a dominating influence on the nutrition of the soybean plant. Growth responses following application of lime to acid soils were formerly attributed to the neutralization of the soil. More recent work shows conclusively that such responses are a t least partially attributable to the addition of calcium as a nutrient. The importance of calcium in the nutrition of the soybean plant has been shown by Horner (1936). Either increase in the calcium level or a higher degree of calcium saturation of colloidal clay resulted in substantially greater growth, nodulation, nitrogen fixation, and calcium absorption during the early growth stages of Virginia soybeans. A close relation between the elements calcium, phosphorus, and nitrogen was indicated. Applications of limestone to Norfolk fine sand were found by Beeson et aE. (1948), in general, to reduce the absorpt,ion of the minor elements, manganese, iron, cobalt, and copper. Reduction in absorption was pronounced when a supply of minor elements had been added to the soils, but was small or entirely lacking in soil not treated with these elements. Toxicity resuking from addition of micronutrients was, therefore, absent in the presence of high calcium concentrations. I n a study pertaining to the effects of magnesium on the early growth stages and nitrogen fixation in soybeans grown on colloidal clay, Graham (1938) found that adequate magnesium facilitated efficient use of the calcium offered. At a given level of calcium, nitrogen fixation increased with higher levels of magnesium and growth was subsequently also increased. No nitrogen fixation occurred in the total absence of magnesium. Early growth responses of non-nodulated Biloxi soybeans grown in sand cultures were reported by C. L. Hamner, (1940) when the cations, calcium, magnesium, and potassium, were varied. A series of nutrient solutions were employed which could be arranged in a triangle, each vertex of which represented maximum concentration of a given ion, and solutions on the adjacent side were entirely deficient in this ion. Of the three cations varied, variations in potassium concentration resulted in greatest vegetative growth responses. With high potassium concentrations active vegetative growth resulted even though concentrations of calcium and magnesium were low. Witsh low potassium, severe chlorosis of the young plants occurred particularly when calcium or magnesium concentrations were high.

94

MARTIN G. WEISS

Application of potassium to colloidal clay resulted in greater nitrogen fixation, increased efficiency of phosphorus removed, and less absorption of magnesium in young nodulated soybean plants (Ferguson and Albrecht, 1941). High carbohydrates in the young plants were found to be closely associated with increased potassium in the substrate, particularly when the plants were not inoculated. The function of potassium in the production of carbohydrates was also noted by Hampton and Albrecht (1944b). Growth of young nodulated Virginia soybean plants was associated with both potassium and calcium levels. Whereas high carbohydrates were considered attributable to potassium additions, variations in nitrogen fixation were associated more closely with calcium concentrations, maximum nitrogen fixation occurring with low potassium-calcium ratios. However, higher nitrogen levels in the young plants were closely associated with increased potassium intake. It is of interest that potassium and phosphorus were absorbed or lost to the substrate by the plant roots depending upon relative concentrations, but in no instance was calcium or magnesium found to move from the plants to the substrate. Nutrient studies in sand cultures with the Morse and Virginia varieties by Allen (1943) showed that varieties may differ substantially in their nutritional requirements. Differences between the two varieties in forage yields were, in general, not appreciable a t low nutrient concentrations. With high concentrations of potassium and magnesium, however, yields of the Morse variety substantially exceeded that of Virginia. Differences were attributed to inability of the latter variety to utilize potassium or magnesium a t as high concentrations as the Morse variety, Less striking differential responses were exhibited by the varieties to levels of nitrogen, phosphorus and calcium. Ralsoy and Ogden, two soybean varieties of similar growth habit and maturity, were reported by Nelson and Hartwig (1948) to interact with fertility level. At high levels Ogden substantially outyielded Ralsoy whereas a t low levels of fertility the yield of the two varieties was similar. Among a number of crop species tested on high lime soils, Bower and Pierre (1944) found soybeans to be intermediate in response to potassium fertilization. Crops responding substantially in growth with the application of potassium, such as corn and sorghum, were observed to contain low concentrations of calcium and magnesium relative to the potassium concentration in their tissue. Sweetclover and buckwheat which gave no response were observed to contain high calcium and magnesium concentrations relative t o their potassium content.. Soybeans were intermediate in this respect. Assuming that absorption of potassium, of which there was an abundance in this soil, is inhibited by high concentrations of

96

SOYBEANS

calcium and magnesium, t.he authors deduce that plants with high requirements for the latter elements will reduce the concentration of them in the substrate adjacent to the roots thereby making potassium available, Although soybeans draw heavily on calcium, their potassium requirement also is high, resulting in an intermediate calcium magnesium : potassium ratio. Response to the application of available potash on high lime soil is, therefore, intermediate to that of other crops. d. Micronutrients. Under certain soil types and conditions, chlorosis attributable to manganese and iron deficiencies readily occurs in soybeans. Manganese deficiency frequently has been observed when soybeans are grown on Maumee loam in Indiana and successful correction with soil or spray applications of manganese sulphate has been demonstrated by Steckel (1947, 1948). On the high-lime areas of the Webster series in Iowa, chlorosis due to iron deficiency is common in soybeans. Spray applications with ferrous sulfate have been found by Nelson (1948) to be entirely satisfactory for control. Corrective treatment in the above instances is further discussed in Section VI-2-a. The ratio of iron to manganese was found by Somers eC al. (1942) to be of greater importance in promoting normal growth in soybeans than concentrations of either element. I n greenhouse studies normal plants were produced only when the ratio of iron t o manganese in the substrate was between 1.5 and 2.5. With lower iron : manganese ratios the chlorosis symptoms generally associated with iron deficiency resulted, which the authors contend are the identical symptoms associated with manganese toxicity. With higher iron : manganese ratios the chlorosis symptoms of manganese deficiency occurred, which are thought to be identical wit.h those of iron toxicity. Even though concentrations of the two elements were increased 100-fold in the substrate, normal plants resulted when the above ratio limits were maintained. As iron and manganese are thought to function as catalysts in cellular respiration, the output of respiratory COZ from the roots of the plants was measured. The highest yields of respiratory COz occurred within the range of ratios giving plants free from symptoms, whereas deviation from this ratio in either direction resulted in lower respiratory Con. Field treatment of young chlorotic soybeans on high lime soils by Nelson (1948) substantiated the above findings. Whereas spray applications of iron compounds increased yields tremendously, and spray applications of manganese compounds showed no effect, spray applications with both elements resulted in yields materially lower than when iron alone was applied. Certain varieties of soybeans were observed by Weiss (1943) t o differ strikingly in e5ciency of iron utiligation when grown on high lime soils,

+

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MARTIN G. WEISS

By growing advanced hybrid progenies of efficient and inefficient varieties on a substrate with a differentiating level of iron, it was determined that the difference in efficiency was conditioned by a single gene. The expressed cell sap of efficient types was of lower p H than that of inefficient plants. The composition of aerial tissues indicated that absorbed iron was precipitated within the inefficient plants when grown on high lime soils and was thereby rendered unavailable to the plant. The concentration of boron necessary for normal soybean growth has been determined by Rogers (1947) to be somewhat less than required by alfalfa or Crimson clover, whereas 0.4 to 0.3 p.p.m. in the substrate was found by Hodgkiss et al (1942) to give visible toxicity symptoms. Optimum concentrations of boron for production of fresh tissue by Harbinsoy soybeans were found by Minarik and Shive (1939) to lie within the range of 0.025 to 1.0 p.p.m. in the substrate. Maximum calcium absorption also occurred within this range. A marked tendency was evident for the moisture content of tissues to decrease with increasing concentrations of boron in the substrate. This occurrence was interpreted as support for the theory that the function of boron may be concerned with the regulation of water absorption by plasma colloids. Boron deficiency in soybeans was found by MacVicar and Struckmeyer (1946) and Struckmeyer and MacVicar (1948) to be accompanied by a marked increase in vascular tissue resulting from abnormal cambium activity. Boron deficiency symptoms and the accompanying cambial activity were particularly noticeable when Biloxi soybeans were grown under long-day conditions. When the Biloxi variety was grown under short days, or Pagoda, a day-neutral variety, was grown with either long or short days, boron requiremen& were considerably lower. The decrease in cambial activity associated with photoperiodic induction of flowering in the latter cases was thought to cause a reduction in severity of boron deficiency symptoms. Varieties of soybeans were found by Earley (1943) to exhibit differential tolerance to zinc concentrations in excess of nutrient requirements in crushed quartz substrate flushed with slightly acid nutrient solutions. Tolerant varieties, such as Hudson Manchu, showed no toxicity symptoms a t concentrations of zinc 8 to 12 times as great as those tolerated by susceptible varieties. Illini soybeans were found by Martin and Trelease (1938) to have greater tolerance for excesses of selenium in the growth medium than tobacco. However, 2 p.p.m. of selenium caused a marked stunting of soybean plants. Selenium absorption was closely associated with its concentration in the substrate. The presence of sulfur, with sublet,hal

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concentrations of selenite, tended to reduce the accumulation of selenium in the plant and also to reduce toxicity symptoms. Effects on soybeans of the chlorate and perchlorate ions, frequently used to sterilize soils and eradicate weeds, were studied by Weaver (1942). Injury symptoms consisting of small, brown necrotic spots distributed uniformly over the older leaves of the plant occurred a t 5 p.p.m. concentration of the chlorate ion in the substrate. Concentrations of 2.5 p.p.m. of the perchlorate ion caused toxicity symptoms consisting of apical meristem restriction in the younger leaves, and thereby resulting in abnormally shaped leaflets. 3. Root Temperatures Optimum root temperatures for yield and height of plant tops were found by Earley and Cartter (1945) to be approximately 22 to 27OC. Temperatures as low as 17" or as high as 37" reduced development of maximum dry weight of tops. No close association between top-root ratios and root temperatures was apparent.

V. EFFECTOF CLIMATEAND LOCATION 1. Location a d Season Effects

Seed composition of the same variety of soybeans grown a t a number of widely separated geographical locations has been observed to vary greatly. Certain investigators have attributed the major portion of the variability t o differences in climate rather than soil types. Several recent investigations have contributed more information on the relative importance of environmental factors on seed composition. Seed of the same variety of soybeans grown a t 11 widely separated locations a t which soil types and climate differed considerably was shown by Viljoen (1937) to vary in oil content from 12.7 to 22.1 per cent, a difference of 9.4 per cent within the same year. Protein content varied from 32.6 to 44.3 per cent, a difference of 11.7 per cent, in t.he same year. When a soybean variety was grown on assembled soil lots from eight widely-separated locations, a t which soil types differed materially, the maximum range of a single variety of soybeans within 1 year for oil and protein contents was 2.9 and 4.2 per cent, respectively. Climatic differences are considered by this worker to have appreciable influence on oil and protein content whereas differences attributable to soil types are relatively small. Comparison of relative influence of climate and soil factors on plant characteristics also can be made when the same varieties are grown a t a number of locations for several seasons. Seasonal differences must be

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largely attributable to climatic differenres. Extensive data on seed composition for 10 variet.ies of soybeans grown in five cornbelt states for the years 1936-1939 and 1936-1940 were reported by Cartter (1941) and Cartter and Hopper (1942), respectively. The components of variance have been calculated from the analyses of variance reported in the latter reference according to the method described by Crump (1946), and are reported in Table 11. Relative to the interaction of location with years, or the second order interaction, variance components attributable to years or locations, with few exceptions, were not of sufficient magnitude to be TABLE I1 Components of Variance for Seed Size and Compositional Characters Attributable to Varieties, Locations, and Years as Calculated from Analyses of Variance a Seed Composition

D.F. Seed Protein Oil weight 70 o/o Sources

g/100

Iodine Ash Phos- Potas- Cal- Crude No. % phonis sium cium fiber % % % %

seed Varieties 9 5.1 Years 4 4 . 1 Locations 4 0.1 Vars. x yrs. 36 5.6 Vars. X locs. 36 0.0 LOCH. X yrs. 16 0.7 Vars. x loc. X yrs.

142

1.2

3.6 1.45 0.4 0.06 0.5 0.13 0.3 0.19 0.1 -0.02 1.4 0.34 2.4

0.51

19.5 .04 2.0 -.02 3.8 .03 1.7 .ll 0.3 .01 5.4 .05 4.2

.04

.0010 -.001 .0001 .002 .0011 .oOg .(NO2 .018 .OM2 .OOO .0014 ,005 .0011

.013

.0029 .OM0

.10 .02 .02 .01

.0007

.OW .0001

.OO

.0007

.03

.0007

.09

'Cartter and Hopper (1942).

statistically significant. Even though the locations were dispersed in five states, effects attributable to soil and climatic factors were insignificant relative to the interaction of these factors. It is of further interest to note that, in general, varieties exhibited a tendency toward greater differential performance with years (climate) than with locations (soil). In soybean variety and date of planting trials conducted in Illinois, Indiana, and Iowa during the years 1940-1942, reported by Weiss, et al. (1949), locations and years had approximately equal influence on yield and on the three seed compositional characters, protein and oil content, and iodine number of oil. The influence of location on lodging and maturity date, however, was greater than that of season. Various components of climate a t widely separated locations were correlated with seed compositional attributes by Viljoen (1937) in an

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effort to determine the causative factors. Precipitation and mean maximum temperatures did not appear to be correlated with protein or oil percentages. Mean minimum temperatures were strongly correlated with high oil content (r = 0.88) and were moderately correlated with low protein content (r = -0.48), both values exceeding the 1 per cent level of probability. Regression coefficients revealed that with every degree Fahrenheit increase in mean minimum temperature, oil content of the beans increased approximately 0.44 per cent and protein content decreased 0.39 per cent. Mean temperatures, which were derived from the daily minimum and maximum temperatures, were also correlated with seed composition but to a lesser degree. Throughout the various locations, negative correlation between oil and protein contents were noted. I n studies including five varieties planted a t five dates for 3 years a t three locations reported by Weiss e t al. (1949) lateness of maturity as conditioned by lateness of planting was found to be correlated with degree of unsaturation of oil. Lateness of maturity as conditioned by varietal differences was not found correlated with drying quality. In an attempt to determine the cause for this association, mean temperatures during the bean developmental period were correlated with iodine numbers. Among varieties and among dates of planting low temperatures were found to be associated with high iodine numbers. Within varieties the degree of association between low temperatures, as conditioned by later planting, and high iodine numbers increased progressively with the genetic lateness of the variety. These findings are in agreement with previous observations by Cartter and Hopper (1942) who noted a tendency for oils with high iodine numbers to be produced at locations with relatively low temperatures. 9. Simulated Hail Damage Hail damage is particularly severe on h l l season crops, such as soybeans, and its occurrence in the western part of the corn belt is of sufEcient frequency to warrant attention. Following hail storms, certain decisions must be made by the grower relative to abandonment of the field or by insurance companies relative to adjustment of damage. These decisions must be based on estimates of the degree of recovery and percentage reduction in yield which can be expected. Variability in yield reductions resulting from hail damage largely is caused by (1) degree of damage, (2) type of damage, and (3) stage of growth when damage occurs. Limited studies on defoliation and removal of parts of stems as reported by Dungan (1939, 1942) and Fuelleman (1944) indicated that reduction in yields was roughly proportional to degree of damage, and that reduction in yields varied greatly when

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damage occurred a t different stages of growth. Provided the degree of damage permitted retention of some primordia, reduction in yields increased progressively with age of plant until the pod development stage when the beans were approximately one-half maximum size. At this stage very severe reductions in yield occurred. Damage a t later stages tended to be less severe in yield reduction. Effect of repeated defoliation on total forage and seed production was reported by Gibson e t al. (1943). Any degree of defoliation decreased seed yields to some extent. Seed production, in general, was inhibited in proportion to the frequency and severity of defoliation. The results of extensive studies relative to the effects of simulated hail damage covering a 4-year period were recently reported by Kalton et al. (1949). The damage was inflicted by removing parts of the plant, and breaking and bruising the plants by beating with light objects. Three degrees of severity were studied. The effects on the plants as measured by several agronomic and seed compositional characters, in general, were in proportion to the degree of severity of the damage inflicted. The least reduction in yields occurred when the damaged plants were 6 to 12 inches tall and the highest reduction occurred a t the time seed development had been initiated in the lower pods. Reduction in yield was further increased by weed growth in hail damaged plots, particularly when such damage was severe and occurred during early stages. Heavy damage inflicted prior to and during the blossoming period delayed maturity as much as 8 days. Damage inflicted later than the “green bean” stage hastened maturity. Reduction in plant height was greatest when the damage occurred during the blossoming period. Seed quality was reduced only a t moderate and heavy degrees of damage, and only when damage was inflicted while pods were maturing. Up to 2.4 per cent reduction in the oil content of the beans resulted from damage to plots prior to ripening of pods, whereas the protein content was unaffected. The drying quality of the oil as measured by the iodine number was increased by all degrees of damage during pod formation and early seed development stages. Certain components of hail damage were studied individually to determine their effects on soybean production. Reduction of stand, which occurs when some plants fail to recover from hail damage, was found to reduce yield in progressively greater amounts when inflicted a t successively later stages of plant growth. Little effect on date of maturity or plant height was apparent. Defoliation was found to reduce yield only slightly when inflicted prior to blossoming. The highest reduction in yield from 100 per cent defoliation during this period was 22 per cent. However, up to 83 per cent reduction in yield occurred with removal of

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all leaves during the critical stage when seed was developing in the lower pods. Defoliation prior to blossoming delayed maturity, whereas, after the green bean stage, removal of leaves hastened ripening. Reduction of plant height was most severe if defoliation occurred during the blossoming period. Seed quality and size were reduced by defoliation during the seed developmental stage. As to seed composition, the protein content was unaffected by defoliation, the oil content was reduced particularly when leaves were removed during the seed developmental periods, and the iodine number of the oil was increased by defoliation during late stages.

PRACTICES VI. EFFECTOF CULTURAL 1. Rotations

The soybean was initially considered an alternative crop for small grains especially throughout the corn belt, and, consequently, was substituted for small grain in the existing rotations. The unsuitability of soybeans as a companion crop for small-seeded legumes and the development of high-yielding, disease-resistant small grain varieties are factors which have changed this system, When grown for bean production, the soybean crop i s a t present largely considered an intertilled crop and, therefore, competes with these crops in the rotation. Throughout the corn belt, Englehorn (1944) st.ates inclusion of soybeans has frequently lengthened the rotation. Whereas corn-corn-small grain-hay mixture was a common corn belt short-term rotation, it now frequently becomes corn-soybeans-corn-small grain-hay mixture. Beeson (1944) lists t,he following rotations for Indiana: For most soils

-corn (1 or 2 years) -soybeans-small grain-hay mixture. For grain farms -corn (1 or 2 years)-soybeans-small grain with sweetclover or clover seed crop. For very fertile soils -corn-soybeans. For southern Indiana-corn-soybeans-winter wheat or barley-lespedeza. He notes that soybeans following corn in the rotation facilit,ates corn borer control in that corn stalks are plowed under in preparing the land for soybeans. I n a l-year survey of 4200 soybean growers in the principal producing areas of Illinois, Indiana, and Ohio, Calland (1946) found corn preceded soybeans in the rotation in 80, 77 and 55 per cent of the fields in the t,hree states, respectively. Soybeans preceded soybeans in 13, 10 and 25 per cent of fields, respectively. I n Indiana and Ohio, oats and winter wheat followed soybeans most frequently, each succeeding soybeans on

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MARTIN G. WEISS

approximately one-third of the farms surveyed. I n Illinois corn and oats followed soybeans on 67 and 25 per cent of farms, respectively. On soils of low productivity, soybeans have by necessity replaced another tilled crop without lengthening the rotation. Such a rotation recommended for sandy soils in Wisconsin by Albert et al. (1947) is soybeans-oat8-legume, hay and seed-legume, seed, or hay and seed. Instead of oats, ensiled corn and a winter grain may be substituted. When grown for hay Trotter (1936) states that soybeans are a t times grown in one-year rotations with winter barley in Missouri. Few data are available at present which would indicate the relative merits of various rotations including soybeans. The effect of soybeans on crop yields in rotations is discussed in Section IX-2. 9. Fertilizers and

Soil Management

a. Response. Soybeans frequently have been classified as a (‘poor land” crop. This concept probably originated from 2 sources: Frequently soybeans yield relatively more than grain crops on soils of low productivity, and the response of soybeans to direct. application of commercial fertilizers is usually disappointing. However, marked yield variations are stimulated by differences in natural productivity of soil or general fertility levels as conditioned by different soil management (Cartter and Hopper, 1942; Lang and Miller, 1942; Norman, 1946; Vittum and Mulvey, 1944 and others). Some evidence is available, as reported by Pierre (1944) and Norman (1946), that increases in soybean yields as stimulated by high fertility levels are similar on a percentage basis to yield increases exhibited by corn. Although the response to direct application of fertilizers relative to other crops has been low, under certain conditions material increases in yield have been obtained. Correction of soil acidity with lime has, in general, resulted in higher yields (Cartter, 1941; Collins et al., 1947; Colwell, 1944; Nelson and Hart.wig, 1948; Pierre, 1944; Prince et al., 1941 ; Vittum and Mulvey, 1944 and others). When soybeans were grown on soils varying in pH from 4.6 to 7.7, Thatcher et al. (1937) found maximum yields resulted at p H 6.8. It is the contention of some workers that the stimulation due to liming is attributable to fertilization with the calcium ion rather t.han to neutralization of the soil. Designation of soybeans as an acid-tolerant crop, according to Albrecht (1944) ,is equivalent tjo intimating that the crop is tolerant to starvation. I n addition to increasing yield, application of lime was reported by Cartter (1941) to increase protein and decrease oil content of the beans. On potash-deficient soils direct application of potash has resulted in increased yields. Striking yield responses to pot,ash application were re-

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ported by Collins e f nl. (1947) and Nelson and Hartwig (1948) on Norfolk loamy, fine sand and Diinbar fine, sandy loam and by Colwell (1944) on Portsmouth and Dunhar silt! loams in North Carolina. Lesser responses were reported by Cartter (1941) on Clermont silt loam in Indiana and by Prince et aE. (1941) on Ondawa loamy fine sand in New Hampshire. In the latter work a slight, decrease in protein and increase in oil percentage of seed were attributed to potassium application. Similar changes in soybean composition as consequence of potassium fertilization of soils in South Africa had previously been reported by Viljoen (1937). Three varieties of soybeans grown on Coxville very fine sandy loam in North Carolina were reported by Nelson et al. (1946) to respond greatly in seed yield to applications of potassium. The yield increase was found to consist of a larger numbw of pods per plant, higher degree of pod filling, greater seed weight, and improved seed quality. I n contrast to other studies discussed later, maturity wag retarded by potassium application. Increased oil content of beans was reported by Colwell (1944) with potash application and hy Adams et nl. (1937) with application of a combination of potassium and nitrogen. Yield responses to phosphate h a w been reported (Cartter, 1941 ; Collins s t al., 1947; Colwell, 1944; Tlang and Miller, 1942) but, in general, responses to phosphate alone Iiavr not been as pronounced as to potassium. Applications of phosphate were even reported by Pierre (1944) to reduce yields under certain conditions. Combinations of phosphate and potassium have been reported by Lang and Miller (1942) to result in higher yields on certain of the light-colored and sandy soils of Illinois. In Indiana, combinations of these elements were found by Vittum and Mulvey (1944) to stimulate yield and result in marked earlier maturity. The components of yield increased by fertilization were found to include number of pods per plant, number of seeds per pod and size of seed. Seed quality also was improved in that number of damaged and purple blotched beans was decreased and germination was increased. Responses to application of certain other elements have, in fewer instances, been reported. Magnesium was found by Nelson et al. (1946) consistently to retard maturity and slightly to increase bean yields of t.wo of three varieties studied. Soil application of manganese sulfate nearly doubled the yield of soybeans on manganese-deficient Maumee loam in Indiana as reported by Steckel (1947, 1948). Early spray applications were found to be equally effective in correcting the deficiency, Severe iron deficiency symptoms in soybeans when grown on high-lime soils of the Webster series in north central Iowa were reported by Weiss (1943). On soils of this series with approximately p H 8 two spray applications of 10 lbs. of ferrous sulfate a t weekly intervals during early

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growth stages were found by Nelson (1948) to result in soybean yields varying between 20 and 35 bushels per acre whereas untreated plots produced virtually no beans. Reports of growth responses of soybeans to incorporation of stran and cornstalks into the soil have not been consistent. I n certain instances (Norman and Krampitz, 1946) ground straw has been incorporated into soil when reduction of available nitrogen has been desired for experimental purposes. This system is based on the premise that the microorganisms which decompose carbonaceous materials compete with the plant roots for available nitrogen. In greenhouse experiments conducted during both winter and summer Pinck et al. (1946) found that the addition of chopped wheat straw to Sassafras sandy loam materially reduced dry plant weight and total nitrogen of soybean plants when harvested in the green bean stage. Addition of the equivalent of 25 Ibs. per acre of urea overcame the nitrogen deficit caused by two tons of straw but 100 lbs. of urea were inadequate to compensate for the depression in available nitrogen accompanying a 4-ton per acre straw application. The above results were not substantiated by field experiments on a prairie soil reported by Englehorn et al. (1947). Plowing under 4 tons of straw per acre or twice the quantity of cornstalks produced by a 60-bushel crop of corn resulted in no decrease in yield or nitrogen content of mat.ure soybeans. Midseason application of nitrogen, however, increased yields and nitrogen content on the plots treated with crop residues. b. Placement. Direct application of fertilizer in contact with soybean seed frequently has been observed to result in poor germination. Phosphate and potash fertilizers, regardless of rate, were found by Probst (1944) to inhibit emergence of soybeans when applied in contact with the seed. Potash decreased emergence more severely than phosphate. When placed in bands 1 inch to either side and a t the same level as the seed, applications of 500 to 750 lbs. per acre of potash and phosphate failed to reduce emergence materially. Applying potash in bands 2 inches to each side and 1 inch below the level of the seed resulted in appreciably greater stands in l-year trials in North Carolina (National Joint Committee on Fertilizer Application, 1947) than when the fertilizer Wac placed 2 inches directly under the seed. The placement of fertilizers in regard to manner of incorporation into the soil has also been found to influence soybean yields. With certain conditions fertilizer broadcast and plowed under has been found t o give materially higher yields of hay and seed than when broadcast and disked in just prior to seeding (Drake and Scarseth, 1941; Enfield, 1943) or when placed in bands to one side of the seed (National Joint Com-

SOYBEANS

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mittee on Fertilizer Application, 1947). I n other areas the results have been less conclusive (Pierre, 1944; Smith, 1943). 3. Seed Inoculation

The soybean, like other legumes, has the faculty of entering into a symbiotic relationship with a species of root nodule bacteria. The species compatible with soybeans is Rhizobium japonicum. When properly nodulated soybean roots may derive a considerable portion of the nitrogen needed by the plant from the nodules which, in turn, is derived by the organisms from atmospheric nitrogen. The amount of nitrogen fixed by the root. nodule bacteria varies with factors such as the quantity of available nitrogen in the soil, and is discussed in greater detail in Section IV-2-a. The manner in which atmospheric nitrogen is transformed into combined nitrogen is still unknown. The transformation is thought t o occur in the nodules on the plant roots, and a number of theories have been advanced as to the chemical reaction which must occur. Searches for intermediate products in the nodules have, in general, proven fruitless. Orcutt (1937), Umbreit and Burris (1938) and others have found nitrogen fractions of soybean nodules similar to those of other parts of the plant with the possible exception of basic non-amino nitrogen which seems slightly higher in nodules. The nitrogen which has been accumlated through the fixation process seems to pass directly into the soybean roots. No evidence of leakage or excretion of nitrogenous substances into the substrate from nodules of Manchu soybeans was detected by Bond (1938) by chemical analysis of the sand in which the plants were growing or by growth response of barley grown in pots with nodulated soybeans. I n certain other legumes excretion of nitrogenous substances from the nodules has, however, been demonstrated. The advantages of inoculation are not always readily apparent to the grower. I n fact, when adequate nutrient nitrogen is present in the soil, increases in yield attributable to nodulation at times are negligible. The nitrogen fertilization experiments described in Section IV-2-a demonstrate that equally high yields can be produced with adequate quantities of soil nitrogen by unnodulated plants as by nodulated plants. On soils where combined nitrogen is a limiting factor for plant. growth, inoculation effects notable changes. On a loess soil with moderate quantities of combined nitrogen Norman and Browning (1943) and Norman (1944a) found the percentage increases of inoculated over uninoculated soybeans of the Mukden variety to be 31 per cent in yield, 11 per cent in protein content of beans, 47 per cent in protein production per acre in beans, 24 per cent

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MARTIN G . W E N S

in oil production per acre, 29 per cent in nitrogen content of straw, and a decrease of 5 per cent in oil content of beans. It was estimated that under these conditions the application of 540 lbs. of ammonium sulfate per acre would be required to equal the effects of inoculation. Nodulated soybean seedlings grown on colloidal clay were found by Hampton and Albrecht (1944a) to be higher in percentage of protein, potassium, calcium, magnesium, and phosphorus than non-nodulated plants. The authors postulated that the roots of nodulated plants, because of higher nitrogen content, have a more efficient physicochemical system for the movement of ions from the substrate into the interior of the root. Nodulated plants were further shown to have a considerably greater top to root ratio, indicating approximately 50 per cent higher efficiency of unit mass and surface of nodulated roots to produce tops than non-nodulated roots. The advisability of inoculating beans to be planted in soil which has grown nodulated soybeans in previous years has received considerable attention. Experiment. stations, in general, have recommended inoculation on fields not recently cropped with soybeans, or on acid soils, in which medium the longevity of rhizobia is thought to be reduced. A survey by Norman (1943) in an Iowa county in which soybeans had been commonly grown and where soils are nearly neutral in reaction showed nodules were absent in 8 per cent and soybeans were poorly nodulated in 13 per cent of the fields sampled. The presence of nodules is, furthermore, not in itself assurance of maximum nitrogen fixation in the plant. Variability in efficiency in nitrogen fixation with R. japonicum has been conclusively demonstrated (Agati and Garcia, 1940; Andrews and Briscoe, 1943; Briscoe and Andrews, 1938 and others). Certain strains have been shown by Andrews and Briscoe (1943) to be relat,ively efficient on limed soils whereas their performance was poor on unlimed soils. Among cultures of rhizobia, abundance of nodules is not closely associated with efficiency in nitrogen fixation. A further source of variation in efficiency of nitrogen fixation is the genetic diversity of the host varieties. Diff erent.ial efficiency of bacterial cultures on soybean varieties was found in the above researches. The reason for inspection of legume inoculants by a number of states immediately becomes apparent. I n tl report of inoculant inspection in Indiana by Quackenbush (1946) it is st,ated that both fixation of nitrogen, as judged by appearance of plants, and nodule formation are used as criteria for classification of commercial cultures, since poor cultures may produce a satisfactory number of nodules but plants may not obtain adequate nitrogen. Total yield and total nit,rogen in soybean plants

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were examined by Andrews and Briscoe (1943) as possible criteria for efficiency indices. The two characters were highly associated when the soybeans inoculated with the bacterial cultures were grown on unlimed noils. On limed soils, however, a low association existed. Efficient nitrogen-fixing strainr of rhizobia, when passed successively through the soybean host, tend to deteriorate in ability t o fix nitrogen, whereas inefficient strains improve in this respect (Umbreit, 1944). Since bacteria occurring in the soil must be assumed to have been derived from one or more such passages, the grower has no assurance that the organisms are highly efficient even though the preceding crop was inoculated with efficient strains. Repeated inoculation, therefore, is considered advisable by most soil bacterio1ogist.s to ensure the presence of adequate quantities of active, efficient rhiaobia and, furthermore, to locate them on the seed where they are needed by the young soybean seedlings.

4. Seed Germinability a. Viability. The longevity of soybeans, relative to ot.her crop seeds, is low. Under storage conditions encountered in Ontario, rapid decrease in viability was found to occur when soybeans were stored for more than three years (Laughland and Laughland, 1939). Viability was maintained for 5 years under Colorado conditions after which time Robertson et al. (1943) found soybeans decreased rapidly in germination. Under the same conditions the viability of small grains diminished only slightly in 10 years. When stored in bins in Illinois, soybeans near the surface were found by Burlison et al. (1940) to decrease appreciably in germination within 1 year from harvest. Within 2 years germinability of beans 4 feet below the surface also had decreased. Relatively rapid loss of viability a t the surface was attributed to greater absorption of moisture. Under the conditions of high temperatures and humidity encountered in Puerto Rico, viability of soybeans stored for more than a few months is consistently low (Stoddard, 1945). High moisture content and high temperatures both have been shown to decrease the longevity of soybean seed (Ramstad and Geddes, 1942; Toole and Toole, 1946). Effert of temperature is readily demonstrable by examination of the findings of Toole and Toole (1946). Soybeans with 13.5 per cent moisture maintained a t 30°C. failed to germinate after 5 months, whereas full viability was maintained after 10 years of storage a t -10°C. Soybeana maintained a t 20°C. with 18 per cent moisture lost all viability in from 5 to 9 months, whereas with 8 t o 9 per cent moisture, 90 per cent germination resulted after 5 years of storage. High moisture content and high storage temperatures of soybeans were found by Tervet

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MARTIN G. WEIISS

(1945) to favor incidence of fungi, particularly Aspergillus spp. Seedling growth was retarded accordingly. High moisture content a t time of harvest in itself is not detrimental to germination. Soybeans harvested with 66.9 per cent moisture content were found by Robbins and Porter (1946) to be germinable. Exposure of immat.ure soybeans to low temperature decreased germination roughly in proportion to the decrease in temperature and degree of immaturity. However, soybeans were found much more tolerant to low temperatures than sorghum. Germinability of soybean seed with 32 per cent moisture content or less was not reduced when frozen for 10 hours a t -20°F. With a moisture content of 50 to 60 per cent, in most varieties seed germination was not decreased by exposure t o temperatures not less than 20°F. . b. Disinfectants and Protectants. The effect of disinfectants and protectants on germination of soybeans has been studied extensively and diverse results have been obtained. Under certain conditions increases in germination have resulted following treatment of the seed with organic mercury or strictly organic compounds (Allington et al., 1945; Davy, 1942; Hildebrand and Koch, 1947a; Johnson and Koehler, 1943; Koehler, 1944a; Melhus et al., 1944; Petty, 1943; Porter, 1944, 194633; Sherwin et al., 1948; and Stoddard, 1945), although under other conditions evidence of control is entirely lacking (Tervet, 1943). Increases in germination generally resulted from seed treatment when the seed was low in viability, damaged in the threshing operation, or when seed of high viability was germinated a t reIatively low temperatures. Beans of moderate viability were found by Sherwin et al. (1948) to give greater response to seed treatment a t germination temperatures of 25OC. than a t higher or lower temperatures. On the other hand, failure of high germination of normally viable seed when germinated under low temperature conditions was found by Porter (1946a, 1946b) to be attributable largely to the occurrence of “baldheads,” which he described as seedlings which failed t o develop normally due to necrosis of the plumule in earl? stages. The occurrence of baldheads was substantially reduced by treatment with protectants. Reported instances of increased yields resulting from seed treatment are less frequent (Hildebrand and Koch, 1947a; Koehler, 1944a). The infrequent occurrence of increased yields from seed treatment is probably due t o the ability of the soybean plant. to compensate for thin stands, as discussed under Section VI-6. Even though less plants developed from the untreated beans, adequate stand for maximum yields still occurred. Considering all data, seed treatment of soybeans would not seem warranted except under the following conditions: When seed is of low viability due to age or severe weather damage (Allington et al., 1945) ; when

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it is desired to plant a t a low rate of planting (Koehler, 1944b) ; or when abnormally low germinating t.emperatures are anticipated (Porter, 1946b). The seed treatment compounds most commonly used in the above researches could be classified as strictly organic protectants, such as Spergon and Arasan, the organic mercury disinfectants, such as New Improved Semesan Jr. and New Improved Ceresan, and others such as Cuprocide (copper oxide) and Fermate (ferric dimethyl-dithiocarbamate). Cuprocide inhibits germination and is considered injurious t o soybean seedlings (Heuberger and Manns, 1943; Johnson and Koehler, 1943). With the exception of Cuprocide, when a substantial response t o seed treatment occurred, some degree of response to all of the above protectants and disinfectants occurred provided adequate quantit,ies were applied. No consistent difference between the mercury and non-mercury compounds was in evidence. The effect of the disinfectants and protectants on nodulat-ion of the soybean has been a subject of considerable interest. I n a study by Appleman (1942) the number of nodules on plants grown from inoculated beans in sterile substrate was not appreciably reduced by prior treatment with either of two organic mercury compounds or a copper oxide disinfectant. However, nodules developed only on lateral roots and not on the tap root as in the untreated check. The aut,hor postulated that with seed treatment a zone of bacteriostatic action was set up around the seed in which area legume bacteria are inactivated t o the degree of preventing their entrance into root hairs. Definite inhibition of nitrogen fixation was reported by Johnson and Koehler (1943) in plants when seed had been treated with Cuprocide. Inhibition of nodulation on the t a p root accompanied treatment with Ceresan and Semesan, Jr., whereas Spergon only partially inhibited t a p root nodulation. Field observations of inoculated seed planted in soil not inhabited with nodule bacteria, in general, have shown that all seed treatments appear to be detrimental to nodulation but not to the extent, of causing nitrogen defficiency symptoms in the plants (Allington e t al., 1945; Koehler, 1944a; Petty, 1943). The organic mercury compounds appear to inhibit nodulation t o a greater degree than the strictly organic protectants (Allington e t al., 1945). I n soils containing the nodule organisms, field observations have failed to detect decreases in nodulation attributable to seed treatment (Allington e t al., 1945; Koehler, 1944a; Petty, 1943; Stoddard, 1945). c. Hormones. Treatment of soybeans with various hormones has been the subject of several investigations (Bartholomew, 1944; Kiesselbach, 1943; Youden, 1940). The hormones studied by the various investigators included indolbutyric acid, napthalene acetic acid, Rootone,

110

MARTIN G. WEISS

Staymone, Grain 0 and Du Bay-120 FF. Stimulation of plant growth as measured by top growth or seed yield was not in evidence. Certain of the substances tended to inhibit germination of the seed. 5. Time of Planting

The effect of time of planting on yield of soybeans varies with geographical location and has been studied wherever soybeans have gained prominence. In general, when adapted varieties are planted at successive intervals throughout the first month following the frost-free date, differences in yields attributttblc to dates of planting are not great. As reported in the corn belt by Burlison et al. (1940) and in the cotton belt by Henson and Carr (1946) successively later plantings result in progressively lower yields. As shown by Weiss et d. (1949) varieties respond differentially in this respect. Early varieties, which do not normally require the entire growing season to mature, may be planted considerably later than adapted varieties without incurring yield reductions. On the other hand, yields of relatively late varieties, which can utilize the entire growing season, may be reduced by any delay in planting following the frost-free date. Time of maturity, in general, is delayed by later planting. However, the delay in time of maturity is not as great as the delay in planting. In South Africa Viljoen (1937) found 4 to 5 days delay in planting retarded maturity 1 day, whereas iq the corn belt Weiss et al. (1949) reported that a 3-day delay in planting retarded maturity 1 day. Varieties differing in earliness do not respond alike in this respect. Maturity of late varieties, which can utilize the entire growing season, is not deIayed by late planting as greatly as in early varieties. I n Mississippi, Henson and Carr (1946) reported that a delay in planting of 50 days resulted in delaying maturity of an early variety 26 days and of a late variety 2 days. Similar but less extreme reports were reported by Weiss et al. (1949). Delayed planting usually results in less plant height, slightly less lodging, and smaller seed size (Weiss et al., 1949), particularly in relatively late varieties, and better seed quality (Henson and Carr, 1946). Composition of soybeans has been shown to be influenced by the time of planting. A decrease in oil content (Viljoen, 1937; Weiss et al., 1949), a slight increase in protein content (Viljoen, 1937), a material increase in iodine number of oil (Weiss et al., 1949), and ti slight decrease in ash content (Viljoen, 1937) accompany delayed planting. Certain associations among characters were reported by Weiss et al. (1949). Among the means of five dates of planting the following attributes were found to be significantly correlated: Large seed size with low iodine number,

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lateness of maturity with low oil content, lateness of maturity with high iodine number of oil, low mean temperature during the seed developmental period with high iodine number of oil, high oil content with low iodine number of oil, and high protein content with low oil content. Little association was obtained between seed size and oil content, seed size and protein content, or maturity date and protein content. 6. Method and Rate of Planting

Methods of planting soybeans alone include broadcast planting, drilling in rows with 7- or 8-inch spacing, and planting in adequately wide rows to permit intertillage. Because of uneven depth of planting and subsequent lack of uniformity in germination, the broadcast planting method has rapidly lost favor in the areas of heavy production. Drilling of soybeans with the conventional grain drill was the first generally accepted practice in the heavy production areas. During the past decade the method of growing soybeans in rows to permit row cultivation has gained favor particularly in the northwestern part of the corn belt. The shift toward rowed planting is undoubtedly attributable to its greater efficiency in weed control, which constitutes a greater advantage in the northern areas where low mil temperatures prevent adequate destruction of weeds prior to planting. Soybean yields obtained when drilled a t various row widths in four corn belt states were summarized by Weber and Weiss (1948) and together with results reported for southeastern Kansas by Zahnley (1942) are illustrated in Fig. 4. The yields are presented as gross yields and no compensation has been made for differential planting rates in the various row widths. No weed control was generally practiced on the drilled beans after their height no longer permitted harrowing, whereas row widths from 21 to 42 inches were additionally intertilled. Highest yields were consistently attained over a period of years with the narrowest, intertilled rows, spaced 21 inches apart. Drilled beans and 28-inch rows gave slightly lower yields, and yields further decreased with wider row widt.hs. Substantiation of these experimental data were obtained by Calland (1946) in a canvass of 4200 growers in 48 principal soybean producing counties in Illinois, Indiana, and Ohio. On the average, narrow rows, spaced 18 to 28 inches, gave highest yields, medium width rows, 30 to 36 inches, yielded slightly less, and wide rows, 38 to 42 inches, and solid drilled fields gave similar yields and ranked lowest among the methods of planting. Lower yields in drilled beans relative to narrow rows was attributed by many investigators t o greater competition by weeds. In seasons during which adequate control of weeds was permitted in the drilled plots, maximum yields with this method of

MARTIN G . WEISS

112

planting frequently resulted. These observations are substantiated by yield data collected in New York by Wiggans (1939). When row widths of 8, 12, 16, 24, and 32 inches were compared for 4 years in plots kept weed-free by hand cultivation, maximum net yields of 38, 35, 34, 33, and 30 bushels per acre, respectively, were obtained. At all row widths approximately six plants per square foot, gave maximum yields, although

{: "

KANSAS SYEARS

7

14

21 28 INCHES BETWEEN ROWS

35

Fig. 4. Effect of row widths on yield of soybeans in five corn belt stat.es (Data of Zahnley, 1942; and Weber and Weiss, 1948).

the author notes that this number would undoubtedly vary for different varieties and growing conditions. Regardless of row width, the number of plants per unit area required to give maximum yields was thought t o be constant. Greatest possible uniformity of distribution was thought to facilitate yield, thereby explaining the decreasing yields with increasing row widths. In most of the corn belt, corn planters and cultivating equipment are used interchangeably on corn and soybeans. Width of soybean rows

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must therefore conform to t.hat of corn which varies from 36 to 42 inches. I n limited areas where soybeans comprise the major intertilled crop or where narrow row crops, such as sugar beets, are grown, equipment for planting and cultivating soybeans in 21- to 28-inch rows is employed. In certain areas alternate wide and narrow row widths have become popular. Reduction of average row width is thereby accomplished and use of 2-row corn planting and cultivating equipment is permitted when exterior sweeps of the 2-row cultivator are removed. Throughout the south interplanting of soybeans with corn is frequently practiced. The growing of soybeans and corn in the same row has been reported by Price (1948) to result in surprisingly high yields at times. Rates of planting giving maximum yields vary greatly with localities and varieties. Certain consistencies are, however, apparent in most rateof-planting data reported. Increased plmting rates result in increased yields only to a certain point beyond which yields remain relatively constant even though the planting rate may be increased three-fold. When reported on a net yield basis (harvested yield less beans planted), therefore, lower yields a t higher planting rates are obtained. As noted by many investigators, the soybean plant has the ability to make wide adjustments to space, which fact accounts for the lack of appreciable differences in yield throughout, wide rates of planting. Planting slightly in excess of the lowest planting rate shown to give maximum yields is recommended by most experiment stations. The heavier rates aid in weed control and soil erosion through provision of a more effective canopy, and ensure adequate stands in the event of formation of pre-emergence soil crusta by heavy rains. Rates of planting for maximum yields vary with various methods of planting. For illustration, under Iowa conditions the recommended rates per acre as reported by Weber and Weiss (1948) are for drilled beans, 120 lbs.; for 21- to 24-inch rows, 75 lbs.; for 30- to 36-inch rows, 60 lbs.; and for 40- to 42-inch rows, 45 lbs. Under Arkansas conditions McClelland (1940) obtained maximum seed yields wit.h the Mammoth Yellow variety a t rates of 21 and 30 lbs. per acre for 36-inch rows and drilling, respectively. Laredo, a small-seeded variety, gave maximum yields with 7 and 30 lbs. per acre, respectively. As noted by Probst (1945) and Weber and Weiss (1948) , dense stands are conducive to increased lodging. A slight deIay in maturity was reported by the latter authors to accompany the increased lodging. Height of plants was reduced and branching stimulated a t lower planting rates, but no effect on protein, oil content., or drying quality of the oil was detectable.

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a.

WEISS

7. Weed Control Initiation of weed control prior to plant,ing is emphasized by most experiment stations (Dunham, 1948; Sherwood, 1946; Weber and Weiss, 1948; and Willard, 1947). Early partial preparation of land, thereby encouraging many weed seeds to germinate and permitting their destruction prior t o planting, is generally recommended. Regardless of method of planting, surface cultivation with the harrow, rotary hoe, or weeder prior to emergence, and repeatedly following emergence is considered good practice. Cultivation with the rotary hoe approximately every 5 days until the soybeans attain a height of 6 inches is recommended by Wallace (1948). Limited injury to soybean plants results if cultivation is accomplished during the heat of the day. Eradication of weeds while in the seedling stage is considered particularly advisable (Weber and Weiss, 1948; Willard, 1947). After soybeans attain adequate size t o shade the soil, they are effective competitors of weeds. As discussed under Section VI-6 the growth of soybeans in rows with adequate separation to permit intert,illage has been found to result in higher average yields. When climatic conditions allow early, effective weed control, row planting8 offer no advantages and usually few cultivations are required. When frequent rains during the critical weedcontrol period prevent early destruction of weeds, however, intertillage permits the destruction of a considerable portion of the weeds which have become established, thereby preventing serious yield reductions. Control of weeds with growth regulators in soybeans has not been found feasible to date. Injury t o soybean plants occurs readily from application of 2,4-dichlorophenoxyacetic acid and related compounds. Concentrations as low as 0.001 g. per square yard (less than 0.01 Ibs. per acre) of 2,4-D were found by Weaver (1946) to cause decreased growth of tissues developed subsequent t o spraying, and to delay the initiation and decrease the amount of pod format.ion. Other workers have found appreciable differences among varieties with respect to susceptibility t o the hormone sprays. Pre-emergence treatment with 2,4-D has at times resulted in excellent cont.ro1 of weeds without severe injury t o soybean plants. I n other cases with identical treatment the incidence of inopportune rains has caused severe injury to the soybean seedlings. At the present writing pre-emergence treatment of soybeans with growth regnlators is considered hazardous. 8. Harueutiny

The combine has become the most popular harvesting machine for soybeans in the corn belt. As compared with cutting with a binder and

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t,hreshing with a stationary machine, cost of harvesting with a combine is approximately one-half as great, and losses in the field are one-third to one-half as severe (Bateman ~t al., 1942; Sjogren, 1939). Combining of soybeans is rwommended when the moisture content) drops below 14 per cent (Rateman et al., 1942; Minn. Agr. Ext. Serv., 1942; Struthers, 1942). Too low moisture content is conducive t o splitting of the beans in the threshing operation. T o minimbe split beans the cylinder speed of threshing machines should be reduced to 300 to 450 revolutions per minute, which is approximately one-half that required for threshing small grain. Most or all of the concaves should usually be removed. Harvesting losses in the field usually result from improper adjustment of the reel, too high cutter bar adjustment, excessive lodging or shattering in the field. Growing desirable varieties and harvesting a t the proper time can largely overcome the latter losses. Lack of severe ridging during cultivation of rowed beans permits lower adjustment of the cutter bar, thereby reducing losses from this source. Adequately low and forward adjustment of the reel has been found to reduce losses in badly lodged fields. VII. GENETICSAND CYTOLOGY

1. Genetic Analysis of Simply Inherited Characters The mode of inheritance of approximately forty simply inherited characters in soybeans has been recorded in the lists assembled by Woodworth (1932, 1933) and Morse and Cartter (1937). As the authority and date of report are conveniently listed in the latter reference, only those studies reported subsequent to 1937 will be cited. Characters, symbols, and dominance status of “qualitative” characters reported in soybeans are listed in Table 111. Among the simply inherited characters in soybeans seed coat color probably affords the most interesting opportunity to study gene interaction. The seed coat color is black (actually intense purple anthocyanin pigmentation), brown, or reddish brown (phlobaphene pigmentation) depending on whether the multiple alleles R1,rl, or rIo, in decreasing order of dominance, are prewnt. It was formerly assumed that the presence of a gene, R2, which was linked with T, a gene conditioning tawny pubescence, with 0 per cent crossing-over, was necessary for the above expression of the R, alleles. However, in line with Williams’ (1938) suggestion, the complementary effect of the R1 locus is considered attributable to a pleiotropic effect of T rather than a separate gene, R2. I n the presence of the gray pubescence allele, t, genotypes carrying rl, or rlo, have buff seed coat colors. The gene, t, also is epi-

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TABLE I11 Simply Inherited Characters Reported in Soybeans -

Characker I. Cotyledon-color 2. Cotyledon-color 3. Cotyledon-color 4. Flower-color

5. 6. 7. 8.

FIower-color Iron utilization efficiency * Leaf-chlorophyll deficiencies Leaf-chlorophyll deficiencies

9. Leafletshapc

10. Leafletshape 11. Leaflet-number 12. Maturity 13. Maturity

14. Pod-color 15. Pod-dehiscence 16. Pod-dehiscencc 17. Pod-number see& ‘ 18. 19. 20. 21. 22. 23. 24. 25. 26.

Pod-number seeds Pubescence-absence Pubescence-absence Pubescence-color Pubescence-erectness Pubescence-sharl)ncss of tip “ Seed-hilum abscission Seed-stcrility Seed coat-bloom

27. Seed coat-cracking 28. Seed coat-defects 29. Seed coat-defects 30. Seed coat-hardness

31. Seed c o a t p i g m e n t h o n

Symbol and dominant + recwsivc? relationship D1-yellow ; dl-green D-yellow; d-green Maternal green (cytoplasmic inheritance) (W, purple w1 1wa purple-blue ( ’. wt with W, nr w-whitc W-intense ; w-dilute Fe-efficient, normal ; fe-inefficient V1-normal; vl-variegation Y1 to Y-normal; yl to y-chlorophyll deficient to varying degfees and a t varying stapes. (@-ovate) ; na-with 0 or o-lanceolate Na- ,o-oval or narrow 0 (with Na)-ovate; o (with Na)-oval. X-more than three ; x-three. S-late maturing, tall ; s-early maturing, short. F-early maturing ; e-late maturing. L-black or dark; 1-light. Shl-non-shattering; shl-shattering. S h h a t t e r i n g (G. ussu&nsiS) ; s b n o n - s h a h tering (G. max, var. Kuradaisu). F*-less than 10 per cent pods 4-seeded; f*more than 10 per cent pods 4-seeded. Lo-intermediate (1.6 to 3) ; lo-lnw (1 to 1.5). P1-glabrous ; pl-pubescent. P-pubescent ; p-glabrous. T-tawny ; t g r a y . A-appressed ; a - e r e r t . B1-sharp ; bl-blunt . N-normal hilum; n-lack of abscission in hilum. St-normal ; st-sterile pollen and ovules. B1, Bz, B-complementary genes-all required for bloom. C,, C-complementary genes-both required for cracking. Del-normal ; del-defective. De-normal; de-defective. H-hard, impermeable ;h-permeable. with T-black (with W1-imperfect black ‘I{with t with wl-buff with T-brown witht-buff ’ with T-reddish brown

I

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SOYBEANS

Character 32. Seed coat-pigmentation 33. Sccd coat-pigmentation 34. Seed roat-pigmentation 35. Reed roat-pigmentationextension of dark pigments 36. Seed coat-pigmentation-

flecking 37. Seed coat-pigmentation-

mottling 38. Stem-branching 39. 40. 41. 42. 43.

Stem-color Stem-fasciation Stem-height Stem-type of growth Stem-type of growth

Symbol and dominant

--$

recessive relationship

L-smoky (diffusion from black pods); 1-no dark pigment. G-green ; g-yellow. D, or D-yellow; dl and d-green. I-romplete inhibition (yellow or green) ; i'-restricted to hilum; ik-restricted to saddle paltern; i-no restriction (black, brown or buff). F1-brown flecks on genetic black; fl-no flecking (a11 black). M-black concentric striping on genetic brown ; m-no mottling (all brown). Sp-spreading, long branches; sp-compact, short branches. W1-purple ; wl-green. F-normal; f-fasciated or flattened. L%tall, late maturing; H t o c k y , early maturing. Dt-indeterminate; dt-determinate. Df-normal ; df-dwarfed.

Weiss (1943). Doming0 (1945). ' Takahashi (1934) dTing (1946). * Not to be confused with the allelomorphs F, f conditioning fasciation. a

static to the R1 allele, but in this instance further differentiation is caused by the purple vs. white flower color alleles. With purple flowers, W1, an imperfect black results whereas with white flowers, wl, a buff pigmentation results which is indistinguishable from the buffs occurring with rl, or rIo. Inhibition of dark pigments (black and brown) is conditioned by an allelomorphic series comparable to the extension series present in rodent coat colors. I n the presence of I, inhibition of the self colors is complete, whereupon a pair of alleles conditioning the plastid pigments green, G, or yellow, g, control seed coat color. An allele, i', causing partial inhibition restricts the dark pigmenhs to the hilum, another, ik,restricts the blacks or brown to a saddle-like pattern, and a fourth allele, i, allows the seed coat to be fully self colored. 2. Linkage

A limited number of linkages among simply inherited characters has been reported in soybeans and appears in Table IV. Four linkage groups have been established. Morse and Cartter (1937) and Woodworth and

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MARTIN Q. WEISS

TABLE IV Linkage Characters in Soybeans Chromosome number 1 1 I 1 2 2

3 4

Crossing over Characters associated

Symbols

%

Defective seed coat and pubescence rolor Maturity and pubescence color Pubescence color and cotyledon color Pubescence color and hardness of seed coat Inhibition of pubescence and black mottling Inhibition of pubescence and brown seed coat Green seed coat and green cotyledons Defective seed coat and inhibition of pubescence Narrow leaflet and high number of seeds per pod Oval leaflet and low number of seeds per pod Indeterminate growth and black pods

t E, T T, d?

0

* Not to be confused with fasciated, also designated

dPI,

T,H

Pi, M PI, rl G, di den, pa na, f* 0, lo Dt, L

0 13 38 18 12

13 2 10 8 36

as f.

Williams (1938) present a provisional chromosome map and give authorities for linkage data. Additional linkages to those listed by Morse and Cartter were reported by Takahashi (1934) between narrow leaflets and high number of seeds per pod, and Domingo (1945) between oval 1eaflet.s and low number of seeds per pod. Although these pairs of characters have been shown by Domingo to constitute different linkage groups, their independence of the four linkage groups previously reported has not been entirely established. Ting (1946) reported loose linkages between pubescence color and seed coat impermeability, and between indeterminate growth habit and black pods. Williams and Williams (1938) discovered a prime type in which reciprocal translocation of segments between two chromosomes had ocourred following irradiation of the seed. I n the segregating generations following crossing with types having normal chromosome complement, plants heterozygous for the prime type could readily be distinguished by the degree of ovule or pollen abortion, as 50 per cent of the gametes were not viable. The authors called attention to the expediency of prime types in linkage determinations. 3. Analgsis of Yield Components

I n an effort to facilitate breeding for high yield, an attempt has been made by several investigators to break this complex character into its component parts. Woodworth (1933) found 26 varieties differed greatly in average numbers of nodes per plant, pods per node, seeds per pod,

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percentage of aborted seed, unit seed weight,, and total yield per plant. I n general, the yield components were independent of one another. However, only relatively low percentage of aborted seed and high unit seed weight were appreciably associated with total yield. Weatherspoon and Wentz (1934) found 237 strains differed in plant height, number of nodes per plant, number of pods per node, number of seeds per pod, percentage of aborted seeds, unit seed weight, and yield. Only plant height and number of pods per plant were appreciably associa€ed with yield. Unit seed weight and percentage of abortive seed showed no association whatever. The yield components, number of pods per plant, percentage of abortive seeds, seed size, number of seeds per pod and percentage of developed pods were all found to differ significantly by M a (1946) in 16 and 24 varieties studied in replicated designs in two successive years. In the first experiment, yield was found to be associated only with percentage of developed pods and number of seeds per pod. I n the second experiment, yield seemed to be associated only with seed size. Although the experiments had 10 varieties in common, associations were not consistent in the 2 years of the study. Among spaced F2 plants Stewart (1925) found height of plant more nearly associated with yield in determinate types than in indeterminate types. The results obtained in various investigations are not in agreement and the extent to which the component characters are associated with yield is still in doubt.

6. Inheritance of Oil and Protein Content The association of high oil with low protein contents among soybean varieties and segregates from crosses has been reported by several breeders. Negative linear correlations varying from -0.31 to -0.58 were reported by Bordakov (1933) among large numbers of soybean selections. Among 195 varieties King and Wang (1935) reported a correlation of -0.26. Viljoen (1937) reported correlations of -0.71, -0.72, and -0.80 among F2 segregates from three crosses, all values attaining a high level of significance. The regression coefficients of oil on protein percentages varied from -0.42 to -0.45. Negative associat*ions also were noted by Weiss et al. (1949) when five varieties were planted a t five dates of plant.ing a t three widely scattered locations. The negative correlations of protein and oil contents were more pronounced and consistent among varieties than among dates of planting. As discussed under Section VII-6, negative correlations were also obtained by Weber (1949) among Fz and F3lines of an interspecific cross. If protein and oil are considered end products of a physiological process within the soybean, negat.ive associations between the attributes would be a reasonable expectation. Development of a variety with high

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oil and high protein contents would be virtually impossible. The common belief that both protein and oil are derived from carbohydrates within the bean is not shared by Wolfe et al. (1942). When expressed on a milligrams-per-average-bean basis, carbohydrates were shown to increase during maturation of the soybean although not as rapidly as protein and oil. They concluded that much of the oil must be formed from materials brought into the bean from other parts of the plant, rather than from carbohydrates within the bean. However, when protein content was calculated on an oil-free residue basis by Weiss et al. (1949), high negative correlations persisted among varieties. Associations between morphological and compositional characters have been less consistent. Among 195 varieties size and weight of beans were found by King and Wang (1935) to have slight positive association with oil content and slight negative association with protein content. Among Fz segregates of three crosses, Viljoen (1937) found large seed size to be correlated with low oil and high protein contents. Among several varieties planted at several dates, large seed size was reported by Weiss et al. (1949) to be correlated with low iodine number of oil but no association with oil or protein contents was obtained. Among varieties lateness of maturity was associated with high oil and low protein content. I n an interspecific cross, discussed under Section VII-6, Weber (1949) reported large seed size among Fz and F3 lines t o be notably associated with high oil content and low iodine number and slightly associated with low protein content. Lateness of maturity was not associated with compositional characters. The wide variation in results indicates lack of consistent association between the agronomic and compositional characters studied in soybeans. The mode of inheritance of oil and protein contents of soybeans in three intervarietal crosses was studied by Viljoen (1937). The crosscs involved parents with oil contents as follows: High x low; high x medium; and medium x medium. Protein percentages of t.he parental varieties were opposite to those of oil. In the high x low and high x medium crosses compositional attributes of the F1hybrids and the mean of the F, populations were very nearly midway between those of the parents, indicating lack of dominance and additive gene action. Limited transgressive segregation occurred in the F, population of the high x low cross whereas parental means were exceeded in the high x medium cross. I n the medium x medium cross the F1 and the mean of the Fz population exceded the high parent in oil content while the respective protein contents were below that of the low parent. Appreciable transgressive segregation occurred for both oil and protein contents.

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Inheritance of compositional characters in interspecific crosses in soybeans is discussed under Section VII-6. 5 . Polyploidy

Colchicine-induced autotetraploid soybeans were found by Tang and Loo (1940) to have rougher and thicker leaves, larger stomata, larger cells and larger nuclei when compared with diploid plants. The autotetraploids of two varieties were found by Porter and Weiss (1948) to have thicker stems, longer internodes, shorter and broader leaves, larger seed and pollen, and later dates of flowering and maturity relative to diploids. Seed yield of the tetraploids was less than 20 per cent th a t of diploids. Lateness of maturity and greater seed size were thought to be largely attributable to the low number of seeds per plant. Seed of the tetraploids had higher protein content, lower oil content, and slightly lower iodine number of oil than the diploid counterparts. The growth rate of autotetraploid plants was lower than that of diploids when either plant height or dry matter of tops was used as a criterion. 6. Interspecific Crosses

Interspecific hybridization among the three Glycine species occurring in Manchuria has been the subject of several investigations. The three species, G. max, G . grasilis, and G. ussuriensis, were found by Fukuda (1933) to be very similar in chromosome number and size, and pollen grain size. Seed size variation within the species was adequately great to result in continuous variation throughout the genus. Karasawa (1936) found the F1 hybrids of G. max x G. ussuriensis t o be intermediate to the two parents in most characters. However, t.hey resembled the G . ussuriensis parent in the twining habit of growth and appressed pubescence. No abnormalities were observed cytologically during sporogenesis of the F1 and fertility was normal. H e stated that the characteristics of the F2 and F3 plants were not much different than those of the F1,which probably indicates that the parental types were not readily recovered in respect to growth habit. The above intcrspecific cross was found by Ting (1946) to be as high in fertility as intraspecific crosses. The mode of inheritance of ten simply inherited Characters, such as pubescence color, flower color, etc., was found to be identical with that of intervarietal crosses in G. max. Height of plants and size and shape of seed were found to be quantitative in inheritance, and positive skewness was evident in the FB distribution but both parental types were recovered in each of the three characters. Partial dominance of the G. ussuriensis parent characters of greater height and longer seed was exhibited by the F1. Seed size of the F1 approached the geometric mean of the parents. Most

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MARTIN 0. WEISS

of 15 crosses of G. mux x G. ussuriemis were reported by Williams (1948) to exhibit 50 per cent pollen and ovule abortion in the F1,which suggests that the species differ by a reciprocal translocation. However, F1 plants of certain crosses gave full fertility. Seed size was found to be geometric in inheritance and neither parental type was recovered in more than 4000 F2 plants, or in the first backcross generation to G. max. The large parental seed size was recovered in the BCl& and BC2 generations with G. max as the recurrent parent. The oil content of the F1 was intermediate but neither the high oil of G. max or the low oil of G . ussuriensis was recovered in the F2. Dominance was exhibited in the F1 for the high protein content of the G. ussuriensis seed, and transgressive segregation for high protein occurred in t*heFz. The prostrate habit predominated in the F2 and F3,and the erect habit of G. max was not recovered even in the first backcross with the erect parent. Maturity was intermediate to that of the parents in the F1and transgressive segregation occurred in the Fz. The same interspecific cross was studied by Weber (1949) as to inheritance of maturity date, seed size, and oil content, iodine number and protein content of the seed. The comparable parental means of these characters were as follows: Maturity date Seed size (g./lOO seeds) Oil percentage Iodine number of oil Protein percentage

0.max Oct. 1

G . ussuriensis Sept. 23

15.7 20.6 129.2 41.2

1.4 6.7

153.8 50.5

Positive skewness was obtained in the Fz and F3 generations for seed size with notable failure to recover the large parent. Gene interaction was found to be geometrically cumulative. It was estimated that a large number of genes conditioned this character. I n the inheritance of oil content and iodine number of oil, lack of dominance, complex inheritance, and a mixture of geometric and additive gene interaction were indicated. Protein content exhibited partial dominance for high protein, evidence for relatively few genes conditioning it, and a tendency toward additive gene action. Maturity gave indications of lack of dominance, extensive transgressive segregation in Fz and F3 generations, additive gene action, and low number of genes involved. Heritability for the various polygenic characters was as follows: iodine number, 47 per cent; seed size, 55 per cent; oil content, 64 per cent; protein content, 70 per cent; and maturity, 86 per cent. Among F2 plants and Fs lines of the interspecific cross, large seed size was notably associated with high oil content, and low iodine

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number of oil, and slightly associated with low protein content. Oil content was negatively correlated with protein content and iodine number of oil, and a slight positive association was evident between protein content and iodine number. Maturity date was not correlated with seed size or any of the compositional att.ributes. VIII. VARIETY IMPROVEMENT 1. Introduction and Selection The soybean industry in t,he United States was established with varieties introduced from eastern Asia. New introductions were made prior to the 20th century but, according to Morse and Cartter (1937, 1939), previous to 1898 not more than eight varieties were grown in this country. Klose (1949) states that introduction by the U.S. Department of Agriculture was initiated with three varieties from Japan in 1900, and by 1920 over a thousand varieties had been collected by the Department. In 1929, when the soybean showed evidence of becoming a major crop, W. J. Morse and P. H. Dorsett were sent t o Japan, Sakhalin, Manchuria, Korea, and China, for an extensive search for additional germ plasm. They returned in 1931 with nearly three thousand varieties. Limitation in the range of adaptation of a particular variety of soybeans stimulated still further introduction by various agencies, and by 1947 over ten thousand varieties ha4 heen introduced by t.he U.S. Department of Agriculture alone. Introduced varieties, or selections from introductions, comprised the major portion of the soybean production in the United States prior to World War 11. Many varieties which had been introduced three decades previously, such as Mammoth Yellow, Biloxi, Manchu and Dunfield, were still grown commercially to some extent. Pure line seIections from introductions, such as Illini, Mukden, and Richland, comprised a major portion of the acreage in the North Central Region. Varieties were, however, found t o differ greatly in agronomic characters, and, under the conditions of their new environment, exhibited weaknesses. I n an attempt to combine the desirable traits of several varieties into one, hybridization programs were init.iated, first in the South, primarily at the DeIta Branch and Tennessee Agricultural Experiment Stations, and later a t the agricultural experiment stations of the north central states. 2. Hybridization

a. Making the Cross. Soybeans are naturally self-pollinated to a high degree. This is largely because fertilization occurs in the advanced bud stage before the flower opens. As minute insects, such as thrips,

124

MARTIN G. WEISS

invade the flower in the bud stage, infrequent cross fertilization occurs when different genotypes are grown in contact with each other. When different varieties were planted in adjacent rows, Woodworth (1922) observed 0.16 per cent natural hybridization, and Garber and Odland (1926b) found 0.14 and 0.36 per cent, respectively, in successive years. Takagi (1927) obtained 0.62 per cent natural crossing when plants of different cotyledon color were grown in contact wit.h each other. When different genotypes were grown alternately in 3- to 4-inch spacing within a row, Cutler (1934) obtained 0.28 per cent crossed seed, although individual varieties varied from 0.08 to 2.43 per cent. He suggested interplanting of parental varieties as a means of obtaining desired crosses. This method has certain disadvantages, however. It necessitates the presence of a dominant marker gene in one parent to enable detection of hybrid plants among the progeny, and the identity of the male parent is sometimes in doubt. When making cross pollinations manually, the flowers found most suitable are those in which anthesis will occur the following morning. The small flower size necessitates removal of the calyx and corolla. Thereafter emasculation may readily be accomplished by removal of the ten anthers with a fine tweezer. A skilled operator may remove most of the anthers concurrently wit.h removal of the keel by grasping the entire keel with tweezers below the level of the anthers and applying very slight pressure during its removal to avoid crushing of the anthers. Pollination may be accomplished a t once by applying pollen from a flower in which anthesis has recently occurred. Gat.hering of flowers of male parents soon after anthesis and storing in a cool, dry place facilitates quality of pollen, as under field conditions insects injure or destroy pollen rapidly. Under refrigerated, dry conditions the author has found pollen to retain its viability for several days. Success in obtaining hybrid seed varies with environmental conditions TABLE V Variability in Setting of Hybrid Pods Following Emasculation and Hybridization Pods set per 100 flowers crossed*

No. flowers Year ~

crossed

Mean

2770 3908 3743

19.2 21.0 40.5

Range among days

Range among operators

~~~

1939 1940 1941

0-30 1-44 19-75

* Soybeans per pod averaged slightly in excess of two in each of

13-33 19-22 3-5 the 3 years.

125

SOYBEANS

and with operators. Crossing results obtained a t Ames, Iowa, for 3 years are summarized in Table V. The climatological measurement which seemed most closely associated with success in crossing was evaporation from a free-water surface. Evaporation during the hybridization season in 1941 was 75 t o 80 per cent as great as the two previous seasons which probably accounts for the higher success in this season. Manual hybridization is frequently more successful when practiced throughout the late hours of the day. Results obtained a t Ames, Iowa, by hourly intervals throughout the day (Central Standard Time) with more than 3000 pollinations in each of 2 years appear in Table VI. TABLE VI Hourly Intervals of the D a y

Success in Crossing During Certain

~

~

~~~

~

Pods set per 100 flowers crossed -.

8-9

Year A.M. 1940 1941

20

-

9-10

10-11

11-12

1-2

A.M.

A.M.

A.M.

P.M.

23 P.M.

34 P.M.

14 39

13 37

14 32

16 29

18 34

19 39

4-5

P.M. 23 47

5 4 P.M. 33 62

6-7 P.M. 37 54

When a dominant character in the male parent enables detection of hybrid plants, emasculation of the female flowers may be omitted. The occurrence of accidental selfs, usually less than 2 per cent with emasculation, during certain seasons has been known to be increased to over 30 per cent when emasculation was omitted. b. Breeding Methods. Methods of breeding employed in the development of new soybean varieties are similar to those used on cereal grains. The bulk method, wherein the entire progeny of a cross is harvested as a bulk lot throughout the segregating generations, and the pedigree method, wherein single plant selections are made during each segregating generation and the progeny of the selections are grown separately, are popular. A combination of these two methods in frequent use consists of growing the crosses a t a central experiment station and employing the bulk system until the F3 or Fq generation, making selections a t this time and classifying them according to maturity groups, and thereafter employing the pedigree system a t outlying experiment stations where the maturity groups are adapted. Improvement of soybean varieties by means of the backcross system of breeding has recently been included in several breeding programs. I n most instances backcrossing has been limited to one generation and thereafter the bulk or pedigree systems were used.

126

MARTIN Q. WEISS

The bulk system of breeding has certain advantages over the pedigree system, such as minimum land and labor requirements during the segregating generations. It also has definite limitations, however, in that it does not permit inheritance studiefi to be carried on concurrently with the practical breeding program, and docs not permit the breeder to learn the breeding potentialities of varieties. I n the bulk system early evaluation of progenies is limited to comparisons between entire crosses, whereas in the pedigree system inferior segregates within a cross may be eliminated during early generations of inbreeding. Progeny testing of segregates within a cross results in the accumulation of performance data on the progenitors of final selections. Such data contribute substantiating evidence of the merits of a selection and thereby shorten the testing period of the final line, resulting in earlier release of the variety to growers. c. Early Generation Testing. I n all systems of soybean breeding evaluation of breeding material early during the period of inbreeding is advantageous in that it permits elimination of inferior germ plasm and, assuming an improvement program has maximum limitations as to size, enhances the probability of finding superior segregates in the remaining material. The problem of immediate interest becomes one of determining how early the various agronomic characters are fixed in segregating populations and whether the evaluation of such characters can be sufficiently accurate. (1) Heterosis. Heterosis in seed yield of F1 soybean hybrids has been frequently noted. Seed yields of limited numbers of F1 plants studied by Wentz and Stewart (1924),Veatch (1930),and Woodworth (1933)were found to exceed those of the higher parent by 117.3,19.6,and 26.1 per cent, respectively. Appreciable differences in the degree of heterosis expressed by various crosses were noted. I n more extensive trials by Weiss et al. (1947s) seed yields of F1plants of 17 crosses exceeded the higher parent by 14.5 per cent when grown in the field and 32.2 per cent when grown in the greenhouse. The F1yields exceeded that of the high parent in each cross when grown in the field and all but one when grown in the greenhouse. Five and ten crosses when grown in the field and greenhouse, respectively, yielded significantly above the higher parent. Crosses differed perceptibly in degree of heterosis expressed and responded differentially when grown in the greenhouse and in the field. Kalton (1948)in a field study of four crosses found the average increase of F1 seed yield over the higher parent to be 26.8 per cent, and also noted that crosses differed in degree of heterosis. I n the characters date of maturity, plant height, and lodging re-

SOYBEANS

127

sist.ance, F1 plants in general were intermediate to their parents in studies by Weiss et al. (1947a). Stewart (1925) found that some heterosis in yield was carried over into the Fa generation of soybean crosses. Fz populations of certain crosses were found by Weiss e t al. (1947a) to exceed their parents in yield. However, heterosis exhibited in the F1 and F2 generations was not consistent among crosses. (2) Selection Among Crosses. YieId performance of bulk hybrid populations in cereals has been shown to be of value in predicting the yield of segregates. Degree of heterosis exhibited by the F1 and early segregating generations is considered indicative of the potentialities of a cross. The bulk population test, therefore, enables the breeder to discard entire crosses in which the probability of obtaining high yielding segregates is low. In soybeans Weiss et al. (1947a) found little association between degree of heterosis as evidenced by seed yield of F1 plants and yield potentialities of 17 crosses. Bulk population tests of the F2 t o F5 generations likewise were unreliable in predicting the yield or maturity date of segregates derived from t.he crosses. Lodging resistance and height of subsequent selections were predicted with a reasonable degree of accuracy. The crosses responded differentially when tested in the bulk F2 to F5 generations in the four characters studied, even though the generations were tested concurrently. An examination of the climatic conditions in the various seasons during which the generations had been advanced ahowed t.hat natural selection had been extremely effective in altering maturity of the populations. As late maturity was shown to be consistently correlated with greater height and more severe lodging, and alternately correlated with high or low yield depending on seasonal conditions, natural selection had also been effective in altering the height, lodging and yield of the populations. Yield of advanced generations, consequently, deviated appreciably from the expected inbreeding depression curve. I n a similar study by Kalton (1948) the unreliability of bulk population tests in predict.ing yield potentialities of crosses was substantiated. Even though the generations were advanced in seasons having similar climatic conditions, yield differences among the bulk crosses were not consistent in the Fz, Fa and F4 generations. However, in this study thc generations were tested in succeeding years and, therefore, interactions of crosses with generations and with seasons were confounded. Differences among crosses in height, lodging resistance, and maturity remained relatively constant throughout the generations. Unreliability of bulk population tests in predicting yield of segregates

128

MARTIN G. WEISS

from soybean crosses would seem largely attributable to the fact that soybeans are a full season crop and therefore are subject to varying natural selection pressures. Unless bulk populations can be grown in the immediate area where a new variety is sought, natural selection pressures can be detrimental to the success of selecting an improved variety. (3) Selection within Crosses. When the bulk system of breeding is employed, plant selection is delayed until all segregates of the cross are relatively homosygous and stable. ?'he individuality of the selected plant is therefore definitely established and the performance of its immediate progeny is unquestionably closely indicative of the merits of a variety developed from it. When the pedigree system is employed plant selection is initiated in the F2 generation, when maximum segregat,ion occurs. When making selections in this and later segregating generations the plant breeder attempts to select the plants which are most desirable froin an agronomic standpoint.. Two questions might be posed: (1) How accurate are evaluations which are based on the characteristics of a single plant or a single row? (2) How early in the inbreeding process are the various agronomic characters adequately fixed to permit accurate selection? I n many other crop species the effect of environment on single plant yields has been shown to be of such magnitude that selection for heritable high yield is virtually futile. Selection for disease resistance, on the other hand, can be very effective provided all plants have been inoculated and suitable environmental conditions prevail for the pathogen. Effectiveness of selection among individual plants is therefore largely dependent on the magnitude of heritable variability relative to variation caused by the environment. I n three soybean crosses Patel (1927) found that seed yields of 249 individual F3 plants were associated to a high degree with the yield of subsequent F4 lines grown in duplicate rows. Associat,ion was also found between yields of F2 plants and F4 lines. Weatherspoon (1933) selected 347 F, plants from a population derived from a single F3plant, grew single progeny rows the following year, and conducted a replicated yield test of the lines in the third year. He found no appreciable association between yields of plant selections or progeny rows and yields of subsequent lines. These results were interpreted to indicate that single plant or single row yields are of limited value for selection. The performance of spaced F2 plants from 17 crosses was compared with the replicated Fa line performance by Weiss et al. (1947a). Individual maturity date determinations of the F2 plants were highly indicative of the maturity date of the subsequent lines. Yield determinations and lodging scores, respectively, gave moderate and poor estimates of progeny performance. The

SOYBEANS

129

method of planting was shown to influence the relative performance of a line. When over 700 F3 lines were compared in space planted and solid planted plots, an interaction between lines and method of planting occurred. I n a study of four soybean crosses Kalton (1948) found plant height. and maturity measurements made on spaced F2 plants provided good estimates of these characters in subsequent F3 and F4 progenies. Seed yield measurements of F2 plants were of little or no value in predicting yields of subsequent lines. Spaced F2 plants and F3 lines of an interspecific cross, G. mu2 X G. ussunemis, were studied by Weber (1949) as to heritability of time of maturity, seed size, and oil content, iodine number of oil, and protein content of seed. As the two species have been shown to be similar cytologically (Fukuda, 1933) and segregate normally for qualitative characters (Ting, 1946), it is assumed that the basic reaction of the polygenes conditioning the characters is similar to that of crosses involving G. mux varieties. Heritability of the characters was as follows: Maturity date, 86 per cent; protein content, 70 per cent; oil content, 64 per cent; seed size, 55 per cent.; and iodine number, 47 per cent. As the two parents in this cross differed considerably more in seed size and seed compositional attributes than varieties of the cultivated species, the heritable portions of these characters are probably in excess of what could normally be expected with intervarietal crosses. The studies reported would indicate that among spaced plants in early generations of soybean hybrid populations, effective selection can be practiced for maturity date and plant height, whereas relatively low heritable variation in seed yield, lodging resistance and seed composition render selection for these characters relatively ineffective. Seed quantities from spaced Fz soybean plants permit testing of the F3 lines in replicated designs, providing plot size is confined to 8-foot rows. However, selection must be based on performance data in a single season. Effectiveness of selection for characters subject to seasonal fluctuations is therefore limited. The question could well be posed: How effective are replicated progeny tests in predicting potential agronomic value of F3 and F4 lines? Patel (1927) noted that the yield of F3 lines grown in single rows was associated with yield of the progenies of F3 select.ions which were grown in duplicate rows the succeeding year. Henson (1928) found the yield of 75 F4 lines grown in duplicate to be slightly associated with the yield of the same lines grown in the Fs generation in a replicated test. As noted above Weatherspoon (1933) found little association between yields of F3lines grown in single rows and yields of the same lines when grown in the Fa generation with replication. F a and F a line replicated

130

MARTIN G. WEISS

tests, regardless of season, provided reliable prediction values for dates of maturity, plant height, and lodging resistance in studies by Weiss e t al. (1947a). As yield was extremely subject t o seasonal fluctuations, negative associations between F3 and F4lines and moderately low associations between F4 and Fslines were obtained, indicating the danger of selecting rigorously for yield among segregating progenies on the basis of results from a single season. Although interseasonal variation persisted in yield in all generations studied, subsequent to the F3 generation interaction of lines with seasons was no greater than obtained among homozygous varieties. More uniform seasons were encountered by Kalton (1948) in a similar study with four crosses. The extent of association between F3 and F4 lines for maturity, height and lodging resistance indicated eff ective selection for these characters could have been made among F3 lines. Yield associations between F3 lines and progenies of F3 plants selected therefrom were low. Studies reported to date would seem to indicate that replicated F3 line tests permit effective selectcion among lines for maturity date, plant height, and lodging resistance. F3 lines are poor and F4 lines moderate in prediction value for yield of subsequent selections. Soybean breeding studies reported to date would seem to warrant these conclusions: (1) When a soybean breeding program is conducted a t a central station and it is desired to develop varieties which are not particularly adapted in earliness to local conditions, the pedigree system of breeding seems superior to the bulk system. Natural selection can be rigorous for adapted maturity types and thereby rapidly eliminate unadapted segregates in a bulk population. (2) I n the pedigree system space planting would seem advisable to permit adequate progeny testing in the succeeding generation. (3) Rigorous selection for desired maturity date and adequate height among F2 plants would seem justified. Selection for disease resishance also would be effective. (4) Replication of the F3 lines using normal rates of planting would seem advisable in that effective selection among lines for lodging as well as additional selection for maturity and height is permitted. (5) Selection for yield a t this stage must be limited to truncated elimination of the poor lines, taking maturity date into consideration if varieties of different maturity dates are desired. Selection for the important seed compositional attribute, oil content, can also be made if practiced with moderate intensity. Space planting of F3 lines in separate plantings permits further selection within the retained lines. (6) Rigorous selection for yield and oil content cannot be accomplished effectively until the Fq and later generations. Yield trials should preferably be located in the areas for which the variety is being developed and a t several locations.

SOYBEANS

3.

131

New Varieties

The establishment of the soybean in American industry has stimulated development of new varieties, particularly varieties suitable for production of beans. Not only yield of seed but suitable composition of seed has been considered paramount in new varieties. I n seed composition the attribute which has received the most emphasis is oil content. High oil content of the seed has consequently been established as a prerequisite for the release of a new variety. Although quality of oil has been studied rather extensively, the use of soybean oil in both edible and drying fields has precluded selection for either high or low iodine number. Recent studies indicate that varieties differ appreciably in refining losses. The event of techniques enabling determination of refining losses on small quantities of seed will undoubtedly add this attribute to those sought for in a new variety. In addition to yield other agronomic characters sought for in a new variety are lodging resistance, shattering resistance, disease resistance, proper maturity, and adequate height t o prevent pods from being borne too near the soil line when planted a t optimum rates. The inherent sensitivity of the soybean to environment seriously limits the latitudinal adaptation of a variety. Although a single variety may be well adapted to large belts in the heavy soybean producing areas, in the marginal producing areas where environmental conditions are varied, varieties are frequently found to respond differentially in areas relatively near each other. Soybean varieties have been classified in nine maturity groups by the U.S. Regional Soybean Laboratory and cooperating agencies. The maturity groups, as described by Morse and Cartter (1949), include the entire maturity range of varieties in the United States; varieties in Groups 0 and I being adapted to the northern states and Group VIII in t.he Gulf Coast region. Maturity of varieties within a group is determined relative to a commercial variety grown in the region of adaptation. A tabulation of new soybean varieties which have come into prominence during the period 1938-1949, their origin and classification as to earliness appears in Table VII. The relative maturities were determined by the US. Regional Soybean Laboratory personnel and cooperators in agricultural experiment stations.

4. Expe.rimentaZ Techniques Numerous researches have been conducted on methods of experimentation in an effort to increase accuracy of comparisons among varieties or heatments. Plot size, shape, and arrangement have been the subject

TABLE VII

w t3

cu

Soybean Varieties which have Come into Prominence During the Period 1938-1949 Variety

Relative maturity

Originating agency

Origin

Use

Group 0 ' Sioux Montreal Manchu Flambeau Kagon Kabott Pagoda Pridesoy Sac Ottawa Mandarin Capital

-7

-2 -7 0 -10 -11 -2 +1

0

-1

U.S. Dept. Agr. T. B. MacCauley, Montreal Wis. Agr. Expt. Sta. Wis. Agr. Expt. Sta. Central Expt. Farm, Ottawa Central Expt. Farm, Ottawa Twin City Seed Co., Minneapolis, Minn. Iowa Agr. Expt. Sta. Central Expt. Farm, Ottawa Central Expt. Farm, Ottawa

1ntro.-P.I. 81021 Se1.-Manchu Se1.-Manchu Unknown Sel.-Manchu Se1.-Manitoba Brown x Mand. Se1.-Norsoy Se1.-P.I. 80462 Se1.-Mandarin Se1.-Strain 171 x A X .

Green vegetable Beans Beans Beans Beans Beans Beans Green vegetable Beans Beans

Group I' Monroe Earlyana Manchukota Ontario Cayuga

+7

+lo +7 +5 -1

Ohio Agr. Expt. Sta. & USRSL' Purdue Agr. Expt. Sta. S. Dak. Agr. Expt. Sta. New York Agr. Expt. Sta. New York Agr. Expt. Sta.

Se1.-Mukden x Mandarin Se1.-Nat. hybrid in Dunfield Se1.-Manchu Se1.-P.I. 65344 1ntro.-P.I. 65393

Beans Beans Beans Beans Hay

Group IIb Harman Richland Bavender Special Hawkeye Korean

+1 0 +4 0 -2

Dominion Expt. Sta., Ontario Purdue Agr. Expt. Sta. Mr. Bavender, Whitten, Iowa Iowa Agr. Expt. Sta. & USRSL" Dominion Expt. Sta., Ontario

Se1.-Manchu 1ntro.-PI. 70502-2 Sel.-Unknown hybrid Se1.-Mukden x Richland Intro.4rient

Beans Beans Beans Beans Beans

?

TABLE VII-Continued Variety

Relative maturity

Originating agency

Origin

Use

~

Granger Seneca Mendota

-3 +2

Adams Lincoln Chief Viking Pennsoy Mingo Scioto

-1

s 100'

+7 -3 0

-

Sel .-Manchu Intr0,F.C. 03654 G Se1.-P.I. 84668

N.J. Agr. Expt. Sta. N.Y. Agr. Expt. Sta. Wis. Agr. Expt. Sta.

Beans Beans and hay Green vegetable

Group IIId

0 +8 +6 +2

0 +7

Iowa Agr. Expt. Sta. and USRSL ' Ill. Agr. Expt. Sta. and USRSL ' Ill. Agr. Expt. Sta. 111. Agr. Expt. Sta. Pa. Agr. Expt. Sta. Ohio Agr. Expt. Sta. Ohio Agr. Expt. Sta.

Se1.-Dunfield x Illini Beans Se1.-Mandarin x Manchu Beans Se1.-Illini x Manchu Beans Se1.-Illini x Manchu Beans Se1.-Natural hybrid in Manchuria 13-177 Beans Sel .-Manchu Beans Se1.-Manchu Beans

u)

0

Group IV" Patoka Gibson Boone Wabash

0 -2

Mo. Agr. Expt. Sta. Indiana Agr. Expt. Sta. Indiana Agr. Expt. Sta. Mo. Agr. Expt. Sta. Indiana Agr. Expt. Sta. and USRSL '

Se1.-Natural hybrid in Illini Se1,P.I. 70218-2 Se1.-Midwest x Dunfield Se1,P.I. 54563-3 Se1.-Dunfield x Mansoy

Beans Beans Beans Beans Beans

Group V I P Ogden Rose Non-Pop Dortchsoy #2 Arksoy 2913 Hale Ogden #12 Ralsoy

0 +2 0 -1 +1 -2

Tenn. Agr. Expt. Sta. W. P. Rose, Goldsboro, N.C. R. L. Dortch Seed Co. Ark. Agr. Expt. Sta. Geo. Hale, Blytheville, Ark. G. H. Banks, Osceola, Ark.

Se1.-Tokyo x P.I. 54610 Se1.-Haberlandt Se1.-Ogden Se1.-Arksoy Se1.-Ogden Se1.-Arksoy 2913

Beans Beans Beans Beans Beans Beans

CI w w

c.i

*

w

TABLE VII-Continued Variety

Volstate Roanoke CNS Palmetto Tanner Pelican Acadian Avoyelles Seminole La. Green Gatan Yelnando JW45

Relative maturity

0 +I

+3 -1 -2

-2 0 +3 +1 +9 -2 0

-3

Originating agency

Origin

Group VII' Tenn. Agr. Expt. Sta. Se1.-Tokyo x P.I. 54610 N.C. Agr. Expt. Sta. Se1.-Mixed seed lot J. E. Wannamaker, St. Matthews, S.C. Se1.-Clemson U.S. Dept. Agr. 1ntro.-P.I. 71587 T. Tanner, Decatur, Ala. Se1.-Natural hybrid in Otootan La. Agr. Expt. Sta. La. Agr. Expt. Sta. La. Agr. Expt. Sta. U.S. Dept. Agr.

Group VIII ' Se1.-Natural hybrid Se1.-Natural hybrid Sel.4tootan 1ntro.-P.I. 93058

La. Agr. Expt. Sta. and USRSL ' Ga. Agr. Expt. Sta. Se1.-Natural hybrid in Otootan Coker Pedigreed Seed Co., Hartsville, S.C. Se1.-Yelredo x Nanda J . E. Wannamaker, St. Matthews, S.C. Se1.-Mixed seed lot

maturity-Days earlier (-1 or later (+) than Ottawa Mandarin. Relative maturity-Days earlier (-) or later (+) than Richland. ' United States Regional Soybean Laboratory. Relative maturity-Days earlier (-) or later ( f ) than Lincoln. ' Relative maturity-Days earlier (-) or later (+)than Gibson. ' Only released variety of newly formed Group V. Relative maturity-Days earlier (-) or later (+) than Ogden. I,Relative maturity-Days earlier (-1 or later (f) than Volstate. ' Relative maturity-Days earlier (-) or later (+) than Acadian. * Relative

Use

Beans Beans Beans Hay and beans Hay

Beans and hay Beam and hay Hay and green manure Green vegetable and beans Beans and hay Hay Hay and beans Beans

F

La ? I

?

3

6 u:

SOYBEANS

135

of several such researches. When soybeans were harvested for either hay or grain Odland and Garber (1928) found that with single rows 30 inches apart, variability of plots 16 feet long was considerably less than that of plots 8 feet long. Increasing the plot length beyond 16 feet decreased variability but not sufficiently to justify the additional land and labor required. Standard errors were decreased substantially by increasing the number of replications up to four when cut for hay and up to three when cut for seed. Higher numbers of replications did not decrease the standard error justifiably, and these authors recommended that under conditions similar to those encountered in this experiment four replications would give optimum results. From results of a uniformity trial conducted a t Nanking, China, Wang (1935) concluded that single rows 30 inches apart and 16 feet long with four replications were adequate to measure soybean yields. H e found &row plots gave little more accuracy than l-row plots. In a study of various plot sizes and shapes in a soybean uniformity trial grown with 2-foot spacing of rows, Weber (1941) found the largest decrease in coefficient of variability resulted when plot size was increased from one row 8 feet long to twice that size. Increase in plot size from twice to three times that of the smallest unit decreased variability moderately, but further increase in size was accompanied by negligible decrease in coefficient of variability. When long, narrow plots were compared with plots of the same size more square in shape, it was found that the coefficient of variation was only slightly lower in the shorter, wider plots. Varieties exhibited differential border effects when single row plots grown 30 inches apart were adjacent to &foot alleys in studies by Probst (1943). Removal of the terminal foot at each end of nursery rows a t harvest time therefore contributed to the accuracy of variety trials. Garber and Odland (1926a) showed that neither height nor yield of soybean varieties grown in rows 30 inches from adjacent varieties appreciably affected the yield of the adjacent variety. Another source of inaccuracy in variety trials is lack of uniformity in stand. Wiggans (1939) demonstrated the ability of soybean plants to make wide adjust.ments for space, and found that yields were not increased by planting more heavily than optimum. Varieties responded differentially to method of planting (28-inch and %inch rows). Although soybean varieties yielded differentially when grown a t various spacings from one to five inches, Probst (1945) suggested that limited variat,ion in stand should not change the relative rank of variety means, Two quasi-factorial types of experimental design, balanced incomplete blocks and lattice squares, were shown by Weiss and Cox (1939) to give

136

MARTIN G. WEISS

greater precision than randomized blocks when soybean variety yield trials were conducted on soils of normal variability. The accuracy of 11 5 x 5 lattice square designs relative to randomized blocks were examined for five agronomic and three seed compositional attributes in soybean variety and date of planting trials by Weiss et al. (1949). Mean relative accuracy varied among characters from 4 to 30 per cent but differences were not consistent. Analyses as lattice squares contributed to the accuracy of certain experiments in all attributes studied. I n a uniformity trial with soybeans, lattice square and triple lattice designs were found by Weber (1941) to give higher precision relative to randomized blocks than simple lattice designs. With long, narrow plots higher relative precisions were obtained than with plots more nearly square. This was attributed to the more nearly square shape of the incomplete blocks which resulted from the long, narrow plots. When estimating the yield and quality of seed in soybean fields prior to harvest, Houseman e t al. (1946) found the optimum size of subsampling units to be approximately 7 square feet and two units per field gave satisfactory prediction values. The development of equipment particularly adapted for the conducting of small plot trials with soybeans has been reported. Weiss (1941a) described a nursery planter employing an endless rubber “v” belt adaptable for planting rows up to 42 feet in length. A portable nursery thresher was described by Probst and Cartter (1941) and a seed cleaner used in conjunction with a thresher in the field was described by Weiss (1941b). IX. EFFECT ON SOILS 1. Erosion

Soybeans, in comparison with other cultivated crops, generally have been considered to leave the soil susceptible to erosion. This concept has arisen largely through observation of the degree of soil erosion on land previously cropped with soybeans in comparison with land on which other intertilled crops have been grown. Controlled experiments, in general, have not substantiated this supposition. As pointed out by many workers, general observation has led to erroneous conclusions largely because in most rotations soybeans follow a cultivated crop. In corn belt rotations soybeans follow one or t,wo years of corn, and are therefore most frequently compared with fields of corn removed from meadow by one less cultivated crop. Effect of the previous crop on the extent of soil erosion when cropped with soybeans was demonstrated by Smith (1947) who reported that, soil loss during the growing season from soybean fields following 1 year of meadow and 1 year of corn was 1.76 and 4.32 tons, respectively.

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Soil on which soybeans have been grown is generally observed to be more loose and friable as compared with soil following other crops. The increased soil aggregation conditioned by soybeans has been noted by many workers and experimental data substantiate general observations. Penetrabilit,y of soil with a steel rod between soybean rows was found greater than between corn rows (Browning e t al., 1943). According to these authors looseness of the soil following a crop of soybeans may be primarily attributable to the following three factors: (1) Canopy effect afforded by the foliage of soybean plants, (2) desiccation of soil due to root concentration, and (3) increased aggregation resulting from decomposition of nodules, roots and tops. The canopy afforded by the broadleafed soybean plant after several weeks of growth is quite effective in preventing compacting of soil due to downpours of rain. On the basis of soil penetrability in field experiments, the above authors found that soil under rowed soybeans was comparable in looseness to soil covered with burlap, and considerably looser than soil under corn. The effectiveness of the canopy afforded by soybeans was demonstrated by Smith (1947), in that the major erosion in drilled soybean plots occurred prior to July 1 before an appreciable canopy was provided. Frequent wetting and drying is conducive to increased aggregation of soil (Browning e t al., 1943). During the growing season, soils under soybeans were notably lower in moisture t o a depth of 15 inches than under corn. The authors attributed this to the less extensive root systems of soybeans with higher root concentrations in surface soils relative to corn. Furthermore, as noted by Browning (1947) , more moisture is required by soybeans than by corn per unit dry weight. Desiccation following rains is, therefore, more rapid and more complete. Soils under drilled soybeans had lower moisture content than those under soybeans planted in rows. Decomposition of nodules and roots of soybeans was found materially to increase soil aggregation in greenhouse experiments. The effect from nodule decomposition was only temporary whereas aggregation caused by root decomposition was greater in extent and duration. Looseness of the soil, as conditioned by the soybean plant, has, in general, not been proven to be associated with soil erosion. Water absorption is definitely increased, particularly in heavy soils, and run-off is thereby diminished. When artificial rain at the rate of 1.75 inches per hour was applied by Van Doren and Stauffer (1944) to dry, strawmulched, permeable prairie soil following single crops of soybeans and corn, the mean percentage run-off on land with 4 per cent slope during the first, second, third and fourth hours was 4, 5, 7, and 9 per cent, re-

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spectively, whereas comparable percentages following corn were 8, 19, 27, and 34. When the soils previously had been saturated with moist,ure, the run-off percentages during the first and second hours were 25 and 34, respectively, following soybeans, and 53 and 62, respectively, following corn. Lower run-off percentages were reflected in lower soil losses. Soil losses caused by 1.75 inches of rainfall falling in one hour on unmulched plots of soybeans and corn during the growing season were 1756 and 3095 lbs., respectively, and 2630 and 3275 lbs., respectively, in the succeeding fall and spring. However, soybean residues ret.urned to the soil were found less effective than corn residues in controlling erosion. Variability in soils as to susceptibility to erosion was noted by Browning (1947). Erosion of soils underlain with tight subsoils may be stimuulated by the loosening action of the top soil by soybeans if heavy rains occur while the topsoil is saturated. A comparison of water run-off and soil loss from corn and drilled soybeans on Putnam silt loam in Missouri on plots 90 feet long with 3 per cent slope as obtained by Smith and Whitt (1948) appears in Table VIII. TABLE VIII Average Run-off and Soil Loss During the Growing Season (April 27 to October 6) under Corn and Soybeans for the Period 1941-1946'' Crop

Corn Soybeans corn Soybeans

Previous crop Corn Corn Grass-clover hay Grass-clover hay

Soil loss (todacre)

Water run-off (inches)

7.1

6.7

6.9

7.2

2.1

5.8 5.9

2 .o

Smith and Whitt (1948).

Planting and cultural operations were up and down the slope. With either corn or grass-clover hay as previous crops, run-off and soil loss under soybeans were not appreciably different than under corn. Water run-off and soil loss from corn and soybeans listed in 40-inch rows on Marshall silt loam were determined by Browning (1948). Regardless of whether rows were planted with the slope or on the contour, appreciably more soil loss occurred on the corn plots. When planted with the slope, water run-off in the corn plots also was greater. Comparable differences in soil loss continued to exist in the contoured plots during the following year in the succeeding crop of oak. Similar soil and water losses under corn and soybeans were obtained in Illinois by Van Doren et aZ. (1948). Both corn and soybeans were

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planted in 40-inch rows. With either contouring or planting with the slope, soil losses under soybeans were substantially less than those under corn. Water losses did not differ great,ly under these conditions. When soybeans are planted with the slope, less water run-off and less soil loss occurs when drilled solid than when planted in rows. I n Missouri, Smith (1943) found drilled soybeans in comparison with soybeans grown in 42-inch rows allowed 89 per cent as much run-off and 46 per cent as much soil loss on land with 4 per cent slope, and 75 per cent as much run-off and 62 per cent as much soil loss on land with 8 per cent slope. Similar result,s were obtained by Browning (1948) in plantinge made both with the slope and contoured.’ I n the plantings with the slope, water run-off and soil loss from drilled plots was approximately onehalf that of the 40-inch rows. I n the contoured plots the differences were less but still substantial. As with other crops, soybeans planted on the contour on land with an appreciable slope result in substantially lower water run-off and soil loss than when rows are planted with the slope. Among the tillage methods used by Browning, it appears that soil conservation attributable to contouring was greatest with wide, listed rows, intermediate with wide surface-planted rows, and least with solid drilled soybeans. Comparable differences in soil losses still occurred in one year when oats followed the above tillage practices. These differences constituted a carry-over of the differential effects of contouring on methods of tillage. Soil losses when soybeans were planted on the contour in Illinois were less than a third of those from plantings with the slope even though the slope was not in excess of 2 per cent. Similarly, water losses were reduced to approximately 40 per cent by contouring. Soil and moisture conservation resulting from contour planting is also reflected in the immediate crop of soybeans. Yields of soybeans planted TABLE IX The Effect of Tillage Methods and Contour Planting on the Yields of Soybeans, Soil Conservation Experimental Farm, Clarinda, Iowa, 1943, 1944, 1946, 1947 Yield of soybeans (bu./acre) Tillage method Listed, 40” rows Surface planted, 40” rows Surface planted, 7” rows a

Browning (1948).

