Advances in Agronomy, Volume 13

ADVANCES IN

AGRONOMY VOLUME 13

This Page Intentionally Left Blank

ADVANCES IN

AGRONOMY Prepared under the Auspices of the AWXUCANSOCZETY OF AGRONOMY

VOLUME 13

Edited by A. G. NORMAN The Uniuersity of Michigan, Ann Arbor, Michigan

ADVISORY BOARD D. G. ALDRICH,JR. C. 0.GARDNER E. G. HEYNE W. H. ALLAWAY J. E. DAWSON F. L. P A ~ ~ S O N R. W. PEARSON

AN ACADEMIC PRESS REPLICA REPRINT 1961

ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich. Publlshers

N e w York

London Toronto

Sydney

San Francisco

This is an Academic Press Replica Reprint reproduced directly from the pages of a title for which type, plates, or film no longer exist. Although not up to the standards of the original, this method of reproduction makes it possible to provide copies of books which otherwise would be out of print.

Copyright 0, 1961, by Academic Press Inc. ALL RIGHTS RESERVED

NO PART OF THIS BOOK MAY B E REPRODUCED I N ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS,

WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS.

ACADEMIC PRESS, INC.

111 Fifth Avenue, New York, New York 10003

United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London N W l 7 D X

Library of Congress Catalog Card Number 50-5598

PRINTED IN THE UNITED STATES OF AMERICA

808182

9 8 7 6 5 4 3 2

CONTRIBUTORS TO VOLUME 13

RUSSELLS. ADAMS,JR., Graduate Assistant, Department of Agronomy, University of Illinois, Urbana, Illinois T. J. ARMY,Soil Scientist, Soil and Water Conservation Research Division, Agricultural Research Seruice, United States Department of Agriculture, Fort Collins, Colorado

K. P. BARLEY,Senior Lecturer, Agronomy Department, Waite Agricultural Research Institute, Adelaide, South Australia JOHNC. BROWN,Soil Scientist, Mineral Nutrition Laboratoy,Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland

ROSCOEELLIS,JR., Professor of Soils, Department of Agronomy, Kansas State University, Manhattan, Kansas P. F. Low, Professor of Soils, Department of Agronomy, Purdue University, Lafayette, Indiana

T. M. MCCALLA,Microbiologist, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Lincoln, Nebraska F. H. W. MORLEY,Geneticist and Agronomist, Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra, A.C.T., Australia ALEX.Mum, Head, Soil Survey of England and Wales; and Head, Pedology Department, Rothamsted Experimental Station, Harpenden, Herts., England

W. F. ROCHOW,Plant Pathologist, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, and Associate Professor of Plant Pathology, Cornell University, Ithaca, New York

V

This Page Intentionally Left Blank

PREFACE It is the function of this series to review research progress in soil and crop science and developments in agronomic practice. The central theme, of course, is soil-plant relationships, but there may be excursions from time to time beyond the generally recognized margins of soil and crop science. Frontiers change as new fields are explored. The final test which the editors employ is whether the material reviewed is likely to be of interest and use to a group of reasonable size. Although science is international, there are aspects of agronomy that are regional. Some practices that are highly effective and economic in one area may not be suitable or acceptable elsewhere. From time to time reviewers have complained that it is primarily North American agronomy that is dealt with in these volumes, and although this may be so, the test of usefulness is the one that should be applied. In the following pages only one article, the comprehensive account of the practice of stubble-mulching, by McCalla and Army, is directly identified with North American agriculture. The practice, found effective in soil and water conservation in our midcontinent states, may well be capable of adaptation or modification, to areas with similar conditions in other parts of the world. Another article, which might be considered to be of a regional nature is that by Morley on subterranean clover in Australia, but the agronomic developments that have followed the widespread use of this legume are so dramatic as to transcend purely regional considerations. Australian workers have approached their peculiar soil and land-use problems with great effectiveness. The remaining chapters cannot be collectively described; all have been prepared by specialists who have become deeply involved in the topics which they treat. The subject matter diversity is so great that there should be something of interest to all.

A. G. NORMAN Ann Arbor, Michigan

July, 1961

Vii

This Page Intentionally Left Blank

CONTENTS Page

........................................ PREFACE .......................................................... CONTRIBUTORS TO

VOLUhlE 13

V

vii

THE PODZOL AND PODZOLIC SOILS

BY ALEX. Mum I. I1. I11. IV . V. VI . VII . VIII. IX . X. XI . XI1.

Introduction ................................................ The Recognition of the Podzol as a Soil Formation . . . . . . . . . . . . . . . . Ortstein and Bleisand ........................................ The Recognition of an Illuvial Horizon .......................... Degree of Podzolization ...................................... The Sod-Forming Process of Soil Formation ...................... Geographical Variants ........................................ The Western Contribution .................................... What Is Podzolization? ....................................... Micromorphology ............................................ The Characteristics of the Russian Podzol and Podzolic Soils ....... Summary and Conclusions .................................... Addendum .................................................. References ..................................................

1 2 11 19 23 24 30 33 38 46 47 51 52 53

SUBTERRANEAN CLOVER

BY F. H. W. MORLEY I. I1. I11. IV. V. VI. VII. VIII . IX . X.

Introduction ................................................. Taxonomy .................................................. Physiological Variation ........................................ Nodulation and Nitrogen Fixation .............................. Ecology .................................................... Agrotechnology .............................................. Diseases and Pests ........................................... The Estrogen Problem ....................................... Agronomic Improvement ...................................... Potential Usage .............................................. References .................................................. ix

58 61 66 72

77 90 103 105 107 113 117

X

CONTENTS

STUBBLE MULCH FARMING BY T. M. MCCALLAAND T. J . ARMY I. I1. I11. IV. V VI . VII . VIII . IX. X XI . XI1 XI11. XIV. XV . XVI. XVII.

. . .

Introduction ................................................. Historical ................................................... Wind Erosion Control ........................................ Infiltration .................................................. Water Erosion Control ........................................ Moisture Conservation ........................................ Soil Temperature ............................................ Soil Structure and Other Physical Properties ..................... Chemical Effects ............................................ Microbial Activity ............................................ Tillage Equipment and Use ................................... Weed Problems .............................................. Insects and Plant Diseases ..................................... Crop Yields ................................................. Mulch Maintenarice .......................................... Use With Other Conservation Practices ......................... Summary and Conclusions .................................... References ..................................................

Page 126 129 132 135 138 144 150 154 159 165 170 175 177 179 187 191 191 194

CONTAMINATION OF SOILS BY PETROLEUM HYDROCARBONS BY ROSCOEELLIS. JR., I. I1. I11. IV. V

.

AND

RUSSELL S. ADAMS. JR .

Introduction ................................................. Effect on Microbial Activities .................................. Effect on Soil Properties ...................................... Effect on Plant Growth ...................................... Summary ................................................... References ..................................................

197 198 203 210 214 215

THE BARLEY YELLOW DWARF VIRUS DISEASE OF SMALL GRAINS BY W . F . ROCHOW I. I1. I11. IV V. VI . VII . VIII . IX.

.

Introduction ................................................ History and Distribution of the Disease ......................... Crops Affected and Importance of the Disease .................... Symptoms of the Disease ..................................... Aphid Vectors of the Virus .................................... Biological Nature of the Virus .................................. Factors That Affect Outbreaks of the Disease .................... Control of the Disease ........................................ Summary and Conclusions .................................... References ..................................................

217 218 220 226 228 231 235 238 244 245

CONTENTS

xi

THE ABUNDANCE OF EARTHWORMS IN AGRICULTURAL LAND AND THEIR POSSIBLE SIGNIFICANCE IN AGRICULTURE BY K . P. BARLEY I. I1. I11. IV. V.

Introduction ................................................ Earthworm Populations under Crops and Pastures . . . . . . . . . . . . . . . . The Influence of Earthworms on Soil Fertility .................... Effects of Earthworms on the Yield of Crops and Pastures . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ..................................................

Page 249 250 256 262 266 266

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION BY PHILIPF. Low

. . . . . . . . . .

I I1 I11 IV V VI VII VIII IX X

Introduction ................................................ Nature of Ice and Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of Ionic Solutions ...................................... Mechanisms of Clay-Water Interaction .......................... Specific Volume of Clay-Adsorbed Water ........................ Viscosity of Clay-Adsorbed Water .............................. Dielectric Properties of Clay-Adsorbed Water .................... Supercooling and Freezing of Clay-Adsorbed Water . . . . . . . . . . . . . . Thermodynamic Properties of Clay-Adsorbed Water . . . . . . . . . . . . . . A Working Hypothesis ....................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

269 269 277 284 287 295 303 306 314 322 323

IRON CHLOROSIS IN PLANTS BY JOHNC . BROWN I. I1. I11. IV. V. .VI.

Introduction ................................................ Earlier Findings on Iron Chlorosis .............................. Factors Involved in Iron Chlorosis .............................. Chelating Agents as Iron Carriers .............................. Chelating and Other Mechanisms in Plants ...................... The Future of the Iron Chlorosis Problem ....................... References ..................................................

329 331 334 351 360 365 366

AUTHOR INDEX ......................................................

371

SUBJECT INDEX.....................................................

383

This Page Intentionally Left Blank

THE PODZOL AND PODZOLIC SOILS

Alex. Muir Soil Survey of England and Wales, Rothamrted Experimental Station, Harpenden, Herts., England

Page I. Introduction ...................................... 11. The Recognition of the Podzol as a Soil Formation . . . . . . . . . . . . . . . . 111. Ortstein and Bleisand ........................................ IV. The Recognition of an Illuvial Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . V. Degree of Podzolization ................................... VI. The Sod-Forming Process of Soil Formation . . . . . . . . . . . . . . . . . . VII. Geographical Variants ........................................ VIII. The Western Contribu .............. IX. What Is Podzolization? ....................................... X. Micromorphology . . , . ............................... XI. The Characteristics of t Podzol and Podzolic Soils . . . . . . . . XII. Summary and Conclusions ................................ References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 11 19

30 33 38 48

47 51 53

1. Introduction

When foreign words are adopted into a language there is quite commonly a change or an extension of meaning which is often signified in dictionaries by little danger signals as a guard against mistranslation. It is a pity that the earlier (and also some more recent) writers on soils did not use a similar system to warn their readers that the descriptions being used as illustrations did not necessarily conform to the original definitions. This is perhaps as true for podzols and podzolic soils as it is for any other soil group. Each successive generation of writers has tended to build on the results of immediate predecessors without considering the possibility of some subtle and even serious change in the meaning of the terms. When one is dependent on translations through a second foreign language the potential difficulties multiply considerably. Although the general history of the development of ideas on podzolic soils has been reviewed in the past, little attention has been given to the earliest work, particularly the Russian; and it is just this that provides some clues to the divergent opinions on this group that still exist. In fact it is not inappropriate to quote some remarks by Dokuchaiev from his “Cartography of Russian Soils”: “As the podzol is a formation so far I

2

ALEX. MUIR

comparatively little studied, and therefore controversial, it is undoubtedly true that the term has different meanings in different parts of Russia, so that in discussing the problem of this soil it is probably best to consider it historically and, to begin with, to take account of exclusively factual data.” It is certainly not true now to say that the podzol has been little studied, but it is still a controversial topic. Glinka (1923) listed some 160 publications in which soils of the Russian podzolic zone were discussed, and this figure could no doubt be trebled or quadrupled by subsequent work. This review deals primarily with the Russian evidence as far as it is accessible to me, but touches on the Western European and American literature to bring out the divergences in usage and point to their origin. The translation of Russian words derived from podzol offers minor difficulties in the presentation of the review. I have consistently rendered podzolistii as “podzolic” and opodzolennii as “podzolized.” As I understand the latter word it means that the soils in question have undergone the process of podzolization but may not qualify for the term podzolic in the Russian sense. I have also used it for soils that would commonly be called podzols in Western Europe or North America. It. The Recognition of the Podzol as a Soil Formation

Since Dokuchaiev is commonly given priority for naming the podzol, it is perhaps as well to give in abbreviated form some extracts from his principal writings on the subject. It is convenient to begin with his ”Cartography of Russian Soils” ( 1879). “It is first of all evident that in various parts of Russia the name podzol is given to by no means identical or even similar formations.. , , [A certain number] are peaty deposits with more or less mineral matter; all have been more or less burned; almost all contain fresh-water shells; all are near rivers and some are covered with river deposits. Such deposits are found everywhere on the alluvial banks of our rivers . . . . Their place among other rocks and their period of formation has been elucidated and it is therefore superthous to retain for them the special name of

poazois. . . .

“The remaining examples of podzols are more or less uniform and correspond in general to the description given by Mr. Solov’ev.” “These are their characteristics. “They lie indifferently in lowlands, at the bottom of slopes, on slopes, or on the tops of hills. * Said by Dokuchaiev to have given the “first sufficiently detailed description of the external features of the podzol” in his “Agricultural Statistics of the Smolensk Government.”

THE PODZOL AND PODZOLIC SOILS

3

“They generally occur in forests, on the sites of former forests or near bogs into which they sometimes gradually merge. “The podzol occurs sporadically, here and there as small islands, and only rarely occupies large areas. “The podzol lies either on the very surface , , . when it is often used as arable land, or it is covered by a thin layer (several inches) of vegetable earth, generally of forest or bog origin when it plays the part of subsoil. “In either case its depth is not great: 6-12 inches, deepening only in places as it approaches bogs. “Podzols are underlain by clays, loams or sands: in all these cases the podzol appears to pass gradually into the underlying rock. “As regards the composition of podzols, all those investigated so far are of a sandy type and contain 8344% SiOz.. . . “Microscopic analysis shows that the examples examined so far consist of phytoliths, diatoms, soft vegetable residues, quartz grains, silty particles, etc. “External characteristics of the podzol are: an ashy colour, generally with yellowish, bluish or greenish tones; it consists of very small particles so that when dry it breaks down into dust, in the wet state it is slimy like clay and because of this the podzol readily holds water and is therefore sometimes the cause of the appearance of bogs. “Such are the general and most important characteristics of the podzol . . . .” [Dokuchaiev explains the origin of the term podzol as arising from the custom of “slash and burn,” a form of shifting cultivation, still practiced in his time. According to him the name was applied to the infertile layer of soil, gravel, sand or heavy clay, which underlay the ashes resulting from burning the forest cuttings. Vosmer (Russisches Etymologisches Worterbuch) states that the element pod is cognate with Ger. Boden, Gr. pedon, Lat. peda; thus podzol simply means ashy soil.] “There are 4 possible modes of formation of the podzol. “1. . , . that it is due to fires . , . . That cannot be agreed to . . . . “2. . . . due to the burning of p e a t , , , does not apply to podzols. “3. From microscopic evidence it might be considered a simple accumulation of vegetable residues, e.g., phytoliths, but this view . . . cannot be upheld. It does not explain ( a ) the exclusive occurrence of the podzol on the surface or under a thin layer of plant remains; nor ( b ) the striking constancy in thickness (6-8 inches); ( c ) it contradicts the gradual transition of podzols to parent material and finally ( d ) phytolith formations may arise by chemical means alone. “4. These considerations (excluding the last) and at the same time

4

ALEX. MuIR

its position in depressions and on the slopes of hillocks, (e) its small thickness, ( f ) the strange discontinuity of the podzol in one and the same peasant’s field, and finally, ( g ) its content of organic, sometimes structured, material-all these taken together will not allow us to accept the conclusion of Krylov that our podzol can be regarded as a kind of chemical precipitate, as a kind of rock, related by its composition (but the clayey podzol?) and micro-structure to foreign siliceous plant bioliths . , . . “It seems to us that all the characteristics (from a to g) of our podzol compel the conclusion that we are here dealing not with rock . . but with soils . . . “It is evident that these sandy, and rarely loamy, podzols are soils of a mixed, vegetable-bog character; all their differences, in mode of formation, from the common northern dry-Zand vegetable loamy and sandy soils, consist in the fact that the latter, in general, are formed in drier places with free access of air and light and the predominance of a dryland herbaceous vegetation, while podzols are formed mainly in forests with a significant participation of bog and forest vegetation. Here, evidently, there was more moisture, less light and probably less access of air to the soil. That is why podzols occur predominantly in forests and near bogs; why they not infrequently pass gradually into these latter formations; why they are coloured with bluish, greenish (vivianite and ferrous iron) and yellowish tints; why one occasionally finds in them ore concretions and still undecomposed plant remains. “It stands to reason that in the formation of different varieties of podzol special conditions operate . . . . “As regards loamy podzols, to my mind, they are none other than the ordinary northern loamy soils, only changed not by dry-land grass vegetation but predominantly by forests and small bogs . . . .” These extracts show clearly enough that the word podzol was restricted to the ashy gray, often superficial mineral layer and had little reference to what was below. This restricted use of the word podzol continued for many years in Russia and is well exemplified by the following quotation from an account by Sibirtzev (1884) of the soils of the Arzamas district (Nizhni Novgorod) in which he describes soils, with the appearance of podzols, occurring in hollows and depressions of some 60-100 meters diameter, in the chernozem plateau. “A Greyish white, slight yellow tinge; when moist fairly compact; on drying becomes an extremely fine grained, loose, light mass, 1-1sft. or more in thickness. B Gradual transition to brownish subsoil, 3 in. C Yellowish brown or yellowish grey clay.”

.

.

THE PODZOL AND PODZOLIC SOILS

5

Locally the A horizon may appear on the surface, but it is often covered by “black e a r t h to several inches or even by peat, when it contains diatoms and phytoliths. “Therefore we feel that it is simplest to regard podzols, in particular the ‘white land’ of Arzamas district, as vegetable-bog soil, in the formation of which both the subsoil brown clay and its moist peaty, boggy cover took part. The brown clay underwent changes in the moist medium, without access of air, in the presence of carbonic acid and acid bog humus: it is not surprising, therefore, that it lost, by leaching, a considerable part of the soluble material, lost its uniform brown colour, became enriched in silica; the presence in the light grey mass, so formed, of diatom shells and phytoliths is in this way understandable; the yellowish or orange spots (in the A horizon) indicate the original parent material of the podzol . . .” (Sibirtzev, 1884). In all this early work the podzol was very carefully distinguished from the so-called “northern loams and sands,” which were considered as “dry-land vegetable soils” and formed the fifth group of the classification used for the Nizhni Novgorod survey. However, it was becoming recognized that there were soils in this “northern loam” category that had some characters of the podzol. A distinction was therefore introduced between the normal and the “podzolic” varieties, and Dokuchaiev quotes several descriptions in his final report on the Nizhni Novgorod survey ( 1886). “From the data of Sibirtzev and Ferkhmin the structure of the normal (northern) sandy loams in their natural state may be represented by the following schematic section: Horizon A: either ordinary sod or forest mat, s-1-2 in., consisting of leaves, roots, twigs, etc., and under it a grey sandy mass, cemented by clay particles; thickness . . . about 3 in. and not more than 5 in. Horizon B: greyish white, sometimes with yellowish brown and reddish veins and spots, fine sandy loamy mass, without nutty structure or only a slight indication of it, in wet weather it is fairly sticky, in dry it is powdery (thickness 2-3 and up to 7 in.). Horizon C: greyish brown, sticky when wet, consisting of sand cemented by reddish brown particles of clay; on rubbing, numerous colourless, reddish and yellowish quartz grains are exposed; in a word this is a sandy clay or sandy loam; imprints of roots fairly frequent.” The podxolic sandy loams have the following structure according to Amalitzky, one of Dokuchaiev’s colleagues (in Dokuchaiev, 1886) :

“A. Soil horizon of a greyish bluish colour, sometimes with a yellowish

6

ALEX. MUIR

tint; the basic mass is very fine, floury to the feel, dust-like, contains sharp, easily visible quartz grains; in the wet state it is sticky and difficult to work. When dried, immediately following rain, it is hard and cracks, but after prolonged droughts it is powdery and dusty, average thickness about 5-7 in. B. Transition horizon: it is still lighter and unevenly coloured; contains residues of roots and clayey granules of a dark grey colour, occasionally porous; thickness 3-5 in. C . Distinctive light ashy podzolic sandy loam, uncoloured by humus, becoming yellower towards the bottom and then effervesces with acid. There are occasional hard granules of reddish brown colour that are only rubbed down with difficulty, also pores in which there are occasional root residues; towards the bottom it gradually passes into the underlying rock; thickness 0 to 2 ft. C1. Most frequently red-brown sandy loam, sometimes stony; in its upper horizons it has the nutty structure so characteristic of the forest loams; below it is the ordinary boulder clay , . . .” The reference to effervescence with acid is unusual and no explanation is given. On the basis of his description and a chemical analysis of the bleached horizon, Amalitzky concluded that the podzolic sandy loam soil just described is very close to the podzols of Smolensk, referred to by Dokuchaiev. During the Nizhni Novgorod survey a close connection was noted between the thickness (the A horizon of today’s nomenclature) of such podzolic sandy loams and steepness of slope provided there was no change in parent material; the depth of soil increased downslope and movement of material was suggested. ”. . . the material from which this soil [a sandy loam] was formed, because of its occurrence on present or former slopes, has undergone alteration by eluvial processes, mainly resulting in the loss of clay and lime and enrichment in sand . . .” (Dokuchaiev, 1886). “The material (podzol) sorted in this way, consisting of quartz and various silicates, poor in clay, will come under the influence of the different atmospheric agents and vegetation-( in this case forest) and undergo new and further weathering. Quartz will remain unaltered, the silicates will decompose forming amorphous silica which will be left; the clay would be carried by flowing water (slope)” (Amalitsky); the result would be an enrichment in silica. It was considered that amorphous silica formed the cement that made the podzol hard when it first dried out and also conferred its coldness in wet weather. That erosion occurred on these soils is mentioned by Dokuchaiev, who says it was sometimes so severe that it became necessary for peasants

THE PODZOL AND PODZOLIC SOILS

7

to abandon their fields to forest. Elsewhere he remarks that erosion on slopes has given rise to a “particular fine, floury, podzolic rock,” which increases in thickness from 4 to 6 inches at the top of the slope to 3 to 4 feet at the base. Georgievsky (1888), who has been cited by Glinka (1914, 1931), provides further interesting information on the podzol as it occurs in the former St. Petersburg and Novgorod governments. On the watershed between the rivers Syas and Tikhvinka (Novgorod government) the following types of soil occur: northern loam, sandy loam, sand, and podzol. “Referring in particular to podzols, we see that they nowhere occupy extensive areas, but are everywhere encountered only as bare spots, broken strips, etc.; in addition the following very characteristic feature of their distribution is observed. “Lying on level, relatively high situations or on slopes the formation we are considering nowhere occurs as a layer of constant, definite, thickness but always shows the well known discontinuity, even over the smaller areas: frequently over a distance of 1-2 arshins ( 2 8 5 6 in.) the podzol wedges out several times; sometimes over a whole peasants’ field it appears only at a few points, a search for it being necessary, the more so in that frequently it cannot be determined if there is podzol present or not at any given point either by the colour of the upper soil horizon ( A ) or by any noticeable change in relief. “It is characteristic that in those cases in which the podzol areas are bounded by adjacent swampy ground, the discontinuity of the podzol decreases in the direction of the swamp and its thickness increases. Thus the surroundings of bogs and various kinds of low places and depressions, even though barely noticeable, yet forming more or less wide swamps, are the most usual sites of the podzol in this area” (Georgievsky, 1888, PP. 8-91. Georgievsky divides his podzolic soils into three types according to their position in the relief. In the first type (Fig. 1, D) on the higher elements of relief, the “upper soil horizon A” of about 6 inches comes immediately under the sod (Russian dern) and passes gradually into the “podzolic horizon B” which may be anything up to 10 to 12 inches thick, but more commonly 3 to 4 inches. Frequently the latter horizon tongues and wedges into the subsoil “(horizon C)”. The second type occurs in depressions and moist spots. In this case the upper soil horizon ( A ) , 3 to 4 inches, takes on a whitish tone and in its properties approximates to the material of the true podzol horizon “B”; the latter is much finer grained and floury when very dry. Thirdly a profile from a bog is described, in which, under a layer of 12 inches or so of black bog earth “A,” the podzolic horizon “B” is of a bluish tint with reddish ochre veins

8

ALEX. MUJR

and spots; it gradually passes into the subsoil “C.” The thickness of the podzolic horizon in this case may be 28 inches. In the examples given above the soils have generally been derived from what were called sandy loams (Russian a p e s ’ ) ,and it is surprising that no comment was made about any illuvial horizon. As far as can be seen from these early writings such a horizon had not been clearly recognized. Amalitzky (cf. p. 6 ) comes near it with his reference to a nutty structure in the “C1”horizon; and Georgievsky, after describing some sandy podzols with ortstein, says that “if the subsoil of the podzol is loamy . . . it passes into the country rock very gradually, separated from it by a yellowish loam; the latter occasionally appears as an independent horizon, replacing as it were the podzol.” With regard to the examples from near St. Petersburg referred to by Glinka (1914, pp. 68-69, 1931, pp. 341-342) and quoted by others (e.g., Robinson, 1949, p. 306), the descriptions are given rather differently by Georgievsky ( 1888, pp. 14-16). Near the river Luga, Georgievsky found on a sandy hillock white, fine, quartz sands underlain by loose, mobile reddish yellow or yellowish “diluvial” sand. “They [the white sands] always appear almost at the surface of the soil and are covered with only a thin layer of dark grey earthy sand, they are discontinuous and occasionally, in the form of long root-like salients, wedge into the subsoil; their thickness is 4 to 7 in., reaching 22 in. in the centre of depressions. In this case two features deserve particular attention: 1 ) on passing up-slope the white sands thin out and finally disappear completely; 2 ) under the white sand lies ortstein, occurring in all stages of development, but wedging out at the top of the slope . . . . The structure [of the ortstein] in a dry depression has the following appearance: “Above lies a dark grey sandy (A) soil (thickness 3 to 4 in.); below comes a white, not always uniformly coloured, sand in which white patches constantly alternate with darker; still deeper the dark colour intensifies and the sand is gradually replaced by ortstein; the upper horizon of the latter, in the form of a thin crust (in sections it appears as a black band which closely follows every undulation of the white sand), separates the overlying sand from the lower compact ortstein rock the colour of which is generally pale yellow (but nevertheless contains 0.65% humus), but there are also dark coloured patches of similarly compacted material. In addition it should be mentioned that the degree of cementation of the ortstein as it passes into the loose yellow subsoil sand always gradually decreases. “As to the depth of the ortstein, it is variable and apparently is not closely related to relief, being about 8 to 28 in., and reaching (in the

THE PODZOL AND PODZOLIC SOILS

9

centre of depressions) 34 in., but in the latter case the ortstein is only weakly cemented. It should also be noted that the surface of the ortstein horizon is extremely uneven . . . .’’ Another example is described from the edge of a bog where the podzol overlies, indifferently, either red boulder clay or sands. ‘When it overlies sand the following interesting section can often be seen.

“A1 Mossy cover-1-2 in. “A Dark-whitish soil, contains 1.7% organic m a t t e r 4 in. “B Sandy mass of a white colour, consisting of sandy fine earth with an admixture of a moderate number of coarse quartz grains; nevertheless hardens on drying; humus content-O.12%; thickness-12 in. “B1 Sandy loamy mass, stained by humus (up to 2.5% ) to a chocolate colour and gradually passing into the subsoil; on ignition first of all lightens in colour and then reddens; contains some concretions strongly cemented by iron oxide which confers the hardness on this horizon. Thickness 7-9 in. “C Yellowish sand with boulders.

“On the clayey subsoil, immediately at its boundary with the podzol, there was also observed a quite peculiar layer of red, hard ferruginous clay, concretions of which occurred in the lower parts of the podzolic horizon. The podzol in this case appeared as a compact mass which, on prolonged drying turned into absolute flour.” Georgievsky’s conclusions on the occurrence of podzols are similar to Dokuchaiev’s: “The influence of climate is shown by the geographical distribution of the podzol. Judging by the available literature this white earth . . . . in contrast to the chernozem, which is characteristic of the open steppe, is encountered (at least in its typical form), exclusively in the north and north west of Russia, i.e. in places with relatively high rainfall, and with an abundance of both forests and bogs. Such a geographical distribution is not fortuitous. It should not be forgotten that just such an abundance of moisture is one of the essential conditions for podzol formation as this soil is most frequently encountered in bogs, on the edges of bogs, in fields adjoining them-in general everywhere that water can stand for long” ( Georgievsky, 1888, pp. 35-36). It is quite obvious from these various descriptions that the idea of the podzol and its variants was confined exclusively to the bleached or A2 horizon of later nomenclature. This is clearly stated by Dokuchaiev

10

ALEX. lMuIR

in a “Short Programme for the Investigation of Soils” (1887-1891) when discussing soil structure: “podzolic, floury structure; the whole mass consists of the finest, dusty, fine earth, almost exclusively quartz, whitish, light yellowish or ashy grey colour; the given structure is especially clearly expressed in the B horizon of typical podzols.” This general attitude to podzolic soils continued up to the turn of the nineteenth century and is summarized by Sibirtzev (1900) in his book “Pochvovedenie.” After contrasting podzolic soils and chernozems and listing the factors influencing podzolization (climate, parent material, etc.) he goes on: “Thus the soils of this type fall naturally into subtypes :

I

Soddy soils, weakly (sometimes not visibly) affected by podzolforming processes. I1 Podzolic soils proper, with a podzolic horizon clearly separated and sharply distinct from the upper horizon. I11 Podxols or soils strongly podzolised (frequently to the surface).” The use of the term “sod-podzolic” is considered to be not altogether appropriate “as the majority of the soils are formed under mixed forest , . . ; but we retain the conventional term ‘soddy’ or ‘sod-podzolic type’ in order to indicate that the degree of podzolization is quite variable (Sibirtzev, 1896). The morphology of the ordinary podzolic soil is summarized thus:

. . .”

“A Upper soddy horizon, from 3 to 5 in. thick, usually structureless, coloured light grey, light cinnamon, or grey. “B Under this lies the podzolic horizon proper, whitish, sometimes almost white, sometimes yellowish and bluish; thickness variable, from 2 in. to a foot or more. “C The podzolic horizon gives way to the subsoil material, e.g. brown clay, brownish bouldery sandy loam, yellowish clayey sand, orange coloured loess-like rock and so on. Frequently the podzol penetrates into the subsoil in ‘pockets and spots,’ i.e. the change to the subsoil is gradual.”

“. . , The common new formation of podzolic soils-ortstein-occurs as segregations of varying size, grains and veins . . .” It is surprising that in none of the writings on soil morphology of the “fathers of pedology” do there appear any clear suggestions that the C horizon of their nomenclature might be the illuvial horizon and the recipient of at least some of the material lost from the bleached layer. Although there is frequent reference to ortstein in the early works, it is

.

THE PODZOL AND PODZOLIC SOILS

11

only in connection with the formation of this layer in sandy soils that the possible fate of the leached substances is considered at all fully. 111. Ortstein and Bleisand

Soils were often described by competent observers in the seventeenth and eighteenth centuries, but none seems to have noticed or recorded the hard humous or ferruginous layer that is so common in heath and moorland soils particularly. The earliest reference given by Glinka is to a paper by Kindler (1836), who observed the bleaching effect of decomposing roots on ferruginous sands. Following this there appears to have been quite a crop of observations in various countries. Thus, Daubrhe ( 1845 ) confirmed Kindler’s observations and accepted Berzelius’ suggestion of crenic acid as the active agent. Sprengel’s description of Bleisand corresponds roughly to that of the early Russian workers of their podzol, i.e., he is concerned only with the bleached layer: “When, under the fine quartz sand, there is so much carbonaceous, hardened, or very resinous, humus that it has a blue-grey colour, it is known in certain localities, e.g. Liineburg area, as Bleisand. This sand is so very infertile that in the Luneburg area it is reckoned the worst of all soils. It is formed both in heaths and in pine forests in which the carbonaceous, resinous humus is formed (Sprengel, 1837; 1844, p. 154). Here we have reference to the bleached layer only. Later in his discussion of ferruginous loams Sprengel refers to “an earthy layer that is very rich in ferric and ferrous oxides; the iron coming partly from the cultivation implements and partly collected from the soil” (Sprengel, 1844, p. 175). Although some analyses of soils and subsoils are given to illustrate their chemical composition, no attempt is made to link them with the soil descriptions. Following Sprengel, a British writer (Gray, 1839) gave a description of “moorband pan” but had no comment to make on a bleached layer. It is not until we come to Senft (1862) that we get a reasonable description of a German podzolic soil. Senft was primarily interested in the influence of humic matter on the formation of limonite deposits, and ortstein therefore figures prominently in his account. His description of a heath podzol is as follows (Senft, 1862, p. 181). “This Ortstein or Ortsand generally occurs in heath soil, especially on the Luneburg Heath. The plant cover consists either of Calluna vulgaris and other Ericaceae or of Nardus stricta, etc. and Vacciniaceae. Directly under the plant cover there generally occurs a 6 to 8-inch-thick layer of sand of leaden-grey to smoky-brown colour due to the heath humus, or a sand-rich layer of brown loam. This layer is followed by one of variable

12

ALEX. MUIR

thickness, of white colour grading into yellowish or greyish white sand, frequently mixed with rounded boulders and small erratic blocks, often cemented with a little loamy material. Below this whitish sand at 2 to 5 ft. depth is a 10-inch to 4$-foot-thick layer of Ortstein distributed in nests or islets or elsewhere in a continuous formation. It is notable that this Ortstein, according to a written communication from Mr. Schultze, never occurs in fully forested areas of the investigated region, but appears some time after careless deforestation is followed by a period of noncultivation. Furthermore, Ortstein never occurs on the ridges of higher hills, but it does occur on slopes even as steep as 35”.” Elsewhere Senft mentions that many ortsteins consist of sand coated with a skin of ferruginous clay or clayey ochre. Of all the early writers on podzolic soils the greatest is undoubtedly P. E. Miiller, whose name frequently appears in reference lists but who appears to have been little read by soil scientists. His work on natural humus forms is a model of scientific observation, and he provides the most detailed account of a podzolic soil given by any nineteenth century writer. In the following paragraphs the word Torf is translated as raw humus, which seems appropriate to Muller’s definition of his use of the term. “The soil in a rather dense beech forest with a raw-humus layer is only scantily covered with the vegetation mentioned above [i.e., Aira flexuosa, Trientalis europea, and mosses]; it carries a poor vegetation; the surface is covered with small branches, twigs and some fallen leaves between the moss and the few and insignificant phanerogamic plants. The loose leaf-layer always covering the mull is conspicuous by its absence from the raw humus soil. The soil is compact and yields underfoot no more than a thick felt layer over a solid basis. The surface is so compact that rainwater will sometimes collect in puddles on the loose sandy soil where the soil is covered with a raw humus layer; if this layer becomes wetted by continued rainfall, it becomes saturated like a sponge, while the underlying soil remains dry. “When the soil is dug, there first appears a tough blackish brown humus layer, the raw humus. Underneath more or less sharply differentiated from the raw humus is a generally loose sand lacking the ochreous colour common in transported formations. Its colour varies between greyish white and grey or dark grey and usually becomes lighter with depth. Beneath is a dark layer of reddish brown or brown colour, which is underlain by sandy loam, sand or an intermediate form of these two. “These layers [Fig. 1, A and B] are of very different thickness and also show other differences. The greyish sand layer, designated Bleisand, under a thin layer of raw humus may occur either as a very fine band

THE PODZOL AND PODZOLIC SOILS

13

FIG. 1. Schematic representations of podzolic soils. A-C after Miiller (1887); D after Georgievsky (1888); E after Afanasiev (1930).

14

ALEX. MUIR

hardly one inch thick with indistinct boundaries, or as a sharply delimited, four-inch thick sandy layer of almost pure white colour, such as is often encountered in forest land adjacent to the North Sea; layers of six inches thickness are less frequent, though in forests on the transported sands in Jutland may attain a thickness of a couple of feet. On a flat terrain, the thickness of the peat and Bleisand layers often show some correlation in so far as the thicker layers of raw humus occur over thicker layers of sand. On more varied relief there are many exceptions to this trend where, in depressions, very thick layers of Bleisand may occur under thinner raw humus layers. The sand is generally very loose, but at the boundary with the underlying layer it sometimes becomes more compact and even assumes the appearance of sandstone in some places [Fig. 1, B]. “The brownish layer underlying the Bleisand, designated as red earth or Ortstein, varies both in thickness and consistency. Ortstein and Bleisand are generally the same thickness when the latter occurs as thin bands hardly one-inch thick, but under thicker layers of the sand the Ortstein may be four to six inches or even eighteen inches thick. The total thickness of the three layers can therefore vary between about four inches and two and a half feet. Thin layers of Ortstein are always loose and earthy, and even deposits of four to six inches often have a similar consistency. But thicker layers of Ortstein are generally compact and develop either into a sandstone-like formation or into the true Ortstein, well known on heathland, such as in the Silkeborg Forests in Jutland.. . . “The subsoil, like that under mull, is thus often compact where the Bleisand and red earth layers are thicker; otherwise the subsoil is very variable and may consist of transitional forms between poor sands low in clay, occurring in bouldery formations, and plastic and micaceous loam characteristic of lignite formations. When the subsoil is sandy or gravelly, the two overlying layers differ little except in thickness; on the other hand, where the subsoil is loamy, three different forms of these layers may be distinguished. “The first form is exemplified by the profile at Store Hareskov (Seeland) [Fig. 1, C]. In a depression and under a not particularly firm raw humus layer 21/, in. thick there was a loam layer 5 in. thick having a uniform very light colour below which was a similar thickness of a uniformly reddish brown loam layer resting on a subsoil of irregularlycoloured ochreous yellow sandy loam. “The second form is shown in a profile occurring at the Teglstrupper Gehege ( Seeland). The profile is in a recently dug ditch in loam on an elevation, falling on three sides by 6-8 feet at distances of about 100-200 feet, Under a 4-inch thick extremely tough layer of raw humus there is a

THE PODZOL AND PODZOLIC SOILS

15

6 or 7 inch thick Bleisand layer of whitish grey colour, underlain by a 16-inch-thick sandstone-like layer of light greyish brown colour with dark brown veins and streaks; a vein of the overlying Bleisand ran downwards through this layer. The subsoil consists, as at the previously described place, of ordinary sandy loam with gravel, sand and glacial boulders. “A third form is shown in the profile of the soil of a high slightly inclined plateau in Laven-Skow, near Silkeborg (Jutland). The very tough raw humus layer of more than 4-inch is underlain by 3 to 4-inches of fine, whitish, micaceous sand, the lowest part being a darker colour and compacted into a very hard but thinner layer. Immediately below this layer is a slightly plastic micaceous clay without any glacial boulders, which undoubtedly belongs to a lignite formation. Though the overlying Bleisand covers the clay in the form of a dense mass without any cracks or fissures, the uppermost part of the clay is full of cracks which divide the soil at many points into larger or smaller clods. The cracks are filled with a blackish brown powdered material which also covers the clods, so that a clean section of the soil appears to be marked with brown veins. If the brown powder covering the clay is scraped off, the surface of the latter is seen to have a greyish colour and a Bleisand appearance and contains less clay; however, at a greater distance from the brown surface (up to 1 inch) the mass is plastic and ochreous-yellow coloured.” After an account of the microscopic appearance of the beech raw humus, Miiller continues: “Mechanical analyses show that the three layers below the raw humus, like those under mull, decrease in stone content with increasing depth, but the clay content may either increase or decrease with depth. In this respect it is particularly noteworthy, that the clay in all profiles in elevated positions increased very markedly with depth, while in the only profile in a very low position the highest amount of clay occurred immediately below the raw humus. It will be seen later that this is due to the effect of water in translocating clay from higher to lower layers of the soil or to lower-lying parts of the relief. In order to ascertain that the process does not take place in the reverse direction and that the sand is not moved by water, as some practical men believe whenever they see a whitish band of sand near the soil surface, a profile about 300 yards long was examined at Store Hareskov. Here the Bleisand was of variable thickness throughout the entire line, but careful levelling showed that it behaved similarly on hilltops, small plateaux and in depressions, showing no removal whatsoever of sand by water. “Further examination of the peculiar structure of Bleisand and Ortstein shows that the former is composed mainly of pure mineral frag-

16

ALEX. MUIR

ments, especially quartz, intermixed with the other components of granite. The humus-like particles, which confer a dark colour on the layer of Bleisand directly underlying the raw humus, as well as containing fine root fibres, consists mainly of small black humus-particles embedded between the grains of sand. Ortstein possesses a quite different constitution: each grain, each solid particle, is surrounded by an apparently structureless substance; each is encrusted with the brown material which confers its colour on the layer, making the entire mass so similar to the topsoil below mull that the uniform colour prevented any differentiation between the structure of Ortstein and of this layer (i.e,, topsoil) when examined under the microscope. A similar incrustation, though less complete and of another composition, occurs in the loamy subsoil.” In a general discussion of forest soils with mull and raw humus, Miiller points out (1887, p. 72) that the whitish gray layer that can occur under mull is not to be regarded as Bleisand for it by no means lacks the encrusting substances characteristic of mull. In other words, we appear to have the first reference to a soil akin to the Parabraunerde or sol lesSiOh.

The heath soil with its ortstein or moorband pan, which provided problems both for the forester and the agriculturalist, gave rise to an extensive literature in Germany and Scandinavia during the later nineteenth century and the early part of the twentieth century. Much of the work was largely repetitive and few adequate soil descriptions were given. Of the various writers, Emeis (1876) and Muller were the most productive and both gave good descriptions with diagrams of heath podzols to which modern horizon terminology can be readily applied. The morphology of the “typical heath soil” is summarized by Muller (1887, pp. 139-140) as follows: “The soil is covered with a layer of compact and tough raw humus, consisting mainly of organic components and residues of heathland plants, densely interwoven with heather roots, fungal mycelia, occasional moss thallus, and roots of other plants. The lowest parts of the heath layer usually contain mineral components, mainly sand, from the subsoil, in amounts increasing with depth, and it assumes the character of a sandy soil containing plant roots strongly intermixed with plant residues, rather than resembling the true raw humus formation on the original soil. Drying, however, also hardens this part of the surface, often making it rock-hard and not sharply distinguishable from the raw humus on the top. The heath layer is underlain by Bleisand in a white layer mixed with humus particles and showing tints of snow white to a greyish black mull-like colour according to the amount of admixture. The boundary between the Bleisand and the heath

THE PODZOL AND PODZOLIC SOILS

17

layer is sharper in proportion as the above-mentioned lower layer of the raw humus is less developed; sometimes this boundary is quite obliterated, but often it is very marked. The layer below the Bleisand is coloured by humic acid and its compounds and is called Ortstein or ‘red earth.’ The mineral constituents of the soil in this layer are covered with material of a humic nature. The layer is usually very thick, permitting only a very slow penetration of water. The Ortstein layer is never sharply delineated at its lower edge; transition to the ochreous sand of the subsoil is indistinct and usually very irregular, so that the dark to blackish brown layer penetrates the subsoil in tongues. The rich variety in the forms of this transition is mainly due to differences in the movement of water. Emeis published very fine drawings of heath profiles, showing all these characteristics. The boundary between the Ortstein and Bleisand is often very sharp, but also frequently indistinct due to accumulation of humic matter in the lowest layers of the sand.” In view of the importance of ortstein Muller gives a detailed account of its nature and origin which is the fullest to be found in the nineteenth century literature. His recapitulation of its nature and occurrence is very complete and does not appear to have been superseded (Muller, 1887, __ pp. 222-224) : “Nature A. Ortstein formed by deposition. 1. Clay Ortstein. A more or less porous, solid and hard mixture of sand and clay of a rather uniform greyish colour. Does not change its consistency on treatment with alkali or dilute mineral acids. 2. Peat-like (Torfige) Ortstein. Dense, earthy to hard, blackish brown to black or bluish black accumulation of carbonized humus containing humic acids and their salts, and more or less mixed with white sand-grains lacking ochreous films. Acid because of the humic-acid content. On treatment with sodium hydroxide easily decomposes into white sand and amorphous humose mud in a black solution. Disintegrates when exposed to air. (Many fine heather roots often occur. ) B. Ortstein formed by absorption. 3. Humus Ortstein. Dense, earthy to hard, yellowish brown to blackish

Occurrence

On mull-covered sandy loam soils it forms continuous layers 8-200 cm. from the surface; it remains even after the soil surface has become covered with raw humus and has consequently undergone changes. Below ,the leached surface of coarse sandy soils on heathland and in forests, best developed and forming continuous layers in fresh to moist or wet soil: immediately overlies the true humus Ortstein from which it is not clearly differen,tiated.

Always occurs in continuous layers, more or less following the relief of the surface,

18

ALEX. MUlR

Nature brown sandstone, consisting of the skeletal components of the soil (especially quartz sand) which are covered with, and cemented together by, humic acids and their salts. Easily decomposes on treatment with sodium hydroxide into sand and fine soil which contains some amorphous humose mud in a black solution. Is only slightly affected by dilute mineral acids, and disintegrates when exposed to air. a. Ferruginous humus Ortstein containing larger amounts of iron than the subsoil below the layer. b. Iron-deficient humus Ortstein (red earth) containing smaller amounts of iron than the subsoil below the layer. C. Ortstein formed by concretion. 4. Iron-sandstone. Dense, hard sand-

stone of yellowish brown colour, consisting of the skeletal components of soil (especially quartz sand) coated and cemented with iron hydroxide (according to Senft intermixed with some lower iron oxide and other substances). Does not disintegrate when exposed to air, or treated with alkali solution, but decomposes on treatment or boiling with dilute hydrochloric acid.

5. Bog iron (Raseneisenstein) . Porous, slag-like ore of a blackish colour and containing 80-95% iron hydroxide (Senft). Behaves like 4 towards air, alkali and acid.

Occurrence

150 cm. below the uppermost leached layer of the top soil (Bleisand) in forests and heathland with raw humus.

In layers, in the uppermost part of the subsoil in sandy soils very poor in clay. In layers, in the topsoil, at variable depths from the surface, of sands richer in clay or of loamy sands and sandy loams. Occurs in the uppermost layer of raw humus covered sandy soil in the form of aggregates of different sizes and shapes. Probably f;orms larger continuous masses where humus Ortstein occurs in moist heath soils.

Slag-like masses occurring in moors and bogs as well as in clay Ortstein occurring near the humose surface and overlying an impermeable subsoil; also in many other partidly or temporarily anaerobic places in the uppermcost layers of soil affected by humose material especially in sands.”

A little earlier than Miiller, Burkhardt (1870) and Wessely (1873) put forward interesting points about ortstein. Burkhardt stated that it was formed only in sandy soils and did not occur even in sandy loams. Wessely asserted that ortstein was always associated with a heathy vegetation, so that heath humus was a condition of its formation and the thicker the ortstein the harder it was cemented. This idea was taken

THE PODZOL AND PODZOLIC SOILS

19

up by Pavlinov (1887) in Russia, who suggested that the phenomenon might be termed “callunism” and the ferruginous deposit (ortsand and ortstein) “callunite.” This paper of Pavlinov’s is also interesting in that, apart from a footnote reference, the word podzol nowhere appears and no attempt was made to connect the soil studied with the podzolic type described by Dokuchaiev and others. Following the German writers, he argues for a movement of clay from the surface to the subsoil, but the “clay fraction” (<0.01 mm.) in the ortstein layer is less than the total sesquioxides extracted by strong acids and his argument is not convincing. Many papers continued to appear dealing with heath and other podzolic soils, but the interest in the Western countries was so concentrated on ortstein that no real progress was made in characterizing the group. This is well seen in Sibirtzev’s treatment already referred to: even when considering the fate of the substances lost from the surface horizons of podzolic soils no mention is made of the possibility that they might be deposited in the “C horizon.” In fact, in referring to ortstein as the common accompaniment of podzolized soils, he states that it “lies in the podzolic horizon, usually in its lower part or directly under it. Small pellets of ortstein sometimes appear in the upper horizon of podzolicloamy or light loamy soils.” IV. The Recognition of an llluvial Horizon

The idea of downward translocation of substances from the upper layers of Bleisandboden to the lower was clearly enough brought out by the earliest West European writers on ortstein and was recognized to be highly probable for this formation by the Russian workers. In general, the transference was thought of as occurring in solution. It has already been mentioned that Pavlinov (1887) suggested the movement of clay particles, and some of Muller’s data would support this argument, but as field estimates of texture were probably somewhat rough and mechanical analyses were rarely extended below 20 p, any textural differences were not recognized. The more general concept of illuviation in the pedological sense was put forward by Vysotzky (1899), who defined illuvium as the material washed out of the surface soil and deposited below to form a definite horizon of accumulation (i.e., the illuvial horizon). He used eluvium, however, in the sense of lateral removal of dissolved substances. It was some years before a clear concept of eluviation and illuviation was established. The problem, from the point of view of morphology, was most acutely felt in dealing with podzolic soils that showed bleaching in patches

20

ALEX. MUIR

(either an incipient bleached layer or the beginnings of degradation in the illuvial horizon). According to Zakharov (1906) it was common to find writers who would relegate to the subsoil the patchy bleached layer of a weakly or soddy podzolic soil. He strongly advocated its recognition as part of the podzolic horizon and proposed that the following letters should symbolize the various horizons: A, upper humous horizon; B, transitional or podzolic-eluvial horizon; C, illuvial (ortstein ); D, parent material. This particular usage of symbols continued among many Russian workers until about 1930, when the form earlier proposed by GlinkaA, eluvial; B, illuvial; C, parent material-with the addition of numeral subscripts for subdivisions became the standard. Once this solution of the division of the podzolic soil profile was accepted, profile descriptions became rationalized and at the same time very much improved. It was recognized that ortstein as defined by Senft and later writers was mainly characteristic of sandy soils whereas in podzolic soils of finer texture rounded concretions in the Az and B horizons were the commoner forms of iron segregation and deposition. Tumin (1912) pointed out that ortstein is confined to the A-B boundary, while concretions were disseminated in all horizons. For a podzol from the Smolensk district he gave the following figures for concretion content: Al, 1.15%; Az, 0.75%; B, 0.29%; C, 0.10%. He found that there tended to be an increase in the number of concretions near depressions, the maximum occurring in the upper 10-15 cm. of the B horizon, but in the soils of depressions no concretions occurred, owing to strong reducing processes. The translocation of clay from the surface to the subsoil had been suggested but inadequately established by early writers. The active agent in podzolization was commonly thought to be crenic acid. However, Glinka (1924, 1931) put forward a strong case for his belief that “in general the podzolic process is basically none other than the leaching from the upper horizons of mobile humus sols of low calcium saturation and, under their protective influence, of fine mineral suspensions, and the deposition of these suspensions in the B horizon together with a small amount of humus.” He rejected the theory that crenic acid was the active decomposing agent in podzolization, but in doing so went still further and assumed that no decomposition of the clay fraction took place in soils of the podzolic type (Glinka, 1931, p. 343), because chemical analyses had shown that there was little “colloidal silica” in the bleached horizon (e.g., Georgievsky, 1888; Gedroiz, 1926) and little free alumina in the B horizon (Gemmerling, 1922). In his discussion he incidentally mentions that the “silica powdering” of degraded loams is

THE PODZOL AND PODZOLIC SOILS

21

nothing more than tiny quartz grains freed from their coatings of soil colloids. This was later confirmed by Rod6 and Feofarova (1955), who also showed that feldspars and small clay aggregates were important constituents of the powder. Good descriptions of the clay coatings on the peds of the B horizon were given by Rod6 (1930) and Gemmerling (1930), who provided the first analyses of the ”crusts,” as he termed them (see below). Rod6’s description of a strongly podzolized soil on varve clay from Lisino Forest, near Leningrad, under a pine-spruce stand (typical Pinetum oxalidostcm) is as follows: “A0

0-1 cm.

Bedding of forest floor, loose, little decomposed. Immediately below lies the principal mass of roots.

‘‘A1

1-9 cm.

Grey, rather pale, medium loam. Above with irregular clods 5-7 mm. in diameter; below the clods increase in size, become angular and acquire a certain schistosity in their disposition. When dry, clods are scarcely perceptible; fragments of soil do not exhibit any apparent tenacity. Rapidly merges into subjacent horizon, but the boundary is indistinct, being diffuse and broken on account of wedges of humus.

“A2

9-26cm.

Whitish with a greyish yellow tint. Medium loam, smears much. In dry condition falls into small and unstable pieces, manifesting an evident tendency toward lamination, well exhibited when moist. Contains an appreciable number of dark brown small ore grains (about 1 mm. in section), on the walls of the section usually appearing in the form of commas (smudged by the spade). Rapidly passes into the subjacent horizon, without, however, showing any distinct boundary line, and in penetrating into that horizon to the depth of 8-10 cm. forms tortuous tongue-like projections and streaks.

“B1 26-35cm.

Pale pinkish yellow heavy loam. When dry, compact and tenacious, of indistinctly expressed usual polyhedral structure with aggregates gradually increasing in size with depth. The aggregates are separated from one another by very fine layers of clay particles of a somewhat paler tint than the aggregates themselves, showing as a kind of “polish” on the surfaces of the latter. In the upper parts contains in some quantity

22

ALEX. MUIR

small ore grains. Gradually passes into the following horizon. ”B2 35-63cm.

Yellowish brown (when fresh darkens to crimsonbrown). In other respects similar to the preceeding. Aggregates are larger. Gradually passes into the subjacent layer.

“C 63-115cm.

Varve clay consisting of summer bands, loose, silty, much staining, of a light yellowish grey colour, 6 7 cm. thick, and of winter bands 1-1.5 cm. thick, dark, crimson-brown with distinct fine laminar structure, clayey, very tenacious, apparently consisting of very fine particles.

“The soil is a strongly podzolized soil on varve clay with all the typical podzol features. The humus horizon Al is not very thick, light coloured, although containing above 4 p.c. of humus. The podzol horizon A2 is fairly thick and has a characteristically friable structure. The presence of ore grains should be noted. In horizon B we meet with the polyhedral structure . . , consisting of angular aggregates. It may be mentioned that the faces of the aggregates are paler, but-a very exceptional case within the forest-they do not exhibit any traces of reduction processes; this is evidently due to the fairly satisfactory aeration of the soil” ( RodB, 1930). Gemmerling’s description of a loamy podzol is from near Moscow under young deciduous forest. “A/O-turf. Depth 6 cm. “A/l-no sub-horizon-if a very narrow greyish stripe, merging with the turf is not considered. “A/2,-thick, whitish, mealy structure. In the upper part a lamellar unstable structure is preserved. Iron concretions occasionally are found (very few), the small ones readily disintegrate, the larger grains are very solid. Penetrates pocket-like into horizon B. Depth 6-23 cm. Pockets up to 33 cm. “B/1-light brown, nutty structure, the nuts throughout are podzolized, dense, the edges of the nuts are covered with a brown crust. Depth 33-45 cm. “B/Sbrown colour, breaks into prismatic units, the edges of which are covered with a reddish crust, dense. Depth 45-90 cm. ”B/&yellowish brown colour, breaks into large prismatic units, the edges of which are covered with a reddish blue-grey crust. In the

23

THE PODZOL AND PODZOLIC SOILS

upper part of the sub-horizon are rather many black spots of manganese, very dense. Depth 90-169 cm. “B/&sandy lentil of yellowish colour. Its depth is over 40 cm.” The characteristic features of these soils are the tendency to a platy structure; the frequent concretions in the A1-AP horizons; the tonguing of the A2 into the B and the nutty or blocky structure and clay skins of the B horizon. The tonguing of the A2 is illustrated in Fig. I, E, which has been drawn after a picture in Afanasiev (1930). The concretions in the A2 are also indicated. The mechanical analyses given by these two workers show strong apparent clay shift, even allowing for small variations in the texture of the drift parent materials. Data given by Gemmerling for the composition of the “crusts” are recorded in Table I. TABLE I Composition of Clay Skins and Peds in Loamy Podzola B3

B2

Component SiO, Fe203

A1203

MgO

+

p205

Si02/A1203(approx. ) a

Clay skin

Ped

Clay skin

Ped

(%) 55.58 7.17 24.27

(%)

(%I

(%I

73.45 5.30 13.46 1.70

56.28 6.88 22.29 2.04

75.48 5.18 13.03 1.33

-

3.8

-

4.5

-

Data from Gemmerling (1930).

Gemmerling states that the thickness of the ”crusts” or clay skins is 0.5 to 2 mm. and “they are much better developed along the vertical than the horizontal sides of the structural units. These crusts are formed from the products, which are washed out from horizon A. In view of the greater density of horizon B, the soil solutions penetrate into this horizon chiefly along the fissures between the structural units; the fine suspensions . . . are gradually deposited on the sides of the units.” While the data do not prove that the material came from above, its composition is obviously close to that of soil clay. Such analyses of more carefully separated clay from clay skins was later reported by Yarilova and Parfenova (1957). Their results were very close to those of Gemmerling, the Si02:A1203 values for podzolic soils being between 3 and 3.5. V. Degree of Podzolization

As mentioned above, Sibirtzev (1900) recognized three main subtypes within the sod-podzolic type: soddy soils, podzolic soils, and pod201s (or strongly podzolic soils). The first attempt to define more closely

24

ALEX. MUIR

the various degrees of podzolization was made by Zakharov (1910,1911). Starting with the relation of soils to relief in which the sod-soils occur on upper parts of slopes and the strongly podzolic toward the foot, Zakharov transposed the concept to a time scale. The position is then: (1)in soddy soils (i.e., the youngest) there is a slight development of an accumulation horizon shown as a graying and browning of the upper layer of the parent material. ( 2 ) As organic matter accumulates the conditions become suitable for the appearance of podzolization, which shows as whitish spots and patches that merge to form a layer-the eluvial horizon; at the same time the brownish ortstein horizon forms. (3) As the podzolic horizon becomes sufficiently developed and thickens, there appears a subdivision of the ortstein horizon into an upper layer, darker red brown when wet, and a lower layer lighter in color. The upper darker part Zakharov considered to be due to humus leached down after the loss of iron from the eluvial horizon. This subdivision for fine-textured materials may be tabulated (Zakharov, 1931) thus: Subtypes Soddy (or cryptopodzolic) Weakly podzolic Podzolic Strongly podzolic Podmls

Degree of development of podzolization Not noticeable As spots Whole horizon (i-e., A, horizon) Whole horizon occupies half the total A horizon Occuuies almost all the A horizon

Such definitions were highly desirable, for the word podzol was loosely used from the start and even long after Zakharov’s proposal. For instance, Glinka (1923, p. 34) describes a “clay podzol” in which the Al horizon was 13-15 cm. thick as against 8-13 cm. for the Aa horizon. The above subdivision, which is applicable to all classes of podzolic soils, is still in general use with minor modifications. Thus: slightly podzolic, Al > 2A2; medium podzolic, Al rr A,; strongly podzolic, A, < % A,. VI. The Sod-Forming Process of Soil Formation

As has been mentioned above, a group of so-called soddy soils was recognized in which little or no podzolization had occurred, Sibirtzev (1896) remarked that “the term sod-podxolic is not altogether appropriate, as the majority of such soils are formed under mixed forest,” but he used the term sod (Russian dernina) and its adjective in the general sense of turf. “In the northern loamy [i.e., podzolic] soils, where moisture is near, the roots and rhizomes of grasses form a shallow, but dense turf horizon” which may contain some 3% organic matter as against about

THE PODZOL AND PODZOLIC SOILS

25

0.5% in the succeeding whitish horizon. However, the influence of the ground vegetation on the amount of organic matter was considered only in relation to forest versus steppe, and variation within the zone of podzolic soils does not seem to have received much attention. The first worker really to emphasize the importance of woody versus grass-herb vegetation seems to have been V. R. Williams. His full treatment appeared in his textbook on pedology, and his ideas were well disseminated by his colleagues at the Timiriazev Institute, Moscow. From the pedological viewpoint, Williams’ theory is so overlain by irrelevant matter and supported by such fragmentary data that, not surprisingly, it found little favor. The botanical aspects received a slashing attack from V. N. Sukachev (1916), a distinguished ecologist, who concluded that the theory lacked any factual basis. Neustruev (1927) remarked that the description of the processes “is dogmatically stated in such a way as to impede critical judgement” although he admits that some instances of the “soddy” process may be observed. In fact, the Dokuchaiev school more or less rejected the theory and little more than formal mention of it is to be found in textbooks of the 1930’s. Reduced to its simplest form the theory states that on opening up a closed-canopy forest with podzolic soils, a grass-herb ground cover may, in course of time, become so dominant that the woody vegetation cannot regenerate. The grass phase will also occur in cutover woodland used for hay or for grazing. As the herbaceous litter is much richer in bases and undergoes (partly for physical reasons) a different type of decomposition, there is an increase in humus content and base status of the soil. Similar views, without reference to Williams, were expressed by Vysotzky (1927), who also recommended practical measures for the reclamation of forest lands to agriculture. Williams (1940, p. 118) gives the following generalized description of a soddy-podzolic soil, this stage being best developed when “loosetufted grasses” ( e.g., Dactylis glomernta, Poa nemoralis, P . sterilis), together with legumes, reach their heyday in the succession. Four horizons are clearly differentiated: “1. Soddy horizon, coloured brownish black from the top by the presence of humin. With depth the colour changes to brown as ulmin predominates below. In its upper part the horizon consists of loosely held stable clods, all interstices between which are filled with living and dead roots and rhizomes, binding the clods into a sod. “With increasing depth the amount of organic residues and amorphous humus gradually decreases; similar changes take place in the degree of clodiness, stability of the clods and colour of the soil, until the upper horizon merges into the second, podzolic horizon.

26

ALEX. MUJR

“2. Podzolic horizon of grey colour, with a much smaller content of organic residues. Cloddiness in the podzolic horizon gradually changes to an aggregate state. Thanks to this the plasticity of the podzol is strongly decreased and the whole horizon assumes a loose loess-like character. “The podzolic horizon gradually passes into the ortstein horizon. “3. The ortstein horizon may have a great variety of tints of yellow, red, brown, black and grey colours. This horizon may or may not effervesce on moistening with acid. Its structure may likewise be most diverse. In the ortstein horizon there is frequently an abundance of roots, alive and dead. “4. Below this, again with a gradual transition comes the gley horizon of grey colour . . . [in it] there are living and dead roots of legumes only and dead roots of trees. “In its lower part the gley horizon begins to break up into angular nutty fragments which are frequently gleyed on the surface but internally retain the colour of the parent material. The surface of the aggregates is covered with a silica dust and coloured with a brown film of ulmin. [With increasing depth the gleying decreases and the transition to the parent material takes place.]” Although Williams had a low regard for soil morphology (“a children’s disease” he called it), his continued emphasis on the sod-forming process had resulted in the recognition by Russian morphologists of a group of soils that had previously been regarded as ordinary podzolic soils, i.e., those now known broadly as sod-podzolic soils. The &st attempt to show that a grassy vegetation following forest would produce at least some of the effects postulated by Williams was that of I. V. Tiurin (1935). In a comparison of two profiles from the Lisino Forest, near Leningrad, one under an old pine-spruce stand, the other in an adjoining old meadow (at least 90 years old) with scattered trees, Tiurin showed that there was a marked increase in the thickness and humus content of the Al horizon under grass and of pH and exchangeable bases in both the Al and Az horizons. Earthworms were extremely active in the meadow soil. No data were given on the over-all changes in the total chemical composition of the soil or its clay fraction. There is thus no evidence of the establishment of an independent soil type; we simply have forest and meadow phases of the same podzolic soil. The descriptions, however are worth quoting:

THE PODZOL AND PODZOLIC SOILS

27

Profile I, under pine-spruce forest (approximately 90 years) with bilberry, cowberry, grasses, and mosses. “Aol Ao”

0 . 5 cm. 0.53 cm.

AoAl

3-5cm.

A1

5-10 cm.

A1/A2 10-16 cm. A2

16-25cm.

B1

25-40cm.

B2

40-80cm.

Undecomposed litter. Brown, partly decomposed litter with fungal mycelia. Transition layer from the litter to A1; grey with a predominance of mineral matter. Light grey with slight yellowish tint, small cloddy, abundant tree roots, loose; gradual transition to Light grey with more definite yellowish tint, more compact than A1. Fewer roots. Whitish podzolic horizon, very compact, small ore grains. Chocolate-brown, very compact, coarse cloddy structure; gradual transition to More uniform, brown, in the lower part some lighter patches; very compact; clayey. At 80 cm. large boulders.”

Profile 2, under meadow adjoining the forest, 150 yards from Profile 1. Grasses include Deschampsia caespitosa, D . flexuosa, Agrostis sp.; herbs and mosses are present. “A0

All

A12

A2

B1 B2 C

0-1 cm. 1-6cm.

Litter of dead leaves. Upper (soddy) part of humus horizon, strongly permeated by grass roots. Grey humose horizon, fairly sharply separated from 6-14cm. lower podzolic layer into which it tongues; well developed small cloddy structure. In spite of drought the horizon is moist, more so than under forest; large number of earthworms. 1424cm. Whitish, with small ortstein concretions; brown spots in the lower part; more compact than Al; wormholes. 2435cm. Brown, with whitish spots; weakly fissured; coarse cloddy; merges into 3Ei55cm. Brown, compact, clayey, illuvial horizon. 55-100 cm. Brown loamy till.”

Although till was encountered in the second pit, it was assumed that both soils were developed in varve clay.

28

ALEX. MUIR

The changes in soil morphology described by Tiurin can be seen in a comparison of an old grassland soil (Park Grass) at Rothamsted (Guide, 1959) and a woodland soil of the Chilterns (Batcombe silt loam; Avery, 1958). There has been an increase in organic matter in the A horizon of the grassland soil, 3.8% organic C as against about 1.5% for the Al of the woodland soil; there is no change in base status, and both the grassland and woodland soils have a pH range of 4 . M down the profile. It appears that the only source of lime and other bases, the underlying Chalk, is completely out of reach of the roots of the sward and of the moderate earthworm population (Satchell, 1953). In Tiurin’s example, roots and invading worms were presumably able to reach the till of higher base status and thus bring nutrients to the surface. The socalled sod-forming process, therefore, does not necessarily lead to an increase in fertility insofar as mineral elements are concerned. In his book on pedology written for foresters, Tiurin (1933) began his discussion of podzolic soils with what he called the “pre-podzolic” or cryptopodzolic stage, which is obviously equivalent to the sod soils of Sibirtzev, Zakharov, and others, and equated them with the forest brown earths of Ramann. Tiurin considered that under primary grassland within the forest zone it was possible for podzolization to occur and, as grass vegetation may invade and colonize a deforested area, one could have two series of podzolic soil which he illustrated thus: Grassland

Forest Cryptopodzolic

Soddy soil

L

(forest brown earths) Weakly podzolic

hlediuni podzolic Strongly podzolic

Sod-weakly podzolic Sod-medium podzolic

&

Sod-strongly podzolic

Podzols

The arrows indicate the possible changes that can occur following changes in the vegetation. The sod-podzolic group as described by Tiurin appear to be essentially what were also called meadow podzolic soils (Glinka, 1923, 1931). Whether, in fact, podzolization can take place under meadow grassland, as distinct from heath grassland, seems not to have been adequately demonstrated.

THE PODZOL AND PODZOLIC SOILS

29

It is probable that some of the soils described in the older literature as meadow podzolic or peaty podzolic are probably essentially surfacewater gley soils and related to pseudogleys; some appear closer to humic gleys. The following description from Glinka (1931, p. 344) illustrates the type from the Amur region.

‘‘A1

“Az

“B1 “C

Almost black when wet, dark grey towards the bottom; structureless. Contains many undecomposed organic remains. Thickness 25 cm. Olive-grey, unevenly coloured; numerous tongues of humus penetrate from upper horizon. Indistinct lamellar structure; porous; hard ortstein concretions of dark brown colour. Thickness 20 cm. Olive-yellow; lamellar stricture breaking down to flat shiny aggregates with pores. Large number of hard ortstein concretions. Thickness 25-30 cm. Brownish yellow sticky clay.”

The dull colors of this profile suggest infiltration of humus and a certain amount of gleying due to impeded drainage. The soil could be described as a humic gley. In proposals for a new soil classification, Gerasimov et al. (1939) included under gray forest soils, brown earth (burozem) and podzolic soils, a subgroup of “sod soils” which was defined as “soils with a greater than typical accumulation of rather saturated humus mostly under grasslands or forest with a well-developed grassy cover.” In other words the sod process was thought of as secondary. This scheme was attacked in principle by S. P. Yarkov (1942), a student and colleague of V. R. Williams, who remarked that “certain pedologists do not recognize the sod-forming process as an independent one.” Following Williams, he considered that the true podzol was found only under close-canopy forest, allowing for variations due to relief and parent material. He considered that the podzolizing process was most vigorous when swamping of the upper layers occurred, but this gave rise to “false podzols” which should be separated from the normal type. The sod-podzolic soils arose as described by Williams, and various subtypes could be distinguished by the thickness of the A2 horizon: weakly ( < 10 cm.), medium (10-20 cm.), and strongly ( > 20 cm.) podzolic, straw-colored podzolic ( Afanasiev ) , and yellow podzolic. However, Yarkov, like Williams, provided no evidence that would enable a satisfactory division to be made between podzolic and sod-podzolic soils apart from the nature of the ground vegetation.

30

ALEX. MULR

As mentioned above, Tiurin (1935) was able to show that there was an increase in total organic matter in the meadow compared with the forest soil, particularly in the Al, but also to some extent in the A2, horizon. This increase was largely due to the more vigorous root system of the grasses. Quantitative differences showed in a smaller content of uronic acids and a higher content of the lignohumus fraction in the meadow soil. He regarded the grassland soil as showing a slight similarity to the chernozem. In more recent work by Kononova (1951) it has been shown that there is a distinct difference in the proportions of humic and fulvic acids in the podzolic and sod-podzolic types (Table 11). TABLE I1 Humic and Fulvic Acid Contents of Podzolic Soilsa Humic acid Soil Strongly podzolic soil (Komi A.S.S.R.) Sod-podzolic soil (Yaroslav region)

Humus A,,

(I)

Fulvic acid (11)

Ratio 1:II

%

%

S 1 2 c m . 2.31 15-20m. 3.49

0.23 0.59

0.55 1.16

0.56 0.51

6.93 1.69 1.00 0.52

1.55 0.25 0.14 0.07

1.96 0.45 0.24 0.14

0.79 0.58 0.57 0.50

B,, A,, 4- 7cm. A,A,, 7-15 cm. A,A,, 15-23cm. A,,, 23-38 cm.