With slope

Contoured

17.3 17.0 16.8

17.9 183 18.1

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in Iowa on contoured plots consistently exceeded those from plots planted with the slope, as shown in Table IX. Contouring was found by Smith (1943) to result in 20 per cent reduction in water loss, 52 per cent reduction in soil loss, and, in general, higher yields over a 6-year period in Missouri. I n 30 tests Browning (1943) found yields were increased by contouring in 21 fields, and failed to differ significantly in the remaining 9 fields. Considering all tests, the mean yield of t.he contoured soybeans exceeded t.hat of the soybeans planted with the slope by 3.2 bushels per acre. 6. Soil Productivity

Soybeans have alternately been regarded as a soil “improving” and “depleting” crop. The ambiguity of such an overall classification is readily demonstrable by examination of the comparable nitrogen additions and losses when the crop is used in different ways. According to Sears (1939) 88, 26, and 16 lbs. of nitrogen are added per acre when the crop is used as green manure, hay with manure returned, or beans with straw returned, respectively. When used for beans with straw removed or hay with no manure returned, however, losses of 3 and 30 lbs. per acre, respectively, occur. Under comparable conditions, corn, oats, and wheat were calculated to remove 40, 26, and 36 lbs. of nitrogen per acre, respectively. Use of the crop is, therefore, important in determining the effect of soybeans on soil productivity. The above additions or losses of nitrogen to the soil were calculated on the basis of 20 bushels of beans or 4500 lbs. of hay per acre. Although the values may be roughly comparable under all conditions, certain exceptions should be noted. The above calculations assume good nodulation of roots, a condition which is not always encountered, as noted by Norman (1943). It was further assumed that approximately twothirds of the total nitrogen in the plant was obtained from the atmosphere. As noted under Section IV-2-a fixation percentages vary greatly with different levels of combined nitrogen in the soil and, as estimated by Norman (1946), on average prairie soils probably range between 20 and 35 per cent. The above calculations, therefore, may be applicable under conditions of relatively low soil productivity. On soils with relatively high nitrogen level, such as used for soybeans grown in the corn belt, nitrogen obtained from the atmosphere may well constitute a smaller portion of the nitrogen in the plant. Under such conditions removal of a seed crop may well result in a net loss of nitrogen to the soil even when straw is returned. As noted previously, soybeans draw heavily on other major and minor soil nutrients. Sears (1939) estimates that a 20-bushel crop of soybeans .

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removes slightly more phosphorus and magnesium, three times more potassium and seven times more calcium than a 40-bushel crop of corn. I n an effort to determine the overall effect of soybeans on soil productivity in relation to other tilled crops, the yields of various crops following soybeans and ,following other crops have been compared by various investigators. Early observations and results from these comparisons have been summarized by Dodd and Pohlman (1935). Reports of additional work are indicative of continued interest in this problem. In 15 tests in Iowa during 1942, Norum (1943) found that corn following soybeans on Webster and Clarion soils yielded 8.3 and 9.5 bushels per acre, respectively, more than corn following corn. On both soil types these increases constituted a 13 per cent higher yield of the succeeding corn crop. Corresponding increases in the succeeding year, according to Pierre (1948), were 8.7 and 3.7 bushels per acre for the Webster and Clarion soil types, respectively, constituting an overall 9.4 per cent higher yield following soybeans in this year. At Ames, Iowa, in 4-year rotations, Norman (1946) reported that the yield of corn following corn was exceeded by the yield of corn following soybeans 4 and 18 per cents in two successive years. Increases, on a percentage basis, were higher in soils of low fertility level than on soils receiving lime and manure in the rotation. On Marshall silt loam in sout.hwestern Iowa, Browning (1948) found that the yields of oats following listed soybeans were 60 and 100 per cent higher than those following listed corn on plantings with the slope and contoured, respectively. I n comparing two rotations over a 4-year period on this same soil type, Browning (1948) found that oat yields in a corn-corn-oats-meadow rotation were 43.5 bushels per acre whereas in a corn-soybeans-oats-meadow rotation, 47.2 bushels per acre were obtained. From data (Wiancko et al., 1941) reported by the Indiana Agricultural Experiment Station, crop yields from a number of comparable rotations are presented in Table X. The rotations in a comparison differ principally in the inclusion of soybeans or substitution of soybeans for and h e r crop. I n these rotations manure and crop residues were returned to the land supplemented with some commercial fertilizer. I n all comparisons cited, however, the rotations not including soybeans were supplemented with more fertilizer than the rotations including soybeans. I n spite of less fertilizer applied, in all instances, the inclusion of soybeans in the rotation was beneficial as measured by the response in yield of the other crops. Phosphate and potash fertilizers were applied, and accordingly the differences in yield of the succeeding crops were probably attributable largely to differences in level of available nitrogen and physical condition of the soil.

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TABLE X Effect of Soybeans on Crop Yields in Comparable Rotations Conducted in Indiana a-d Corn (bu./acre)

Rotation

Small Grain" (bu./acre)

Hay (1bdaci-e)

Brookston-Crosby Silt Loam, LaFayette, Indiana, 1916-1940 58.8 272 I. Corn-corn-wheat-clover 2. Corn-soybeans-wheat-clover 65.2 33.6 3. Corn-corn-soybeans-wheat-clover 64.6 342

4050 4208 4238

Newton Fine Sandy Loam, Wanatah, Indiana, 1925-1940 I. Corn-corn-oats-mixed hay 29.9 28.4 2. Corn-soybeans-oats-mixed hay 32.3 37.1

2460 2575

Plainfield Fine Sand, Culver, Indiana, 1928-1940 31.3 9.8 37.2 12.2

1. Corn-oats-wheat-clover 2. Corn-soybeans-wheat-mixed hay

1809 2172

Clermont Silt Loam, Jennings County, Indiana, 1922-1940 58.6 20 .o 1. Corn-wheat-clover 2. Corn-soybeans-wheat-clover 62.6 20.6

3113 3226

* Wiancko, Mulyey and Miles (1941). Wiancko, Walker and Miles (1941). ' Wiancko, Walker and Robbins (1941). Wiancko, Walker and Warran (19411, "Small grain crop common t o all rotations within a comparison.

Immediately after harvesting soybeans, available nitrogen in the soil is low (Dodd and Pohlman, 1935). Thereafter, however, the nitrate content increases rapidly. This change in nitrate content of the soil was reflected in yields of wheat planted after soybeans which were removed a t various dates. Removal of soybeans during August resulted in higher yields of the succeeding crop than if removed in September shortly before planting the wheat. The effect of soybeans on stands and yields of succeeding legume forages seeded with a companion crop has been a matter of considerable controversy. I n certain areas, such as Central Indiana, alfalfa and clover failures have been thought more prevalent following soybeans. I n other areas, such as southwestern Iowa, stands of seedings following soybeans have been observed to be better than following other tilled crops such as corn. The factors affecting establishment of meadows have been thoroughly discussed by Thatcher et al. (1937). As pointed out by Frank (1947), several factors may contribute to greater seeding failures following soybeans than following other cultivated crops. As discussed under

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Section IX-1, soybeans in comparison with other intertilled crops favor aggregation of the soil, which results in a loose and friable condition. With improper management greater desiccation may occur thereby eliminating young forage seedlings. A second factor which may contribute to seedling failures is greater competition from the companion crop. Based on the above citations it becomes quite evident that small grains when following soybeans yield more than when following other intertilled crops. The small grains, commonly used as companion crops, also attain greater height and vigor following soybeans, thereby resulting in greater competition to the young seedlings. The observations of poorer stands of forage crops following soybeans have not been substantiated in controlled experiments. As evident in Table X yields of hay following soybeans were higher than when following other crops. Extensive stand counts of alfalfa and red clover seedlings in companion crops of wheat and oats following soybeans and corn were reported by WiIlard and Thatcher (1947). Although seedling counts following soybeans tended to be slightly lower, only in one field, described by the authors as “the most seriously run down” field, were stands substantially lower. Yields of hay following soybeans were actually slightly higher than following corn. The authors concluded that when stand reductions follow soybean cropping, nutrient deficiencies are probably the causative factors. The effects of fertilizer on yields of clover seeded in oats and wheat following corn and soybeans were studied in central Indiana by Frank (1947). Regardless of fertilizer application or companion crop, hay yields following soybeans exceeded those following corn. The fertilizer was equally effective in stimulating hay yields on land previously cropped by soybeans or corn. No evidence is available which would indicate that failures of seedings are more numerous following soybeans than following other tilled crops when good soil management practices are observed. X. DISEASES AND INSECT PESTS 1. Diseases

Compared with other widely grown crops, soybeans in the United States have been relatively free of diseases causing serious widespread reduction in yield. This sihation, unfortunately, is probably only temporary and is attributable largely to the comparative newness of the crop in this country. The soybean plant, indigenous to the Orient, was introduced into an entirely new habitat and, through care of importing agencies, not all of the pathogens which preyed upon it in the Orient were introduced with it. Furthermore, the soybean has not been grown in its

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new habitat sufficiently long for many new pathogens to develop and become adapted to this new host. Some pathogens were undoubtedly imported with the seed brought into this country. I n this connection it is of interest. to note th a t a considerable number of the soybean diseases reported in this country carry over into the next generation of plants in or on the seeds of the soybean plant. Certain other pathogens which were prevalent on crops already grown in this country, were found to also attack soybeans. An excellent summary of soybean diseases reported in the United States prior to 1943 has been made by Johnson and Koehler (1943). This reference has been used freely in the following discussion. Only the presently more important diseases will be discussed herein. a. Stem Diseases. Several diseases of increasing importance on stems and pods are included in this section. Diaporthe phaseolorum var. batatatis (Hart. and Field) Wehmeyer, reported by Welch and Gilman (1948) as causing a stem canker, gives evidence of becoming one of the most serious of the stem pathogens. It is prevalent throughout the corn belt states. Field infection is characterized by the occurrence of scattered premat,urely-killed plants throughout the field during the period of pod formation. Most of the leaves and pods remain on the infected plants. Close examination reveals that the stem has been girdled by a lesion, usually diagonal to the axis of the stem, and th a t parts of the plant above this lesion are dead. This lesion, considered the point of infection, usually occurs a t the junction of a branch or petiole with the stem, most. frequently near the base of the plant although girdling a s high as 2 feet above the soil sometimes occurs. No fruiting bodies are found to be associated with this type of lesion. A closely related fungus, Diaporthe phaseolorum var. sojae (Lehman) Wehmeyer, causes the pod and stem blight of soybean. This fungus is only weakly parasitic, attacking senescent plants or those weakened from other causes. Pycnidia of the fungus may be found aligned in rows on the stems or uniformly scattered over the pods of diseased plants, usually late in the growing season. The occurrence of brown stem rot was increased by the repeated cropping of land in the corn belt with soybeans during World War 11. The causal organism, thought by Presley and Allington (1947) to be a Cephalosporium, was identified as Cephalosporium gregatum by Allington and Chamberlain (1948). The organism invades the pith and xylem of the stem below the ground level, moves upward with only slight external symptoms, and suddenly causes interveinal chlorosis, subsequently necrosis, and death of the top leaves of the plant. In the corn belt the external symptoms usually appear in late August. Retention of these

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dead leaves causes the field as a whole to resemble the symptoms attributable to an early autumn frost. Subsequently, the rotted, weakened stems result in excessive lodging. Bccausc serious infections of brown stein rot seem limited to soils on which soybeans have been grown repeatedly, adequate control appears possible through crop rotation. The organism is thought to bc soil borne. Brown stem rot is prevalent throughout the corn belt and, according to Hildebrand (1948), has recently been found in Ontario. High temperatures were found by Allington (1946b) to be highly unfavorable for the development of this fungus. Lack of severe damage in 1947 was attributed by Chamberlain (1948) to high mean temperatures during the month of August. No varietal resistance has been found to this parasite. Premature killing of plants is also caused by the anthracnose fungus, Glmerella glycinas (Hori) Lehm. and Wolf. The symptoms are similar to those of Diaporthe pod and stem blight. Differentiation of the two diseases is possible by examination of the small black fruiting bodies which, in the case of anthracnose, occur irregularly over the stems rather than in rows. On magnification the antliracnose fruiting bodies appear very spiny. Originally considered a disease prevalent only in southeastern United States, it has recently been reported in the corn belt. Although symptoms described in this country pertain to plants in the pod development stage, it was reported by Ling (1940) to attack soybeans in the seedling stage in China. It overwinters in infected stems, is seed borne, and in China is thought to overwintcr in soils. Bud blight is one of the three virus diseases of soybeans reported in the United States. According to Allington (1946a) the causal virus is indistinguishable from the tobacco ring-spot virus. Plants infected early usually develop only rudimentary pods and are somewhat stunted in growth. Late infection of plants results in production of distorted, shrunken pods and defective beans. At harvest time these plants are conspicuous in that. they do not mature until killed by frost. One of the characteristic symptoms of this disease is the “shepherd’s crook” assumed by the stems due to the necrosis of the apical bud. Pith discoloration, particularly at the nodes of the plant, usually accompanies the disease. Bud blight occurs quite commonly throughout the United States and southwestern Ontario. During years of heavy infection this parasite has caused severe losses. No insect vector or wild host has been found. Seed transmission of the virus is still open to question for, according to Hildebrand and Koch (1947b), seed from infected plants gave rise to only slightly more diseased plants than seed from apparently healthy plants. The pathogen, however, can be transmitted by mechanical means. As yet host resistance has not been found in soybeans.

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b. Root and Crown Diseases. Important among soybean diseases are 5 fungus pathogens which invade and attack the roots or the stem near the ground level. Subsequent girdling of the stem results in death of the plant above the infected area. The diseases and causal organisms are charcoal rot, Macrophomina phaseoli (Maubl.) Ashby, sclerotial blight., Sclerotium rolfsii Sacc., Fusarium blight, Fusarium oxysporum f. tracheiphilum (E.F.S.) Snyder and Hansen, stem rot, Sclerotinia sclerotiorum (Lib.) Massee, and Pythium rot, Pythium debaryanum Hesse. Charcoal rot occurs generally in the central Mississippi Valley, sclerotial and Fusarium blight in the sandy soils of the south, and stem and Pythium rots are believed to have general distribution. All are thought to be soil borne. Charcoal rot can be identified by numerous small, black sclerotia uniformly distributed in the subepidermal layer of the stem. The epidermis must be removed to examine them. The symptoms for sclerotial blight are similar but the sclerotia are larger, rounder, and brown in color. Infection with stem rot results in large black sclerotia developing on and within the stem. This pathogen is most severe during early summer when cool wet weather prevails. I n most, instances damage is confined to scattered small areas in which young plants have been killed. In Ontario, however, the almost complete destruction of more than three acres of soybeans was reported by Hildebrand (1948). Brown or black discoloration of the xylem tissues of the root or stem identifies infection with Fusarium, and Pythium infection can usually be identified by typical “damping-off” symptoms and root necrosis. None of the above organisms is restricted to attacks on soybeans. The causal organism of Fusarium blight has been proven to be indistinguishable from that causing cowpea wilt. The other organisms attack a wide variety of plants. Resistance to Fusarium blight has been found in field varieties of soybeans but a high type of resistance to the other pathogens has not been found. Root knot ranks as one of the most serious diseases of the south. Caused by the soil-borne root knot nematode, Heterodera marioni (Cornu) Goodey, its chief symptom is the formation of galls in the roots of the plants. The tops of the plants are thereby reduced in vigor, size and green coloration. Differences in degrees of infection have been observed among varieties of soybeans. c. Foliage Diseases. Probably the most common and conspicuous of the soybean diseases are three bacterial leaf spots, bacterial blight, bacterial pustule, and wildfire. All are generally common throughout the soybean production area, overwinter in diseased plant tissue, and are also seed borne. Bacterial blight, caused by the organism Pseudomows glycinea (Coerper) Stapp, is characterized by small, angular, yellow spots

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on che leaves, which soon bccome dark brown to bIack in color with thc initiation of necrosis, and frequently drop out, t,hen giving the leaf a ragged effect. The symptoms of bacterial pustule, caused by Xanthomoms phaseoli var. sojensis (Hedges) (Starr) Burkholder, are similar to those of bacterial blight. The brown areas are more irregular in size and are surrounded by yellow margins. This disease occurs particularly in the southern United States. Symptoms of both of the above diseases may also occur on the pods. Genetic resistance to bacterial pustule is available, particularly in the variety CNS. Chamberlain (1948) reported that resistance to bacterial blight is also available. From among 1100 varieties tested, 50 showed relatively little infection and three appeared highly resistant. Wildfire, caused by Pseudomonas tabaci (Wolf and Foster) Stevens, a8 described by AIlington (1945), is characterized by light brown, variable-sized, necrotic spots which are nearly always surrounded by a distinct, wide, yellow halo. The symptoms develop somewhat later than those of the two preceding bacterial diseases. The leaf spot diseases, brown spot and frog-eye, caused by the fungi imperfecti Septork glycines Hemmi and Cercospora daizu Miura, respectively, are prevalent in the southeastern states. Both organisms overwinter on diseased leaves and stems and are also seed borne. Brown spot is recognized by angular brown or reddish-brown lesions which occur on the soybean leaves, particularly of young plants. Severe infection results in basal defoliation. Moderate resistance is available to brown spot. Round, dark-margined spots are the characteristic symptoms of frog-eye on soybean leaves. Symptoms on stems and pods are less conspicuous and appear late in the season. Late varieties usually show heavier degrees of infection than early varieties. A seed-borne transmissable virus has been found to be the causative agent of certain types of crinkling in soybean leaves, usually known as soybean mosaic. Delayed maturity and interveinal, dark green puckers are typical of the disease. In certain instances the puckers are aligned in rows contiguous to the veins whereas in other instances they occur uniformly over the leaflet. A moderate type of resistance is available in certain field varieties while certain vegetable varieties and the wild soybean, Glycine ussuTiensi8, Regal and Maack, are particularly susceptible. The causative virus of soybean mosaic has been designated by Conover (1948) as Soja virus I . Symptoms were found to be influenced by air temperatures, being severe a t 18.5"C. and largely masked a t 29.5"C. Soja virus 1 was found to produce systemic infection only on soybeans. A conspicuous yellow mottle of soybean leaves was found by Conover to be caused by another virus, Phaseolus virus 2. This disease,

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called yellow mosaic, induces systemic mottling on numerous species. Yellow mosaic was not found to be seed transmitted and all soybean varieties bested reacted similarly. Downy mildew, caused by Peronospora manchurica (Naoum.) Syd., is sometimes found on soybean foliage during damp periods. The symptoms consist of grayish brown to dark-brown lesions surrounded by chlorotic margins, and grayish masses of conidiophores on the under surface of the lesions. This phycomycete has been shown by Jones and Torrie (1946) to be systemic. The mycelia proceed up the stem in the pith and phloem parenchyma tissues. Infected beans frequently have milky-white crustaceous masses of the oospores on their surface. Oospores also develop within the vegetative tissue, constituting a source of infection the following year. Several Manchu selections, Mandarin, Habaro, Dunfield, and Mukden exhibited resistance to the disease whereas Richland and Illini were found susceptible. Yield reductions due to infection during a season in which the disease was epidemic were not serious. d. Control. Differential varietal reaction to some of the diseases has been cited in the preceding sections. To certain diseases a high type of resistance has been found whereas to other diseases the differential reaction consists mainly of varying degrees of susceptibility. Varietal resistance is an ideal means of disease control. To a few diseases control through the use of resistant varieties is presently possible. I n other cases resistance to certain diseases is being transferred to agronomically desirable varieties t.hrough hybridization and selection. Many other diseases have not been studied adequately to permit accurate host genotype classification as to resistance. Rapid progress is being made and development of varieties resistant to many of the diseases seems highly possible. Many soybean disease organisims overwinter in diseased plant residues or in soil. This mode of overwintering permits effective control of the disease through crop rotation. Certain diseases, such as brown stem rot, seldom make their appearance except in fields cropped with soybeans in two or more consecutive years. Seed treatment offers a means of control of those pathogens which carry over on the surface of the seed. Response to seed treatment is discussed under Section VI-4-b. Successful treatment for “deep” infection of the seed by certain organisms such as the one causing downy mildew, pod and stem blight, etc., is difficult if not impossible, and planting of disease-free seed is likely to be the only effective means of control.

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2. Insects

Severe damage to snybeans by insects has not been widespread in the inaj or producing areas. However, in local areas severe yield reductions have been caused by insects during certain years. Heavy infestations of grasshoppers have completely destroyed sizeable fields in the northwestern part of the corn belt. Blister beetles also have caused substantial tlefoliation damage in this area and in southern states, and Japanese beetles have inflicted similar damage in the eastern states. The green clover worm, which is the caterpillar of a moth, Plathypena scabra Fab. is a defoliating insect widespread throughout the South Atlantic Coastal Plain and Eastern Corn Belt. Alt,hough this insect constitutes a constant menace, it falls prey to a considerable number of insect parasites, and has therefore not caused widespread damage. Complete failure of soybean crops in eastern North Carolina has been caused by caterpillars (Kulash, 1948). The principal caterpillars are the velvetbean caterpillar, Anticarsia gammatiles Hbn., which is a ravenous feeder on soybean foilage, and the corn earworm, Heliothis armigera (Hbn.), which feeds on the green pods. Other caterpillars commonly found attacking soybeans in North Carolina are the yellow striped armyworm, Prodenia ornitholgalEi Guenee, the fall armyworm Laphygma frugiperda S. and A. and species of the Autographa group of caterpillars. Excellent control was provided by application of benzene hexachloride or DDT. Symptoms of the grapevine colaspis, Colaspis brunnea Fubr. are usually noticeable in the corn belt during late June. Branches with shrunken stems and withered leaves are symptoms of the stems hollowed out by this insect. Leafhoppers of several types cause damage to varieties lacking upright pubescence. Differential damage is usually noticeable when several varieties of soybeans are exposed to a population of insects. As explained by Snelling (1941) in a review of literature on plant resistance to insects, diferential damage to varieties may be the result of clearcut resistance, or may merely constitute differential levels of susceptibility or tolerance. This is well illustrated in soybeans. Upright, dense pubescence, such as occurs on most field varieties, was found by Johnson and Hollowell (1935) to constitute a high type of resistance to leafhoppers, Empoasca fabm, Harr. Glabrous varieties, on the other hand, were severely damaged by the sucking insects, whereas variet.ies with appressed pubescence were intermediate in resistance and sustained only slight damage. Although Coon (1946) reported differential damage by Japanese beetles, Popilliu japonica, Newm., to a group of soybean varieties, the differences merely constituted varying degrees of susceptibility.

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XI. THEREGIONAL APPROACH TO SOYBEAN RESEARCH Prior to the past decade research in soybeans was conducted largely by individual agrirult,urnl experiment stations of states in which soybeans gave promise of becoming a major crop. Many of these studies were in cooperation with t,he Division of Forage Crops and Diseases, Bureau of Plant Industry, U. S. Department of Agriculture. Cultural and varietal recommendations frequently differed in adjacent states having similar soil and climatic conditions. Regionalization of agronomic research in soybeans was made possible in 1936 with the organization of the U. S. Dept. of Agr. Regional Soybean Industrial Products Laboratory (1947). Organized as a coopcrativc laboratory between the U. S. Department of Agriculture and tlic agricultural experiment stations of t.he 12 states in the North Central region, it cnabled coordination of research in these states and augmented the projects of the expcriment stations with leadership and facilities not prcviously available. Considerable impetus was given to agronomic research on soybeans by the organization of the Laboratory. Of great immediate usefulness were the regional variety tests, organized and implemented by the agronomy section of this Laboratory. Regional evaluation rapidly revealed the merits of varieties, such as Earlyana, Patoka, Gibson, Chief, Viking, and Boone, which were subsequently distributed to farmers by the states where these varieties are best adapted. The efficiency of the extensive iegional testing program was again aptly demonstrated when hybridization programs, conducted cooperatively by several state agricultural experiment stations and the Laboratory, attained the selection-testing stage. As described by Cartter (1947), reduction in the period required for testing was thereby made possible. Whereas development of varieties of normally self-pollinated crops is generally considered to require 14 to 15 years, some of the varieties emanating from the regional program, such as Hawkeye (Weiss et al., 1947b), were released to growers within 10 years from t.he time the cross was made. The material reduction in the time required for development was permitted by the combination of early generation testing, as discussed under Section VIII-2-cJ and the extensive regional testing program. The cooperative hybridization programs have resulted in the development and distribution of several superior varieties such as Lincoln, Hawkeye, Adams, Monroe, and Wabash. Expansion of the U. S. Regional Soybean Laboratory in 1942 to include cooperative research with 12 southern states resulted in equivalent advantages for this region. The regional testing program promptly revealed the superiority of Ogden, a variety already developed but not,

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extensively grown. The development of superior oil-type varieties for the south is well underway. Continued benefits from the coordinated testing programs in the entire soybean production area are to be expected. Emphasis on the composition of soybeans was permitted largely by the analytical facilities of the Laboratory. As a consequence of thorough analytical examination, commercial varieties with unsuitable composition were rapidly eliminated from the experiment station lists of recommended varieties and high oil content, suitable protein content and iodine number of oil are required attributes of new varieties. The exchange of breeding material between states has been strongly stimulated by regional cooperation. Segregrating populations and selections are furnished by the states with substantial cooperative breeding programs t o those states in which the soybean acreage has not justified extensive breeding programs. Final selection of varieties adapted to the local conditions is thereby possible. Materially improved varieties in the areas marginal to the regions of heavy production may be expected in the near future. The soybean disease investigations of the Division of Forage Crops and Diseases have been closely coordinated with the U. S. Regional Soybean Laboratory. As described by Morse and Johnson (1946) the disease studies are closely integrated with the breeding programs to facilitate the product,ion of improved, disease-resistant varieties. Although the pathological investigations are relatively new, much progress has been made in the identification of pathogens, the determination of relative damage attributable to the various diseases, the search for resistant host germ plasm, transmission of the parasites, and control measures. These cooperative pathological investigations will undoubtedly contribute greatly toward sustaining high production of soybeans in the United States. Cultural methods are again claiming the attention of various research agencies. Preliminary cultural studies were conducted by many agricultural experiment stations when soybeans were introduced as a new crop. Thereafter, researches were largely directed in other channels. Wit,h the mechanization of the soybean crop and development of varieties more resistant t o lodging, the need for additional information on cultural methods has been realized and several intensive projects have been initiated by experiment stations. Researches in weed control have also gained momentum and are being coordinated through the Regional Weed Control Conferences. Although numerous studies have been conducted on the effect of soybeans on succeeding crops, relatively few data have been reported as to the merits of various cropping systems when soybeans are included in the rotation. Long-time rotational studies involving soybeans have been

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initiated by several agricultural experiment stations and data on this subject will be forthcoming. I n conjunction with the above studies, more information is needed concerning the effective use of fertilizers in rotations including soybeans. The author wishes to express his appreciation to Dr. I. J. Johnson for reviewing this manuscript and to Dr. J. M. Crall for reviewing the section on diseases.

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The Clay Minerals in Soils J . E . GIESEKING University of Illinois. Urbana. Illinois CONTENTS

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I . Introduction . . . . . . . . . . . . . . . . . . . . . . . I1 Historical Development of Clay Mineralogy . . . . . . . . . . . 111 Crystal Structure of the Clay Minerals in Soil Clays . . . . . . . . 1. The Montmorillonitic Minerals . . . . . . . . . . . . . . 2 . The Illitic Minerals . . . . . . . . . . . . . . . . . . . 3. The Kaolinitic Minerals . . . . . . . . . . . . . . . . 4. The Interstratified Clay Minerals . . . . . . . . . . . . . Other Imperfections in Clay Mineral Crystals . . . . . . . . . IV Qualitative Identification and Quantitative Estimation of the Clay Minerals 1. Chemical Methods . . . . . . . . . . . . . . . . . . . 2 . Optical Methods . . . . . . . . . . . . . . . . . . . . 3 . Thermal Dehydration Methods . . . . . . . . . . . . . . 4 . Electron Microscopic Methods . . . . . . . . . . . . . . . 5. X-Ray Diffraction Methods . . . . . . . . . . . . . . . . V. Distribution of the Clay Minerals in Soils . . . . . . . . . . . . VI The Configuration of the Clay Mineral Crystals as Related to Their Properties VII . The Physicochemical Reactions of the Clay Minerals . . . . . . . . 1. Polar Sorption Reactions . . . . . . . . . . . . . . . . . a . The Montmorillonitic Minerals . . . . . . . . . . . . . b . The Illitic Minerals . . . . . . . . . . . . . . . . . c. The Kaolinitic Minerals . . . . . . . . . . . . . . . 2. Ionic Sorption Reactions . . . . . . . . . . . . . . . . . a . Base Exchange or Sorption of Cations . . . . . . . . . . The Fixation of Cations in Difficultly Exchangeable Forms . . b . Anion Exchange . . . . . . . . . . . . . . . . . . 3. Dispersion, Flocculation, and Gel Formation . . . . . . . . . VIII . Functions of the Clay Minerals . . . . . . . . . . . . . . . . IX Conclusions . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

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I. INTRODUCTION The clay fraction of soils is involved in many physicochemical reactions which are important in the development of soils and in their functioning as a medium for plant growth . The active minerals of clays 169

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undergo instantaneous changes upon contact with water, plant residues, soil amendments, excretory and secretory products of organisms, or with soluble substances released from weathered minerals and decaying organic materials. Other changes in these minerals occur when water and nutrient elements are removed from them. Changes in the arrangement of the clay particles may result from tilling operations, the impact of raindrops, alternate freezing and thawing, or from pressures exerted during the growtli of organisms. The many reactions and changes which clays may undergo have led Baver (1948) to designate appropriately the clays as the active fraction of the soil. Clays perform vital functions in soils that are used as a medium for plant growth. They impart cohesive properties to soils which are valuable in preventing wind and water erosion and in providing anchorage for plant roots. They promote the weathering of resistant minerals by sorbing the soluble products of weathering equilibria. They are active throughout the year as agents which hold and conserve, in available form, water and plant nutrient elements even though these sorbed substances may be required only during a short growing season. The clays, therefore, serve as liaison agents in soil-plant relationships. Clays may also impart undesirable properties to soils. They may impede the movement of air and water through the soil; under certain condit.ions they may prevent root penetration; they may fix water and plant nutrient elements in forms unavailable to plants; or they may be responsible for poor tilth, especially in poorly managed soils. Since the properties of the clay minerals can be changed by numerous reactions, however, opportunity is often provided whereby the desirable properties of the clay minerals may be enhanced and the undesirable properties suppressed.

11. HISTORICAL DEVELOPMENT OF CLAYMINERALOGY The more obvious properties of clays, such as plasticity, cohesiveness, and capacity to sorb water, have been recognized for many centuries, Approximately a century ago Thompson (1850) and Way (1850) reported that soils had a tendency to sorb bases or cations from salts in water solutions. Thompson, with the help of Huxtable, showed that the NH4+ ion was sorbed by soils. These results were confirmed by Way, who extended his investigations to show that Na+ and K + ions were sorbed by soils in a similar manner and that C a + + ions were released to associate with the anions which originally were associated with the sorbed Na+ and K+ cations. Further experiments by Way showed that burnt clays lost their sorptive capacities for cations. The realization that the clays were responsible for these significant physicochemical re-

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actions of soils led to much effort to establish the nat.ure of the components of soil clays. Since the clays are mixtures of microscopic and sub-microscopic mineral particles, the usual laboratory techniques of the early investigators were not well adapted to a discriminating study of their components. Erroneous conclusions were often drawn by comparing the composited and blended properties of these mixtures with the properties of minerals which were known to occur in relatively pure forms. For example, the blend of properties from the mixt.ure of clay minerals in many clays resembled in many ways the properties of china clays. The china clays were known to be composed mostly of kaolinite and i t was, therefore, often loosely assumed by many investigators that most soil clays were high in kaolinite. Somewhat later the zeolites with their high cation exchange capacities were discovered. After the discovery that the soil clays had cation exchange capacities, it was again erroneously assumed t,hat these clays were high in zeolites. Mattson (1930) in his early work, considered soil clays to be amorphous isoelectric precipitates of hydrated sesquioxides and silicic acid. The true nature of the clay minerals was not revealed until after Pauling’s (1930a) work on the crystal structure of the micas by x-ray diffraction methods. 1Vit.h the development of x-ray diffraction techniques for powdered materials, a method of observation became available that could be used to differentiate between groups of minerals in extremely finely divided mixtures. These methods were soon applied to soil clays by Hendricks and Fry (1930) and by Kelley et al. (1931). These investigators showed that colloidal soil clays contained crystalline substances which had previously been considered amorphous by many soil scientists even though Hadding (1923) had shown conclusively that the fine clays, which he studied by x-my diffraction methods, contained crystalline components. Pauling’s work was followed by the use of x-ray diffraction methods by Gruner (1932) in determining the crystal structure of kaolinite and by Hofmann et al. (1933) in determining the crystal structure of montmorillonite. It was soon discovered by numerous investigators, by a combination of x-ray diffraction, chemical, optical, and differential thermal dehydration methods, that montmorillonitic or closely related groups of minerals and kaolinite were commonly found in the d a y s of many soils in the temperate regions of the world. I n working with soil clays and closely related clays many workers have noted the presence of a group of mica-like clay minerals. This group of minerals has been referred to by such names as “hydrous mica” (Hendricks and Alexander, 1939; Hosking, 1940), “mica-like clay mineral” (Maegdefrau and Hofmann, 1937), “sericite-like mineral” (Grim, 1935), and “potash-

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bearing clay mineral” (Ross and Kerr, 1931). Grim e t al. (1937) have suggested the name “illite” for this group of minerals together with a proposed general crystal structure for these minerals. In this paper this group of minerals will be referred to as the illit,ic minerals, but it must be remembered that they are called hydrous micas by many clay mineralogists. Gruner (1934), Bray (1937), Hendricks and Jefferson (1938), Hendricks and Alexander (1939) , and Grim (1942) have concluded that it is possible for various clay minerals to crystallize in mixed crystals by interstratifications along the c axes of the crystals. Chlorite, which can be classified as an interstratified mineral, has been reported in soils by Jeffries and Yearick (1948). The work of Grim and Rowland (1942), Marshall and Bergman (1942), and Bradley (1945a) shows that montmorillonitic and illitic minerals interstratify. From numerous chemical, optical, x-ray, differential t.herma1 dehydration and electron microscopic data, it has been concluded that the clay minerals commonly found in clays developed in the temperate regions belong to the groups of minerals designated as montmorillonites, illites, kaolinites, and interstratified clay minerals. These are the minerals that impart to the clays their unique physicochemical properties. Varying amounts of accessory minerals and organic matter, however, are always found in soil clays. I n isolated instances, some of the accessory minerals or organic materials may be of prime importance. Clays usually contain large quantities of quartz (Si02). They may also contain cristobalite (Si02) and amorphous silica, hydrargillite A1 (OH)3, boehmite (A10 (OH)), limonite (Fe (OH)3), goethite (FeO (OH)), hematite (Fe203), magnetite (Fe0.Fe20s), rutile (Ti02), ilmenite Fe0.Ti02), zircon (ZrSi04) and leucoxene (CaTiSi05). Resistant feldspars have also been found in soil clays. The hydrated sesquioxides will be considered furt.her in Section VII-2-b.

111. CRYSTAL STRUCTURE OF THE C L A Y MINER.4LS IN SOIL CLAYS The clay mineral crystals are reactive crystalline solids. The nature of this activity and their degree of activity, therefore, must be governed by the crystal structure of t,hese minerals and the amount and kind of surface which they exhibit. It is, therefore, necessary to study the crystal structure and the surficial characteristics of the clay minerals in order to understand their unique physicochemical properties. Marshall (1936, 1937) and Wear e t al. (1948) have constructed and published photographs of scale models and Marshall has published scale drawings which clearly show the arrangement and relative sizes of the various ions in the common clay minerals found in soil clays. Grim (1942) has

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assembled many diagrams showing the arrangements of the ions and the coordination between neighboring ions for most of the known clay minerals. Davidson et al. (1943), have illust,rated the montmorillonite crystal by means of a diagram in which successive layers of the crystal are cut away. The relationship of the layer lattice group of clay mineral

Fig. 1. Arrangement of the ions in two adjoining units of a montmorillonitic clay mineral crystal. Oxygen ions are represented by white spheres, and hydroxyl ions are represented by gray spheres. Sorbed ions and molecules are represented by the spheres between the crystallographic units. Small silicon and aluminum ions reside in some of the interstitial spaces between the oxygen and hydroxyl ions (Courtesy of C. E. Marshall).

t o the other silicate minerals has been shown by Jackson et al. (1948). From a very casual observation of a scale model of a clay mineral crystal (Figs. 1, 2, 3 ) , it appears that the crystal is composed of layer upon layer of a mixture of 0-- and OH- ions, both of which have the same size. It will also be noted that there are two kinds of layers. I n the one layer there are only 0-- ions and the center of each 0-- ion is a t the corner of two adjoining hexagons, each hexagon being formed by six 0-- ions. This leaves a space in the center of each hexagon equivalent to the size of an 0-- ion. This layer will be referred to later as the perforated oxygen layer. The other type of layer is composed of either

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a mixture of 0-- and OH- ions or in some cases it is composed entirely of OH- ions. These latter layers will be referred to as the solid oxygenhydroxyl layers. A closer examination of a scale model of clay mineral crystals will show that the sinall S i + + + + ions and the slightly larger A l + + + , F e + + + , or M g + + ions are found in the interstitial spaces left between t,he regularly spaced large 0-- and OH- ions. Consequently, these small positive ions do not affect the overall dimensions of the clay mineral crystals. It will also be noted that these small positive ions are arranged in definite three-dimensional patterns. The clay mineral crystals, therefore, are held together by nttractions beween neighboring positive and

Fig. 2. Arrangement of ions in an illitic clay mineral cryatal. Oxygen ions are rcpresented by white spheres, hydroxyl ions by gray spheres and potassium ions by black spheres. Silicon and aluminum ions reside in interstitial spaces between oxygen and hydroxyl ions (Courtesy of C. E. Marshall).

negative charges on the ions and by multiple charged ions sharing their influence with two adjoining layers. This interlacing of forces between neighboring ions and layers of ions is interrupted at the edges and sides of all crystals and this unbalanced condition becomes the basis for some of the physicochemical reactions of t,heir surfaces. As will be discussed in more detail in Sections VI and VII-1-a, these unbalanced forces a t the edges of crystals are magnified to enormous proportions in the clay mineral crystals due to the shape and finely divided nature of these minerals. 1 . The Montmorillonitic Minerals

The 0-- ions of the inontmorillonites are stacked as follows: One perforated layer, two solid layers, another perforated layer followed by a,

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variable spacing in which is sorbed exchangeable ions, water molecules, or other polar molecules. This whole sequence is then repeated over and over until the thickness of the montmorillonite crystal is attained. The small S i + + + + ions fit into the interstit.ia1 spaces between the outside perforated layers of 0-- ions and the adjoining internal solid mixed layer of 0-- and OH- ions. A l + + + or some other substituting positive ion fills two-thirds of the interstitial spaces between the two solid 0-layers. The perforated oxygen layer contains 0-- ions arranged in groups of threes. The centers of each of these 3 ions form three corners of a regular tetrahedron. The Si+ + + + ions reside in the centers of these tetrahedra, which are completed through sharing with an 0-- ion in the adjoining solid oxygen layer. Two 0-- ions and 1 OH- ion, of one of the solid oxygen-hydroxyl layers, form a reguIar octahedron with 2 0-- ions and 1 OH- ion in the adjoining solid oxygen-hydroxpl layer. The 4 0-- ions in these octahedra share corners with their respective adjoining tetrahedra as pointed

Fig. 3. Arrangement of the ions in a ksolinitic clay mineral crystal. Oxygen ions are represented by white spheres and hydroxyl ions are represented by gray spheres. Silicon ions are represented by the smallest lower layer of ions in the interstitial spaces left between the tetrahedral oxygen and hydroxyl ions. Aluminum ions are represented by the upper layer of small spheres in the interstitial octahedral positions. The aIuminum ions are somewhat larger than the silicon ions (Courtesy of C. E. Marshall).

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out in the preceding discussion on the oxygen tetrahedra. The interstitial space a t the center of the oxygen-hydroxyl octahedra is somewhat greater t,han the space at the center of the oxygen tetrahedra. Al+++ predominates in the interstitial space at the centers of the octahedra, but there is also sufficient space in this position for F e + + + , Mg++, or Li+ ions. If these latter ions are present when the montmorillonite crystal is formed, some of them may occupy this octahedral position. Starting with one flat side of the montmorillonite crystal, therefore, there is a silica tet.rahedra1 layer upon which is stacked, and with which is interlaced, a sesquioxide octahedral layer. The top side of the octahedral layer is

9.6

- 21.4 i*

n H20

MONTMORlLLONlTE tOHLAl. S 1 * 0 ~ ~ h. n0

Fig. 4. The crystal structure of montmorillonite as presented by Hofmann e t al. (1933) (Courtesy of R. E. Grim).

interlaced with another silica tetrahedral layer. Upon this part of the crystal lattice there is a variable space which will accommodate exchangeable ions, water, and other polar molecules. All these foregoing layers taken as a unit are repeated over and over again until the flat sides of the crystal are reached. The smallest unit of the ideal montmorillonite crystal, or the unit crystal cell, according to Hofmann et al. (1933), must contain the following layers with the following numbers of ions (see Figs. 1 and 4 ) : (1) Variable spacing containing sorbed ions and molccules. (2) 6 0--.

(3) 4 Si++++ (interstitial).

(4) 4 0-- and 2 OH-. ( 5 ) 4 A1+++(insterstitial), (6) 4 0-- and 2 OH-. (7) 4 Si++++ (interstitial). ( 8 ) 6 0--.

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Edelman and Favejee (1940) suggest that the following structure would more nearly account for the properties of the montmorillonitic minerals (see Fig. 5 ) :

aOH)

2

51

60 21

4iOHI.20 P AI

4 (OH). 2 0

2 56

60 2 5, 2(0Hl

b-AXIS

-

YONTMORlLLONlfE cOHl,a

AI.

S1.0,a'n Ha0

Fig. 5 . The crystal structure of montmorillonite as presented by Edelman and Favejee (1940) (Courtesy of R. E. Grim). Variable spacing containing sorbed ions and molecuIes. 2 OH- (protruding). 2 sit+++(interstitial).

6 0--. 2

si"++(interstitial).

(6) 4 OH- and 2 0--. (7) 4 Al+++(interstitial). (8) 4 OH- and 2 0--. (9) 2 Si++++(interstitial). (10) 6 0--. (11) 2 Si++++ (interstitial). (12) 2 OH- (protruding).

The montmorillonites are built, up by three dimensional extensions of these units. The montmorillonitic minerals found in nature seldom, if ever, contain the exact ions mentioned above, but instead there is replacement of the positive ions in the tetrahedral and octahedral layers, according to Marshall (1935b) by other ions having approximately the same size and coordination. I n the tetrahedral layer Al+ + + can substitute for Si++ + +. This leaves a deficiency of one positive charge which must be supplied by a charge from an exchangeable cation on the clay mineral crystal or, as will be pointed out in Section 111-2, by addition of a potassium ion to form a micaceous mineral. In the octahedral layer F e + + + , Mg++,

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or Li+ can substitute for A l + + + . When M g + + and Li+ substitute for A l + + + , there is also a deficiency of positive charges which must be satisfied by exchangeable cations or by extension of the lattice into another mineral, such as the micas. These replacements have been called isomorphous replacements by Marsha11 (1935b) due to the fact that they occur without any overall changes in the dimension of the crystal lattice units. Isomorphous replacements provide an important mechanism by which clays acquirk cation exchange and polar sorptive capacities. 2. The Illitic Minerals

The crystal structure of the illites was proposed by Grim et al. (1937'). These minerals like the montmorillonitic minerals are composed

r

10.0

i

b-AXIS ILLITE (OHLKy (AL.Fs4.M&.MgeI

( S I . ~ A I ~ ) O ~ ~

Fig. 6. The crystal structure of illite as presented by Grim et al. (1937) (Courtesy of R. E. Grim).

of a silica tetrahedral layer, an aluminum octahedral layer, and another silica tetrahedral layer, but in place of the variable spacing of montmorillonite, there is a K + ion. This combination of layers comprises the smallest crystallographic unit along the c axis and the illitic minerals are formed by repeating these units over and over again until the flat edges of the crystals are reached. The K + ions balance deficiencies in positive charges which, according to these workers, arise from the isomorphous substitution of A l + + + for S i + + + + in the silica tetrahedral layers. Substitution of M g + + for A l + + + in the octahedral layer is

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responsible for the cation exchange capacities of these minerals. The ratios of the ions in the various layers of the illites as given by Grim e t al. (1937) are as follows (see Figs. 2 and 6) : 6 0--. 4-y Sit+" and y Al+tt (insterstitial). 4 0-- and 2 OH-. 4 Al"' (interstitial) with possible substitutions by Fe+++or Mg++. 4 0-- and 2 OH-. (6) 4-y Si"" and y Al+++(interstitial). (7) 6 0-(1) (2) (3) (4) (5)

(8) Y K'.

The illite crystals are built up by three dimensional extensions of these units. 3. The Kaolinitic Minerals The unit crystal of the kaolinites is composed of one silica tetrahedral layer and one alumina octahedral layer. The octahedral layer in

b-AXIS

-

KAOLlNlTE (OH). Al.

L,

O,o

Fig. 7. The crystal structure of kaolinite as presented by Gruner (1932) (Courtesy of R. E. Grim).

kaolinite contains 4 more OH- ions and 4 less 0-- ions per unit crystal than the montmorillonites and the illites. The ratios of the ions in the various layers are as follows (see Figs. 3 and 7) : (1) 6 0--. (2) 4 Sit+" (interstitial). (3) 4 0-- and 2 OH-.

(4) 4 Al"' (interstitial). ( 5 ) 6 OH-.

The kaolinite crystals are three-dimensional extensions of t.hese units. From the numerous data available on the clay minerals, it appears that isomorphous replacements in the kaolinite minerals are uncommon.

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4. The Interstratified Clay Minerals Pauling (1930a, 1930b) showed that t.he layer lattice tninerals containing silica tetrahedral layers and sesquioxide octahedral layers have unit crystal faces of approximately the same size. He suggested that this should permit different clay minerals t o form mixed interst.ratified crystals. He also showed that the chlorites had a mica structure which had the potassium layer removed and the structure expanded sufficiently a t this point to accommodate a positively charged hydrated magnesia or brucite octahedral layer in place of the K+ ions. Chlorites have been found by Jeffries and Yearick (1948) in a number of Pennsylvania soils. Chlorite-mica interstratified minerals are known and according to Pauling these are intermediates in which only part of the K + ions of the micas have been substituted by posit,ively charged brucite layers. Hendricks (1939) showed that faratsihite is an interstratification of layers of kaolinite and nontronite ( a montmorillonite in which Fe+ + + substitutes for Al+++ in the octahedral layer). Bradley (1945a) has shown conclusively that montmorillonitic-illitic interstratifications are present in bravaisite. Since the clay minerals formed in soils crystallize in extremely heterogenous systems and under conditions which are constantly changing, i t appears reasonable to assume that these clay minerals should show numerous combinations of interstratifications of different minerals. This concept and the work of Pauling and Hendricks have resulted in feasible yet inconclusive suggestions by Bray (1937), Hendricks and Alexander (1939), and Grim (1942) that soil clays commonly contain interstratified clay minerals. As far as the aut.hor is aware, the only interstratified clay mineral conclusively shown to be present in soil clays is chlorite. Other Imperfections in Clay Mineral Crystals. Certain imperfections in the clay mineral crystals contribute to the unique properties of these minerals. The substitution of an ion with one valence for an ion with another valence and the influence of these substitutions on the properties of the clay mineral crystals has been ment.ioned in Section 111-1. Substitutions involving whole layers have been mentioned in the discussion on interstratified clay minerals, Section 111-4. From electron micrographs and from sorption-desorption studies, it appears that tthere are other physical imperfections in the clay mineral crystals which probably involve relatively large spaces in the crystal lattice which have been left vacant. Wiegner (1935) has shown that ions sorbed in vacant spaces in the crystal 1att.ices of kaolinitic and bentonitic clays c m actually blockade incoming ions. These vacant spaces or voids

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in clay mineral crystals would be expected to offer a variety of possibilities with respect to shape. They could be channels, wide angled coneshaped pores, narrow angled cone-shaped pores, bottle-necked pores, crevices, cracks, or they might be ragged edges on crystals. Since pores offer possibilities of directing attractive forces from many directions toward sorbed molecules or ions, it is evident that these internal surfaces can be responsible for the sorption and fixation of ions and molecules which may not be easily removed once bhey become sorbed.

IV. QUALITATIVEIDENTIFICATION AND QUANTITATIVE ESTIMATION OF THE CLAYMINERALS Clays are most commonly composed of mixtures of one or more of the clay minerals with free oxides of silicon and free sesquioxides. Much of the material in these mixtures is composed of crystalline particles too small to be observed by the best optical microscope. Such mixtures are difficult to analyze. Any of the known qualitative procedures for the identification of minerals has very definite limitations when applied to mixtures of finely divided crystals of the types found in clays. When attempts are made to devise quantitative met.hods for the estimation of the individual minerals in the clays, still other limitations arise. It is often necessary to use data obtained from several procedures to identify and quantitatively estimate the minerals in these mixtures. When the limitations of the various methods do not overlap, i t is possible to obtain fairly conclusive evidence concerning the kind and amount of certain of the minerals in the clays. The methods used in the identification and estimation of the clay minerals are: (1) chemical, (2) optical, (3) thermal dehydration, (4) electron microscopic, and ( 5 ) x-ray diffraction. 1. Chemical Methods Total chemical analyses have long been used in the study of the clays and the clay minerals. They have been very useful in studies on deposits of clay minerals which contain chiefly one component, but they are limited in their usefulness when applied to the mixtures of minerals found in the soil clays. The total amount of potassium in clays has been widely accepted to be an index of the amount of mica-like minerals in soil clays. While the illites vary considerably in their potassium content, they have been generally assumed to contain 6 per cent potassium. Pearson and Ensminger (1948) and Buehrer e t al. (1948) calculated the percentage of the illites in clay samples by assuming that these minerals contain 6 per cent potassium. This method of determining the amounts of the illites

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in clays is based on the assumptions that the illites always contain 6 per cent of potassium and that no other potash-bearing minerals are to be found in the clays. These asfiiimptions are not strictly valid but this method appears to be the best met.hod available for the estimation of the illites. Numerous attempts have been made to estimate the amounts of free silica in clays by treating them with weak NazCOs solutions. These methods are not widely accepted as specific for free silica. Nascent hydrogen, from a mixture of oxalic acid, potassium oxalate, and magnesium ribbon, has been used by Jeffries (1946) to remove free iron oxides from soil fractions in preparing these fractions for mineralogical examination. The amount of iron removed from clays has been considered to represent the amount of free iron oxides in the clays. This method, however, is open to question for use on soil clays. Iron-bearing clay minerals lose their crystalline structure, as revealed by x-ray diffraction methods, when they are treated with oxalic acid. Even though other soluble materials may not be removed from t.he clay minerals by oxalic acid solutions, iron may be removed from these minerals by this treatment. Cation or base exchange capacity measurements often provide valuable supplemental evidence concerning the nature of the clay minerals in clays. It is pointed out in Section VII-2-a, that the montmorillonitic clay minerals have high base exchange capacities, the illitic minerals have intermediate capacities, and the kaolinitic minerals have low capacities. When the base exchange capacity bf an organic free clay is high, a high percentage of montmorillonitic minerals is expected in the sample. Low base exchange capacities do not necessarily indicate, however, the presence of kaolinitic minerals. The illitic minerals, when combined with high percentages of the miscellaneous oxides and hydrated oxides, would also give a low cation exchange capacity. 2. Optical Methods

Optical methods are among the oldest methods applied to the study of the clay minerals. These methods were especially valuable in the early investigations on the more or less pure kaolinitic minerals. The kaolinitic minerals often occur in relatively large crystals to which the early optical methods could be adapted. Adaptations of optical techniques were made by Marshall (1935s) and by Bray et al. (1935) which permitted the use of these methods on fine fractions of the clay minerals, The optical methods are not especially well adapted to the study of the clay minerals in soil clays since they give average refractive indices for mixtures of minerals. These methods are often useful, however, in sup-

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plementing and confirming the results obtained by other methods. The various optical properties of the clay minerals have been tabulated by Grim (1939). 3. Thermal Dehydration Methods

Thermal methods are the oldest and most widely used methods for the identification of the clay minerals. Kelley et al. (1936) found that the clay minerals and related minerals lose water at critical temperatures which are characteristic of the type of mineral. Orcel (1926), Orcel and Caillere (1933), and Agafonoff (1935) used the differential thermal method of LeChatelier (1887) in devising a simple method of determining the nature of the loss of water from clays at various temperatures and the nature of the reactions involved in the loss of the water from the crystal structure. In this procedure an unknown sample and an inert standard anhydrous aluminum oxide sample are placed in separate wells in a furnace which is heated at a constant rate to 1000°C. The difference in temperature between the unknown sample and the sample of inert aluminum oxide is continuously measured by a recording device attached to two opposing thermocouples, one of which is inserted in the unknown and the other in the inert standard. Loss of water or any other crystallographic change in the unknown will either lower the temperature of the unknown or raise its temperature with respect to the inert standard. In case the temperature of the unknown is raised, an exothermic reaction has occurred, but if the temperature is lower, an endothermic reaction has occurred. The temperature of the inert sample is also continuously recorded so that the temperature ranges can be determined over which t.he endothermic and exothermic reactions occur. Detailed descriptions of the apparatus and procedures for differential thermal analyses of clays and typical differential dehydration curves are given by Norton (1939) and Grim and Rowland (1942). Allaway (1948) has used the differential thermal decomposition and oxidation of piperidine sorbed on clay minerals to characterize these minerals. He has obtained characteristic differential thermal oxidation curves for the piperidine combinations with the various clay minerals in soil clays. Allaway (1948) and Bradley and Grim (1948) have found that amines sorbed on clays are oxidized in a step-wise fashion. The hydrogen of the amine is oxidized first leaving carbon, which requires a higher temperature for oxidation. This method appears to offer promise in supplying additional confirmatory evidence concerning t.he nature of the clay minerals occurring in soil clay mixtures. Buehrer et al. (1948) have devised a method whereby they can obtain differential thermal curves of clay samples and a t the same time weigh

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the sample to determine the amount of water lost a t various stages of dehydration. These weighings can be made at any predetermined temperature ranges. Buehrer and his associates consider the crystal lattice water (water lost above 300°C.) of t.he montmorillonitic and illitic clay minerals to be 5 per cent and that of the kaolinitic minerals to be 14 per cent. This is a refinement in differential thermal techniques that can be very valuable in working with certain clay mineral mixtures. The differential thermal method is usually suitable for qualitative and rough quantitative estimations of the components of soil clays, if the clays contain little or no organic matker and if the various mineral component,s of the clay do not give coincidental endothermic and/or exothermic reactions. The thermal methods of clay mineralogical analysis are very valuable supplements to the other methods of analysis.

4. Electron Microscopic Methods The electron microscopic methods have been extremely useful in showing the sizes and shapes of the crystals of the clay minerals found in relatively pure deposits. The electron micrographs of the soil clays have not shown the regularity of crystalline form that has been exhibited by specimens of the clay minerals from geological deposits. This finding alone is probably the most valuable contribution made by the electron microscope in elucidating the structure of t.he soil clays. This confirms the suggestion that the soil clays are heterogeneous mixtures of minerals formed in a more or less fortuitous manner. 5. X - R a y Diffraction Methods

X-ray diffraction methods have been responsible for the decided progress in t*he study of clay minerals since 1930. Pauling (1930a, 1930b) published the structure of the layer lattices of the micas and the chlorites which led to further work and publication of the crystal structure of the other clay minerals, as related in Section 11. The progress through the use of x-ray diffraction methods has not come as a result of the lack of limitations in these methods but because of the fact bhat good diffraction patterns from these methods can usually hc interpreted in a conclusive manner. Furthermore, x-ray diffraction techniques, while they are usually most effective on single crystals, are, nevertheless, more effective than other physical methods of studying finely divided crystalline particles. Preferentially oriented aggregates of the flat sheet-like crystals of the clay mineral crystal have been used by Clark et al. (1937) as a substitute for single crystals. In these preferentially oriented aggregates the individual clay mineral crystals may be rather perfectly oriented

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along their c axes or in a parallel, flat-face to flat-face manner. Orientat>ionof the crystals along the other axes, however, will be random. The arrangement of clay mineral crystals in these aggregates can be compared to a deck of cards thrown in a heap on a table. The cards will preferentially orient along an axis perpendicular to their faces, but the corners will not match and they will be randomly oriented except for the preferred orientation along a single axis. Fortunately, the clay minerals differ only in the nature and order of stacking of t,he various units within each crystal along the c axis (Section 111-1,2,3,4). Preferential orientation along the c axes of the clay mineral crystals in aggregates, therefore, provides a valuable aid in the identification and quantitative estimation of these minerals by x-ray and optical methods. The ions in crystalline substances are arranged in definite threedimensional patterns. Upon inspection of a model of a crystal, i t will be observed that the ions in the crystal can be placed in numerous planes of ions. It will be observed that these planes may be thickly or sparsely populated with ions and that they cut across the crystal in many directions. It will be further observed that every plane of ions of a given composition will be repeated over and over again forming a series of equally distant parallel planes. Any parallel series of planes of ions always bears the same angular orientation with any other series of parallel planes of ions irrespective of the part of the crystal selected, and these same constant relationships hold for all crystals of any substance as long as it exists in only one crystalline form. Two different substances which have exactly the same interplanar relationships described above are said to be isomorphous. Isomorphous substances usually arise from the substitution of one ion by another ion having the same coordination number, approximately the same size, and carrying a charge of t.he same sign but not necessarily an equal charge. The two end members of such an isomorphous series are represented by crystals in which either one or the other of the two ions completely fills the spaces allotted to these ions. Int*ermediate members of the isomorphous series may have any degree of replacement between the two end members of the isomorphous series. Examples of isomorphous replacement were given in Section 111-1, 2. The interplanar distances and planar orientations are constant and specific for any given crystal and its isomorphous relatives, in case it belongs to an isomorphous series. Measurements of these constant distances and planar orientations of the ions, therefore, form an excellent basis for the identification of crystalline substances. The distances between the more prominent planes in a crystal can be measired by the diff rnction of monochromatic x-rays in s way similar

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to the measurement of distances between closeIy spaced fine parallel lines on a transparent or reflecting plate by diffracted monochromatic light. Since constant interplanar distances are characteristic for a given crystalline substance or its isomorphous series, if such arc formed, x-ray diffraction methods are useful for the qualitative estimation of crystalline substances. Rough quantitative estimations of the components of mixtures of crystalline substances have been made by measuring the intensities of diffracted x-radiation from the various components and by comparing these results with the results from suibable standards. When fine crystalline powders are irradiated with a sharply collimated monochromatic beam of x-rays, the reinforced diffracted rays come out as sides of hollow cones with the various sized cones having a common vortex a t the point where the undiffracted collimated beam hits the sample. The angles, a t which the sides of the various cones of reinforced diffracted x-rays come out from the sample, depend on the wave length of the x-rays and the interplanar distances of highly reflective denser planes of ions in the crystal. The edges of t,he bases of the cones of the diffracted x-rays will form concentric rings. The position of these concentric rings can be recorded on a photographic film or by means of a Geiger-Muller counter. The undiffracted x-ray beam will strike the common center of the concentric bases of the various diffraction cones. By knowing the distance of the recording device from the sample and the distance of the sides of cone-bases of the various diffraction cones from the undiffracted x-ray beam, the various angles between the common center of the cones and the sides of the cones can be calculated. This angle is twice the diffraction angle 8. The edges of the base of each diffraction cone form the lines on a common powder diffraction x-ray pattern, when a strip of photographic film is used across the bases of the cones to record the diffraction effects. Each line is characteristic of a single value for the distance between the members of a series of parallel ionic planes in a crystal lattice. The value for this distance d can be nh calculated from tlie Bragg cquation: d = 2 sin 8 I n this expression .n represents the order of reflection. It is always a small whole number, and it can be determined after all calculations are made by using 1 for its value. Lambda is the characteristic wave length of the most intense x-rays generated by the x-ray tube. This wave length is characteristic of the target in the x-ray tube. The characteristic wavc length from a copper target is 1.54A and from iron i t is 1.93A; these being the most common wave lengths used in x-ray diffraction studies of clay minerals. The d values for montmorillonite, illite, and kaolinite

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are given by the American Society of Testing Materials, Philadeplphia, Pennsylvania, August, 1945. Under carefully controlled conditions with proper standardization the intensity of x-ray diffracted radiation can be used to estimate the amount of the various clay minerals in clay samples. Detailed procedures for such estimations are given by Favejee (1939a, 1939b), Hellman and Jackson (1944), Aldricli et al. (1944), White and Jackson (1946), and MacEwan (1944, 1946). The x-ray spectrometer recently developed by the North American Philips Company, New York, has attracted much attention among clay mineralogists. This instrument permits the measuring of diffraction angles and intensity of diffracted x-radiation by means of a Geigerf i l l e r tube. The intensity of the diffracted lines can be determined by manual counting or intensities can be recorded automatically on a Brown recorder. Further details on this procedure are given by Jeffries and Anthony (1948). Like all other methods for the identification and estimation of the clay minerals in soil clays, the x-ray diffraction methods have certain limitations. The characteristic diffraction lines for the clay minerals arise from the basal spacings along the c axes of the minerals. Diffraction patterns of the clay minerals in samples of soil clays often show diffuse, weak, or no basal diffraction lines which arise from spacings along the c axes of the crystals. These spacings for the montmorillonitic, illitic, and kaolinitic minerals are 13&, 10, and 7.2A., respectively. On the other hand, the diffraction lines arising’ from the prismatic spacings in these clay mineral crystals coincide. These latter lines from soil clay mixtures are usually well defined. This can result from a mixture of small percentages of the clay minerals with amorphous or poorly organized sesquioxides and oxides of silicon ; from poor organization or random interstratification of minerals along the c axes of the crystals; from crystals which are split into sheets which are too thin to give well defined basal x-ray interferences; or from any combination of these conditions. It must, therefore, be concluded that all methods of identification and estimation of the complex mixture of clay minerals in the soil clays invite the confirmation of results by other methods.

V. DISTRIBUTION OF THE CLAYMINERALS IN SOILS Methods for the identification and quantitative estimation of the clay minerals are somewhat inadequate for the complex mixtures which often occur in the clay fractions of soils. Consequently, there is insufficient data from which broad accurate generalizations can be drawn concerning the occurrence of the clay minerals. There is also considerable

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controversy over the conditions which are favorable for the formation of the various clay minerals. The work of Schachtschabel (1938), Alexander et al. (1939) Kelley et al. (1939), Kelley, Dore (1939), Sedletzky (1939a, b, c, 1940), Sedlet.zky and Yussupova (1940), Russell and Haddock (1940), Hosking (1940), Nagelschmidt et al. (1940), Kelley et al. (1941), Sideri and Liamina (1942), Whiteside and Marshall (1944), Coleman and Jackson (1945), Peterson (1946b), Jeffries and Anthony (1948), Jeffries and Yearick (1948), Pearson and Ensminger (1948), Buehrer et al. (1948) gives certain indications concerning the relationships between some of the soil-forming factors and the formation of the clay minerals. Clay minerals with high exchange capacities are widely distributed throughout the humid temperate regions of the world, This would indicate that these minerals are montmorillonitic minerals except for the fact that they often fail to show the characteristic basal x-ray diffraction spacings of montmorillonitic minerals. Montmorillonitic minerals interstratified with other minerals or other imperfections in crystallographic organization could account for these resu1t.s. I n soils where montmorillonitic minerals have been definitely shown to be a n important component, it appears that slightly weathered parent material having relatively high p H values and, in some cases, large amounts of organic matter have been favorable for the formation of this clay mineral. The author has obtained numerous unpublished data on the loessial soils of the Mississippi valley which generally show the presence of considerable montmorillonite. Erickson and’ Gieseking (unpublished) have recently obtained data which show that the dark-colored grassland loessial soil types contain more montmorillonite than the corresponding c l o d y associated timbered types. The illitic clay minerals are very widely distributed in soils. Jeffries and Anthony (1948) give 20 per cent as the average mica content of the sediment of the eart.h. It appears that much of the illitic fraction of clays i e derived from the micas of parent materials, but there is no conclusive evidence to show that these minerals cannot be formed as a result of soil developmental processes. There is general agreement that the kaolinitic minerals occur in highly weathered, leached, well-drained, and acidic soils. While these may be the ideal conditions for the formation of kaolinitic minerals, it is probably not necessary for all of these conditions to be fulfilled in order to have the kaolinitic minerals formed. Chlorite has been found in a number of Pennsylvania soils by Jeffries and Yearick (1948). Pearson and Ensminger’s (1948) description of an unidentified clay mineral in a number of Alabama soil clays fits that

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of chlorite. It seems probable that chloritic minerals have been overlooked in many clays. No11 (1932,1935,1936) has been able to synthesize a number of d a y minerals and related minerals in the laboratory. Among these have been montmorillonite, sericite mica, and kaolinite. These syntheses were carried out by heating water suspensions of A1203 and SiOz under pressure and under various conditions. He found that montmorillonite was formed when slightly alkaline suspensions contained small amounb of sodium, potassium, magnesium, and/or calcium ions. Sericite mica was formed when the suspensions were strongly alkaline and when they contained sufficient potassium ions to form the mica. Kaolinite was formed when the suspensions were acid. These experiments are of interest because they parallel some of the natural soil conditions under which these minerals appear to be formed. There is evidence to show that the illitic and montmorillonitic minerals are not resistant to intensive weathering. According to Grim (1942)montmorillonitic and kaolinitic minerals may form from the illitic minerals and according to Kelley, Woodford, et al. (1939), kaolinitic minerals can be formed from montmorillonitic minerals. The author has some unpublished studies on the stability of the montmorillonitic minerals in some very young calcareous loessial clays developed under grass vegetation. These studies were made by comparing the x-ray diffraction patterns of the clays after electrodialyzing for several months with the x-ray diffraction patterns of these clays before electrodialysis. Before electrodialysis these clays gave intense characteristic basal spacings for the montmorillonitic group of minerals, but after electrodialysis no sharp intense characteristic basal spacings were obtained. The electrodialysed clays gave x-ray diffraction patterns similar to those obtained from similar but older more highly weathered clays. They still retained relatively high cation exchange capacities after electrodialysis. From these experiments it seems that the more perfect montmorillonitic crystals are rather unstable, and it may be postulated that they partially decompose to form a type of pseudo-montmorillonitic crystal under the acidic destructive forces of electrodialysis. These rapid destructive forces of laboratory methods should not necessarily be assumed to duplicate the extremely slow orderly reactions occurring in nature. They may be indications, however, of the trends of weathering reactions when somewhat similar conditions are met in nature. The distribution of the various clay minerals in soils has not been adequately studied. The meager available data, therefore, do not permit definite conclusions concerning the factors which influence the formation of the various clay minerals.

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VI. THECONFIGURATION OF THE CLAYMINERAL CRYSTALS AS RELATED TO THEIR PROPERTIES It has long been known from optical and x-ray investigations of the clay mineral crystals that these crystals are plate-like and that they readily assume a position of preferred orientation in which the flat-face of one platelet tends to rest upon the flat-face of an adjoining platelet. Recent developments in the field of electron microscopy have enabled soils investigators to obtain more definite information on the configuration of the clay mineral crystals. Humbert and Shaw (1941),Humbert (1942),Marshall et al. (1942),Shaw (1942),and Jackson et al. (1946) have shown that various clay mineral crystals may exist in extremely thin flimsy, film-like crystals. Ardenne et al. (1940) and Shaw (1942) have reported clay mineral crystals which have a thickness spproximating the unit cell height (1 millimicron). This configuration of the clay mineral crystals gives them very high specific surfaces (surface per unit weight) which in turn has a profound effect on the physicochemical properties of these minerals. The amount of surface in a few grams of finely divided clay can best be appreciated by considering the increase in surface if a one-centimeter cube of massive mica were split into sheets 10 millimicrons in thickness. This operation would increase the surface of the mica cube from 6 sq. cm. to 2,000,004sq. cm. or an area of approximately 1/20 of an acre. If these thin sheets of mica should be cut into pieces a few hundred millimicrons in length and width, we would have essentially a size fraction of a mineral that we would expect to find in a fine illitic clay, Since the original 1 centimeter cube of mica has essentially the same crystal structure as the illitic clay minerals, the large oxygen and hydroxyl ions will occupy the bulk of the volume of the cube with the other ions filling interstitial spaces in the same way as in the clay minerals. The original mica cube will have 0.00015 per cent of its oxygen and hydroxyl ions in iix surface layers. After the cube is reduced to sheets 10 millimicrons in thickness, 5 per cent of the oxygen and hydroxyl ions of the original cubic centimeter of mica will be in the surface area of the crystals. If the mica crystals were divided further into sheets 1 millimicron in thickness, 60 per cent of the oxygen-hydroxyl framework would be in the surface of the crystals. This same trend would follow for the kaolinitic clay minerals. The montmorillonite clays, however, always have 50 per cent of the oxygen-hydroxyl framework in either external surfaces or in the internal surfaces of the expansible layers irrespective of the size of the crystal aggregates. The crystals in a few grams of finely divided clay minerals have com-

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hined surface areas which can be most conveniently expressed in terms Thiv explains why there are no other inorganic substances which have water-holding capacities that approach the water-holding capacities of the clays. It explains why 6 t o 8 feet of silty clay soils usually have sufficient water-holding capacity to hold the water equivalent to 40 inches of rainfall, It explains why the negligible sorptive capacity of the original l-centimeter cube of mica can be changed by subdivision to a capacity which is great enough to have an important role in sorbing inorganic ions, organic ions, sugars, starches, and other polar compounds from solutions. Clay mineral crystals are to inorganic chemistry what the proteins are to organic chemistry. Both groups of substances can be considered to be large charged molecules which can be extended on and on. As a result, they sorb ions and polar molecules. Due to their configurations, they have enormous surfaces and since their surfaces are charged, they can orient and sorb enormous quantities of water. Certain interparticle arrangements allow them to sorb, orient, and confine water molecules in inter-particle three dimensional associations t o form gels. The proteins are amphoteric and have both cation and anion sorbing capacities. The clay minerals have well-known cation sorbing capacities, and recently Dean and Rubins (1947) and Coleman and Mehlich (1.948) have shown that they have anion sorbing capacities. The work of Schofield (1940) shows that the clay minerals are amphoteric and that their crystals have both positive and negative spots. There is a great variety of molecules, called polar molecules, in which one end or one side of the molecule is weakly positively charged and another end or side is weakly negatively charged. The oxygen and divalent sulfur-bearing compounds belong to this group of compounds. These molecules, when dispersed in liquids, tend to form associations among themselves in order to neutralize these weak charges, or if clay minerals are in contact with the polar molecules, associations can form around the charged spots on the clay mineral crystals. I n these associations the positive end of one polar molecule will orient towards the negative end of a neighboring molecule or clay mineral crystal. Exchangeable ions will also tend to orient these polar molecules in which case the negative end of polar molecules will be oriented towards cations, and the positive end of the molecules will be oriented towards anions. I n water suspensions of clay mineral crystals, water would be expected to be oriented around the charged spots which holds exchangeable ions and also around the exchangeable ions themselves. Furthermore, in these thin film-like crystals it would not be expected that all charges would be exactly balanced by an opposite charge especially since the of acres.

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surfaces of the crystals are covered with large oxygen and hydroxyl ions. There would be expected, therefore, weak widely distributed residual charges over the surface of the clay mineral crystals. These charges would be expected to sorb and orient polar molecules, and they would account for the capacity of the clay minerals to sorb polar liquids and gases, as will be pointed out in Section VII-1-a. The clay mineral crystals are extremely flexible, flimsy; and fragile. To appreciate this the thinnest clay mineral crystals might well be likened to wet sheets of paper with torn edges and with haphazard holes punched in them. The thicker crystals could be likened to cardboard with similar imperfections. When wet with thin films of water or other polar liquids, these crystals tend to orient themselves and stick t o other clay mineral surfaces or t o other surfaces. Since these crystals are charged, t.hey also attract, orient, and organize the molecules of thin films of liquid on their surfaces. These oriented semi-rigid films of liquids serve as lubricants, and they become responsible for the high degree of plasticity exhibited by large masses of wet finely divided and highly charged clay mineral crystals. If clay mineral crystals are dispersed in water and allowed to flow through a porous medium, such as a soil profile, they gradually clog the pores and channels of the soil even though these voids are many times larger than the greatest dimension of the clay mineral crystals. Jenny and Smith (1935) have shown that clays flocculated by electrolytes or hydrated iron oxide sols or clays aggregated by dehydrat.ion are effective in clogging columns of coarse sand. The sheet-like nature of the clay mineral crystals enhances their tendency to form claypans in soil profiles. The clay mineral crystals that associate large amounts of water with their crystal surfaces tend to form gels in rather dilute systems. The swelling bentonites, which are composed of montmorillonitic clay minerals, form gels in concentrations as low as 1to 2 per cent. This tendency of certain clays to form gels is enhanced by the sheet-like nature of the clay mineral crystals. Hydrous crystals of this type form more tenacious cells in which oriented water molecules are confined. The clay mineral crystals would be expected to show many structural and voided imperfections due to pressures exerted upon them and due to the fact that they are formed from extremely dilute solutions. Voids could, therefore, result from a lack of sufficient ions a t the time the crystals were forming. Mechanical breakage and frayed edges of the frail clay mineral crystals certainly result from such forces as biological activity, alternate freezing and thawing, and from the cultivation of soils. Oulton (1948) has found that activated montmorillonite cracking

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catalysts are extremely porous. He found that the pores in these catalysts are completely interconnecting without “dead” ends and without cylindrical sides. It must be rernembered, however, that the activation process may‘ be responsible for rendering the catalyst more porous. The voids in the clay mineral crystals should exhibit the same type of reactions that are characteristic of the edges of the crystals, but in the case of small voids, they should enter into these reactions with more vigor. These voids should be especially favorable for the energetic sorption of ions which have the proper size and coordination to fit in the vacant places in the crystal. They could well be the places in which the plant nutrient elements are fixed in difficultly available forms. The sheetlike crystals of the clay minerals have a marked influence on the physical properties of soils. When they are randomly oriented throughout soil structure part.icles, they are efficient binding agents for the structure particles because each crystal extends its influence over a relatively great distance along its a and b axes. I n this way the randomly oriented clay mineral sheet.s provide a three-dimensional interlacing of binding forces for soil structural aggregates but still do not greatly interfere with the movement of air and water when there are channels and pores between aggregates. When clayey masses are caused to flow either in the moist plastic state or in suspension, the individual sheetlike clay mineral crystals tend to orient in a preferred direction with the longest axis parallel to the direction of flow. This occurs on a wet clayey furrow slice while i t is slipping over the moldboard of a plow. If the water content of the mass is high during the time that the clay mass is flowing, organic materials, hydrated sesquioxides, and extremely small grainlike mineral crystals will be gradually worked from spaces between the sheets of the clay minerals. This will result in an arrangement of multiple sheets of overlapping clay mineral crystals. If the water content of the clay mass is then reduced, the sheets of clay mineral crystals will adhere toget,her or to faces of other crystals much the same as moist bits of paper adhere together. The moist clay mass will now be much more plastic and the overlapping of layers of clay mineral sheets will interfere with the movement of water and air. If the clay mass is dried, it will be hard and consistent. The overlapping crystals will tend to clog channels and pores in clayey masses much like bits of paper will clog N wet sieve. These are some of t,he essentials of a puddled condition in clayey soils. Parallel-oriented clay mineral sheets can most easily be returned to a condition of random orientation when they are surrounded by large amounts of water containing organic materials or other charged colloidal substances. The formation of ice crystals bet.ween the layers of clay

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mineral crystals during alternate freezing and thawing is also very effective in rearranging these crystals in a condition of random orientation.

VII. THE PHYSICOCHEMICAL REACTIONS OF THE CLAYMINERALS It has been pointed out in Section VI that the clay minerals possess extremely large surface areas. These surfaces provide soils with vast capacities t o store sorbed water, organic compounds, and plant nutrient elements. The clay minerals hold these valuable substances against the forces of nature in forms which later may be used by microorganisms and plants. For convenience of discussion, the sorption reactions of the clay minerals are divided into two types. The one type of sorption involves associations with polar molecules. The other type of sorption involves the attraction of ions oppositely charged from the spots upon which they are sorbed on the clay mineral crystals. Fixation of some of the plant nutrient elements and the dispersion, flocculation, and gel-forming properties of clay minerals are closely associated with these sorption reactions. I . Polar Sorption Reuctions According to Debye (1929) many iionionic inurganic and organic molecules are dipoles due to a lack of symmetry of electron distributions within individual molecules. These molecules act as if they carried both centers of positive charges and centers of negative charges. Clay mineral crystals are also polar. When these polar crystals are in contact with liquids, solutions or suspensions containing other polar substances, the negative centers on the clay mineral crystals attract the positive centers on polar substances in surrounding liquid phases, and positive centers on the clay mineral surfaces attract negative centers on surrounding polar particles. These attractions provide the forces by which polar substances are sorbed by the clay minerals. Water is by far the most important polar compound which is sorbed and conserved by the soil clays. The oxygen atom in water, according to the work of Bernal and Fowler (1933), and Cross et al. (1937) tends to direct its attractive forces toward the four corners of a regular tetrahedron. I n the water molecule, most of the attractive force of the oxygen atom will be directed towards two corners of the tetrahedron where the hydrogen atoms will reside, but a small residual force will be directed towards the other two corners of the tetrahedron. The total positive attractive force on the 2 hydrogen atoms should exactly equal the total negat.ive attractive force on the divalent oxygen atom. For steric reasons, according to Bernal and Fowler, the two hydrogens cannot exactly neutralize the four spots on the oxygen atom so that the spots where the

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hydrogen atoms reside will carry a slight reeidual posit,ive charge and the other two corners of the tetrahedron should carry an equally small residual negative charge. Water molecules, therefore, tend to attract or sorb each other. I n this way a molecule of water is subjected to less strain if the residual positive side of the molecule has this posit.ive charge neutralized by the negative side of a neighboring molecule. This process, whereby residual positive hydrogen-rich spots of one molecule neutralize residual negative spots on neighboring molecules, is called hydrogen bonding. According to Bernal and Fowler, hydrogen bonding does not stop with the union of only 2 molecules of water in the liquid phase, but from x-ray diffract.ion patterns of water, Fowler and Bernal (1933) have concluded that a few tens or hundreds of water molecules tend to be hydrogen bonded. The x-ray data indicate that 4 water molecules are grouped around a fifth molecule, thus forming a tetrahedron. These units are assumed to be propagated on and on until broken by the kinetic nature of the water molecules. The edges of oriented, organized, pseudocrystalline clumps of water molecules would still not have all the residual charges neutralized, but there would be both negative and positive spots remaining on the sides and edges of the pseudocrystal. a. Montmorillonitic Minerals. The residual positive and negative spots on water molecules or clumps of molecules are important in the clay mineral-water sorption relationships. The oxygen ions in the surface of the clay mineral crystals (Figs. 1, 2, 3 ) , like the oxygen atoms in the water molecules, will also be weakly charged. These layers of oxygen ions will direct most of their forces backward into the crystal towards t,he positive silicon or metallic ions in the interstitial spaces of the clay mineral crystal. Weak residual negative charges will result on the outside layer of oxygen atoms in the clay mineral crystals, which will be free to take part as donors in hydrogen bonding. Each oxygen atom, if free from other sorbed substances, should attract the hydrogenated or positive side of a water molecule. Hendricks et al. (1940), however, have suggested that water molecules on hydrated clay layers arrange themselves in a hexagonal network in which the individual molecules are three angstroms apart. Such an arrangement would allow each water molecule to reside in the depression of the base of each tetrahedron (a ratio of 1 water molecule to 1% oxygen ions), which is composed of 3 oxygen ions in the surface of the silica tetrahedral layers. Hendricks (1941) gives four distinct steps for the hydration of t,he surface layers of dry montmorillonite. They are: (1) hydration of the exchangeable cations on the external and on the internal surfaces of the

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expansible part of the montmorillonite crystal. According to Debye and Falkenhagen (1928) water molecules orient between cations and anions, (2) hydration of the remaining surface wit.h a hexagonal network of water molecules as described above, (3) a t somewhat higher relative humidities, a second hexagonal network of water molecules will be sorbed on t,he first oriented layer due t o the propagation of polar attractive forces by the preferentially oriented first layer of molecules, (4) when the relative humidity approaches 100 per cent, water will condense on the surfaces. As the water films become thicker and thicker, it is reasonable to assume that they gradually change to an organization in which five molecules of water form a terahedron. Tetrahedral groups of this type tend to propagate themeselves into a structure similar to that of quartz, as shown by Bernal and Fowler (1933), for the liquid phase of water. The water molecules in the hexagonal network, as proposed by Hendricks et al. (1940), are not closely packed. Convincing support is provided for the above theories of water sorption by montmorillonitic clay minerals by data presented by Nitzsch (1940). He found that the first water added to dried clay materials assumed a much greater volume than ordinary liquid water and that as more and more water was added to the clays, the specific gravity of the added water gradually rose to 1. Hendricks (1941) and Grim et al. (1947) have shown that large organic cations interfere with the sorption of water by montmorillonitic clays. Gieseking (1939) reported that montmorillonite clays lost their tendency to swell by water sorption when saturated with a variety of large organic cations. The author also has some unpublished data to show that large organic cations sorbed on montmorillonitic clays decreases their water-holding capacity as measured by their moisture equivalents. Other polar compounds or sorbed hydrated sesquioxides have been observed to decrease the tendency of the montmorillonite clays to form gels. These observations seem to indicate that positively charged colloids and organic cations sorbed on the predominantly negatively charged montmorillonitic crystals in the place of inorganic cations tend to break up the network of forces responsible for the polar hydration of these crystals and thereby decrease their hydration tendencies. Edelman and Favejee (1940) have proposed a structure for the montmorillonites in which 4 hydroxyl ions replace 4 oxygen ions in the montmorillonite structure of Hofmann e t al. (1933) (see Figs, 4 and 5 ) . Two of these hydoxyl ions are in the octahedral layer and the other two, according to the proposal, protrude from the tetrahedral layer. They conclude that the protruding hydroxyl groups ionize to give replaceable hydrogen ions and that they are centers for hydrogen bonding with water.

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The author has some unpublished observations on the action of acetyl chloride on Wyoming bentonite which supports the contention of Edelman and Favejee. Acetyl chloride is an extremely effective reagent for the destruction of hydroxyl groups. Wyoming bentonite, a very lyophilic substance, which forms gels in dilute suspensions, loses its tendency to swell and form gels after complexing with acetyl chloride. This indicates that destruction of the hydroxyl groups on the crystals of these montmorillonite crystals greatly reduces the tendency of these crystals to sorb water. X-ray diffraction patterns of the acetylated montmorillonite indicate that a t least part of the acetyl chloride is complexed within the expansible portion of the crystal. The pure montmorillonitic clay minerals form thixotropic gels (thixotxopic gels are gels that are solid when undisturbed but easily liquified by mechanical disturbance) even in dilute suspensions. These gels form as a result of a n organization of the clay mineral crystals in which a continuous three-dimensional framework of crystals completely surrounds and traps large quantities of preferentially oriented water molecules. Suspensions containing as little as 2 per cent of a montmorillonitic clay mineral with a high exchange capacity may form a thixotropic gel upon standing a short time in an undisturbed condition. Fortunately, the montmorillonitic clay minerals which have an extreme tendency to gel do not exist long under most soil-forming conditions. Such clay minerals are very active and as soon as they come in contact with basic nitrogenous organic substances or positively charged hydrated sesquioxides, they sorb these substances and thereby lose much of their gel-forming tendency. If this were not the case, small amounts of the montmorillonite clay minerals would render soils practically impervious to water. MacEwan (1946) and Bradley (1945b) have shown that there are many organic molecules which mutually sorb clay mineral crystals by hydrogen bonding. The carbohydrates, alcohols, and proteins contain groups which enter into polar sorption reactions by this method. Myers (1937) suggested that a chemical union resulted from combinations of organic components of composts with a number of acid soil clays. The proteins and other basic nitrogenous organic compounds can also be sorbed by ionic exchange (see Section V I I - 2 4 ) . Compounds of this type tend to interfere with the sorption of large quantities of water by the clay mineral crystals. I n this way they reduce the tendency of the clay minerals to form gels and while they may actually reduce the capacity of the clay minerals to hold water, they may render a clayey soil more permeable and more useful as a place for water storage. The edges of the montmorillonite crystals have broken valencieb which must be satisfied by ionic and polar sorption. Kelley et al. (1936)

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have found broken bonds to be very effective sources of sorptive capacities in finely ground minerals which do not have layer lattices and consequently have much of their surface area composed of broken bond cleavages. On t,he other hand, their results with B variety of montmorillonites, which have only a small percentage of their surfaces in positions of broken bond cleavage, showed that only about 5 to 15 per cent of the polar xorptivc c:tp:icity of these minerals could be assigned to broken bond edges of the cyvstal. This is to be expected in the sheetlike crystals of the montmorillonites since they have planar surfaces completely populated with ions whose valencies are mostly satisfied within the crystal.

b. The Zllitic Minerals. The illitic clay minerals sorb polar compounds by mechanisms similar to those operative in the montmorillonitic clay minerals, except that the former have no expansible layer in which sorption can take place. Consequently, the broken bond forces in the illitic minerals, as has been shown by Kelley et al. (1936) for finely ground biotite and muscovite, account for a higher percentage of the total polar sorption than in the montmorillonitic minerals. Due to the loss of internal surfaces, however, the illitic minerals have less total polar sorptive capacities than the montmorillonitic minerals.

c. The Kaolinitic Minerals. The kaolinitic clay minerals usually exist in relatively large crystals. The units in the kaolinite crystal are considered to be held together by an ideal system of hydrogen bonding in which the bonding occurs between each out,side hydrogen atom of the hydroxyl ions in the octahedral layer serving as the acceptors and each outside oxygen ion of the silica tetrahedral layer acting as a donor. The bonding energy from this ideal type of arrangement is too great to be supplanted by polar molecules. Consequently, the kaolinitic clay minerals exhibit still smaller sorptive capacities for polar compounds than is exhibited by the illitic clay minerals. 2. Ionic Sorption Reactions

The clay minerals have the unique property of sorbing, through exchange reactions, both cations and anions. These reactions can provide plant nutrient elements for maximum plant growth. Without these reactions, plants would be forced to feed on whatever nutrient elements might become available from organic decomposition and the weathering of minerals. During periods of little or no consumption of these released products, they would be lost by leaching. During periods of peak plant nutrient requirements, in many cases the plants would not have enough

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of these elements for proper development. By cation exchange reactions, however, nutrient elements released by weathering are sorbed on tshe clay mineral crystals. These sorbed nutrient elements are available to plants and during periods of peak requirements the sorbed supply of plant nutrient elements serves as a reservoir which prevents starvation of plants during the critical periods of their growth and reproductive cycles. I n t.his way the clay minerals conserve plant nutrient elements until they are needed in the various biological r p r l e c involved in the use of soils as a medium for plant growth. a. Base Exchange or Sorption of Cations. In Section 111-1 it has been pointed out that substitutions in the tetrahedral and octahedral layers of the clay mineral crystals may leave an overall excess of negative charges on the surfaces of these crystals. The hydrogens in the hydroxyl groups on the broken bonds a t the edges of the crystals may also ionize to give negative spots, especially when the p H of the clays is high. These negative charges are considered the source of the cation exchange capacities of the clay minerals. Each negative charge on the clay mineral crystals attracts a monovalent cation or two negative charges will share a divalent cation. The hydrogen-saturated clay minerals are analogous to the inorganic acids. When the hydrogen is replaced by other cations, the resulting combinations are analogous t o the corresponding inorganic salts. This analogy is shown by the following series of reactions involving the common exchangeable ions found in slightly acid, neutral, or alkaline soils:

+ + + + +

2HC1+ Ca(0H)z + CaCls 2H20 2HC1+ Mg(OH), + MgCL 2H20 HCl + KOH --$ KCl HzO HCl + NH,OH + NH4C1 HzO HCI + NaOH --$ NaCl HBO

ZH-Clay 2H-Clay H-Clay H-Clay H-Clay

+ Ca(0HL + Ca-Clay + 2Hz0 + Mg(OH), + Mg-Clay + 2Hs0 + KOH + K-Clay + HaO + N E O H + NHa-Clay + HzO + NaOH -+Na-Clay + HzO

A single clay mineral crystal has many sorbed exchangeable ions. These ions most commonly are: C a + + , Mg ++ , H+, K+, NH4+, and a trace of N a + except in alkali soils where N a + is a prominent sorbed ion. The order of abundance of these ions in most productive soils will usually be as given above. The amounts of these exchangeable ions in different soils depend on the nature of the clay mineral components, the nature of their parent material, and the history of their development. The inorganic exchange capacity of soils may vary from practically no exchange capacity to 60 milliequivalents per 100 g. of soil. The following amount8 of exchangeable catione in 100 g. of productive soil can he considered typical values:

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Ca Mg H

K NH4 Na

15 milli equivalents 5

5 0.25 trace trace

Since the cation exchange capacity of soils is concentrated in the clay mineral fraction, the capacity of some of the pure clay minerals must be higher than the total of the typical values given above. Grim (1942) gives the following exchange capacities for some of the common clay minerals in soils: Montmorillonite 60-100 milliequivalents per 100 g. IlIite 20-40 Kaolinite 3-15 Most of the cation exchange reactions are equilibrium reactions. This may be illustrated by considering the following reaction between sorbed calcium and the potassium in potassium chloride: Ca-Clay f2KC1# ZK-Clay f CaClp

When this reaction has reached equilibrium all of the KCI will not have been sorbed nor will all of the calcium have been replaced. The point of equilibrium can be influenced by changing the concentrations of the soluble salts on either side of the equation. The addition of more KCl or the removal of CaClz from the reaction will tend to make the reaction go more and more to the right. The removal of KC1 or the addition of CaC12 will make the reaction go more to the left. Most of the reactions of soluble fertilizers with clays and the reactions involving the nutrition of plants are equilibria exchange reactions of this type. They can be represented by making appropriate substitutions in the above equilibrium. The cation exchange reaction that results from the application of lime to an acid soil clay is not an equilibrium reaction. This reaction may be illustrated as follows: ZH-Clay

+ CaCOs +Ca-Clay + HzO + COz

Water, a nonionized substance, is formed and C 0 2 is lost from the soil. This amounts to the removal of two products from the right hand side of the equation. Consequently, it goes to complet,ion and it is not an equilibrium reaction. Since the exchange of sorbed cations provides a source of plant nutrient elements for plant nutrition, the ease of the release of these ions has been widely st,udied. The early investigations of Jenny (1932) and

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Gieseking and Jenny (1936) have dealt with the ease of release of a single cation from clay mineral systems in which the ion to be studied was artificially made to occupy essentially all of the cation exchange spots on the mineral surfaces. B y various combinations with various concentrations of replacing ions the ease of release of the agriculturally important exchangeable cations has been found to decrease in the following order: Na>K>NH4>Mg >Ca>H More recent studies have dealt with the ease of release of cations from the clay mineral surfaces when two or more cations occupy the exchange spots on the mineral surfaces. Bray (1942) used a very low concentration of hydrogen ions to exchange the naturally sorbed cations from a group of soils containing montmorillonitic and illitic minerals. He found the same order of ease of release in these heterionic systems as was found by Gieseking and Jenny for homoionic systems, except that the NH4+ ion was not included in Bray’s experiments. Schachtschabel (1940) has demonstrated t.hat various clay minerals differ in the ease with which they release sorbed cations. H e worked with minerals which he considered pure clay minerals. He showed that Ca+ + and Mg+ + ions are the most difficult of the plant nutrient cations t.o release from montmorillonite and kaolinite and H + ions are somewhat easier to release. He showed that ground micas hold H+, K + , and NHI+ ions very tenaciously and release C a + + and M g + + ions more easily. He extended his work to studies on cation exchange reactions of the humic acids and found that these acids held C a + + ions much more tenaciously than NH4+ ions. Schachtschabel’s results emphasize the importance of determining the amounts of the various components in soil clays for the interpretation of their cation exchange relationships. Wiklander (1946) has studied the interrelationships of competing cations in exchange reactions. H e has found that the ease of release of an ion depends not only on t.he nature of the ion itself but also upon the nature of the complementary ions filling the remainder of the exchange spots and on the degree to which the replaced ion saturates the exchange spots. Jenny and Ayers (1939) have obtained similar results. Wiklander’s results show t.hat, as the amount of exchangeable calcium on the clay mineral crystals becomes less, the calcium becomes more and more difficult to release. Sodium, on the other hand, becomes easier to release as the degree of saturation with sodium ions becomes less. The magnesium and potassium are not affected by degree of saturation to the extent that calcium and sodium are affected. Wiklander suggest8 this as an explanation for the fact that calcium generally occuoies most

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of the exchange spots and sodium occupies the least number while exchangeable magnesium and potassium are intermediate in abundance on the clay mineral crystals in nature. This distribution of exchangeable ions usually holds irrespective of the nature or the composition of the parent material from which the clay minerals are formed. The effect of rontact exchange on surface migration of exchangeable ions and upon the sorption of nutrient ions by plants has been investigated by Jenny e t al. (1939) and by Jenny and Overstreet (1939a, 1939b). According to the rontact exchange theory, since exchangeable ions are subject to continuous thermal agitation, when neighboring oscillations overlap, there should be opportunity for the exchangeable ions in these neighboring spots to exchange without the aid of oppositely charged ions in solution. This means that ions could migrate on exchange surfaces by jumping from spot to spot, provided there is another ion of like charge simultaneously jumping in the opposite direction. Jenny and his coworkers have also shown by the use of radioactive tracer ions that ions may exchange by contact of the oscillation volumes of exchangeable ions on clay mineral surfaces with the oscillation volumes of exchangeable ions on plant root surfaces. They have shown that plants may sorb ions from exchangeable ions on clays or clays may deplete plants of their sorbed ions, as was also suggested by Kelley (1927), depending on concentrations of the ions in the two systems. They consider that exchangeable ions may be moving from clays to plant roots in one area, and the same kind of ions may be moving from plant roots to clay in another area by contact exchange. These investigators have shown in other experiments that radioactive rubidium and sodium were more readily sorbed by decapitated barley plants from clay surfaces than from bicarbonate solutions of these elements. These experiments and the observations by Albrecht (1946b) show the many possibilities whereby plants may feed on sorbed nutrient elements and adjust themselves to unfavorable nutrient concentrations by mutual exchanges with the clay minerals. There is evidence to show that clays may take ions from water suspensions of very insoluble substances and resistant minerals by means of ionic sorption reactions. Lemberg (1876) showed that finely powdered leucite, a potassium feldspar, could be changed to analcite, a sodium aeolite, and vice versa, by Na and K exchange reactions in water suspensions. Bradfield (1932) found sodium-saturated clays were able to take enough barium from barium sulfate (an extremely insoluble substance) to fill one-sixth to one-fifth of the cation exchange capacity of the clay. Peech and Bradfield (1934), Graham (1941), and Albrecht (1946a) show that clay minerals are effective in sorbing cations from

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resistant minerals. These resistant. minerals in water suspensions are in equilibrium with traces of ions which dissolve from their surfaces. The clays destroy this equilibrium by sorbing the ions in solution. If the equilibrium is maintained, ione mud, move from the resistant mineral into solution and on to the clay until a new equilibrium is established which involves the resistant mineral, the water solution, and the clay mineral. The activity of the exchangeable ions, on the clay mineral crystals, determines the extent to which the clay minerals enter into the many dynamic soil processes. Marshall and McLean (1947) and their associates have developed clay membrane electrodes which permit the measurement of activities of single cations when sorbed on clays. McLean and Marshall (1948) have extended these studies to clay systems containing two cations. Their results on these more complex systems are encouraging, and it appears that these investigators may be able to realize their goal, namely the measurement. of activities of sorbed ions in samples of whole soil. The clay minerals also enter into exchange reactions with the organic cations. The most common organic cations contain basic amino groups. These cations are ammonium ions in which one or more of the hydrogens have been substituted by organic groups. These cations have been found by Gieseking (1939), Ensminger and Gieseking (1939, 1941, 1942), Hendricks (1941), Ensminger (1942), Bradley (1945b), Erickson (1948), and by Allaway (1948) to be very strongly sorbed by the montmorillonitic clay minerals. Ensminger and Gieseking (1942) and Erickson (1948) have found the proteins and amino acids to be more resistant to enzymatic attack in the sorbed state than in the free state. As a rule these large cations are difficult to replace by means of small cations, but Gieseking (1939) has found them to be more easily replaced by other large organic cations. It has been pointed out in Section VIII-1-a, that the sorption of organic cations on the clay minerals interferes with the tendency of these minerals to sorb large amounts of water. The organic cation-clay complexes do not swell to form gelatinous impervious masses. It appears that the organic cations destroy the undesirably high watersorbing capacity of the montniorillonitic clay minerals, thereby improving water-clay-plant relationships in soils containing these minerals. The Fixation of Cations in Difficultly Exchangeable Forms. Clay minerals can be shown to sorb, in difficultly exchangeable form, many of the exchangeable cations found in soils. While these difficultly exchangeable cations may have various degrees of availability to plants, i t is generally accepted that the fraction of easily exchangeable cations are readily available. A measure of these 2 fractions of exchangeable cations

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in a soil is, therefore, by no means a perfect method of estimating their availability to plants. A knowledge of the amounts of these fractions of exchangeable cations in soils, however, and a knowledge of the tendency of one fraction to change to the other is essential for the best management of soils. Because of the general economic importance of potash fertilization, potassium has been by far the most widely studied cation with respect to fixation in soils in a difficultly exchangeable form. Numerous conflicts have appeared in the interpretation of results of the earlier experiments on potassium fixation. Stanford (1947) has found that the various clay minerals differ in their fixation of potassium with respect to amount of fixation and conditions under which fixation occurs. H e has properly attributed the early conflict,s on these problems to insufficient knowledge of the clay mineral composition of the various soils studied. The investigations of Chaminade (1936) and Wiklander (1949) have shown bhat high pH was conducive to the fixation of potassium in t,he clay minerals with which they were working. Volk (1938), Page and Baver (1940), Attoe and Truog (1946), Martin et al. (1946), Raney and Hoover (1946), Attoe (1947), and Joffe and Levine (1947) have shown that fixation is increased by drying. Hoover (1945) and Raney and Hoover (1946) have shown that montmorillonitic soils fix much more potassium than kaolinitic soils. Wiklander and Gieseking (1949) have shown montmorillonitic clays and illitic clays to be more effective in potassium fixation than kaolinitic clays. Stanford (1947) has found illite to be effective in fixing potassium in the moist state when the p H of the clay system was high and that drying increased the fixation of potassium by illite and montmorillonite. Bray and DeTurk (1939) , Wood and DeTurk (1941), and DeTurk et al. (1943) have shown that an equilibrium exists in soils between the various fractions of exchangeable and nonexchangeable soil potassium. Since the ammonium ion is very similar to the potassium ion in its properties with respect to easily exchangeable cations, it is interesting to find that Chaminade (1940), Page and Baver (1940), and Stanford and Pierre (1947) have demonstrated the fixation of the ammonium ion by clays in difficultly exchangeable form. The latter workers have concluded that the ammonium ion and the potassium ion are fixed by soils by the same mechanism.

b. Anion Exchange. The investigations on the anion exchange reactions of soils and clay minerals have been associated almost entirely with the sorption of the phosphate ions by these materials. It has been shown by Mattson (1931) , Ravikovitch (1934), Scarseth (1935), Toth

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(1937), Murphy (1939), Stout (1940), Nitzsch and Czeratzki (1940), Bray and Dickman (1941), Dickman and Bray (1941), Kelly and Midgley (1942), Coleman (1944), Kurtz et al. (1946), Sieling (1946), Dean and Rubins (1947), Low and Black (1947), Ensminger (1948), and Perkins (1948) that the minerals common to many soil clays exhibit anion exchange reactions. Many of these investigators have shown conclusively that some of the hydroxyl groups in the hydrated sesquioxides can be exchanged with phosphate ions by reactions analogous to the cation exchange reactions. A number of these investigators have emphasized the importance of the kaolinitic minerals in phosphate and other anion exchange reactions. There has been considerable argument, however, concerning the mechanism of anion exchange in the systems containing kaolinite. Some investigators have concluded that the hydroxyl groups of the kaolinitic minerals were replaced by phosphate ions in the same manner that they are replaced from the hydrated sesquioxides. Other investigators have concluded that the procedures used for demonstrating anion exchange capacity have been drastic enough to decompose the kaolinitic minerals forming one of the hydrated sesquioxides, namely hydrated aluminum oxide. According to this latter argument, the hydrated aluminum oxide formed by the destruction of the kaolinite crystals was then responsible for the phosphate exchange reactions. The recent work of McAuliffe et al. (1947), however, has shown conclusively that anion exchange reactions take place readily between the surfaces of kaolinitic mineral surfaces and surrounding solutions as well as bet.ween the surfaces of crystals of some of the hydrated iron and aluminum oxides. They found that radioactive phosphate ions in solutions replaced inactive phosphate ions from surfaces of minerals of several soils. In additional experiments with hydrated iron and aluminum oxides and kaolinitic minerals, they found exchange between duterium tagged hydroxyl ions in solution and ordinary hydroxyl ions on the hydrated oxide and clay mineral surfaces. These results explain some of the uncertainties resulting from the inadequate procedures which the earlier investigators on anion exchange were forced to use on clay minerals. An account of phosphate exchange and fixation by soils by L. A. Dean will be found elsewhere in this volume (see p. 391). 3. Dispersion, Flocculation, and Gel Formation

When clays are completely dispersed in water, each clay mineral crystal is entirely surrounded by water. Dispersed clay minerals move with water movements. When they move, they tend to clog pores and channels in the soil, and they cement neighboring soil aggregates to-

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gether. Under these conditions soils become impervious to air and water, and they become unfavorable media for the growth of plants and microorganisms. Kelley (1927) has discussed the marked effect of exchangeable sodium in dispersing natural soils and its effect on the permeability of soils. According to Kelley, the dispersed and swollen sodium clays are responsible for the clogging of the pores in alkali soils. When clay suspensions are flocculated, the individual clay mineral crystals are not separated entirely from neighboring crystals in the suspension, but they are associated in various sized groups or floccules. Floccules of clays do not move as readily as dispersed clays, and they cont,ribute to more stable soil aggregates and to more permeable conditions in soil masses than dispersed clays. Jenny and Reitemeier (1935) and Jenny and Smith (1935) have found that water suspensions of a clay saturated with an easily exchangeable ion is more difficult to flocculate than the same clay saturated with a more difficultly exchangeable cation. This means that hydrogen or calcium clays are most easily flocculated, sodium clays are most difficult to flocculate, and magnesium, ammonium, and potassium clays take intermediate positions. These studies explain the effective action of polyvalent cations, positively charged hydrated sesquioxides, and positively charged organic ions in flocculating clays which carry predominately negative charges. When clays form gels, the clay mineral crystals associate to form an interlacing, three-dimensional string-like network upon which clumps or globules of water molecules are sorbed. According to Hauser (1939), the water molecules are an integral part of such systems. Gels form in high concentrations of lyophilic clays. Clay gels resist the movement of water and air when they are formed in soils. The montmorillonitic clay minerals have the greatest tendency t o form gels, especially when t,hey are free of sorbed organic matter and hydrated sesquioxides. The kaolinitic minerals do not form gels, and the illitic minerals take an intermediate position between the montmorillonitic and kaolinitic minerals.

VIII. FUNCTIONS OF THE CLAYMINERALS The functioning of the clay minerals as a reservoir for water and available plant nutrient elements and the functioning of these minerals in the intricate reactions involved in water-nutrient-plant relationships have been described in Section VI-1-2. The clay minerals have another important function-they serve as binding agents for soil masses. Through their binding action, t,hey provide anchorage for plant roots, and they hold small masses of soil together in the form of structural aggregates. Stable structural aggregates provide clayey soils with chan-

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nels for the movement of air and water, and they prevent movements of soil materials by wind and water erosion. The most effective soil aggregates are those that resist destruction by mechanical means and by physicochemical reactions. I n order for soil aggregates to be st.able, they must meet the following conditions: 1. They must be held together by effective binding agents. 2. They must resist swelling and dispersion on contact with water. 3. They must resist coalition with neighboring aggregates when wet.

Russell (1934, 1935) has suggested a plausible mechanism by which aggregates are bound together by clays to fulfill the first of the above conditions. According to Russell, the exchangeable cations, since they are positive, attract the negative ends of water molecules postulated in the Bernal-Fowler (1933) theory of hydrogen bonding (see Section VII-1). This leaves the positive spots on the water molecules pointed away from the exchangeable ions. These positive spots are attracted by the negatively charged neighboring clay mineral crystals. When the water films become thinner by drainage or evaporation, the sheetlike crystals of the clay minerals are pulled closer and closer together, with many of their ffat faces oriented parallel to each other. The interlacing of chains of overlapping clay mineral crystals forms an effective matrix for holding clayey masses together. This theory agrees with the work of Sideri (1936) and Henin (1937, 19381, who have found evidence of preferred orientation in the clay mineral crystals of soil structural aggregates. The observations of Gorkova (1939) and Peterson (1944, 1946a, 1947) give further support to these theories concerning the binding of soil aggregates by clay minerals. They found that montmorillonite, with its greater surface and negative charge, was more effective than kaolinite as a binding agent for structural aggregates. The work of Du tt (1947, 1948a) suggests the possibility of soluble silicates serving as binding agents for aggregates. Organic residues appear to be responsible for the second and third conditions necessary for the formation of stable aggregates in soils. Russell (1935), Myers (1937), McCalla (1945), Martin (1946), and Kroth and Page (1946) have suggested that polar organic compounds serve as important binding agents in soil aggregates. According to the hydrogen bonding theory of Bernal and Fowler (1933), these polar substances could take the place of water molecules as linkages between exchangeable cations and clay surfaces in the mechanism proposed by Russell (1935). Gieseking (1939) found that Wyoming bentonite and gelatin, two extremely lyophylic substaiices, reacted to f o m a clay gelatin complex which was lyophobic. The dried aggregates from this complex were extraordinarily difficult to crush and did not swell

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when they were again placed in contact with water. A complex of this type is probably formed by attract,ions between many positive amino spots on the gelatin molecule and the many negative spots on the montmorillonite crystals. In this way linkages with much bonding energy could be formed which could be propagated on and on throughout t.he system. The author has unpublished data on a number of aggregates from various soil types which were treated with water solutions of several different organic cations. After these aggregates were dried, they were found to be very difficult to crush, even after placing them again in contact with water for long periods of time. It appears, therefore, that certain organic molecules react with the clay minerals to give complexes which resist dispersion. The activity of microorganisms in soils is conducive to the formation of stable aggregates. Myers and McCalla (1941) and Peele and Beale (1941) noted that the maximum effect of microorganisms on soil structure stability lagged behind the maximum microbial activity. They interpreted this to mean that excreted and/or secreted metabolic products of the organisms were responsible for the beneficial effect on stabilization of soil aggregates. These observations have been confirmed by McHenry and Russell (1944), Martin (1945, 1946), and Kroth and Page (1946). Kroth and Page emphasize the point that these products form physicochemical complexes with the clay minerals. McCalla (1945) has observed that gums, waxes, and fats produced by microorganisms are instrumental in stabilizing soil aggregates. Martin (1945) found that polysaccharides, arising from bacterial growth are important stabilizing agents for soil aggregates. The casts of earthworms were reported by Dutt (1948b) to be very stable. Certain plants appear to be responsible for the production of organic substances which are sorbed on the clay mineral surfaces of soil aggregates and thereby render these aggregates resistant to the attack of water. Numerous investigations have shown that any cropping system which adds organic matter to soils, especially systems involving sod crops with their fibrous root systems, are responsible for highly stable soil aggregates. Among these investigations have been those of Elson (1940), Woodruff (1940), Johnston e t al. (1942), Gelzer (1943), Wilson and Browning (1945), Feng and Browning (1946) , Olmstead (1946) , and Shauffer (1946). Norman (1946) has suggested that the enormous activity of rhizosphere bacteria around grass roots may be responsible for this effect of the sod crops. There has been much discussion concerning the importance of binding agents in soil aggregate stabilizations. The clay minerals and organic materials serve well as binding agents. More emphasis needs to be placed

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on mechanisms whereby neighboring aggregates are held apart. The most important aspect of the mechanism of soil aggregate stabilization is the stabilization of the clay minerals on the surface of the aggregates so that they will not exert an attraction or binding force between neighboring aggregates. The first, step toward aggregation must be the development of weakened cleavage zones in the clayey mass of the soil material. After this has been accomplished by the growth of plant roots, by animal activity, by the formation of ice crystals, by shrinkage from dehydration, or by other mechanical means, it. would seem t h a t the negative clay minerals in the faces along these cleavage zones might become inactivated by the sorption of positively charged hydrous oxides and organic substances. This should prevent the dispersion of the clay mineral crystals in the zones of incipient cleavage and the subsequent coalition of neighboring aggregates after removal of the original mechanical condition responsible for the first cleavage. Sideri (1936, 1938) suggests that humus coats the surfaces of soil aggregates. Kroth and Page (1946), however, found the organic materials in soil aggregates to be quite uniformly distributed throughout the aggregate. They compared the nitrogen contents of the shells of a number of aggregates with the nitrogen contents of their centers and found in every case a slightly higher percentage of nitrogen in the shells. The significance of this small difference is questionable, but their results seem to indicate that the clay minerals in the surface of soil aggregates are stabilized by sorbed organic cations. More conclusive evidence is needed t o establish the certainty of this mechanism of surface stabilization of soil aggregates. There have been several suggestions concerning the stabilization of soil aggregates by means of irreversible -colloidal cementing agents. Williams (1935), Sideri (1936), and Kubiena (1938) give the general impression that freshly formed hydrated organic substances are sorbed on mineral surfaces, and when these organic substances become dehydrated, they will not again hydrate on later contact with water. Thus, these irreversible organic substances serve as cementing agents between mineral particles. Lutz (1936) has proposed a somewhat analogous role for the hydrated iron oxides.

IX. CONCLUSIONS Clay mineralogy and clay physicochemistry have developed almost entirely during the last two decades. I n this short period, remarkable progress has been made towards a better understanding of the clay minerals and their intricate activities. There still remains much to be accomplished, however, in the way of more refined and more precise studies of these minerals.

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The reactions of the clay minerals are determined by the amount and nature of their external, internal, and voided surfaces. More refined methods are needed to determine these properties of the clay mineral crystals. The clay minerals vary in the vigor with which they enter into physicochemical reactions. They also vary in their capacities to hold various sorbed substances. More precise methods for the quantitative estimation of the clay minerals are needed in order to be able to predict the nature of the react*ionsof the clay mineral mixtures commonly found in soils. Combinations between the clay minerals and sorbed substances have properties which are often greatly different from the properties of the components. Further data is needed to reveal the importance of sorbed substances in changing the properties of the clay minerals. The clay minerals have been shown to be extremely important in plant-soil relationships. More studies are needed to reveal the nature, mechanism, and extent of the physicochemical reactions of the clay minerals which influence plant growth. Some of these reactions are desirable and others are undesirable. Studies need to be extended to show how the undesirable reactions can be siippressed and how the desirable reactions can be enhanced. Past developments in clay mineralogy have very closely paralleled the development of physical methods of analysis. Some of the newer techniques involving x-ray and electron diffraction can be expected to be helpful in furthering research on the clay minerals. The use of radioactive and mass “tagged” isotopes in studying the physicochemical reactions of the clay minerals promises ta give results which will permit more definite interpretations than have been possible with some of the older techniques.

REFERENCES Agafonoff, V. 1935. Trans. Intern. Congr. Soil Sci. 3rd Congr., Oxjord 3, 74-78. Albrecht, W.A. 1946a. Soil Sci. 61,265-271. Albrecht, W.A. 1946b. Soil Sci. 62, 23-31. Aldrich, D. G.,Hellman, N. N., and Jackson, M.L. 1944. Soil Sci. 57, 215-231. Alexander, L. T.,Hendricks, S. B., and Nelson, R. A. 1939. Soil Sci, 48,273-279. Allaway, W.H. 1948. Soil Sci. SOC.Am., Proc. 13. Am. SOC. Testing Materials. 1945. Card Index of X-Ray Diffraction Data for Chemical Analysis, Philadelphia, Pennsylvania. Ardenne, M., Endell, K., and Hofmann, U. 1940. Ber. deut. keram. Ges. 21, 209-227. Attoe, 0. J. 1947. Soil Sci. SOC.Am., Proc. 11, 145-149. Attoe, 0.J., and Truog, E. 1946. Soil Sci. SOC. Am., Proc. 10, 81-86. Raver, L.D. 1948. Soil Physics. 2nd ed. Wiley, New York.

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Bernal, J. D., and Fowler, R. H. 1933. .I. Chem. Phys. 1, 515-548. Bradfield, R. 1932. J. Phys. Chem. 36, 340-347. Bradley, W. F. 1945a. Am. Mineral. 30, 704-713. Bradley, W. F. 1945b. J. Am. Chem. SOC.67, 975-981. Bradley, W. F., and Grim, R. E. 1948. J. Phys. Colloid Chem. 52, 1404-1413. Bray, R. H. 1937. Soil Sci. 43, 1-14. Bray, R. H. 1942. J. Am. Chem. Soc. 64, 954-963. Bray, R. H., and DeTurk, E. E. 1939. Soil Ski. SOC.Am., Proc. 3, 101-106. Bray, R. H., and Dickman, S. R. 1941. Soil Sci.SOC. Am., Proc. 6, 312-320. Bray, R. H., Grim, R. E., and Kerr, P. F. 1935. Bull. Geol. SOC.Am. 46, 1909-1926. Buehrer, T. F., Robinson, D. O., and Deming, J. M. 1948. Soil Sci. SOC.Am., Proc. 13. Chaminade, R. 1936. Ann. agron. 6,818-830. Chaminade, R. 1940. Compt. rend. 210, 264-266. Clark, G. L., Grim, R. E., and Bradley, W. F. 1937. 2.Krist. A96, 322-324. Coleman, N. T., and Mehlich, A. 1948. Soil Sci. SOC.Am., Proc. 13. Coleman, R. 1944. Soil Sci. SOC.Am., Proc. 9, 72-78. Coleman, R., and Jackson, M. L. 1945. Soil Sci. SOC.Am., Proc. 10, 381-391. Cross, P. C., Burnham, P., m d Leighton, P. A. 1937. J . Am. Chem. SOC.59, 11341147. Davidson, R. C., Ewing, F. J., and Shiite, R. S. 1943. Natl. Petroleum News, 35, 318-321. Dean, L. A., and Rubins, E. J. 1947. Soil Sci. 63, 377-406. Debye, P. 1929. Polar Molecules. Chemical Catalogue Co., New York. Debye, P., and Falkenhagen, H. 1928. Physile. 2. 29, 121-132. DeTurk, E. E., Wood, L. K., and Bray, R. H. 1943. Soil Sci. 55, 1-12. Dickman, S. R., and Bray, R. H. 1941. Soil Sci. 52, 263-273. Dutt, A. K. 1947. Soil Sci.SOC.Am., Proc. 12, 497-501. Dutt, A. K. 1948a. Soil Sci. 65, 309-319. Dutt, A. K. 1948b. J. Am. SOC.Agron. 4 4 407-410. Edelman, C. H., and Favejee, J. Ch. L. 1940. 2.KrisL. 102, 417-431. Elson, J. 1940. Soil Sci. SO, 339-355. Ensminger, L. E. 1942. Sbil Sci. 54, 191-197. Ensminger, L. E. 1948. Soil Sci. SOC.Am., Proc. 13. Ensminger, L. E., and Gieseking, J. E. 1939. Soil Sci. 48, 467-473. Ensminger, L. E., and Gieseking, J. E. 1941. Soil Sci. 51, 125-132. Ensminger, L. E., and Gieseking, J. E. 1942. 53, 205-209. Erickson, A. E. 1948. Doctor’s Thesis, Univ. of Illinois. Favejee, J. Ch. L. 1939a. 2.K G t . 101, 259-270. Favejee, J. Ch. L. 1939b. 2.Krist. 100, 425-436. Feng, C. L., and Browning, G. M. 1946. Soil Sci. Moc. Am., Proc. 11, 67-73. Fowler, R. H., and Bernal, J. D. 1933. Trurts. Faraday SOC.29, 1049-1056. Gelzer, F. J. 1943. Pedology U.SS.R. 9-10, 62-71. Gieseking, J. E. 1939. Soil AS&. 47, 1-13. Gieseking, J. E., and Jenny, H. 1936. Sozl Iclci. 42, 273-280. Gorkova, I. M. 1939. Pedology U.S.S.R. 10, 65-83. Graham, E. R. 1941. Soil Sci. 51, 65-71. Grim, R. E. 1935. Bull. Am. Cerawb. SOC.14, 113-119, 129-134, 170-176. Grim, R. E. 1939. Ill. Slate Qeol. Survey, Circ. No. 49. Grim, R. E. 1942. J. Geol. SO, 225-275.

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Grim, R. E., Bray, R. H., and Bradley, W. F. 1937. Am. Mineral. 22, 813-829. Grim, R. E., and Rowland, R. A. 1942. Am. Mineral. 27, 746-761, 801-818. Gruner, J. W. 1932. Z. Krist. 83, 75-88. Gruner, J. W. 1934. Am. Mineral. 19, 557-575. Hadding, A. 1923. 2.Krist. 58, 108-112. Hauser, E. A. 1939. Colloidal Phenomenon. McGraw-Hill, New York. Hellman, N. N., and Jackson, M. L. 1944. Soil Sci. SOC.Am., Proc. 8, 135-143. Hendricks, S. B. 1939. Am. Mineral. 24, 529-539. Hendricks, S. B. 1941. J . Phys. Chem. 45, 65-81. Hendricks, S. B., and Alexander, L. T. 1939. Soil Sci. 48, 257-271. Hendricks, S. B., and Fry, W. H. 1930. Soil Sci: 29, 457-479. Rendricks, S. B., and Jefferson, M. E. 1938. Am. Mineral. 23, 851-862. Hendricks, S. B., Nelson, R. A., and Alexander, L. T. 1940. J . Am. Chem. Soc. 62, 1457-1464.

Henin, S. 1937. Compt. rend. 204, 1498-1499. Henin, 8. 1938. fitude physico-chemique de la stabilite structurale des terres. Monograph National Center of Agronomic Research, Paris, 50. Hofmann, U., Endell, K., and Wilm., D. 1933. 2.Krist. 86, 340-348. Hoover, C. D. 1945. SoilSn'. SOC.Am., Proc. 9, 66-71. Hosking, J. 8. 1940. J . Council Sci. Znd. Research 13, 206-216. Humbert, R. P. 1942. Bull. Am. Ceram. SOC.21, 260-263. Humbert, R. P., and Shaw, B. 1941. Soil Sci. 52, 481-487. Jackson, M. L., Mackie, W. Z., and Pennington, R. P. 1946. Soil Sci. SOC. Am., Proc. 11, 57-63. Jackson, M. L., Pennington, R. P., and Mackie, W. Z. 1948. Soil Sci. Soc. Am., Proc. 13. Jeffries, C. D. 1946. Soil Sci. s b c . Am., Proc. 11, 211-212. Jeffries, C. D., and Anthony, R. D. 1948. Proc. Am. SOC.Hort. Sci. 5 1 , 271-286. Am., Proc. 13. Jeffries, C. D., and Yearick, L. G. 1948. Soil Sci. SOC. Jenny, H. 1932. J. Phys. Chem. 36, 2217-2258. Jenny, H., and Ayers, A. D. 1939. Soil Sci. 48,443-459. Jenny, H., and Overstreet, R. 1939s. J . Phys. Chem. 43, 1185-1196. Jenny, H., and Overstreet, R. 1939b. Soil Sci. 47, 257-272. Jenny, H., Overstreet, R., and Ayers, A. D. 1939. Soil Sci. 48, 9-24. Jenny, H., and Reitemeier, R. F. 1935. J. Phys. Chem. 39, 593-604. Jenny, H., and Smith, G. D. 1935. Soil Sci. 39, 377-389. Joffe, J. S., and Levine, A. K. 1947. Soil Sci. 63, 241-247. Johnston, J. R., Browning, G. M., and Russell, M. B. 1942. Soil Sci. SOC.Am., Proc. 7, 105-107. Kelley, W. P. 1927. Proc. First Intern. Congr. Soil Sci. 4, 483-489. Kelley, W. P., Dore, W. H., and Brown, S. M. 1931. Soil Sci. 31, 25-55. Kelley, W. P., Dore, W. H., and Page, J. B. 1941. Soil Sci. 51, 101-194. Kelley, W. P., Dore, W. H., Woodford, A. O., and Brown, S. M. 1939. Soil Sci. 48, 201-255.

Kelley, W. P., Jenny, H., and Brown, S. M. 1936. Soil Sci. 41, 259-274. Kelley, W. P., Woodford, A. 0 , Dore, W. H., and Brown, S. M. 1939. Soil Sci. 47, 175-193.

Kelly, J. B.,and Midgley, A. R. 1942. Soil Sci. 55, 167-176.

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Kroth, E. M., and Page, J. B. 1946. Boil Sci. SOC. Am., Proc. 11, 27-34. Kubiena, W.L. 1938. Micropedology. Collegiate Press, Ames, Iowa, p. 156 Kurtz, T., DeTurk, E. E., and Bray, R. H. 1916. Soil Sci. 61. 111-124. LeChatelier, H. 1887. 2. physik. Chem. 1, 396-402. Lemberg, J. 1876. 2. deut. geol. Ges. 28, 535-547. Low,P.F.,and Black, C. A. 1947. Soil Sci. SOC.Am., Proc. 12, 180-183. Lute, J. F. 1936. Soil Sci. SOC.Am., Proc. 1, 43-45. McAuliffe, C. D., Hall, N. S., Dean, L. A., and Hendricks, S.B. 1947. Soil Sci. SOC. Am. Proc. 12, 119-123. McCalla, T.M. 1945. Soil Sci. 59, 287-297. MacEwan, D. M. C. 1944. Nature 154, 577-578. MacEwan, D.M. C. 1946. J . SOC.Chem. Ind. London 65, 298-306. McHenry, J. R.,and Russell, M. B. 1944. Soil Sci. 57, 351-357. McLean, E.O.,and Marshall, C. E. 1948. Soit Sci. SOC.Am., Proc. 13. Maegdefrau, E., and Hofmann, U. 1937. 2. Krist. 98, 31-59. Marshall, C. E. 1935a. 2. Krist. 90, 8-34. Marshall, C.E. 1935b. 2. Krkt. 91, 433-449. Marshall, C.E. 1936. Science Progress 119, 422-433. Marshall, C.E. 1937. J . Phys. Chem. 41,935-942. Marshall, C. E.,and Bergman, W. E. 1942. J . Phys. Chem. 46, 52-61. Marshall, C. E.,Humbert, R. P., Shaw, B. T., and Caldwell, 0. G. 1942. Soil Sci. 54, 149-158. Marshall, C. E., and McLean, E. 0. 1947. Soil Sci. SOC.Am., Proc. 12, 172-175. Martin, J. C., Overstreet, R., and Hoagland, D. R. 1946. Soil Sci. SOC.Am., Proc. 10, 94-101. Martin, J. P. 1945. Soil Sci. 59, 163-174. Martin, J. P. 1946. Soil Sci. 61, 157-166. Mattson, S. 1930. Soil Sci. 30, 459-495. Mattson, S. 1931. Soil Sci. 32, 343-365. Murphy, H. F. 1939. Hilgardia 12, 341-382. Myers, H. E. 1937. Soil Sci. 44, 331-359. Myers, H. E.,and McCalla, T.M. 1941. Soil Sci. 51, 189-200. Nagelschmidt, G., Desai, A. D., and Muir, A. 1940. J . Agr. Sci. 30, 639-653. Nitzsch, W. v. 1940. Kolloid-2. 93, 110-115. Nitzsch, W. v., and Czeratzki, W. 1940. 2. Pjlunz. Diin. 18, 1-40. Noll, W. 1932. Hydrothermale Synthese des Muscovits. Ein Beitrag zur Frage dw Serieitbildung in Tonschiefern. Fachgruppe IV (Geologie und Mineralogie) Nr 20, Weidmannsche Buchhandlung, Berlin. Noll, W. 1935. Neues Jahrb. Mineral., Geol. A70, 65-115. Noll, W. 1936. Mineralog. u. petrog. Mitt. 48, 210-247. Norman, A. G. 1946. Soil Sci. SOC.Am., Proc. 11, 9-15. Norton, F. H. 1939. J . Am. Cerarn. Soc. 22, 54-63. Olmstead, L.B. 1946. Soil Sci. SOC.Am., Proc. 11, 89-92. Orcel, M. J. 1926. Compt. rend. 183, 565-567. Orcel, M. J., and Caillere, S. 1933. Compt. rend. 197, 774-777. Oulton, T.D. 1948. J. Phys. Colloid Chem. 52, 1296-1314. Page, J. B., and Baver, L. D. 1940. Soil Sci. SOC.Am., Proc. 4, 150-155. Pauling, L. 1930a. Proc. Natl. Acad. Sci. U S . 16, 123-129. Pauling, L. 1930b. Proc. Natl. Acad. Sci. U.S. 16, 578482. Pearson, R. W., and Ensminger, L. E. 1948. Soil Sci. SOC.Am., Proc. 13.

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Peech, M., and Bradfield, R. 1934. A m . Soil Survey Assoc. Bull. 15, 101-106 Peele, T.C.,and Beale, 0. W. 1941. Soil SCi. SOC.Am.. Proc. 6. 176-182. Perkins, A. T. 1948. Soil Sci. 65, 185-191. Peterson, J. B. 1944. Soil Sci. SOC.Am., Proe. 9. 37-48 Peterson, J. B. 1946a. Soil Sci. 61,247-256. Peterson, J. B. 1946b. Soil Sci. 61, 465-475. Peterson, J. B. 1947. Soil Sci. Soc. AWL.,Proc. 12, 29-34. Raney, W. A.,and Hoover, C. D. 1946. Soil Sci. SOC.Am., Pror. 11, 231-237. Ravikovitch, S. 1934. Soil Sci. 38, 219-239,279-290. Ross, C.S.,and Kerr, P. F. 1931. J . Sediment. Petrol. 1, 55-65. Russell, E.W. 1934. Trans. Roy. SOC.London A233, 361-389. Russell, E.W. 1935. Trans. Intern. Congr. Soil Sci., 3rd Congr. Oxford 1, 26-29. Russell, M.B., and Haddock, J. L. 1940. Soil Sci. Sop. Am., Proc. 5, 90-94. Scarseth, G. D. 1935. J. A m . SOC.Agron. 27, 596-616. Schachtschabel, P. 1938. Forschungsdienst 6,319-332. Schachtschabel, P. 1940. Kolloid-Beiheftc 51, 199-276. Schofield, R. K. 1940. Trans. Brit. Cwam. SOC.39, 147-158. Fedletsky, I. D. 1939a. Compt. rend. arad. sn'. U.R.S.S. 25, 207-209. Sedletzky, I. D. 1939b. Compt. r m d . acad. sci. U.R.S.S. 23, 258-262. Sedletzky, I. D. 1939~.Compt. rend. acad. sci. U.R.S.S. 22, 510-514. Sedletzky, I. D. 1940. Compt. rend. acad. #xi.U.R.S.S. 26, 154-155. Sedletzky, I. D.,and Yussupova, S. 1940. Compt. l e n d . nmrl. nri. U.R.S.S. 26, 244246. Shaw, B. T. 1942. J . Phys. Chem. 46, 1032-1043. Sideri, D.I. 1936. Soil Sci. 42, 461-481. Sideri, D.I. 1938. Soil Sci. 46, 129-137,267-271. Sideri, D.I., and Liamina, A. N. 1942. Soil Sci. 54, 83-100. Sieling, D.H. 1946. Soil Sci. SOC.Am., Proc. 11, 161-170. Stanford, G. 1947. Soil Sd.Soe. Am., Proc. 12, 167-171. Stanford, G., and Pierre, W. H. 1947. Soil Sci. SOC.Am., Pror. 11, 155-160. Stauffer, R.S. 1946. J . Am. SOC.Agron. 38, 1010-1017. Stout, P. R. 1940. Soil Sci. Soc. Am., Proc. 4, 177-182. Thompson, H. S. 1850. J. R o y . Agr. Sor. England 11, 68-74. Toth, S. J. 1937. Soil Sci. 44,299-314. Volk, G. W. 1938. Soil Sci. 45, 263-276. Way, J. T. 1850. J. Roy. Agr. SOC.England 11, 313-379. Wear, J. I., Steckel, J. E., Fried, M., and White, J. L. 1948. Soil Sci. 66, 111-117. White, J. L.,and Jackson, M. L. 1946. Soil Sci. SOC.Am., Proc. 11, 150-154. Whiteside, E.P.,and Marshall, C. E. 1944. M o . AQT.Expt. Sta. Research Bull. 386. Wiegner, G. 1935. Trans. Intern. Congr. Soil Sci., 3rd Congr. Ozford 3, 5-28. Wiklander, L. 1946. Ann. Roy. Agr. Coll. Suwden 14. Wiklander, L. 1949. Unpublished. Wiklander, L.,and Gieseking, J. E. 1949. Unpublished. Williams, W. R. 1935. Pedology U.S.S.R. Nos. 5 and 6,755-762. Wilson, H.A.,and Browning, G. M. 1945. Soil Sci. SOC.Am., Proc. 10, 51-57. Wood, L. K.,and DeTurk, E. E. 1941. Soil Sci. SOC.Am., Proc. 5, 152-161. Woodruff, C. M. 1940. Soil Sci. SOC.Am., Proc. 4, 13-19.

Alfalfa Improvement WILLIAM J . WHITE Dominion Forage Crops Laboratory. University of Saskatchewan] Saskatoon. Saskatchewan CONTENTS

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I . Introduction . . . . . . . . . . . . . . . . . . I1. Seed Setting and Production . . . . . . . . . . . 1. Tripping and Its Necessity . . . . . . . . . 2. Self- and Cross-Pollination and Seed Setting . . . . 3 . Tripping and Cross-Pollinating Agencies . . . . . . a . Rain, Wind. Antomatic and Mechanical Tripping b . Tripping Insects . . . . . . . . . . . . . 4 . Factors Influencing Bee Visitation . . . . . . . . . 5 . Soil. Climatic and Vegetat.ive Growth Factors . . 6. Injurious Insects . . . . . . . . . . . . . . a . Lygus Bugs . . . . . . . . . . . . . . . b . Control of Lyguw Bugs . . . . . . . . . . c . Other Insects . . . . . . . . . . . . . . I11. Progress in Methods of Breeding . . . . . . . . . 1. Breeding Characteristics . . . . . . . . . . . 2 . Utilizing Hybrid Vigor . . . . . . . . . . . . 3. Methods of Testing for Combining Ability . . . . . 4 . Selection Procedures for Certain Characteristics . . . IV . Conquering Some Diseases . . . . . . . . . . . . 1. Bacterial Wilt. . . . . . . . . . . . . . . . 2. Black Stem . . . . . . . . . . . . . . . . V . Summary and Conclusions . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .

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I . INTRODUCTION Medicago sativa L., known by its Arabic name. alfalfa. in the United States and Canada but commonly called lucerne in other parts of the world. is generally regarded as one of the world’s most valuable cultivated forage crops . Few if any crops are equal to it in capacity to produce heavy yields of highly nutritious palatable feed . The excellent soil-improving ability of the crop is also generally recognized . A combination of desirable attributes as a forage plant and adaptation to a wide diversity of soil and climatic conditions has led to the use of alfalfa in the world to an extent probably exceeding th a t of any other single 205

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legume or grass species. It is utilized as a cultivated crop on every inhabited continent and in many countries extending from near polar regions to the tropics. With such a wide distribution and use under extremely diversified environmental conditions the problems of production and utilization are many and varied. Some problems are more or less local or regional in nature, such as cold resistance or soil nutrient deficiencies, while others, of which seed sett.ing is a good example, are more universal in occurrence. Diseases and insect pests are universal problems. Solution of some of the problems by cultural or management practices or by breeding better varieties has already resulted in expanded utilization of the crop. Further expansion and increased production will undoubtedly follow as research on the factors limiting production and utilization establishes ways and means of elimination or control of the problems. Investigations involving alfalfa cover a wide diversity of subjects and the literature is indeed voluminous. Consequently in the preparation of this review limitation of space necessitated a choice between a sketchy coverage of many topics or a more comprehensive consideration of a few selected phases. The latter alternative was chosen. The subjects selected are those on which there has been rather extensive investigation and noteworthy advances in the past decade, but by no means does the selection of subjects represent only those fields in which recent advances have been made. Other recent or fairly recent reviews, however, have dealt with topics not covered in this review. Atwood (1947) has summarized the cytogenetic literature on the crop. Klinkowski (1933) has reviewed the early and modern history of the distribution and utilization of the crop in the world. An abstract review of the alfalfa literature for the period 1925 to 1930 covering several subjects has been presented by the Imperial Bureau of Plant Genetics: Herbage Plants (1931). Tysdal and Westover (1937) have dealt with earlier improvement work.

11. SEEDSETTINGAND PRODUCTION Alfalfa is notoriously erratic in respect to seed production. I n many extensive areas where the crop is widely utilized for hay and pasture the yields of seed are so low and undependable that practically no acreage is devoted to seed production. Thus dependence for a large portion of the seed requirements of a region, nation, or continent generally falls upon relatively few, often rather restricted areas where for some reason or reasons yields are comparatively dependable. Stewart (1926) emphasizes this fact by stating that from “80 to 90 per cent of all alfalfa seed in North America is grown in eleven areas. Six of these are small

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and concentrated; the remaining five are more extensive but production of seed is less intensive.” Even within major seed growing areas violent interannual and interfield yield fluctuations occur. I n Utah, for example, in 1926, the production amounted to 20,000,000 lbs., but in each of several recent years it has only been about 4,000,000 lbs. (Tysdal, 1946). Investigations over the past several decades have served to reveal the multiplicity of factors influencing seed setting and seed yield, and contributing to the variability from area to area, field to field, and year to year. T o understand and interpret the role of various factors it has been necessary first to gain a knowledge of the biology and functioning of the alfalfa flower. To this fundamental information the influences of soil, climate, beneficial and injurious insects, disease, and management practices can be added. 1. Tripping and I t s Necessity

The anthers, anther filaments, stigma, style and ovary, collectively called the staminal or sexual column, are enclosed by the two keel petals which are united along one edge and held firmly together along the other two free edges. The filaments of nine of the ten anthers are united t o form a tube which practically surrounds the ovary and style and exerts a strong forward pressure. Whenever a force separates the two keel petals even slightly along their free edges, the restraining mechanism is released and the staminal column is violently snapped (tripped) forward from the pressure exerted by the tube. Upon tripping the upper end of the staminal column makes a strong impact with the standard (banner) petal and comes to rest on it several degrees from the original upright position. The process of release of the staminal column from the keel is known as tripping. Although tripping has been observed for many decades the fundamental nature of the process to seed setting has been a matter of controversy even fairly recently. Carlson (1935) and Brink and Copper (1936) maintained that a considerable proportion of flowers set seed without tripping. Recently Tysdal (1946) drew attention to the fact that t.he procedure used by Brink and Cooper was open to question. Ufer (1932), Armstrong and White (1935), Hadfield and Calder (1936), Knowles (1943), and Tysdal (1940, 1946) concluded that a t most only a very small percentage of flowers set pods without first tripping. Both Tysdal (1940) and Knowles (1943) report on detailed observations covering many individual flowers on large populations of p1ant.s over extensive periods of time and a variety of soil and climatic conditions. Their data show that about 1 per cent of untripped flowers may set pods. These observations and conclusions are further supported by the high

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correlations found between percentage of flowers tripping and setting pods (Tysdal 1940, 1946; Knowles 1943). Vansell and Todd (1946) observed one plant on which 10 per cent of the flowers examined had the sexual column growing out of the top of the keel. Carlson (1946) recorded some pod setting without tripping and by histological examination established that pollen tubes and embryos were present in 13 of 84 untripped flowers. He, however, considered that the occurrence of pod setting without tripping was not high. Tysdal (1946) pointed out that it was possible to select rare plants in which tripping was unnecessary for pod setting. The progeny of one such plant was in fact included in the study reported by Knowles (1943). The accumulated masr of evidence establishes the fact that tripping is almost an obligatory requisite to seed setting. The extent to which tripping occurs is consequently the fundamental factor in determining seed setting and seed yield. As Tysdal (1940) states "although tripping will not insure seed production at least seed will not set to any great extent without tripping.'' The essential function which tripping performs in rupturing the stigmatic membrane has been shown by Armstrong and White (1935). They established that in untripped flowers a membrane covering the stigma retained the stignlatic fluid. Upon tripping, the impact of the stigma against the standard petal or other obstacle ruptured the membrane and released the fluid thus inducing pollen germination. Occasionally the membrane may rupture in untripped flowers as indicated by the observations of Vansell and Todd (1946) and Carlson (1946), referred to above. Seed setting wit.hout tripping results in self-pollination, the consequences of which will be discussed in Section 111-1. Conversely while tripping does not insure cross-pollination it is essential for its occurrence. 8. Self- and Cross-Pollination and Seed Setting

The functioning in fertilization of pollen from the same plant is known as self-pollination as contrasted to cross-pollination which involves the functioning of pollen from another unrelated plant. As early as 1914 Piper et al. showed that cross-pollination resulted in more seeds than self-pollination. Investigations reported by Hadfield and Calder (1936), Tysdul (1940), Cooper and Brink (1940), Jones and Olson (1943), and Bolton (1948) all have shown that on the average crosspollination results in a t least three to four times as much seed as does self-pollination. These studies have revealed that the higher seed yield upon crossing is due to the combined effect of a higher proportion of flowers sett.ing pods and a larger number of seeds per pod.

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Plants vary widely in the extent to which they will set seed upon selfing. Tysdal and Kiesselbach (1944) have shown th a t the interplant variation in percentage of flowers setting pods upon selfing ranges from 0 to 100 per cent. Bolton and Fryer (1937) present data showing a similar range. When, however, the number of seeds per pod is taken into account there seems to be no reported case in the literature of a plant which sets an equal or higher amount of seed on selfing than on crossing. I n the general population of plants as indicated above crosspollination results in a markedly higher seed yield. The explanation for the higher seed setting upon crossing as contrasted to selfing was established by Cooper and Brink (1940). They conducted a very detailed study of the progress of pollen tube growth, fertilization, and embryo development in seven plants. Upon selfing 14.6 per cent of the ovules were fertilized as compared to 66.2 per cent upon crossing. Restricted pollen tube penetration of thc ovary and failure of pollen tubes to enter the ovules accounted for the low percentage of fertilized ovules on selfing. Furthermore they found that 34.4 per cent of the fertilized ovules collapsed in the selfed series within 144 hours after pollination whereas only 7.1 per cent collapsed in the crossed series. I n their study the combined effect of these two factors resulted in about 5.5 times as much seed setting on crossing as on selfing. They concluded that “one of the basic phenomena involved in reproduction in alfalfa is partial self-incompatability.” A study by Brink and Cooper (1939) revealed that the rate of endosperm development was significantly higher on crossing than on selfing. They postulated that compet.ition for nutrients occurred between the inner integument and the developing endosperm, and that when the growth rate of the latter was slow, as it is on selfing, the balance in the competition was tripped in favor of the integument which resulted in a hyperplasia in the latter tissue and in time terminated the ovule development. Ovule collapse due to this course of events was termed somatoplastic sterility by these authors. Structurally and functionally the alfalfa flower is thus adapted to tripping and cross-pollination. The extent to which seed setting is dependent on tripping has been shown in the previous section of this paper. While in a random population of plants seed setting will take place to a certain degree from self-pollination, yet high seed setting is dependent upon a high cimount of cross-pollination. The extent to which crossand self-pollinstion occurs under natural field conditions will be discussed more fully in Section 111-1. Briefly, however, it has been shown that the crop is naturally cross-pollinated to a high degree.

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3. Tripping and Cross-Pollinating Agencies a. Rain, Wind, Automatic and Mechanical Tripping. Tripping may

be induced by a number of factors. The role and relative importance of wind, rain: temperature, insect activity, and mechanical treatment have been the subject of a number of investigations. Knowles (1943) and Tysdal (1946) noted that during rain a certain amount of tripping occurred. Tysdal (1946) showed that the extent varied with the intensity of the rain but as an average of five rains only 8.3 per cent of the flowers were tripped. B y sprinkling to simulate rain and artificially tripping Tysdal (1946) demonstrated that sprinkling materially reduced pod setting. Sprinkling, then tripping followed by sprinkling, a sequence of events similar to rain tripping, resulted in only 21 per cent as much pod setting as did tripping with self-pollination in the absence of sprinkling. The above sprinkling treatment gave only 14 per cent as much pod setting as did cross-pollination without sprinkling. Knowles (1943) also observed that rain tripping resulted in low pod setting. In tripping induced by rain no provision is made for crosspollinat.ion and, therefore, the number of seeds per pod set is low. As a consequence of the low percentage of rain tripped flowers which set pods and the low number of seeds per pod, which is likely to result, tripping by rain is undoubtedly of insignificant importance in seed production. Wind action also appears to be of very minor significance as a tripping agent. Tysdal (1946) records that numerous observations have failed t o show any appreciable degree of tripping due to this factor. Knowles (1943) found that no correlation existed between wind velocity and percentage tripping. I n common with rain tripping, there is lit.tle or no opportunity for cross-pollination following wind tripping. The occurrence of automatic (self) tripping has been frequently observed over a considerable period of time. Armstrong and White (1935) and Wexelson (1946) have concluded that a high amount of automatic tripping occurred. By excluding tripping insects by means of screen cages, paper or cotton bags, however, it has been possible to measure the extent of automatic tripping. Knowles (1943) reported that 26 per cent of the flowers inside cages set pods as compared to 55 per cent of flowers of the same plank outside cages. It should be noted that a number of the plants included in this study were selected for ability to trip automatically. Tysdal (1940) found that from 2 to 4 per cent of flowers inside nainsook cotton bags set pods as contrasted to 15 to 35 per cent outside. Hughes (1943) observed 5.4 per cent of flowers setting pods inside of cages. Lejeune and Olson (1940), Silversides and Olson (1941), and Vansell and Todd (1946) reported very low pod setting

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inside cages. Carlson (1946) found from 4.6 to 12.1 per cent of flowers setting pods in paper bags and Tysdal (1946) reported an average of 7 per cent pod-setting one year and 5.8 per cent the next year inside paper bags. Vansell and Todd (1946) also gave data showing a low pod setting inside cages as compared to outside. Furthermore, individual flower histories based on frequent or continuous observation have been reported by Tysdal (1940) to show a low incidence of automatic tripping. While about 5 per cent of flowers setting pods due to tripping of this nature is of some consequence in seed setting and yield it is of relatively minor significance in relation to the potential pod setting when tripping insects populations are adequate to trip 70 to 100 per cent of the flowers. Although in the general population t.he incidence of automatic tripping is low, certain individual plants may be found which trip freely (Armstrong and White, 1932; Carlson, 1946). Tysdal (1946) states that less than 1 per cent of the population have this characteristic. Tysdal 1942, 1944, 1946) has repeatedly stressed that highly self-tripping-self-fertile plants are undesirable for use in a breeding program because of the high degree of selfing which occurs and the resulting depressing effect on the progeny yield. The work of Stevenson and Bolton (1947) with such plant material has borne out Tysdal’s contention. Theoretically automatic tripping could result in cross-pollination, assuming that the pollen was wind borne or deposited by insects and lodged on t.he standard petals of untripped flowers. Hadfield and Calder (1936) found an average of 28.7 pollen grains per square inch on greased slides. Unpublished studies a t Saskatoon have shown that comparatively little pollen is wind-borne and has indicated that the quantity of pollen adhering to standard petals is totally inadequate to effect cross-pollination to any significant degree. Knowles’ (1943) data showing that pods set outside of cages contained twice as many seeds as pods set inside cages from automatic tripping is evidence that automatic tripping results in selfing. He showed further that in a random lot of 17 plants under field conditions hand tripped flowers, which would correspond to flowers automatically tripped, set 0.42 seeds per flower tripped as compared to 2.55 seeds per flower tripped by bees on t.he same plants. This differential corresponds closely with that shown to exist upon self- as compared to cross-pollination. Acceptance as a fact that rain, wind and automatic tripping result very largely in self-pollination and that under natural conditions a high degree of cross-pollination occurs, evidence of which will be discussed in a later section, leads to the conclusion that there is a low incidence of tripping from the above causative factors under average field conditions.

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In the absence of tripping and cross-pollination from other caiises the seed set and seed yield therefore will be low. Recognition of the necessity of tripping and the general deficiency of tripping agents has led to the investigation of the effectiveness of mechanical tripping by such mean3 as ropes, chains, harrows or specially constructed devices drawn through the fields. Hadfield and Calder (1936) reported that with the peveral implements tried the results were negative. Silversides and Olson (1941) showed that while tripping was increased the seed yield was not increased by mechanical treatment. Undoubtedly the explanation of the failure of mechanical manipulation lies in the fact that the indeterminate type of flowering habit of the crop necessitates repeated treatment and also that any treatment severe enough to induce tripping causes considerable injury (Silversides and Olson, 1941; Jones and Olson, 1943). In addition no mechanical device so far evolved mekes provision for cross-pollination. b. Tripping Insects. Some of the earlier and many of the more recent investigations have produced a wealth of data showing the fundamental part which insects play in tripping. The higher tripping and pod setting which occurs outside as compared to inside cages and bags is evidence of the part which insects play. Knowles (1943) reported 0.11 seeds per flower observed inside and 0.90 seeds per flower observed outside cages. The presence of bees outside and their exclusion inside the cages is the major treatment difference in his study. From detailed observation of individual flowers Tysdal (1940) concluded that relatively little tripping occurred except from insect activity. Further evidence of the important role of insects in tripping is found in the high positive correlations between percentage tripping and population of tripping bees (Knowles, 1943; Peck and Bolton, 1946). The latter authors found the multiple .78one correlation between tripping and population of all bees to be .63 the next year, bot,h of which values were highly signifiyear and cant. The significance of bees in tripping has seemed almost incredible to many since frequently their numbers appear very low. Actually many investigators have agreed that the bee populations are inadequate and have concluded that low seed yields are the consequence. When it is realized, however, that several efficient tripping species visit from 10 to 20 flowers per minute (Knowles, 1943; Peck and Bolton, 1946; Linsley, 1946; Vansell and Todd, 1946; Linsley and MacSwain, 1947), and trip 80 to 100 per cent of the flowers they visit, their role and importance can be more fully appreciated. With suitable weather for their activity over a period of time on plants in a thrifty condition and in the absence of insects or disease destruction of buds, flowers, pods or seed, a relatively

+

+

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few bees can be responsible for a considerable amount of seed. Knowles (1943) estimated that one bee working for 100 hours during the flowering season could effect sufficient tripping and cross-pollination to set one pound of seed. Yet 200 t o 300 bees dispersed aver an acre could be almost unnoticed to the casual observer. Tripping bees are undoubtedly of fundamental importance as crosspollinating agents as well. In the act of tripping the stamina1 column generally strikes them and pollen is deposited on their bodies and is transferred from flower to flower. Their habits in respect to concentrating on the flowers of one raceme or one plant as contrasted to skipping rapidly between racemes and plants may influence the degree of crosspollination. Intergeneric differences in the working habit of bees have been noted bv Linsley and MacSwain (1947a) and Vansell and Todd (1946), and have been considered to be a possible factor influencing the extent of cross-pollination. While a wide variety of insects visit alfalfa flowers i t has been observed and generally accepted that only those in search of pollen are instrumental in tripping to any appreciable extent. Many nectar gatherers can attain their end without disturbing the flowers enough to cause tripping. A few large insects, such as some of the bumble bees, may occasionally induce tripping apparently by their weight or clumsiness. Butterflies, thrips, and flies, while often present, are generally conceded to be “unable to trip or at the most very unimportant as trippers” (Linsley, 1946). A wide variety of pollen-collecting bees are recognized R S being primarily responsible for tripping. The species of bees which are found tripping varies widely from area to area and even from field to field within a relatively small area. Linsley (1946) and Linsley and MacSwain (1947a) reported that in California the following species tripped alfalfa flowers: leaf cutter bees (Megachile s p . ) , bumble bees (Rombus sp.), alkali bees (Nomia sp.), metallic sweat bees (Agapustemon s p . ) , true sweat bees (Halactus sp.) and (Lasioglussum sp.) , cotton bees (Anthidium sp.) , osmiine bees (Diceratosmia sp.) , long horned bees (Melissodes s p . ) , anthophorid bees (Exornolapsis s p . ) , furred bees (Anthophora sp.) , carpenter bees (Xylcopa sp.), and honey bees (Apis inellifera L.). Tysdal (1946) lists the following additional genera : Auguchlora, Andrenids, and Calliopsis. Crandall and Tate (1947) drew attention to the efficiency of species of the latter genera. Peck and Bolton (1946) reported in addition Osmia s p . , Coelioxys sp.; and Psithyrus as of some value as trippers. Tysdal (1946) also noted that the soldier beetle (Chauliognathus basalis) had been observed to trip flowers. The leaf-cutter bees are widely distributed in North America (Tysdal,

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1946; Knowles, 1943; Peck and Bolton, 1946; Linsley, 1946). Bumble bees are also widely distributed, although Tysdal (1940) considers them to be more important in the eastern United States than eleswhere. Nomk sp. were reported by Tysdal (1940) to be partirularly important, pollinators in Wyoming, Idaho, Utah and Oregon. The crop apparently is attractive to certain species of a genus and not to others of the same genus. Peck and Bolton (1946) reported that certain leaf-cutter species were not found in alfalfa fields. Between genera and also between species within genera there are marked differences in speed of flight, rate of flower visitation, efficiency in tripping, rapidity of transfer from raceme to raceme and plant to plant, length of working day, etc., as shown in various aspects by studies reported by Tysdal (1940,1946),Knowles (1943), Peck and Bolton (1946), Linsley (1946), and Linsley and MacSwain (1947a). The above and possibly other considerations complicate the evaluation of the variour: species as tripping agents. However, more intensive research on wild bee populations would appear to be warranted. Interannual fluctuations in populations of wild bees in alfalfa fields have been observed in California by Linsley and MacSwain (1947) and in Saskatchewan by Knowles (1943) and Peck and Bolton (1946). The importance of the honey bee in tripping has been one of the most controversial topics. Tysdal (1940, 1946), Knowles (1943), Peck and Bolton (1946), Linsley (1946), Wexelsen (1946), Akerberg and Lesins (1946) and Harrison et al. (1945) have reported them as being frequently present in very large numbers but effecting little or no tripping. Lejeune and Olson (1940) noted that over a period of 2 days a relatively small number of honey bees tripped up t,o 28 per cent of the flowers visited, but the following day 16 bees observed failed to trip a single flower, and no honey bee tripping was observed for the balance of the season. On the other hand, Hare and Vansell (1946) and Vansell and Todd (1946) have shown the honey bee to be an important. tripper in the Delta area of Utah. Knowlton and Sorenson (1947) have also stressed their value in Utah. I n contrast to their 1945 studies Linsley and MacSwain (1947a) considered the pollen-collecting honey bees to be of major importance in 1946 in California. Alfalfa plants in cages in which honey bees have been confined have shown considerable seed setting (Hadfield and Calder, 1936; Dwyer and Allman, 1933). The confusion in respect to the value of the honey bee probably is due largely to the diversity of ecological and environmental factors in various areas. The most important single ecological factor is undoubtedly the abundance of competing preferred pollen sources for the bees. I n California Linsley and MacSwain (1W7a) have concluded “that of

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all bees important in alfalfa pollination in these areas the honey bee is most readily diverted from alfalfa by this particular series (sweet clover, mustard, carrot, tamarisk, sunflower, blue curl, and arroweed) of competing pollen plants.” I n contrast these authors recorded that alfalfa is the preferred nectar source both for wild and honey bees. Ecological, environmental and humanly controlled factors influence the abundance of competing pollen sources geographically and seasonally, and contribute to the lack of agreement on the value of honey bees as trippers. The desirability of utilizing honey bees for tripping and cross-pollinating has occurred t o many since their populations are controlled so readily by man. The primary problem in so doing, as indicated above, is t o force them to forage for pollen on the crop. I n areas where conditions lend themselves to reduction or elimination of competitive sources by the use of selective herbicides, mowing, or by other means, the possibility seems to warrant further investigation. The further possibility exists of influencing pollen collection by manipulation of the pollen supply in the colony by means of pollen traps (Rubnev, 1941). However, Linsley and MacSwain (19474 have indicated that such treatment, through adverse effects on brood development, may defeat its purpose.

4. Factors Influencing Bee Visitation Competing pollen sources have already been cited as influencing the visitation of honey bees. This has been shown by the work of Linsley (1946), Linsley et al. (1947a), Hare and Vansell (1946), and Vansell and Todd (1946). The plant species involved vary with the area and the season and need to be determined for each locality. The preference of wild bees for certain plants other than alfalfa has been observed by Knowles (1943), Peck and Bolton (1946), Vansell and Todd (1946), and Linsley and MacPwain (19474. The desirability and possibility of reducing or eliminating competition has been pointed out by these authors. Its beneficial effect was demonstrated by Linsley and MacSwain (1947a). It remains as a possible practical effective means to be more fully explored. In eliminating or reducing the competitive flora Peck and Bolton (1946) and Linsley and MacSwain (1947a) have drawn attention t o the necessity of providing food sources for bees during those seasons of the year when alfalfa is not in flower. This demands a more thorough knowledge of the life cycle and nesting habits of many bees than is now available. Proximity of nesting sites to fields may be of importance in visitations, as has been shown by Vansell and Todd (1946) in the case of the alkali bee. I n the case of one field adjacent to nesting sites they

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estimated there were 14,520 bees per acre and noted that even the partly unfolded flowers were being tripped. This raises the question of the possibility of artificial propagation of wild bees in or adjacent to fields, and also the effect of culturak and irrigation practice on insect populations. Peck and Bolton (1946) have demonstrated t.he possibility of attracting certain leaf-cutter bee species to holes drilled in logs and have cited references on the successful propagation of bumble bees. Bohart (1947) records that bumble bees can be induced to nest in artificial domiciles and considers that establishment and transfer should be possible. Crandall and Tate (1947) described the nesting sites used by Calliopsis sp., and indicated the possibility of encouraging them to nest in and around fields. Linsley (1946) described tthe nesting sites of many species he observed, and drew attention to the possible effect of cultivation and irrigation practices. Of the wild bees, all of those so far reported as trippers, except bumble bees, are solitary and it would seem that propagation of tIiem would be more difficult than that of the colonial bumble bee. Individual alfalfa plants have been noted to differ very markedly in their attractiveness to wild bees (Knowles, 1943; Vansell and Todd, 1946). The latter authors stated that no plant differences in attractiveness to honey bees had been observed. The reason for the differences in attractiveness have not been explained. It may involve quality or quantity of pollen or nectar. An intervarietal difference in sugar content of nectar has been recorded by Vansell (1943). Linsley and MacSwain (1947a) point out that pollen-collecting bees require nectar to supply their body needs. Therefore nectar quantity and quality conceivably could be of flignificance in attractiveness. I n breeding the crop this characteristic seems to warrant considerat.ion as a possible means of improving seed yield. Soil moisture level has been shown to influence the sugar concentration of the nectar and its attractiveness to bees. Vaneell (1943) found a range in nectar sugar concentration of from 11 to 38.3 per cent in plants growing on wet and dry soil respectively. Vansell and Todd (1946) also noted that a wide difference in sugar concentration was associated with soil moisture level. Their data on honey bees showed that the population of nectar collectors was positively correlated with degree of succulence, but that, in the case of pollen collectors, a negative correlation existed. In cases of production under irrigation, within limits, succulence may be controlled, and the above cited evidence indicates that it may be of significance in influencing pollen collection. Temperature is obviously a dominant governing factor in bee activity and foraging. There is some evidence that relative humidity is also of

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significance. Temperature and possibly humidity affect the ease of tripping (Hughes, 1943; Tysdal, 1946), and thus exert a dual influence. In Nebraska Tysdal (1940) noted a marked increase in number of flowers visited and tripped as the temperature rose from 70 to 100" F. Tysdal (1946) related maximum temperatures and minimum humidity to percentage of flowers forming pods. H e concluded that low maximum temperatures and high minimum humidity during the seed setting period resulted in a low percentage of flowers forming pods. He noted t h a t during cool, wet weather insect activity invariably came to a halt. Knowles (1943) established that a highly significant positive correlation existed between Fercentage of tripping and temperature, Linsley and MacSwain (1947a) also established a positive relationship between temperature and low humidity and insect activity, but their observations showed that above a certain temperature and below a certain relative humidity further changes in these climatic factors had a depressing effect on populations of both nectar and pollen collectors. Certain species of bees are influenced to a greater degree by temperature than others. The leaf-cutter bee has been noted by Tysdal (1940, 1946) and Peck and Rolton (1946) to cease actsivity at higher temperatures than bumble bees. Pollen-collecting honey bees have been shown by Vanseli and Todd (1946) to work a t lower temperatures than leaf-cutters. While intergeneric differences in this respect exist, humidity and particularly temperature are nndoubtedly the dominant factors in the activity of all species. Competition between species of bee may in certain circumstances determine the visiting species. Vansell and Todd (1946) have recorded a case where the Nomia population was so high that honey bees were not present in the field even although a large apiary was nearby. They also found bumble bees disappearing as Nomia became abundant. I n general, however, the poplation of any one species is not sufficiently abundant to provide severe competition, and various species usually work the same field and the same plant in apparent harmony. The relationship between the visitation of bees and the control of injurious insects by DDT and other insecticides needs further clarification. Vansell and Todd (1946) have shown that tripping was higher than elsewhere on plots in which lygus and thrips were controlled. Linsley and MacSwain (1947a) established that dusting when the crop was in bloom caused an immediate decrease in population, and that 3 or 4 days were required for the population t o build up to the predusting level. It is possible that dusting in the prebloom stage would control the injurious insects without affecting the beneficial species. This topic will be discussed further under Section II-6- (b) .

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6. Soil, Climatic and Vegetative Growth Factors

Soil fertility may be a limiting factor in seed setting and production by stunting growth and limiting flower production a t low fertility levels, or by stimulating excessive vegetative growth and lodging a t high levels. Applications of sulfur-bearing fertilizers have been shown by Bentley and Mitchell (1946) t o result in heavy increases in seed yield on sandy soils in northern Saskatchewan. Without fertilizer vegetative growth was very unthrifty. Boron deficiency has been shown by Piland et al. (1941, 1944) and Grizzard and Mathews (1942) to be a limiting factor in seed production in the southern United States. At high fertility levels, when moisture is not limiting growth, lodging frequently occurs. The work of Tysdal (1946) demonstrated the depressing effect of this plant condition on seed yield. The highest seed yields are obtained between the extremes of fertility level. Drought sufficient to inhibit vegetative growth seriously also inhibits reproductive development and seed yield. Grandfield (1945a) concluded that soil moisture somewhat below the optimum for best vegetative growth was most conducive to seed setting. I n a greenhouse study Tysdal (1946) showed that the highest seed yield was secured a t the highest moisture level, and concluded that moisture itself actually increased rather than suppressed the inherent seed setting capacity. I n a field test Tysdal (1946) showed that, when plants were widely spaced, high soil moisture did not depress yield, but that in a close-spaced planting a high moisture level did depress yield. In a previous section it has been shown that bee activity, incidence of tripping, and ease of tripping increase as temperature increases. Grandfield (1945a) has further established that temperature, independently of these other factors, affects the physiological process of reproductive development. He found t.hat the percentage of tripped selfpollinated flowers setting pods increased as the temperature increased from 60" to 80"F., and then declined somewhat up to 100°F., above which it dropped off drastically and failed a t 120°F. H e pointed out, however, that plants could be hardened to high temperatures, after which 120°F. was not the upper limit for pod development. Sexsmith and Fryer (1943) found a linear relationship between pollen tube growth and temperature. At 50°F. no pollen germination occurred. The influence of relative humidity on reproductive development was studied by Graudfield (1945a). Pod setting of tripped self-pollinated flowers was not significantly influenced by relative humidities of from 10 to 50 per cent, but at 70 and 90 per cent a highly significant reduction was found.

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Grandfield (1945a) investigated the effect of organic reserves on seed setting, and found that high organic reserves resulted in increased seed production. The greatest influence of reserves occurred when the soil moisture was low. Summing up his studies on organic reserves, soil moisture, temperature and humidity, Grandfield concluded that “moderate air temperature, low humidity and soil moisture below optimum produced the type of vegetative growth of alfalfa plants that was conducive to storage of high organic reserves, resulting in a physiological condition favorable to seed setting.” I n an extensive study of the effect of lodging, Tysdal (1946) found that upright plants produced from 2 to 10 times as much seed as artificially lodged plants. Lodging reduced seed setting most markedly in the thicker plantings and with heavy watering. I n explanation of these results Tysdal considered that the sparse bee population preferred the upright growth, while the injurious insects preferred the lodged growth, and also that possibly the heavy new growth following lodging may have diverted the necessary nutrient supply away from the lodged growth. 6. Injurious Insects

Alfalfa, like the majority of other crops, is host to a number of injurious insects. All are unquestionably harmful to some degree but certain species are particularly serious pests because of their widespread occurrence, generally heavy infestation, and the heavy damage they cause. Lygus spp. in relatively recent years have been shown to be particularly deleterious, and t.his review will deal mainly with them. The species of Lygus have been assigned various common names. There has been a trend, however, now generally adopted in the literature, t o use the generic name as the common name and that nomenclature will be followed herein. Adelphocoris spp. as they affect seed setting and yield have been studied in detail by Hughes (1943a, 1943b). In respect to type of damage done, nature of damage, and possibly to methods of insecticidal control, members of this genus closely resemble Lygus s p p . In most seed growing areas it appears that lygus populations considerably overshadow those of Adelphocoris sp. and thus, assuming equal or nearly equal effect per insect, lygus are generally more serious. a. Lygus Bugs. I n North America, alfalfa is a host plant, of a t least three lygus species, namely, Lygus hesperus Knight, L. elisus Van Duzee, and L. oblineatus (Say). According to Stitt (1940) , the latter species is common in the eastern United States but also occurs in California and Arizona. The two former species apparently predominate in the Western United States and Canada (Sorenson, 1939; Stitt, 1940; Salt, 1945;

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Bolton and Peck, 1946). I n respect to symptoms, nature and type of damage, population trends, etc., the various investigators have generally not made any distinction between species. The species therefore will be treated collectively in the following sections. It should be pointed out, however, that Stitt (1944) has demonstrated that lygus species differ in respect to the degree of damage they do. Detailed life histories and descriptions of the species are given by Sorenson (1939) and Stitt (1940) and will not be reviewed here. Although alfalfa is one of the preferred plants, lygus have a very wide range of hosts among both cultivated and noncultivated plants and are able to feed on most succulent plants. Sorenson (1939) states that in Utah the insect has been collected from “nearly all field, truck, nursery, and orchard crops; from various ornamentals and most flowers; from meadows and other grasslands; from many nat.ive plants and introduced weeds.” Bolton and Peck (1946) found it rare on oats, barley, flax, and peas, but common on lambsquarters (Chenopodium album L.). The native flora or cultivated crops thus harbor the pest, and from there it can invade newly established alfalfa. Carlson (1940) records a case of a field sown on virgin land with no known alfalfa within a 30-mile radius, which, in the year after seeding, had a lygus populat,ion as high as found in repiesentative fields in older seed-producing regions. The general distribution of the numerous hosts complicates the problem of control. Intra-annual lygus population trends in alfalfa fields have been studied by Sorenson (1939)) Stitt (1940, 1941), and Smith and Michelbacher (1946). There is general agreement, that with the advent of spring the population is a t the lowest level and is comprised entirely of adults. From this low point numbers increase rapidly, reaching a peak a t full bloom or shortly thereafter. Cutting the crop results in a substantial reduction in population, after which it builds up again. When the crop is not cut throughout most of the growing season Sorenson (1939) states “that favorable conditions are provided for uninterrupted reproduction.” Smith and Michelbacher (1946) , however, showed that a marked population decline occurs after the full bloom stage of the crop. These latter authors point out that variations in moisture and temperature influence the speed of the build-up by affecting the host and the insect. Populations may be influenced materially by migration, such as from cut to uncut fields or portions of fields. Population intensity may reach as high as 20 to 50 adults and nymphs per sweep of a standard insect net. Populations per field or region may vary widely from year to year (Sorenson, 1939: Stitt, 1941; Bolton and Peck, 1946). The symptoms of damage are: a whitish-yellow appearance of the

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tops of plants due to the presence of blasted racemes of buds; the rosetting of the racemes of buds which are generally discolored; dwarfing of plants ; excessive branching and appearance of stringiness; presence of discolored and shrunken seeds. Lack of pod setting, while not a specific symptom of lygus, is nevertheless a symptom (Sorenson, 1939; Stitt, 1940, 1941; Carlson, 1940; Jeppson, 1946). The symptoms are obviously expressions of damage caused by the insect. Stitt (1940) states that lygus prefer buds, flowers and tender terminal parts of the plant. Flower buds are favored for feeding and egg laying. Buds turn white and fail to develop when lygus feeds on racemes of buds in the pre-elongation stage. Complete racemes of buds may be damaged. Sorenson (1939) showed that caging 1 bug per 40 buds for 5 days resulted in 81.82 per cent of the buds being destroyed. Bud blasting is responsible for the whitish appearance of the tops of plants when populations are high. Flower fall may result from lygus feeding as well as from failure of fertilization or other causes. With controlled infestations Sorenson (1939) established that, as the population of bugs increased in relation to the flower population, the amount of flower fall increased. Stitt (1940) also found a high positive correlation between percentage fall and lygus population, and observed that with high insect populations over one-half the flowers normally expected to seed pods were lost. Sorenson (1939) likewise showed that there was a relationship between bug population and loss of pods. Brown shrunken seeds occur as a consequence of the feeding of the insect upon the pods (Sorenson, 1939; Stitt, 1940, 1944; Carlson, 1940; Smith and Michelbacher, 1946; Bolton and Peck, 1946). The, extent of seed damage depends upon the population and the amount of shrivelling depends upon the stage of seed development at which feeding occurs. Seed damage may reach high proportions under natural field conditions. For example, Bolton and Peck (1946) recorded that in one field 52 per cent of the seed was brown and shrunken. As an average from several fields the latter aut.hors showed that the very light fraction of brown seed had a germination capacity of only 3 per cent while the damaged but heavier fraction germinated 48 per cent. Reduction in the growth rate due to feeding by the insect has been demonstrated by Sorenson (1939), Carlson (1940) and Jeppson (1946), and accounts for the characteristic stunting which occurs under heavy infestation. More profuse branching is commonly displayed by damaged plants. Crinkled and misshapen leaves were observed by Carlson (1940) and Jeppson (1946). The former investigator recorded that 21.8 per cent

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of the leaves of plants infested under control displayed this characterifitic as contrasted to 1.8 per cent on lygus-free plants. Histological studies conducted by Carlson (1940) and Jeppson (1946) on buds, flowers, and bud initials have revealed the nature of the damage. The point and path of penetration of the mouth part of the insects was traced in some instances, although in others it was obscure, there being as a rule little or no disintegration of the tissues along the path of penetration. At the point of feeding, however, relatively large areas of disintegration were apparent, in some cases a considerable part of the ovary being involved apparently as a consequence of one feeding. Both authors considered the disintegration to be due in part to mechanical injury from the act of feeding, and also to the secretion by the insect of a toxic or irritating substance. Jeppson (1946) showed that lateral bud primordia were substituted for injured terminal buds. Both authors considered that the crinkling and deformity of leaves noted above arose as a consequence of insect feeding on the leaf primordia. Further damage is undoubtedly done by egg-laying in buds and the upper part of stems. b. Control of Lygus Bugs. Insecticidal control of lygus proved economically impractical prior to the advent of DDT, benzene hexachloride, and sabadilla. Sorenson (1939) reported tests involving sulfur, paris green, pyrethrum, cyanogas, lethane dust, nicotine sulphate, calcium cyanide and gypsum, alone or in combinations, and none were sufficiently effective to justify the cost of application. Published acccunts of extensive tests wihh DDT in recent years have reported its effectiveness in controlling lygus (Sorenson and Carlson, 1946; Lieberman, 1946; Smith and Michelbacher, 1946; Munro, 1948; Pederson, 1948). The well-known residual action of this insecticide undoubtedly is an important factor in its effectiveness. Sorenson and Carlson (1946) in a study entailing weekly and semi-weekly applications noted that after treatments started “practically no lygus nymphs were captured on those (plots) treated with DDT, indicating that either oviposition had not occurred on them, or if it had, the newly hatched nymphs failed to develop.” I n experimentally treated plots involving only a portion or portions of fields migration of adults occurs into the treated portion. The concentration of DDT and rate of application most economical and effective have not been standardized as yet. Smith and Michelbacher (1946) used a variety of concentrations and rates, and found good control when the dosage of D D T was between 1 and 1.5 Ibs. per acre. They believed that best results would be obtained by dust.ing with a 5 per cent dust a t 30 lbs. per acre. The extensive usage and research

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with DDT a t the present time will undoubtedly throw further light on dosages in the near fut.ure. The populaticn level above which it is economically feasible to treat for lygus control is in need of clarification. While it is to be expected that a rigid formula applicable t o all situations cannot be determined because of the number of factors operating and interacting, yet, for a given set of circumstances, more extensive research will undoubtedly establish more clearly when treatment can be justified. Smith and Michelbacher (1946) concluded that “under California conditions dusting to control this pest was probably not justified unless the number of lygus bugs (adults and nymphs) reach a peak of 15 per sweep.” This suggested level, however, would appear to be unduly high for general acceptance. Sorenson (1939) has shown severe flower loss and other extensive damage from considerably lower populations. With a population level of about 8 per sweep a t the time of dusting Lieberman (1946) found the seed yield of D D T treated plots to average 385 lbs. per acre, while one check plot averaged 23 and the other 179 lbs. per acre. These two references serve to stress the necessity for clarification of the economic population level. With respect to the most desirable time of application in relation t o blooming of t.he crop, the question of the killing or repelling action of the insecticide on wild and honey bees as well as the control of injurious insects must be considered. I n general, investigators have considered that prebloom application was most desirable, in that the possible injurious effect on bees would be minimized. Smith and Michelbacher (1946), however, consider that dusting should be done after the population reaches a t least 10 adults and nymphs per sweep irrespective of the stage of flowering of the crop, basing their conclusion partially on the observation that under field conditions DDT did not appear to be harmful to wild or honeybees. Linsley and MacSwain (1947b), studying the effect of DDT on bees (mainly honeybees), found that there was an almost immediate decrease in population of bees following dusting, and that 3 or 4 days were required for the build-up to predusting level. Although they found that honeybees captured within a few hours after dusting exhibited a high mortality within 24 hours they considered the depression pattern following dusting could possibly be due to some repellent action of t.he insecticide. They state that “large scale mortality under field conditions has not yet been demonstrated experimentally.” They conclude, however, that until further facts are available DDT should “be applied as early in the growth of the plant as lygus populations warrants and that a second dusting be applied only where absolutely necessary.” In areas where cutting for hay one or more times is possible before

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the seed crop is allowed to develop, the proposal of trap-strip dusting made by Scholl and Medler (194713) would seem to warrant further investigation. Their procedure entails leaving a trap-st.rip or strips to which the lygus and Adelphocoris sp. migrate when the balance of the field is cut. The trap-strip only is dusted. Aside from economy in dusting, the trap-strip procedure has merit, as pointed out by Scholl and Medler (1947b), in reducing or elirninahg the hazards to whrm-blooded animals of DDT Isesidues on the crop. The occurrence of such residues 40 days after dusting alfalfa a t even quite low rates of Itpphation was demonstrated by Eden and Arant (1948). These latter authors also reviewed the literature on toxicity of DDT to warm-blooded animals, and their review points out the dangers involved to man and animals in feeding crops carrying DDT residues. Control of lygus with DDT has overshadowed the managerial control method described by Stitt (1941). This latter control is based upon community action in removing the preseed hay crop on a uniform time schedule. Stitt established that clean cutting of the crop under conditions of high temperature (90 to 100°F.) resulted in a nymph mortality of 80 to 95 per cent following which 2 to 3 weeks elapsed before even a small population increase occurred. He demonstrated that adoption of a uniform community cutting schedule reduced damage, and resulted in improved yield and quality of seed. It would seem that a community effort in this respect associated with DDT dusting might be more effective than the use of the insecticide alone. The effect of sabadilla has been compared to DDT in certain investigations. Sorenson and Carlson (1946) showed that 10 per cent sabadilla compared favorably with 3 per cent DDT in insect control and seed yield of treated plots. Smith and Michelbacher (1946) considered results wit.h sabadilla encouraging but that the compound is less effective than DDT. I n respect to sabadilla Linsley and MacSwain (1947b) noted that “the visible (injurious) effect upon (honey) bees was greater than of the other materials studied.” The similarity in kind, nature and extent of injury to alfalfa between Lygus spp. and Adelphocoris sp. has already been noted. The latter genus, however, has been shown by Hughes (1943) to overwinter in the egg stage near the base of alfalfa stems. Hughes (1943a, 1943b) demonstrated that clean burning of the stubble in the early spring was effective in markedly reducing populations and in increasing seed yields. Incomplete burning was relatively ineffective and cultivation had no effect. c. Other Insects. I n addition to Lygus spp. and Adelphocoris spp. Tysdal (1946) lists the following other insects as affecting seed setting in alfalfa: Says p!ant bug (C‘hlorochroa sayii), chalcis fly (Bruchophayus

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funebris),alfalfa weevil (Hypera postica), potato leaf hopper (Ernpoasca fabae Harris) and grasshoppers. According to Scholl and Medler (19474 the spittle bug (Philaenus leucophth alrnus) occurred in large populations in many eastern Wisconsin seed fields in 1946. Rotenene, sabadilla, and nicotine were found to be ineffective in their control, and the effect of D D T was doubtful. Thrips are also commonly present and often abundant. Reduction in their numbers bv DDT treatment was noted by Sorenson and Crtrlson (1946) and Lieberman (1946). Aphids were also noted to be practically eliminated by DDT (Lieberman, 1946). Poos 11945) and Scholl and Medler (1947b) have noted the effectiveness of DDT in reducing potato leaf hopper populations. 111. PROGRESS I N METHODSOF BREEDINa The breeding approach to the solution of many of the problems limiting or inhibiting the use of alfalfa has demonstrated its effectiveness. Efficient progress in improvement is contingent upon the development of satisfactory breeding systems and techniques. I n recent years noteworthy progrew has been made in this regard. 1. Breeding Characteristics

Certain fundamental facts which have a bearing upon the development of breeding methods and techniques have become established during the past few years. Some of the breeding characteristics of significance and upon which knowledge has advanced are (1) the degree of natural cross-pollination, (2) the cross- and self-fertility relationships, (3) the effects of inbreeding, and (4) the expression of hybrid vigor upon crossing. Although some of the earlier studies indicated that about 50 per cent of natural crossing occurs in alfalfa, more recent work has shown that a much higher degree of crossing takes place. Tysdal et al. (1942) and Tysdal (1942) report an average of 89.1 per cent crossing in three tests. Data presented by Knowles (1943) and by Bolton (1948) show natural crossing to about the same degree as cited above. Interplant, location, and seasonal variations in the extent of crossing do, of course, occur, as has been shown by Bolton (1948) and Tysdal and Crandall (1948). The latter authors point out that there is an inverse relationship between degree of natural crossing and self-fertility. Although interplant differences do occur, there is no particular problem in selecting plants which naturally cross-pollinate to the extent of 90 to 100 per cent. An almost complete range from total self-sterility to very high selffertility may be found (Tysdal and Kiesselbach, 1944). Tysdal (1947) estimates that about 15 per cent of the plant population could be classed as highly self-sterile. Selection of such plants insures that under open-

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pollination conditions a very high proportion of the seed set will be of crossed origin. Thus high self-sterility may be utilized in a breeding system to force crossing, in much the same way that detasseling is used in hybrid corn breeding. The desirability of selecting for high selfsterility has been repeatedly stressed by Tysdal since 1942. Tysdal and Kiesselbach (1944) compared the hay yield of open-pollinated progenies of' nine highly self-fertile plants, nine medium self-fertile plants and seven highly self-sterile plants, and found the yields to average 6.04, 6.35, and 6.59 tons per acre for the three groups. The lowest yielding individual progeny was in the self-fertile group and the highest yielding in the self-sterile group. Tysdal and Crandall (1948) report a significant correlation coefficient of - 0 . 4 0 between the self-fertility of the parent and open-pollinated progeny yield of 34 clones. While there is a wide range in self-fertility in the crop a considerable mass of evidence has accumulated demonstrating the desirability of selecting for high selfsterility. The explanation for the lower yields obtained from open pollinated progenies of self-fertile plants undoubtedly lies in the fact that selfing (inbreeding) results in a marked loss of vigor. The work of Kirk (1927, 1933), Tysdal et aE. (1942), Tysdal (1942) and others has shown that upon inbreeding, forage yield, and particularly seed yield, declines drastically and progressively with each advance in generation of selfing up until a t least the seventh or eighth. A marked reduction in self-fertility upon inbreeding has recently been demonstrated by Wilsie and Skory (1948). Tysdal and Kiesselbach (1944) have shown that a population can contain a certain proportion of inbred plants without depressing hay yield, but there can be no doubt that above a certain level the proportion of inbreds does have a depressional effect on yield. A further breeding characteristic of alfalfa which has been established in recent years is that a marked expression of hybrid vigor may be secured by crossing certain plants or certain lines. Tysdal et al. (1942) gave seed and forage yields of a number of hand-pollinated crosses some of which yielded as high as 139 per cent of the average forage yield of three st.andard varieties. Expressed in percentage of the checks the seed yield of the hybrids ranged as high as 257 per cent. Tysdal (1947), Tysdal and Crandall (1948), Bolton (1948) , Wilsie and Skory (1948) all present further data demonstrating conclusively the occurrence of hybrid vigor to rather marked degrees. Study of these data suggests that heterosis is expressed more strongly in seed yield than in forage yield. Nevertheless hay yield increases of 25 to 30 per cent over check varieties have been obtained, and in improvement programs attainment of such yield superiority is highly desirable.

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Hybrid vigor. is, of course, not expressed in all crosses. In alfalfa, as in corn and in nnimals, certain individuals cross with other individuals to give superior yield or performance. Such individuals are said to combine or nick well. Ot.her individuals do not possess this capacity and are said to be “poor combiners.” Data given by Tysdal and Kiesselbach (1944) serves to illustrate the differences between plants in combining ability. They showed that the F1 of one particular plant crossed as a male with three female plants gave an average yield of 1003 g. of green weight per plant. I n t.he case of three other male plants all crossed into the same three female plants the FI yielded 483, 659 and 754 grams. Bolton (1948) selected 13 plants and intercrossed them in all possible combinations. The average seed yield of the F1progenies of one plant crossed with each of the 12 others ranged between 309 and 166 lbs. per acre. Further evidence of a similar natsure has been presented by Tysdal ef al. 1942),Stevenson and Bolton (1947),and Wilsie and Skory (1948). In a breeding program, in order to utilize the characteristic of heterosis, it is essential to test for combining ability of the selected plants. Methods of testing for this behavior will be covered in Section 111-3. 2. Utilizing Hybrid Vigor

Recognizing the close similarity between the breeding characteristics of alfalfa and corn, Tysdal e t al. (1942) and Tysdal (1942) proposed a breeding system for alfalfa similar to that employed in breeding hybrid corn. Application of the hybrid corn breeding system, or variations of it, represents in the writer’s opinion a most outstanding and promising advance in methods of breeding alfalfa. By means of this system of improvement it is possible to capitalize upon hybrid vigor in the crop as utilized by the farmer grower, and at the same time to maintain a degree of uniformity for such characteristics as disease resistance, insect resistance, and quality that is not obtainable by any other method of breeding. Certain fundamental differences between corn and alfalfa necessitate an alternative procedure in the application of the hybrid breeding system. Firstly, in corn the male and female organs are carried on different part of the plant. This makes it possible to emasculate the female or seed parent of a hybrid by the relatively simple process of detasseling. I n alfalfa the male and female organs are contained in the same flower and mechanical emasculation on any extensive scale is impossible. I n Section 111-1, however, it was shown that about 15 per cent of alfalfa plants are highly self-sterile, and that when such self-sterile plants are in association with other plants under natural field conditions a very high proportion of their seed is of crossed origin. Thus high self-sterility

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is the inherent characteristic which Tysdal et al. (1942) envisioned as being used to force crossing as an alternative to detasseling in corn. The degree of control of crossing is obviously not absolute in alfalfa as it is in corn. Inbreeding or brother-sister mating (sibbing) is necessary in the annual corn in order to maintain and propagate the selected lines from year to year. There are of course other reasons for selfing in corn. I n alfalfa the perennial habit of the crop eliminates the necessity of selfing or sibbing to maintain the selected unit. The selected alfalfa plants can he readily propagated asexually by stem cuttings (Tysdal, 1942; Tysdal et al., 1942; White, 1946; and Grandfield et al., 1948). Therefore, in the system of breeding hybrid alfalfa proposed by Tysdal e t al. (1942) selected single plants propagated clonally (by cuttings) become the basic units used as parents in producing single crosses, in contrast to lines maintained by inbreeding or sibbing in corn. The breeding procedure evolved by Tysdal et al. (1942) entails rigid selection of single plants for self-sterility, high combining ability, resistance to disease and insects, and any other characteristic desired. For the production of single crosses two of the selected plants are propagated clonally and clones of the two plants are established in an isolated crossing plot. Such a plot produces single cross seed. Because of the labor required in clonal propagation and transplanting it is unlikely that it will prove practical to produce single cross seed in sufficient volume to supply the demand from farmers who wish to use the hybrid for hay or pasture purposes. The exponents of this scheme of breeding thus propose the prodiiction of double cross hybrid seed. This simply entails the establishment of a second isolated single crossing plot comprised of clones of two other selected plants. The seed from the two single crosses is sown in alternate rows or mixed in a third isolated field, from which the double crow seed is harvested. In the production of the single crosses the high self-sterility of the two clonally propagated parents insures that a high proportion of the seed will be crowed. I n the production of the double cross, however, any one plant may self, may cross with another plant or plants of the same single cross (sib). or may cross with a plant or plants of the other single cross. The latter type of cross is the desired one. The extent of selfing in producing the double cross is not likely to be any or much greater than in producing the single crosses. Wilsie and Skory (1948). have shown that on crossing plants of low x low self-fertility the F, was low in selffertility. Nor is the proportion of sibbed seed likely to be high. Bolton (1948) has shown that seed-setting upon sibbing is only 60 per cent of that following out-crossing. Tysdal (1942) has indicated that it max

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be possible to select plants the F1 of which would be inter-sterile (sibsterile). Furthermore, the proportion of sibbed plants likely to occur in the double cross progeny will probably not seriously reduce yield. Tysdal and Kiesselbach (1944) have shown that at, least 25 and possibly 50 per cent of selfed seed may be mixed with open-pollinated seed without significantly reducing the yield of the open-pollinated variety. As an alternative to the extensive use of clonally propagated plants as parent.al units for the production of single crosses Bolton (1948) suggested the use of inbred lines. His plan would entail clonal propagation of the parent selections and space isolation of each to produce inbred seed, and would probably necessitate use of material somewhat more selffertile than would be the case in following the procedure outlined above. While Bolton’s plan has not. been fully tested with alfalfa, essentially the same procedure is practiced in the production of commercial hybrids in sunflower, which is also an insect pollinated crop (Unrau and White, 1944; Unrau, 1947). The breeding of synthetic varieties has also been suggested by Tysdal et al. (1942) as a means of utilizing hybrid vigor. Tysdal (1947) has described a synthetic variety as one “that is developed by crossing, composking or planting together two or more unrelated strains or clones, the bulk seed being harvested and replanted in successive generations. B y intercrossing the unrelated strains or clones are synthesized into a new variety.’’ This breeding system demands that rigid selection for high combining ability and other desirable characteristics be practiced just as would be the cme if single or double crosses were to be produced. It largely elminates the necessity of extensive vegetative propagation. Experimentally produced synthetic varieties have demonstrated their superiority over standard varieties. Tysdal and Crandall (1948) have presented data showing that certain synthetics in their first generation of synthesis yield as much as 16 per cent more hay than standard varieties, and they point out t.liat in the second generation of synthesis the yield was almost exactly the same as the first. These results provide grounds for optimism for the successful use of this system of breeding. While hybrid alfalfas and synthetics have been produced only experimentally as yet, the results have indicated that the evolution of breeding systems which embody the utilization of hybrid vigor afford a means of improvement not attainable in the breeding systems previously employed. 3. Methods of Testing for Combining

Ability

It has been shown in Section 111-1 that plants differ markedly in capacity to combine in crosses with other plants to produce high yielding progeny. It is impossible to assess combining ability by the appearance

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or the yield performance of a plant itself as it is dependent upon how the genes of one plant complement those of another. It is thus necessary to cross and test the crossed progeny to evaluate this characteristic. The proportion of plants in any population possessing high combining ability is likely to be m a l l and consequently to find such plants involves the testing of relatively large numbers. To hand cross on an extensive scale is a slow and expensive procedure, and, if not impossible, is impractical. For example, to intercross 100 plants in all possible combinations would require making and testing 4950 crosses, disregarding reciprocals. The polycross procedure proposed by Tysdal et al. (1942) provides an alternative means of making crosses on an extensive scale. Tysdal and Crandall (1948) state that “polycross seed is the seed produced on selected clones inter-pollinated a t random in isolation.” The technique is comparable to the top-cross met.hod used in corn (Hayes and Immer, 1942). The polycross procedure simply consists of choosing plants for any one or more major characteristic and low self-fertility and placing them in an isolated nursery to intercross naturally among themselves. To provide that each plant has an 0pportunit.y of crossing with several other plants, the selections are cloned and replicated a t random several times through the nursery. Seed is collected from each clone of each plant and all the seed from a plant is bulked. Because the plants are relatively self-sterile the seed produced is largely of crossed origin and has a number of different male parents. Tysdal and Cranda!l (1948) have compared the yield, bacterial wilt resistance, leaf hopper resistance, and cold resistance of polycrosses of a number of plants with that of single crosses involving the same plants. Their data show that the polycross progenies gave essentially the same ranking as did the single crosses, thus demonstrating that the polycross test provides a dependable means of determining general combining ability. It is of interest to note that Tysdal and Crandall (1948) found that top-crosses of selected clones unto a standard variety, and also polycrosses unto a standard variety, gave progenies performing essentialIy similarly to that of the conventional polycross and the single crosses. This finding appears to the writer to throw doubt upon the necessity of isolating the polycross nursery. I n the conduct of a breeding program the polycross technique is used to determine those plants having superior combining ability. It is then necessary or highly desirable to test these superior plants in single cross combinations. Having very materially reduced the numbers of plants being worked with by the polycross test it becomes practical to make and to test the single cross combinations.

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4. Selection Procedures for Certain Characteristics In earlier breeding programs with alfalfa the general procedure was to inbreed and select in inbred progenies (Kirk, 1927; Stewart, 1931; Kirk, 1932; Tysdal and Clarke, 1934; and Dwyer, 1936). There has been, however, a definite trend in recent years towards selection of openpollinated plants and utilization of them without inbreeding (Tysdal et al., 1942; Tysdal and Kiesselbach, 1944; Bolton, 1948; and Reitz et al., 1948). The results obtained to date in breeding for increased yield of hay and seed, bacterial wilt resistance, and black stem resistance indicate the improvements which may be made without resorting to inbreeding. Reitz et nl. (1948), for instance, secured as high a level of resistance for black stem disease in open-pollinated material as in selected selfed lines. These latter authors point out that the high degree of self-sterility, the low vigor of inbreds and t,he labor involved reduce the value of inbreeding in an improvement program. In spite of the acknowledged advances, however, which have been made by selection of open-pollinated material without resorting to inbreeding, it would seem desirable to continue to explore the possibilities of increasing homozygosity for t.he particularly desired characteristic through inbreeding. In breeding for improved seed yield, plants which trip automatically to a high degree and are self-fertile occasionally may be selected. Such plants set seed in the absence of tripping and cross-pollinating insects. It is a strong temptation to utilize them in the breeding program. Tysdal has repeatedly warned against the selection and use of such material. Stevenson and Bolton (1947) have presented data on the hand-crossed single cross performance of such plants showing that certain F1combinations yielded four to six times as much seed as Grimm. However, when open-pollinated progenies of eleven F1 plants were compared with the clones, selfed progenies, and with Grimm as a check, the seed yield of the open pollinated progenies was only slightly more than t.hat of the selfed progenies of the same plants. These data clearly demonstrate that under open pollination such self-tripping self-fertile plants self-pollinate rather than cross, and that the following generation is decidedly inferior. I n improving alfalfa for seed production Bolton (1948) has followed t.he procedure of selecting on the basis of large pod size and heavy pod production and in the field. This was followed by a test of cross- and self-fertility und.er greenhouse conditions. B y selecting those plants which were highly cross-fertile in the greenhouse test he found that the average of all eingle crosses involving any one select,ed plant exceeded the seed yield of Grimm and Ladak. The average of the single crosses

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from certain plmts outyielded the checks by over 100 per cent. This procedure is obviously efficient for isolating superior seed yielding plants. I n the selecting for disease resistance it is desirable to create controlled epidemics of the disease rather than to depend upon the often sporadic natural infection. The methods of testing for bacterial wilt resistance have now become fairly well standardized (Brink et al., 1934; Jones, 1934; Peltier and Tysdal, 1934; Weimer and Madson, 1936; and Jones, 1940). A procedure for use in selection for black stem resistance has been described by Reitz et al. (1948). Cormack (1948) has worked out. inoculation techniques in testing for winter crown rot. Laboratory methods of testing for cold resistances have been developed by Peltier and Tysdal (1932). The reliability of their method is indicated by results reported by Tysdal and Crandsll (1948). They found a significant correlation of +.62 between resistance in laboratory cold tests at Lincoln, Nebraska and cold resistance under field conditions a t Saskatoon, Saskatchewan. IV. CONQUERING SOMEDISEASES Among the many factors restricting utilization and limiting production of alfalfa certain diseases rank high in significance. The diseases of the greatest obvious seriousness are those which cause killing of plants and severe stand reductions either suddenly or over a period of time. Bacterial wilt causes damage of this nature. There are, however, many more or less insidious diseases the effect of which on stand establishment or maintenance, or on yield or quality is less apparent but none the less of very considerable importance. Several leaf diseases and at least one seedling disease belong in the latter category. A mult.iplicity of organisms find the alfalfa plant a suitable host. Chilton et al, (1943) have presented a lengthy list of fungi found on the genus Medicago, to which could be added several diseases caused by viruses and bacteria. To deal adequately with even the majority of the major diseases would be beyond the scope of this review. Bacterial wilt and black stem have been chosen for discussion as representative of diseases upon which considerable work has been done and progress made in control. 1. Bacterial W i l t Of all the diseases attacking alfalfa on the North American continent bacterial wilt is undoubtedly the most serious. The causative organism now designated as Corynebacterium insidwsum (McCull) Jensen was first identified by Jones in 1925, and the disease and organism was more fully described by Jones and McCulloch (1926). Recognition of the disease in many alfalfa growing areas soon followed its discovery.

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According to Tysdal (1947) it has now been found in every major alfalfa producing state in the United States. In addition it has been discovered in each of the three prairie provinces of Canada. I n regions in which the disease is prevalent it has been amply demonstrated that stands of susceptible varieties survive only 3 to 5 years (Jones and McCulloch, 1926; Tysdal and Westover, 1937; Weihing et al., 1938; and Grandfield, 1945b), whereas prior to the advent of the disease longevity of st.ands was much greater. Speaking of the United States, Tysdal and Westover (1937) state that “Bacterial wilt annually destroys hundreds of thousands of acres,” and they point out that resistant strains which would extend the life of stands even 2 years would save millions of dollars. The disease usually does not manifest itself until plants are about 3 years old. The characteristic symptoms are dwarfing and profuse branching associated with yellowing and small leaves. The tap and larger branch roots, when cross-sectioned, display a partial or complete ring of yellowish or pale-brown discoloration immediately below the bark. The discoloration and a slimy appearance are apparent when the bark is peeled back. In the advanced stages plants wilt and die. The damage is caused by the bacteria plugging the vasrular conductive tissue of the plant (Jones and McCulloch, 1926). Certain control measures were suggested by Jones and McCullocli (1926). These mainly involved sanitary precautions. Tysdal and Westover (1937) , however, report that “Considerable preliminary work indicated that cultural practices in general would not control the disease. The only avenue of approach that offered possibilities was a breeding program.” Recognition of the disease and its seriousness immediately touched off an extensive search for resistant material. Hundreds of lots of seed were collected from many parts of the world, and tested a t several points in United States It was found that a reasonably high level of resistance was present in some strains obtained from Turkestan or adjacent areas (Wilkins and Westover, 1934; Weimer and Madson, 1936; Tysdal and Westover, 1937; and Weihing et al., 1938). Seed secured from a Nebraska farmer but tracing back probably to Turkestan origin was found to possess resistance, and was assigned the variety name of Hardistan by Kiesselbach et al. (1930). Anot.her strain secured from France but thought t o have originated from Turkestan was named Kaw by Salmon (1932). According to Wilkins and Westover (1934) Turkestan alfalfa strains in general were more susceptible t o leaf spot diseases and inferior in yielding ability to commonly grown domestic varieties. I n varieties and strains other than those of Turkestan origin various levels of resistance have been found and breeding has yielded new

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varieties. From common alfalfa Grandfield (1945b) developed the resistant variety Buffalo which was released in 1943. Weimer and Madson (1936) and Wilson (1947) have also shown that highly resistant lines could be obtained from this type. Among the variegated varieties Ladak has frequently been shown to possess a fairly high degree of resisthnce. Even Grimm, which once was considered almost completely susceptible, has been shown to be a source of some immune and resistant plants (Jones and Smith, 1947). By compositing lines selected out of Cossack, Turkestan, and Ladak, Tysdal developed the Ranger variety which was released in 1942 (Hollowell, 1945). I n the development of the Buffalo and Ranger varieties a high degree of resistance to wilt has been attained, combined with a higher degree of resistance to leaf spot diseases than was possessed by varieties of Turkestan origin (Grandfield, 1945b; Hollowell, 1945; Tysdal, 1947). To illustrate the sliperiority of these new varieties in respect to stand maintenance and yielding ability, data given by Grandfield (1945b) may be cited. I n a tert a t Manhattan, Kansas comparing the varieties Buffalo, Kansas Common, Grimm, Oklahoma Common, and Dakota Common the stands ranged between 95 and 100 per rent in 1939 but by 1942 had been reduced to 6 t o 25 per cent for the wilt-susceptible varieties while that of Buffalo showed no reduction. The stand reduction was reflected in hay yield. Buffalo was not superior in yield in 1939 but by 1942 it yielded 3.26 ad compared to 2.53, 2.50, 2.46 and 2.85 tons per acre for the varieties liuted in the order above. In the fourth year of a test at Ames, Iowa, Buffalo yielded 2.54, Ranger 2.34, Kansas Common 0.60 and Grimm 0.84 tons of hay per acre. These results serve to illustrate the outstanding progress which has been made through development of resistant varieties. Varieties even more resistant than those presently available will undoubtedly be forthcoming. Jones and Smith (1947) have described certain selected plants as immune to this disease. Wilson (1947) has isolated one gene for high resistance, and plants possessing it in the homozygous condition were found to be 72 per cent healthy in artificially inoculated tests. Wilson (1947) has pointed out that in similar tests conducted in Nebraska and Wisconsin Hardistan Rhowed 19 per cent and Ranger 37 per cent. healthy plants. The genetics o f resistance to bacterial wilt has been found to be complex. Brink et al. (1934) concluded that resistance behaved as an intergrading character and that a factorial interpretation of their data was impossible. Wtiiner aiid Madson (1936) also found t.hat transmission of resistance to selfed and open-pollinated progenies was complex. Wilson (1947) , however, isolated “three and possibly four partially dominant

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genes differing in strength of resistance.” As shown above, one of these genes when homozygous affords a high level of resistance. 2. Black Stern

Black stem caused by Ascochyfa iinperfecta Peck is widely distributed in North America and Europe (Toovey e t al., 1936; Remsberg and Hungerford, 1936; Peterson and Melchers, 1942; Cormack, 1945; and Reitz e t al., 1948). I t s occurrence, distribution and intensity of attack is favored by relatively cool humid conditions. It is thus of less economic importance in t,he dry land agriculture of semi-arid or arid regions than under irrigat.ion or in humid areas. The appearance of small, dark brown or black spots on the leaves and stems is tlit! common early symptoms of infection. As the disease progresses the lesions on the leaves enlarge and coalesce, and the leaves become chlorotk and die. The progress of infection on the stem is similar, and results in a smooth black discoloration often involving a considerable portion of the stem. Lesions may also occur on petioles, racemes and pod6 Cormack (1945) showed that 50.5 per cent of the seed samples he examined carried the disease organism although displaying no symptoms of disease. Death of axillary buds in early spring and death of shoots in severe epidemics is a further, although not specific, symptom of the disease. The damage caused by the disease is more or less indicated by the above described symptoms. Defoliation due to leaf and petiole infection is probably thz commonest injury. Peterson and Melchers (1942) reported a loss of over 15 per cent of the leaves in some plots under conditions in which the infection was not particularly severe. Undoubtedly the loss of Ieaves under certain circumstances is much higher. Since the leaves are higher in protein and carotene (Tysdal, 1947) than the stems, the injury and loss of leaves causes a reduction in forage quality and nutritive value. It is likely that the lesions which also develop on the stems also adversely affect forage quality. Under favorable conditions for infection death of shoots and stems and whole plants occur (Johnson and Valleau, 1933; Toovey e t d.,1936; Reitz et al., 1948). Richards (1934) recorded that t.he yield of the first cut of severely attacked varieties was reduced by 40 to 50 per cent. Cormack (1945) has shown that the organism causes a reduction in seedling emergence. Reduction in the incidence of the disease by management practices has been suggested. Johnson and Valleau (1933) noted that early spring grazing by sheep removed the dead growth and reduced the primary infection in the new spring growt,h. It has frequently been observed that in any one season t,he first crop is more severely infected than the second

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or third. Toovey et al. (1936)advised cutting the first crop early before injury becomes severe. While management practices afford a means of reducing the damage, control undoubtedly is contingent upon developing resistant varieties. While no such varieties have been developed as vet, progress has been made towards that end. Johnson and Valleau (1933)noted that varieties differed in degree of infection. Richards (1934) recorded marked intervarietal differences in susceptibility. Toovey et al. (1936) recorded that a t Cambridge a strain from Iraq was highly susceptible and that in Norfolk varieties were observed to differ in susceptibility. Peterson and Melchers (1942) found M . falcata and M . ruthenica more resistant than common alfalfa. Reitz et al. (1948) reported that varieties, strains, and species differed significantly in resistance both to natural field infection and to artificial inoculation in the greenhouse. The occurrence of interviarietal and interspecific differences in disease reaction is evidence of inherent variation for resistance and also is suggestive of the possibilites of breeding resistant strains and varieties. Reitz et al. (1948)established that significant differences in resistance existed between lines of Kansas Common and also between the selfed progenies of reistant. and susceptible selections out of several different varieties. They noted a “significant tendency for the inbred progeny to react to black stem in the same manner as the parent had reacted.” A significant correlat,ion of +.716 was found between the infection indices of the first and second generation inbred progenies. Selection was shown to be effective in progressively increasing the level of resistance. They noted that the highest levels of resistance achieved by inbreeding with selection was matched by selecting from open-pollinated varieties. Although immunity to the disease was not observed a few highly resistant plants were isoleted. Inheritance of resistance to the disease was examined by Reitz et al. (1948). The F1of crosses between highly susceptible and highly resistant inbred plants were found to be quite uniform for a level of resistance nearly as low as that of the inbred progeny of the resistant parent. The Fz of crosses involving two resistant plants was shown to be significantly more resistant than the Fz of crosses of two susceptible plants or of one resistant plant They concluded that “inheritance of resistance is definite but not simple.” While the breeding of resistant varieties is as yet in the more or less preliminary stages, yet the work to date, particularly that of Reitz et al., (1948),has shown conclusively that the attainment of such an objective is possible. It should be noted that control of black stem by the breeding approach is complicated by the occurrence of physiologic races of the

ALFALFA IMPROVEMENT

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fungus (Reitz et al., 1948). Nevertheless, that this fact need not be a deterrent has been amply demonstrated in other diseases, the case of stem rust of wheat being a well-known example.