Data from Kononova (1951),

Both soils occurred in spruce forest, that from Komi with an undergrowth of spruce, birch, and cedar and a well-developed raw humus ( 8 cm.); that from Yaroslav with a mixture of birch and ash, and ground cover of herbs and grasses (no mention of an A0 horizon). The differences are quite striking, particularly in the apparent mobility of the fulvic acid in the Komi soil, which is probably texturally similar to the sod-podzolic soil, both being formed on silt-loam drifts. Such differences are considered to be due to the presence or absence of the grassy ground vegetation, but no data are available for a comparison of open- and closedcanopy forest in the same region. The ratio of fulvic and humic acids has been suggested by Duchaufour (1957b) as a criterion to distinguish lessiuS and podzolized soils. VII. Geographical Variants

Perhaps we may take as the first recognition of the fact that there might be geographical variants of “podzolic” soils, the suggestion by Glinka (1911) that the Ramann “Braunerde” was really a slightly podzolized soil that had through cultivation lost its bleached layer. The

THE PODZOL AND PODZOLIC SOILS

31

latter, according to Glinka, was unlikely to be strongly developed mainly because in western Europe, with its higher temperature, longer growing period, and higher rainfall, the decomposition of organic residues would be more energetic. He considered that the “Braunerde” of western Europe was, “as it were, the last stage of the podzolic (acid) type of weathering, intermediate between the latter and the more southern krasnozems and terra rossa.” Glinka compared these “Braunerden” with the soils of the Novo-Alevandrovsk Experiment Station which were undoubtedly podzolic in character both from morphology and total chemical composition. The first to provide some evidence for Glinka’s theory was Afanasiev (1926a), who, in discussing the soils of Bielorussia (White Russia), pointed out that the strongly podzolic soils on clays could be distinguished from those to the north and east of that country. “The most important and interesting characteristics of these soils . , . are (1) the straw-yellow tint of the podzolic horizon; ( 2 ) obvious and deeply penetrating tongues of siliceous formations and ( 3 ) weak development of the underlying, illuvial B, horizon; in comparable soils to the north-east (outside Bielorussia) the podzolic horizon is usually light grey and whitish and the B horizon sharply defined.” Afanasiev, like other before him (e.g., Zakharov, 1910, 1911; Tumin 1912), felt that it was too much to expect uniformity of morphology over the whole of the enormous area shown as podzolic soils on the soil map: as well as variations by latitude, there must also be longitudinal changes. He suggested that the Moscow region might be taken as the central area, with variants to a still more continental climate toward the east and to a more oceanic type toward the west. The latter set of variants he named yellow podzolized in his account of Russian soil classification ( Afanasiev, 1927), and a visit to Czechoslovakia confirmed him in his view (Afanasiev, 1926b). After the 1st International Congress of Soil Science, when an opportunity was afforded of viewing the soil changes from Canada to the southern United States, he could write: “If we compare the colours of the forest soils of temperate latitudes with examples from Western Europe and the United States, it is evident, as should have been expected, that the forest soils of these latitudes are, in the colour of their A horizons, transitional; the grey tones on passing to the south take on a brownishness, a straw-colour, a yellowing, changing very gradually into zhetozems” ( Afanasiev, 1930). He equated his “straw-coloured podzolic soils with the gray brown podzolic soils. This point was not taken up at the time by his colleagues although he showed on the soil map of Bielorussia (1926a) quite large areas of “straw-coloured podzolic soils in the vicinity of Senno (south of Vitebsk), to the east of Vitebsk, and to

32

ALEX. MUIR

the west of Slutsk. On the recent soil maps of the U.S.S.R. this soil type has been merged in the sod-podzolic group (e.g., the 1:4M soil map of 1954), perhaps with some justification, as Nogina (1952) has shown that the evidence in favor of Afanasiev’s suggestion is contradicted by other facts. In her view these soils are simply variants of the sod-podzolics that occur in other regions. However, the geographical aspect of the group of podzolic soils had been raised by Zavalishin (1944) during consideration of podzolic soils of the Trans-Ural region. He pointed out that two groups were in general recognized: (1) Typical podzolic or peaty podzolic soils with a raw humus or peaty horizon. These had a higher acidity in the upper part of the profile, the maximum being usually in the A0 or AoAl horizon. Leaching had been strongest in the upper horizon (i.e., A1). This soil type is associated with “the root system and leaf-fall of spruce forests and their moss cover.” ( 2 ) The sod-podzolic soils had a more or less humic Al horizon and a great profile thickness due to the development of this horizon, a depression of the level of the podzolic A2 horizon, and an increase in thickness of the B horizon. They were also acid, but the maximum acidity was usually at a depth of 30-40 cm.from the surface. The most impoverished horizon was not the Al but the Az. This type occurred in the southern taiga with mixed and broad-leaved forest together with a grassy ground flora, as well as mosses. Zavalishin pointed out that there is no sharp boundary between the two types and asked: “Can one regard strongly podzolic raw humus soils of north-eastern European Russia or Trans-Ural as having the same degree of podzolization as a sod-strongly podzolic soil at the southern border of the podzolic zone under broad-leaved forest. Or is it more correct to regard them as independent series of varying degree of podzolization increasing in some or other directions (e.g., to the north or to the south)?” The point made by Zavalishin about the differences in the way in which the acidity varied down the profile was also discussed by Rod6 (1944), who concluded that the differences were of degree rather than of kind. However, some previous results obtained by Zavalishin for the pH values of soils in the Kuznetz basin, and quoted by Rod6, show that in the more strongly podzolized soils the minimum pH occurred in the B horizon. In soils from Lisino (near Leningrad) the minimum p H was in the surface layer. The type of variation in acidity referred to by Zavalishin is shown in Table I11 where pH data for various kinds of podzolic soils are given. The gradual decrease in acidity with depth is characteristic of sandy podzols described in western Europe and North America. The first two soils in Table I11 are sandy clay loams in the surface and loams and clay

33

THE PODZOL AND PODZOLIC SOILS

loam in the subsoil; the sod-podzolic soils have sandy loam surface soils over loam subsoils. From an examination of the field data provided by Ufimtseva (1955) and Vadkovskaya (1955) it appears that the soils with close-canopy forest, whether it be coniferous or broad-leaved, tend to be most acid in the A, horizon; in more open forest, particularly with a grassy sward, the pH is at a minimum in the A2B or B horizon. TABLE I11 Variations in Acidity in Podzolic Soils A, soil (pH) Podzolic soil (Leningrad) 4 4.0 3.4 Podzolic soil (Archange1)b S o d - p d d i c soil ( IVZUIOV)~ 5.6 5.4 Sod-podzolic soil (Vo1ogda)b Sod-podzolic soil (Moscow)~ 4.3 Gray brown podzolic soil6 6.3 Gray-wooded soile 6.5 0 Rod15 (1937). b Uht se v a (1955). 0 Vadkovskaya ( 1955 ) . d Brown and Thorp (1942). e Newton et al. (1959).

A2

B,

B2

B,

CorD

(pH) 5.0 4.5 5.3 5.4 4.8 5.9 5.8

(pH) 5.1 4.5 4.9 6.3

(pH) 5.4 4.8 4.9 6.9 5.2 5.2 4.8

(pH)

(pH) 6.9 7.2

4.9

5.5 5.6

-

4.1

-

6.4 5.3

-

7.3 7.4 7.6 7.3

Zavalishin (1954; Zavalishin and Nadezhdin, 1957) returned to this question of distinguishing the main subtypes of soils in the forest zone of European U.S.S.R. by a study of the Baltic region. He regarded the sod process as the “transformation” of soils under the intluence of a “meadow grassy meso-hydrophyl vegetation” with a consequent concentration in the accumulation horizon (A,) of humified organic matter, more or less saturated with bases and sesquioxides. However, he concluded that the sod process in the western Russian plain was stimulated by calcareous conditions in the ground-water and parent material. Some of the soils described were extremely acid throughout even under oak-hornbeam forest; cultivated variants showed both apparent clay shift and a minimum pH in the B or BC horizon. However, no adequate data were given to enable a separation of podzolic and sod-podzolic soils to be made. VIII. The Western Contribution

In the nineteenth century, theories regarding the problems of the podzol stemming from investigations in Russia and elsewhere developed concurrently in many parts of the world, although with differences in nomenclature. The Russians quite early recognized the similarity of their

34

ALEX. MUIR

podzols to the Bleisand of Senft and Muller, but the reverse process seems to have been a much slower one. When the Russian ideas and the name first spread further is not clear, but in the main it took place through Germany. This is not entirely surprising in view of the proximity of the two countries and the attraction of German laboratories for Russian workers. The first of the Dokuchaiev school to study in Germany appears to have been V. R. Williams, who carried the Russian ideas to the Wollny laboratories at Munich. It is to Ramann ( 1911), however, that we are most indebted for adopting the Russian term podzol and equating it with Bleisandboden, which he thought was a misleading name. Bleicherde he suggested should be used as a group term to cover podzol soils, Grauerden, and others that were depleted in iron and had become bleached. The Russian ideas were adopted surprisingly slowly although information about them had been widely disseminated. V. R. Williams had organized the Russian agricultural exhibit at the Chicago Fair in 1894 for which Dokuchaiev wrote an account in English of Russian soils. Later, Sibirtzev presented a paper in French on the soils of Russia to the 7th Geological Congress and Dokuchaiev’s soil map of the world was shown at the Paris exhibition in 1901. About the same time an excellent English translation of one of Sibirtzev’s accounts of Russian soils appeared in the Experiment Station Record ( Fireman, 1901) . Although in Germany the Bleisand with its associated ortstein was taken as equivalent to the podzol, little consideration was given to the possibility of the “Molkenboden” being similar to the more commonly found heavier-textured podzols of the Russians. Sibirtzev ( 1898) had pointed out that the Russian term podzol nearly corresponds to the German expression Bleisand, but is also applied to loamy and clayey soils if they are clearly affected by the chemical leaching processes characteristic of Bleisand. However, German workers devoted most of their attention to the ortstein layer as it was undoubtedly a major hindrance to soil utilization, particularly when cementation had taken place. The use of the term podzol was thus limited to what w e n nssentially tbsandier varieties. The influence of the German emphasis on ortstein and orterde was reflected in a lecture by C. F. Marbut (1928) when he distinguished the Lakewood, Cecil, and other soils as being podzolic soils developed under the influence of a relatively weak operation of the podzolic process. Although they had light-textured A horizons and heavier-textured B horizons, the latter differed from the ortstein of the “true podzol” in having a percentage of organic matter no higher than was present in the A horizon and in not being indurated as a rule, A similar statement could equally well have been made of many of the true podzols of the Russians.

35

THE PODZOL AND PODZOLIC SOILS

A selection of data for humus and nitrogen contents of Russian podzolic soils is given in Table IV. Only the peaty sandy podzol shows B humus-illuvial horizon, and the figures for the strongly podzolic soil under pine forest are similar to those for the sod-podzolic soil. TABLE IV Organic Matter and Nitrogen in Podzolic Soils

Horizon Soil Strongly podzolic soil on till under pinea Strongly podzolic soil on till under meadowa Peaty sandy podzol, humus illuvial; forestb Clayey podzol on till0

Sandy straw-colored podzolic soild a

b 0

d

Constituent

*I

Humus ( % ) N (%I C:N Humus ( % ) N (%I C:N Humus ( % )

3.81 0.14 15.7 4.84 0.18 15.6 37.55 0.99 22 7.92 0.32 14.3 2.0

N

(%I

C:N Humus ( ”/o ) N (%I C:N Humus ( % )

A2 0.81 0.04 11.7 1.22 0.09 8.0 0.36 0.02 10 0.61 0.031 11.4 1.4

Bl 0.41 0.05 4.8 0.41 0.05 4.8 0.92 0.04 9.6 0.42 0.035 7.0 0.7

2.41

0.06 17.4 0.43 0.048 5.2 0.3

Tiurin ( 1935). Rod6 ( 1937). Kossovich and Krasiuk (cited from Rod6, 1937) : first layer A,A,; last layer C,. Nogina (1952): layers 3 and 4, A,B and B,, respectively.

The advent of Glinka’s “Die Typen der Bodenbildung (1914) stimulated interest in the main groups of soils, but its description of podzolic soils and podzols on clay and loess did little to alter the prevalent idea that such soils were confined to sandy parent materials: the later work of Frosterus (1914) and Tamm (1920) rather confirmed it, although Tamm (1930) described a “mull podzol” which, from its morphology, could equally well have been called a sod-podzolic soil. During the 1930’~~ both in the United States and Britain the analysis of the clay fraction of soils became a standard procedure in soil studies and at once provided a fairly certain distinction between podzols and brown earths (including gray-brown podzolic soils). This aspect received strong emphasis by Robinson (1930, 1949), who showed that in the Welsh podzols there was a marked change in the composition of the clay in passing from the A to the B horizon in comparison with the relative constancy in the whole profile of the brown earths. This received confirmation in the work of Anderson and Byers (1931), who examined

36

ALEX. MUIR

the clay fraction from American podzols, gray-brown podzolic, and redyellow podzolic soils. There were even more marked differences in the clays from their podzols than in Robinson’s, whereas in the other soils the composition was almost constant. The use of the clay fraction in defining soil groups was extensively used in Britain (e.g., Kay, 1934, 1939; Muir, 1934, 1935; Muir and Fraser, 1940; Robinson, 1935). Muir showed that the separate ratios of alumina to iron oxide and of silica to alumina were of more value than the ratio of silica to sesquioxides, as the marked differentiation of iron and aluminium in podzols was a characteristic feature (Table V ) , The method for distinguishing between podzols and brown earths using acid-oxalate extraction introduced by Tamm (1920) was applied by him and by Aaltonen (1935) in Finland to elucidate the rate of podzol development. Tamm’s method was used by Lundblad (1934) to distinguish brown earths and podzols and, together with clay analyses, was used by Muir (1935) to separate podzols and a variety of brown earth common in Britain. The method is not satisfactory for heavier textured soils (cf. Tamm, 1934), but this difficulty has been overcome by the use of dithionate (Deb, 1950; cf. Duchaufour, 1956). These variations in clay fraction ratios, however, give no indication of the mineral constituents of the various horizons. MacEwan (1948) showed for some Scottish podzols that there generally appeared to be an increase in the illite content at the expense of biotite, hydrobiotite, and chlorite. Tedrow (1954) showed an increase in the quartz content of the clay from Az horizons of podzols; the dominant minerals in the soils were interstratified 2:l layer silicates and illite. Recent studies by Jackson and his co-workers (e.g., Brown and Jackson, 1958) have shown that in some sandy podzols montmorillonite may form in the A2 horizon at the expense of a vermiculite-chlorite that is present in the B and C horizons. None of these workers produced evidence for mineral synthesis in the B horizon. In the case of the gray-brown podzolic soils, McCaleb (1954) found that illite was the dominant mineral in all horizons; and in the more base-saturated profiles, some vermiculite and montmorillonite. The more acid soils had some kaolin. In soils of a similar character Avery et d. (1959) showed a tendency to increase in vermiculite content-possibly at the expense of clay mica. On the morphological side it was generally accepted that raw humus was characteristic of the podzols while mull was one of the principal features of brown earths in the widest sense (Robinson, 1949). The latter soils lacked any marked deposition of iron oxides in the subsoil, the profile having a much more uniform appearance, although in the more strongly leached varieties there might be a clear textural and structural

TABLE V Variation in Clay Fraction Ratios of Podzols and Podzolic Soil Soil Brown eartho Soil (England) Podzol ( Scotland)b

Podzol (U.S.A.)O

Ratio

A,

A2

A3

Si02/A1203

3.17 2.81

2.94 2.90 3.03 5.85 3.22 5.72 3.15 3.98

-

d2°3/F%?03

Si02/A1203 A1203/Fe203

-

M2O3/Fe2O3 Si02/A1203 A1203/Fe203

-

sio,/&o3

Cray-brownd Podzolic (USA.) a Kay (1939). Muir (1934). 0 Anderson and Byers ( 1931). d Brown and Thorp (1942).

*

-

3.23 3.86

-

3.11 3.64

Horizon Bl

3.04 3.00 2.41 1.58 1.85 0.84 3.05 3.66

+I

2 B,

B3

C

3.10 3.05 1.80 2.58 1.83 2.40 3.70 3.12

-

3.38 3.02 1.38 5.90 2.12 3.71 3.21 3.10

1.03 4.07 -

3.13 3.13

8

F 5 8

3

0

E n

cn

;

v1

38

ALEX. MUIR

change with depth. Such varieties were represented by the gray-brown podzolic soils and what were later to be called sols lessiue’s by French workers (cf. Duchaufour, 1956, 1960) and the Purubraunerde of the German workers (cf. Altemuller, 1956; Kubiena, 1956). The microscope had often been used in soil mineralogy, but its applications by Kubiena (1938,1953) to the examination of soil fabrics was a revolutionary stage in the study of soil morphology. Kubiena’s techniques, particularly that of thin sections, have now been widely used and, as applied to the groups of podzols and podzolic soils (lessive’, etc.), have provided more definite morphological criteria for their separation. However, although Kubiena gives Dokuchaiev credit for priority in the use of the term podzol, it is certain that many of the soils described by Dokuchaiev and his colleagues would not qualify for inclusion in Kubiena’s group of podzols. IX. What Is Podzolization?

The discussion of the geographical variants of podzolic soils has shown that, while there was a general realization of differences in both morphology and chemistry between podzolic and sod-podzolic soils, the kind of data that it had been customary to collect did not satisfactorily distinguish between the two types if they were to be regarded as more than variants. This is not entirely surprising, for from the beginning the terms podzol and podzolic had been loosely used. Both Dokuchaiev and Sibirtzev criticized this tendency, but with little effect. Thus, in an excellent account of the aspen groves of the Voronezh steppe, Popov (1914) repeatedly refers to podzolized solonetz. Such usage, at least in the earlier period, was not unnatural as the folk term referred solely to the bleached layer and neither Dokuchaiev nor Sibirtzev had ever clearly distinguished an illuvial horizon except in the sandy varieties. However, following Vysotzky’s definition of the illuvial horizon the terms podzol and podzolic were normally restricted to the soils found in the taiga zone. From the time of Senft and Dokuchaiev it was agreed that the basic process involved was the decomposition of the primary minerals with subsequent removal of sesquioxides together with more or less humus from the surface soil. Residual quartz and secondary silica formed the gray or white surface layer. The analyses quoted by Dokuchaiev and his co-workers were almost all confined to the bleached layer, and they invariably showed an extremely high silica content. Early Russian data for the other horizons are scarce, but analyses given by Sibirtzev (1898) are shown in Table VI.

THE PODZOL AND PODZOLIC SOILS

39

TABLE VII Podzol from Tikhvin: Total Chemical Analysis5 Sample Soil covering to pOdZ0l Podzol Subsoil of the

wdzol 5

Loss on ignition

Hygr. water

Humus

CaO

MgO

5.86 0.9

1.22 0.24

2.8 0.3

1.192 0.79

0.378 0.24

2.4

0.97

-

1.03

0.34

Data from Georgievsky (1888).

‘%O3

SiU,

p205

1.84 0.67

7.032 4.79

81.02 90.68

0.085

1.62

7.21

84.5

Trace

Fe203

0.059

w

5

B

41

THE PODZOL AND PODZOLIC SOILS

Later Russian workers carried out more complete chemical analyses, and many hundreds, if not thousands, must be in print. They were usually supplemented by determinations of exchangeable bases and organic matter and sometimes by analyses of the HC1, H2S04,KOH, and water extracts. Mechanical analyses when made were only rarely extended below 20 p. Bulk chemical analyses, of course, provided clear evidence of apparent movement of material from the surface layer to the subsoil, and the distribution of exchangeable bases showed the leaching of the more mobile ions. The KOH extraction seems first to have been used by Georgievsky (1888) as an improvement on the soda extraction introduced by Il’enkov (1869) in an attempt to study the state of the silica in the “podzol.” Il’enkov and later Amalitsky had considered “amorphous” silica the cause of the adverse agricultural properties of the soil. A selection of Georgievsky’s data for a sandy loam podzolic soil is given in Tables VII and VIII. Georgievsky concluded from his results TABLE VIII Analysis of KOH Extracta Sample SiO, Fe,O, Podzol 0.112 1.45 Subsoil of podzd 0.99 0.06 a Data from Georgievsky (1888).

A1,0, 0.56 1.52

CaO 0.05 0.02

MgO Trace Trace

that, although some amorphous silica might be present, the removal of alumina, etc., suggested that aluminosilicate materials had been attacked in the treatment. He thought that the small amounts of silica extracted could not appreciably affect the soil properties, as had been suggested. The 5% KOH extraction was later used by Gedroiz (1926) to distinguish between podzolic and solodized soils. The latter invariably showed a large excess of extractable silica over the amount required to form kaolin with the alumina simultaneously extracted; podzolic soils showed little or no excess. No development in the laboratory study of podzolic soils was made until in the early 1930s A. A. Rod6 began to publish the results of his researches on the soils of the Lisino Forest near Leningrad. These results, together with others, formed the basis of his monograph on the podzol-forming process (Rode, 1937), which was partly an extension and partly an amplification of 0. Tamm’s work on Swedish soils, with a wider survey of the available literature. In addition to detailed total chemical analyses of podzolic soils, Rod6 also gave a range of clay analyses, which if they had been more extensive might have provided

42

ALEX. MUJR

the basis for a distinction between the ‘‘true” and the “soddy” podzolic soils. Rod6 (1944) concluded that “the nature of the podzol-forming process, as regards the mineral part of the soil, consists in the complete breakdown of all minerab of the parent material, except quartz, and the removal of all products of this decomposition from the upper layers of the parent material.” He considered the most characteristic feature of the process to be the instability of the secondary clay minerals. In a later account, Rod6 (1955) again stressed these aspects and in regard to the sod-forming process remarked that indications of it can be found in all three zonal subdivisions of podzolic soils, even in the absence of a grassy vegetation. He therefore, concluded that there is no strict division between podzolic and sod-podzolic soils, a view that would not find general acceptance with Russian pedologists. Rod6s definition of podzolization implies differentiation of the clay fraction, but this is not so in many sod-podzolic soils which thus cannot be regarded as “true” podzolic soils (see p. 44 and Table XI). Yarilova and Parfenova (1957) suggested that clay is resynthesised in the B horizon of podzolic soils, a view to which Rod6 also subscribes. This, however, is not substantiated, although not excluded, by their data, which could equally well be given by clay that had been moved from the A horizon. Other Russian pedologists (Minashina, 1958; Fridland, 1958; Karpachevsky, 1960) accept clay movement. TABLE IX Clay Fraction Data for Lisino Podzolic SoiP Fraction

Fraction 2.5-0.25 p

< 0.25 p

Amount (%)

SiO,/ A1,0,

A1,0,/ Fe203

Amount

Horizon

(%)

SO,/ A1,0,

A1,0,/ Fe,03

A1 A2 B C

26.5 29.2 33.0 39.5

4.32 4.12 3.79 3.89

5.63 5.26 3.89 4.46

1.7 1.3 23.2 17.7

2.26 2.04 2.49 2.49

3.74 4.70 3.01 2.94

(1

Data from Rod6 (1937).

Clay fraction data obtained by Rod6 are given in Table IX. No standard clay fraction has ever been adopted, so that results by different authors are not always strictly comparable. Rode’s coarser clay probably contained quartz, which makes the Si02/A1203 value for it somewhat high, particularly in the upper layers. The relative uniformity of the si1ica:alumina ratios does not suggest alteration of the clay to the extent indicated by the data for the podzolized soils given in Table V. The differentiation of iron oxide and alumina is, however, clear in both coarse

THE PODZOL AND PODZOLIC SOILS

43

and fine fractions. The ratios for the fine clay shows that it is the more aluminous and ferruginous and is the main fraction that is migrating. However, on the clay fraction criteria mentioned above, this soil cannot be regarded as strictly comparable with the sandy podzols of the United States and Great Britain. Clay mineral analyses have been made on Russian podzolic soils with results that are probably similar to those obtained elsewhere for this general group. The results obtained by Rod6 and Sedletzky (1939) for sandy podzols indicate an interstratified mineral or vermiculite as the dominant constituent of all horizons, with little variation with depth. Yarilova and Parfenova (1957, 1959) have published similar data on B horizon samples from Russian podzolic soils. Skrynnikova ( 1958 ), from a consideration of pedogenic processes in the forest zone, concluded that podzolization is no longer an active process, except perhaps in some northern podzolic soils under conifers. She believes that podzolization was initiated at the end of the Ice Age under a colder and moister climate than the present. The advent of broad-leaved forest in the south of the forest zone has caused a reversal of the earlier processes, leading to “regradation.” She does not clearly define podzolization, but implies decomposition and leaching of mineral matter so that the Al is the most impoverished horizon. Under broadleaved forest and meadows there is not only an improvement in base status in the A horizon, but an increase in clay content in the A1 horizon following decomposition of mineral-rich litter. This higher clay content in the Al horizon is very common in what have been called sod-podzolic soils, and in many it is a very striking feature. A few examples are given TABLE X Clay Content of some Podzolic Soils ( ”/o of Air-Dry Soil)@

I I1 I11 IV V VI A, 4.47 3.39 14 12 9 15.9 5.13 0.04 9 6 8 15.5 A2 13.65 15 8 6 16.4 -42B 9.80 16.39 20 13 22.7 Bl B2 21.87 14.45 28 13 36.9 C 19.34 11.97 21 14 14 21.3 5 I, Strongly podzolic soil, Leningrad ( Rod&, 1937 ) : < 2 c~ clay. 11, Loamy podzol, Moscow (Gemmerling, 1930) : < 1 ~1 clay. 111, Podzolic soil, Archangel ( Ufimtseva, 1955) : < 1 P clay; last layer CD. IV, Sod-strongly podzolic soil, Vologda ( Ufimtseva, 1955) : < 1 p clay. V, Straw colored sod-podzolic Bieloxussia (Nogina, 1952) : < 1 clay: last layer CD. VI, Gray-brown podzolic (Brown and Thorp, 1942) : < 2 p clay; third layer is Horizon

’43.

44

ALEX. MUIR

in Table X with “true” podzolic and gray-brown podzolic soils for comparison. Some western European soils of the lessive’ type also show a somewhat higher clay in the A1 than in the A? horizon (cf. Duchaufour, 1957a; Avery, 1958) although none of these is as strongly bleached as the Russian soils. Results more comparable with those for sod-podzolic are given by Muckenhausen (1957) for a pseudogley. The same feature is seen in the gray-wooded soils of Canada (Newton et al., 1959), but it does not seem to occur in the gray-brown podzolic type. The distribution of podzolic soils with lower clay in the A2 horizon is not clear, but it is possible that they represent cases of extreme degradation of former “chernozemic” soils as described by Glinka (1924). From data given by Zavalishin and Firsova (1960), it appears that in the zone north of the 60th parallel podzolic soils should not show this feature, but exceptions occur as can be seen from the data for the Leningrad and Archangel soils in Table X. These presumably reflect purely local conditions. There is no very clear connection between the occurrence of this very depleted A2 horizon and vegetation, and it may occur in the presence or absence of raw humus. Of the soils listed in Table X, I and I11 occur under a raw humus layer; Zavalishin and Firsova (1960) describe a “strongly podzolic raw humus soil” from Komi which shows a decrease in clay in the lower A?. Russian ideas on podzolization have undergone a change since Fridland (1958) introduced the term and concept of “illimarization,” which is the exact equivalent of the “lessivage” of Duchaufour. The process of illimarization he considers to operate in brown forest soils, yellow podzolic soils, etc., and partly in the sod-podzolic group, whereas podzolization is the effective process in “typical podzolic” soils, gley podzolic, and others. The distinguishing feature is the presence or absence of differentiation in composition of the clay fraction down the profile. Fridland also recognizes that there are intermediate phases in which podzolization may accompany illimarization. The question of what are “typical podzolic” soils is left unanswered. The issue was somewhat confused by Gerasimov (1959) when he stated that “soils of a gley-pseudopodzolic type have been called sols lessiue’s and pseudogleys in Western Europe. Thus they have been rightly distinguished from the true podzolic soils characteristic of Eastern Europe, but have been wrongly regarded as genetic formations contrasting with and of equal status to that group.” Gerasimov does not define his “true podzolic soil” but he accepts Rodd‘s definition of podzolization which, as we have seen, implies a marked differentiation in the clay complex. A selection of Russian clay fraction data is given in Table XI which shows that there is no great differentiation in

45

THE PODZOL AND PODZOLIC SOILS

these sod-podzolic soils. The Al horizon tends to be somewhat more siliceous than the A2 horizon, which may indicate a certain breakdown of the clay minerals, but this is also shown by some soils that would be called Zessiue' by Duchaufour's definition. The difference could equally well be, and is more probably, due to the removal of the finest clay, as indicated by Rode's data. The figures in Table XI given by Karpachevsky TABLE XI Variation in Clay Fraction Ratios for Podmlic Soils Horizon

soil Sod-podzolic Moscow regiona Moscow regiona Bielorussiab Bielorussiab Moscow regionc Moscow regiond Podzolic Kama Rivers a b

e

Ratio

A,

A2

B

C

SiO,/Al,O, A1,O3/Fe,O3 Si0,/A1,03 A1203/Fe203 Si02/A1,03 Al2O3/Fe2O3 SiO,/AI,O, A120,/Fe,03 SiO,/AI,O, A1203/Fe203 SiO,/AI,O, A1,03/Fe203

4.98 4.32 4.26 3.70 3.72 2.58 4.56 2.96 3.26 3.26 4.7 4.5

4.13 3.90 3.09 4.43 3.59 2.61 4.12 2.64 3.63 3.44 4.0 3.5

4.08 3.68 3.00 4.23 3.20 2.80 3.90 2.34 3.12 3.05 3.5 3.5

3.99 3.72

3.33 2.67 4.4 2.5

Si0,/A1203 AI2O3/Fe2O3

-

5.55 5.45

2.47 3.84

2.87 6.75

Morozov (1940). Nogina (1952) Vadkovskaya (1955). Karpachevsky (1960). Rod6 and Sedletzky ( 1939 )

-

-

.

(1960) for a soddy medium podzolic soil of loamy texture under oak show a more marked differentiation of iron oxide and alumina between the A1 and A2 horizons than most of the other results. He nevertheless believes that Rode's argument for the breakdown of secondary minerals is wrong and that translocation of unaltered clay minerals can occur in both light- and heavy-textured podzolic soils. A more extreme example is that of the Kama River soil which contains only about 1.5% clay in the B horizon and is thus comparable with the American podzols referred to in Table v. Thus, on the available evidence of clay composition no clear distinction can be made between the two main Russian classes of podzolic soil. They both show a marked apparent transfer of clay to the subsoil, but

46

a E X . MUIR

without any strong differentiation of its constituents. This does not preclude transformation from one clay mineral to another (e.g., possible increase in vermiculite or illite in the A horizon), but it does not support RodGs contention unless one assumes equal rates of removal of the main weathering products and that the new minerals formed in the subsoil have the same chemical composition. The very high losses, particularly of silica, from podzolic soils shown by Rod6 (1937) could well be accounted for by the breakdown of primary minerals. There would inevitably be some loss of the weathering products before resynthesis of clay was complete. In any case the change from, say, feldspar (SiO2/ A1203 = 6 ) to a clay mineral (SiOz/A1203 = 2-4) involves a significant loss of silica. It would seem that, on the basis of the evidence adduced so far, the Russian podzolic soils on heavier textured materials have much in common with the sols lessive‘s (and Parabraunerde) of western Europe as well as the gray-brown podzolic and gray-wooded soils of North America. As many of the first group were formerly considered to be brown earths, Glinka’s suggestion that they formed the western end of a sequence through from the podzolic soils of eastern Europe was nearer the truth than at one time seemed to be the case. So too with Afanasiev’s equation of gray-brown podzolic soils and his straw-colored podzolics. Thus, it appears that the concept of podzolization as it applies to the great majority of Russian soils described as podzolic is simply an extreme expression of “lessivage.” X. Micromorphology

The use of thin sections to investigate micromorphology has only recently begun in Russia, and little information on soil fabrics is yet available. Karpachevsky ( 1960), Minashina ( 1958), Kundler ( 1959), and P. Bullock ( 1960, unpublished) have described the micromorphology of some podzolic soils in sufficient detail for comparisons to be made with Kubiena’s and other criteria. Yarilova and Parfenova (1957) printed excellent photographs suggesting clay deposition in B horizons, but gave no indication of the soils from which the specimens came. Two sandy soils, one from Leningrad (Bullock) and one from Moscow (Karpachevsky), showed all the features of Kubiena’s humus or humusiron podzols. P. Bullocks description is as follows:

A2 B1

Largely clear quartz grains, no evidence of iron oxide or humus; little clay. Largely quartz sand; little clay; a few iron oxide concretions. Quartz grains coated with humus which forms bridges be-

THE PODZOL AND PODZOLIC SOILS

BC

47

tween the grains and shrinks to form a network of fissures; local accumulations of humus pellets. Mainly quartz grains with little iron oxide; some humus coatings locally.

Sod-podzolic soils were described by all the authors mentioned; the description by Kundler is as follows: Al

A2

B

Porous crumbly fabric; finely dispersed organic matter mixed with minerals, many fine plant remains; frequent small dark brown concretions; the clay is unoriented. Consists of a series of roughly parallel layers (0.5-3 mm. thick) of a light yellow tint; the fine material is “unoriented; conspicuous dark brown concretions. Soil mass in section is yellowish brown and interspersed with a network of strongly birefringent platy minerals and mineral aggregates; the skins on the peds and linings to pores show layering and strong birefringence.

Bullock’s descriptions of a loamy podzol and a loamy strongly podzolized soil from the Moscow region correspond quite closely to that of Kundler, and they both show much strongly birefringent clay in the channels and pores of the B horizon. The schistosity in the A2 mentioned by Kundler was not seen in Bullock‘s specimen. It is unfortunate that the heavier-textured podzolic soils so far examined in thin section are those occurring in the sod-podzolic zone. However, Karpachevsky considers his sandy soil to be a sod-podzolic. It thus appears that sandy soils in the sod-podzolic zone can have the micromorphology of the western and American podzols. The heaviertextured podzolic soils have all the attributes of strongly developed sols lessiv6s or bleached Parabraunerde. Whether an examination of heaviertextured soils from areas shown on the soil map as unqualified podzolic soils will prove that they are likewise of the lessive‘ type remains for the future. XI. The Characteristics of the Russian Podzol and Podzolic Soils

Gorshenin ( 1958) recommended that the podzolic and sod-podzolic soils should be recognized as independent soil groups at the highest category of classi6cation. Tiurin (1960) has written that soils called ilovka and gley soils by Sibirtzev “can now without difficulty be recognized as pseudogleys (or surface gley podzols) .” He also mentions that Sibirtzev had commented on “the yellowish colour of the podzolic horizon

48

ALEX. MUIR

in sod-podzolic soils on the loess-like loams of Bielorussia and the plains of Central Europe. Such soils are now known as sod-straw-yellow podzolic (or sols lessivds).” However, this statement does not go far enough, for Nogina’s data showed that there was really little difference between the ordinary sod-podzolic and the straw-yellow variety. It has been shown above that in the absence of an adequate definition of a podzolic soil the present course seems to be to make a subdivision as already done in the West: i.e., the sandy soils of the north (and perhaps in the southern taiga also) correspond to the humus and iron-humus podzols of Kubiena whereas the podzolic soils on medium and heavytextured materials belong to the lessive‘ or Parabraunerde group. On the available data it is possible to distinguish two types within this broad group which correspond to Russian division into podzolic” and “sodpodzolic,” but it is doubtful if they are independent groups as Gorshenin suggested. They are, rather, expressions of degree of development ( RodB, 1944) or alteration due to human interference, as indicated by Tiurin (1935), or to difference in age. The sod-podzolic perhaps should have a subdivision for those soils that show a relic humus horizon in the lower Az. Kubiena (1953) has suggested that his Molkenpodzol is equivalent to Georgievsky’s clayey podzol although the description of the latter could well apply to soils that are probably of the lessive‘ type but are influenced by surface waterlogging. This effect of surface-waterlogging can be recognized in many of the heavier-textured Russian podzolic soils and podzols and has been commented on by many Russian workers. Some, like Yarkov (1954, 1956), have put considerable emphasis on anaerobic reduction processes in the formation of podzolic soils. There is no doubt that these must be responsible in part for concretion formation in the Az horizon, but there is no evidence that they are essential for the translocation of ferruginous clay. Once the textural and structural B horizon is well formed, temporary waterlogging in cracks and fissures can readily occur and give rise to the bluish gray colors that are sometimes seen on clay skins. It may be that waterlogging in the surface contributes to the breakdown of the structure aggregates at the top of the lower B horizon (cf. Grossman et al., 1959, p. 75) which results in the powdering that is such a common feature of the well-developed Russian podzolic soils (cf. p. 20). Soils showing the more extreme effects of surface waterlogging are usually separated as a subtype which approximates to the pseudogley (cf. Tiurin, 1960, quoted above ) In the description of the characters of the various Russian groups of podzolic soils attention has been concentrated on the better-drained varieties. Russian workers recognize some 20 subtypes and varieties of

.

THE PODZOL AND PODZOLIC SOILS

49

podzolic soils, some of which have a large literature devoted to them. The range of chemical analyses made is very wide, and only a selection of results that appear to be useful for characterization is mentioned in the following paragraphs.

A. Sandy podzolic soils, The podzol is probably the commonest variety. Usually associated with pine woods or heaths. 1. Field morphology. Raw humus, or moder, forms the A. horizon; the thickness is variable, 5-10 cm. The Al horizon is not well developed and if present is normally only slightly humic; its structure is weak. The A2 horizon is variable, 10-20 cm. thick; structure weak or absent. Usually sharp change to B horizon which may be more humic in its upper part ( Bl ); usually well cemented. 2. Micromorphology . Depending on the degree of development of the soil the sand grains in the A horizon may be coated to some extent with humic matter or be quite clean. The B horizon is characterized by the pronounced coating of the grains with humus or humus-iron complexes. These coatings tend to crack and flake off. Segregations of iron oxide may be present. 3. Chemistry. Sandy podzols are commonly strongly acid, this acidity decreasing gradually with depth; the minimum pH is usually in the A, or A2. Clay translocation may be shown by mechanical analysis, but in very sandy varieties this can be accounted for by “free” sesquioxides. Clay fraction shows marked differentiation of silica and alumina as well as alumina and iron. Organic matter content in B horizon normally higher than in A2 and has a high C:N ratio ( > 10). B. Loamy podzolic soils. This group appears to consist mainly of strongly podzolized soils and podzols, i.e., the A, horizon is poorly developed or absent. They occur under coniferous forest often with an ericaceous and mossy ground flora. 1. Field morphology. Raw humus or moder forms the A. horizon: the thickness varies and some varieties have been termed peaty. The bulk of roots occur in the raw humus and the Al, if present. Most of the humus in the Al is infiltrated and not due to decomposing roots. The structure of the A1 is weak, but sometimes shows a platiness; when dry breaks down to floury consistency. The A2 horizon is well developed and may be up to 15-20 cm. thick and shows a platy structure which is unstable when dry, Concretions of iron oxide are common. Humus content normally low but may reach 2 3 % . Frequently this horizon tongues into the illuvial layer, residues of which may form an AZB transition, which in part consists of a “powdering” on the B horizon peds. The B horizon is, however, clearly defined. It shows a strong increase

50

ALEX. MUIR

in clay content and has a well-developed blocky structure, the size of the aggregates increasing with depth. Clay skins on the peds are common. Concretions are usually present in the A2B, but may be absent below. Toward the bottom the structure becomes massive and passes into the parent material (varve clay, till, loesslike loam). Soil faunal activity is low. 2. Micromorphology. No data. 3. Chemistry. The soils are usually strongly acid with the minimum pH in the Ao, or Al horizon when present: the pH rises steadily with increasing depth. Maximum humus in Al horizon, decreasing rapidly below, but a few soils have been described in which there is a second accumulation at the top of the B horizon ( 2 Molkenpodxol). When this second humus horizon is present it has a high fulvic acid content. The clay shows a fairly uniform Si02/A1203 ratio, but the A1203/Fe203values suggest differentiation of the sesquioxides. The finer clay fraction is the more sesquioxidic. No data for “free” iron oxide. C. Sod-podxolic soils. This group shows the whole range of development from slightly podzolic to strongly podzolic. Podzols have also been described in which the Al horizon is vestigial, but the soils have the other properties of the group. 1. Field morphology. There may or may not be a raw humus layer, depending on the density of the forest canopy. When present it may be up to 2 inches thick; under more open conditions leaf fall is rapidly mineralized. The Al horizon usually contains abundant roots forming the “sod” which gives its name to the group. It is medium to dark gray in color, has a well-defined crumbly structure, and shows evidence of earthworm activity. Concretions are scarce. The transition to the A2 horizon is often gradual, allowing an AlA2 horizon to be separated. The A2 horizon i s commonly a slightly yellowish gray color when wet but dries very light gray. It is usually clearly lighter textured than the A1 and contains numerous concretions. The structure is usually platy. Tongues of Az penetrate in the B, the upper part of which shows a “powdering” on the peds. The powdering may extend down fissures into the B horizon. The B horizon is brown to strong brown or reddish brown in color and shows a marked increase in clay content. The subangular blocky structure is well developed, the peds increasing in size with depth until the structure is massive. Clay skins on the peds are well developed and sometimes thick. Concretions are uncommon, but segregations of manganese oxide may be present. In some soils the peds break horizontally to form coarse platy aggregates. Soils in the southern taiga (broad-leaved and mixed forest) often

THE PODZOL AND PODZOLIC SOILS

51

show a patchy second, “relic,” humus horizon in the middle or lower part of the A2. 2. Micromorphology. In the less strongly developed soils there may be some humus staining of the sand in the Al horizon and some unoriented clay. Concretions or segregations of iron oxide are present in both A1 and A2. The latter may show schistosity. The B horizon in thin section is yellowish brown with much birefringent material and oriented clay films in fissures and pores which decrease in frequency with depth. No information is available on the fabric of the second humus horizon. 3. Chemistry. The soils are moderately to strongly acid, the pH of the A1 horizon commonly being higher than that of the A2. The minimum pH is in the A2B or B in the more typical examples. The humus content is highest in the Al and usually falls sharply in the A2, decreasing still further with depth. The content of fulvic and humic acids is proportionate to the humus content. The fulvic acid is less mobile than in the podzolic soils. The clay has fairly uniform silica:alumina ratio throughout the profile and there is little differentiation of iron oxide and alumina. Determination of so-called “mobile” iron by the Kirsanov (1937) method (extraction in cold with 0.05 N HC1) gives lower amounts in the A2 than in the Al, the values of the latter being similar to those for the B horizon. The humus content of the second humic horizon is about 1% (cf. Ufimtseva, 1955) and is said to have a humic acid content suggesting its origin from earlier grassland conditions. XII. Summary and Conclusions

It has been shown that in the early Russian work the term podzol was applied solely to the bleached horizon that occurred at or near the surface of soils in the taiga zone. The word was by no means restricted to soils that were well drained. The appreciation of “illuviation” as an important feature of soils with bleached surface horizons led to the application of the word podzol and its derivatives to a wide variety of soils. The distinctions between many of these podzolic soils have never been clearly made, but two main groups of the better-drained varieties have been recognized: podzolic and sod-podzolic. The podzolic soils can be subdivided into those with and those without a humus B horizon. The former are commonest in sandy soils but apparently may occur in heaviertextured materials. From the available descriptions, the sod-podzolic soils may or may not carry a raw humus or moder cover. On the basis of laboratory data the sandy podzolic soils are similar to what have been called podzols or humus-iron podzols, etc. in western Europe and North America. The podzolic soils of heavier texture and

52

ALEX. MUIR

the sod-podzolic soils show a relatively uniform clay composition which, together with thin section evidence, suggests that the textural B horizon results from clay movement. In the sod-podzolic soils of the sprucemixed forest zone, however, there is usually a distinctly lower clay content in the A2 than in the Al horizon. The latter feature might serve to distinguish soils that are close to pseudogleys from those that are strongly bleached sols lessivds or Parabraunerde. The apparent disintegration of the B horizon is clear in both cases. The following tentative correlations can be suggested, although further work is essential before they can be considered as established. Russia

Sandy podzolic soils Sandy podzolic soils with illuvial humic horizon Podzolic soils Podzolic soils with illuvial humic horizon Sod-podzolic soils

W. Europe and N . America Iron or iron-humus podzols Iron-humus or humus podzols Sols lessivds or sols podzoliques (bleached Parabraunerde)

Molkenpodzol Bleached Parabraunerde, pseudogleys, or gray wooded soils

ACKNOWLEDGMENTS

I wish to thank Dr. I. Sladits for making the translations from the German, Mr. D. V. Jones for making the diagrams, P. Bullock for the thin sections, and Dr. D. A. Osmond for much helpful discussion.

Addendum

In their textbook on Soil Science, published at the end of 1960, Gerasimov and Glazovskaya (1960) accept the idea of lessiuage, i.e., translocation of clay with little or no decomposition, but consider that gleying is an important accompaniment and suggest that the complex of processes giving rise to sols lessiv6s and related soils should be called “pseudopodzolization” (cf. Gerasimov, 1959). They contrast with this podzolization-“the podzolic process”-which, they say, “is accompanied by intense decomposition of the mineral part of the soil under the influence, principally, of humic materials, and the removal from the upper layers of the soil mass of the most diflicultly mobile products of weathering and soil formation-the hydrated sesquioxides.” Following leaching of the readily mobilizable bases as humates and fulvates, there takes place “the dissolution of substances entering into the nucleus of the colloidal particles. In particular, colloidal hydrates of iron, aluminium and manganese oxides are dissolved by reaction with fulvic acids” which also attack minerals coarser than clay. “Gradually from the upper horizons,

THE PODZOL AND PODZOLIC SOILS

53

lying directly under the forest litter, all the mobile products of the exchange between the mineral part of the soil and organic acids are leached out; the layer becomes impoverished in colloids and sesquioxides. Quartz, being most resistant to decomposition, accumulates as the residual product.” The more mobile constituents may be removed from the soil completely, “the less mobile fulvates (or crenates) of iron and aluminium (and also colloidal solutions of hydrated silica) precipitate from solution somewhat below the podzolic horizon and form a distinct ore-like or illuvial horizon.” It is considered that the various hydrated oxides may crystallize or combine to form secondary alumo- and ferrisilicates in the illuvial horizon. In discussing the podzolic soils of the taiga forests (predominantly coniferous), in the middle zone of which the typical podzol is developed, Gerasimov and Glazovskaya repeat the above arguments, but unfortunately use data for a sod-strongly podzolic soil to illustrate the typical podzolic profile. “The sod-podzolic soils are formed by the superimposition on the podzolic soil forming process of a more or less developed process of humus accumulation.” The explanation of this process is similar to that given by Williams and others, but one is left with the impression that sod-podzolic soils are in the main simply a phase or variant. REFERENCES Aaltonen, N. T. 1935. Conimuns. Inst. forest. Fenn. 20, No. 6. Afanasiev, J. N. 1926a. Zapiski Belaruskai DzarzhaunaK Akad. ScZ’skae i Lyasnoe Gaspadurki No. 1, 92-126. Afanasiev, J. N. 1926b. Pochvovedenie No. 2, 84-89. Afanasiev, J. N. 1927. Rzcss. Pedol. Invest. No. 6. Afanasiev, J. N. 1930. “Basic Outlines of the Earth‘s Soil Cover.” (Bielarusskaya Akademiya Navuk, Minsk). AltemuUer, H.-J. 1956. 2. P@nzenem?hr. Diing. Bodenk. 72, 152-157. Anderson, M. S., and Byers, H. G. 1931. U. S. Dept. Agr. Tech. Bull. 228. Avery, B. W. 1958. J. Soil Sci. 9, 210-224. Avery, B. W., Stephen, I., Brown, G., and Yaalon, D. H. 1959. J. Soil Sci. 10, 177195. Brown, B. E., and Jackson, M. L. 1958. Clays and Clay Minerals, Proc. 5th Natl. Conf. 1956, Washington, pp. 213-226. Brown, I. C., and Thorp, J. 1942. U. S. Dept. Agr. Tech. Bull. 834. Burkhardt, H. 1870. “Saen und Pflanzen.” (4 Aufl. Hannover). Quoted from Pavlinov ( 1887). DaubrBe, A. 1845. C m p t . rend. mad. sci. 20, 1775-1780. Deb, B. C. 1950. J . Soil Sci. 1, 212-220. Dokuchaiev, V. V. 1879. Cartography of Russian Soils. In “Collected Works,” Vol. 2, p. 226 et seq. ( Acad. Sci. U.S.S.R., Moscow.) Dokuchaiev, V. V. 1886. Materials for the evaluation of the lands of Nizhny Novgorod government. In “Collected Works,” Vol. 5, p. 510 et seq. (Acad. Sci. U.S.S.R.,Moscow.)

54

ALEX. Mum

Dokuchaiev, V. V. 1887-1891. Short programme for the investigation of soils. In “Collected Works,” Vol. 7, p. 318 et seq. Duchaufour, Ph. 1956. “PBdologie: Applications Forestihres et Agricoles” ( Bcole Nat. des Eaux et For&, Nancy). Duchaufour, Ph. 1957a. “Tableaux descriptifs et analytiques des sols” (Ecole Nat. des Eaux et For&, Nancy). Duchaufour, Ph. 195713. Compt. rend. ucad. sci. 246, 1154-1157. Duchaufour, Ph. 1960. “Prbcis de PBdologie.” Masson, Paris. Emeis, C. 1876. “Waldbauliche Forschungen und Betrachtung” Berlin. Fireman, P. 1901. Expt. Sta. Record 12, 704, 807. Fridland, V. M. 1958. Pochvovedenie N o . 1, 27-38. Frosterus, B. 1914. Geologiska Kommission i Finland. Geoteknisk Meddeknden No. 14. Gedroiz, K. K. 1926. Nosovsk. Se1.-Khoz. Opyt. Stu. Otdel ugron. No. 14. Gemmerling, V. V. 1922. Rws. Pochvoved No. 4-6; quoted from Glinka (1924). Gemmerling, V. V. 1930. Guide-book for the Excursion of the 2nd Intern. Congr. Soil Sci., Leningrad 11, 40-46. Georgievsky, A. 1888. Materialy PO izuchenie russkikh pochv, No. 4. Gerasimov, I. P. 1959. Izvest. Akad. Nauk S.S.S.R., Ser. Geogruf. No. 3, 20-30. Gerasimov, I. P., and Glazovskaya, M. A. 1960. “Fundamentals of Pedology and the Geography of Soils.” Gosudavstvennoye Izdatel’stvo Geograficheskoi Literatury, Moscow (in Russian), Gerasimov, I. P., Zavalishin, A. A., and Ivanova, E. N. 1939. Pochvovedenie No. 7, 10-43. Glinka, K. D. 1911. Pochvovedenie 13, 17-48. Glinka, K. D. 1914. “Die Typen der Bodenbildung.” Borntraeger, Berlin. Glinka, K. D. 1923. “Soils of Russia and Adjacent Countries.” State Publishing House, Moscow (in Russian). Glinka, K. D. 1924. “Disperse Systems in Soils.” Kultumo-Prosvetitel’noye Trudovoye Tovarishchestvo “Obrazovaniye.” Leningrad ( in Russian). Glinka, K. D. 1931. “Pochvovedenie” (text-book), 4th ed. State Agric. Publishing Office, Moscow. Gorshenin, K. P. 1958. Pochoovedenie No. 9, 114. Gray, R. 1839. Prlze Essays G Tram. Highland and Agr. SOC. Scot. 12, 286-287. Grossman, R. B., Stephen, I., Fehrenbacher, J. B., and Beavers, A. H. 1959. Soil Sci. SOC. Am. PTOC. 23, 73-75. Guide, 1959. Guide to the Experimental Farms. Rothamsted Experimental Station, Harpenden. Il’enkov, -, 1869. Russkoye Sel’skoye Khozuistuo 2, 118 (quoted from Georgievsky, 1888). Karpachevsky, L. 0. 1960. Pochvmedenie No. 6, 43-52. Kay, F. F. 1934. Univ. Reading Fuc. Agr. Bull. 48. Kay, F. F. 1939. Univ. Reading Fac. Agr. Bull. 62. Kindler, A. 1836. Ann. Physik. [NF] 37, 203-206. Kirsanov, A. T. 1937. Trudy Pochvennogo Inst. im. V. V. Dokuchaeva, &ad. Nauk. S.S.S.R. 14, 5-29. Kononova, M. M. 1951. “Problems of Soil Humus.” Acad. Sci. U.S.S.R., Moscow (in Russian). English translation by T. Z. Novakovsky to be published by Pergamon Press. Kubiena, W. L. 1938. “Micropedology.” Iowa Collegiate Press, Ames.

THE PODZOL AND PODZOLIC SOILS

55

Kubiena, W. L. 1953. “The Soils of Europe.” Murby, London. Kubiena, W. L. 1956. Eiszeitalter u . Gegenwart 7, 102-112. Kundler, P. 1959. 2. Pftanzaemiihr. Dung. Bodenk. 86, 16-36. Lundblad, K. 1934. Soil Sci. 37, 137-155. McCaleb, S. B. 1954. Soil Sci. 77, 319-333. MacEwan, D. M. C. 1948. Velre et silicates i d . 13, 41-46. Marbut, C.F. 1928. Soils, their genesis, classification and development. U . S. Dept. Agr. Mimeograph. Minashina, N. G. 1958. Pochvovedenie No. 4, 90-96. Morozov, S. S. 1940. Pochvovedenie No. 8, 43-66. Muckenhausen, E. 1957. Wiss. Schriftenreihe des Auswertungs- und Informations dienst, Bad Godesberg. Hft. 14. Muir, A. 1934. Forestry 8, 25-55. Muir, A. 1935. Forestry 9, 116-123. Muir, A., and Fraser, G. K. 1940. Trans. Roy. SOC. Edinburgh 60, 233-341. Muller, P. 1887. “Studien uber die naturlichen Humusformen.” Springer, Berlin. Neustruev, S. S. 1927. Russ. Pedol. Invest. No. 3. Newton, J. D., Bentley, C. F., Toogood, J. A., and Robertson, J. A. 1959. Univ. Alberta, Fac. Agr. Bull. 21 (5th rev. ). Nogina, N. A. 1952. Pochvovedenie No. 2, 132-144. Pavlinov, N. 1887. Alaterialy P O izuchenie russkikh pochv, No. 3. Popov, T. I. 1914. Trudy Dokuchaievskogo Pochvennogo Komiteta No. 2. Ramann, E. 1911. “Bodenkunde.” Springer, Berlin. Robinson, G. W. 1930. J. Agr. Sci. 20, 618-639. Robinson, G.W. 1935. Empire Journal of Experimental Agriculture 2, 258-273. Robinson, G. W. 1949. “Soils, Their Origin, Constitution and Classification,” 4th ed. London. (First published in 1932.) Rod&, A. A. 1930. An excursion to the Lisino Experimental Forest. I1 Congress Guide: Dokuchaiev Inst. Soil Sci. Leningad. Rod&, A. A. 1937. “The Podzol-forming Process.” Moscow-Leningrad, Acad. Sci. U.S.S.R. (in Russian), Rod&, A. A. 1944. Pochvovedenie No. 4-6, 159-179. Rod&, A. A. 1955. “Pochvovedenie” ( text-book ) . Goslesbumizdat, Moscow. Rod&,A. A., and Feofarova, I. I. 1955. Pochvovedenie No. 9, 58-60. Rod&,A. A., and Sedletsky, I. D. 1939. Trudy Inst. Pochv. Dokuchueva 19, 81-141. Satchell, J. E. 1953. Studies on Earthworms. Ph.D. Thesis, University of London. Senft, F. 1862. “Die Humus-, Marsch-, Torf- und Limonitbildung.” Engelmann, Leipzig. Sibirtzev, N. M. 1884. Soils of the Arzamas distriot. In “Selected Works” Vol. 2, p. 109 et seq. Izdatel’stvo Sel’skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1896. Soils of the Previslan Region. In “Selected Works,” Vol. 2, p. 363 et seq. Izdatel’stvo Sel‘skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1898. Zap. Novo-Alexandrov. In&. Se1.-Kh. 11, vyp. 3; also in “Selected Works,” Vol. 2, p. 308 et seq. Izdatel’stvo Sel’skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1900. “Pochvovedenie” ( text-book), Quotations from 2nd ed. 1909, Fralova, St. Petersburg; also in “Selected Works,” Vol. 1. Skrynmkova, I. N. 1958. Pochvovedenie No. 4, 1-13. Sprengel, C. 1844. Die Bodenkunde oder die Lehre vom Boden, Nebst einer vollershdigen Anleitung zur chemischen Analyse der Ackererden ( 1st ed. 1837). Muller, Leipzig.

56

ALEX. MUIR

Sukachev, V. N. 1918. Pochvovedenie No. 2, 1-28. Tamm, 0. 1920. Medd. Statens Skogsforsoksanst, 17, 49-300. Tamm, 0. 1930. Svenska Skogsvdrdsforen Tidskr. 28, pp. 1-30. Tamm, 0. 1934. M e d . Statens Skogsforsoksanst. 27, 1-20. Tedrow, J. C. F. 1954. Soil Sci. SOC. Am. PTOC. 18, 479-481. Tiurin, I. V. 1933. “Kurs Pochvovedeniya.” Gosydarstvennoye Izdatel’stvo Kolkhoznoi i Sovkhoznoi Literatury, Moscow. Tiurin, I. V. 1935. “Jubilee Volume for Academician V. R. Williams,” pp. 263-287. Sel’Khozgiz, Moscow. Tiurin, I. V. 1960. P o c h v o v e h b No. 7, 1-2. Tumin, G. 1912. Zhur. Opyt. Agron. 13, 321-348. Ufimtseva, K. A. 1955. Trudy Pochvenmgo Inst. im. V. V. Dokuchaeva, Akad. Nauk S.S.S.R. 46, 5-77. Vadkovskaya, 0. A. 1955. Trudy Pochvennogo Inst. am. V. V. Dokuchueva, Akad. Nauk S.S.S.R. 46, 78-135. Vysotzky, G. N. 1899. Pochvovedenie N o . 1, 36-44. Vysotzky, G. N. 1927. Byull. Pochvoveda pp. 5-13, 74-82, 104-119. Wessely, J. 1873. “Der Europaische Flugsand und seine Kultur.” Wien. Williams, V. R. 1940. “Pochvovedenie” (text-book), 4th ed. Sel’Khozgiz, Moscow. Yarilova, E. A., and Parfenova, E. I. 1957. Pochvovedenie N o . 9, 37-48. Yarilova, E. A., and Parfenova, E. I. 1959. Pochvovedenie N o . 2, 75-79. Yarkov, S. P. 1942. “Memorial Volume to V. R. Williams,” pp. 99-119. Acad. Sci. U.S.S.R., Moscow. Yarkov, S . P. 1954. The formation of podzolic soils. Comm. to V Intern. Congr. Soil Sci. (Russian and French). Acad. Sci. U.S.S.R., Moscow, Yarkov, S. P. 1958. Pochvovedenb No. 6, 30-44. Zakharov, S. S. 1908. Zhur. Opyt. Agron. 7 , 388-477. Zakharov, S. S. 1910. Pochvovedenie, 339-366. Zakharov, S . S. 1911. Pochvovedenie, 49-72. Zakharov, S . S. 1931. “Kurs Pochvovedeniya” 2nd ed. Gosudarstvennoye Izdatel’stvo Sel’sko-Khozaistvennoi i Kooperativnoi Literatury, Moscow. Zavalishin, A. A. 1944. Pochvovedmb No. 4-6, 180-204. Zavalishin, A. A. 1954. Sbornik Rabot Centralnogo Muzeya Pochvovedeniya 1, 101159. Zavalishin, A. A., and Firsova, V. P. 1980. Shornik Rabot Centralnogo Muzeya Pochvovedeniya 3, 7-95. Zavalishin, A. A., and Nadezhdin, B. V. 1957. Sbornik Rabot Centralnogo Muzeya Pochuovedeniya 2, 27-56.

SUBTERRANEAN CLOVER

. . .

F H W Morley Division of Plant Industry. Commonwealth Scientific and Industrial Research Organization. Canberra. Australia

I. Introduction ................................................. I1. Taxonomy .................................................. A . Distribution ............................................. B. Morphology ............................................. C . Cytogenetics ............................................. D. The Breeding System ..................................... I11 Physiological Variation ........................................ A. Embryo Dormancy ........................................ B . Physiology of Flowering .................................. C . Growth Rate ............................................ D . Chemical Composition .................................... IV. Nodulation and Nitrogen Fixation .............................. A . Nitrogen Economy under Sub. Clover ........................ B . The Symbiotic Relationship ................................ V . Ecology .................................................... A . Maturity or Flowering Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Seed Conservation ........................................ C . Seed Size ................................................ D. Competition ............................................. E . Weed Control .......................... ....... F. Limits to Distribution in Australia . . . . . . . . . VI . Agrotechnology .............................................. A . Establishment ............................................ B . Management of Sub. Clover Pastures ........................ C . The Role of Sub. Clover in Ley Farming .................... D . Sub. Clover and Soil Conservation ........................... E . Seed Production ......................................... VII . Diseases and Pests ........................................... A . Fungous Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Virus Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Pests ................................................... VIII. The Estrogen Problem ........................................ IX. Agronomic Improvement ...................................... A . The Varietal Scene ....................................... B. Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Polyploidy ..............................................

.

57

Page 58 61 61 61 64

65 66 66 68 69 71 72 72 73 77 77 79 80 81 87 90 90 95 99 100 101 103

104 104 105 107 107 108 108

58

F. H. W. MORLEY

D. Induced Mutation ........................................ E. Evaluation .............................................. X. Potential Usage ............................................. References .................................................. 1.

109 110 113 117

Introduction

Not much more than half a century ago, Mr. A. W. Howard, a farmer in South Australia, wrote to the Adelaide press stating that he had found a plant which, he believed, would go far to solving the problem of introducing nitrogen into the soil (Hill, 1936). This seemingly extravagant claim was apparently received with scepticism, but Howard, undaunted by indifference, continued to observe his discovery and to press for its recognition as a plant of great agricultural potential. His shrewd observations had not led him astray. The plant referred to by Howard, Trifolium subterraneum L. (hereinafter called sub. clover), was recorded as naturalized in Victoria in 1887 (Audas, 1921). It was observed on a South Australian farm in the Mount Barker district by Howard in 1889. In what now seems a classic understatement, Maiden (1896), the Government Botanist in New South Wales, wrote “it is not an introduction which need render us uncomfortable.” Gardner and Dunne (1933) reported that the species was naturalized in Western Australia even prior to 1893, probably having been accidentally introduced in ryegrass (Lolium spp.) seed from Europe. In 1909 Howard stated that this clover was regarded as sari insignificant weed growing in poor soils” and was “unknown to any seedsman in Europe or Australia as a fodder plant.” The first requirement for the evaluation of a plant, the production of sufficient seed for trials on an adequate scale, was met by Howard, who wrote in his diary “30 lbs of clean clover seed to Messrs E. and W. Hackett and Company on 18th January, 1906, at 2s.6d. [then about 50 cents] per pound.” By the mid-thirties 200 or more tons of seed was being produced annually; today the average production is over 3000 tons of certified seed annually. Sub. clover is established on 20 to 30 millions of acres, with a potential of the order of 100 million acres ( Davies, 1952). This article attempts to trace the development of this plant from a roadside weed to its present role as one of the most valuable plants in Australia, and to describe its cultivation and usage, its taxonomy, physiology, and techniques of improvement. It is not intended to include an exhaustive list of references. Nutritional aspects will be largely omitted because recent reviews by Anderson (1956) and Stephens and Donald (1958) have covered these. The general background, the climate, soils,

SUBTERRANEAN CLOVER

59

and grassland agronomy of Australia have been described by Trumble (1952) and will not be considered in detail here. Davies (1951) stated that “there is no evidence that our agricultural scientists played any part in this development until the plant was very well on its way.” Sub. clover was obviously recognized as a plant of great potential, and it had became widely established, at least in South Australia and Victoria, by the early 1920’s. Perhaps this was largely a result of Howard’s propaganda in the early part of the century, for its use was not officially advocated until Mullett (1922) discussed its value on various Victorian farms judged in a farm competition, and also suggested methods of management. He referred to a number of tests which had been conducted by the Agricultural Department of Victoria. The first sowings were being made about this time on the southern tablelands of New South Wales. Results were disappointing until the sowing of clover was accompanied by topdressing with superphosphate According to Elliott et al. (1948) there were only 25,000 acres under sown pastures in Western Australia in 1923, a figure which sets the upper limit for clover acreage in that State. There was thus a considerable delay between the first recognition of the possibilities of the plant by Howard and its wide adoption by farmers as the basis of improved pastures. But it is by no means clear that Davies is justified in singling out for criticism agricultural scientists any more than other sections of the community (e.g., banks, finance and pastoral companies). Indeed it seems he could scarcely have expected faster progress in that period. There are probably several reasons for this lag in adoption. The role of nutrition (especially phosphorus) was not fully appreciated. Until most soils in the areas climatically suitable for sub. clover were topdressed with superphosphate, the potential of sub. clover could not be visualized. World War I, in which Australia was involved from 1914 to 1918, and various economic recessions in the 1920’s and 1930s, diverted the capital, expended the means, and destroyed the incentives for pasture improvement. Seed was not available in large quantities, and techniques for harvesting on a commercial scale were yet undeveloped. Establishment methods required investigation, and genetic variation was neither known nor appreciated in relation to the range of unexploited environments. In the years following World War I1 the crest of postwar expansion of plantings was not reached until the mid-fifties, even though the financial and social climate favored the establishment of improved pastures and technical knowledge was usually adequate. Transport, labor, equipment, and fertilizer were all in short supply and imposed limits on expansion for years after hostilities ceased.

60

F. H. W. MORLEY

At present certified seed production is of the order of 3500 to 4000 tons annually. Assuming that 1 acre of pasture is established for each 3 to 4 pounds of certified seed produced, the current rate of expansion would be of the order of two million acres per annum. At this rate it will be many years before the area on which sub. clover might profitably be grown is fully planted. This slow relative increase reflects not so much a failure of agricultural science, but rather social and financial barriers to rapid expansion. The picture in New Zealand is similar to that in Australia, but on a reduced scale. Hamblyn (1937) stated that the species had been known in that country since the 1880'~~ but it was not exploited until the 1930's. He estimated that 15 million acres of the North Island alone could benefit from its establishment. Saxby (1956)) distinguished four stages of development. Between 1906 and 1925 sub. clover was regarded as a weed of little value relative to white clover. From then until 1934 it was being tried out in many localities, but it was not sown on a large scale. During this period the importance of phosphate was slowly appreciated. The use of sub. clover was restricted early in the period by lack of knowledge, later by lack of finance. From 1934 to 1947, sowings were expanded on the topographically more favored sites, but during and following World War I1 expansion was curtailed by lack of fertilizer and seed. Since 1947 expansion has been rapid, especially following the advent of aerial seeding and topdressing. The probable level of use in 1956 was about 200 tons of seed per annum. The utilization of sub. clover in other countries has not advanced to the same level as that in Australia and New Zealand. Several forms are cultivated in the subtropical zones of U.S.S.R., most of these being local ecotypes (Ivanova, 1938). In South Africa it appears to be promising in certain areas (Smit, 1943; Vorster, 1943; Penzhorn, 1944). In view of the extent of areas with a Mediterranean-type climate, it is to be expected that sub. clover will play an increasingly important role in South Africa. Sub. clover was introduced into North America about 1920 (Mestre, 1948) but has only recently been cultivated there in Oregon (Rampton, 1952), Texas (Crouch and Jones, 1945), and California (Bentley, 1946). In Texas it was judged inferior to white clover (Trifolium repens) and other species. In California, Love (1952) and Love and Jones (1952) stated that rose clover ( TrifoZium hirtum) is more widely preferred. In Israel sub. clover has become commercialized. According to Katznelson (private communication), 200 acres have been artificially established as a cover crop in deciduous orchards. Seed supply is at present the main limitation to expansion. In 1958 there were only two small nurseries of sub. clover, but this species was present on fairly extensive

SUBTERRANEAN CLOVER

61

areas of range land. Perhaps improved understanding of its nutrition and management will enable this species to enhance production over extensive areas of range in the Mediterranean region. There have been reports from many countries of trials that include sub. clover, but most of these are of a preliminary nature only. The possible role of sub. clover in these regions will be evident only after further experiments, so that most of these reports will not be discussed here. II. Taxonomy

A. DISTRIBUTION

Trifoliurn subterruneurn is largely but not wholly Mediterranean in its distribution, as is shown in Fig. 1. This map was compiled by Mr. J. Katznelson, Agricultural Experiment Station, Neveh Yaar, Israel, from collections, about 30 floras from Mediterranean and Middle Eastern countries; monographs from Spain, Russia, and Italy; herbarium specimens in Jerusalem and Paris; and including some from the Food and Agriculture Organization of the United Nations. The boundaries of its area of distribution are necessarily inexact, but they clearly include a wide range of climates and presumably other environmental variables.

B. MORPHOLOGY Many descriptions have been published, for example by Gardner and Dunne ( 1933) and Aitken and Drake (1941). Some plant parts are illustrated in Fig. 2. The main features of interest are: 1. Annual, self-fertilizing, winter-growing, prostrate habit. 2. Inflorescence of 3 to 7, usually 4, perfect papillionate flowers, usually white, which form the outer part of the inflorescence. The inner part is developed after fertilization as a cluster of barbed processes, which are the calyxes of many abortive flowers. These processes form a bur enveloping the seeds. 3. Following fertilization peduncles elongate toward the ground, burying a variable proportion of burs in the soil (Yates, 1957, 1958). 4. The ovary contains two ovules, but usually only one develops. In some varieties both frequently develop, forming “twin,”hemispherical seeds. 5. Seed weight varies between and within strains. Average seed weight may be from less than 5 mg to more than 8 mg. Seed color is usually black, but white and amber forms occur. 6. A wide range exists of leaf markings, anthocyanin marks on leaves, stipules, and calyxes, and pigmentation of stems and petioles.

FIG.1. Distribution of sub. clover in original habitat (inset is the Canary Islands). Broken shading indicates uncertain whether present. (Compiled by J. Katznelson, Agricultural Experiment Station, Neveh Yaar, Israel, as part of a research project supported by the Palestine Endowment Fund of New York.)

SUBTERRANEAN CLOVER

63

FIG.2. Morphology of sub. clover. A. Flowering runner from a sterile F, plant. x 0.25. B. Vegetative runner. x 0.25. C. Leaf. x 1. D. Flower. x 5. E-E3. Inflorescence development to formation of bur; approximately weekly intervals between stages. F. Pod. G. Seed.

64

F. H. W. MORLEY

Varieties are distinguished by characteristic combinations of leaf marks, calyx colors, seed colors, and flowering times, which are of great value for seed certification. Some hundreds of lines, discovered in Australia or collected in the original habitat, have been assembled in Canberra. Of these Aitken and Drake ( 1941) have described fifty, mostly Australian isolates, found either by chance or in a systematic search for variants. According to C. M. Donald (quoted by Frankel, 1954) no lines introduced from the Mediterranean were identical with any from elsewhere, or with any Australian lines. The number of genotypes must be legion, but few have been evaluated from the agronomic viewpoint. C. CYTOCENETICS Wexelsen (1928) reported 2n = 16, but Yates and Brittan (1952) found chromosome counts of both 2 n = 16 and 2n= 12, the latter number being characteristic of a race from Israel, the former of six Australian varieties. Attempts to cross the two groups have so far been unsuccessful. Brock ( 1953) obtained counts of collections from Australia, England, Malta, Morocco, and Portugal, all of which were 2n = 16 whereas Israel material was 2n = 12. Zohary and Katznelson (1957, 1958) studied the distribution and morphology of the two chromosomal races in Israel, where they are sympatric in some localities. Since the races are clearly distinguishable by flower and fruit morphology, they considered that, both on morphological and genetical grounds, the Israel race should be regarded as a distinct species. Accordingly they named it T . israeliticum and published a description. They suggested that this new species, because of its earliness of flowering and seed development and its adaptation to relatively dry places (even on calcareous bedrock), could be a useful legume where drought escape was especially necessary for survival. Morley et al. (1956) found pollen abnormalities and high levels of sterility in several F1 and F2 plants (see Fig. 1 ) . In some Fa plants vegetative abnormalities were also present. Gross chromosomal rearrangements accounted for less than 40 per cent of observed pollen sterility, Morley ( 1 9 5 8 ~ )reported that, on the basis of F1 fertility, 20 Australian strains could be divided into three groups, one with 16 strains the others with 2 strains each. The largest Australian group, which includes TALLAROOK, MT. BARKER, BACCHUS MARSH, and DWALGANUP, produced highly or fully fertile (judged by pollen examination) F1 plants in crosses with 15 of 33 strains from Portugal, Morocco, Algeria, Turkey, and Greece, One Moroccan strain was fully fertile with another group (CLARE, WENIGUP),

SUBTERRANEAN CLOVER

65

but no strain fertile with the third group (YARLOOP, YABBA NORTH) has yet been found, although many F1plants have been examined. Since then the wide distribution of the TALLAROOK group has been confirmed. In addition a Western Mediterranean group of several strains from Portugal, Morocco, Algeria, and the Canary Islands has been shown to constitute a fourth group, one from Portugal a fifth, and one from Israel a probable sixth. It seems likely that the TALLAROOK group, being distributed very widely in the known habitat, is the oldest of the “subspecies,” others having arisen from time to time, these eventually becoming more or less widely distributed, depending on their adaptability and perhaps on the rate of speed dispersal. The Western Mediterranean group is presumably quite ancient, but less so than the TALLAROOK group. Other explanations are, however, possible, since area of distribution is not necessarily wholly determined by age. It is perhaps of significance that, to the present, no representative of the Western Mediterranean group has been found in Australia and that a far higher proportion of the Australian varieties belongs to the TALLAROOK group than is found in the collections from any locality in the Mediterranean area. This suggests either that the TALLAROOK group is composed of much more invasive strains than other groups or that the Australian strains came from a relatively restricted area, probably not from the Western Mediterranean region. The extreme variation within this group makes it unlikely that more than a small proportion of Australian strains has arisen de nouo. Further examination of collections is proceeding. From the plant improvement viewpoint this subspeciation is a nuisance. The recombination of desirable characters is made more difficult because many hybrids are highly infertile. Admittedly fertile segregants may be obtained in later generations, but this involves growing larger populations and selection for additional characters.

D. THEBREEDING SYSTEM Because of the cleistogamous flowers and the stability of lines from generation to generation (despite absence of measures to prevent crossing), sub. clover has usually been regarded as wholly self-fertilized. Symon (1954) suggested that the frequency of seedlings with chlorophyll aberrations found by him, approximately 1 and 1500, indicated either a total mutation rate of the order of 1 in 700 to 800 for all genes capable of such mutation, or the presence of heritable variation (presumably as a result of occasional hybridization). This is not necessarily so for, even assuming that the majority of the aberrants were genetically determined, and were completely lethal, a frequency of the order observed would be expected if approximately 150 loci with mutation rates

66

F. H. W. MORLEY

about were involved-a figure that is by no means excessive. Further, it is evident that the frequency of aberrants of this type must depend on the mutation rate, not on the breeding system, if the population is maintained in an equilibrium between mutation and elimination. Also the proportion of heterozygosity for recessive lethals is kept relatively low by even moderate (20 per cent) levels of selfhg (Wright, 1931), so that the imposition of increased levels of selfing would be unlikely to markedly raise the frequency of aberrants. Therefore Symon’s results seem to indicate no more than that a moderately large number of mutational blocks can affect chlorophyll content. This is supported by Carpenter’s (1958) results on irradiation of sub. clover and is consistent with numerous investigations on chlorophyll defects in other species. Chlorophyll defects comprise a large fraction of either natural or induced mutations. Frankel and Williams (1958) used the dominant marker, redleaf, to demonstrate the occurrence of natural crossing in the field. They emphasized that since the recessive parents were segregates of a cross ( YARLOOP BURNERANG) in which many plants were highly infertile, their results might not represent those to be expected from normally fertile plants growing together in the field. I have observed honeybees working sub. clover swards and noted that they made brief visits to many flowers, but these flowers were at a stage at which fertilization would already have taken place. Nevertheless the possibility certainly exists that occasional hybridization occurs. Indeed, as discussed elsewhere (Morley, 1960), a certain amount of out-crossing has probably been of great significance to the evolution of this species. However, I have no evidence that cross fertilization is other than very rare in Australia, Some pollinating agent may be present in the natural habitat, but no evidence for this has been reported. However, Katmelson (private communication) observed in Israel that some flowers were damaged by a beetle before fertilization, and this may have increased the chances of cross pollination.

x

111.

Physiological Variation

Sub. clover is present in a wide range of natural habitats and therefore might be expected to possess a wide range of physiological adaptations to these habitats. In this section present knowledge of the physiology of this species is summarized. A. EMBRYO DORMANCY Germination may be inhibited by hardseededness, which prevents absorption of water. This must be distinguished from embryo dormancy,

SUBTERRANEAN CLOVER

67

which may be defined as the inhibition of germination in viable, fully imbibed seeds. Woodforde (1935), Toole and Hollowell ( 1939), and Loftus Hills (1942b, 1944b, c, d, e ) showed that high temperatures inhibit germination, as is found in many winter growing species. Large differences between varieties were observed. Ballard ( 1958) showed that carbon dioxide, even at less than 1per cent by volume, relieved dormancy in sub. clover and in several other annual legumes in which germination is inhibited by high temperatures (Grant Lipp and Ballard, 1959). Morley (1958b) found differences between strains and between plants of the same strain, the latter presumed to be environmentally determined. He showed that the genotype of the embryo was implicated and that there was a tendency for high dormancy to be inherited as a dominant. Dormancy has been shown in these publications to be relieved by low temperatures, carbon dioxide, activated carbon, hydrogen peroxide, and removal of the testa. Per cent germination is also higher if seeds are almost wholly immersed in water, rather than being placed on moist paper. Dormancy is high immediately after harvest but decreases with time at rates differing with variety. I have found that differences in dormancy between and within varieties may persist for five or more years but may not be evident unless tests are made at sufficiently high temperatures (e.g., 25" to 28°C.). Although pH within the range normally encountered in soil does not appreciably affect germination ( Aitken and Davidson, 1954), I have found that exposure for an hour or two to HC1, H2SO4,or a wide range of organic acids at a pH in the region of 2.5 to 3.0 stimulates extrusion of the radicle within 2 to 4 hours. This is usually, but not invariably, followed by germination unless toxic effects have supervened. The extrusion of the radicle may be inhibited by calcium, but not by magnesium. The present evidence is consistent with the theory that germination may be prevented by the absence of one or more promoters, or the presence of inhibitors. Inhibitors may be prevented from diffusing away by the testa, or may be destroyed by acids or other chemicals. But gas exchange affecting the synthesis of promoters must also be considered and seems at present equally probable. Embryo dormancy is clearly not a simple phenomenon. Nor should it be expected to be simple since germination involves a number of steps that may be blocked by different mechanisms. Thus, I have been able to separate initial cell elongation as shown by radicle protrusion from subsequent steps that require cell division. The clarification of the nature of genetic differences and of the influence of environment may depend very largely on a separation of the component processes. Genetic and biochemical tools may both be required.

68

F. H. W. MORLEY

B. PHYSIOLOGY OF FLOWEFUNG The period from germination to flowering may conveniently be divided into two intervals: from germination to flower initiation, and from initiation to the appearance of the flower. The first of these intervals is subject to a variety of physiological processes, each influenced by environmental stimuli. Sub. clover is a long-day plant (Aitken, 1955a); that is, flower initiation is promoted by increased day length. It shares with many other long-day plants a requirement for low temperatures during the vegetative phase (vernalization). This low temperature requirement is greatly modified by day length. Evans (1959) showed that, in continuous light, vernalization was not required by any of the varieties tested. Both he and Aitken (1955a) also showed that plants of some varieties, when exposed continuously to low temperatures, initiated flowers even in continuous darkness. In the normal range of growing conditions a genetically variable amount of vernalization was necessary for initiation. Flowering time in the field seemed to depend on the interaction of vernalization with day length. As the vernalization process continues, the requirement for long days decreases until the critical day length corresponds with the prevailing day length. Initiation then takes place. Unfortunately, the facts were not found to be in full accord with this theory. Morley and Davern (1956) described large genotype-environment interactions in the field. Attempts to explain these in terms of differing day length and vernalization requirements were not wholly convincing. The demonstration in the laboratory (Evans, 1959) and in the field (Morley and Evans, 1959) of a third process, one that is necessary for initiation and that is promoted by high temperatures, offered a more satisfying explanation of varietal behavior. In cold climates, such as in Canberra, flower initiation may be prevented by the retarding effect of low temperatures on a promotive process which follows vernalization. This process is evidently also of significance in the flowering of lettuce and wheat. Other processes may be operative, but the three described seem sufficient to explain varietal behavior in different environments. Earliness in all environments demands small vernalization and long-day requirements and the capacity to initiate at low temperatures. Lateness may be determined by requirements for extended vernalization, for long days, or for high temperatures for initiation, or some combination of these, The time of flowering is thus a function of the particular physiological limitations and of the environment in which tests are conducted.

SUBTERRANEAN CLOVER

69

Flowering time is polygenically determined, approximately 90% of the total variance being additive genotypic (Davem et al., 1957). Requirements for different stimuli (e.g., vernalization) appear to be continuously distributed and are therefore probably determined by many genes with small effects. They are not “all-or-none,” discontinuously distributed, as if determined by few genes with large effects.

C. GROWTH RATE The iduence of temperature and light energy on the growth rate of sub. clover was examined in two adjoining papers by Black (1955a) and Ozanne (1955) using very different methods and leading to widely different conclusions. Black made 52 consecutive weekly measurements of the growth rate of widely spaced plants of the BACCHUS MARSH variety. These were grown in the open with ample moisture and nutriment, in the relatively mild climate of Adelaide, South Australia. Growth w& determined, on a series of plantings, between two points; the first when plants reached the second trifoliate leaf stage, the second 1 week later. Light energy received was measured on an integrating recorder. Mean temperatures were obtained from thermograph charts in a Stephenson screen. Black then estimated partial regression coefficients, using various measures of growth rate as the dependent variable; light energy, mean temperature, maximum and minimum temperatures were included as independent variables. He found positive associations of light energy and maximum temperature, and a negative association of minimum temperature, with net assimilation rate. Mean temperature was not significantly correlated with net assimilation rate. Relative growth rate was significantly correlated with light energy received, but not with temperature. Black constructed an equation relating plant weight to light energy received. He continues to maintain that light energy was the principal factor affecting seasonal growth (Blackman and Black, 1959) and that temperature is of minor significance, except perhaps in extreme conditions. Ozanne (1955),in a study of the effect of day length and light intensity on the response to zinc, grew plants for 6 to 8 weeks in natural daylight in a greenhouse, subjecting them to different light intensities and day lengths by screening. His results showed a fairly consistent effect of day length and light intensity. The growth rate at 1400 foot-candles light intensity and 11.75 hours daylight, was about equal to that at 8000 foot-candles light intensity and the same day length. If one substituted the appropriate values for the independent variable of this experiment into Black‘s prediction equation, one would predict a negative growth rate. Ozanne’s findings are thus very much at variance with Black‘s con-

70

F. H. W. MORLEY

clusions. However, it is not valid to conclude from these results that the optimum light intensity for growth of sub. clover lies near 1400 footcandles, for leaf temperature may have been above optimum in the highintensity treatment. Millikan ( 1957) demonstrated the effect of light intensity (below 1220 foot-candles ), day length, temperature, and variety on the growth rate of plants grown in the Earhart Laboratory. Mitchell (1956a, b ) showed that MT. BARKER sub. clover has, at rather low light intensities, a relatively low optimum temperature for growth compared to T . repens or Lotus major. Morley (1958a) compared growth rates of five varieties at three temperatures in temperature-controlled greenhouses, at relatively high light intensities. Temperature and strain effects and strain by temperature interactions were significant for relative growth rate and net assimilation rate (production per unit leaf area), especially for the former. Black (1955a) and Blackman and Wilson (1951) have both emphasized the limitations imposed by light energy, and Black discounts the effect of temperature. In contrast, the other authors have found maximum or high growth rate in spaced plants at relatively low light energy values and have demonstrated clear temperature responses. How can these conflicting conclusions be reconciled? The statistical technique used by Black, multiple regression, might readily give false conclusions, with relatively few degrees of freedom, if two or more of the independent variables were strongly correlated and if responses were nonlinear. Light energy is strongly correlated with temperature, even more so with day length (which Black did not consider). The response of growth to temperature is clearly nonlinear. The environment prior to his first harvest could also have influenced his results. Black's conclusions must therefore be rejected. They are subject to several criticisms and give results incompatible with those obtained by other workers with techniques less open to question. The suggestion by Blackman and Wilson (1951) that sub. clover is ill adapted to shading because adjustment of the leaf area:plant weight ratio to shade is slight may nevertheless be entirely valid. Seasonal variations in sward productivity may be influenced by light energy, temperature, and day length, to a degree depending on the density of plants. Shortly after germination, growth rate will depend largely on the variety, temperature, and day length. As a leaf canopy forms, mutual shading will cause light intensity to become increasingly limiting for the lower leaves in the canopy. Light energy must then become the main factor determining sward producton. But, at this advanced stage, livestock production is unlikely to be greatly influenced by growth

SUBTERRANEAN CLOVER

71

rate, since the production of the pasture is far greater than can be consumed. In southern Australia the value of the direct contribution of sub. clover to livestock nutrition will depend on the rapidity with which the photosynthetic factory is built. Contrary to the opinion expressed by Donald and Black (1958), light is unlikely to be so much the “operative limiting factor to the level of production” as are the reaction of varieties to low temperatures and short days and the capacity for producing and conserving sufficient seed to assure high density (Davidson and Donald, 1958). As Davidson and Philip’s ( 1956) calculations show, the greater the leaf area index, the more sensitive is total stand photosynthesis to changes in illumination. At small leaf area indexes, when the amount of fodder available to grazing animals limits animal production, increases in light intensity are unlikely to increase growth rate to an important extent. On the other hand the production of swards destined for harvesting for fodder conservation will be profoundly affected by light intensity whereas, at this time, temperature is likely to be nearly optimum and days long. This subject is discussed further in Section V, D. I have perhaps overemphasized the differences between viewpoints of different workers, but this will serve to draw attention to the necessity for further investigation of the role of temperature, day length, light energy, and light intensity in determining the contribution of sub. clover to animal production. Techniques for selection for growth in critical environments may impart logic and speed to plant improvement. But it is necessary first to be certain what factors of the environment are limiting. Such knowledge may also lead to improvements in chemical treatment to raise production at certain seasons. The increase in winter production following use of gibberellic acid reported by Scurfield and Biddiscombe (1959) may be an initial step in this direction.

D. CHEMICAL COMPOSITION The chemical composition of the whole plant, and of its parts, is expected to vary greatly according to degree of maturity. Hardwick (1954) found that the content of free and reducing sugars was high in the young plant, but lowest in the mature plant. May and Davidson (1958) found that nonstructural carbohydrates decreased after defoliation but that a period of recovery followed this decrease. They postulated that this decrease results largely from the use of these materials as respiratory substrates. Beck (1952) found that the crude fiber content rose from 10 to 12 per cent in early growth, to 30 to 35 per cent in late spring. The proportion of lignin and the ratio of lignin to cellulose increased during

72

F. H. W. MORLEY

the later stages of the life cycle. Shapter (1935) found 23 per cent of fiber at flowering time. The main interest in chemical composition has become focused on estrogens, which are discussed in Section VIII, and on protein content. Shapter (1935) and Beck (1952) found 20 to 29 per cent of protein in the leaves petioles of DWALGANW, but this diminished to 10 to 12 per cent in the late spring. The composition of seeds is of special interest because of their role as a fodder reserve. Shapter (1935) and Franklin and Powning (1942) reported about 40 per cent protein and 6 and 15 per cent fat in seed, but Beck (1952) reported only 28 per cent protein. Rossiter (private communication, 1960) found values similar to those of Shapter in seed of DWALGANUP, YARLOOP, and BACCHUS MARSH. The fats are mostly unsaturated, with more than 10 carbon atoms per molecule of fatty acid (McArthur, 1942). The amino acid composition suggests the presence of cysteine, and tryptophan (Lugg deficiencies in methionine, cystine and Clowes, 1945) and also isoleucine and threonine (Holmes, 1953), but the arginine content is apparently ample (Holmes, 1951). Williams and Moir (1951) fed Merino lambs a basal diet of chopped oaten hay and wheat grain supplemented with various sources of nitrogen. They estimated the biological value of the protein in sub. clover seed to be about the same as that of linseed. However, since 60 per cent of the nitrogen in the diet came from the basal ration (probably quite high in methionine), this result is scarcely critical. It is by no means clear that sub. clover plant or seeds are deficient in any amino acid or other chemical component required for ruminant nutrition. Since sub. clover would seldom be the only item in the diet, any deficiency may not be serious if it is made good by the nonclover portion of the ration. Differences between genotypes in amino acids or other chemical compounds remain to be explored.

+

+

IV. Nodulation and Nitrogen Fixation

A. NITROGEN ECONOMY UNDER SUB.CLOVER The most important contribution of sub. clover to pastures is the fixation of nitrogen, through symbiosis with Rhizobium trifolii, in root nodules. This nitrogen may not become available to associated grasses until the clover plant has virtually completed its life cycle (Trumble and Strong, 1937; Trumble and Shapter, 1937a,b) except as nitrogen returned to the pasture after passage through the grazing animal. However, having been fixed, it accumulates in quantities sufficient to main-

SUBTERRANEAN CLOVER

73

tain a highly productive grass-clover sward in nitrogen equilibriumprobably requiring a turnover of 50 to 100 pounds of nitrogen per acre per annum. As a large proportion of this nitrogen would be returned to the soil in plant debris, or after animal passage, the actual amount fixed each year is diEcult to estimate. It seems unlikely to approach figures such as 400 pounds per acre per annum quoted by Russell (1950)), or found by Sears (1951) in New Zealand pastures, but it may be much greater than 100 pounds. The nitrogen economy of Australian soils has been inextricably bound with their phosphate status, and without phosphate, nitrogen is seldom fixed in appreciable amounts in the higher-rainfall districts. Cook (1939) found at Kybybolite, South Australia, an increase in soil nitrogen of the order of 1000 pounds per acre in the top 6 inches after 15 years under sub. clover which was topdressed annually with 90 pounds of superphosphate per acre. Neglecting the small loss of nitrogen from sale of livestock products and losses from leaching, erosion, and denitrification, this figure represents the fixation of about three-fourths of a pound of nitrogen for each pound of superphosphate applied. Donald and Williams (1954) estimated that 0.76 pounds of nitrogen were fixed per pound of superphosphate applied to sub. clover pastures in the Crookwell district on the southern tablelands of New South Wales. The agreement between these estimates is probably fortuitous since neither figure is likely to be near the potential. If, as Jensen (1947), Greaves and Jones (1950), and Gibson and Nutman (1960) have suggested, the presence of nitrogen inhibits nodulation, and perhaps nitrogen fixation, it seems that fixation would be below maximum, especially in the later years. There is no indication of a decline in the Crookwell results, and both estimates were obtained from predominantly sub. clover pastures in which the nitrogen level would be high for most of the period under consideration. It is nevertheless possible, since yields of clover were increasing with time (and phosphate status), that soil nitrogen would have inhibited, to some extent, the fixation of nitrogen per unit of pasture grown, Further, the rate of build up of soil organic matter, and thus nitrogen, was probably limited at Crookwell by sulfur deficiency (Williams and Donald, 1957). It is relevant that most of the extra nitrogen observed by Cook lay in the top 4 inches of soil; leaching may not have been an important cause of nitrogen loss in this instance.

B. THE SYMBIOTIC RELATIONSHIP Certain rhizobia that were effective on sub. clover were shown by Strong (1937) to be ineffective on T. pratense and T . repens, and vice versa. Since then strains of clover rhizobia have been further investigated

74

F. H. W. MORLEY

by Purchase and Vincent ( 1949), Vincent (1954a, b, 1956), and others. Isolates from the coast or wetter localities were generally effective with T. repens or T. pratense, but not with sub. clover. Isolates from the drier inland were mostly effective on sub. clover and T. glomerutum. Vincent (1954b) placed sub. clover with T. incarnatum, T. glorneratum, and T . alexandrinum in a group with broadly similar nodulation reactions. Nutman (1956) proposed the inclusion of T. ornithopodoides in this group. He suggested that the species within groups tended to be related ecologically rather than taxonomically, possibly reflecting cross inoculation in native habitats. Nutman (1956) and Vincent (1954b, 1956) have reviewed all but the most recent literature on root nodule symbiosis in sub. clover, so that much earlier material will not be referred to again here. However, the factors that limit nitrogen fixation by sub. clover will be considered briefly. These may be classified by their effect on the host plant, on the rhizobia, or on the processes of nodulation and nitrogen fixation which involve both host and rhizobia. The amount of nitrogen fixed may be limited by poor growth of the host plant. The most obvious and important limitation in Australia is inadequate water. It is illogical to expect appreciable nitrogen fixation in regions where plant growth is restricted by lack of moisture; and a sward which is sparse, whether because of lack of water or some other deficiency, is unlikely to contribute useful amounts of nitrogen to soil, pastures, or crops. For this reason sub. clover may not always benefit non-irrigated pastures in semiarid regions, nor contribute to livestock production in such areas. In such places water is more likely to limit livestock production than is nitrogen, although soils in such regions may be nitrogen deficient. Nutritional deficiencies, especially of phosphorus (Anderson and McLachlan, 1951; and many others), sulfur (Anderson and Spencer, 1950), and trace elements have seriously limited nitrogen fixation by a direct effect on the host plant. Nodulation is dependent on the presence of a healthy and dense population of rhizobia in the rhizosphere. Purchase and Nutman (1957) found that the nodulating capacity of a clover root is saturated by a rhizosphere population of lo4 bacteria per milliliter, such densities being attained by rapid multiplication in the rhizosphere under suitable conditions. Soil acidity may be more critical for the growth of rhizobia than for that of sub. clover: indeed sub. clover flourished at pH4.5 although rhizobia failed to grow (Loneragan, 1958, 1960; Loneragan and Dowling, 1958). Anderson and Moye (1952) showed that harmful effects of low

SUBTERRANEAN CLOVER

75

pH on sub. clover nodulation and pasture establishment could be overcome by relatively light dressings (224 pounds per acre) of lime, applied so as to be in close contact with the seed. This approach was taken further by Loneragan et al. (1955a, b ) , who obtained satisfactory nodulation and establishment of sub. clover from 3 pounds of seed pelleted with 5 pounds of lime per acre, results being quite comparable to those obtained from drilling 224 pounds of lime per acre with the seed. In practice this development has often, but not always, given satisfactory results (Paton, 1960; Cass Smith and Holland, 1958; Fitzpatrick, 1958). Some soils may be so deficient that they are unsuitable for renewed nodulation and nitrogen fixation in following years unless heavy dessings are applied. The viability of rhizobia on seed seems to be somewhat precarious (Vincent, 1958), these organisms being particularly sensitive to contact with superphosphate. The addition of equal parts of lime to the fertilizer reduced the decline in viable organisms fivefold. Lime pelleting of seed may be beneficial by aiding both the survival of rhizobia and their multiplication. The population density of rhizobia in soil apparently depends largely on characteristics of the soil in relation to type of organism. Survival of rhizobia effective on sub. clover is no problem in some soils, but requires some soil amendment (e.g., adjustment of pH) in others which support different types of rhizobia (Vincent, 1954a). In still other soils there may be competition due to rhizobia that are ineffective (Harris, 1953, 1954) or to unidentified soil microorganisms that may be antagonistic or competitive (Hely et al., 1957; Cass Smith and Holland, 1958). In such soils effective nodulation has been obtained by 100- to 1000-fold normal doses of inocula or by soil fumigation or sterilization; these measures would be impractical on a commercial scale, although calcium cyanamide treatment, as proposed by Cass Smith and Holland, might be a practical proposition. Bergersen et a,!. (1958) proposed pelleting seed with a nutrient material to enable an increase in inoculation level around the seed during germination. A bentonite-dried blood pelleting greatly improved nodulation at three localities, Trace elements and insecticides may also decrease rhizobial populations (Vincent, 1958; Braithwaite et al., 1958). The factors involved in gradual improvement in nodulation without lime (Anderson and Moye, 1952) are not clear. In view of Loneragan’s (1958) findings, this probably does not result from the supply of calcium to rhizobia, through superphosphate, but phosphorus may be involved. It is possible, although evidence is yet inadequate, that selection may occur of rhizobia capable of surviving and nodulating effectively under the generally unfavorable conditions (Vincent, 1956). If this were so

76

F. H. W. MORLEY

these could be valuable for use in similar problem areas. Possibly a gradual increase in numbers of effective organisms may 'determine the change in nitrogen status (Paton, 1960). The process of nodulation may fail because specific nutritional requirements are not met (Loneragan, 1960) or because of the inability of strains of rhizobia to complete certain stages of infection or nodule formation (Bergersen, 1957). Combined nitrogen may inhibit nodule formation, as discussed earlier, but it is not clear whether such inhibition is important in old, nitrogen-rich, pastures. The processes of nitrogen fixation may be restricted by nutritional deficiencies (review by Nutman, 1956). The calcium requirement for nitrogen fixation seems to be higher than that for growth of the host plant (Loneragan, 1958), moderate calcium deficiency resulting in extensive nodule degeneration. The special role of molybdenum in nitrogen fixation has been reviewed by Anderson (1956). Small amounts are required for nitrate reduction in the host plant. Larger amounts are needed for symbiotic nitrogen fixation: plants with moderate molybdenum deficiency may thus be nitrogen deficient. An increase in soil pH may correct molybdenum deficiency by rendering the element available to clover. Other trace elements may play a critical role (Hallsworth et al., 1960). High temperatures may also inhibit nitrogen fixation in sub. clover (Meyer and Anderson, 1959). A. Gibson (private communication, 1960) has found that different strains of rhizobia have different temperature optima and digerent ranges of adaptation to temperature. Whether temperature tolerance of rhizobia is a factor limiting nitrogen fixation in the field is yet to be determined. The presence of variety-strain interactions has been demonstrated in number of nodules formed in the field (Vincent, 1956; Waters, 1956). Nutman (1961) discounted the existence, in sub. clover, of strain-variety interaction in effectiveness, but recently A. Gibson (private communication, 1960) has demonstrated such interactions in tube cultures. The investigation of interactions in the field is difficult, but the use of a genetic indicator may be a useful aid to such studies. The redleaf gene may be used as an indicator of effectiveness of nodulation in media deficient in nitrogen (Brockwell, 1956). Backcrossing of this gene into different genetic backgrounds may prove of value for rapid determination of field response, but this technique has not yet been exploited. There are thus many factors that may inhibit the survival of rhizobia, nodulation, and effective nitrogen fixation. Many environments are unfavorable to a satisfactory sub. clover-rhizobium symbiosis (Vincent, 195413, 1956). Therefore inoculation of seed with strains of known effectiveness, in addition to precautions against nutritional deficiency or

SUBTERRANEAN CLOVER

77

unfavorable pH, are to be recommended generally unless local experience has clearly shown these steps to be unnecessary. V. Ecology

Sub. clover, as discussed earlier, is naturally distributed around the Mediterranean region in areas having predominantly wet winters and dry summers. Within this area of distribution it may be found in habitats from sea level to over 1500 meters, and with annual precipitation from 350 to over 1100 mm. Therefore it may be expected to exhibit considerable variation in those attributes that are of major significance in adaptation. Unfortunately the physiological and morphological bases of adaptation in this, or any other, species still largely elude us. An attempt is being made to examine this experimentally (Morley and Frankel, 1959; Morley, 1960), but results must await the action of natural selection and the subsequent assessment of changes in different characteristics.

A. MATURITYOR FLOWERING TIME Flowering date, or maturity grade, has attracted the interest of most workers associated with sub. clover, probably because of its constancy within any variety at any location and its obvious significance in adaptation. Thus Donald and Smith (1937) emphasized the effect of maturity grading on seed yield of spaced plants, and Rossiter (1959) in spaced plants and in swards. These workers, and results from our spaced plants in Canberra, showed that the seed yield of spaced plants increased with lateness of flowering. A curvilinear association was noted by Donald and Smith, apparently caused by lack of moisture late in the season. TALLAROOK appeared to be an exception among late varieties in possessing the ability to produce seed under unfavorable conditions. In swards Rossiter (1959) observed a decline in seed production with maturity grading (lateness), the steepness of decline varying with location and year. In swards at Canberra the trend has been, if anything, in the reverse direction. As Rossiter suggested, the seed production potential in his swards was presumably reduced by insufficient moisture in late spring, a limitation of much less significance in the irrigated plots at Canberra. In general one would expect early flowering to be important, as a mechanism of drought escape, in the arid parts of the area of distribution. But the advantage of late flowering in the moister parts of the area of distribution is by no means obvious. Morley et al. (1961) proposed an explanation based on the higher proportion of leaf in later strains relative to early strains, as demonstrated by Lawson and Rossiter (1958) and others, The early varieties would tend to be deprived of light

78

F. H. W. MORLEY

by the leafier, later varieties and would thus tend to be eliminated from the mixed sward. Examination of strains collected by C. A. Neal-Smith over a range of habitats in the Mediterranean region disclosed a rather loose relationship between flowering time at Canberra and moistness of habitat. The results set forth in the tabulation were obtained. Maturity Habitat Dry Moist

Late

Very early

Early

Midseason

8

7 11

10

1

7

15

-

Number of strains 26 33

However, such data are not especially critical. The flowering time in a cool locality such as Canberra may not reflect the flowering time in the natural habitat. A variety might be late flowering at Canberra because of limitations on a process requiring high temperatures (Evans, 1959; Morley and Evans, 1959) whereas in a warm environment, where such limitations are not imposed, it may be early. Examples were given by Morley and Davern (1956). In a study discussed by Frankel (1954), a wide range of maturity was often found in plants from the same locality. Although this does not necessarily reflect a wide range in flowering time in the natural habitat, the relationship between maturity and aridity is by no means clear cut, yet the general pattern is unmistakable. Perhaps heterogeneity within localities results from microclimatic variation as well as from variation from season to season. My own observations of natural populations show that such heterogeneity is not confined to flowering time. Since the correlation is imperfect in the natural habitat, it is scarcely surprising that neither Donald and Smith (1937), nor Aitken and Drake (1941), were able to demonstrate a clear relationship, such as that proposed by Trumble (1937), between flowering time and the distribution of strains in various climatic regions of Australia. With prolonged natural selection, a relatively clear pattern can be expected to emerge only in areas with a regular and distinctly Mediterranean pattern of rainfall, and observations by Rossiter (private communication, 1960) appear to conform to this expectation. Otherwise, irregularity of rainfall pattern from season to season, coupled with the conservation of hard seeds over several seasons, and the influence of other traits such as rate of seed maturation (Millington, 1956), might be expected to maintain a degree of heterogeneity which may itself be an adaptation to a fluctuating environment ( Morley, 1960). This compromise situation suggests that in

SUBTERRANEAN CLOVER

79

all but extreme situations a fairly wide range of maturity types should be tested, and perhaps included in pasture mixtures.

B. SEEDCONSERVATION Conservation of seed may be achieved by two mechanisms: hardseededness and embryo dormancy. Aitken (1939) showed that hardseededness depends on the continuity of an impermeable suberized thickening. This may be broken by physical or chemical agencies. Under natural conditions, softening of seed is probably a consequence of fluctuating temperatures. We have found that over 90 per cent of mature seeds of several varieties are impermeable to water if they have been handled very carefully. Threshing will reduce the proportion of hard seed to levels that apparently depend largely on the size of seeds and severity of treatment. Embryo dormancy may be defined as failure of fully imbibed and viable seed to germinate. Loftus Hills (1942b, 1944b, c, d, e, f ) examined both dormancy and hardseededness in seed samples from different varieties grown in different sites. He showed that dormancy, which was maximized by high temperatures, differed strongly among varieties, in contrast with hardseededness, in which varietal differences could not be demonstrated. The physiology of embryo dormancy was discussed in detail in Section 111, A. Donald (1959) showed that, in Canberra, 92 per cent of the seed crop which germinated did so in the year following its production, 6.3 per cent in the second year, values falling to 0.07 per cent in the fifth year, after which germination was nil or negligible. There was a regular seasonal pattern, the peak of germination occurring in the late summer and autumn both in the year following seed formation and in subsequent years. The relative contributions of hardseededness and dormancy to delay in germination could not be assessed from Donald's results. The fact that a large proportion of the total germination occurred shortly after seed formation, at a time when dormancy would still be maximum, suggests that neither mechanism was very effective. Nevertheless imbibed, but highly dormant, seeds may not germinate in soil at high temperatures, although moisture is ample. They will remain viable despite several repeated cycles of wetting and drying (Morley, 1958b). Further, I have found that seed five years old may still be highly dormant at 30" C. The fact that the most dormant strains are to be found in the coolest parts of the area of natural distribution suggests that dormancy is probably an important ecological adaptation serving to diminish seed losses. The mortality in the young seedlings which emerge after summer

80

F. H. W. MORLEY

rains, and which frequently encounter conditions unfavorable for survival, would thus be avoided. Adams (1943) showed that hard seeds of the variety DWALGANUP survived with unimpaired germinating capacity from 1929 until 1942. Meadley (1947) found no decline in viability in seed stored in closed, but not sealed, containers after nine years. I have found no indications of loss of viability in samples stored in envelopes in laboratory drawers for six years, and approximately 40 per cent viable seeds from samples stored in cans for sixteen years, compared with over 95 per cent viability in fresh samples. Although maintenance of viability under such conditions may not be indicative of the viability to be expected of seed in soil, these results do suggest that the virtual absence of germination observed by Donald, after five years, was not caused by loss of viability but indicated that practically all seeds had germinated. His results are consistent with a germination of about 90 per cent of seeds of BACCHUS MARSH, MT. BARKER, or TALLAROOK and 80 per cent of DWALGANUP for each year in the ground. Since the seed yields obtained by him were unusually light, the observed numbers of plants germinating per unit area in the fifth and subsequent years might be appreciably less than that to be expected under normal stands. However, it is possible that a large proportion of seeds never germinated, since Rossiter (private communication, 1960) found that only 40 per cent of the original seeds could be accounted for after five or six years in the ground. The various mechanisms for maintenance of viability thus provide insurance for survival over a number of consecutive seasons. They are also the means of maintaining genotypic diversity-in itself an adjustment to the ecological range encountered within any locality.

c. SEED SIZE Seed size and weight changes with germination have been the subject of several papers by Black (1955b, 1956, 1957a, b, 1958) and Lawson and Rossiter (1958) and of a review by Black ( 1959). Seed size largely determines the growth rate of the plant (but not the relative growth rate) and thus influences the time at which competition becomes operative in swards of given density. Black (1956) concluded that seed size was important also in limiting the maximum hypocotyl elongation, and hence emergence at any depth of sowing (or natural burial). Once cotyledons had emerged seed reserves were of no further significance, the growth rate being determined by cotyledon area, which depended on seed size, not on depth of planting. However, it is possible that growth rate could equally depend on meristem size, which is perhaps a function of seed size, so Black's contention that growth rate depends on cotyledon area

SUBTERRANEAN CLOVER

81

is not yet wholly proved. The competitive advantage of seed size in dense swards was revealed by Black (1958), who found that the proportion of the total leaf area contributed by plants from small seeds rapidly diminished. There would thus be natural selection for large seed size, especially in dense communities. The ecological significance of the marked strain variations in seed size found in sub. clover remains obscure. Two opposing influences may be visualized. As Salisbury (1942) emphasized, large fruit size is advantageous in closed communities, whereas the production of large numbers of small seeds is preferable in open communities. In sub. clover there appears to be a tendency for large seed size to be more frequent in the warmer parts of the area of distribution, especially Morocco and Israel, than in the cooler parts. This may reflect intensity of competition, but it seems more likely to result from selection for rate of root growth, this being sufficient to keep pace with drying out of surface layers of the soil in seedlings from large seeds, perhaps inadequate in those from small seeds, In our natural-selection plots, large seed size does not seem to have conferred any overwhelming advantage. There are considerable varietal differences, almost independent of seed size, in the proportions of testa, cotyledons, and hypocotyl of seeds (Morley and Rogers, unpublished, 1957). Apparently these differences in proportions have little or no influence on seedling growth rate, as judged by results from five rather contrasting varieties (Morley, 1958a). D. COMPETITION Intraspecific competition in sub. clover has been examined by a number of workers in relation to yield of swards of different density (Donald, 1951, 1954; Lawson and Rossiter, 1958), the effects of seed size (Black, 1958), and petiole length (Black, 1960). The essential theory of these studies was examined by Davidson and Philip (1956). In the presence of adequate moisture, and in normal growing temperatures, competition between plants for light begins at a stage of growth that is dependent on density of planting and increases as the ratio of leaf area to ground area (leaf area index = LAI) increases. The interception of light by leaves is almost complete at an LA1 of approximately 5 (depending somewhat on light intensity). At a higher LA1 many leaves receive insufficient light to maintain essential functions, and these must degenerate. According to Davidson and Philip (1956), at LA1 values above optimum, sward production will decrease because increasing numbers of leaves in the lower layers become parasitic. However, as Saeki (1960) suggested, the evidence from translocation studies indicates that, since such leaves do not receive assimilates from other leaves, they do not become para-

82

F. H. W. MORLEY

sitic. Nevertheless, since their losses from respiration exceed gains from photosynthesis, these leaves would lose weight. The rate of increase of plant material in the sward would thus decline as LA1 increases above optimum, although the rate of decline might be appreciably less than that proposed by Davidson and Philip. At high LA1 values genotypes at a disadvantage in competing for light may be rapidly eliminated. Thus Black (1960) showed that the variety YARLOOP, which has very long petioles, rapidly suppressed varieties with shorter petioles when grown in mked swards at high density. Competitive ability is, however, a function of many characteristics. In our natural selection experiments the most predominant variety, MT. BARKER, in those plots where plant density has been consistently high, is not blessed with large seed size or with long petioles. Presumably it has some compensating advantages, but what these are is yet obscure. Interspecific competition follows the same general principles as intraspecsc competition, but of course competing species may differ more than do competing varieties. Sub. clover has competed successfully with endemic species and with many introduced species over many millions of acres. This is no accident. Davies et al. (1934) and Trumble and Donald (1938a) demonstrated the effect of sub. clover on yield, nitrogen status, and the proportion of grass to clover in native and sown pastures. Anderson and McLachlan (1951) and Donald and Williams (1954) have described the characteristic disappearance of endemic species under the impact of sub. clover and superphosphate, the result being initially a sward composed predominantly of sub. clover with some introduced grasses if these had been sown. But after several years of superphosphate application to pure sub. clover swards, grasses and other nonleguminous species, such as thistles, invade the swards and may even at times dominate them. The appearance of thistles and other nonlegumes in old sub. clover pastures is generally ascribed to improved soil fertility, and especially to increased nitrogen content, which provides an environment more favorable for such species (Fig. 3). Control of the invaders, or at least their replacement by more useful species, may often be achieved by sowing grasses, especially Phalaris tuberosa and Lolium rigidurn, or by cropping with cereals ( Meadley, 1946). The mechanism of control has been assumed to be related to a decrease in the nitrogen levels in the soil. Willoughby (1954) suggested that two conditions must be met in order fully to exploit nitrogen fixation. First, an associate grass should be grown to acquire and store nitrogen, thereby lowering the level of nitrogen in the soil and thus reducing the inhibition of further nitrogen fixa-

SUBTERRANEAN CLOVER

83

tion. Second, the nitrogen status under the sward should be periodically reduced either by a harvest of hay or silage or by a cropping phase. Williams and Donald (1957) and Cook (1939) reported that the nitrogen contents in the top few inches of soils under old sub. clover

FIG.3. A sward of sub. clover established without a companion grass becomes dominated by thistles and volunteer annuals (here Hordeum Zeporinum) after a few years.

pastures were about 2000 pounds per acre. The former authors showed that the carbon:nitrogen ratio was relatively constant, and they therefore concluded that almost all the nitrogen was combined with carbon in soil organic matter. Therefore, it would not be immediately available to

84

F. H. W. MORLEY

plants, but would become available at a rate that would depend on the rate of decomposition of organic matter. A grass growing in a sub. clover pasture might acquire nitrogen as fast as it becomes available and would store a certain amount in roots and other organs. The amount so stored, which might be 50 pounds per acre, would simply be transferred from one kind of store to another and would scarcely affect the total nitrogen content of the soil. The grass could thus affect the level of available nitrogen, but not the amount stored. The evidence for a significant inhibition of nitrogen fixation by high levels of soil nitrogen is by no means convincing. Results of Donald and Williams (1954) show increases in soil nitrogen linear with time or phosphate, unaffected by nitrogen level. These results, as they pointed out, do show some decline in quantity of nitrogen fixed per unit weight of clover grown. But if inhibition did occur, it was far from complete. Any increase in the rate of nitrogen fixation per unit weight of clover, which might result from the removal by grass of available soil nitrogen, would surely be more than counteracted by the decrease in weight of clover in the sward which results from competition by the grass. As Willoughby showed, at the highest level of superphosphate application the proportion of clover in the sward is reduced to about 20 per cent. The amount of nitrogen removed in animal products would be approximately 8 per cent of the weight of greasy wool, 3 per cent of the weight of undressed carcasses. Even at high levels of animal production the loss of nitrogen would therefore be little more than 10 pounds per acre per year, although dairy farming might remove more than twice this figure. Since nitrogen fixation in highly productive stands of pure clover may approach 100 pounds per acre per annum, the nitrogen balance of grazed swards may be maintained by a relatively small clover component, although other elements (e.g,, potassium) may become limiting, especially if hay crops are removed (Williams and Lipsett, 1960). A crop such as wheat might remove up to 50 pounds of nitrogen per acre (Purchase et d.,1949),a crop of hay a similar, or perhaps a slightly larger, amount. The amount of nitrogen stored under old sub. clover pastures is thus unlikely to be reduced greatly by an occasional harvest of hay or cereals, for a single year of nitrogen fixation by a good sub. clover pasture would provide ample nitrogen for a crop, and a margin for losses. Hence rotations that include a year or two of sub. clover for every two crops should maintain the nitrogen content of the soil at high levels. Grasses in old sub. clover-grass pastures may show symptoms resembling those of nitrogen deficiency, despite the presence of large amounts of organic nitrogen in the soil. These symptoms seem more

SUBTERRANEAN CLOVER

85

common in early spring and tend to disappear with the arrival of warmer weather. They probably indicate that the rate of uptake required for current maximum growth by the grass exceeds the rate at which nitrogen is becoming available through decomposition of organic matter in the soil. This process presumably proceeds relatively slowly at low temperatures, such as in winter or early spring. Suppression of thistles and other invaders might therefore seem to be achieved by competition for available nitrogen, even though the total amount of nitrogen in the soil may be far greater than that required for plant growth. But this conclusion appears to be refuted by the fact that thistles flourish in many old sub. clover pastures, despite the presence of a dense cover of volunteer annuals such as Hordeum Zeporinum. Thus the effect of perennial grasses in closing the sward, and in competing for light and moisture, seems to offer a better explanation for the control of weeds in established pastures. From the practical viewpoint the result is satisfactory, no matter what the mechanism happens to be. If phosphate is adequate, the proportions of grass and clover in a sward might be expected to reach an equilibrium. Improved nitrogen status would result in grass dominance, hence reduced fixation of nitrogen, leading in turn to decreased growth of grass and temporary increase in the proportion of clover. If phosphate is inadequate the growth of grass appears to be limited more than that of clover, and clover dominance results. This has been a major cause of animal infertility, through high intake of estrogen (see Section VIII), Grazing may influence grassclover relationships through preferential consumption, by the removal or deposition of nutrients, and by the differential effects of defoliation on competition for light. Rossiter (private communication, 1960) has sought, but not found, evidence for preferential grazing by sheep, but further studies of intake are required. Heavy grazing, accompanied by deposition of excreta, may mobilize fertility and increase the available nutrients in the soil. But if excreta are not deposited uniformly one would expect the heavily manured areas to become grass dominant. Heavily grazed areas, especially those on which excreta are not deposited, might well become clover dominant. Species with relatively erect growth habits, such as most grasses, probably have considerable advantages over clover in competition for light, Therefore lightly grazed pastures are likely to become grass dominant (Fig. 4).If there were any tendency for the more erect species to be preferred by livestock, heavily grazed pastures might well become clover dominant. Further, Blackman and Wilson (1951) suggested that sub. clover is ill adapted to shading, so that it would be prone to succumb in competition for light in lightly grazed swards.

86

F. H. W. MORLEY

Competition for light may well explain the results of Rossiter and Pack (1956),who found that a heavily grazed area became dominated by sub. clover and Cryptostemma calendula (cape weed). In an adjacent enclosure the percentage of clover dropped below 10, and the sward became grass dominant.

FIG.4. A grass-dominated sward of sub. clover and Phaluris tuberosa several years after establishment.

The effect of rainfall on competition with sub. clover depends on the time of the year and the competing species. Sub. clover is stated to make more rapid growth in early autumn than Lolium rigidum (Willoughby, 1954). Early autumn rains therefore tend to be followed by aclover years.” On the other hand, late “opening” rains tend to be “grass years.”

SUBTERRANEAN CLOVER

87

This may not be true if sub. clover is growing with a vigorous perennial, such as Phalaris tuberosa. Personal observations suggest that the same general tendency is present but is less marked than with annual grasses. Whether this seasonal variation in grass:clover ratio reflects nutritional influences, or arises from differential responses to temperature and day length, is not clear at present. Presumably decomposition of soil organic matter would release not only nitrogen, but phosphorus, sulfur, and other nutrients. The differential effects on grass and clover of the amounts and balance of nutrients made available might account for some of Willoughby’s observations. Where sub. clover and white clover ( T . repens) are growing in association the effect of moisture on the balance between the two is very marked. Bartels et aZ. (1932) found that sub. clover dominated infrequently watered pastures, whereas white clover dominated pastures in which natural rainfall was supplemented by more than four watering of 3 or more inches. In the northern tablelands of New South Wales the balance obviously changes with rainfall; improved pastures resemble pure stands of white clover in wet years, but sub. clover is dominant in years when the rainfall is less, or when rain falls mostly in the winter.

E. WEEDCONTROL One of the outstanding contributions of sub. clover to Australian agriculture lies in the control of weeds of pasture and cultivation. Stening (1933) in New South Wales, and Clarke (1934) and Orchard (1951) in South Australia, discussed control of H ypericurn perforatum by competition with sub. clover. Moore and Cashmore (1942) demonstrated almost complete control of this weed by sub. clover, especially if combined with PhaZaris tuberosa. It has also been recommended for control of ConvoZvuZus arvensis (Pearson, 1946), Senecio jacobea (Hexter, 1950), NUsella trichotoma (Green, 1956), and Chondrilla juncea (Cameron, 1952; Tindale, 1955). The replacement of weeds of wheat fallows by sub. clover and Wimmera ryegrass, is now commonplace. The ecological control of weeds by sub. clover pastures is probably dependent on the fact that most weeds of pastures and fallows are able to compete, under conditions of depleted fertility, more aggressively than species adapted to high levels of fertility. Since most agricultural plants fall in the latter category the raising of fertility favors them at the expense of weeds. Not all weeds are controlled by sub. clover, for example, Erodium botrys and Cryptostemma calendula ( Meadly, 1946; Rossiter and Pack, 1956) and Wimmera ryegrass, which is in some respects a weed in certain wheat-growing areas. It seems that plants which respond markedly to high fertility are unlikely to be dominated by sub. clover,

88

F. H. W. MORLEY

but they may be dominated by other aggressive species, such as Phalaris tuberosa, which are highly responsive to increased fertility.

F. LIMITSTO DISTRI~UTION IN AUSTRALIA The biological limits to the persistence of sub. clover are far from precisely understood. Donald ( 1960b) described three climatic boundaries of the area of distribution, these being determined by aridity, heat, and cold. In the markedly Mediterranean climates of the southern comer of Western Australia and portion of South Australia the persistence of sub. clover is chiefly a matter of drought escape, largely by early flowering (Donald and Smith, 1937; Trumble, 1937). Donald suggested that 12.8 million acres of Western Australia are climatically suitable to the late midseason variety MT. BARKER, 38.4 million to the early variety DWALGANUP.

In the southeastern States the field evidence mentioned by Donald is far from convincing. It is difEcult, if not impossible, to separate the influence of aridity from edaphic and temperature effects. In recent years a succession of good seasons has enabled large seed reserves to accumulate in some marginal areas, so that we now find midseason varieties persisting in areas formerly considered doubtful for even the earliest varieties (see results from Deniliquin in Morley et al., 1961). There seems to be no powerful reason why sub. clover might not extend as far as species such as Medicago hispicla, M . minima, and M . laciniata, well beyond the recognized sub. clover zone. Perhaps edaphic factors, strongly correlated with rainfall in this region, are critical. The observations of Yates (1957, 1958) on seed burial are probably also relevant. He found that seed setting was considerably better when development occurred at or below the soil surface or when burs were enclosed in sponge rubber. In high temperatures and low humidity, such as are commonly experienced in early spring in the drier inland areas, seed-set above ground was poor relative to that in buried burs. In general, varieties with a strong tendency to bury burs tended to have poor seed setting above ground, this presumbaly being a reflection of past selection. In addition, Hutton and Peak (1954a) found that germination of pollen grains was adversely affected by high temperature, and especially by low humidity. Both pollen germination and seed-set may be limited by the microenvironment of the inflorescence, which may be of greater significance than general aridity in the eastern region. The warm boundary of the area of distribution will depend in part on vernalization requirement, as was first suggested by Aitken (1955b). Donald considered that the boundary for MT. BARKER would correspond with the 55" F. mean July isotherm. The warm boundary for varieties

SUBTERRANEAN CLOVER

89

which require little vernalization-DwALGmp, YARLOOP, cLm-is probably determined by rainfall distribution rather than temperature per se. The warm areas to the north usually have winters that are too dry for sub. clover. Further, wet summers, which are usual in these areas, favor the growth of competing species at critical times, such as germination and seed setting, in addition to providing conditions unfavorable for survival of seed during summer. Jenkins (1957) showed that in subtropical regions such as at Lismore, New South Wales (see Donald, 1960b, for climatic data) the late varieties set relatively little seed and over 90 per cent of this was destroyed or germinated (equally fatal) during summer. The survival picture was little better for early varieties, but seed production was at a much higher level. The seed production of the variety BACCHUS MARSH, which requires a moderate amount of vernalization (Morley and Evans, 1959), was as high as that of early varieties. The difference in vernalization requirement between this variety and MT. BARKER, although small, nevertheless seems to be critical. On the basis of these results MT. BARKER and TALLAROOK could not be recommended for the Lismore region (Arthur and Jenkins, 1955; Jenkins, 1957). The presence of a cold boundary of the area of distribution was first proposed by Donald (1959) on the basis of meteorological data and differences among seed yields of certain varieties in four years. He interpreted his results as indicating that “frosts during flowering were apparently the principal factor governing seed production at Canberra in the four seasons under study.” The seed production observed was unusually low, and other climatic factors could have operated, so that one may not unreservedly accept this interpretation. But there can be little doubt of the validity of his conclusion that lateness of flowering is an adaptation to cold climates, as is the vernalization requirement ( Morley and Davern, 1956). These mechanisms ensure that flowering does not take place before winter, thereby avoiding frost damage to the highly vulnerable floral primordia. Whether this adaption affects the distribution in Australia is open to doubt. Some varieties, especially CLARE and some from Morocco, suffer severe frost damage even when vegetative (personal observations). At high elevations in southern Australia, precipitation is sufficient to promote competition to sub. clover, by spring- and summer-growing species, at the critical period for seed setting. Germination of seed which did set would be encouraged by the high moisture and low temperatures of elevated localities, at a time when competition would be most severe. At altitudes over 4500 feet snow cover would probably severely prejudice the survival of a winter annual. But whatever the mechanism or combination of mechanisms that de-

90

F. H . W. MORLEY

termine the precise cold boundary of the distribution, in fact the extent of the area beyond this boundary is small relative to the actual or potential total. Thus, pushing back this boundary is unlikely substantially to increase the total area suitable for sub. clover. In any case, white clover is likely to be more productive in such localities. Finally, the edaphic limits of the distribution should be considered. In the natural habitat the species is usually restricted to soils on the acid side of neutrality, and in Australia it has been most successful on acid soils. Whether this distribution depends on nutritional or other properties of soils is yet uncertain. Trumble and Donald (1938b) proposed that symptoms shown by sub. clover on calcareous soils indicated a nutrient deficiency. This suggestion was confirmed by the discovery by Higgs (1958) that CLARE, the variety best adapted to alkaline soils, differed from BACCHUS MARSH in responses to zinc and manganese. Thus, the supply and balance of trace elements may be important. Extensive investigations of the role of nutrition in adaptation seem to be called for. If edaphic limitations can be overcome by supplying trace elements, attemps to select for edaphic adaptation would seem unjustified. VI. Agrotechnology

A. ESTABLISHMENT Several techniques of pasture establishment are used, with results that depend greatly on the particular soil and season and especially on the incidence of rain after sowing (Toms, 1958). The technique favored is a compromise between economy and probability of establishment. It may depend very much on topography and the state of development of the land as regards clearing and cultivation. Probably 30 to 50 per cent of the sown areas have been established by broadcasting from the air or ground (e.g., Parish and Dillon, 1956). This is cheap but unreliable (Hudson, 1935; Harrison, 1936; Hardy, 1949) because good results are unlikely unless prolonged moist weather, and other conditions favoring germination and establishment, follow the initial sowing. Broadcasting remains the best, often the only practicable, method of establishment on steep, rocky, or partly cleared country. It is worth gambling the cost of seed and fertilizer if land can be obtained cheaply or is of little value without pasture improvement, provided it is known to be of reasonable potential productivity. The chances of successful establishment are increased if sub. clover is sown into prepared seed beds (Fig. 5 ) , but the amount of preparation is often slight compared to that usual in crop husbandry. Frequently the seed is only scratched into an unimproved pasture with a drill or similar

SUBTERRANEAN CLOVER

91

machine (Figs. 6 and 7 ) . This is inexpensive and usually adequate. Some pastures are established under cover crops such as oats (Donald and Smith, 1938). Very seldom is a seed bed prepared thoroughly, and it is doubtful if thorough preparation is justified except for the benefit of the cover crop According to Davies (1953) careful preparation is usually

FIG.5. A lightly grazed sward of sub. clover with Phalaris tuberosa and volunteer annuals 18 months after sowing in a prepared seedbed.

a waste of time and money. However, as Tiver (1957) indicated, the extra cost may be worth while if seed of the most expensive varieties, such as CLARE, is being used. The extra expense may also be offset in part by better grazing during the first year. Successful establishment has been obtained in stony and tussocky country by sowing the clover, with fertilizer, in shallow grooves scratched by chisel points (Fig. 8 ) . Sub. clover spreads naturally over extensive areas, probably mainly

92

F. H. W. MORLEY

by animal passage. Hardy (1949) suggested that animal passage might be used to establish the clover on new areas by transferring livestock from a well-seeded pasture. This is likely to result only in very patchy establishment. Moreover topdressing with fertilizer is almost invariably necessary, so that this method is unlikely to eliminate broadcasting operations. It is not sufficiently reliable to be generally recommended, al-

FIG.6. Sub. clover and a grass mixture were scratched into this degenerate, tussocky pasture 6 months previously. The furrows were 1.5 feet apart.

though “naturally pelleted seed may start with a slight advantage over broadcast seed. Nevertheless, seed has become widespread over extensive areas by animal passage, and good establishment may eventually follow with no more treatment than the application of fertilizer. As Davies (1953) stressed, the main limitation to establishment over large areas is nutritional, and it is of little advantage to disseminate seed if nutritional deficiencies are not corrected. Seeding rates vary greatly depending on variety, seedbed preparation, cost of seed, and the value of land. Trumble (1935b) recommended

SUBTERRANEAN CLOVER

93

5 pounds per acre; Dunne ( 1936) and Tiver (1957), 4 pounds per acre for prepared seedbeds, sown to % inch deep, or on the surface if covered lightly by harrows. One or 2 pounds are probably sufficient, under a cover crop, to set sufficient seed to establish a useful stand the following season. Two to 4 pounds is usual when broadcasting, especially with later varieties. If conditions are satisfactory for establishment

FIG.7. Sub. clover pasture similar to that shown in Fig. 6, but 12 months later. The sown species are competing with the tussocks.

this will be sufficient. Heavy seed rates will not compensate for unsatisfactory climatic conditions following seeding. It is important to realize that the fertility and texture of soils in native pastures seldom favor establishment, and repeated failures to establish do not mean that a dense sward will not regenerate itself. Sub. clover plant debris (Williams et d.,1954) and more favorable rhizobia, soil nitrogen, and soil texture all favor regeneration in old pastures. The problems of establishment of clover into the relatively dense pastures of high-rainfall areas have called for a novel method of seeding.

94

F. H. W. MORLEY

Breakwell and Jenkins (1953) developed a machine for overseeding paspalum pastures which are too dense to be readily penetrated by seed drills. This machine, called a sod-seeder, places seed and fertilizer in shallow furrows cut into the sod. Before sod-seeding, the pasture must be closely grazed or mown, thus diminishing competition encountered by the newly emerged seedlings. This procedure avoids the cost of preparing land, which would be heavy in very dense swards, and it scarcely interrupts the normal utilization of pastures.

FIG.8. The pasture on the right was sown with sub. clover 18 months previously. The pasture on the left has not been sown. Note the tussocks remaining close to the fence line, where the clover could not be sown.

The best time of sowing is relatively simple to determine in regions with a strictly Mediterranean rainfall pattern. Otherwise it is a gamble between higher yield from early sowings if the rainfall is favorable, and greater certainty of establishment but lower average yield from later sowings. Later sowings are more likely to obtain sufficient moisture for establishment, but growth will be inhibited by cold to such an extent that seed yield may be unsatisfactory in the first year. Management of the freshly established pasture should be directed to obtain maximum yield of seed. In general, grazing should be avoided, but it may at times be useful in aiding control of weeds and grasses.

SUBTERRANEAN CLOVER

95

Stands established in early autumn, if they have formed a nearly continuous ground cover, may be grazed heavily in the first season. But such results are unusual. Grazing by cattle is preferable to that by sheep during the first season, or until a good stand is established. The companion grass, if any, may be sown at the same time as the clover or, as shown by Fricke (1939), may be readily sown into a prepared seed bed on land which has been brought to a reasonably high level of fertility by some years of clover. In general, sowing the grass with clover is likely to be more economical, although growth of grasses may be unsatisfactory until appreciable amounts of nitrogen become available. If the soil is moderately fertile, seedling losses of grasses during the first year may be avoided by sowing grasses a year or two before sowing clovers (D. F. Smith, private communication, 1960). The variety, species, or mixture of companion grasses cannot be generally recommended. Probably the most common grass is Wimmera ryegrass (Lolium rigidurn), which has often been introduced accidentally. Phalaris tuberosa is outstanding in many environments, and perennial ryegrass (Lolium perenne) is generally favored in higher rainfall areas. There is no evidence favoring any particular grass, although Andrew and Neal-Smith (1956) proposed that the inclusion of an additional perennial grass ( such as Agropyrum obtusiusculum, Bromus coloratus, Bromus inermis, Dactylis glomerata, or Festuca arundinacea) might be advantageous in a Phalaris-sub. clover pasture. B. MANAGEMENT OF SUB. CLOVER PASTURES The outstanding problems of management of an established sub. clover pasture are, first, the amount and kind of fertilizer to apply and, second, the intensity and management of stocking. As the first lies in the realm of plant nutrition it will be discussed only in outline, without detailed references. Australian soils are rich only in the diversity and intensity of their deficiencies. Correction of these deficiencies is fundamental to the exploitation of sub. clover or other exotic species. Many deficiencies can be induced in artificial culture, and these have been described and illustrated by Millikan ( 1953). In the field calcium, nitrogen, phosphorus, potassium, sulfur, molybdenum, manganese, zinc, and copper deficiencies have been found alone or in various combinations. The most important of these, Ca, P, S, and N, can be rectified by the combination of superphosphate with rhizobial nitrogen fixation. Other deficiencies require application of suitable quantities of chemicals, alone or in fortified superphosphate. As has been repeatedly emphasized, phosphorus is deficient on all but

96

F. H. W. MORLEY

a few soils, and sub. clover will seldom flourish unless phosphorus is applied. Ten pounds of phosphorus per acre (as superphosphate) give handsome dividends, but on most soils responses to increasing amounts are almost linear up to several hundreds of pounds of superphosphate per acre. If finance permits, heavy initial applications (400 to 500 pounds per acre) may be advisable except on some soils in which phosphorus is rendered unavailable. If finance limits the amount that can be applied, the best policy may be to sow seed over a wide area with 100 pounds superphosphate per acre, thus establishing a pasture that will respond rapidly to heavy applications in the following years. Since the costs of clearing and preparation of the land, the purchase of seed, and the closing of the area during establishment, are likely to be far greater than that of the fertilizer, it is probably false economy to restrict superphosphate. The maximum stocking rate of a pasture used for wool production depends on the number of animals the pasture will sustain during the season in which the fodder available is minimal. Supplementary feeding is seldom considered economical except as a last resort to keep animals alive during a severe drought. Two periods of scarcity, late winter and early autumn, are usual. The former is almost invariably critical because it is the season of minimum fodder availability, and at this time late pregnancy and lambing are demanding a rising or high plane of nutrition. The term “sustain” is used advisedly for reasons initially advanced by Moore et al. (1946) and Davies (1946) and recently given increased emphasis by Willoughby (1959a). The period during which the amount of pasture available is adequate for high production is short, so that the production of animal matter per acre is largely determined by the number of animals that can be placed on the pasture at this time. The growth of the pasture during this period is far in excess of the demands at any practicable intensity of stocking. The number of animals that a sub. clover-based pasture will sustain is probably minimal after rains in winter, following a dry autumn. Any dry fodder will have been greatly reduced in value by the rains, and the amount of new pasture available is insufficient to provide even a minimal maintenance ration. Note, it is not necessarily the growth rate of the pasture that is critical at this time, but the amount of fodder available to the sheep. Willoughby (1959b) emphasized the point that, if the amount available were lower than 1400 pounds of dry matter (as green material) per acre, animal production would be below maximum because of the inability of sheep to gather a full ration. Nevertheless Willoughby’s (1959a, Fig. 3) results show that sheep may maintain body weight, at 8 sheep per acre, on the production of somewhat less than 200 pounds green material, plus a substantial amount

SUBTERRANEAN CLOVER

97

of dry residues. Presumably the high protein content of the young pasture would have aided the digestion of the low quality roughage. The relative growth rate of a sub. clover-Phalaris pasture in winter conditions similar to those in Canberra may be determined approximately by using Willoughby’s figures obtained from a pasture which was left unstocked in July, the coldest month. The figure obtained, 0.05 g./g./day, is comparable with a determination of my own on several sub. clover varieties. This was 0.04 g./g./day at 8°C. day, 3°C. night, 10% hours day length, in growth cabinets. If 1% pounds dry matter per day is sufficient to maintain a sheep, 300 pounds dry material per acre, as green plant tops, could produce sufficient for 10 sheep per acre, if they were able to harvest this quantity, Willoughby’s results indicate that they might not be able to do so, but the exact points of equilibrium between consumption, production, and animal requirements must be very difficult to determine. If these calculations are approximately correct, a pasture presenting sufficient material (probably less than 500 pounds per acre) to enable a sheep to harvest its needs will produce considerably more than sufficient for the demands of any practical stocking intensity, even during winter months, provided water and nutrition are not limiting. If the amount of available material is less than this, the ability of the sheep to harvest would limit intake to submaintenance levels at any stocking intensity. High stocking intensity will prolong the submaintenance period. It may be possible to increase stocking intensity for the whole year by removal of stock from the pasture until a sufficient photosynthetic factory is built, which may occupy 4 to 6 weeks after opening rains. The cost of conserving, storing, and using fodder to do this may not be offset by the gains from increased stocking, but the practicality and economics of such practices deserve examination. The effects of stocking rate on reproductive performance, growth rate, and longevity, and the bionomics of alternate forms of utilization, await further investigation. The food required by a sheep on a maximum ration for 4 months, and on little above maintenance for the rest of the year, is approximately 750 pounds dry matter per annum. The amount of dry matter that might be produced by a good sub. clover-based pasture is not known, but Donald (1951) suggested 4 tons per acre as a maximum harvest. For reasons discussed by Brougham ( 1956) and Davidson and Philip (1956), this figure may well be far short of maximum production under continuous harvest, even during the sub. clover growing season. Production, especially by perennials, at other seasons could add substantially to this figure. Stocking rates of twelve sheep per acre, with careful management, may not be entirely impossible, but they may not be the most economic.

98

F. H. W.

MORLEY

Willoughby’s experiments have demonstrated ( C.S.I.R.O., 1956), and other evidence is confirming, the possibility of stocking rates of up to eight sheep per acre on unirrigated sub. clover based pastures. The significant idea emerging from this discussion is that the usual stocking rate of two to four sheep per acre on sub. clover-based pastures is well below the maximum possible in the higher rainfall areas. Maximum production per acre is more likely to result from a high intensity of stocking at a relatively low plane of nutrition for most of the year than from a low intensity of stocking at levels of nutrition perhaps only slightly higher. But removal of stress from one season may impose greater stress in another. The problem of the nutritive value of dried residues of sub. clover pastures, discussed by Donald and Allden (1958) and Allden (1958), may be as crucial to productivity in a Mediterranean environment as is winter production on the tablelands of New South Wales. As these authors and Fels et al. (1958) showed, the dry residues may be as deficient in quality as are the winter pastures in quantity. But in either case a decrease in stocking rate may not offer a real solution to the deficiency. The observation by Ferguson et al. (1949), that wool production is more efficient at lower levels of intake, is highly relevant to the problem of stocking rate. One of the most valuable attributes of sub. clover is that it lays down large quantities of seed. After a good spring, as much as a ton of seed per acre may be lying on the surface or in the top inch of soil. This represents a substantial drought reserve, sufficient to keep eight sheep alive for 6 months or more, even if they are able to harvest only 80 per cent of this material. However, if a pasture consists of only 20 per cent clover the seed yield, and hence fodder reserves, will be greatly diminished. Therefore light stocking, which will result in grass dominance, may not be the best method of avoiding losses in drought. Indeed it may result in inadequate fodder reserves at a time when the need is most urgent. Clover dominance must be avoided because of the dangers of estrogens, and probably low winter productivity. Grass dominance must be avoided because it may result in inadequate reserves of seed. A compromise is probably best achieved by ample phosphate, perennial grasses, and intense stocking. Obviously such arguments may not apply to dairy farming, and perhaps only in part to beef cattle production or even sheep operations mainly intended for meat production. In these a high plane of nutrition must be maintained for sufficient time to fatten the animal for marketing. Such forms of utilization may make demands on the pasture very difEerent from those made by wool production. It is perhaps necessary to consider the use of rotational grazing. The

SUBTERRANEAN CLOVER

99

results of Moore et al. (1946), and the discussion of these by Davies ( 1946), seem to have disposed of arguments favoring rotational grazing. The concepts of leaf area and its relationship to pasture production, considered by Davidson and Philip (1956) and Davidson and Donald (1958), indicate that there is no known theoretical basis, or reported result, supporting the expectation that rotational grazing will increase production. Rossiter’s (1952) and Willoughby’s (1959%b) results have provided further experimental confirmation, if such were needed, of the lack of value of rotational grazing. It is to be hoped that future investigations on pasture management will be directed to more profitable topics. “Autumn saving,” or conserving growth at one period for use at another period when livestock demands are more acute, may have an important place in increasing productivity. This form of management should not be confused with rotational grazing, and its value should not be taken as an indication of the value of rotational grazing. In the few decades that sub. clover has been an important contributor to pastures a considerable body of information on such topics as the nutrition, the distribution, the physiology, and the ecology of this species has been accumulated. Knowledge of the utilization of the extra fodder, the nitrogen, and the energy produced through sub. clover has not kept pace. There are reasonable grounds for believing that the value of many existing pastures based on this species could be doubled by the integration of plant and animal sciences. C. THE ROLEOF SUB. CLOVER IN LEY FARMING Since writers such as Cook (1935 and other papers) and Hayman (1945) referred to the value of sub. clover for mixed cereal and fat lamb farming in southern Australia, the urgency of problems of declining soil fertility, increased erosion, and decreased yields of cereals have been emphasized by Forster ( 1939), Cornish ( 1949), Sims and Jardine ( 1949), Kaleski (1956), and others. The reports of experiments in Victoria on the effect of sub. clover leys have been summarized in a series of papers delivered in 1949 to a specialist conference on “Plant and Animal Nutrition in Relation to Soil and Climatic Factors” (H.M.S.O., London). As this publication is not widely available, the reader is referred to Morrow et a2. (1948), Bath (1951), and Cameron (1952). The evidence indicates that, despite the use of superphosphate and the advent of better varieties, yields were declining over wide areas of the wheat belt in South Australia and in Victoria. Deterioration in soil structure and loss of soil nitrogen, the inevitable consequence of wheatfallow-wheat rotations, seem to have been the cause of this decline. Sub. clover leys, associated with increased emphasis on livestock, offer a

100

F. H. W. MORLEY

solution to the problems of declining fertility and have been credited with increases of 8 to 10 bushels per acre in wheat yields (Bath, 1951; Gruen, 1956) as well as substantial increases in livestock production on wheat farms. There are some arguments as to the rate of improvement in fertility status which probably arise from variations in seasons, phosphate status, soils, and the degree of deterioration at the time of commencement, but the general picture is not questioned. It seems worth while to emphasize that rapid improvement can scarcely be expected unless a heavy sward has been grown. Trumble (1935a) suggested that approximately 43 pounds of nitrogen is fixed for every ton of dry clover herbage. This is sufficient to supply the needs of a crop, but it will not restore high fertility to a soil impoverished by years of abuse. The advent of sub. clover in wheat rotations has not been a wholly unmixed blessing. High fertility levels have, especially in wet seasons, created a severe problem because of lodging, so that resistance to lodging may need increased attention in breeding programs of the future. The encouragement of Wimmera ryegrass has resulted in severe competition to the young wheat. Finally, there are indications (Butler, 1959) that some fungous diseases may flourish in sub. clover leys. These are problems that can and will be met by modification of techniques and the development of new varieties. Such difficulties scarcely constitute an argument against the use of sub. clover for conserving soil and fertility. A development of special interest has been the use of sub. clover in orchards as a cover crop and source of nitrogen (Davey, 1942; Gayford, 1947; Botting, 1957) and as a “sod culture” for control of soil loss (Pascoe, 1946; Nicholas, 1952). The essential feature of sod culture is the encouragement of a dense sward, in deciduous orchards, during autumn and winter, and the frequent mowing of this in spring. The advantage of sub. clover is that it sets seed, dies, and forms a surface mulch during summer and early autumn, and thus does not compete with the trees for moisture, while it provides in spring a substantial supply of nitrogen for use by the trees. D. SUB. CLOVER AND SOILCONSERVATION Cameron (1959a, b) recently reviewed the role of sub. clover in soil conservation. The ability to establish a sward as dense as permitted by moisture supply, to raise soil fertility to a level where nonlegumes can flourish, and to improve soil structure so that water intake is increased, and runoff and gully erosion minimized, make sub. clover invaluable in control of erosion. On steep or recently cleared land, sub. clover can be established even though preparation of a seedbed would be impossible (McCaffrey, 1952). On badly eroded land it can be established in gullies (Fig. 9) as well as on adjacent areas (Kaleski, 1956), and on arable land

SUBTERRANEAN CLOVER

101

it will help prevent erosion and also aid reclamation through improved soil fertility and structure ( Lightfoot, 1948). Cameron ( 1959b) very reasonably does not give unqualified approval of sub. clover, pointing out that, during the summer, land on which sub. clover is dominant may be vulnerable to erosion unless care is taken to conserve some surface trash. He strongly recommends the establishment of perennial grasses with sub. clover to minimize this erosion hazard.

FIG. 9. An eroded gully filling in and becoming grassed in a heavily stocked sub. clover pasture.

Sub. clover is not the panacea for all problems of erosion in southern Australia (and similar areas) but, where the climate is suitable, it seems to provide an economic and practical basis for erosion control, The key to erosion control lies in soil fertility and surface cover rather than in mechanical earthworks, and for much of southern Australia sub. clover has demonstrated its value as builder of the fertility necessary for production of good surface cover.

E. SEEDPRODUCTION Sub. clover seed production, as first described by Howard in 1906 (Hill, 1936), has undergone many changes in technique. The harvesting equipment discussed by Duggan (1940), and that described by Meadley

102

F. H. W. MORLEY

(1945), seems crude in comparison with the recently developed machinery now used for large-scale harvesting operations ( Ballard, 1956). The essential features of seed production consist in removal of top growth by rakes (this may be threshed if it contains much seed), loosening of the surface soil by some kind of cultivator, collection of the burs by rakes and by road sweepers or sheepskin rollers, threshing the burs, decoating the seeds, and cleaning the seed by grading, winnowing, and gravity separation. Modem equipment includes machines specially developed for each process, rather than machines designed for other purposes and adapted to sub. clover seed harvesting. The crude sheepskin roller, drawn by a horse, has been replaced by gangs of rollers or sweepers, with hydraulic unloading devices, drawn by tractors or self-propelled. If a large proportion of the seed is not buried, as may frequently be the case with later varieties, direct harvesting by a header fitted with pea lifters may be more economical. Wilkie (1946) stated that a suitably adjusted machine would harvest and thresh at one operation two bags of field-dressed seed per acre. Thaine (1949) using this method obtained 750 pounds machine-dressed seed per acre the first year after sowing, and 1260 pounds in the second year. It is doubtful whether this method could be used on all varieties, but it would often seem to be far more economical to “skim the cream” than to attempt to harvest much of the buried seed. The yield of seed per acre varies greatly with variety, season, and year. Average yields of clean seed are less than 500 pounds (Duggan, 1940) although more than 1000 pounds per acre may frequently be obtained. Most commercial operations probably leave some hundreds of pounds of seed in the ground. In small experimental tests we have obtained seed yields of 1200 to 1800 pounds per acre in the first year after sowing, in both early and late varieties. The relationship between seed yield, maturity grade, and climate has been discussed in Section V, A. The production of certified seed in 1956 to 1959 in the main seedproducing Australian States is shown in Table I. These figures, kindly supplied by my colleagues in State Departments of Agriculture and Dr. R. C. Rossiter, probably represent over 80 per cent of the total seed crop. The value of this crop would be approximately J?,A1,000,000 ($2.24 million) at 1960 prices. The actual cost of production is difficult to estimate, but probably 2 shilling (22 cents) per pound is near the irreducible minimum. The present levels are probably fairly stable, the output having declined from a peak in the middle 1950’s when production was almost twice that shown in Table I. High wool prices and taxation in the early

103

SUBTERRANEAN CLOVER

1950's provided the capital and the incentive for expansion of pasture improvement. Superphosphate and equipment were readily available, in contrast to the 1940's, and seasons were favorable for seed production TABLE I Certified Production of Subterranean Clover Seed Averaged over 1956 to 1959 (tons/annum) Variety State Western Australia South Australia Victoria New South Wales

DWALGANUP

894

343 1247

MT.