V. SUMMARY AND CONCLUSIONS Seed-setting investigations have established that tripping of the flowers is almost obligatory if seed is to set and that a high degree of cross-pollination is required for satisfactory seed yields. Both of these essential functions are performed by wild bees and under certain circumstances by honey bees. Inadequate populations of wild bees are considered to be the major ecological factor limiting production in many areas. General recognition of this fact has recently stimulated investigations on the domestications of wild species, or their attraction to artificially prepared nesting sites. I n addition attention is being directed t o cultural and management practices which encourage natural population increases. The presence of competing sources of pollen and nectar has been shown to be an important factor influencing foraging particularly by honey bees, and the possibilities of reducing or eliminating competition of this nature is being explored. In the preliminary stages of study there is some evidence which indicates that the populations of wild bees can be brought under control to some degree as can also the foraging of wild and honey bees. Attainment on a practical scale of such objectives would provide effective means of increasing seed yields and production. Insecticidal control of the very injurious lygus bug wibh DDT, already in the stage of farmer usage, promises to be of much value in increasing seed production. Although demonstrated experimentally, the breeding of superior seed yielding varieties has as yet not advanced to the point of practical application of the knowledge and material but this avenue of approach holds much promise for the future. Through advances in the phases outlined above and such other factors as fertilizer treatment, control of disease, control of other insects, there is ample grounds for optimism that in the not too distant future seed yield and production will be materially stabilized. The possibilities of solution of certain disease and insect problems through breeding for resistance has been clearly demonstrated experimentally. Furthermore, the existence of highly significant differences between plants and strains in such important nutrient factors as protein and carotene content has been well established. Except for the development of bacterial wilt resistant varieties the potential advances in thc above respects have not reached as yet the point of practical application. Given adequate financial support, however, there can be no question that through the cooperation of breeders, plant pathologists, entom-

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WM. J. WHITE

ologists, chemists and others, very great strides will be made in the Droduction of varieties much more capable of resisting the ravages of disease and insects and superior in feeding value as well as yielding ability. It must be recognized that the perennial nature and other characteristics of the crop are impedimenbs which demand more time to attain objectives than is the caw with annual crops. Progress in breeding is to R large measure dependent upon the development of suitable techniques and systems. For many years alfalfa breeders have been searching for adequate tools. The recent evolution by Tysdal and his coworkers of a breeding system essentially similar to that so successfully employed in the breeding and production hybrid corn represents a noteworthy advance. I n its application alfalfa breeders today are a t the stage reached by corn breeders some two decades ago. But the evidence to date seems to warrant confidence that the application of these principles will enable improvements t o be made in alfalfa closely paralleling the epoch making progress made with corn.

REFERENCES Akerberg, E., and Lesins, K. 1947. Acta Agr. Suecanu 2, 249-251. Anonymous. 1931. Imp. Bur. Plant Uenetics IZerb. Plants Bull. 4, 40. Armstrong, J. M., and White, W. J. 1935. J. Agr. Sci. 25, 161-179. Atwood, 8.S. 1947. Advances in Genetics 1, 1-67. Bentley, F., and Mitchell, J. 1946. Univ. of Saskatchewan Ert. Bull. 122. Bohart, G. E. 1947. Farm and Home Sci. 8, 13-14. Bolton, J. L. 1948. Sci. Agr. 28, 97-126. Bolton, J. L., and Fryer, J. R. 1937. Sci. Agr. 18, 148-160. Bolton, J. L., and Peck, 0. 1946. Sci. Agr. 26, 130-137. Brink, R. A., and Cooper, D. C. 1936. Am. J . Botany 23, 678-683. Brink, R. A., and Cooper, D. C. 1939. Science 90, 545-546. Brink, R. A., Jones, F. R., and Albrecht, H. R. 1934. J. Agr. Research 49, 635-642. Carlson, J. W. 1935. Utah Agr. Expt. Sta. Bull. 258, 48. Carlson, J. W. 1940. J . Agr. Research 61, 791-816. Carlson, J. W. 1946. J. Am. Boc. Agron. 38, 502-514. Chilton, S. J. P., Henson, L., and Johnson, H. W. 1943. U.S. Dept. Agr. Misc. Pvh. 499. (:ooper, D. C., and Brink, R. A. 1940. J. Agr. Revearch 60, 455-472. Cormack, M. W. 1945. Phytopath. 35, 838-855. Cormack, M. W. 1948. Can. J . Research c26, 71-85. Crandall, B. H., and Tate, H. D. 1947. J . Am. SOC.Agron. 39, 161-163. Dwyer, R. E. P. 1933. Herbage. Rev. 1, 135-136. Dwyer, R. E. P. 1936. Herbage Rev. 4, 1-8. Dwyer, R. E. P., and Allman, 8. F. 1933. Agi. Gaz. N . Is. Wales Misc. Pub. 2915. Eden, W. G., and Arant, F. 8. 1948. J . Ecun. Entomol. 41, 383-387. Grandfield, C. 0. 19458. J. Agr. Research 70, 123-132. Grandfield, C. 0. 194513. Kans. Agr. Exp. Sta. Circ. 226,

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Grandfield, C. O., Hansing, E. D., n.nd Hackerot,t, H. 1,. 1948. .I. Am,. Soc. Agron. 40,716-720. Griazard, A. L., and Mathews, E. M. 1942. J. Am. SOC.Agron. 34, 365368. Hadfield, J. W., and Calder, R. H. 1936. N.Z.J . Sci. Tech. 17, 577-594. Hare, Q. A., and Vansell, G. H. 1946. J . Am. Soc. Agron. 38, 4 8 2 4 9 . Harrison, C. M., Ketly, R. H., and Rliimer, C. 1945. Quart. Bull. Mich. Agr. Expt. Sta. 28, 85-89. Hayes, H. K., and Immer, F. R. 1942. Methods of Plant, Breeding. McGraw-Hill, New York. Hollowell, E. A. 19-15. J . Am. SOC.Agron. 37, 649-652. Hughes, J. H. 1943a. Univ. Minn. Tech. Bull. 161. Hughes, J. H . 1943b. Comm. Iron Range Resources-Repor1 of Ini~entiqntioiis No. 2. Jeppson, L. R. 1946. Hilgardia 17, 165-181. Johnson, E. M., and Valleau, W. D. 1933. K y . Agr. Expt. Sta. Riill. 339. Jones, F. R. 1925. Phytopath. 15, 243-244. Jones, F. R. 1934. J. Agr. Research, 48, 1085-1098. Jones, F. R. 1940. Rep. 8th Alfalfa Jmpr. Conf. U.S. Dept. Agr. Div. Forage Crops and Diseases, pp. 10-14. Jones, F. R., and McCulloch, 1,. 1926. J. Agr. Research 33, 493-521. Jones, F. R., and Smith, W. K. 1947. J. Am. SOC.Agron. 39, 423-425. Jones, L. M., and Olson, P. J. 1943. Sci. Agi. 23, 315-321. Kiesselbach, T. A., Anderson, A., end Peltier, G. T,. 1930. .I. Am. Soc. Agron. 22. 189-190.

Kirk, L. E. 1927. Sci. Agr. 8, 1-40. Kirk, L. E. 1932. Imp. Bur. Plant Genetics Herb. Plants Bull. 7, pp. 7-13. Kirk, L. E. 1933. Proc. Worlds Grain Ezhibition and Conf. 2, 159-167. Klinkowski, M. 1933. Imp. Bur. Plant Genetics Herb. Plants Bull. 12. Knowles, R. P. 1943. Sci. Agr. 24, 29-50. Knowlton, G. F., and Sorenson, C. J. 1947. Utah State Agr. Coll. Ezt. Bull. 150. Lejeune, A. J., and Olson, P. J . 1940. Sci. Agr. 20, 570-572. Lieberman, F. V. 1946. J. Am. SOC.Agron. 38, 489-494. Linsley, E. G. 1946. J. Econ. Entomol. 39, 18-29. Linsley, E. G., and MacSwain, J. W . 1947a. J. Econ. Entomol. 40, 349-358. Linsley, E. G., and MacSwain, J. W . 194713. J. Econ. Entomol. 40, 358-363. Munro, J. A. 1948. N. Dak. Agr. Expt. Sta. Bimonthly Bull. 10, 114-115. Peck, O., and Bolton, J. L. 1946. Sn' Agr. 26, 388-418. Pederson, C. E. 1948. Quart. Bull. Mich. Agr. Expt. Sth. 30, 298-308. Peltier, G. L., and Tysdal, H. M. 1932. J . Agr. Research 44, 429-444. Peltier, G. L., and Tysdal, H. M. 1934. Neb. Agr. Expt. Sta. Bull. 76. Peterson, M. L., and Melchers, L. E. 1942. Phytopath. 32, 590-597. Piland, J. R., and Ireland, C. F. 1941. J. Am. Soc. Agron. 33, 938-939. Piland, J. R., and Ireland, C. F., and Reisenauer, H. M. 1944. Soil Sci. 57, 75-84. Piper, C. V., Evans, M. W., McKee, R., and Morse, W. J. 1914. U.S. Dept. Agr. Bull. 75. Poos, F. W. 1945. J. Econ. Entomol. 38, 197-199. Reitz, L. P., Grandfield, C. O., Peterson, M. L., Goodding, G. V., Arneson, M. A., and Housing, E. D. 1948. 3. Agr. Research 76, 307-323. Remsberg, R., and Hungerford, C. W . 1936. Phytopath. 26, 1015-1020. Richards, B. L. 1934. Phytopath. 24, 824-827.

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Rubnev, V. Z. 1941. Sozial. Zern. Khozh. No.2, 141-144 Salmon, S. C. 1932. J. Am. SOC.Agron. 24, 352-353. Salt, R. W. 1945. Sci. Agr. 25, 573-576. Scholl, J. M., and Medler, J. T. 1947a. J. Econ. Entomol. 40, 446-448. Scholl, J. M., and Medler, J. T. 1947b. J. Econ. Entomol. 40, 448-450. Sexsmith, J. J., and Fryer, J. R. 1943. Scs'. Agr. 24, 145-151. Silversides, W. H., and Olson, P. J. 1941. Sci. Agr. 22, 129-134. Smith, R. F., and Michelbacher, A. E. 1946. J . Econ. Entomol. 39, 638-648. Sorenson, C. J. 1939. Utah Agr. Expt. Sta. Bull. 284. Sorenson, C. J., and Carlson, J. W. 1945. Farm and Home Sci. 6, 5, 11. Sorenson, C. J., and Carlson, J. W. 1946. J. Am. Soc. Agron. 38, 495-501. Stevenson, T. M., and Bolton, J. L. 1947. Empire J . Expt. Agr. 58, 82-88. Stewart, G. 1926. Alfalfa-growing in the Unitpd States and Canada. MarMillan Company, New York. Stewart, G. 1931. Science 74, 341-343. Stitt, L.L. 1940. U.S. Dept. Agr. Tech. Bull. 741. Stitt, L. L. 1941. US.Dept. Agr. Bur. Entomol. Plant Quar. Processed B d l . E.546. Stitt, L. L. 1944. J . Econ. Entomol. 37, 709. Toovey, F. W., Waterston, J. M., and Brooks, F. F. 1936. Ann. Applied Biol. 23, 705-717. Tysdal, H.M. 1940. J. Am. SOC.Agron. 32, 570-585. Tysdal, H.M. 1942. Mich. Stnte Coll. Dapt. Farm Crops, Spragg Memorial Lertures. Tysdal, H.M. 1946. J. Am. SOC.Agron. 38, 515-535. Tysdal, H.M. 1947. U.S. Dept. Agr. Yearbook Agr. pp. 433-438. Tysdal, H.M., and Clarke, I. 1934. J. Am. Soc. Agron. 26, 773-780. Tysdal, H. M., and Crandall, B. H. 1948. J . Am. SOC.Agron. 40, 293-306. Tysdal, H.M.,and Kiesselbach, T. A. 1944. J. Am. Soc. Agron. 36, 649-667. Tysdal, H.M., Kiesselbach, T. A., and Westover, I,. I,. 1942. Neb. Agr. Rxpt. Stn. Research Bull. 124. Tysdal, H. M., and Westover, H.L. 1937. U.S. Dept. Agr. Yearbook Agr. pp. 11221153. Ufer, M. 1932. Zuchter 4, 282-286. Unrau, J. 1947. Sci. Agr. 27, 414-427. Unrau, J., and White, W. J. 1944. Sci. Agr. 24, 516-525. Vansell, G. H. 1943. Am. Bee J . 83, 106-107. Vansell, G. H.,and Todd, F. E. 1946. J. Am. SOC.Agron. 38, 470-488. Weihing, R. M., Robertson, D. W., and Coleman, 0. H. 1938. Colo. RtotP Coll. Tech. Bull. 23. Weimer, J. L., and Madson, B. A. 1936. J. Agr. Reseurch 52, 547-555. Wexelson, H. 1946. Tidsskr. Norske Landbr. 53, 125-161. White, W. J. 1946. Sci. Agr. 26, 194-197. Wilkins, F.R., and Westover, H.I,. 1934. J . Am. SOC.Agron. 26, 213-222. Wilsie, C. P.,and Skory, J. 1948. J. Am. SOC.Agron. 40, 698-706. Wilson, M. C. 1947. J. Am. POC.Agron. 39, 570-583.

Soil Microorganisms and Plant Roots FRANCIS E . CLARK

U 8. Department of Agriculture and Iowci Agricu1t.iu a l h’xperirnent Statiori * Ames. Iowa CONTENTS

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . .

. .

1 Historical Summary . . . . . . 2 Characterization of the Rhizosphere

Paye

. . . . . . . . . . . . . . . . . . . . . . . . . .

I1. Types of Relationships Between Microorganisms and Plant Roots . . . . . . . . . . . . . . . . . . . . . . . . . 1. Symbiotism 2 . Parasitism . . . . . . . . . . . . . . . . . . . . . 3. Commensalism . . . . . . . . . . . . . . . . . . . . . I11. The Rhiaosphere Microflora in Relation to the Growth of Higher Plants . 1. Influences on the Availability of Nutrient Elements . . . . . . . a . On the Availability of Combined Nitrogen . . . . . . . . . b . On the Fixation of Free Nitrogen . . . . . . . . . . . . c . The Transformation of Elements Other Than Nitrogen . . . . d . In the Solution of Relatively Insoluble Minerals . . . . . . e . Mycorrhizae and Bacteriorrhizae in Relation to Plant Nutrition . 2. Some Influences on Plant Growth and Welfare . . . . . . . . . a . Production of Plant Growth Substances in the Rhieosphere . . b . Influence of the Rhizosphere Flora on the Incidence of Root Pathogens . . . . . . . . . . . . . . . . . . . c . The Rhizosphere Flora in Relation to the Formation of a Stable Soil Structure . . . . . . . . . . . . . . . . . . IV . The Numbers of Microorganisms Aeociated with Plant Roots . . . . . 1. Methods of Study . . . . . . . . . . . . . . . . . . . . 2. Density of the Microbial Population Within the Rhieosphere . . . 3 . Some Factors Affecting Rhieosphere Populations Determined Culturally . . . . . . . . . . . . . . . . . . . . . . . a . Influence of Soil Moisture Content a t Sampling . . . . . . . b . Influence of Type and Stage of Plant Growth . . . . . . . c. Influence of the Region of the Rhizosphere Studied . . . . . V . The Kinds of Microorganisms Found on Plant Roots . . . . . . . . 1. Relative Occurrence of the Major Groups of Soil Microorganisms . . 2. The Fungal Flora of the Rhizosphere . . . . . . . . . . . . 3. The Bacterial Flora of the Rhizosphere . . . . . . . . . . . . a . Physiological and Morphological Characterization . . . . . . b . The Incidence of Individual Genera and Species . . . . . . . *Paper No . 5-1612, Project 965, of the Iowa Station 241

.

212 243 244 247 247 248 248 249 250 250 252 254 255 257 260 260 261 263 264 264 266 268 268 269 269 270 270 271 271 271 273

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FRANCIS E. CLARK

VI. Modification of the Root Surface Microflora . . . . . . . . . . . 1. By Seed or Soil Inoculations . . . . . . . . . . . . . . . . 2. By Soil Treatment . . . . . . . . . . . . . . . . . . . . 3. By Plant Treatments . . . . . . . . . . . . . . . . . . . 4. By Application of Herbicides and Insecticides . . . . . . . . . VII. Influences of the Rhieosphere Flora on Succeeding or Aswciated Plants . 1 . Persistence of the Rhieophilic Flora in Field Soils . . . . . . . . 2. Persistence of Changes in the Soil Environment Brought About by the Rhizosphere Flora . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . .

274 274 275

276 276 278 278 279 282

I. INTRODUCTION For many years the plant root has been regarded as an absorbing and anchoring organ, between which and the surrounding soil there exist numerous relationships. Concepts concerning these relationships must be subjected periodically to scrutiny. Certain clarifications have already been made. I n the uptake of nutrients by plant roots, it was formerly considered that mineral salts entered the cell sap of epidermal cells and root hairs by processes of simple diffusion from the soil solution. Such processes have been found inadequate to account for the transfer of nutrient materials into the root, particularly since solutes may be encountered in the cell solution in higher concentration than in the soil solution; accordingly, other physical as well as metabolic explanations are now offered. Further, with the discovery of the phenomenon of base exchange, it has become recognized that cations come largely from absorption sites in the clay lattice and not simply from the soil solution. A concept relatively little emphasized is that plant roots are in contact not merely with a physicochemical environment, but with a microbiological environment as well. Root hairs, as well as root surfaces, are covered with a mantle of microorganisms, which affect the growth and welfare of the plant in many ways, and the activities of which in turn are subject to influences of higher plants. The concentration of soil microorganisms upon plant root surfaces is intense, and a t times may reach such proportions that the roots themselves possess no direct contact with the soil solution. Such is the case with tree roots that become encompassed by mycorrhizal-forming fungi. For other roots, the ensheathment is less complete, but, nevertheless, the region of contact between plant roots and the surrounding soil remains a region of microbiological interest. It is here that the soil microorganisms may exert their most direct influences upon plants. Here any beneficial products of microbiological decompositions or syntheses are in immediate contact with the root absorbing surfaces; here also, any toxic or injurious substance of microbial origin, or any com-

SOIL MICROORGANLSMS AND PLANT ROOTS

243

petition by microorganisms for nutrients, directly challenges t,he growing plant. It is beyond the purpose of this review to consider either the manifold activities involved in microbiological transformations of organic and inorganic subst.ances in soil, or the physiological responses of plant root$. Its purpose primarily is to consider the extent t o which the root surface microflora, by reason either of its composition or its proximity, influence plant growth and welfare. There is also offered a brief characterization of the microflora encouraged by growing roots, and a discussion of the persistence of this microflora under field conditions. 1. Historical Summary

Interest in the microbiology of plant roots dates rather sharply from Hellreigel and Wilfarth’s report in 1888 on the symbiosis between root nodule bacteria and leguminous plants. Successful explanation of the fixation of atmospheric nitrogen by symbiotic bacteria served not only t o stimulate research in soil microbiology as a branch of science, but it also led microbiologists to expect that similar relationships, equally clear-cut and dramatic, might be established between soil bacteria and other plants. I n the decade following Hellreigel and Wilfarth’s contribution, claims were made that nonlegumes could be inoculated profitably with diverse bacteria. Although such claims failed to win general acceptance, they have not failed to attract enthusiastic supporters even to this day. A further debate, more academic than that concerning the bacterial inoculation of nonlegumes, developed at the opening of the twentieth century as to whether there was a specialized microflora associated with plant roots or whether there was merely a stimulation of the general soil microflora. This controversy was nonproductive excepting for its emphasis upon the association of microorganisms and plant roots, and the resulting designation of this ecologic region by Hiltner in 1904 as the rhizosphere. The rhizosphere is defined as that soil region inside which the soil is subject to the specific influence of plant roots. Knowledge concerning the microflora of the rhizosphere developed slowly during the first quarter of the current century. There were scattered observations that the bacteria associated with roots might affect plant development. The possibility was suggested that the root microflora, by its contribution to the carbonic acid production in soil, might affect the solution and availability of mineral salts. It was also admitted that the rhizosphere population might compete directly with plants in the assimilation of nutrient elements from soil, and there was observed a depression of mineralization of nitrogen in soil under the influence of plant roots.

244

FRANCIS E. CLARK

A series of papers by Starkey (1929a-l929c, 1931a, b) in the period 1929-1931 called attention to the many microbiological problems afforded by the rhizosphere. Stmarkeyboth reviewed the earlier literature, and presented his own extensive data concerning microbial numbers and activity in the root zone and the unequal stimulation by roots of different microbial types. He observed that type of plant, age and condition of any given plant, and proximity to roots influenced microbial activity in soil; he also emphasized the possible importance of the rhizosphere flora to the growing plant. More recent investigations on the rhizosphere have been fostered by a diversity of interests. Russian microbiologists have developed an extensive program of seed treatment wit,h nonsymbiotic bacteria, claiming that such inoculation increases yields; they have also been interested in the relation of the rhizosphere microflora to the formation of a stable soil structure. Some workers in t,he U S . Department of Agriculture have studied the saprophytic microflora of the rhizosphere and its relationship to differences in resistance of plant roots to soil-borne plant pathogens. Canadian investigators have been similarly intereeted, and t.hey have also considered the rhizosphere in relation to the nutritive requirements of soil bacteria. Workers in several countries have considered the role of the rhizosphere flora in the uptake or availability of nutrient. materials to plants. Krassilnikov (1940) has emphasized the extent to which bacteria overgrow root surfaces and has reviewed the literature concerning influences of microorganisms on the growth of plants. Kabznelson et aZ. (1948) have reviewed the more recent literature concerning the microflora of plant roots, particularly that dealing with t,he preferential stimulation by higher plants of certain types of soil organisms. 2. Characterization of the Rhizosphere

The diversity of interests in rhizosphere microbiology has led to a profusion of experimental techniques and to differences in expression and interpretation of results. It becomes desirable, therefore, to attempt some orientation of the biology of the rhizosphere before undertaking to discuss, firstly, t,hose activities of microorganisms that may affect plant welfare, and secondly, those alterations within the soil microflora that are occasioned by plant growth and development. Hiltner’s definition of the rhizosphere, stated above, refers primarily to soil adjacent to plant) roots. Nevertheless, nearly all microbiological studies of the rhizosphere by cultural procedures have included both roots and soil as material for study. Referring indirectly to the rhizosphere, Thorn and Smith (1939) speak of “that ball of earth, filled by the roots

245

SOIL MICROORGANISMS AND PLANT ROOTS

of a particular plant, with the microorganisms that accompany them." Photomicrographs of Cholodny slides recovered after burial within the root zone show large numbers of microorganisms to be associated intimately with the root surfaces, very much as an enveloping sheath, with the surrounding soil relatively sparsely colonized. Both Starkey (1938) and Linford (1942) have published excellent photomicrographs of associations of microorganisms and roots as shown upon glass slides exposed within the root zones of growing plants. Cultural data also show the importance of proximity to roots upon the magnitude of microbial populations encountered. Representative data for the cotton plant rhizosphere are shown in Table I. Starkey (f931a) has published microbiological data for other plants which serve equally well to emphasize the immediate localization of microorganisms on root surfaces. Perot,ti (1926) , in attempting to establish boundaries for the rhizosphere, has considered it to be bounded on one side by the general soil region, or edaphosphere, and on the other, by the root tissues, or histosphere. Although a sharp line cannot be drawn between the edaphosphere and the rhizosphere, practically, the boundary may be considered to be reached wherever plant roots can no longer be shown to have an observable influence on the soil flora. I n Table I, with sample (a) considered as representative of t,he soil flora apart from and uninfluenced by roots,

TABLE I Occurrence of Certain Types of Bacteria in Relation to Proximity to Cotton Roots"

Total bacteria Description of sample (a) Soil, 10 to 15 cm. distant from roots . . . . . . . . . . . . . . . . . . . . (b) Soil, 5 to 10 cm. distant from roots . . . . . . . . . . . . . . . . . . . (c) Soil, 2.5 to 5 cm. distant from roots . . . . . . . . . . . . . . . . . . (d) Soil, 0.5 to 2.5 cm. distant from roots . . . . . . . . . . . . . . . . . . (e) Soil, 0 t o 0.5 cm. distant from roots . . . . . . . . . . . . . . . . . . . ( f ) Root surface scrapings . . . . . a

b

I

Dyetolerant bacteria

I1

Ratio

II/I

Spores of bacillus b Ratio

111

III/I

52.6

7.4

0.141

17.0

0.323

47.9

9.0

0.188

18.0

0.376

45.6

7.7

0.165

17.0

0.373

54.7

15.0

0.274

11.0

0203

129.9 510.0

91.3 440.0

0.702 0.863

13.7

0.105
0.1

After Clark (1940). Hundred thousands per g. of air-dry soil or per g. of root surface scrapings.

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FRANCIS E. CLARK

samples (b) and (c) would not be included in the rhizosphere on the basis of microbiological data presented for them. If appreciable cultural or direct microscopic differences are not required as criteria in delimit,ing the rhizosphere, then it must be considered to be the entire field soil at least to the depth penetrated by plant roots, and perhaps even further, inasmuch as such soil is, in the main, under the influence of roots. Past experience and usage is justifiably against such an extensive definition. The histosphere, which in Perotti’s scheme serves as t.he inner boundary of the rhizosphere, has in the past been considered normally to be free of microbial cells. Exceptions, to be discussed shortly, occur in parasitism, in mycorrhizae, in bacteriorrhizae and the nodulation of legumes, and, of course, during the terminal disintegration and decay of the root system. Some recent work makes it appear probable that the interior tissues of plant roots are more commonly colonized by microorganisms than has heretofore been recognized. Perotti (1926), and almost wit,hout exception other writers publishing on the rhizosphere, include under that term the det.ritus on the root surface, sloughed or dying cells, and the microorganisms closely attached to the root surfaces. Inasmuch as nutrient material coming from the roots is primarily responsible for this immediate microbial concentration, this mantle of microorganisms attached to the root surface is undoubtedly the most important part of what has been called the rhizosphere microflora. Accepting the root surfaces as responsible for the bulk of the microorganisms found in those combined soil and root samples taken collectively as the rhizosphere, it would be more logical to speak of this microflora in terms of the root surfaces than in terms of the soil adjacent to the root# of plants. This goal can be obtained by the use of the term, rhizoplune. The rhizoplane is defined as the external surfaces of plant roots together with any closely adhering particles of soil or debris. The introduction of the term, rhizoplane, by shifting the microbiological emphasis from the soil around the root to the root surfaces themselves, provides those working either with roots or root surface scrapings with a means of considering their data on other than a soil basis. The frequent employment in the literature of the phrase “the microorganisms associated with root surfaces” indicates an unwillingness to ascribe t o the surrounding soil the microbes present on root surfaces. Certain workers (Graf, 1930; Poschenreider, 1940) have used the terms “outer rhizosphere” and “closer rhizosphere” in describing sites of microbial concentration. Berezova (1941) considered there were two distinct zones in the rhizosphere supporting appreciably different microbial populations - o n e zone was the adjacent soil, the second, the root surface itself. The writer is of the opinion that by using the term rhizosphere to describe

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the Soil region adjacent to plant roots, and the term rhizoplane to denote the plant root surfaces, much of the uncertainty and confusion in the literature can be avoided.

11. TYPESOF RELATIONSHIPS BETWEEN MICROORGANISMS AND PLANT ROOTS Microorganisms associated with plant growth cannot be classified rigidly as to whether they occur within, on the surface of, or only at a distance from plant roots. The site a t which any given microorganism occurs may vary with changes in environmental or physiological conditions. Nor can soil microorganisms be classified rigidly as to the type of relationship exist,ing between them and plant roots. These relationships may range all the way from chance association to symbiotism or to parasitism. 1 . Symbiotism Symbiotism between microorganisms and plant roots, in the sense of mutualism or benefit to both partners of the associat,ion, is best known in mycorrhizal formations and in the nodulation of legumes by Rhizobium. A mycorrhiza is defined as the symbiotic association of a fungus with the roots of a seed plant, with the symbiosis probably of mutual benefit, though not unquestionably proved to be so. Where the hyphae of the fungus form an interwoven mass investing the root tips, they constitute an ectotrophic mycorrhiza, where they penetrate the parenchyma of the roots, t.hey constitute an endotrophic mycorrhiza. A mycorrhiza is the morphologic combination of a fungus and a root. The term mycotrophy denotes the processes of plant nutrition by means of mycorrhizae, or the tendency of fungi and roots to form associations of functional benefit. The literature on mycorrhizae and mycotrophy is voluminous. A number of review articles have been published; it will suffice here to mention those of Rayner (1927), Hat.ch (1937), and Schmidt (1947). Bacterial associations with roots, analogous to those formed by fungi in mycotrophy, are termed bacteriorrhizae. Very little is known concerning the actual existence or extent of bacteriorrhizae, apart from the root hair invasions and the resulting localized nodule formations by rhizobia. Nodules on legumes caused by rhizobia may be considered to represent a localized bacteriorrhizal condition. Because of their practical importance, the rhizobia have been studied more extensively than any other group of soil microorganisms. They are commonly referred to as the legume bacteria, and their symbiotic relationships with their host plants are generally acknowledged. Monographs on the nitrogen-fixing

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bacteria have been prepared by Fred et al. (1932), and by Wilson (1940). Whether actinomycetes form symbiotic associations with roots similar to those formed by fungi and bacteria is unknown. There remains the possibility of the eventual discovery of an “actinorrhiza.” Lutman and Wheeler (1948) observed invasions of cells of potato tubers by microbial filaments, which they first thought to be actinomycelial in nature, but which yielded bacilli upon culture. Sanford (1948) has recently reported that the interior tissues of the stems and tap roots of potato, lucerne, and sweet clover contain a mixed bacterial flora. T e y e t and Hollis (1948) also have encountered bacteria in healthy storage organs of potato and other plants.

2 Parasitism The occurrence of fungi, actinomyces, bacteria, and viruses injurious to plant roots has long been recognized. References and discussions concerning the phytopathogenic microflora of plant roots are available in the numerous textbooks and monographs on plant diseases. 3. Commensalism

Because of the excretion of inorganic and organic substances, and the sloughing of root caps, root hairs, cortical and epidermal cells, all of which supply available energy material for microbial utilization, a great number of microorganisms are encouraged to develop immediately upon the root surfaces, and the majority of these microorganisms are undoubtedly commensalistic in their relationship to the higher plant, living a t the expense of secreted or excreted material, but not invading the root tissues with either injurious or beneficial results. Certain workers have emphasized the scavenger role of microorganisms on root surfaces ; others, the existence of a successive symbiotic effect, rather than a true or simultaneous symbiotism. Undoubtedly the relationships between microorganisms and plant roots are variable, depending upon environmental and physiological conditions. Microorganisms normally commensalistic may become parasitic with decreased vigor of plant growth. Similarly, mycorrhizal invasions vary from absolute parasitism to complete symbiosis, depending upon t.he vitality of the fungus and the health of the higher plant (McArdle, 1932). Thornton (1935) has suggested that eventually it will be possible to trace an evolutionary gradient from the purely ecological relationships that exist between free-living soil bacteria and higher plants through that too little studied population of organisms existing outside but in close contact with the roots of higher plants, to those microorganisms such as mycorrhizal fungi and rhizobia that live symbiotically within

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the root tissue. Since the determining factors in such a gradient have not yet been clearly defined, it remains necessary to speak rather generally of the physiological relations between plant roots and bacteria, and quite frequently to employ physical site of occurrence, rather than type of physiological relationship, for microbial characterization.

111. THERHIZOSPHERE MICROFLORA IN RELATION TO THE GROWTH OF HIGHER PLANTS Soil microorganisms have been considered capable of affecting the growth of higher plank in numerous ways. Waksman and Starkey (1931) state summarily that in the soil microorganisms may generally (1) act as general agents of decomposition of organic constituents of the soil leading to the formation of water, carbon dioxide, ammonia, sulfates, and phosphates; (2) act as transformers of such mineral constituents of the soil as ammonia and sulfur, oxidizing them to nitrate and sulfate, respectively; (3) act as agents of assimilation of nutrients which are thus removed a t least, temporarily from the zone of absorption of plants; (4) act as agents lowering the oxygen concentration in the soil system and thus create conditions unfavorable to root growth; ( 5 ) produce toxic substances or reduce such substances as nit,rates and sulfates to gaseous nitrogen and sulfides, thus rendering them unavailable; (6) act as solvent agents through the organic and inorganic acids which are produced in various transformations ; (7) act as nitrogen-fixing organisms, the nitrogen sooner or later becoming available to higher plants. Close t o the root systems these processes may either be accentuated or diminished, because of increases in numbers of microorganisms about roots or because of the preferential stimulation of certain types. In the two sections immediately following, some effects on the growth of higher plants of the activities of microorganisms within the rhizosphere will be considered. For discussion, t,hese effects are divided into two broad categories: (a) those which directly affect the availability of plant nutrients, and (b) those which affect plant weIfare by influences other than those of nutrient transformations. Under the former heading, influences of the root microflora upon the availability of combined nitrogen and the fixation of free nitrogen, and the bringing into solution and chemical transformation of elements other than nitrogen are discussed; under the latter heading, there are considered influences of the rhizosphere flora in the production of plant-accelerating substances, in the incidence of disease, and in the formation of a desirable soil structure.

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1. Influences on the Availability of Nutrient Elements a. On the Availability of Combined Nitrogen. The amount of nitrogen available in soil is very frequently the limiting factor in crop growth : consequently, the rate a t which it becomes mineralized from organically combined, unavailable nitrogen, is a question of prime importance. Numerous workers (Brown, 1912; King and Whitson, 1900; Lyon et al., 1911, 1913, 1920, 1923, 1924, 1928; Wilson and Wilson, 1925) have noted the greater accumulation of nitrate nitrogen after the growth of legumes than after nonlegumes. This accumulation doubtless can be ascribed to the higher nitrogen content of the legume residues, inasmuch as the rate of mineralization of plant material in soil is dependent upon the carbon/nitrogen ratio of the residues involved. It is generally believed that nitrogen in the soil is transformed primarily by microbial activity, and that claims for photonitrification processes have not as yet been adequately substantiated. The work of Broadbent and Norman (1947) has suggested t*hatmicrobiological transformation of nitrogen in soil may be limited a t times by the absence of available energy material to support a vigorous microbial population. Nitrogen-balance experiments on legume-cropped soils have been rendered impracticable by the fact that legumes in symbiosis with nodule bacteria are able to fix atmospheric nitrogen. Several studies have been made of the influence of the growth of nonlegumes on nitrogen mineralization in soil. Mineralization of nitrogen under such crops appears to differ in rate from that in fallow soil. I n 1914 Russell reported that, at the end of a growing season, land cropped to wheat and barley contained less nitrate than fallow land, even though an accounting was made of the nitrogen removed by the crop. Lyon et al. (1923), in experiments with oats and maize, demonstrated that the accumulat*ionof nitrate under these crops was less than in corresponding fallow soil. Allowance was made for the nitrogen taken up by the crop. Prescott (1920) noted depression of nitrification in similar experiments with maize and wheat. It remained unsettled whether the rate in cropped soil was variable for different crops or for differing growth stages of a single crop. Nor was it determined whether there was an actual depression of mineralization or an apparent one caused by denit,rification and loss of gaseous nitrogen to the air. There have been occasional claims that cropping may result in a loss of mineral nitrogen not entirely accounted for by that taken up by the crop or by changes in the organic nitrogen content of the soil. Pinck et al. (1945) suggested that there could be loss of nitrogen due to metabolic processes occurring within growing plants. Not all workers have agreed that crops depress mineralization.

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Greaves et al. (1917) and Lohnis (1926) believed that plant growth favored mineralization of nitrogen. Lyon e t al. (1923) believed that maize might stimulate nitrogen mineralization during the earlier stages of growth. Starkey (1931b) found that nitrates accumulated more rapidly in soils taken from near the roots than in soils taken some distance away; he concluded that cropped soils have a greater nitrifying capacity than do fallow soils. Soil taken from near the roots, however, may contain more organic material than that taken some distance away, and consequently the release of mineral nitrogen would be greater from soil from within the root zone simply because such soil contains more readily decomposable material. The results would not give true est.imates of the nitrifying capacities of soils under growing crops. Starkey suggested that the influence of plant growth on nitrification may have been due to the addition of organic matter with narrow carbon/nitrogen ratio to the soil resulting in local liberation of ammonia on decomposition. Goring and Clark (1949) have recently studied the mineralization of nitrogen in soil under the influence of a number of growing crops, and have reported nitrogen mineralized in soil cropped for 13 weeks to be approximately half th at mineralized in aliquots of the soil maintained fallow. I n fallow soil, nitrogen mineralized averaged 30 mg. per kg. of soil; in cropped soil, with allowance for the nitrogen taken up by the growing plant (tops and roots), 14 mg. of nitrogen per kg. were mineralized. Mineralizations determined for the several crops were as follows: timothy, 20.4; wheat 19.4; brome grass, 18.6; tomato, 17.6; oats, 15.0; tobacco, 11.6; rye, 7.6; and Sudan grass, 1.4 mg. per kg. The depression of mineralization was positively correlated with total weight of roots in the cropped soil lots and also with total number of bacteria contributed by the roots of the crop grown, but negatively correlated with the nitrogen content of the harvested roots. These correlations were interpreted to indicate an immobilization of nitrogen by the rhizosphere microflora. This int,erpretation was supported by a subsequent observation that during 16 weeks of incubation in the laboratory, nitrogen mineralization in soil lots previously cropped exceeded th a t in soil lots previously fallowed. I n fact, for the majority of crops, the accelerated mineralization following cropping approximated the previously observed depression of mineralization. With longer incubation, it is possible that the few soil lots for which exception must be made would likewise have made up their previous deficits. It was also observed that following cropping, the rates of nitrogen mineralization were surprisingly uniform for the several soil lots previously cropped, regardless of the crop grown. The numbers of nitrifying bacteria were found unaffected by crop

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growth. Other investigators have also failed t o find differences in numbers of nitrifying bacteria in rhizosphere soil and the surrounding soil. Starkey (1932a), Graf (1930) , and Katznelson (1946) noted no appreciable influence of crop growth on numbers of ammonia-oxidizing and nitrite-oxidizing bacteria. Numerous investigators have noted that in the rhizosphere there are increased numbers of bacteria which under suitable conditions are capable of bringing about denitrification, but there has been no direct evidence that nitrogen loss to the atmosphere does occur. To the possibility that qualitative shifts in the microflora may be sufficient to alter the rate of nitrogen mineralization must also be added the possibility that with crop growth differences may be established in aeration or in moisture that in turn affect nitrogen transformations. Willis and Green (1949) have reported a more fairorable total nitrogen balance in flooded soil cropped to rice than in flooded soil maintained fallow. I n summary, mineralization of nitrogen appears depressed during crop growth. At present no adequate explanation can be given for the data recorded in the literature. It does appear plausible, however, that there is a microbiological immobilization of nitrogen during crop growth, and that this factor may account for the apparent difference in mineralization. Undoubtedly, nitrogen transformations in soil present a fertile field for the application of isotopic techniques. b. On the Fixation of Free Nitrogen. Following Beijerinck’s discovery (1901) of the Azotobacter group, Hiltner (1904) stressed that these nitrogen-fixing bacberia were stimulated by growing roots. Beij erinck and Van Delden (1902) found Azotobacter in the vicinity of roots especially.in conjunction with B . radiobacter. Greaves (1918) reported Azotobacter more abundant in cropped than in virgin soils. Several Russian workers (Rokitzkaya, 1932; Sheloumova and Menkina, 1936; Sidorenko, 1940) have noted favorable response by Azotobacter to roots of certain plants. Starkey (1931a) cited scattered references that Azotobacter is especially favored by cruciferous plants, but he himself did not find appreciable stimulation of that microorganism by plant roote. Poschenreider (1929) examined the roots of crucifers and found Azotobacter present; he then extended his work and found this bacterium associated with the roots of a wide variety of plants (1930). His observations have been cited in support of the contention that Azotobacter is particularly adapted to the rhizosphere. Careful reading of Poschenreider’s papers, however, shows that he used enrichment cultures only, and therefore his results can hardly be considered as quantitative. I n fact, a t certain seasons of the year, he could not find Azotobacter in the closer rhizo-

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sphere, even though the organism was present in the adjacent soil. Truffaut and Vladykov (1930) reported Azotobacter generally present in the rhizosphere of wheat, but again, the criticism may be made that their observations are of doubtful value, as they did little more than record the presence of some easily cultured and easily recognized soil organisms. Several of the bacteria they named in addition to Azotobacter can hardly he considered as rhizophilic organisms. Lipman and Starkey (1935) stated that the bulk of the available evidence showed that Azotobacter does not appear more frequently in the rhizosphere than in soil apart from roots. Most studies since 1935 have been in agreement with their summarizing statement. Neither Clark (1940) nor Jensen (1940) found Azotobacter in the rhizosphere of wheat in soils generally free of Azotobacter. Jensen and Swaby (1940) failed to find Azotobacter stimulated by the roots of legumes. Krassilnikov (1934) reported that Azotobacter was depressed by the roots of plants. Katznelson (1946) failed to note any positive response to mangels by Azotobacter, even t,hough other microbial groups were markedly stimulated. Recently, Clark (1948a) has reported that Azotobacter introduced into the rhizosphere of tomatoes disappeared rapidly. The disappearance of Azotobacter was found to be more rapid in cropped than in uncropped soil. Addit,ion of tomato root fragments to soil did not affect the Azotobacter content of Webster loam normally containing Azotobacter. The Azotobacter flora of soil normally containing these bacteria was no greater when such soil was cropped to tomatoes, to soybeans, to broccoli, or to mustard than when it was maintained fallow. Paralleling the disagreement on the occurrence of Azotobacter in the rhizosphere, there has been controversy concerning the value of crop inoculation wit.h Azotobacter. Several early investigators claimed benefit from inoculation of nonlegumes, but others failed to duplicate their successes, and prior to 1925 there was very little inoculation of nonlegumes (Allison, 1947). Within the past two decades, extensive claims have been made by a number of Soviet scientists that yields of nonleguminous crops are superior following seed or seedling inoculation with Azotobacter. Field inoculation with this bacterium in the U.S.S.R. reached 5 million acres in 1942, and is estimated to have tripled since that time. Allison (1947) has recently reviewed t,he Russian literature, and it is unnecessary to duplicate his citations here, Although few workers outside the U.S.S.R. believed that Azotobacter could be used profitably for crop inoculation, the extent of the Russian claims has prompted some furt,her investigation. Only negative results have been reported. Allison and coworkers (1947) failed to obtain increased crop yields or increased nitrogen content in crops receiving

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Azotobacter inoculations in greenhouse experiments. Gainey (1946) has reported negative results from Kansas field experiments carried on for 20 years. Timonin (1949) has reported failure to obtain increased yields in field experiments conducted in Canada under widely differing conditions of soil and climate. Although anaerobes capable of fixing nitrogen in culture solution have been found in association with the roots of rice plants (Sen, 1929), reports concerning the incidence of anaerobic, free-nitrogen-fixing organisms in the rhizosphere are too few and too incomplete in character either to justify any summarizing statements concerning them or to ascribe to them any importance in fixation of nitrogen in the rhizosphere. Nor does there appear any satisfactory evidence that algae capable of fixing atmospheric nitrogen are of any especial importance in the nitrogen economy of cropped soils. I n summary, there are a t present neither adequate microbiological data, nor acceptable crop yield data, to warrant a positive st,atement that there is any increased fixation of atmospheric nitrogen in the rhizosphere, with or without attendant benefit to t,he growing plant, by either anaerobic or aerobic microorganisms other than rhizobia. c. The Chemical Transformation of Elements Other Than Nitrogen. In soil, microorganisms transform many elements in addition to nitrogen. I n general, microbial transformations affecting plant growth involve either the oxidation or reduction of inorganic compounds, the decomposition of organic compounds, or the assimilation of materials into microbial tissue. The availability of iron, sulfur, phosphorus, potassium, and of other elements, as affected by microbial transformations apart from t,he rhizosphere, has been discussed by Waksman and Starkey (1931). It was established early that manganese could be rendered unavailable to plants by the oxidizing action of soil bacteria. Beijerinck (1913) and Sohngen (1914) demonstrated that both soil bacteria and fungi were capable of converting available manganese into the unavailable form. It was also noted that various plants grown under conditions of manganese deficiency developed characteristic diseases. The grey-speck disease of oats is illustrative. The role of manganese in the etiology of this disease has been discussed by Samuel and Piper (1929) and Gerretsen (1937). Recognizing that susceptibility of oats to grey-speck disease is varietal, Timonin (1947) compared the rhizosphere microfloras of resistant. and susceptible varieties. It was found that the rhizosphere of the susceptible variety of oats harbored a denser population of manganeseoxidizing, casein-hydrolyzing, and denitrifying bacteria than did the

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rhizosphere of a resistant, variety grown in the same soil under identical conditions. Fungi on the other hand were most numerous in the rhizosphere of the resistant variety. With application of soil fumigants, the bacteria capable of oxidizing manganese were greatly reduced or completely eradicated, and susceptible plants grown in such treated soil showed lowered incidence of disease or were disease-free. There was a positive correlation of 0.939 between severity of disease and number of manganese-oxidizing and cellulose-decomposing organisms. Such a study is of interest as an example of the microbiological factors involved in manganese transformations. Fuj imoto and Sherman (1948b) have discussed chemical aspects of the manganese cycle in soil. Activities of microorganisms in phosphorus transformations in soil have been summarized by Pierre (1948) under the following headings: ( a ) mineralization of organic phosphorus; (b) immobilization of available phosphorus; and (c) effects on the solubi1it.y of organic and inorganic phosphorus compounds. Organic phosphorus compounds, which make up a significant fraction of the total phosphorus of soils, are readily subject to microbial attack. Thompson and Black (1948) and Thompson et al. (1949) noted close correlat,ion between the mineralization of phosphorus, nitrogen, and carbon in soils during incubation. Only a few scattered references exist concerning the role of the rhizosphere flora in mineralization of phosphorus from organic compounds. Verona and Luchetti (1931) ascribed the ability of cruciferous plants to use more phosphorus to a favorable microflora in their rhizosphere. Some Russian microbiologists have isolated microorganisms which break down lecithin and nucleic acid in culture and have claimed that seed inoculation with such types gives increased crop yields (Menkin, 1946). Such inoculation claims have not been substantiated. Phosphorus immobilization by microbial populations in soil has been shown by Thompson et al. (1949). The problem of phosphorus immobilization in the rhizosphere has not been investigated. Whether there occurs immobilization commensurate with the size of the root system and the magnitude of the microbial population in the rhizosphere has not been shown. At least it can be said with certainty that if nitrogen immobilization by the microbial population of the rhizosphere occurs, then phosphorus immobilization also occurs, inasmuch as neither element can be immobilized in microbial tissue independently of the other. d. I n the Solution of Relatively Insoluble Minerals. I n the soil, carbon dioxide may facilitate the release of nutrients from relatively insoluble soil minerals. Therefore, i t is appropriate to preface a discussion of the effects of the rhizosphere flora upon the solubility of

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minerals with a few remarks on the role of microorganisms in carbonic acid production in soil. Carbon dioxide in cropped soil is produced almost entirely from biological activity, either directly from root respiration, or from microbial attack upon soil organic matter and upon organic materials coming from plant roots. Carbon dioxide production has been observed greater from planted than from unplanted soil (Neller, 1922; Headdon, 1927) and bicarbonates have been found in much greater concentration near plant roots than in soil apart from roots (Metzger, 1928). St.arkey ( 1 9 2 9 ~ ) has shown that soil taken from near roots yielded more carbon dioxide than did distant soil. Such an observation doubtless can be explained by the greater organic matter content of the root-adjacent soil, together with its sharply higher numbers of microorganisms. Attempts have been made to evaluate the relative importance of rhizosphere microorganisms in the formation of carbon dioxide about plant roots. Following the experiments of Liebig (1858) and Sachs (1860) , who demonstrated that marble and ostheolite were etched by plant roots, Fred and Haas (1919) showed that the etching of marble by roots of peas was markedly less in sterile cultures than in cultures contaminated with bacteria. They concluded that microorganisms played an important role in etchings obtained. Data of Lundeglrdh (1924) indicate that nearly half of the carbon dioxide arising from plant roots growing in unsterilized sand is produced by their accompanying microorganisms. Root respiration in unsterilized and in sterilized sand amounted to 5.57 mg. and 3.05 mg. carbon dioxide per hour, respect.ively. Barker and Broyer (1942) noted that 63 per cent of the carbon dioxide produced by squash roots in aerobic culture came from bacterial respiration. Stille (1938) concluded that microorganisms were responsible for roughly 35 per cent of the carbon dioxide evolved from a sand and solution substrate in which mustard plants were being grown. Parker (1924, 1925) questioned whether carbon dioxide was of any value in the feeding power of plants. He noted no correlation between carbon dioxide in the soil and mineral content of plants grown. The removal of carbon dioxide by continuous aeration did not influence plant composition. Truog (1927), and McGeorge (1938) , however found that carbon dioxide increased the availability and absorption of phosphorus by plants, at least in calcareous soils. Gerretsen (1948) recently has published interesting data showing an effect on plant nutrition by microorganisms in the rhizosphere. Oats, mustard, sunflower, and rape were grown under sterile and nonsterile conditions in pot culture in order to determine the influence of micro-

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organisms on the top growth and the phosphate intake of plants. Data in Table I1 are illustrative of the order of increases in yield and in phosphorus content for plants grown with microorganisms present in their rhizospheres. Gerretsen pointed out that even though the presence of bacteria considerably increased the quantity of phosphorus in the plants grown, nevertheless the plants with sterile rhizospheres also absorbed notable amounts of phosphorus from tricalcium phosphate and bone meal. I n view of t,he negligible solubility of phosphorus in such compounds, he assumed that plant roots are able to mobilize some phosphorus independently of microbial activity, and he considered it possible that carbon dioxide production by microorganisms was the effective mechanism in the increased phosphate intake observed. Whatever the explanation, his evidence supports the view that the rhizosphere flora increases the phosphate intake by plants. Gerretsen’s experiments were performed with basic phosphates. I n acid soils, in which the inorganic phosphates of iron and aluminum tend to accumulate, it is possible that dissimilar results might be obtained. His observation that in an iron-deficient substrate, the increased phosphorus brought into solution when bacteria were present caused precipitation of the small amounts of available iron, and that as a result, much poorer plant growth was obtained in the presence of bacteria than in their absence. These aspects of nutrient intake certainly warrant further investigation. The solubility of elements other than phosphorus may be affected by microbial activity in the rhizosphere. Calcium, magnesium, potassium, and iron may be influenced by carbonic acid of microbial origin or by other acids arising as products of nitrification, sulfur oxidation, and fermentation.

e. Mycorrhizae and Bacteriorrhizae i n Relation to Plant Nutrition. Although Frank (1885) early suggested that mycorrhizal-forming fungi are beneficial to the plants on which they occur, there was for many years controversy concerning the mutualistic nature of the association (Schmidt, 1947). Within the past dozen or so years, several workers have presented convincing data that mycorrhizae benefit the higher plant. Hatch (1936,1937) compared pine seedlings in seedbeds with and without mycorrhizal fungi, and found that the mycorrhizal plants absorbed 86 per cent more nitrogen, 75 per cent more potassium, and 234 per cent more phosphorus than did non-mycorrhizal plants. Young (1936) obtained similarly positive results, as did Mitchell et al. (1937). McComb (1938, 1943) and McComb and Griffith (1946) noted increased phosphorus intake with mycorrhizae present on pine seedlings. Rosendahl (1942) found that mycorrhizae increased intake of potassium from a

TABLE I1 Influence of Microorganisms on the Yield and Phosphate Intake of Plantsa

Treatment

Total dry weight of plants

Increase in yield with microbes present, compared with sterile

g.

%

PsO, absorbed by plants mg.

Increase in P,06 absorbed with microbes present, compared with sterile

%

Oats, Avena sativa

Blank, without phosphate Ferrophosphate, sterile infected Algerian phosphate sterile infected CaHPO' sterile infected CadPO,). sterile infected Bonemeal sterile infected

2.0b

-

18

-

2.5 43

72

63 13.9

120

25 4.8

92

2.6 11.0

324

3.1 8.9

187

28.5 66.0

124

3.4 9.8

188

36.4 80.1

120

52 12.4

138

42.0 75 2

79

83.0 162.5

94

668 1215

82

Mustard, Sinapk albo Ca4PO')I sterile infected

108 225

108

Sunflower, Helicrnthus annuus Ca3(P04). sterile infected a

After Gerretsen (1948).

173 38.3

121 b

Mean values only from original tables.

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259

sand orthoclase medium. Rout,ien and Dawson (1943) reported that mycorrhizae enabled pine roots to absorb Ca, Fe, K, Mg, and P a t lower levels of base saturation of the clay substrate than was possible in their absence. As such reports indicate, most of the recent workers on mycorrhizae suggest that such formations are of importance in the intake of nutrient salts, but do not claim that mycorrhizae enhance the nitrogen nutrit.ion of the plants on which they occur. Hatch (1937) concluded that mycotrophy is not a special adaptation for acquiring nitrogen, as mycorrhizal organisms neit,her make available combined organic nitrogen, nor do they fix free nitrogen. Allison et al. (1934) failed to find mycorrhizal fungi capable of fixing appreciable quantities of free nitrogen. The possibility that mycorrhizae benefit the plant by affecting its carbohydrate nutrition is likewise not well supported. Schmidt (1947) has contrasted Bjorkman’s view that mycorrhizal fungi invade the plant and benefit it by removing excess carbohydrate with the reports of Falck (1923) , Francke (1934) and MacDougal and Dufrenoy (1944, 1946) that mycorrhizal fungi make additional energy material available to the plant. The suggestion of Perotti (1926) that bacteriorrhizae mobilize carbohydrates for transportation within the plant has not been substantiated. Both the experimental data on tree growth and composition, as cited above, and the greatly increased absorptive area provided in mycotrophy (mycorrhizal roots may expose several hundred times as much absorbing surface as nonmycorrhizal roots) support the proposit,ion of Stahl (1900) that mycorrhizal formations are of benefit because such organs increase nutrient salt absorption from the soil. It is interesting to note that essentially the same benefits are ascribed to the nonsymbiotic rhizosphere microflora as to mycorrhizal fungi. Preceding sections have indicated that combined nitrogen is not made availale more rapidly by microorganisms in the rhizosphere and also that there is no acceleration of nitrogen fixation therein by nonsymbiotic nitrogen-fixing organisms, but that there is good possibility that the rhizosphere flora increases the availability of mineral salts to the plant. Thus far, however, the experimental evidence concerning the role of the nonsymbiotic root-surface bacterial flora in plant nutrition is far more limited than that. for the mycorrhizal fungi, and it is only within the last decade or so that mycotrophy has been accepted generally aB of practical significance (Schmidt, 1947). I n rhizosphere microbiology, further studies such as the one by Gerretsen (1948) on phosphorus intake are clearly needed. Until factual data are a t hand, the role of the rhizosphere microflora in plant nutrition cannot accurately be assessed.

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2. Some Influences on Plant Growth and Welfare

a. Production of Plant Growth Substances in the Rhizosphere. In 1914 Hoffman reported that growth of individual species of bacteria in soil produced changes in the soil solution manifested by an increased or decreased development of plant seedlings when grown in extracts made from such soils. There soon followed a number of papers concerning the occurrence, nature, and action of plant growth substances. It is now generally considered that such substances are essential to the best growth and development of green plants. General reviews on plant growth hormones have been given by Boysen-Jensen (1936) and by Thimann and Bonner (1938). A general review of the role of vitamins in plant development has been prepared by Bonner (1937). Soil microorganisms can produce effective quantities of growth stimulating substances. Bacterial synthesis of various growth substances used by plants has been shown by McBurney et al. (1935), West and Wilson (1938), West (1939), Roberts and Roberts (1939), and Thompson (1942). That such syntheses by microorganisms are of more than academic interest has also been indicated. Clark (1930) and Clark and Roller (1931) found that sterile organic extracts added to nonsterile medium accelerated the rate of reproduction in Lemna, but were without influence when added to sterile medium. Thimann (1939) expressed the opinion that growth substances produced by rhizobia are of significance in nodule formation on legume roots. Wilson (1940) lists vit,amin B-1, vitamin C, nicotinic acid and biotin as of probable importance in the symbioses between leguminous plants and rhizobia. Isakova (1936, 1939, 1940) and Isakova and Smirnova (1937) concluded that bacteriorrhizal types are stimulatory to germinating seedlings, and that such stimulation extends throughout plant development. Brown (1946) concluded that some seeds will not germinate in the absence of external supplies of plant growth accelerators. Berezova et al. (1938) believed that the success of seed or seedling inoculation with Azotobacter was due to auxin production by the introduced organisms. Allison (1947) considered the most plausible explanation of the beneficial effect of Azotogen on crops to be the production of growth accelerating substances, which promote plant growth a t the maximum rate. It is also possible that toxic as well as stimulatory factors may be produced by rhizosphere bacteria. Schreiner and Shorey (1909) reported y’ears ago that decomposition products in the soil had harmful effects upon plant growth. More recently, McCalla and Duley (1948) have demonstrated that extracts of decaying sweet clover inhibit the

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growth of maize embryos. They suggested that the coumarin may be one of the constituents responsible for the inhibition of germination and growth. The question is raised whether there is any connect,ion between their observation and the reduced stands sometimes encountered in stubble mulch farming. Steinberg (1947), growing tobacco seedlings in aseptic culture in the presence of diffusates of some common soil bacteria, noted different types of chlorosis in the aerial parts, simulating various mineral deficiencies ; he also observed various types of malformed leaves. Such toxic effects, if present, are not widespread in nature, and the beneficial effects ordinarily far outweigh the toxic. Johnson (1919) found that the toxicity to plants of heat-sterilized soils was noticeable especially under sterile conditions, but that bacteria had an ameliorating influence. According to Fujimoto and Sherman (1948a), certain soils high in manganese may become toxic following heating because of t3he liberation of an excessive amount of available manganese. Colonization of such sterilized soils by manganese-oxidizing bacteria should prove beneficial. b. Influence of the Rhizosphere Flora on th,e Incidence of Root Pathogens. As a rule, root-invading fungi are more destructive in their attack upon susceptible plants grown in a sterile substrate than upon plants grown in aliquots of the same substrate openly exposed to a mixed microflora. Within recent years, there have been several attempts, with a t least partial success, t o exploit the principle of microbiological antagonism to obtain biological control of soil-borne root diseases. Certain of these studies have emphasized the importance of the rhizosphere flora in the incidence of root diseases. Eaton and Rigler (1946) have shown that corn, normally immune to Phymatotrichum root rot, is rapidly attacked and killed when grown in sterile culture without its normal complement of root surface bacteria. It is also known that improper host nutrition, rather than lack of competitive saprophytic microorganisms, may be responsible for infection. Corn grown in nutrient solution, with magnesium withheld, is also subject to lethal attack by Phymatotrichum (Gott and Goldsmith, 1946). Stumbo et al. (1942), Clark (1942), and Garrett (1941) have found that control of the take-all disease of wheat by application of organic manures, previously suggested as biological control (Fellows, 1929, Garrett, 1938), was largely a question of proper host plant nutrition. Some caution must be observed, therefore, in evaluating the effective mechanisms in controls achieved by use of organic manures. A general discussion of the root disease fungi has recently been prepared by Garrett (1944). Justification of the inference that the rhizosphere flora may in some manner be connected with disease incidence is seen in the occurrence of

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differences in microbial populations on roots of suscept.ible and resistant varieties of crop plants. Timonin (1940b) has observed that the rhizosphere of varieties of flax and tobacco plants susceptible to soil-borne fungi, even though the roots are entirely free from disease, harbor greater numbers of bacteria than do those of corresponding resistant varieties. His observations have been corroborated by other work (Lochhead, 1940; Lochhead et al., 1940; West and Lochhead, 1940a). Timonin (1941) also has demonstrated that a wilt-resistant variety of flax excreted sufficient hydrocyanic acid into its culture medium to alter the fungus flora of the rhizosphere. He found from 25 to 37 mg. HCN per plant in the solution after growth of the resistant variety, whereas the solution after growth of the susceptible variety contained only a trace. Allowed to diffuse from a collodion membrane, the former solution permitted growth of Trichoderma viride, a common soil fungus, but inhibited the pathogenic types, Fusarium and Helminthosporium. The possibility then arises. that the hydrocyanic acid may be directly responsible for differences in varietal susceptibilities, but there is also the possibility that it selectively stimulates T. viride, an “antagonistic” fungus regarded by many as import,ant in controlling the phytopathogenic fungi in the soil. That chemicals secreted by the host influence host disease resist,ance was first shown by Link and associates (1929a, 192913, 1933; Walker et al., 1929; Angel1 et al., 1930). The resistance of colored onions to smudge (Colletotrichum circinans) and neck rots (Botrytis a& and B. byssoidea) was found due to toxic phenolic subst,ances identified as protocatechuic acid and catechol. The occurrence of toxic secretions, although explaining some instances of disease resistance, fails to explain why susceptible varieties of some plants, even though grown entirely free from disease, harbor greater numbers of bacteria in the rhizoplane than do the corresponding resistant varieties. It was shown by Hildebrand and West (1941) and West and Hildebrand (1941) that soil treatments which control root rot of strawberries affect the qualitative nature of the rhizosphere microflora, as revealed by the nutritional requirements of large numbers of organisms isolated from the root surfaces. Clark and Thom (1939) reported that organic amendments applied to soil have no a,pparent effect on the root surface microflora. Although accepting the import.ance of biological control in the absence of the host plant, they considered antagonistic phenomena of lesser importance in the presence of the host plant. It is entirely possible that colonization of a root by one microorganism may affect subsequent colonization by a second. Virtanen and Linkola (1947) reported that initial exposure of legumes to ineffective strains of rhizobia prevents successful inoculation at a later date with effective st.rains. The

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factors involved in root disease incidence appear sufficiently numerous and complex so that it remains unwise to ascribe any predominant role to the rhizosphere flora until a considerably greater fund of information is available. c. T h e Rhizosphere Flora in Relation to the Formation of a Stable Soil Structure. Within the past dozen years, there has been increasing recognition of the importance of microbiological factors in soil aggregation. In Russia, publications by Kanivetz and Korneeva (1937), Kanivetz and Prikker (1940) and Radziewski (1940) stressed the importance of biological agents for obtaining a desirable degree of soil aggregation. In the United States, Waksman and Martin (1939), Peele (1940) and Peele and Beale (1940) contributed to the earlier knowledge concerning the role of microbiological factors in soil aggregation. Following these pioneer contributions, a number of other workers (Martin, 1945 ; Martin and Waksman, 1940, 1941; McCalla, 1942, 1944; Mishustin and Gromyko, 1946; Mishustin and Pushkinskaya, 1942; Myers and McCalla, 1941 ; Stocker, 1946) have evaluated the extent t o which microbiological activity contributes to aggregation phenomena in soil. The value of perennial grass vegetations in building desirable soil structure has long been recognized. It appeared logical that if microorganisms are instrumental in soil aggregation, then the rhizosphere, as a region in which there exists an extremely large microbial population, should be characterized more strongly by aggregate formation than soil apart from roots, Kanivetz (1940) was perhaps the first to emphasize that the formation of a desirable soil st.ructure was promoted by the joint action of two forces, namely, the growth and decomposition of the root system, and the activity of microorganisms in the root zone. Dorokhova (1942) believed t.hat fungi in the rhizosphere especially are important in a stable structure, and that inoculation procedures for this purpose may be beneficial. The data on inoculation are not convincinginoculated plots were cropped to barley, whereas the uninoculated plots were maint,ained fallow. That differences in aggregation resulted is not surprising. Sekera and Brunner (1943) likewise believed that living root hairs and fungi were responsible for the stability of soil aggregates. Geltser (1943a, b, 1945) concluded from studies extending over several years that the formation of a stable soil structure during the growth period of perennial grasses is caused mainly by the development of bacteria on plant roots. Considerable emphasis was placed on the importance of the products of bacterial autolysis. Nikolsky (1942) expressed the opinion that the main factors determining the formation of aggregates in chernozem soils were the living plant roots together with the undecomposed residues of dead p1ant.s. The humus produced by the enhanced

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biological activity of the rhizosphere initiated the aggregateion of finely divided soil particles. In the United States, Hubbell and Chapman (1946) have attempted to evaluate the joint. action of microorganisms and growing roots in the formation of a stable soil structure. Microorganisms were found capable of initiating aggregation, and roots contributed to the process only after microbial initiation. Tot,al aggregation was not affected by the presence of roots, but the soil aggregates were larger when roots were present. Contrary to Martin (1945), Hubbell and Chapman failed to find that the main effect of microorganisms in aggregate formation was attributable to cell secretions. Water-stable aggregates were observed only in the presence of living microorganisms, and compound microbial structures were held together by roots. I n exploratory studies with an elect.ron microscope, Jackson et al. (1947) likewise have demonstrated the importance of living microorganisms in aggregate formations. Most of the aggregates observed were centered around bacteria and other forms of microorganisms and organic matter. Norman (1947) has emphasized the probable superiority of a fibrous root system together with its rhixosphere bacteria to the simple process of turning under crop residues for securing improvement in soil structure. Application of organic manures may encourage initially greater microbial populations, but the period of active decomposition is relatively short. The stabilizing effects of microorganisms on structure under such conditions are also transitory. Plant root systems with their continued support of microbial growth provide greater opportunity for the operation of any struchre-forming effects of microorganisms that may exist. IV. THENUMBERS OF MICROORGANISMS ASSOCIATED WITH PLANT ROOTS

I. Methods of Study Discussion of quantitative data of plant root microbiology necessarily involves some consideration of t.he various procedures which have been used for the collection and expression of such data. There appears to be general agreement among different laboratories concerning methods of collecting and handling root samples for microbiological studies. There is less agreement concerning the basis to be employed for expression of microbial numbers. To illustrate the range of possibilities, it may be noted that densities of microorganisms in the rhizosphere have been expressed as numbers (a) per gram of root surface scrapings; (b) per gram of gross sample, that is, whole roots with adhering soil; (c) per gram of adhering soil fraction of the gross root sample; and also (d) as num-

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bers in successive washings of the root sample and (e) as numbers per square centimeter of root surface. The two last named procedures have not been widely used. Starkey (1929-1931) preferred to use root surface scrapings for microbiological study of the rhizoplane. Clark (1940) and Mitchell et ul. (1941) have occasionally employed root surface scrapings, but for the most part this group (Clark, 1939-1948b; Clark and Thom, 1939; Stumbo et al., 1942; Thom and Humfeld, 1932) has used samples containing whole roots with adhering soil. For fibrous rooted plants, such as grasses, the whole root system, or a reasonable aliquot thereof, may be employed; for larger tap-rooted and fleshy-rooted plants, only the secondary or feeding roots are collected. The attempt is made t o secure as much root surface as possible, and to avoid including in the sample fleshy or bulky portions, which contain a large amount of material not considered a part of the rhizoplane. When large or fleshy roots are being studied, root surface scrapings certainly should be employed in preference to the whole roots. Samples of composition similar to the ones recommended have been employed by workers in Canada (Kat,znelson, 1946; Lochhead et ul., 1940; Timonin, 1940a, b), but whereas many American workers have expressed microbial numbers per gram of gross sample, the Canadian investigators generally have determined the soil fraction of each gross sample, and have computed numbers of organisms per gram of oven-dry “rhizosphere soil.” There are a few exceptions, for example, Tyner (1948), also working a t Ottawa, has expressed microbial numbers per gram of gross sample, oven dry basis. According to Katznelson (1946), the “rhizosphere effect” is expressed by means of the rhizosphere-soil ratio (R:S), that is, the number of organisms in the rhizosphere soil divided by the number in the soil a t a distance from the root. The desirability of expressing microbial populations on root surfaces as numbers per gram of adhering soil, regardless of the amount of roots included, is open to serious question. For soil taken near the roots, or with but small amount of root material included, observations may well be reported per gram of soil. For roots with very little soil adhering, it appears reasonable to express the numbers per gram of sample material. I t is generally conceded that the roots or materials coming therefrom are responsible for the increased microbial populations observed in t.heir vicinity. Clark (1948b) has recently shown that when both root weight and soil weight in a series of rhizosphere samples from a number of plants were determined by recovery procedures and oven-drying, there was significant, correlation between magnitude of microbial population and root, content of the sample, but no significant correlation between

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soil content and microbial population. Katznelson et al. (1948) have suggested that for root samples from which as much superfluous soil as possible has been removed, the recorded data include dry weights of both roots and rhizosphere soil in the material studied. This suggestion appears commendable. 9. Density of the Microbial Population Within the Rhizosphere Plate count estimates of the soil population vary widely; some workers consistently obtain only a very few millions per gram, while others obtain much higher figures. Thom (1938) has discussed the range of findings recorded in the literature, and has preferred to accept a general estimate of 50 millions per gram. Populations in excess of this estimate are nearly always reported for rhizosphere samples. For the rhizoplane of corn, mangel beets, and beans, Starkey (1931a) obtained populations of 653, 427, and 199 millions, respectively, while the second year growth of clover, mangel beets, and table beets yielded counts of 3,470, 583, and 485 millions per gram. Considering all the plants together, there were 24.8 times as many bacteria on root surfaces as in soils close to roots. Exclusive of the legumes, bacteria were 12.1 times as numerous on roots; for the legumes alone, they were 50.3 times as numerous as in soil. Other workers generally have reported rhizosphere populat.ion/soil population ratios of approximately this same order. Timonin (1940a) reported increases in total microbial numbers of from 7 to 71 times for roots of wheat, oats, alfalfa, and peas. Lochhead (1940) reported increases of from 2 to 45 times for a variety of crops; Clark (1940) from 2 to 24 times for cotton and wheat; Zukovskaya (1941), up to 100 times for potatoes, flax, and clover; and Adati (1939), nearly 200 times for peas, with lesser increases for other plants. Such figures show the range of values reported from several different laboratories. These values are of course all subject to the many errors of the standard plate count (Harmsen, 1940; James and Sutherland, 1939). Accepting Thom’s estimate of 50 millions microorganisms per gram of soil, and accepting a rhizosphere/soil microbial population ratio of 10 as representative of reports in the literature, then roughly 500 millions of microorganisms are to be expected per gram of rhizosphere sample. This value merely provides a perspective-it will be pointed out shortly that many factors, such as type of plant, stage or condition of plant growth, and environmental influences, produce extreme fluctuations in the actual value to be determined. At this time brief comparison may be made of the magnitude of microbial counts on roots and those of other plant materials occurring in nature. Humfeld and Smith (1932) reported plate counts of from

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5 to 8 billions for the decomposing layer of a green manure in soil. Clark et al. (1948) reported counts as high as five billion, primarily of a single species, for late season cotton fibers in bolls prematurely opened by frost. Starkey (1931a) reported a maximum population of approximately 3.5 billions per gram for roots of sweet clover; the majority of the plants which he studied yielded microbial populations for the rhizoplane of approximately one-half billion. For the root system generally, although a t times the populations thereon may approach those encountered in decaying materials in the field, the microbial populations encountered ordinarily do not equal those on plant residues subject to immediate and complete decay in soil. The fact that roots can support comparatively dense populations without undergoing destruction and decay attests to the continuing supply of organic material sloughed or excreted by the roots. Some writers consider that the magnitude of the root population is such that it is indicative of secretory or excretory products being supplied by the roots in considerable quantity, as the sloughing of dead parts alone appears inadequate as a source of stimulation. The question may be raised concerning the proportion of the soil that consists of microbial tissue and the relative contribution of the root microflora to the total microbial mass. It is probable that the total microbial tissue in a fertile soil comprises no more than 0.3 per cent of the soil weight. This estimate may be reached from data given for the total number of microorganisms in soil, as revealed by direct microscopic count. Using a fluorescence microscope for direct examination of soil, and employing acridine orange staining to differentiate living and dead organic matter, Strugger (1948) recently has reported from 1,038 millions to 8,640 millions bacteria per gram of soil on the dry basis. Kendall (1928) has placed the volume of an average bacterial cell a t 15.7 x mm.3, its density a t 1.04 and its moist weight a t 16.3 x 10-lo mg. If the mg., then dry weight of a single bacterium is taken as 3.26 times according t o the data cited from Strugger (1948), the total bacterial mass in soil ranges from 0.03 to 0.28 per cent. This estimate appears plausible in the light of certain observations on the relative organic phosphorus contents of soil and of microorganisms. Converting the estimate just reached from a percentage to a p.p.m. basis, bacterial tissue contributes roughly 300 t o 3000 p.p.m. to the soil mass. If microbial tissue contains approximately 3 per cent phosphorus (Porter, 1947), then such tissue may account for from 9 to 90 p.p.m. of phosphorus in soil. The observation that nucleic acid and microbial tissue show a similar course of mineralization (Pearson et al. 1942; Thompson and Black, 1948) has led the latter authors to conclude that the nucleic acid

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fraction of phosphorus in soil and the microbial fraction roughly are identical. Bower (1949) places the fraction of organic phosphorus occurring a5 nucleic acid as approximately 50 per cent. The values for microbial phosphorus given above, therefore,, indicate an organic phosphorus content in the soils of from 18 to 180 p.p.m. These values appear reasonable in view of current concepts of organic phosphorus in soils (Pierre, 1948). Krassilnikov (1944) has estimated the microbial content of noncropped soil a t 0.03 to 0.06 per cent, and of soil cropped to legumes, a t 0.18 to 0.27 per cent by weight. His estimates agree rather closely with those calculated from direct microscopic counts. They also indicate that the total microbial mass in soil is markedly higher in the presence of plant roots than in their absence. Although on a dry basis, microorganisms may account for only a fractional per cent of the soil mass, in their living state in soil, because of their high water content, and relatively low density in comparison to the mineral fraction of the soil, microorganisms may comprise a much higher percent,age of the soil volume. 3. Some Factors Affecting Rhizosphere Populations Determined Culturally

a. Influence of Soil Moisture Content a t Sampling. I n the course of rhixosphere studies with flax grown a t two differing soil moisture contents, Timonin (1940b) noted higher microbial populat,ions in the rhizosphere of plants in soil maintained at 30 per cent moisture holding capacity than a t 60 per cent. Similarly, Clark (1940) noted increased bacterial numbers in the rhizoplane of wheat as soil moisture content a t sampling was decreased from 24.5 to 12 per cent. Thus in both laboratories, higher populations were encountered a t the lower moisture contents employed. A dissimilar observation commonly is made in microbiological studies on field soils apart from the rhixosphere. For such samples, decreasing soils moisture content within the range of maximum water retention to the permanent wilting percentage is accompanied by decreasing bacterial populations. I n a recent study of the effect,of moisture on the rhixosphere population of soybeans grown in the greenhouse, Clark (1948b) again noted that microbial numbers were higher for roots taken from drier than from wetter soils. The greater weights of the gross root samples collected from t.he wetter soils, the macroscopic appearance of such samples, and the smaller percentage of root material determined as present in them, make it appear plausible that the microbiological differences encountered

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resulted largely from the adhesion of more soil to those roots taken from the wetter soils. The further observation that when paired containers in which plants were being grown were brought to low moisture simultaneously, one container then being sampled while dry, the other after wetting to near saturation just prior to collection of root, material, reduced microbial numbers were found for roots from the wetter soil, also provided evidence of a mechanical or sampling influence. There is little reason to believe that there would be an act.ual reduction in bacterial numbers with heavy watering; such a treatment certainly is not bacteriocidal, a t least not within a short interval of t,ime. b. Influence of Type and Stage of Plant Growth. Environmental and sampling effects are not responsible for many differences in microbial populations reported for plant roots. Starkey (1929b) noted t.hat rhizosphere populations of individual crops differed a t successive stages of plant growth. H e found relatively small numbers of organisms in the early growth stages, increased numbers after the plants had reached considerable size, and decreased numbers after fruiting. Greatest numbers were usually noted a t the stage of maximum vegetative growth and a t fruiting. Graf (1930), Krassilnikov et al. (1936a, b) and Timonin (1940a) also noted that rhizosphere populations differ during differing stages of plant growth. Rhizosphere populations also differ for different groups of plants. Almost without exception, microbiological studies of random series of plants, including both legumes and nonlegumes, show that the former support higher rhizosphere populations. In due time, differing relative effects may be determined for other plant groups or species, but until procedures of sampling and cultural methods of study are refined, one cannot attempt to arrange the common agricultural plants according to their relative stimulation of microorganisms in the rhizosphere. Different varieties of a single plant species may harbor dissimilar rhizosphere populations. The occurrence of greater numbers of bacteria in the rhizospheres of varieties of plants susceptible to soil-borne plant pathogens, even when grown entirely free from disease, than in the rhiaospheres of resistant varieties has been discussed in Section III-2-b. c. Influence of the Region of the Rhizosphere Studied. The several parts of the root system of a single plant harbor dissimilar rhizosphere populations, a t least insofar as t.hose populations are revealed by cultural studies. Some of the effects noted may be due to differences in the composition of the samples employed. Whatever the reason, greater rhizosphere populations commonly are noted for the central or crown porbion of the root and for that portion of the root system in the upper soil profile than for the more distal portions (Clark, 1939; Sabinin and

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Minina, 1932). Direct microscopic st.udies, however, show that even the root hairs carry clusters of microorganisms upon them (Linford, 1942; Rossi e t al. 1936; Starkey, 1939). Inasmuch as the upper horizons in the soil profile normally contain higher numbers of bacteria and more organic matter, it is of interest that with roots of wheat, the greatest numbers of microorganisms were found associated with those roots growing in the uppermost 4 inches of soil, regardless of whether the greatest soil population was established in the lst, 2nd, or 3rd four inches of soil by means of heavy appIications of manure (Clark, 1939). Concentration of bacteria in the upper portion of the rhizosphere, therefore, appears independent of the normally occurring microbial concentration in the upper portion of the soil profile. Further studies are needed concerning t.he distribution of microorganisms in the individual root systems of plants. V. THEKINDSOF MICROORGANISMS FOUND ON PLANT ROOTS 1 . Relative Occurrence of the Major Groups of Soil Microorganisms

During the time of the initial investigations on types of organisms present on plant roots, there was controversy concerning whether the general soil microflora was simply encouraged e n mmse by the presence of plant roots, or whether certain types were especially favored. Gottheil (1901) believed that the general soil flora const.ituted the bulk of the root microflora; Lohnis (1910) was in agreement with this opinion. Various other workers have contended that there exists a specialized microbial population on plant roots. With the development of microbiological technique and the accumulation of a more extensive literature, it has become increasingly apparent that not only are bacteria stimulated by roots to a greater extent than are fungi and actinomycetes, but, also that particular groups or species of bacteria are disproportionately encouraged or depressed. Katznelson et aE. (1948) classify investigations on the qualitative nature of the rhizosphere microflora along t.wo broad lines; (a) those in which numbers of specific groups or species are determined on selective media by plating or dilution methods, and (b) those in which a nonselective medium is used and all colonies on a plate or a representative sector of a plate are picked and studied from the point of view of cult*ural, morphological, and physiological behavior. They have also noted that the study of types of bacteria adhering to buried slides has yielded useful information. Inasmuch as the literature resulting from the application of such methods of study has become too extensive to permit reviewing separately each contribution, only summary attention will be

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given to many observations, and some isolated and unconfirmed reports will be ignored altogether. The increased microbial flora within the rhizosphere is predominantly bacterial. Starkey’s (1931a) data on sweet clover show that bacteria are 200 times more numerous in the rhizoplane than in root-free soil, and that fungi and actinomycetes are 10 times more numerous. Thom and Humfeld (1932) found that the roots of alfalfa, rye, and vetch stimulated bacteria, fungi, and actinomycetes in the order named. Adati (1939) noted increases in these groups in the same relative order. Frequently, t.he rhizosphere effect on fungi and actinomycetes is reported as negligible. There is direct microscopic evidence that protozoa and microphagow nematodes are more numerous within the root zone. By cultural means, Katznelson (1946) has found a twenty-fold increase of protozoa on roots of mangels grown in manured soil. Further attempt6 to determine whether other plants exerted a favorable influence on protozoa were not successful (Katznelson et al., 1948). The fact that protozoa quite frequently increase in numbers following increases in bacterial populations makes i t probable that the accumulation of bacteria on roots is aceompanied by increased numbers of protozoa. 6. The Fungal Flora of the Rhizosphere

It remains unsettled whether certain species of fungi are preferentially encouraged by plant roots. Timonin (1940a) failed to find significant differences in the fungal flora of the rhizosphere of seedling wheat, oats, alfalfa, and clover. Later, he (1941) reported that flax varieties susceptible to disease preferentially encouraged certain genera of fungi. West and Hildebrand (1941) also noted qualitative differences in the fungal flora of strawberry roots grown under different treatments for root rot control, and Clark (1942) found that certain groups of fungi responded t o cotton roots variously treated. Inasmuch as these observations are complicated by plant treatment and by the factor of disease, there does not yet appear sufficient evidence to name individual species of fungi as rhizophilic in habit. Even less is known about t,he response of individual species of actinomycetes, although the direct microscopic as well as the cultural evidence indicates some increased growth of this group about plant roots. 3. T h e Bacterial Flora of the Rhizosphere a. Physiological and Morphological Characterization. According to Thom (1935), the microorganisms associated with root surfaces belong to species active in the decomposition of fresh organic matter in soil, and not to species associated with the breakdown of humus residues. The

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former type of flora is considered to consist, of rapidly growing organisms with specialized saccharolytic or proteolytic abilities, the latter, of slower growing forms less specialized in their feeding habits. Norman (1947) has stated that the soil populat.ion consists almost exclusively of organisms that have a wide range of activities and which possess a great diversity of enzyme systems. Lochhead (1940), following single-culture studies of a number of soil and rhizosphere isolates, concluded that isolat.es from the rhizosphere showed greater physiological activity than those from soil in that a greater percentage of them liquefied gelatin and fermented glucose. Krassilnikov et al. (1936a) similarly reported that the predominant organisms in the rhizosphere consisted of rods with pronounced ammonifying capacity ; cellulolytic types also were numerous. West and Lochhead (1940a) reported that the bacteria of the rhizosphere of flax and tobacco possessed more complex nutribive requirements than did those of the corresponding control soils. Differing requirements for thiamine, biotin, and amino nitrogen were evident. These authors (1940b) subsequently proposed that the equilibrium existing between nutritional groups in soil or about roots be expressed as the “Bacterial Balance Index,” which was calculated by assigning a negative value to the percentage of gram-negative bacteria growing readily in a simple inorganic medium, and a positive value to the bacteria requiring amino acids and growth factors. The sum of the two values represented the “Bacterial Balance Index.” Hildebrand and West (1941) used such an index in the characterization of the rhizosphere flora established in certain root rot disease treatmenh. Lochhead and Chase (1943) and Lochhead and Thexton (1947) have used such an index in soil bacteriology studies, but investigators other than these Canadians have not adopted this index as a means of characterizing the rhizosphere flora. Gram-negative bacteria of simple morphology are either predominant in the rhizosphere or relatively more abundant therein than in soil. Both cultural and direct microscopic studies have shown spore-forming rods generally less abundant in the rhizosphere than in soil. The relative numbers of different morphological types on roots as compared to those in soil have been shown by a study (Clark, 1940) of four hundred isolates from cotton roots and from the surrounding soil. Gram-negative rods accounted for 67 per cent of the root isolates, but for only 20 per cent of the soil isolates. Gram-poskive coccoids, on the contrary, accounted for only 12 per cent of the root isolates, and for 40 per cent of the soil isolates. Spore-forming rods were even more sharply depressed in number by the presence of roots, as they accounted for only 1 per cent of the root isolates, but for 30 per cent of the soil isolates.

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b. The Incidence of Individual Genera and Species. The bacterial genera most frequently named as responding favorably to root development have been Radiobacter (now called Agrobacterium, Breed et al., 1948) and Pseudomonas. Some very marked increases in numbers have been reported for the Radiobacter group in the presence of plant roots; Starkey (1931a), for example, stated that this group is 112 times more numerous on roots of legumes than in soil. He pointed out, however, that in dilution plate cultures, Radiobacter is indistinguishable from Rhizobium, and therefore counts, as commonly recorded, undoubtedly include the Rhizobium population also. Although Radiobacter and Rhizobium are markedly increased in number in the presence of leguminous plants, knowledge concerning the response of this group to nonlegumes is meagre. Fluorescent gram-negative rods of the genera Pseudomonas and Xunthomonas are commonly encountered in greater abundance on roots rather than in soil (Clark, 1940; Starc, 1943). The genus Mycoplanu is considered to be more numerous in the rhizosphere than in surrounding soil (Lochhead, 1940). Clostridium is also reported to be encouraged by root development (Katznelson, 1946; Sen, 1929; Velich, 1903). The incidence of Azotobacter, and of Nitrosomonas and Nitrobacter, upon the surfaces of plant roots has been discussed in preceding sections (III-1-a and III-1-b), and it will suffice here to state summarily that these genera fail to show a positive response to the presence of plant roots. Proactinomyces, and soil corynebacteria, which Conn and Dimick (1947) recently proposed be placed in the genus Arthrobacter, are depressed in number by root development,. The genus Bacillus is also less frequently encountered in the rhizosphere than in soil apart. from roots (Clark, 1940; Krassilnikov, 1934; Krassilnikov et al., 1936a; Lochhead, 1940; Lochhead et al., 1940). Very few studies have been made of the occurrence of individual species of bacteria within the rhizosphere. Although the literature is quite fragmentary, it does suggest that certain species, as well as certain genera, are more frequently encountered on root surfaces than in soil generally. Within the genus Arthrobacter, Lochhead (1948) has noted a greater incidence of chromogenic types in the rhizosphere than in adjacent soil. Within the genus Bacillus, Clark (1940) has noted that B. brevis, B . circulans, and B. polymyxa constitute more important fractions of the bacillus population in the rhizoplane than they do in soil. Within the fluorescent Pseudomonas group, single culture studies have indicated that the types encountered in soil are different from those associated with root surfaces. The majority of the soil isolates did not utilize sucrose or st.arch, whereas the majority of the root isolates fermented these materials.

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Taxonomic studies, although limited, are sufficient to indicate qualitative differences in the bacterial floras of the rhizosphere and of the soil, even though for the most part genera and species of bacteria cannot yet be catalogued as to whether they are encouraged by, indifferent to, or inhibited by root development. VI. MODIFICATION OF THE ROOTSURFACE MICROFLORA 1. B y Seed or Soil Inoculations Soil bacteriologists long have attempted to control the root surface microflora by seed or soil inoculations. The most notable success has been in the inoculation of legumes with rhizobia, a practice which dates from the work of Hellriegel and Wilfarth (1888). Subsequent investigators of the symbiotic nitrogen-fixing organisms have been more concerned with the selection and maintenance of effective strains and with consideration of factors which affect the persistence of strains of rhizobia in soil after being introduced therein than with the feasibility of inoculation. Problems of soil reaction, of bacteriophage, of the effect of various intervening crops, and of the applications of fertilizers and lime have been studied. Inoculation of nonlegumes, as recommended initially by Caron (1895) and Stoklasa (1908) generally has not been found beneficial, although in this connection possibly exception should be made for the Russian claims on Azotogen and allied preparations. This controversy has been discussed in Section III-1-b above. Apart from seed and soil treatments with Azotobacter, certain inoculation attempts have been made in connection with root rot studies that are pertinent here. Morrow et al. (1938) were of the opinion that both molds and bacteria could be successfully established on cotton roots by use of proper inocula on seeds or seedlings. Recovery of an inoculated microorganism from the rhizosphere, wit.hout demonstration that it is of more than chance occurrence therein or simply capable of a passive survival for a period of time, may be misleading. The writer remains in agreement with Greaves (1918) that a few organisms placed in a new environment seldom gain ascendency over the organisms naturally present and struggling for countless generations to adapt themselves to that environment. Exception of course must be made for phytopathogens and for the nodule bacteria, which are capable of invading the root tissue or histosphere, and in consequence, enjoy a special advantage over the noninvading types. Even the plant pathogens, however, are not always easily established upon roots when subject to competition from the soil microflora, as is shown by several comparatively recent experiments on the take-all disease of

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wheat, wherein, in order to establish experimental infection, the fungus (Ophiobolus graminis Sacc.) was introduced into the soil along with the organic substrate on which it had been grown. With such procedure, there frequently was failure to establish infection. Stimulation of growth of the antagonistic flora in the soil by the added organic material has been considered responsible for the rapid disappearance from soil of t,he fungal parasite introduced (Garrett, 1944). 2. B y Soil Treatment

Inasmuch as organic manures are known to affect the incidence of certain root-infecting fungi, numerous studies have been made of the effect of such treatments on the rhizosphere microflora. I n experiments in which manured, untreated, and steam-sterilized soil samples were established in containers cropped to wheat, Clark (1939) found that the root flora was largely independent of the soil flora. Clark and Thom (1939) believed that the effects of organic manuring were primarily evident on the soil microflora, and that the root microflora of the plants themselves was relatively little affected. In view of an observed independence of the root microflora from manurial treatments capable of affecting differences in disease incidence, Stumbo et al. (1942) expressed the opinion that factors of host nutrition were of greater importance than microbial antagonisms in the rhizosphere in the control of take-all disease of wheat. Timonin (1940b), Katznelson and Richardson (1943) and Mitchell et al. (1941) also showed that organic manures did not greatly affect the rhizosphere microflora. In some instances, however, there do exist indirect influences of manurial treatments. Katznelson (1946) has pointed out that manuring may affect the growth rate, vigor, and maturity of the plant, and that stage of plant development is known to influence t.he flora of the rhizosphere. Hildebrand and West (1941) found that manurial treatment affecting the incidence of Ontario rot of strawberries also affected the relative incidence of nutritional groups within the rhizosphere. Differences in host plant nutrition following organic fertilization may induce changes in the rhizosphere microflora, which in turn may affect disease incidence. Differences in plant nutrition may also affect incidence of disease. Following primary invasion by root parasites, there typically is secondary invasion by members of the soil population, and in such circumstance there is abrupt disturbance of the rhizosphere flora. Frequently it is difficult properly to separate cause from effect. There appear instances in the root rot literature where this has been attempted without sufficient information on the many factors involved in incidence of root diseases.

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Various inorganic and physical treatments have been reported to affect a t least certain fractions of the root microflora. I n experimental soil initially of pH 5.0, Pohlman (1946) noted that nodules on roots of alfalfa were largely concentrated in the soil layer receiving a high-lime treatment, irrespective of whether this was the 8 to 16 or 16 to 24 inch layer. Timonin (1947) found that various soil fumigants affected numbers of manganese-oxidizing bacteria on the roots of oats. Katznelson and Richardson (1943) noted that although soil steaming markedly reduced microbial numbers in the soil in which tomato plants were grown following the steaming, the numbers on healthy roots of the tomato plants were as high as the bacterial numbers on roots in untreated, control soil. Such data indicate that steaming, as organic manuring, has little effect on the root surface microflora. 3. By Plant Treatments

Inasmuch as stage and condition of plant growth have been found to affect the rhizosphere microflora, it may be expected that plant treatments which affect the vigor or condition of plant growth may be used to modify the rhizosphere flora. Such modifications have been accomplished experimentally. Eaton and Rigler (1946) found that with plant mutilation (half-leming of fruiting plants us. de-flowering of fully leafed plants), differences in the carbohydrate levels of cotton plants could be established, and simultaneously, differences occurred in the surface microfloras of the roots. For field cotton, plant mutilation has been found to affect the incidence of saprophytic fungi in the rhizosphere (Clark, 1942).

4. B y Application of Herbicides and Insecticides Applications of plant growth regulators or herbicides, by affecting plant development, can also be expected to alter root surface microfloras. To date most of the investigations conducted on the microbiological effects of herbicides have been concerned with their effects on the soil population rat,her than on the rhizosphere population. Smith et al. (1946) reported that 2,4-dichlorophenoxyacetic acid (2,4-D) had no effect on numbers of fungi, actinomycetes, or of total bacteria when applied to soil a t concentrations up to 0.05 per cent, although there was injury to nitrifying bacteria with applications of 0.01 per cent. Stevensen and Mitchell (1945) found 0.02 per cent of 2,4-D to be bacteriostatic in culture. Newman and Norman (1947) reported no effects of 2,4-D on numbers of microorganisms present in soil, or on rates of nitrification, until great excesses of the compound were added. Jones (1948) found that although 2,4-D a t the rate of 25 pounds per acre had no effect on

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nitrification in soil to which no extra nitrogen had been added, applications a t the rate of 15 pounds per acre to soil with added nitrogen (urea. or sodium nitrate) gave temporary inhibition of nitrate formation. Lewis and Hamner (1946) found that Rhizobium cultures were not affected by 2,4-D, and concluded that the amounts of 2,4-D reaching the soil from normal rates of application would have no important effect on microorganisms in the soil. Some subsequent workers, however, have reached dissimilar conclusions. Employing rates of application of 2,4-D sublethal for plant growth, Payne and Fults (1947) found that 0.009 lb. per acre mixed with the soil reduced the nodulation of beans grown in such soil, and that nodulation was entirely prevented by 0.075 lb. rates. Carlyle and Thorpe (1947) found that 2,4-D salts present in the soil solution a t the rate of 0.5 p.p.m. (0.21 lbs. per acre) would seriously restrict germination, limit growth, and practically inhibit nodulation of legumes. In culture, however, rhizobia were not seriously inhibited in growth until 2,4-D was applied a t a rate corresponding to 200 lbs. per acre. Apparently, the inhibition of nodulation by 2,4-D must be ascribed to an effect on the physiology of the plant or on the joint physiology of the two symbionts, rather than to any direct bacteriocidal or bacteriostatic action on Rhieobium. The writer is unaware of any report on the effect of lethal or sublethal doses of 2,4-D on the rhizosphere flora of nonlegumes. A study in this laboratory has indicated that for tomato plants treated with 2,4-D, there is an increased rhizosphere flora in comparison with untreated plants, and that this increased population persist-s in the rhizosphere for from two to three weeks, after which there follows a general microbial invasion and disintegration of the entire root system. Knowledge concerning the effects of recently developed insecticides upon the rhieosphere flora is also limited. Wilson and Choudhri (1946) reported no effect of dichlorodiphemyltrichloroethane (DDT) on nodulation of legumes, nor were soil populations nor pure cultures of various microorganisms adversely affected. Appleman and Sears (1946) likewise failed to find intereference with nodulation of legumes with DDT applied at the rate of 100 Ibs. per acre, but Payne and Fults (1947) reported that applications of 103 Ibs. of DDT per acre greatly reduced the number of nodules on bean roots. Wilson and Choudhri (1948) found that benzene hexachloride, applied in the field a t rates recommended for the control of wireworms, did not interfere with the nodulation of legumes. More complete studies of the effects of herbicides and of insecticides on the rhizosphere populations of both legumes and nonlegumes are indicated.

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VII. INFLUENCES OF THE RHIZOSPHERE FLORA ON SUCCEEDING OB ASSOCIATED PLANTS I n the preceding sections, the discussion for the most part has considered the microorganisms of the rhizosphere in relation to the plants with which they are associated. It is also possible that organisms developing upon the roots of ine plant may influence another plant, growing either successively or simultaneously in the same soil area. These microbiological influences may be exerted either through the establishment and persistence in soil of the microorganisms themselves, or through changes in the soil environment which persist even after the microflora initially responsible for them no longer is alive. The concluding section of this review considers (1) the persistence of the rhizophilic flora in field soils, and (2) the persistence of changes in the soil environment brought about by this flora. 1 . Persistence of the Rhizophilic Flora in Field Soils

There is little information available as to whether the microflorae developing in association with plant roots persist in soil for any appreciable length of time after the death of the plants. The bulk of the available evidence indicates that the large numbers of microorganisms associated with a growing crop do not long persist. Brown (1912) found that the crop present on the soil was of more importance from the bact,eriological standpoint than the previous cropping of the soil. A rotation of crops caused the development of greater numbers of organisms in soil than did continuous cropping. For soils cropped to potatoes or oats, Starkey (192913) noted a marked decline in microbial numbers subsequent to maturity and death of the plants. It. is generally known that higher microbial populations are observed for only brief periods following organic matter additions to soil. Smith and Humfeld (1931), in a study of the decomposition of green manures added to soil under glasshouse conditions, noted t,hat the effects of added rye and vetch residues on counts of microorganisms disappeared within a few days following addition. Where plants were grown on the soil, and both tops and roots returned as a green manure, secondary increases in numbers of soil microorganisms were observed after 14 to 21 days. These effects disappeared after an additional 7 to 14 days. Such data emphasize the rapidity with which quantitative changes in the soil flora become obliterated. Under field conditions, where as a rule conditions of moisture and temperature are less favorable for microbiological activity than t.hose in the greenhouse, a longer period of time would be required. I n field plots in Nebraska, Dawson et al. (1948) found that the stimulatory effect of

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crop residues on numbers of soil microorganisms was pronounced during the first few months following addition, but had mostly disappeared by nine months. That qualitative changes accompany the quantitative has been shown by Krassilnikov and Nikitina (1945), who found that an abrupt change in types of microorganisms takes place during the decomposition of plant roots. Doughty (1941) has called attention t o differences in the rate of decomposition of plant roots, as measured by recovery of coarse material and by production of carbon dioxide. Although no lasting quantitative changes in microbial populations are observed following crop growth, certain rhieophilic microorganisms have been recovered a t varying intervals following crop removal. Smith (1928) noted increased numbers of B. radiobacter in soil for several weeks after the growth and harvesting of legumes. Nodule-forming bacteria inoculated on appropriate seed have been reported to become established in soil and to persist therein for from one to inariy years following removal of the crop (Albrecht and Turk, 1930; Deherain, 1900; Fred et al. 1926; Nobbe and Hiltner, 1898). Appleman and Sears (1947), following study of numbers of nodule bacteria in field plots, concluded that the largest numbers of rhieobia are found where the appropriate host plants have recently been grown. Their data indicate that thc presence of nodule bacteria in plots on which host plants have not been grown recently should be attributed to applications of farm manure containing the organisms and not to any prolonged survival of nodule bacteria in soil. 2. Persistence of Changes in the Soil Environnient Brought

about by the Rhizosphere Flora It has long been known that one species of plant may have a beneficial or detrimental influence upon other species of plants that are growing in close proximity to i t or that follow it in succession. General discussions of mixed cropping and of crop sequence effects have been given by Nicol (1934), Miller (1938), and Ripley (1941). There is evidence that both beneficial and deleterious effects upon accompanying or following crops may result from changes in the soil environment accomplished by the microflora accompanying certain crops. The beneficial effect of a leguminous crop on another following frequently can be explained by the fixation of atmospheric nitrogen during the growth of the legume. Although i t may be impossible to show change in the total nitrogen content of a soil after legumes have been grown in that soil for several years, the nitrate-supplying power of soils on which legumes have previously been grown is generally greater than that of

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soils cropped to nonlegumes. Newton et al. (1939, 1940) found no difference in the total nitrogen of soil after alfalfa, timothy, brome grass and western rye grass had been grown for periods of 1, 3, and 5 years. There was, however, a greater accumulation of nitrates in the soil after alfalfa than after grasses. Inasmuch as the effects of crops on the physical condition of the soil may persist from one to several years, an altered soil structure a t times may account for the influence of one crop on another following. Grass crops particularly have been found valuable in increasing the aggregat.ion of soil, and the inclusion of a grass crop in a rotation is widely recommended. Although the beneficial influence of grasses is not observed within short periods of growth, the improved structure once obtained is more persistent than the improvement obtained by applications of readily decomposable organic matter to soil. Desirable effects on aggregation of an extended bluegrass culture were noted by Woodruff (1940) to be still in evidence even after six years of continuous fallow. Browning and coworkers (Feng and Browning, 1947; Gish and Browning, 1949; Johnston et al., 1943; Wilson and Browning, 1946) have found clovers and grasses to give benefits extending into the following cropping seasons. Breazeale (1924) believed that the unfavorable effect of a sorghum crop on another following might be due to the development of a poor state of structure under sorghum. He suggested that cyanide excretion by sorghum roots depressed microbiological activity and sufficiently lowered the carbon dioxide content of the soil so that calcium became less active, and that its replacement in zeolite by sodium led to soil deflocculation. Recognition of the factor of parasitism in the influence of one crop on another is usually but not always a simple matter, and in a number of crop sequence effects, this factor doubtless can be considered as unimportant. However, the recent work of Valleau et al., (1942, 1943, 1944) and of Diachun and Valleau (1946) showing that certain bacteria, pathogenic to tomato and tobacco, overwinter on the roots of cover crops and of weeds, emphasizes that concepts of host range and of control of plant disease by crop rotation may need to be reconsidered. Valleau et al. (1944) stated that bacteria capable of causing angular leaf spot and wildfire in tobacco are not primarily tobacco plant pathogens, but are organisms apparently adapted to a life on the surface of small rootlets of several plants. The question may be raised as to the extent to which, in a legumegrass mixture, the root microflora associated with one plant species may influence the growth and welfare of the second plant species. Certainly

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the microbiological factor is not the only factor responsible for the success of legume-grass mixtures. Such factors as maximum utilization of moisture, available nutrients, and of light intensity, or even the protection of one species by another in reducing ground freezing and heaving may a t times be of predominant importance. Other than extensive work concerning improved nitrogen nutrition of the nonlegume in a grass-legume mixture, there is a limited literature concerning the microbiological factors in such mixtures. This does not necessarily mean that such factors are not operating. Nicol (1935) found that the roots of alfalfa and of grass growing in sand were difficult to separate when these plants were grown together, but that no such difficulty was encountered with the roots of contiguous plants grown in single crop culture. Microbiological studies have not been made of the intermingled root systems of legumes and grasses. Possibly, with present imperfections of microbiological technique, the attempt to characterize a mixed rhizosphere and to differentiate it from the rhizospheres of the component species grown separately would be unsuccessful. But inasmuch as differing plants encourage specific microfloras, and particularly since the rhizospheres of legumes harbor greater numbers of microorganisms than do those of nonlegumcs, it is possible that a combined rhixosphere would offer advantages, either in increased availability of nutrients or in increased production of growth accelerating substances, to the nonleguminous component of the mixed crop. It is also possible that the combination of two root systems with their accompanying microfloras can act on the physical condition of the soil to a differing degree than either separately. Page and Willard (1947) have stated that from the standpoint of soil structure, the combination of a deeprooted legume and a grass seems almost ideal. These several possibilities indicate the existence of other microbiological factors in addition to those of symbiotic nitrogen fixation in legume-grass mixtures, and emphasize the need for broader microbiological study of mixed legiime-grass rhizospheres. I n general summary of the preceding subsections, there appears little evidence that the characteristic microflora of a given crop can long persist in soil after the removal of that crop, but it does appear possible that such a microflora effects changes in the physical or chemical conditions of the soil, and that these changes are sufficiently lasting to influence a following crop. I n mixtures of legumes and grasses, symbiotic nitrogenfixing bacteria associated with the former apparently increase the supply of nitrogen available to the latter; otherwise, there is practically no evidence of the existence of direct microbiological effects by the microflora of one plant upon the growth and development of an associated

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plant. This lack of evidence does not mean necessarily that such effects do not exist. A dozen years ago, Loehwing (1937) stated that the mutual interest and cooperative participation of agronomists, microbiologist,s, and biochemists in questions of root interaction had produced practical results otherwise unachievable, but that “further factual research on root excret,ions and interactions is required for the complete integration of the physiology of the root with that of the shoot and for the proper evaluation of the many edaphic factors influencing plant development.” I n this review an attempt has been made to emphasize some of the more recent contributions of the many soil microbiologists who have become intereded in the relationships of soil microorganisms and plant roots. Although progress in this field of study has in no way been sensational, nevertheless it is encouraging to note that as individual problems in rhizosphere microbiology are more intensively studied, their relationships to each other and to the welfare of growing plants are becoming more clearly established. REFERENCES Adati, M. 1939. J. SOC.Trop. Agric. Taihoku Imp. Univ. 11, 57-65. Albrecht, W. A., and Turk, L. M . 1930. Mo. Agr. Expt. Sla. Res. Bull. 132. Allison, F. E. 1947. Soil Sci. 64, 413-429. Allison, F. E., Gaddy, V. L., Pinck, L. A., and Armiger, W. H. 1947. Soil Sci. 64, 489-497.

Allison, F. E., Hoover, S. R., and Morris, H. J. 1934. J. Agr. Research 49, 1115-1123. Angell, H. R., Walker, J. C., and Link, K. P. 1930. Phytoputh. 20, 431-438. Appleman, M. D., and Sears, 0. H . 1916. J. Am. SOC.Agron. 38, 545-550. Appleman, M.D., and Sears, 0. H . 1947. Soil Sci. SOC.Am. Proo. 11, 252-254. Barker, H. A., and Broyer, T. C. 1942. Soil Sci. 53, 467-477. Beijerinck, M.W. 1901. Zentr. Bakt. Parasitenk. I 1 7, 561-582. Beijerinck, M. W. 1913. Bolia Microbiol. 2, 123-125. Beijerinck, M. W., and Van Delden, A. 1902. Zentr. Bakt. Pn,rtisit~nk.I1 9, 3-43. Berezova, E.F. 1941. Microbiology (U.S.S.R.) 10, 918. Berezova, E. F., Naumova, A. N., and Rasnizina, E. A. 1938. Compt. w n d . ncod. S C ~ ,U.S.S.R. 18, 357-361. Bonner, J. 1937. Botan. R e v . 3, 616-640. Bower, C.A. 1949. Iowa Agr. Expt. Sto. Res. Bull. (in press). Boysen-Jensen, P. 1936. Growth Hormones in Plants. MrGraw-Hill, NPWYork. Breazeale, J. F. 1924. J . Am. Soc. Agron. 16, 689-701. Breed, R. S., Murray, E. G. D., and Hitchens, A. P. 1948. Bergey’s Manual of DPterminative Bacteriology. 6th ed., Williams and Wilkins, Baltimore. Broadbent, F. E., and Norman, A. G. 1947. Soil Sci. 8 o c . Am. Proc. 11, 264-267. Brown, P. E. 1912. Iowa Agr. Expt. Stn. Res. Bd1. 6. Brown, R. 1946. Nature 157, 65-69. Carlyle, R . E., and Thorpe, J. D. 1947. J . Am. Soc. Agron. 39, 929-936. Caron, A. 1895. Landw. Vers. Sta. 45, 401-418. Clark, F. E. 1939. Trans. Kansas Acad. Sci. 42, 91-96.

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Clark, F. E. 1940. Trans. Kansas Acad. Sci. 43, 75-84. Clark, F. E. 194. U.S.D.A. Tech. Bull. 835. Clark, F. E. 1948a. Soil Sd.65, 193-202. Clark, F. E. 1948b. Soil Sd.SOC.Am. Proc. 12, 239-242. Clark, F. E., Hervey, R. J., and Blank, L. M. 1948. U 3 . D . A . Tech. Bull. 935. Clark, F. E.,and Thom, C. 1939. Trans. Third Comm. Intern. Soc. Soil Sci. A, 94100. Clark, N. A. 1930. Science 71, 268-269. Clark, N.A.,and Roller, E. M. 1931. Soil Sci. 31, 299-310. Conn, H.J., and Dimmick, I. 1947. J. Bact. 54, 291303. Dawson, R. E., Dawson, V. T., and McCalla, T. M. 1948. Neb. Agr. Expt. Sta. Res. BUZZ. 155. Deherain, P. P. 1900. Expt. Sta. Record 11, 218. Diachun, S.,and Valleau, W. D. 1946. Phytopath. 36, 277-280. Dorokhova, M. A. 1942. Bull. acad. sci. U.R.S.S. 36, 219-234. Doughty, J. L. 1941. Sci. Agr. 21, 429-432. Eaton, F. M.,and Rigler, N. E. 1946. J. A ~ TResearch . 72, 137-161. Falck, R. 1923. Mykol. Unters. Ber. 2, 38-72. Fellows, H. 1929. Phytopath. 19.103. Feng, C.L.,and Browning, G. W. 1947. Soil Sci. Soc. Am. Proc. 11, 67-73. Prancke, H. L. 1934. Flora 129, 1-52. Frid~,A. B. 1885. Ber. deut. botan. Ges. 3, 128-145. Fred, E. B., Baldwin, I. L., and McCoy, E. 1932. Root Nodule Bacteria and Leguminous Plants. Univ. Wisconsin Press, Madison. Fred, E. B.,and Haas, A. R. C. 1919. J. Gen. Physiol. 1, 631-638. Fred, E. B., Whiting, A. L., and Hastings, E. G. 1926. Wisc. Agr. Espt. Sta. Res. Bull. 72. Fujimoto, C. K., and Sherman, G. D. 1948a. J. Am. Soc. A g ~ o n 4 . 0, 527-534. Fujimoto, C.K.,and Sherman, G. D. 1948b. Soil Sci. 66, 131-146. Gainey, P. L. 1946. Kans. Agr. Expt. Stn. Rept. 1944-1946,p p . 17-18. Garrett, 8. D. 1938. Biol. Rev. Cambridge Phil. Soc. 13, 159-185. Garrett, S.D. 1941. Ann. Applied Biol. 28, 14-18. Garrett, S.D. 1944. Root Disease Fungi. Chronica Botanica Co., Waltham, Mass. Gelteer, F. V. 1943a. Dokl. Akad. S. Kh. Nauk. 3, 38-40. Geltser, F. V. 1943b. Pedology (U.S.8.R.) 9-10,62-74. Geltser, F. V. 1946. Pedology (U.SS.IZ.) 8, 421-426. Gerretsen, F. C. 1937. Ann. Botany 1, 207-230. Gerretsen, F. C. 1948. Plant and Soil 1, 51-81. Gish, R. E.,and Browning, G. M. 1949. Ahil Sci. Soe. Arne).. Proc. 13, (in press). Goring, C. A. I., and Clark, F. E. 1949. Soil Sci. SOC.A m . Proc. 13, (in press). Gott, C. L. T., and Goldsmith, G. W. 1946. Phytopath. 36, 667-670. Gottheil, 0. 1901. Zentr. Bakt. Parasitenk. I I 7, 481-484. Graf, G. 1930. Zentr. Bakt. Parasitenk. I I 82, 44-69. Greaves, J. E. 1918. Soil Sci. 4 163-217. Greaves, J. E.,Stewart, R., and Hirst, C. F. 1917. J . A y i . Research 9, 293-341. Elarmsen, G. W. 1940. Antonie vun Leruwejrhoek 1. Microbiol. Serol. 6, 178-199. Hatch, A. B. 1936. J. Forestry 34, 22-29. Hatch, A. B. 1937. Black Rock Forest Bull. 6. Headdon, W. P. 1927. Colo. Agr. E’xpt. Sta. Bull. 319.

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Weed Control A. S. CRAFTS AND W . A . HARVEY Uniueraiiy of California. Davis. California CONTENTS

Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . 289 I1. Tillage. Cropping. and Competition in the Control of Weeds . . . . 290 I11. Chemical Weed Control . . . . . . . . . . . . . . . . . . 293 IV . Principles of Chemical Weed Control . . . . . . . . . . . . . 293 V. Herbicidal Action . . . . . . . . . . . . . . . . . . . . . 295 VI . Molecular Properties of Herbicides . . . . . . . . . . . . . . 296 VIT . EmuIsions and Emulsion Stabilizers . . . . . . . . . . . . . . 298 VIII . Selectivity of Herbicides . . . . . . . . . . . . . . . . . . 299 IX . The 2.4-D Herbicides . . . . . . . . . . . . . . . . . . . 300 X . Uses of 2.4-D . . . . . . . . . . . . . . . . . . . . . . 303 1 . General Contact Spray . . . . . . . . . . . . . . . . 303 2 . Selective Contact Spray . . . . . . . . . . . . . . . . 303 3. Translocated Spray . . . . . . . . . . . . . . . . . . 304 4 . Temporary Soil Sterilization . . . . . . . . . . . . . . . 305 5. Permanent Soil Sterilization . . . . . . . . . . . . . . . 307 XI . Nitro- and Chloro-Substituted Phenols . . . . . . . . . . . . . 307 XII.Oils . . . . . . . . . . . . . . . . . . . . . . . . . . 308 XI11. Other Organic and Inorganic Chemicals . . . . . . . . . . . . 310 1 . I P C (Isopropyl Phenylcarbamate) . . . . . . . . . . . . 310 2 . TCA (Trichloroacetic Acid) . . . . . . . . . . . . . . . 310 3. PMAS (Phenylmercuric Acetate) . . . . . . . . . . . . . 311 4 . Cyanamid and Cyanate . . . . . . . . . . . . . . . . . 311 5 . CS,, DD, Prochlors . . . . . . . . . . . . . . . . . 311 6. Arsenic, Borax, ChIorate . . . . . . . . . . . . . . . . 312 XIV. Water Weed Control . . . . . . . . . . . . . . . . . . . 312 XV . Herbicide Application Equipment . . . . . . . . . . . . . . . 312 XVI . Drift, Volatilization, Blowing of Herbicides . Secondary and Residual 312 Effects . . . . . . . . . . . . . . . . . . . . . . . . XVII . Flame Cultivation . . . . . . . . . . . . . . . . . . . . 314 XVIII . The New Agronomy . . . . . . . . . . . . . . . . . . . . 314 References . . . . . . . . . . . . . . . . . . . . . . . 315

I. INTRODUCTION Chemical weed control has recently come to the forefront. as an important phase of modern scientific agriculture . With the introduction of 2,4-dichlorophenoxyacet.ic acid (2,4-D), tremendous strides have been 289

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made and millions of crop acres have been benefited. Although misuse, resulting from lack of knowledge, has caused a certain amount of injury to sensitive crops, the benefits gained to date so outweigh the damage that the balance is far on the favorable side. Any chemical t.hat within a brief 4 years finds use in quantities approaching 20 million lbe. on almost as many acres must be of outstanding significance. Offering more aid to the farmer than any material introduced since the advent of artificial fertilizer, 2,4-D has found a definite place in mechanized agriculture. The most apparent need today is for more education in the use of thip revolutionary tool. Not only has 2,4-D itself become a vital force in modern crop production; the interest which i t has aroused is being reflected in a vigorous search for other new chemical tools. Many of the older methods, both rhemiral and cultural, are finding renewed acceptance. The agronomist, like the farmer, needs no introduction to weeds and weed problems. If he has worked in the field he recognizes weed specie*, and he understands the ways in which they interfere with crop production. For the student, the laboratory man, and others not familiar with weeds, a number of general referenres are given; this paper is principally concerned with the recent developments in weed control. For many, chemicd weed control started with the introduction of 2,4-D, and chemical methods involve simply those used in the application of this herbicide. This review will treat many chemical methods in the hope than an ordered consideration of their uses will lead to a more balanced view. Discussing these methods from the standpoint of the basic chemical and agronomic principles involved, an attempt will be made to designate the role of each in a broad and comprehensive field practice.

11. TILLAGE, CROPPING, AND COMPETITION IN THE CONTROL OF WEEDS It should be obvious that the adaptation of the simple tillage operation,s required for seedbed preparation to the control of weeds is one of the easiest methods for handling these pests. For example, the use of the moldboard plow equipped with a rolling coulter and jointer will provide a clean, weed-free seedbed in which seeds of many crops will germinate quickly, grow rapidly, and produce heavily. I n contrast, a disk plow or stubble plow will leave the seedbed filled with trash and partially buried weed seedlings that interfere with growth of the crop. On level, well-drained soils, not subject to erosion, good seedbed preparation may be synonymous with good weed control. On rolling lands where water erosion is a problem, and in semi-arid regions where winds carry soils, the situation is entirely different; stubble mulch may be essential

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29 1

to erosion control and weeds must be handled by other means. A complete consideration of the methods of cultural, tillage, and competition methods of weed control is given in Robbins et al. (1942). Further reference material will be found in Korsmo (1930), Muenscher (1935), Herbage Publication Series Bulletin 27 (1940), Reports of the Cooperative Weed Investigations of the U.S.Department of Agriculture, Timmons (1941), etc. It should also be obvious that the simplest method of controlling weeds in row and checkrowed crops is the frequent shallow tillage so commonly practiced. A point that has not been so well recognized is that, a t least in some soils, frequent tillage destroys the structure of soil colloids, rendering the soil less pervious to air and water, and hence less favorable as a medium for plant growth. Harvey (1944) observed that 2 seasons of tillage operations resulted in alteration of soil structure to the extent that seedbed preparation was difficult and irrigation furrows tended to wash out. Many citrus growers in California have adopted nontillage as a preferred program in their groves and all weed control is carried on with chemical sprays. Parker and Jenny (1945) have made preliminary investigations on nontilled and compacted soils, finding that tillage restricts percolation. Whereas nontillage may be readily pract.iced in an orchard, the necessity for seedbed preparation requires a certain amount of manipulation of the soil for many field crops and frequent tillage for weed control has been considered an essential feature in the production of row crops. If tillage is as harmful to soil structure as some of the experimental work indicates, the substitution of chemical weed control for cultivation can be readily made, a t least in row crops. I n view of the benefits claimed for nontillage, the advantages of growing perennial forage crops such as alfalfa and clover may come, partly a t least, from the absence of tillage during relatively long periods. The role of competition in cropping methods of weed control has been studied in Canada by Godel (1935, 1938), Pavlychenko and Harrington (1934, 1935), Pavlychenko (1940), in Germany by Rademacher (1940), and in the United States by Timmons (1941), and others. The work up to 1942 is reviewed in Robbins et al. (1942). Although the advent of the dinitro compounds, 2,4-D and related compounds, and other selective herbicides has lowered costs of weed control in cereal crops to a level that rules out many cultural and cropping procedures, the sowing of clean seed, the prompt control of new infestations, the elimination of weeds on fence lines, roadsides, ditchbanks, and obher noncropped areas, and other preventive measures are still economiaally sound. Research on competition and cropping methods is necessarily time-consuming and costly; publications, therefore, are not so numerous

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as in the field of chemicals. This in no way reflects upon the relative value of the former methods. I n any educational work on weed control the merits of t.hese “good farming” methods should be stressed. One real point of departure from the older methods included under “good farming” is t.he changed view regarding frequent tillage that has been expressed above. It has been proved that the periodic stirring of the soil that constitutes the essential feature of “summer fallow,” “black fallow,” and mechanical weed control in trees, vines, and row crops does, a t least in semi-arid regions, have a deleterious effect on the colloidal structure of the soil. Furthermore, such tilled soils are often subject to severe wind or water erosion. In tree and vine crops sprays are proving a satisfactory and effective substitute for tillage (Aldrich, 1948; Johnston and Sullivan, 1949; Moore, 1945a, b; Puffer, 1947; Puffer and Yarick, 1948; Sullivan and LaRue, 1947; Yarick, 1947). I n some row crops such as sugar cane, corn, milo, soy beans, and cotton, experimental work in progress promises some relief from the deleterious effects of cultivation (Anderson and Ahlgren, 1947; Anderson and Wolf, 1947; Crafts, 1948b; Harvey and Robbins, 1947; Lee, 1948a, b; Leonard et al., 1947, 1948; Van Overbeek and Velez, 1946a, b; Van Overbeek, 1947; and Willard, 1948). Work on the substitution of chemical control for fallow has just started (Pavlychenko, 1947). Much more will have to be done before satisfactory substitutes for all fallow procedures will be found. Furthermore, there are undoubtedly soils that are definitely benefited by tillage. A case in point is a heavy tideland soil in Puerto Rico which was reclaimed by drainage and irrigation with a good quality irrigation water. By plowing in bagasse and sugar cane trash this land was gradually reclaimed and brought into a very productive condit.ion. Hester and Isaacs (1948) report a case in Pennsylvania where carrot production was much lower on nontilled than on tilled land. I n this situation it should be emphasized, however, that one season of nontillage will not compensate for harm done by years of frequent cultivation. Biological methods of weed control are reviewed in Robbins, et al. (1942). Reports in recent years indicate the successful control of St. Johnswort in Australia (Currie and Garthside, 1932; Wilson, 1943). Of interest to Westerners is the fact that, Chrysolinu hyperici and C . gernellata, introduced in California in 1945 by Smith and Holloway (Holloway, 1948), are increasing, and give all indications of producing satisfactory results on St. Johnswort under Pacific Coast conditions. This is encouraging to the stockmen because in the four states of Idaho, Washington, Oregon, and California upward of a million acres have become infested with St. Johnswort. This weed takes over the open productive areas of the range and become so dense that little or no forage is

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produced. Because of the wide dissemination of this species, and the fact that it infests low- to intermediate-value lands, biological control is the only hope for holding it in check.

111. CHEMICAL WEEDCONTROL Early reviews on chemical weed control include Bolley (1908)) Cook and Halferdahl (1937), Long (1934) , Long and MacDowall (1935), Crafts and Raynor (1940), and Robbins et al. (1942). More recent reviews have been written by Mitchell and Marth (1947), Avery et al. (1947), Akamine (1948), Avery and Thompson (1947), and Van Overbeek (1947). Current papers on weed control, particularly concerned with the hormone weed killers are appearing in Botanical Gazette. Of special interest is the group of contributions from the Special Projects Division, Chemical Warfare Service, Camp Detrick, Maryland, published in Bot. Gaz. 107,476-634, 1946, and the paper on “Growth Regulating Substances as Herbicides’’ by Kraus and Mitchell (1947). T h e JournaZ of the American Society of Agronomy, Science, American Journal of Botany, and other scientific periodicals are including papers on weed control. The trade journal, Agricultural Chemicals, carries articles on weed control and has many advertisements describing the current products being offered for sale. The organ “Down to Earth” of Dow Chemical Company also carries timely articles on pest control, including weeds. Many research and extension publications are coming from Agricultural Colleges both in this country (e.g., Weeders Readers, Oregon State College of Agriculture, Extension Service) and abroad. Mimeographed reports of regional weed control conferences contain many valuable data (Anonymous, 1948b; North Central Weed Control Conf., 1947; North Eastern Weed Control Conf. Minutes, 1947; Western Weed Control Conf. Minutes, 1946, 1947, Proc. 1948). OF CHEMICAL WEEDCONTROL IV. PRINCIPLES

There are many mechanisms by which the cells of plants may be killed (Aberg, 1947; Crafts, 1939b, 1948c; Crafts and Reiber, 1945; Harvey, 1931; Offord and d’Urbal, 1931; Van Overbeek, 1947) ; salts such as sodium chloride will take water osmotically causing plasmolysis and death; strong acids or bases cause violent shifts in cell reaction from which cells die; some herbicides such as sodium arsenite are protoplasmic poisons that denature the proteins of cells; some partake of the nature of enzymes or stimulators (e.g., the dinitro compounds seem to accelerate respiration) ; many new chemicals such as the naphthoxy-, phenoxy-, and benzoic acids and phenyl carbarnates have hormone properties causing

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disturbances in the basic metabolism of plants; the unsaturated hydrocarbons occurring in petroleum have a compatibility and a reactivity with protoplasm that enables them to enter cells readily and cause a breakdown of their semi-permeable properties (Crafts and Reiber, 1948) ; substituted benzenes seem particularly effective (Crafts, 1945b). Because of the number of reagents used and the variety of actions noted, it seems obvious that different herbicidal properties should result. Not only do the large general groups named above have different mechanisms of herbicidal action, but individual compounds within these groups have specific toxic properties and different plant species, or even different individuals wit,hin a species may react differently. And, finally, variations in environmental conditions cause wide differences in response of plants to herbicides, often making for success or failure of a given treatment. Because of the variations in response of plants to herbicides chemical weed control is complicated and often confusing; on the other hand these same variations result in the high degree of specificity that permits selective use of herbicides. An example is the selective use of dinitro compounds and chlorophenoxy compounds against broad-leaved weeds in cereal crops. An even finer degree of selectivity is that of 2,4-D in killing mustards, Amsinckia, and knotweed in flax, or the use of Stoddard Solvent to control mixed weeds including both grasses and broad-leaved annuals in crops of the carrot family. As research continues, even finer degrees of selectivity are found; 2,4,5-trichlorophenoxyaceticacid is more toxic than 2,4-D on potatoes and a number of woody species; mixtures of Stoddard Solvent and kerosene will kill wild oats in flax; and combinations of chemicals such as 2,4-D and I P C (isopropyl phenylcarbamate) have proved successful in controlling mixed weeds including grasses in Ladino clover. I n discussing herbicides, it has been found convenient to group them into the following categories : Contact herbicides Translocated herbicides Soil “sterilants”

1

General contact Preplanting Preemergence Selective contact Hormonelike

{ Nonhormonelike Temporary Pre- and post-emergence Permanent (relatively)

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While the above grouping of herbicidal mechanisms is convenient and provides a useful terminology, it is not strictly applicable to the reagents used; many of the chemicals serve several purposes. As an example, arsenic in the form of sodium arsenite may be used as a general contact spray. I n crops it may be used either before planting, after planting the crop and before its emergence, or to kill the crop plants before harvest, as in potato-top killing. It is very effective on roadsides, fence lines, and other noncropped areas. In the proper concentration it may be used as a selective spray to kill broad-leaved weeds in grain (Crafts, 1946b). I n the form of an acid-arsenical solution it will be translocated into the roots of deep-rooted perennials (Crafts, 1933a). And, applied at proper rates, it will give either temporary or permanent soil sterilization of the soil (Crafts et al., 1941). More recently, 2,4-D has been used as a preharvest treatment to kill growing weeds (e.g., Lactuca scariola) in ripe grain; as a cotton defoliant; as preplanting and preemergence sprays in corn (Anderson and Ahlgren, 1947; Crafts, 1948b), milo (Harvey and Robbins, 1947), and sugar cane (Crafts, 1948a, b; Van Overbeek and Velea, 1946a, b) ; as a selective spray against mustard and other broad-leaved annuals in grain and flax (North Central Weed Control Conf., 1947; Pavlychenko, 1947) ; as a translocated herbicide to control woody (Anonymous, 1946; Barrons and Coulter, 1948) and deep-rooted perennial weeds (Kephart and Evans, 1948; U.S. Dept. Agr., 1939) ; and as a temporary soil sterilant, both pre- and post-emergence. These examples illustmte the complex nature of weed-control problems and the difficulties involved in the recommendation of specific uses and dosages.

V. HERBICIDAL ACTION In the successful response of weeds to herbicidal treatment several processes are involved. First, following application, the chemical must penetrate the treated portion of the plant. In the case of certain organic compounds such as 2,4-D, dinitro herbicides, and oils this consists of solution in and diffusion through the cuticle (Crafts, 1 9 4 8 ~ ) . Other herbicides such as strong acids and some alkaline sprays must break down or denature the cuticle before they can enter the leaves (Crafts, 193313). Soil fumigants (e.g., (2%) and “sterilants” (2,4-D, arsenic, chlorate) act on the roots. They may enter from the gaseous phase of the soil or from the liquid phase. Second, in the case of translocated herbicides, the chemical must move through the plant tissues, often for distances of several feet, and accumulate at critical regions in sufficient concentration to kill the tissues. I n the case of the hormonelike chemicals such as 2,4-D, this transport

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takes place through living cells and seems to be correlated with food movement (Mitchell and Brown, 1946; Mitchell et al., 1947; Rice, 1948; Weaver and DeRose, 1946). Movement through shorter distances may take place by normal diffusion (e.g., spread throughout a leaf following application to localized regions in t.he form of a dust) or by polarized movement a8 occurs with auxin. For reviews of auxin and 2,4-D movement, see Crafts (1939a) and Crafts et al. (1949). Finally, having entered the plant, and having moved (in the case of translocated herbicides), the chemical must kill the cells. Several of the modes of physiological action have already been mentioned. Others will undoubtedly be found as the search for new and better herbicides is carried on. I n all cases, whether it be a violent upset of cell reaction, a coagulation of cell proteins, or a more subtle disorganization of metabolism, the final action usually involves denaturing of the sensitive protoplasmic system responsible for normal plant function. The effectiveness of a given plant poison depends largely upon the nature of its molecules.

VI. MOLECULAR PROPER TIE^ OF HERBICIDES All chemicals may be divided into two groups-polar and nonpolar compounds. The polarity of a compound depends upon the electrical properties of its molecules or ions. All ions are highly polar; being electrically charged, they are surrounded by strong fields of force. Many molecules in solution have asymmetric force fields; these are polar. Molecules having symmetrical force fieIds are nonpolar. Solvents vary in polarity; water having asymmetric molecules is highly polar ; aliphatic hydrocarbons such as hexane have symmetric molecules, they are nonpolar; compounds having OH, COOH, CHO, COC, and similar groups are somewhat polar (West, 1946). For example, the polarity of the molecules becomes less and less through the series : acet,ic acid, acetaldehyde, ethylene, ethane. Another similar series would be : benzoic acid, phenol, toluene, benzene. A general rule is that polar compounds are soluble in polar solvents; nonpolar compounds are soluble in nonpolar solvents. This is because polar molecules have strong intermolecular forces of attraction for each other but do not attract nonpolar molecules; the same applies to nonpolar molecules. The strongly polar solvent, water, having a high dielectric constant, tends to saturate the force fields between oppositely charged ions in a crystal; hence the ions are less strongly attracted to each other; they wander out into the solvent, become hydrated, and with the water molecules become virtually a new substance, the so1ui:on. Molecules of the nonpolar substance, paraffin, have little attraction for and are little attracted by water. On the other hand, a nonpolar sol-Jent

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like decane readily dissolves paraffin because there are mutual forces of attraction between the molecules of solute and solvent. The application of these principles to herbicidal action comes through the fact that the surfaces of leaves are coated with the nonpolar substance, cutin, and bark is largely composed of a similar substance, suberin. Hence, to penetrate easily the leaves and bark of plants, a herbicide should be nonpolar in character. I n contrast, the roots of plants are specialized to absorb ions or polar molecules; in addition, nonpolar molecules can enter through the lipoid portions of the surfaces of root hairs and through the suberized bark of secondary roots. Furthermore, distribution and availability of herbicides applied through the soil are dependent upon their solubility in the soil moisture. Many of the toxic properties of herbicides depend upon the above relations; oils and oil soluble herbicides are soluble in and readily absorbed by the oillike cuticle. Water and water-soluble compounds are readily taken up by roots (Crafts, 1 9 4 8 ~ ) . On the other hand, reactivity of organic compounds often increases with increased polarity, and toxicity may depend upon polar groups attached to relatively nonpolar molecules. To illustrate these relations, consider the substituted phenols. Toxicity increases through the series : phenol, nit.ropheno1, dinitrophenol (Crafts, 1945b). Among the chlorophenols toxicity increases regularly from phenol to pentachlorophenol. Since all of these compounds are oil soluble, it seems probable that the increases in toxicity, following as they do increases in polarity, are related to the reactivity of the compounds with the protoplasm. Considering the nitrophenols, toxicity increases regularly through the series : dinitrophenol, dinitromethylphenol (cresol) , dinitroethylphenol, dinitropropylphenol, dinitrobutylphenol. The amyl subst,ituted compound is slightly less toxic than the butyl. Through this series the compounds pass from a crystalline substance of relatively high melting point to a substance that melt,s a t about room temperature. The amyl compound is a dark, oily liquid a t ordinary temperatures. I n other words, the compounds become more oillike with increase in length of the substituted side chain. From the above consideration it seems evident that compromise is often necessary to obtain a certain result with herbicides. This is illustrated by the toxicity relations of the various salts of the substituted phenols. For instance, toxicity increases with the decrease in polarity through the series sodium salt, ammonium salt, parent compound, ester. The series stops with the parent compound in the case of nitro- and chloro-substituted phenols and if water solubility is necessary, as when differential wetting is required for selectivity, the ammonium salt or

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amine salt is preferred, I n the phenoxy compounds a further compromise is effected in the case of the ester by using it as an emulsion where water is required as a carrier. Finally, considering the oils, toxicity increases with polarity through t.he series : aliphatic, naphthenic, olefinic, aromatic hydrocarbons (Crafts and Reiber, 1948). Since even the aromatics are oils, however, they penetrate and hence are satisfactory contact herbicides. To give a clearer picture of toxicity relations, however, it is well to compare the effectiveness of some of the naturally-occurring aromatics in an oil with one of the more polar substituted compounds. I n gasoline one might isolate methylbenzene (toluene) or diniethylbenzene (xylene) . I n diesel fuel one might find propyl and butyl benzenes. These and similar compounds are required in quantities equalling 15 to 25 per cent of the total weight of an oil contact herbicide in order to give a satisfactory toxicity. In contrast, pentachlorophenol a t 1/2 to 1 per cent and dinitrosecondary butyl phenol a t 1/4 to 1/2 per cent may be equally or even more toxic.

VII. EMTJLSIONS AND EMULSION STABILIZERS Although oils, because of their low surface tension, high covering power, and cuticle compatibility are excellent herbicides, their limited supply and high cost prohibits their wide use. As explained above, substituted phenols are much more effective toxicants than are the aromatics contained in oils. One iniprovement is to add one of the phenols such as pentachlorophenol or dinitro secondary butyl phenol to the oil and increase its toxicity. Such a fortified oil may be so toxic that it is required only as an extremely thin film on weeds; very much lower dosages of such fortified oils are required to kill weeds. One further step in the same direction is to fortify an oil and then to prepare from it an emulsion. I n such a fortified oil emulsion the fortifying agent does the killing, the oil forms a thin film that holds the fortifier in contact with the plant and the water forms the dispersion medium serving as a diluting agent and in the applied spray providing driving force that aids in penetrating dense foliage. To be successful, such an emulsion should be well stabilized, and the modern petroleum sodium sulfonates and higher alcohol detergents have proved most effective as stabilizers (Anonymous, 1938, 1943, 1944, 1945a, b, 1948h, 19481). Mention has been made of nonpolar and polar compounds. There exist compounds that combine a nonpolar and a polar group within the same molecule. Sodium oleate, a constituent of common soap, is such a compound; the stabilizers mentioned above have similar properties. When such compounds are dissolved in water their molecules tend to orient themselves with- their polar ends in the water and with the non-

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polar groups in the surface layer. At an oil-water interface the polar group is in the water, the nonpolar in the oil. I n this way they overcome the inherent incompatibility of these interfaces so that when the oil is broken up into minute droplets there is little or no tendency for the droplets to run together and form a Pepurat,e phase. I n other words, the emulsion is stable. Even in such stabilized emulsions the stability is a relative state; if the oil particles are large enough, the force of gravity will cause them to accumulate and eventually to coalesce. I n the case of light oils the droplets cream out on top and form a layer over the water; in the case of oils of high density the layer may form beneath the water. Two factors are concerned in the long-time stability of such emulsions: (1) the density of the oil, if i t is close to unity the tendency to separate is less; (2) the droplet size is also concerned, if the droplets are so small that they are violently agitated by thermal agitation they have less tendency t o coalesce and form a separate layer. Homogenization of the emulsion therefore is often of an advantage by reducing droplet size of the oil below a certain threshold value so that they stay permanently dispersed in a uniform concentrRtion throughout the mass of the dispersion medium. VIII. SELECTIVITY OF HERBICIDES The various mechanisms of herbicidal selectivity have been considered in detail (Crafts, 194613). It suffices here t o enumerate the types of selective herbicidal treatments and illustrate them with examples from the field. Selective herbicidal action may result from : (1) Differential wetting, (2) morphological selectivity, (3) biochemical selectivity, (4) selective spray placement, ( 5 ) selective soil sterilization, (6) differential life span of plants. Differential wetting is illustrated by Sinox application to kill mustards and fiddleneck in flax. Leaves of the weeds are broad and readily wetted; those of flax are small and difficult to wet. Peas are another crop that illustrates this phenomenon. Morphological selectivity results from differences in the location of the vital tissues in plants. For exanple, in cereals, onions, and lilies the growing points of the plants are located a t the base near or below the ground level; they are also protected by the surrounding leaves. In contrast, most broad-leaved weeds have exposed growing points which can readily be wet by the spray. The selective action of herbicides containing iron or copper salts, sulfuric arid, or Sinox is due to a combination of these two mechanisms.

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Biochemical selectivity depends upon inherent differences in the makeup of plants with respect to their tolerance of certain poisonous chemicals. When weeds in crops of the carrot family are sprayed with Stoddard Solvent, all the plants are thoroughly wet; only the weeds die. When mustard in a wheat crop is dusted with 2,4-D or given a low volume spray treatment with an ester formulation of 2,4-D in oil, the selectivity is biochemical; all plants are exposed to the chemical, only the weeds die. When a robust crop plant such as corn, milo, or sugar cane attains a height of 12 to 18 inches, it has proved feasible to spray small weeds around the base with a general-contact herbicide, the growing tissues of the crop being protected from the spray by the bases of the older leaves. B y using such sprays i t has been possible to bring a crop to maturity, completely eliminating the frequent tillage commonly used (Crafts, 1948b). Under Home conditions this may be less expensive than the common tillage practice; under some it is preferable because it minimizes erosion and prevents the formation of cultivator sole. This latter is important in the semi-arid west where such sole interferes with irrigation and where impervious layers in t.he soil are not broken up by freezing of the soil during the winter. The same biochemical selectivity shown by the tops of certain crops and weeds is reflected in their roots. For instance, the salts of 2,4-D in the soil will kill mustards at concentrations t,hat cause little or no injury to cereal crops. For this reason 2,4-D salts may be used either as preemergence or post-emergence treatments in small grains and corn, providing soil moisture conditions are right and dosage is carefully controlled. Certain perennial crops may be treated during their dormant period or between cuttings. This is exemplified by alfalfa that may be sprayed with a general contact herbicide to rid it of weeds during the winter or early spring, or immediately after a cutting during the summer. This is becoming common practice in the western states (Harvey and Riddle, 1946; Raynor, 1947). The foregoing discussion presents a number of the types of herbicidal action that are now being widely used in the field of chemical weed control. The remainder of this paper will be devoted t o description of the various chemicals and review of current literature concerning them.

IX. THE2,4-D HERBICIDES The advent of 2,4-D has so rapidly and so profoundly changed the practices of chemical weed control that the terms 2,4-D and herbicide are considered synonymous by many. While a proper balance in em-

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phasis should be maintained with respect to the different herbicides available, a material that has reached a production of around 20 million lbs. by the fourth year of its use in the field deserves recognition. Particularly is this true when one realizes that in many cases one pound or less of this chemical is used per acre. For the first time, chemical weed control may be recommended as an economical substitute for tillage or cropping practices. Selective weed control has assumed a major position among pest-control practices. Apparently 2,4-D was discovered independently in England and in the United States during the war (Blackman, 1945; Kraus and Mitchell, 1947; Nutman et al., 1945; Slade et at., 1945). After its announcement (Hamner and Tukey, 1944; Mitchell and Hamner, 1944), a wave of interest spread across the land and thousands of field tests were made by investigators and private individual farmers before adequate experience in its use had been gained. Early experiments with 2,4-D and similar chemicals are reported by the British workers, Slade et al., (1945), Nutman et al., (1945), and Blackman (1945). Hamner and Tukey (1944) were the first to publish experimental work in this country; Kraus and Mitchell (1947) later described many experiments performed during the war and carried out under restriction. These firmly established the herbicidal properties of 2,4-D and similar growth regulators. Subsequent work has been carefully reviewed by Akamine (1948). Readers are referred to this paper where scientific references as well as government publications, popular articles, and commercial reports are listed. Popular aspects are al3o treated by Mitchell and Marth (1947) a n d by Avery et al. (1947). 2,4-Dichlorophenoxyacetic acid is a white crystalline substance formed by reacting dichlorophenol with monochloroacetic acid (Wellman, 1948). It is only slightly soluble in water but may be readily dissolved in alcohol and acetone (Anonymous, 1948j). Early 2,4-D formulations involved the use of Carbowax (polyethyleneglycol) as an intermediary but it was soon realized that the sodium and ammonium salts were soluble in water and convenient to use. More recent formulations involve the amine salts, the principal advantage of these being that they are liquid a t high concentrations, which makes for easy mixing by dilution. Some of these formulations contain a quantity of the salt equivalent to four pounds or more of acid per gallon. The esters of 2,4-D are somewhat more effective than the salts since they are less polar in nature. As these compounds lose in polarity, however, they lose correspondingly in water solubility and most of the formulations involve the ester or t.he ester in solution in oil, plus a large

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amount of emulsion stabilizer. Thus the material may be dissolved in oil and applied as a very fine spray, or emulsified in water with application possible over a wide range of volume dosages. Though appreciably more toxic on an acid equivalent basis (up to twice), some esters have the drawback of being volatile and problems of injury to adjoining sensitive crops have been more serious with ester formulations than with the salts (Brown et al., 1948; Dunlap, 1948; Pryor, 1948). Because 2,4-D is available in such a variety of forms, workers have agreed to recommend and use it on an acid equivalent basis. In order to compute formulations, niolecular weights of the various 2,4-D compounds are necessary. Table I from Weeders Readers of November 1, TABLE I Concentrations and Weights of 2,4-D Materials

Substance

2,4-D Acid 2,4-D Ammonium Salt 2,4-D Sodium Salt (anhydrous) 2,4-D Sodium salt (monohydrate) 2,4-D Diethanolamine salt 2,4-D Triethanolamine salt 2,4-D Methyl ester 2,4-D Ethyl ester 2,4-D Isopropyl esters (2, same weight) 10. 2,4-D Butyl ester (4, same weight) 11. 2,4-D Amy1 esters (about 15, all with same weight) 12. 2,4-D Isopropanolamine salt 1. 2. 3. 4. 5. 6. 7. 8. 9.

Molecular weights

Per cent 2,4-D

Units required to contain 100 units of 2,4-D acid

220 237 242 260 325 369 234 248

100 93 91 85 68 60 94 89

100 108 110 119 147 166 106 112

262 276

84 80

119 125

290 296

76 74

132

135

1947, gives these values. Structural formulas are given by Freed (1946). From what has been said, it is apparent that 2,4-D is unique among herbicides. Having growth-regulating properties (Synerholm and Zimmerman, 1945, 1947; Zimmerman and Hitchcock, 1942), it i8 effective at dosage rates that are phenomenally low. Only the nit.ro- and chlorophenols approach 2,4-D in toxicity and while they have definite uses in the field they do not approach 2,4-D in multiplicity of herbicidal characteristics.

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2,4-D

Because 2,4-D has hormone properties, it differs from the older herbicides in its physiological action on plants. Upon coming in contact with leaves or roots, it rapidly migrates to other parts of the plant; this is the action that makes possible low-volume and dust treatments as contrasted with most other herbicides that are required in much greater volume t o effect sufficient cover. In spite of this remarkable quality, 2,4-D may be used in essentially the same way as the older contact herbicides such as sodium arsenite, chlorate, and dinitro compounds. Therefore, in what follows, where 2,4-D is being used as a contact herbicide it is so labelled even though it may move within the plant after its application. 1. General Contact Spray While not widely used as such, 2,4-D has found valuable use as a general-contact killer. I n California it has been used to kill full grown pigweeds, radish, and Chinese lettuce in ripe grain to facilitate harvest by combining. It is being widely used in the Imperial Valley to kill weeds on ditch banks. And, odd as it may seem, it has proved effective in killing the leaves and stems of cotton preparatory to harvest with mechanical pickers. 2. #elective Contact Spray

The outstanding application of 2,4-D has been as a selective spray in cereal crops. Here the relatively low susceptibility of t8hegrass plants is made use of in the control largely of broad-leaved plants. I n 1948, many million acres of cereal crops have been treated (Anonymous, 194%). Application has been by ground sprayer and by airplane. Ester and amine salt formulations have been predominant mainly because of convenience; sodium and ammonium salts have been used in lesser quantities. I n addition to the cereal crops, wheat, barley, and oats, corn has been sprayed to the extent of over one million acres (Lee, 1948a, b; Stahler, 1948; Willard, 1948), rice t o several hundred thousand acres (Anonymous, 1948f; Brown, 1947a; Higgins, 1947; Tullis, 1948), sugar cane t o a like extent (Brown, 1947b; Hance, 1948a, b; Hanson, 1948; Nolla, 1948; Shaw et al., 1947; Van Overbeek and Velez, 1946a, b ; White and Mangual, 1948; White and Villafane, 1946), milo to many thousands, and flax has been treated on a considarable acreage. Grass seed crops have been profitably sprayed (Anonymous, 1947a, d ; Marth and Mitchell, 1946a ; Marth et al., 1947), and many thousands of acres of pastures and range

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A. HARVEY

have been treated (Kephart and Evans, 1948; North Central Weed Control Conf., 1947; Savage and Costello, 1948; Savage et al., 1948). Lawns and fairways have been successfully weeded (Ahlgren and Cox, 1947; Marth and Mitchell, 1946a, b, 1947; Mitchell and Marth, 1947). Experimental work has been done on potatoes (Ennis et al., 1946; Smith e t al., 1947), asparagus, strawberries (Carlson, 1947a), and a number of other crops. 3. Translocated Spray Probably the most remarkable characteristic of 2,4-D is the fact that it is readily translocated in plants. Being relatively nonpolar, i t is absorbed through the cuticle of leaves; the acid and ester forms are most readily taken in. Arriving in the leaf, this chemical is apparently subject to the same mechanisms of transport as auxin, namely polar transport in parenchyma cells and rapid movement in the assimilate stream along with organic foods. Literature on such transport is reviewed by Crafts (1938, 1939a) and Crafts e t al. (1949). Polar 2,4-D movement shows a positive geotropic pattern; taking place through living parenchyma cells, it probably accounts for the complete destruction of the leaves and stems of annual weeds when application is by dust to localized areas or by low-volume application to a relatively small portion of the total surface of the plant. Transport of 2,4-D with the assimilate stream accounts for the movement of this toxicant deep into the root systems of perennial weeds. The widespread and generally successful control of wild morning-glory, poison oak, sage brush, and a host of other perennial weeds depends upon this movement from the tops of the treated plants into the root systems underground. Killing of the roots of wild morning-glory to depths of 15 or 20 feet has been noted on deep alluvial soils of California and other states. The observation that the vertical tap root is often more completely killed than are lateral roots is probably a manifestation of polar movement; geotropic movement in parenchyma cells would result in accumulation in the cortex of the first few centimeters of a lateral and the remainder of the root would not be killed. Experiments by Mitchell and Brown (1946), Weaver and DeRose (1946), and Rice (1948) have shown that 2,4-D transport takes place most rapidly and effectively in plants in which photosynthesis and food movement are going on. Evidently 2,4-D is carried along with foods in the phloem of plants and attains a similar distribution in the roots. It has been a matter of some confusion to plant physiologists that 2,4-D gives the most satisfactory root kills on wild morning-glory, hoary cress, chicory, artichoke thistle, St. Johnswort, and similar perennial

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weeds if applied just before or during the early stages of blossoming, whereas maximum translocation and storage of food is known to occur somewhat later. The answer lies in the &ate of the roots at this time. Van Overbeek (1947) has pointed out that 2,4-D accumulates in and effectively kills meristematic tissues. During the earlier growth stages the roots of perennials are still meristemat,ic and are using assimilated foods in the production‘of new tissue. Later, the bulk of the assimilates is accumulated in mature storage parenchyma, cells that can absorb 2,4-D (Mitchell e t al., 1947) but t,hat do not respond by rapid death as does meristem. Hence, during the early blossoming period 2,4-D absorbed by leaves is rapidly taken to the meristems of roots and shoots, later it is taken t o storage parenchyma where it has less killing action. Although t.he translocation of 2,4-D into the roots is essential to the killing of perennial weeds, a similar transport in annuals probably explains the extreme effectiveness of this chemical as a selective spray. The control of many aquatic weeds such as Alisrna and Sagittaria species in rice, nutgrass, t.ules, and cattails in wet lands and ditches, water hyacinth, yellow primrose, and alligator weed in waterways, and a number of others illustrates further the wide herbicidal usefulness of 2,4-D. These instances all involve translocation, though to a lesser extent than in wild morning-glory. The successful control of brushy and woody plants on pastures and in right-of-way maintainance is of great credit t o 2,4-D (Anonymous, 1946; Ashbaugh, 1948; Barrons and Coulter, 1948; Hamner and Tukey, 1946; Savage and Costello, 1948; Savage et al., 1948). Successful treatment of such species during active growth undoubtedly results from transport of the 2,4-D with the assimilate st,ream. The recent finding that many such plants also respond to treatment with the esters of 2,4-D in oil during the dormant period (Barrons and Coulter, 1948) indicates some other mechanism of movement, possibly slow cell-to-cell transport in living parenchyma.

4. Temporary Soil Sterilization Although 2,4-D is outstanding in the uses noted above, probably its unique role is that of a temporary soil “sterilant.” Such compounds do not sterilize the soil in the sense that all biological life is killed. The word “sterilant” is employed by weed men for compounds that prevent germination of seeds or establishment of seedlings. I n contrast to arsenic, borax, chlorate, and others of the older sterilants that are required in amounts of hundreds of pounds per acre (Crafts, 1935, 1936; 1939c; Crafts et al., 1941), 2,4-D performs many useful functions when applied at rates of 1to 3 or 5 Ibs. per acre. This depends on two facts: (1) 2,4-D

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is not strongly fixed nor rapidly altered by contact with the soil (DeRose, 1946; Hanks, 1946; Kries, 1947; Taylor, 1947) ; (2) it is from 10 to 100 times as toxic to roots as to the tops of the plants. For these reasons it will kill weed seedlings in concentrations of a few parts per million in the soil. The same relatively high tolerance shown by the tops of grasa plants is also present in the roots (Mitchell et al., 1947). Methoxone (4 chloro-2 methyl phenoxyacetic acid) and 2,4-D have been tested by the British workers (Nutman et al., 1945; Slade et al., 1945) and shown to be effective when applied through the soil (see also DeRose, 1946). I n contrast with resu1t.s in the eastern states and Britain, California farmers have experienced deleterious effects from 2,4-D residues in the soil (Crafts, 1946a, 1949). Studies have shown that decomposition of 2,4-D takes place most rapidly in warm, moist soils of acid reaction. Seldom are all of these factors favorable for breakdown in California soils a t any one time (DeRose, 1946; DeRose and Newman, 1947; Kries, 1947; Mitchell, 1948; Taylor, 1947). Because 2,4-D breaks down too rapidly in soils to serve as a means for controlling deep-rooted perennials by soil applications, its most logical role is that of a selective herbicide. There are 3 general methods for using 2,4-D as a selective temporary soil sterilant: (1) as a preplanting treatment to provide a weed-free seedbed; (2) as a preemergence treatment to provide a weed-free soil medium, and (3) as a postemergence treatment to eliminate weeds that have come up with the crop (Norman, 1948). Where soil and climatic conditions favor rapid 2,4-D breakdown it is possible that this chemical might be used as a general preplanting treatment for all crops, including 2,4-D susceptible ones; however, since a period of 4 to 6 weeks is usually required to free the soil of 2,4-D, the most logical use would be as a selective treatment in tolerant crops. One obvious advantage of the soil treatment method is the possibility of controlling grass seedlings that would not succumb to contact spray treatment (Anderson and Ahlgren, 1947; Mitchell and Brown, 1946). Considering the use of 2,4-D as a soil treatment, there are 4 rather distinct climatological conditions that might determine its effectiveness (Crafts, 1948b): (1) no rainfall, that would result in failure because the chemical would not be washed into.the soil around the roots of the weeds; (2) very light rainfall or foggy drizzly weather, that might cause serious injury because the chemical would be localized a t the soil surface in high concentration; (3) moderate rainfall, that should be most favorable for selective action, and (4) heavy rainfall or flood t.hat would leach the chemical excessively and allow the weeds to survive. Results of trials have shown that the soluble salts of 2,4-D are most effective during

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periods of light rainfall; 2,4-D acid may be used when heavy rainfall i s anticipated (Anderson, 1948; Crafts, 1948b). Preemergence treatment with 2,4-D has tremendous possibilities for reducing the labor of tillage so commonly practiced in row crop agriculture. Not only will it lower costs and reduce drudgery, it will minimize cultural operations that tend to break down soil colloids and render soils impervious to water and air. Under some conditions it will appreciably increase yields. It may also be used to control weeds in a mulch-covered soil so that wind and water erosion are minimized. 6. Permanent Soil Sterilization

Because of its tendency to leach and to break down in soils, 2,4-D offers little promise as a permanent soil sterilant. Possibly in desert regions it might be used around power poles or on fences and around structures. In agricultural areas the hazards from leaching into cropped areas or of being blown on dust are too great. Permanent soil sterilization is apparently the least useful category into which 2,4-D may fall; arsenic, chlorate, and borax would seem to be more logical materials to use. However, 2,4-D may be useful to kill deep-rooted perennial weeds growing in sterilized soil that are not killed by the shallow chemical treatment of permanent soil sterilants. XI. NITRO-AND CHLORO-SUBSTITUTED PHENOLS Sodium dinitro-ortho cresylate (Sinox) was introduced into the United States in 1935 and soon became the most widely used selective spray (Westgate and Raynor, 1940). Iron sulfate and sulfuric acid sprays were displaced and selective weed killing was introduced in a number of new crops such as flax, peas, onions, corn, alfalfa, lawns, etc. Within a short period the use of acid salts to increase toxicity (activation) was introduced and thousands of acres of crops were sprayed annually. Instead of the injury so often experienced with sulfuric acid, Sinox proved beneficial to many crops and though occasional burning of leaf tips occurred, in most instances the crops were actually stimulated. This was particularly true of small grains the yields of which were often significantly increased over and above those resulting from elimination of weeds (Westgate and Raynor, 1940). Following chemical studies the mechanism of Sinox activation was described (Crafts and Reiber, 1945) and the suggestion was made that the ammonium salt be used instead of the sodium salt. Further studies along this line proved that the parent phenols in oils could be used as general conttact sprays, that emulsions of such fortified oils were efficient and inexpensive herbicides both for pre- and postemergence treatment.,

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and that pentachlorophenol and its salts could be used as direct substitutes for the dinitrocresols (Barrons, 1947; Crafts, 1945b, 1947b, 1948a, b; Crafts and Reiber, 1945). Because dinitrocresol proved more toxic than dinitropheno1, screening tests were made on the ethyl, propyl, butyl, and amyl substituted phenols. These proved that toxicity increased up to the butyl substitution, t,hat the ortho compounds were more toxic than the meta or para compounds, and that dinitro substitution was more effective than mononitro (Crafts, 1945b). With chloro substitutions on the phenol ring, toxicity increased regularly to the penta compound. At present the ammonium salt of dinitro secondary butyl phenol is being used extensively as a selective herbicide (Dow Selective, Sinox W) . Dinitro-secondary butyl phenol (Dow General) and amyl phenol (Sinox General) are widely used to fortify oil sprays and to prepare fortified oil emulsion sprays for general contact weed control, for potato top killing (Anonymous, 1947b) and as a selective spray in sugar cane (Crafts, 1948a, b; Crafts and Emanuelli, 1948; White and Mangual, 1948), corn, milo, and similar crops (Crafts, 1948b) by selective placement of the spray. Pentachlorophenol has recently been introduced as an oil fortifier (Anonymous, 1948e; Crafts, 1947b, 1948a, b; Crafts and Emanuelli, 1948; Crafts and Reiber, 1945, 1948; Hance, 1948a). Sodium pentachlorophenate is used in large quantities as a preemergence treatment and as a selective soil sterilant in pineapples in Hawaii (Anonymous, 1948m). There are still many opportunities for introduction of nitro-, chloro-, and nitro-chloro substituted phenols as selective and general contact herbicides as relatively few of such compounds have been tested as to their special selectivities. Combinations of nitro- and chlorophenol contact herbicides with 2,4-D have proved very effective in certain situations where weeds susceptible to both types of toxicants are present. Work on such combinations is presented by White and Villafane (1946), Mangual (1948), White and Mangual (1948), Crafts (1948a, b), Nolla (1948), and Hance (1948).

XII. OILS Petroleum fractions have long been used as weed killers. At first waste products such as acid sludge, waste engine oil, and similar materials were used; later smudge pot oil, Edeleanu extract, and diesel fuel were adopted (Robbins et al., 1942). More recently stove oil has been used as a selective weed killer in crops of the carrot family (Crafts, 1947a; Crafts and Reiber, 1944, Grigsby, 1946; Lachman, 1945; Raynor, 1943; Sweet, 1945; Sweet e t al., 1944, 1945; Warren, 1946; and Warren and Hanning, 19461, and since actual research on the herbicidal properties

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of oils has been (harried on (Crafts and Reiber, 1948) new weed oils have been introduced. In the selective field Stoddard solvent or similar light fractions have been used with eminent success (Crafts, 1947a; Lachman, 1945; Sweet, 1945; Warren, 1946). Standard Weedkiller #I, Shell Weedkiller #lo, and a number of cleaning solvents and paint thinners have been placed on the market, and in California alone (Crafts, 1947a, b) well over a million gallons per year are used. Recently a mixture of Stoddard solvent and kerosene (Standard Weedkiller #11) has been successfully used to kill wild oats and other grasses in flax (Crafts and Reiber, 1948; Herbert, 1948). Recent work in several eastern states has proved that seedlings of coniferous trees tolerate oil fractions (Eliason, 1948) in the same way as plants of the carrot family. Oil sprays are proving useful in handling conifer nurseries and thousands of gallons of Stoddard Solvent and special weed oils have been used during the past two seasons. During the war much work was done in perfecting methods of weed control in guayule using oils. Stove oil and diesel oil were used and guayule proved highly resistant to them compared with weeds (Benedict, 1944). In fact, by the time the guayule work was abandoned, practically all weeding operations were being carried on with oil. Oils have also been suggested for use in onions (Crafts and Raynor, 1944; Crafts and Reiber, 1948). I n the field of general contact herbicides, a number of aromat.ic oils have been introduced. The following list includes the weed oils registered for sale in California during the year 1947-48 (Anonymous, 19488). Some of these are straight-run distillates; many are high in aromatics: Avon Weed Killer, Chapman-Gilbert Weed Killer #5, Cox Hykil Weed Oil, Cox Standard Weed Oil, Foothill Oil Weed Killer, General Weed Exterminator, G & H Weed Oil, Harold Preston’s Weed Oil, Home Oil Anaheim Weed Killer, Kem-Kill W, Richfield A, Shell Weedkiller #20, Standard Weedkiller #2, Union 40-60 Distillate. The highly aromatic oils in this list are excellent weed oils, being extremely toxic and effective against such grasses as Bermuda grass, quack grass, Kikuyu grass, Johnson grass, and others. They are also useful as oil bases in fortified oil emulsion herbicides (Crafts, 194713, Crafts, 1948a, b; Crafts and Reiber, 1948; Hance, 1948a, b). Since the use of such emulsions greatly extends the service of a given volume of oil, they will probably become widely used as their properties become known, and as oil becomes scarce. The use of straight oil for weed hilling seems even now to be justified only where difficult-to-kill grasses constitute a fair proportion of the weed population.

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XIII. OTHERORGANIC AND INORGANIC CHXMWALS

IPC (Isopropyl Phenylcarbamate) Several new chemicals have been recently tested for herbicidal Imperties. Probably the most widely known is I P C (O-isopropyl-Nphenyl carbamate), which was first reported as being toxic to cereals biit iiot to certain broad-leaved plants by Templeman and Sexton (1945, 1946) in Great Britain. This work was verified a t Camp Detrick (Allard et al., 1946). Many workers following these early leads tested the effectiveness of I P C on numerous grasses and broad-leaved plants (Anonymous, 1 9 4 7 ~ ) . In general, this chemical exhibits a selective toxicity toward grasses, although there are notable exceptions. Because of its low water solubility, i t has usually been applied dry with sand as a carrier, or as a wettable powder. I P C appears to be noneffective when applied as a foliage spray prior to elongation of the internodes (Ennis, 1947). Most of the experimental work has been concerned with applications to the soiI, either before or immediately after emergence of grass seedlings. Early work on quack grass (Carlson, 1947b; Mitchell and Kephart, 1947) indicated that IPC had definite possibilities but such use has not yet proven satisfactory (Derscheid and Stahler, 1948, Grigsby, 1948). Although IPC has not complctely lived up to early expectations (Freed, 1948), it is still an important herbicide. Probably its greatest use will be in the control of weedy grasses in legumes such as alfalfa and ladino clover (Tucker, 1948). The responses of fifty-two crop species to this compound have been described by Ennis (1948). Much more information is needed on formulation, application, and species tolerance before I P C finds its proper place among modern herbicides. 1.

2. T C A (Trichloracetic Acid)

The sodium and ammonium salts of trichloroacetic acid are presently receiving wide attention as grass killers. There is less information currently available on these materials than on IPC. The early work indicates, however, that TCA is less selective as a grass killer than I P C but is more toxic to certain species. Some success has attended its use on Bermuda and Johnson grass and as a control for annual grasses which appear after cotton is laid by (Evans, 1948). Both salts of the acid are readily soluble in water and are usually applied as sprays. Action through leaf absorption as well as through the soil has been noted with somewhat less selectivity when applied to the tops. Again, more information is necessary before TCA finds its proper place in weed control.

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3. PMAS (Phenyl Mercuric Acetate)

Recently phenyl mercuric acetate has received attention as a selective crab grass killer (DeFrance, 1947). Most of the work reported to date has been on lawns, turfs and golf greens, but successful development of the material should be a boon in truck gardens and pastures. PMAS is usually applied as a spray although its action, a t least in part, is through the soil. I t s selective action on crabgrass may result from the rather superficial root system of this species. Developmental work is under way with two other chemicals (Evans, 1948) sodium isopropyl xanthate and ally1 chlorophenyl carbonate, but little information is as yet available on them. )

4. Cyanamid and Cyanate Calcium cyanamid has long been used as a temporary soil sterilant to rid soils of weed seeds preparatory to planting lawns (DeFrance, 1948), tobacco seedbeds (Anonymous, 1948c), and various vegetable and field crops (Wolf, 1948). Cyanamid dust lias been applied t o cereal crops as a selective herbicide wherever dew is sufficient to provide moisture to dissolve the chemical on the leaves of weeds. More recently potassium cyanate sold under the name of Aero Cyanate has proved effective as a selective spray in onion and other bulb crops (Anonymous, 1948d; Evans, 1948). Upon breakdown in the soil both of these materials leave residues that are high in nitrogen and hence valuable as fertilizers. 5. CX2, DD, Prochlow The Irerbicitlal properties of CS2 are well described by Hannesson et al. (1945). DD (dichloropropane-dichloropropene mixture) proved effective in the killing of deep-rooted perennial weeds but dosage was many times that required for nematode control. Since the advent of 2,4-D this material has been limited to the latter use. Prochlors (chlorinated propane-propene mixture) have been tested as weed killers (Freed, 1947) but have not been widely used. They offer some advantages over CS, but cost considerably more than 2,4-D in the control of perennial weeds, While generalization as to new chemicals in a field as fluid a s that of herbicides is not without risk, nevertheless certain trends do appear. Specific selectivities, undreamed of a few years ago, have aIready emerged and become an integral part of the agricultural scene. More such selective chemicals are in the offing, bringing further advances in weed control. Also, new developments point to better grass killers both as

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selectives and general toxicants and the resultant decrease in the use of oil for this purpose. The major impact of herbicides on agricultural practices is yet to be felt. 6. Arsenic, Borax, Chlorate Because litt.le has been added to the information given in Robbins et al. (1942) on these herbicides, they will receive only passing mention here. There are still many situations in which they are useful, but full knowledge concerning their properties is required for their intelligent handling. XIV. WATERWEEDCONTROL Of importance to irrigation agriculture are the new chemical methods for controlling semi-submersed and submersed water weeds. I n place of chaining, dragging, and cutting by various means, methods that tended to subdivide the plants and spread the infestations, treating the irrigation water with chlorinated benzenes or solvent naphthas has proved effective and relatively inexpensive (Crafts, 1945a; Hirst, 1947; Moran and Shaw, 1948). The chemicals, mixed with sufficient emulsion stabilizers to insure immediate emulsification, are sprayed under the surface of the flowing water in the ditch. They form milky white emulsions and the toxicants are absorbed by the leaves of the weeds, resulting in rapid plasmolysis and death. The chlorinated benzene killers are heavier than water and they tend to settle into the mud in the ditch bottom, resulting in a slight residual effect. The solvent naphthas, being lighter than water, rise to the surface when the emulsions break. XV. HERBICIDE APPLICATION EQUIPMENT The rapid expansion of chemical weed control has created a big demand for equipment. One company sold over two million nozzles during the 1948 spraying season and this represents only a small fraction of the total business. Because of the great demand, many State Agricultural Experiment Stations have printed bulletins or circulars describing equipment. The following references provide a number that will be found convenient (Akesson and Harvey, 1948; Anonymous, 1948k ; Derscheid and Stahler, 1948; Price et al., 1946; Sylvester and Bakke, 1947).

XVI. DRIFT,VOLATILIZATION, BLOWING OF HERFXCIDES. SECONDARY AND RESIDUAL EFFECTS As a corollary to the introduction of new pest control chemicals come abuses, misuse, and accidents. Already 2,4-D has damaged thousands of acres of cotton, tomatoes, beans, sweet potatoes, melons, and grapes,

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and many people are ready to prohibit its sale and use. To those who remember t.he numerous cases of arsenic poisoning, chlorate burns and other accidents, this is simply evidence of the lag of popular education behind the promotion and sales of these materials. Most of the cases of serious damage such as those occurring in Texas and Louisiana (Brown, 1947s; Dunlap, 1948; Staten, 1946), and that of the San Joaquin Valley in California in 1948 (Pryor, 1948) are obviously examples of misuse. They can be prevented in t.he future by proper instruction of growers in the precautions essential to the use of so potent a chemical. Of more interest and concern to the agronomist are certain cases involving secondary effects. I n one instance in California, either bull thistle or milk thistle plants sprayed with 2,4-D proved fatal to lambs. Several people have been reported as exhibiting allergic symptoms as a result of contact with 2,4-D. One instance of allergic reaction to mayweed sprayed with 2,4-D has been noted. These cases are serious and deserve careful study. On the other side, curly dock plants in a ladino clover pasture that had received a 2,4-D spray were so relished by sheep that they were eaten clear into the ground. And many tests in the greenhouse with 2,4-D in soils show stimulation of crop plants by as much as 100 per cent after the chemical has broken down in the soil (Crafts, 1949; Tullis, 1948) . Breakdown of 2,4-D in soils has been shown to be related to the activities of microorganisms (DeRose and Newman, 1947; Newman, 1947). Recent work indicates that the soil microflora may be altered by the presence of 2,4-D so that subsequent applications of the chemical are destroyed more rapidly than the initial treatment. This is of great significance in preemergence practice as too rapid disappearance of the chemical may reduce the effectiveness. Another prominent effect of 2,4-D use is the build-up of grassy weed populations. Already some growers are complaining about the increase in wild oat populations in their grain fields. One case has been reported in California where elimination of bur clover by 2,4-D treatment has seriously reduced the value of the stubble as sheep pasture. These examples illustrate the far-reaching effects that can be expected from sudden changes as drastic as that of weed elimination. I n all tests on new herbicides the investigator should be constantly on the watch for such secondary effects. Only by vigilance can serious accidents be prevented and full usefulness of the new materials be exploited.

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XVII. FLAME CULTIVATION The use of weed burners to control weeds selectively in growing crops has received attention in several areas.. Probably the widest agronomic interest has been in cotton where mechanization has been a prime motive in the development of flame weeding (Neely and Brain, 1944). The selectivity of the method is based on the greater resistance to heat of cot.ton stems which become woody more rapidly than the stems of succulent weeds. Thus it is possible to find a time of exposure to a particular temperature which will kill young weeds but not permanently injure cotton. Such selectivity is only relative and too long an exposure to flaming when cotton is too small will injure the crop. The method is still in the developmental stage (Grigsby, 1948), although some plantations are flaming a substantial acreage. Other crops on which flaming shows promise include corn and sugar cane, where the enfolding outer leaves protect the stem from injury. A few truck crops (i.e.J onions) and ornamentals (lilies) also withstand some flaming. The use of flame as a preemergence treatment has also received attention. In this respect, flaming competes directly with herbicides and the choice may become one of cost and convenience. XVIII. THENEW AGRONOMY Under the above title we should like to discuss some of the ideas that are evolving with the use of the new, highly effective herbicides. Many practices involving rates of seeding, spacing of drill rows, harrowing and fertilization have grown up around the problems of weed control. The width of rows and the spacing of seeds in the rows of intertilled crops have been dictated by the length of the single-tree or the prescribed spacing of implements on the tractor drawbar (Norman, 1948). Since the introduction of 2,4-D the eminent success in the control of weeds has suggested to many the possible alteration of such practices with increased crop production as the goal, using mechanized pest control and mechanized harvesting in place of conventional methods. I n recent t.rials, in which 200 bushels or'more of corn per acre were produced (Anonymous, 1948i), several of the successful growers used 20-inch rows with 6 to 9 inch spacing within rows. This provides a more uniform environment. Similar spacing trials are being tested in cotton, soy beans, and other intertilled crops (Leonard et al., 1947, 1948). By using close spacing, mechanized pest control, and increasing the fertilieer applicat.ion, not only are crop yields being increased, but tillage with it,s attendant root pruning and its deleterious effects on soil colloids

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is reduced to a minimum. This allows the soil to attain as nearly its virgin fertility as is possible under a cropping regime, and it appreciably lowers losses from erosion and leaching. Use of segmented or pelleted seed and precision planting are adding t o the degree of control which the grower is able t o exercise over his crops (Bainer, 1947, 1948). These two features alone may cut the need for hand weeding almost t o zero. Furthermore, the various organic materials used in pest control are broken down in the soil, with a resultant increase in fertility (Zobel, 1946). Many of the toxicants, after breaking down, seem to have a stimulatory effect on the crops (Crafts, 1949). How far the grower can go in combining these various effects to increase yields is difficult to foresee, but it. seems certain that when they are used t o their maximum advantage many of the soil losses that have been stressed by soil conservation workers will be eliminated. This will mean that instead of facing a future of constantly diminishing returns from the soil, crop production may be maintained a t least a t its present level and possibly may be raised to an appreciable extent. REFERENCES Aberg, B. 1947. Royal Agr. Coll. Sweden Ann. 15, 37-107. Ahlgren, G. H . and Cox, H. R. 1947. N.J. Agr. Expt. Sta. Bull. 725. Akamine, E. 1948. Univ. of Hawaii Agr. Expt. Sta. Circ. 26. Akesson, N. B., and Harvey, W. A. 1948. Agr. Eng. 29, 384-389. Aldrich, D. G. 1948. Am. Cyanagrams 1 (7), 2-4. Allard, R. W., Ennis, W. B., DeRose, H. R., and Weaver, R. J. 1946. Bot. Gaz. 107. 589-596. Anderson, J. C. 1948. Down to Earth 3, 13. Anderson, J. C., and Ahlgren, H. 1947. Down to Earth 3 ( l ) , 16. Anderson, J. C., and Wolf, D. E. 1947. J. Am. SOC.Agron. 39, 341342. Anonymous. 1938. The Aerosol Wetting Agents. Leaflet 554. American Cyanamid co. Anonymous. 1943. Table of surface-active agents. Ind. Eng. Chem., Ind. Ed. 35, 126-130 Anonymous. 1944. Oronite Wetting Agents. Tech Bull. 1-12-45. Oronite Chem. Co., San Francisco. Anonymous. 1945a. Oronitc Sodium Sulfonatcs. 'I'crh.H d Z . 8-1-46. OroIiite Chrm. Co., San Francisco. Anonymous. 194513. Spans and Tweens. Ind. Cheni. Dcpl., Atlas Powder Co Wilmington, Dela. Anonymous. 1946. Doum to Earth 2 (31, 2-6. Anonymous. 1947a. Down to Earth 2 (41, 2. Anonymous. 194713 Down to Earth 3 (21, 14-15. Anonymoils 1947~. IPC. J. T. Raker Chemical Cu. Intorni. Leaflet. Anonymous. 1947d. Oregon State Coll. and U.S. Dept. Agr. Coop. Ext. in Agr , Weeders Readers #15, p. 14-15.

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Anonymous. 1948a. Agr. Chem. 3 (3), 21 (Editorial). Anonymous. 1948b. Agr. Chem. 3 (3), 43-53. Anonymous. 1948c. Am. Cyanugrams 1 (6), 6. Anonymous. 1948d. Am. Cyanagrams 1 (6). 11. Anonymous. 1948e. Destroy Weeds with Santophrn 20. Monsanto Progress Rept. 7 PP. Anonymous. (Tullis, E. C.) 1948f. Down to Earth 4 ( I ) , 8. Anonymous. 1948%. Economic Poisons. Calif. State Dept. Agr. Bur. Chem. Special Publ. 229. Anonymous. 1948h. Facts about Vatsol Wetting Agents. Agr. Chem. Div. LeafEet 905. American Cyanamid Co. Anonymous. 1948i. Farm J. (Dec.), p. 41. Anonymous. 1948j. Monsanto Tech. Bull. 0-50. 20 pp. Anonymous. 1948k. Oregon State Call. and U.S. Dept. Agr. Coop. Ext. in Agr., Weeders Readers No. 17. pp. 7-16. Anonymous. 19481. Surface Active Agents in Insecticides. Ind. Chem. Drpt. Mimeo. Atlas Powder Co., Wilmington, Dela. Anonymous. 1948m. Weed Control with Santobrite. Monsanto Progress Rept. 8 pp. Ashbaugh, F. A. 1948. Down to Earth 3 (4), 10-12. Avery, G. S., Johnson, E. B., Addoms, R. M., and Thomson, B. F. 1947. HormoncAs and Horticulture. McGraw-Hill, New York, N.Y. Avery, G. S., and Thomson, B. F. 1947. Econ. Bot. 1, 176-187. Bainer, R. 1947. Agr. Eng. 28, 49-54. Bainer, R. 1948. Agr. Eng. 29, 477-479. Barrons, K. C. 1947. Down to Earth 3 (31, 2-4. Barrons, K. C., and Coulter, L. L. 1948. Down to Earth 4 (21, 4. Benedict, H. M. 1944. Winter Res. Progress and Plans Conf. Guayule Proj. Mimeo. Blackman, G. E. 1945. Nature 155, 500-501. Bolley, H. L. 1908. N . Dak. Agr. Expt. 8ta. Bull. 80. Brown, C. A. 1947a. Am. J. Bot. 34,20a. Brown, C. A. 1947b. Down to Earth 2 (4), 6-10. Brown, C. A., Holdeman, Q. L., and Hagood, E. S. 1948. La. State Univ. and A g r . and Mech. Coll. Bull. 426. Carlson, R. F. 1947a. Am. SOC.Hort. Sci. Proc. 49, 221-223. Carlson, R. F. 1947b. Mich. Agr. Expt. Sta. Quart. Bull. 29, 274-280. Cook, W. H., and Halferdahl, A. C. 1937. Nat. Res. Council, Canada, Bull. 18. Crafts, A. S. 1933a. Hilgardia 7 , 361-372. Crafts, A. S. 1933b. Hilgardia 8, 125-147. Crafts, A. 8. 1935. Hilgardia 9, 461-498. Crafts, A. S. 1936. Hilgardia 10, 343-374. Crafts, A. S. 1938. Plant Physiol. 13, 791-814. Crafts, A. S. 1939a. Bot. Rev. 5, 471-504. Crafts, A. S. 1939b. 1. Agr. Res. 58, 637-671. Crafts, A. S. 1939c. Hilgardia 12, 177-200. Crafts, A. S. 1944. Experiments on General Contact Herbicides. Calif. Coll. Agr. Div. Bot. Mimeo. Crafts, A. S. 1945a. Control of Aquatic Weeds. Calif. Agr. Coll. Div. Bot. Mimeo. Crafts, A. S. 1945b. Science 101, 417-418. Crafts, A. S. 1946a. Calif.Stale Dept. Agr. Mo. Bult. 35, 35-36. Crafts, A. S. 1946h. Plant Physiol. 21, 345-361.

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Crafts, A. S. 1947a. Calif.Agr. Ext. Serv. Circ. 136. 12 pp. Crafts, A. S. 1947b. Calif.Agr. Ext. Serv. Circ. 137. 16 pp. Crafts, A. S. 1948a. Science 107, 196-197. Crafts, A. S. 1948b. Agr. Chem. 3, 25-27, 81, 83, 85. Crafts, A. S. 1948c. Science 108, 85-86. Crafts, A. S. 1949. Hilgardia. 19, 141-158. Crafts, A. S., Bruce, H. D., and Raynor, R. N. 1941. Calif. Agr. Expt. Sta. Bull. 648. Crafts, A. S., Currier, H. B., and Stocking, C. R. 1949. Water in the Physiology of Plants. Chronica Botanica, Waltham, Mass. Crafts, A. S., and Emanuelli, A. 1948. Bot. Gaz. 110, 148-154. Crafts, A. S., and Raynor, R. N. 1940. Principles of Chemical Weed Control. Herbage Public Ser. Bull. 27. Imp. Bur. Pastures and Forage Crops, Aberystwyth, Great Britain. Crafts, A. S., and Raynor, R. N. 1944. West. Grower and Shipper 15 (9), 10-11, 26-28. Crafts, A. S., and Reiber, H. G. 1944. Toxicity of Oils to Carrots and Weeds. Calif. Agr. Coll. Div. Bot. Mimeo. Crafts, A. S., and Reiber, H. G. 1945. Hilgardia 16, 487-500. Crafts, A. S., and Reiber, H. G. 1948. Hilgardia 18, 77-156. Currie, G. A., and Garthside, S. 1932. Austral. Council Sci. and Ind. Res. Pamphlet 29. DeFrance, J. A. 1947. The Greenskeepers’ Reporter. Jan.-Feb. DeFrance, J. A. 1948. A m . Cyanugrams 1 (61, 8-10. DeRose, H. R. 1946. Bot. Gaz. 107,583-589. DeRose, H. R., and Newman, A. S. 1947. Soil Sci. SOC.A m . Proc. 12, 222-226. Derscheid, L. A., and Stahler, L. M. 1948. S. Dak. Agr. Expt. Sta. Agron. Dept. Circ. 69. Dunlap, A. A. 1948. Phyfopath. 38, 638-644. Ekdahl, I. 1947. Royal Agr. Coll. Sweden. Ann. 15, 113-172. Eliason, E. J. 1948. The Use of Oil Sprays for the Control of Weeds in Coniferous Nurseries. N.Y. State Conservation Dept. Mimeo. Ennis, W. B., Jr. 1947. Science 105, 95. Ennis, W. B., Jr. 1948. Bot. Gaz. 109, 473-493. Ennis, W. B., Jr., Swanson, C. P., Allard, R. W., and Boyd, F. T. 1946. Bot. Gaz. 107, 568-574. Evans, L. S. 1948. Western Weed Control Conf. Tenth Ann. Proc. pp. 10-14. Freed, V. H. 1946. Oreg. State Coll. and U.S.D.A. Coop. Ext. in Agr., Weeders Readers No. 13, p. 7-10. Freed, V. H. 1947. Western Weed Control Conf. Ninth Ann. Minutes, p. 30-32. Freed, V. H. 1948. Ore. State Coll. and U.S.D.A. Coop. Ext. in Agr., Weeders Readers No. 19, p. 10. Godel, G. L. 1935. Sci. Agr. 16, 165-168. Godel, G. L. 1938. Sci. Agr. 19, 21-32. Grigsby, B. H. 1946. Mich. Agr. Exp.S f a . Quart. Bdl. 28, 3. Grigsby, B. H. 1948. Agr. Chem. 3 (6), 33-34. Hamner, C. L., and Tukey, H. B. 1944. Science 100, 154-155. Hamner, C. L., and Tukey, H. B. 1946. Bot. Gaz. 107, 379-385. Hance, F. E. 1948a. Science 108, 278-279. Hance, F. E. 1948b. Hawaiian Planters Record 52, 93-112.

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Hanks, R. W. 1946. Bat. Gaz. 108, 186-191. Hannesson, H. A., Ragnor, R. N., and Crafts, A. S. 1945. (‘nlif. Agr. Rrpt. Stn. Bull. 693. Hanson, N. S. 1948. Hawaiian Planters Rerord 52, 113-154. Harvey, R. B. 1931. J . Am. SOC.Agron. 23, 481-489. Harvey, W. A. 1944. Wash. Agr. Expt. Sta. Bull. 448. Harvey, W. A., and Riddle, 0. C. 1946. Controlling Alfalfa Weeds with Chemiral Sprays. Calif. Agri. Coll. Div. Bot. Mimeo. 7 pp. Harvey, W. A,, and Robbins, W. W. 1947. Calif. Coll. Agr. Ext. Serv. Circ. 133 Herbage Publication Series. 1940. Bull. 27, The Control of Weeds. Imp. Bur. of Pastures and Forage Crops, Aberystwyth, Great Britain. Herbert, F. B. 1948. Report in Western Weed Control Conf. 10th Ann. Proc. p. 56-57.

Hester, J. B., and Isaacs, R. L. 1948. Science 108, 635. Higgins, F. H. 1947. Rice J. 50 (121, 8-10, 25-32. Hirst, H. W. 1947. Reclamation Ein 33 (41, 76-77. Holloway, J. K. 1948. Report in Western Weed Control Conf. 10th Ann. Proc. p. 19-20. Johnston, J. C., and Sullivan, W. 1949. Calif. Agr. Ext. S e w . Circ. 150. Kephart, L. W., and Evans, L. S. 1948. Killing weeds with 2,4-D. U.S.D.A. Bur. Plant Ind. Soil and Agr. Engr. Div. Cereal Crops and Diseases. Mimeo. Korsmo, E. 1930. Unkrauter im Ackerbau der Neuzeit. Julius Springer, Berlin. Kraus, E. J., and Mitchell, J. W. 1947. Bot. Gaz. 108, 301-350. Kries, 0. H. 1947. Bot. Ghz. 108, 510-525. Lachman, W. H. 1945. Mass. State Coll. Ext. Serv. Special Circ. 120. Lee, 0. C. 1948a. Down to Earth 4, 6-7. Lee, 0. C. 1948b. Indium Agr. Erpt. Sta. Circ. 335. Leonard, 0. A., Ark, H. F., and Harris, V. C. 1948. Miss. Farm Res. 11 ( l ) , 2. Leonard, 0. A,, Harris, V. C., and Arle, H. F. 1947. Miss. Farm Res. 10 (9), 1, 2. Long, H. C. 1934. Weed Suppression by Fertilizers and Chemicals. Orchard Road, Hook, Surbiton, Surrey, England. Long, H. C., and MacDowall, R. K. 1935. J . Roy. Agr. SOC.England 96, 22-44. Mangual, J. C. 1948. Science 107, 66. Marth, P. C., and Mitchell, J. W. 1946a. Rot. Gaz. 107, 417-424. Marth, P. C., and Mitchell, J. W. 1946b. Science 104, 77-79. Marth, P. C., and Mitchell, J. W. 1947. Bot. Gaz. 108, 414-420. Marth, P. C., Toole, V. K., and Toole, E. H. 1947. Am. SOC.Agron. J . 39, 426-429. Mitchell, J. W. 1948. Agr. Chem. 3 (31, 28-30, 81. Mitchell, J. W., and Brown, J. W. 1946. Bot. Gaz. 107, 393-407. Mitchell, J. W., and Hamner, C. L. 1944. Bot. Gaz. 105, 474-483. Mitchell, J. W., and Kephart, 1,. W. 1947. Effect of IPC on quack grass. U.S.D.A. Bur. Plant Indus., Soils and Agr. Engr. Mimeo. Mitchell, J. W., and Marth, P. C. 1947. Growth Regulators for Garden, Field, and Orchard. Univ. of Chicago Press, Chicago, Ill. Mitchell, J. W., Wood, J. W., Wolfe, W. C., and Irving, G. W. 1947. Science 106, 395-397.

Moran, W. T., and Shaw, J. M. 1948. Reclamation Era 34 (5), 81-83, 93. Moore, E. C. 1945a. Calif. Cult. 92, 310-311. Moore, E.C. 1945b. Citrus Leaves 25 (61, 30-31. Muenscher, W. C. 1935. Weeds. Macmillan, New York, N.Y.

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Neely, J. W., and Brain, S. G. 1944. Miss. Agr. Expt. S I ~ Ciic. . 118. Newman, A. S. 1947. Soil Sci. SOC.Am. Proc. 12, 217-221. Nolla, J. A. B. 1948. Science 108, 112-113. Norman, A. G. 1948. J. Am. SOC.Agron. 40, 111-119. North Central Weed Control Conf. 1947. Rept. of Res. Committee and Suppl. to Proc. of Ann. Meeting. North Eastern Weed Control Conf. First Ann. 1947 Minutes. 18 pp. Nutman, P. S., Thornton, H. G., and Quastel, J. H. 1945. Nature 155, 498-500. Offord, H. R., and d'Urbal, R. P. 1931. J. Agr. Research 43, 791-810. Parker, E. R., and Jenny, H. 1945. Soil Sci. 60, 353-376. Pavlychenko, T. K. 1940. Imp. Bur. Pastures and Forage Crops, Herbage Pub. Scr. Bull. 27, pp. 9-26. Pavlychenko, T. K. 1947. Agricultural Uses of Selective Herbicides. Mimeo. Res. Lab. of Plant Ecology, Univ. of Sask. Pavlychenko, T. K., and Harrington, J. B. 1934. Can. J . Research 10, 77-94. Pavlychenko, T. K., and Harrington, J. B. 1935. Sci. Agr. 16, 151-160. Price, F. E., Lunde, N. R., Bursik, J., and Freed, V. H. 1946. Recommendations for Weed Spraying Equipment. Oreg. State Coll. and Agr. Expt. Sta. Agr. Eng. Dept. Mimeo. Pryor, M. 1948. Calif. State D e p t . Agr. Weed Circ. 36. Puffer, R. E. 1947. Calif. Citrograph 32, 274-275. Puffer, R. E., and Yarick, B. E. 1948. Calif. Agr. 2 (8), 7. Rademacher, B. 1940. Imp. Bur. Pastures and Forage Crops, Herbage Pub. Ser. Bull. 27, pp. 9-26. Raynor, R. N. 1943. Chemical Weeding of Carrots with Stove Oil Sprays. Calif. Agr. Coll. Div. Bot. Mimeo. Raynor, R. N. 1947. D o w n to Earth 3 (3), 8-9. Rice, E. L. 1948. Bot. Gaz. 109, 301-314. Robbins, W. W., Crafts, A. S.,and Raynor, R. N. 1942. Wccd Control. McGrawHill, New York, N.Y. Savage, D. A., and Costello, D. F. 1948. U.S. Dept. Agr. I'enrbook, pp. 531-534. Savage, D. A., Harlan, J. R., Brown, A. L., and McIlvain, E. H. 1948. Mechanical and Chemical Control of Range Brush and Weeds. U.S. Dept. Agr. Bur. Plant Indus., Forage Div., US. Southern Great Plains Field Sta. Minieo. Shaw, H. R., Conrad, P. F., Amundsen, R. F., and Tutton, S. M. 1947. Hawaiian Planter's Record 51, 155-175. SiIversides, W. H., and Thornton, B. J. 1942. Selected Bibliography on Weeds. W. H. Silversides, McFayden Seed Co. Ltd., Winnipeg, Man. Slade, R. E., Templeman, W. G., and Sexton, W. A. 1945. Nature 155, 497-498. Smith, H. S. 1947. Ent. Soc. Wash. Proc. 6, 169-170. Smith, O., Baeza, M. A., and Ellison, J. H. 1947. Rot. Gaz. 108, 421-431. Stahler, L. M. 1948. 2,4-D pre- and post-emergence spraying. Talk before Hybrid Corn Div. A.S.T.A. Chicago. Staten, G. 1946. J. Am. SOC.Agron. 38, 536-544. Sullivan, W . , and LaRue, R. G. 1947. Clean Culture Nontillage of Citrus. Calif. Agr. Ext. Serv. Mimeo. 13 pp. Sweet, R. D. 1945. Oils for Weeding Carrots. N.Y. State Coll. Agr., Dept. of Vpg. Crops. Mimeo. Sweet, R. D., Kunkel, R., and Raleigh, G. J. 1944. Am. SOC.Hort. Sci. Proc. 45, 440-444.

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Sweet, R. D., Raleigh, G. J., and KunkeI, R. 1945. N.Y. State COIL Agr. Ext. Serv. Mimeo. Vul. V-33. Sylwester, E. P., and Bakke, A. L. 1947. Iowa Farm Sci. 2 (6), 7-9. Synerholm, M. E., and Zimmerman, P. W. 1945. Boyce Thompson Znst. Contrib. 14, 91-103.

Synerholm, M. E., and Zimmerman, P. W. 1947. Boyce Thompson Inst. Contrib. 14, 369-382.

Taylor, D. L. 1947. Bot. Uaz. 108, 432-445. Templeman, W. G., and Sexton, W. A. 1945. Nature 156, 630. Templeman, W. G., and Sexton, W. A. 1946. Proc. Roy. SOC.London B.133, 480484.

Timmons, F. L. 1911. Kana. Agr. Expt. Sta. Bull. 296. Timmons, F. L. 1948. Controlling Weeds with 2,4-D in the Southern Great Plains. U.S.D.A. Bur. Plant Indus. Soils and Agr. Eng. Div. Cereal Crops and Diseases. Mimeo. Tucker, W. B. 1948. Oreg. State Coll. and U.S.Dept. Agr. Coop. Ext. in Agr., Weeders Readers No. 19, p. 8. Tullis, E. C. 1948. Rice J . 51 (41, 9-10, 28-31. U.S. Dept. Agr. 1938. Progress Rept. of Coop. Weed Investig. U.S.D.A. Bur. Plant Indus. Div. Cereal Crops and Diseases. Mimeo. Van Overbeek, J., and Velez, I. 1946a. Znst. Agr. Trop. Univ. Puerto Rico, Bull. 1 . Van Overbeek, J., and Velez, I. 1946b. Science 103, 472-473. Van Overbeek, J., Gregory, L. E., and Velea, I. 1946. Am. SOC.Hort. Sci. Proc. 47, 434-438.

Van Overbeek, J. 1947. Econ. Bot. 1 (a), 446-459. Warren, G. F. 1946. Am. SOC.Hort. Sci. Proc. 47, 415-420. Warren, G . F., and Hanning, F. 1946. Am. SOC.Hort. Sci. Proc. 47, 407-414. Weaver, R. J., and DeRose, H. R. 1916. Bot. Gaz. 107, 509-521. Wellman, R. H. 1948. Chem. Indust. Separate, compliments of Carbide and Carbon Chemicals Ltd. pp. 15-16. August. West, E. S. 1946. Physical Chemistry for Students of Biochemistry and Medicine. Macmillan Co., New York, N.Y., p. 44. Western Weed Control Conf. Eighth Ann. 1946. Minutes, 68 pp. Western Weed Control Conf. Ninth Ann. 1947. Minutes, 59 pp. Western Weed Control Conf. Tenth Ann. 1948. Proc. 62 pp. Westgate, W. A., and Raynor, R. N. 1940. Calif. Agr. Expt. Sta. Bull. 634. White, D. D., and Mangual, J. C. 1948. Sugar 43 (4), 31-35. White, D. D., and Villafane, A. G. 1946. Agr. in the Americas 6, 126-128. Willard, J. C. 1948. Ohio State Univ. Agr. Ext. Serv. Bull. 293. Wilson, F. 1943. Austral. Council Sci. and Znd. Res. Bull. 169. Wolf, D. E. 1948. Am. Cyanagrams 1 (5), 4-6. Yarick, B. E. 1947. Calif.Citrograph 32, 452-453. Zimmerman, P. W., and Hitchoock, A. E. 1942. Boyce Thompson Znat. Contrib. 12, 321343.

Zobel, C. E. 1940, Bact. Rev. 10, 1-49.

Boron in Soils and Crops* K . C . BERGER CONTENTS

Page

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . 321 I1. Boron Determination . . . . . . . . . . . . . . . . . . . . . 323 1. Titrimetric Procedures . . . . . . . . . . . . . . . . . . 323 2. Spectroscopic Methods . . . . . . . . . . . . . . . . . . 323 3. Colorimetric Methods . . . . . . . . . . . . . . . . . 324 a . Turmeric Test . . . . . . . . . . . . . . . . . . . 324 b . Hydroxyanthraquinone Tests . . . . . . . . . . . . . . 325 c . Quinalizarin Test . . . . . . . . . . . . . . . . . . 325 4. Biological Methods . . . . . . . . . . . . . . . . . . . 326 I11. Boron Availability in Soils . . . . . . . . . . . . . . . . . . 327 1 . Conditions Favoring Availability . . . . . . . . . . . . . . 327 a . Organic Matter . . . . . . . . . . . . . . . . . . 328 b . Lack of Leaching . . . . . . . . . . . . . . . . . . 329 c. Texture . . . . . . . . . . . . . . . . . . . . . . 329 2. Conditions Favoring Fixation or Loss . . . . . . . . . . . . 329 a . Alkalinity and Change in pH . . . . . . . . . . . . . 330 b . Calcium-Boron Ratios . . . . . . . . . . . . . . . . 332 c. Leaching . . . . . . . . . . . . . . . . . . . . . . 332 d . Drying . . . . . . . . . . . . . . . . . . . . . 333 e . Crop Removal . . . . . . . . . . . . . . . . . . . 334 3. Boron Cycle . . . . . . . . . . . . . . . . . . . . . . 334 IV. Boron Requirement of Plants . . . . . . . . . . . . . . . . . 336 1. Function of Boron in Plants . . . . . . . . . . . . . . . . 337 2. Interrelations with Other Elements . . . . . . . . . . . . . 338 a . Calcium-Boron Ratios . . . . . . . . . . . . . . . . 339 b . Potassium-Boron Ratios . . . . . . . . . . . . . . . . 340 c. Nitrogen-Boron Relationship . . . . . . . . . . . . . . 341 d . Other Elements . . . . . . . . . . . . . . . . . . . 341 3. Symptoms of Boron Deficiency . . . . . . . . . . . . . . . 342 4. Boron Requirements of Plants . . . . . . . . . . . . . . . 344 V . Summary . . . . . . . . . . . . . . . . . . . . . . . . . 317 References . . . . . . . . . . . . . . . . . . . . . . . . 348

I . INTRODUCTION Although borax. under the name of Tincal or Tincar. was exported to Europe from central Asia and was used before the middle of the sixteenth

* Contribution from the Department of Soils. University of Wisconsin. Madison. Wis . Published with the permission of the director of the Wisconsin Agricultural Experiment Station . 321

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century (Agricola, 1556), boron was not discovered in plants until 1857 when Wittstein and Apoiger obtained boric acid from the ash of the seeds of Maesa picta, an Abyssinian plant belonging to thc order Myrsinaceae. I n 1910 Agulhon published results showing increases in dry weight of wheat, oats, and radish with the addition of boron to sand cultures. MazB, however, in 1915 was the first to report t.hat boron was essential for the growth of corn. Warington ( 1 923) first showed conclusively that boron is not only a stimulant but that in its absence certain members of t.he order Leguminosae develop characteristic deficiency symptoms. According to Stiles (1946)) Nakamura in 1903 had reported increased growth of peas and spinach as a result of adding boron to soils. During the first World War much attention was given to boron injury in plants due largely to damage caused to crops by the use of American potash fertilizer contsaining considerable boron. From 1925 to 1930 further proof of the essential nature of boron for plant life was obtained. Because Warington failed to get satisfactor!r evidence of the necessity of boron for grains, and because the results of field tests were inconclusive, it was felt by many investigators that thc boron requirement of legumes was further evidence of their specializeci mode of nutrition. I n 1926, however, Sommer and Lipman, working with water cultures, demonstrated increased growth of sunflower, cotton, barley, buckwheat, castor bean, flax and mustard due to a trace of boron in t,he cultures. Johnston and Dore (1928) demonstrated the need of potato and tomato for boron and McMurtrey (1929) provided similar evidence for tobacco. Brandenburg (1931) , working with sugar beets and mangolds, WILS the first to demonstrate clearly in the field that heart and dry rot oi these two crops were due to boron deficiency and could be prevented by the application of boron. This work greatly stimulated interest in boron and it was quickly proved essential for a number of crops. I n a surprisingly short time borax became one of the accepted commercial fertilizer materials. A great number of investigations were carried on shortly after this, and during the period 1938 to 1942 over 800 papers on boron were published. Several methods for determination of boron in soils and plants were refined and developed during this time, and, because these methods were more accurate and rapid than those previously used, research on the fate of boron in soils was greatly stimulated. Most of the publirations of this period, however, deal with the results of field experiments with various crops. Since 1942 the areas of boron deficiency have been defined, particu-

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larly in the United States, and emphasis has been placed on crops with high boron requirements, such as alfalfa and beets, that frequently show deficiency in the field. The interest in boron fixation, availability and movement in soils has increased greatly.