TALLA-

BACCHUS

BARKER

ROOK

MARSH

207 319 61 834 1421

1

7 56

64

CLARE YARLOOP

6 26 260 167 459

1 5 34 8 48

191 1

192

Total

1299 352 362 1418 3431

and pasture establishment. But this boom seems to have passed, perhaps because of lower wool prices-certainly not because all land suitable for improvement is already improved. VII. Diseases and Pests

A. FUNGOUS DISEASES 1. Rust Infection of sub. clover by rust (Uromyces trifolii) was observed in Tasmania by Radel (1935), who noted that the variety WENIGUP was resistant but MT. BARKER was susceptible. Henrick (1935) stated that in a warm, moist spring MT. BARKER was heavily attacked, the foliage being completely lost within a week. Levy and Gorman (1937) noted varietal differences in resistance. These observations were extended by Loftus Hills (1942a, 1944a), who also presented Fz results suggesting that susceptibility was dominant. Latter (1953) found three races of rust, with differential patterns of host varietal resistance. Crofts et al. (1955) reported that Misses B. Carne and H. Philpotts found a number of additional races in collections from five States of Australia. Only three varieties, WENIGUP, BAULKAMAUGH NORTH, and an introduction from Spain, were found resistant to all. The importance of rust to productivity of sub. clover is difficult to evaluate, and I have yet to be convinced that resistance should be a major objective in a breeding program. Although defoliation is severe and spectacular in certain years, these are generally years of plenty. However, the effect on fodder conservation could be critical.

104

F. H. W. MORLEY

2. Anthracnose Walker (1956a, b ), described outbreaks of "scorch" caused by Kabatiella caulivora, resulting in loss of yield and reduced seed setting. He also found differences between strains in resistance. The disease has apparently been unimportant in more recent years but could cause losses in the future. Other fungous diseases have been recorded, such as those caused by Rhixoctonia solani and Corticum praticola, reported by Ludbrook et al. (1953), but these appear to be only of minor or negligible importance.

B. VIRUSDISEASES 1. Phaseolus Virus 2 Pierce (Bean Mosaic) Aitken and Grieve (1943) and Watson (1949) described a mosaic disease which was identified by Hutton and Peak (1 9 5 4 ~)as the Phaseolus virus 2 Pierce. They found lethal necrotic reactions in some varieties, but mild mosaic reactions in others. Lethal reactors were resistant in the field, and the hypersensitive reaction was inherited as a dominant in most crosses. The disease may be transmitted mechanically or by aphid vectors. Although it may not be very important in the field, the disease is a major nuisance in spaced plants. No attempt is being made to incorporate resistance into commercial varieties. 2. Sub. Clover Stunt This disease, first noticed by various agronomists in the early 1950's, was described by Grylls and Butler (1956, 1959), who demonstrated transmission by aphids. These insects are persistent carriers once they have become infected from a diseased plant. OLoughlin (1958) found a high degree of resistance in the variety TALLAROOK, which was confirmed by Grylls and Peak (1960), who also located other resistant genotypes. Resistance of Fz plants was found to lie between that of the parents. The disease may affect growth in late autumn and winter, and thus could adversely influence production. It is also the cause of serious losses in experimental spaced plantings. Breeding programs to incorporate resistance in commercial lines of sub. clover are proceeding ( Grylls and Peak, 1960) . C. PESTS Ludbrook et al. (1953) reported damage due to nematodes (Meloidogyne spp.) in South Australia, but whether damage is frequent or widely distributed is not known. Andrewartha (1945) and Came (1948, 1951) reported serious damage from the cockchafer scarab, Aphodius

SUBTERRANEAN CLOVER

105

howitti. This may constitute an erosion hazard (Cameron, 1959b). Swan (1934) and Norris (1944) reported damage by the red-legged earth mite Halotydeus destructor. Sub. clover seems to be especially attractive to this pest. Control of both cockchafers and mites may be achieved by mixing 3 to 4 ounces of lindane per acre with superphosphate applied in autumn. Disease is not yet a major problem with sub. clover, but this happy state may not continue indefinitely. The existence, year after year, of large populations of similar genotypes offers ideal conditions for the evolution of parasites. Perhaps it is not a coincidence that the most widespread variety, MT. BARKER, appears to be highly susceptible to all diseases of any importance. In view of the probable increase in importance of diseases, our own breeding program includes the production and maintenance of many agronomically acceptable lines derived from a wide variety of crosses. If new diseases should have drastic effects on productivity, any disease resistance in the species stands a fair chance of being already present in some useful material. VIII. The Estrogen Problem

In the early 1940’s reproductive disorders and infertility in ewes and abnormal development of the mammary and accessory sex glands of wethers, when grazing sub. clover-dominant pastures, were reported (Bennetts, 1944, 1946; Bennetts et al., 1946). The symptoms, which appeared after a few weeks of grazing on the potent pasture, were suggestive of the action of an estrogen or estrogenlike substance, since they resembled lesions produced by prolonged administration of stilbestrol (Bennetts, 1946,1947). Curnow et al. (1948) found that crude ether extracts from artificially dried DWALGANUP sub. clover produced symptoms similar to those following injection of estradiol. East et al. (1949) and East (1950) seemed to clinch the matter neatly by demonstrating that male sex hormones prevented symptoms of estrogenic action of sub. clover from appearing in castrated male guinea pigs or sheep. This also offered an explanation for the nonappearance of symptoms in rams. Here was a serious economic problem. Lambing percentages sometimes fell to as low as 10 per cent; neonatal deaths of lambs and difficulties associated with parturition caused severe losses. Infertility was shown to be permanent ( Schinckel, 1948; Underwood and Shier, 1951). Further, experiments by Underwood and Shier (1952) and Underwood et al. (1953, 1959) showed that, even at low levels (0.01 mg. stilbestrol per day) of estrogen administration, reproductive performance was adversely affected, although not in any obvious way. At somewhat higher rates of administration, reproduction was severely affected, although the

106

F. H. W. MORLEY

estrous cycle did not appear to be abnormal. This suggests that important losses could be sustained at relatively low intakes of estrogen. Unless special investigations were made, these might not be diagnosed. A technique of bioassay of estrogenic activity of sub. clover, by uterine weights of spayed guinea pigs, was developed by Alexander and Watson (1951). They found that, although dehydration of clover reduced its estrogenic activity, little change took place in stored material. However, Bennetts and Underwood (1951) found that wilted or dry clover did not maintain uterine weight in spayed ewes. Rossiter (private communication, 1960) states that there is ample evidence that such material has a low estrogen content. On the other hand, Bickoff et al. (1960) reported estrogenic activity in sun-cured forages, including sub. clover from Australia, and many other dehydrated legume meals. So it seems that at least some estrogens are reasonably stable and persistent in forage. The estrogenic potency of sub. clover was not affected by various minerals added to superphosphate, but clover from plots to which superphosphate had not been recently applied was significantly more estrogenic than clover from recently fertilized plots (Alexander and Rossiter, 1952). Bradbury and White (1951) identified an isoflavin derivative, genistein, with estrogenic activity one-fifth that of estrone, in potent sub. clover. Biggers and Curnow (1954) found that genistein, which is of relatively low estrogenic activity, is a proestrogen, a compound which, when absorbed into the body, gives rise to true estrogenic substances in the course of metabolism. Curnow (1954) described techniques for isolation and quantitative estimation of genistein, and suggested that the quantities found in sub. clover are sufficient to account for the symptoms observed. He found similar and high levels of genistein in four varieties of sub. clover, but no such substances were found in T . repens, T . fragiferum, T . pratense, or Melilotus alba. Curnow and Rossiter (1955) found appreciable amounts of genistein in all of 123 strains of sub. clover, and in T. israeliticum and T. globosum. Both T. pratense and T . campestre gave suspicious reactions, while T. repens and thirteen other species were negative. Production of genistein-free strains by breeding therefore seems improbable. In view of the problems of assay, the use of radiation or other mutagenic agents is unlikely to solve these difficulties. It seems that genistein is not the only estrogen in pasture legumes. Others, including biochanin A (Pope and Wright, 1954), formononetin ( Bradbury and White, 1951), and coumestrol (Lyman et al., 1959), have been found, the last in several pasture legumes. The significance of these in reproduction of sheep is not known; nor is it known whether estrogenic compounds in sub. clover affect grazing animals other than sheep. This would seem possible, even probable.

SUBTERRANEAN CLOVER

107

Symptoms have been reported from widely separated localities in southern Australia usually, but not invariably, in sheep grazing on pastures consisting largely of sub. clover. There can be little doubt that reproductive disorders in sheep will be a normal consequence of high intake of sub. clover. Control of the disease is then a question of limiting intake of clover. As Shier and Rossiter (1949) suggested, prevention is largely dependent on preventing clover dominance of pastures, or on providing some alternative grazing. The condition was first noted on sub. clover pastures deprived of superphosphate because of shortages during World War 11. As discussed elsewhere (Section V, D), low phosphate status and heavy grazing pressure have generally resulted in clover dominance. Measures to correct clover dominance, such as the application of additional superphosphate, have been followed by disappearance of obvious symptoms. The disease is no longer considered so serious as it was formerly. But since symptoms are reported from time to time, it may still cause hidden but important losses. Nevertheless, since very satisfactory breeding performances are being recorded from ewes fed on sub. clover-based pastures, clover intake can apparently be reduced to safe levels by management of sheep and pastures. IX. Agronomic Improvement

A. THE VARIETAL SCENE Agronomic improvement of sub. clover has been, to the present, a matter of selection among varieties, largely on the basis of maturity. Prior to 1941 four varieties were in commercial use. TALLAROOK, a latematuring variety, was recommended for cooler and moister regions. MT. BARKER, a midseason variety, was grown widely throughout areas rean earlyceiving 20 or more inches annual precipitation. DWALGANUP, maturing variety, was suited to the drier portions of the distribution, especially in Western Australia where its earliness provided a means of drought escape. The early midseason BACCHUS MARSH filled a niche between MT. BARKER and DWALGANUP. The varietal situation prior to 1940 was summarized by Aitken and Drake (1941). YARLOOP was described by Elliott and Gardner (1947) from the Yarloop district in Western Australia, where it was apparently able to resist prolonged immersion. CLARE,described by Walker and Neal Smith (1959), is of particular interest since it seems to be adapted to alkaline soils. Other varieties included recently in certification schemes are WOOGENELLUP ( Quinlivan, 1958) and MARRAR ( Godden and Cuthbertson, 1960), both early midseason varieties, and GERALDTON ( Millington, 1960),

108

F. H. W. MORLEY

an early variety which is said to mature seed much faster than DWALGANUP, and therefore to be better adapted to dry conditions. Seed production of the last three is as yet only a few tons annually. The extension of the use of sub. clover beyond southern areas, which have predominantly winter rainfall and acid soils, to the summer rainfall regions further north and to regions with more alkaline soils, together with improved technology of pasture management and increasing incidence of diseases, will call for a greater array of varieties to recombine the virtues of different parents. It is thus likely that the creation and selection of new varieties will displace the present selection among existing lines. The older criterion of flowering time will be replaced by more complex criteria arising from specific demands or developed to exploit some newly discovered attribute. Millington ( 1956) has described his program of recombination in Western Australia. Agronomic improvement in our laboratory is based on selection of recombinants, not selection among introduced lines.

B. HYBRIDIZATION Donald and Smith (1937) referred to what was probably the first artificial cross in sub. clover, made by Trumble in 1929, between MT. BARKER and DWALGANUP. However, nothing seems to have come of the segregants. McMillan (1937) described a technique of crossing and produced crosses which were used in studies of inheritance of rust resistance by Loftus Hills (1942a, 1944a). The technique in present use at this laboratory, which was developed by Hutton and Peak (1954a), consists of emasculation by removal of the corolla, and pollination 24 to 48 hours later. Results vary greatly among female parents, almost half the crosses being successful with some varieties, none with others. The position is complicated by the intraspecific sterility barriers, discussed elsewhere, which apparently render a proportion of hybrids inviable. The most important requirements for success seem to be maintenance of high humidity and cool temperatures.

C. POLYPLOIDY The use of polyploidy in the improvement of sub. clover was studied by Hutton and Peak (195413). Polyploidy was induced by applying to seedlings a solution of 0.4 per cent colchicine with a wetting agent. In row trials the tetraploids gave increased dry weights in one variety, DWALGANUP, but decreases in four other varieties. Seed setting was reduced and seed yield per plant was very much less in tetraploids than in diploids, except for DWALGANUP, where the digerence was less marked. In this species, which relies on seed production for replacement each

SUBTERRANEAN CLOVER

109

year, and on density of seedlings for early production, a reduction in seed yield is unlikely to be compensated for by any increases in growth rate that may accompany polyploidy.

D. INDUCED MUTATION Carpenter (1958) X-rayed dormant (9 to 10 per cent moisture) DWALGANUP seed with doses of 0 to 120 kr at approximately 10 kr intervals. The 50 per cent survival dose was 60 to 70 kr. When chlorophyll mutations and obvious morphological aberrations were scored in Xz, mutation rates of over 50 per cent were obtained with doses of 57 kr or greater. Twenty-eight per cent of all mutations scored were dwarf or small plants, and 42 per cent were chlorophyll aberrations. No dose effect on the fertility of mutants was revealed. Thirty-one per cent were fertile and viable, 41 per cent semilethal or semisterile, 28 per cent lethal or sterile. Brock and Latter (1960) studied the effect of seed irradiation on the flowering-time of TALLAROOK. The genetic variance was increased. Selection of groups of the earliest and latest individuals resulted in a symmetrical response of 12 days, corresponding to a realized heritability of 59 per cent. A response of 4 to 5 days, which was asymmetrical and corresponded to a realized heritability of 34 per cent, was found in the control population. They considered that irradiation offered an attractive alternative to wide hybridization, allowing artificial selection to be concentrated on the particular quantitative character of interest. Although this viewpoint would be justified in many other species where past selection has combined many agronomic virtues in the one variety, this is scarcely true yet in sub. clover. Since genotypes later and earlier than the extreme commercial varieties are available, hybridization and selection might still offer the simplest technique for production of even more extreme commercial varieties and could simplify the inclusion of the distinguishing characteristics necessary for identification and certification. Irradiation might be of special value in adjusting the flowering time of varieties with very special attributes. For example, CLARE, which seems well adapted to alkaline soils, flowers later than would be desirable in many of the areas where such soils are found. Hybrids between CLARE and early varieties are difficult to make and are highly sterile. Repeated backcrosses would present many difficulties. Selection for early-flowering mutants may be the most practical technique, especially if large populations can be treated and examined en masse.

110

F. H. W. MOlUEY

E. EVALUATION Sub. clover may play several roles in agriculture. The most important of these is the enhancement and maintenance of the nitrogen content of the soil, thereby stimulating increased production of forage as clover, grass, or forbs. This improvement of soil fertility may result in increased yields of pastures (especially in winter) and of crops in ley farming systems. The clover itself may dominate swards so that pasture productivity depends on the growth rate of the clover, and not that of a mixed sward. The dried tops, and especially the seed, provide fodder reserves in dry summers and through droughts (Millington, 1956), which may be more critical than the growth made in cooler or wetter seasons. However, evaluation of growth in different seasons may be a complex problem, as discussed by Willoughby ( 1959a). Finally the vigorous growth enables the species to suppress many weeds, to colonize areas denuded of vegetation, and, through improved soil structure, fertility, and vegetative cover, to prevent soil and moisture loss. These roles must determine the breeding objectives and the criteria of strain evaluation. Performance at high levels of fertility, freed from competition or protected from grazing, may be a useful test for a certain component of productivity, but not for total commercial value. Ability to establish, and to maintain, dense populations is a sine qua non. This ability is a reflection of the physiological and morphological adjustment to the environment; it may not be reflected in maximum yields at any time. Many of the introduced lines that have been tested at Canberra lack the ability to establish dense swards. They are therefore discarded except possibly for use as breeding material to incorporate some desired character. It is perhaps of significance that the locally recommended varieties have consistently and rapidly succeeded. The ability to establish and persist has probably been far more critical in the race to commercialization than has the possession of any other especially valuable productive characteristics. The fact that MT. BARKER, at present the most widely grown variety, rapidly dominated mixed swards over a wide range of environments supports this suggestion (Morley et al., 1961). The characteristics that are necessary for establishment and survival are by no means understood. The main criterion in present use is flowering time or “maturity grade.” As discussed in Section V, A, the importance of drought escape often, but not always, justifies selection of early-flowering lines for more arid areas and late-flowering lines for

SUBTERRANEAN CLOVER

111

moister districts. In regions where rainfall distribution is less sharply seasonal such a classification may discard many useful lines. Thus midseason varieties have survived, and have even become dominant in mixed swards, in regions considered marginal for the earliest strains (Morley et al., 1961). Donald and Smith (1937) found that, even in distinctly unfavorable conditions, the variety TALLAROOK produced far more seed than would be expected from the regression of seed production on maturity. Recent work in South Australia has drawn attention to the role of minor elements in adaptation to certain soils. Higgs (1958) showed that the variety CLARE would grow well on certain alkaline soils whereas BACCHUS MARSH would not, unless supplemented with zinc and manganese. The extent to which the present distribution of sub. clover reflects such requirements is a matter for conjecture. Numerous attributes such as seed size, leaf size, petiole length, pilosity, and anthocyanin markings have doubtless contributed to survival in natural populations, but their agronomic significance is as yet obscure. Embryo dormancy appears to be a mechanism of significance for seed conservation (Morley, 195813) and may be of importance to persistency in pastures. Perhaps a high degree of embryo dormancy should be incorporated in lines intended for cool environments, especially if these experience sporadic rains without follow-up falls in summer. Genetic studies indicate that this should not be difficult. Disease resistance may become increasingly important. Most pastures based on sub. clover include only one or two varieties. There may thus be lo6 or lo8 plants of the one biotype per acre. Such populations, which are renewed in most years, seem to offer every opportunity for parasites to evolve. The remarkable fact is that, as yet, diseases due to fungi, bacteria, or viruses seem to prevail only in certain years and seldom constitute a serious threat to production. However, recent observations suggest that the virus disease “sub. clover stunt” may be sufficiently serious to justify breeding for resistance, a program which is now being undertaken in Canberra. The outline in Table I1 presents the sequence of tests now used or planned in the program at Canberra. In common with other programs for agronomic improvement this attempts to select most intensely in the early stages when tests are relatively cheap and simple. It should, however, be admitted that stage 5, the extensive trials involving response by livestock, has never, in my knowledge, been undertaken with sub. clover, or any other species. One might accept the principle that the evaluation of a pasture plant is valid only if it is exposed to the kind of grazing pressure and treatment

112

F. H. W. MORLEY

expected in normal commercial use. Unfortunately we never h o w in advance just how an improved variety might influence normal use, for example by an increase in stocking rate. Moreover the practicable levels of stocking may well be twice those in commercial use. TABLE I1 The Sequence of Testing Subterranean 'Clover Lines Stage Characters evaluated 1. Spaced plant in field 2. Greenhouse and growth cabinets

3. Small swards (heavy seedings or hand planted) 4. Large swards (seeding rate moderately high to overcome contamination) sown with companion grass at different localities in N-deficient soil 5. Extensive trials on semicommercial scale in Merent localities after commercial release; low seeding rates

Maturity grade, disease resistance, seed yield, size, dormancy, special characteristics Specific disease susceptibility, temperature response, phasic development Special yield features, e.g., winter growth, seed yield Establishment and survival, yield at specific times, nitrogen fixation, resistance to overgrazing, seed yield Invasiveness, persistence under various managements, response of livestock, commercial value and regions of adaptation soils, climates, and usage

But it is clear that, apart from its indirect influence on productivity through soil fertility, an increase in the direct contribution of sub. clover to livestock fodder would be most valuable during winter (see arguments in Section VI, B). This should be measured by an assessment of the number of sheep that can be kept alive and reasonably strong during that season. The purist would require tests of such size and complexity that they would be quite impractical. Therefore some kind of compromise must be found to enable numbers of strains to be tested without invoking any unreasonable or many doubtful assumptions. Our present technique is to test yield in stages 1through 4 of Table I1 by close cutting of quadrats or strips of sward and obtaining the seed yield of small quadrats. The animal is introduced only in stage 4 to graze swards heavily during winter, when grazing pressure is expected to be maximum. Resistance to heavy grazing is then determined by measuring recovery, the new growth from the heavily grazed swards. We plan to release only lines that show ability to establish and maintain swards, have good winter growth, are resistant to important diseases, and can withstand really heavy grazing pressure during winter. The eventual test, livestock production, and nutritive value in all seasons, cannot be undertaken at present.

SUBTERRANEAN CLOVER

113

A testing program such as this, while it places the correct emphasis on relieving limitations to livestock production, neglects the possibility of replacing or supplementing winter growth by conserved fodder. The economics of such operations are at present uncertain but, as pressure for increased intensity of land use rises, and improved machinery is developed, fodder conservation may become increasingly important. Accordingly we have determined yields in spring as an index of hay production, although such data have scarcely been considered when selecting lines. But these figures may have an additional value as indexes of nitrogen fixation. While little of the nitrogen currently fixed is available to companion grasses until after harvest of the annual legume (Trumble and Strong, 1937; Trumble and Shapter, 1937a, b ) except by returns through the animal, the growth of the pasture as a whole is likely to indicate the efficiency of the legume component in improving soil fertility, provided other elements ( especially phosphorus) have not been limiting. The evaluation of varieties is thus a difficult and disconcertingly complex problem. For this reason the measurement of components of productivity under reasonably controlled environments should be attempted. The production of a pasture depends on the leaf area and the net assimilation rate (production per unit leaf area). Therefore a variety which rapidly develops a high leaf area index and which has a high net assimilation rate ( E a ) should most rapidly remove limits to animal production. Such rapid development might be achieved by a combination of high density of plants, a high Ea, and a high leaf area:plant weight ratio. Morley (1958a) found large differences in Ea between varieties, and the implications of these were examined in small swards by Black (1960). Differences between varieties were related to differences in Ea, although Black found the ratio of leaf area:plant weight differed from that in Morley’s spaced plants. Further examination of such determinants of productivity may develop techniques of measurement to supplement field observations. X. Potential Usage

Any estimate of the potential area on which sub. clover might be profitably used must be highly speculative. Davies (1952) estimated the area of “reasonably well-watered” land in Australia (excluding the Northern Territory) and concluded that over 300 million acres of land was relatively undeveloped (cf. 12.8 million acres in improved pastures in 1946 to 1948). According to the Bureau of Agricultural Economics (1960) there are now about 34 million acres in improved pastures. The area of which sub. clover is a major component is probably about the

114

F. H. W. MORLEY

same as, or perhaps 50 per cent higher than, that of pastures topdressed with artificial fertilizers-almost wholly superphosphate (Donald, 1960a). This is of the order of 25 million acres. Estimates of the area in sub. clover in each State, kindly provided by my colleagues in State Departments of Agriculture and C.S.I.R.O., are presented in Table 111. TABLE I11 Present and Potential Acreage (million acres) of Subterranean Clover in Australia

State New South Wales Victoria Queensland South Australia Western Australia Tasmania

Improvable Now in sub. Potential for (Davies, 1952) clover pastures sub. clover 4.5 10.0

78.9 23.3 193.0 11.3 41.5 7.5 -

-

-

355.5

-

23.9

90.1

66.2

7

2.5 6.3 0.6

39.5 17.5 ? 8.5 20.8 3.8

Residual awaiting sub. clover 35.0 7.5 ? 6.0 14.5 3.2

Using Davies’ figures for improvable land, and assuming that sub. clover would not be grown on improvable land in all Queensland, half New South Wales, Tasmania, or Western Australia, and one-quarter of Victoria and South Australia, one arrives at the estimates of actual and potential area given in Table 111. The figures for sub. clover pastures in existence do not include many millions of acres on which sub. clover is present, having spread by natural means, but on which an improved sward has not formed probably because fertilizer has not been applied. Also sub. clover grows, or could make a useful contribution, on a large proportion of some 16 million acres sown to cereals. Indeed it already does so, and inclusion of these areas would probably bring the present total to well over 30 million acres. Is it likely that the potential will be approached? There are no obvious technological reasons why the area under sub. clover in Australia should not exceed 90 million acres. In some apparently suitable areas sub. clover is almost universal, in others it is yet rare. Parish and Dillon (1956) described a district in southeastern New South Wales where 44 per cent of the area suitable for improvement was under improved pastures (94 per cent sub. clover with grasses) in 1955, and farmers planned to expand improvement. In that district the area under sown grasses in 1955 had increased by a factor of 4.7 since immediately before World War 11. The area more than doubled in the six years from 1950 to 1955, but the rate of increase has now declined. The barriers to still further expansion are likely to be social and economic; they depend on

SUBTERRANEAN CLOVER

115

land pressure, taxes, and wool prices, as much if not more than on technical knowledge or agricultural skill, although plenty of technical problems await solution within the present sub. clover areas. Planting of the potential area will not be without difficulties such as trace element deficiencies, genetic adjustment, and failure of nodulation. Solution of such problems may in turn expand the potential. There is thus good reason to think that sub. clover will yet increase the soil fertility and agricultural production of over 90 million acres in Australia alone. The full utilization of the potential of the planted areas requires further knowledge of the management of animals and pastures. The increase in production of the 90 million acres could be as great as or even greater than, the present total agricultural production of Australia. The available evidence indicates that stocking and cropping of sub. clover pastures could be greatly intensified, but intensification will generate many problems. The production, health, and reproduction of animals on heavily stocked pastures, the nutrition and management of such pastures, the utilization and availability of surpluses, soil conservation and fertility, and the breeding of crops (and perhaps animals) to exploit the new environment, are examples of subjects that must be examined. Further, alternate systems of animal and plant production, and the impact of these on the socioeconomic world, as well as on the agricultural scene, need to be investigated. There is no shortage of subjects for research. It may seem strange that this species had made so little contribution in its native habitat. Thus Chevalier (1953) considered that the wild forms in France have no practical significance, but admitted with Mestre (1948) that no serious attempt to cultivate sub. clover had been made. The statements of the limitations of sub. clover in Italy by Rose (1949) and Gasparini (1949) cannot be accepted as conclusive in view of the Australian experience. Beliz and Abreu (1951) described an area near Elvas in Portugal where sub. clover was one of the dominant species and suggested that it may play an important part in pastures of this area. My own impressions of this locality support this contention. Sub. clover is widespread in that region, but it obviously suffers from nutrient (probably phosphorus) deficiency. In Greece, h4argaropoulos ( 1958 ) found that, at medium altitudes, topdressing with superphopshate increased the proportion of sub. clover and other legumes from 10 per cent to 50 to 60 per cent of the sward. Such results parallel those obtainable in many parts of Australia. Sub. clover is an unimpressive plant in grass gardens compared with many other pasture legumes. It should not be judged by its appearance, but by its ability to produce energy, to fix nitrogen, to enrich the soil,

116

F. H. W. MORLEY

and to establish and persist despite heavy grazing and unfavorable seasons. These attributes, and its potential contribution to agriculture, are not readily assessed in grass gardens. Yet few tests in the native habitat have gone beyond the nursery row stage of appraisal. Sub. clover will undoubtedly continue to be the preferred legume for in many other habitats, but this will not be realized without considerable work on nutrition, nodulation, seed production, genetic adjustment, and grazing management. As discussed earlier these problems are still important in Australia and, without wishing to belittle the pioneering contribution by A. W. Howard, the efficiency of utilization and the extent of expansion of sub. clover did, and still does, depend on their being solved. Today, even with familiar crops, the exploitation of new material is a slow process. To the agriculturalist of 1910, sub. clover was a plant of obscure origin and uncertain requirements. He had no background knowledge of its value, or of its culture in other countries. The fact that the main features affecting the establishment and use of sub. clover were evident by the middle 1930’s is indeed a tribute to both the farmers and the agricultural scientists who acquired that knowledge. Sub. clover will undoubtedly continue to be the preferred legume for establishment of improved pastures in a large proportion of the better rainfall areas of southern Australia, but it is doubtful whether it will maintain pride of place among legumes in older established pastures, except perhaps where the rainfall pattern is strictly Mediterranean. Much of the area of adaptation receives substantial rains in summer, often as heavy thunderstorms, followed by hot weather. These are usually of little value to sub. clover pastures. Indeed the accompanying leaching of dried residues may precipitate a feed shortage. I think that, in such areas, alfalfa, and perhaps other perennial legumes, will replace sub. clover as the main legume. The two will usually be associated, if only because sub. clover is difficult to eradicate, but sub. clover may become of secondary importance. It is certainly not the only legume deserving serious consideration, but to the present it has been, by far, the most successful. The potential for profitable expansion of the area under sub. clover will remain large for many years, and the advent of aerial agriculture and irrigation, the amelioration of nutrient deficiencies, and increased pressure on land will serve to intensify the demand on this pioneering species in Australia. In other countries similar influences are at work, often with great intensity, as population pressure overtakes agricultural production. Whether sub. clover has any part to play will depend on a complex of economics, biology, and politics. But, whatever the future, already this erstwhile neglected plant of waste spaces, this botanical

SUBTERRANEAN CLOVER

117

curiosity, has filled a prominent niche in agriculture. Howard's wisdom and foresight have been amply confirmed. REFERENCES Adam, A, B. 1943. J . Dept. Agr. W . Australia [21 20, 155. Aitken, Y. 1939. Proc. Roy. SOC. Victoria [N.S.] 61, 187-210. Aitken, Y. 1955a. Australian J. Agr. Research 6, 212-244. Aitken, Y. 195%. Australian J. Agr. Research 6, 245-257. Aitken, Y., and Davidson, B. R. 1954. J. Atmtralian Inst. Agr. Sci. 20, 253-256. Aitken, Y., and Drake, F. R. 1941. Proc. Roy. SOC. Victoria rN.S.1 63, 342-393. Aitken, Y., and Grieve, B. J. 1943. J . Australian Inst. Agr. Sci. 9, 81-82. Alexander, G., and Rossiter, R. C. 1952. Australian J. Agr. Research 3, 24-28. Alexander, G., and Watson, R. H. 1951. Australian J. Agr. Research 2, 47-493. Allden, W. G. 1958. Austmlian J . Agr. Research 10, 219-236. Anderson, A. J. 1956. Advances in Agron. 8, 163-202. Anderson, A. J., and McLachlan, K. D. 1951. Australian J. Agr. Research 2, 377400. Anderson, A. J., and Moye, D. V. 1952. Australian J . Agr. Research 3, 95-110. Anderson, A. J., and Spencer, D. 1950. Australian J. Sci. Research Ser. B 3, 431449. Andrew, W. D., and Neal-Smith, C. A. 1956. Australian J. Agr. Research 7, 367376. Andrewartha, H. G. 1945. J. Dept. Agr. S. Australia 49, 11-16. Arthur, J. M., and Jenkins, H. V. 1955. Agr. Gaz. N.S. Wales 66, 136-138. Audas, J. W. 1921. J. Dept. Agr. Victoria 19, 650-660. Ballard, J. H. 1956. Agr. Gaz. N . S. Wales 67, 114-122. Ballard, L. A. T. 1958. Australian J. Biol. Sci. 11, 246-260. Bartels, L. C., Beruldsen, E. T., and Morgan, A. 1932. J. Dept. Agr. Victoria 30, 187-206. Bath, J. G. 1951. J . Dept. Agr. Victoria 49, 601-607. Beck, A. B. 1952. J . Dept. Agr. W . Australia [31 1, 257-259. Beliz, J. M., and Abreu, J. P. 1951. Melhoramento 4, 75-122. Bennetts, H. W. 1944. J. Dept. Agr. W . Australia [2] 21, 104-109. Bennetts, H. W. 1946. Australian Vet. J . 22, 70-78. Bennetts, H. W. 1947. Australian Vet. J . 23, 10-15. Bennetts, H. W., and Underwood, E. J. 1951. Australian J. Exptl. Biol. Med. Scl. 29, 249-253. Bennetts, H. W., Underwood, E. J., and Shier, F. L. 1946. Australian Vet. J . 22, 2-12. Bentley, J. R. 1946. California Forest and Range Expt. Sta., Forest Research Note No. 48 (mimeo); see Herbage Abstr. 18, 137. Bergersen, F. J. 1957. Australian J. Biol. Sci. 10, 233-242. Bergersen, F. J., Brockwell, J., and Thompson, J. A. 1958. J. Australian Inst. Agr. Sci. 24, 158-160. Bickoff, E. M., Livingston, A. L., Booth, A. N., Hendrickson, A. P., and Kohler, G. 0. 1960. J. A n i d Sci. 19, 189-197. Biggers, J. D., and Curnow, D. H. 1954. Biochem. J. 68, 278-282. Black, J. N. 1955a. Australian J. Biol. Sci. 8, 330-343. Black, J. N. 1955b. Australian J . Agr. Research 6, 203-211. Black, J. N. 1956. Australian J. Agr. Research 7, 98-109.

118

F. H. W. MORLEY

Black, J. N. 1957a. Australian J. Agr. Research 8, 1-14. Black, J. N. 1957b. Australian J. Agr. Research 8, 335-351. Black, J. N. 1958. Australian J. Agr. Research 9, 299-318. Black, J. N. 1959. Herbage Abstr. 29, 235-2.41. Black, J. N. 1960. Australian J. Agr. Research 11, 277-291. Blackman, G. E., and Black, J. N. 1959. Ann. Botony (London)[N.S.] 23, 131-145. Blackman, G. E., and Wilson, G. L. 1951. Ann. Botany (London) [N.S.] 16, 373408. Botting, G. W. 1957. J. Agr. S. Australia 60, 387-390. Bradbury, R. B., and White, D. E. 1951. J. Chem. SOC. pp. 3447-3449. Braithwaite, B. M., Jane, A., and Swain, F. G. 1958. J. Australian Inst. Agr. Scl. 24, 155-159. Breakwell, E. J., and Jenkins, H. V. 1953. J. Australian Inst. Agr. Sci. 19, 109-110. Brock, R. D. 1953. Nature 171, 939-940. Brock, R. D., and Latter, B. D. H. 1960. Third Australian Conf. Radiobwl., Sydney, 1961. (in press). Brockwell, J. 1958. J. Australian Inst. Agr. Sci. 22, 260-265. Brougham, R. W. 1956. Australian J. Agr. Research 7, 377-387. Bureau of Agricultural Economics. 1960. Quart. Reu. Agr. Econ. 13, 98-100. Butler, F. C. 1959. Ann. Appl. Biol. 47, 28-36. Cameron, D. G. 1952. J. Soil Consem. N . S . Wales 8, 28-39. Cameron, D. G. 1959a. J . Soil Consem. N . S . Wales 12, 5-18. Cameron, D. G. 1959b. J . Soil Conseru. N . S . Wales 12, 105-124. Came, P. B. 1948. J. Council Sci. Ind. Research 21, 1-6. Carne, P. B. 1951. Australian J. Agr. Research 2, 429-434. Carpenter, J. A. 1958. J. Australian Inst. Agr. Sci. 24, 39-44. Chevalier, M. A. 1953. Compt. rend. acad. agr. France 16, 750-752. Clarke, G. H. 1934. 1. Dept. Agr. S. Australia 37, 695-699. Cook, L. J. 1935. J. Dept. Agr. S . Australia 38, 730-744. Cook, L. J. 1939. J . Dept. Agr. S. Australia 42, 791-808, 851-866. Cornish, E. A. 1949. Australian J . Sci. Research Ser. B 2, 83-137. Crofts, F. C., Sullivan, G. D., and McMillan, J. R. A. 1955. Uniu. Sydney Faculty Agr. Rept. No. 1, 55. Crouch, E. K., and Jones, J. H. 1945. Texas Agr. Expt. Stu. Bull. No. 666. C.S.I.R.O. 1956. Rural Research in C.S.I.R.O. No. 16. C-OW, D. H. 1954. Biochem. J . 68, 283-287. Curnow, D. H., and Rossiter, R. C. 1955. Australian J. Exptl. Blol. Med. Sci. 89, 243-2A8. Curnow, D. H., Robinson, T. J., and Underwood, E. J. 1948. Australian J. Exptl. Biol. Med. Sci. 26, 171-180. Davem, C. I., Peak, J. W., and Morley, F. H. W. 1957. Australian J. Agr. Research 8, 121-134. Davey, M. 1942. New Zealand J. Agr. 66,285. Davidson, J. L., and Donald, C. M. 1958. Australian J. Agr. Research 9, 53-72. Davidson, J. L., and Philip, J. 1956. UNESCO Symposium Arid Zone Research in Climato2. (Australia) pp. 181-187. Davies, J. C. 1946. Australia, Commonwealth Council Sci. Ind. Research Bull. No. 201. Davies, J. G. 1951. J. Australian Inst. Agr. Sci. 17, 54-68. Davies, J. G. 1952. J. Australian Inst. Agr. Sci. 18, 60-67.

SUBTERRANEAN CLOVER

119

Davies, J. G. 1953. Proc. Intern. Grassland Congr., 6th Congr., Uniu. Park, Penna., 1952 1, 433-442. Davies, J. G., Scott, A. E., and Fraser, K. M. 1934. Australia, Commonwealth Council Sci. Ind. Research Bull. No. 83. Donald, C.M. 1951. Australian J. Agr. Research 2, 355-376. Donald, C. M. 1954. Australian J . Agr. Research 6, 585-597. Donald, (C. M. 1959. Australian J. Agr. Research 10, 771-787. Donald, C.M. 1960a. J . Australian Inst. Agr. Sci. 26, 134-152. Donald, C. M. 196Ob. Herbage Abstr. SO, 81-90. Donald, C. M., and Allden, W. G. 1958. Australian J . Agr. Research 10, 199-218. Donald, C.M., and Black, J. N. 1958. Herbage Abstr. 28, 1-6. Donald, C. M., and Smith, C. A. Neal. 1937. J . Council Sci. Ind. Research 10, 277290. Donald, C. M., and Smith, C. A. Neal. 1938. Australia, Commonwealth Council Sci. Ind. Research BuU. No. 122. Donald, C. M., and Williams, C. H. 1954. Australian J. Agr. Research 6, 664-687. Duggan, W. C. 1940. Agr. Gaz. N . S. Wales 61, 420-424. Dunne, T. C. 1936. J. Dept. Agr. W . Australia [2] 13, 67-69. East, J. 1950. Australian J. Exptl. Biol. Med. Sci. 28, 449-458. East, J., Underwood, E. J., and Bennetts, H. W. 1949. Australian J. Exptl. Biol. Med. Sci. 27, 105-113. Elliott, H. G., and Gardner, C. A. 1947. J. Dept. Agr. W . Australia [2] 24, 228-231. Elliott, H. G., Shier, F. L., Lightfoot, L. C., Underwood, E. J., and Rossiter, R. C. 1948. J. Australian Inst. Agr. Sci. 14, 119-124. Evans, L. T. 1959. Australian J . Agr. Research 10, 1-16. Fels, H. E., Moir, R. J., and Rossigr, R. 1C. 1958. Australian J. Agr. Research 10, 237-247. Ferguson, K. A., Carter, H. B., and Hardy, M. H. 1949. Australian J . Sci. Research Ser. B 2,4241. Fitzpatrick, E. N. 1958. J . Dept. Agr. W . Australia [3] 7, 505-510. Forster, H. C. 1939. J. Dept. Agr. Victoria 37, 130-133. Frankel, 0.H. 1954. Caryobgh 6, Suppl., 600-619. Frankel, 0. H., and Williams, J. D. 1958. J. Australian Inst. Agr. Sci. 24, 162-163. Franklin, M. C., and Powning, R. F. 1942. J . Council Sci. Ind. Research 16, 191-

200. Fricke, E. F. 1939. Tasmanian J. Agr. 10, 1-16. Gardner, C. A., and Dunne, T. C. 1933. J . Dept. Agr. W . Australia [2] 10, 40-48. Gasparini, M. 1949. Ital. agr. 86, 107-124 Gayford, G. W. 1947. J . Dept. Agr. Victoria 46, 81-83. Gibson, A. H., and Nutman, P. S. 1960. Ann. Botany (London) 24, 420-433. Godden, G. R., and Cuthbertson, E. G. 1960. Agr. Gm. N . S. Wales 21, 204-205. Greaves, J. E., and Jones, L. W. 1950. Soil Sci. 69, 71-76. Green, K. R. 1956. Agr. Gaz. N . S. Wales 67, 123-126. Gruen, F. H. 1956. Rev. Marketing Agr. Ecm. ( N . S. Wales) 24, 196-221. Grylls, N. E., and Butler, F. C. 1956. J . Australian Inst. Agr. Sci. 22, 73-74. Grylls, N. E., and Butler, F. C. 1959. Australian J . Agr. Research 10, 145-159. Grylls, N. E., and Peak, J. W. 1960. Australian J. Agr. Research 11, 723-733. Hallsworth, E. G., Wilson, S. B., and Greenwood, E. A. N. 1960. Nature 187, 7980. Hamblyn, C. J. 1937. New Zealand J. Agr. 64, 25-29.

120

F. H. W. MORLEY

Hardwick, N. E. 1954. Australiat J. Agr. Research 6, 372-391. Hardy, W. D. 1949. Agr. Gaz. N . S. Wales 69, 617-619. Harris, J. R. 1953. Nature 172, 507. Harris, J. R. 1954. Australiat J. Agr. Research 6, 247-270. Harrison, J. E. 1936. J. Dept. Agr. Victoria 34, 273-277. Hayman, R. H. 1945. J. Dept. Agr. Victoria 43, 409-414. Hely, F. W., Bergersen, F. J., and Brockwell, J. 1957. Australian J. Agr. Research 8, 24-44. Henrick, J. 0. 1955. Tasmanian J . Agr. 6, 28-29. Hexter, G. W. 1950. J. Dept. Agr. Victoria 48, 217-218. Higgs, E. 1958. 1. Dept. Agr. S . Australia 61, 267-272. Hill, R. 1936. J. Dept. Agr. S . Australia 40, 322-330. Hills, K. Loftus. 1942a. J. Council Sci. Ind. Research 16, 272-274. Hills, K. Loftus. 1942b. J. Council Sci. Ind. Research 16, 275-284. Hills, K. Loftus. 1944a. J. Council Scf. Ind. Research 17, 74-78. Hills, K. Loftus. 194413. J. Council Sci. Ind. Research 17, 186-190. Hills, K. Loftus. 1944c. J. Council Sci. Ind. Research 17, 191-196. Hills, K. Loftus. 1944d. J . Council Sci. Ind. Research 17, 242-250. Hills, K. Loftus. 1944e. J. Council Sci. Ind. Research 17, 251-254. Hills, K. Loftus. 1944f. J. Council Sci. Ind. Research 17, 255-257. Holmes, P. 1951. Australian J . Exptl. Biol. Med. Scl. 29, 63-66. Holmes, P. 1953. Australian J. Exptl. Biol. Med. Sci. 31, 595-602. Hudson, E. R. 1935. Tasmanian J. Agr. 6, 167-168. Hutton, E. M., and Peak, J. W. 1954a. Australian J. Agr. Research 6, 271-278. Hutton, E. M., and Peak, J. W. 195413. Australian J. Agr. Research 6, 356-364. Hutton, E. M., and Peak, J. W. 1954c. Australian J. Agr. Research 6, 598-607. Ivanova, K. V. 1938. Rukovod Approb. seljshokoz Kulit. 4, 54; see Herbage Abstr. 10, 311. Jenkins, H. V. 1957. Univ. Sydney School Agr. Publ. 2, 18-24. Jensen, H. L. 1947. Proc. Linnean SOC. N . S. Wales 72, 265-291. Kaleski, L. J. 1956. J. Soil Conserv. N . S . W&s 12, 3-13. Latter, B. D. H. 1953. Proc. Linnean SOC. N . S. Wales 77, 337-356. Lawson, E. H., and Rossiter, R. C. 1958. Australian J . Agr. Research 9, 286-298. Levy, E. B., and Gorman, L. W. 1937. New Zealand J. Agr. 64, 82-94. Lightfoot, L. C. 1948. J. Australian Inst. Agr. Sci. 14, 121. Lipp, A. E. G., and Ballard, L. A. T. 1959. Australian Agr. Research 10, 495-499. Loneragan, J. F. 1958. Australian J . Biol. Sci. 12, 26-39. Loneragan, J. F. 1960. J. Australian Inst. Agr. Sci. 26, 26-31. Loneragan, J. F., and Dowling, E. J. 1958. Australian J. Agr. Research 9, 464-472. Loneragan, J. F., Meyer, D. R., Fawcett, R. G., and Anderson, A. J. 1955a. J . Australian Inst. Agr. Sci. 21, 254-265. Loneragan, J. F., Meyer, D., Fawcett, R. G., and Anderson, A. J. 1955b. J . Australian Inst. Agr. Sci. 21, 285-2437. Love, R. M. 1952. I . Range Management 6, 120-123. Love, R. M., and Jones, B. J. 1952. California Agr. Expt. Stu. Circ. 371. Ludbrook, W . V., Brockwell, J., and Ricernan, D. S. 1953. Australian J. Agr. Research 4, 403-414. Lugg, J. W. H., and Clowes, G. J. 1945. Australian 1. Exptl. Bwl. Med. Sci. 23, 75-79. Lyman, R. L., Bickoff, E. M., Booth, A. N., and Livingston, A. L. 1959. Arch. Bioc h . 80, 61-67.

SUBTERRANEAN CLOVER

121

McArthur, A. M. 1942. Proc. Roy. SOC. Victoria 64, 243-244. McCaffrey, L. A. H. 1952. J. Soil Conserv. N . S . Wales 8, 97-103. McMillan, J. R. A. 1937. I. Council Sci. lnd. Research 10, 167-168. Maiden, J. H. 1896. Agr. Gaz. N . S . Wales 7, 741. Margaropoulos, P. 1958. Bull. fdde‘ration fmw. econ. Alp. 8, 339344; see Herbage Abstr. 29, 177. May, L. H., and Davidson, J. L. 1958. Australian J. Agr. Reseurch 9, 767-777. Meadley, G. R. W. 1945. J. Dept. Agr. W . Australia [2] 22, 353-357. Meadley, G. R. W. 1946. 1. Dept. Agr. W . Australia [Z] 23, 85-123. Meadley, G. R. W. 1947. I. Australian Inst. Agr. Sci. 13, 1513-160. Mestre, L. 1948. La Potasse 22, 72. Meyer, D. R., and Anderson, A. J. 1959. Nature 183, 61. Millikan, C. R. 1953. J. Dept. Agr. Victoria 61, 215-225. Millikan, C. R. 1957. Australian J. Agr. Research 8, 225-245. Millington, A. J. 1956. J. Australian Inst. Agr. Sci. 22, 111-119. Millington, A. J. 1960. J. Dept. Agr. W . Australia [4] 1, 137-144. Mitchell, K. G. 1956a. N e w Zealand 1. Sci. Technol. A37, 395-413. Mitchell, K. G. 1-b. Proc. Intern. Grassland Congr., 7th Congr., Palmerston North, New Zealand, 1956 pp. 58-69. Moore, R. M., and Cashmore, A. B. 1942. Australia, Commonwealth Council Scl. lnd. Research Bull. No. 161. Moore, R. M., Barrie, N., and Kipps, E. H. 1946. Australia, Commonwealth Council Sci. I d . Research Bull. No. 201. Morley, F . H. W . 1958a. Australian J . Agr. Research 9, 745-753. Morley, F. H. W. 195813. Australian 1. Biol. Sci. 11, 261-274. Morley, F. H. W. 195%. Proc. Intern. Congr. Genet., 10th Congr., Montreal 1958 2, 194-195. Morley, F. H . W . 1960. Cold Spring Harbor Symposia Quant. Bwl. 24, 47-56. Morley, F. H. W., and Davem, C. I. 1956. Australian J. Agr. Research 7, 388-400. Morley, F. H. W., and Evans, L. T. 1959. Australian J. Agr. Research 10, 17-26. Morley, F. H. W., and Frankel, 0. H. 1959. I n “Ecological Research in Australia” (F. S. Bodenheimer, ed.). The Hague, Junk. Morley, F. H. W., Brock, R. D., and Davem, C. I. 1956. Australian J. Biol. Sci. 9, 1-17. Morley, F. H. W., Davem, C . I., Rogers, V. E., and Peak, J. W. 1961. Proc. Roy. SOC.Victoria (in press). Morrow, J . A., Kileen, N. C., and Bath, J. G. 1948. J. Dept. Agr. Victoria 46, 13-20. Mullett, H. A. 1922. J. Dept. Agr. Victoria 20, 739-761. Nicholas, H. T. 1952. J. Soil C m e r u . N . S . Wales 8, 72-76. Norris, K. R. 1944. Australia, Commonwealth Council Sci. lnd. Research Bull. No. 183.

Nutman, P. S. 1956. Biol. Revs. C u m b d g e Phil. SOC. 31, 109-151. Nutman, P. S. 1961. Australian J. Biol. Sci. (in press). O’hughlin, G. T. 1958. J. Dept. Agr. Victoria 66, 385-393. Orchard, H. E. 1951. J. Dept. Agr. S . Australia 64, 536-540. Ozanne, P. G. 1955. Australian J. Biol. Sci. 8, 344-353. Parish, R., and Dillon, J. L. 1956. Reu. Marketing Agr. Econ. N . S . Wales 24, 74100. Pascoe, W. B. 1946. J. Soil Conserv. N . S . Wales 2, 242-243. Paton, D. F. 1960. J. Australian Inst. Agr. Sci. 26, 32-37. Pearson, A. 1946. Agr. Gaz. N . S . Wales 67, 227-228.

122

F. H. W. MORLEY

Penzhorn, K. E. W. 1944. Famtng in S. Africa 19, 101-104. Pope, G. S., and Wright, H. G. 1954. Chern. G I n d . (London) 33, 1019-1020. Purchase, H. F., and Nutman, P. S. 1957. Ann. Botany (London) [N.S.] 21, 439454. Purchase, H. F., and Vincent, J. M. 1949. Proc. Linnean SOC. N . S . Wales 74, 227238. Purchase, H. F., Vincent, J. M., and Ward, L. M. 1949. J . Australian In&. Agr. S C ~ 16, . 112-117. Quinlivan, B. J . 1958. J. Dept. Agr. W . Australia [3] 7, 553-554. Radel, L. H. 19%. Tasmanian J. Agr. 6, 16-24. Rampton, H. H. 1952. Oregon State College, Agr. Expt. Sta. BuU. No. 432 (revised). Rose, Dona dalle, 1949. Ital. agr. 86, 865-668. Rossiter, R. C. 1952. Australian J. Agr. Research 3, 148-159. Rossiter, R. C. 19%. Australian J. Agr. Research 10, 305-321. Rossiter, R. C., and Pack, R. J. 1958. J . Australian Inst. Agr. Sci. 22, 71-73. Russell, E. J. 1950. “Soil Conditions and Plant Growth,” 8th ed. Longmans Green, New York. Saeki, T. 1960. Botan. Mag. (Tokyo) 73, 55-63. Salisbury, E. J. 1942. “The Reproductive Capacity of Plants.” G. Bell and Sons, London. Saxby, S . H. 1958. New Zealand J . Agr. 92, 518-527. Schinckel, P. G. 1948. Australian Vet. J. 24, 289-294. Scurfield, G., and Biddescombe, E. F. 1959. Nature 183, 1196-1197. Sears, P. D. 1951. “Plant and Animal Nutrition in Relation to Soil and Climatic Factors,” Proc. Speciaclist Canf. Agr., pp. 409-418. H.M.S.O., London. Shapter, R. E. 1935. J. Council Sci. Ind. Research 8, 187-194. Shier, F. L., and Rossiter, R. C. 1949. J. Dept. Agr. W . Australia [2] 26, 111-116. Sims, H. J., and Jardine, R. 1949. “Plant and Animal Nutrition in Relation to Soil and Climatic Factors,” Proc. Specialist Conf. Agr., pp. 438-447. H.M.S.O., London. Smit, N. L. 1943. Funning in S. Africa 18, 133-136. Smith, V. P. Cass, and Holland, A. A. 1958. J. Dept. Agr. W . Australia 131 7, 225231. Stening, H. C. 1933. Agr. Guz. N . S . Wales 44, 360-362. Stephens, C. G., and Donald, C. M. 1958. Advances in Agron. 10, 167-256. Strong, T. H. 1937. J. Council Sci. Ind. Research 10, 12-16. Swan, D. C. 1934. I . Dept. Agr. S. Australia 38, 353-367. Symon, D. 1954. Australian J . Agr. Research 6, 614-616. Thaine, R. 1949. N e u Zealand J . Agr. 78, 51. Tindale, E. 1955. Agr. Grrz. N . S . Wales 66, 649-651. Tiver, N. S. 1957. J. Agr. S. Australia 60, 339-341. Toms, J. 1958. J. Dept. Agr. W . Australia [3] 7, 555-557. Taole, E. H., and Hollowell, E. A. 1939. J. Am. SOC. Agron. 8, 604-619. Trumble, H. C. 1935a. J. Australian Inst. Agr. Sci. 1, 117-119. Trumble, H. C. 193513. 1. Agr. S. Australia 38, 1460-1487. Trumble, H. C. 1937. Trans. Roy. SOC. S. Australia 61, 41-62. Trumble, H. C. 1952. Adwnces in Agron. 4, 1-65. Trumble, H. C., and Donald, C. M. 1938a. Australb, Conimmwealth Council Sci. Id.Research Bull. No. 116. Trumble, H . C., and Donald, C. M. 193813. 1. Australian Inst. Agr. Sci. 4, 206-208.

SUBTERRANEAN CLOVER

123

Trumble, H. C., and Shapter, R. E. 1937a. Australia, Commonwealth Council Sci. Znd. Research Bull. No. 106, 25-36. Trumble, H . C., and Shapter, R. E. 193713. Australia, Commonwealth Council Sci. Ind. Research Bull. No. 106, 37-40. Trumble, H. C., and Strong, T. H. 1937. Australia, Commonwealth Council Sci. Ind. Research Bull. No. 106, 11-24. Underwood, E. J., and Shier, F. L. 1951. Australian Vet. J. 27,63-67. Underwood, E. J., and Shier, F. L. 1952. Australian Vet. J. 28, 157-158. Underwood, E. J., Shier, F. L., and Peterson, S. E. 1953. Australian Vet. J. 29, 206-211. Underwood, E. J., Shier, F. L., Davenport, N., and Bennetts, H. W. 1959. Australian Vet. 1. 36, 84-87. Vincent, J. M . 1954a. Australian J. Agr. Research 6, 55-60. Vincent, J. M. 1954b. Proc. Linnean SOC. N . S . Wales 79, 4-32. Vincent, J. M. 1956. Proc. Intern. Grassland Congr., 7th Congr., Palmerston North, New Zealand, 1956, pp. 179-189. Vincent, J. M. 1958. In “Nutrition of the Legumes” (E. G. Hallsworth, ed.) Butterworths, London. pp. 108-123. Vorster, P. W. 1943. Farming in S. Africa 18, 197-198. Walker, A. J. K., and Neal-Smith, C. A. 1959. J. Australian Inst. Agr. Sci. 26, 18-22. Walker, J. 1956a. Agr. Gaz. N . S. Wales 67, 353-357. Walker, J. 1956b. J. Australian Inst. Agr. Sci. 22, 288-2Ql. Waters, L. M. 1956. 1.Australian Inst. Agr. Sci. 22, 141-142. Watson, M. A. 1949. J. Australian Inst. Agr. Sci. 16, 76-81. Wexelsen, H. 1928. Univ. California (Berkeley) Publs. Agr. Sci. 2, 355-376. Wilkie, D. R. 1946. New Zealand J. Agr. 73, 582-583. Williams, C. H., and Donald, C. M. 1957. Australian J. Agr. Research 8, 179-189. Williams, C.H., and Lipsett, J. 1960. Austmlian J. Agr. Research 11, 473-484. Williams, V. J., and Moir, R. J. 1951. Australian J. Sci. Research Ser. B. 4, 377-387. Williams, W. A., Lenz, J. V., and Murphy, A. H. 1954. Agron. J. 46, 95-96. Willoughby, W. M. 1954. Awtralian J . Agr. Research 6, 157-180. Willoughby, W. M. 1959a. Australian J. Agr. Research 10, 248-268. Willoughby, W. M. 195913. Australian SOC. Animal Production Proc. 2, 42-45. Woodforde, A. H. 19%. Tasmanian J. Agr. 6, 126-127. Wright, S. 1931. Genetics 16, 97-159. Yates, J. 1957. Australian J. Agr. Research 8, 433-443. Yates, J. 1958. Australian J. Agr. Research 9, 754-766. Yates, J., and Brittan, N. H. L. 1952. Australian 1. Agr. Research 3, 300-304, Zohary, D., and Katznelson, J. 1957. Bull. Research Council Israel Sect. B. 6, 284. Zohary, D., and Katznelson, J. 1958. Australian J. Botany 6, 177-182.

This Page Intentionally Left Blank

STUBBLE MULCH FARMING'

. .

T M McCalla and

T. J . Army

Soil and Water Conservation Division. Agricultural Research Service. United States Department of Agriculture. Lincoln. Nebraska. and Fort Collins. Colorado

I. Introduction ................................................ A. Definition of Stubble Mulching ............................ B. Development and Use of Practice ........................... C . Purpose of Review ....................................... I1. Historical ................................................... A. Use of Mulches .......................................... B. Kinds of Mulches ........................................ C . Research and Development ................................ III. Wind Erosion Control ........................................ A. Control of Wind Erosion .................................. B . Mulch .................................................. IV. Infiltration .................................................. A. Reduction of Water Intake ................................ B. Value of Mulch .......................................... V . Water Erosion Control ....................................... A. Runoff .................................................. B . Soil Loss ................................................ VI . Moisture Conservation ........................................ A. Intake and Evaporation ................................... B . Field Results ............................................ VII . Soil Temperature ............................................ A Fluctuations with a Mulch ................................ B. Factors ................................................. C . Biological Effects ......................................... VIII. Soil Structure and Other Physical Properties ..................... A . Aggregation ............................................. B. Porosity and Density ...................................... C . Other Physical Properties .................................. IX. Chemical Effects ............................................ A. Ammonia Losses and Formation ............................ B. Nitrate and Nitrite ........................................ C . Phosphorus .............................................. D. Manganese .............................................. E . Calcium and Potassium ................................... F . Organic Matter ..........................................

.

Page 120 120 127 128 129 129 130 130 132 132 132 135 135 137 138 138 140 144 145

146 150 150 150 153 154 154 155 158 159 159 159 163 163 163 163

1 Contribution from Soil and Water Conservation Research Division, Agricultural Research Service, U. S . Department of Agriculture, Nebraska Agricultural Experiment Station cooperating.

125

. .

. .

T M MCCALLA AND T J ARMY

126

X. Microbial Activity ........................................... A Influence of Fluctuating Environment ........................ B. Numbers ................................................ C . Toxic Condition .......................................... XI. Tillage Equipment and Use ................................... A. Machinery Requirements .................................. B . Seedbed Preparation and Seeding for Small Grains ............ C. Row Crops .............................................. D . Seeding Grasses and Legumes .............................. XI1 Weed Problems ............................................. A . Problem Areas ........................................... B. Control ................................................. XI11. Insects and Plant Diseases .................................... A. Insects ................................................. B. Plant Diseases ........................................... XIV. Crop Yields ................................................. A . Grain .................................................. B. Protein Content and Mineral Composition .................... C Straw .................................................. D. Roots ................................................... XV. Mulch Maintenance .......................................... A Decomposition ........................................... B. Effect of Tillage Equipment ............................... C . Resistant Mulching Material ............................... D Estimating Crop Residues in the Field ...................... XVI Use with Other Conservation Practices .......................... XVII. Summary and Conclusions .................................... A . Accomplishments ......................................... B Needs .................................................. References ..................................................

.

.

.

.

.

.

.

1

.

185 165 188 189 170 170 171 174 175 175 175 178 177 177 177 179 179 179 184 187 187 187 188 190 190 191 191 191 193 194

Introduction

.

A DEFINITIONOF STUBBLEMULCHING The term “stubble mulching.” as used in this review. describes a system of crop residue management using tillage. generally without any soil inversion. usually with blades or V-shaped sweeps. Sweeps vary in width from a few inches to several feet. Straight blades usually are 8 to 18 feet long. The goal of subsurface tillage is to leave a desirable quantity of plant residue on the surface of the soil for erosion control at all times. The residue used is generally that remaining from previous crop or crops. and amounts may vary from a few hundred pounds to several tons per acre. The Soil Conservation Service of the U . S . Department of Agriculture defines stubble mulching as “the managing of plant residues on a yew-round basis in which harvesting. tilling. planting. and cultivation operations are performed in such a way as to keep protective amounts of vegetative material on the surface of the soil until the time of seeding the

STUBBLE MULCH FARMING

127

next crop.” In the Great Plains area of the United States and Canada, stubble mulching generally applies to the use of small grain residue, usually wheat, grown in monoculture, or to a limited extent to sorghum residues in a wheat and sorghum rotation. However, all kinds of crop residues may be used, and the stubble mulch system of farming can be applied to most cropping systems.

B. DEVELOPMENT AND USEOF PRACTICE The development and expansion in the use of stubble mulching was the result of the erosion-conscious era in the 1930’s following in the wake of a disastrous wind erosion period in the Great Plains of the United States and Canada. Anchored crop residues seemed to fulfill the requirement of keeping the soil in place despite the unpredictable and erratic climatic conditions of the Great Plains. Stubble mulching was soon found to be equally as good for water erosion control as for wind erosion control. Its merits for soil erosion control were investigated at a number of locations in the central and eastern parts of the United States. It had long been noted that a soil covered with grass or other dense vegetation seldom had much soil erosion. Likewise, a dense covering of leaves or trees in forested areas gave good soil erosion control. A mulch of plant residues has also been found to protect the soil against the beating action of raindrops and to keep the surface of the soil open. This protective action may increase infiltration over that of a bare soil. Despite this fact, the practice has been somewhat disappointing as a moisture conservation measure. Part of this lack of moisture conservation is due to high potential evaporation and to the large number of small rains characteristic of the Great Plains. The use of crop residues in the stubble mulch system necessitated the development of a complement of machinery to stir the soil for weed control without inversion. Seeding equipment was modified to plant the seed in the soil through a layer of mulch. Even though the stubble mulch system has much merit in the control of soil erosion, its use by farmers in the final analysis depends upon the effects of surface residues on crop yields. Yields with stubble mulching are often reduced, particularly in the more humid areas of the United States. This apparently is due to a number of factors such as: ( 1 ) lack of proper equipment and knowledge of how to manage it; ( 2 ) low soil temperature in early spring, particularly in northern areas of the United States; ( 3 ) weed problems; (4)lower nutrient availability, such as lower nitrate production; and ( 5 ) changes in the microbial status of the soil. In some semiarid areas yields are often increased by stubble mulching.

128

T. M. MCCALLA AND T. J. ARMY

Also, stubble mulching sometimes prevents a crop from being lost by wind erosion. Despite the low yields frequently obtained in subhumid and humid areas, stubble mulching apparently is serving a definite need as a practice with United States farmers. In 1959 the U. S. Soil Conservation Service reported over 20 million acres of stubble mulching with an increase of 3 million acres of stubble mulching above the 1958 figure, as shown in Table I. Stubble mulching is now most extensively practiced in the drier areas of the West and particularly within the Great Plains. TABLE I Amount of Stubble Mulching in Sections of the United States5 Area of United States Northeast Corn Belt Great Plains West

Increased acreage in 1959b ( acres ) 17,327 54,044 2,379,956 822,919

Amount now on land ( acres ) 331,486 315,443 17,590,733 2,125,279

3,274,246

20,362,941

From a report of Soil Conservation Service on 1959 accomplishments. (Courtesy, Soil Conservation Service, U. S. Department of Agriculture.) Fiscal year, July 1, 1958, to June 30, 1959. a

It was soon realized that the use of stubble mulching alone was not enough for good soil conservation. Its use in combination with other proved mechanical and conservation cropping practices was necessary for a more effective job of soil and water conservation.

c.

PURPOSE OF &VIEW

About two decades of research have been completed on the use of stubble mulching. In light of this experience, it is well to take inventory of the available information and appraise the merits and faults of the system, There is still a considerable lack of information on many aspects of stubble mulching. Its influence on insect, disease, and weed control practices has not been thoroughly studied. The effect of the system on the physical and chemical properties of the soil is not fully understood. Improvements continue to be needed in the development and use of tillage and seeding equipment. Incorporation of stubble mulch tillage in a well-balanced conservation system of farming apparently has not been economically evaluated. This review is limited, except for occasional comparative evaluations,

STUBBLE MULCH FARMING

129

to a discussion of crop residues grown in place, left on the surface of the soil during tillage without inversion, and used primarily to protect the land against erosion by wind and water. There will be no attempt to summarize all the literature pertaining to stubble mulching developed in the last 22 years. Only pertinent and illustrative data will be used to describe the areas of application and merits and faults of the stubble mulch system as the authors interpret the data. Where essential technical information is lacking, the deficiency will be called to the reader’s attention. II. Historical

According to Jacks et al. (1955), the English word mulch, which has been used as a noun since the seventeenth century, was probably derived from the German word molsch, meaning “soft, to decay.” This apparently referred to the gardeners’ use of straw and leaves spread over the ground as a mulch. Spreading of a mulch on the surface of the soil has been referred to in English as “mulching” since 1802.

A. USEOF MULCHES Mulches of various types have been used in farming for a long time. Orchards and truck garden crops were probably the first crops to be mulched. Mulches were often several inches thick. The Chinese, many hundreds of years ago, used stone mulches in their agriculture. Early Roman writers appreciated the value of stones on the surface of the soil in moisture conservation (Jacks et al., 1955). On grasslands the dead and living grass serves as a mulch to protect the land against erosion by wind or water. Before the white man came, much of the Great Plains was protected in this manner. Where vegetation was sufficiently dense, effective erosion control was probably attained. However, since the word “Missouri” is an Indian name meaning “big muddy,” the Missouri River apparently carried a lot of eroded soil material under native prairie conditions. This material was probably from three sources: (1) eroding badland areas; ( 2 ) streambank erosion; and ( 3 ) areas denuded by drought, fires, or overgrazing locally by wild animals. Land in forest often has a mulch of leaves on the soil that is effective in reducing erosion. In deserts there are pads of algae with microorganisms on the surface of the soil that help to keep the soil in place. Even desert pavement is effective in erosion control.

130

T. M. MCCALLA AND T. J. ARMY

B. KINDS OF MULCHES Lamb and Chapman ( 1943), at Ithaca, New York, studied the effects of surface stones on erosion, evaporation, soil temperature, and soil moisture. They found stone mulch effective in reducing runoff, soil loss, and evaporation. Soil temperatures in the surface inch of soil under a stone mulch were raised above that of bare soil. Horticulturists have long been interested in the use of mulches such as plant residues, paper, and more recently, plastic materials. Plant residue mulches used in horticultural work generally consisted of heavy amounts of residues amounting to several tons per acre, giving a cover depth of up to 6 inches or more on the surface of the soil. Such mulches kept the soil considerably cooler through the summer and warmer in the winter than bare soil. This amount of carbonaceous material temporarily depressed nitrate formation. However, long use of a heavy mulch resulted in high nitrate formation. Soil structure was greatly improved through increased amounts of larger-sized, water-stable aggregates with mulching. The potash content of the soil was often very high as a result of the potash leached from straw mulches into the soil. Use of paper mulches, particularly black paper, was an effective method of warming the soil because of heat absorption from the sun’s rays. The paper mulches were also effective in controlling weeds. Such mulches have been used in the pineapple industry and with other special crops. The use of clear and colored plastics as mulches to reduce evaporation has been practiced only in recent years. Axmy and Hudspeth (1960) found that clear or translucent plastic covers helped in establishing grass on seedbeds of limited areas, such as earth dams, waterways, and terrace channels. The use of films of chemicals on the surface of the soil to control evaporation in the soil has also been tried (Hanks et al., 1961). The practical and economical value of such treatments is yet to be determined. Other mulches used include glass wool, cloth, metal foil, sugar-cane trash, manures, leaves, peat, litter, and dust mulches. Dust mulches were at one time thought to be important in conserving soil moisture. Natural mulches are snow and volcanic dust. Snow mulch is valuable in protecting crops such as wheat against cold or sudden changes in temperature during the winter. C. RESEARCHAND DEVELOPMENT Use of stubble mulching for economy of operation and for erosion control was attempted at a number of locations in the Great Plains in the early years of the twentieth century. For example, Carter and McDole

.

STUBBLE MULCH FARMING

131

( 1942) reported that “gopher plowing” (leaving residues on the surface of the soil) of summer-fallow land in western United States was used in 1910. Gopher plowing was done with the moldboard of the plow removed. The first intensive research was started at Lincoln, Nebraska, in 1937 by the Nebraska Agricultural Experiment Station in cooperation with the Research Division of the Soil Conservation Service. Dr. F. L. Duley and Professor J. C. Russel were in charge of this research (Duley, 1959). At about the same time, similar work was started in Canada by Noble of Nobleford, Alberta. As a result of Noble’s work, the Noble sweep machine was developed. There was considerable exchange of ideas between the Nebraskan and Canadian groups. About 1941 a number of investigators at several locations in the United States started field work with stubble mulching: notably, Peele in South Carolina (Peele et aZ., 1946), Browning in Iowa ( Browning et d., 1943), McKay in Idaho (McKay and Moss, 1944), Englehorn in North Dakota (Englehorn, 1946), G. M. Horner at Pullman, Washington, and Whiffield in Texas (Zingg and Whiffield, 1957). These stations were primarily involved with the effects of stubble mulching on crop yields. Some of these locations collected data on the influence of stubble mulching on moisture conservation, nitrate formation, and control of water and wind erosion. Machinery development for the system was also of major concern. Improvements in the practice subsequently were made as a result of the work at different experiment station locations. A number of the machinery improvements came from machinery companies interested in stubble mulching. Credit should be given to the Chase Plow Company, Lincoln, Nebraska, for its adaptation of the sweeps used for bindweed eradication into suitable sweeps for stubble mulch farming. The oldest continuously subtilled plots in the United States today are those at Lincoln, Nebraska, now in their 22nd year (Duley, 1960). These plots, in a corn-oats-wheat rotation, serve to illustrate the long-time effects of the system on some chemical, biological, and physical properties of the soil. The results of observations and laboratory studies on these plots showed that the stubble mulch system is not the same as plowing with respect to its effects on the chemical, physical, and biological properties of the soil. At the Lincoln, Nebraska, location additional experiments were started in 1941 to investigate the effects of stubble mulching on the number and kinds of soil microorganisms.

132

T. M. MCCALLA AND T. J. ARMY

111. Wind Erosion Control

A. CONTROL OF WINDEROSION The urgent need for an effective means to control soil erosion by wind stimulated the development of a stubble mulch farming system. Basically there are two methods used to control wind erosion: (1) create a soil condition resistant to erosion; and (2) shelter the soil from wind. The second method of control, among other practices, includes stubble mulching. However, when residues are scarce because of lack of crop production or dissipation of residues due to decomposition or tillage, then attention to soil condition as well as cover is necessary for soil protection (Chepil, 1958). Results to date indicate that stubble mulch farming is the most practical way to control wind erosion.

B. MULCH 1. Amount Field studies in the Great Plains under conditions of severe wind erosion have indicated that varying amounts of residues anchored to the soil are required to reduce wind erosion to a “permissible level.” Chepil (1960) has tentatively set 15 tons per acre per annum of soil loss as the permissible level. Zingg and Whitfield ( 1957), citing work of Chepil and Englehorn in eastern Kansas during March 1950, reported that 1 ton of crop residues fastened to the soil was essential for wind erosion control. In the spring of 1954 Chepil and Woodruf€ (1955), in studies made in western Kansas, found that on cloddy soils as little as 500 pounds of anchored and standing wheat residues gave adequate wind erosion control. For highly erodible soils the quantity required was much greater. Estimates of minimum residue requirements are often at variance because soils of different textures require different amounts of residues (Table 11) for erosion control. Sandy and clay soils require more residues than medium-textured soils such as loam, silt loam, clay loam, and silty clay loam. 2. Factors That Influence Erosion With a portable wind tunnel, Zingg (1950), at Amarillo, Texas, found that the factors responsible for wind erosion of the soil surface were: (1)dry condition of the soil; ( 2 ) roughness of the ground surface; and ( 3 ) the amount of protective vegetal cover either in the form of growing crop or plant residues. Factors that are related to dry structure of the soil are: (1)soil texture; ( 2 ) organic matter; ( 3 ) water-stable structure; (4) soil moisture and raindrop effect; ( 5 ) calcium carbonate; ( 6 ) alkali and

133

STUBBLE MULCH FARMING

other water-soluble salts; and ( 7 ) nature of colloidal fraction. Other major factors that affect wind erosion are surface crust, surface barriers, width of field, wind velocity, and wind direction (Chepil, 1959; Englehorn et al., 1952). TABLE I1 Minimum Amount of Residue Required for Wind Erosion Control on Different-Textured Soils in the Semiarid Regions of the Great Plains6 Minimum amounts of residue requiredb

Kinds of soil

Wheat stubble and growing wheat (lb./acre)

Coarse textured soil (sand, loamy sand, loamy fine sand) Moderately coarse and fine textured soils (fine sandy loam, sandy loam, silty clay, clay) Medium and moderately fine textured soils (verv - , fine sandy loam, loam, silt loam, clay loam, sandy clay loam, silty clay loam)

Sorghum ( lb./acre)

1750

3500

1250

2500

750

1500

0 Soil Conservation Service, U. S. Department of Agriculture, Technical Standards and Specifications. b About twice these amounts are required if the residue is completely flattened.

3. Equation

The relative quantity of wind erosion has been recently expressed by the generalized equation (Chepil, 1959) E = IRKFBWD in which E = relative quantity of erosion; I = soil cloddiness factor; R = crop residue factor; K = ridge roughness equivalent factor; F = soil abradability factor; B = wind barrier factor; W = width of field factor; D = wind direction factor. Erodibility as determined from Z and F is soil erodibility; that based on additional factors R and K measures the relative surface erodibility; and that based on all the above factors measures the relative field erodibility. In addition to the above-mentioned factors, wind velocity and soil moisture also influence wind erosion. The higher the wind velocity and the lower the soil moisture level for a given region, the greater is the quantity of crop residue required. Chepil and Woodruff (1955) and Chepil (1959) have developed a technique on how to measure the relative erodibility for any field from the major measurable factors. This was done with a series of alignment

134

T. M. MCCALLA AND T. J. ARMY

charts and tables. The charts and tables can be used in reverse to determine what conditions and specifically what amounts of residue are necessary to reduce the relative erodibility to any degree under different conditions. 4. Function of Residues The principal function of crop residues maintained on the surface of the soil is to decrease the force of the wind on the soil itself. Zingg (1954) gave quantitative data on the effect of crop residues in reducing the force of the wind on the soil. Field studies of different amounts, kinds, and arrangement of crop residues showed 5 to 99 per cent of the wind action on soil surface could be eliminated by the use of anchored residues. The forces causing saltation movement of erodible materials are reduced when the force of the wind is transferred to plant residue cover. Zingg et al. (1952) found that if rows of sorghum were transverse to the wind, the relative velocity and the drag of the wind on the soil surface were decreased. Thus, soil erosion was decreased. Chepil (1944) and Zingg et al. (1953) indicated that there are several properties of plant residues in addition to kind and weight that are important in wind erosion. Density, height, and orientation of the residues at the surface of the soil are important. Pound for pound, the finer the residue, the more protection it gives to the soil, provided it is equally distributed and anchored. Residue in an upright position shelters the soil better than in a flat position. Long or tall crop residue or stubble is more effective than an equal weight of short residue. Sorghum stubble about 1 foot high is effective under most conditions if there is sufficient density to cause the wind to flow over the stubble rather than through it. The amount of leaves on the stubble is important in increasing the density of the stubble. Wheat stubble for summer fallow usually should not be disturbed until after spring winds in order to maintain maximum cover. Occasionally it may be necessary to undercut wheat stubble in the fall when weed growth is excessive. At the Soil Conservation Experiment Station near Amarillo, Texas, Zingg (1950) found that plots were more susceptible to wind erosion in the spring than they were in the fall. In general, this was due to a decrease in residue cover and plot roughness and to a marked increase in the proportion of soil material of the smaller erodible sizes. Of the plots tested in the spring, the order of erodibility was clean fallow > continuous sorghum > continuous wheat > rotated sorghum > rotated wheat > stubble mulch fallow. When a field has adequate cover of crop residues that are anchored to the soil, good wind erosion control is achieved. As experimental results indicate, arriving at a suitable amount of residues for soil protection is

STUBBLE MULCH FARMING

135

involved and complex. The kind of soil, roughness of surface, amount of residues, and wind velocity must be considered. In general, in the Great Plains area good protection against wind erosion can be obtained with well-placed and anchored residues. The effectiveness of residues is supplemented if the land has a growing plant cover and if stubble is used in conjunction with some other conservation practices such as shelter belts and stripcropping. Wind erosion control is sufficient justification in itself for stubble mulch farming in the Great Plains. IV. Infiltration

A. REDUCTION OF WATER INTAKE When raindrops hit a bare soil, they have a dispersive action (Ellison, 1944, 1947). This is due to two factors: ( 1 ) a dry soil when wetted will slake; and ( 2 ) falling waterdrops beat loose soil particles. A tremendous

FIG.1. A photomicrograph showing a %-inch vertical section of soil. A compact layer is shown at the surface which was due to the effect of raindrops altering the soil structure. This compact layer prevents rapid intake of water and is the principal cause of excessive runoff from bare soils. (From Duley and Kelly, 1939.)

TABLE I11 Comparison of Straw-Mulched Soil and Cultivated Bare Surface on the Rate and Total Intake of Different Soil Types, Lincoln, Nebraskaa

subsoil Sandy, uniform

Treatment

Duration of application ( hours )

Mulched Bare

12.8 4.7

45.83 8.09

28.89 3.48

Infiltration rate at end of application (inches/hour) 0.92 0.32

Knox silt loam

Silty, uniform

Mulched Bare

15.9 9.3

28.52 13.85

24.58 5.90

0.98 0.25

Marshall silt loam heavy subsoil phase ( A-slope)

Silty clay

Mulched Bare

14.0 9.6

28.39 15.33

24.46 6.27

1.20 0.21

Butler silty clay loam

Claypan

Mulched Bare

13.5 7.0

41.14 12.47

21.89 2.74

0.50 0.15

Pawnee clay loam

Claypan

Mulched Bare

10.6 7.0

34.31 10.93

20.17 4.85

0.38 0.29

Dickinson sandy loam

Silty clay

Mulched Bare

6.5 4.0

20.29 7.01

11.12 2.23

0.48 0.24

12.2 6.9

33.08 11.28

21.85 4.25

0.74 0.24

Character of

Soil type Lancaster sandy loam

Average mulched Average bare a

Data from Duley and Kelly ( 1939).

Total water applied ( inches )

Total intake (inches)

xn n

c

137

STUBBLE MULCH FARMING

amount of energy is generated when an inch of rain falls on an acre of soil. The collision of a falling raindrop with a soil aggregate standing in water is much like an explosion. The resulting impact and the splash of the water set in motion by the falling waterdrop may dislodge soil particles from aggregates. Clay, silt, and sand are sorted at the soil surface. Suspended soil particles are either carried away in excess water to lower land, or carried down into the soil with percolating water. When water flows over the surface, the finer particles fill in the spaces around the larger ones to form a relatively nonpervious layer. Duley (1939) indicated that the rain-compacted layer at the surface of the soil has a greater bulk density and contains coarser material and less organic matter than the layer immediately beneath. Consequently, a compact layer is formed at the surface of the soil (Fig. 1) and water infiltration into the soil may be greatly reduced. Duley and Kelly (1939) further pointed out in their studies with small plots, using an artificial rain machine, that the surface condition was the important factor that controlled intake. However, internal barriers, such as heavy subsoils, may also be important in limiting water intake in some soils. B. VALUEOF MULCH When a mulch of protective material is used on the surface of the soil, the forces of the falling waterdrops acting on the surface of the soil are reduced. Only the slaking factor is operative. Consequently, infiltration of water into a mulched soil is maintained at a higher rate for a longer period of time. A bare, dry soil, however, may take water at a rapid rate for a 10- to 20-minute period (Duley and Kelly, 1941). When rains are of a sufficient duration and intensity, a gain in infiltration of water may be obtained on mulched soil as compared to plowed soil (Tables I11 and IV). If a rain is of too high an intensity, even a mulched TABLE IV Average Infiltration in Inches in First 60 Minutes as Influenced by Surface Cover, Urbana, Illinoisa Infiltration Crop and cover

Tune 1941

October 1941

April 1942

Inches per 60 minutes ~~

Soybeans Bare Wheat straw mulch Corn Bare Wheat straw mulch a

0.91 1.66

0.47 1.68

0.47 1.71

0.88 1.70

0.23

0.39

1.56

1.68

Data from Kidder et d. ( 1943).

138

T. M. MCCALLA AND T. J. ARMY

soil may not take in all the water. Runoff will then occur, and if it is of sufficient volume and intensity, serious erosion may result even with a mulched soil. Also, when some soils become wet, water intake may be reduced regardless of whether the surface is mulched. Barnes and Bohmont (1958), in Wyoming, found that the water intake at the end of 1 hour for bare fallow, grassland, and stubble mulch surface condition was 0.3, 1.20, and 2.26 inches per hour, respectively. Research in Arizona also showed that protecting the soil surface either by plants or litter prevented sealing and increased water infiltration (Beutner and Anderson, 1943). McCalla (1943), in emphasizing the importance of mulches on soil structure, showed that a surface mulch is more important than soil organic matter in increasing water infiltration. A subsoil devoid of organic matter and unmulched had an intake of 0.44 inch per hour after 3 hours of sprinkling, while mulched subsoil had an intake of 0.76 inch per hour for the same period of sprinkling. Infiltration in a good topsoil unmulched and mulched was 0.55 and 1.62 inches, respectively, after 3 hours of sprinkling. Citing work on infiltration in a number of locations in the world, Jacks et al. (1955) reported that mulches generally increased water intake into the soil. There seems to be little doubt that when a soil is mulched with plant residues, water infiltration into the soil is increased. V. Water Erosion Control

A. RUNOFF When land is covered with crop residues anchored in the surface of the soil, runoff following rains is usually reduced. Each piece of crop residue acts as a dam to slow down the water running over the soil. The mulch then increases infiltration rate and surface storage. Also, since a protected soil does not seal as rapidly as a bare soil, more water is taken into the soil and less is available for runoff. The decrease in runoff from using mulches at Lincoln, Nebraska; Cherokee, Oklahoma; and Pullman, Washington, is shown in Table V. The stubble mulched field retained ?4 to 1 inch more water than did the moldboard-plowed fields. Van Doren and Stauffer (1943) reported that soybean, cornstalk, and wheat straw mulches were effective in reducing runoff at Urbana, Illinois. Wheat straw was particularly effective as a mulch, and cornstalks provided greater bulk and more complete coverage of the surface than soybean residues. Wheat straw and cornstalks were more effective in reducing runoff than soybean residues. The effectiveness

139

STUBBLE MULCH FARMING

of residues appears related to amount of surface cover and to the persistence of mulch. According to Kidder et al. (1943) mulches of plant residues protect the soil against beating action of raindrops and create small ponds of water on the surface of the soil. As a result of these ponds of water, there are deeper boundary films of water on the mulched as compared to the unmulched soil. Consequently, on a mulched soil, moving water is a greater distance from the ground surface and erosion is reduced. TABLE V Comparative Annual Surface Runoff for Mulched and Moldboard-Plowed Surfaces at 3 Locationsm Average annual runoff when surface was: Period Location Lincoln, Nebraska Cherokee, Oklahoma Pullman, Washington a

Crop sequence

( years )

Mulched (inches )

( inches )

Plowed

Corn, oats, wheat

6

0.70

2.09

Continuous wheat

10

3.92

4.28

Alternate wheat and fallow, with 1 ton per acre residue on surface of mulched plots

10

0.99

1.93

Data from Zingg and Whitfield ( 1957).

The effectiveness of stubble mulching as compared to moldboard plowing may vary considerably with individual storms. Recorded results of two storms at Cherokee, Oklahoma, in 1950, are given in Fig. 2 (Zingg and Whitfield, 1957). With a dry soil, the total runoff from the rain of August 15 was about 5 inch more from plowing than from stubble mulching. In the second storm on August 17, at which time the soil was still wet from the August 15 rain, there was more runoff from the stubble mulching than from plowed land. The totals for both storms, however, showed less runoff from the stubble mulching. Although there was more runoff from the stubble mulching during the second storm, the runoff was delayed by the crop residues. Timing of runoff as influenced by stubble mulching or plowing should be taken into consideration when evaluating the influences of farming practices on flooding of land by small streams. Runoff from plowed land is of the flash-type closely associated with rainfall intensity. Residues on the surface of the soil retard and delay runoff and peaks of runoff are not as sharp as those obtained under plowing. Coover et al. (1953), at Bushland, Texas, applied water to land with

140

T. M. MCCALLA AND T. J. ARMY

different degrees of cover and concluded that runoff would decrease with the use of residue cover. B. SOIL Loss According to Stallings (1953), water causes erosion by detaching soil particles from the soil surface and then transporting them mostly downRAINSTORM AUG. 15,1950 4.

PREVIOUS RAIN 1.47" JULY 31, I950

-

RAIN FALL Role TOtOl

-

RATE OF RUNOFF Plowed Mulched TOTAL RUNOFF Plowed MUlCkd ---

--------

3-

----

3

-

T I M E (hours)

FIG.2. Runoff from plowed and stubble mulched plots at Cherokee, Oklahoma, during two storms (1950), showing contrast where soil was dry at time of storm on August 15 (upper graph) and soil was wet at time of storm on August 17 (lower graph). Runoff data are the composite of 4 plots of 2 to 3 acres for each condition. (From Zingg and Whitfield, 1957.)

141

STUBBLE MULCH FARMING

hill. Water has its most destructive action when the soil is bare. The factors influencing erosion are amount, intensity and duration of rainfall, amount and velocity of surface flow, kind of soil, surface cover, soil slope, and other factors. Soil erosion by water is determined by the complex interaction of these factors mentioned. After soil is dislodged from the surface it is splashed, rolled, slid, or carried in suspension across the surface. These actions are a result of raindrop splash, turbulence of moving water caused by raindrop splash, and moving water. The two important factors in erosion during a rain are the falling raindrops and flowing water. Drop size and rainfall intensity are important in determining the kinetic energy in a falling drop. All but a small part of the kinetic energy of raindrops is expended instantly against the soil. According to Mihara (1951) there is enough kinetic energy in a drop 2.5 mm. radius to raise 46 g. of material 1 cm. He also reported that the kinetic energy increases with intensity at the “rate of 1.2 powers of the intensity.” The use of a mulch can remove much of the raindrop splash effect as a factor in soil erosion by dissipating the drop energy above the soil surface. The flowing water still remains a factor in soil erosion with stubble mulching. However, unless the rain reaches a high intensity for a long time, the soil may take in a considerable amount of water before flow takes place. Subsequently erosion may occur because the water may concentrate in rills and even stubble mulch cannot withstand concentrated water flow. The effectiveness of mulched soil as compared to plowed bare land over a 6-year period at Lincoln, Nebraska, and over a 10-year period at Pullman, Washington, is shown in Table VI. About 1ton per acre of crop residues was present on the surface of the soil on the mulched plots in TABLE VI Comparative Annual Average Amounts of Erosion for Mulched and Moldboard-Plowed Surfaces at 2 Locationsa Soil losses in runoff when surface was: Location Lincoln, Nebraska Pullman, Washington

(I

Crop sequence Corn, oats, wheat

Alternate wheat and fallow, with 1 ton per acre of straw on surface of mulched plots compared with no straw on surface of plowed plots

Data from Zingg and Whitfield (1957).

Period

Mulched

Plowed

( years )

( ton/acre )

( ton/acre )

6

1.26

6.02

10

3.63

17.93

142

T. M. MCCALLA AND T. J. ARMY

these studies. As a result of using mulch, the soil loss was only one-fifth as much as on plowing. The effect of different amounts of mulch on controlling soil loss was shown by results from Pullman, Washington (Zingg and Whitfield, 1957). Mulches at rates of 0, 1, and 2 tons per acre were applied for a 6-year period under sweep tillage methods. Average soil

FIG.3. Upper: Plot on corn land where stalks have been removed. With 9.78 inches of rainfall in the spring there was a loss of 1.95 inches of water by runoff and 10.14 tons per acre of soil by erosion. Lower: On this plot the stalks were returned to the soil. With the same rainfall there was a loss of only 0.51 inch of water by runoff and 0.54 ton per acre of soil by erosion. The bare soil lost nearly 4 times as much water by runoff and 19 times as much soil by erosion as did the plot with the cornstalks. (From Duley and Russel, 1948.)

losses for the period were 10.2, 3.6, and 0.8 tons per acre, respectively, for the different amounts of residues. Reduction of runoff on mulched versus unmulched plots at other locations is shown in Fig. 3 and Table VII. The amount of erosion at Tetonia Experiment Station, Idaho, after thundershowers in 1945, shows the effectiveness of the residues and tillage methods. When the residues were burned, soil loss by erosion was

TABLE VII Soil Erosion Losses bv Treatment Variables. 1944. 1945. 1946. and 1948. at Tetonia Exoeriment Station. Idahoa Rod weed Rod weed Year and Stubble immediately when Spring Fall erosion burned after plowing necessary disk disk Average ( ton/A 1 (ton/A) (ton/A) (ton/A) (ton/A) ( ton/A ) cause Method of plowing 5 2.4 1 1 1 4 Melting snow, Moldboard 0 2 2 2.0 spring, 1944 One-way disk 5 1 0 3 1 1.6 1 3 Modified moldboard 0 0.2 Sweep plow 0 0 0 1 0 1 1 1.2 Modified moldboard, fall 3 1 Average 2 1.2 1.8 1.5 0.2 2.2 Two thunder Moldboard showers, One-way disk total rainfall Modified moldboard 1.64 inches, Sweep plow summer, 1945 Modified moldboard, fall Average

3.9 2.8 3.1 2.9 3.3 3.2

2.2 1.2 1.2 0.9 1.3 1.4

2.1 0.8 1.1 0.5 1.2 1.1

Melting snow, Moldboard spring, 1946 One-way disk Modified moldboard Sweep plow Modified moldboard, fall Average Melting snow, Moldboard 1948 One-way disk Modified moldboard Sweep plow Modified moldboard, fall

2.3 1.8 1.3 1.3 0.4

2.0 1.2 0.8 0.6 0.2

1.8 0.0 0.8 0.0 0.7

~

~

1.4

a

0.7

10 9 12 7 3 8.2

2 0 2 9 6 3.8

2.9 1.9 1.8 1.5 2.0 2.0

2.5 1.3 0.6 0.0 0.3

2.1 0.5 0.9 0.3 0.0

2.1 1.0 0.9 0.4 0.3

0.9

0.8

3m

m r M

0.9

~___

16 15 16 13 13 ~

Average Data from Siddoway et al. (1956).

~

1.0

1.8 1.6 2.1 2.2

2.8 2.5 1.9 1.7 2.0 2.2

3.3 2.3

14.6

22 1 4 20 6 10.6

17 4 11 17 4

13 6 9 13 6

__

-

10.6

9.4

F 4

0

144

T. M. MCCALLA AND T. J. ARMY

highest. Spring and fall disking caused the next highest soil loss. The least soil loss occurred with the rod weeding treatment (Siddoway et al., 1956). Browning et al. (1943), however, showed that plowing with looseground listing gave better erosion control with corn than did stubble mulching at the Clarinda Experimental Farm in Iowa (Table VIII). TABLE VIII The Effect of Cultural Treatments on Loss of Soil and Water from Corn, January 1 to October 30, 1943; Precipitation during Period, 30.11 Inches; Clarinda, Iowa, Experimental Farma Treatment

Soil loss (ton/acre)

Runoff (%)

Plow and loose-ground list Plow and surface plant Subsurface tillage and surface plant

2.3 34.1 9.9

1.0 20.2 24.2

a

Data from Browning et al. (1943).

Stubble mulching resulted in better erosion control than plowing followed by surface planting of corn. Peele et al. (1946), in South Carolina, showed that in 1944-1945 oats following lespedeza as a mulch lost 40 pounds per acre of soil by erosion, while oats following disking of lespedeza lost 2170 pounds. At Wooster, Ohio, the average loss of runoff water on 10 and 12 per cent slope was 4.1 inches on unmulched soil and 1.8 inches on manure-mulched soil. Soil loss by erosion was reduced from 12.2 tons to 0.5 ton per acre by mulching with manure (Borst and Mederski, 1957). Similar results have been reported by Fenster (1960a) from Nebraska and McKay and Moss (1944) from Idaho. In western Nebraska in 1959, soil erosion was 12,630 pounds per acre on black fallow wheat and only 1492 pounds per acre on stubble mulched wheat. In Idaho, stubble mulching reduced soil erosion from 8.7 to 2.0 tons per acre of soil as compared to plowing. All these data show that stubble mulching with adequate amounts of residue affords protection to the land and results in reduced soil erosion by water. Water erosion, however, is not completely eliminated. Thus, it may be necessary to use additional soil conservation measures to control soil erosion adequately. The chief merit of the use of stubble mulching, nevertheless, is control or reduction of water and wind erosion of the soil. VI. Moisture Conservation

The use of mulches has generally been associated with moisture conservation. Many of the results in the literature are from artificially applied mulches, generally at high rates of 50 to 100 tons per acre, and often to

STUBBLE MULCH FARMING

145

a depth of 6 inches or more on the surface of the soil. Such amounts of mulch require a considerable amount of water to wet the residue. Heavy mulches, in regions of high rainfall, may result in moisture conservation. However, with stubble mulch, as usually practiced in semiarid areas, the amounts of residue generally are much less. At rates of 1 to 5 tons per acre of plant mulches, moisture conservation is considerably less than with heavier mulches. A. INTAKE AND EVAPORATION If rains fall at a sufficiently high intensity to exceed the intake rate on plowing, but not to exceed that on stubble mulching, the net results will be a greater intake of water on stubble mulching than on plowing. If the rainfall intensity is lower than the intake rate of the plowed plots, water intake under the two conditions will be the same. A mulch of plant residues is generally considered to slow down evaporation from the soil surface. But according to Jacks et al. (1955), evaporation is reduced by a mulch only where the soil surface is maintained at a high moisture content by frequent rains or a high water table. Russel (1940) also has pointed out that mulches conserve moisture only in periods of frequent rains but are of little value during extended dry periods. Gardner (1959) has recently shown that attempts to limit evaporation by a surface mulch or other treatment may have little long-range benefit over a bare soil unless the lower initial evaporation rate permits greater downward percolation of water. Field and laboratory studies by Army et al. (1961) have shown conclusively that mulches can decrease evaporation significantly from the immediate surface only as long as the soil surface remains wet. After the soil near the surface dries, the rate of waterflow upward within the soil becomes increasingly important in soil moisture losses, and surface conditions, including the effects of mulch, decrease in importance. During a drying cycle of a Pullman silt loam at Bushland, Texas, where mulches were compared with clean tillage, residues on the soil surface had little effect on soil moisture content at soil depths greater than 2 inches. The moisture content of the soil surface (0- to ?&inchdepth) on mulched plots remained high for at least a week following a rain. In view of these recent data, it appears under field conditions that only if rains are frequent can stubble mulching be expected to improve moisture storage materially by reducing evaporation and subsequently increasing depth of water percolation. If the rains are infrequent, the cumulative moisture loss on a mulched soil will lag behind a bare soil but eventually will reach approximately the same moisture content in the total profile. With the high evaporation potential that occurs during the lengthy fallow periods in the Great Plains, residues on the soil

146

T. M. MCCALLA AND T. J. ARMY

surface cannot be expected to increase moisture storage materially by reducing evaporation except during the rare years when rains are of frequent occurrence and above average in amounts. The conclusions, although minimizing the role of mulches in reducing evaporation during the fallow period, do not detract from the value of stubble mulch tillage for runoff and erosion control in the Great Plains. Furthermore, the reduced drying rates of the soil surface that are found with residues improve moisture conditions for germinating seeds and growing crops. In dryland farming, establishment of small-seeded grasses and some small grains is a major problem. Keeping the soil surface wet with a mulch also aids seedling emergence by preventing surface crusting, especially in fine-textured soils.

B. FIELDRESULTS Under field conditions in the dryland farming areas of the West, the storage of additional water under stubble mulching has been rather disappointing, as shown in Tables IX and X from Montana and Idaho. TABLE IX Average Per Cent of Moisture to a Depth of 5 Feet of Soil in Summer Fallow at Seeding Time during Tears 1941 through 1947 and an Average of the 7 Years at Froid, MontanaaJ Per cent soil moisture by years Fallow methodc Sweep S.S. tiller Blade S.S. tiller M.B. plow, D.F. h1.B. plow, R.W. One-way 0

b

c

1941

1942

1943

1944

1945

1946

1947

Avg.

13.3 12.3 11.9 12.3 11.4

11.9 12.5 11.3 10.2 12.1

15.5 14.8 15.6 15.1 14.1

13.3 11.6 12.8 11.4 11.9

13.4 14.8 12.9 16.0 13.2

15.1 14.4 12.2 12.5 13.9

13.0 15.0 13.8 14.2 14.1

13.6 13.6 12.9 13.1 13.0

Data from Aasheim (1949). Samples taken from four locations on each plot; plots run in duplicate. S.S. = subsurface; D.F. = duckfoot; R.W. = rod weeder; M.B. = moldboard.

Russel ( 1940), in Nebraska, has shown good moisture conservation during the spring and late summer (Table XI). There was essentially no moisture conservation during the period of May 24 to August 9. Daniel et al. (1956), with wheat studies in Oklahoma, found that moisture content of soil tilled by plowing, basin listing, or by the stubble mulching method was the same at beginning of treatment, after seeding, after spring growth, or after harvest. Englehorn (1946) reported that soil moisture storage was the same for stubble mulching and plowing under continuous cropping or summer fallow at Langdon and Edgeley, North Dakota. This observation was based on data for 6 years.

147

STUBBLE MULCH FARMING

McKay and Moss ( 1944), in studies in southern Idaho, over a 5-year period, found that tillage did not generally affect moisture conservation. Because of lack of effective rainfall in summer, stubble mulching summer fallow did not result in a moisture increase except in abnormally wet years. Krall et al. (1958) at Havre and Froid, Montana, in stubble mulch TABLE X Inches of Soil Moisture for 6-Foot Profile in the Spring and Fall on Fallow Plots at the Tetonia Branch Experiment Station, Idaho, 1945a.b ~

__

~~

Method of plowing and residue treatment

Moisture spring ( inches )

Moisture fall ( inches )

Moldboard Stubble burned Rod weed immediately Rod weed when necessary Spring disk Fall disk

19.0 18.4 18.3 18.5 17.6

-

16.6 16.8 17.2 17.2 17.0

-

-2.4 -1.6 -2.1 -1.3 -0.4

18.6

17.0

-1.6

19.4 18.2 19.0 18.5 17.7

17.5 17.1 17.8 16.7 17.1

Average

One-way disk Stubble burned Rod weed immediately Rod weed when necessary Spring disk Fall disk Average

Sweep Stubble burned Rod weed immediately Rod weed when necessary Spring disk Fall disk Average 0,

b

Difference ( inches )

-

-

-

18.6

17.2

-1.9 -1.1 -1.2 -1.8 -0.6 -1.4

17.9 19.4 18.0 17.8 18.6

-

16.6 17.3 17.0 16.8 17.5

-

-1.3 -2.1 -1.0 -1.0 -1.1

18.3

17.0

-1.3

-

Data from Siddoway et al. (1956). 13.5 Inches of rainfall received between sampling dates.

studies from 1943 to 1955, found no difference in moisture storage among plow, one-way, or blade tillage methods. Lemon ( 1956), in experiments at College Station, Texas, with mulched and bare plots, failed to show any moisture conservation on the mulched plots. Similar results were found at Bushland, Texas ( Wiese and Army, 1958). Brown ( 1956) found that stubble mulch fallow land stored considerably more water at Hays, Kansas, than plowed or one-wayed fallow during a period when rainfall

148

T. M. MCCALLA AND T. J, ARMY

TABLE XI Comparative Evaporation Losses on Three Summer-Fallow Plots at Lincoln, Nebraska, during Three Intervals of Sampling, and Total Losses for the Summer-Fallow Season from April 23 to September 8, 1938a Moisture disposition Plot description and sampling interval April 23-May 24; rainfall 6.23 inches Subsurface tillage 2 tons straw Contour basin-listing Plowed black fallow May 24-August 9; rainfall 5.60 inches Subsurface tillage 2 tons straw Contour basin-listing Plowed black fallow August %September 8; rainfall 6.07 inches Subsurface tillage 2 tons straw Contour basin-listing Plowed black fallow April 23-May 24 and August 9-September 8; rainfall 12.30 inches Subsurface tillage 2 tons straw Contour basin-listing Plowed black fallow April 23-September 8; rainfall 17.90 inches Subsurface tillage 2 tons straw Contour basin-listing Plowed black fallow a

Rainfall lost by evaporation ( % )

Storage

Runoff

( inches )

( inches )

Evaporation (inches )

4.57 2.39 2.52

Trace 0 Trace

1.66 3.84 3.71

36 62 59

0.05 0.29 -0.34

Trace 0 0.52

5.55 5.31 5.42

99 95 97

5.10 2.27 1.53

Trace 0 1.49

0.97 3.80 3.05

16 63 50

9.67 4.66 4.05

Trace 0 1.49

2.63 7.64 6.76

21 62 55

9.72 4.75 3.71

Trace 0 2.01

8.18 12.95 12.18

46 72 68

Data from Russel (1940).

was unusually heavy (Table XII). Rainfall during September 1955 was 7.53 inches. Prior to the September rainfall, soil moisture storage for the three fallow methods was the same. The additional moisture storage under stubble mulch increased wheat yields. Staple et al. (1960) found that 37 per cent of winter precipitation at Swift Current, Saskatchewan, was conserved in small grain stubble fields and only 9 per cent in bare

149

STUBBLE MULCH FARMING

fallow fields. Rainfall and snowfall were conserved equally well in stubble, but conservation in bare fallow was mostly from rainfall. Bond et al. (1961) reported that standing wheat stubble under some conditions traps blowing snow. Melting of the trapped snow may supply additional increments of moisture. TABLE XI1 Moisture Depths, Stored Moisture, and Fallow Wheat Yields From Three Methods of Fallow. Haw. Kansas. 1955-1956a

Fallow method

Moisture depth 10-31-55 ( inches )

Available stored moisture to 6-foot depth ( inches )

Plow One-way Stubble mulch tillage

26 25 35

6.0 5.3

a

7.1

Wheat yields (bu./acre) 6.7 9.1 13.8

Data from Brown (1956).

Research from the Corn Belt States and other areas of similar rainfall generally shows that stubble mulching improves moisture storage. Verma and Kohnke (1951), in Indiana, indicated that wheat straw, corn stover, and glass wool applied at the rate of 3000 pounds per acre between soybean rows after first cultivation increased the available moisture content of surface and subsurface soil an average of 3.3 per cent over that of an unmulched soil. Alderfer and Merkle (1943), in Pennsylvania, used 5 tons per acre mulch of manure, charcoal, sawdust, oak leaves, straw, pine needles, and corn fodder in 1939 and one-half of this amount in succeeding years on uncropped plots. They found that at no time during the 3 years did the moisture content of the soil drop below 20 per cent. The mulched plots contained more moisture than did the plots with incorporated residues. The use of manure, straw, sawdust, oak leaves, and pine needles as mulching materials resulted in the highest soil moisture content all 3 years. Borst and Mederski (1957), at Wooster, Ohio, over a 5-year period, showed that the percentage of soil moisture to a depth of 6 inches in manure- and straw-mulched soil cropped to corn ranged from 1 to 9 per cent higher than in unmulched soil. Free (1953), at Marcellus, New York, obtained similar results with mulching. Schaller and Evans (1954), in Iowa, did not find any difference in moisture content of soil planted to corn in stubble mulching or plowing. In regions of higher rainfall, stubble mulching seems to be more effective than plowing in storing moisture. Under some conditions in the drier areas, stubble mulching may result in more moisture storage than

150

T. M. MCCALLA AND T. J. ARMY

plowing. However, moisture conservation by stubble mulching in semiarid areas has not been generally increased over that with bare fallow. Ways by which stubble mulching might be used to increase moisture conservation urgently need investigation. VII. Soil Temperature

A. FLUCTUATIONS WITH A MULCH The temperature regime of a soil under a mulch is different from that of a bare soil. At Alliance, Nebraska (Table XIII), the soil under a mulch TABLE XI11 Temperature at 3-Inch Depth, Box Butte County, Alliance, Nebraskaa Plowed High Date March 19-25 March 26-April 1 April 2-8 April 9-15 April 16-22 April 23-29 April 30-May 6 May 7-12 August 22-28 August 29-September 4 September 5-1 1 September 12-18 September 19-25 September 264ctober 1 Q

Low

( O F . )

( O F . )

49 53 42 47 56 56 67 60 82 73 72 75 74 71

40 43 40 41 49 46 54 53 63 59 59 59 57 58

Stubble mulched Range ("F.)

High (OF.)

Low (OF.)

Range (OF.)

9 10 2 6 7 10 13 7 19 14 13 16 17 13

45 47 41 45 54 54 62 58 80 74 74 77 75 72

39 40 40 39 45 45 51 51 68 66 64 62 61 60

6 7 1 6 9 9 11 7 12 8 10 15 14 12

From McCalla (1959) and C. R. Fenster (unpublished data, 1958).

was cooler than a plowed soil in the spring and summer. However, during the latter part of the summer a reversal occurred and the plowed soil became cooler than the mulched soil. This relationship continued through the winter until early spring when the plowed soil again became warmer than the mulched soil. Temperatures did not fluctuate as much under the mulched plot as under the plowed plot, and the range between minimum and maximum temperatures was 0 to 6" F. greater in the plowed soil than in the mulched soil.

B. FACTORS 1. Amount of Mulch Many factors influence the effect of a mulch on soil temperature. Amount and kind of mulch are important. The temperature under an

STUBBLE MULCH FARMING

151

8-ton mulch of straw was as much as 17.7" C. lower at the l-inch depth than in plowed soil at Lincoln, Nebraska, in July (McCalla and Duley, 1946). For a period of 3 to 4 months after the application of a straw mulch at the rate of 2 to 3 tons per acre, soil temperatures were reduced from 3 to 6" C. at the l-inch depth and from 2 to 4" C. at the 4-inch depth. During the winter the soil seldom froze under an 8-ton mulch, while a plowed soil froze to the 1- to 2-foot depth. Jacks et al. (1955) reported that in Minnesota minimum winter temperatures at the %inch depth were 27" F. under a 3-inch peat mulch, 24" F. under a 3-inch mulch of straw, 32" F. under 6 inches of snow, and -2" F. in bare ground. Lemon (1956), in experiments at College Station, Texas, with mulched and bare plots, obtained continuous soil temperature records from July to October 1953. He showed that soil temperatures were lower at the 3-inch depth under a mulch of chopped cornstalks applied at the rate of 10 tons per acre but were slightly increased at the 6- and 12-inch soil depths. He concluded that mulched plots conserved more heat than plowed plots. Englehorn (1946), at Edgeley, North Dakota, found that soil temperature at the l-inch depth between 4 and 5 P.M. on May 30, 1945, was 99.5" F. under plowing, 97.3" under disking, 96.8" under field cultivating, and 88.7" under stubble mulching. Soil temperature was inversely related to amount of mulch present. Borst and Mederski (1957), at Wooster, Ohio, in 1951 and 1952 reported soil temperatures at the 4-inch depth in plots mulched with 4 inches of straw and manure to be 2 to 3" F. lower than in plowed plots throughout the growing season, June through August. In 1955 the temperature differences were only 1 to 2" lower for stubble mulching than for plowing. Verma and Kohnke ( 1951), in Indiana, also lowered soil temperatures consistently by applying a mulch of 3000 pounds of wheat straw, corn stover, or glass wool to soybeans. 2. Decomposition

At Lincoln, Nebraska, a 2-ton mulch of straw will mostly decompose in 6 to 9 months and the temperature differences between plowing and stubble mulching become negligible ( McCalla and Duley, 1946). Soil temperatures of mulched soil lagged behind air temperatures and this difference reached a maximum on clear days between 1 and 2 P.M. Both soil and air temperatures reached the lowest point for the day at about 5 A.M. in the summer. At this time the mulched and plowed soils had about the same temperature. After considerable decay of the mulch

T. hf. MC CALLA AND T. J. ARMY

152

had taken place, midday temperature differences were only a few degrees lower on the mulched soil.

3. Light and Energy Reflection Light reflection appears to be an important factor in influencing the temperature of a soil under a mulch. Bright straw, for example, may reflect up to 80 per cent more light than bare, dark soil (Table XIV). TABLE XIV Light Reflected from Straw Mulch Applied to Plots Fallowed through the Summer, with Straw Applied May 16, 1946a-b

Months

Light reflection from unmulched plot

2 tons

4 tons

8 tons

May June July August

82 82 73 87

52 52 23 23

65 65 33 37

65 68 36 37

a

b

Increase in light reflection due to different applications of straw mulch in tons per acre

Data from McCalla (1947). Values are mean readings by months in candlepower per square foot.

Increasing the amount of mulch present until complete coverage is attained increased light reflection ( McCalla, 1947). However, as a straw mulch becomes dark from weathering, less light and energy are reflected. Lemon (1956) made radiation measurements over a bare soil and one covered with a mulch of 10 tons per acre of chopped cornstalks. Thc amount of radiant energy absorbed by the ground was the same whether the soil was bare or mulched; but slightly before and after sundown, radiation of heat from the bare plots exceeded that from the mulched soil. Lemon concluded that because of emissivity characteristics of the bare and mulched surfaces, the latter acted as a heat pump. This type of action would be reflected in a higher soil temperature on the mulched plot, according to Lemon. Recent field studies by Hanks et al. (1961), at Manhattan, Kansas, have shown that during the early part of the spring and summer season net radiation on straw-covered plots (4 tons of wheat straw per acre) was lower than on the bare soil. At the end of the season, however, net radiation was highest on the straw-covered plots (Table XV). Darkening of the straw with time apparently resulted in greater energy absorption as the fallow season progressed. It is interesting to note that Hanks et al. (1961) found no direct relationship under the conditions of their research between net radiation and evaporation. Apparently the factors within the soil, especially after the surface dried, were

153

STUBBLE MULCH FARMING

more important than incoming energy in determining water losses by evaporation during the fallow season. TABLE XV Influence of Soil Surface Condition on Net Radiationa Radiation in Langleys/day Net radiation

Time May 28-30, 1958 June 17-19, 1958 July 12-14, 1958 August 25-30, 1958 0

Total radiation 683 600 530 563

Check (bare soil) 321 288 300 269

Straw ( 4 tons wheat straw per acre) 283 237 310 277

Data from Hanks et al. (1961).

C. BIOLOGICAL EFFECTS In cooler parts of the United States, such as in the Corn Belt, in the spring of the year the cool temperatures under a mulch may adversely affect plant growth. Van Wijk et al. (1959) measured soil temperatures at the 4-inch depth in Iowa, Minnesota, Ohio, and South Carolina. Their data suggest that early corn growth was decreased by low temperatures of the soil under the mulch in Minnesota, Iowa, and Ohio. In South Carolina the soil temperature was sufficiently high so that the soil temperatures with stubble mulching did not appreciably influence growth rates. The influence of warming a mulch soil on corn yield is shown in Table XVI (Willis et al., 1957). TABLE XVI Corn Yields As Affected by Controlled Soil Temperature in a Mulch in Iowaa

Treatment Bare-unheated Mulched-unheated Bare-heated, 75" F. (thermoregulator setting) Mulched-heated, 75" F. (thermoregulator setting) Mulched-heated, 71" F. (thermoregulator setting)

116.2 114.8

Soil temperatureh (OF.) 73.3 71.6

109.2

79.9

127.4

74.2

118.0

75.9

Yield (bu./acre)

C.V. = 4.16% L.S.D. (0.05) = 9.6 a b

Data from Willis ct al. (1957). Average temperature for the period June 8 to August 16, at the 4-inch depth.

154

T. M. MCCALLA AND T. J. ARMY

To avoid some of the adverse effects of low soil temperature under mulches in northern areas, seeding may be delayed. Southern areas of the United States seem to have less adverse effects of mulching on plant growth due to low soil temperature. Soil temperature without doubt is an important factor in evaluating the effects of stubble mulching on evaporation, microbial activity, nutrient availability, and plant growth. The interrelationships of stubble mulching with soil temperature and the chemical, physical, and biological properties of soil have not been adequately determined. VIII. Soil Structure and Other Physical Properties

A. AGGREGATION Beale et al. (1955), at Clemson, South Carolina, found that waterstable aggregates greater than 0.2 mm. were approximately the same with plow or mulch tillage 1 year after the start of a cover crop and tillage method experiment (Table XVII). During the next 2 years, the TABLE XVII The Effects of Tillage Methods and Cover Crops on Soil Aggregationa Degree of aggregation

Tillage methods Cover crops Mulch Vetch and rye Vetch and rye Plowed L.S.D. (0.05)

1943 39 36 7

1945 45 34 7

1949 44 35 2

1951 45 40 4

Mulch Crimson clover Plowed Crimson clover L.S.D. (0.05)

35 38 9 40

36 32 9 29

39 31 3 22

40 32 5 27

Plowed 5

None

L.S.D. (0.05) (years) 4 6

5 8 9

Data from Beale d al. (1955).

aggregation of mulch-tilled plots of vetch and rye increased considerably and was greater than the aggregation of the plowed vetch and rye plots. In the last 2 years of the experiment the crimson clover mulch-tilled plots had a higher degree of aggregation than the crimson clover cover and the plowed plots. In Illinois a mulch of wheat straw, soybean plants, and cornstalks favored the formation of larger water-stable soil aggregates ( Stauffer, 1946). Dawson (1945) and Stephenson and Schuster (1945) have reported that the percentage of water-stable aggregates can be increased by mulching with plant residues. According to McCalla ( 1959), decomposing mulches increase water stability of soil aggregates at the surface

155

STUBBLE MULCH FARMING

(Table XVIII). However, the surface soil of the subtilled plots may contain more material of sizes easily eroded than do plowed plots. Chepil ( 1955) found that decomposing vegetative matter (wheat straw or green alfalfa), when mixed with the soil, increased the proportion of waterstable aggregates and slightly decreased erodibility of the soil by wind. However, he concluded that far greater protection to the soil from wind erosion could be achieved by keeping residues on the surface. TABLE XVIII Effect of Subsurface Tillage and Plowing on Waterdrop-Stable Aggregates under Five Cropping Systems in the 0- to l-Inch Layera No. of waterdrops per 0.1 gm. of air-dry soil to break down lump Crop rotation Wheat, sweetclover, sorghum Wheat, sweetclover, corn Sweetclover, sorghum, corn Sweetclover, sweetclover, sorghum Sweetclover, sweetclover, corn

Subtilled

Plowed

41

36 34 31

39

35 38 41

33 32

Data from McCalla (1959).

Although stubble mulching generally increases water-stable aggregates, such may not be the case with dry aggregates. Zingg and Whitfield ( 1957) reported that dry-aggregate determinations on experimental plots at Cherokee, Oklahoma, showed stubble mulching had only 39.9 per cent dry clods greater than 0.84 mm. in size as compared to 48.8 per cent for plowing. The dividing line between erodible and nonerodible fractions for mineral soils is generally considered to be about 0.84 mm. (Chepil, 1958). Dry clods greater than 0.84 mm. are considered resistant to wind erosion. Although the results of Zingg and Whitfield (1957) show that plowing increases cloddiness and thereby would tend to decrease erodibility by wind, the loss of protection from burying of the residues by plowing is usually greater than the gain from increased cloddiness. Where residues are meager or lacking, it usually is advantageous to plow or chisel to bring clods to the surface. With the present increase in row crops, such as sorghum, where residues are in short supply and it is difficult to maintain adequate protective cover, clods and residues probably will have to be considered as a unit of protection rather than as separate identities in erosion control.

B. POROSITY AND DENSITY The use of sweeps that do not invert the soil raises the question immediately of whether the physical condition of the surface soil is

156

T. M. MCCALLA AND T. J. ARMY

improved or damaged. Page et al. (1946), in Ohio, reported that the use of sweeps resulted in seedbeds with a lower noncapillary porosity and a higher soil density, as indicated by 26.0 strokes of a penetrometer required to penetrate 3 inches of soil compared with 14.2 strokes for plowed soil (Table XIX). At Lincoln, Nebraska, there appears to be no difference between stubble mulching and plowing with respect to bulk density,

FIG. 4. Undisturbed surface soil, taken August 28, 1959, from corn, oats, and wheat plots in corn, either continuously subtilled or plowed for 20 years, at Lincoln, Nebraska. Upper, subtilled; lower, plowed. Note more open structure and presence of residues on stubble mulch soil. Magnification x 15. (From Turelle and McCalla, 1961.)

specific gravity, total porosity, and compaction ( McCalla, 1959), However, soil structure at the surface of plots continuously subtilled or plowed for 20 years at Lincoln, Nebraska, seem to show that the subtilled plots had a more open structure at the immediate surface than plowed plots, as shown in Fig. 4 (Turelle and McCalla, 1961). Seedbeds prepared with stubble mulching appear as “loose” as those

157

STUBBLE MULCH FARMING

TABLE XIX Physical Properties of Soils under Different Tillage Treatments, End of Season, October 10, 1944, 3- to 6-Inch Dept1ia.h

Tillage treatment Standard Sod plow Rototiller Subsurface Surface only Standard plus mulch

Noncapillary porosity

(%)

(70)

Average number times weight had to be dropped to drive in 3-inch sampler

56.3 56.0 56.9 54.8 49.1 55.8

25.9 24.9 24.2 19.2 14.2 26.9

14.2 14.4 18.8 26.0 25.5 13.0

Total porosity

Data from Page et al. ( 1946). The differences in physical condition resulted entirely from the effects of the preparation before and at planting time. a b

TABLE XX Bulk Density of Plowed and Mulch-Tilled Soils, Sampled August 1949, July 1954, and June 1955a 3-Year averages

Depth (inches) 0-2 2-4 4-6 a

Plowed ( g. icc. ) 1.26 1.28 1.26

Mulch-tilled ( g./cc. 1 1.23 1.34 1.36

Data from Borst and Mederski (1957). TABLE XXI Bulk Density of Soil under Different Tillage Practices, August 1949a.b Depth below surface

Tillage and cropping practice

0 to 3 inches ( g./cc. )

3 to 6 inches (g./cc.)

6 to 9 inches (g./cc.)

9 to 12 inches ( g./cc. )

Continuous wheat One-way Subtillage Moldboard

1.36 1.38 1.41

1.41 1.43 1.52

1.63 1.66 1.73

1.76 1.76 1.78

Wheat on fallow Subtillage One-way

1.42 1.37

1.56 1.50

1.74 1.71

1.83 1.81

Virgin prairie Untilled

1.40

1.51

1.74

1.80

a b

Data from Johnson (1950). Determinations made on oven-dry soil.

158

T. M. MCCALLA AND T. J. ARMY

prepared by plowing. Borst and Mederski (1957), in Ohio, and Johnson (1950), in Texas, in measuring the bulk density of plowed and mulchtilled soils also found very little difference due to tillage systems (Tables XX and XXI). In the Texas study, soils of continuous wheat culture had about the same bulk density whether one-wayed or subtilled and were less dense than the virgin prairie or moldboard-plowed soils. The prairie and moldboard-plowed soils were similar in bulk density. The use of subtillage equipment apparently did not result in the formation of a plow sole in this study. Unpublished data from Earl Burnett at the Big Spring Experiment Station, Big Spring, Texas, indicate that tillage pans may be found under some conditions with stubble mulching.

C. OTHERPHYSICAL PROPERTIES Schaller and Evans ( 1954), in Iowa, concluded that tillage methods had no effect on total porosity and pores occupied by air at sampling time but did influence pores drained by 50 cm. of water tension. The average percentage of pores drained for plowing and stubble mulching was 6.1 and 7.3, respectively. Johnson (1950), in Texas, found that a surface mulch of wheat residues on a fine-textured soil prevented excessive surface crusting. The importance of mulches in crust retardation with subsequent improvement in emergence of small grain and grass seedlings has recently been pointed out by Army et al. (1961). Verma and Kohnke (1951), in Indiana, reported that soils under mulches remain loose and friable throughout the growing season. However, Albrecht and Uhland (1925), in Missouri, found that soil mulched with 2 and 6 tons of straw per acre and unmulched plots were of equal moisture content but the mulched plots were plastic, sticky, and of poor tilth. The soil on the unmulched plot “worked well.” Jacks et al. (1955) indicated that dew formation was increased with a mulch. These authors also reported that some of the beneficial effects of mulching on plant growth were due to the electric-potential difference between soil and atmosphere produced by mulching. Mulching did not greatly affect potential of soil, but the diurnal fluctuations were less on mulched soil. Mulching slightly reduced oxygen content of the soil in Ohio studies (Borst and Mederski, 1957). Stubble mulching appears to change some of the physical properties of some soils. Water-stable aggregates may be increased on stubble mulching over that obtained on plowing. Amount of dry aggregates may decrease on stubble mulching. However, most of the commonly measured

STUBBLE M U L C H FARMING

159

physical properties appear to be about the same on stubble mulching as on plowing. The paucity of results available indicates the need for additional research on the influence of stubble mulching on the physical condition of the soil. IX. Chemical Effects

A. AMMONIALOSSESAND FORMATION Stubble mulching causes changes in the chemical nature of soil that are apparently different from those associated with plowing. Amounts of ammonia in the soil have been reported to be greater under mulching than with plowing (Albrecht and Uhland, 1925). At Columbia, Missouri, these workers found that ammonia nitrogen, as measured from early spring to December, ranged from 10 to 45 pounds per acre on plots mulched with 2 to 6 tons of straw. On comparable unmulched plots ammonia nitrogen ranged from only 2 to 35 pounds per acre. Ammonia loss from stubble mulched land has been studied at Lincoln, Nebraska ( McCalla, 1959). When crop residues such as wheat straw were left on the surface of the soil to decompose, there was no difference in the amount of ammonia losses from stubble mulching or plowing, and the loss in both cases was low. When sweetclover was allowed to decompose on the soil surface, about 5 to 10 pounds per acre of nitrogen as ammonia were lost with stubble mulching, with only traces of loss when sweetclover was plowed under. B. NITRATEAND NITRITE Numerous experiments have shown that the use of crop residues on the surface of the soil generally results in depressed nitrate formation (Scott, 1921; Browning et al., 1943; Gamble et al., 1943; McKay and Moss, 1944; Englehorn, 1946; McCalla and Russel, 1948; Schaller and Evans, 1954; Jacks et al., 1955). Typical results are shown in Tables XXII and XXIII. The immediate and intense depression of nitrate as obtained when residues are plowed under, however, is usually not evident. Apparently the depressive effects of residues are spread over a longer period of time with stubble mulching than is the case with plowing. In general, if nitrate is measured in the field any time in the spring or early summer, nitrate content is 5 to 7 per cent less on stubble mulching than on plowing. If soil samples are taken in the fall or winter, nitrate content may be higher under stubble mulching than with plowing. Higher soil temperatures in the fall under stubble mulching as compared to plowing, as well as stage of decomposition of mulch, may partially

160

T. M. MCCALLA AND T. J. ARMY

account for the increased nitrate content in the fall. McCalla (1959) found that when soils from stubble mulched or plowed plots were brought into the laboratory and incubated under favorable moisture and temperature conditions, there was no difference in nitrate formation. TABLE XXII Influence of Tillage on the Development of Nitrate Nitrogen in the Field Following Different Legumes at Lincoln, Nebraskan Method of handling: residue Legume used

Subtilled

None Annual sweetclover Partridge pea Lespedeza Vetch Biennial sweetclover

64 92 93 98 125 115

Plowed

Lb. of NO,-N, 0 to 6 feet

5

74 90 90 1@4 158 179

Data from McCalla (1959).

TABLE XXIII Nitrate Nitrogen in Pounds per Acre at Time of Planting Winter Wheat in a Corn-Oats-Wheat Rotation in Relation to Disposition of Crop Residues and Manner of Seedbed Preparation, 3 Years (1939, 1941, and 1942) at Lincoln, Nebraskaa.b Crop residues and seedbed preparation

Depth of sampling (feet) 0 to 0.5

0 to 1

0 to 3

0 to 6

Residues Subtilled Disked in Basin listed Plowed under

23.6 25.2 27.8 32.8

35.9 38.6 42.6 52.5

70.9 73.7 91.8 112.1

112.4 123.7 126.5 152.6

No residues Plowed

32.6

53.5

116.0

153.7

a b

Data from McCalla and Russel ( 1943). Values are means of three seasons expressed in pounds per acre.

Johnson (1950), working with Pullman silt loam at Bushland, Texas, reported that large amounts of nitrate nitrogen were produced on summer-fallowed stubble mulched land. Albrecht and Uhland (1925), in Missouri, reported that 2- and 6-ton per acre rates of straw mulch depressed nitrate production. Lack of aeration was suggested as a factor in retarding nitrification. Mooers et al. (1948a, b ) , in Tennessee, using wheat-straw mulch at the rate of 5 tons per acre in lysimeters, found nitrate depressed by mulch. They rejected

STUBBLE MULCH FARMING

161

aeration as the prime factor. Instead, they believed that the soluble organic compounds in the mulch were the prime cause of lower nitrate production. The reduction in nitrate content with stubble mulching, regardless of the causative factors, undoubtedly accounts for some of the adverse effects of stubble mulching on crop yields, particularly under higher rainfall conditions. Applications of nitrogen fertilizer may minimize or overcome yield reductions associated with reduced nitrate supplies. For example, at Cherokee, Oklahoma (M. B. Cox and B. B. Tucker, unpublished data), in 1958 and 1959, under continuous wheat where no nitrogen was applied, wheat yields with clean tillage averaged 10.3 bushels per acre higher than where stubble mulch tillage had been used, i.e., 25.1 versus 35.4 bushels per acre. When 40 pounds of nitrogen were applied, the average difference in grain yields attributable to tillage practices was only 1.7 bushels per acre in favor of clean tillage. Under continuous stubble mulch tillage, 40 pounds of nitrogen increased wheat yields by 15.0 bushels per acre, while with clean tillage the nitrogen increased yields 6.4 bushels per acre. In another experiment in 1960 at Cherokee, Oklahoma (M. B. Cox and B. B. Tucker, unpublished data), using 0, 20, 40, and 80 pounds of nitrogen, the yield of wheat on stubble mulching was 10.2, 14.9, 16.7, and 23.1, and on clean tillage 17.5, 21.2, 21.6, and 22.7 bushels per acre, respectively. Similar results have been obtained for wheat in the Pacific Northwest. At Pendleton, Oregon, nitrogen applications increase wheat yields on stubble mulched land; but for a given rate of application, grain yields have usually been lower with stubble mulching than with plowing (M. M. Oveson, personal communication, 1960). Winterlin et al. (1958) at Lincoln, Nebraska, found that the nitrogen content of plant tissue of corn, oats, and wheat produced under stubble mulch tillage tended to be lower than that produced with plowing. Nitrogen fertilizer largely overcame this effect. Field experiments in Iowa also showed that surface applications of low-nitrogen residue without added nitrogen fertilizer decreased nitrogen uptake and grain yield of corn. When nitrogen fertilizer was added with the residue, yields were increased above the unmulched check (Parker et al., 1957). The amounts of nitrite found in stubble mulched or plowed soil were the same and were low, regardless of the method of tillage, at Lincoln, Nebraska ( McCalla, 1959). No information apparently is available on post-fertilizer application losses of nitrogen with stubble mulch tillage compared to unmulched soils.

TABLE XXIV Effect of Stubble Mulching and Plowing on the Amount of Acid-Soluble and Adsorbed Phosphorus in the Soil at Lincoln, Nebraskaa Sampling depth in inches Stubble mulched Rotation

Oto 1

1 to 3

3 to 6

Plowed 0 to 6

0 to 1

1 to 3

3 to 6

0 to 6

Parts per million of acid-soluble phosphorusb Corn, oats, wheat in corn Corn, oats, wheat in wheat Sweetclover, sweetclover, wheat, oats, corn in wheat

22.8 25.5 24.5

18.2 21.3 22.3

13.4 14.6 17.9

16.6 18.7 20.5

19.4 20.8 18.5

19.1 22.5 18.1

19.5 15.7 17.4

19.3 18.8 17.8

Mean

24.3

20.6

15.3

18.6

19.5

19.9

17.5

18.6

L.S.D. (0.05) between depths L.S.D. (0.05) between mean depths within and between tillaEe

5.8 3.2 Parts per million of adsorbed phosphorusc

Corn, oats, wheat in corn Corn, oats, wheat in wheat Sweetclover, sweetclover, wheat, oats, corn in wheat

5.5 6.9 7.1

4.8 5.1 5.9

3.2 4.6 3.6

4.1 5.1 5.0

4.7 4.7 4.8

4.1 4.8 4.7

3.9 5.0 4.3

4.1 4.9 4.5

Mean

6.5

5.2

3.8

4.7

4.7

4.5

4.4

4.5

L.S.D. (0.05) between depths L.S.D. (0.05) between mean depths within and between tillage a b c

1.1 0.6

Data from Withee and McCalla (1954). 0.1 N HC1 was used for extraction. 0.5 N ammonium fluoride solution, adjusted to pH 7.0, was used for extraction.

STUBBLE MULCH FARMING

163

C. PHOSPHORUS Extractable phosphorus reportedly is increased with stubble mulching. Withee and McCalla (1954) found a significant increase in acidsoluble and adsorbed phosphorus in the surface inch of soil with stubble mulching as compared with plowing (Table XXIV). In the surface 6 inches of soil, however, there was no difTerence in extractable phosphorus. At Amarillo, Texas, Johnson (1950) reported only a slight increase in soluble phosphorus in the top 3 inches of the soil under stubble mulching as compared to plowing. He concluded that tillage caused no “noteworthy differences” in soluble phosphorus on the Pullman soil. Zingg and Whitfield (1957) in reviewing limited results from Stillwater, Oklahoma, and Lincoln, Nebraska, concluded that “available phosphates tend to be concentrated nearer the surface on stubble mulched plots than on plowed plots.” They postulated that early seedling growth and root development of wheat will respond to this increase in available phosphorus with stubble mulch tillage. D. MANGANESE Jacks et al. (1955) indicated that the use of a mulch on soil with excess manganese would reduce the soluble manganese su5ciently SO that crops would grow. Sherman and Fujimata (1946), on Hawaiian soils, found that the use of a mulch which lowered the surface soil temperature and maintained a moist soil to the surface decreased the amount of exchangeable manganese in the soil to about one-fifth of that in the bare soil. Timonin (1946), at the Central Experiment Farm at Ottawa, Canada, found that an application of a straw mulch 2 inches thick resulted in a denser population of manganese-oxidizing and cellulose-decomposing organisms, more severe symptoms of manganese deficiency of oats, and lower yield of grain than on untreated soil. E. CALCIUM AND POTASSIUM Mulches have no apparent material effect on the soluble calcium in the soil (Stephenson and Schuster, 1945).Stephenson and Schuster ( 1945, 1946), in Oregon, reported that on a plot mulched with straw to a depth of 6 inches for a 5-year period, soluble potassium in the soil increased. However, Schaller and Evans (1954), in Iowa, stated that potassium available to plants was frequently lower with stubble mulching than with plowing. F. ORGANIC MATTER Soil samples taken at Lincoln, Nebraska, from a corn-oats-wheat rotation were analyzed for readily oxidizable organic matter, per cent carbon,

164

T. M. MCCALLA AND T. J. ARMY

and nitrogen (Norstadt and McCalla, 1960). Table XXV shows that readily oxidizable organic matter, carbon, and nitrogen were slightly higher in the surface inch of the subtilled plots than on the plowed plots. The same trends were evident at the lower depths, but differences due to tillage methods were less. Calculated values for the carbon-nitrogen ratios were not different for the subtilled and plowed plots, thus suggesting that no substantial change had taken place in the nature of the soil organic matter insofar as this ratio may indicate. TABLE XXV The Effect of 17 Years of Stubble Mulching on the Carbon and Nitrogen Content and Carbon:Nitrogen Ratios of Soil in a Corn-Oats-Wheat Rotation at Lincoln, Nebraskan Sampling depth in inches Tillage

0 to 1

1 to 6

6 to 12

0 to 12

Readily oxidizable organic matter (milliequivalents per gram of soil) Plowed Subtilled

4.92 5.39

4.96 5.04

Plowed Subtilled

1.84 1.99

1.89 1.90

3.88 4.06

4.42 4.58

Per cent carbon

1.31 1.37

1.60 1.64

Per cent nitrogen Plowed Subtilled

0.163 0.171

0.164 0.166

0.128 0.135

0.146 0.151

Carbon :nitrogen ratio Plowed Subtilled a

11.3 11.5

11.4 11.4

10.3 10.3

10.8 10.9

Data from Norstadt and McCalla (1960).

Recent work by L. K. Porter and B. A. Stewart (unpublished data, 1960) with soil from the Lincoln, Nebraska, corn-oats-wheat rotation plots, however, indicates that organic nitrogen fractions of the soil may not be entirely similar with stubble mulching and with plowing. The amino acid fraction (determined by subtracting the ammonium N released during hydrolysis in 6 N HC1 for 9 hours at 95" C. from the total N released during hydrolysis) in the 0- to 3-inch depth of soil after 21 years of cropping was 748 p.p.m. on the stubble mulched plots as compared to 684 p.p.m. on the plowed plots. Tillage effects were statistically significant at P = 0.01. Tillage had no effect on N fractions insoluble in 6 N HC1 or on ammonium released on acid hydrolysis using 6 N HCl at 95" C. for 9 hours. There were no measurable differences in the N fractions in the 3- to 6-inch depth of the stubble mulched and plowed plots.

165

STUBBLE MULCH FARMING

The higher organic matter content with stubble mulching as compared with plowing has been reported by several workers (Stephenson and Schuster, 1945; Beale st al., 1955; Johnson, 1950; Moody et al., 1952; Zingg and Whitfield, 1957). The higher organic matter content of the stubble mulch soil may in effect be a less rapid decline in organic matter rather than a build-up in many instances. The less rapid decline or build-up in organic matter with stubble mulching is probably the result of the residues decomposing at the soil surface at a reduced rate. Results from Clemson, South Carolina, are illustrative of the increase in organic matter in a mulch-tilled soil (Table XXVI) (Beale et al., 1955). With a cover crop, organic matter TABLE XXVI Effect of Tillage Methods and Cover Crops on Soil Organic Matter Content at Clemson, South Carolinam Percentage of soil organic matter

Tillage methods

Cover crops

L.S.D. (0.05) ( years )

1942

1945

1949

1951

Mulch Vetch and rye Plowed Vetch and rye L.S.D. (0.05)

1.50 1.20 0.58

1.76 1.39 0.44

1.89 1.55 0.49

2.06 1.61 0.45

Mulch Crimson clover Plowed Crimson clover L.S.D. (0.05)

1.59 1.64 0.74

1.72 1.51 0.57

1.84 1.43 0.64

1.95 1.79 0.59

0.52 0.74

Plowed

1.20

1.24

1.13

1.23

0.91

None

0.41 0.57

-

Data from Beale et nZ. (1955).

and nitrogen content increased significantly in a 4-year period. Organic matter and nitrogen content of the clean-tilled soil without a cover crop did not increase as much. At Newell, South Dakota (Zingg and Whiffield, 1957), the organic matter in the top 12-inch portion of the soil profile averaged 1.04 per cent and 0.92 per cent for mulched and plowed plots, respectively. Unless residue amounts are exceedingly high, stubble mulching apparently results in only a slightly higher soil organic matter content in the surface inch of the soil than with plowing. X. Microbial Activity

A. INFLUENCE OF FLUCTUATING ENVIRONMENT

The use of a mulch on the soil surface reduces diurnal and seasonal fluctuations in soil temperatures. Similarly, a mulch as compared to clean tillage tends to keep the soil surface wet longer after a rain. These and

166

T. M. MCCALLA AND T. J. ARMY

other alterations of the microenvironment of the surface soil by stubble mulching affect microbial activity. For example, a soil that is alternately dried and wetted will produce about twice as much carbon dioxide and nitrate as a soil maintained in a nonfluctuating moisture and temperature condition, Drying reduces the number of microorganisms and increases the formation of spores. When the soil is rewetted, the spores germinate and vigorous new growth takes place. Also, the chemical composition of the soil organic matter apparently changes. All of this results in more carbon dioxide and nitrate being formed (McCalla, 1958, 1959).

B. NUMBERS With stubble mulch tillage most of the population of microorganisms in the soil is often concentrated at the surface adjacent to the residues. Dawson et ul. ( 1948) have shown (Table XXVII) that the numbers of TABLE XXVII Mean Density of Microbial Populations within the Plow-Depth Layer of Soils in Which Residues Were Subtilled and P1oweda.b

Depth 0 to 1 inch 1 to 6 inches

Subtilled 231 227

Plowed

Mean difference

Fungi in thousands per gram of soil 67c 164 226 1

Standard error of mean difference 16 13

Bacteria plus actinomycetes in millions per gram of soil 0 to 1 inch

1 to 6 inches

37 30

21 29

16c 1

4 2

Data from Dawson et al. (1948). Of the twelve tests included, six were obtained from rotation plots, of which one received oats straw, and five received wheat straw. The remaining six observations were from sweetclover plots. Intervals of time ranging from 5 days to 14 months between applications of residue and sampling for microbial analyses are represented. c Significant at the 1 per cent level. a b

bacteria and fungi in the surface inch of soil with stubble mulching were greater than those found with plowing. In the lower depths of the profile, there is very little difference in number of microorganisms with the two tillage systems. The number of denitrifiers also appeared to be higher in the surface inch of mulched soil than on plowed soil. In the 1-to 6-inch depth, however, there apparently was no difference. Gamble et ul. (1943), in Virginia, also found that stubble mulching stimulated the growth of fungi more than plowing in the 0- to 6-inch depth of soil. There was no significant difference in bacterial population

167

STUBBLE MULCH FARMING

due to tillage. Schaller and Evans (1954) showed that differences in microbial numbers between mulched and plowed soils in Iowa varied with the time of sampling. On some sampling dates (June 26-30 and July 16) there was no significant difference in the number of microorganisms due to tillage. On June 10-12 the 0- to 3-inch portion of the profile under a mulch contained 44.3 per cent more organisms than the comparable plowed soil. Dawson ( 1945), in Maryland, showed that neither bacterial nor fungal population of the 0- to 6-inch depth of soil was significantly altered by mulching with 1.5 tons per acre of wheat straw or legume residues. Apparently the influence of stubble mulching on the number of soil microorganisms is variable and is determined by the amount of residue, soil conditions, and environmental factors. Organisms such as earthworms and nematodes are favored by the cool and moist conditions usually more prevalent with stubble mulching, as shown in Tables XXVIII and XXIX. Slater and Hopp (1947) found that mulching the soil through the winter was important in maintaining high earthworm population in soils of Maryland, West Virginia, Ohio, MichiTABLE XXVIII Effect of Mulching Field Plots with Straw on the Number of Earthworms and Their Activity at Lincoln, Nebraska5 Amount of straw mulch (tons/acre)

Earthwormsb per acre (thousands)

Air-dry worm casts producedc (tons/acre)

0 2 4 8

13 103 169 263

1.3 18.7 29.1 41.5

Wormholesd per square foot of area

1 8 18 25

Depth of wormholesd ( inches ) 12 to 18 10 to 12 8 to 10 6 to 10

Data from Teotia at al. ( 1950). Mean of biweekly samples for 13 consecutive months. c Total production of wormcasts for 1 year. d Number and depth of wormholes formed per square foot under various treatments during August, 1948. TABLE XXIX Influence of Stubble Mulching on the Number of Nematodes in Land Seeded to Wheat, September 29, 1955 (Sharpsburg silty clay loam soil)& 0

b

Tillage treatment

Depth 0 to 1 inch

1 to 6 inch

Number per 50 grams of soil Plowed Subtilled 0

Data from McCalla (1939).

55 226

10 31

TABLE XXX Percentage Distribution of the Different Species of Earthwormsa in the Subtilled and Plowed Plots of the Three Rotations During the Different Months of the Year ( 1948-1949)b Sweetclover-corn-corn

Sweetcloverc-sorghum-corn

Subtilled

Month and Year April 1948 May June July August September October November December January 19,9d Februaryd March Mean

Plowed

Subtilled

Sweetclover-wheat-corn-oats Subtilled

Plowed

Plowed

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

70 75 90 90 100 100

25 10 10 10 0 0

5 15 0 0 0 0

65 75 85 100 100 100

15 5 0 0 0

20 20 15

0 0 0 0

75 100 100 95 90 100

10 0 0 5 10 0

15 0 0 0 0 0

30 50

5 0 0 0 0

5

85 90 100 100 100 90

5 5 0 0 0 5

10 5

0

20 25 0 0 0 0

65 50 50

0 0 0

80 75 100 100 95 100

95 100 0 0 0

5 0 0 0 0

0 0 0 0 0

85 90

5 5

10 5

100 100

0 0 0

0 0 0

0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

100 100 0 0 0

0 0 0 0 0

0 0 0 0 0

2

6

_ - -

6

6

5

_ _ -

4

3

5 0

_ _ -

_ _ _

0 0 0

0 0 0 0 0 0 7 5 0 2

0 5

100 100 0 0 9

6

7

6

7

5

1

0

2

1

100

100 80

50

0 0 15

_ - -

95 100 0 0 0

5 0 0 0 0

5 8 1 4 1

0 0 0

5

- - -

7

0

1

2

a Subheads 1, 2, and 3 refer, respectively, to the species Hebdrilus calignosus (Sav.), O ctohium Zacteum (Orley), Diplocardia riparia ( Smith). b Data from Teotia et nl. (1950). c All land was in sweetclover in 1947. d Ground frozen-no worms found to 1-foot depth.

>

3

169

STUBBLE MULCH FARMING

gan, and Wisconsin. Teotia et al. (1950) showed that species of earthworms are also different with mulching as compared to plowing (Table -)* Growth of algae and mosses on mulched soils was more prevalent than on bare soils during cool, moist weather at Lincoln, Nebraska ( McCalla, 1943). Other organisms play a role in decomposing mulches. For example, Jacks et al. (1955) reported that termites would consume a mulch in Uganda, Africa, in 3 months and leave the soil in good tilth. Millepedes were also active in decomposing sugar-cane mulch in the Tropics. C. TOXICCONDITION With stubble mulching, there is evidence of retardation of plant growth. Laboratory studies by McCalla and Duley (1948) showed that when corn kernels were soaked in sweetclover residue and distilled water, inhibition of germination, and top and root growth occurred (Table -1). The toxic substance in sweetclover residues was identiTABLE XXXI Influence on Germination and Growth of Soaking Corn Seeds for 24 Hours with Sweetclover Extracta Parts of plant material

to distilled water Sweetclover Sweetclover Sweetclover Sweetclover Sweetclover Control a b

1:5 1:10 1:20 1:80 1: 100 -

Seedling growth after 3 dayso (length in cm. )

Germination (%)

Tops

Roots

33 52 75 92 87 95

0.3 0.5 0.7 1.8 1.6 2.8

0.8 0.9 1.7 3.8 4.0 6.4

From McCalla and Duley (1948); reproduced from Science with permission. Based on the number of germinated seeds.

fied as coumarin. Microorganisms were also found to produce substances that influenced the direction of root growth and decreased top growth of corn (McCalla and Duley, 1950). Recent studies by McCalla (1959, 1960) have shown conclusively that plant residues contain substances and that microorganisms produce substances that affect the germination and growth of plants. The compounds toxic to plant growth are as yet unidentified.