11. BORONDETERMINATION The early methods of analysis for boron were designed chiefly for large amounts and involved separation and then estimation by gravimetric or volumetric procedures. Typical of these procedures are those which involve distillation of the methyl ester of boric acid followed by titration or weighing. It was soon found that the amounts in soils and plants were minute and that it was necessary to develop methods that were far more sensitive. A number of these methods were worked out by various investigators as follows. 1. Titrimetric Procedures

An accurate titration procedure involves first, separation of the boric acid from the material under test, and then its concentration in relatively pure form. This separation is accomplished by the distillation of boron as the methyl ester of boric acid or by the precipitation of interfering substances in the test solution. After relatively pure boric acid has been obtained, it is activated by the addition of mannitol or one of several other polyhydric alcohols, and then titrated with a base either electrometrically or in the presence of an indicator. The boric acid complex titrates as a monobasic acid. Procedures have been described in detail by Wilcox (1932, 1940), Scharrer and Gottschall (1935), and Cook and Millar (1938). Titration procedures are accurate when relatively large arn0unt.s of boron are to be determined, but because of the small amounts of boron found in soils and plant materials, it is necessary to use very weak solutions of alkali to titrate the boric acid solutions. These weak solutions are easily affected by carbon dioxide and traces of buffering material which are difficult to remove from the test solution. Also, the necessity for distillation of boric acid from the test material makes the method long and laborious. For these two reasons, this method is not generally used to determine boron in soils and plants. 2. Spectroscopic Methods

The spectroscopic method for the determination of small amounts of boron has been used by Goldschmidt and Peters (1932), Foster and Horton (1937), McHargue and Calfee (1932, 1937) and Melvin and O’Connor (1941) and others. Foster and Horton determined boron in fresh

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plant material by the spark method, using the gold line as an internal standard. With their method, however, there was considerable variation in the replicate determinations, and the limit of sensitivity of the method was not clearly defined. McHargue and Calfee (1932) determined the boron in a flame spectra optically, but it was necessary first to convert the boron to the methyl ester of boric acid by distillation. Later Calfee and McHargue (1937) determined boron by excitation in an oxygenmethane flame, polarized the light admitted and compared it with the standard boron solution. Their later method was an improvement on the former but even so there was considerable variation in the results. Melvin and O’Connor (1941) used the arc method for the simultaneous determination of boron, copper and manganese in fertilizers, but the method lacks the sensitivity needed for accurate determinations of boron in soils and plant materials. So far, it has not been found possible to estimate boron polarographically. I n general, these methods are laborious and time-consuming, require rather elaborate equipment, and are not too well suited to the accurate determination of t.he small amounts of boron normally found in soils and plants. 3. Colorimetric Methods Usually, when small amounts of element are to be determined, a greater degree of sensitivity and accuracy can be attained by the use of a colorimetric method than by gravimetric or volumetric methods. In the determination of small amounts of boron a number of different colorimetric methods have been proposed.

a. Turmeric Test. Faraday (1821) demonstrated that turmeric paper dipped in a solution of boric acid would change from yellow to red and that dilute solutions of boric acid would produce this change. This method was used and improved by a number of investigators since its proposal by Faraday. Small strips of paper impregnated with the turmeric solution are dipped in the test solution (Bertrand and Agulhon, 1910), or the color is developed in solution (Cassal and Gerrans, 1903). Naftel (1939) refined the solution method so that quantities of boron down to 0.5 gamma could be determined. Later, Haas (1944) further refined the method using a Fisher electrophotometer to determine the color change. The turmeric procedure is long, and precise control of temperature of water baths and preparation of materials is necessary. This makes the method quite long and cumbersome. McHargue and Hodgkiss (1942) found that t.his method was not as satisfactory as was the quinalizarin method later to be described.

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b. Hydroxyanthruquinone Tests, It has been known that the addition of boric acid to a number of hydroxanthraquinones in concentrated sulfuric acid could be used as a test, for boron. For this purpose, Komarovski and Poluektoff (1934) used p-nitrobenzene azochromotiopic acid, Zorkin (1936) used carmine red, Scharrer and Gottschall (1935) used purpurin and quinalizarin, Feigl and Krumholz (1929) tried purpurin, alizarin S and quinalizarin and found quinalizarin to be the most sensitive. Quinalizarin is the test substance now most widely used for the determinat,ion of boron in soils and plants. c. Quinatizarin Test. Smith (1935) found t h a t it was necessary t o have a t least 44 per cent by weight sulfuric acid present in the solution in order to produce a visible color change in quinalizarin due to boron. H e found that the sensitivity or color change increased as the concentration of sulfuric acid increases until about 93 per cent of acid by weight is present. Beyond this the sensitivity decreases. Smith indicates that the method is sensitive to 0.001 mg. of boric acid. He applied the method to the determination of boron in alloys. Berger and Truog (1939) further refined the quinalizarin test and applied it to the determination of available and total boron in soils and to the boron in plant tissue. The available boron in soils is determined by refluxing 20 g. of soil with 40 ml. of distilled water for 5 minutes. An aliquot of the filtered extract is then made alkaline and evaporated t o dryness. The residue is ignited t o destroy organic matter and nitrates, and then taken up with dilute acid. A 1 ml. aliquot of this solution is placed in the tube and 9 ml. of 98.5 per cent sulfuric acid are added. After cooling, 0.5 ml. of quinalizarin reagent is added and the color which develops is compared with a standard. Berger and Truog (1944) found later that it was possible to simplify the procedures by adding the quinalizarin directly to the sulfuric acid. The quinalizarin-sulfuric solution is made up of 98 per cent by weight sulfuric acid containing 5 mg. of quinalizarin per liter. Olson and Berger (1946) found that the sensitivity of the method was greatly increased by increasing the amount of quinalizarin from 5 mg. to 25 mg. per liter. I n the determination of total boron in soils and silicates, a sodium carbonate fusion is made and the resulting melt is dissolved at pH 5.5 to 6.0 so as to leave insoluble most of the silica and sesquioxides. The bulk of the sulfate is thrown out of solution by the addition of alcohol. The filtered solution is made alkaline, evaporated to dryness, and the residue ignited. After dissolving in dilute acid, the quinalizarin test is applied. I n the determination of the total boron of plants, the plant tissue is

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ignited to a gray ash which is taken up with dilute acid. The quinaliearin test is then applied to some of the clarified extract. The color change in the quinaliearin test is from a reddish color in the blank solution to a bluer color, with increasing amounts of boron. The addition of 98 per cent sulfuric acid-quinalizarin solution to the water solution of the unknown generates a considerable amount of heat, and it was found that the higher the temperature, the redder is the color of the test solution. Maunsell (1940) found, however, t.hat the final color of the test solution is the same after cooling, regardless of the temperature reached when the quinalizarin is added. Therefore, it was found possible to read unknown solutions in a colorimeter as soon as they are cooled down to a uniform temperature. This greatly speeds up the procedure so that a determination of boron for plants can now be made in from 15 to 20 minutes. The quinalizarin colorimetric determination for boron has t,he advantages of being rapid, accurate, and sensitive to low amounts of boron. It has the disadvantage in that it is necessary to use strong sulfuric acid which requires care in handling. At the present time, t,he quinaliearin procedure is widely used in laboratories all over the world. Although dilute acid treatments of soils extract much more boron than do water extractions, as shown by Woodbridge (1940) , Berger and Truog (1940) , and Whetstone et al. (1942), there is much better correlation between water-soluble boron and plant response than there is between acid-soluble boron and plant response. Good correlations between hot-water-soluble boron and amounts of boron found in the leaves of table beets were obtained by Berger and Truog (1940).

4. Biological Methods Colwell and Baker (1939) and Schuster and Stephenson (1940) proposed a biological method for the determination of boron in soils using the sunflower as an indicator plant. The method was later refined by Colwell (1943, 1946). I n this method sunflower plants are grown in one pound of soil, and the criterion of the boron status of the soil is the agc of the culture when the initial stages of boron deficiency symptoms become apparent. These are compared with a set of quartz sand cultures to which different increments of boron are added. This would seem to be an excellent method in principle for determining available boron in soils but the method has two serious drawbacks. One is that it takes a considerable length of time to make a single determination. Secondly, when a soil is placed in it pot in the greenhouse, the soil temperat,ure is elevated above what it would be under field conditions

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and its moisture content may also be increased or maintained at a high constant level. This causes a greater biological activity with subsequent greater release of boron in some cases.

111. BORONAVAILABILITY IN SOILS Although it has been found that. boron availability is decreased in many cases by excess of lime, the greatest areas of boron deficiency are found in the humid regions where soils are quite generally acid. Beeson (1945) has shown the occurrence of boron deficiency to be largely in these humid regions. I n regions of low rainfall the boron content of the soil is high (Eaton, 1935; Whetstone et al., 1942; Haas, 1944). Boron in these soils probably exists largely as sodium-calcium borates, both of which are highly soluble. I n these regions the boron content of the water is sometimes great enough so that the water cannot be used for irrigation purposes. Schofield and Wilcox (1931) found sufficient boron in irrigation waters of southern California to cause injury to citrus and walnuts. Eaton (1935) and Eaton et al. (1941) made detailed studies on the effect of excess boron in irrigation water on many crops. Magistad and Christiansen (1944) found 0.14 p.p.m. of boron in the water of the lower Colorado River and Yuma, Arizona, and 0.71 p.p.m. of water in a well of the Coachella Valley, California. They divide the irrigation waters of the southwestern United States into three classes on the basis of their sodium and boron content, and consider water having over 2 p.p.m. of boron to be unsuitable for irrigation. Therefore, most of the problems of boron availability and fixation are problems of humid regions where the amounts of available boron in soils are of the order of 0.1 to about 2.5 p.p.m. 1. Conditions Favoring Availability Although boron is a component of about 56 minerals, in the humid regions the only widespread boron mineral is tourmaline. Tourmaline, a borosilicate containing varying amounts of iron, aluminum, magnesium, manganese, calcium, lithium, sodium, and potassium, has the general formula MZ0B2Si4Oz1, in which M consists of the various bases listed. This mineral is very hard, highly refractive, and resistant to weathering and is one of the heavy minerals. Dennis and O’Brien (1937) associated the prevalence of brown heart of turnips with the absence of tourmaline in certain Scottish soils examined by Elder and McCall (1936). They doubted the availability of boron in tourmaline but believed other more available boron compounds were associated with it. Eaton and Wilcox (1939) ground mine tailings containing tourmaline to pass a 60-mesh screen and found the boron

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present in them to be partially readily available. They did not believe that this test proved conclusively the availability of boron in tourmaline, however, because of the presence of other materials. Berger and Truog (1940) ground pure tourmaline crystals to pass a 100-mesh screen and found that even with the unwashed material, sunflowers could not obtain boron from it at a fast enough rate to continue growth. From these results it seems plausible that in the humid regions the available boron in the soil may be in a form other than tourmaline, but that tourmaline, as i t gradually decomposes, replenishes the supply of available boron. If leaching is very rapid in acid soils or if large amounts of lime are added so as to cause fixation, or crop removal is great, the replenishment from tourmaline will not be fast enough to prevent boron deficiency in crops with a high requirement. If, on the other hand, conditions favoring availability of boron are good, the amount of boron supplied by tourmaline will probably be adequate for normal growth of crops.

a. Organic Matter. I n acid soils, availability of boron is apparently correlated with the organic matter in the soil, the higher qmounts of available boron being found in soils of higher organic matter content4 (Berger and Truog, 1945). They also found that in alkaline soils, soil reaction and available calcium seem to have more effect on availability than does organic matter. They analyzed about 180 different samples from horizons down to the 24 inch layer. Correlation coefficients bet,ween organic matter and available boron in both virgin and cultivated surface samples were highly significant. Partial correlation coefficients between organic matter and available boron if pH was held constant were highly significant in the surface layer of cultivated soils below p H 7.0, while in those soils above pH 7.0, the partial correlation coefficient was highly significant for pH and available boron if organic matter was held constant. The data showed also t,hat available boron decreased with increasing acidity, probably due to the fact t.hat organic matter also decreased with increasing acidity. The effect of organic matter on these soils is to keep the boron in a more available form. The final effect of organic matter on the availability of boron is, however, not as great as tshat of pH. This is particularly true where the pH is above 7.0. I n these cases a highly significant negative correlation between pH and available boron was obtained. I n Wisconsin, high organic matter soils having a pH below 7.3 were found usually to contain adequate supplies of available boron. Ferguson and Wright (1940), in their work correlating the available boron content

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of various horizons of 7 soil types with the amount of cork observed in apples, show that below the 6-inch layer the amount of available boron declines rapidly. I n these soils the organic matter content is, of course, lower also in the lower horizons than in the surface layers. In Mississippi on acid soils, Coleman (1945) found also the available boron content of subsoils to be much lower than that of surface soils. Woodbridge (1940) found 0.57 p.p.m. of boron in the surface 6 inch of a sandy soil and only 0.09 p.p.m. in the 24 to 30-inch layer. Piland et al. (1943) found less boron in the subsoil than in the top soil in a great number of North Carolina soils. All these data show t.hat in humid regions the available boron is held largely in the organic fraction. It indicates that soluble boron salts are leached out easily, thus causing these soils to be generally low in available boron.

b. Lack of Leaching. In dry land soils, leaching is not a factor and subsoils are often higher in available boron bhan are surface soils. I n this case, available boron apparently exists in the form of sodium and calcium salts. Haas (1944a) has analyzed soils in the Coachella Valley of California the subsoils of which a t a depth of 2 to 3 feet contain as much as 4.5 p.p.m. of available boron. Purvis and Hanna (1938), in a field experiment, found that applications of boron did not injure a second crop of snapbeans planted three months after a first crop had been planted and injured-by an application of 50 lbs. of borax per acre. A similar experiment in pots resulted in injury to both crops, showing that the boron was leached in the field and not fixed. c. Texture. Although there has not been much work done on correlations between available boron in soils and texture, usually in the humid regions the lighter textured soils contain less available boron in the plow layer than do slightly acid, silt and clay loams. Lehr (1940) has shown that clays, particularly those of marine origin, are high in boron and in sands are low. He estimates the amounts of total boron in the various types of Dutch soils as follows: marine clays about 100 p.p.m. boron; river clays about 20 p.p.m. boron; sandy soils average about 6 to 25 p.p.m. boron in tourmaline, and 1 to 2 p.p.m. boron in organic matter. Kubota et al. (1948) also show that sandy soils contain less available boron t.han do soils with heavier textures. The effect of texture, however, has much less influence on the amount of available boron than does organic matter or pH. 2. Conditions Favoring Fixation or Loss

It has been known for a long time that boron deficiencies occur frequently on alkaline soils in humid regions. The natural assumption

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was to attribute this to over-liming. It was suggested by Naftel (1938) that the boron deficiency as a result of over-liming may be due to absorption of the boron by the increased population of soil microorganisms, which might accompany the change in reaction caused by liming. It is quite generally accepted now that this is not correct because of the speed with which reaction takes place. A number of conditions favor fixation in, and loss of boron, from soils.

a. Alkalinity and Change in p H . One of the first indications that boron is fixed in soils in a form unavailable to plants was obtained by Bobko and Syvorotkin (1935) when they noticed that small additions of boron to soils counteracted the harmful effect of overliming. Naftel (1937) obtained similar results. Wolf (1940) added sodium, potassium and magnesium hydroxides to a soil of p H 5.9. He found t,hat magnesium causes the greatest reduction in availability of soil boron, with calcium, sodium, and potassium hydroxides having lesser effects in the order named. He based his conclusions on the amount of boron found in the roots and tops of radishes grown on the soil. The work is subject to crit,icism because of the upset physiological condition of the plants due to the wide variation in calcium-magnesium ratios, caused by the addition of large amounts of magnesium hydroxide. Tulin (1940) found that reduction in plant yield due to the addition of calcium carbonates was reduced by the addition of magnesium carbonates, probably due to bringing about a more favorable calcium-magnesium ratio. Olson and Berger (1946), investigating the effect of alkalinity on boron fixation, found that the effect of boron fixation in soils is closely related to the clay cont,ent and soil reaction as changed by the addition of sodium hydroxide, calcium hydroxide, or hydrochloric acid. The cations of the bases had little influence on the boron fixation but the alkalinity produced by them resulted in fixation. With a prairie soil, calcium hydroxide was more effective in inducing boron fixation than was sodium hydroxide, while with a wooded soil, Spencer silt loam, the bases were equally effective. Even a t pH values of 9.5 or higher, however, not more than 40 per cent of the added or native available boron in the soil was fixed. Olson and Berger also found that upon oxidation of soil organic matter, there was a slight decrease in boron fixation, and that of the inorganic fractions of the soil, the clay separate fixed the greatest amount of boron. Midgley and Dunklee (1939, 1940) and Dunklee and Midgley (1943) found that very large amounts of boron were fixed by soils when the pH was changed greatly. Acid leaching, and then liming to neutrality, caused boron fixation up to 90 per cent of the boron added. This was up

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to four times as niiich fixation as on the unlimed soils. They state that both the organic and inorganic fract
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plants but can be released by the action of strong mineral acids and also probably is released by the decomposition of organic matter. b . Calcium-Boron Ratios. Since Brenchley and Warington (1927) first indicated that there was an association between boron and calcium absorption by plants, numerous workers have studied boron-calcium ratios in plants and the interelationships of various other element8 with boron. Marsh and Shive (1941), working with corn, found that the metabolically effective calcium which is maintained in the soluble state in the active plant tissues is directly correlated with the supply of available boron in these tissues. Later, Reeve and Shive (1944) found that as the calcium content of a nutrient solution was increased, more boron was required to prevent boron deficiency in the plant, and that more boron could be added to the solution without the development of symptoms of boron toxicity. Jones and Scarseth (1944), working with a number of Indiana farm crops, in greenhouse experiments on limed and unlimed soils, found t,hat plants would take up varying quantities of calcium and boron depending upon the availability of these elements in the soil. Analysis of the plant showed that normal growth would occur only when a certain balance in the intake of calcium and boron existed. They found low tolerance for boron with low calcium in the plant, and a high requirement for boron with high calcium. Drake et al. (1941) have found that growth of Turkish tobacco on a Norfolk sand appeared normal when the calcium-boron ratio in t,he plant did not exceed 1340:l. A calcium-boron ratio of 1500:l in the plants was correlated with severe boron deficiency. From these data it is obvious that not only is boron fixed in alkaline soils in a form temporarily unavailable to plants but that more boron is required by plants growing on these high calcium soils. c. Leaching. Another factor that contributes to the loss of boron in some soils, and consequently contribut,ing to deficiency, is that of leaching. One of the first field investigations on boron movement was conducted in New Zealand by Askew and Thomson (1937) and Askew et al. (1938), who found that boron applied to an apple orchard 2 years previously had moved to a depth of 30 inches in this period. A similar study by Woodbridge (1940) indicated that greatest movement was found in the lighter soils 2 years after application and the least movement in the heavier soils. Kriigel et al. (1937, 1938) applied water in successive liter portions to soils until the leachates were relat,ively free of boron. They found that approximately 78 per cent of the applied boron was recovered in the leachate. The highest recovery, 94 per cent, was obtained from clay

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loam and the lowest, 63 per cent, from a heavy loam. No difference in the recovery was found following application of boron either as boric acid or borax. White-Stevens (1941) has found that boron is readily leached out of acid Long Island soils by heavy rainfall, and recommended heavier borax applications in wet years than in dry ones. Reeve e t al. (1944) applied water equivalent to one-fourth of the average annual rainfall of New Jersey to soils to which borax had been applied a t the rate of 20 lbs. per acre. They found that about 85 per cent of the boron leached from a sandy soil and 75 per cent of the applied boron leached from loam and silt loam soils. I n a comprehensive study of Wisconsin soils, Kubota et al. (1948), working in the field and laboratory, found that the rate of boron movement was related primarily to the soil texture. Where the soil was uniformly light-textured throughout the profile, much of the applied boron moved to a depth of 24 inches or deeper in 6 months. I n the heavier soils, little of the boron moved below the Winch layer. In laboratory experiments it was found that the bulk of applied boron moved downward in mass rather than in portions. The movement of boron was found to lag behind the movement of the water, and was also found to be independent of the movement of sodium in the borax. Following application of borax and leaching with 2 inches of water, most of the sodium was found to be in exchangeable form in the surface 3 inches of a sand and silt loam soil, while the boron was found at greater depths. Liming reduced the rate of boron movement. Calcium applied as calcium chloride decreased the rate, but less than did lime. From these studies it can be seen that leaching, particularly in acid soils, is a very imporbant factor in the loss of boron from soils. d. Drying. Drying has been shown by Parks (1944) and others t o cause increased fixation of boron in the laboratory. Olson (1947) found that the drying of soil after boron was added increased the amount of boron fixed. The increase was greater in the case of limed soils than with unlimed soils Drying at 60°C. increased fixation more than drying a t 20°C. These findings confirm those of Parks, who found that drying a t 85°C. almost doubled the amount of boron fixed when soil was dried a t 26°C. Walker et al. (1944) observed that boron deficiencies were more severe in places in the fields where soil dried out excessively in dry years. Latimer (1941) showed that drought in June and July was the most important factor in the cause of internal cork in New Hampshire apple orchards. It is doubtful whether the plow layer in the field will dry out enough in most years so as to cause appreciable boron fixation. I n the first place,

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if boron was fixed to any extent, most soils would be very deficient in boron particularly those in the arid regions. This boron would not be released in available form in wet years. This is obviously not the case. Rather, it is suggested that the reason for more boron deficiency in dry years than in wet years is that, as has been previously shown in Section II-1-a, most of the available boron is found in the surface organic layer. When this layer becomes relatively dry, plants feed in it but little. Thus it is necessary that they feed off the lower horizons of the soil which are usually low in available boron and organic matter. This causes boron deficiency in crops during dry years because their supply of available boron has been reduced, not so much by fixation, but by lack of ability of the plant roots to feed in the surface horizon because of a lack of water. It is quite possible that there is some boron fixation due to drying in surface soils in extremely hot and dry weather, but is very doubtful if much boron is fixed below the 2-inch depth.

e. Crop Removal. Another way in which boron is lost from soils is by crop removal. Because boron is found in such small quantities in plank, this is not often considered to be a serious loss. Reeve et a!. (1944), however, have shown that, in New Jersey, alfalfa hay contained boron equivalent to nearly 2 lbs. of borax per ton when grown on soils that were adequately supplied with the element. I n most humid region soils, however, the boron removal from soils by alfalfa is much less than this. Crop removal is a factor that is important and this is evidenced by the increasing number of areas in the world in which boron deficiency has been observed. 3. Boron Cycle A cycle of boron in nature has been diagrammed by Dennis (1937). This cycle implies the permanent removal of boron into coal formations, iron and manganese ores, and borosilicate minerals. This cycle is not detailed as far as soils and plants go, because of lack of information when it was proposed. A more detailed cycle of boron in humid region soils is given in Fig. 1. This cycle is a summary of Section 11, of this article. As can be seen, boron is removed from the soil by leaching, and by removal in plants. Boron is added to the soil by fertilization with boron fertilizers and by decomposition of tourmaline and small amounts of other primary soil boron minerals. The available soil boron is in two forms: organic and inorganic. These are in equilibrium with each other and with unavttilable organic and inorganic soil boron. Available boron enters the plants during growth, and again appears in an available form

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boron

Microbial growth followed by death

Decay t o form calcium borate

Boron fertilizers

Temporarily

free Ca

boron

acidity

!

Leaching

Tourmaline and small amounts of other primary soil boron minerals

I n acid soils, much of the boron remains in the inorganic form and is leached out. This cau8es acid sandy soils to be low in available boron. In general in the humid region, soils low in available boron are lightcolored acid soils and alkaline soils regardless of the organic matter content. I n arid and semi-arid regions, because of the absence of leaching, quantities of inorganic soil boron occur probably as sodium and calcium salts.

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IV. BORON REQUIREMENT OF PLANTS As was stated in the introduction, there has been a tremendous amount of work on boron in soils and plants, and since the first field proof of boron deficiency was given shortly after 1930, nearly 2000 papers on boron have been published. The major part of these have dealt with the boron requirement of plants. Shive (1945) gave an excellent historical survey of boron in plant life in which he pointed out that, although Wittstein and Apoiger discovered boron in plant tissue in 1857, the significance of the discovery was not realized until years later when four different men between 1888 and 1890 detected boron in wines and certain fruits and in leaves and tissues of a great variety of plants. Finally Jay (1895), in a rather comprehensive investigation, concluded that boric acid is generally dist%ributedthrough the earth’s crust, that both cultured and wild plants take up boric acid from the soil and from water, and that when boric acid is introduced in the stomachs of animals it is not assimilated but is nearly all excreted. Jay further stated that boron occurs universally in autotrophic plants, and is known to be distributed in at least some of the heterophytes such as mushrooms. It was also discovered that plants differ widely in their ability to absorb boron from soils and water. Even a t this early date, it was recognized that the agricultural monocotyledonous plants, wheat, rye, oats, barley, corn and others, have a much lower capacity for absorbing boron than have many of the dicotyledenous plants. Stiles (1946) reported that Nakamura in 1903 obtained increased growth of peas and spinach as a result of adding boron to soil. I n 1910, Agulhon obtained increased yield of wheat, oats, and turnips by the use of boron in nutrient solutions. He suggested that the boron is a useful element for higher plants and included it with manganese as an element occurring only in very minute amounts, and exercising a function which he regarded as catalytic. Maz@ (1915) was the first to demonstrate that boron was essential for the normal growth of the corn plant. By the use of new and greatly improved met.hods of experimentation, he produced evidence which led him t o the conclusion that boron was essential for corn. He also concluded that other elements not previously considered essential might be found indispensable for green plants. Warington (1923) showed that boron was indispensable for the broadbean and that in the absence of boron, this plant did not complete its life cycle but died prematurely with characteristic symptoms. Preliminary results wit.h other species such as barley, crimson clover and several species of beans lead to the general belief that boron is essential to these species also, but the evidence was held to be inconclusive.

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Stiles (1946) listed the name of the first worker to call definite attention to the favorable effect of boron on the growth of the species concerned. This was given whether the worker regarded boron as essential for the species or not. Thus, Nakamura in 1903, reported increased growth of peas and spinach as a result of adding boron to the soil but i t was not until 1915 that M a d claimed the essential nature of boron for plant growth. 1, Function of Boron in Plants Warington (1923) first showed that meristematic activity was markedly affected in the broadbean and that both roots and stem tissues were abnormal in the absence of boron. Since this work, there have been a great number of studies on the function of boron in plants. Johnston and Dore (1929) found t.hat plants grown in a boron deficient nutrient solution showed four distinct types of injury: (1) death of the terminal growing point of the stem; (2) breakdown of the conducting tissues of the stem; (3) a characteristic brittleness of the stem and petiole and (4)extremely poor growth of roots which develop a brownish unhealthy color. The total sugars and starches were more abundant in the leaves and stems of the boron deficient plants while a greater amount of benzene-insoluble matter was found in the leaves of normal plants and in the stems of boron deficient plants. Haas and Klotz (1931) concluded that boron is essential to cell division in the meristematic tissues and in the cambium. In the absence of sufficient boron the cambium and portions of the phloem were observed to disintegrate and gum up, some of which found its way to the exterior through a split in the cortex. When there was any xylem disintegration, the amount was small. A normal accumulation of carbohydrates in the leaves of boron deficient, plants was observed and ascribed to the disintegration of the phloem with consequent interference with translocation. The addition of boron to the culture solution resulted in a reduction in the total sugar content of the leaves and a restoration of the vigor of the plant. Shive (1941) believed there was considerable experimental evidence that boron is an important factor in the processes involved in organic synt.hesis. He found that plants grown in boron deficient regions yielded strong positive tests for pectins and negative tests for fats. Lohnis (1940), studying the influence of boron deficiency on the anthers of several small grains, found the primary effect of boron deficiency to appear in the cell nucleus where division was inhibited in the early stages of the boron deficiency. Working with alfalfa, Scripture and McHargue (1943) found that soluble nitrogenous compounds and reducing sugars

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were present in greater proportions in expressed sap from boron deficient plants than in that from normal plants. Later (1945), working with radishes they found that direct reducing sugars, sugar hydrolyzed by invertase, and alcohol-insoluble carbohydrates were all present in excess amounts in the tops of boron deficient plants. The rook of the radishes contained less direct reducing sugars but more of the other carbohydrate fractions than did the normal ones. They concluded that boron must function in metabolism and translocation of carbohydrates but whether directly or indirectly through its role in nitrogen metabolism remained uncertain. Smith (1944) found that in squash leaf cells approximately 50 per cent of the boron was immobilized in the cell wall or intracellular substance. Along with the boron, he found 70 per cent of the calcium. He suggests that boron is of importance in the cytoplasm and in the wall but not in the chloroplast or vacuole. From the evidence above it can be seen that boron is very import,ant in cell division and is apparently a necessary component of the cell wall. Boron also plays an important role in the synthesis of proteins in the plant as shown by the fact that in its absence, nitrogen compounds and sugars accumulate while meristematic tissues die. Further work is necessary before more definite roles can be ascribed. 2. Interrelations with Other Ekments

The relationship of boron to calcium in plant nutrition, as well as the relationship between boron and numerous other elements, has been the subject of much study. There have been many conflicting reports as to the effect of boron on the other elements, particularly on t*he base elements. There have been cases where, because boron was the limiting factor, the content of other elements in the plant have been increased. When boron was adequately supplied, the amounts of the other elements in the plant declined. Under conditions of boron toxicity, the normal physiology of the plant can be so upset as to cause either abnormal accumulations, or lowered amounts of both cations and anions. I n other cases if an element such as phosphorus is limitsing, the boron content of the plant might be high due to a stunting because of a lack of phosphorus for normal growth. I n such a case, if the phosphorus supply is adequate the amount of boron in the plant will probably decline due to the greater growth of the plant when supplied with phosphorus. The same relationship could hold true of any of the other nutrient elements. When the effects of various bases are studied, it should be remembered that on a chemical equivalent basis plants tend to maintain R

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constant amount of bases. Therefore, if the supply of potassium is increased in a plant, the amount of calcium or other bases will probably he lower. It should also be remembered that plants require a certain balance of one nutrient element to another for normal growth and if the balance is greatly upset the plant will be abnormal. Thus if the calcium supply was increased three or four-fold above normal, the result could be magnesium deficiency even though the amount of magnesium present would be sufficient under normal conditions. It is important, therefore, when studying the relationship of boron to some of the bases, to keep this fact in mind. I n nearly all experiments, it has been found that the amount of boron in the plant tissue increases with the amount supplied to the plant whether the plant is growing in soil or nutrient solution. This is true even when the amounts of boron are supplied in toxic concentrations. This build-up in the pIant can be as much as ten-fold or more, as shown by Parks, et al. (1944). The following are some of the factors that. should be taken into consideration when relationship between boron and the other nutritive elements are studied.

a. Calcium-Boron Ratios. .Much of the work on calcium-boron ratios has already been discussed under Section II-2-b. The relationship betweeen calcium and boron apparently is a very real one and, as shown by Reeve and Shive (1944), when plants have access t o increasing amounts of calcium they require more boron to prevent deficiency. With high amounts of calcium, plants are able to withstand larger amounts of boron without it becoming toxic. They found t>hata t high boron levels there is a marked decrease in both total and soluble boron in the plant tissue with increase in the calcium concentrations in the nutrient solut.ion. The calcium accumulation in the tissues is largely determined by the calcium concentration in the growing medium and appears to be independent of boron. This is in line with observations by other workers. Not only is a certain definite amount of boron needed with calcium in the plant to build cell walls, as well as for the other functions of boron in the plant, but it appears that excess of boron above t,hese needs combines with calcium to form compounds no longer toxic to the plant. Because the symptoms of boron deficiency and of calcium deficiency which appear in the growing point are very similar, i t appears logical that these two elements are related in their function in plant growth. Further evidence that their function is related was given by Smith (1944), who found 50 per cent of the boron and 70 per cent of the calcium immobilized in the cell wall or intracellular spaces.

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b. Potassium-Boron Ratios. The main work on potassium-boron ratio has been done by Reeve and Shive (1944). Along with their work on calcinm-boron ratios, they grew tomato plants in water culture solutions with five different potassium levels and four different boron levels. They found that the external symptoms of boron toxicity a t high boron levels and the deficiency symptoms a t low boron levels were progressively accentuated with increasing potassium concentrations in the nutrient solution. Increasing t.he potassium concentration had the effect of increasing the boron content of the plants especially a t the higher boron levels. They found that calcium and potassium were similar in their capacity to accentuate the symptoms of boron deficiency with increasing concentrations of these cations in the nutrient solution, but they found boron toxicity at the high boron levels decreased markedly with increasing concentrations of calcium but not with potassium. This might largely be due to the constant ion effect in plants which is now well known, whereby, on the equivalent basis, t.here tends to remain for any given species a rather constant total amount of cations in the plant. When one of the cations is increased in the plant it does so a t the expense of some other. I n this case increasing the potassium content would have the effect of reducing the amount of calcium in the plant. and thus the effect of high potassium would be the same as that of low calcium. The results seem to agree with this. Another effect of potassium in the plant might, be that of increasing the permeability of the roots. The fact that boron concentrations in the plant are increased with increasing amount8 of potassium would seem to substantiate this. If, when the potassium concentrations was increased, greater growth resulted, boron deficiency symptoms would be accentuated a t low boron levels because of t,he increased growth of the plant. This probably is a factor in some cases in the field, where potassium deficiency and potassium responses often occur. Chapman et al. (1940) suggested a reciprocal relationship of calcium and potassium when they found that lemon plants grown in potassium deficient cultures supplied with boron a t the rate of 1 p.p.m. developed symptoms of boron toxicity. Normal lemon leaves contained 20 to 40 p.p.m. of boron in the dry matter but in these plants the boron concentration was nearly 200 p.p.m. It appears that the relationship between potassium and boron in the plant is of much less importance than that of calcium and boron. In many cases probably the effect of potassium on boron in plants is an indirect one, where adding potassium to the plant decreases the uptake of calciiim, thus upsetking the calcium-boron ratio in the plant.

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c. Nitrogen-Boron Relatiomhip. Because the region of the meristem is one of the first to be affected when boron deficiency exists, it seems likely that boron not only is an important factor in cell wall formation but also that it might be tied up with protein formation in the protoplasm. There is considerable evidence in the literature to show that when boron is deficient, there is an accumulation of nitrogen compounds and simple carbohydrate compounds in the plant. This would indicate an inability of the plant to complete protein synthesis in the absence of boron. Schropp and Arene (1942) in a study of the nitrogen content and metabolism in plants found more nitrogen and a smaller proportion of protein-nit,rogen in boron deficient plants than in normal plants. Baumeister (1941) working with Phaseolus vulgaris in water cultures, with and without boron, found that when the plants were grown with only a small amount of calcium sulfate present, the nitrogen being supplied by the cotyledons only, the plants accumulated excess nitrogen in shoots, leaves, and roots, in comparison with the parallel series in which boron was also supplied. When nitrogen was supplied to the nutrient solution, disturbances in the nitrogen content and metabolism of boron deficient plants were seen earlier in the stems and roots than in the leaves. Scripture and McHargue (1943-1945), working wit,h alfalfa, radishes, and spinach, found that soluble nitrogenous compounds and reducing sugars were present in greater proportions in the expressed sap from boron deficient alfalfa and radishes than in that from normal plants. The radish roots from deficient plants contained less direct reducing sugars but more of the other oarbohydrate fractions than did those from normal plants. Further support for the theory t.hat boron is used in protein formation is found in their work on, spinach. The proportion of protein-nitrogen to alcohol-soluble-nitrogen in spinach leaves increased with increasing boron supplies. Working with nasturtiums, Briggs (1943) found a progressive decrease in nitrate absorption in boron deficient plants as compared with that of normal plants. Ammonia-nitrogen, soluble organic nitrogen, and carbohydrates accumulated in such plants. In plants showing boron toxicity, the amount of protein nitrogen was increased and the amount of soluble nitrogen lowered. d . Other Elements. Parks et al. (1944) studied the relationship of boron to a number of other elements. Their data show that differences between the content of various elements as affected by the boron supply could not be correlated with the type of ion (cation or anion), the valence of the ions, or the total growth of the plant. This is probably due to the

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fact that most of their treatments contained toxic quantities of boron. I n their paper, they list conflicting reports as to the effect of the boron supply on the magnesium, phosphorus, and iron contents of various plants. As has been previoiisly stated, many of these results have been obtained due to abnormalities in tlie physiological functions of the plant due to exheme deficiencies or excesses of boron in the plant, or have been clue to the fact that some one of the nutrient elements has been limiting plant growth, thus causing accumulation of other elements. I n the normal physiological functions of the plant it appears to be established that there is a direct relationship between calcium and boron and between nitrogen and boron. Although other elements have been considered to have an effect on boron in the plant it seems t,hat under normal conditions their effect is mostly indirect by influencing the uptake of either calcium or nitrogen. 3. Symptoms of Boron Deficiency

Visual symptoms of boron deficiency have been the subject of much investigation and have been summarized recently in useful form by Dennis and O’Brien (1937), Dennis and Dennis (1943) , Dennis (1948) , and McMurtrey (1948). One striking thing about boron deficiency is that invariably it affects the terminal growth which indicates that boron is not translocated in the plant., but is fixed in insoluble compounds, and that it is needed in cell division. McMurtrey (1948) lists the visual symptoms of a number of crops and in nearly all of them, the main visual symptom of boron deficiency is that terminal growth ceases, internodes become shortened, and the plant in many cases acquires a rosetted appearance. These symptoms are nearly alike in such widely divergent species as apple, beet, and alfalfa. Of course, because the terminal growth dies, flowers are often blasted and fruit and seeds frequently fail to form. Calcium deficiency symptoms in most plants are much like those of boron deficiency in that the terminal growth is commonly affected first, and yet are usually differentiated by different types of chlorosis of the leaves. It is essential to remember that with boron, as with phosphorus and other plant nutrient elements, deficiency may be present long before visual deficiency symptoms occur, and increases in yield may be obtained t.hrough the application of borax. This was shown to be true by Berger and Truog (194413). They worked with red beets and sugar beets and obtained statistically significant increases in yield in 5 out of 6 trials in which boron deficiency symptoms were absent. I n only 1 of the 6 trials were boron deficiency symptoms present when an increase in yield was

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obtained. Significant increases in yield were only obtained when tlie soil had less than 1 p.p.m. of available boron. As is true with other deficiency symptoms in any given plant, a number of different visual symptoms of boron deficiency occur, depending upon the severity of deficiency. The first of these symptoms is internal, as shown by Walker (1944), who found that when boron deficiency of garden beet and cabbage is brought about gradually in sand cultures the progressive effects on the histology of the plant can be studied in successive tertiary rings and that long before any external symptoms develop, various profound histological changes occur. Probably the next stage of boron deficiency in most species is a necrosis of terminal gr0wt.h and shortening of the apical internodes. As the deficiency becomes more severe chlorosis often appears, flowers blast and fruit fails to form. Finally the entire plant dies, Thus there are a whole series of boron deficiency symptoms with the mildest manifested only by reduced growth and certain chemical changes, the second st.age by cellular changes, and next a series of microscopic changes leading to premature death of the entire plant. It is interesting to know that some boron deficiency symptoms were thought to be diseases and were named and described as such long before their causes were understood. Thus, in 1924, Foster and Weber described a nonparasitic disease of celery which they named “cracked stem” and ascribed as its probable cause a combination of climatic factors, use of an unbalanced fertilizer, and excessive use of lime. It was not until 1935 that Purvis and Ruprecht t,raced the cause of these symptoms to a deficiency of boron. It is interesting, however, that Foster and Weber (1924) exactly described the conditions which caused the boron deficiency. In some cases a great many years elapsed between the recognition of the disease and the final discovery that it is caused by boron deficiency. This is what Atwater (1941) refers to as the “ancient history of boron deficiency symptoms.’) A considerable number of these socalled nonparasitic diseases associated with boron deficiehcy have been reported and described. Among them are top sickness of tobacco, heart rot of beet, cork disease of apple, brown rot of cauliflower, cracked stem of celery, “raan” of swedes, and many others. Along with boron deficiency symptoms, a great deal of work has been done on boron toxicity. This was summarized recently by Eaton (1944) who has grouped plants into t.hree classes : sensitive, semi-tolerant, and tolerant. It is interesting to note from tables in this publication that sensitive plants, in general, contain high amounts of boron when grown in a solution containing 5 p.p.m. of boron. Semi-tolerant plants, grown in the same solution, contain an intermediate amount of boron, while

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the lowest amount of boron is found, in general, in the tolerant plants. For the most part tolerant plants not only have a high boron requirement, but also a high calcium requirement.

4. Boron Requirements of Plants Although there has been a tremendous amount of work done on boron fertilization of crops in the field, it is still difficult t o classify many orops according to their boron requirement. I n order to do this properly, it is necessary to have knowledge of the boron content of the crop when grown in a normal soil, the response obtained with the crop on boron deficient soils, and the extent of the deficiency found in various parts of the country. When the boron content of the growing medium is high, there is no relationship between boron needs and boron content of the plant. When the boron content of the medium is similar to that in which the plants normally grow however, the boron content of plants grown in such a medium is an indication of t.he boron need of the crop. Crops with a low boron content will have a low requirement for boron, and in general, the crops with a high content of boron will have a relatively TABLE I Amounts of Boron Found in Topa of Plants Grown in Two Soils Kind of plant Barley Rye Leek Wheat Corn Spinach Black nightshade Endive Pea White mustard Plantain Carrot Tobacco Sainfain Cabbage Soy bean Lentil

Boron content of dry matter p.p.m. 2.3' 3.1" 3.1" 33' 5.0" 10.4" 11.O' 13.1" 21.7" 222" 225' 25.0' 26 .on 362' 37.1' 373' 41.4'

Kind of plant Kidney bean Turnip Black mustard Radish Beet Dandelion Spurge POPPY Meadow grass Onion Flax Celery Mallow Potato Broad bean Tomato Alfalfa

Data of Bertrand and De Weals (1936). Data of Bertrand and Silherstsin (1937)

Boron content of dry matter p.p.m. 43.0' 49.2' 53.3" 645" 75 6 ' 80.0' 93.0' 94.7' 3.2b 4Ab 7.1b 11.Qb 13.7b 139b 15.4b 15.0b 25 .Oh

--___

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high requirement for boron. In Table I are given the boron content of plants grown in one soil by Bertrand and De Waals (1936) and in another Foil by Bertrand and Silberstein (1937). As can be seen, the boron content vrtried from 2.3 p.p.m. in barley to 94.7 p.p.m. in poppy. The results given in this table are representative of boron contents found in plants grown in humid region soils in various other parts of the world. In Table I1 an attempt is made to arrange plants according t o their boron requirement. The information from which this listing was made TABLE I1 Boron Requirement of Some Common Field and Vegetable Crop Plants Probable available boron content of soils (p.p.m.1 required for optimum growth Plants with high requirement > 0.5 p.p.m.

Plants with medium requirements 0.1 to 0.5 p.p.m.

Plants with low requirement <0.1 p.p.m.

Apple Alfalfa Red clover Crimson clover White clover Sweet clover Red beets Sugar beets Mangolds Turnips Cabbage Broccoli Cauliflower Asparagus Sunflower Radish Brussels sprouts Celery Rutabaga Burr clover

Tobacco Tomato 1,etture PWch Cherry Olirc Pecan Cotton Sweet potato Peanut Carrot Walnut Filbert Onion Pear

Wheat Oats Rye Barley Buckwheat Corn Soybeans Peas Green beans Lima beans Navy beans Strawberry Citrus Raspberry White potato Blue grass Brome grasa Other grasses Flax

was obtained not only from the boron contents of the plants but also from the field experience of a great number of workers in various parts of the world. It is impossible in a paper of this kind to list all the sources. There will probably be changes in this classification, not only from one category to another, but also by addition and deletion as more information is obtained. An attempt is also made to give an approxi-

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mation of the available boron content of soils required for the optimum growth of the various crops. It is realized of course that this will vary considerably from area to area, will change with soil and weather conditions, and that it is only ti rough approximation. It should be remembered that in a soil producing half of a normal crop, because of a lack of water or plant food elements, the amount of boron needed for normal growth will also be only about one-half of the normal. Furthermore, to obtain maximum growth of the plants, it will be necessary to have more boron in a soil high in available calcium than in one low in calcium. There is also a difference in ability of various plants to feed on boron which is strikingly shown up in the difference between red beets and sugar beets. When grown in the same soil both of them have about the same content of boron in the leaves although it is necessary to have more available boron in the soil for red beets to prevent boron deficiency than it is for sugar beets simply because of the smaller root system of the red beets. When plants become deficient in boron, they contain a certain amount of the element. Death of the plant will occur before this amount will be lowered. There has probably been more work done on alfalfa than on any other crop to determine the amounts of boron needed for normal growth and methods for detecting boron deficiency. Rogers (1947) has TABLE I11 Critical value for the boron content of alfalfa plants as reported by various investigators ' Investigators

Amount of boron reported in deficient plants or plants with need for boron p.p.m.

McLarty, Wilcox, and Woodhridgr Rprger and Truog Haddock and Vandrcavryr Powers Dregne and Powers Jordan and Powers Dunklee and Midgley Brown, Munsell, and King Whetstone, Robinson, and Byws

Dawson and Gustafson Munsell and Brown Rogers (1947).

6.9 8.0 10.0 10.0 7.0 to 11.5 (normal plants 12.0 to 22.5 p.p.rn. B) 12.0 15.0 17.0 (Also 17.0 with no response to B) 13.0 to 17.0 response to boron 12 to 19 No response 20 23 in leaves, yellows

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347

listed the amount of boron found by various investigators in deficient plants or in plants with a need for boron. These are given in Table I11 and it can be seen that there is considerable discrepancy. Rogers states that, if alfalfa, crimson clover, or burr clover contain less than 10 p.p.m., boron response to additions of borax is indicated on the coarse textured red and yellow podzolic soils of Alabama. The same limiting amount has been found to be true with alfalfa in Washington, Oregon and Wisconsin. There undoubtedly is a certain critical minimum boron content in plants as there is a certain maximum boron content above which toxicity symptoms appear. As stated previously, these symptoms vary with species and various growing conditions. With the exception of alfalfa, little work has been done on this problem.

V. SUMMARY Although borax has been in use for about 4 centuries, it was only about 90 years ago when it was first discovered in plants and only a little over 30 years ago when it was first claimed to be essential for plant growth. The largest share of the work on boron has been accomplished in the last 15 years. Very satisfactory chemical methods are now available for the determinat.ion of boron in plant tissues and for the determination of total and available boron in soils. The amount of available boron in soils can also be determined by growing plants in soils and comparing the growth with those grown in nutrient cultures with known amounts of boron. This method is fairly accurate but is slow, requiring two to eight weeks for completion of a test. Boron availability and fixation have been the subject of much recent work. Because available boron is lost from soil most rapidly by leaching, problems of boron deficiency are largely found in soils of the humid regions. I n arid regions, soils not leached are usually high in available boron and irrigation waters often contain considerable quantities of this element. The available soil boron is apparently in two foriris, organic and inorganic, ant1 these are in equilibriiim with the fixed forins of boron. Boron is supplied to the soil largely from tourmaline but the rate of decomposition is slow. In heavily cropped soils, boron is often depleted through crop removal. Free calcium in soils tends to hold boron from leaching and in alkaline soils, in the presence of free calcium, boron is fixed in a temporarily unavailable form, partially by organic matter : m l partially by soil minerals, the activity of which predominates in the clay fraction.

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C. BERGER

This fixed boron can be readily released by the action of strong mineral acids. I n light-textured acid soils the available boron content tends to be low because of leaching and a lack of calcium to prevent removal of boron. Boron added to soils as sodium borate tends to move downward in t.he soil in mass, independent of the sodium applied with it, and lagging behind the movement of the water. Apparently all higher plants need boron for normal growth. The amounts of boron needed vary considerably with the species but in general monocotyledonous plants require only about one-fourth as much boron for normal growth as do dicotyledonus plants. The function of boron in the plant is not understood as yet. The similarity between boron and calcium deficiency symptoms indicate that they may be involved in the same function in the plant. It has been shown by several workers that as the supply of calcium is increased, the plant needs more boron for normal growth, and a t high boron levels can withstand larger quantities of boron without developing toxic symptoms. When boron deficiency starts the area affected first is invariably the actively growing region of the plant The normal development of cell walls is upset when boron becomes deficient. There may also be an accumulation of soluble nitrogen and of carbohydrate in the plant and a reduction of the amount of proteins formed. Boron deficiency symptoms are similar for many plants in that the youngest. growing part of the plant becomes necrotic. This is often followed by disintegration of tissues with chlorosis in leaves and pigmentation of roots and stems. These symptoms represent extreme deficiency. Much work remains to be done on the function of boron in the plant, particularly as regards the compounds that boron enters into in the plant and relation of boron to other elements necessary for plant growth. Furt.her information is required as to the mechanism of boron availability and fixation in soils and on the chemistry of the boron compounds present in soil. More work is also needed on the boron requirements of many crops and on methods, rates, and time# of applications of borax for the most profitable yields of these crops on the many different soil aeries on which they are produced.

REFERENCES Agricola, G . 1556. Libri X I I . Basileae. Agulhon, H. 1910. Compt. rend. 151, 1382-1383. Askew, H. O., and Thomson, R. H. K. 1937. N . 2. J . Sci. Tech. 18, 661-664.

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Askew, H. O., ThornRon, R. H. K., and Chittenden, E. 1938. N . Z. J . Sci. Tech. 20A, 74-78. Atwater, C. G. 1941. -1. Am. SOC.Agron. 33, 939-942. Baumeister, W. 1941. Phytoputh. Z . 13, 481-502. Beeson, K. C. 1945. Soil Sci. 60, 9-13. Berger, K. C., and Truog, E. 1939. Ind. Eng. Chem., Anal. Ed. 11, 540-545. Berger, K. C., and Truog, E. 1940. J. Am. Soc. Agron. 32, 297-301. Herger, K. C., and Truog, E. 1944a. Soil Sci. 57, 25-36. Berger, K. C., and Truog, E. 1944b. Wis. Acad. Sci. Arts LettPrs 36, 421-425. Berger, K. C., and Truog, E. 1945. Soil Sci. SOC.Am., Proc. 10, 113-116. Bertrand, G.,and Agulhon, H. 1910. Bull. 9oc. Chim. 7, 90-92. Bertrand G., and De Waals, H. L. 1936. Ann. inst. Pasteur 57, 121-126. Bertrand, G., and Silberstein, L. 1937. Ann. inst. Pusteur 59, 442-444. Bobko, E. V., and Syvorotkin, G. S. 1935. Khim. Sotsinl. Zemeid 8, 17-22. Brandenburg, E. 1931. Phytopath. Z . 3, 499-517. Brenchley, W.E., and Warington, K . 1927. Ann. Bot. 41, 167-187. Briggs, G.B. 1943. Plant Phgsiol. 18, 415-432. Calfee, R. K., and McHargue, J. S. 1937. Znd. Eng. Chem., Anal. Ed. 9 , 288-290. Cassal, C. E., and Gerrans, H. 1903. Chcm. News 87, 27-30. Chapman, H. D., Liebig, G. F., and Vanselow, A. P. 1939. Soil Sci. Soc. Am., Proc. 4, 196-200. Coleman, R. 1945. Better Crops with Plunt Food 29, 18-20, 48-50. Colwell, W. E. 1943. Soil Sci. 50, 71-94. Colwell, W. E. 1946. Soil Sci. 62, 43-49. Colwell, W.E., and Baker, G. 0. 1939. J . A m . SOC.Agron. 31, 503-512. Colwell, W.E., and Cummings, R. W. 1944. Soil Ski. 57, 37-49. Cook, R. I,., and Millar, C, E. 1938. Soil Sci. SOC.Am., Proc. 3, 146-152. Dennis, R. W. G. 1937. Science Prog. 32, 58-69. Dennis, R. W . G. 1948. Boron and Plant Life VZ. Dennis, A. C., and Dennis, R. W. G. 1943. Boron and Plant Life V . Dennis, R. W. G., and O’Brien, D. G. 1937. West Slcot. Agr. Coll. Plant Husb. Dept. R e s . Bull. 5, 9-16. Drake, M., Sieling, D. H., and Scarseth, G. D. 1941. J . Am . Soc. Agron. 33, 454-462. Dunklee, D. E., and Midgley, A. R. 1943. V t . Agr. Expt. Sta. Bull. 501. Eaton, F. M. 1935. U.S. Dept. Agr. Tech. Bull. 448. Eaton, F. M. 1944. J . Agr. Research 69, 237-277. Eaton, F. M., McCallum, R. D., and Mayhugh, M . S. 1941. U.S. Dept. Agr. Tech. Bull. 746. Eaton, F. M., and Wilcox, L. V. 1939. U . S . Dept. Agr. Tech. Bull. 696, 1-57. Elder, S., and McCall, R. J. S. 1936. 1. Agr. Sci. 26, 1-21. Faraday, M. 1821. Quunt. J. Sci. 11, 403-405. Feigl, F.,and Krumholz, P. 1929. Mikrochernic Pwgl Pestschi. 77-80. Ferguson, W., and Wright, L. E. 1940. Sci. Agr. 20, 470-487. Foster, A. C., and Weber, G. F. 1924. Fln. Agr. Rzpt. Sta. Bull. 173. Foster, J. S., and Horton, C. A. 1937. I’roc. R o y . Soc. London B123, 422-430. Goldschmidt, V. M., and Peters, G. 1932. Gesell. d e i Wiss. Zu. Gottingen, Math. Phys. K5,402-407. Haas, A. R. C. 1944a. Proc. Am. SOC.Hort. Sci. 44, 34-42. Haas, A. R. C. 1944b. &il Sn‘. 58, 123-137. Haas, A. R. C., and Klotz, L. J. 1931. Hilgurdia 5, 175-196.

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,Jay, H. 1895. Compt. rend. 121, 896-899. Johnston, E. S. 1928. Soil Sci. 26, 173-175. Johnston, E. S., and Dore, W. H. 1928. Science N . S. 67, 324. Johnston, E. S., and Dore, W. H. 1929. Plant Physiol. 4, 31-62. Jones, H. E., and Scarseth, G. D. 1944. Soil Sci. 57, 15-24. Komarovski, A. S., and Poluektoff, N. S. 1931. Mikrochemie 14, 317-320. Kriigel, C., Dreyspring, C., and Lotthammer, R. 1937. Das Superphosphnl. 13, 99-104. Kriigel, C., Dreyspring, C., and Lotthammer, R. 1938. Sup~rphosphate11, 161-166. Kubota, J., Berger, K. C., and Truog, E. 1948. Soil Sci. Soe. Am. Proc. 13. In prc-. Latimer, L. P. 1941. Proc. Am. SOC.Hort. Sci. 38, 63-69. Lehr, J. J. 1940. Thesis, Utrecht. Lohnis, M. P. 1940. Meded. LandbHoogesch. Wageningen 44, 3-36. McHargue, J. S., and Calfee, R. K. 1932. Ind. Eng. Chem., Anal. Ed. 4, 385-394. McHargue, J. S., and Hodgkiss, W. S. 1942. J . Assoc. Oficial Agr. Chem. 25, 311-313. McMurtrey, J. C. 1929. J. Agr. Research 38, 371-380. McMurtrey, J. E., Jr., 1948. Diagnostic Techniques for Soils and Crops. Am. Potash Institute, Washington, pp. 238-251. Magistad, 0. C., and Christiansen, J. E. 1944. U. S . Dept. Agr. Circ. 707. Marsh, R. P., and Shive, J. W. 1941. Soil Sci. 51, 141-151. Maunsell, P. W. 1940. N . Z. Jo. Sci. Tech. 22, 100B-lllB. M a d , P. 1915. Compt. rend. 160, 211-214. Melvin, E. H., and O’Connor, R. T. 1941. Ind. Eng. Chem., Anal. Ed. 13, 520-524. Midgley, A. R., and Dunklee, D. E. 1939. Soil Sci. SOC.Am., Proc. 4, 302-307. Midgley, A. R., and Dunklee, D. E. 1940. V t . Agr. Espt. Sta. Bull. 460, 3-22. Naftel, J. A. 1937. J. Am. SOC.Agron. 29, 761-771. Naftel, J. A. 1938. Am. Fert. 89, 5-8, 26. Naftel, J. A. 1939. Znd. Eng. Chem., Anal. Ed. 11, 407-409. Olson, R. V. 1947. Thesis, Univ. of Wis. pp. 26-28. Olson, R. V., and Berger, K. C. 1946. Soil Sci. SOC.,Proc. 11, 216-220. Parks, R. Q. 1944. Soil Sci. 57, 405-416. Parks, R. Q., Lyon, C. B., and Hood, S. L. 1944. Plant Physiol. 19, 404-419. Parks, R. Q., and Shaw, B. T. 1941. Soil Sci. SOC.Am., Proc. 6, 219-223. Piland, J. R., Ireland, C. F., and Reisenauer, H. M. 1943. Soil Sci. 57, 75-84. Purvis, E. R., and Hanna, W J. 1938. Soil Sci. SOC.Am., Proc. 3, 205-209. Purvis, E. R., and Ruprecht, R. W. 1935. Flu. Agr. Ezpt. Sta., Ann. Rept. 63. Reeve, E., Prince, A. L., and Bear, F. E. 1944. N . J . R a . Bull. 709. Reeve, E., and Shive, J. W. 1944. Soil Sci. 57, 1-14. Rogers, H. T. 1947. J . Am. SOC.Agron. 39, 914-928. Scharrer, K., and Gottschall, R. 1935. 2. Pfkznz. Dun. 39, 178-197. Schofield, C. S. and Wilcox, L. V. 1931. U.S. Dept. Agr. Tech. Bull. 264. Schropp, W., and Arenz, B. 1942. Bodenk. PflErnahr. 26, 198-246. Schuster, C. E., and Stephenson, R. E. 1940. J . Am. SOC.Agron. 32, 607-621. Scripture, P. N., and McHargue, J. S. 1943. J. Am. SOC.Agron. 35, 988-992. Scripture, P. N., and McHargue, J. 8. 1944. J . Am. Soc. Agron. 36, 865-869. Scripture, P. N., and McHargue, J. S. 1945. J . Am. Sor. Agron. 37, 360-364. Shive, J. W. 1941. Plant Physiol. 16, 435-445. Shive, J. W. 1945. Soil Sci. 60,41-51. Smith, G . S. 1935. Analyst 60, 735-739.

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Smith, M. E. 1944. Aust. J . Ezpt. Biol. 22, 257-263. Sommer, A. I., and Lipman, C. B. 1926. PZant Physiol. I, 231-249. Stiles, W. 1946. Trace Elements in Plants and Animals. Cambridge Univ. Press, London. Tulin, A. S. 1940. Pedology 3, 39-53. Walker, J. C. 1944. Soil Sci. 57, 51-54. Walker, J. C., Schroeder, W. T., and Kuntz, J. E. 1944. Better Crops with Plant Food 28, 19-21, 49-50. Warington, K . 1923. Ann. Bot. 37, 629-672. Whetstone, R. R., Robinson, W. O., and Byers, H. G . 1942. U.S. Dept. Agr. Tech. Bull. 797, 1-32. White-Stevens, R. H. 1941. Proc. A m . SOC.Hort. Sci. 39, 367. Wilcox, L. V. 1932. In. Eng. Chem., Anal. Ed. 4, 38-39. Wilcox, L. V. 1940. In. Eng. Chem., Anal. Ed. 12, 341-313. Wittstein, A., and Apoiger, F. 1857. Ann. Chem. Pharmacie Liebig 103, 362-365 Wolf, B. 1940. Soil. Sci. 50, 209-216. Woodbridge, C. G. 1940. Sd.Agr. 20, 257-265. Zorkin, F. P. 1936. J . Applied Chem. i7.SS.R. 9, 1505-1506.

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Potato Production ORA SMITH Cornell University, 1 thaca, N . Y . CONTENTS PflQI.

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . 353 11. Breeding and Improving Potato Varieties . . . . . . . . . . . . 355 111. Chemical Weed Control . . . . . . . . . . . . . . . . . . . 357 IV. Fertilizer Practices . . . . . . . . . . . . . . . . . . . . . 360 V. Rotations and Green Manures . . . . . . . . . . . . . . . . 363 VI. Response to Nitrogen Fertilization . . . . . . . . . . . . . . . 365 VII. Response to Phosphorus Fertilization . . . . . . . . . . . . . . 366 VIII. Response to Potassium Fertilization . . . . . . . . . . . . . . 367 IX. Effects of Magnesium, Liming and Soil Reaction . . . . . . . . . 369 X. Minor Elements . . . . . . . . . . . . . . . . . . . . . . 371 XI. Time and Method of Application of Fertilizers . . . . . . . . . . 372 XII. Relation of Yield and Tuher Composition to Plant and Soil Analyws . 374 XIII. Killing Potato Vines . . . . . . . . . . . . . . . . . . . . 377 XIV. Recent Developments in Insect Control . . . . . . . . . . . . . 381 XV. Recent Developments in Disease Control . . . . . . . . . . . . 385 References . . . . . . . . . . . . . . . . . . . . . . . . 386

I. INTRODUCTION The potato, Solanurn tuberosum L., is one of the most important economic crops and is grown commercially in most countries of the world. In many countries it comprises a substantial portion of the diet of the people and in most instances it is an inexpensive source of food. It also contributes an appreciable amount to livestock feeds in some areas as well as being raw material for alcohol, flour and starch manufacture. Total annual production for the United States for many years averaged approximately 360 million bushels; recently production has increased sharply. The average production for the six-year periods are as follows: 1919-1924, 359 million; 1925-1930, 348 million; 1931-1936, 368 million; 1937-1942, 362 million; and 1943-1948, 430 million. Contrary to t.his, acreage devoted to potato production in the United States has been declining during this same period. Acreage for the six-year period 1919-1924 averaged 3,430,000 compared with 2,587,000 acres for the last 353

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six-year period, 1943-1948. Despite this acreage reduction through the years, production has been increasing. This, of course, has been possible largely owing to the improved methods of potato growing which have resulted in larger yields per acre. The average production per acre for the six-year period 1919-1924 was 105.1 bushels; for the period 19431948 it increased to 169.6 bushels per acre. Simultaneously with these increased yields and increased total production there has been a steady decline in per capita consumption of potatoes in the United States. Average annual consumption per capita for the six-year period 19191924 was 155 Ibs. and for the past six-year period, 1943-1948, it has declined to 127 lbs. With the present population of the United States this amounts to a reduction in consumption of approximately 69 million bushels. As the supply of potatoes increases and the demand declines, marketing problems appear to become more acute. While it is not within the province of this paper to discuss the problems of marketing, it appears pertinent a t this time to present some of the reasons for the decline of consumer demand for potatoes and later, perhaps, some of the possible methods of preventing further decline. Olne of the important reasons for the decline in consumption of potatoes, particularly in recent years, is the relatively low quality of potatoes offered to the consumer in many markets. Experiment station workers, growers and others have been more successful apparently in increasing production per acre than in improving the quality of the product. In addition events have occurred which have not encouraged growers and packers to package a product with as high quality as the consumer desires. The best, most economical, most practical and permanent way to supply consumers with the quality of potatoes which they desire is to produce fewer undesirable, off-grade or cull potatoes. There is as yet no satisfactory commercial outlet for low grade potatoes, hence many appear on the market, instead of high grade stock. The following defects are often found on potatoes marked U.S. No. 1 in many markets today: mechanical injuries such as bruised, skinned and cut, second growth, growth cracks, undersise, sun greened, scab, rhizoctonia, late blight rot, storage rots, injury from wireworm, millipede, scab gnat, snails and slugs. I n addition, there often are internal defects such as blackspot, mahogany browning, vascular discoloration, stem end browning, net necrosis, etc. Even when the consumer is fortunate enough to obtain potatoes with none of the above-mentioned defects, often the tubers when cooked are soggy when mealiness may be desired, or they may slough and fall apart during cooking or turn gray or dark a t the stem end just after cooking. Most of these troubles can be avoided

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and a good research program of breeding, production, storage and marketing undoubtedly will disclose methods of further reducing these defects. Several comprehensive studies of consumer demands for potatoes have disclosed that consumers want potatoes of clean surface, smooth skin, few eyes, regular shape and no blemishes. They desire potatoes of medium size which are mealy after cooking and do not discolor and large potatoes for baking.

11. BREEDINGAND IMPROVING POTATO VARIETIES At the turn of the cent.ury the leading potato varieties in this country were Irish Cobbler, Bliss Triumph, Spaulding Rose No. 4, Early Ohio, Rural and Green Mountain. Thirty years later nothing of much value had been added to the list except Russet Burbank and Russet Rural. In the meantime, however, the need for varieties of improved culinary and storage quality, higher yield and with resistance to heat, drought, insects and diseases was recognized and breeding programs were initiated for the specific purpose of producing better varieties. The past 15 years have been very productive along these lines and the future looks even brighter. Many of the older varieties were deep-eyed, dark-skinned and relatively unattractive on the market. Since the housewife buys her potatoes principally as a result of eye appeal it is stimulating to sales to produce varieties of bright skin, shallow eyes and regular shape, such as Katahdin, Chippewa, Hoiima and Sebago. I n addition to these varieties others of high yielding ability such as Pontiac and Sequoia have been introduced and adopted in certain areas. The Mohawk, a high starch content variety, matching Green Mountain in mealiness, also is now grown in some areas. Since the control of diseases in potatoes is one of the most costly operations in potato product,ion it is natural that breeding programs have had as one of their objectives the production of varieties resistant to certain diseases, There now are varieties such as Erie and Teton which are resistant to ring rot; the Erie is also somewhat resistant to scab and the Teton to latent mosaic. The Ontario is very resistant to scab and is il promising variety, especially on high lime soils. Breeding for resistance to late blight has received considerable dtention and in New York State alone the following twelve new late blight resiskant varieties have been named within the past three years by Dr. D. Reddick: Ashworth, Chenango, Snowdrift, Cortland, Virgil, FilImore, Empire, Essex, Placid, Harford, Madison and Glenmeer. Many more unnamed varieties appear promising and undoubtedly soon will be introduced to growers for trial, The Kennebec, a high yielding, attractive appearing variety, resistant t o late blight, has been introduced recently by the United States Department

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of Agriculture. Wit,h a breeding program well under way in several sections of the country during the next few years we should see the introduction of many varieties of improved quality, appearance, yielding ability and resistance to diseases and insect injury. Akeley e t al. (1948) state bhat late blight probably is the most widespread and serious of all potato diseases. Serious losses occur every year despite the fact that control measures are practiced more generally than ever. Losses in the United States in one season from this disease have been estimated as high as 31 million bushels. The first indication of resistance to late blight obtained by workers in the National Potato Breeders Program was found in a cross of two susceptible varieties, Chippewa and Katahdin (Clark and Stevenson, 1935). One of these selections, B127 was crossed with 96-56, a cross related to the German W races, from which was obtained a seedling, Kennebec, a late maturing, high yielding, blight resistant variety which apparently is well adapted to a wide range of growing conditions and areas. I n the extensive investigations of Reddick (1943) the seedling plants are inoculated soon after emergence from the ground and every selection made thereafter is sampled and inoculated at least once each year. The whole success of a program of breeding for blight resistance depends upon elimination of all plants which react even slightly to the parasite. Immunity of aerial parts does not necessarily extend to the tubers. Reddick (1943) found tuber rot repeatedly under plants that were entirely free from foliage infection; adjacent infected plants were the source of infection. It is now more than a century since potato blight, Phytophthora infestans first appeared in North America and was disastrous in Europe. About a century ago efforts were made on both continents to find varieties that were resistant to this disease. Solanum demissum, which is highly resistant or immune to late blight, and which has never been found native outside of Mexico, is the species being used wherever work on the problem of developing resistant varieties is in progress. Solanurn polyadenium, a tuberous Mexican species, has good vigor, is immune to blight and is very resistant to the attack of some of the insects destructive to potat,oes. No cross involving this species has been made apparently after many attempts. Although S. demissum has 72 chromosomes, if used as the female parent, it crosses easily with domestic varieties with 48 chromosomes. After a first cross is effected, backcrossing t o domestic varieties is employed. Acceptable progeny may be obtained by further backcrossing satisfactory backcrosses to domestic varieties. Hawkes (1945) described Solanunz demissuni as valuable because of its resistance to late blight, Colorado beetles and frost rand because of its fertility when selfed and crossed with the tetraploids S. andigenum and S. tube-

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rosum. S. andigenum, the most promising cultivated species, contributes high vigor, yield, protein, vitamin C and starch content together with a greater range of flavor and cooking qualities than that of the ordinary European domestic potato. It also is highly fertile. Potato scab, Streptomyces scabies (Thax) , GUSSOW, Waksman and Henrici, is one of the diseases that has been only partially controlled by any of the methods that have been used. This disease usually does not occur in soils which are sufficiently acid. There are many areas, however, in which scab is one of the principal limiting factors in potato production and where scab-resistant varieties would make potato production more practicable and profitable. As a result of the National Potato Improvement program workers in New York have cooperated with those of the United States Department of Agriculture in breeding and testing scab resistant seedlings (Blodgett and Stevenson, 1946). Three scab-resistant varieties have arisen from this work, Ontario, Cayuga and Seneca. Ontario, the most promising of the three, is a selection from the cross Richter’s Jubel and U.S.D.A. seedling 44537. Cayuga and Seneca are from the cross Hindenburg, a German variety with good scab resistance, and Katahdin. I n cooperation with workers a t the Michigan Agricultural Experiment Station, the United States Department of Agriculture plant breeders have selected and named another scab resistant variety, the Menominee (Wheeler, e t al., 1944). The Menominee is from the same cross as the Ontario. In breeding potatoes for resistance to common scab the usual testing procedure has been to grow the entire population of seedling families in a scab-test plot heavily infested with Streptomyces scabies. Krantz and Eide (1948)found that crosses between parent.s of the same clonal resistance resulted in progenies which differed widely in the percentage of reFistant seedlings. Crosses between parents derived from the same resistant ancestor seemed to produce progeny of slightly lower resistance t.han where the two parents had different resistant ancestry. There are varieties available now which are highly resistant to one or more of the following diseases and insects: mild mosaic, rugose mosaic, latent mosaic, net necrosis as a result of leafroll virus, yellow dwarf, late blight, common scab, ring rot, potato wart, leafhopper, flea beetle and aphid injury. 111. CHEMICAL WEEDCONTROL The control of weeds in potatoes with chemicals is perhaps the most vital topic in potato production today. The potentialities of this phase of potato growing are tremendous arid it has promise of being the biggest labor saving practice since mechanized potato growing arrived. Weeds

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take a heavy toll annually in reducing yields of potatoes although much time and expense are involved in cultural practices devoted to the control of weeds. Conventional methods of soil stirring for weed control injure and destroy some potato roots. Control of weeds with chemicals will make it possible to avoid this root injury. The idea of weed control in potatoes with chemicals is extremely new and this work is still in the experimental stage. Some of the chemicals adapted to this purpose are classified as dinitro compounds, such as Dow Contact Herbicide and Sinox General, certain petroleum products, pentachlorophenol and its sodium salt, several salts of trichloroacetate and growth regulators, such as 2,4-D. Weeds may be killed by application of contact herbicides which are nonselective, killing the portions of the weeds with which they come in contact. Selective herbicides kill some types of plants, leaving others uninjured. Soil application of chemicals appears to have the greatest potentialities in the control of weeds in potatoes, although this may be supplemented with subsequent application of selective weed killers. During 1946 in New York it was shown that potatoes could be grown successfully without any cultivation whatever. No tool except the sprayer for insect and disease control touched the land between planting and harvesting operations. 2,4-dichlorophenoxyacetic acid (2,4-D) was applied a t rather high rates two weeks after planting but before potato emergence (Smith, Meadows and Marshall, 1949). This method of treatment for control of weeds has been designated “pre-emergence application.” I n 1947 further experiments on a more extensive scale were made. Applications of about 20 chemicals, singly and in combination, a t several concentrations, were made a t various intervals after planting. Good control of grasses and broadleaved weeds was obtained with no further stirring of the soil. Yields of these plots were equivalent to those receiving the accepted cultivation procedures. Some of these chemicals were 2,4-D, Sinox General, Dow Contact Herbicide and several petroleum products such as Aromatics HB and Heavy Aromatic Naphtha No. 132. I n 1948, Smith, Meadows and Marshall found several additional chemicals which, when applied a t “pre-emergence” resulted in excellent weed control with no injury to potatoes. Some of these are sodium pentachlorophenate and several petroleum products applied either as 30 per cent emulsions a t the rate of 100 gallons to t,he acre or undiluted a t the rate of 30 gallons to the acre. Ammonium and sodium trichloroacetate were excellent for controlling weeds and grasses but resulted in reduction of potato yields, Assuming that chemical control of weeds will be as widely practiced in the near future as application of fertilizers are at present, it is necessary to look into the future to determine what this may do to other

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potato cultural practices. The main reason for spacing rows of potatoes 32 to 36 inches apart. is to furnish space for the cultivator in weed control. Assuming that weeds will be controlled by chemical sprays it no longer will be necessary to waste this space. Smith, Marshall and Meadows (1949) conducted experiments where potatoes were planted 11 x 10 inches over the entire area except for passage of sprayer wheels. This area could be planted also for airplane or helicopter application of insecticides and fungicides. Weeds were controlled by pre-emergence application of 2,4-D instead of by hand or tractor cultivation. Of course, by this close spacing, the number of plants per acre are multiplied by three, hence more nutrients are needed to grow the crop. Consequently fertilizers were applied at rates as high as 5000 lbs. of 5-10-15 per acre. Since moisture supply may be the limiting factor, irrigation also was provided. The use of blight-resistant varicties aids in the control of late blight in the close planted areas. Yields of No. 1 size tubers were not influenced to a significant degree by any of the treatments. Number 2 size potato yields were affected to a highly significant degree, primarily by spacing. Significant differences in yield of No. 2 tubers also occurred between 2,4-D treated plots and wheel-hoed plots when both were planted at normal distances. 2,4-D treated plots with close spacing a t low fertilizer rate had more No. 2’s than any other closely spaced plots regardless of weed control method or rate of fertilizer application. Lowest yields of No. 2’s were in plots wheel-hoed and spaced normally. Highly significant differences in total yield were obtained by several of the combinations of treatments. Lowest yields resulted from normal spacing, low rate of fertilizer application and 2,4-D application. Highest yields, 936 bushels to the acre, occurred in closely spaced, wheel-hoed plots with high rate of application of fertilizer. There were indications that 2,4-D applications reduced total yields and perhaps yields of U.S. 1 size as compared with wheel-hoeing or tractor cultivation. Yields of U.S.1 size were practically the same a t the two spacing distances, but total yields and yields of No, 2 size were much higher on closely spaced plots. Slight but consistent increases of U.S. 1 size and total yield occurred at, the higher rate of application of fertilizer. Growers of table stock potatoes are interested in methods of growing potatoes which might result in changes in mealiness of cooked potatoes as measured by the specific gravity metihod. Seed stock growers desire some method of growing potatoes which will reduce the average size of tubers since these can be more economically prepared for planting and are in greater demand than the larger tubers. Application of 2,4-D did not influence the specific gravity of tubers. In every comparison, high

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rat.e of fertilizer application resulted in lowered specific gravity although significant in only one case. There were highly significant differences in the sizes of U. S. 1 tubers as indicat,ed by weight per tuber and number per bushel. Spacing greatly affected tuber size; the smaller tubers occurred a t the close spacing. Neither application of 2,4-D nor rate of fertilizer applicat,ion influenced the weight or size of tubers.

IV. FERTILIZER PRACTICES Fertilization of the potato crop varies considerably from state t o state and within states especially with respect to the analyses of fert,ilizers used and their rates of application. These practices usually are based on experimental evidence and vary between areas because of differences in soil type, native fertility of the soils, rotations used, soil reaction, rainfall and temperatures, varieties used, and other factors. Results of a survey by the National Fertilizer Association (1946) on the use of fertilizer on 28,992 farms in 40 states and in Canada show a very wide range in the analyses and amounts applied. The least average amount of fertilizer used on potatoes in any one state was 449 lbs. per acre and the most was 2864 Ibs. In pounds of plant food the variation was from 24.8 lbs. N, 120.9 Ibs. Pz05 and 1.7 Ibs. KzO per acre in the state using the least to 149.5 Ibs. N, 273.8 lhs. Paor, and 261.2 lbs. K 2 0 per acre in the state with the highest application. Cordner '(1943) working in Oklahoma on a light sandy soil found that 600 lbs. of 4-8-4 per acre is about the right amount of fertilizer for potatoes. Production was not increased by the application of more than 24 lbs. KzO to t,he acre. Houghland and Parker (1948) working on the Eastern shore of Virginia with several row and seed spacings and fertilizer rates found that yields of tubers were highest a t the 2000-lb. fertilizer rate. Results from year to year were extremely variable. Chucka et al. (1942) found that approximately a 1-2-2 ratio of nitrogen, phosphoric acid and potash produced the highest yields as an average on three widely separated farms in Maine. Under fair cultural methods on relatively old land, 1000 Ibs. of a 4-10-4 fertilizer proved most practical on the Cumberland Plateau of Tennessee (Bird, 1942). Under adequate insect and disease control, 2000 lbs. per acre produced economical returns. Bushnell (1942) st.ates that with early potatoes on sandy loam soil in southern Ohio a 6-8-8 fertilizer a t the rate of 1000 lbs. per acre proved ample for yields of 300 bushels per acre. A somewhat heavier rate of application is suggested if a higher yield is anticipated. B. A. Brown (1944) found that under continuous culture for six years on Merrimac fine sandy soil in Connecticut, maximum yields of potatoes were obtained from annual applications of 100 lbs. N, 80 lbs. PzOsand 120 lbs. KzO per acre.

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Addition of carriers of calcium, magnesium, sodium, manganese, zinc, boron and chlorine to a fertilizer containing only N, P, and K did not increase the yields. Hibbard (1943) states that for Missouri conditions, fertilizer mixtures analyzing 4 to 5 per cent nitrogen, 10 to 16 per cent phosphorus and 4 to 6 per cent potash are most generally advised. If liberal quantities of manure are used, a material carrying only nitrogen and phosphorus is satisfactory. An application of 300 to 600 lbs. with an analysis such as 4-12-4 or 4-16-4 can be used economically. Smith (1944) recommended the use of 1200 to 2200 Ibs. to the acre of such analyses as 5-10-5, 5-10-10, 4-8-12 or 3-9-15 in New York. The amount and analyses depended upon soil type, location, rotation and use of cover crops and manure. Since that time most rates have been increased. B. E. Brown (1943) presented data to show that potato yields in 171 experiments in seven eastern states averaged 73 bushels to the acre more in fertilized than in unfertilized soil. Bennett (1942) stated that responses to fertilizers in Idaho have been variable, some soils responding favorably and others giving no results. All experiments so far in the potato-growing areas of that state have given no response from the addition of potash. There is some indication that small applications of nitrogen and phosphorus will increase yields. Chucka et aZ. (1943) demonstrated the importance of crop rotation and of adding organic matter in addition to chemical fertilisers. The effect of varying amounts of nitrogen, phosphoric acid and potash in potato fertilizers was studied over a period of 15 years. As compared with the yield produced by a 4-8-7 standard mixture, leaving out nitrogen, reduced the yield by 84 bushels, and leaving out potash, reduced . the yield 248 bushels, Increasing the amounts of nitrogen, phosphoric acid and potash over the amounts contained in a 4-8-7 mixture resulted in a small increase in yield from additions of nitrogen, no increase from additional phosphoric acid, and a rather large increase from additional potash. Of the minor elements thus far only magnesium deficiency has been observed on the plots receiving synthetic salt mixtures. I n a comparison of acid and neutral fertilizers it was found that acid fertilizers produced as high yields as the same mixtures made neutral by the addition of either high calcium or dolomitic limestone. Teakle et aE. (1942) report that the most profitable potato fertilizer per acre for the growing conditions in Western Australia is a mixture supplying 200 lbs. of ammonium sulfate, 800 lbs. of 22 per cent superphosphate and 100 lbs. of potassium sulfate. Teakle and Morgan (1943) found that on virgin peat soil 800 lbs. of superphosphate was adequate for potatoes; on old potato land 400 lbs. gave optimal yields, while on land that had been under pasture, 1600 lbs. were found necessary for the

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best yields. From 100 to 200 lbs. of ammonium sulfate gave improved yields, I n only one case out of three was potassium sulfate beneficial. Carolus (1944) conducted experiments on a sassafras sandy loam on the Eastern Shore of Virginia, the results of which indicated that after dry seasons in which much of the applied nitrogen is not utilized by t,he plant but is prevented from leaching by green manure cropping, a reduction in the amount of nitrogen applied for potatoes is highly desirable. On acid soils large quantities of phosphorus in potato fertilizers are apparently necessary, regardless of previous weather conditions, until the phosphorus-fixing capacity of the soil is adequately satisfied. Under these conditions calcium does not appear to be an important factor in the growth and yield of potatoes. Davies and Fagan (1944) in England found that where fertilizers are applied to acidic bracken areas in the absence of ground limestone, they have little, if any, effect on the yield of the tubers unless calcium is also a constituent, but mixtures supplying the plant with calcium have a very appreciable effect. Phosphoric acid has the greatest effect on yield in the presence of ground limestone. Nitrogen and potash effect a further improvement in yield where adequate quantities of limestone and phosphoric acid have been included. Lyons, Russel and Rhoades (1944) obhained increased yields of potatoes when superphosphate was applied to calcareous soils. After a period of cropping where little or no manure was used, superphosphate also was beneficial when applied a t 100 to 150 lbs. per acre; on noncalcareous soils, from 70 to 100 lbs. per acre. A comparatively few tests showed that western Nebraska soils were not deficient in potassium. Addit.ions of magnesium, manganese, iron, copper, zinc and boron were not necessary. Terman and Hawkins (1947) state that considerably more phosphorus and potash fertilizer is commonly used in Aroostook County, Maine, for potatoes than is necessary for efficient production. Fertilizer applications range as high as 160 lbs. N, 320 lbs. P205,and 320 lbs. K 2 0 per acre. Tests show that 160 to 299 lbs. of PZOSand K20 are adequate on practically all the soils. A 2-3-3 ratio appears to be a more satisfactory ratio on most soils than any now mixed for the potato farmer. Applications of 120 lbs. N and 180 lbs. P 2 0 5 and KzO per acre are indicated. Smith and Kelly (1946) obtained increases of over 50 bushels per acre in yields of U. S. 1 size potatoes from applications of 2400 lbs. 5-10-10 per acre compared with 1200-lb. applications. Other factors such as source of potash and method of placement of fertilizer greatly influenced the yields obtained from any one rate of application. The correlation of nitrogen content of potato petioles with yields of pot,atoes throughout the season was highIy significant. One of the largest single factors causing increased yields from 2400-lb. per acre applications over

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t.he 1200-lb. rate was the additional available nitrogen in the higher rate. Manurial and other factors that induce deficiency symptoms in potato plants relating to potassium, phosphate, and calcium, respectively, were observed by Cowie (1942) in field experiments. Leaf scorch and the other potash-deficiency symptoms on the aerial part of the plant are normally induced by nitrogen-phosphorus treatment and not by nitrogen treatment. The presence of leaf scorch on nitrogen plots is correlated with a high level of available phosphate in the soil. B y the increase of nitrogen in nitrogen-phosphorus plots the potassium deficiency symptoms of the plants were intensified. An interaction between nitrogen and phosphat,e is the primary factor inducing potash-deficiency symptoms in the aerial part of the plants. Phosphate deficiency is induced by nitrogen and especially by nitrogen-potash treatments. Phosphate-deficiency symptoms are produced on nitrogen plants by low phosphates and low potassium in the soil. Calcium deficiency symptoms appeared on poor sandy soils with a p H of 4.5-5.0. Some increases in yield of potatoes may be secured with the use of starter solutions, but the increase is too small t o warrant the extra expense of using these materials. Although it has been suggested by some that radioactive materials applied as fertilizers increase the yields of potatoes, results from field experiments’in 1948 indicate that no benefits were derived from the application of radioactive materials in the fertilizer for potatoes.

V. ROTATIONS AND GREENMANURES

It has been shown repeatedly that pot,atoes grow better in soils relatively high in organic matter than in soils of low organic matter content. I n some instances increased yields obtained from applications of farm manures are due more to the organic matter content than to their content of nutrients. Rotations in which potatoes follow a green manure crop or one contributing plant residues usually result in increased potato yields, Greatest response to green manures in potato yields usually is obtained in soils relatively low in organic matter content. Ware (1943) conducted experiments in Alabama on the value of organic matter and irrigation in the production of potatoes. The addit,ion of Lespedeza sericea and the plowing under of vetch and cowpeas markedly increased yields above those from plots receiving no organic matter. With the addition of irrigation water, yields from t.he organicmatter plots were further increased. Winter barley, seeded in August, proved to be a very satisfactory cover crop in Southern Ohio (Bushnell, 1942). Bird (1942) mentioned t,hat the most common potato rotation practiced by Plateau growers in Tennessee consists of potatoes, corn, and

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Lespedeza 2 years. Crimson clover as a cover crop has proved well suited to the potato rotation. Chucka and Harrington (1942) compared the yields of eight different green-manure crops for potatoes. Sunflowers yielded the largest amount of organic matter, followed in order by corn, millet, buckwheat, Sudan grass, crimson clover, oats and soybeans. In another experiment corn produced about twice as much organic matter as crimson clover. Pot.ato yields following the crimson clover, however, were somewhat higher than those following corn. Yields also were higher where all the fertilizer was applied to potatoes and none to the greenmanure crop, than they were where part of the fertilizer was applied to the green-manure crop and the remainder to the potato crop. Potatoes following potatoes yielded lower than potatoes following either of the green-manure crops. B. A. Brown (1943) obtained an average of 30 bushels per acre increase in the first season of potatoes following a greenmanure crop. In the second season following green manures, there were no increases over potatoes grown every year. The greatest differences between rotated and nonrotated potatoes have been obtained in the driest seasons. Redtop and crimson clover have been followed by high yields of potatoes, whereas soybeans and millet have resulted in lower yields than redtop and crimson clover. Smith and McCubbin (1940) obtained highest yields in two sections of New York State in rotfitions in which potatoes were grown every year and which received 12 tons of manure to the acre annually, in addition to 1000-1200 Ibs. of 5-10-5 fertilizer. The highest yielding 2-year rotation was one in which potatoes followed a l-year sod of clover and timothy grown for green manure. Lowest yields were obtained in the rotation in which potatoes followed dry shell beans or soybeans for hay in a 2-year rotation. I n another section of the state, no significant differences in yields were obtained between any of the rotations during the first 2 years of the experiment. During the third year highest yielding rotations were of 2 years’ durat,ion, potatoes following corn for green manure and potatoes following dry shell beans. Manure applications in this section did not significantly increase yields over similar rotations to which manure was not applied. Extremely low yields were obtained in the Northern New York plots when potatoes were grown every year in the same Soil without commercial fert,ilizer, manure or cover crop. In the Central-Western New York plots such rotations during the first two years yielded as well as any other plot,s. Prince et al. (1940) conducted two 3-year rotation experiments of potatoes, oats and mixed hay and potatoes, oats and clover with all of the fertilizer applied to the potato crop. These rotations failed to maintain the organic matter level which existed a t the beginning of the experiment. High-phosphorus treatment

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appeared to reduce the organic carbon loss. Omission of phosphorus resulted in a loss of organic carbon more than twice that from the highphosphorus treatment. Liming had a favorable effect on the organic carbon level. Bcrkner e t al. (1939) stated that under economic conditions prevailing at that tiiiic in Germany, growing any crop only to be plowed under for green manure was not to be encouraged. A nonhitter variety of lupine was equal to the old varieties in all respects; when fed and t.he manure returned to the soil, its net value was greater than if plowed under. Crimson clover was even more valuable as it was a good hay crop and the roots remaining in the soil contributed about four times as much organic matter and nitrogen as did lupine roots and stubble. Potatoes did especially well after crimson clover. I n irrigated portions of western Nebraska highest yields and best quality tubers are produced in rotations with potatoes following sweet clover or alfalfa and planted only once in six or seven years on the same land (Werner, 1947). Yields are lower when potatoes follow sweet clover or alfalfa in a 4-year rotation and in 2- or 3-year rotat.ions yields of potatoes are very low. Application of manure in any irrigated rotation increased the yield of potatoes. A rotation system is less essential on dry land than in irrigated areas according to Werner; the important factor is moisture conservation. A 5-year rotation of barley, winter wheat 2 years, fallow and potatoes usually is satisfactory.

VI. RESPONSE TO NITROGEN FERTILIZATION The potato plant responds markedly to available nitrogen in the soil. Potato plants producing 400 to 500 bushels per acre contain 120 to 160 lhs. nitrogen per acre. Depending upon the native fertility of the soils, soil moisture and other factors potatoes receive nitrogen applications in commercial fertilizers ranging from none in certain portions of the west to as much as 160 lbs. nitrogen per acre in the northeast. Much controversial evidence exists as to the relative advantages of inorganic and organic sources of nitrogen. Some of this will be presented here. Cox and Odland (1940) grew potatoes for 2 years on plots which had received nitrogen for more than 40 years from either nitrate of soda or animonium sulfate. All yields were considerably better following ammonium sulfate on the well-limed plots but only small differences between t.he two nitrogen carriers were noticeable on the plots without lime. With sulfate of ammonia there were marked increases in yield by the use of lime, whereas the yields after nitrate were as great, if not somewhat greater, on the unlimed plot. B. E. Brown (1944) states that in 27 field tests on six soil types in

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Maine, New York, Pennsylvania, and Virginia a slightly but not significantly higher average yield of potatoes was obtained with ammonium nitrate than with either ammonium chloride, ammonium sulfate, sodium nitrate or urea. Smith and Kelly (1946) obtained no significant differences in yields when four-fifths of the nitrogen was from ammonium sulfate, one-fifth from uramon compared with four-fifths from ammonium sulfate and one-fifth from ammonium nitrate. Daines and Martin (1940) compared sources of nitrogen in a 4-8-7 fertilizer. They found that nitrogen supplied from a combination of organic and inorganic forms, together with the treatment in which the nitrogen used consisted of two units each of sodium nitrate and ammonium sulfate, produced the highest yields. There was no correlation between yields and acidity or basicity of the fertilizer used except where sodium nitrate supplied all the nitrogen, in which case the alkaline mixture outyielded the acid combination in three of four tests. The continued use of nitrate of soda as the sole source of nitrogen resulted in a higher percentage of scabbed potatoes. Allison (1940) states that about 50 per cent of the nitrogen in a mixed fertilizer for potatoes may be derived from urea instead of from natural organic sources without appreciably affecting the yield when t,he remainder is supplied by a combination of nitrate of soda and sulfate of ammonia. The use of natural organic materials as 20 per cent of the nitrogen source when the balance of this element is supplied by a mixture of equal parts of urea and sulfate of ammonia did not affect the yield of tubers as compared with either the check treatment or the all-urea mixtures.

VII. RESPONSE TO PHOSPHORUS FERTILIZATION Response of potatoes to phosphorus applications varies tremendously between potato growing areas. These differences are due largely to the native supply of available phosphorus in some of the areas in the middle and far west compared with the east and south and also to the large accumulated supplies of phosphorus in some of the eastern soils as result of large annual applications of high quantities of phosphorus. Very little difference exists between the values of various readily available sources of phosphorus for potatoes; slowly available sources are of less value. As shown by field experiments by Jacob and Armiger (1944) in a number of states, superphosphate made with either clear sulfuric acid or spent acid from the manufacture of high-octane gasoline by the alkylation process, had similar effects on the growth of potatoes, Brown and Hawkins (1944) report that superphosphate is a more efficient source of

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phosphorus for potatoes in Aroostook County than either colloidal phosphate or Tennessee raw rock phosphate. The yields produced by the two latter materials were sometimes no greater than those obtained in the no-phosphorus experiments. The investigation affords definite proof of the need of a readily available source of P205in potato fertilizers for Maine. White-Stevens (1942) compared six sources of phosphorus in the production of potatoes on Long Island in applications of 2,000 Ibs. 5-10-5 fertilizer to the acre. The following five sources were equally efficient in providing adequate phosphoric acid: superphosphate, nitrophoska, soft phosphate, potassium metaphosphate, and monocalcium chlorophosphate. Ammophos, on these low reaction soils, was not as suitable as the others. The results of Chucka et al. (1942) indicate that more phosphoric acid is being used in potato fertilizers in Maine than is necessary t o obtain maximum yields. Bear and Toth (1942) pointed out that the continuous use of large amounts of soluble phosphates on a soil results in marked changes in the physicochemical properties of that soil. Among these changes are an increase in exchange capacity and a lowering of the p H a t which iron and aluminum come into solution. Any increase in exchange capacity results in a greater retention of the nutrient cations which will have been applied in combination or in association with the phosphate. These facts help explain why it is possible to continue t o produce high yields of potatoes on soils which for the control of scab must be maintained a t pH values approximating 5.2. Bushnell (1943) found no differences in yields of potatoes in 1941 on Chenango sandy loam soil in Ohio from applications of 1500 Ibs. per acre of 6-8-8, 6-4-8 and 6-0-8 fertilizers. He suggested that where the test for available phosphorus shows more than 200 Ibs. per acre a field trial should bc conducted comparing standard fertilizers with one containing 40 lbs. PzOa per acre. If the field trial confirms the chemical test, Bushnell recommends 1000 Ibs. 8-4-12 t o the acre for potatoes in Ohio. VIII. RESPONSETO POTASSIUM FERTILIZATION Potash fertilization for potatoes in the United States varies from none in Nebraska and other western areas to as much as 450 lbs. K 2 0 per acre in some portions of the northeast. Considerable evidence is advanced indicating that the chlorine in muriate of potash has a slight depressing effect on starch content and in some instances also on yields compared with sulfate of potash. Brickley (1943) found there was a close correlation between very low potassium availability in the soil, as determined by chemical methods and the incidence of browning of potato foliage. Excess nitrogen in the soil was more potent tllan excess phosphorus in inducing potash-deficiency

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symptoms in the foliage. Development of the deficiency was favored also by applications of phosphorus to soils of very low potassium content but high in nitrogen, Browning occured earlier, with consequent reduction in yield, when the potassium fertilizer used in small quantities was broadcast on the soil and harrowed-in before drilling than when it, was applied in localized strips near the potato in the drill. Normal potato foliage contained 1.2 per cent K20and abnormal foliage only about 0.50 per cent. When the levels of exchangeable soil potassium were above 220, potatoes did not respond further to additions of potash to soils in Tennessee (Winters, 1946). Yields of potatoes were influenced by the source of potash in the fertilizer (Smith and Kelly, 1946). Plots with one-half of the potash from KC1 and one-half from sulfate of potashmagnesia consistently yielded higher than either those treated with onehalf KC1, one-half KzS04or all from KCl. Plants receiving all the potash in the form of KC1 matured earlier than those containing a portion of the KzO as sulfate of potash-magnesia. Asdonk and Jacob (1940) in a survey of 3678 experiments found an average increase of 19.2 kg. of tubers or 3.5 kg. of starch per 1 kg. of KzO. Additions of 80 kg. of KzO gave an average increase in yield of 1730 kg. of tubers; fertilizing with 120 kg. of KzO gave an increase of 2130 kg., 160 kg. of K 2 0 gave an average increase of 2800 kg. of tubers, Of the various potassium forms Patenkali and sulfate of potash proved preferable to 40 and 50 per cent KzO fertilizers as far as tuber yields were concerned and still more as far as starch content was concerned. The latter fertilizers produced an average of 1.9 kg. of starch per 1 kg. of KzO, whereas sulfate of potash produced 4.2 kg. No correlation could be found between the potassium content of the soil and the increased yield. Manure does not appreciably reduce the effect of chlorine on the dry matter of tubers (Cowie, 1943). The percentage of starch in the dry matter is depressed by chloride-containing potash fertilizers. Under certain conditions kainite leads to the production of potatoes with a soapy texture and a tendency to blacken after boiling. Results of Chucka et al. (1942) in Maine show that, fertilizers with 10 per cent potash produced slightly higher yields than did fertilizers with 8 per cent potash. Raising the potash above 10 per cent did not increase potato yields. Fertilizers with sulfate of potash and with combination sources of potash produced somewhat higher yields than did fertilizers with muriate of potash as the only source of potash. Thun (1942) noted that on neutral soils the chloride-containing fertilizers and on acid soils potash-magnesia gave better tuber yields. I n reference to the starch yield the sulfate-containing fertilizers show the best effect. On the neutral soils sulfate of potash and

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on acid soils potash-magnesia, are superior. Nemec (1940) found that marked reduction in the starch content of potatoes fertilized with kainite and 40 per cent potash fertilizers appear with heavy precipitation. The decrease is heightened by high chloride content. and a strongly acid reaction of the soil. Fertilizing with chlorine-free potash fertilizers generally increases the starch content.

IX. EFFECTS OF MAGNESIUM, LIMINGAND SOILREACTION Many soils in which potatoes are grown in the northeast and along the Atlantic Seaboard are deficient in available magnesium for optimum growth of potatoes. These soils predominantly are highly acid in reaction. Additions of magnesium in available form result in yield increases. Some of these acid soils do not need additional magnesium b u t do respond to additions of calcium in the form of limestone or hydrated lime. Thowands of acres of good potato soils of this type exist especially in eastern areas of the country. Many potato growers refuse to make lime applications even on highly acid soils for fear of aggravating the common scab problem on potatoes. On light, strongly acid soils Selke (1942) found that dolomitic gray limestone (16.34per cent MgO) was superior to white limestone when used on potatoes. On these soils the dolomitic limestone is suited for the rapid and lasting correction of niagnesium deficiency. Favorable results obtained with concurrent applications of MgS04. 7Hz0and limestone indicated that the better action of the dolomitic limestone was not due solely to its effect on soil acidity but also to its nutrient action. Favorable results on potatoes with dolomitic limestone are assumed to be due to the effect of the siliceous residue of this material. Limestone has less effect on soil reaction during the first year than in the next, twcJ years. Limestone applications, however, sharply diminished the exchange acidity during the first year, Magnesium sulfate had no significant effect on tlie soil acidity but it did appear, indirectly, to affect the neutralizing action of the limestones. Calcined lime arid calcined dolomitic lime on a slightly acid, heavy-weathered soil, gave about tlie same results. Compared with limestone, calcined limestone had the greater effect on the soil acidity the first year. Collins and Skinner (1942)state that fertilizers containing dolomitic limestone formulated to be nonacid-forming, gave significantly higher yields of potatoes than did acid-forming fertilizers on coastal plain soils in North Carolina. The nonacid-forming fertilizer maintained soil pH a t approximately the original level, increased the total magnesium in the plant and the available magnesium in the soil of the root zone, and increased the total calcium in the plant with no appreciable change in the

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ultimate available calcium content of the soil or the potash content of the plant, which indicates no appreciable liberation or fixation of the soil and fertilizer potassium. Berger (1948) states that when soils are below p H 5.0 finely ground dolomitic limestone should be applied to add available calcium and magnesium to the soil and to reduce the amounts of soluble manganese present in some Wisconsin soils. Additional soluble magnesium in the fert,ilizer often is advisable. Smith and Kelly (1946)found a highly significant correlation between magnesium content of petioles and yields of potatoes. Where sulfate of potash-magnesia was contained in the fertilizer, the content of magnesium in the petioles was highest and the yields were largest. That the increased yields were due to the magnesium and not to the sulfate form of potash in the sulfate of potash-magnesia was shown by the consistently higher content of magnesium in the petioles and the higher yields of potatoes from sulfate of potash-magnesia than from sulfate of potash. Application of sulfate of potash-magnesia supplying 52.5 Ibs. MgO per acre was sufficient for optimum absorption of magnesium by the plant and for maximum yields of potatoes. Wallace et al. (1941) found that the incidence of magnesium deficiency in potatoes in relation to t,he fertilizer treatments was correlated with the magnesium content of haulm samples from the respective treatment.s. Corum (1941) presented data on the effect of p H on the initiation of growth of potato tubers. A bimodal curve of growth was obtained. For the sprout,ing of the potato tubers, the minimum in the curve is a t pH 7.0. This effect of p H on the initiation of growth is considered to be directly related to the effect of H-ion concentration on the hydration of the biocolloids of the protoplasm of the plant tissues concerned. The problem of maintaining soils a t the optimum reaction for highest yields of potatoes without making conditions favorable for scab infection is of great importance to potato growers. The fact that tuber development occurs in a limited portion of the soil suggests the differential lime treatment of the tuber zone and the deeper zone of root development as a possible solution to the problem. Nelson and Brady (1943) grew potatoes in the greenhouse under three liming treatments: (1) dolomitic limestone mixed in the surface ten inches of soil a t the rate of 2.5 tons per acre, (2) the same amount of lime placed ten inches below the surface of the soil, and (3) no lime. Subsurface application of limestone resulted in higher yields than either of the other treatments. The uptake of calcium and potassium was greater in the subsurface lime treatment than in the other treatments. Scab infection was increased markedly when lime was mixed in the surface soil. Subsurface applications did not increase scab infection.