170

T. M. MCCALLA AND T. J. ARMY

XI. Tillage Equipment and Use

A. MACHINERYREQUIREMENTS To put a system of stubble mulching into operation, it is necessary to have a complement of machinery such as that required in any tillage system. There are several methods by which mulches may be handled in a fallow field so that most of the residue remains on the surface. A V-sweep or blade of varying width or length may be used to lift the soil for loosening and killing weeds (Fig. 5 ) . Ordinarily, in the Great

FIG. 5. Stubble mulch tillage with use of wide sweeps, 4- to 8-foot blade, and rod weeder machine. (Courtesy of Soil Conservation Service, US. Department of Agriculture. )

Plains, if the soil is dry the sweep is operated to a depth similar to plowing depth of about 5 to 6 inches in the first operation and at shallower depths for subsequent operations. In the dry farming areas of the Pacific Northwest where small grain straw production is usually higher than in the Great Plains, it may be necessary to increase the depth of the second sweep operation to avoid serious plugging of the equipment. For the first tillage, lister-bottom plows from which the moldboards and shares have been removed, or modified moldboard plows from which the moldboard has been removed, also have been used ( McKay and Moss, 1944). If unusually heavy residues are present, a disk-type implement may be used

STUBBLE MULCH FARMING

171

which will cover part of the residue and hasten decomposition. Stubble pulverizers or busters may also be used in the first operation with heavy residues (Jacks et al., 1955). Fenster et al. (1958) reported that the following implements work well in keeping residues on the surface of the soil: ( 1 ) sweeps-24 inches wide or wider; ( 2 ) rod weeders with semichisels or small sweeps; ( 3 ) straight-blade machines; ( 4 ) chisel plows; ( 5 ) one-way (generally should not be used more than once, usually the first operation in the spring); and ( 6 ) rod weeders. Draft requirements are generally less for stubble mulching than for plowing at comparable depths of tillage. Browning et al. (1944), at Clarinda, Iowa, on Marshall silt loam soil found the relative labor input and power requirements in the preparation of seedbed and planting were 100 for plowing and 72 for stubble mulching. Daniel et al. (1956) reported that the draft for a plow and blade tiller was 7.20 and 5.67 pounds per square inch, respectively. Promersberger and Pratt (1958), at the North Dakota Experiment Station, in an extensive study of the power requirement of various implements used in summer fallow indicated the power requirements of various machines are influenced by several factors, such as size of machine, depth of operation, travel speed, texture, moisture, and slope of soil. In separate studies they indicated that moldboard plow and field cultivator (sweeps), operating at 5 to 7 inches in clay soils, required 11.6 to 12.6 and 1.8 to 5.4 horsepower hours per acre, respectively. The Noble blade operating 6 to lO?h inches deep required 5.8 to 7.2 horsepower hours per acre. A detailed description of each implement that may be used with stubble mulching and its use in different areas is given by Jacks et al. (1955), Ryerson (1950), Duley (1954), Duley and Russel (1941, 1942a, b ) , Nutt and Peele (1947), and Ackerman and Ebersole (1945). A more recent description of the various types of machinery and their use in stubble mulch farming is given by Fenster (1960b). It is readily apparent that there are many possible ways to accomplish stubble mulching. The essential part of any technique is the preservation of sufficient residues on the surface to ensure adequate erosion control from harvest to harvest. Many tillage implements have been developed that will do a satisfactory job of stubble mulching if properly used. AND SEEDING FOR SMALLGRAINS B. SEEDBED PREPARATION

In areas of limited rainfall it is necessary to leave the land idle for a season to store sufficient moisture for a crop. This practice is called summer fallow. During the summer fallow period, it is necessary to keep the land free of weeds. In stubble mulch farming during summer fallow,

172

T. M. M C C AL L A AND T. J. ARMY

weeds are controlled with implements such as sweeps, rod weeders, oneways, skew treaders, or other appropriate weed control practices that will maintain crop residues on the surface of the soil. Following stubble mulch tillage operations for weed control, either in summer fallow or continuous cropping, the soil may need packing or smoothing for a seedbed. However, evidence from the Pacific Northwest (T. R. Horning and M. M. Oveson, unpublished data, 1960) indicates that in areas where winter rainfall predominates, smoothing or packing of the seedbed is not necessary to obtain satisfactory wheat yields (Fig, 6).

FIG.6. Seedbed at wheat planting time in the fall in the Pacific Northwest. (Courtesy of Soil Conservation Service, US. Department of Agriculture.)

When some packing is desirable, it can be done with a treader which is somewhat similar to a rotary hoe run with the tongues reversed so

that the fingered sprockets pack the soil instead of penetrating and tearing up the mulch-filled soil surface. If there are many weeds, two of the treaders are usually hooked together at a skew and used to destroy weeds (Fig. 7 ) . At the Pendleton, Oregon, Agricultural Experiment Station, treaders have been used successfully in dryland wheat farming to break heavy stubble, spread bunched stubble, break surface crusts, and control small weeds, including cheatgrass ( Bronzus tectorum L. ) . Treaders also have a tendency to break up clods and, therefore, must be used with caution where stubble is light. As soon as a satisfactory seedbed is prepared, free of weeds but provided with a residue cover, the next step is to seed. For small grains it

STUBBLE MULCH FARMING

173

is necessary to use a drill that will plant the seed in a clean furrow through light to heavy crop residues (Fig. 8 ) . There are a number of different kinds of drills that will do this. One drill which is commonly

FIG.7. A skew or mulch treader which is used to pack the soil and to eradicate weeds following the subsurface tiller. It puts the soil in good condition for later planting. It operates through residue without clogging (Duley and Russel photo, 1954).

FIG. 8. Drilling wheat through stubble mulch. (Courtesy of Soil Conservation Service, U S . Department of Agriculture. )

174

T. M. MCCALLA AND T. J. ARMY

used has a stub runner and a disk to part or cut the residues prior to dropping the seed in the soil. In the Pacific Northwest deep furrow drills are used in the heavy mulches usually encountered. Hoe drills also are generally considered more satisfactory than disk drills in the Great Plains. Wide spacing and staggering of standards reduces clogging of drills (Robins and Blakely, 1960).

C. Row CROPS Seedbed preparation for row crops, such as sorghum or corn, requires the same technique as with small grains. Seed is placed in a clean furrow with the residues to the side between drill rows (Fig. 9). Fenster ( 1960b)

FIG.9. Narrow lister for throwing out narrow furrows without covering much of the residue between the rows (Duley and Russel photo, 1954).

planted row crops successfully with a planter equipped with furrow openers or with a lister operated at shallow depth. If a lister is used, it need not scour. The furrows should be deep enough for a clean seedbed in the row, but not deep enough to cover the residues between the rows. Residues should be about 3 to 4 inches from center of row. Poynor (1950) has described a planter with a stub runner for row crops. The first cultivation of row crops may be done with a rotary hoe or skew treader. Later cultivation may be done with sweeps, 18 inches or wider, attached to cultivators to keep the residues on the surface of the soil (Fig. 10). Some weeds may also be kept under control with chemicals.

STUBBLE MULCH FARMING

175

D. SEEDINGGRASSESAND LEGUMES

Grasses and legumes have been seeded in stubble mulching by using a seeder box in front of a treader. Seeds are scattered ahead of the treader, and the treader covers some of the seed. A preferable method of seeding is to use a drill equipped with double furrow openers and depth bands which is capable of handling rough seeds and drilling through stubble. By this method the soil can be kept protected against erosion, and moisture conditions more favorable for germination and emergence can be partially maintained.

FIG.10. Cultivation of corn with cornstalk residue, showing how the land can be tilled for eradication of weeds without burying the residue (Duley and Russel photo, 1945).

XII. Weed Problems

A. PROBLEM AREAS Control of cheatgrass or downy brome (Bromus tectorum L.) is one of the principal weed problems with stubble mulching in the winter wheat areas of the Great Plains area. Because downy brome is a winter annual grass, as is winter wheat, the problem becomes acute with continuous wheat. At Cherokee, Oklahoma, in the subhumid area, serious trouble was encountered with cheatgrass and other weeds in wheat, as shown in Table XXXII. Control of cheatgrass under stubble mulching

176

T. M. MCCALLA AND T. J. ARMY

also has been a serious problem in the Palouse area of Washington and the Columbia Basin of Oregon. Yield reductions usually result when a cheatgrass infestation is not controlled. TABLE XXXII Weed Production in Continuous Wheat with Four Methods of Seedbed Preparation at Cherokee, Oklahomaa Weeds per acre (Ib.)

Method of tillage Stubble mulched Moldboard plowed Listed Basin listed a

254 45

64 60

Data from Daniel et al. (1956).

In certain areas of the United States, other weeds may be more prevalent on stubble mulching than on plowing. Cook and Peikert (1950), in Michigan, found that weed growth in beans and corn following bromegrass was a problem with stubble mulching. The yield of beans following the use of a rotary-type tiller was 21.4 bushels per acre, as compared to 33.4 bushels per acre for conventional cultivation. Weed competition was thought to account for the yield reduction. In eastern Nebraska, crabgrass (Digitnria sanguinalis L. Scop. ), green foxtail (Setaria viridis L a ) ,and sandburs (Ce'iachrus) are a serious weed problem with stubble mulching. Also, volunteer wheat is difficult to kill during wet seasons. Aasheim ( 1949), in Montana, indicated that shallow-rooted weeds were more of a problem to control on stubble mulching than on plowing or one-waying. Green foxtail, wild buckwheat ( Polygonum convolvulus L.), and small Russian thistle (Salsola pestifer) were the hardest to control with stubble mulch tillage. Englehorn (1946) reported that in North Dakota the control of weeds on summer fallow was more difficult with stubble mulch tillage than on fields tilled by other methods. Differences in weed growth in the following crop were not usually apparent. B. CONTROL Weeds are more difficult to control on stubble mulching than plowing because: (1) weed seeds are not buried as with plowing; and ( 2 ) the soil and seedlings are not inverted. Soil remains in contact with the roots and the plants remain upright so that they continue to grow. The longer roots are severed, but this does not necessarily kill the weed. If a rain follows tillage with sweeps, or the soil is wet at the time of tillage, weeds are seldom killed. The proper use of a skew treader, however, can do

STUBBLE MULCH FARMING

177

much to control weeds in a stubble mulch system of farming. The skew treader will remove most of the soil from the root, exposing it to drying. When a crop follows a perennial grass, it is generally necessary to plow before seeding. The use of chemicals combined with stubble mulch tillage for weed control appears to hold promise (Wiese et al., 1960). Krall et al. ( 1958) obtained good results for control of weeds on summer fallowing at Havre and Froid, Montana, with 2,4-D (ester formulation of 2,4-dichlorophenoxyacetic acid) for broad-leaved weeds. Dalapon (sodium salt of ) 2,2-dichloropropionic acid) and Fenuron (3-phenyl-1,l-dimethylurea appeared promising for the control of grassy weeds. However, they withheld recommendations pending information on residual toxicity. Chemical weed control on stubble mulching, however, needs much additional study before practical recommendations can be made. XIII. Insects and Plant Diseases

Only an extremely limited amount of information is available on insects and diseases associated with stubble mulch tillage. A. INSECTS Spawn (1940, 1941), at the South Dakota Agricultural Experiment Station, showed that moldboard plowing was more effective than sweeps for controlling grasshoppers (Table XXXIII). Spawn (1945) further indicated that the order of decreasing effectiveness in limiting grasshoppers were plowing, disking (various types ) ; duckfoot-type subsurface cultivation; listing; and straight blade-type subsurface cultivation. Zingg and Whitfield (1957) indicated that counts of greenbugs (Toxoptera graminum (Rond.) ) were no higher on stubble mulched than on plowed land. Jacks et al. (1955), in reviewing stubble mulch literature, concluded that insect control on stubble mulching did not appear any more difficult than on plowing. Hessian fly damage actually may be reduced with stubble mulch tillage because delayed seeding is made possible. On plowed land it is often necessary to seed early in order to get cover for wind erosion protection.

B. PLANTDISEASES General observation indicates that plant diseases are no more of a problem with stubble mulching than with plowing. However, as reported by Zingg and Whitfield (1957), root rot of wheat was increased with stubble mulching at Cherokee, Oklahoma. Field counts at Cherokee, Oklahoma, showed an average of approximately eight times as many

178

T. M. MCCALLA AND T. J. ARMY

whiteheads in wheat grown on subtilled land as that grown on plowed land (Table XXXIV) . Jacks et nl. (1955) indicated that wheat mosaic damage may be reduced on stubble mulching because seeding may be delayed. Tillage TABLE XXXIII Influence of Tillage on Grasshopper Control. Valentine Sand, Hecla Area, South Dakotaa Fall tillage Tillage method Moldboard plowing Subsurface cult. ( B ) (duckfoot type) One-way disking (Wheatland plow) Double disking One-way disking plus drilling of grain Single disking Double tandem disking Subsurface cult. ( straight-blade type) Plowing with moldboard removed a

Number of tests

Average percentage control

2 1

96.8 83.2

2

72.7

1 1

71.4 67.5

1 1 1

51.1 50.2 4.0

2

No control

Data from Spawn (1941). TABLE XXXIV Whiteheads of Wheat Produced on Plots As Influenced bv Tillaee Methodsa Whiteheads of wheat per acre Method of tillage Stubble mulched Mo1,dboard plowed Listed Basin listed

0

(%) 5.3 0.6 1.2 1.4

From Daniel et d. (1956).

practices where crop residues were left on the soil surface were associated with a decreased incidence of root and stalk rot of corn in Iowa (Parker and Burrows, 1959). After 20 years of research and experience with stubble mulching, disease or insect control problems appear to be no greater than those found with plowing.

STUBBLE MULCH FARMING

179

XIV. Crop Yields

A. GRAIN The early widespread beliefs in the Great Plains that stubble mulching, because of improved moisture conservation, would result in a marked yield increase were soon generally dispelled. Since stubble mulching kept the surface of the soil from compacting, it was thought that moisture conservation would logically follow. This, except in occasional instances, has not been borne out by field experiments. A comparison of seedbed preparation by stubble mulching, plowing, and one-waying (Table XXXV ) showed that stubble mulching produced slightly lower wheat yields than did plowing on an average for the Great Plains. In the Pacific Northwest wheat yields with stubble mulching were markedly reduced when compared to yields with plowing. At Lincoln, Nebraska (Duley, 1960), long-time studies were made comparing stubble mulching to plowing, using wheat and oats in different rotations with and without legumes. Results are shown in Table XXXVI. Oats generally yielded less with stubble mulching than with plowing in all rotations. Wheat yielded about the same under all tillage practices in the rotations with a legume. In the nonlegume rotation, plowing resulted in somewhat better wheat yields. In most cases stubble mulching gave inferior corn yields as compared to plowing. In wet years the reduction in yields with stubble mulch tillage was considerable. In some dry years stubble mulching yielded more than plowing. Differences in yield of wheat on stubble mulched land versus plowed land have been related to climate. Zingg and Whitfield (1957) reported that the ratio of yield from stubble mulching to that from plowing was related to the P-E index (Thornthwaite’s precipitation to evaporation index). This ratio exceeded 1 where the P-E index was less than 32, and was less than 1 when the P-E index exceeded 32 (Table XXXVII). The trend (Fig. 11) is for stubble mulching to yield more than plowing when the climate is of semiarid to arid type and less than plowing when the climate is subhumid to humid.

B. PROTEIN CONTENT AND MINERAL COMPOSITION Because mulches of plant material contain inorganic nutrients, mulches can be expected to have an influence on the composition of plants, Mulches, during decomposition, may also tie up elements which subsequently may be released in a form available to plants. The protein content of wheat (Zingg and Whitfield, 1957) from 7 representative Great Plains locations is shown in Table XXXVIII. At 6 of

TABLE XXXV Summary of Average Annual Yields of Wheat under Various Tillage Methods at 15 Experimental Locationsa Yield of wheat following fallow or in rotation when plots were Location Akron, Colorado St. Anthony, Idaho Hays, Kansas Froid, Montana Havre, Montana Moccasin, Montana Lincoln, Nebraska North Platte, Nebraska Mandan, North Dakota Cherokee, Oklahoma Stillwater, Oklahoma Pendleton, Oregon Newell, South Dakota Amarillo, Texas Pullman, Washington

Period Years

12 14 11 11 12 5 16 4 22 10 12 13 22 11

10 -

Mean

*

Zingg and Whitfield (1957).

Yield of continuous wheat when plots were

Mulched (bu./acre)

Plowed (bu./acre)

One-wayed (bu./acre)

23.2 25.2

23.1 23.5

-

-

23.9

20.5

25.9 17.4 25.2 26.6 16.3

27.5 17.1 26.9 25.1 17.5

19.4 -

-

-

-

21.2

22.0

26.6

16.8 14.4

15.6

15.1

19.3 -

-

-

-

-

-

21.8 31.0 20.1 18.3 28.9 23.3

-

-

-

26.1 16.6

-

21.6 36.1 19.7

21.0 30.2

34.6 -

16.0

24.8

-

22.9

Mulched (bu./acre)

-

11.0 -

16.6

Plowed (bu./acre)

-

-

-

-

-

19.4

One-wayed (bu./acre) 20.6

-

-

9.6

15.1

181

STUBBLE MULCH FARMING

the 7 locations, the protein content was lower with stubble mulching than with plowing. The values, however, are not much different under the two tillage systems. The over-all average of the 7 locations covering TABLE XXXVI Yields of Crops0 on Subtilled versus Plowed Land in Different Rotations, Agronomy Farm, Lincoln, Nebraskab Number of years

Bushels per acre Rotation

Subtilled

Plowed

Corn-oats-wheat Sweetclover 2 years, wheat, corn, oats Sweetclover 2 years, oats, wheat, corn Sweetclover 2 years, corn, corn Sweetclover 2 years, corn, corn Sweetclover, COM, corn Sweetclover, corn, corn Brome alfalfa 3 years, corn, corn, oats, wheat Brome alfalfa 3 years, corn, corn, oats, wheat

44.4 54.6 56.3 59.5 43.3 52.5 49.7

47.9 56.6 57.0 57.6 45.3 52.2 51.9

43.1

43.9

47.8 50.1

49.3 51.3

Corn yields

18 10 10 10 10 12 10 10

9 Mean

Wheat yields

19 10 10 10

Corn-oats-wheat Sweetclover 2 years, wheat, corn, oats Sweetclover 2 years, oats, wheat, corn Brome alfalfa 3 years, corn, corn, oats, wheat

26.1 37.6 32.3

28.1 38.3 31.9

27.1

28.3

30.8

Mean

31.7 Oats yields

18 10 10 9

Corn-oats-wheat Sweetclover 2 years, wheat, corn, oats Sweetclover 2 years, oats, wheat, corn Brome alfalfa 3 years, corn, corn, oats, wheat

Mean a b

30.8 36.5 53.9

32.8 36.1 60.5

29.5 37.7

32.7 40.5

Italic type indicates crop reported. Data from Duley ( 1960).

52 years of data, shows a reduction in protein content of wheat of only 0.6 per cent with stubble mulching. Jacks et al. (1955) reported a variety of effects of mulches upon mineral composition of plants depending upon type and amount of mulching material, stage of decomposition, soil and climatic conditions,

TABLE XXXVII Annual Crop-Season Precipitation, P-E Index, Years of Yield Increase and Decrease, and Average Yield Ratio, under Stubble-Mulch and Clean-Tillaee Methods at 8 Locations"

Location Havre, Montana

Number of Years of record 12

Amarillo, Texas

11

St. Anthony, Idaho Pendleton, Oregon Hays, Kansas Cherokee, Oklahoma Lincoln, Nebraska Pullman, Washington

14 13 11 10 16 10

5

Crop sequences Spring wheat on fallow (Winter wheat on fallow 1 ]Winter wheat, continuous 1 Winter wheat on fallow Winter wheat on fallow Winter wheat, continuous Winter wheat, continuous Winter wheat in 3-year rotation Winter wheat on fallow

Data from Zingg and Whitfield (1957).

Average precipitation, July-June (inches) 11.29

Number Of years when yield was

P-E index 24.56

17.70

24.85

13.74 17.56 24.27 27.01 28.33 21.53

38.49 43.74 46.28 47.92 56.56 69.66

Increased 8

(10 1 9 9 2 5 2 5 0

Decreased 4 0 1 4 11 6 8 11 10

Average yield ratio of stubble mulch to clean tillage 1.05 1.14 1.15 1.07 0.85 0.95 0.75 0.94 0.84

5

183

STUBBLE MULCH FARMING

and kind of plant. According to these authors, one of the most marked effects of heavy mulching on the mineral content of the soil is an increase in available potassium. This is apparently the result of leaching of readily I. 5

0

PULLMAN, WASH. AMARILLO, TEX.

Q

HAVRE, MONT.

0

0 -

I-

*.

2 I S 0 ST.ANTHONY,IDAHO PENDLETON,OREG. 2 0.5 - 0 LINCOLN, NEBR. @

W

F

@

\

+ \ v

e

v H A Y S , KANS. A CHEROKEE,OKLA.

0.f

... A

I

0.7

A

AVERAGE VALUES AT EACH LOCATION

I

0.f

+--ARID & SEMIARID

lo

_._I I

I

1.5

I.

SUBHUMID p

0

0

B

0

B

W

; 0.8

0

0

I

2 0.9

HUM I D --+

1 1 --

0

0

0

V 00

n

B as

0

----o

10

ANNUAL VALUES AT EACH LOCATION I I 16 se P-E

n 0

64

100

INDEX

FIG.11. Graphical plotting of P-E index and wheat-yield ratios, that is, (yield from stubble mulch system) /( yield from clean-tillage system), at several locations in the West, showing: ( A ) average values at each location; ( B ) annual values at each location. (From Zingg and Whitfield, 1957.)

water-soluble potassium compounds from the mulch material. In Iowa, however, stubble mulching on some soils has resulted in potassiumdeficient corn plants (Bower et al., 1944; Lawton and Browning, 1948).

184

T. M. MCCALLA AND T. J. ARMY

Reduced soil aeration with stubble mulch tillage was proposed as the cause for the reduced potassium absorption by corn. Under the subhumid conditions of Nebraska, there was no significant difference in the K, Ca,

Location St. Anthony,

Idaho

Havre, Montana Moccasin, Montana Froid, Montana Lincoln, Nebraska North Platte, Nebraska

TABLE XXXVIII Protein Content of Wheat Grain at 7 Locationsa Protein in grain when surface was Period Mulched Unmulched Crop sequence ( years ) (%I (%I Wheat, fallow

12

14.1

14.3

Wheat, fallow

12

16.4

16.5

Wheat, fallow

5

12.0

12.6

Wheat, fallow

7

14.4

14.9

b

5

13.5

13.4

Continuous wheat; wheat, fallow

4 4

12.0 12.4

12.3 13.2

3

12.8

15.3

13.5

14.1

Newell, South Dakota Continuous wheat Total years of data: Simple averages:

* b

52

-

-

-

From Zingg and Whitfield (1957). Protein values are averages of 28 crop-treatment-year comparisons.

or Mg content of the corn, oats, or wheat due to tillage methods (Table Xxxur). However, the nitrogen content of these crop plants tended to be lower with stubble mulch tillage than with plowing.

c.

STRAW

In summarizing the results from 6 locations in the Western States, as shown in Table XL, Zingg and Whitfield (1957) reported that when wheat was grown in a rotation with corn, oats, and sweetclover, straw production was reduced with stubble mulching at Lincoln, Nebraska. Similarly, at Cherokee, Oklahoma; Newell, South Dakota; and Amarillo, Texas, straw production from continuous wheat was reduced with stubble mulch tillage as compared to plowing. However, there was a slight increase in straw production with stubble mulching as compared to plowing under alternate wheat-fallow at Newell, South Dakota, and

TABLE XXXIX Effect of Tillage Practices and Fertilizer on Nutrient Content (August 24, 1955) and Yields of Corn, Lincoln, Nebraska; Corn follow in^ Wheat after Coma Dry

Tillage and fertilizer treatment Subtilled, no fertilizer Subtilled, fertilizerb Plowed, no fertilizer Plowed, fertilizerb L.S.D. (0.05) between tillage Mean, subtilled Mean, plowed 0

b

weight of 10 plants

Composition in per cent

N 0.93 1.04 1.06 1.15

P 0.21 0.18 0.20 0.18

K 5.12 4.99 5.03 4.99

Ca 0.22 0.22 0.21 0.21

Mg

(g.)

0.14 0.15 0.13 0.14

6218 7182 7770 7830

0.07 0.99 1.11

N.S. 0.20 0.19

N.S. 5.06 5.01

N.S. 0.22 0.21

N.S. 0.15 0.14

N.S. 6700 7800

Data from McCalla (1959). Fertilizer equivalent to 100 pounds of 40+40+0 per acre was applied at planting time.

Corn yield ( bu./acre)

RR

27.4 26.9 28.3

5Q

29.3 N.S.

1 2!0

TABLE XL Wheat-Straw Production and Ratios of Straw to Grain at 6 Locationsa Straw produced Location

Crop sequence

Lincoln, Nebraska

Corn, oats, wheat Oats, sweetclover, wheat Mandan, North Dakota Wheat, fallow Stillwater, Oklahoma 3 years wheat, oats, sweetclover Cherokee, Oklahoma Wheat, continuous Newell, South Dakota Wheat, fallow Wheat, continuous Amarillo, Texas Wheat, fallow Wheat, continuous Total years of data Simple averages a

Data from Zingg and VVhiffield ( 1957).

Ratio of straw to grain when Plots were Mulched Unmulched

Period (years)

(lb./acre)

(lb./acre)

16 9

2573 4276

2856 4558

1.71 1.99

1.78 2.06

15

2120

2125

1.84

1.80

12 10 20

2156 2328 2300 961 2235 1691

2154 2720 2155 1029 2083 1796

1.65 2.69 1.91 1.31

1.66 2.34 1.82 2.09

1.78 1.98

1.93 2.38 1.98

4

7 8 101

-

-

-

-

2294

2386

1.87

.+I

5

E r

5 +I

Y

ti

5

STUBBLE MULCH FARMING

187

Amarillo, Texas. An average of all data covering 101 crop-year comparisons of the 6 locations indicates a small decrease of 3.9 per cent in straw production with stubble mulching as compared to plowing. The ratio of straw to grain in stubble mulching and plowing is given in Table XL. In 6 of the 9 comparisons, the straw-grain ratio (ratio of weight of straw to weight of grain) is lower under the mulched condition than under unmulched. The average for mulched condition was 1.87 compared with 1.98 for unmulched. These figures and relationships of straw production to grain production can be expected to vary considerably with fertility level, variety, and growing conditions.

D. ROOTS Jacks et al. (1955), in a comprehensive review of the literature, concluded that, with heavy mulches, roots tended to accumulate at the surface of the soil and beneath the mulch. The concentration of roots in this layer was attributed to moisture conditions and protection against trampling. Yocum (1937) working with heavy mulches found that root systems of apple trees on heavily mulched plots in Nebraska had a pronounced shallow and lateral development. The main roots under straw were more numerous, smaller in diameter in proportion to length, had less fibrous branches, and were straighter than roots of trees under clean cultivation. Roots and tops of trees under sod mulch were dwarfed in comparison to trees with clean cultivation. Beckenbach and Gourley (1932), in Ohio, found that apple tree roots after 35 years of heavy mulching become very thick in the mulching materials at the surface of the soil. There is no available information in the literature on the effect of stubble mulching on the development of roots of the common crop plants of the Great Plains. XV. Mulch Maintenance

A. DECOMPOSITION Decomposition of crop residues left on the surface of the soil takes place as a result of the action of numerous organisms. Since the amount of crop residues available for protecting the cultivated land is limited, the rate of decomposition and means of speeding it up or slowing it down are of vital interest in the successful management of crop residues for protection of the land. The rate of decomposition is important in the development or maintenance of soil structure and in the production of available nutrients, especially nitrogen. The rate of disappearance of crop residues through decomposition is greatly influenced by temperature, moisture, mineral

188

T. M. MCCALLA AND T.

J. AIIMY

nutrients, chemical composition, state of maturity of residues, and number as well as kind of microorganisms. Results from Lincoln, Nebraska ( McCalla and Duley, 1943), showed that when straw was used as a mulch in the field for 6 months, a 2-ton application of straw lost two-thirds, a 4-ton application lost one-half, and an 8-ton application lost one-third of the added material. After 18 months there was little residue left except with the 8-ton application. When 2 tons of cornstalks were left on the surface, about two-thirds of the material was left after 6 months. After 18 months some cornstalks still remained. Of the fungi decomposing wheat straw in a laboratory study with pure cultures, the genera Penicillium, Fusarium, Alternuria, Hormodendrum, Trichoderma, and Chaetomium as a group decomposed straw more rapidly than Aspergillus, Rhizopus, and some species of Trichoderma (Dawson, 1949). These organisms bring about decomposition when the soil is well aerated. In actual practice the amount of protection afforded the land with stubble mulch tillage is inversely related to the amount of decomposition, while the release of nutrients from plant material is directly proportional to the decay process. From the standpoint of protecting the land, crop residues that are resistant to decomposition are desirable. For the release of nutrients, a rapid decomposition process is desirable. Between these two needs, a compromise must be made.

B. EFFECT OF TILLAGEEQUIPMENT In the management of plant residues for soil and moisture conservation, deterioration of the mulch is inevitable. Unnecessary mixing of the residue with the soil and excessive residue fragmentation with tillage machinery should be avoided (Table XLI). Any tillage operation that causes residue fragmentation hastens decomposition. Running of machinery over residues, fragments the mulch and also hastens decomposition (Table XLII ) . Numerous factors have been reported to affect the conservation of residue. Anderson ( 1953) indicated that at Lethbridge, Canada, stubble height had a definite infiuence on the amount of trash conserved. After using a one-way disk, there was 60 per cent of 12- to 14-inch stubble left as compared to about 35 per cent conservation with 6- to 8-inch stubble. Depth and speed of equipment operation also were important in residue conservation. A 12- to 14-inch stubble tilled at a depth of 3 and 5 inches with a one-way disk had 70 and 40 per cent of residue left, respectively. At high and low speed of operation the residue remaining was 58 and 40 per cent for the 12- to 14-inch stubble. Type of equipment is extremely important in residue maintenance.

189

STUBBLE MULCH FARMING

According to Anderson (1953),after four operations a blade cultivator left as much as 70 per cent of original residues. But with a treader after three operations, only 40 per cent of the residues remained. A heavy chisel after two operations left 70 per cent of the residues. A duckfoot TABLE XLI Residue Reduction with Tillage Equipmenta Expected amount of remaining residues to be covered in each operation

(%I

Type of equipment

10 50 25 20 30 70 50 25 50 25 10 5-10

Blade type, 30 inches or longer Blade and treader Heavy-duty cultivator, shanks 12 inches apart Duckfoot cultivator, shanks 9 inches or less apart One-way disk, 2 to 3 inches in depth One-way disk, 6 to 7 inches in depth Tandem disk Chisels spaced 9 to 12 inches apart One-way disk and tandem disk Flexible disk harrow Sweeps, 32 inches or wider Rod weeder (plain rotary rod) Rod weeder with semichisels or small sweeps

1215

a Data from Anderson ( 1953), Anonymous (1958), Woodruff and Chepil (1958), Fenster ( 1960b).

TABLE XLII The Brittleness of Straw Mulch after Different Lengths of Time of Decay in the Fielda

Kind of straw

Time exposed as mulch (months)

1 Time

5 Times

10 Times

Bright Partly decayed Partly decayed Partly decayed

0 2 3 6

0.0 3.6 6.4 36.0

0.0 21.6 24.8 93.6

7.2 27.2 36.0 97.6

0

Per cent of straws broken when wheel passed over straw

Data from McCalla ( 1944).

cultivator after two operations left 60 per cent of the residues. The oneway disk covered up to 50 per cent of residues with one operation. There were 47, 25, and 12 per cent of the residues remaining after one, two, and three one-way disk operations, respectively. The flexible one-way disk harrow, however, left 20 per cent of residues remaining on the surface after three operations. In Kansas studies (Woodruff and Chepil,

190

T. M. MCCALLA AND T. J. ARMY

1958), a maximum of about 50' per cent of the total wheat straw residue remained on the surface with shallow depth ( 2 inches) or small-angle (43 degrees) one-way tillage after the first operation. This compared to about 85 per cent after the first sweep operation. At the end of winter after a summer fallow operation, three to seven times as much residue remained on the soil surface with subsurface sweep as with one-way tillage. Fenster et al. (1958), in Nebraska, reported that straight blades or wide sweeps may leave 50 to 75 per cent of residues in the surface after four operations. Rod weeders with small sweeps or semichisels will usually leave 40 to 60 per cent of residues after four operations. Straightrod weeders leave about the same amount of residues as wide sweeps or straight-blade machines. Chisel plows may leave 50 to 70 per cent of the residues on the surface after two operations. Narrow sweeps or chisels bury more residues than wide sweeps. The drier the residues and more frequently the ground is worked, the greater the loss of residues. Beale et al. (1955), in South Carolina, found tillage equipment important in the distribution of residue within the surface horizon of the soil profile (Table XLIII). TABLE XLIII Distribution of Undecomposed Vetch and Rye and Other Plant Residues in 0- and 6-inch Soil Depths in Relation to the Various Tillage Methods, 1952a Plant residues in soil depths, tons per acre Tillage methods

0 to 2 ( inches )

( inches )

4 to 6 (inches)

0 to 6 ( inches )

Mulch Plowed Plowed (no cover crop)

2.41 1.41 0.68

1.63 1.38 0.83

0.65 0.67 0.58

4.69 3.46 2.09

a

2 to 4

Data from Beale ct al. ( 1955).

C. RESISTANTMULCHINGMATERIAL In a search for a natural material resistant to decomposition, studies were made of the decomposition characteristics of Carex: filifolia (thread leaf sedge) (McCalla, 1948). From studies of this plant material, the duration of the roots in the field was estimated at 35 to 40 years. Haas (1958) found 500 pounds of this sedge in the upper 12 inches of soil 35 years after it had been plowed under. Corncobs were found to be more resistant than wheat straw, cornstalks, or sorghum stalks.

D. ESTIMATING CROPRESIDUES

IN THE

FIELD

The amount of crop residues in the field may be estimated by comparison with pictures of known amounts of residues (Duley, 1958). Crop

STUBBLE MULCH FARMING

191

residues also may be screened or floated from the soil and weighed ( McCalla et al., 1943). No standardized procedure has been developed to measure accurately the amount of residue under field conditions. Lack of standardization in measuring techniques has without doubt resulted in a considerable amount of confusion and misinterpretation of stubble mulch experimental data. XVI. Use with Other Conservation Practices

The chief merit of stubble mulching appears to be its effect in reducing runoff and erosion by wind and water. Also, its value for snow catch and overwinter moisture storage efficiency should not be overlooked. Because of the inability to keep adequate cover of plant residues or growing crops at all times on the land, it is necessary to use stubble mulching along with other good conservation practices. Stubble mulching should frequently be used in a system of conservation farming with practices such as terraces, contouring, grassed waterways, stripcropping, and other proved cropping systems. When stubble mulching is incorporated into a system of farming employing patterned windbreaks, good cropping and fertilizing practices, cover crops, contouring, and rotations using grass and legumes, a good job of protecting the land against erosion by wind and water usually can be accomplished. In some areas it may be possible occasionally to use stubble mulching alone or with one or two simple conservation practices and do a reasonably good job of soil and water conservation. In some areas of the Pacific Northwest, topography may be such that present types of terraces and contouring may not be practical. Then stubble mulching must be used alone. Even here it can be combined with rotations. Under such circumstances, a great deal of effort and skill are necessary if adequate residue cover is to be maintained on the land at all times. Under some conditions where there is a limited amount of residues it may be necessary to use emergency wind erosion control practices such as chiseling to produce clods which will give the land temporary protection. XVII. Summary and Conclusions

A. ACCOMPLISHMENTS Stubble mulching is a system of farming primarily developed in the arid and semiarid areas of the United States in wheat monoculture. It has been adopted to some extent in other more humid areas. The system refers to tillage of the soil without inversion and the maintenance of crop

192

T. M. MCCALLA AND T. J. A R M Y

residues on the soil surface for protection against erosion by wind and water. Stubble mulching has been demonstrated to be of practical value in reducing soil erosion by wind and water and in resisting surface runoff. Under some conditions the practice of stubble mulching may reduce soil losses from intense rains to only a portion of those occurring with plowing. Corresponding runoff reductions are also obtained. Field studies also have shown that various amounts and types of residues may reduce wind force at the surface of the soil from 5 to 99 per cent. The moisture content of soil profile with stubble mulching may not be greatly different from that with plowing. On the other hand, it is not too difEicult to demonstrate increased soil moisture storage on stubble mulched plots shortly after a substantial rain. Recently seeded or growing crops may be able to take advantage of this increased moisture. In the more humid areas, the use of stubble mulching may often result in greater moisture conservation but this excess moisture may be undesirable. Even under semiarid conditions heavy stubble on land may delay seedbed preparation or fallow initiation in the spring. In the drier regions of the Great Plains stubble mulching usually results either in increased wheat yields or in little change in yield. In more humid areas of the Great Plains and in the Pacific Northwest there may be substantial grain yield decreases with stubble mulching. These yield reductions with stubble mulch tillage are apparently due to decreased nutrient availability, particularly nitrogen, inadequate weed control, or a combination of both factors. The amount of wheat straw produced under stubble mulch tillage also may be slightly less than with plowing. Comparative data for 101 crop-years at 6 locations in the Great Plains of the United States showed a 3.9 per cent decrease in straw yields with stubble mulching. The average ratio of straw to grain for stubble mulching and plowing was 1.87 and 1.98, respectively. These figures could be expected to vary considerably with fertility level, variety, and growing season conditions. The protein content of wheat at 7 locations in the Great Plains, covering 52 crop-year comparisons, showed a reduction in protein content of wheat of 0.6 per cent for stubble mulching as compared to plowing. The use of stubble mulching may decrease nitrate formation as compared to plowing. Organic matter and total nitrogen may be increased slightly in the surface inch of the stubble mulch soil. The concentration of acid soluble and adsorbed phosphorus has been found to be higher in the surface inch of stubble mulch plots. The number of bacteria, actinomycetes, fungi, nematodes, and denitriflers may be slightly higher

STUBBLE MULCH FARMING

193

in the surface inch of stubble mulch soil when adequate residues are present as compared to plowing. Percentage of water-stable aggregates appears to be slightly higher on stubble mulching than on plowing. Percentage of nonerodible air-dry soil aggregates is generally lower on stubble mulching than on plowing. There is not much difference in bulk density of soil due to tillage. Soil temperatures are lower in spring and early summer but higher in fall and winter on stubble mulching than on plowing. Diurnal fluctuation is usually less with stubble mulching. The average temperature difference due to tillage throughout the whole year may not be much different in the two tillage systems. Crop residues are necessary for protection against erosion by wind and water, but their decomposition is essential for the release of nutrients. Thus, a compromise must be made in the use of crop residues. Considerable care must be exercised in residue management in order to retain suitable amounts of crop residues for adequate soil protection at planting time. Cheatgrass and other weeds are a serious problem with stubble mulch tillage in some wheat-growing areas. The incidence of insects and disease with the use of stubble mulching does not appear to present any particular problem. There are many machines available commercially for the application of stubble mulching to the land. Because of the variability of climatic and soils conditions, it is difficult to prescribe standard tillage requirements. B. NEEDS Much more information is needed on the influence of stubble mulching on the chemical, physical, and biological properties of the soil. There is a paucity of information on how to use the system more effectively with other conservation practices. Much more information is needed on how the system affects the forms and availability of soil nitrogen, the availability of minor elements, the ecology of soil microorganisms, and the types of organic compounds produced during the decomposition of crop residues on the surface of the soil. An economic evaluation of stubble mulch tillage should be made to determine accurately the cost of all operations and the capital gain value of conservation farming in the dryland region. More research is also necessary, particularly with amounts of residues, including those of row crops, that can be practically produced and maintained by dryland farmers. Emphasis should be placed on the combined use of clods and residues for soil erosion control. If stubble mulching is to be widely used in humid areas, substantial improvements must be made in it as a practice.

194

T. M. MCCALLA AND T. J. ARMY

REFERENCES Aasheim, T. S. 1949. Montana Agr. Expt. Sta. Bull. 468, 1-38. Ackeman, F. G., and Ebersole, J. C. 1945. Agr. Eng. 26, 249-250. Albrecht, W. A,, and Uhland, R. E. 1925. Soil Sci. 20, 253-267. Alderfer, R. B., and Merkle, F. G. 1943. Soil Sci. SOC. Am. Proc. 8, 79-86. Anderson, D. T. 1953. Agr. Inst. Rev. 8, 13-14. Anonymous. August 1958. Soil Conserv. Sew. Mimeo. U . S . Dept. Agr. Agronomy Workshop, Scottsbluff, Nebraska. Army, T. J., and Hudspeth, E. B., Jr. 1960. Agron. J. 62, 17-22. Army, T. J., Wiese, A. F., and Hanks, R. J. 1961. Soil Sci. SOC. Am. Proc. (in press ) . Barnes, 0. K., and Bohmont, D. W. 1958. Wyoming Agr. Expt. Sta. Bull. 358, 1-20. Beale, 0. W., Nutt, G. B., and Peele, T. C. 1955. Soil Sci. SOC. Am. Proc. 19, 244247. Beckenbach, J. R., and Gourley, J. H. 1932. Proc. Am. SOC. Hort. Sci. 29, 203-204. Beutner, E. L., and Anderson, D. 1943. Agron. J. 36, 393-400. Bond, J. J., Van Doren, C. E., and Army, T. J. 1961. J. Soil and Water Conscro. (in press). Borst, H. L., and Mederski, €1. J. 1957. Ohio Agr. Expt. Sta. Research Bull. 796, 1-19. Bower, C. A., Browning, G. M., and Norton, R. A. 1944. Soil Sci. SOC. Am. Proc. 9, 142-146. Brown, P. L. 1956. Kansas Agr. Expt. Sta. Circ. 346, 17-21. Browning, G . M., Norton, R. A,, and Shedd, C. K. 1943. Soil Sci. SOC. Am. Proc. 8, 424-431. Browning, G. M., Norton, R. A,, Collins, E. V., and Wilson, H. A. 1944. Soil Sci SOC. Am. Proc. 9, 241-247. Carter, L. S., and McDole, G. R. 1942. U.S. Dept. Agr. Farmers’ Bull. 1917, 1-24. Chepil, W. S. 1944. Sci. Agr. 24, 307-319. Chepil, W. S. 1955. Soil Sci. 80, 413-421. Chepil, W. S. 1958. U . S. Dept. Agr. Tech. Bull. 1186, 1-40. Chepil, W. S. 1959. J. Soil and Water Conserv. 14, 214-219. Chepil, W. S. 1960. Soil Sci. SOC. Am. Proc. 24, 143-145. Chepil, W. S., and Woodruff, N. P. 1955. Kansas Agr. Expt. Sla. Circ. 318, 1-11. Cook, R. L., and Peikert, F. W. 1950. Agr. Eng. 31, 211-214. Coover, J. R., Van Doren, C. E., and Whitfield, C. J. 1933. Texas Agr. Expt. Sta. Mkc. Publ. 97, 1-10. Daniel, H. A,, Cox, M. B., and Elwell, H. M. 1956. U . S. Dept. Agr., Agr. Research Service, Production Research Rept. 6, 1-44. Dawson, R. C. 1945. Soil Sci. SOC. Am. Proc. 10, 180-184. Dawson, R. C. 1949. Soil Sci. 67, 467-479. Dawson, R. C., Dawson, V. T., and McCalla, T. M. 1948. Nebraska Agr. Expt. Sta. Research Bull. 165, 1-26. Duley, F. L. 1939. Soil Sci. SOC. Am. Proc. 4, 60-64. Duley, F. L. 1954. Nebraska Ext. Circ. 64-100,3-16. Duley, F. L. 1958. U. S. Dept. Agr. Handbook 136, 1-31. Duley, F. L. 1959. J. Soil and Water Conserv. 14, 7-11. Duley, F. L. 1960. Nebraska Agr. Expt. Sta. Research Bull. 190, 1-53. Duley, F. L., and Kelly, L. L. 1939. Nebraska Agr. Expt. Sta. Research Bull. 112, 1-16.

STUBBLE MULCH FARMING

195

Duley, F. L., and Kelly, L. L. 1941. U.S. Dept. Agr. Circ. 608, 1-30. Duley, F. L., and Russel, J. C. 1941. Soil Sci. SOC. Am. Proc. 6, 484-487. Duley, F. L., and Russel, J. C. 1942a. Agr. Eng. 23, 2. Duley, F. L., and Russel, J. C. 194213. U.S. Dept. Agr. Misc. Publ. 494, 1-16. Duley, F. L., and Russel, J. C. 1948. U.S. Dept. Agr. Farmers’ Bull. 1997, 1-32. Ellison, W. D. 1944. Agr. Eng. 26, 131-136, 181-182. Ellison, W. D. 1947. Agr. Eng. 28, 197-201. Englehorn, C. L. 1946. North Dakota Agr. Expt. Sta. Bull. 341, 1-35. Englehorn, C. L., Zingg, A. W., and Woodruff, N. P. 1952. Soil Sci. SOC. Am. Proc. 16, 29-33. Fenster, C. R. 1960a. Nebraska Agr. Expt. Sta. Quart. 6, 3-4. Fenster, C. R. 1960b. Soil Sci. SOC.Am. Proc. 24, 518-523. Fenster, C. R., Duley, F. L., and Swinbank, J. C. 1958. Nebraska Ext. Circ. 58-123, 1-3. Free, G. R. 1953. Soil Sci. SOC.Am. Proc. 17, 165-170. Gamble, S. J. R., Edminster, T. W., and Orcutt, F. S. 1943. Scil Sci. SOC. Am. Proc. 16, 267-269. Gardner, W. R. 1959. Soil Sci. SOC.Am. Proc. 23, 183-187. Haas, H. J. 1958. Agron. J. 60, 5-9. Hanks, R. J., Bowers, S. A., and Bark, L. D. 1961. Soil Sci. (in press). Jacks, G. V., Brind, W. D., and Smith, R. 1955. Commonwealth Bur. Soil Sci. (Gt. Brit.) Tech. Ccnnm. N o . 49. Johnson, W. C. 1950. U.S. Dept. Agr. Circ. 860, 1-18. Kidder, E. H., Stauffer, R. S., and Van Doren, C. A. 1943. Agr. Eng. 24, 155-159. Krall, J. L., Power, J. F., and Massee, T. W. 1958. Montana Agr. Expt. Stn. Bull. 540, 1-33. Lamb, J., Jr., and Chapman, J. E. 1943. Agron. 1. 35, 567-578. Lawton, K., and Browning, G. M. 1948. Soil Sci. SOC.Am. Proc. 13, 311-317. Lemon, E. R. 1956. Soil Sci. SOC.Am. Proc. 20, 120-125. McCalla, T. M. 1943. Trans. Kansas Acad. Sci. 46, 52-56. hlcCalla, T. M. 1944. Soil Sci. SOC.Am. Proc. 8, 258-262. McCalla, T. M. 1947. Agron. J. 39, 690-696. McCalla, T. M. 1948. Soil Sci. SOC.Am. Proc. 13, 284-286. McCalla, T. M. 1958. J. Soil and Water Conseru. 13, 255-258. hlccalla, T. M. 1959. Nebraska Agr. Expt. Sta. Bull. 453, 1-31. McCalla, T. M. 1960. SOC. Am. Bacteriologists Proc., 30. McCalla, T . M., and Duley, F. L. 1943. Agron. J . 35, 306-315. McCalla, T. M., and Duley, F. L. 1946. Agron. J. 38, 75-89. McCalla, T. M., and Duley, F. L. 1948. Science 108, 163. hlcCalla, T. M., and Duley, F. L. 1950. Soil Sci. SOC. Am. Proc. 14, 196-199. hlccalla, T. M., and Russel, J. C. 1943. Nebraska Agr. Expt. Sta. Research Bull. 131, 1-21. McCaUa, T. M., and Russel, J. C. 1948. Agron. J. 40, 411-421. h/lcCalla, T. M., Duley, I?. L., and Godding, T. H. 1943. Soil Sci. 66, 159-166. McKay, H. C., and Moss, W. A. 1944. Idaho Agr. Expt. Sta. Bull. 266, 1-20. Mihara, Y. 1951. Natl. Inst. Agr. Sci. Bull. Ser. A, 1, 1-51, Tokyo, Japan. Moody, J. E., Lillard, J. H., and Edminster, T. W. 1952. Soil Sci. SOC. Am. PTOC. 16, 190-194. Mooers, C. A., Washko, J. B., and Young, J. B. 1948a. Soil Sci. 66, 307-315. Mooers, C. A,, Washko, J. B., and Young, J. B. 194813. Soil Sci. 66, 399-400. Norstadt, F. A., and McCalla, T. M. 1960. Agron. J . 62, 477-479.

196

T. M. MCCALLA A N D T. J. ARMY

Nutt, G. B., and Peele, T. C. 1947. Agr. Eng. 28, 391-393. Page, J. B., Willard, C. J., and McCuen, G. W. 1946. Ohio Agr. Expt. Stu. Agron. Mimeo. 102, 1-5. Parker, D. T., and Burrows, W. C. 1959. Agron. J. 61, 414-417. Parker, D. T., Larson, W. E., and Bartholomew, W. V. 1957. Soil Sci. SOC. Am. PTOC. 21, 608-612. Peele, T. C., Nutt, G. B., and Beale, 0. W. 1946. Soil Sci. SOC. Am. Proc. 11, 356360. Poyner, R. R. 1950. Agr. Eng. 31, 509-510. Promersberger, W. J., and Pratt, G. L. 1958. North Dakota Agr. Expt. Sta. Tech. Bull. 416, 1-33. Robins, J. S., and Blakely, B. D. 1960. U.S. Dept. Agr. Yearbook, 142-147. Russel, J . C. 1940. Soil Sci. SOC.Am. Proc. 4, 65-70. Ryerson, G. E. 1950. Agr. Eng. 31, 506-508, 510. Schaller, F. W., and Evans, D. D. 1954. Agr. Eng. J. 36, 731-734, 736. Scott, H. 1921. Agron. J. 13, 233-258. Sherman, D. G., and Fujimata, C. K. 1946. Soil Sci. SOC. Am. Proc. 11, 206-210. Siddoway, F. H., McKay, H. C., and Klages, K. H. 1956. Idaho Agr. Expt. Sta. Bull. 262, 1-46. Slater, C. S., and Hopp, H. 1947. Soil Sci. SOC. Am. Proc. 12, 508-511. Spawn, G. B. 1940. South Dakota Agr. Expt. Sta. Ext. Pam. 1, 1-8. Spawn, G. B. 1941. South Dakota Agr. Expt. Sta. Ext. Pam. 2, 1-4. Spawn, G. B. 1945. South Dakota Agr. Expt. Sta. Bull. 379, 1-16. Stallings, J. H. 1953. U.S. Dept. Agr. SCS-TP 118, 1-18. Staple, W. J., Lehane, J. J., and Wenhardt, A. 1960. Can. J. Soil Sci. 40, 80-88. Stauffer, R. S. 1946. Agron. J. 58, 1010-1017. Stephenson, R. E., and Schuster, C. E. 1945. Soil Sci. 69, 219-230. Stephenson, R. E., and Schuster, C. E. 1946. Soil Sci. 61, 219-224. Teotia, S. P., Duley, F. L., and McCalla, T. M. 1950. Nebraska Agr. Expt. Sta. Research Bull. 186, 1-20. Timonin, M. I. 1946. Soil Sci. SOC.Am. Proc. 11, 284-292. Turelle, J. W., and McCalla, T. M. 1961. Soil Sci. SOC. Am. Proc. (in press). Van Doren, C. A., and Stauffer, R. S. 1943. Soil Sci. SOC. Am. Proc. 8, 97-101. van Wijk, W. R., Larson, W. E., and Burrows, W. C. 1959. Soil Sci. SOC. Am. Proc. 23, 428-434. Verma, A. B. S., and Kohnke, H. 1951. Soil Sci. 72, 149-156. Wiese, A. F., and Army, T. J. 1958. Agron. J. 60, 465-468. Wiese, A. F., Bond, J. J,, and Army, T. J. 1960. W e e d s 8, 284-290. Willis, W. O., Larson, W. E., and Kirkham, D. 1957. Agron. J . 49, 323-328. Winterlin, W. L., McCalla, T. M., and Luebs, R. E. 1958. Agron. J. 60, 241-243. Withee, L. V., and McCalla, T. M. 1954. Soil Sci. SOC. Am. Proc. 18, 285-286. Woodruff, N. P., and Chepil, W. S. 1958. Trans. Am. SOC.Agr. Engrs. 1, 81-85. Yocum, W. W. 1937. Nebraska Agr. Expt. Sta. Bull. 96, 1-55. Zingg, A. W. 1950. Soil Sci. SOC.Am. Proc. 16, 11-17. Zingg, A. W. 1954. Trans. Am. Geophys. Union 36, 252-258. Zingg, A. W., and Whitfield, C. J. 1957. U.S. Dept. Agr. Tech. Bull. 1166, 1-56. Zingg, A. W., Woodruff, N. P., and Englehom, C. L. 1952. Agron. J. 44, 227-230, Zingg, A. W., Chepil, W. S., and Woodruff, N. P. 1953. Bur. of Plant Zndusty, Soils and Agr. Eng., Mimeo. U. S . Dept. Agr., 1-18.

CONTAMINATION OF SOILS BY PETROLEUM HYDROCARBONS' Roscoe Ellis, Jr., and Russell S. Adams, Jr. Departments of Agronomy, Kansas State University, Manhattan, Kansas and University of Illinois, Urbano, Illinois

I. Introduction ................................................ 11. Effect on Microbial Activities .................................. A. General Discussion ....................................... B. Stimulation of Microbial Growth ........................... C. Kinds of Microorganisms Involved .......................... D. Occurrence of Hydrocarbon-Oxidizing Organisms . . . . . . . . . . . . . E. Environmental Conditions ................................. F. Assimilation of Hydrocarbons by Microorganisms . . . . . . . . . . . . . . G. Use as a Prospecting Tool ................................. 111. Effect on Soil Properties ...................................... A. Chemical ................................................ B. Physical ................................................ IV. Effect on Plant Growth ....................................... V. Summary ................................................... References ..................................................

Page 197 198 198 199 200 201 201 202 203 203 203 208 210 214 215

1. Introduction

Contamination of soils by petroleum hydrocarbons causes drastic changes in microbiological, chemical, and physical properties of soil. Growth of vegetation on contaminated areas may be prevented, retarded, or accelerated depending on changes taking place in soil properties. Because of the changes in soil properties and effects on plant growth, the problem concerns agronomists as well as the petroleum industry. Crude oil and natural gas are the two commonest petroleum hydrocarbon contaminants of soils. Contamination with crude oil may occur in rather large areas of agricultural land in the vicinity of oil wells or pipelines. Contamination with natural gas is usually confined to much smaller areas, since lateral penetration of gas in soils is generally not extensive. However, contamination around pipelines in urban areas can 1 Contribution number 711, Department of Agronomy, Kansas State University, Manhattan.

197

198

ROSCOE

ELLIS, JR., AND

RUSSELL S.

ADAMS, JR.

be a serious problem since damage to valuable lawns, shrubs, and trees is often involved. There is good evidence that contamination of soils with crude oil and natural gas are related problems. Plice (1948) compared effects of crude oil and natural gas and found that parallel effects of nitrogen fixation and organic matter formation occurred in soils. Davis (1952) discovered that treating soils with natural gas would produce characteristics comparable to those of naturally occurring “paraffin dirt.” Thus, crude oil and natural gas appear to produce similar effects in many soils. The chemical and microbiological processes involved have not yet been completely explained. Additional research work is needed to ascertain the similarities and differences involved in the decomposition and assimilation processes that occur when crude oil and natural gas are added to soils. It. Effect on Microbial Activities

A. GENERAL DISCUSSION Microorganisms may be responsible for many of the conditions observed in soils that have become contaminated with petroleum hydrocarbons. Conditions that retard or alter the routine of microorganisms may affect soil structure adversely (Hubbell and Gardner, 1948). Bacteria generally create reducing conditions in media from which oxygen is excluded (ZoBell, 1946c). When natural gas or petroleum displace the oxygen from the soil atmosphere, vigorous microbial activity may result in an increase of substances in a chemically reduced state. This is particularly true of iron and manganese (Broadbent, 1957). The unusual characteristics of “ p a r a n dirt” are attributed to microorganisms and the colloidal organic materials produced by them (Davis, 1952,1956; Milner, 1925; ZoBell, 1945). Both Milner and Davis considered the microorganisms in “paraffin dirt” beds capable of utilizing methane. Davis (1952) established, by mass spectrometer analysis of samples of soil atmosphere collected at a depth of 6 feet in a “para5n dirt” bed, that methane was present. He was able to produce “paraffin dirt” in the laboratory by passing natural gas through two ordinary surface soils for several months. A marked increase in the organic carbon content and the number of microorganisms resulted as the gas flow continued. The increase in nitrogen and organic carbon in soils contaminated with petroleum hydrocarbons must be due primarily to the activities of microorganisms. Some nitrogen may even be fixed by some of these organisms. Pine and Barker (1954) found that Methanon-bacterium omelianskii was capable of nitrogen fixation, although inefficient. Anderson and Byers (1933) suggested that the accumulation of

SOIL CONTAMINATION BY PETROLEUM HYDROCARBONS

199

microbial cells may be responsible for retarded wetting in soils. Studying the nature of organic colloids, they found it almost impossible to wet soil that was 9.1 per cent impregnated with fungus cells. Although petroleum and natural gas seepages in nature are rare, those that occur must have existed for centuries. Despite this constant seepage, no lakes of surface oil occur and the possibility that natural gas could have passed into the air or that evaporation of petroleum could have occurred to any extent is questioned because of the low hydrocarbon content of the atmosphere. This phenomenon can be explained only by the constant assimilation of hydrocarbons by microorganisms in the earth's surface.

B. STIMULATION OF MICROBIAL GROWTH At least a portion of the soil microbial population appears to be stimulated by the addition of petroleum hydrocarbons. Baldwin (1922) reported one-half to threefold increases in aerobic microorganisms in soils treated with various amounts of crude petroleum. These increases were greatest with the largest applications of petroleum. Although the total number of aerobic microbes increased in petroleum-treated soils, the number of microbial types was greatly reduced by larger applications of crude petroleum. Plice (1948) made microbial counts on soil to which he had applied large amounts of oil. Prior to treatment the soil contained 8.1 million microbes per gram of soil; shortly after treatment the microbial count fell to 7.3 million per gram, This reduction was considered to be due either to partial sterilization or to incomplete dispersion of the oiled soil. At the end of two years the microbial population had risen to 94 million per gram of soil in cultivated plots and 110 million per gram in uncultivated plots. Harris ( 1959) suggested that high bacterial populations found in soil near asphalt-coated pipelines indicated microbial attack on the coatings. Matthews (1924) observed a sharp rise in microbial numbers in soils treated with various hydrocarbons. The number of microorganisms increased to a maximum and then slowly decreased. She reported the same effect in the field, with more time required to reach maximum numbers. She postulated that the disturbance in soils in the field might easily be extended 400 to 500 days, but that the microbial population should eventually return to normal. Baldwin (1922), or Plice's (1948) experiments were of shorter duration or they might have observed similar results.

200

ROSCOE ELLIS,

JR.,

AND RUSSELL S. ADAMS,

JR.

C. KINDSOF MICROORGANISMS INVOLVED Miyoshi ( 1895) observed paraffin decomposition and Beerstecher (1954) reported that Stahl and Scheiko noted bacterial action on kerosene in 1901. However, the first major studies of the microbial oxidation of hydrocarbons were by Kaserer (1906) and by Sohngen (1906), who published their results on the bacterial utilization of methane. These investigations were immediately followed by many others so that today over 100 species of bacteria, yeasts, and molds, representing 31 genera, have been described which are known to attack one or more kinds of hydrocarbons. Genera represented include: Achromobacter, Actinomyces, Aspergillus, Bacillus, Bacterium, Botrytis, Corynebacterium, Debaryomyces, Desulfovibrio, Endomyces, Escherichia, Gafkya, Hansenia, Methanomonas, Methanobacterium, Micrococcus, Micromonospora, Monilia, Mycobacterium, Mycoplana, Nocardia, Penicillium, Proactinomyces, Pseudomonas, Sarcina, Serratia, Spirillium, Thiobacillus, Torula, Torulopsis, and Vibrio (Beerstecher, 1954; Pine and Barker, 1954; ZoBell, 1945, 1946a). With the exception of the Methanobacterium species, species in the above genera known to utilize hydrocarbons are aerobic. The species proved capable of utilizing hydrocarbons, as well as those reported to predominate in soils contaminated with petroleum hydrocarbons, have been largely aerobic. Stone et al. (1942) divided bacteria found in petroleum-contaminated soils into three groups: Pseudomonas aeruginosa, Achromobacter, and Alcaligenes radiobacter, all aerobic. Haas ( 1942) found approximately 66 per cent of the hydrocarbon oxidizers in ordinary soils were Pseudomonas species. Adams (1959) reported three genera identified in gas-saturated soils that were aerobic: Aspergillus, Penicillium, and Mycobacterium. Baldwin (1922) reported no change in anaerobes in petroleum-contaminated soils, but large increases in aerobic organisms. It seems feasible that anaerobic conditions might be created immediately below the soil surface around gas or petroleum seepages, for here at least a large portion of the soil atmosphere probably would be displaced by gas or petroleum during the exposure period. Harper (1939) observed a shift from aerobes to facultative anaerobes in the microbial population of a soil exposed to natural gas. In a soil sample taken near a leaking gas main at a depth of 18 inches, Harper succeeded in identifying two species of obligate anaerobes: Clostridium sporogenes and Clostridium bifermentans. Large numbers of facultative anaerobes were found, including such species as Aerobacillus polymyxa, Aerobacillus macerans, Bacillus brevis, Bacillus circulans, and Bacillus amylolyticus. His comparisons were made on the basis of the number of microorganisms normally found

SOIL CONTAMINATION BY PETROLEUM HYDROCARBONS

201

in the soil in this area, and not on a study of a control sample from near the gas-contaminated soil. OF HYDROCARBON-OXIDIZING ORGANISMS D. OCCURRENCE Microorganisms capable of utilizing petroleum hydrocarbons in their metabolism are widely distributed in nature. Large numbers are found in the surface soil in oil fields, in natural seepage areas, around refineries, along asphalt roads, and around leaking pipelines. They are found in crude oil in pipelines, oil wells, and storage tanks, in separation tanks, gasoline storage tanks, and sedimentation pools, particularly where they come in contact with water. These microorganisms also have been found in areas not directly associated with petroleum or natural gas. For example, hydrocarbon-oxidizing organisms have been found in water from Supply and Sulfur Springs at Yellowstone National Park, in freshwater wells, hay, leaves, manure, peat, sewage, garden soils, and river and marine muds (Beerstecher, 1954; Davis and UpdegraE, 1954; Haas, 1942; Haas et al., 1941; Porter, 1948; Stone et al., 1942; ZoBell, 1945, 1946a, c) Harris ( 1960) has recently reported isolation of asphalt-utilizing bacteria from normal soils and from soils near coated pipelines in all areas tested throughout central and western United States.

E. ENVIRONMENTAL CONDITIONS Widespread occurrence of hydrocarbon-oxidizing organisms indicates that they have the ability to function throughout a wide range of environmental conditions. Many of these organisms have been uninjured by hydrostatic pressures up to 150,000 pounds per square inch (ZoBell, 1945) and have been active at temperatures as low as 0°C. and as high as 85°C. (Beerstecher, 1954; Matthews, 1924; ZoBell, 1945). Most hydrocarbon oxidizers appear to require oxygen in small amounts. The optimum partial pressure of oxygen is 10 to 40 per cent for Methanomonas, for example, but much smaller amounts stimulate growth (Beerstecher, 1954; Haas, 1942; Hutton and ZoBell, 1949; ZoBell, 1946a). A partial pressure of carbon dioxide of 5 to 10 per cent has been found to enhance the growth of methane oxidizers (Dworkin and Foster, 1958; Hutton and ZoBell, 1949). These same organisms grow best in atmospheres containing high concentrations of methane (Hutton and ZoBell, 1949; ZoBell, 1946a). Optimum pH for the utilization of hydrocarbons varies greatly with organisms, but a pH near neutral appears to be most suitable. However, pH’s between 5 and 9 generally are not considered critical (Beerstecher, 1954; Bushnell and Haas, 1941; Hutton, 1948; Hutton and ZoBell, 1949; Strawinski, 1943; ZoBell, 1945). The optimum pH for hydrocarbon con-

202

ROSCOE ELLIS, JR., AND RUSSELL S . ADAMS,

JR.

sumption may not be the optimal pH in terms of product yield, as Strawinski (1943) found the yield of oxidation products increased with the initial pH of the media over a limited range. Strawinski ( 1943) also found that the oxidation-reduction potential was less in the laboratory when Pseudomonas was grown on unaerated media containing naphthelene. Adams (1959) found the same phenomenon in soils contaminated with natural gas. However, ZoBell (1946b) and Hutton and ZoBell ( 1949) considered low oxidation-reduction potentials detrimental to optimum growth of hydrocarbon-oxidizers. Again this may not correspond to product formation. The assimilation of petroleum hydrocarbons may be retarded by the presence of large amounts of other organic materials (Beerstecher, 1954; Davis, 1956; Strawinski and Tortorich, 1955; ZoBell, 1946a). Presumably, these organic materials are preferentially attacked; nevertheless, the presence of small amounts of organic materials may prime the utilization of hydrocarbons. This may be particularly true in the case of the oxidation of gaseous hydrocarbons, where a greater degree of specificity has been observed (Dworkin and Foster, 1958; Strawinski and Tortorich, 1955). The presence of certain heavy metals, hydrogen sulfide, or specific oxidase inhibitors has been found to prevent microbial oxidation of hydrocarbons, thus indicating the presence of an essential enzyme. Hydrocarbon-oxidizers also require a physiologically balanced supply of mineral salts, as well as a nitrogen source such as nitrate or ammonia (ZoBell, 1946a, 1950).

F. ASSIMILATIONOF HYDROCARBONS BY MICROORGANISMS Almost any hydrocarbon, from methane to the heaviest paraffins or asphaltic residues, can be attacked by microorganisms, although the heavier oils are less susceptible to attack as the viscosity and molecular weight increase. This may be due in part to the size of the molecule and in part to the fact that more viscous oils are harder to disperse, exposing less surface to microbial attack. Hydrocarbons ranging in size from 10 to 16 carbons are most readily attacked (Beerstecher, 1954; Bushnell and Haas, 1941; Haas, 1942; Stone et al., 1942; Strawinski and Stone, 1940; ZoBell, 1945, 1946a). Thus, in paraffinlike oils, the fractions from kerosene up to medium-weight lubricating oil are most readily attacked. Very little is known regarding the mechanism by which hydrocarbons are attacked by microorganisms. Carbon dioxide and cell substances are considered the principal products formed during microbial dissimilation, although organic acids, ketones, aldehydes, alcohols, and modified hydrocarbons also have been detected as intermediate products. More detailed

SOIL CONTAMINATION BY PETROLEUM HYDROCAREONS

203

accounts of the assimilation of hydrocarbons have been given by Appert (1952), Beerstecher (1954), Davis and Updegraff (1954), Haas (1942), Hessel (1924), Osnitskaya ( 1946), Schwartz and Mueller (1953), Tausson ( 1929), and ZoBell ( 1945,1946a, 1950). More recent w7ork has been concerned primarily with studies of the intermediary products. Results of such work have been reported by Brown and Strawinski (1958), Kester and Foster (1960), Leadbetter and Foster ( 1958), Pine and Barker ( 1954), Raymond and Davis (1960), Rogoff ( 1957), Stewart and Kallio (1957,1959), and Stewart et al. ( 1959).

G. USEAS

A

PROSPECTING TOOL

Large numbers of microorganisms that oxidize volatile hydrocarbons may be found in the surface soil in the vicinity of oil fields. This is because a continuous, although small, amount of gaseous hydrocarbons passes into this soil and becomes available to these organisms. The demonstration of these bacteria or their fossil remains in the surface soil in defmite distribution patterns forms the basis for the so-called “geomicrobiological” prospecting method ( ZoBell, 1945). Beerstecher ( 1954), Davis (1956), Davis and Updegraff (1954), and Strawinski (1955) described methods that have been used in the search for new oil deposits. Little commercial success by use of these methods to discover new fields has been reported in scientific journals. 111. Effect on Soil Properties

A. CHEMICAL Soils that have been contaminated with natural gas or crude oil show large increases in organic matter, total carbon, and nitrogen compared with normal soils. In the case of natural gas, total carbon content might be directly increased by absorption of the gas by a soil. Adams (1959) fixed carbon in soils by passing natural gas through soil held at -1°C. Apparently, none of the carbon fixed by this method was stable, for it had disappeared within 2 weeks after the soil was brought to room temperatures. Therefore, any permanent increases in organic matter, total carbon, and nitrogen would appear to be due to the activity of microorganisms in converting hydrocarbons and atmospheric nitrogen into organic materials. Data comparing the organic matter and nitrogen contents of normal soils and soils contaminated by natural gas are shown in Table I. They were obtained by Harper (1939) for soils near Oklahoma City, Oklahoma. The average nitrogen content of soils exposed to natural gas was approximately two and one-half times greater than that from adjacent normal soils, and the organic matter content of the “gassed soils was nearly three

TABLE I Organic Matter and Nitrogen Content of Soil Samples Collected from Normal Soil and from Areas Exposed to Natural Gas from Leaking Pipelines in or near Oklahoma City, Oklahomaa Total nitrogen

Organic matter

~

soil number 1

2 3 4

5 6 7 8 9 10 11 12

Location West Reno Street u ) Ft. wth of No. 1 SW% Sec. 36, T12N, R 4 W Portland Ave. and Frisco R.R. Portland Ave. and 23rd Street Portland Ave. and 56th Street Portland Ave. and 63rd Street E%Sec. 22, T 10 N, R 4 W 400 Ft. south of No. 8 63rd Street and Santa Fe 1 Mi. N. of Cushing, Oklahoma 1 Mi. N. of Cushing, Oklahoma Average

0

Data from Harper ( 1939).

Normal soil

Gassed soil

Normal soil

Gassed soil

0.125 0.126 0.102 0.060

2.75 2.75 2.47

10.60 10.40 7.30 7.96

0.036 0.101

0.297 0.291 0.197 0.357 0.129 0.258 0.157 0.260 0.157 0.134 0.092 0.106

1.60 267 1.10 2.60 5.40 1.40 1.24 2.75

0.084

0.203

2.39

(%I

0.044

0.112 0.028 0.073 0.151 0.044

(%I

(%I

1.95

(%I

4.60

8.25 5.60 3.37 5.60 4.80

4.71 3.70 6.41

Organic matter: nitrogen ratio

5

88 P

Normal soil

Gassed soil

"G

21.8 21.8 24.2 32.5 36.4 23.8

35.7 35.7 37.1 22.3

5

35.7 32.0

m

39.3 35.6 35.8 31.8 34.4 27.2 30.4

35.7 13.0 35.7 35.8 51.2 34.9 33.7

1

9 Pr

In

g

$

m

3

SOIL CONTAMINATION BY PETROLEUM HYDROCARBONS

205

times that of the normal soils. Similar results for increases in total carbon have been reported by Adams and Ellis (1960) for soils that had been exposed to natural gas. A “paraffin dirt” examined by Davis (1952) contained 17.6 per cent organic carbon and 1.2 per cent total nitrogen. Plice (1948) found large increases in organic matter in soils that had been contaminated with crude oil. Large changes in the oxidation-reduction potential occur during the decomposition and assimilation processes that take place when saturated hydrocarbons are added to soils. Taggart (1948) made the following statement in his patent for using the oxidation-reduction potential of the top soil as a method to prospect for petroleum: “By the expression ‘oxidation-reduction characteristic’ is meant a characteristic which is imparted to the soil by reason of there having taken place in the soil a reaction involving both oxidation and reduction. The net effect of this reaction generally gives the soil a predominantly reducing characteristic with respect to soil in which no oxidation-reduction has occurred. By reason of correlation of the results of the method of the present invention with the results of examination of soils for bacteria, it may be assumed that the characteristic which is relied upon in the practice of the present invention is connected with the bacterial action. It may not be stated definitely what product of the bacterial action imparts the reducing characteristic to the soil, but it can be assumed that among these products will be unsaturated hydrocarbons and nascent hydrogen.” Plice (1948) observed that the redox potential of soils saturated with natural gas was lower than that of surrounding soils. Adams (1959) found that gas-saturated soils were in a highly reduced condition as compared with adjacent normal soils. The redox potential was a negative 982 millivolts in one gas-saturated soil compared with a positive 833 millivolts in a normal soil. The effect which the large change in the oxidation-reduction potential of soils contaminated with saturated hydrocarbons has on chemical properties of a soil, and on the growth of vegetation, is of particular interest to agronomists. Compounds of manganese and iron are more soluble in the reduced state. Schollenberger ( 1930) found that the exchangeable manganese content was much greater in a soil contaminated with natural gas than in uncontaminated soils, A detailed comparison of the amounts of exchangeable manganous and ferrous ions in normal and gas-contaminated soils is shown in Table I1 ( Adams and Ellis, 1960). These data show tremendous increases in exchangeable manganous ion in the gas-saturated soils, with noticeable increases in the exchangeable ferrous ion. Exchangeable ferric iron determinations in the same investigation are shown in Table 111. The soils contaminated with natural gas showed a large increase in exchange-

TABLE I1 Comparison of Exchangeable Manganous and Ferrous Ions in Normal and Gas-Saturated Soilsa Exchangeable Mn+ + Gas Depth ( inches ) (p.p.m. ) Site A 04 4.96 612 11.18 12-18 10.22 18-24 30.34 24-30 55.52 30-36 63.63 B 04 0.59 &12 3.42 12-18 7.14 18-24 20.58 24-30 24.84 30-36 25.83 D 0-6 7.87 13.86 612 12-18 19.43 18-24 15.49 24-30 15.43 30-36 4.85 0 Data from Adams and Ellis ( 1960) .

Normal (p.p.m.) 0.00 0.00 0.00 0.00 0.00 1.17

Trace Trace Trace Trace Trace 0.74 1.36 0.73 1.38 1.32 1.19 0.82

Exchangeable Fe+ + Gas (p.p.m.) 77.21 151.40 119.10 119.55 116.60 111.60 20.09 17.36 30.78 38.76 35.10 33.32 29.29 37.53 53.19 31.82 52.74 51.68

Normal (p.p.m. 1 18.43 24.23 23.02 19.51 26.32 26.50 15.83 16.60 25.56 16.37 16.92 25.09 24.57 17.85 27.26 41.84 37.47 32.73

Fe+ + :Mn+ f ratio Gas 15.8 13.6 11.6 3.8 2.1 1.7 34.1 5.1 4.3 1.8 1.4 1.2 3.7 2.7 2.7 2.1 3.4 10.7

Normal

22.7 -

34.9 18.2 24.5 19.8 31.7 31.5 40.0

L

8 P 0 0

$

TABLE I11 Comparison of Exchangeable Ferric Iron in Gas-Saturated and Normal Soilsa Site A

Site B

Site C

5 2

Site D

Depth (inches)

Gas (p.p.ni.)

Normal (p.p.m.)

Gas (p.p.m.1

Normal (p.p.m.1

Gas (p.p.ni.)

Normal (p.p.m.)

Gas (p.p.m.)

Normal (p.p.m.1

0-6

820 2140 1670 1890 2250 -

130 130 130 170 130

280 1550 1780 700 570

160 170 180 130 160

290 340 420 420 270

170 200 230 260 260

320 320

350

200

150 160 160 160 160 160

6-12 12-18 1%24 24-30 3036 a

-

Data from Adams and Ellis ( 1960).

-

-

360

520 520 620

81: z 'd

3 !zi 0

3s 8

208

ROSCOE ELLIS, JR.,AND RUSSELL S. ADAMS,

JR.

able ferric iron compared with normal soils. The pH titration curves for the same soils indicated greater activity of ferric iron in the gas-saturated soils. Soils contaminated with natural gas show a distinct change in pH compared with adjacent normal soils. Schollenberger ( 1930) reported a “gassed” soil with a pH of 7.1 compared with a pH of 6.2 for the adjacent normal soil. Adams and Ellis (1960) found that soils contaminated with natural gas generally had pHs around the neutral point. For acid soils, the pH values were generally increased by gas saturation; for the one alkaline soil, the pH decreased in the gas-saturated soil. This suggests the possibility that products of gas saturation may tend to buffer the soil toward neutrality rather than making the soil more basic. That a gassaturated soil tends to become more basic when the adjacent normal soil is acid appears not to support the theory that nascent hydrogen is released as one of the products in gas-saturated soils. This is one of the many points that needs to be clarified by additional research. Adams and Ellis (1960) observed that the available phosphorus content was greater in most gas-saturated soils than in normal adjacent soils. This could be explained on the basis of the more favorable pH, and that some phosphorus could be brought into solution by reducing conditions that make iron phosphates more soluble.

B. PHYSICAL Soils contaminated with saturated hydrocarbons develop striking physical characteristics. In severe cases, soil aggregates are broken down and dispersion results. The soil has a hard, waxy appearance when dry and resists wetting from the surface. However, once these soils are “wetted,” they tend to remain wet. Schollenberger ( 1930), while sampling soils contaminated with natural gas, observed that the “gassed” soil difFered somewhat in texture from the normal soil. The crumb structure of the normal soil was largely lost and the soil was considerably puddled. He observed that the “gassed” soil was more moist than the normal soil at the time of sampling and concluded that this was due to the more vigorous growth of the crop on the normal soil. Plice (1948), studying “deeply oiled” soils that had been cultivated, observed that they were subject to blowing during hot, windy weather and that aggregation had broken down even in clay soil. Adams and Ellis (1960) made physical measurements to compare normal soils and soils that had been saturated with natural gas. Moisture tension curves were determined to compare the water retention of gassaturated soils with adjacent normal soils. These curves for one site are shown in Figs. 1 and 2. In the 0- to 6-inch layer of soil, the water reten-

I30 120

+= qas saturated, fresh sample -*=gas saturated, air dried

100

.-B=

normal soil, air dried

10 1

0

16

4 0 12 Atmospheres of Tension

FIG.1. Water retention curves for fresh unground soil samples of gas-saturated soil and for air-dried samples of gas-saturated soil and normal soil of the 0- to 6-inch layer at site A. From Adams and Ellis ( 1980).

1

i

'\ \ ,

40%

\.

20

\\

\.

\d,

.'

'.

\

\.

'.&----*-

& .-.-.-.

@

Q-

Atmospheres of Tension

FIG.2. Water retention curves for air-dried samples of gas-saturated and normal soils of the 6- to 12-inch layer at site A. From Adams and Ellis (1960).

210

ROSCOE ELLIS, JFi., AND RUSSELL

S. ADAMS, JFi.

tion of the gas-saturated soil was increased markedly over that of the normal soil (Fig. 1). This increase was still present but not so pronounced in the 6- to 12-inch layer (Fig. 2 ) . The increase in water retention was reflected both at field capacity (+$ atmosphere tension) and wilting point (15 atmospheres tension). If the amount of water retained between field capacity and the wilting point is accepted as the amount available for use by plants, there was more water available in the gas-saturated soil than in the normal soil. Results of the bulk density and porosity determinations are shown in Table IV. Site A had a marked change in bulk density and porosity. Most of the increase in porosity was in the micropores. The bulk density of the gas-saturated soil was greatly reduced at site A, the value for the surface soil being less than 1. The decrease in bulk density and the increase in micropores must have been due to the great increase in organic matter in the gas-saturated soils. IV. Effect on Plant Growth

Investigators agree in general that contamination of soils with petroleum hydrocarbons has a pronounced effect on plant growth. The effect of varying amounts of crude oil added to a soil on the growth of soybeans is shown in Table V (Carr, 1919). Growth of soybeans was improved somewhat by adding a small amount of oil (30 cc. per gallon), and it took a rather large amount of oil to repress growth of soybeans. Schollenberger ( 1930), investigating the effect of contamination of soils by gas from a leaking pipeline, found that injury to oats could be noticed 2 weeks after planting and was evidenced by yellowing of the seedlings. Germination of the grain was not affected, but the most seriously injured oats soon ceased to grow and died shortly afterward. The plants in a 3- or 4-foot circle around the gas leak were entirely killed, and surrounding this zone was an area of stunted plants, many reaching a height of only 4 or 5 inches, with only 1 or 2 leaves, and producing a single kernel at maturity. When pulled, the badly stunted plants revealed a very small root system confined to the upper inch of soil. As the distance from the leak increased, the plants appeared more nearly normal, until at 12 to 15 feet no injury was apparent. In the same investigation, small flower pots were filled with normal and “gassed” soil and seeded to wheat. All the wheat germinated and grew well, but the growth of the wheat in the “gassed” soil was noticeably better. Harper (1939) noted that Bermudagrass and wheat failed to grow above gas leaks. He conducted a greenhouse experiment to determine the availability of nitrogen accumulating in the “gassed soil, and observed

s

R TABLE IV Comparison of Bulk Densities and Porosities between Gas-Saturated and Normal Soilsa Macropores Micropores Bulk density Depth

Gas

Normal

Gas

Normal

Site

( inches )

(%I

(%I

(%I

(%I

A A B B

0-6 &12 0-6 612

7.0 10.8 8.3 11.9

8.1 6.6 6.0 9.4

63.3 53.5 46.9 41.8

47.0 48.9 47.8 45.4

a

Data from Adams and Ellis ( 1960).

Gas ( g./cc.

Normal (g./cc.

0.739 1.055 1.248 1.324

1.353 1.350 1.289 1.396

8

8

9 6"

TABLE V Growth of Soybeans in Soil Containing Varying Amounts of Crude Oil0

Oil per Pot number 1 2 3 4 5 6

7 8 9 10 11 12 13 14 a

gallon of soil (cc.)

Blank 5 10 20 30

Total number of pods produced 40

40 50

60 70 80

90 100 120 160

Data from Carr (1919).

39 32 33 84

35 33 32

2s 18 31 15 15 3

Total weight of pods (g.) 14.0 16.0 12.1 12.5 38.0 14.0 12.5 13.0 8.0 4.5 10.0 5.0 4.3 0.4

Average weight per pod (g.) 0.35 0.41 0.38 0.38 0.45 0.40 0.38 0.41 0.32 0.25 0.32 0.33 0.29 0.13

Number of stalks 3

Nodules per stalk 25

3 3 3 3 3 3 3 3 3 3 3 3 3

20 30 28 108 48

48 96 29 23 32 12 30 7

Y

% of Oil

DTY weight of 3 stalks (g.1

added to soil

25 26 24 24 61 26 24 23 14 15 26 9 9 8

0.00 0.12 0.25 0.50 0.75 1.oo 1.25 1.50 1.75 2.00 2.25 2.50 3.00 4.00

Oil added per acre (2,000,OOO lb.) (1b.) 0 2,200 4,400 9,000 13,400 18,000 a400 27,000 31,400 36,000 40,400 45,000 54,OOO 72.000

P

Ro P

"E

Y

9m Pr

SOIL CONTAMINATION BY PETROLEUM HYDROCARBONS

213

that the oat leaves were darker and that more forage was produced bv plants in the “gassed soil than by plants in the normal soil. Plice (1948) studied soils contaminated with crude oil and observed that soil previously saturated 4 feet deep remained boggy and barren for several years. However, after 7 years, one of the oil-soaked soils he examined appeared to be more productive than the surrounding normal soil. Numerous other examples of the effects on plant growth of contamination of soils with petroleum hydrocarbons confirm the general pattern of those reviewed here. When soils are contaminated severely with either natural gas or crude oil, vegetative growth usually is eliminated for a period of time. After the source of contamination is removed, this situation is corrected and growth of vegetation on contaminated soils has frequently been found to be greater than on adjacent normal soils. Analyses of the effects on plant growth through effects on the microbiological, chemical, and physical properties of soil seem to be appropriate. When a soil is saturated with natural gas, soil air is replaced by the gas. This would eliminate plant growth through inability of plant roots to obtain oxygen. When a soil has been contaminated with either natural gas or crude oil, reducing conditions develop with the resulting accumulation of manganous and ferrous ions. Adams and Ellis (1960) found that gas-saturated soils contained over 60 p.p.m. of exchangeable manganous ions and nearly 120 p.p.m. of exchangeable ferrous ion. Black (1957) reported that as little as 4 p.p.m. of exchangeable manganese would depress the yields of lespedeza, soybeans, and barley and that corn would show intolerance when 15 p.p.m. or more were present. Morris and Pierre (1949) found that 1 to 10 p.p.m. of exchangeable manganese damaged five legume stands studied. Thus, reducing conditions and the resulting increase in exchangeable manganese to toxic levels seem to explain much of the damage to plant growth. That plant growth returns to normal after the source of contamination has been eliminated and the soil has been cultivated and aerated is explained by the return of the soil to a normal oxidation state, with accompanying decreases in manganous and ferrous ions. That plant growth has been found to be improved, compared with normal adjacent soils, several years after soils were contaminated with natural gas or crude oil, also can be explained by effects on the microbiological, chemical, and physical properties of soils. The increased microbiological activity due to contamination by petroleum hydrocarbons results in a great increase in organic matter and nitrogen. An increase in

214

ROSCOE ELLIS,

JR.,

AND RUSSELL S . ADAMS,

JR.

organic matter changes the physical properties of the soil, a large increase in water-holding capacity being one of the significant changes. An increased supply of nitrogen, and perhaps water, offer a feasible explanation for the increase in plant growth on these soils. V. Summary

Contamination of soils by petroleum hydrocarbons has pronounced effects on the microbiological, chemical, and physical properties of soils and on the growth of vegetation. Microbiological activity is enhanced after contamination of soil with petroleum hydrocarbons. Some 31 genera and over 100 species of microorganisms are known to assimilate hydrocarbons under a wide variety of environmental conditions. Presumably, the increased organic matter and nitrogen content of soils contaminated with petroleum hydrocarbons is due to the accumulated cells of these hydrocarbon-oxidizers or to the byproducts of their metabolism. Most of the microorganisms known to be capable of utilizing hydrocarbons in their metabolism are aerobic. However, anaerobic conditions are very likely to exist during the period when soils are being contaminated with nautral gas or petroleum. Thus, the possibility of an active participation of anaerobic microorganisms in producing the changes observed in contaminated soils remains strong. Reducing conditions accompany the decomposition and assimilation processes of the microorganisms in these soils, and exchangeable manganous and ferrous ions show large increases. Severely contaminated soils may exhibit deflocculation and loss of structure, and consequently be more subject to erosion. The large increase in organic matter increases the water-holding capacity of contaminated soils. Growth of vegetation is usually eliminated for some period on soils which have been severely contaminated. This can be explained on the basis of the displacement of soil air by gas in case of saturation by natural gas. The reducing conditions and the increase of exchangeable manganese to toxic levels during the decomposition and assimilation processes offer an explanation for the elimination of vegetation for a considerable period. Cultivation and aeration help to return the soil to a normal oxidation state which allows vegetation to be re-established. A possible explanation for the increase in plant growth noted by several investigators following contamination is the increased supply of nitrogen, and perhaps of available water. Additional research is needed to clarify numerous points of the problem, In fact, the whole area of contamination of soil by petroleum hydro-

SOIL CONTAMINATION BY PETROLEUM HYDROCARBONS

215

carbons has been neglected to a great extent so far as the field of agronomy is concerned. Workers in the petroleum industry also feel that this area of research has been neglected. Beerstecher (1954), considering methods for prospecting for petroleum, made the following statement: “If the chemistry of the soil and its flora overlying producing formations had been studied as extensively as has, for instance, the chemistry of marine sediments, it might be predicted that better prospecting methods would now be available. Such an apparent denial of the vast effort that has gone into geochemistry takes into account the fact that few if any geochemical studies gave consideration to the biochemical and biophysical aspects of the problem.” The authors hope that this article will stimulate some interest in additional research in these areas. REFERENCES Adams, R. S., Jr. 1959. M.S. Thesis, Kansas State University, Manhattan, Kansas. Adams, R. S., Jr., and Ellis, R., Jr. 1960. Soil Sci. SOC. Am. Proc. 24, 41-44. Anderson, M. S., and Byers, H. G. 1933. U . S. Dept. Agr. Tech. Bull. 377. Appert, J. 1952. Reu. inst. fraw. pdtrole et Ann. combustibles liquides 7 , 170-180. Baldwin, I. L. 1922. Soil Sci. 14, 465-475. Beerstecher, E., Jr. 1954. “Petroleum Microbiology.” Elsevier, Texas. Black, C. A. 1957. “Soil-Plant Relationships.” Wiley, New York. Broadbent, F. E. 1957. Yearbook Agr., U. S. Dept. Agr. pp. 151-157. Brown, L. R., and Strawinski, R. J. 1958. Bacteriol. Proc. pp. 122. Bushnell, L. D., and Haas, H. F. 1941. J. Bacteriol. 41, 653-673. Cam, R. H. 1919. Soil Sci. 8, 67-68. Davis, J. B. 1952. Bull. Am. Assoc. Petrol. Geologists 36, 2186-2188. Davis, J. B. 1956. Bacteriol. Reus. 20, 261-264. Davis, J. B., and Wpdegraff, D. M. 1954. Bacterwl. Revs. 18, 215-238. Dworkin, M., and Foster, J. W. 1958. J . Bacterial. 76, 592-603. Haas, H. F. 1942. Ph.D. Thesis, Kansas State College, Manhattan, Kansas. Haas, H. F., Yantzi, M. F., and Bushnell, L. D. 1941. Trans. Kansas Acad. Sci. 44, 39-45. Harper, H. J. 1939. Soil Sci. 48, 461-466. Hams, J. 0. 1959. Kansas Agr. Expt. Sta. Tech Bull. 102. Harris, J. 0. 1960. Corrosion 16, 113-121. Hessel, F. A. 1924. Mat. grasses 16, 5936-6940. Hubbell, D. S., and Gardner, J. L. 1948. J . Am. SOC. Agron. 40, 832-840. Hutton, W. E. 1948. Ph.D. Thesis, University of California at Los Angeles, Los Angeles. Hutton, W. E., and &Bell, C. E. 1949. J . Bacteriol. 68, 463-473. Kaserer, H. 1906. Zentr. Bakteriol. Parasitenk. Abt. I I . 15, 575-576. Kester, A. S., and Foster, J. W. 1960. Bacteriol. Proc. pp. 168. Leadbetter, E. R., and Foster, J. W. 1958. Bacteriol. Proc. pp. 122. Matthews, A. 1924. J. Agr. Sci. 14, 1-57. Milner, H. B. 1925. Mining Mag. (London) pp. 73-85 (as reviewed by D. C. Barton). 1925. Bull. Am. Assoc. Petrol. Geologists 9, 1118-1121. Miyoshi, M. 1895. Jahrb. wiss. Botan. 28, 269-289,

216

ROSCOE ELLIS, JR., AND RUSSELL S. ADAMS,

JR.

Moms, H. D., and Pierre, W. H. 1949. Agron. J. 41, 107-112. Osnitskaya, L. K. 1946. Mikrobiologiya 16, 249-263. Pine, M. J., and Barker, H. A. 1954. J. Bucteriol. 68, 589-591. Plice, M. J. 1948. Soil Sd.SOC. Am. Proc. 13, 413-416. Porter, J. R. 1948. “Bacterial Chemistry and Physiology.” Wiley, New York. Raymond, R. L., and Davis, J. B. 1960. Bucteriol. Proc. pp. 158-159. Rogoff, M. H. 1957. Bacteriol. Proc. pp. 133-134. Schollenberger, C.J. 1930. Soil Sci. 29, 281-286. Schwartz, W., and Mueller, A. 1953. World Oil 136(4), 95-96. Sohngen, N. L. 1.906. Zentr. Bakteriol. Parusitenk., Abt. I1 16, 513-517. Stewart, J. E., and Kallio, R. E. 1957. Buctmiol. Proc. pp. 134. Stewart, J. E.,and Kallio, R. E. 1959. Bucteriol. Proc. pp. 118. Stewart, J. E.,Kallio, R. E., Stevenson, D. P., Jones, A. C., and Schissler, D. 0. 1959. J. Bact. 78, 441-448. Stone, R. W., Fenske, M. R., and White, A. G. C. 1942. J. Bucteriol. 44, 169-178. Strawinski, R. J. 1943. Ph.D. Thesis, Pennsylvania State ,College, State College. Strawinski, R. J. 1955. World Oil 141(6), 104-115. Strawinski, R. J., and Stone, R. W. 1940. J. Bucteriol. 40, 461. Strawinski, R. J,, and Tortorich, J. A. 1955. Bucteriol. Proc. pp. 27. Taggart, M. S. 1948. U. S. Patent 2,442,476. Tausson, W. 0. 1929. Plunta 7, 735-758. ZoBell, C.E. 1945. Science 102, 364-369. ZoBeU, C. E. 1946a. Bucteriol. Revs. 10, 1-49. ZoBell, C. E. 1946b. Bull. Am. Assoc. Petrol. Geologists 30, 477-513. ZoBell, C.E. 1946~.Oil Weekly 120( 12), 30-36. ZoBell, C.E. 1950. Advances in Enzymol. 10, 443-486.

THE BARLEY YELLOW DWARF VIRUS DISEASE OF SMALL GRAINS

. .

W F Rochow Cornell University. Ithaca. New York

. .

Page

1 Introduction ................................................ 11 History and Distribution of the Disease .......................... A Discovery of the Virus in 1951 ............................. B. Early Records ........................................... C Distribution ............................................. 111 Crops Affected and Importance of the Disease . . . . . . . . . . . . . . . . . . . A Crops Affected ........................................... B Yield Losses ............................................. C Importance .............................................. IV Symptoms of the Disease ..................................... A External ................................................ B Internal ................................................. V Aphid Vectors of the Virus ................................... A Known Vectors .......................................... B Virus-Vector Relationships ................................. VI Biological Nature of the Virus ................................. A Relation to Other Plant Viruses ............................ B Kinds of Variability among Isolates .......................... C Relationships among Isolates ............................... VII . Factors That Affect Outbreaks of the Disease .................... A Aphid Species Transmitting the Virus ...................... B. Source of Virus Inoculum ................................ C Aphid Movement ......................................... D Aphid Feeding Habits .................................... E Crop Age When Infected .................................. VIII Control of the Disease ........................................ A. Use of Crop Varieties Resistant to the Virus . . . . . . . . . . . . . . . . . . B. Use of Insecticides ....................................... C Cultural Practices ........................................ D Possible Use of Crop Varieties Unacceptable to Vectors . . . . . . . . IX . Summary and Conclusion ..................................... References ..................................................

.

. .

.

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

.

.

. . . . .

.

I

217 218 218 219 220 220 220 224 226 226 226 228 228 228 230 231 231 232 234 235 235 236 236 237 238 238 238 242 243 244 244 245

Introduction

Study of the barley yellow dwarf virus disease of oats. barley. wheat. or rye involves many problems arising from the interaction of two biological systems. a plant and a virus. and the effect of a third factor. the 217

218

W. F. ROCHOW

weather, on this interaction. This disease is more complex than most because it also includes the interaction of these systems with still another, the aphid vector. Since aphids are the only known means by which the virus can be transferred from plant to plant, they are the key to the disease as it occurs in nature. The importance of aphid vectors places the barley yellow dwarf virus disease in a category quite different from other widespread diseases of small grains. Consideration of aphids as vectors of a virus is completely different from consideration of them as direct pests; much confusion about the barley yellow dwarf virus disease results from failure to appreciate this fact. The numbers of aphids and the extent of the infestation are important with respect to direct damage by aphids, but neither of these factors is necessarily important in the aphid transmission of viruses. One moving aphid is more important in spreading virus than a hundred stationary ones. Moreover, the aphid species important as a vector may not be the species that predominates in the crop; in fact, the vector could be an aphid that does not even colonize the crop in question (Kennedy, 1950). Barley yellow dwarf is not a new disease. Only the discovery of its cause by Oswald and Houston (1951) is new. All workers agree that the disease was present for many years before the 1951 outbreak in California focused the attention of Oswald and Houston on the problem. In the ten years that have elapsed since that time, much work has been done on some aspects of this disease. More than 200 research reports, at least four Ph.D. theses, and a special supplement ( N o . 262) of the Plant Disease Reporter have dealt with the disease, its causal virus, or the vectors. One of the main tributes to the excellent initial work of Oswald and Houston is the fact that most of these reports have merely confirmed or extended many of their findings. Some recent general treatments of the disease or of certain aspects of it have appeared (Jedlinski, 1959; Rochow, 1959d; Slykhuis, 1960; Browning et al., 1960). A monograph on the disease is in preparation by G. W. Bruehl. It is the purpose of this review to summarize current knowledge about barley yellow dwarf virus (BYDV) and the disease it causes and to examine this knowledge critically in the light of accumulated experience of many workers with aphid-transmitted viruses of other crops. II. History and Distribution of the Disease

A. D~~COVERY OF THE VIRUS IN 1951 In April, 1951, many fields of barley in California turned brilliant yellow within a week. Early in May a severe yellowing was found in almost every barley-producing county of the State. Young barley plants were

BARLEY YELLOW DWARF

219

either killed or stunted so severely that they failed to head. A chlorosis of wheat and a reddening of oats soon developed. In this dramatic way, the disease now known to be caused by BYDV suddenly became destructive, and Oswald and Houston (1951, 1952, 1953a, b) began their efforts to find its cause. Because virus diseases do not usually affect crops so uniformly, a virus cause was probably not the first possibility considered. The possible effect of climatic factors, nutrient unbalance or inadequacy, and fungus root rots all were eliminated as explanations for the condition. The presence of at least a few green plants in each field and the large aphid population led Oswald and Houston to make tests of the possible role of aphids in the disease. Four aphid species were soon found to be capable of transmitting a virus that caused the disease; a fifth species was later found to be a vector (Oswald and Houston, 1951, 1952). As Oswald and Houston (1951) pointed out, the disease differed from other virus diseases of cereals in three ways: (1) since it was a yellows-type disease, it had symptoms and characteristics quite different from mosaic virus diseases already known in cereals; (2) the virus was not mechanically transmissible; and (3) the virus was readily transmitted by grain-infesting aphids. Oswald and Houston (1952) had pointed out the similarity between the disease caused by BYDV in oats and the oat disease referred to as red leaf. Takeshita ( 1956a) (name later changed to Endo) further established the fact that the red leaf disease of oats is caused by BYDV. Since the virus was first isolated from barley that was yellowed and dwarfed, it was named barley yellow dwarf virus by Oswald and Houston ( 1951) . We now know that the red leaf symptom in oats is only one manifestation of infection by BYDV; the disease in oats is usually referred to as the BYDV disease of oats or as yellow dwarf of oats caused by BYDV. I t is important to remember that there are numerous “yellow dwarf” and “red leaf” diseases of other crops. Because none of these other diseases is caused by BYDV, confusion can be eliminated by use of the full name of the causal virus in reference to the BYDV disease of small grains. B. EARLY RECORDS Oswald and Houston (1951) pointed out in their first report that the disease was not new in California. Possibly the earliest record of its occurrence in the United States is the observation by Galloway and Southworth (1890) on a “mysterious oat disease” in the Eastern and Central States. Peck (1890, 1891) had noted a peculiar condition of oats in New York in the same year. Extension records at Cornell University for 1907 contain four separate reports from New York growers describing a red

220

W. F. ROCHOW

leaf condition of oats. Manns (1909) described oat losses in Ohio and reviewed early observations on an abnormal condition that now appears to have been caused by BYDV. The reports on possible causes of this condition were conflicting. Bacteria were cited frequently, but descriptions of the disease, frequent mention of the importance of aphids, and the colored plate published by Manns (1909) all suggest that BYDV was involved. Bruehl et al. (1959) believe the disease occurred in Washington at least as far back as 1918. It was present in Germany at least as early as 1932 (Rademacher, 1932; Rademacher and Schwarz, 1958). Barrus (1937) described a condition of oats in New York that undoubtedly was caused by BYDV. The disease was particularly widespread in the central United States in 1949 (Caldwell et al., 1959b; Browning et aE., 1959; Sechler et al., 1959; Wilson and Murphy, 1953). Although this disease has been present in the United States for many years and had undoubtedly caused some serious losses in earlier years, such as 1907 and 1949, it was largely overlooked until 1951, probably because it usually occurred only on scattered plants, it was normally insignificant in comparison with several other diseases, numerous factors had been listed as possible causes, and it represented a kind of problem so different from those known in small grains that workers had no experience in dealing with it. C. DISTFUBUTION Allen and Houston (1956) and Takeshita (1956a) showed that the disease occurs in at least 15 States; its presence is now fairly well established in all areas of the United States from Alaska (Dickson et al., 1957) to Maine (Blackmon, 1957) to Florida and Texas (Rochow, 1959~). Transmission tests have definitely established the occurrence of BYDV in The Netherlands (Oswald and Thung, 1955), Great Britain (Watson and Mulligan, 1957), Germany (Rademacher and Schwarz, 1958), Norway (Slykhuis, 1958), Finland ( Slykhuis, 1958; Ikaheimo, 1960), Sweden (Lindsten, 1959), and New South Wales (Butler et al., 1960). Plants with symptoms of the disease have been observed in other areas of the world. BYDV undoubtedly is the most widely distributed virus affecting cereals. It would be difficult to find a small grain area where the disease does not occur. 111. Crops Affected and Importance of the Disease

A. CROPSAFFECTED 1. Naturally Znfected Crops Oats and barley are the small grains most severely affected by the disease. Many of the earliest disease observations were for oats; recent

BARLEY YELLOW DWARF

221

outbreaks have been more severe on oats than on barley. Although wheat and rye are also susceptible to the virus, they seem to have more tolerance to infection and usually develop symptoms less readily than do oats and barley. It may be that infection in wheat, for instance, is more prevalent than is generally assumed and that the relatively mild symptoms are often overlooked. Many grasses are also susceptible. Grasses that have been shown to be naturally infected include the following: bluegrass, Poa pratensis L. (Takeshita et al., 1956; Slykhuis et al., 1959b); timothy, Phleum pratense L. (Slykhuis, 1958; Slykhuis et al., 1959b); smooth brome, Bromus inermis Leyss. (Slykhuis et al., 1959b); red fescue, Festuca rubra L. (Slykhuis et al., 195913); perennial ryegrass, Lolium perenne L. (Wit, 1956; Slykhuis, 1958; Slykhuis et al., 1959b) ; intermediate wheatgrass, Agropyron intermedium (Host) Beauv. (Slykhuis et al., 1959b); and ripgut grass, Bromus rigidus Roth (Allen and Houston, 1956). Interest in the susceptibility of such grasses, many of which develop no obvious symptoms of infection, focuses largely on their possible importance as sources of v i r u s for spread to small grains. Almost no attention has been given to the possible role of BYDV as the cause of a disease of forage grasses, as a factor in seed production by certain grasses, or as an agent affecting susceptibility of grasses to other pathogens. As Kreitlow (1959) has recently stated, virus infection of perennial grasses is potentially more destructive than that of annual cereals.

2. Host Range of the Virus Oswald and Houston (1953b) tested 55 grass species for susceptibility to BYDV. Twenty species developed symptoms, 16 became infected but did not develop symptoms, and 19 were immune from the virus isolate used in their tests. The results of an extensive host range study made by Bruehl and Toko (1957) with 2 isolates of the virus from Washington disagreed in several respects with those of Oswald and Houston. Bromus inermis, found to be susceptible by Oswald and Houston, was immune from both virus isolates used by Bruehl and Toko. On the other hand, Phleum pratense was immune from the isolate used by Oswald and Houston but susceptible to both virus isolates used in Washington. In fact, some species were susceptible to one Washington isolate but not to the other. Similar discrepancies have been noted among results of other host range studies (Rochow, 195913; Watson and Mulligan, 1960b). Two factors influencing these differences are the variability among different isolates of the virus and the genetic variability among individual grass plants. This problem will be considered further in Section VI, B, 3. It is certain, however, that BYDV has an extensive host range among

222

W. F. ROCHOW

TABLE I Grass Species Reported to be Susceptible to One or More Isolates of Barley Yellow Dwarf Virus Host Aegilops triuncialis L. Agropyron cristatum (L.) Caertn. Agropyron inemte (Scribn. & Smith) Rydb. Agropyron intermedium (Host) Beauv. Agropyron trachycaulum (Link) Malte Agropyron trichophurum (Link) Richt. Agrostis tenuis Sibth. Alopecuncs agrestb L. Alopecurus pratenris L. Andropogon barbinodis Lag. Anthoranthum odoraturn L. Aristida oligantha Michx. Avenu barbata Brot. Avena fatw L. Avenu sativa L. Beckmannia syzigachne ( Steud. ) Fernald Boutelow curtipendub (Michx.) Torr. Bromus arvensis L. Bromus b r i z u e f m i s Fisch. & Mey. Bromus carinatus Hook. & Am. Bromus catharticus Vahl Bromus commutatus Schrad. Bromus erectus Huds. Bromus i m i s Leyss. Bromus iaponicus Thunb. Bromus mollis L. Bromus racemoms L. Bramus rigidus Roth Bromus rubens L. Bromus secalinus L. Bromus stedlis L. Bromus tectorum L. Bromus tomentellus Boiss. Chlo7is gayanu Kunth Cynodon dactylon (L.) Pers. Cynosum cristatus L. Cywsurus echinutus L. Dactylb glomerata L. Deschampsia caespitosa (L.) Beauv. Deschumpsia dunthonwides ( Trin. ) M u m ex Benth. Digitariu sanguinalis (L.) Scop. Elymus caput-medusae L. Elymus condensatus Presl

Referencea

O&H

R

B&T B&T, R O&H B&T W&M W&M W&M O&H O&H, W&M O&H O&H O&H, B&T, W&M O&H, B&T, R, 0, W&M B&T O&H W&M B&T B&T O&H, B&T B&T B&T O&H, R B&T O&H, B&T, R B&T O&H, B&T O&H B&T B&T, W&M O&H, B&T B&T O&H O&H W&M O&H O&H, B&T, R, W&M B&T B&T 0 O&H, B&T B&T

223

BARLEY YELLOW DWARF

TABLE I (Continued) Host

Referenma

OSrH E l y m w triticoides Buckl. O&H, B&T, W&M Festuca arundinacea Schreb. B&T Festuca elatior L. B&T Festuca idahoensis Elmer. O.&H Festuca myuros L. B&T, W&M Festuca ovinu L. W&hl Festuca pratensis Huds. O&H, B&T Festuca rejlexa Buckl. B&T, 0, W%M Festuca rubra L. Gastridium ventricosum (Gouan) O&H Schinz and Thell O&H Hordeurn brachyanthmm Nevski B&T Hordeurn brevisubulutum (Trin. ) Link B&T Hordeurn bulbosum L. O&H Hordeurn hystrix Roth O&H, B&T Hordeurn bporinum Link O&H, B&T, R, 0, W&M Hordeum vulgare L. B&T KoeleTia cristata (L.) Pers. O&H, B&T, W&M Lolium multiflorum Lam. B&T, R, 0, W&M Lolium perenne L. B&T Lolium remotum Schrank B&T Lolium temulentum L. W&M Oryza sativa L. O&H Panicum capillare L. B&T Phalaris arundinacea L. O&H Phalaris paradoxa L. O&H Phalaris tuberosa L. B&T, R, 0, W&M Phleum pratense L. B&T, R P w ampla Merr. O&H, W&M Poa annua L. B&T Poa canbyi (Scribn.) Piper O&H, B&T, 0,W&M Poa pratensis L. W&M Poa tdvialis L. W&M Secale cereab L. B&T, 0 Setaria lutescens (Weigel) F. T. Hubb O&H, B&T Sitanion hystrix (Nutt.) J. C. Smith B&T Sitmion jubatum J. G.Smith O&H Sorghum sudanense (Piper) Stapf O&H Sorghum vulgare Pers. B&T Stipa c m t a Trin. and Rupr. O&H, B&T, R, W&M Triticum aestivum L. W&M Zea maw L. a References to host range studies are indicated as O&H (Oswald and Houston, 1953b), B&'T (Bruehl and Toko, 1957), R (Rochow, 1959b), 0 (Orlob, 1959), and W&M (Watson and Mulligan, 1960b).

224

W. F. ROCHOW

grass species. On the basis of greenhouse tests, some 84 plant species are known to be susceptible to one or more isolates of the virus (Table I). All known hosts are monocotyledonous plants; nothing is known about susceptibility of dicotyledonous plants. B. YIELD LOSSES

1. Estimated Losses The outbreak of the BYDV disease in California in 1951 was estimated by Oswald and Houston (1953a) to have caused a 10 per cent loss in the barley crop. Many other workers have attempted to estimate the yield loss, particularly for oats and barley, due to natural infection by the virus. Most reports deal with the years of heavy outbreaks of the disease such as 1907, 1949, and 1959, Manns (1909) estimated a 24 per cent reduction in the Ohio oat crop for 1907. Wilson and Murphy (1953) listed a 15 per cent loss for Iowa oats in 1949. In 1959, the BYDV disease was the most destructive disease affecting oats in the United States. This outbreak was associated with large populations of greenbugs, which caused direct feeding damage in some localities. Although there were record losses in some areas, the national loss was somewhat less than that resulting from the widespread occurrences of Victoria blight and crown rust in earlier years. Moreover, the disease may actually have been more severe in 1907 and 1949 than in 1959 (Murphy, 1959). Sechler et al. ( 1959) estimated a 37 per cent loss for Missouri, one of the States hardest hit in 1959. Caldwell et al. (195913) found the estimated 27.5 per cent reduction in the oat crop to be the most severe attack on record for Indiana. Browning et al. (1959) estimated a 12 per cent reduction in Iowa. Sill et al. (1959) estimated the loss in Kansas to be over 5 million bushels or 25 per cent of the 1959 oat crop. The average damage in Wisconsin was 5 per cent (Amy and Shands, 1959). State-wide estimates are useful, but they do not reflect the variation that may occur within one State. Although the 1959 State loss was only 12 per cent in Iowa, the BYDV disease was more devastating in certain fields than either Victoria blight or crown rust had been previously (Browning et d.,1959). In Illinois many oat fields were replanted to other crops or not harvested at all (Jedlinski and Brown, 1959). Losses in most parts of the country have been severe only occasionally. In parts of the Pacific Northwest, however, the disease has usually been severe in recent years. Bruehl et al. (1959) estimated an annual loss of 1million dollars or more for oats, barley, and wheat in 1955, 1957, and 1958 in Washington. Losses in Oregon have been severe most years

BARLEY YELLOW DWARF

225

since 1957; the 1959 loss for all Oregon small grains was estimated at more than 1million dollars (Raymer and Foote, 1959).

2. Measured Losses Four general approaches have been used for the measurement of yield losses due to BYDV. Oswald and Houston (1953a) tagged individual field plants when symptoms appeared and then compared the yield of such plants with that of uninfected ones. In their tests, yield losses in barley were 95 per cent when plants became infected in the seedling stage but 15 to 25 per cent when infection occurred during the heading stage. This finding of the importance of plant age at the time of infection has since been confirmed by many workers. Rothman et al. (1959) used a similar method to record losses of 14 to 46 per cent among different oat varieties. A second method involves inoculation of field plots by infesting them artificially for certain periods with aphids known to be carrying BYDV and then comparing yields of such plots with yields of plots not infested with aphids. Suneson and Ramage (1957) tested oats, wheat, and barley in this way over a 6-year period. Average yield losses ranged from 26 to 80 per cent depending on age of plants when infected. Endo and Brown (1957a) similarly tested three varieties of oats inoculated at intervals during the season. The yield loss in CLINTLAND oats, for instance, was 94 per cent when plants were inoculated in the 3-leaf stage and 22 per cent when they were inoculated in the boot stage. Comparable results have been obtained for oats and barley by Slykhuis et al. (1959a) and for oats, wheat, and barley by Watson and Mulligan (1957, 1960b). Endo and Brown (1960) used controlled inoculation resulting from artificial aphid infestation to study the importance of fall infection on yield and winter killing of winter oats and barley. Infection during October resulted in more winter killing as well as a greater yield loss of both oats and barley than did infection during November. The high incidence of infections, and their uniform distribution, occurring in plots inoculated by controlled aphid infestation is not representative of most naturally occurring infections. The third and fourth approaches have been used in a limited way to attempt to measure losses more closely resembling those that occur naturally. In a third method, Bruehl and Damsteegt (1959) caged oats in the field in an attempt to keep control plants uninfected for comparison with those that became infected naturally. Interpretation of their finding of a 10-bushel loss per acre is complicated by the fact that the control plants were shaded. Making use of a fourth method, Browning et al. (1959) compared the yields of oat varieties in 1959, when the incidence of infection by BYDV

226

W. F. ROCHOW

was high, with the yields of the same varieties grown at the same location in 1958, when the disease was not present. Since both years were excellent for oat production and since other diseases did not complicate the comparison, these can be assumed to be reasonable measurements of the effect of the disease on yield. In 1959 the varieties PUTNAM and NEWTON yielded 25 per cent less grain than in 1958; the varieties CLINTLAND and SAUK yielded 95 per cent less. Bruehl et al. (1959) similarly used 1956 ( a year free of yellow dwarf) as a basis for estimation of natural losses in western Washington.

C. IMPORTANCE The numerous observations on yield losses show that BYDV can be a major threat to production of small grains. The virus is important not only because of the damage it can cause to individual plants, but also because it occurs so commonly and is so widely distributed around the world. The virus must surely be ranked potentially as destructive as any cereal virus yet discovered; it infects oats, barley, wheat, many forage grasses, and numerous wild grasses, and it is transmitted efficiently by aphids that occur commonly on these plants in many parts of the world. The general availability of inoculum for natural infection in one area is illustrated by the fact that in the last five years the writer has failed to find a single New York field of oats beyond the seedling stage that did not contain at least a few plants with symptoms of infection by BYDV. IV. Symptoms of the Disease

A. EXTERNAL Symptoms vary with the plant species, with the crop variety, with environmental conditions such as temperature and light (Endo, 1957b), with the strain of BYDV, with the age of the plant at the time of infection, and with the physiological condition of the plant. Despite the importance of such factors, most descriptions of symptoms are in general agreement (Oswald and Houston, 1953a; Allen and Houston 1956; Takeshita, 1956a; Rademacher and Schwarz, 1958). Stunting is the one general symptom that may occur with all plants. It may be the only symptom noticed, as has been observed for rye (Oswald and Houston, 1953a; Takeshita, 1956a). Although plants inoculated in the seedling stage may be severely stunted, those inoculated at a later stage of growth may be only slightly stunted, if stunted at all. Roots are stunted as well as shoots; this is particularly important when moisture or nutrients become limiting (Oswald and Houston, 1953a; Raymer and Foote, 1959). Tillering may be increased as is the case for BLACKHULLESS barley infected in the seedling stage, or it may be suppressed as has been

BARLEY YELLOW DWARF

227

observed for wheat (Oswald and Houston, 1953a). Elongation of leaves and initiation of new leaves may be inhibited (Esau, 1957b). The most conspicuous symptom usually is loss of green color of leaves. In barley, for instance, leaves start to turn yellow at the tips about 2 weeks after inoculation. The yellowing progresses down the leaf and eventually covers the entire blade. The color is not the sickly chlorotic yellow associated with nitrogen deficiencies and other physiological disturbances, but usually is a brilliant yellow golden shade. Perhaps the most characteristic feature of the leaf symptom is the appearance of irregular blotches or longitudinal stripes preceding the development of the solid yellow coloration. Similar changes occur in oats, but the characteristic color is light green to straw tan, rather than the brilliant yellow of barley. In oats, and in some barley varieties, a red or purple coloration may develop. Confusion has resulted from the fact that this reddening in oats, although common, is not always present. It varies with the oat variety and particularly with the temperature; cool temperatures seem to favor development of the red pigment. The characteristic water-soaked areas and the blotches and streaks which develop as the disease progresses are the most distinguishing leaf symptoms in oats as well as in barley. Similar leaf symptoms develop in wheat; colors from yellow to red have been noted (Oswald and Houston, 1953a; Caldwell et al., 1959b; H. C. Smith, 1959). Grasses may develop similar symptoms, but leaf discolorations are less common among grasses than among oats and barley. Green areas of infected plants may be darker green than normal, particularly under conditions of high fertility (Oswald and Houston, 1953a). Additional leaf symptoms include serrations along leaf borders, an effect noted mostly in greenhouse studies. H. C. Smith (1959) considered such serrations to be the most reliable greenhouse symptom in wheat. Oswald and Houston (1953a) noted serrations so severe that whole leaf tips were severed; this occurred particularly in oats, in which many plants with half-leaves were found. Leaves on plants infected by BYDV may be more erect and stiffer than those on uninfected plants. A sugary exudate may form on infected leaves ( Esau, 1957b). Blasting of florets is a common symptom, particularly in oats. It may occur in all florets of a panicle, or only the basal florets may be blasted. Because almost any physiological disturbance of the oat plant can cause blasting (Johnson and Brown, 1940; Empson, 1958), this symptom alone is not a particularly diagnostic one. Diagnosis of the BYDV disease solely on the basis of symptoms is difficult and often unreliable. Transmission of the virus by means of aphids is the only proof of infection. Such transmission tests are complicated by variability among isolates of the virus and among clones of

228

W. F. ROCHOW

at least one vector ( Rochow, 1960b). It has been the experience of workers familiar with the disease, however, that symptoms, together with the distribution of affected plants in a field, can be used to identify the disease of oats, barley, and wheat with reasonable certainty. Diagnosis is particularly difficult for oats, since many other factors have been listed as possible causes of reddening in oats (Sprague, 1936, 1939; McKinney, 1950; McKinney et al., 1952). Since the studies on the nonvirus factors that cause reddening in oats were carried out before BYDV was discovered, it is possible that the virus may have been involved in some of these early investigations. B. INTERNAL Esau (1957a,b) investigated the histological changes that occur in infected oats, barley, wheat, and some grasses. The primary change occurs in phloem tissue of leaves and roots. Phloem degeneration involves necrosis of sieve elements, companion cells, and neighboring parenchyma cells. The onset of such degeneration is related to maturation of the first sieve elements of a given vascular bundle. Necrosis may occur also in the xylem and mesophyll of leaves. Esau’s suggestion that the virus is transported in the phloem, probably in mature sieve elements, was supported by the data of Allen (1957b), who used three methods to obtain evidence that BYDV occurs in phloem of infected plants. Infection by BYDV also results in pronounced physiological changes, especially in susceptible varieties ( Orlob, 1959). V. Aphid Vectors of the Virus

A. KNOWNVECTORS In addition to the five vectors originally reported by Oswald and Houston (1953a), four more aphid species are now known to transmit the virus (Table 11). Since aphid taxonomy is complex even for specialists, not all workers have used the same name for the nine known vectors. Hille Ris Lambers (1960) and Richards (1960) have recently discussed the identity and name of Rhopalosiphum padi, the source of some confusion. Aphids of the species in question, submitted by the writer to aphid taxonomists, have been identified sometimes as R. padi and other times as R. fitchii; most recent advice from entomologists indicates that R. padi is the correct name. For the sake of consistency in this review, the name R. padi will be used for the aphids variously referred to as R. padi, R. fitchii, or R. prunifoliae, regardless of the name used in the original work cited. It is likely that the same aphid species is involved despite the use of three different names. Two other species that have been referred to by different names are listed in Table 11.

TABLE I1 Aphid Species Known to be Vectors of Barley Yellow Dwarf Virus with Reference for First Report Aphid vectora

Reference

1. Rhopalosiphum padi ( L. ), oat bird-cherry aphid Rhopalosiphum ftchii ( Sand. ), apple grain aphid Rhopubsiphum prunifolim ( Fitch ) , apple grain aphid 2. Rhopalosiphum midis (Fitch), corn leaf aphid 3. Rhopalosiphum pone ( Gill. ), bluegrass aphid 4. Macrosiphum granarium ( Kby. ), English grain aphid Macrosiphum avenue (Fab.), English grain aphid Sitobium avenue ( Fab. ), grain aphid 5. Macrosiphum avenue s-sp. miscanthi Takahashi 6. Macrosiphum dirhodum (Walker), grass aphid Metapolophium dirhodum (Walker), rose-grain aphid 7. Toroptera graminum ( Rond. ), greenbug 8. Neomyzus circumflexus ( Buckt. ) 9. Sitobium fragariue (Walker), Rubus aphid

Watson and hlulligan ( 1957) Bruehl and Toko (1955) Oswald and Houston ( 1953a) Oswald and Houston (1953a) Orlob (1959) Oswald and Houston (1953a) Slykhuis et al. (1959a) Watson and Mulligan (1960a) Butler et al. (1960) Oswald and Houston (1953a) Watson and Mulligan (1957) Oswald and Houston (1953a) Watson and Mulligan (1960a) Watson and Mulligan (1957)

0 Names indented under 1, 4, and 6 have been used by some authorities to identify species here referred to only by name numbered.

E 5

9 E

2

230

W. F. ROCHOW

Not all vectors are equally effective in transmitting BYDV. In one comparison, for instance, Toko and Bruehl (1956) observed 99 per cent transmission by R. padi and only 41 per cent by M . granarium. Differences among aphid species are even more marked in the transmission of different strains of the virus, as will be discussed in Section VI, B, 1. All nymphal instars and adults of at least two vectors are capable of transmitting virus (Toko and Bruehl, 1959). Not only do different species differ in efficiency as vectors, but two kinds of variability within one species are known. Of six forms of R. padi collected from different winter hosts in the fall and spring by Orlob and Arny (1960), only two transmitted BYDV after feeding on infected barley; however, the results may have been affected by failure of some forms transferred from winter hosts (Crataegus sp. and Malus sp.) to feed well enough on barley to acquire virus. Another kind of variability was reported by Rochow (1960b). When greenbugs from three locations were compared as vectors of BYDV, those from two locations transmitted two isolates of the virus efficiently, and a third isolate occasionally, but greenbugs from the third location essentially never transmitted any of the virus isolates. This was considered evidence for physiological specialization among greenbugs in the transmission of BYDV. There is no evidence at present of such specialization of other vectors of BYDV, but similar variability among clones of aphids is known for species that transmit other plant viruses (Rochow, 1960b) Since aphids are the only known means by which BYDV can be transmitted from plant to plant, they play the central role in transmission of the virus in nature, as will be considered in Section VII. I

B. VIRUS-VECTOR RELATIONSHIPS Knowledge of the relationship between the virus and its vector is necessary for an understanding of how the virus is spread in the field. Oswald and Houston (1953a) showed that BYDV is one of a relatively small group of plant viruses that persist in their aphid vectors. All subsequent work has confirmed this finding and filled in some of the details. R. padi must feed on an infected plant for some hours to acquire the virus. Orlob (1959) found a period of at least 7 to 9 hours to be necessary. Toko and Bruehl (1959) reported that an acquisition feeding period of 24 hours was necessary for efficient transmission. Watson and Mulligan (1960a) found that a few aphids acquired virus in 30 minutes, but that 32 hours was needed for about half the potential transmitters to acquire and to transmit virus. In some cases viruses that persist in their vectors have been reported to undergo a latent period in the vector during which time the virus

BARLEY YELLOW DWARF

231

cannot be transmitted to plants. Watson and Mulligan (1960a) found some evidence for a short latent period for BYDV in R. padi, but the evidence was not conclusive. If a short latent period does exist, it is of variable length and undoubtedly depends on conditions used, plant species, and similar factors (Toko and Bruehl, 1959; Orlob, 1959). After acquiring BYDV some aphids can transmit it to plants after feeding for a period as short as 15 to 30 minutes (Orlob, 1959; Watson and Mulligan, 1960a). Longer feeding periods are usually necessary for efficient transmission. Toko and Bruehl (1959) found that transmission was rare unless feeding was for 4 to 8 hours. The importance of this inoculation feeding period is illustrated by tests made with aphids that were allowed to feed on Systox-treated plants; although the aphids did not live much longer than 2 hours, they were able to transmit BYDV to most plants they fed upon (Pizarro and Amy, 1958). Once an aphid has acquired BYDV and is able to transmit, the aphid may continue to transmit virus as long as it lives. Many workers have confirmed the persistence of BYDV in its aphid vectors. The virus persisted in single individuals of R. padi for 21 days and in single aphids of M. granariurn for 12 days in one series of tests ( Rochow, 195913). Persistence is good evidence that this virus is not merely carried mechanically by the aphid, for the aphid usually can continue to transmit even after a molt. VI. Biological Nature of the Virus

A.

RELATION TO OTHER

PLANT VIRUSES

About half of the several hundred known plant viruses are transmitted by aphids. The kinds of relationships that exist between these viruses and their aphid vectors cover a wide spectrum. The basis for the differences rests with the virus, and not with the aphids. At one extreme are nonpersistent viruses, which can be acquired and transmitted in a few minutes or seconds, but are lost by the aphid within a short time after it leaves the source plant. Most of the aphid-transmitted viruses fall into this group. At the other end of the spectrum are the persistent viruses, of which BYDV is an example. Only about a dozen viruses of this group are known, potato leaf roll virus being the best-known one. Virologists are interested in persistent viruses for many reasons, but a main reason is the fact that the persistent virus-vector relationship indicates some kind of specific biological interaction. A comparable group of plant viruses transmitted by leafhoppers contains some viruses that can multiply in the insect vector as well as in the plant. From the point of view of basic biology, then, persistent aphid-transmitted viruses, such as BYDV, are of great interest apart from their economic importance.

232

W. F. ROMOW

B.

KINDS OF

VARIABILITY

AMONG ISOLATES

A main development in the knowledge of BYDV during the last five years has been the elucidation of several kinds of variability among different isolates of the virus. These different isolates occur widely in nature, differ in several ways, and may be of great importance to an understanding of the disease in nature and to the success of attempts to control the virus by breeding resistant plants, A characterized isolate, one that retains one or more distinctive properties when studied in the greenhouse, is called a strain by most workers. 1. Vector Specificity

When Toko and Bruehl (1957) tested 34 Washington field collections by means of R. padi and M . granarium, they found that the virus was transmitted from 32 of them by both aphid species but that it was transmitted from one collection only by R. padi and from another collection only by M. granarium. Similar specificity was found in New York (Rochow, 1958a), but the presence of a strain of virus transmitted by M . granarium and not by R. padi was the rule, not the exception as in Washington. Cooperative studies by Bruehl (1958) and Rochow (1958b) showed that the virus, not the aphid, was responsible for the differences. Vector-specific strains described to date fall into several groups. First, there are the strains transmitted efficiently by R. padi but with varying efficiency by M . granarium. These strains seem to form a series extending from those not transmitted by M . granarium (Toko and Bruehl, 1959) through those occasionally transmitted by M . granarium (Rochow, 1959b), to those readily transmitted by both aphid species. Much of the early work on BYDV seems to have involved strains of this group. A second group includes strains transmitted efficiently by M. granurium but rarely, if at all, by R. padi. The vector specificity of this group is much more marked than is that of the first group when transmission is from plants infected by only one virus strain, but the specificity may break down when the source plants are infected by more than one virus strain (Rochow, 1959a). Such strains seem to be common in the northeastern United States (Rochow, 1960c), and they have been found in other parts of the United States (Toko and Bruehl, 1959; Rochow, 1959c), in Canada (Slykhuis et al., 1959a), and in Great Britain (Watson and Mulligan, 1960a). Watson and Mulligan (1960a) found such strains to be transmitted also by M . dirhodum. Rochow (1961) found what appears to be a third group of strains transmitted with fair efficiency by R. muidis but not transmitted regu-

BARLEY YELLOW DWARF

233

larly by either R. padi or M . granarium. Such strains appeared to be less common than those of the other two groups in New York during 1959 and 1960. The existence of this group provided an explanation for failure in earlier tests involving only R. padi and M . granarium to transmit BYDV from some field samples showing symptoms typical of infection by the virus. Virtually nothing is known about the mechanism of specificity, about the possible existence of additional vector-specific strains, about the role of such strains in nature, or to what extent they complicate the problem of disease control. 2. Virulence Isolates of BYDV also differ in the severity of the disease they cause. Although such differences have been noted between isolates transmitted by R. padi and M . grannrium (Toko and Bruehl, 1956), similar digerences occur among isolates transmitted by one aphid species. Most studies have involved isolates transmitted by R. padi. Takeshita et n2. (1956) found the differences in virulence among virus isolates sufficient to require attention in a program of evaluating the resistance of oat varieties to the virus. Symptoms may develop 3 to 7 days earlier in plants infected by virulent isolates than in those infected by mild isolates (Takeshita, 1956b). Allen (1957a) studied virus isolates transmitted by R. padi not only on the basis of severity of disease caused, but also by the differential reaction of 4 small grain varieties. The 43 isolates investigated could be separated into 16 strains, which were grouped into 7 types. As Endo (1958) has pointed out, it is fortunate that the highly virulent isolates of the v i r u s do not seem to be as common in nature as do the less virulent ones. 3. Host Range Although isolates of BYDV differ in host range, as indicated by reports from at least five laboratories (see Section 111, A, 2, and Table I ) , it is not clear how much of the difference actually reflects variability among virus isolates and how much is based on other factors. Virus isolates similar in other ways do not necessarily have similar host ranges. Watson and Mulligan (1960b) suggested that there is more agreement among results from different laboratories if only the reactions of wild grass species are compared. Results of host range studies are difficult to interpret because of variations in the readiness with which aphids feed on different grass species, possible genetic differences among plants of any one grass species, and variability in the reaction among individual plants of any one species to any one virus isolate. Host range is not a suitable criterion for identification of BYDV, and

234

W. F. ROCHOW

identifications based on host range are open to question. For example, Yu et al. (1957,1958) and Pei and Hsu ( 1958), who described a virus disease of foxtail millet ( Setaria italica (L.) Beaux.) that is similar to the BYDV disease, may have placed undue emphasis on host range in identifying the causal virus. The virus was named millet red leaf virus, for they considered it distinct from BYDV mainly because it was transmitted to four hosts found to be immune from BYDV by Oswald and Houston (1953b). The limited usefulness of this criterion is illustrated by the fact that two of these hosts (Digitaria sanguinalis and Zea mays, Table 11) have since been found to be susceptible to other isolates of BYDV.

C. RELATIONSHIPSAMONG ISOLATES Many of the differences among isolates of BYDV are so marked that the characterized isolates have been called strains by most workers. It is far from clear, however, whether these isolates actually are strains of a single virus or whether they are distinct viruses. In fact, results of crossprotection tests, which are widely used for determining strain relationships, fail to support the position that the isolates are closely related strains (Allen, 1957a; Toko and Bruehl, 1959; Rochow, 1959b; Watson and Mulligan, 1960b). The issue, which is academic in some respects, will remain unresolved until chemical, physical, and serological criteria can be applied to the problem. Basic studies on the nature of the virus itself and on the relationship among isolates have not been made because attempts to transmit BYDV from plant to plant by mechanical means have failed (Oswald and Houston, 1953a; Takeshita, 1956a; Butler et al., 1960). Recent application of two techniques by which aphids can be made to acquire BYDV from liquid extracts shows promise in making possible fundamental studies. One method involves allowing aphids to feed through a membrane on liquid extracts made from BYDV-infected plants; this procedure works well for M. granarium and the strain it regularly transmits but not with any other combination tried (Rochow 1960a). The other method is based on injection of extracts into aphids by means of a fine glass needle (Mueller and Rochow, 1961). When the vectorspecific strains of BYDV were injected into R. padi or M . granarium, each species transmitted only the strain normally transmitted by it from plants. In other words, the vector specificity of these two virus strains obtains whether acquisition is by feeding or by injection.

BARLEY YELLOW DWARF

235

VII. Factors That Affect Outbreaks of the Disease

A. APHID SPECIESTRANSMITTING THE VIRUS It is unlikely that all the vectors of BYDV are of equal importance in nature. R. padi, M . granarium, and R. maidis are generally cited as the main vectors in most parts of the United States. In 1959 the greenbug was considered by many workers to be the principal vector in some areas. Most reports on aphids that are responsible for transmitting BYDV are based on field observations; virtually all evidence for the role of different aphid species is indirect. Many of these observations are based on the assumption that the most common aphid species is the most important vector. Since this assumption is known to be false in many cases involving aphid-transmitted viruses (Kennedy, 1950; Broadbent, 1953; K. M. Smith, 1958), it may be equally misleading in some instances for BYDV. An experiment by the writer illustrates the point. In the fall of 1959, a winter barley plot near Ithaca was virtually covered with R. maidis. Some of the infested plants also showed symptoms of infection by BYDV. When such leaves were tested for virus by means of four aphid species (Rochow, 1 9 5 9 ~ BYDV )~ was recovered from three of four plants. All transmissions were by M . granarium only, not by R. m i d i s . Thus, despite the huge population of R. maidis, the predominating virus strain was not one that can be transmitted regularly by R. maidis, but instead was one transmitted by M . granarium, a species present in much smaller numbers. Several lines of evidence suggest that M . granarium is the main vector in New York (Rochow, 1 9 6 0 ~ ) . A few direct attempts have been made to determine which aphids are important in nature. Stetson et al. (1958) collected aphids in Maine fields and caged them individually on test plants to determine how many were carrying virus. Thirteen of 47 specimens of R. padi and 4 of 10 specimens of M . granarium transmitted BYDV. Orlob ( 1959) collected 466 aphids of R. pndi and M . granarium over a two-year period in Wisconsin, but obtained transmission in only 3 cases. None of 19 field collections of several aphid species transmitted virus in tests made by Hebert et al. (1959) in North Carolina. Direct evidence for the occurrence of naturally viruliferous aphids was obtained by Slykhuis et al. (1959a) in Canada and Jedlinski and Brown (1959) in Illinois. Many more such direct tests are needed before the role of different aphid species can be evaluated. The major vector of one season may be different from that of another. In 1958 in Ontario, Canada, R. padi appeared to be the most important vector; in 1959 in the same region, M . granarium probably was the most

236

W. F. R O M O W

important vector (Slykhuis et al., 1959b). The sequence of occurrence of different species during any one season might be a further complicating factor. B. SOURCEOF Vmus INOCULUM The source of virus, and its relation to the vectors, is another major factor in spread of the virus. This general relationship is known, at least in part, for some regions. In the 1951 California outbreak, the warm, wet winter favored growth of wild grasses as well as the aphids that infested them. When a sudden long dry period occurred, the aphids moved from the grasses to young barley plants in nearby fields. Since many of the grasses were infected by BYDV, the virus was transmitted to the barley (Oswald and Houston, 1953b). Perennial grasses are considered to be a main source of virus in many other regions. Although naturally infected grasses have been detected in many areas, it does not necessarily follow that any one grass species serves as a virus source merely because it has been found to be infected by BYDV. The significant thing is whether the aphids that are active in the area feed or overwinter on the grass species in question. Even if aphids feed on a certain grass species, it may be of less consequence as a virus source and play less of a role in nature than do some other grass species that are more effective as virus sources (K. M. Smith, 1958). Winter grains, which are another possible source of BYDV in some regions, may serve in this role more commonly than do wild grasses. The greatest need is for knowledge on life cycles of the aphids and for information on how such cycles are related to the possible sources of BYDV in nature. A beginning in such studies was made by Coon ( 1959), who tested the adaptation of R. padi and M.granarium to 59 grass species in the greenhouse. Infected plants can be important as sources even though they are some distance from the susceptible plants.

C. APHID MOVEMENT The movement of aphids, not their numbers, is the critical factor in virus spread. Many workers have found no correlation between aphid counts on plants and spread of aphid-transmitted viruses (Kennedy, 1950; Broadbent, 1953; Hille Ris Lambers, 1955). With BYDV, for instance, Rothman et al. (1959) noted that aphid infestations were about equal on oats and barley but that only the oats were damaged appreciably. One possible explanation is that the vectors moved from plant to plant more readily in the oats than in the barley. Long-distance aphid movement is of particular si@cance for BYDV

BARLEY YELLOW DWARF

237

because of the persistent virus-vector relationship. Aphids can be carried great distances in air currents. There is no known reason why BYDV could not be spread over large areas in this way. Spring migrants from the south were important in spreading BYDV over a three-year period in Wisconsin (Orlob, 1959; Arny and Orlob, 1960). Such long-distance movement, as well as local aphid movement, is greatly affected by wind and other weather conditions. Local movement appears to be involved in some areas. In the Pacific Northwest, for instance, the mild winters usually favor aphid survival so that large aphid populations are close to most small grain fields at the start of the season (Raymer and Foote, 1959; Bruehl and Damsteegt, 1959; Bruehl et al., 1959). The ability of aphids to overwinter may be a key to the disease in many parts of the world. Since such overwintering is often controlled by the weather, it may some day be possible to predict outbreaks of this disease in areas where BYDV is spread mostly by aphids moving over short distances. Progress is now being made in the United States and Canada in the use of trapping records to determine the movement of small grain aphids. Yellow water traps are used frequently although there is evidence that yellow is not as efficient for aphids that infest small grains and grasses as it is for other species (Eastop, 1955). Wind traps, light traps, and sticky traps are also being used. In some areas traps are operated in conjunction with trap plants which are used in attempts to learn which aphids are carrying the virus and when most virus spread occurs. Much laborious work of this kind is needed to reveal details of aphid movements and to learn how such movements are related to the spread of BYDV. D. APHID FEEDING HABITS Many observations, particularly by workers in the North Central Region of the United States, have shown that, in a typical field, infected plants occur predominantly along the margins and in circular spots in the field, The writer and H. C. Murphy have frequently noted that this is not the typical pattern of infection in fields in northeastern United States. In the Northeast, infected plants are generally scattered uniformly throughout oat fields, Moreover, differences between patterns of the outbreak of the disease in the North Central Region in 1959 and in the Northeast in 1960 suggest that there are other significant differences between the two areas in the occurrence of BYDV. The explanation for such differences undoubtedly is based on the activity and feeding habits of the aphids involved and on their distribution in fields, which may be influenced by topography and wind conditions. M. granarium appears to

238

W. F. ROCHOW

be the principal vector in one part of the Northeast (Rochow, 1 9 6 0 ~ ) ; R. pudi may be more important in the North Central States than in the Northeast. Perhaps some of the differences observed are merely reflections of the differences in feeding habits and behavior of these two aphid species. The scattered distribution of M . grunurium in oat fields in the Northeast, an observation that has also been made for this aphid in other areas (Summers and Bowman, 1953), supports the possibility. E. CROP AGE M’HEN

INFECTED

Since losses are severe only when young plants become infected by BYDV, the relationship between age of plant and time of aphid infestation is a critical one. If aphids usually do not develop until the crop is nearly mature as in parts of Europe (Oswald and Thung, 1955; Rademacher and Schwarz, 1958), there is little chance of serious loss. On the other hand, if virus-carrying aphids are present when the crop is in the seedling stage, serious losses can be expected. The early appearance of aphids and the early season weather conditions might be a basis for forecasting the disease. When Schafer et al. (1959) observed a serious outbreak of aphids the second week in May, they correctly predicted that BYDV would be more severe than usual in Indiana in 1959. The writer obtained direct evidence for presence of viruliferous M . grunarium in New York as early as May 3, 1960; this was at a time when spring oats were just emerging. Since the cool damp weather of May and early June favored aphid development, the disease became more severe than usual in the following months. If forecasting were possible before planting time, growers in some regions might be able to plant some other crop that would be less likely to suffer serious losses. VIII. Control of the Disease

A. USE OF CROPVARIETIESRESISTANT TO

THE

Vmus

All workers have agreed with the suggestion of Oswald and Houston (195313) that eventual control of BYDV lies in development of resistant varieties. Perhaps the largest single research effort involving BYDV has been the search for sources of resistance and related studies on the incorporation of resistance into acceptable varieties of oats and of barley. Early progress has been more rapid with barley than with oats. In both cases the available sources of resistance might be described as tolerant to the virus. The best lines are susceptible to infection by BYDV, but they produce grain despite the infection (tolerance), tend to escape infection in nature, or combine both of these characteristics. Since the disease was present long before its cause was known, selec-

BARLEY YELLOW DWARF

239

tion undoubtedly has been made in the past for resistance to BYDV. ALBIONoats, now known to have some tolerance to BYDV, resulted from a selection made by L. C. Burnett at Ames, Iowa, in 1906 (Stanton, 1955), a year when BYDV may have influenced the selection (Manns, 1909). Good tolerance has been observed over a three-year period in New York in a progeny from an oat cross (CRAIG x ALAMO) made and selected by N. F. Jensen before the cause of the disease was known but during a time when it was present. 1. Sources of Tolerance

a. Oats. A major program to evaluate the reaction of oats to infection by BYDV was initiated by R. M. Endo and extended by H. Jedlinski, in cooperation with C. M. Brown, in Illinois. Over 4000 oat selections were tested during a three-year period by means of controlled inoculations made in the field (Endo, 1957%c ) . No immune or highly resistant oats (C.I. 1915) were found. ALBION(C.I. 729), C.I. 4918, and FULGHUM possessed the best tolerance among hexaploid oats (C.I. refers to accession numbers of the Cereal Crops Research Branch, Crops Research Division, U. S. Department of Agriculture). SAIA,a selection of Avena strigosa Schreb., had the best resistance. Although only one strain of virus was used in the preliminary screening, the most promising selections were further tested with four strains of BYDV that varied in virulence (Endo, 1958). The four lines mentioned above were among the eleven resistant selections detected in these additional tests. Seven of the eleven resistant selections belonged to the 21-chromosome group. In addition to being tolerant to the virus, some of the most promising selections appeared to have a tendency to escape infection. ALBIONhas been used as a source of resistance in the breeding program in Illinois. Arny (1959a) listed 12 oat lines that have shown some tolerance in Wisconsin. These included several selections that were tolerant in Illinois. Bruehl and Damsteegt (1959) evaluated over 3500 oat lines exposed to natural infection by BYDV in Washington. SAIAwas again more tolerant than ALBION, but so were over 130 other lines, including VICTORY; further, 306 lines were equal to ALBION. Raymer et al. (1960) found 7 lines that approached or equaled the performance of VICTORY in an evaluation of the first 1000 entries in the active USDA oat collection in Oregon. Many workers observed oats that appeared to have resistance during the 1959 outbreak in the United States. Some thirteen articles in Plant Disease Reporter Supplement 262, issued in December 1959, refer to possible sources of resistance in oats. The tolerance of ALBION,FULCHUM, NEWTON, PUTNAM, BEEDEE, and TONKA in the North Central Region in 1959 was noted by several workers.

240

W. F. ROCHOW

b. Barley. Oswald and Houston (1953a) listed four barley varieties (C.I. 1227, C.I. 1237, C.I. 2376, and ABATE) that appeared to be highly resistant and two additional lines (ROJO and HANNCHEN) that had some tolerance to BYDV. Schaller (1960) has since evaluated 6728 barley varieties for their reaction to BYDV infection at Davis, California. Plants were inoculated by infesting them artificially with aphids carrying naturally occurring virus strains. Only 28 of the varieties were highly resistant; 34 were resistant. All of these were considered suitable as parental material for breeding. Each of the resistant varieties came from Abyssinia or had Abyssinian types in its parentage. The 39 entries found to have some tolerance in Wisconsin by Amy (1958, 1959b) included some varieties, such as C.I. 3906-1, C.I. 3906-2, C.I.3908-1, and C.I. 2376, also found tolerant in California. When Bruehl and Damsteegt (1960) exposed 318 barleys found tolerant by Schaller, along with 46 suggested by Amy, to natural infection in Washington, 75 lines were considered worth further testing. The best varieties were C.I. 3208-1, C.I. 3208-2, and C.I. 3208-4. Foote et al. (1960) tested 321 of the best lines from Schaller’s tests by exposing them to natural infection in Oregon. The 20 best lines all came from Abyssinia. ABATE,C.I. 2376, C.I. 1227, and C.I. 1237 ranked 7, 9,10, and 13, respectively. Although there are differences among the results of tests carried out at the various locations, there is general agreement on the fact that many of the most tolerant barleys are lines that came from Abyssinia. c. Wheat. Little attention has been given to reactions of wheat varieties to BYDV. Oswald and Houston (1953a) listed SONORA 37 as the most tolerant of the California wheats. Bruehl and Toko (1955) found IDto be tolerant to one virus strain in Washington. H. C. Smith (1959) has initiated a survey for resistance in wheat in New Zealand. 2. Problems in Evaluation of Tolerance to BYDV Results of tests for tolerance, both in barley and oats, are not in complete agreement. ALBION,for instance, is one of the most tolerant oat varieties in Illinois, whereas many lines appear superior to it in Washington. The discrepancies are not particularly surprising in such a complex disease about which so little basic information is available. There are at least four major factors that might explain some of these discrepancies. First, the method of inoculation is usually different at each location. Controlled inoculation resulting from artificial aphid infestation reduces the chance of a line escaping infection and eliminates variability due to ages of plants at time of infection. It has been suggested (Shands and Cruger, 1959) that such inoculations may be too severe and that varieties

BARLEY YELLOW DWARF

241

with some field resistance might be overlooked in tests of this type. In tests relying on infection under natural conditions, there is no control over virus strain used, identity or prevalence of vector, or age of plant when inoculated. In addition, differences in acceptibility of ditFerent varieties to the vectors might be important as well as the prevalence of other diseases. Tests such as those carried out in California, where a combination of controlled and natural methods is used, introduce further obvious complications. Second, the method of evaluating the reactions of varieties differs among locations. Endo and Brown (1957b) found that it was not possible to predict yield of a particular oat line on the basis of a diseaseseverity reading for leaves, an estimate of the number of leaves with symptoms, the amount of stunting, or an estimate of the amount of blasting. Other workers have found such factors useful in evaluating oat lines. Since most workers use their own evaluation system, this undoubtedly influences the variation among results. Third, the relative adaptation of varieties at different locations would be expected to vary. This might affect the performance of a particular variety directly as well as influence its relative reaction to infection by BYDV. Delay in maturity, for example, might be expected to increase the damage caused by BYDV (Endo and Brown, 1957a). Fourth, the possible role of strains of BYDV not only in the evaluation of sources of resistance, but also in further development and use of resistant varieties, could be most significant. This factor has not been considered to any great extent to date, but as Jedlinski and Brown (1959) have pointed out, it is one aspect of breeding that now requires further study. The existence of strains of BYDV is well established; whether variations among these strains are important in nature has not been well established, but several observations suggest that they are. Hebert et al. (1959) reported that DAVE barley appeared more susceptible when inoculated by controlled aphid infestation than when exposed naturally in the field. W. B. Raymer (personal communication) found BONNEVILLE barley to be very resistant to a strain of BYDV from eastern Oregon but badly damaged by a strain collected in western Oregon. Bruehl and Toko (1955) found evidence that some varieties tolerant to one virus strain were less tolerant to other strains. Observations such as these suggest that variation in the pathogen, so well known for rusts, might be equally critical in the breeding of varieties tolerant to BYDV. A striking example of the possible importance of strains of the virus or of the interaction of virus strains with other factors is the fact that Ab 101 (C. I. 7232) was consistently found to be one of the most susceptible oats tested in Illinois by Jedlinski (1960,personal communica-

242

W. F. ROCHOW

tion) whereas Coffman (1960) noted apparent tolerance of this line in Idaho.