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Albrecht and Schroeder (1942) found greater mobilization of the nutrients into the crop when the soil was acid than when it was neutral. Schroeder and Albrecht (1942) show that tops of potato plants contain large quantities of calcium whereas potassium is high in the tubers. Importance is attached to the relation of calcium to potassium while other nutrients are held constant. Soil acidity as a means of reducing the incidence of potato scab is apparently effective because of the increased mobilization of certain cationic plant nutrients by the presence of the H ion. Cook and Nugent (1942) concluded from experiments conducted in Virginia that the amount of scab is closely correlated with soil reaction, and is only indirectly correlated with the fertilizer reaction or calcium content of the fertilizer to the extent that the fertilizer may change the soil reaction. They also concluded that calcium itself has no effect in the development of scab and that calcium compounds affect it only to the extent that they may change the soil reaction. Cook and Houghland (1942) stated that when fertilizers in which the potential acidity was, respectively, completely neutralized and one-third neutralized were used continuously for 7 years, they ultimately affected the occurrence and amount of potato scab, principally through their effect on the degree of acidity of the soil. The incidence of scab and the pH value of the soil were highest with the completely neutralized fertilizer.

X. MINORELEMENTS Most soils used in potato production apparently have a sufficient supply of available minor or trace elements and hence do not respond to additions of these elements. On a few soil types, however, and in some limited areas, a deficiency occurs and the potato plant responds t o applications of one or more minor elements. These responses may be in the form of increased yields or the prevention of certain abnormalities in leaf, stem or tuber. Teakle et aZ. (1941) found on neutral sandy swamp soils in western Australia that potatoes gave excellent responses fo copper sulfate used at the rate of 5-10 lbs. per acre. Potato yields on marly soils were increased by additions of manganese and further increased by copper additions, although copper without manganese had no beneficial effect. Ten pounds of manganese sulfate and 5 Ibs. of copper sulfate are recommended for potatoes on these soils. Zinc sulfate, magnesium sulfate and borax did not increase yields. Hawkins et al. (1941) found in Aroostook County, Maine, that comparatively small amounts of boron and nickel added to fertilizer may be toxic to potatoes. Manganese, iron and copper had little or no effect on potato yields. Neiiweiler (1941) reported im-

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proved quality of Flava and Ackersegen potatoes from boron treatment in Switzerland. Large applications of lime probably should be avoided as they tend to produce manganese and boron deficiency symptoms where these symptoms have not previously been observed (Keese, 1942). Chandler (1944) found that stem-end browning of potato tubers was produced in the greenhouse on low boron solutions. Applications of zinc sulfate, manganese sulfate, copper sulfate, and borax have had no measurable effect on potato yields on the Cumberland Plateau of Tennessee (Bird, 1942). No minor element deficiencies for potatoes have been detected in Connecticut (B. A. Brown, 1943). Dostal (1947) found that out of 12 minor elements in Hoagland’s A-Z solution, only boron and zinc considerably increased the growth of potato seedlings in solution cultures. Zinc stimulated the growth of the stems, and boron that of the roots and tubers. Katalymov (1946) found an increase in boron uptake of potatoes along with an increase in yield, varying from 53 to 106 g. per hectare. Potatoes were grown a t four locations by MacVicar et al. (1946) in soils with varied contents of soluble boron. No plant-deficiency symptoms were shown a t any of the places. The range of soluble boron was from 0.36 ppm in sandy loam to 2.10 ppm in peat. The ash and the boron contents of leaves and tubers of four varieties of potatoes showed no outstanding differences. The boron content of the leaves, and, in most cases, of the tubers increased progressively in the following order of soil type: Vilas sandy loam, peat, Antigo silt loam, Waukesha sandy loam. These variations may be correlated with soluble boron content only on soils similar in texture. Applications of borax in excess of 50 lbs. per acre depressed yields of tubers in Miami silt loam soil. The unfertilized plots produced leaf tissue with 41 ppm of boron and tuber tissue with 11.2 ppm. Where 150 lbs. per acre of borax was added the boron contents increased to 450 ppm in leaves and 121 ppm in tubers. The soils used in the various tests were all acid in reaction.

XI. TIMEAND METHODOF APPLICATION OF FERTILIZERS For a number of years the conventional method of applying fertilizers to potatoes through t.he planter has been in equal depth bands along both sides of the seed and a t a slightly greater depth. Results of experiments in several areas during recent years indicate that there are methods better than this. One of these is the split application in which approximately one-half of the quantity of fertilizer is broadcast before plowing, and the remainder is applied in the usual manner through the planter a t planting time. I n New York on soil with abundant organic matter in a well fer-

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tilized rotation and where the rate of fertilizer application has been high, this method has resulted in consistent increases. Gray (1944) presents evidence from results of experiments conducted in Pennsylvania indicating that applying a portion of the fertilizer in the plow furrow and the remainder in bands results in higher yields than where all is applied in bands. Smith et al. (1943) have reported the results of a study in which consideration was given to the amounts of fertilizer applied, methods of fertilizer placement, sizes of seed pieces planted, distance of spacing seed pieces in the row and the responses of two varieties to these treatments. With the equal-depth band application of 1600 lbs. 5-10-10 fertilizer at time of planting, the yields were no higher than with the 800 Ibs. application; in fact, with Sebago, it was somewhat lower. This indicates possible injury to the plants with the high rate of applicat,ion under the conditions of this experiment. This is further indicated by the much larger yields obtained at the 1600-lb. application from the high-low band method and the one-half broadcast-one-half band application than from the equaldepth band placement. A t the 800-lb. per acre rate of application, highest yields of Sebagos were obtained from equal-depth band application and lowest from the one-half broadcast-one-half band method; whereas, with the Rural, highest yields were obtained from the high-low band, and lowest from the broadcast-band method. With the Sebago there was indication of some possible injurious action of the 1600-lb. per acre application when all was applied in equal-depth bands. There is clearly some merit in the deeper application of commercial fertilizers for potatoes a t least on certain soil types and under certain environmental conditions. Heavier applications of fertilizer may be applied profitably when the fertilizer is placed deeper in the soil or farther from the seed-piece as compared with equal-depth band placement. More recent work substantiates earlier evidence that yields of potatoes are greatly influenced by method of placement of the fertilizer (Smith and Kelly, 1946). Placing one-half the fertilizer broadcast, then plowing, and the remainder in equal depth bands a t planting resulted in higher yields than either all in bands a t planting or one-half on the plow sole and the remainder in bands through the planter. Plants receiving all the fertilizer in bands a t planting matured earlier than those receiving split applications. Placement of all the fertilizer in bands is not as efficient for the uptake of nitrogen by the plant as when a portion of the fertilizer is broadcast before plowing or applied on the plow sole. Berger (1948) found in Wisconsin that application of 800 lbs. of 3-12-12 in the row increased the yield about 60 bushels per acre over the unfertilized. More than this in the row was not beneficial but an addi-

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tional broadcast application of 1200 lbs. 6-6-18 gave a further increase of 80 bushels. On the other hand, Chucka et al. (1942) applied 8-16-20 and 8-12-20 at rates of 1000 and 1500 lbs. per acre in equal depth bands compared with high-low bands, and found that all methods of placement yielded approximately the same.

XII. RELATION OF YIELDAND TUBEE COMPOSITION TO PLANT AND SOILANALYSES Considerable progress has been made in the use of plant analyses in diagnosing mineral deficiencies and in determining nutrient requirements. With some nutrients such as nitrogen there is a close relationship between the nitrogen content of cer'tain portions of the plant and the response of the plant to additions of this element to the soil or nutrient solution. With other nubrients such as phosphorus the relationship is not so definite. The relationship which exists is dependent on the native fertility of the soil and especially on the level of other available nutrients in the soil or culture solution. The application of fertilizers is reflected in the lower petioles of the plant by the methods of tissue analyses employed by Hill and Cannon (1948). Tissue analyses indicated that if plants grown on muck soil contain less than 3500 to 4000 ppm of potassium, yield will be depressed. From the lowest) level of phosphorus up to 70 ppm there was no relationship with yield but with levels above 70 ppm there was a negative relationship. The negative relationship held only if the potassium level was below 3000 ppm. With nitrogen up to 200 ppm there was probably a positive relationship with yield, while with nitrogen from 200 to 1000 ppm there was a negative relationship with yield. There was a significant negative relationship between the combined nit.rogen and phosphorus unit on potassium levels and the effects of the higher nitrogen or phosphorus levels on yield were largely conditioned by the potassium level. A negat,ive relationship existed between levels of potassium and magnesium and between pot,assium and calcium. Low potassium in the tissue was associated with an accumulation of magnesium and calcium. Lorenz (1944) found that nitrogen was the predominating fertilizer element affecting yield of potatoes on sandy loam soils in Kern County, California. Phosphorus gave slight response but potassium had no effect. Analyses of soluble nutrients in the petiole tissues indicated that if the plant contained less than 800 ppm of nitrate approximately one month after emergence the yield would be decreased. Nitrate decreased rapidly as the plants matured. During the period of most rapid growth

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in the plants on plots without added phosphorus, soluble phosphorus was less than 70 ppm. Soluble potassium was never less than 10,000 ppm. The starch content decreased from 17 per cent in tubers produced without nitrogen to less than 13 per cent in tubers grown on plots receiving 210 lbs. nitrogen per acre. Throughout the season nitrogen was lower in all parts of the plants grown on the plots lacking nitrogen than in plants on plots receiving nitrogen, Phosphorus was low both in plants grown without nitrogen fertilizer and in those without phosphorus. OmiPsion of nitrogen from the fertilizer greatly reduced the absorption of phosphorus and the amount in all parts of the plant analyzed. Potassium was high in all samples and was unaffected by any of the fertilizer treatments. Calcium was highest in plants grown without nitrogen and lowest in plants grown without phosphorus. Lorenz (1947) showed later that the largest amount of growth and the greatest intensity of nutrient absorption occurred between about 75 and 110 days after planting or between 45 and 80 days after emergence. A yield of 658 bushels per acre absorbed 139 Ibs. of nitrogen, 36 of phosphoric acid and 253 of potash. The tubers alone removed 106 IbR. of nitrogen, 31 of phosphoric acid and 196 of potash. Hawkins (1942) reporting from Maine, states that potato plants capable of producing upwards of 400 bushels per acre, absorb about 120 to 160 lbs. of nitrogen, 25 to 30 lbs. of phosphoric acid, 200 to 250 lbs. of potash, 60 llp. of calcium oxide, 30 Ibs. of magnesium oxide and about 10 to 12 Ibs. of sulfur per acre. The Green Mountain variety absorbed about 70 per cent of its total consumption of plant nutrients between 50 and 80 days after planting. The earlier varieties, Cobbler and Chippewa, usually emerge and make more rapid growth early in tqheseason than do the later-maturing varieties, and the period of more rapid absorption of nutrients occurred with these two varieties about one week earlier than for the Green Mountain variety. The magnesium content of the tops and roots of the Cobbler variety was considerably less than that of the other varieties, but there was comparatively little difference between varieties in the magnesium content of the tubers. During the first 50 days after planting, the Green Mountain variety absorbed 9 per cent of the total major nutrient elements while making 3 per cent of the growth for the season. The later varieties, Green Mountain and Smooth Rural, each absorbed about 100 lbs. more total major nutrient elements during the season than did the earlier varibties, Cobbler and Chippewa. The later varieties absorbed considerably more nitrogen, potassium and calcium, and, as compared with the Cobbler variety, particularly more magnesium. The Cobbler variety absorbed 22 per cent of its nitrogen requirements by the 50th day as compared with 8 per cent for the Rural

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variety. By t,he 70th day, Cobblerv had absorbed 86 per cent of the nitrogen absorbed during the season BS compared to 52 per cent for the Rural and 56 per cent for the Green Mountain varieties. Hawkins (1946) later 'reported that the proportion of the nutrients absorbed that were translocated into the tubers was approximately as follows: 80 per cent of the Pz05,67 per cent of the N, 60 per cent of the S, 50 per cent of the KzO,40 per cent of the MgO, and about 5 per cent of the CaO. Regardless of rate of fertilizer application, 1200 and 2400 lbs. 5-10-10 to the acre, method of placement, source of nitrogen or source of potash, Smith and Kelly (1946) found no significant difference in phosphorus content of leaf petioles at any time during the growing season. Petioles of plants of all treatments contained large quantities of potassium and evidently sufficient potassium was taken up by all treatments to result in high yields. Correlation of nitrogen and magnesium content of petioles with yields of potatoes was highly significant at all treatments. Jones and Plant (1942) found, however, that deficiency symptoms of potassium and magnesium agreed with chemical analysis of the leaves. Soil analyses, especially variations of the so-called quick soil tests, have been made in attempts to establish relationship between such analyses and subsequent potato yields. These have been reasonably successful and show considerable promise 8s a tool for detecting deficiencies of soil nutrients. Sparks and McLean (1946), working with alkaline soils in Colorado, found a close correlation between the results obtained by soil analyses and yields following applications of nitrogen, phosphorus and potassium. Peech (1945) reported a marked accumulation of readily soluble phosphorus in all soils studied from the important potato-producing areas along the Atlantic Coast. This accumulation varied in different soils. I n general, the amount of readily soluble phosphorus increased with the increasing degree of saturation of the soil with phosphate. I n the light-textured soils containing large amounts of readily soluble phosphorus in the surface layer, there has been appreciable downward movement of phosphorus into the subsoil. Despite the low cation-exchange capacity and the low p H value, the exchangeable potassium content of many of these soils has been greatly increased by fertilization. This accumulation is relatively small as compared with the total amount of potassium applied over a period of years. The majority of the soils had p H values below 5. The amounts of exchangeable calcium and magnesium were very low. I n some areas the soils were extremely deficient in magnesium. The organic matter content was low and was quite variable in many of the soils even within the same series, I n some of the areas the organic matter content of the soils has been increased, whereas in other areas it has been decreased by cultivation.

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Hawkins (1945) found that in Maine and North Carolina the increase in yield of potatoes per pound of PzOa applied was usually lower a t locations where there were high amounts of residual phosphate in the soils. In Maine no significant increases in yield were obtained from additional potash above the 100 lb. base rate. Analyses of virgin and cultivated Northern Wisconsin soils show that when potatoes are grown on these soils from 10 to 30 years the available phosphorus and soluble manganese content and acidity are increased, while the available potassium, caIcium and magnesium contents are seriously depleted (Berger, 1948). Nelson and Hawkins (1947) made a study to show the relationship between the amounts of readily soluble phosphorus and exchangeable potassium in the soil and the response of potatoes to applications of these nutrients. Results from North Carolina shoved that applied phosphorus gave significant increases in yield a t all six test locations. Significant increases were obtained on 8 of the 9 experiments in Maine. The degree of yield response to applications of P z O ~ was related to the amount of readily soluble phosphorus in the soil. Yield increases from the first 80 lbs. of P205applied decreased as the amount of readily soluble phosphorus in the soil increased. The phosphorus content of the leaves in the North Carolina experiments was related to the amount of readily soluble phosphorus in the soil and to the amount of phosphorus applied. Phosphorus was particularly important in influencing the number of tubers per hill on soils low in readily soluble phosphorus. Significant increases in yield from applied KzO were obtained in all experiments in North Carolina and in 5 out of 8 experiments in Maine. The weight of potatoes resulting from the first 60 lbs. of KzO tended to decrease as the amount of exchangeable KzO in the soil increased. I n the North Carolina experiments the KzO content of the leaves was related to the amount of KzO in the soil, and to applied KzO up to 120 lbs. per acre. The amount of potassium extracted from the rachises of the potato plants in the Maine experiments was related to the exchangeable K20content of the soil and to the amount of K20 applied.

XIII. KILLINGPOTATO VINES During the past few years potato growers have shown growing interest in killing potato vines preparatory to harvest.. The use of chemicals as killing agents has increased steadily each year since the practice was started on an appreciable scale. This may be attributed in part to the increasing importance of varieties which remain veget.ative and green over a longer period of time and to the intensive spraying programs now practiced by good commercial growers which reduce insect and disease injury to a relative minimum. It usually is the desire of any grower

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to maintain strong, vigoroue top growth as long as feasible in order to obtain greater total photosynthesis, translocation and increase in tuber size. It may be desirable or necessary, however, to destroy the above ground vegetation for one or more of the following reasons: (1) to avoid the risk of having potatoes left in the ground after extreme weather conditions arrive, (2) to avoid oversize tubers, (3) to prevent late virus spread in fields certified for seed, (4) to prevent second growth of tubers, (5) to prevent late blight infection of tubers, (6) to enable harvesting in time to meet a favorable market, (7) to increase the proportion of tubers within the size limits for good seed, (8) to enable completion of harvest with the labor and equipment available, and (9) to destroy late weed growth along with the vines. When it is desirable to begin harvesting before natural death of the yines occurs, some means of destroying top growth is needed because: (1) commercial digging equipment will not move heavy vines with green leaves and succulent stems over the conveyor belt in a satisfactory manner, (2) tubers will stick or hang onto green vines thereby making picking extremely difficult, and (3) tubers will not mature until after the vines have died and will skin or bruise easily when in an immature state. Wilson and Boyd (1947) and Wilson e t al. (1947) found that certain tar-acid compounds provided the most efficient noncorrosive substitute for sulfuric acid which is the killing agent of most widely accepted usage in England. Sodium chlorate gave fair leaf kills, but poor stem kills. Calcium cyanamid dust provided inefficient kills even when used a t very heavy rates of application. Dinitrocresol derivatives were only moderately effective while copper sulfate and sodium chloride gave fairly efficient kills on senescent haulms only. Young haulms kill readily but become more resistant as the season advances and finally again susceptible during senescence. Varietal differences in susceptibility were attributed largely to differences in stage of maturity. Complete kill was rarely accomplished in less than 10 days. Comprehensive tests indicated no differences in flavor, texture, nr internal color of tuber attributable t n vine killing treatments. Bates and Martin (1935), MacDowell (1935), Main and Grainger (1947),Small (1935), and some British workers (Anonymous, 1945), have reported experimental results which indicate that destruction of vines by chemical means is very important in the control of tuber infection by late blight. Chemical destruction is preferable to cutting of the vines (Small, 1936). Bonde (1935) reported satisfactory kills with 6 to 7 per cent sulfuric acid, and later found Sinox with ammonium sulfate to be very effective as a vine killer in the control of late blight infection of tubers (Bonde and Schultz, 1945). Schultz et al. (1944) found that 2 gallons of Sinox

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plus 10 lbs. of ammonium sulfate per 100 gallons of water a t 135 gallons per acre could be used as a less tedious substitute for pulling the vines in early harvested seed plots in Maine. Such early harvesting materially reduced the spread of virus infection as compared to plants remaining green to a later date. Findlay and Sykes (1936, 1937) reported no significant effect of vine killing treatments on yield in 1935, but a reduction of yield in 1936. Kraus (1944) reported little, if any, reduction in yield from vine killing in 1943 and 1944, but stated that this would depend primarily on relative maturity. Samuel (1944) found that killing of the vines may reduce potato yields and states that even in September, crops with green tops can put on as much as half a ton of tubers per acre per week. He considered 10 days or more necessary between killing date and harvest date for satisfactory setting of the skins. Idaho Experiment Station workers (Anonymous, 1940) also concluded after microscopic examination of tubers dug a t frequent intervals that normal ripening changes occurred after killing of the vines and that potatoes were sufficiently matured in 10 days to prevent abnormal surface damage during harvesting. Steinbauer (1945) reported that sprays were more effective as vine killers than dusts under Maine conditions. Dilute sulfuric acid was the most rapid killing agent studied, but Dowspray 66 Improved and Sinox gave fairly satisfactory rates of kill. Varieties such as the Sebago were more difficult to kill than others. Plants making rapid growth with abundant soil moisture and moderately high temperatures were more readily killed than when they were toughened by low soil moisture or low temperature. A yellowish discoloration of the xylem vessels was reported in tubers from Sinox-killed vines. Greenhouse tests indicated that this did not impair planting behavior. Some tuber growth takes place after vines are sprayed with a slow acting vine killer and the killing date may be advanced a few days with such chemicals without large yield reductions. I n lQter work, Steinbauer (1947) reported that oil preparations were much more expensive than most other vine killers with ordinary sprayers and using 125 t o 150 gallons per acre. Products mch as Stoddard Solvent were fairly satisfactory as defoliants but not for complete kill of vines. Of the compounds studied those with a high content of aromatics were most effective. Otis (1946) stated that maturing yellow vines were more easily killed than green vigorously growing ones. Low humidity or low temperature a t time of application was unfavorable to both dusts and sprays. Varietal differences did not govern speed and completeness of kill as much as maturity and size of vine growth. Kunkel e t al. (1948) state that killing the vines prematurely resulted in lower specific gravity and increased stem end discoloration.

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Hoyman (1947) found in field experiments in North Dakota that discoloration in and near the vascular tissue of tubers was positively correlated with rapidity of kill and was less when applications were made later in the season. Such discoloration may or may not be a direct effect of vine killing chemicals. Tubers from vines cut a t ground level showed the same type of discoloration as those from chemically killed vines. The effectiveness of Dowspray 66 Improved was increased by the use of aluminum sulfate as an activator, Brentzel (1944)obtained unsatisfactory kills with dusting treatments in North Dakota, but a 95 to 99 per cent kill resulted from 2 gallons of Sinox plus 10 lbs. of ammonium sulfate in 100 gallons of water applied a t the rate of 173 gallons per acre. Fernow and Smith (1944) stated that experience of New York growers indicates that the vines die more rapidly when application is made during comparatively warm weather or is followed by fairly high temperatures. Ennis et al. (1946) found that 2,4,5-trichlorophenoxyaceticacid applied to the foliage of potato plants caused pronounced stunting and distortion of vegetative growth and scab-like injury on the tubers. Smith et al. (1947) found potatoes resistant to spray applications of 10 ppm of 2,4-D and 10,000ppm naphthaleneacetic acid methyl ester when applied during August. Injury similar to that reported by Ennis et al. occurred on many tubers from plants which had been sprayed with the latter material. Callbeck (1948) found that tubers from untreated cut vines showed a greater incidence of stem-end discolorat,ion than tubers from untreated plants or from plants destroyed by slow acting herbicides. Tuber vascular discoloration appears to be correlated with rapidity of kill of the tops. The amount and intensity of discoloration in tubers from plants killed a t different stages of development with dinitro ortho secondary butyl phenol increased quite regularly with the age of the plants; those killed late in the season exhibited the greatest injury. This is in accordance with the work of McGoldrick and Smith (1948)and of McGoldrick (1948). Hoyman (1947), in North Dakota, found less discoloration when the applications were made later in the season. Callbeck (1948)detected no differences in flavor or texture of the cooked tubers from the several treatments, McGoldrick and Smith (1948) and McGddrick (1948), however, found that those killing agents which destroyed top growth most efficiently and rapidly reduced specific gravity of the tubers compared to poorer kills and controls. Discoloration of the vascular region of tubers was decidedly increased by killing injury to top growth. Further results of comprehensive field and greenhouse experiments by McGoldrick (1948) showed that Penite 6 plus an activating compound resulted in the most efficient kill of vines. Dowspray 66 Improved,

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Penite 6 without activator, Sinox General plus diesel oil, and sizz flame treatment also gave satisfactory kills. 2,4-dichlorophenoxyacetic acid a t the rate of 6% lbs. plus 118 gallons diesel oil to the acre or 125 lbs. sodium nitrite to the acre resulted in satisfactory kills on relatively senescent plants at the, last killing date only. Fairly satisfactory vine destruction, especially of leaves, resulted from spray applications of Cyanamid x-1 and x-5. The following materials at concentrations named were worthless as vine killers: 2,4-D, 6% and 25 lbs. to the acre; methyl ester naphthaleneacetic acid, 21/2 gallons per acre, and a petroleum product a t 6% gallons to the acre. 2,4,5-trichlorophenoxyaceticacid as a spray a t the rate of 25 lbs. to the acre caused some leaves to turn yellow and die alt.hough there was no death of the plants. Increase in tuber size during late stages of maturity warrants delaying the application of vine killers as long as possible unless greater size is objectionable. Some discoloration of the vascular region was found in tubers from untreated plants as well as in those from treated plants. This browning of the vascular bundles, however, was decidedly increased by killing injury to top growth. Sprouting and shrinkage loss other than sprouting in subsequent storage was not significantly affected by killing agents. Early kill resulted in significantly greater loss than later kill. Neither killing agents nor application dates appeared to have a significant effect on the value of tubers as a source of seed the following season. More effective kills were obtained a t 100 per cent relative humidity than a t approximately 75 per cent. Drying and shriveling of killed tissue, however, was faster a t the lower humidity level. Artificial rainfall applied a t intervals up to 10 hours after killing treatment reduced the effectiveness of Sinox General and Dowspray 66 Improved. Recent experience with machines such as the “Roto-Beater,” which chop the vines to small pieces, indicates that this method of vine destruction will gain wide acceptance by growers.

XIV. RECENT DEVELOPMENTS IN INSECT CONTROL The potato crop is infest,ed with many insects which result in tremendous losses in yield and quality. Programs for combating them cost millions of dollars annually. The most important insects are aphids, leafhoppers, flea beetles, Colorado potato beetles and wireworms. Several of these insects were not satisfactorily controlled until after the general adoption of DDT as an insecticide for potatoes. This occurred mainly in 1945 and 1946. After an amazingly short period of testing, this one material has largely repIaced the various insecticides formerly recommended for potato insect,s throughout North America. Several other in-

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secticides introduced even more recently than DDT are especially lethal to aphids and appear very promising for their control in potato fields. I n many areas the potato leafhopper, Ernpoasca fabae Harris, probably is the worst pest of potatoes. Leafhopper injury results in premature death of vines and a resultant decrease in starch content, mealiness and yield. Until the advent of DDT, control of this insect was erratic and incomplete. Under such conditions it was extremely difficult to evaluate potato varieties for anything other than leafhopper resistance. It has been found repeatedly that almost perfect control can be obbained with DDT without injury to the plants (Granovsky, 1944; Apple and Arnold, 1945; Wilson and Sleesman, 1945; Heuberger and Stearns, 1946 ; Wolfenbarger and Heuberger, 1946 ; Wilson and Sleesman, 1947). Potato varieties vary widely in their susceptibility to hopper-burn (Sleesman and Bushnell, 1937; Allen and Rieman, 1939; Allen et al. 1940; Sleesman and Stevenson, 1941 ; Sleesman and Bushnell, 1945; Maughan, 1947) ranging from the very susceptible Bliss Triumph and Pontiac to resistant Sequoia. There is no agreement, however, as to why some varieties are more severely injured than others. It has been claimed that early varieties are more susceptible than later varieties but Allen et al. (1940) state that relative earliness or lateness is not the prime factor; early varieties such as Bliss Triumph and Warba were more attractive to the insects than the later varieties, Katahdin and Houma. The correlation between earliness and susceptibility may not be a true relationship as it is difficult to differentiate between natural maturity and that caused by leafhoppers (Sleesman and Stevenson, 1941). Allen and Rieman (1939) d a t e that leafhopper tolerance of Katahdin and Houma might account in part for the heat, and drought resistance attributed to these varieties. Varietal response to leafhopper control has been measured by adult and nymphal populations, severity of hopperburn and yield increases with but little emphasis on increase in starch content of the tubers. Sleesman and Wilson (1943) report a high negative correlation between nymphal populations and yield, between dead foliage and yield, and a high positive correlation between nymphs and dead foliage. Apple and Arnold (1945) found a highly significant correlation coefficient. of -0.87 between nymphal populations and specific gravity of the tubers. Linn et al. (1948) measured the effects of leafhopper control with D D T on length of season, quality and yield of seventeen new and old varieties. Control of leafhoppers increased the length of season most in the case of very early varieties. Yields of all varieties were increased except Erie and Sequoia in one location. Control of leafhoppers also increased the specific gravity of tubers of most varieties. Post et al. (1948) obtained significant increases in yields from spray

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applications of emulsions of DDD or DDT, and dust applications of 5 per cent DDT, and highly significant increases from 50 per cent powder D D T as a spray or 3 per cent DDT as a dust. Chemicals such as piperonyl butoxide, piperonyl cyclohexanone, benzene hexachloride, chlordane, aryl alkyl thionophosphate, toxaphene, hexaethyl tetraphosphate and iriethoxychlor compositions did not increase yields significant.ly. Pepper ef al. (1947) observed less leafhopper injury to potato plants growing in soil treated with benzene hexachloride for the control of wireworms. The plants in treated plots remained greener much longer than did those in untreated plots. Apparently some substance toxic to leafhoppers was taken up by the plants from the benzene hexachloride. The fluctuations in increased yields cannot be explained by comparable reductions of insect populations. Many instances are quoted of plots treated with certain insecticides resulting in high yields despite only moderate control of such insects as Colorado potato beetle, fleabeetles and leafhoppers. I n order to account for these differences in yield, further research must be done to determine the possible stimulating or depressing effect caused to plants by some of the newer insecticides. Morofsky and Muncie (1948) reported the use of insecticides and fungicides on the Menominee variety in Michigan. Most consistent control of insects in general was obtained by applications of DDT. Of the new materials, parathion as a dust was the most promising, particularIy in the control of leafhoppers. Five per cent DDT gave almost complete control of the six-spotted leafhoppers, flea beetles and tarnished plant bugs. Benzene hexachloride, as a dust, ranked with DDT in the control of tarnished plant bugs and flea beetles but for leafhoppers it was less effective. Among the insecticides used by Campbell and Pepper (1948) parathion was outstanding in its control of aphids and resulted in the highest yield. Plots sprayed with benzene hexachloride, DDT emulsion or wettable DDT were similar in yield and not significantly lower than the yield from those with parathion applications. DDT emulsion gave better control of aphids than D D T powder whereas DDT powder gave best control of flea beetles. Toxaphene caused severe plant injury and resulted in poor insect control. Chlordane and Rhothane resulted in lower yields than parathion. Gyrisko (1946) found a mixture of DDT isoiners, D D D and D D T equally effective in their toxicity to flea beetles and leafhoppers. A new insecticide dust containing piperonyl cyclohexanone added to pyrethrum marc (PCH) and a three way dust containing rotenone, sulfur and pyrethrins gave good initial kills but did not possess the lasting qualities of DDT and related compounds. Lethane B-71 was ineffective against flea beetles and was only fair for the control of leafhoppers.

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Benzene hexschloride has been used on a limited scale for the control of several insects on potatoes. It has been reported promising or very effective in wireworm control on potatoes by Greenwood (1947), Pepper et aE. (1947) and Wolfenbarger (1948). The problem of taste contamination has entered and each author has mentioned the off-flavor of tubers from benzene hexachloride treatments. Commercial lots of potatoes have been rejected as a result of off-flavor imparted by benzene hexachloride. The constituent of benzene hexachloride causing the off-flavor is not fully u:iderstood; one of the isomers, alpha, beta or delta is suspected. The gamma isomer is the most effective insecticidal constituent of crude benzene hexachloride. Refined high gamma content benzene hexachloride from which the other isomers are absent or present only in reduced content has been used with the possibility of eliminating off-flavored tubers. Greenwood (1947) reported that practical and effective wireworm control could be obtained with from 2 to 2.5 lbs. per acre of crude benzene hexachloride of 10 per cent gamma isomer, applied with a grain drill in the form of a dilute dust. Benzene hexachloride also was used as a seedpiece treatment and as an ingredient in poison baits with less success. Pepper et al. (1947) reported further that resulk of wireworm control with benzene hexachloride were outstanding. Fall application of 1.6 lbs. of the gamma isomer per acre and spring application of 2.0 lbs. of the gamma isomer per acre gave good control. Sprays and dusts of benzene hexachloride applied to foliage also reduced the amount of wireworm injury to tubers. Some off-flavor, however, was detected in potatoes from plots treated with benzene hexachloride. Wolfenbarger et al. (1948) found wide variations in the abilities of individual tasters to detect and determine off-flavor of boiled tubers from treatments of soil, treated seed pieces or fertilizer combinations with benzene hexachloride and tubers from untreated lots. A positive relationship was observed in which samples of potatoes from the higher applications of gamma benzene hexachloride per acre were more frequently recorded as off-flavor than those of lower concentrations or of those untreated. Because of their importance in the transmission of virus disease, aphids are of considerable concern wherever seed potatoes are grown. Certain weeds and other plants are alternate hosts for the aphids and eradication of such hosts is now advocated as a step toward control of insect-transmitted diseases. The use of insecticides is depended upon in most cases for the control of aphids. Bronson et al. (1946) and Bronson, Smith and Simpson (1946) in Maine found a zinc nicotinyl fluosilicate dust, nicotine sulfate spray, a benzene hexachloride dust and DDT all effective. Treated plots remained green later in the season than

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did the untreated. Aphids were attracted to the green foliage which resulted in more leafroll in the t.reated than in the untreated plants. Gyrisko e t al. (1946) reported DDT to be a better insecticide in New York than such materials as nicotine, rotenone and the thiocyanates. I n recent tests hexaethyl tetraphosphate and ot,her esters of phosphorus acids proved superior to nicotine for the control of aphids (Ludvik and Decker, 1947, and Roark, 1947). During the past two growing seasons parathion has been found to be a very satisfactory aphicide.

XV. RECENTDEVELOPMENTS IN DISEASE CONTROL Bordeaux mixture for years has been considered the most effective spray for controlling potato foliage diseases, and especially for late blight caused by Phytophthora infestam (Mont.) DeBary. Criticisms of Bordeaux mixture often cited, however, are that it causes injury to plants, is difficult to mix and is corrosive to spraying equipment. Certain organic fungicides lack some of the objectionable qualities as well as exhibiting desirable fungicidal properties. Many new fungicides have appeared in recent years, especially organic fungicides of the dithiocarbamate group. Disodium ethylene bisdithiocarbamate (Dithane) has been applied t o potatoes in tests for its fungicidal and insecticidal activities for several years (Dimond e t al. 1943). It soon was found to be good for control of late blight in Florida (Ruehle, 1944). Heuberger and Manns (1943) discovered that zinc sulfate and lime added fungicidal activity to Dithane and raised it to the level of a promising material for late blight. Horsfall and Turner (1947) reported that Dithane is not much if any better than Bordeaux mixture in the control of late blight but it permits the potatoes to set a larger yield. It is suggested that this is due to a lower level of phytotoxicity which, in turn, is due to the low lime content of Dithane. Davidson and Rich (1947) compared the performance of these organic materials with Bordeaux mixture in the field on Green Mountain during two seasons of severe late blight infection. I n both seasons Bordeaux mixture was better than any of the other 12 materials tested. Phygon, Fermate, Dithane, Zerlate and G-11 (bis 3,5,6 trichloro-2-hydroxyphenylmethane) were all better than the unsprayed treatments. I n both seasons, less foliage injury was caused by the organic materials than by the sprays containing copper. The failure of some of the chemicals t o control late blight was due partially to the poor adherence of the sprays to the leaves. DDT did not appear to possess any fungicidal activity. Organic fungicides such as Phygon, Karbam Z and Dithane were found to be promising for potatoes in Maine (Bonde and Snyder, 1946). Field tests with numerous chromates have been conducted for a number of years and tests on potatoes as early as 1942 demonstrated that with

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suitable proportions of zinc in the complex there was better control of tipburn than was obtained with simple copper chromates or other forms of copper fungicides, probably due to the control of leafhoppers. Campbell and Pepper (1948) reported the use of Dithane D14 plus zinc sulfate and lime resulted in the highest yield although not significantly higher than Zerlate, Parzate and tri-basic copper. P1ot.s sprayed with copper oxychloride sulfate returned the lowest yield but not significantly lower than plots sprayed with Bordeaux, Phygon or DDT alone. Thurston et al. (1948) conclude from field experiments conducted in Pennsylvania, West Virginia and Ohio that copper zinc chromate is the most promising new inorganic potato fungicide introduced in recent years. It is effective against early blight, Alternuria and late blight, Phytophthora and is compatible with DDT. Dithane applications also resulted in high yields and alternating application of Parzate and Zerlate ranked next best, It is very important in the study of virus disease to be able to transmit the symptoms from an infected to a healthy plant. Leafroll is one of the most important and widespread of the virus diseases which are insect transmitted. The potato is not. satisfactory as a test plant in transmission studies of leafroll virus because of the long period of time required for symptom development. Symptoms developed from current season infection in New York are often unrecognizable (Kirkpatrick, 1948). To determine infection it is necessary to grow the potatoes to maturity and to index the tuber progeny. The ideal indicator plant for transmission studies with an insect-transmitted virus should develop distinct symptoms in a short time whenever inoculated with a single insect. It should be grown from true seed, useable in the seedling stage and a food plant for the insect vector. Physalis floridana, Rydberg, P . angulata L. and Datura 8 t r a ~ o n i u ~ L. have characteristics which make them more favorable than the potato as test plants for transmission studies with leafroll virus. They develop symptoms within 15 to 30 days following inoculation. Transmission percentages from single insect feedinge of Myzus persicae Sulzer range from 70 to 100 per cent for P. floridana (Kirkpatrick, 1948). REFERENCES

Akeley, R. V., Stevenson, F. J., and Schultz, E. S. 1948. Am. Potato J . 25, 351-361. Albrecht, W. A,, and Schroeder, R. A. 1942. Soil Sci. 53,313-327. Allen, T. C., and Rieman. G. H. 1939. Am. Potato J. 16, 130-142. Allen, T. C., Rieman, G . H., and McFarlane, J. S. 1940. Am. Potato J . 17, 283-286. Allison, R. V. 1940. Am. Fertiliier 92, 8-9. Anonymous. 1940. Idaho Agr. Expt. 6ta. Bull. 239, 34-36. Anonymous. 1945. Gt. Britain J. Ministry Agr. 52, 215-217.

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Apple, J. W., and Arnold, C. Y. 1945. Am. Potato J . 22, 339-343. Asdonk, T., and Jacob, A. 1940. Bodenk. PjlEmahr, 20,107-122. Bates, G. H., and Martin, L. D. 1935. Gt. Britain J . Ministry Agr. 42, 231-236. Bear, F. E., and Toth, S. J . 1942. I d . Eng. Chem. 34, 49-52. Bennett, E. R. 1942. Idaho Agr. ColI. Ext. Bull. 141. Berger, K . C. 1948. Am. Potato J. 25, 377-386. Berkner, F., Weise, E., and Newrzella, B. 1939. Bodenk. PjZEmuhr, 13, 370-384. Bird, J. J. 1942. Tenn. Agr. Expt. Sta. Bull. 181. Blodgett, F. M., and Stevenson, F. J. 1946. Am. Potato J . 23, 315-329. Bonde, R., and Snyder, E. G. 1946. Am. Potato J . 23, 415-425. Bonde, R. 1935. Maine Agr. Expt. Sta. Bull. 380. Bonde, R., and Schultz, E. S. 1945. Am. Potato J . 22, 163-167. Brentzel, W . E. 1942. North Dakota Agr. Expt. Sta. Bimonthly Bull. 7, 27. Brickley, W. D. 1943. Eire Dept. Agr. J . 4 4 149-161. Bronson, T. E., Smith, F. F., Shands, W. A., and Simpson, G. W. 1946. Me. Agr. Expt. Sta. Bull. 442, 134-140. Bronson, T. E., Smith, F. F., and Simpson, G. W. 1946. J . Econ. Entomol. 39, 189194.

Brown, B. A. 1943. Conn. Vegetable Growers’ Assoc., Proc. 30, 51-52. Brown, B. A. 1944. Am. Potato J . 21, 163-169. Brown, B. E. 1944. Am. Potato J. 21, 1-5. Brown, B. E., and Hawkins, A. 1944. Am. Fertilizer 100. (8), 5-7. Bushnell, J . 1942. Ohio Agr. Expt. Sta. Bimonthly Bull. 27. 63-70. Bushnell, J. 1943. Am. Potato J . 20, 153-155. Callbeck, L. C. 1948. Am. Potato J . 25, 225-233. Campbell, J. C., and Pepper, B. B. 1948. Am. Potato J. 25, 82-86. Carolus, R. L. 1914. Am. Potato J . 21, 199-203. Chandler, F. B. 1944. J. Am. SOC.Agron. 34, 894-901. Chucka, J. A., and Harrington, J. L. 1942. Me. Agr. Expt. Sta. Bull. 411. Chucka, J. A., Hawkins, A., and Brown, B. E. 1943. Me. Agr. Ezpt. Sta. Bull. 414. Chucka, J. A., Hawkins, A., and Harrington, J. L. 1942. Me. Agr. Expt. Sta. Bull. 411. Clark, C. F., and Stevenson, F. J. 1935. .US.D.A. Circ. 374. Agron. 34, 894-901. Collins, E. R., and Skinner, J. J. 1942. J. Am. SOC. Cook, H. T., and Houghland, G. V . C. 1942. Am. Potato J . 19, 201-208. Cook, H. T., and Nugent, T. J. 1942. Va. Agr. Expt. &a. Bull. 108, 1785-1795. Cordner, H. B. 1943. Okla. Agr. ExptSta. Tech. Bull. T-18. Corum, C. J. 1941. Ohio J. Sci. 41, 389-396. Cowie, G. A. 1942. Ann. Applied Biol. 29, 333310. Cowie,G. A. 1943. Emp. J . Expt. Agr. 11, 23-32. Cox, T. R., and Odland, T. E. 1940. Am. Potato J. 17, 105-113. Daines, R. H., and Martin, W. H. 1940. Hints to Potato Growers N.J . 20, 1-6. Davidson, R. S., and Rich, A. E. 1947. Am. Potato J. 24, 35-39. Davies, R. O., and Fagan, T. W. 1944. Emp. J. Expt. Agr. 12, 54-60. Dimond, A. E., Heuberger, J. W., and Horsfall, J. G. 1943. Phytopath. 33,1095-1097. Dostal, R. 1947. Shornik Ceskoslov. Akab. Zemedelake 19, 32-39. Ennis, W. B. Jr., Swanson, C. P., Allard, R. W., and Boyd, F. T. 1946. Botan. Gaz. 107, 568-575.

Fernow, K. H., and Smith, 0. 1944. Cornell Univ. Agr. Ext. Bull. 653. Findlay, D. H., and Sykes, E. T. 1936. Gt. Britain J . Ministv Agr. 43, 457-460. Findlay, D. H., and Sykes, E. T. 1937. Gt. Britain J . Ministry Agr. 44, 546-552.

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Granovsky, A. A. 1944. Am. Potato J . 21, 89-91. Gray, S. D. 1944. Better Crops with Plant Food 28(2), 24-26, 42-43. Greenwood, D. E. 1947. J. Econ. Entomol. 40, 724-727. Gyrisko, G. G. 1948. J. Econ. Entomol. 39, 262-263. Gyrisko, G. G., Wene, G. P., and Rawlins, W. A. 1946. .I. Econ. Entomol. 39, 205208.

Hawkes, J. C. 1945. Emp. J. Expt. Agr. 13, 11-40. Hawkins, A. 1945. Soil Sci. SOC.Am. Proc. 10, 252-256. Hawkins, A. 1946. J. Am. SOC.Agron. 38, 667-681. Hawkins, A., Chucka, J. A., and Brown, B. E. 1941. Am. Potato J . 19, 234-239. Hawkins, A. 1942. E. I . du Pant de Nemourn & Co., Pub. Relations Dept. Agr. News Letter 10, 13-17. Heuberger, J. W., and Manns, T. F. 1913. Phytopath. 33, 1113. Heuberger, J. W., and Steams, L. A. 1946. J . Econ. Entomol. 39, 287-268. Hibbard, A. D. 1943. Mo. Agr. Expt. Sta. Bull. 464. Hill, H., and Cannon, H. B. 1948. Sci. Agr. 28, 185-199. Horsfall, J. G I and Turner, T. 1947. Am. Potato J . 24, 103-110. Houghland, G. V. C., and Parker, M. M. 1948. Am. Potato J. 25, 393-406. Hoyman, W. G. 1947. Am. Potato J . 24, 110-118. Jacob, K. D., and Armiger, W. H. 1944. J. Am. SOC.Agron. 36, 281-286. Jones, J. O., and Plant, W. 1942. Ann. Rept. Agr. Hort. Research Sta., Long Ashton, Bristol, 44-45. Katalymov, M. V. 1946. Compt. rend. acad. sci. U.R.S.S. 53, 821-825. Keese, H. 1942. Bodenk. PfZErniihr. 27,116-134. Kirkpatrick, H. C. 1948. Am. Potato J . 25, 283-290. Krants, F. A., and Eide, C. J. 1948. Am. Potato J. 25, 294-300. Kraus, J. E. 1944. Idaho Agr. Expt. 9ta. Circ. 88. Kunkel, R., Edmundson, W. C., and Binkley, A. M. 1948. Am. Potato 3. 25, 371378.

Linn, M. B., Apple, J. W., and Arnold, C. Y. 1948. Am. Potafo J . 25, 315-328. Lorens, 0. A. 1944. Am. Potato J. 21, 179-192. Lorens, 0. A. 1947. Am. Potato J . 24, 281-293. Ludvick, G. F., and Decker, G. C. 1947. J. Econ. Entomol. 40, 97-100. Lyons, E. S., Russel, J. C., and Rhoades, H. F. 1914. Neb. Agr. Ezpt. Sta. Bull. 365. MacDowall, R.K. 1935. Scot. J. Agr. 18, 243-249. MscVicar, R., Tottingham, W. E., and Rieman, G.H. 1946. Soil Sci. 62, 337-340. McGoldrick, F. 1948. Ph. D. Thesis, Cornell University. McGoldrick, F., and Smith, 0. 1948. Proc. Sac. Hort. Sci. 51, 401-405. Main, A. D. C., and Grainger, J. 1947. Scot. J. Agr. 27, 14-17. Maughan, F. B. 1947. Am. Potato J . 14, 157-161. Morofsky, W. F., and Muncie, J. H. 1948. Am. Potato J . 25, 255-259. National Fertilizer Assoc. 1948. Fertilizer Rev. 21(2), 7-10. Nelson, W. L.,and Brady, N. C. 1943. Soil Sci. SOC.Am. Proc. 8, 313-316. Nelson, W. L., and Hawkins, A. 1947. J. Am. Sac. Agron. 39, 1053-1067. Nemec, A. 1940. Bodenk. PflErnlihr. 20,84-106. Neuweiler, E. 1941. Rev. Applied Mycol. 20, 334. Otis, C. E. 1948. Am. Potato J . 23, 333-336. Peech, M. 1945. Soil Sci. Sac. Am. Proc. 10, 245-251. Pepper, B. B., Wilson, C. A., and Campbell, J. C. 1947. J. Econ. Entomol. 40, 727730.

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Post, R. L., Colberg, W. J., and Munro, J. A. 1948. Am. Potato J. 25, 334-339. Prince, F. S., Blood, P. T., Coates, W. H., and Phillips, T. G. 1940. N.H. Agr. Ezpt &a. Bull. 324. Reddick, D. 1943. Am. Potato J. 20, 118-126. Roark, R. C. 1947. U.S.D.A. Bur. Entomol. Plant Quur. E-721. Ruehle, G. D. 1944. Flu. Agr. Expt. Sta. Press Bull. 598. Sfimuel, G. 1944. Gt. Britain J. Ministry Agr. 51, 277-280. Schroeder, R. A., and Albrecht, W. A. 1942. Soil Sci. 53, 481-488. Schultz, E. S., Bonde, R., and Raleigh, W. P. 1944. Maine Agr. Expt. Sta. Bull. 427. Selke, W. 1942. Bodenk. PflErnahr. 28, 193-215. Sleesman, J. P., and Bushnell, J. 1937. Am. Potato J. 14, 242-245. Sleesman, J. P., and Bushnell, J. 1945. Ohio Agr. Ezpt. Sta. Bimonthly Bull. 30, 7375. Sleesman, J. P., and Stevenson, F. J. 1941. Am. Potato J. 18, 280-298. Sleesman, J. P., and Wilson, J. D. 1943. Ohzo Agr. Expt. Sta. Bimonthly Bull. 28, 173-183. Small, T. 1935. Ann. Applied Biol. 22, 469-479. Small, T. 1936. Gt. Britain J. Ministry Agr. 43, 1162-1168. Smith, 0. 1944. Cornell Univ. Agr. Ezt. Bull. 551. Smith, O., Baeza, M. A., and Ellison, J. H. 1947. Botan. Gaz. 108, 421-432. Smith, O., Hommel, R. F., and Kelly, W. C. 1943. Am. Potato J . 20, 267-277. Smith, O., and Kelly, W. C. 1946. Am. Potato J. 23, 107-135. Smith, O., and McCubbin, E. N. 1940. Am. Potato J. 17, 235-243. Smith, O., Marshall, E. R., and Meadows, M. W. 1949. Northeastern States Weed Control Conf. Proc. 1949, 126-130. Smith, O., Meadows, M. W., and Marshall, E. R. 1949. Northeastern States Weed Control Conf. Proc. 1949, 98-113. Sparks, W. C., and McLean, J. G. 1946. Proc. Bm. Boc. Hort. Sci. 48, 449-457. Steinbauer, G. P. 1945. Maine Agr. Expt. Sta. Bull. 438, 526-530. Steinbauer, G. P. 1947. Maine Agr. Expt. Sta. Bull. 449, 325. Teakle, L. J. H., Burvill, G. H., and Morgan, E. T. 1942. J. Dept. Agr. West Australia 19, 182-194. Teakle, L. J. H., and Morgan, E. T. 1943. J. Dept. Agr. West Australia 20, 119-123. Teakle, L. J. H., Morgan, E. T., and Turton, A. G. 1941. J. Dept. Agr. West Australia 18, 96-125. Terman, G. L., and Hawkins, A. 1947. Better Crops with Plant Food 31(8), 23-26; 41-44. Thun, R. 1942. Mitt. Landw. 57, 298. Thurston, H. W. Jr., Leach, J. G., and Wilson, J. D. 1948. Am. Potato J . 25, 406409. Wallace, T., Croxall, H. E., and Pickford, P. T. H. 1941. Ann. Rept. Agr. Hort. Research Sta., Long Ashton, Bristol, 33-38. Ware, L. M. 1943. Am. Potato J. 20, 12-23. Werner, H. 0. 1947. Nebr. Agr. Expt. Sta. Bull. 384. Wheeler, E. J., Stevenson, F. J., and Moore, H. C. 1944. Am. Potato J. 21, 305311. White-Stevens, R. H. 1942. Am. Potato J. 19, 81-90. Wilson, A. R., and Boyd, A. E. W. 1947. Gt. Bm'tain J. Ministry Agr. 54, 201-205. Wilson, A. R., Boyd, A. E. W., Mitchell, J. G., and Greaves, W. S. 1947. Ann. Applied Biol. 34, 1-33.

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ORA SMITH

Wilson, J. D., and Sleesman, J. P. 1945. Proc. Ohio Vegetable Potato Growers’ AsSOC.

31, 193-208.

Wilson, J. D., and Sleesman, J. P. 1947. Am. Potato J . 24, 260-266 Winters, E. 1946. Soil Sci. Soc. Am. Proc. 10, 162-167. Wolfenbarger, D. 0. 1948. F h . State Hort. Soc. Proc. 1947, 116-121. Wolfenbarger, D. O., and Heuberger, J. W. 1946. Am. Potato J . 23, 389-395. Wolfenbarger, D. O., Decker, P., and Rawlins, W. A. 1948. Am. Potato J . 25, 413417.

Fixation of Soil Phosphorus L. A. DEAN U . S. Department. of Agricultuw. Beltsuille, Maryland CONWANTS

Page 391 392 111. Phosphorus Fixation by Soils, Clay Minerals, and Hydrous Oxidcfi . . . 393 IV. Chemically Precipitated Phosphorus . . . . . . . . . . . . . . . 397 1. Acid Soil Systems . . . . . . . . . . . . . . . . . . . . 398 2. Calcium-Soil Systems . . . . . . . . . . . . . . . . . . . 399 V. Fixation of Phosphorus by Surface Reactions . . . . . . . . . . . 400 1. Adsorption . . . . . . . . . . . . . . . . . . . . . . 400 2. Metathetical Reactions and Anion Exchange . . . . . . . . . . 402 . . 406 VI. Biological Fixation of Phosphorus in Soils . . . . . . . . . 1. Distribution of Organic Phosphorus in Soil . . . . . . . . . 406 2. Identification of the Organic Phosphorus Compounds in Soils . . . 407 a. Nucleic Acids and Derivatives . . . . . . . . . . . . . 408 b. Phytin and Inositol Phosphates . . . . . . . . . . . . . 409 . . . 409 References . . . . . . . . . . . . . . . . . . . . . .

I. Introduction . . . . . . . . . . . 11. Accumulation of Phosphorus in Soils

. . . . . . . . . . . . . . . . . . . . . . . . . . .

~

I. INTRODUCTION The term phosphorus fixation is a general one which usually implies the conversion of phosphorus to a more insoluble form. I n some instances, however, this term has been used to designate a change in the degree of availability* of soil phosphorus. Changes in availability are not sufficiently specific to warrant quantitative interpretation. It does not necessarily follow that a change in solubility will mean a change in availability. For the purposes of this discussion, fixed phosphorus will be defined as the soil phosphorus which has become attached t o the solid phase of soils. Thus a n example of phosphate fixation would be the disappearance of phosphate ions from a solution placed in contact with soil. *Available phosphorus pertains to those forms of phosphorus in soils which are usable by plants. Usually thought of in relative terms such as readily or slowly available. 391

L. A. DEAN

392

11. ACCUMULATION OF PHOSPHORUS IN SOILS In many soils fixed phosphorus probably embraces most of the phosphorus which is available for plant absorption. In parent materials such as igneous rocks the phosphorus exists primarily as apatite and as inclusions in many silicate minerals. During t.he processes of soil formation this phosphorus enters the soil solution and is subsequently converted into fixed phosphorus or absorbed by plants and eventually deposited on the soil surface as plant or animal residues. Thus as a soil matures t.he phosphorus accumulates in the surface layers and in the clay fraction. This point is demonstrated by the data presented in Table I. TABLE 1. Accumulation of Phosphorus at the Surface and in the Clay Fraction

Horizon

A1 A,

BI C

Chester loam Percent PPOS Depth Whole Colloid inches soil only 0-2 2-10 10-34 34-60

0.16 0.09 0.12 0.10

0.53 0.26 0.15 0.20

Appling sandy loam Percent PnOs Depth Whole Colloid Horizon inches soil only A, A2

BI

B* C

0-1 1-9 9-14 14-28 28-60

0.09 0.05 0.04 0.09 0.03

0.45 025 0.18 0.26 0.15

_ _ _ _ _ _ _ ~ ~~ _~ _ _ _ _ ~

“Brown and Byers (1938).

In addition to the changes in soil phosphorus during the soil genesis, extensive changes have been brought about in cultivated soils through the long continued use of fertilizers and manures. Cummings (1945) discussed the use of phosphatic fertilizers in the United States and presents a table showing that in 1943 farmers in 22 states applied more phosphorus as fertilizer than was removed by harvested crops. I n 12 states more than three times as much phosphorus was applied than was removed by crops. Soil studies have shown the extent of the increase of total and soluble phosphorus in soils resulting from this intensive use of phosphatic fertilizers (Anderson et al., 1927; Blair and Prince, 1936; Bryan, 1933; Hester, 1937; Peech, 1939, 1945). As much 8s fivefold increases in the amounts of total phosphorus in soils are reported. Soil properties have been shown to influence the forms of phosphorus that persist in soils. Bray and Dickman (1941) measured the effect of additions of superphosphate and rock phosphate on t.he amounts of acid soluble and neutral ammonium fluoride soluble phosphorus in Illinois

FIXATION OF SOIL PHOSPHORUS

393

soils. Superphosphate was rapidly converted to the forms soluble in ammonium fluoride except on limed soils. Additions of rock phosphate increased the amounts of acid soluble phosphate (presumably this material remained relatively unchanged). Conversion of rock phosphate to ammonium fluoride soluble forms was found to occur on acid soils, pH 4.8 to 5.0. A study of soils, p H 6.2, from a 33-year-old experiment a t Blacksburg, Virginia, by Rich e t al. (1947) showed more than 50 per cent of the residual superphosphate to be in a sodium hydroxide soluble form, whereas none of the residual phosphorus from rock phosphate was converted to this form. Dean (1938) has reported the distribution of phosphorus in the classical plots a t Rothamsted and Woburn. These soils have different degrees of acidity, and two sets of soil samples were available from each location, taken a t intervals of 45 years. The results show distinct differences in t.he distribution of phosphorus. The Rothamsted soil (pH 7.48.0) had higher amounts of acid soluble phosphorus, while the Woburn soil (pH 4.8 to 6.0) had higher amounts of alkali soluble phosphorus. There is an indication that when nitrogen and phosphorus were in abundance with correspondingly favorable crop growth there was an accumulation of organic phosphorus. The insoluble phosphorus (insoluble in an alkali followed by an acid extraction) comprises a large proportion of the total soil phosphorus and remains unchanged after years of cropping and fertilization.

111. PHOSPHORUS FIXATION BY SOILS,CLAYMINERALS, AND HYDROUS OXIDES The relative ability of soils, the clay minerals, and hydrous oxides to fix phosphorus has been determined by a variety of methods (Davis, 1943; Ford, 1933; Fraps, 1922; Heck, 1934b; Hibbard, 1935; Lohse and Ruhnke, 1933; McCool, 1928; McGeorge, 1939; Perkins and King, 1943). There is no generally accepted method. By and large two variations are used: (1) measuring the decrease in concentration of aqueous phosphate solutions equilibrated with soils, and (2) by measuring the amounts of phosphorus extractable from soils equilibrated with phosphatic compounds as compared with untreated soils. I n the first instance no attempt is made to define the nature of the combination between soil and soluble phosphate. Soil or similar material is introduced into a solution containing phosphate ions and the decrease in concentration noted. Since it is conceivable that a part of the phosphorus taken up from solutions will be available to plants, i t has been argued that this method of studying phosphorus fixation does not give adequate information. Consequently, other methods have been devised which entail incorporating phosphorus with a soil, allowing fixation to take place, and subsequently extracting

394

L. A. DEAN

the soil using plants or a reagent capable of removing the readily soluble phosphorus. The assumption is made that the added phosphorus remaining in the soil is in a slowly soluble form and consequently is relatively unavailable. Radioactive phosphorus has been used in phosphate fixation studies (Ballard and Dean, 1940; Neller and Comar, 1947). The obvious advantage in such procedures lies in being able to distinguish between the phosphorous added and that already in the soil. However, the validity of such an assumption is subject to question. It has been shown that P3*04ions added to soils will undergo an isotopic exchange with some of the native soil phosphorus (McAuliffe et al., 1947). If the order of magnitude of this exchange is appreciable under the experimental condition for measurement of phosphorus fixation it would be necessary to account for losses by this mechanism. What appear to be discrepancies arise through the use of different methods for determining phosphorus fixation. For example, the effect of the degree of saturation of soils with calcium on the fixation of phosphorus depends on the method of determination. Heck (1934~)concluded that “a low degree of base saturation tends to give a soil a greater capacity for fixing phosphorus in difficultly available form than if the soil is more fully saturated with bases.” The method used was to add a phosphate solution to soil, evaporate to dryness, and extract the readily soluble phosphorus with 0.002 N HzSOa a t pH 3, the phosphorus remaining in t,he soil being considered fixed phosphorus. When the phosphate fixing capacity is measured by observing the decrease in concentration of phosphorus in a soil-water system, however, the fixation of phosphorus is found to increase with increasing degree of saturation in respect, to calcium (Benne et al., 1936; Davis, 1946). The methods determining t.he phosphorus fixation by soils are empirical. It is necessary to rigidly control such factors as concentrations of phosphorus added, time of reaction, temperature and pH in order to obtain reproducible results. Hibbard (1935) stated “soils have no definite fixing power.” The resu1t.s of phosphorus fixation measurements are frequently reported as of percent fixation: the percentage of added phosphorus that had been fixed. When considered as relative values these results have shown interesting differences in the properties of soil and related material. Measurement of the penetration of phosphorus in soils has also been used as an index of fixation. Fraps (1922) placed soils in tubes 2 inches in diameter and 14 inches long, mixed 1 g. of superphosphate with the top 3 inches of soil, added 100 ml. water and allowed the column to stand 24 hours. Then water was percolated through the column until

FIXATION OF SOIL PHOSPHORUS

395

1 liter was collected. With soils having a high fixing capacity almost no phosphorus was found in the percolate. Several refinements to this basic procedure have been used in studying the effect of fixation and fertilizer properties and the penetration of phosphorus (Conrad, 1939; Heck, 1934a). Henderson and Jones (1941) demonstrated the excellent possibilities of using radio phosphorus in movement studies. Field investigations (Midgley, 1931 ; Stephenson and Chapman, 1931) have shown that little or no penetration takes place with heavy soils, but with light. textured soils appreciable movement may be expected. Early in the history of the study of phosphate fixation attention was turned toward ascertaining what soil components had the property of retaining phosphate ions. Fraps ( 1922) found a significant correlat,ion between the amounts of iron and aluminum dissolved from soils by strong acid and the phosphorus fixing capacity of Texas soils. Gile (1933) and Scarseth and Tidmore (1934) found an inverse relationship between the silica-sesquioxide ratio and the phosphate fixing capacity of soil colloids. The naturally-occurring hydrated oxides such as goethite, limonite, diaspore, and bauxite were shown to have phosphate fixing power comparable with that of soils (Dean, 1934; Ford, 1933; Weiser, 1933). Hematite does not show a noticeable phosphate-fixing capacity. Such investigations have established the importance of iron and aluminum compounds, especially the hydrous oxides, in the fixation of phosphorus by soils. The effect of the removal of iron from soils or clays on their ability to fix phosphorus has been one approach to the consideration of the role of iron compounds in the fixation of phosphorus (Allison and Scarseth, 1942; Black, 1942; Chandler, 1941; Coleman, 1942, 1944; Metzger, 1940, 1941; Romine and Metzger, 1939; and Toth, 1937). Invariably, treatments which remove a part of the iron result in a reduction in phosphate fixation. Such treatments, however, do not completely eliminate significant fixation by the residual materials. For example, Toth (1937) removed the free iron oxides from a Cecil colloid by the method of Drosdoff and Truog (1935), thus reducing the fixation of phosphorus from 0.370 to 0.205 m.e. per g., and with a Sassafras colloid from 0.275 to 0.125 m.e. per g. This approach does not completely clarify the role of iron in the fixation of phosphorus. Some of the free iron oxides in soil colloids exist as coatings on the clay particles. The relation of this coating to the properties of clays such as kaolinitjr is not known. It, is not improbable that. the iron coatings mask the phosphate fixing properties of the clays themselves. Black (1942) discussed the role of the kaolinite and iron oxide components of Cecil clay, and suggests that kaolinite is only of importance in fixing phosphorus from solutions of high concentrations. There is little information available on the phosphorus fixing capacity

396

L. A. DEAN

of the mechanical soil separates or the relation of the surface area of specific materials to the fixing capacity. Perkins, et a2. (1942)measured the phosphorus fixation of separates from a Wabash soil. As the particle size decreased phosphate fixation decreased when expressed as PzOafixed per square meter of surface but increased when expressed on a weight basis. Dean and Rubins (1947) show the anion exchange capacity of Sassafras soils in millimols per 100 g. to be roughly proportional to the specific surface when expressed as square meters per g. The clay minerals common to soils such as kaolinite, halloysite, montmorillonite and illite all exhibit phosphorus fixation. It can be questioned, however, whether the order of magnitude is sufficient to materially contribute to the fixation capacity of many soils. Studies have shown that particle size, pH, concentration of phosphate solutions, and time of contact all have effects on the fixation of phosphorus by the clay minerals (Black, 1942; Murphy, 1939; Scarseth, 1935; and Stout, 1939). The amount of phosphorus fixed by natural unground kaolinite is relatively low. Ball mill grinding, however, greatly increases this capacity. In light of recent studies by Laws and Page (1946) on the effect of grinding kaolinite, it would be reasonable to conclude that the prop-

- - ------

Bentonite Colloid

Upshur Colloid Miami H Colloid

---

b

1

1

3

I

I

I

I

5

7

9

II

PH Fig. 1. Phoshorus retention by bentonite and soil colloids as a function of pH (Steele, 1936).

FIXATION OF SOIL PHOSPHORUS

397

erties of this material may not be analogous necessarily to natural kaolinite. Experiments with finely ground kaolinite have shown very high retention of phosphate with a maximum a t about p H 4. Fixation decreases rapidly as the p H approaches neutrality. When natural kaolinites are used t.his does not necessarily hold. Black (1942) has shown natural kaolinites to have much lower fixing capacity. A change in the type of fixation was noted when the time and concentration of phosphate solution are varied. With dilute solutions there was no evidence of maximum fixation a t p H 4, but as the concentration and time of contact are increased greater fixation occurs in the acid range. Similar results were obtained by Coleman (1944) using kaolinite isolated from the C horizon of an Orangeburg soil. Fixation studies with bentonite by Scarseth (1935) and Stout (1939) have shown a maximum retention a t p H 6. Black (1942) concurs; however, when using high concentrations the point of maximum fixation shifted to pH 5. The curves by Steele (1935) shown in Fig. 1 illustrate the relation between pH and phosphate retention by bentonite and two soil colloids. Bentonite shows characteristic maximum fixation a t about p H 6. Near neutrality the behavior of the soil colloids appears similar to bentonite. However, they also show a point of maximum fixation in the acid range. Presumably this is attributable to the iron oxides associted with these materials.

IV. CHEMICALLY PRECIPITATED PHOSPHORUS Probably the oldest theory pertaining to the mechanism of phosphate fixation is that phosphate ions in solution are precipitated, thus becoming a part of the solid phase. For the purposes of this discussion the term chemically precipitated phosphorus will be limited to those compounds which are formed as chemically homogeneous particles from ions in solution. Such a definition is intended t o eliminate from consideration chemically precipitated layers on surfaces of the soil constituents. Quite obviously no single mechanism will account for the decrease in concentration of phosphate that takes place when all soils are brought in contact with phosphate solutions. Bradfield et al. (1935) postulate three separate mechanisms which possibly overlap. A t p H 2 t o 5 the retention is chiefly due to the gradual dissolution of iron and aluminum oxides which are then reprecipitated as phosphates. At p H 4.5 to 7.5 phosphates are fixed on the surface of clay particles, and a t pH 6 to 10 phosphate is precipitated by divalent cations if present.

398

L. A. DEAN

1 . Acid Soil Systems

I n acid soil systems iron and aluminum appear to be the most likely soil constituents to fix phosphorus by chemical precipitation. The precipitation of phosphorus by iron and aluminum has been the subject of a systematic and comprehensive study (Gaarder, 1930; Gaarder and Graehl-Nielson, 1935). This work has shown that when iron and phosphorus are combined in equivalent quantities minimum solubility occurs between p H 2 and 3. In the presence of an excess of iron, however, there is a tendency to extend the range of minimum solubility to about p H 4. When aluminum and phosphorus are combined in equivalent quantities minimum solubility occurs at about pH 4 but when an excess of aluminum is present the range of minimum solubility extends from p H 4 t o 7. Considering these possibilities there is no reason to dispute the possibility of the formation of chemically precipitated iron and aluminum phosphate in soils. There is considerable evidence available, however, which indicates that such compounds do not exist in soils in important quantities. The availability for plant growth of precipitated iron and aluminum phosphates (Marais, 1922; McGeorge and Breazeale, 1932; Truog, 1916) suggests that these materials are more available to plants than much of the fixed phosphorus in soils. Electrodialysis (Dean, 1934) and acid extraction (Heck, 1934a) studies have indicated that precipitated iron and aluminum phosphates are more readily extracted from soils than much of the fixed phosphorus. These same studies did indicate that dufrenite, a naturally-occurring basic iron phosphate, had analogous properties t o much of the fixed phosphorus in soils. Mineral specimens of dufrenite are usually in the form of hard nodules, however, and intensive grinding is necessary to prepare samples having a surface area similar to the precipitated iron and aluminum phosphates. No indicat.ion is given about the relative surface of the dufrenite used in these studies. The concept that basic iron and aluminum phosphates are precipitated in soils still persists. The amounts of water soluble iron and aluminum in soils are very low in comparison to the amounts of phosphorus that soils are capable of fixing. Bear and Toth (1942) in discussing phosphate fixation by a Colts Neck soil show the following: this soil has a phosphorus fixing capacity of 1.2 g. P205per 100 g. yet prolonged electrodialysis of the soil only removed 5.6 mgm. of iron and 3.4 mgm. of aluminum. On the basis of these amounts of iron and aluminum it is hard to conceive how any appreciable part of the fixing capacity of this soil can be accounted for on the basis of the formation of chemical precipitates. On the other hand, Metzger (1940, 1941) presents evidence to support the conclusion

FIXATION OF SOTL PHOSPHORUS

399

that the phosphorus fixing capacity of acid prairie soils can be accounted for largely by precipitation phenomena. These conclusions were based on the amounts of iron extracted from soils with 0.002 N H2S04 and the reduction in phosphorus fixing capacity observed when soils were extracted with this reagent. The solubility of the iron and aluminum associated with montmorillonitic and kaolinite clays separated from soils was measured by Coleman (1944). Samples of clay were shaken for a month with solutions prepared by adjusting a dilute solution of hydrochloric acid to different reactions with ammonium hydroxide. An appreciable amount of iron and aluminum was dissolved a t tshe more acid reactions, but not sufficient to account for the total fixing capacity of the clays. It is not improbable that if the iron and aluminum were precipitated immediately upon entrance into solution this would enhance the dissolving of these substances from soil colloids. Such a mechanism is suggested by Low and Black (1947) on the basis of studies on phosphate induced decomposition of kaolinite. I n considering the fixation of phosphorus by acid Hawaiian soils Davis (1935) dismissed the possibility of any large part of the phosphate fixation being by the formation of double decomposition precipitates because for any given equilibrium phosphate concentration the amount of phosphate fixed varies very nearly a? the ratio of soil to solution. 2. Calcium-Soil Systems

Considering the complexity of the system H20-C02-Ca0-P205soil scientists have spent little time on the stable forms of calcium phosphates that are formed and persist in soils. It is not improbable that a part of the calcium phosphate combinations that exist are of uncertain composition. This is in line with the early suggestion of Cameron and Bell (1907) that phosphoric acid and lime exist as a series of solid solutions. Bassetk (1917) suggested hydroxy apatite as the only stable compound that can exist under soil conditions. McGeorge and Breazeale (1931) have concluded that in calcareous soils the phosphate of low availability is a carbonate-phosphate compound in which one mol of calcium Carbonate is combined with three mols of tricalcium phosphate. MacIntire and Hatcher (1942) discuss evidence to support. a theory that some of the superphosphate incorporated into limed soils will ultimately be reverted to a fluorphosphate similar in characteristics to raw rock. This is an interesting theory in that it provides a mechanism by which it would be possible for soil components to alter a fertilizer in situ. One usually envisions fixation taking place by the phosphate ions diffusing away from the fertilizer particles and reacting a t the soil surfaces.

400

L. A. DEAN

This theory, however, could provide for the basic soil constituents surrounding a superphosphate particle inducing reduction in solubility without the phosphate ions ever leaving the fertilizer particle. The continuous fertilizer plots a t the Rothamsted Experimental Station have provided excellent material for obtaining actual evidence of the formation of calcium fluorphosphates in soils which received long continued applications of superphosphate. Prior to the time the differential fertilizer treatments were started (over a century ago) the Broadbalk field had received a heavy application of chalk. Many of the particles were large and some of the original material still persists. Nagelschmidt and Nixon (1944) selected chalk fragments (0.5-2 mm. diameter) from the superphosphate plot and determined phosphorus and fluorine. Analyses of samples taken 63 years apart are given below.

PzOa 1881 sample 1944 sample

per cent

F per cent

0.44 2.80

0.032 0.250

Fragments from the 1944 sampling were heated to 800'C. and the calcium oxide removed with sucrose solution. The residue gave the X-ray powder diagram of apatite.

V. FIXATION OF PHOSPHORUS BY SURFACE REACTIONS 1. Adsorption The tendency for phosphorus to concentrate a t the interface between the liquid and solid phase of the soil system is the phenomenon of adsorption. This term supplies no implication which alludes to the nature of the binding forces contributing to the phenomenon. Adsorption is a sufficiently general term to include several kinds of surface reactions. The simple statement that the phosphorus fixation is an adsorption frequently leads to confusion since this can hardly be construed as defining a specific mechanism. A distinction between the kinds of adsorption is of as much interest as t,he amounts of phosphorus involved. Davis (1935) considers the retention of phosphorus a t the surface as involving forces akin to those studied in organic chemistry. H e envisions the phosphate ions penetrating the liquid-solid interface to form new compounds with the hydrated minerals, and that these compounds are in equilibrium with the hydrated minerals. Such a theory is not inconsistent with many of the experimental observations pertaining to the fixation of phosphorus.

401

FIXATION OF SOIL PHOSPHORUS

The amount of phosphorus taken up by soils is proportional to the concentration. It has been pointed out by Russell and Prescott (1916) and others (Davis, 1935; Kurtz et al., 1946) that the adsorption of phosphorus by soils can be described by the equation of Freundlich

$=kCn.

Fisher (1922) pointed out that compliance of data with this equation cannot be considered as a criterion of adsorption. Nevertheless it can be considered as corroborative evidence. It is well known that some of the phosphorus associated with the solid phase of soils which have been brought to equilibrium with phosphate solutions is water soluble. A dist.inction is frequently made between this phosphorus and the phosphorus ions more tightly associated with the solid phase. Mattson and Karlson (1938) have distinguished colloid-bound phosphate as ions that have become a nondiffusable structural unit in the colloidal aggregate, and saloid-bound phosphate as ions in the diffusable ionic atmosphere held as compensation to ions of opposite charge. These two forms of binding are named micellar binding in contrast to extra-micellar binding, which is precipitation of phosphate by another ion, both being outside of the soil micelles. The fixation of phosphorus as a function of pH by sodium and calcium-saturated bentonite was measured by Scarseth (1935). Calcium ions greatly increased the phosphorus fixing capacity of the clay. The

Frederick Colloid

Miami Colloid -5

2 2

4

6

PH

8

u

2

6

4

8

PH

Fig. 2. Phosphorus retention by sodium and calcium-soil colloids (Allison, 1943).

402

L. A. DEAN

theory proposed to explain this observation was that the phosphate ions were held by the calcium present as exchangeable calcium. Similar experiments were undertaken by Allison (1943)and are illustrated in Fig. 2. It may be seen that there is always a greater fixation of phosphorus by the calcium saturated clay. Between p H 4 and 6 there is a tendency for the Ca and Na curves to be parallel, but at about p H 6 the curves break in opposite directions. This is the point where precipitation of calcium phosphate probably becomes an effective mechanism in phosphate fixation. The role of calcium in the fixation of phosphorus within the range pH 4 to 6 has not been adequately clarified. This is probably what Mattson and Karlsson (1938) have termed saloid-bound phosphorus, the mechanism being that the calcium ions share valences with the clay micelles and phosphate ions. Davis (1945) also concludes that some of the H20-insoluble phosphate retained by soils may be in the form of a double layer or a soil micelle-calcium-phosphate linkage. 2. Metathetical Reactions and Anion Exchange

Metathesis may be defined as the substitution of one ion for another. In a broad sense it may be considered as a chemical react.ion without the stigma of the laws of mass action and stoichiometry. It describes a reaction involving chemical forces and affinities by which an ion in solution may become associated with the solid phase. The fixation of phosphorus by the exchange or substitution of phosphate ions for hydroxyl ions has been suggested by Mattson (1930), Demolon and Bastisse (1934), Scarseth (1935), Toth (1937),Stout (1939),Kelly and Midgley (1943),and Coleman (1944). When a clay suspension and a phosphate solution of similar pH are mixed, an increase in pH accompanies the disappearance of phosphate ions from solution. This can be t.aken as evidence that phosphate ions are replacing hydroxyl ions associated with the solid phase. Quantitative estimates of the hydroxyl ion release with phosphorus retention have also been reported. Other corroborative evidence of this phenomenon is the decrease in phosphorus retention with increasing pH and the release of phosphorus from soils extracted with dilute sodium hydroxide. The exchange of phosphate for hydroxyl ions can be accounted for in several ways. It can be postulated that it entails an exchange for the hydroxyl ions associated with surface of the clay minerals. Since the order of magnitude of the phosphate fixing capacity and the inorganic base exchange capacity are similar, all of the phosphate retention can be accounted for on the basis of a surface reaction. This exchange of surface hydroxyl ions is a rateher plausible mechanism when considering the fixation by minerals such as kaolinite and montmorillonite. However,

FIXATION OF SOIL PHOSPHORUS

403

evidence of silica release (Low and Black, 1947; Toth, 1937) accompanying phosphorus retention is not explained by this mechanism. The fixation by the hydrated oxides of iron and aluminum can also be ascribed to a simple substitution of phosphate for hydroxyl ions. Heck (1934a) discussed the fixation of phosphorus in terms of it being unnecessary for iron and aluminum to be in solution in order to have iron and aluminum phosphates formed. It is not clear whether the iron or aluminum phosphates formed are chemical precipitates or surface phosphates. Possibly this distinction is overemphasized. I n any event the chemical bonding between the iron or aluminum and phosphate ions is probably similar. Both Mattson (1930) and Davis (1935) foresaw that the fixation of phosphorus could increase the base exchange capacity of soils. Ample evidence is now available to demonstrate this increase in base exchange capacity. This increase in base exchange capacity is not inconsistent with the theory that phosphate ions are fixed by an exchange with surface hydroxyl ions, and would be applicable to both the hydrous oxides and the silicate minerals. However, Toth (1939) has shown that deferrated colloids may adsorb phosphorus without altering the base exchange capacity. Anion exchange may be defined as the reversible substitution of one anion for another. The term anion exchange immediately brings to mind an analogy with t.he well-known base exchange reactions. As compared with cations, only a limited number of anions having rather specific properties are considered as taking part in anion exchange reactions. Since the nitrates, chlorides, and sulfates are only sparingly retained by soils, any concept of anion exchange must exclude these ions. Phosphate ions which are fixed by soils can be displaced by hydroxyl, fluoride, silicate, arsenate, and possibly other anions (Dean and Rubins, 1947; Dickman and Bray, 1941; Stout, 1939; Weiser, 1933). If this exchange involves a reversible equilibrium of ions in solution with surface ions the possibility of an anion exchange reaction presents itself. Kolthoff (1936) describes an exchange adsorption between lattice ions in the surface of precipitates and foreign ions from the solution. It was shown that less restricted ions a t the corners and edges of ionic crystals may enter into exchange reactions with ions in the surrounding solution. When crystalline barium sulfate was placed in a solution containing chromate ions, there was an equivalent exchange between sulfate and chromate ions. Dean and Rubins (1947) have postulated that a similar reaction takes place when soils are placed in a solution containing phosphate ions. I n this instance ph0sphat.e ions in solution exchange for the hydroxyl ions associated with the surface of the clay mineral or hydrous

404

L. A. DEAN

oxides. Thus a reversible equilibrium would be established between hydroxyl ions of the soil surface and phosphate ions in solution and vice versa. If chemical forces and affinities are involved in this reaction, nitrate, chloride, and sulfate ions would not necessarily be expected to participate. Piper (1942) has suggested a method for determining the anionexchange capacity of soils. It is essentially a counterpart of the usual methods for determining the base exchange capacity. Soils are saturated with phosphate by treating with N ammonium phosphate at pH 4, washed with alcohol to remove the excess, the adsorbed phosphate displaced by sodium hydroxide and subsequently determined to give the anion-exchange capacity. Dean and Rubins (1947) sought to determine the anion exchange-capacity with respect to several different oombinations of anions. For example, soils were saturated with arsenate by treating with 0.5 M sodium arsenate p H 5.7, the excess removed with alcohol, the adsorbed arsenate dispIaced by 0.5 M sodium phosphate p H 5.7 and subsequently determined to give the anion-exchange capacity. Furthermore, it was also possible to proceed one step further and with the same soil sample displace the phosphate with arsenate. This ability of soils to become alternately saturated with phosphate and arsenate ions has been used as evidence of anion exchange. However, this does not completely preclude the possibility that chemical precipitates were not being formed. When soils are saturated with arsenate and phosphate by using 0.5 M solutions a t pH 5.7, more total phosphorus is adsorbed than arsenic. Also, all of the adsorbed arsenate is displaced by phosphate but all of the adsorbed phosphate is not displaced by arsenate (see Table 11). It would be interesting to know whether the differential adsorption was simply attributable to ionic size or whether other ionic properties are TABLE 2. Phosphate and Arsenate Adsorbed by Soils and Displaced by Anion Exchange a Results in millimols per 100 grams

Soil type

Total phosphorus adsorbed

Phosphorus displaced by arsenate

Total arsenate adsorbed

5.4 15.8 25.7

4.1

2.3

9.1 13.8 15.0

4*Q 9.1 112

Atwood h e sandy loam Sassafras loam Chester clay loam Davidson clay loam a

Dean and Rubim (1947).

32.0

Arsenate displaced by phosphate 3.8 3.9 10.0

10.0

405

FIXATION OF SOIL PHOSPHORUS

involved. There is also no indication as to whether mono- or divalent anions are involved. Another approach to the exchange equilibrium of phosphate and hydroxyl ions between surface and solution is through the use of isotopic exchange with Ps204 (McAuliffe et al., 1947). If a soil is suspended in water an equilibrium should be established between the phosphate ions in solution and those associated with the solid phase. Then, if Ps204 ions are introduced into the system without materially altering the total phosphorus concentration it should be possible to measure the extent of the equilibrium between the phosphate ions in solution and on the surface in accordance with the following reaction.

-

Pszo4f ps104 solution surface

PS204 f

surface

Ps104 solution

This is a simple isotopic exchange and the equilibrium constant should be equal to 1. Thus, knowing the total Ps204and measuring the P3204 and P3'04 in solution, it is possible to calculate the P3l04 on the surface that is in equilibrium with the P3lo4in solution. Such experiments have been performed. The rate of equilibration of PS2o4in solution wit,h

1

0

I

50

I

I I50

100

I

200

Time, Hours

Fig. 3. Changes in the ration P% surface to €% '

solution with time (McAuliffe

e t al, 1948).

Ps204 ions on the surface is indicated in Fig. 3. Apparently there are two distinct reactions. The first rapid reaction reaches equilibrium in approximately 32 hours whereas the second reaction shows no indication of a change in rate at the end of 200 hours. Thus two easily separable steps were observed in the phosphate interaction with soil surfaces. The

L. A. DEAN

406

extent of the reaction taking place in 32 hours was taken as the first step and the amount of surface P31 involved was calculated. This readily exchangeable surface P31 was compared with the NaOH soluble phosphorus (see Table 111). Only a small fraction of the phosphorus, of these TABLE 3. Relationship between the Surface P Readily Exchangeable with P and that Soluble in NaOH' ____~

Fraction Soil type Psssurface/ and number F soh. Moyock 451586 Caribou 451588 Caribou 451589 Caribou E969 Davidson E787 a

11

71 220 100 210

P3'soil solution ppm P 2.6 0.27 0.029 0.026 0.005

PS1 surface lbs. P per 2,000,000 Ibs.soi1

Na OH-soluble phosphorus lbs. P per 2,000,000 lbs. soil

surface/ NaOH-P x 100 = yo

290 190 64 26 10

1,800 2,700 1,700 1,150 520

16.0 7.O 3.8 2.3 2.o

Pa

McAuliffe et al. (1947).

soils, that was presumed to be held as exchangeable anions appears to be in direct equilibrium with the phosphorus in the soil solution. These experiments make it difficult to conceive of any close analogy between base exchange and anion exchange of soils, since the exchangeable bases of soils are all in equilibrium with the bases in the liquid phase.

VI. BIOLOGICAL FIXATION OF PHOSPHORUS IN SOILS Soil phosphorus adsorbed by plants is, in part, converted into organic compounds of phosphorus. When the plant products are returned to the soil they provide a source of energy for the soil microflora which in turn synthesize organic compounds of phosphorus. Thus, on the basis of prima facie evidence, i t may be concluded that the organic phosphorus of soils is contained in compounds and derivatives of compounds syn€hesized by plants and microorganisms, biological fixation of phosphorus being the process by which these compounds are formed. The presence of relatively large amounts of organic phosphorus compounds in soils is now well established. Until relatively recently, however, there was insufficient evidence to substantiate this assertion. 1. Distribution of Organic Phosphorus in Soils

The methods which have been proposed for the determination of the organic phosphorus in soiIs are indirect procedures. Two systems have

FIXATION OF SOIL PHOGPHOPUS

407

been applied, namely; the treatment of soils with hydrogen peroxide to oxidize the organic matter, followed by measurement of the increase in acid soluble phosphorus (Dickman and DeTurk, 1938; Peterson, 1911) , and the extraction of the organic with alkalies. The organic phosphorus is assumed to be the difference between the total and tGheinorganic phosphorus in the extract (Dean, 1938; Pearson, 1940; Potter and Benton, 1916; Schollenberger, 1918; and Wrenshall and McKibben, 1937). Certain objections could be raised pertaining to the accuracy of these methods; however, their use does permit useful generalization concerning the overall distribution of organic phosphorus in soils. Schollenberger (1920) studied the organic phosphorus contents of virgin and cultivated soils representative of 12 soil types of Ohio. When the organic phosphorus was expressed as per cent of the total phosphorus, the virgin and cultivated soils contained very nearly the same ratio of organic to total phosphorus. The range in organic phosphorus found in the surface soils was 18 to 52 per cent of the total phosphorus. Dean (1938) measured the organic phosphorus content of 34 surface soils from widely separated parts of the world. The organic phosphorus content of these soils was correlated with their carbon content and varied from 8 to 50 per cent of the total phosphorus. A study of the distribution of organic phosphorus in seven Iowa soil profiles by Pearson and Simonson (1939) has shown the amounts to range from 205 to 393 p.p.m. in surface soils to as low as 8 p.p.m. in the C horizons. The ratios of organic phosphorus t o organic carbon and nitrogen varied considerably within the individual profiles and from one soil type to another. Wrenshall and Dyer (1939) found 75 to 85 per cent of t.he total phosphorus in black muck soils and approximately 50 per cent of the phosphorus in podsol soils to be in organic combination. 2. Jdentification of the Organic Phosphorus Compounds in Soils

The efforts to identify and characterize the organic phosphorus in soils have centered about nucleic acids, phytin, and t,heir derivatives. The ether-soluble phosphorus fraction accounts for only about 1 per cent of the total organic phosphorus (Wrenshall and McKibbin, 1937). The common approach to characterizing the organic phosphorus compounds of soils has been to isolate phosphorus-rich fractions of the soil organic matter and study the properties of these fractions. Virtually complete extraction of the organic phosphorus can be facilitated by leaching soils with dilute hydrochloric acid to remove the calcium, followed by an extraction with hot sodium or ammonium hydroxide. When these alkali extracts are made slightly acid, the alpha humus precipikates and may be removed by filtration. Yoshida (1940)) working with sodium hy-

408

L. A. DEAN

droxide extracts from Hawaiian soils, showed that very little of the organic phosphorus was retained as part of the alpha humus. On the other hand, Dyer and Wrenshall (1941a), working with ammonium hydroxide extracts, showed that with the majority of the Canadian soile studied only about 10 per cent of the organic phosphorus passed into the filtrate from the alpha humus. Additions of ammonium oxalate before acidifying the alkali extract increased the organic phosphorus in the alpha humus filtrate to as much as 52 per cent of the total, These diecrepancies have remained unexplained. The elimination of the alpha humus without loss of important quantities of organic phosphorus is an important step in the isolation procedures used to date especially if quantitative data are sought. An additional concentration of the organic phosphorus can be effected by precipitation with alcohol of the filtrate from the alpha humus. Yoshida (1940) prepared ash-free soil organic phosphorus by adsorption on charcoal and desorbing with dilute ammonium hydroxide. The product obtained contained 7.4 per cent phosphorus, all in organic combination.

a. Nucleic Acids and Derivatives. Upon total hydrolysis nucleic acids yield phosphoric acid, pentose sugars, pyrimidine and purine compounds. The identification of nucleic acids in soils has been based upon the hydrolysis products obtained when organic phosphorus products prepared as described above are subjected to acid hydrolysis. Shorey (1913) identified pentose surgars, hypoxanthine, and adenine; Bottomley ( 1919) identified adenine and uracil. Wrenshall and McKibbin (1937) and Wrenshall and Dyer (1941) identified pentose sugars, adenine, uracil, xant.hine, and guanine, but Yoshida (1940) was unable to identify either purine or pyrimidine componnds. This evidence tends to support thc contention that some of the organic phosphorus in soils exists as nucleic acids. There is no evidence to indicate, hawewr, whnt proportion of t h e total organic phosphorus this comprises. Dyer and Wrenshall (1941b) compared thc rate of cieconiposition in soils of known nucleic acid and nucleotidr preparations with a preparation from soils which was presumed to contain nucleotide. These materials were mixed with soils which were in turn inciihatd and extracted with diluted acid. The increase in ac.id-sohrhle phosphorus was used as an index of decomposition. The soil preparations of organic phosphorus were found to be highly resistant to dephospharizstion as compared with the known nucleotides. It is commonly believed that a considerable part of the organic phos-

FIXATION OF SOIL PHOSPHORUS

409

phorus is made up nucleic acids. The evidence available, however, does not seem to wholly support this generalization. b. Phytin and Inositol Phosphates. Phytin is the calcium-magnesium salt of phytic acid (inositol hexaphosphate) ; intermediate derivatives such as inositol tri- and monophosphate are also known. Upon hydroly4 s the inositol phosphates yield phosphoric acid and inositol. Yoshida 11940) isolated and identified inositol in the hydrolysates of soil organic phosphorus preparations which had failed to reveal the presence of nucleic acid derivatives. Thus the conclusion that inositol phosphates were present. The phosphorus: ammonia-nitrogen ratio of the product suggested the presence of the ammonium salt of inositol monophosphate. Quantitative measurement of the amount of inositol and phosphoric acid liberated by hydrolysis indicated that only a part of the total soil organic phosphorus could be accounted for as inositol-phosphate compounds. Wrenshall and Dyer (1941) added ferric chloride to a N/6 hydrochloric acid solution containing soil organic phosphorus and separated a ferric salt having a P:Fe ratio similar to an authentic ferric phytate precipitated under similar conditions. By a similar procedure Bower (1945) separated a soil ferric phytate preparation which on hydrolysis yielded an inositol : inorganic phosphorus ratio corresponding to inositol hexaphosphate. Another fraction, isolated from the filtrate of the iron salt, gave an inositol : phosphorus ratio indicating the presence of derivatives of phytic acid. Quantitative estimates showed that approximately 35 per cent of the organic phosphorus of Carrington and Webster soils and 26.5 per cent of that of a Fayette soil occurred as phytic acid. I n addition, these same soils were found to contain 11.4 to 14.1 per cent of the organic phosphate as derivatives of phytic acid. The contention that an important part of the organic phosphorus of soils is present as inositol phosphates seems to be well substantiated. The observation by Wrenshall and Dyer (1941) that Fe- and Al-phytates are virtually immune to the action of phytase provides a possible explanation for persistence of phytin in soils. The writer is not aware of any reports indicating that microorganisms synthesize phytin or its derivatives, the implication being that the phytin in soils originated in the plant materials returned to the soil.

REFERENCES Allison, L. E. 1943. Soil Sci. 55, 333-342. Allison, L. E., and Scarseth, G. D. 1942. J. Am. SOC.Agron. 34, 616-623. Anderson, P. J., Morgan, M. F., and Nelson, N. T. 1927. Conn. Agr. Expt. Sta., Tobacco Sta. Bull. 7.

410

L. A. DEAN

Ballard, 5. S., and Dean, L. A. 1940. J . Applied Phys. 11, 386-370. Bassett, H. 1917. J . Chem. SOC.111, 620-642. Bear, F.E.,and Toth, S. J. 1942. Znd. Eng. Chem. 34, 49-52. Benne, E.J., Perkins, A. T., and King, H. H. 1936. Soil Sci. 42, 29-38. Black, C. A. 1942. Soil Sci. SOC.Am., Proc. 7, 123-133. Blair, A. W., and Prince, A. L. 1936. N J . Agr. Expt. Bta. Bull. 604. Bottomley, W. B. 1919. Proc. Roy. SOC.London B90, 39-44. Bower, C. A. 1945. Soil Sci. 59, 277-285. Bradfield, R.,Scarseth, G., and Steele, J. G. 1935. 3rd Intern. Cony. Soil Sci. 1, 7475. Bray, R. H., and Dickman, 5. R. 1941. h i 1 Sci. Soc. Am., Proc. 6,312-320. Brown, I. C., and Byers, H. G. 1938. US. Dept. Agr. Tech. Bull. 609. Bryan, 0. C. 1933. Soil Sci. 36, 245-259. Burd, J. S. 1948. Soil Sci. 65, 227-247. Cameron, F. K.,and Bell, J. M. 1907. U.S. Dept. Agr. Bur. of Soils Bull. 41. Chandler, W. V. 1941. J . Am. SOC.Agron. 33, 1-12. Coleman, R. 1942. Soil Sci. SOC.Am., Proc. 7, 134-138. Coleman, R. 1944. Soil Sci. SOC.Am., Proc. 9,72-78. Conrad, J. P. 1939. J . Agr. Research 59,507-518. Cummings, R.W. 1945. Sbil Sci. SOC.Am., Proc. 10,240-244. Davis, F. L. 1943. Soil Sci. 56,457-478. Davis, F. L. 1945. Soil Sci. 60, 481-489. Davis, F. L. 1946. Soil Sci. 61, 179-190. Davis, L.E. 1935. Soil Sci. 40, 129-158. Dean, L. A. 1934. Soil Sci. 37, 253-266. Dean, L. A. 1938. J . Agr. Sci. 28, 234-244. Dean, L.A,, and Rubins, E. J. 1947. Soil Sci. 63, 377-387. Demolon, A., and Bastisse, E. 1934. A m . Agron. 4, 53-76. Dickman, S. R.,and Bray, R. H. 1941. Soil Sci. 52, 263-273. Dickman, S.R.,and DeTurk, E. E. 1938. Soil Sci. 45, 29-40. Drosdoff, M.,and Truog, E. 1935. J. Am. SOC.Agron. 27, 312-317. Dyer, W. J., and Wrenshall, C. L. 1941a. Soil Sci. 51, 159-170. Dyer, W. J., and Wrenshall, C. L. 1941b. Soil Sci. 51, 323-329. Fisher, E.A. 1922. Trans.Faraday SOC.17, 305-316. Ford, M. C. 1933. J . Am. Soc. Agron. 25, 134-143. Fraps, G. S. 1922. Texas Agr. Expt. Sta. Bull. 304. Gaarder, T. 1930. Medd. Vestlandets foist. Forsdcssta. 14, 1-140. Gaarder, T., and Graehl-Nielson, 0. 1935. Medd. Vestlandets forst. Forsdcssta. 18, 1-107. Gile, P. L. 1933. US. Dept. Agr. Tech. Bull. 371. Heck, A. F. 1934a. Soil Sci. 37, 343-355. Heck, A. F. 193413. Soil Sci. 37,477-482. Heck, A. F. 1934c. Soil slci. 38, 463-470. Henderson, W. J., and Jones, U. 5. 1941. Soil Sci. 51, 283-288. Hester, J. B. 1937. J . Am. SOC.Agron. 29, 10-16. Hibbard, P.L. 1935, Soil Sci. 39, 337-358. Kelly, J. B., and Midgley, A. R. 1943. Soil 8ci. 55, 167-176. Kolthoff, I. M. 1936. J . Phys. Chem. 40,1027-1040. Kurtz, T , DeTurk, E. E., and Bray, R. H. 1946. Soil Sci. 61, 111-124. Laws, W.D.,and Page, J. B. 1948. Soil Sci. 62, 319336.

FIXATION OF S O L PHOSPHORUS

411

Lohse, W. H., and Ruhnke, G.N. 1933. Soil Sci. 36, 303-316. Low,P. F.,and Black, C. A. 1947. Soil Sci. Soc. Am., Proc. 12, 180-184. McAuliffe, C. D., Hall, N. S., Dean, L. A., and Hendricks, S. B. 1947. Soil Sci. SOC. Am., Proc. 12, 119-123. McCool, M. M. 1928. Proc. 1st Intern. Soe. Soil Sci. 2. McGeorge, W. T., and Breazeale, J. F. 1931. Ariz. Agr. Expt. Sta. Tech. Bull. 36. McGeorge, W.T.,and Breaseale, J. F. 1932. Ariz. Agr. Expt. Sta. Tech. Bull. 40. McGeorge, W. T. 1939. Ariz. Agr. Expt. Sta. Tech. Bull. 82. MacIntire, W.H.,and Hatcher, B. W. 1942. Soil Sci. 53, 43-54. Marais, J. S. 1922. Soil Sci. 13, 355-409. Mattson, S. 1930. Soil Sci. 30, 459-495. Mattson, S.,and Karlsson, N. 1938. Ann. Agr. Coll. Sweden 6, 109-157. Metzger, W.H. 1940. J . Am. Soc. Agron. 32, 513-526. Metsger, W.R. 1941. J. Am. Soe. Agron. 33, 1093-1099. Midgley, A.R. 1931. J . Am. Soc. Agron. 23, 788-799. Murphy, H.F. 1939. Hilgardia 12, 343-382. Nagelschmidt, G.,and Nixon, H.L. 1944. Nature 154. 428-9. Neller, J. R.,and Comar, C. L. 1947. Soil Sci 64,379-387. Pearson, R.W. 1940. Ind. Eng. Chem., Anal. Ed. 12, 198-200. Pearson, R. W., and Simonson, R. W. 1939. Soil Sci. SOC.Am., Proc. 4, 162-167. Peech, M. 1939. Flu. Agr. Expt. Sta. Bull. 340. Peech, M. 1945. Soil Sci. Soc. Am, Proc. 10,245-251. Perkins, A. T.,and King, H.H. 1943. Soil Sci. SOC.Am., Proc. 8, 154-158. Perkins, A. T.,Wagoner, C. E., and King, H. H. 1942. Soil Sci. 53, 37-41. Peterson, P. P. 1911. Wisc. Agr. Expt. Sta. Research Bull. 19. Piper, C. S. 1942. Waite Agr. Research Inst. Monograph. Potter, R. S., and Benton, T. H. 1916. Soil Sci. 2,2914. Rich, C.I., Obenshain, S. S., and McVickar, M. H. 1947. Soil Sci. SOC.Am., Proc. 12,270-274. Romine, D. S., and Metsger, W.H. 1939. J. Am. Soc. Agron. 31, 99-108. Russell, E.J., Prescott, J. A. 1916. J . Agr. Sci. 8, 65-110. Scarseth, G.D. 1935. J . Am. Soc. Agron. 27, 596-616. Scarseth, G.D., and Tidmore, J. W. 1934. J . Am. SOC.Agron. 26, 138-151. Schollenberger, C.J. 1918. Soil Sci. 6,365-95. Schollenberger, C.J. 1920. Soil Sci. 10, 127-141. Shorey, E.C. 1913. US.Dept. Agr. Bur. of Soils Bull. 88. Steele, G . J. 1935. Abstracts of Doctors’ Dissertations, No. 16, Ohio State Univ. Press, 203-217. Stephenson, R. E.,and Chapman, H. D. 1931. J. Am. SOC.Agron. 23, 759-770. Stout, P.R. 1939. Soil Sci. Soc. Am., Proc. 4, 177-182. Toth, S. J. 1937. Soil Sci. 44, 299-314. Toth, S.J. 1939. Soil Sci. 48,385-401. Truog, E. 1916. Wisc. Agr. Expt. Sta. Res. Bull. 41. Weiser, V. L. 1933. Vt. Agr. Expt. Sta. Bull. 356. Wrenshall, C. L.,and McKibbin, R. R. 1937. Can. J . Research B15,475-79. Wrenshall, C. L.,and Dyer, W.J. 1939. Can. J. Research B17, Wrenshall, C. L.,and Dyer, W. J. 1941. Soil Sci. 51, 235-248. Yoshida, R. I<. 1940. Soil Sci. 50, 81-89.

This Page Intentionally Left Blank

Author Index Names in parenthesev indicate cuauthom of the references and are included t o assist in locating reference6 where a particular name is not on a given page. Ezample: Adams, A. J., 46 (see Curtis), 73 (SF Cueis) means that Curtis et al. will be mentjoned on page 48, the et caE: accounting for Adams. This article can he located under Curtis in the list of references. Numbers In italics refer to the pages on which references are listed in bibliographies at the end of each article.

A Aberg, B., 293, 316

Adams, A. J., 46 (see Curtis), 78 (see Curtis) Adams, J. E., 103,162,274 (see Morrow), 686 (see Morrow) Adams, J. R., 51 (see Robs, Whittaker), 76 (see Ross), 76 (see Whittaker) Adati, M., 266, 271, 282 Addams, R. M., 293 (see Avery), 301 (see , Avery), 316 (see Avery) Agati, J. A., 106, 162 Agafonoff, V., 173, 200 Agricola, G., 322, 348 Agulhon, H., 322, 324, 3@, 349 (see Bertrand) Ahi, S. M., 24, 31, 36 Ahlgren, H., 292, 293, 295, 306, 316 Akamine, E., 293, 316 Akeley, R. V., 356, 386 Akerberg, E., 214, 238 Akesson, N. B., 312, 316 Albert, A. R., 102, 162 Albrecht, H. R., 234 (see Brink), 238 (see Brink) Albrecht, W. A., 94, 102, 106, 162, 163 (see Ferguson, Hampton), 192, 200, 279,282,371,986,389 (see Schroeder) Aldrich, D. G., 177, 200, 292, 916 Alexander, E. D., 69, 72 Alexander, L. T., 161, 162, 170, 178, 185 (see Hendricks), 186 (see Hendricks), 200,602 (see Hendricks) Allard, H. A., 85, 86, 87, 162, 163 (see Garner) Allard, R. W., 304 (see Ennis), 310, 916, 317 (see Ennis), 380 (see Ennis), 987 (see Ennis)

Allaway, W. H., 60, 73 (see Fitts), 173, 186 (see Grim), 193, 200, 202 (see Grim) Allen, D. I., 94, 162 Allen, T. C., 382, 386 Allington, W. B., 108, 109, 144, 145, 147, 162, 166 (see Porter) Allison, F. E., 104 (see Pinck), 166 (see Pinck), 250 (see Pinck), 253, 259, 260, 282, 286 (see Pinck) Allison, L. E., 32 (see Reeve), 34,37 (see Reeve), 395, 401, 40.9 Allison, R. V., 366, 386 Allman, S. F., 214, 298 (see Dwyer) Alway, F. J., 48, 72 Aniundsen, R. F., 303 (see Shaw), 319 (see Shaw) Anderson, A , 233 (see Kiesselbach), 239 (see Kiesselbach) Anderson, J. C., 292, 295, 306, 307, 916 Anderson, P. J., 392, 409 Anderson, W. S., 19 (see Leonard), 37 (see Leonard) Andrews, W. B., 56, 58, 72, 91, 106, 107, 162 Angell, H. R., 262 (see also Link, Walker), 282, 284 (see Link), 987 (see Walker) Anthony, R. D., 177, 178, 202 (see Jeff ries) Apoiger, F., 322, 336, 561 (see Wittstein) Apple, J. W., 382 (see also Linn), 387, 388 (see Linn) Appleman, M. D., 109, 162, 277, 279, 282 Arant, F. S., 224, 238 (see Eden) Ardenne, M., 180, 200 Arenz, B., 341, 360 (see Schropp) Arle, H. F., 292 (see Leonard), 314 (see Leonard), 318 (see Leonard)

413

414

AUTHOR INDEX

Armiger, W. H., 46 (see Hill), 51, 73 (see also Hill), 253 (see Allison), 259 (see Allison), 2886 (see Allison), 366, 388 (see Jacob) Armstrong, J. M., 207, 208, 210, 211, 238 Arneson, M. A., 231 (see Reitz), 232 (see Reitz), 235 (see Reitz), 236 (see Reitz), 237 (see Reitz), 139 (see Reitz) Arnold, C. Y., 382 (see also Linn), 387 (see Apple), 388 (see Linn) Arnon, D. I., 19, 36 Asdonk, T., 368, 387 Ashbaugh, F. A., 305, 316 Askew, H. O.,332, 848, 349 Attoe, 0. J., 194, BOO Atwater, C. G., 343, 3-49 Atwood, S. S., 206, 238 Avery, G. S., 293, 301, 316 Ayers, A. D.,8, 11 (see Magistad), 14, 16, 17 (see Magistad), 22, 25 (see Magistad), 26, 27, 28, 30, 31 (see also Magistad), 32,36,97 (see Magistad), 38 (see Wadleigh), 191, 192 (see Jenny), 202 (see Jenny)

B Baeza, M. A., 304 (see Smith), 319 (see Smith), 380 (see Smith), 389 (see Smith) Bainer, R., 315, 316 Baker, G. O.,48, 76 (see Toevs), 326, 349 (see Colwell) Bakke, A. L., 312, 320 (see Sylwester) Baldwin, I. L., 248 (see Fred), 183 (see Fred) Ballard, S. S., 394, dl0 3arker, H. A., 256, 182 Barrons, K. C., 295, 305, 308, 316 Bartholomew, R. P., 60, 73, 109,162 Baskervill, W. H., 48 (see Copson), 7 3 (see Copson) Baslavskaja, S. S., 17, 36 Bassett, H., 399, 410 Bastisse, E., 402 (see Demolon), 410 (see Demolon) Batchelor, L. D., 67, 75 (see Parker) Bateman, H. P., 115, 162 Bates, G. H., 378, 387

Baumeister, W., 341, S@ Baver, L. D., 160, 194, ,900, 203 (see Page) Beale, 0. W., 198, 204 (see Peele), 263, 286 (see Peele) Bear, F. E., 15 (see Wallace), 38 (see Wallace), 333 (see Reeve), 334 (see Reeve), 360 (see Reeve), 367, 387, 398, 410 Beeson, K. C., 93, 162, 327, 3@ Beeson, K. E., 101, 162 Beijerinck, M. W.,252, 254, ,982 Bell, J. M., 399 (see Cameron), 410 (see Cameron) Bell, R. E., 42, 73 Benedict, H. M., 316 Benne, E. J., 394, 410 Bennett, E. R., 361, 387 Bennett, H. W., 70, 74 (see Means) Bentley, F., 218, 238 Benton, T. H., 407 (see Potter), 411 (see Potter) Berezova, E. F., 246, 260, 288 Berger, K. C., 326, 326, 328, 3% (see Kubota), 330, 331, 333 (see Kubota), 342, 349, 360 (see Kubota, Olson), 370, 373, 377, 387 Bergman, W. E., 162,103 (see Marshall) Berkner, F., 365, 587 Bernal, J. D., 184, 185, 186, 197, 201 (see also Fowler) Bernstein, L., 26 Bertrand, G., 324, 344, 345, 349 Binkley, A. M., 379 (see Kunkel), 388 (see Kunkel) Bird, J. J., 360, 363, 372, 887 Bizzell, J. A,, 250 (see Lyon), 251 (see Lyon), 286 (see Lyon) Black, C. A., 195, $03 (see Low), 255,267, 287 (see Thompson), 395, 396, 397, 399 (see Low), 403 (see Low), 410, 411 (see Low) Blackman, G. E., 301, 316 Blair, A. W., 392, 410 Blair, W. M., 27, 36 (see Hayward) Blank, L. M., 267 (see Clark), 283 (see Clark) Blaser, R. E., 43, 73 (see Bledsoe) Bledsoe, R. W., 43, 73 Blodgett, F. M., 357, 387

AUTHOH, INDEX

Blood, P. T., 102 (see Prince), 103 (see Prince), 165 (see Prince), 364 (see Prince), 389 (see Prince) Blumer, C., 214 (see Harrison), 239 (see Harrison) Bobko, E. V., 330, 349 Boggs, H. M., 103 (see Adams), 162 (see Adams) Bohart, G. E., 216, 238 Bollen, W. B., 260 (see McBurney), 286 (see McBurney) Bolley, H. L., 293, 916 Bolton, J. L., 208, 209, 211, 212, 213, 214, 215, 216, 217, 220, 221, 225, 226, 227, 228, 229, 231, 238, 239 (see Peck), 840 (see Stevenson) Bond, G., 105,162 Bonde, R., 378 (see also Schultz), 385, 387, 389 (see Schultz) Bonner, J., 260, 282, 887 (see Thimann) Bordakow, P. P., 119, 162 Borthwick, H., 86, 87 (see also Scully), 88 (see also Parker), 89 (see also Heinze), 162, 163 (see Heinze), 166 (see Parker, Scully) Bortner, C. E., 44 (see Karraker), 74 (see Karraker) Bottomley, W. B., 408, 410 Bower, C. A., 9, 19, 36, 94, 162, 268, 882, 409, 410 Bowling, J. D., Jr., 17 (see Garner), 36 (see Garner) Boyd, F. T., 304 (see Ennis), 3i7 (see Ennis), 378, 380 (see Ennis), 387 (we Ennis), 989 (see Wilson) Boynton, D., 16, 36, 68, 69 (see Fisher), 73 (see also Fisher) Boysen-Jensen, P., 260, 989 Bradfield, R., 192, 901, 904 (see Peech), 397, 410 Bradley, W. F., 162 (see also Grim), 168 (see Grim), 169 (see Grim), 170, 173, 174 (see Grim), 187, 193, 201 ( S P C also Clark), 202 (see Grim) Brady, N. C., 370, 388 (see Nelson) Brain, S. G., 314, 319 (see Neely) Brandenburg, E., 322, 349 Bray, R. H., 162 (see also Grim), 168 (see Grim), 169 (see Grim), 170, 172, 191,194 (see aIso De Turk), I96 (see

415

also Kurtz), 201 (see also De Turk, Dickman, Grim), LO2 (see Grim), 203 (see Kurtz), 392, 401 (see Kurtz), 403 (see Dickman), 410 (see also Dickman, Kurtz) Breazeale, J. F., 9, 19, 20, 36, 37 (see McGeorge), 280, 282, 398 (see McGeorge), 399 (see McGeorge), 410 (see McGeorge) Breed, R. S., 273, 282 Brenchley, W. E., 332, 349 Brentzel, W. E., 380, 387 Brickley, W. D., 367, 387 Bridger, G. L., 45, 46, 76 Briggs, G. B., 341, 3-49 Briggs, G. M., 102 (see Albert), 162 (see Albert) Brink, R. A., 207, 208, 209, 234, 838 Briscoe, C. F., 106, 158 (see also Andrews), 168 Broadbent, F. E., 250, 882 Bronson, T. E., 384, 387 Brooks, F. F., 235 (see Toovey), 236 (see Toovey), 240 (see Toovey) Brooks, 0. L., 54, 7 3 Brown, A. L., 304 (see Savage), 305 (see Savage), 319 (see Savage) Brown, B. A., 62, 73, 360, 364, 372, 387 Brown, B. E., 49, 51 (see Whittaker), 73, 76 (see Whittaker), 365, 366, 371 (see Hawkins), 387, 388 (see Hawkins) Brown, C. A., 302, 303, 313, 316 Brown, 1. C., 392, 4 l O Brown, J. G., 15 (see Lilleland), 28 (see Lilleland), 34 (see Lilleland), 37 (see Lilleland) Brown, J. W., 7, 18, 27, 31, 296, 304, 306, 318 (see Mitchell) Brown, P. E., 250, 278, 282 Brown, R., 260, 282 Brown, S. M., 64 (see Chapman), 67 (see Chapman), 73 (see Chapman), 161 (see Kelley), 173 (see Kelley), 178 (see Kelley), 187 (see Kelley), 188 (see Kelley), 202 (see Kelley) Browning, G. M., 105, 137, 138, 139, 140, 141, 162, 166 (see Norman), 198 (see also Johnston, 201 (see Feng), 202 (see Johnston), 20.1 (see Wilson), 280

416

AUTHOR INDEX

(see also Johnston), 283 (see Feng), 284 (see Johnston), 288 (see Wilson) Brayer, T. C., 256, 282 (see Barker) Brunner, A., 263, 286 (see Sekura) Bruce, H. D., 295 (see Crafts), 305 (see Crafts), 316 (see Crafts) Bryan, 0. C., 392, 4lO Buehrer, T. F., 171, 173, 174, 178, 201 Ruffum, B. C., 20, 21, 25, 55, 37 (see Slosson) Bull, H. B., 16, 37 (see Moyer) Burd, J. S., 410 Burgess, P. S.,4, 36 Burkhart, L., 103 (see Nelson), 166 (see Nelson) Burlison, W. L., 107, 110, 163 Burnham, P., 184 (we Cross), 201 (see Cross) Burrell, A. B., 16, 36 (see Boynton) Burrell, R. C., 120 (see Wolfe), 167 (see Wolfe) Burris, R. H., 105, 166 (see Umbreit) Bursik, J., 312 (see Price), 319 (see Price) Burton, J. C., 288 (see Wilson) Burvill, G. H., 361 (see Teakle), 389 (see Teakle) Bushnell, J., 363, 367, 382, 387, 389 (see Sleesman) Byers, H. G., 326 (see Whetstone), 327 (see Whetstone), 331 (see Whetstone), 367 (see Whetstone), 392, 410 (see Brown) C

Caillere, S., 173, 203 Calder, R. H., 207, 208, 211, 212, 214, 239 (see Hadfield) Caldwell, A. C., 60, 62, 73 Caldwell, 0. G., 180, 203 (see Marshall) Calfee, R. K., 323, 324, 349, 360 (see McHargue) Calland, J. W., 101, 111, 163 Callbeck, L. C , 380, 387 Cameron, F. K., 31, 36, 399, 4l0 Camp, A. F., 68, 73 Campbell, J. C., 49 (see Houghland), 73 (see Houghland), 383 (see Pepper), 384 (see Pepper), 386, 387, 388 (see Pepper)

Cannon, H. B., 374, 388 Carlson, J. W., 207, 208, 211, 220, 221, 222, 224, 225, 238, 240 (see Sorenson) Carlson, R. F., 30'4, 310, 316 Carlyle, R. E., 277, 282 Carolus, R. L., 362, 387 Caron, A., 274, 282 Carr, R. S., 110, 163 (see Henson) Carter, D. G., 115, 162 (see Bateman) Cartter, J. L., 97, 98, 99, 102, 103, 115, 117, 118, 123, 131, 136, 150, 163 (see also Earley), 164 (see Morse). 166 (see Probst) Cassal, C. E., 324, 349 Chamberlain, D. W., 144, 145, 147. 16.2 (see Allington), 163 Chambers, R., 16, 96 Chamblee, D. S., 70 (see Woodhouse), 76 (see Woodhouse) Chaminade, R., 194, 201 Chandler, F. B., 372, 387 Chandler, R. F., Jr., 48, 49, 53, 73 Chandler, W. V., 395, 410 Chang, S. C., 57, 74 (see Jackson) Chapman, C. J., 102 (see Albert), 152 (see Albert) Chapman, J. E., 264, 284 (see Hubbell) Chapman, H. D., 19, 36, 56, 57, 64, 67, 73, 340, S49, 395 (see Stephenson), 411 (see Stephenson) Chase, F. E., 272, 284 (see Lochhead) Chenoweth, 0. V , 64 (see King), 74 (see King) Chermezon, H., 6, 36 Chilton, S. J. P., 232, 238 Chittenden, E., 332 (see Askew), 349 (see Askew) Choudhri, R. S., 277, 288 (see Wilson) Christiansen, J. E., 30, 32, 33, 34, ST (see Magistad), 327, 360 (see Magistad) Chucka, J. A., 360, 361, 364, 367, 368, 371 (see Hawkins), 374, 387, 388 (see Hawkins) Clark, C. F., 386, 387 Clark, F. E., 245, 251, 253, 255 (see Thompson), 261 (and see Stumbo), 262, 265 (see also Mitchell, Stumbo), 266, 267, 268, 269, 270, 271, 272, 273, 275 (see also Stumbo, Mitchell), 276,

417

.4UTHOR INDEX

282, $83 (see Goring), 186

(see Mitchell), 287 (see Stumbo) Clark, G. L., 174, 201 Clark, K. G., 48, 49 (see also Houghland), 51, 73 (see Brown, Fuller, Houghland), 74 (see Madorsky) Clark, N. A., 260, 283 Clarke, E. J., 16, 38 (see Walsh) Clarke, I., 231, 240 (see Tysdal) Coates, W. H., 364 (see Prince), 389 (see Prince ) Colberg, W. J., 382 (see Post), 389 (see Post) Coleman, N. T., 178, 181, 195, 201 Coleman, 0.H., 233 (see Weihing), 2440 (see Weihing) Coleman, R., 329, 8.49, 395, 397, 399, 402, 410 Collander, R., 15, 36 Collins, E. R., 102, 103, 163, 369, 387 Colwell, W. E., 103 (see also Nelson), 163, 164 (see Nelson), 326, 331, 349 Comar, C. L., 394 (see Nellar), 411 (see Nellar ) Conover, R. A., 147, 153 Conn, H. J., 273, 283 Conrad, J. P., 13, 36, 395, 410 Conrad, P. F., 303 (see Shaw), 319 (see Shaw) Cooil, B. J., 7, 27, 31 Cook, H. L., 55 (see Scarseth), 59 (see Scarseth), 76 (see Scarseth) Cook, H. T., 371, 387 Cook, R. L., 65, 76 (see Peikert), 323, 3.49 Cook, W. H., 293, 316 Coon, B. F., 149, 163 Cooper, D. C., 207, 208, 209, 238 (see Brink) Copson, R. L., 46 (see Curtis), 48, 73 (see also Curtis) Cordner, H. B., 387 Cormack, M. W., 232, 235, 238 Corum, C. J., 370, 387 Costello, D. F., 304,305,319 (see Savage) Coulter, L. L., 295,305,316 (see Barrons) Cowie, G. A., 363, 368, 587 Cox, G. M., 135, 167 (see Weiss) Cox, H. R., 304, 916 (see Ahlgren) Cox, T. R., 71, 73, 365, 387 Crafts, A. S., 291 (see Robbins), 292

(see also Robbins), 293 (see also Robbins), 294, 295, 296, 297, 298, 299, 300, 304, 305, 306, 307, 308 (see also Robbins), 309, 311 (see Hannesson), 312 (see also Robbins), 313, 315, 316, 318 (see Hannesson) Crtlndall, B. H., 213, 216, 225, 226, 229, 230, 232, 238, 24O (see Tysdal) Cross, P. C., 184, 201 Croxall, H. E.,,370 (see Wallace), 389 (see Wallace) Crump, S. L., 98, 16Y Cummings, R. W., 54, 73, 331, 349 (see Colwell), 410 Currie, G. A., 292, 817 Currier, H. B., 296 (see Crafts), 304 (see Crafts), $17 (see Crafts) Curtis, H. A., 46, 73 Cuthbert, F. L., 186, 10.2 (see Grim) Cutler, G. H., 124, 163 Cseratzki, W., 195, 203 (see Nitzsch)

D Daines, R. H., 366, 387 Daugherty, 16 Davidson, R. C., 163, 201 Davidson, R. S., 385, 387 Davies, R. O., 362, 387 Davis, E. H., 64, 73 Davis, F. L., 393, 402, 410 Davis, L. E., 399, 400, 401, 403, 4 l O Davy, R. H., 108, 163 Dawson, R. E., 278, 283 Dawson, R. F., 259,286 (see Routien) Dawson, V. T., 276 (see Smith), 278, 283, 286 (see Smith) Dayton, W. A., 78, 164 Dean, L. A., 181, 195 (see also McAuliffe), 201, 203 (see McAuliffe), 393, 394 (see McAuliffe, Ballard), 395, 396, 398, 403, 404, 406 (see McAuliffe), 407, 410 (see also Ballard), $11 (see McAulXe) Debye, P., 184, 186, 201 Decker, G. C., 385, 388 Decker, P., 384 (see Wolfenbarger), 390 (see Wolfenbarger) De France, J. A., 311, 317 Deherain, P. P., 279,283

418

AUTHOR INDEX

Deming, J. M., 171 (see Buehrer), 173 (see Buehrer), 178 (see Buehrer), 901 (see Buehrer) Demolon, A., 402, 4i0 Dennis, A. C., 342, 34.9 Dennis, R. W. G., 327, 334, 342, 34.9 De Rose, H. R., 296, 304, 306, 310 (see Allard), 313, 316 (see Allard), 317 Derscheid, L, A,, 310, 312, 317 Desai, A. D., 178 (see Nagelschmidt), ,803 (see Nagelschmidt) De Turk, E. E., 194, 195 (see Kurtz), 201 (see also Bray), 203 (see Kurtz), $04 (see Wood), 401 (see Kurtz), 407 (see Dickman), 410 (see Dickman, Kurtz) De Waals, H. L., 344,345, 3.49 (see Bertrand) Diachun, S., 280 (see Valleau), 283, 287 (see Valleau) Dickey, R. D., 68, 73 Dickman, S.R., 195,201 (see also Bray), 392 (see Bray), 403, 407, 410 (see also Bray) Dickson, A. D., 262 (see Link), 284 (see Link) Dimmick, I., 273, 283 (see Conn) Dimond, A. E., 385, 387 Dobson, S. H., 69, 73 Dodd, D.R., 141, 142, 163 Domingo, W. E., 117, 118, 163 Doolas, G.Z., 91, 163 Dore, W. H., 161 (see Kelley), 178 (see also Kelley), 202 (see Kelley), 322, 337, 360 (see Johnston) Dorokhova, M. A., 263,283 Dorsett, P. H.,123 Dostal, R., 372, 387 Doughty, J. L., 279, 283 Drake, M.,62, 73, 104, 163, 332, 3@ Dreyspring, C., 332 (see Krugel), 360 (see Krugel) Drosdoff, M., 68 (see Dickey), 73 (see Dickey), @O Dufrenoy, J., 259, 286 (see MacDougal) Duley, F. L.,260, 286 (see McCalla) Dungan, G. H., 99, 163 Dunham, R. S., 114, 163

Dunklee, D. E., 330, 34.9, 360 (see Midgley) Dunlap, A. A., 302, 313, 917 Dutt, A. K., 197, 198, 201 Dwyer, R. E. P., 214, 231, 238 Dyer, W. J., 407, 420, 412 (see Wrenshall) Dyke, K., 57, 73

E Earley, G. B., 96, 97, 163 Eaton, F. M., 12, 17, 19, 25, 26, 28, 33, 36, 261, 276, 283, 327, 343, 3.49 Edelman, C. H., 167, 186, 187, 201 Eden, W.G., 224, 238 Edmundson, W. C., 379 (see Kunkel), 388 (see Kunkel) Edwards, F. E., 56, 58 (see Andrews), 79 (see Andrews) Eide, C. J., 357, 388 (see Krantz) Ekdahl, I., 317 Elder, S., 327, 34.9 Eldredge, J. C.,100 (see Kalton), 164 (see Kalton) Eliason, E. J., 309, 317 Ellison, J. H., 304 (see Smith), 319 (see Smith), 380 (see Smith), 389 (see Smith) Elson, J., 198, 201 Emanuelli, A,, 308, 317 (see Crafts) Endell, K., 161, 166 (see Hofmann), 180 (see Ardenne), 186 (see Hofmann), 200 (see Ardenne), 202 (see Hofmann) Enfield, G. H., 104, 163 Englehorn, A. J., 101, 104, 163 Ennis, W. B., Jr., 304, 310 (see also Allard), 316 (see Allard), 317, 380, 387 Ensminger, L. E., 171, 178, 193, 195, 201, 203 (see Pearson) Erickson, A. E., 193, 201 Evans, L. S, 295, 304, 310, 311, 317, 328 (see Kephart) Evans, M. W., 208 (see Piper), 23Q (see Piper) Ewing, F. J., 163 (see Davidson), 901 (see Davidson)

ATJTHOR lNDLX

F

419

Fullmer, F. S., 64, 74 (see McCollam) Fults, J. L., 277, 286

Fagan, T. W., 362, 387 (see Davies) Falck, R., 259, 283 G Falkenhagen, H., 186, 201 (see Debye) Faraday, M., 324, 349 Gaarder, T., 398, 410 Favejee, J. Ch. L., 167, 177, 186, 187, 201 Gaddy, V. L., 104, 166 (see Pinck), 250 (see also EdeIman) (see Pinck), 253 (see Allison), 259 Federer, W. T., 136 (see Houseman). (see Allison), 282 (see Allison), 154 (see Houseman) 286 (see Pinck) Feigl, F., 325, 349 Gainey, P. L., 254, 261 (see Stumbo), Fellows, H., 261, 283 265 (see Stumbo), 275 (see Stumho), Feng, C. L., 198, 201, 280, 283 283, 287 (see Stumbo) Ferguson, C. E., 94, 163 Garber, R. J., 124, 135, 163, 156 (SCP Ferguson, W., 328, 349 OdIand) Fernow, K. H., 380, 387 Garcia, E. H., 106, 168 (see Agati) Findlay, D. H., 379, 387 Gard, L. E., 138 (see Van Doren), 156 Finn, R. F., 257, 286 (see Mitchell) (see Van Doren) Fireman, M., 12, 14, 20, 28, 36, 38 (see Gardner, R., 4, 36 Wadleigh) Garman, W. H., 56, 58, 73 Fisher, E., 69, 73 Garner, W. W., 17, 36, 85, 86, 87, 152 Fisher, E. A., 401, 4 0 (see Allard), 163 Fitting, H., 6, 36 Garrett, S. D., 261, 275, 283 Fitts, J. W., 60, 78 Garthside, S.,292, 317 (see Currie) Forbes, I., Jr., 51 (see Armiger), 73 (see Gauch, H. G., 11 (see Magistad), 14, 15, Armiger) 16, 17 (see also Magistad), 18, 25 Ford, M. C., 393, 395, 410 (see Magistad), 28, 30, 32, 32 (see Foster, A. C., 343, 349 Wadleigh), 33 (see also Wadleigh) Foster, J. S., 323, 349 36, 37 (see Magistad), 58 (see WadFowler, R. H., 184, 185, 186, 197, m1 leigh) (see also Bernal) Geddes, W. F., 107, 165 (see Ramstmad) Francke, H. L., 259, 288 Gedroix, K, K., 19, 36 Frank, A. B., 257, 283 Geltser, F. V., 263, 283 Frank, F. A., 142, 143, 169 Gelzer, F. J., 198, 201 Fraps, G. S., 393, 394, 395, 410 Gerrans, H., 324, 349 (see Cassal) Fred, E. B., 90, 92, 163, 166 (see UmGerretsen, F. C., 254, 256, 257, 258, 259, breit), 248, 256, 279, 283 283 Freed, V. H., 302, 310, 311, 312 (see Gesue, G., 270 (see Rossi), 286 (see Price), 317, 319 (see Price) Rossi) Freeman, J. F., 44 (see Roberts), 61, 74 Gibson, R. M., 100, 163 (see Karraker), 76 (see Roberts) Gieger, M., 19 (see Leonard), 37 (see Fried, M., 162 (see Wear), 204 (see Leonard) Wear) Gieseking, J. E., 186, 191, 193, 194, 197, Fry, W. H., 161, 202 (see Hendricks) 201 (see also Ensminger) Fryer, J. R., 209, 218, 238 (see Bolton), Gilbert, S. G., 95 (see Somers), 156 (See 240 (see Lexsmith) Somers) Gile, P. L., 395, do Fuelleman, F. R., 99, 163 Gilman, J. C., 144, 167 (see Welch) Fujimoto, C. K., 255, 261, 183 Gish, R. E., 280, 283 Fukuda, Y., 121, 129, 163 Godel, G. L., 291, 317 Fuller, W. H., 51, 78

420

ATJTHOR INDEX

Goldschmidt, V. M., 323, 349 Goldsmith, G. W., 261, 283 (see Gott) Gooding, G. V., 231 (see Reitz), 232 (see Reitz), 235 (see Reitz), 236 (see Reitz), 237 (see Reitz), 23.9 (sep Reitz) Goring, C. A. I., 251, 283 Gorkova, I. M., 197, 201 Gortner, R. A,, 7 (see Harris), 36 (see Harris) GOSS,A., 4, 36 GOSS,W. H., 85, 166 (see Shollenherger) Gott, C. L. T., 261, 283 Gottheil, O., 270, 283 Gottschall, R., 323,325,360 (see Scharrer) Graehl-Nielson, O., 398 (see Gaarder) , 410 (see Gaarder) Graf, G., 246, 252, 269, 283 Graham, E. R., 93, 163, 192, 201 Grainger, J., 378, 388 (see Main) Granberg, W. J., 46, 73 Grand, C. G., 16 (see Chambers), 36 (see Chambers) Grandfield, C. O., 218, 219, 228, 231 (see Reitz), 232 (see Reitz), 233, 234, 235 (see Reitz), 236 (see Reitz), 237 (see Reitz), 238, 239 (see Reitz) Granovsky, A. A., 382, 388 Gray, L., 93 (see Beeson), 152 (see Bee-

son) Gray, S. D., 373, 388 Greaves, J. E., 251, 252, 274, 283, 378 (see Wilson), 389 (see Wilson) Green, J., 64, 74 (see Jones) Green, V. E., 252, 288 (see Willis) Greenwood, D. E., 384, 388 Gregory, L. E., 320 (see Van Overbeek) Griffin, H. H., 4, 36 (see Goss) Griffith, D. T., 42, 73 (see Bell) Griffith, J. E., 257, 286 (see McComb) Grigsby, B. H., 308, 310, 314, 317 Grim, R. E., 161, 162, 166, 167, 168, 170, 172 (see Bray), 173, 174 (see Clark), 179, 186, 190, 201 (see also Bradley, Bray, Clark), 202 Grizzard, A. L., 218, 239 Gromyko, E. P., 263,286 (see Mishusten) Gruner, J. W., 161, 162, 168, 202

(hiest, P., 274 (see Morrow), 285 (see Morrow) Gyrisko, G . G., 383, 385, 388

H Haas, A. R. C., 256, 283 (see Fred), 324, 321, 329, 337, 349 Hackerott, H. L., 228 (see Grandfield), 239 (see Grandfield) Hackleman, J. C., 107 (see Burlison), 110 (see Burlison), 162 (see Burlison) Hadding, A., 161, 202 Haddock, J. L., 178, 204 (see Russell) Hadfield, J. W., 207, 208, 211, 212, 214, 239 Hageman, R. H., 18 (see Heller), 36 (see Heller), 96 (see Hodgkiss), 164 (see Hodgkiss) Hagood, E. S., 301 (see Brown), 316 (see Brown) Halferdahl, A. C., 293, 316 (see Cook) Hall, N. S., 195 (see McAuliffe), 203 (see McAuliffe), 394 (see McAuliffe), 405 (see McAuliffe), 406 (see McAuliffe), 4f 1 (see McAuliffe) Hamilton, J., 68 (see Dickey), 73 (see Dickey) Hammons, J. G., 56, 58 (see Andrews), 72 (see Andrews) Hamner, C. L., 87, 88, 93, 163, 277, 284 (see Lewis), 301, 305, 317, 318 (see Mitchell) Hamner, K. C., 93 (see Beeson), 162 (see Beeson) Hampton, H. E., 106, 163 Hance, F. E., 303, 308, 309, 317 Hanks, R. W., 306, 318 Hanna, W. J., 329, 860 (see Pun&) Hannesson, H. A., 311, 318 Hanning, F., 308, 3.20 (see Warren) Hansen, P. L., 82, 163 Hansing, E. D., 228 (see Grandfield), 289 (see Grandfield) Hanson, N. S., 303, 318 Hare, Q. A., 214, 215, 239 Harlan, J. R., 304 (see Savage), 305 (see Savage), 319 (see Savage) Harley, C. P., 18, 56 Harmsen, G. W., 266, 283

AUTHOR INDEX

Harper, H. J., 17, 27, 36' Harrington, J. B., 291, 319 (see Pavlychenko) Harrington, J. L., 360 (see Chucka), 361 (see Chucka), 364,367 (see Chucka), 368 (see Chucka), 374 (see Chucka), 387 (see Chucka) Harris, F. S., 4, 21, 22, 25, 89, 30, 31, 32, 33, 34, 36 Harris, J. A., 7, 17, 56 Harris, V. C., 292 (see Leonard), 314 (see Leonard), 318 (see Leonard) Harrison, C. M., 214, 239 Harter, L. L., 23, 26, 36 (see also Kearney) Hartmann, E. L., 18 (see Heller), 36 (see Heller) Hartwig, E. E., 94, 102 (see also Collins), 103 (see also Collins), IbC3(see Collins), 164 (see Nelson) Harvey, R. B., 318 Harvey, W. A., 291, 292, 293, 295, 300, 312, 516 (see Akesson), 518 Hastings, E. G., 279 (see Fred), 983 (see Fred) Hatch, A. B., 247, 257, 259, 283 Hatcher, B. W., 44 (see MacIntire), 45 (see MacIntire), 74 (see MacIntire), 399 (see MacIntire), 411 (see MacIntire) Hauser, E. A., 196, 202 Hawkes, J. C., 356, 388 Hawkins, A., 49 (see Houghland), 73, 360 (see Chucka), 361 (see Chucka), 366, 367 (see Chucka), 368 (see Chucka), 374 (see Chucka), 375, 376, 377, 382, 387 (see also Brown), 388 (see also Nelson), 389 (see Terman) Hayes, H. K., 230, 239 Hayward, H. E., 11, 15, 17, 18, 22, 25, 26, 27, 30, 32, 34, 36 (see Ayers), 36 Haelewood, B. P., 70, 73 Headden, W. P., 19, 36, 256, 283 Heck, A. F., 393, 394, 395, 398, 403, 4 l U Heilbrunn, L. V., 16, 56 Heinze, P. H., 89, 162 (see Borthwick), 163

Heller, V. G., 18 Hellman, N. N., 177 (see also Aldrich), 200 (see Aldrich), 202

421

Hellreigel, H., 243, 274, 984 Henderson, W. J., 395, 410 Hendricks, S. B., 88 (see Parker), 156 (see Parker), 161, 162, 170, 178 (see Alexander), 185, 186, 193, 195 (see McAuliffe), 200 (see Alexander), 202, 903 (see McAuliffe), 394 (see McAuliffe), 405 (see McAuliffe), 406 (see McAuliffe), 411 (see McAuliffe) Hendrickson, A. H., 13, 36, 37 (see Veihmeyer) Henin, S., 197, 202 Henson, L., 232 (see Chilton), 238 (see Chilton) Henson, P. R., 110, 129, 165 Herbert, F. B., 309, 318 Hervey, R. J., 267 (see Clark), $83 (sev Clark) Hester, J. B., 65, 66, 73, 292, ,918, 392, $10 Heuberger, J. W., 109, 163, 382. 385 (wt' also Dimond), 387 ( W C Ihxtiond). 388, 390 (see Wolfenbdiarr) Hibbard, A. D., 361, 388 Hibbard, P. L., 4, 56, 393, 394, 4lU Higgans, I,. J , 102 (see Prince), 103 (see Princr), lbii (see Prince) Higgins, F. H.. 303. 318 Hignett, T. P., 44, 73 Hildebrand, A. A., 108, 145, 146, 163, 154, 262, 271, 272, 275,284, 287 (see West) Hilgard, E. W.,4, 9, 29, 36 Hill, H., 374, ,388 Hill, W. L.. 45 (see Jacob), 46, 73, 74 (see Jacob) Hiltner, L., 243, 244, 252, 279, 284, 286 Hinkle, D. A,, 45, 48, 73 Hirst, C. F., 251 (we Greaves), 283 (see Greaves) Hnst, H. W., 312, 318 Hitchcock, A. E., 302, 3dO (see Zimmerman) Hitchens, A. P., 273 (see Breed), 282 (see Breed) Ho, Chung, 267 (see Pearson), 286 (see Pearson) Hoagland, D. R., 194 (see Martin), $03 (see Martin) Hodgkiss, W. S., 96, 164, 324, J5U (see McHargue) Hoffman, C., 260, 284

422

AUTHOR INDEX

Hoffman, W. F., 7 (see Harris), 17 (see Harris), 36 (see Harris) Hofmann, U., 161, 166, 180 (see Ardenne), 186, 200 (see Ardenne), 202, 203 (see Maegdefrau) Holdeman, Q. I,., 302 (see Brown), 316 (see Brown) Hollingsworth, C. A., 43, 76 (see Whitney) Hollis, J. P., 248, %87 (see Tervet) Hollowell, E. A., 149, 164, 234, 239 Holloway, J. K., 292, 318 Hommel, R. F., 373 (see Smith), 389 (see Smith) Hood, S. L., 339 (see Parks), 341 (see Parks), 360 (see Parks) Hopper, T. H., 98, 99,102,163 (see Cartter) Hooten, D. R., 265 (see Mitchell), 275 (see Mitchell), 286 (see Mitchell) Hoover, C. D., 194,202,204 (see Raney) Hoover, S. R., 259 (see Allison), 282 (see Allison) Horner, G. M., 93, i64 Horsfall, J. G.,385 (see also Dimond), 387 (see Dimond), 388 Horton, C. A., 323, 343, 349 (see Foster) Hosking, J. S., 161,178,202 Houghland, G.V. C., 49,73, 371, 387 (see Cook), 388 Houseman, E. E., 136, I64 Housing, E. D., 231 (see Reitz), 232 (see Reitz), 235 (see Reitz), 236 (see Reitz), 237 (see Reitz), 239 (see Reitz) Hoyman, W. G., 380, 388 Hubbell, D. S., 264, 284 Hubbuch, T. N., 44,73 Hughes, J. H., 210, 217, 219, 224, 239 Humbert, R. P., 180, 202, 203 (see Marshall) Humfeld, H., 265, 266, 271, 278, S84, 286 (see Smith), 287 (see Thom) Hungerford, C. W., 235, 939 (see Remsberg) Hutchings, T. B., 92, 164 I Ignatieff, V., 286 Iljin, W. S., 7, 36

Immer, F. R., 230, 239 (Bee Hayes) Ireland, C. F., 218 (see Piland), 239 (see Piland), 329 (see Piland), 360 (see Piland) Irving, G. W., 296 (see Mitchell), 305 (see Mitchell), 306 (see Mitchell), 318 (see Mitchell) Isaacs, R. L., 292, 318 (see Hester) Isakova, A. A., 260,284 J

Jackson, M. L., 54, 74, 163, 177 (see also Aldrich), 178,ZOO (see Aldrich, Coleman), 202 (see also Hellman), 204 (see Wear), 264, 284 Jacob, A., 368, 387 (see Asdonk) Jacob, K. D., 42, 45 (see also Reynolds), 46 (see Hill), 52,73 (see Hill), 74, 76 (see Reynolds), 366, 388 James, N., 266, 284 Jardine, F. A. L., 68, 74 Jay, H., 336, 360 Jefferson, M. E., 162,202 (see Hendricks) Jeffries, C. D., 162, 170, 172, 177, 178, 202

Jenny, H., 173 (see Kelley), 182, 187 (see Kelley), 188 (see Kelley), 190, 191, 192, 196, 201 (see Gieseking), SO2 (see also Kelley), 291,319 (see Parker) Jensen, H. L., 253, 284 Jeppson, L. R., 221, 222, 239 Joffe, J. S., 194, 20.8 Johnson, C. M., 19, 36 (see Arnon) Johnson, E. B., 293 (see Avery), 301 (see Avery), 316 (see Avery) Johnson, E. M., 235, 236, 239, 280 (see Valleau), 287 (see Valleau) Johnson, H. W., 108, 109, 149, 151, 164 (see also Morse), 232 (see Chilton), 238 (see Chilton) Johnson, J., 261, 284 Johnston, J. C., 292, 318 Johnston, J. R., 137 (see Browning), 162 (see Browning), 198, 202, 280, 284 Johnston, E. S., 322, 337, 360 Jones, H. E., 332, 360 Jones, F. R., 148, 164, 232, 233, 234 (see also Brink), 238 (see Brink), 239 Jones, H. E., 276, 284

AUTHOR INDEX

Jones, J. O., 376, 388 Jones, L. M., 208, 212, 2% Jones, R. A., 64, 74 Jones, R. J., 45, 54, 55, 70, 74 Jones, U. S., 395 (see Henderson), 410 (see Henderson) Jordan, H. V., 54, 55, '74,274 (see Morrow), 285 (see Morrow) Junkins, J. N., 46 (see Curtis), 73 (see Curtis)

K Kalton, R. R., 100, 126 (see also Weiss)? 127 (see also Weiss), 128 (see Weiss), 129, 130 (see also Weiss), 150 (see Weiss), 164, 166 (see Weiss) Kanivetz, I. I., 263, 284 Karasawa, K., 121, 164 Karlsson, N., 401 (see Mattson), 402 (see Mattson), 4ll (see Mattson) Karraker, P. E., 44, 61, 74 Katalymov, M. V., 372, 388 Katunsky, V. M., 88, 164 Katenelson, H., 244, 252, 253, 265, 266, 270, 271, 273, 275, 276, 284

Kearney, T. H., 23, 29, 30, 31, 32, 33, 34, 36

Keese, H., 372, 388 Keller, B., 7, 36 Kelley, W. P., 4, 19, 27, 34, 36, 161, 173, 178, 179, 187, 188, 192, 195, 196, 202 Kelly, J. B., 402, 410 Kelly, W. C., 362, 366, 368, 370, 373 (see also Smith), 376, 389 (see Smith) Kelsey, H. P., 78, 164 Kendall, A. I., 267, 284 Kennedy, A. L., 57, 74 Kent, G. C., 108 (see Allington), 109 (see Allington), 162 (see Allington) Kephart, L. W., 295, 304, 310, 318 (see also Mitchell) Kerr, P. F., 162, 172, (see Bray), 201 (see Bray), 204 (see Ross) Ketly, R: H., 214 (see Harrison), 239 (see Harrison) Kidder, E. H., 138 (see Van Doren), 166 (see Van Doren) Kiesselbach, T. A., 109, 164, 209, 225 (see also Tysdal), 226 (see also Tysdal), 227 (see also Kiesselbach), 228 (see

423

Tysdal), 229 (see Tysdal), 230 (see Tysdal), 231 (see also Tysdal), 233, 239, 240 (see Tysdal) King, A. S., 64, 74 King, F. H., 250, 284 King, H. H., 393 (see Perkins), 394 (see Benne), 410 (see Benne), 411 (see Perkins) King, S. P., 119, 120, 164 Kirk, L. E., 226, 231, 239 Kirkpatrick, H. C., 386, 388 Klinkowski, M., 206, 239 Klose, N., 123, 164 Klotz, L. J., 337, 349 (see Klotz) Knowles, R. P., 207, 208, 210, 211, 212, 213, 214, 215, 216, 217, 225, 239 Bnowlton, G. F., 214, 239 Koch, L. W., 108, 145, 154 (see Hildebrand) Koehler, B., 108 (see also Allington), 109 (see also Allington), 152 (see Allington), 163 (see Johnson), 154 Koehn, C. J., 69, 74 Kolisch, M., 8 Kolthoff, I. M., 403, 410 Komarovski, A. S., 325, 350 Kopac, M. J., 16 (see Chambers) Korneeva, N. P., 263, 284 (see Kanivetz) Korsmo, E., 291, 318 Krampitz, L. O., 92, 104, 166 (see Norman) Kwntz, B. A., 54, 55 (see also Ohlrogge, Scarseth), 59 (see Scarseth), 60, 74, 76 (see Ohlrogge, Scarseth) Krantz, F. A., 357, 388 Krassilinikov, N. A., 244, 253, 268, 269, 272, 273, 279, 284

Kraus, E. J., 293, 301, 318 Kraus, J. E., 379,388 Kries, 0. H., 306, 318 Kriss, A. E., 269 (see KrassiIinikov), 272 (see Krassilinikov), 273 (see Krassilinikov), 284 (see Krassilinikov) Kroeger, H., 107 (see Robertson), 165 (see Robertson) Kroth, E. M., 197, 198, 199, 203 Krugel, C., 332, 350 Krumholz, P., 325, 349 (see Feigl) Kubiena, W. L., 199, 203 Kubota, J., 329, 333, 360 Kulash, W. M., 149, 164

424

AUTHOR INDEX

Kunkel, R., 308 (see Sweet), 319 (see Sweet), 380 (see Sweet), 379, 388 Kuntz, J. E., 333 (see Walker), 361 (see Walker) Kurtr, T., 195, 203, 401, 410

L .

Lachman, W. H., 308, 309, 318 Lang, A. L., 72, 74, 102, 103, 164 La Rue, R. G., 292, 319 (see Sullivan) Latimer, L. P., 360 Laughland, D. H., 107, 164 Laughland, J., 107, 164 Lawrence, J. V., 7 (see Harris), 17 (see Harris), 36 (see Harris) Laws, W. D., 396, 410 Lawton, K., 104 (see Englehorn), 163 (see Englehorn) Leach, J. G., 386 (see Thurston), 389 389 (see Thurston) Leavitt, F. H., 52, 58, 74 Le Chatelier, H., 173, 203 Lee, 0. C., 292, 303, 318 Lefebvre, C. L., 108 (see Sherwin), 166 (see Sherwin) Lehr, J. J., 16, $7, 329, 360 Leighton, P. A., 184 (see Cross), 201 (see Cross) Lejeune, A. J., 210, 214, 239 Lemberg, J., 192, 203 Leonard, 0. A., 19, 37, 292, 314, 318 Lesage, P., 26, ST Lesins, K., 214, 238 (see Akerberg) Leukel, R. W., 108 (see Sherwin), 166 (see Sherwin) Levine, A. K., 194, 202 (see Joffe) Lewis, R. D., 102 (see Thatcher), 142 (see Thatcher), 166 (see Thatcher) Lewis, R. W., 277, 284 Liamina, A. N., 178, 204 (see Sideri) Lieberman, F. V., 222, 223, 225 Liebig, G. F., Jr., 19, 36 (see Chapman), 340 (see Chapman), 349 (see Chapman) Liebig, J., 256, 284 Lilleland, O., 15, 28, 34, 37 Lindner, R. C., 18, 36 (see Harley) Linford, M. B., 245, 270, 284 Ling, L., 145, 164

Link, K. P., 262 (see also Walker, Angell), 288 (see Angell), 884, 887 (see Walker) Linkola, T., 262, 287 (see Virtanen) Linn, M. B., 382, 388 Linsley, E. G., 212, 213, 214, 215, 216, 217, 223, 224, 239 Lipman, C. B., 322, 861 (see Sommer) Lipman, J. G., 253, 284 Litvinov, M. A , 269 (see Krassilinikov), 272 (see Krassilinikov), 273 (see Krassilinikov), 284 (see KrassiIinikov) Lochhead, A. G., 244 (see Katznelson), 262, 265, 266 (see also Katznelson), 270 (see Katznelson), 271 (see Katrnelson), 272, 273,284 (see also Katznelson), 287 (see West), 688 (see West) Lohnis, M. P., 337, 360 Loehwing, W. F., 282, 286 Lohnis, F., 251, 270, 286 Lohse, W. H., 393, 411 Long, E. M., 12, 17 (see also Hayward), 18 (see Hayward), 25, 26, 27 (see also Hayward), 35 (see Hayward), 36 (see Hayward), 37, 87,164 Long, H. C., 293, $18 Loo, W. S., 121, 166 Lorenr, 0. A., 46,74, 374, 375, 388 Lotthammer, R., 332 (see Krugel), 360 (see Krugel) Loughridge, R. H., 29, 32, 35, 37 Love, K. S., 51 (see Ross), 52, 76 (see Ross), 76 (see Yee) Lovvorn, R. L., 70 (see Woodhouse), 76 (see Woodhouse), 100 (see Gibson), 163 (see Gibson) Low, P. F., 195, 203, 399, 403, 411 Lowery, J. C., 69, 74 Luchetti, G., 255,287 (see Verona) Ludvick, G. F., 385, 388 Lunde, N. R., 312 (see Price), 319 (see Price) Lundeghdh, H., 16, 37, 256, $ti% Lundatrom, F. O., 61 (see Armiger), 7s (see Armiger) Lute, A. M., 107 (see Robertson), 166 (see Robertson) Lutman, B. F., 248, 886

AUTHOR INDEX

Lutr, J. F., 199, 203 Lyon, C. B.,339 (see Parks), 341 (see Parks), 560 (see Parks) Lyon, T.L., 250, 251, 286 Lyons, E.S.,362,388

425

McKibbin, R. R., 407 (see Wrenshall), 410 (see Wrenshall) Mackie, W. Z., 163 (see Jackson), 180 (see Jackson), 202 (see Jackson), 264 (see Jackson), 284 (see Jackson) MacLachlan, J. D., 68, 74 McLean, E. O., 193, 203 (see also MarM shall) Ma, R., 119, 164 McLean, J. G., 376, 388 (see Sparks) McArdle, R. E., 248, 286 McLemore, P., 72, 74 McAuliffe, C. D., 195, 203, 394, 405, 406, McMurtrey, J. C., 322, 360 McMurtrey, J. E., Jr., 17 (see Garner), 4l1 McBurney, C. H., 260, 286 36 (see Garner), 342,360 McCall, R. J. S., 327, 345 (see Elder) MacSwain. J. W., 212, 213, 214, 215, 216, McCalla, T. M , 197, 198, 203 (see also 217, 223, 224, 239 (see Linsley) Myers), 260, 263, 278 (see Dawson), MacVicar, R., 96, 164, 155 (see Struck983 (see Dawson), 286 see also meyer), 372, 388 Myers) McVickar, M. H., 393 (see Rich), 4 1 McCallum, R. D., 327 (see Eaton), 345 (see Rich) see Eaton) Madorsky, S. L., 48, Y4 McClelland, C. K., i64 Madson, B. A., 232, 233, 234, 240 (see McCollam, M. E., 64, 74 Weimer) McComb, A. L., 257,286 Maegdefrau, E., 161, 203 McCool, M. M., 393, .@I Magistad, O., 2, 4, 11, 14 (see Wadleigh), McCoy, E., 248 (see Fred), 983 (see 15 (see Hayward), 17, 25, 27 (see Fred) Ayers), 30, 31 (see also Gauch), 32, McCubbin, E. N., 364, 389 (see Smith) 33,34,36 (see Ayers), 36 (see Gauch, McCulloch, L., 232, 233, 239 (see Jones) Hayward), 37, 38 (see Wadleigh), MacDougal, D. T., 259,286 327, 360 MacDowall, R. K.,293, 318 (see Long), Main, A. D. C., 378, 388 378, 388 Malan, P.F., 68, 74 MacEwan, D. M. C., 177, 187, a03 Malloch, J. G., 280 (see Newton), ,986 McFarlhne, J. S., 382 (see Allen), 586 (see Newton) (see Allen) Mangual, J. C., 303, 308, 318, 320 (see McGeorge, W. T., 19, 20, 36 (see BrearWhite) eale), 37, 63, 64,74, 256,286, 393,398, Manns, T. F., 109, 165 (see Heuberger), 399, 411 385, 388 (see Heuberger) McGoldrick, F., 380, 388 Mansfield, G. R., 42, 7'4 MacGregor, J. M., 60 (see Caldwell), Marais, J. S., 398, 411 02 (see Caldwell), 73 (see Caldwell) Marsh, R. P., 332, 360 McHargue, J. S., 96, 164 (see Hodgkiss), Marshall, C. E., 162, 164, 165, 167, 168, 323, 324, 337, 341, 549 (see Calfee), 172, 178, 180, 193, 903, 204 (see 360 (see also Scripture) Whiteside) McHenry, J. R., 60, 7 3 (see Fitts), 198, Marshall, E. R., 358 (see Smith), 359 203 (see Smith), 389 (see Smith) MacIntire, W.H., 44, 45, 74, 399, 411 Marth, P. C., 293, 301, 303, 301, 318 McIlvain, E. H., 304 (see Savage), 305 Martin, A. L., 96, 154 (see Savage), 319 (see Savage) Martin, J. C., 194, 197, 198, 203 McKee, R., 208 (see Piper), 239 (see Martin, J. N., 108 (see Melhus), 164 (see Melhus) Piper)

426

AUTHOR INDEX

Martin, J. P., 263, 264, 286, 287 (see Waksman) Martin, L. D., 378, 387 (see Bates) Martin, W. H., 366,387 (see Daines) Masaewa, M., 15, 37 Matthews, E. M., 218, 239 (see Grizzard) Mattson, S., 194, 203, 401, 402, 403, 411 Maughan, F. B., 382, 388 Maunsell, P. W., 326, 360 Mayhugh, M. S., 327 (see Eaton), 3.49 (see Eaton) Mae&,P., 322, 336, 337, 360 Meadows, D. T., 50 (see Volk), 76 (see Volk) Meadows, M. W., 358 (see Smith), 359 (see Smith), 389 (see Smith) Means, R. H., 70, 74 Medler, J. T., 224, 225, 240 (see Scholl) Mehlich, A., 181, 201 (see Coleman) Mehring, A. L., 40, 52, 74 (see also Jacob) Melchers, L. E., 235, 236, 239 (see Peterson) Meldrum, H. R., 104 (see Englehorn), 163 (see Englehorn) Melhus, I. E., 108, 164 Melvin, E. H., 323, 324, 360 Menkin, R., 255, 286 Menkina, P., 252, 286 (see Sheloumova) Merrill, R. M., 56, 61, 74 Metzger, W. H., 256, 286, 395 (see also Romine), 398, 41f (see also Romine) Michelbacher, A. E., 220, 221, 222, 223, 224, 240 (see Smith) Midgley, A. R., 62, 74, 195, 202 (see Kelley),-330, 343 (see Dunklee), 360, 395, 402 (see Kelly), 410 (see Kelly), 411 Miles, S. R., 141 (see Wiancko), 142, 167 (see Wiancko) Miller, C. E., 60, 61, 74, 323, 349 (see Cook) Miller, E. C., 279, 286 Miller, H. F., 44 (see Karraker, Roberts), 74 (see Karralcer), 76 (see Roberts) Miller, L. B., 102, 103, 164 (see Lang) Minarik, C . E., 96, 154 Minina, E. G., 270, 286 (see Sabanin)

Mishusten, E. N., 263, 286 Mitchell, H. L., 257, 986 Mitchell, J., 218, 238 (see Bentley) Mitchell, J. G., 378 (see Wilson), 389 (see Wilson) Mitchell, J. W., 276, 287 (see Stevensen), 293, 296, 301, 303, 304, 305, 306, 310, 318 (see also Kraus) Mitchell, R. B., 265, 275, 286 Mogen, C. A., 28, $7 Moore, E. C., 292, 318 Moran, W. T., 312, 318 Morgan, E. T., 361, 371 (see Teakle), 389 (see Teakle) Morgan, M. F., 392 (see Anderson), 409 (see Anderson) Moore, H. C., 357 (see Wheeler), 389 (see Wheeler) Morofsky, W. F., 383, 388 Morris, H. J., 259 (see Allison), ,982 (see Allison) Morrow, M. B., 274,286 Morse, W. J., 78, 79, 80, 115, 117, 118, 123, 131, 151, 164, 166 (see Piper, Ricker), 208 (see Piper), ,939 (see Piper) Moss, E. G., 17 (see Garner), 36 (see Garner) Moulton, R. W., 46, 74 Moyer, L. S., 16, 87 Muenscher, W. C., 318 Muir, A., 178 (see Nagelschmidt), 203 (see Nagelschmidt) Mulvey, R. R., 102, 103, 142, 166 (see Vittum) Muncie, J. H., 383, 388 (see Morofsky) Munro, J. A., 248, 239, 382 (see Post), 389 (see Post) Munsell, R. I., 62, 73 (see Brown) Murneek, A. E., 86, 164 Murphy, H. C., 108 (see Melhus), 164 (see Melhus) Murphy, H. F., 195, 203, 396, 411 Murray, E. G. D., 273 (see Breed), 28P (see Breed) Musgrave, R. B., 48, 49, 53, 73 (see Chandler) Myers, H. E., 187, 197, 198, 283, ,985

427

AUTHOR INDEX

N Naftel, J. A., 324, 330, 360 Nagelschmidt, G., 178, 203, 400, 411 Naumova, A. N., 260,B 2 (see Berezova) Neely, J. W., 314, 319 Neller, J. R., 256, 286, 394, 411 Nelson, L. B., 94, 95, 164 Nelson, N. T., 392 (see Anderson), 4OY (see Anderson) Nelson, R. A., 178 (see Alexander), 185 (see Hendricks), 186 (see Hendricks), 200 (see Alexander), 802 (see Hendricks) Nelson, W. L., 102 (see also Collins), 103 (see also Collins), 104, 163 (see Collins), 166, 370, 377, 388 Nemec, A., 369, 388 Nesom, G. H., 48, 72 (see Alway) Neuweiler, E., 371, 388 Newcomb, G. T., 64 (see King), 74 (see King) Newman, A. S., 276, 286, 306, 313, 317 (see De Rose), 319 Newrzella, B., 365 (see Berber), 387 (see Berkner) Newton, J. D., 280,986 Newton, R., 280, 286 Nicol, H., 279,281, 286, 287 (see Thornton) Nielsen, C. S., 86, 166 Nikitina, N. I., 279, 284 (see Krassilnikov) Nikolsky, N. N., 263, 286 Nissley, C. H., 64, 76 Nitzsch, W. v., 188,195,203 Nixon, C. L., 400 (see Nagelschmidt), 4 1 (see Nagelschmidt) Nobbe, F., 279, 286 Noll, W., 179, 203 Nolla, J. A. B., 303, 308, 319 Norman, A. G., 90, 92, 102, 104 (see also Englehorn), 105, 106, 140, 141, 163 (see Englehorn), 166, 198, 203, 250, 264, 267 (see Pearson), 276, 286 (see also Newman), 282 (see Broadbent), 286 (see Pearson), 306, 314, 319 Norton, F. H., 173, 605 Norum, E. B., 141, 166

Nugent, T. J., 371, 887 (see Cook) Nutman, P. S., 301, 306, $19 0

Obenschain, S. S., 393, 411 (see Rich) O’Brien, D. G., 327,342,349 (me Dennis) O’Brien, R. E., 44, 47, 76 O’Connor, R. T., 323, 324, 360 (see Melvin) Odland, T. E., 60, 76 (see Rich), 124, 135, 163 (see Garber), 166, 365, $87 (see Cowie) Offord, H. R., 293, 519 Ogasa, T., 24, 37 Ohlrogge, A. J., 55 (see also Scarseth), 59 (see Scarseth), 76 (see also ScarSeth) Olmstead, L. B., 198, 203 Olson, P. J., 208, 210, 212, 214, 239 (see Jones, Lejeune), 240 (see Silversides) Olson, R. V., 325, 330, 331, 333, 360 Orcel, M. J., 173, $03 Orcutt, F. S, 105, 166 Otis, C. E., 379, 338 Oulton, T. D., 182, $03 Overstreet, R., 192 (see also Jenny), 194 (see Martin), 208 (see Jenny), 803 (see Martin)

P Page, J. B., 178 (see Kelley), 194, 197, 198, 199, 602 (see Kelley), 203 (see also Kroth), 281,286,396 (see Laws), $10 (see Laws) Palmer, G., 44 (see MacIntire), 45 (see MacIntire), 74 (see MacIntire) Parberry, N. H.,62, 76 Park, J. B., 120 (see Wolfe), 167 (see Wolfe) Parker, E. R., 07, 76, B91, 319 Parker, F. W., 256, $86 Parker, M. M., 388 Parker, M. W.,86, 87 (see also Scully), 88, 89 (see also Heinre), 166 (see Borthwick), 163 (see Heinre), 166 (see also Soully) Parks, R. Q., 331, 333, 339, 341, 360 Patel, P.L., 128, 129, 166

428

AUTHOB INDEX

Pauline, L., 161, 170, 174, 803 Pavlychenko, T. K., 291, 292, 295, 319 Payne, M. G.,277, 286 Pearson, R. W., 171, 178, 208, 267, 286, 407, 411 Peck, O., 212, 213, 214, 215, 216, 217, 220, 221, ,238 (see Bolton), 239 Pederson, C . E., 222, 239 Peech, M.,192, $04, 388, 392, 411 Peele, T. C.,198, 204, 263, 286' Peikert, F. W., 65, 75 Peltier, G. L., 232, 233 (see Kiesselbach), ,239 (see also Kiesselbach) Pennington, R. P., 163 (see Jackson), 180 (see Jackson), 208 (see Jackson), 264 (see Jackson), B 4 (see Jackson) Pepper, B. B., 383, 384, 386, 587 (see Campbell), 388 Percival, G. P., 102 (see Prince), 103 (see Prince), 156 (see Prince) Perkins, A. T., 195, $04, 393, 394 (see Benne), 396, 410 (see Beme), 41 Perotti, R., 245, 246, 259,2880 Peters, G., 323, 349 (see Goldschmidt) Peterson, D. F., Jr., 32 (see Reeve), 37 (see Reeve) Peterson, J. B., 178, 197, 204 Peterson, M. L., 231 (see Reitz), 232 (see Reitz), 235 (see also Reits), 236 (see also Reita), 237 (see Reits), $39 (see also Reits) Peterson, P. P., 407, 411 Peterson, W., 19, 37 (see Stewart) Petty, M. A., 108, 109, 166 Phillips, T. G., 364 (see Prince), 589 (see Prince) Pickford, P. T. H., 370 (see Wallace), $89 (see Wallace) Pierre, W. H., 94, 102, 103, 105, 141, 162 (see Bower), 166, 194, $04 (see Stanford), 255, 268, 286 Piland, J. R., 218, 233, 329, 360 Pinck, L. A., 104, 166, 250, 253 (see Allison), 259 (see Allison), M (see Allison), 286 Pinkerton, C., 45 (see Jacob), 74 (see Jacob) Piper, C . S., 264, 286, 404, 421

Piper, C. V., 79, 166, 208, 239 Pittman, D. W., 22, 25, 32, 36 (see Harris) Plant, W., 316, 588 (see Jones) Pohlman, G. G., 63, 76, 141, 142, 163 (see Dodd), 276, 286 Pole, G. R., 48 (see Copson), 73 (see Copson) Poluektoff, N. S., 325, 360 (see Komarovski) Poos, F. W., 225, 839 Porter, J. R., 287, 286 Porter, K. B., 121, 166 Porter, R. H., 108, 109, 166 (see also Robbins) Poschenreider, H., 246, 252, 886 Post, R. L., 382, 389 Potter, R. S.,407, 411 Powers, W. L., 4, 24, 31, 3b (see Ahi), $7, 64, 68, 76 Prescott, J. A., 260,286,401 (see Russell), 411 (see Russell) Presley, J. T., 144, 166 Preston, C., 18, $7 (see Stewart) Priceb C . W., Jr., 113, 166 Price, F. E., 312, 319 Prince, A. L., 66, 76, 333 (see Reeve), 334 (see Reeve), 360 (see Reeve), 392 (see Blair), 410 (see Blair) Prince, F. S., 102, 103, 166, 364, 383 Pritzker, N. Y., 263, 284 (see Kaniveta) Probst, A. H., 98 (see Weiss), 99 (see Weiss), 104, 110 (see Weiss), 113, 119 (see Weiss), 120 (see We&), 135, 136 (see also Weiss), 166, 167 (see Weiss) Proebsting, E. L., 63, 75 Pryor, M., 302, 313, 318 PulTer, R. E., 292, 319 Purvis, E. R., 329, 343, 360 Pushkinskaya, O., 263, 285 (see Mishustin)

8 Quackenbush, F. W., 106, 166 Quastel, J. H., 301 (see Nutman), 306 (see Nutman), 319 (see Nutman)

AUTHOR INDEX

E Rademacher, B., 291, 319 Rader, L. F., Jr., 45 (see Reynolds), 76 (see Reynolds) Radziewski, G. G., 263, 286 Raleigh, G. J., 17, 37, 66, 76, 308 (see Sweet), 319 (see Sweet), 820 (see Sweet) Raleigh, W. P., 378 (see Schultz), 389 (see Schultz) Ramstad, P. E., 107, 166 Raney, W. A., 194, 20.4 Rasnizina, E. A., 260, 282 (see Berezova) Ratner, E. I., 9, 19,37 Ravikovitch, 8, 194, 204 Rawlins, W. A., 384 (see Wolfenbarger), 385, 388 (see Gyrisko), 390 (see W olfenbarger) Rayner, D. S., 64 (see Chapman), 67 (see Chapman), 73 (see Chapman) Rayner, M. C., 247, 286 Raynor, R. N., 291 (see Robbins), 292 (see Robbins), 293 (see also Robbins), 295 (see Crafts), 300, 305 (see Crafts), 307, 308 (see also Robbins), 309, 311 (see Hannesson), 312 (see Robbins), d l 7 (see Crafts), 318 (see Hannesson), 319 Reddick, D., 355, 356, 388 Reeve, E., 332, 333, 334, 339, 340, 560 Reeve, R. C., 20, 32, 36 (see Fireman), 87

Reiber, H. G., 293, 294, 298, 307, 308, 309, 317 (see Crafts) Reisenauer, H. M., 218 (see Piland), 239 (see Piland), 329 (see Piland), 360 (see Piland) Reitemeier, R. F., 34, 87 (see Magistad), 196, 802 (see Jenny) Reitz, L. T,231, 232, 235, 236, 239 Remsberg, R., 235, I39 Retzer, J. L., 28, 87 Reynolds, D. S., 45, 76 Rhoades, H. F., 57, 76, 362 (see Lyons), 388 (see Lyons) Riccardo, S., 270 (see Rossi), I86 (see Rossi) Rice, E. L., 296, 304, 319

429

Rich, A. E., 60,76, 385, 387 (see Davidson) Rich, C. I., 393, 411 Richards, B. L.,235, 236, 23.9 Richards, L. A., 13, 14, 37 Richardson, L. T., 275, 276, 284 (see Katznelson) Iticker, P. L., 78, 166 Riddle, 0. C., 300, 318 (see Harvey) Riede, W., 81, 165 Rieman, G. H., 372 (see MacVicar), 38%. 386 (see Allen), 388 (see MacVicttr) Rigler, N. E., 19, 36 (see Eaton), 261, 276, 283 (we Eaton) Ripley, P. O., 279, 286 Roach, W. A., 68, 76 Roark, R. C., 385, 388 Robbins, C., 142, 167 (see Wiancko) Robbins, W. A,, 108, 166 Robbins, W. W., 291, 292, 293, 295, 308, 312, 318 (see Harvey), 319 Roberts, E., 260, 286 Roberts, G., 44, 76 Roberts, J. L., 260, 274 (see Morrow), I86 (see Morrow), 2236 Robertson, D. W., 107, 165, 233 (see Weihing), 2.40 (see Weihing) Robinson, D. O., 171 (see Buehrer), 173 (see Buehrer), 178 (see Riiehrer), 201 (see Buehrer) Robinson, W. O., 326 (see Whetstone), 327 (see Whetstone), 331 (see Whctstone), 361 (see Whetstone) Rogers, H. T., 61, 76, 96, 165, 346, 347, 350

Itokitzkaya, A. I., 252, 286 Roller, E. M., 103 (see Adams), 162 (see Adams), 260, 283 (see Clark) Romine, D. S., 395, 411 Rosendahl, R. O.,257,285 (see Mitchell), 286 Rosene, H. F., 25, 37 Ross, C. S., 162, 204 Ross, W. W., 61, 76 Rossi, G., 270, 286 Rost, C. O., 60 (see Caldwell), 62 (see Caldwell), 73 (see Caldwell) Routien, J. B., 259, 286 Rowland, R. A,, 162, 173,202 (see Grim) Rubins, E. J., 181, 195, 201 (see Dean),

430

AUTHOB INDEX

396 (see Dean), 403 (see Dean), 404

(see Dean), 410 (see Dean) Rubnev, V. Z., 215, 240 Rudolfs, W., 21, 23, 37 Ruehle, G. D., 08, 76, 385, 388 Ruhnke, G . N., 393, .&I2 (see Lobe) Ruprecht, R. W., 313, 360 (see Purvis) Russel, J. C., 362 (see Lyons), 388 (see Lyons) Russell, E. J., 250, 286, 401, all Russell, E. W., 197, 198, 204 Russell, M. B., 137 (see Browning), 162 (see Browning), 178, 198 (see Johnston), $02 (see Johnston), 203 (see McHenry), 204, 280 (see Johnston), 284 (see Johnston) Ryzhikov, N., 80, 166 8

Sabanin, D. A,, 269, 286 Sachs, J., 256, I 6 Salmon, S. C., 233, 240 Salmon, W. D., 69, 74 (see Koehn) Salt, R. W., 219, 940 Samuel, G., 254, 286, 379, 388 Sanford, G . B., 248, 286 Savage, D . A,, 304, 305, 319 Sayre, C. B., 88, 69, 76 Scarseth, G. D., 55 (see also Ohlrogge), 59, 76 (see also Ohlrogge), 104, 165 (see Drake), 194, 204, 332 (see also Drake), 3@ (see Drake), 360 (see Jones), 395 (see also Allison), 396, 397 (see also Bradfield), 401, 402, 410 (see Bradfield), 4ll Schachtschabel, P., 178, 191, 204 Scharrer, K., 323, 325, 360 Schermerhorn, L. G., 65, 76 (see Tiedjens) Schmidt, E. L., 247, 257, 259, 886 Schofield, C. S., 327, 360 Schofield, R. K., 181, 204 Scholl, J. M., 224, $40 Scholl, W., 51, 76 Schollenberger, C. J., 407, .&I1 Schreiner, O., 260, 886 Schroeder, E. W., 62, 76 Schroeder, R. A., 371, 386, 389

Schroeder, W. T,333 (see Walker), 361 (see Walker) Schropp, W., 341, 360 Schultz, E. S., 356 (see Akeley), 378, 386 (see Akeley), 387 (see Bonde), 389

Schuphan, W., 17, 37 Schuster, C. E., 326, 360 Scofield, C . S., 29, 30, 31, 32, 33 34, 36 (see Kearney) Scripture, P. N., 337, 341, 360 Scully, N. J., 87, 88 (see also Parker), 166 (see also Parker) Sears, 0. H., 90, 140, 166, 277, 279, 282 (see Appleman) Sedletzky, I. D., 178, 204 Sekera, F., 263, 286 Selke, W., 369, 389 Sen, J., 254, 273, 286 Sexsmith, J. J., 218, 225, 240 Sexton, W. A., 301 (see Slade), 306 (see Slade), 310, 319, 320 (see Templeman) Shands, W. A,, 381 (see Bronson), 387 (see Bronson) Shaw, B. T., 180, 208 (see Humbert), 203 (see Marshall), 204, 331,360 (see Parks) Shaw, G. W., 33, 37 Shaw, R.R., 303, $19 Shaw, J. M., 312, 318 (see Moran) Shawl, R. I., 115, 162 (see Bateman) Sheloumova, A., 252, 1 6 Sherman, G . D., 255, 261, 283 (me Fujimoto) Sherwin, H. S., 108, 166 Sherwood, L. V., 114, 166 Shive, J. W., 21, 37, 96 (see Somers), 96, 164 (see Minarik), 166 (see Somers), 332, 336, 337, 339, 340, 360 (see a l ~ o Reeve, Marsh) Shollenberger, J. H., 85, 166 Shorey, E., 260, 286 (see. Schreiner), 411 Shute, R. S., 163 (see Davidson), 201 (see Davidson) Sideri, D. I., 178, 197, 199, so4 Sidorenko, A. I., 252, ,986 Rieling, D. H., 196, 204, 332 (see Drake), @!3 (see Drake) de Sigmond, A. A. J., 4, 16, 37

AUTHOR INDEX

Silberstein, L., 344, 345, 3.49 (see Bertrand) Silversides, W. H., 210, 212, 240, 319 Simonson, R. M., 407 (see Pearson), 411 (see Pearson) Simpson, G. W., 384 (and see Bronson), 387 (see Bronson) Sjogren, J. W., 166 Skinner, J. J., 369, 387 (see Collins) Skodvin, K., 69 (see Fisher), 73 (see Fisher) Skory, J., 226, 228, 9@ (see Wilsie) Slade, R. E., 301, 306, 319 Sleesman, J. P., 389, 390 (see Wilson) Slosson, E. E., 21, 37 Small, T., 378, 3819 Smirnova, A., 260, 984 (see Isakova) Smith, A. M., 64, 76 Smith, B. V., 100 (see Gibson), 153 (see Gibson) Smith, D. D., 105, 136, 137, 138, 139, 140, 166

Smith, E. V., 71, 76 (see Swingle) Smith, F. F., 384 (see also Bronson), 1587 (see Bronson) Smith, F. W., 60, 75 Smith, G. D., 182, 196, 202 (see Jenny) Smith, G. S., 325, 339, 360 Smith, H. S., 3119 Smith, M. E., 338, 361 Smith, N. R., 244, 266, 276, 278, 279, 284 (see Humfeld), 987 (see Thorn) Smith, O., 304, 319, 358, 359, 361, 362, 364, 366, 368, 370, 373, 376, 380, 387 (see Fernow), 388 (see McGoldrick), 589

Smith, R. F., 220, 221, 222, 223, 224, 240 Smith, W. K., 234, 939 (see Jones) Snelling, R. O., 149, 166 Snyder, E. G., 385, 387 (see Bonde) Snyder, W. E., 87, 166 Sohngen, N. L., 254, 286 Somers, I. I., 95, 166 Sommer, A. I., 322, 361 Sorenson, C . J., 214, 219, 220, 221, 222, 223, 224, 225, 239 (see Knowlton), $40

Sparks, W. C., 376, 389 Spurr, W. B., 11, 17, 25, 26, 27, 34, 36 (see Hayward)

431

Btahl, E., 259, 286 Stahler, L. M., 303, 310, 312, 317 (see Derscheid), 319 Stanford, G., 194, SO4 Stanganelli, M., 270 (see Rossi), 286 (see Rossi) Starc, A., 273, 286 Starkey, R. L., 244, 245, 249, 251, 252, 253, 254, 256, 265, 266, 267, 269, 270, 271, 273, 278, 284 (see Lipman), 286, 287 (see Waksman) Staten, G., 313, 319 Stauffer, R. S., 137,156 (see Van Doren), 198, 204 Stearns, L. A., 382, 388 (see Heuberger) Steckel, J. E., 95, 103, 166, 162 (see Wear), 204 (see Wear) Steele, G. J., 396, 397, 411 Steele, J. G., 397 (see Bradfield), 410

(see Bradfield) Steinbauer, G. P., 379, 389 Steinberg, R. A,, 261, 287 Stephenson, R. E., 326, 350 (see Schuster), 395, 411 Stevensen, E. C., 276, 287 Stevenson, F. J., 356 (see Akeley, Clark), 357 (see also Wheeler), 382, 386 (see Akeley), 387 (see Blodgett, Clark), 389 (see Sleesman, Wheeler) Stevenson, T. M., 211, 227, 231, $40 Steward, F. C., 18, 37 Stewart, G., 206, 231, 240 Stewart, J., 21, 23, 3Y Stewart, R., 19, 37, 251 (see Greaves), 283 (see Greaves) Stewart, R. T., 119, 126, 166, 157 (see Wentz) Stiles, W., 322, 336, 337, 3661 Stille, B., 256, 287 Stitt, L. L., 219, 220, 221, 224, $40 Stocker, 263 Stocking, C. R., 296 (see Crafts), 304 (see Crafts), 3617 (see Crafts) Stockli, A., 287 Stoddard, D. L., 107, 108, 109, 166 Stoklasa, J., 274, 287 Stout, P. R., 195, 204, 396, 397, 402, 403, 4611 Strand, G. G., 83, 156 Strong, D. C., 14 (see Wadleigh), 32

432

AUTHOR INDEX

(see Wadleigh), 33 (see Wadleigh), 38 (see Wadleigh) Strong, W. F., 68, 74 (see MacLachlan) Struckmeyer, B. E., 96, 164, 166 Strugger, S., 267, 8 7 Struthers, D. K., 115, 166' Stumbo, C. R., 261, 265, 275, 287 Sullivan, W., 292,318 (see Johnston), 31.9 Sutherland, M . L., 266, 284 (see Jzimes) Swaby, R. J., 253, 284 (see Jensen) Swanson, C., 15 (see Lilleland), 28 (see Lilleland), 34 (see Lilleland), ST (see Lilleland) Swanson, C.P., 304 (see Ennis), 317 (see Ennis), 380 (see Ennis), 387 (see Ennis) Sweet, R. D., 308, 309, 319, 320 Swingle, H. S., 71, 76 Sykes, E. T., 379, 387 (see Findlay) Sylwester, E. P., 312, 320 Synerholm, M. E., 302, 320 Syvorotkin, G. S., 330, 349 (see Bobko)

T Tagawa, T., 25, 87 Tahgi, M., 124, 166 Takahashi, N., 117,118,166 Tang, P. S., 121, 166 Tate, H. D., 213, 216, ,938 (see Crandall) Taylor, D. L., 306, 320 Teakle, L. J. H., 361, 371, 389 Templeman, W. G., 301 (see Slade), 306 (see Slade), 310, 319 (see Templeman), B O Terman, G. L., 44, 76, 362, 389 Tervet, I. W., 107, 108 (see also Allington), 109 (see Allington), 162 (see Allington), 168, 248, 287 Thatcher, L. E., 102, 142, 143, 166, 167 (see Wiggans) Thexton, R. H., 272, 284 (see Lochhead) Thimann, K. V., 260, 287 Thorn, C., 244, 262, 265, 266, 271, 275. 283 (see Clark), 282' Thomas, E. E., 27, 35, 36 (see Kelley) Thompson, H. S., 160, 204 Thompson, L. M., 265, 260, 267, 987 Thomson, B. F., 293 (see also Aveuy), 301 (see Avery), 316' (see A w r y )

Thomson, R. H. K., 332, 348 (see Askew), 840 (see Askew) Thorne, D. W., 19, 87 Thornton, B. J., 310 Thornton, G.D., 92, 166 Thornton, H. G., 248, 287, 301 (see Nutman), 306 (see Nutman), 319 (see Nutman) Thorpe, J. D., 277, 282 (see Carlyle) Thun, R., 368, 889 Thurston, H. W., Jr., 386, 389 Tidmore, J. W., 50 (see Volk), 76 (see Volk), 395 (see Scarseth), @1 (see Scarseth) Tiedjens, V. A,, 65, 66, 76 (see also Prince) Timonin, M. I., 244 (see Katznelson), 254, 262 (see also Lochhead), 265 (see also Lochhead), 266 (see also Katznelson), 268, 269, 270 (see Katznelson), 271 (see also Katznelson), 273 (see Lochhead), 275, 276, 284 (see Katznelson, Lochhead), 287 Timmons, F. L., 291, 320 Ting, C. L., 117, 118, 121, 129, 166 Tinsley, J. D., 4, 37 Todd, F. E., 208, 210, 211, 212, 213, 214, 215, 216, 217, 9-40 (see Vansell) Todd, J. R., 44 (see Karraker), 74 (see Karraker ) Toevs, J. L., 48, 76 Toole, E. H., 107, 168, 303 (see Marth), 318 (see Marth) Toole, V. K., 107, 166, 303 (see Marth), 318 (see Marth) Toovey, F. W., 235, 236, 240 Torrie, J. H., 148, 164 (see Jones) Toth, S. J., 15 (see Wallace), 38 (ser Wallace), 194, ,804, 367, 387 (see Bear), 395, 398 (see Bear), 402, 403, 410 (see Bear), 411 Tottingham, W. E., 17,37,372 (see MacVicar), 388 (see MacVicar) Trelease, H. M., 16, 87 Trelease, S. F., 16, 37, 96, 164 Trotter, I. P., 102, 166 Truffaut, G., 253, 287 Truog, E., 194, 200 (see Attoe), 256, 287, 325, 326, 328, 329 (see Kubota), 331, 333 (see Kubota), 342, 3@ (see

433

AUTHOR INDEX

Berger), 350 (see Kuhota), 395, 398, 4 0 (see Drosdoff), 411 Tucker, W. B., 310, 320 Tukey, H. B., 301, 305. 517 (see Hsmner) T d i n , A. S., 330, 351 Tullis, E. C., 303, 313, 32U Turk, L. M., 19, 35 (see Rower), 279, 282 (see Albrecht) Turnell, K., 64, 75 Turner, T., 385, 388 (see Horsfall) Turrentine, J. W., 52, 76 Turton, A. G., 371 (see Teakle), 589 (see Teakle) Tutton, S. M., 303 (see Shaw), 319 Tyner, L. E., 265, 287 Tysdal, H. M., 206, 207, 208, 209, 210, 211, 212, 213, 214, 217, 218, 219, 224, 225, 226, 227, 228, 229, 230, 232, 233, 234, 235, 238, 239 (see Peltier), 240

U Ufer, M., 207, 240 Uhvits, R., 17 (see Hayward), 18 (see Hayward), 23, 24, 27 (see Hayward, 33, 35 (see Hayward), 36 (see Hayward), 37 Umbreit, W. W., 90, 105, 107, 166 Unrau, J., 229, $4 Uphof, J. C., Th., 6, 26, 37 d’Urbal, R. P., 293, 319 (see Offord)

V Valentine, A. T., 7 (see Harris), 36 (see Harris) Valleau, W. D., 235, 236, 239 (see Johnson), 280, 283 (see Diachun), 287 Van Delden, A., 252, 282 (see Beijerinck) Van Doren, C. A., 107 (see Burlison), 110 (see Burlison), 137, 138, 155 (see Burlison), 156 Van Hallie, Th. B., 19, 57 Van Overbeek, J., 292, 293, 295, 303, 305, 320

Vansell, G. H., 208, 210, 211, 212, 213, 214, 215, 216,217, 239 (see Hare), 240

Vanselow, A. P., 340 (see Chapman), 349 (see Chapman) Veatch, C., 126, 166 Veihmeyer, F. J., 13, 35 (see Conrad), 36 (see Hendrickson) , 37 Velea, I., 292, 295, 303, .720 (see Van Overbeek) Velich, A., 273, $87 Verona, O., 255, 287 Viljoen, N. J., 97, 98, 103, 110, 119, 120, 156

Villafane, A. G., 303, 308,320 (see White) Virtanen, A. I., 262, 287 Vittum, M. T., 102, 103, 166 Vladykov, V., 253, 287 (see Trriffaut) Volk, G. W., 47, 60, 63. 76, 194, 204 Volk, N. .J., 50, 54, 70 W

Wadleigh, C. H., 9, 11 (see Magistad), 12, 14, 15, 16, 17 (see also Magistad), 18, 25 (see Magistad), 27 (see Ayers), 28, 30, 31 (see Magistad), 32, 33, 36 (see Ayers, Bower), 36 (see Gauch), 37 (see also Magistad), 38 Wagner, R. E., 51 (see Armiger), 73 (see Armiger) Wagoner, C. E., 396 (see Perkins), 411 (see Perkins) Waksman, S. A., 249, 254, 263, 286 (see Martin), 287 Walker, G. P., 141 (see Wiancko), 142, 157 (see Wiancko) Walker, J. C., 262 (see also Link, Angell), 282 (see Angell), 284 (see Link), 287, 333, 343, 361 Wallace, A., 15, 58 Wallace, H. M., 51, 75 (see Scholl) Wallace, T., 370, 389 Wallace, W., 114, 156 Walrath, F. J., 69, 76 Walsh, T., 16, 38 Walthall, J. H., 45, 46, 76 Wang, C. C., 119, 120, 154 (see King) Wang, S., 135, 166 Wang, Tsu Kao, 270 (see Rossi), ,986 (see Rossi) Ward, A. S., 286

434

AUTHOR INUEX

Ward, F. N., 45 (see Jacob), 46 (see Hill), 73 (see Hill), 74 (see Jacob) Ware, L. M., 363, 389 Warington, K., 322, 332, 336, 337, 349 (see Brenchley), 361 Warren, C., 141 (see Wiancko), 142, 167 (see Wiancko) Warren, G. F., 308, 309, 320 Waterston, J. M., 235 (see Toovey), 236 (see Toovey), S.40 (see Toovey) Way, J. T., 160, 804 Waynick, 56, 57, 76 Wear, J. I., 162, SO4 Weatherspoon, J. H., 119, 128, 129, 166 Weaver, L. R., 13, 14, 37 Weaver, R. J., 97, 114, 166, 296, 304, 310 (see Allard), 316 (see Allard), 320 Weber, C. R., 98 (see Weiss), 99 (see Weiss), 100 (see Kalton), 110 (see Weiss), 111, 112, 113, 114, 119 (see also Weiss), 120 (see also Weiss), 122, 126 (see Weiss), 127 (see Weiea), 128 (see Weiss), 129, 130 (see Weks), 135, 136 (see also Houseman, Weiss), 150 (see Weiss), 164 (see Houseman, Kalton), 166, 167 (see Weiss) Weber, G. F., 343, 349 (see Foster) Weidemann, A. G., 60, 76 Weihing, R. M., 233, %#I Weimer, J. L., 232, 233, 234, $40 Weise, E., 365 (see Berkner), 387 (see Berkner) Weiser, V. L., 395, 403, 411 Weiss, M. G., 95, 98, 99, 103, 110, 111, 112, 113, 114, 117, 119, 120, 121, 126, 127, 128, 130, 135, 136, 150, 165 (see Porter), 166 (see also Weber), 167 Welch, A. W., 144, 167 Wellman, R. H.,320

Wene, G.P., 385,388 (see Gyrialro) Wentr, J. B., 119, 126, 166 (see Weatherspoon), 167 Weneel, M. E , 276 (see Smith), 286 (see Smith) Werner, H. O., 365, 389 West, E. S., 296, 1 0 West, P. M., 260, 262 (see also Lochhead), 265 (see Lochhead), 271, 272, 273 (see Lochhead), 276, 1 4 (see Lochhead), $87,288

Westgate, W. A,, 307, 390 Westover, L. L., 206, 225 (see Tysdal), 226 (see Tysdal), 227 (see Tysdal), 229 (see Tysdal), 230 (see Tysdal), 231 (see Tysdsl), 233, 8.40 (see TYPdal, Wilkins) Wexelson, H., 210, 214, 2.40 Wheeler, E. J., 357, 389 Wheeler, H. E., 248, 286 (see Lutman) Wheeting, L. C., 46, 76 Whetstone, R. R., 326, 327, 331, 361 White, D. D., 303, 308, 3gO White, J. L., 162 (RCP Wear), 177, 204 (see also Wear) White, W. J., 207, 208, 210, 211, 228, 229, 238 (see Armstrong), 240 b e also Unrau) White-Stevens, R. H., 333, 361, 367, 38.9 Whiting, A. L., 279 (see Fred), 2885 Whitney, W. T., 43, 76 Whiteside, E. P., 178, ,904 Whitson, A. R., 250,284 (see King) Whitt, D. M., 138, 166 (see Smith) Whittaker, C. W., 51, 76 Whyte, R. O., 86, 164 (see MurnrPk) Wiancko, A. T., 141, 142, lrir Wiegner, G., 170, 30.4 Wiggans, R. G., 112, 135, 167 Wiklander, L., 191, 194, 904 Wilcox, L. V., 323, 327, 349 (see Eaton), 360 (see Schofield), 361 Wilfarth, H., 243,274,884 (see Hellreigel) Wilkins, F. R., 233, 840 Willard, C. J., 102 (see Thatcher), 114, 142 (see Thatcher), 143, 166 (see (Thatcher), 167, 281, 286 (see Page) Willard, J. C., 292, 303, 880 Williams, G.S., 118, 167 Williams, L. F., 98 (see Weiss), Y9 (RW Weiss), 110 (see Weiss), 115, 118, 119 (see Weiss), 120 (see Weiss), 122, 138 (see Weiss), 167 (see also Weiss, Woodworth) Williams, R. J., 260 (see McBurney), 286 (see McBurney) Williams, W. R., 199, SO4 Willis, W. H., 252, 288 Wilm, D., 161, 166 (see Hofmann), 186 (see Hofmann), $02 (see Hofmann) Wilsie, C. P., 226, 228, $40

435

AUTHOR I N D E X

Wilson, A. R., 378, 389 Wilson, B. D., 250 (see also Lyon), 251 (see Lyon), 1 6 (see Lyon), ,988 Wilson, C. A., 383 (see Pepper), 384 (see Pepper), 388 (see Pepper) Wilson, F., 292, 320 Wilson, H. A., 198, 204, 280, 288 Wilson, J. D., 382, 386 (see Thurston), 389 (see Thurston, Sleesman), 390 Wilson, J. K., 250, 277, 988 Wilson, M. C., 234,240 Wilson, P. W., 92 (see Fred), 153 (see Fred), 248, 260, 288 (see West) Winterberg, S. H., 44 (see MacIntire), 45 (see MacIntire), 74 (see MacIntire) Winters, J. D., 369, 382, 390 Wittstein, A., 322, 336, 861 Wolf, B., 72, 76, 330, 361 Wolf, D. E., 292, 311,316 (see Anderson), 3.20 Wolfe, A. C., 120, 167 Wolfe, W. C.,296 (see Mitchell), 305 (see Mitchell), 306 (see Mitchell), 318 (see Mitchell) Wolfenbarger, D. O., 382, 384, 390 Wood, J. W., 296 (see Mitchell), 305 (see Mitchell), 306 (see Mitchell), 318 (see Mitchell) Wood, L. K., 68, 76 (see Powers), 194 (see also De Turk), ,901 (see De Turk), 204 Woodbridge, C. G., 326, 329, 332, 361 Woodford, A. O., 178 (see Kelley), 179, 202 (see Kelley) Woodhams, G. E., 67, 76

Woodhouse, W. W., Jr., 70, 76 Woodle, H. A., 09, 76 Woodruff, C. M., 198, S04, 280, 288 Woodworth, C. M., 115, 117, 118, 124, 126, 167 Wrenshall, C. L., 407, .@0 (see Dyer), 411

Wright, L. E., 328, 349 (see Ferguson) Wuhrmann, K., 26, 38 Wyatt, F. A., 286 Wyss, O., 92 (see Fred), 163 (see Fred)

Y Yarick, B. E., 292, 319 (see Puffer), 320 Yearick, L. G., 162, 170, 178, 202 (see Jeffries) Yee, J. Y., 51 (see Ross), 52, 76 (see Ross), 76 Yoder, R. E., 59,60,76 Yoshida, R. K., 407, 409, 411 Youden, W. J., 109,167 Young, H. E.,257, 288 Young, R. S., 280 (see Newton), 286 (see Newton) Yussupova, S., 178, ,904 (see Sedletzky) 2:

Zahnley, J. W., 111, 112, 167 Zimmerman, P. W., 302, 32U (see also Synerholm) Zobel, C. E., 315, 380 Zorkin, F. P., 325, 361 Zukovskaya, P. W., 266, ,988

Subject Index A Agrobacterium, 273 Alfalfa, bacterial wilt, 232-235 black stem, 235-237 breeding methods, 225-232 cross pollination, 208-215 diseases, 232-237 hybrid vigor, 237-229 injurious insects, 219-225 osmotic pressures of sap, 7 role of bees in tripping, 212-217 seed setting, 206-225 self pollination, 208-209 synthetic varieties, 229 tripping, 207-208, 210-217 tripping by rain, 210 tripping insects, 212-217 Alkali soils, influence on plant growth,

calcium-boron ratios, 332, 339 colorimetric determination, 324-326 cycle in nature, 334-335 determination in soils and plants, 323 determination by quinalizerin, 325-326 determination, spectroscopic, 323-324 determination, titrimetic, 323 fixation in soils, 330, 331 function in plants, 337-338 irrigation water, 327 leaching in soils, 329, 332-333 nitrogen-boron ratios, 341 potassium-boron ratios, 340 requirements of crop plants, 336-337, 344347

turmeric test, 324 Boron deficiency, 322, 329, 330, 342344 symptoms, 342-344

19-20

Alkali tolerance, physiological basis, 9-10 Alkalization, 5 Amberlites, 10 Ammoniated superphosphate, 51 Ammonium nitrate, as fertilizer, 51 Anhydrous ammonia, 52, 56-59 Anion exchange, 402, 403, 404 Apatite, 392, 400 Ascochyta imperjecta, 235-237 Auxin, 304 Azotobacter, 252-254, 260, 273 Azotogen, 274

B Bacterial wilt, alfalfa, 232-235 Bacteriorrhiaae, 246, 247, 257-259 Base exchange, 242 Bees, alfalfa tripping, 212-217 Bentonite, 397 Benzene hexachloride, 383-384 Birdsfoot trefoil, salt tolerance, 8 Bordeaux mixture, 385-386 Boron, availability in soils, 327 486

U

Calcium cyanamid, 71, 311, 378 Calcium fluorphosphate, 400 Carbowax, 301 Cephalosporium gregatum, 144 Chlorite, 162, 178 Chlorophenols, 297, 307, 308 Clay minerals, 159-199 aggregation, 198 anion exchange, 194-195 cation exchange, 189-193 cation fixation, 193-194 crystal configuration, 180-184 crystal structure, 162-171 distribution in soils, 177-179 electron micrographs, 174 flocculation, 195-196 functions in soil, 196-199 identification, 171-177 interstratified, 170 ionic sorption reactions, 188-195 isomorphous replacement, 168, 175, 176 phosphorus fixation, 393-397 polar sorption, 184-188

437

SIJBJECT INDEX

thermal dehydration, 173-174 X-ray diffraction, 174-177 Colloidal clay, 93 Commensalism, 248 Contact exchange theory, 192 Corn, yield responses t o nitrogen, 54 Corynebacteriuw insidiosum, 232-235 Cristohalit,e, 162

D D.D.T., as potato insecticide, 381-385 fate in soils, 277 lygus bug control, 217, 222-225 Diaporthe phaseolorum, 144 Dicalcum nitraphosphate, 50-51 Dinitro-secondary butyl phenol. 297, 308 Dithane, 385, 386

H €hematite, 162 Halophytes, 6, 26 Herbicides, contact, 294, 298 selective, 299 translocated, 294, 295 Heterosis, in soybeans, 126-7 Histosphere, 246, 274 Hybridization, in soybeans, 1236 Hybrid vigor, alfalfa, 227-229 Hydrous mica, 161 Hydroxy apatite, 399 I Illite, 162, 171, 172, 174, 177, 178, 179, 188, 190, 191, 194, 196, 396

P

cryPtal structure, 164, 168-169 Inositol phosphates, 409 Iron chlorosis, 16 Irrigated land, acreage of, 2 Isomorphous replacement, 188, 175, 176 Isopropyl phenylcarbamate, 294,310

Fertilizer consuiiiption in US., 40 Fertilizers, application to irrigation water,

K

E Edaphosphere. 245

63

defoliant, 71 farm-ponds, 71 foliage sprays. 67 liquid, 63-66 mixed, 53 pasture applications, 61-63. 69-71 phosphatic, 392 plow-sole placement, 59 potash, 52 requirements of soybeans, 102-105 starter solutions, 65-66 subsurfncp placement for sod crops, 6163 Flame cultivation, 314 Fungicidps, potato, 385-386 0

GYomerella glycines, 145 Glycine grasilis, 79, 121 Glycine max., 78, 121, 122, 129

Glycine soja, 78-9 Glycine Ussuriensis, 79, 121, 122, 129, 147

Kaolinite, 161, 169, 170, 172, 174, 177, 178, 179, 188, 190, 191, 194, 195, 196, 395, 396, 397, 399, 402 crystal structure, 169

L Limonite, 162, 395 LYYUS SW., 219-225 control by insecticides, 222-225

M Manganese oxidation, 254-255, 276 Medicago sativa, 205 Metaphosphates, 46-49 calcium, 46-48 potassium, 48-49 MetatheBia, 402 Methoxone, 306 Micas, 161 Montmorillonite, 161, 163, 164-168, 170, 174, 177, 178, 179, 180, 182, 185, 186,

438

SUBJECT INDEX

187, 190, 191, 194, 196, 396, 399, 402 crystal structure, 163, 164-168 sorption of water, 186 Mycorrhizae, 246, 247, 248, 257-9

N Nitrogen fixation, 252-254 symbiotic, 243, 248 Nitrogen mineralization, 250-252 Nitrophenols, 297, 307, 308 Nitraphosphate, dicalcium, 50-51 Nontronite, 170 Nucleic acids, 407-409 0

Oil, soybean, 85 Olivine, 45, 46

P Pentachlorphenol, 297, 308, 358 Phenol, dinitro-secondary butyl-, 297, 308 Phenoxyacetic acid, 2,4-dichloro-(2,4-D), 276, 277, 289, 294, 295, 300, 301, 302, 303, 304, 305, 306, 307, 308, 311, 312, 313, 358-360, 381 breakdown in soils, 313 , esters, 301, 302 injury to soybeans, 114 preemergence treatment, 294, 306, 307 transport, 304, 305 Phenoxyacetic acid, 2, 4, 5.-trichloro(2, 4, 5-T), 380, 381 Phenyl mercuric acetate, 311 Phymatotrichum root rot, 261 Phytin, 407-409 Phytophthora infestans, 356, 385, 386 Phosphate fertilizers, 41-51 Phosphate, diammonium, 50 Phosphate rock-magnesium silirate glass, 45 Phosphates, defluorinated, 43-45 Phosphorus, accumulation in soil, 392-393 immobilization, 255 organic in soils, 406-409 precipitation by oxides, 397-400 radioactive, 394, 405 Phosphorus fixation, 391-409 anion exchange, 402-406

adsorption, 400-402 biological, 406-409 Polyploidy, in soybeans, 121 Potassium cyanate, 311 Potato blight, 356 Potatoes, breeding and improvement, 355-357 chemical weed control, 357-360 fertilizer practices, 360-377 insect control, 381-385 killing vines, 377-381 minor element requirements, 371-372 nitrogen fertilization, 365-366 phosphorus fertilization, 366377 tuber composition, 374-377 potassium fertilization, 367-369 production in U.S.,353-354 virus diseases, 386 Potato leaf hopper, 382, 386 Potato scab, 355-357 Prochlors, 311 8 Quartz, 162 R Red clover, salt tolerance, 8

Rhizobiurn japonicum, 105, 106 Rhizobia, 247, 273 Rhiroplane, 246,265, 266, 268, 271 Rhizosphere, 243-282 bacterial flora, 271-274 definition, 244 fungal flora, 271 growth substances in, 260 microbial counts, 266-268 soil aggregation, Q63,261 Root respiration, 256

a Saline soils, 5, 11-19 Salinization, 2, 5 Salt tolerance, physiological basis, 5-9 relationship to stage of growth, 20-29 specificity, 29 Seed disinfectants, 108-109 Seed germination, saline conditions, 2024 Serpentine, 46

SUBJECT INDEX Sinox, 299, 307, 358, 378, 379, 380, 381 Sodium arsenite, 293, 295, 303 Soil aggregation, 198, 263-264 caused by soybeans, 137 role of micro-organisms, 198 Soils, alkali, 19-20 black alkali, 9 boron availability, 327 clay minerals distribution, 177-179 distribution of clay minerals, 177-179 saline, 5, 11-19 Soil erosion, following soybeans, 137-40 Soil inoculation, 274 Soil sterilants, 294, 295, 305, 307, 308 Solanum demissum, 356 Solanum tuberosum, 353 Soybeans, acreage in U.S., 81-83 bacterial blight, 146-147 brown stem rot, 144-145 calcium requirements, 93-95 diseases, 143-148 effect on soils, 136-143 floral initiation, 85-90 genetics, 115-123 hail damage, 99-101 hybridization, 123-5 insect damage, 149 interspecific crosses, 121 micronutrient requirements, 95-97 nitrogen fixation, 91 nitrogen nutrition, 90-92 nodulation, 91, 109 oil, 85 planting rate and spacing, 111-113 response to fertilizers, 102-105 response to photoperiod, 86-88

439

seed composition, 97-99 seed inoculation, 105-107 seed viability, 107 simply inherited characters, 116-117 time of planting, 110 utilization in U.S., 83-85 variety improvement, 123-136 virus diseases, 145, 147 world production, 80 Stoddard solvent, 294, 300, 309, 379 Stove oil, 308, 309 Streptomyces scabies, 357 Sulphur deficiency, 43 Superphosphate, 366-367, 392, 393, 399, 400 ammoniated, 51 concentrated, 41, 42, 44 Symbiotism, 247 Synthetic varieties, alfalfa, 229

T Tobacco, chloride toxicity, 17 Tourmaline, 327, 328, 329 Trichloracetic acid, 310, 358 Trichoderma viride, 262

u Urea-form, 51

W Weathering, 160 Z

Zeolites, 161

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