3. Inheritance of Tolerance Incorporation of increased tolerance to BYDV into new lines of spring grains has progressed farther for barley than for oats. Suneson (1955) noted the resistance of some F2 barley lines from a cross of ROJO and CALIFORNIA MARIOUT during the outbreak of the disease in California in 1951. The tolerance of ROJO,found to involve a single recessive gene, was set in barley plants after four backcrosses. Rasmusson and Schaller (1959) also obtained evidence for the existence of one gene conditioning tolerance to BYDV in each of four barleys (C.I. 1227, C.I. 1237, C.I. 2376, and ABATE). Their evidence showed that the four varieties derived tolerance from the same gene, which was incompletely dominant. The symbol Y d , was assigned to this gene, and ydl was suggested as a symbol for the recessive gene in ROJO reported by Suneson (Rasmusson and Schaller, 1959). For oats the most advanced program is probably that of Brown and Jedlinski (1960). Although their selections are not yet of good agronomic type and have little rust resistance, the average yield of 14 fifth- or sixthgeneration selections ( ALBION x FAYETTE) after heavy natural infection in 1959 was 80 bushels per acre compared with an average yield of 69 bushels for ALBION and 50 bushels for the susceptible variety FAYETTE (Jedlinski and Brown, 1959). Brown and Jedlinski (personal communication) have some indications that one or relatively few major genes, with possible modifiers, are involved. Coffman (1960) studied progeny from crosses of BUCK MESDAG with Ab. 101 (C.I. 7232), which appeared to be tolerant to BYDV at Aberdeen, Idaho, in 1959. His data indicated that probably two or more factors for tolerance were involved in this cross between the hexaploid and the derived tetraploid. Although the inheritance of resistance is not completely understood, it is clear that both in oats and in barley the incorporation of increased tolerance is a workable approach to control of the BYDV disease. B. USE OF INSECTICIDES In an excellent review on use of insecticides in the control of virus diseases, Broadbent (1957) pointed out that attempts to control insecttransmitted viruses by applying insecticides have generally failed; sometimes application of an insecticide has actually increased the disease despite “control” of the insect. Such failures are related to the basic fact that spread of plant viruses is not necessarily proportional to the extent or to the duration of an insect infestation. Failures have been greatest

BARLEY YELLOW DWARF

243

with nonpersistent aphid-transmitted viruses. Some success has been achieved in the control of persistent aphid-transmitted viruses when the source of virus is within the crop; less success has occurred when the aphids that enter a crop are already carrying virus. Results of attempts to control BYDV by means of insecticides are in agreement with these general results obtained for other viruses and other crops. Orlob (1959) noted some reduction of secondary spread of BYDV in oat and barley plots sprayed with Systox or parathion. In other Wisconsin tests, some reduction in the incidence of disease and occasional indications of yield increases were obtained ( Amy, personal communication). Dickason et al. (1960) tested several insecticides in various kinds of treatments. Some treatments resulted in initial suppression of aphids and some delay in virus infection, but 100 per cent of the barley plants eventually became infected; the treated and untreated barley did not differ in yield or quality. Jedlinski and Brown (1959) used malathion to reduce aphid populations and direct feeding injury of greenbugs, but the prevalence of BYDV was equal in sprayed and unsprayed plots. Pizarro and Arny (1958) used Systox in several ways; they found some protection from aphid infestation but no protection from BYDV. Seed treatments with insecticide in Michigan (Kiesling, 1959) and in other areas had no effect on incidence of BYDV infection. In contrast to most other reports, Caldwell et al. (1959a) found striking control of BYDV in Indiana with one spray of Dimethoate. If this material or some other new insecticide should continue to prove useful, application of insecticides may be of value, at least for treatment of breeding nurseries and special fields. It may be possible to treat overwintering hosts or plants that serve as sources of virus when more information is available. At present, however, there seems to be little hope that use of insecticides would be economical on any wide scale for control of BYDV. C. CULTURAL PRACTICES In some areas the time of seeding may influence control of the disease. Many workers have advised early seeding of spring grains so that plants will likely be beyond the seedling stage when aphids become active (Endo and Brown, 1957a; Rademacher and Schwarz, 1958; Jedlinski, 1959; Orlob, 1959). In some regions, such as the Pacific Northwest, it is doubtful whether the seeding date can be used with any assurance of success (Bruehl and Damsteegt, 1959), although early seeding may alleviate damage in some seasons (Bruehl et al., 1959). In California, the recommendation for oats is to avoid very early (October 1) or very late (February 15) seeding because aphid populations are usually higher in early fall and early spring ( Suneson et al., 1959). Uniform rate and depth

244

W. F. ROCHOW

of planting, proper fertilization, and good management are additional factors that have been found to contribute to reduced destruction by BYDV. D. POSSIBLE USE OF CROPVARZETIES UNACCEPTABLE TO VECTORS Use of varieties tolerant to the virus appears promising as a major control method for BYDV; however, there are certain dangers inherent in the use of tolerant varieties. For instance, their widespread use perpetuates an enormous pool of inoculum. It seems worth while, therefore, to continue to explore other possible control measures. One such possibility is the use of small grains that would be unacceptable to the vector. As Sylvester (1958) has suggested, it might be possible to modify epidermal tissues of plants enough to prevent an aphid from doing anything more than testing the tissues for acceptability. Since aphids need to feed for more than a few minutes to transmit BYDV, such modification should prevent transmission of this virus. Modification of the epidermal tissues might be brought about by chemical treatments or through breeding. A program of exploring the breeding possibilities by evaluating the acceptability of small grain varieties for some of the vectors has been initiated by B. F. Coon at Pennsylvania State University. IX. Summary and Conclusion

Although the barley yellow dwarf virus disease of small grains has been present for many years, its virus cause was not discovered until 1951. The disease occurs in all parts of the United States and in many regions of the world where oats, barley, or wheat are grown. Yield losses due to infection are usually greater in oats and in barley than in other small grains. At least 84 grass species are susceptible to infection. The wide host range and general occurrence of barley yellow dwarf virus make it potentially the most destructive virus known to affect small grains and grasses. Symptoms of the disease, which include stunting and characteristic leaf discolorations, often are not sufficiently diagnostic for certain identification of the disease. Transmission of the virus by means of at least one of nine known aphid vectors is the only means of confirming diagnosis. Barley yellow dwarf virus is a yellows-type virus representing a relatively small group of plant viruses that persist in their aphid vectors. A main recent development is the discovery that isolates of the virus vary with regard to the aphid species that serve as a vector, the severity of disease caused, and host range. Since aphids are the only known means by which the virus can be

BARLEY YELLOW DWARF

245

transmitted from plant to plant, they play the central role in outbreaks of the disease. One of the areas where knowledge is most needed is that of the relative importance of different aphid species as vectors and the relationships between life cycles of the important vectors and the plants that serve as sources of virus. Losses due to barley yellow dwarf virus can be reduced by proper management practices coupled with use of varieties tolerant to infection. Excellent progress has been made with barley and oats in the incorporation of increased tolerance into potential new varieties, but the final outcome of such efforts is clouded by the possible importance of strains of the virus and by the general lack of fundamental knowledge about the virus and the aphids responsible for its spread. ACKNOWLEDGMENTS This review is part of a cooperative investigation of Crops Research Division, Agricultural Research Service, U.S.D.A., and Gomell University Agricultural Experiment Station; supported in part by a Public Health Service research grant, E-2540, from the National Institutes of Health.

REFERENCES Allen, T. C., Jr. 1957a. Phytopathology 47, 481-490. Allen, T. C., Jr. 195713. Phytoputhology 47, 1-2. Allen, T. C., Jr., and Houston, B. R. 1956. Plant Disease Reptr. 40, 21-25. Amy, D. C. 1958. 1957 Bailey Newsletter 1, 76. Amy, D. C. 1959a. 1958 Oat Newsletter 9, 67-68. Amy, D. C. 195913. 1958 Barley Newsletter 2, 83-84. Amy, D. C., and Orlob, G. B. 1960. 1959 Barley Newsletter 3, 17-18. Amy, D. C., and Shands, H. L. 1959. Plant Disease Reptr. Suppl. 262, 376. Barrus, M. F. 1937. Plant Disease Reptr. 21, 359-361. Blackmon, C. A. 1957. Maine Farm Research 6, 18. Broadbent, L. 1953. Biol. Revs. Cambridge Phil. SOC. 28, 350-380. Broadbent, L. 1957. Ann. Rev. Entomol. 2, 339-354. Brown, C. M., and Jedlinski, H. 1960. lllinois Research 2( 4 ) , 16. Browning, J. A., Wheat, J. G., and Frey, K. J. 1959. Plant Diseuse Reptr. Suppl. 262, 336-341. Browning, J. A., Peters, D. C., and Frey, K. J. 1960. Iowa Farm Sci. 14, 6-8. Bruehl, G. W. 1958. Plant Disease Reptr. 42, 909-911. Bruehl, G. W., and Damsteegt, V. D. 1959. Plant Disease Reptr. Suppl. 262, 369370. Bruehl, G. W., and Damsteegt, V. D. 1960. 1959 Barley Newsletter 3, 71-72. Bruehl, G. W., and Toko, H. 1955. Plant Disease Reptr. 39, 547-549. Bruehl, G. W., and Toko, H. V. 1957. Plant Disease Reptr. 41, 730-734. Bruehl, G. W., McKinney, H. H., and Toko, H. V. 1959. Plant Disease Reptr. 43, 471-474. Butler, F. C., Grylls, N. E., and Goodchild, D. J. 1960. J . Australian lnst. Agr. Sci. 26, 57-59.

246

W. F. ROCHOW

Caldwell, R. M., Wilson, M. C., and Schafer, J. F. 1959a. Plant Disease Reptr. SUppl. 262, 334-335. Caldwell, R. M., Schafer, J. F., Compton, L. E., and Patterson, F. L. 1959b. Plant Disease Reptr. Suppl. 262, 333. Coffman, F. A. '1960. 'i959 Oat Newsletter 10, 14-15. Coon, B. F. 1959. J. Econ. Entomol. 62, 994-996. Dickason, E. A., Raymer, W. B., and Foote, W. H. 1960. Plant Disease Reptr. 44, 501-504. Dickson, J. G., Logsdon, C. E., and Taylor, R. L. 1957. Phytopathology 47, 7-8. Eastop, V. F. 1955. Nature 176, 936. Empson, D. W. 1958. Plant Pathology 7, 85-87. End'o, R. M. 1957a. Phytopathology 47, 9. Endo, R. M. 1957b. Phytopathology 47, 520. Endo, R. M. 1957c. Phytopathology 47, 520. Endo, R. M. 1958. 1957 Oat Newsletter 0, 44-46. Endo, R. M., and Brown, C. M. 1957a. Agron. J. 49, 503-505. Endo, R. M., and Brown, C. M. 195%. 1956 National Oat Newsletter 7, 5-7. Endo, R. M., and Brown, 6.M. 1960. Phytopathology 60, 84. Esau, K. 1957a. Am. J. Botany 44, 245-251. Esau, K. 1957b. Hilgardia 27, 15-69. Foote, W. H., Raymer, W. B., and Sieveking, W. E. 1960. 1959 Barley Newsletter 3, 65-66. Galloway, B. T., andSouthworth, E. A. 1890. J. Mycul. 6, 72-73. Hebert, T. T., Kline, D. M., and Toler, R. W. 1959. Plant Disease Reptr. Suppl. 262, 361-363. Hille Ris Lambers, D. 1955. Ann. Appl. Biol. 42, 355-360. Hille Ris Lambers, D. 1960. Virology 12, 487-488. Ikaeimo, K. 1960. Maataloustieteelllnen Aikakauskilja 32, 62-70. Jedlinski, H. 1959. Dept. Plant Pathol., Univ. Illinois Circ. 101, 2 pp. Jedlinski, H., and Brown, C. M. 1959. Pknt Disease Reptr. Suppl. 262, 326-332. Johnson, T., and Brown, A. M. 1940. ScZ. Agr. 20, 532550. Kennedy, J. S. 1950. Nature 166, 1024-1025. Kieshg, R. L. 1959. Plant Disease Reptr. Suppl. 262, 347. Kreitlow, K. W. 1%9. Western Grass Breeders Work Planning Cmf. Rept. 15, 19-20. Lindsten, K. 1959. Phytopathol. 2. 36, 420-428. McKinney, H. H. 1950. Plant Disease Reptr. 34, 151-154. McKinney, H. H., Specht, A. W., and Stanton, T. R. 1952. Plant Disease Reptr. 36, 450-458. Manns, T. F. 1909. Ohio Agr. Expt. Stu. Bull. 210, 91-167. Mueller, W. C., and Rochow, W. F. 1961. Virology 14 (in press). Murphy, H. C. 1959. Plant Disease Reptr. Suppl. 262, 316. Orlob, G. B. 1959. Ph.D. Thesis, Univ. of Wisconsin, Madison, Wisconsin. Orlob, G. B., and Amy, D. C. 1960. Virology 10, 273-274. Oswald, J. W., and Houston, B. R. 1951. Plant Disease Reptr. 36,471-475. Oswald, J. W., and Houston, B. R. 1952. Plant Disease Reptr. 36, 182-183. Oswald, J. W., and Houston, B. R. 1953a. Phytopathology 43, 128-136. Oswald, J. W., and Houston, B. R. 195313. Phytopathology 43, 309-313. Oswald, J. W., and Thung, T. H. 1955. Phytopathology 46, 695. Peck, C. H. 1890. 43rd Rept. New York Stute Museum pp. 49-97. Peck, C. H. 1891. 44th Rept. New York State Museum pp. 115-187.

BARLEY YELLOW DWARF

247

Pei, M. Y., and Hsu, H. K. 1958. Acta Phytoputh. Sinica 4, 93. Pizarro, A. C., and Amy, D. C. 1958. Plant Diseuse Reptr. 42, 229232. Rademacher, B. 1932. Arch. Pjianzenbau 8, 510-513. Rademacher, B., and Schwarz, R. 1958. Z. Pflanzenkrankh. u. PfEnnzenschutz 66, 641-650. Rasmusson, D. C., and Schaller, C. W. 1959. Agron. J. 61, 661-664. Raymer, W. B., and Foote, W. H. 1959. Plant Diseuse Reptr. Suppl. 262,365-368. Raymer, W . B., Foote, W. H., and Sieveking, W. E. 1960. 1959 Oat Newsletter 10, 63. Richards, W. R. 1960. Can. Entomologist 92 (Suppl. 13): 1-51. Rochow, W. F. 1958a. P h Diseuse Reptr. 42, 36-41. Rochow, W. F. 1958b. Plunt Disease Reptr. 42, 905-908. Rochow, W. F. 1959a. Phytopathology 49, 548. Rochow, W. F. 1959b. Phytopathology 49, 744-748. Rochow, W. F. 1959~.Plant Diseuse Reptr. Suppl. 262, 356-359. Rochow, W. F. 1959d. New York Farm Research 26, 6. Hochow, W. F. 1960a. Virology 12, 223-232. Rochow, W. F. 1960b. Phytopathology 60, 881-884. Rochow, W. F. 1960c. Plant Disease Reptr. 44, 940-942. Rochow, W. F. 1961. Phytopathology 61 (in press). Rothman, P. G., Bowman, D. H., and Ivanoff, S. S. 1959. Plant Disease Reptr. Suppl. 262, 348-350. Schafer, J. F., Caldwell, R. M., Cartwright, W. B., and Gallun, R. L. 1959. Plant Diseuse Reptr. 43, 1052. Schaller, C. W . 1960. Phytopathology 60, 653. Sechler, D. T., Poehlman, J. M., Whitehead, M. D., and Calvert, 0. H. 1959. Plant Diseuse Reptr. Suppl. 262, 351-353. Shands, H. L., and Cruger, L. G. 1959. Plant Diseuse Reptr. Suppl. 262,371-374. Sill, W. H., Jr., King, C. L., and Heyne, E. G. 1959. Plant Diseuse Reptr. Suppl. 262, 342-345. Slykhuis, J. T. 1958. F A 0 Plant Protect. Bull. 6, 129-134. Slykhuis, J. T. 1960. Cereal News 6, 7-11. Slykhuis, J. T., Zillinsky, F. J., Hannah, A. E., and Richards, W. R. 1959a. Plant Dkeuse Reptr. 43, 849-854. Slykhuis, J. T., Zillinsky, F. J., Young, M., and Richards, W. R. 1959b. Plant Diseuse Reptr. Suppl. 262, 317-322. Smith, H. C. 1959. New Zealand Wheat Reu. 1958-1958 No. 7, pp. 51-56. Smith, K. M. 1958. Ann. Rev. Entomol. 3, 469-482. Sprague, R. 1936. Plant Diseuse Reptr. 20, 114-115. Sprague, R. 1939. Oregon Agr. Expt. Sta. Circ. Inform. 208, 1-3. Stanton, T. R. 1955. U.S. Dept. Agr. Tech. Bull. 1100, 1-206. Stetson, B. J., Johnson, B. E., and Simpson, G. W. 1958. Maine Farm Research 6, 14-16. Summers, T. E., and Bowman, D. H. 1953. Plant Diseuse Reptr. 37, 142-147. Suneson, C. A. 1955. Agron. J. 47, 283. Suneson, C. A., and Ramage, R. T. 1957. Agron. J. 49, 365-367. Suneson, C. A., Miller, M. D., and Houston, B. R. 1959. California Agr. Expt. Sta. Ext. Circ. 481, 1-23. Sylvester, E. S. 1958. Proc. 10th Intern. Congr. Entomol. (Montreal, 1956) 3, 195200. Takeshita, R. M. 1956a. Phytopathology 46, 436-440.

248

W. F. ROCHOW

Takeshita, R. M. 195613. Phytopathology 46, 28. Takeshita, R. M., Brown, C. M., Bonnett, 0. T., and Bever, W. M. 1956. 1955 National Oat Newsletter 6, 24-28. Toko, H. V., and Bruehl, G. W. 1956. Plant Disease Reptr. 40, 284-288. Toko, H. V., and Bruehl, G. W. 1957. Phytopathology 47, 536. Toko, H. V., and Bruehl, G. W. 1959. Phytopathology 49, 343-347. Watson, M. A., and Mdligan, T. 1957. Plant Pathol. 6, 12-14. Watson, M. A., and Mulligan, T. 1960a. Ann. Appl. Biol. 48, 711-720. Watson, M. A., and Mulligan, T. 1960b. Ann. Appl. Biol. 48, 559-574. Wilson, V. E., and Murphy, H. C. 1953. Plant Disease Reptr. 37, 21-23. Wit, F. 1956. Euphyticu 6, 119-129. Yu, T. F., Pei, M. Y., and Hsu, H. K. 1957. Actu Phytopath. Sinica 3, 1-18. Yu, T. F., Hsu, H. K., and Pei, M. Y. 1958. Acta Phytopath. Sinica 4, 5-7.

THE ABUNDANCE OF EARTHWORMS I N AGRICULTURAL LAND AND THEIR POSSIBLE SIGNIFICANCE IN AGRlCULTURE K. P. Barley Waite Agricultural Research Institute, Adelaide, South Australia

Page I. Introduction ................................................ 249 11. Earthworm Populations under Crops and Pastures . . . . . . . . . . . . . . . . 250 A. Measurement of Number and Weight ....................... 250 B. Population Determinants .................................. 251 C. Population Data .......................................... 253 111. The Influence of Earthworms on Soil Fertility .................... 256 A. Consumption of Soil and Organic Material . . . . . . . . . . . . . . . . . . 256 B. Influence on the Supply of Plant Nutrients . . . . . . . . . . . . . . . . . . . 257 C. Modification of the Physical Properties of Soils . . . . . . . . . . . . . . . . 259 D. Effects on Other Organisms in the Soil ...................... 261 IV. Effects of Earthworms on the Yield of Crops and Pastures . . . . . . . . . . 262 A. Earthworms as Pests ...................................... 262 B. Experiments in Artificial Cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 C. Field Experiments ........................................ 264 V. Conclusion .................................................. 266 References .................................................. 266

1.

Introduction

Productive soils frequently contain macroscopic animals that feed upon plant litter or prey upon the various litter feeders. On agricultural land earthworms are often the most conspicuous group and may constitute from half to three-quarters of the total weight of the fauna. Because the earthworms are conspicuous, naturalists are apt to exaggerate their effects on the soil. Although it is qualitatively true to say that earthworms incorporate and help to decompose plant litter and dung and that their tunnel making helps to keep soils open and porous, what we need to know is the quantitative effect of such processes on productivity. The word “earthworm” has been used to describe many terrestrial oligochetes, but authors often exclude or neglect small species. In this review the word “earthworm” is taken to mean an earth-living oligochete of any size. 249

250

K. P. BARLEY

The geographical distribution of the oligochetes has been described by Stephenson (1930), who concluded that earthworms may be found anywhere except in polar and desert areas. Dim0 (1938) gave an example of the ability of earthworms to grow and reproduce under difficult conditions in the arid soils of Georgia, U.S.S.R. Earthworms were moderately abundant even though drought and cold restricted their activity to a period of 50 to 60 days in spring. The distribution of the earthworms has been greatly modified by transport in soil used as ship’s ballast and in soil accompanying potted plants. In many parts of the Southern Hemisphere the introduction of lumbricids, and of the Allolobophora caliginosa complex (Gates, 1958) in particular, has led to the establishment of an abundant earthworm fauna adapted to feed on the kind of litter and dung returned to the soil from exotic pastures and crops. Colonization by introduced peregrines usually occurs during or some years after the disappearance of the endemic earthworm fauna, the loss of the endemic fauna being caused by failure of the original food supply, rather than by competition with the introduced species ( Miller et al., 1955). Michaelsen’s ( 1907) collections in southwestern Australia, for example, showed that the peregrine A. caliginosa ( Sav.) had been widely distributed through the agricultural zone and had become numerous near towns within seventy years of the first settlement by European man. As Abbie ( 1954) suggests, it is of great interest to h d out whether the introduction of a new earthworm fauna to such areas has changed, or is likely to change, the fertility or rate of denudation of the soils. II. Earthworm Populations under Crops and Pastures

A. MEASUREMENT OF NUMBER AND WEIGHT Digging out and hand sorting is the best method for recovering worms longer than 2 cm. from easily crumbled soils. A known number of marked worms should first be added to the sample to provide a check on the efficiency of sorting (Nelson and Satchell, 1960). Cocoons and small worms are missed, but these contribute little to the weight of the population. Sampling methods of this kind have been reviewed by Wilcke (1955). When the soil is difEicult to crumble, or where there is a mat of organic matter, a high recovery can be obtained by washing followed by hand sorting and flotation in dense solutions (Raw, 1960). When vermifuges, such as formalin (Raw, 1959) or potassium permanganate (Evans and Guild, 1947), are applied some of the worms come to the surface, but such methods give a low recovery compared with hand sorting (Svendsen, 1955). Overgaard Nielsen ( 1953) has described an effective method of extracting enchytraeids from soil cores by wetting and heating.

EARTHWORMS IN AGRICULTURAL LAND

251

Electrical methods of extraction have been described by Doeksen (1950) and by Satchel1 ( 1955b). When an alternating current ( A.C.) of several amperes is discharged into moist soil some of the earthworms emerge from the area within a radius of 1 meter from the electrode. Direct currents or intermittent low watt-sec discharges from an induction coil are ineffective compared with 50 C.P.S. A.C. High energy intermittent discharges have not yet been tested. The distance from the electrode at which extraction is most efficient depends upon the moisture content of the soil and upon the kind and size of worm. If the fresh weight of worms is to be determined, the method of handling the animals must be carefully standardized. Earthworms are never fully hydrated in unsaturated soils, and their water content can vary between wide limits. Thus Grant (1955a) found that the water content of A. caliginosa could vary from 570 to 140 per cent of the dry weight, the lower lethal limit being similar for both mature and immature worms. The worms become fully hydrated when immersed in aerated water or when kept on a pad of saturated blotting paper. Water uptake may continue for 5 hours or more. After most of the contents of the gut have been voided, the weight remains steady for some days (Bahl, 1947). Although most of the soil in the gut is voided within 4 days, small amounts may remain in the gut for more than 4 weeks (Roots, 1956). According to Wolf (1940), rolling the worms on filter paper removes not only excess water from the body surface, but also the water stored in the nephridia. As much as one-eighth of the water in fully hydrated worms is removed by rolling. B. POPULATION DETERMINANTS

1. Reproduction, Length of Life, and Dispersal The rate of cocoon production varies greatly with species, temperature, moisture, and food supply. Evans and Guild (1948) measured the cocoon production of a number of lumbricids fed with dung during a period when the mean soil temperature was 11°C. The number of cocoons per adult per month ranged from 0.3 to 12.0. In most of the species examined only one young worm emerged from each cocoon. As these figures suggest, the birth rate of earthworms is low compared with that of many other soil animals. Little is known about the age distribution of earthworms in nature. Individual worms may be kept alive for several years in the laboratory, but in the field many deaths are caused by predators, such as birds, moles, centipedes, and staphylinid beetles, and by parasites, such as gregarine protozoa and certain nematodes. Hamblyn and Dingwall (1945) found that the rate of advance of

252

K. P. BARLEY

the margin of populations of A. caliginosa (Sav.) into recently limed grasslands was of the order of 10 m. per year. Rapid dispersal in broad areas must therefore depend on transport by man and other agents. Cocoons are laid near the surface of the soil and can be transported on the hooves of livestock (Lee, 1958) or on cultivating implements. Both worms and cocoons can be transported by water flowing along streams and channels. 2. Food Small lumbricids and enchytraeids live and feed chiefly within surface litter, and their excrements consist almost entirely of litter fragments (van der Drift, 1951). Larger species come to the surface to feed on dead leaf or dung, but they also tunnel through the soil and feed on dead roots. The young of A. caliginosa (Sav.) prefer dung to leaf litter when offered a choice; but they can gain weight on a diet either of dung or of grass and clover leaves providing that the material has decayed to a soft condition. Fresh dung or leaf is not ingested (Waters, 1951). Mycelium and other microbial tissue in the decaying material ingested may itself be an important constituent of the diet. In a field experiment Watkin (1954) found that preventing the return of dung to pasture land substantially decreased the weight of the lumbricids present.

3. Soil Conditions As a working rule, where temperate grasses can grow, some species of earthworm can also grow. A few weeks' drought is sufficient to kill both worms and cocoons of some enchytraeids (Overgaard Nielsen, 1955), but many earthworms in a quiescent state can withstand several months' drought. Data of Evans and Guild (1948) suggest that lumbricids become inactive when the soil moisture suction exceeds 1 atmosphere. Death may be caused if low partial pressures of oxygen persist for more than a few hours; however many earthworms can tolerate high carbon dioxide concentrations providing that the supply of oxygen is maintained (Stephenson, 1930). Although earthworms may be found on soils of any texture, species that make tunnels are excluded from very loose soils and enchytraeids are unable to penetrate layers that lack wide pores. Earthworms are killed if the temperature falls below freezing point for more than a few hours. Although conditioning has a considerable influence on the upper lethal limit, the earthworms cannot tolerate high temperatures. Even after conditioning for a month at 22'CC.,the lumbricid A. caliginosa and the megascolecid Pheretima hupeiensis cannot withstand temperatures higher than 25°C. for more than a few days (Grant,

EARTHWORMS IN AGRICULTURAL LAND

253

1955b). Within the lethal limits, temperature greatly influences the activity of the worms. For example, the rate of cocoon production of the lumbricids studied by Evans and Guild (1948) increased four times over the range from 6 to 16°C. Species differ widely in their salt tolerance. Some enchytraeids and a few members of other families are euryhaline, but tunnel-making lumbricids are rarely found where the salinity of the soil solution exceeds 0.07 M for long periods. Data on reaction to chloride solutions of different concentration are given by Parker and Metcalf ( 1906). Few earthworms are found in acid soils where the pH is below 4.5, although small species such as Lumbricus rubellus (Hoff.) may be abundant within the litter on top of highly acid soils. Satchel1 (1955a) found that Allolobophora chlorotica (Sav.) did not enter soils of pH less than 4, and Laverack (1960) showed that solutions having this acidity caused considerable irritation of sense organs in the body wall of the less acidtolerant species. Correlations found between mineral nutrient status and the abundance of earthworms may account for much of the variance observed in worm numbers (Jefferson, 1956). However earthworms probably obtain most of their mineral nutrients from ingested leaf litter or dung, and correlations of this kind often depend upon an effect of the soil factor on the yield of vegetation and hence the food supply. C. POPULATION DATA On agricultural land earthworms are most abundant where productive pastures have been grown for a number of years. Most workers have sampled populations by digging and hand sorting. The various estimates for the abundance of worms comparable in size to the lumbricids range from 100 to 1200 per square meter, the mean body weight being 200 to 400 mg. In addition, 10,000 enchytraeids of mean body weight 1 to 3 mg. are often found when suitable methods of extraction are employed (von GrafF, 1953). At Palmerston North in New Zealand, where the worms remain active throughout the year, the pastures support 150 pounds body weight of worms per 1000 pounds annual production of dry herbage (Waters, 1951). By comparison, in a seasonal climate at Adelaide, South Australia, where the worms have to withstand 5 months’ drought each summer, the pastures support about 80 pounds of worms per 1000 pounds of dry matter production (Barley, 1959a). Evans and Guild (1948) found that when old pasture land at Rothamsted, England, was cultivated and cropped earthworm numbers were maintained for 6 months but subsequently declined. After the fifth cereal crop, numbers had fallen to a quarter of the value under pasture. It had previously been noticed (Cernosvitov and Evans, 1947) that the per-

254

K. P. BARLEY

manganate method of extraction used at Rothamsted gave poor results on soils disturbed by cultivation. The changes reported by Evans and Guild could therefore have been due to the failure of their sampling technique. However, measurements made by core sampling and hand separation have also shown that cultivation and cropping reduce the abundance of earthworms (von Graff, 1953). On intensively cropped land, populations of moderate size, say 20 g. per square meter of lumbricids and 5 g. per square meter of enchytraeids, may be maintained if farmyard manure is applied from time to time, but exhaustive cropping without the addition of organic manure reduces populations to a negligible level. When land is cropped on a rotation including pasture ley, the abundance of earthworms fluctuates with the phase of the rotation. Ponomareva (1950) measured the number of worms in each phase of a wheat, cereal rye, two years’ grass rotation near Moscow, U.S.S.R., and found that numbers were highest in the second year of the ley. In a similar experiment at Adelaide, Australia, where land was cropped on a rotation of fallow, wheat, two years’ pasture ley, Barley (1959a) found that the weight present after the fallow was only 25 per cent of that found on permanent pastures, but by the end of the ley the weight was at least 70 per cent of that on permanent pasture. Although the weight of earthworms present after a ley has been established for several years can be as great as that found on old grassland, the kind of worms present may nevertheless be different (Evans and Guild, 1948). The effects of fertilizers, and of various insecticides and herbicides commonly used in agriculture, on the abundance of earthworms have recently been reviewed by Satchel1 (1955c, 1958) and will not be further described here. The most reliable data on seasonal fluctuations in the abundance of earthworms are those given by Waters (1955) for pasture land at Palmerston North, New Zealand (see Fig. 1). Waters sampled to the full depth of the population, and his estimates included both active worms and worms in diapause. Samples were taken at monthly intervals for 30 months on an old pasture of perennial rye grass and white clover, which produced about 15,000 pounds per acre of dry matter per year. The weight of worms fell to a minimum of 140 g. per square meter in summer, then increased to a maximum of 300g. per square meter in the early winter. Waters suggested that the seasonal fluctuations in weight in this equable environment were chiefly caused by changes in the supply of food. In a climate with a hot dry summer at Adelaide, Australia, Barley (1959a) found that the number and weight of earthworms increased after the break of the season until midwinter but decreased during spring. Any deaths caused by desiccation during summer were balanced by the

255

EARTHWORMS IN AGRICULTURAL LAND

emergence of young worms from cocoons soon after the opening rains. Data given by Hopp (1947) for Maryland, United States, are not reliable, as samples were taken to a depth of only 7 inches and the proportion of the population extracted is likely to have varied considerably during the year. Evans and Guild (1947) investigated seasonal changes at Rothamsted using the permanganate method of extraction, which is

2p

350

1400

250

1000

200

000

I50

600

I I-

9 LL

0

c

NUMBER WEIGHT

400

---- ---

-

\ u)

B 2 cc

2 8 5m

3 -J

i? 200

J S D +1951-I 952

'W'SP'SU

M

J

A

'

S

D

M

J

S

-1953-

W

SP'SU

D

1954

'

A

I

W

I

SP'SU

'

FIG.1. Seasonal fluctuations in the abundance of earthworms found in pasture land at Palmerston North, New Zealand (Waters, 1955). The standard errors shown refer to weights. W = winter. SP = spring. SU = summer. A = autumn.

known to give a low recovery when the soils are dry. The apparent seasonal changes in population that were observed may largely have been due to changes in the efficiency of the sampling method. Seasonal variation in the abundance of enchytraeids on grassland is described by Shindo (1929) for Tokyo and by Overgaard Nielsen (1955) for localities in Denmark. The vertical distribution of earthworms in the soil changes during the

256

I(.

P. BARLEY

year as the animals travel up and down in response to temperature or moisture gradients. Overgaard Nielsen (1953, 1955) found that enchytraeids could rapidly move through soils with coarse pores in response to a temperature gradient, but they showed less ability to escape desiccation by moving deeper, probably because uneven drymg caused the small worms to retreat into moist pockets. The observations of Kuhnelt (1950) and Guild (1955) show that, among the lumbricids, Lumbricus rubellus (Hoff.), Bimastus spp., and Dendrobaena spp. spend most of their life within litter on or near the top of the soil and that species of this group are not usually found on arable land unless there is a well-defined layer of plant litter. Other kinds, such as Allolobophora caliginosa (Sav.) and Eisenia rosea (Sav.), construct their tunnels in all directions through the topsoil as they feed; and others again, for example Lumbricus terrestris (L.) and Octolasium lacteum (Oerley), live in deep, vertical tunnels but come to the surface to feed and to copulate. Even on small areas that appear to be homogeneous, earthworm populations are not randomly distributed. When all worms are considered, a pattern of overlapping aggregates is usually found. When the adult and young worms are considered separately, the adult worms may be randomly distributed, but the populations of young worms are highly aggregated ( Overgaard Nielsen, 1954; Satchell, 1955a). Thus the aggregating effect of reproduction may often override the randomizing effect of deaths and dispersal. Areas should be stratified, before sampling, according to some relevant but independent feature of the environment, Seasonal changes in the population can be measured with the greatest efficiency by taking successive samples near permanent markers within each zone. 111. The Influence of Earthworms on Soil Fertility

A. CONSUMPTION OF SOIL AND ORGANIC MATERIAL When earthworms eat a mixture that is mostly mineral soil, the weight of feces cannot be very digerent from the weight of material ingested. After measuring the total number of worms present and the rate of casting of certain species, Evans (1948) estimated that the earthworms in pasture land at Rothamsted ingested from 5 to 9 kg. of oven dry soil per square meter per year. Even when litter is freely available many species continue to ingest a large amount of mineral soil. The peregrine A. caliginosa ( Sav. ) behaves in this way, and worms of this kind continue to ingest 200 to 300 mg. dry weight of earth per gram body weight per day even when litter and dead roots are provided ad libitum. The ingested earth moves through the gut in about 20 hours (Barley, 1959b). Lindquist (1941), Franz and Leitenberger ( 1948), van der Drift

EARTHWORMS I N AGRICULTURAL LAND

257

(1951), and Guild (1955) measured the rate at which samples of litter or dung lost weight in the presence of earthworms, but they did not determine how much of the loss was caused by microbial action. When allowance is made for losses due to other organisms, it appears that worms of body weight 0.1 g. eat as much as 80 mg. of dry matter per gram body weight per day, and that larger worms of body weight 1 g. may eat as much as 40 mg. per gram body weight per day.

B. INFLUENCE ON THE SUPPLYOF PLANT NUTRIENTS 1. Availability of Nutrients in the Excreta The concentration of plant nutrients in the casts of earthworms often exceeds that present in the total soil. This has been demonstrated by Lunt and Jacobson (1944), Nighawan and Kanwar ( 1952), and others, but such comparisons merely show that the worms selectively feed on those parts of the soil which are rich in organic matter. The rate of excretion of nitrogen is greatly influenced by the condition of the worms. When they are gaining weight and feeding on material of low to moderate protein content, the rate of excretion of ammonium and other available forms of nitrogen is less than 5 pg. per gram body weight per hour (Barley and Jennings, 1959). However, if the worms are fasted for several days, there is a marked increase in the rate of excretion of nonprotein nitrogen (Cohen and Lewis, 1949). Robertson (1936) showed that calcium ions in solution in the gut were absorbed and that the surplus over body requirements was excreted into the gut as calcium carbonate. This process together with breakdown of organic matter within the gut affects the pH of the excreted soil, and hence the solubility of various nutrients in the casts. Salisbury (1924) found that, when earthworms were feeding on a mixture of organic material and mineral soil, the pH tended to approach neutrality irrespective of whether the soil was initially acid or alkaline. 2. Contribution to the Decay of Organic Material Although a number of digestive enzymes including a plentiful supply of a cellulase and a chitinase (Tracey, 1951) have been found in the gut wall of earthworms, little is known about the amount of organic material that can be digested. Franz and Leitenberger (1948) tried to measure the amount of digestible material in leaf litter by extracting with acetyl bromide both the litter provided for food and the feces of the worms. The assumption that acetyl bromide dissolved partly decomposed litter without dissolving humified material was not examined. Moreover the feces were known to be contaminated with an unde-

258

K. P. BARLEY

termined amount of mineral particles. Evidence of surprisingly efficient digestion of proteins in leaf litter was obtained by Needham (1957), who found that negligible amounts of nitrogen appeared in the feces of certain lumbricids when the worms were fed with a low-protein diet of elm litter. Measurements of the amount of oxygen consumed by particular organisms provide an estimate of their direct contribution to decay. Data for various animals living in forest soils in Denmark were obtained by Bornebusch (1930), who concluded that the annual oxygen consumption of the complete fauna amounted to at least a quarter of the oxygen needed for the combustion of the annual leaf fall. Although the accuracy of Bornebusch's estimates is uncertain, his work suggests a practicable method by which the action of the various decomposers in agricultural soils can be compared. Mellanby (1960) points out that as the rate of uptake of oxygen by lumbricid worms in respirometers is only about 50 pl per gram body weight per hour, and that as the rate in nature is probably much less than this, the direct contribution which the earthworms make to decay cannot be very large. On pasture land the earthworms themselves are unlikely to account for more than a tenth of the oxygen used in decay. Apart from the direct effects of the worms, we have to take into account the influence that earthworms may have on the decomposition brought about by other organisms. Darwin (1837) first drew attention to the way in which earthworms mixed plant residues and dung with the underlying soil. In addition to this, comminution of soil aggregates in the gut may expose fresh surfaces to microbial attack (cf. Rovira and Greacen, 1957 ) Evidence of earthworms promoting other decomposers in a culture experiment is given by Barley and Jennings (1959). When earthworms were removed from the cultures the rate of oxygen consumption dropped by 20 per cent, but not more than half this decrease could be accounted for by the oxygen consumption of the worms themselves. Wollny (1890) made the fmt attempt to measure the effects of earthworms on the rate of ammonium and nitrate accumulation in soils. In his experiment two lots of a mixture of soil and humus, one with and the other without earthworms, were kept moist for 3 months before being dried and analyzed. Unfortunately Wollny did not measure the change in weight of the worms, and, although he found increased amounts of ammonium and nitrate where they were present, this may have been caused by the release of nitrogen from the bodies of the worms, The amounts of nitrogen added in the worms in Wollny's experiments were equivalent to as much as 470 pounds of nitrogen per acre.

.

EARTHWORMS IN AGRICULTURAL LAND

259

Russell (1910) allowed for the maximum possible release of nitrogen from the bodies of the living worms by adding an equal weight of dead worms to the controls. Although this procedure biased his experiment against any favorable influence which the earthworms might have on the production of available nitrogen, the action of the living worms increased the net rate of mineralization of the nitrogen of herbage which had been mixed with the soil at the start of the experiment. After 28 weeks at 10 to 15"C., the soil with living worms contained 77 p.p.m. of mineral nitrogen, compared with the control soil which contained 47 p.p.m. Russell did not attach any importance to his results because he accepted von Hensen's (1877) estimates of the weight of worms in field soils and supposed that the concentration of earthworms added to the cultures far exceeded that found in nature. In fact the concentration of earthworms in the cultures (1 g./kg.) lay well within the range encountered in the field. Although the evidence suggests that earthworms can sometimes accelerate decay, we cannot entirely neglect the syntheses that they perform. Their tissues are rich in nitrogen (12 per cent of dry weight), and noticeable amounts may be retained during periods when the worms are increasing in weight. For example, the increase in the total weight of earthworms that occurs during the autumn on New Zealand pastures (see Fig. l ) , entails the transformation of 30 pounds of nitrogen per acre to worm tissue.

PHYSICAL PROPERTIES OF SOILS 1. Porosity When animals construct tunnels and void earth on top of the ground they help to counteract consolidation caused by livestock and implements. The amount of soil transported to the surface by tunnel-making species of earthworms can be very large. Measurements made on grassland by Darwin (1881) and by more recent workers suggest that the average rate of casting at 14 localities in western Europe is about 4 kg. per square meter per year. This corresponds to 3 mm. of soil per year at an apparent density of 1.4 g. per cubic centimeter. However, many of the measurements reported in the literature have been made for only one year on single quadrats. The work of Evans and Guild (1947) is the most thorough, collections having been made from replicated, fixed quadrats at intervals of 3 or 4 days for two years. On the old pastures examined by these workers at Rothamsted, the rate of casting was 3 kg. per square meter per year. Not all species cast on top of the ground, and worms may be active below ground without casts being seen at the surface.

C. MODIFICATION OF

THE

260

K. P. BARLEY

The effects of earthworms on pore size distribution have been overrated by Baver (1956), who quoted data of Wollny (1890) to show the favorable influence of earthworm action on the proportion of readily drained pores. Wollny had found that earthworm tunnels raised the “Luftkapazitat”-the air space present after rapid drainage-of loamy soils from 8 per cent of the apparent volume to 30 per cent; however Wollny’s measurements were made after six large earthworms had been enclosed for 2 months in a 5-cm. diameter column of soill Nevertheless earthworms may sometimes have a significant influence on the air space in field soils, and Stockli (1949) claimed that the volume of earthworm tunnels under grass was as much as 0.05 cc. per cubic centimeter of soil. Dim0 (1938) found 1400 worm tunnels per square meter of an irrigated lucerne field, and von Finck (1952) found from 100 to 300 tunnels per square meter on pasture land. These observations would have been more informative if the workers had described the methods used to identify the tunnels. 2. Texture Diflerentiation Darwin (1881) described the way in which earthworms stratified soils by selectively ingesting small particles and transporting them toward the surface. He noticed that heavy and light objects sank into the soil at similar rates, and he concluded that the subsidence resulted from undermining and covering with voided earth. On five areas of grassland which Darwin examined in England, thin layers of cinders or marl had been covered with soil at an average rate of 5 mm. per year to depths of up to 15 cm. The depth of overburden approached the limit set by the depth of tunneling at a diminishing rate, because the earthworms mostly tunneled near the surface. Although transport by wind or water or by other burrowing animals could have contributed to the burial, Darwin considered that earthworms were the main agents in the areas which he examined. The upper limit to the size of the particles ingested depends on the size of the earthworm. The small enchytraeids, at one extreme, do not ingest much mineral soil and have little influence on either texture differentiation or porosity. At the other extreme, the casts formed by fully grown megascolecids may contain particles as large as coarse sand (Nighawan and Kanwar, 1952). At Rothamsted, Evans (1948) found that the top 4 inches of soil under old grassland contained a low proportion of particles wider than 0.2 mm. The old grassland supported large populations of earthworm species of a kind which habitually cast at the surface. Although the action of these worms may well have led to a reduction in the content of coarse particles, it should be noted that the fields which Evans took for comparison with the old grassland had all

EARTHWORMS IN AGRICULTURAL LAND

261

been cultivated, and the difference observed could also have been caused

by implements bringing coarse particles to the surface. Grinding in the gizzard of earthworms is said to cause some mechanical comminution (Meyer, 1943), but there is very little quantitative evidence to support this belief. Blank and Giesecke (1924) found a small increase in the clay fraction and a decrease in the silt fraction of a loam in which earthworms had been kept for two years. No details were given about the method of dispersion used in the mechanical analysis. 3. Stability in Water Where the ingested mineral soil is mixed with a good supply of decomposable organic matter, the casts of earthworms are often more stable in water than the bulk of the soil from which they have been formed ( Gurianova, 1940). Microbial action rapidly changes the stability of the voided earth. Thus, Teotia et al. (1950) found that fresh casts became more stable in water during the first 15 days of incubation. The stability then decreased, but the casts were still twice as stable as control aggregates after 30 days' incubation. However, unless the stability of earthworm casts is remarkably persistent, the animals are unlikely to have much effect on the stability of the whole topsoil as not more than a tenth of the bulk is ingested by the worms each year.

4. Denudation or Gradual Erosion Over geological periods of time, earthworms and other animals that bring soil to the surface contribute to the denudation of grass-covered slopes. Darwin (1881) made careful observations on the downslope movement of worm casts, and he believed that the gradual reduction of relief of grassy slopes depended largely on the downslope movement of voided earth.

D. EFFECTS ON OTHERORGANISMS IN THE SOIL Interactions with other decomposers have been described in Section III. We note in addition that the earthworms are an important source of food for some animals, such as moles, which can considerably disturb the soil. Microorganisms that lack tough outer coats, like yeasts and algae, and which happen to be ingested by earthworms are killed by the time they reach the crop ( Aitchberger, 1914). The most critical work on the fate of microorganisms in the worm gut has been carried out by Day (1950), who showed that the microbial count could be either increased or decreased depending upon the kind of microorganisms present, the time of sampling, and the nature of the soil. Apart from selective killing in the gut, earthworms may influence microbial populations by trans-

262

K. P. BARLEY

porting organisms. Hutchinson and Kame1 (1956) found that the rate of spread of fungi through a sterile soil was much higher when earthworms were present. However, there is no evidence as yet that either selective killing or dispersal by earthworms influences microbial populations in field soils. IV. Effects of Earthworms on the Yield of Crops and Pastures

A. EARTHWORMS AS PESTS The larger earthworms sometimes pull growing leaves into their tunnels, and Zicsi (1954) states that earthworms occasionally attack the epigeal parts of plants. Waters (1955) could not detect any damage even when live shoots and roots were the only organic materials available. Although enchytraeids may cause further damage to plants already injured by other organisms, they are not known to attack healthy plants. Several instances in which earthworms have been suspected of damaging plants have been reviewed by Stephenson (1930), but direct damage to growing plants is unusual. There are a few reports of excessive casting by earthworms leading to poaching of the soil by livestock (Evans and Guild, 1947; Lee, 1959), and of wormcasts being deposited sufficiently high in cereal stooks (shocks) to make it difEcult to obtain clean grain when thrashing (Rept. Rothamsted Expt. Sta. 1956, p. 155). B. EXPERIMENTS IN ARTIFICIAL CULTURES Some of the changes to which earthworms contribute in soils, such as modification of texture, occur too slowly to produce any detectable effect on plant growth in trials of a few months’ or even a few years’ duration. The growth of plants during short-term experiments may, however, be influenced by any acceleration of mineralization or by improvements in air and water regimes accompanying an increase in porosity. The numerous pot-culture experiments of Wollny ( 1890) together with trials conducted in various artificial cultures by Chadwick and Bradley (1948), Baluev (1950), Joshi and Kelkar (1952), and Nielson (1953) were inconclusive because these workers did not allow for the possible effects of nutrients released from worms that lost weight or died during the experiments. In a yield trial, as in his comparison of mineralization rates (see p. W9), Russell (1910) allowed for release of nutrients by adding dead worms to the controls. In a treatment where live lumbricids were added at the rate of 0.5 g. body weight per kilogram of soil, dry matter yields were as much as 25 per cent higher than on the controls even though

EARTHWORMS IN AGRICULTURAL LAND

263

the plants on the controls absorbed more nitrogen. (Twice this concentration of worms was added in Russell's mineralization experiment where the live worms increased the rate of nitrate production.) In the yield trial the increase in yield brought about by the living worms was probably due to an improvement in some physical condition. Russell noted, in particular, that less evaporation occurred from the pots with live worms, probably because they covered the top of the soil with a loosely packed layer of casts. In experiments that followed those of Russell (1910), Kahsnitz ( 1922) found that the addition of dead earthworms to a garden soil made little difference to the yield of peas or oats, but the addition of living worms increased the yield of tops by 70 per cent. However, the concentration of worms in the pots (14 g./kg.) was much higher than that found in field soils, and the physical changes were extraordinarily large, the apparent volume of the soil in the pots being increased by as much as 7 per cent. Hopp and Slater (1948, 1949) compared the effects of dead and live worms on the yield of herbage plants grown on poorly structured soils. In their first experiment, a clay subsoil was packed into large barrels and the soil was covered with animal manure and herbage clippings. Grasses and legumes were sown and lumbricids were added at the rate of 120 worms per square meter. The yield of air-dry herbage over a period of 5 months was equivalent to 0.3 tons per acre where dead worms had been added, compared with 1.4 tons where live worms had been added. Where N-P-K fertilizer had been mixed with the soil, the yields were 0.6 tons per acre with dead worms and 2.0 tons with live worms. In their second experiment, Hopp and Slater packed boxes with a heavily fertilized soil, which had either been puddled to destroy the aggregates or broken into crumbs to produce a good structure. Living or dead worms were added to the puddled soil at the rate of 600 per square meter. After 4 months soybeans were grown in the boxes. Yields were very low in the puddled soil except where living worms had been added. Yields were then as great as on the well-structured soil. The best way to check on the net exchange of nutrients between earthworms and soil is to weigh the worms at the beginning and at the end of a trial. In an experiment of Waters (1951) with A. caliginosa (Sav.) the worms were allowed to feed for 8 weeks in a soil that contained 30 g. per kilogram of dry dung. The worms increased in weight during this period and were therefore unlikely to have lost any net amount of nutrients to the medium. After the worms had been removed, the soil was crushed and screened to standardize its physical condition, transferred to small pots, and planted with ryegrass. The grass was also grown on similarly prepared samples of medium that had not contained

264

K. P. BARLEY

any worms. Where earthworms had been working, the yield of tops was doubled. The experiments described above show that earthworms can significantly increase yield, when working in the artificial environment of pot or tank cultures, and that the increases are not merely due to the addition of nutrients from the bodies of the worms. Finally we will examine the few experiments which have been conducted to test whether earthworms can influence productivity in the field. C. FIELDEXPERIMENTS Early field experiments, for example that of Ribaudcourt and Combault (1907) in which the yield of crops was increased by the addition of earthworms to small field plots, were inconclusive because the crops could have obtained appreciable quantities of nutrients from worms which lost weight or died during the trial. Moreover, where earthworms are scarce, it will usually be found that soil conditions are not suitable or that food supplies are limited by the system of land use. When this is so, adding more worms will not lead to any permanent increase in the population, and even if a yield response is detected, it is unlikely to be anything more than a transient phenomenon. However, when previously unsuitable lands are rapidly changed in ways that make them suitable for earthworms, or when new genotypes are transported to an area, there is an opportunity to observe long-term effects on yield. For example, when previously arid lands are irrigated, or when previously acid lands are limed, conditions suitable for large earthworm populations are established, but many years may elapse before the arrival of well-adapted species. There is some evidence from field trials in New Zealand to show that the introduction and spread of earthworms of European origin have led to increases in the yield of sown pastures. In the New Zealand experiments earthworms of the A. caliginosa complex were deliberately introduced into lands to which they had not previously been transported by accidental means. Either the areas concerned were isolated, or else the soils were naturally acid and had recently been limed. The experiments were of the following kind: areas of pasture which had previously received uniform applications of lime and fertilizer were inoculated with five or more colonies of A. caliginosa, each colony comprising 25 worms, Four years after inoculation, areas of a few meters diameter around each point of liberation had become greener and more densely covered with vegetation than the rest of the paddock. After eight years the area in which earthworms were active and in which the pasture was more vigorous had spread 100 meters from these points. The first of these trials

265

EARTHWORMS IN A G R I C U L W L LAND

began in 1926, and beneficial effects have been reported at Raetihi (Hamblyn and Dingwall, 1954), at Glenledi (Richards, 1955), and at Hindon (Stockdill, 1959). Quantitative data obtained by Stockdill (1959) show the changes in herbage yield and soil characteristics produced at Hindon after five years. Part of Stockdill's data is shown in Tables I and 11. The yield data TABLE I Effects of A. caliginosa on the Yield of Herbage at Hindon, New Zealando Dry matter production (lb./acre) Treatment Without lumbricids

18/~/5422/ix/54

22/ix/54-

18/x/54

18/x/541l/xi/54

Total

970

660

910

2540

1700

1450

1270

4420

Inoculated with A. caliginosa in 1949 a

Data from Stockdill (1959). TABLE I1 Effects of A. caliginosa on the Distribution of Lime and Organic Matter at Hindon, New Zealanda Soil characteristics, May, 1955

Depth Treatment (inches) Without lumbricids 0-1 13 3-6

Inoculated with A. caliginosa in 1949 a b

0-1

13 3-6

C.E.C.b (meq. % 1 28 17 16

Exchangeable Ca (meq. k ) 20 9 5

24 20 16

10 7

15

N (%)

0.62 0.29 0.23

0.50 0.34 0.25

Data from Stockdill (1959). C.E.C. = cation exchange capacity.

refer to herbage cuts obtained during the winter and spring of 1954 within five exclosures placed over the sites where worms had been liberated in 1949 and over reference points 40 meters away from each site. The soil data show that the earthworms had mixed lime and organic matter lying on and near the surface through the top 6 inches of soil. Similar experiments on recently limed land have been in progress in Lancashire, England since 1954 (Rept. Rothamsted Erpt. Sta. 1958, p. 141). In this area a small population of lumbricids was present on the moorland before the experiment began, and populations were prevented

266

K. P. BARLEY

from increasing on control plots after lime had been applied by spraying with lead arsenate. This technique probably interfered with other members of the fauna as well as the earthworms, and results that may be obtained will be difficult to interpret. V. Conclusion

In pot cultures earthworms can bring about a number of changes that benefit plant growth. Although the amount of evidence is small, data from trials in New Zealand suggest that in some places earthworms can also increase productivity in the field. Earthworms are conspicuous in soils and may easily be caught and cultured. However, we should not overrate the importance of this group of animals just because they have been studied in more detail than other members of the soil fauna. Many kinds of animals contribute to the breakdown of dung and plant litter and modify soil structure on agricultural land. The ecological relationships of the various animals concerned have been assessed by Birch and Clark (1953) for forest soils, and the system which these authors describe may be applied in principle to the fauna of soils used for agriculture. Our existing knowledge of the abundance and action of earthworms in agricultural lands is derived very largely from European work. Surprisingly few investigations have been made in North America or the Southern Hemisphere, even though the earthworm fauna is known to have been transformed by human migration and agriculture. REFERENCES

Abbie, A. A. 1954. Australian J . Sci. 17, 1-9. Aitchberger, R. 1914. Kleinwelt 6, 53-58, 69-72. Bahl, K. N. 1947. Bio2. Revs. 22, 109-146. Baluev, V. K. 1950. Pedology Leningrad pp. 219-227. Barley, K. P. 1959a. Australian J . Agr. Research 10, 171-178. Barley, K. P. 195913. Australian J . Agr. Research 10, 179-185. Barley, K. P., and Jennings, A. C. 1959. Australian J . Agr. Research 10, 364-370. Baver, L. D. 1956. “Soil Physics,” 2nd ed. Wiley, New York. Birch, L. C., and Clark, D. P. 1953. Quart. Rev. Biol. 28, 13-36. Blank, E., and Giesecke, F. 1924. Z. Pfianzenemiihr. u. Dung. 9, 198-210. Bornebusch, C. H. 1930. Forsstlige Fors&vaesen Dun. 11, 1-224. Cemosvitov, L., and Evans, A. C. 1947. “Synopses of the British Fauna, Lumbricidae.” Linnean SOC.,London. Chadwick, L. C., and Bradley, K. 1948. Proc. Am. SOC. Hort. Sci. 61, 552-562. Cohen, S., and Lewis, H. B. 1949. 1. Bwl. Chem. 180, 79-92. Darwin, C. R. 1837. Trans. Geol. SOC. London 2nd Ser. 6, 505-509. Darwin, C. R. 1881. “The Formation of Vegetable Mould Through the Action of Worms,” 1st ed. John Murray, London. Day, G. M. 1950. Soil Sci. 69, 175-183.

EARTHWORMS IN AGRICULTURAL LAND

267

Dimo, N. A. 1938. Pedology Leningrad pp. 524-526. Doekson, J. 1950. Trans. Fourth Intern. Congr. Soil Sci., Amsterdam 2, 129-130. Evans, A. C. 1948. Ann. A w l . Bwl. 36, 1-13. Evans, A. C., and Guild, W. J. 1947. Ann. Appl. Bid. 34, 307-330. Evans, A. C., and Guild, W. J. 1948. Ann. Appl. Biol. 36, 485-493. Franz, H., and Leitenberger, L. 1948. Usterr. zool. Z. 1, 498-518. Gates, G.E. 1958. Ann. Mag. Nut. Hist. 1, 3-44. Grant, W.C. 1955a. Ecology 36, 400-407. Grant, W. C. 195%. Ecology 36,412-417. Guild, W. J. 1955. In “Soil Zoology” (D. K. McE. Kevan, ed.), pp. 83-98. Butterworths, London. Gurianova, 0. Z. 1940. Pedology Leningrad pp. 99-108. Hamblyn, C. J., and Dingwall, A. R. 1945. New Zealand J. Agr. 71, 55-58. Hopp, H. 1947. Soil Sci. SOC.Am. Proc. 12, 503-507. Hopp, H., and Slater, C. S. 1948. Soil Sci. 66, 421-428. Hopp, H., and Slater, C. S. 1949. J. Agr. Research 78, 325-341. Hutchinson, S. A., and Kamel, M. 1956. J. Soil Sci. 7, 213-218. Jefferson, P. 1956. J. Sports Turf Research Inst. 9, 166-179. Joshi, N. V., and Kelkar, B. V. 1952. Indian J. Agr. Sci. 22, 189-196. Kahsnitz, H. G. 1922. Botan. Arch. 1, 315-331. Kiihnelt, W. 1950. “Bodenbiologie.” Verlag Herold, Wien. Laverack, M. S. Quoted by Satchell, J. E. 1960. New Scientist 7, 79-81. Lee, K. E. 1958. New Zealand J. Agr. Research 1, 998-1002. Lee, K. E. 1959. New Zealand Dept. Sci. Ind. Research Bull. 130, 486 pp. Lindquist, B. 1941. Svenska Skogwdrdforen. Tidskr. 39, 179-242. Lunt, H. A., and Jacobson, H. G. 1944. Soil Sci. 68, 367-375. Mellanby, K. 1960. Soils and Fertilizers, Commonwealth Bur. Soil Sci, 23, 8-9. Meyer, L. 1943. Bodenk. u. Pfinzenerniihr. 29, 119-140. Michaelsen, W. 1907. “Die Fauna Sudwest-Australiens, Oligochaeta.” Fischer, Jena. Miller, R. B., Stout, J. D., and Lee, K. E. 1955. New Zealand J. Sci. Technol. 37, 290-313. Needham, A. E. 1957. J. Exptl. Bwl. 34, 425-446. Nelson, J. M., and Satchell, J. E. ( I n Press) In “Proceedings of the Colloquium on Research Methods in Soil Zoology.” Butterworths, London. Nielson, R. L. 1953. New Zealand J. Agr. 86, 374. Nighawan, S. D., and Kanwar, J. S . 1952. Indian J. Agr. Sci. 22, 357-375. Overgaard Nielsen, C. 1953. Oikos 4, 187-196. Overgaard Nielsen, C. 1954. Oikos 6, 167-178. Overgaard Nielsen, C. 1955. Oikos 6, 153-169. Parker, G. H., and Metcalf, C. R. 1906. Am. J. Physiol. 17, 55-74. Ponomareva, S. I. 1950. Pedology Leningrad pp. 476-486. Raw, F. 1959. Nature 184, 1661-1662. Raw, F. 1960. Nature 187, 257. Ribaudcourt, E.,and Combault, A. 1907. Bull. SOC. for Belg. 14, 212-223. Richards, J. G. 1955. New Zealand J. Agr. 91, 559. Robertson, J. D. 1936. J. Exptl. Biol. 13, 279-297. Roots, B. I. 1956. J. Exptl. Bid. 33, 29-44. Rovira, A,, and Creacen, E. L. 1957. Australian 1. Agr. Research 8, 659-673. Russell, E. J. 1910. J. Agr. Sci. 3, 246-257. Salisbury, E. J. 1924. J. Linnean SOC. (Bot.) London 46, 415-426.

268

K. P. BARLEY

Satchell, J. E. 1955a. In “Soil Zoology” ( D . K. McE. Kevan, ed.), pp. 180-190. Buttenvorths, London. Satchell, J. E. 1955b. In “Soil Zoology” ( D . K. McE. Kevan, ed.), pp. 356-364. Buttenvorths, London. Satchell, J. E. 19.55~. Soils and Fertilizers, Commonwealth BUT. Soil. Sci. 18, 279285. Satchell, J. E. 1958. Soils and Fertilizers, Commonwealth Bur. Soil. Sci. 21, 209219. Shindo, B. 1929. 1. Coll. Agr. Imp. Univ. Tokyo 10, 159-171. Stephenson, J. 1930. “The Oligochaeta.” Oxford University Press, London. Stockdill, S. M. J. 1959. New Zealand J. Agr. 98, 227-233. Stockli, A. 1949. Z. Pflanzenerniihr. Dung. Bodenk. 46, 41-53. Svendsen, J. A. 1955. Nature 176, 864. Teotia, S. P., Duley, F. L., and McCalla, T. M. 1950. Nebraska Agr. Exptl. Sta. Research Bull. 166. Tracey, M. V. 1951. Nature 167, 776. van der Drift, J. 1951. Tijdschr. Ent. 94, 1-168. von Finck, A. 1952. Z. Pflanzenerniihr. Diing. Bodenk. 68, 120-145. von Graff, 0. 1953. Z. Pfkznzenemiihr. Dung. Bodenk. 61, 72-77. von Hensen, R. 1877. Z. wiss. Zool. a8, 354-364. Waters, R. A. S. 1951. Proc. 13th Conf. New Zealand Grassland Assoc. pp. 168175. Waters, R. A. S. 1955. New Zealand J. Sci. Technol. 36, 516-525. Watkin, B. R. 1954. 3. Brit. Grassland SOC. 9, 35-46. Wilcke, D. E. 1955. Z . Pflnnzenemiihr. Diing. Bodenk. 68, 44-49. Wolf, A. V. 1940. Physiol. Zool. 13, 294-308. Wollny, E. 1890. Forsch. Gebiete Agr.-Physik 13, 381-395. Zicsi, A. 1954. Agrcirtudomdnyi Egyetem Agron. Kar Kiadvanya 1, 1-20.

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

Philip F. low Department of Agronomy, Purdue University, Lafayette, Indiana

I. 11. 111. IV. V. VI. VII. VIII. IX. X.

Introduction ................................................ Nature of Ice and Water ...................................... Nature of Ionic Solutions ..................................... Mechanisms of Clay-Water Interaction .......................... Specific Volume of Clay-Adsorbed Water ........................ Viscosity of Clay-Adsorbed Water .............................. Dielectric Properties of Clay-Adsorbed Water .................... Supercooling and Freezing of Clay-Adsorbed Water .............. Thermodynamic Properties of Clay-Adsorbed Water .............. A Working Hypothesis ....................................... References ..................................................

1.

Page 269 269 277 284 287 295 303 306 314 322 323

Introduction

Clay and water are two of the most common substances in the earth's crust. Separately and together they influence our daily lives. Frequently the mutual interaction between clay and water controls the formation of clouds and the infiltration of rain water into the soil. It is often responsible for the failure of buildings and highways and the decreased production of oil wells. It affects the quality of paper coatings and ceramics. And yet our knowledge of clay-water interaction is limited. Despite many investigations a satisfactory concept of this important phenomenon is only just beginning to develop. This paper is intended to be a critical review of the recent literature on clay-water interaction. From this review certain facts will emerge. Usually these facts can be interpreted in more than one way. The author will give his interpretation of the facts. He will also give alternative interpretations and his reasons for not accepting them. It is hoped that this procedure will stimulate research on the part of the reader so that, in time, speculation will yield to knowledge. II. Nature of Ice and Water

The nature of clay-water interaction cannot be discussed intelligently unless we understand the nature and properties of the two components. Most of the readers of this article have an understanding of clay. How269

270

PHILIP F. LOW

ever, there may be some who are not so familiar with the nature and properties of water. Therefore, the following discussion is included. According to Bernal and Fowler (1933), the water molecule consists of a V-shaped arrangement of the atomic nuclei, the internuclear 0-H distances being 0.96 A. and the internuclear angle being 103 to 106 degrees, which is very close to the tetrahedral angle of 109 degrees. In the molecule there are four regions where the density of the outer electrons is maximal (Bernal and Fowler, 1933; Lennard-Jones and Pople, 1951). Two of these regions are associated with the 0-H bonds and coincide with the positions of the protons; the other two are associated with lone pairs of electrons and are located above and below the plane of the atomic nuclei on the opposite side of the oxygen nucleus from the protons. Therefore, the net charge distribution in the water molecule resembles a tetrahedron with two positive and two negative corners. The resultant center of positive electricity, midway between the protons, is separated from the resultant center of negative electricity near the oxygen nucleus on the side next to the protons. Hence the water molecule has a dipole moment. It is equal to 1.83 x electrostatic units. When two water molecules approach each other there is electrostatic attraction between a positive tetrahedral corner of one molecule and a negative tetrahedral comer of the other; i.e., there is electrical interaction between the proton of the former and the lone electron pair of the latter. According to Lennard-Jones and Pople ( 1951) this interaction has little effect on the electron distribution of the lone pair so that the attraction remains essentially electrostatic. However, Frank ( 1958 ) believes that the electron distribution of the lone pair is so distorted by the field of the proton that these electrons may be regarded as shared by the proton. Thus, a covalent character is imparted to the bond between the two molecules. In either event, it is obvious that the proton of the hydrogen is involved. For this reason the bond is called the hydrogen bond. Each water molecule can form four hydrogen bonds, one at each tetrahedral corner. Therefore, in an assembly of water molecules there is a tendency for every molecule to be hydrogen bonded to four neighboring water molecules which surround it tetrahedrally. At temperatures below 0" C., the water molecules exist in fixed positions in the ice lattice. For this reason it is possible to determine their molecular arrangement by means of X-ray analysis. The results show that each molecule is tetrahedrally coordinated to four others and that the oxygen nuclei are 2.76 A. apart (Pauling, 1945; Owston, 1951, 1958). Infrared and Raman spectra (Ockman, 1958) show that the vibrational stretching frequency of the O-H bond in ice is only slightly different from that in water vapor, indicating that the O-H distances are nearly

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

271

the same. The calculated distance for ice is about 1.00 A. Consequently, the hydrogen atom is not midway between the oxygen atoms of the bonded molecules but is 1.00 A. from one oxygen atom and 1.76 A. from the other. It appears, therefore, that the individual water molecules retain their identity and are held together by hydrogen bonds. The strength of these bonds has been estimated (Pauling, 1945) to be 4.5 kcal. per mole. However, it is probable that not all the bonds in ice are intact. As a consequence, ice has residual entropy (Pauling, 1945). Further, the water molecules can undergo restricted rotation in an alternating electric field (Smyth and Hitchcock, 1932). And the protons in ice are capable of movement because ice conducts a direct electric current ( R . S. Bradley, 1957). Both the dipole rotation and the electrical conductance of ice increase with increasing temperature, suggesting that hydrogen bonds are broken as the thermal energy of the molecules increases. Possibly, this is why ice flows more readily at higher temperatures (Glen, 1958). When ice melts there is an increase in density from 0.917 for ice to nearly 1.00 for water. The magnitude of this density increase is very revealing. If all the hydrogen bonds were broken in the process of melting, water would have a close-packed arrangement, i.e., each water molecule would be in close contact with twelve others. Then, provided the molecular radius remained at 1.4 A., it would have a density of 1.84. Conversely, for a density of 1.00 the molecular radius would have to be 1.72 A. In the words of Bernal and Fowler (1933), ‘We have therefore the choice of assuming either that water is a simple close-packed liquid in which the effective molecular radius has changed from 1.4 A. in the solid to 1.72 A. in the liquid, or that the radius is still approximately 1.4 A. but that the mutual arrangements of the molecules are far from that of a simple liquid.” The X-ray evidence of Katzoff (1934) and Morgan and Warren (1938) indicate the correct choice. These investigators determined the radial distribution function for water at several temperatures. The radial distribution function gives the probability of finding the center of a water molecule in a spherical shell a distance T from a given central molecule, Analysis of this function showed that near 0” C. each water molecule has slightly more than four nearest neighbors at a distance of 2.90 A. and a marked concentration of next-nearest neighbors at a distance of 4.5 A. Recall that ice has four nearest neighbors at a distance of 2.76 A. Its next-nearest neighbors are at 4.5 A. Hence, not only is the radius of the water molecule nearly the same as in ice, precluding the above alternative that water is a simple liquid, but the structure of water must be similar to that of ice, at least for short distances. On the other hand, if the structure of water were identical

272

PHILIP F. LOW

with that of ice, the slight increase in internuclear distances would lead to a density for water of 0.78 (Pople, 1951). Obviously, water does not retain the ice structure in detail. As the temperature of the water increases, there is a decrease in the sharpness and intensity of the peaks in the radial distribution function. These changes correspond to a closer packing of water molecules and a breakdown in the degree of order in the water structure. Because every hydrogen bond is shared by t w o water molecules, the bond energy associated with each of these molecules is half of the total. Therefore, if all the water molecules in ice form hydrogen bonds with their four neighbors, the bond energy per molecule is twice the energy of the hydrogen bond. On a molar basis this is 9.0 kcal. But the heat of fusion of ice is only 1.44 kcal. per mole. Consequently, no more than 16 per cent of the maximum number of hydrogen bonds are broken on melting. And it may be that these bonds are bent instead of broken. Pople (1951) claims that the bending of a bond (the movement of either the hydrogen atom or the lone pair of electrons out of the 0-0 line as a result of molecular rotation) requires a much smaller supply of energy than its complete rupture, so that bond bending should be of considerable importance near the freezing point. As the temperature is raised, additional bonds are bent or broken. Between 25" and 90°C. the average number of hydrogen bonds is estimated to be somewhat greater than half the number possible (Cross et al., 1937). Using the assumption that water is a mixture of ice and a normal close-packed liquid, each possessing its normal volume at the given temperature, Grjotheim and Krogh-Moe (1954) calculated that the per cent of broken hydrogen bonds in water increases from 55 to 77 as the temperature is raised from 0" to 100"C. However, from heat of vaporization data, Haggis and associates (1952) calculated that the per cent of broken bonds increases from 9 to 20 in the same temperature range. Regardless of which values are correct, it is apparent that liquid water retains a high degree of hydrogen bonding and that this bonding decreases with increasing temperature. Water has unusual properties relative to those of similar compounds such as H2S, H2Se, H2Ti, CH4. For instance, water has an abnormally high melting point, boiling point, heat of fusion, heat of vaporization, and specific heat. The high values of these quantities are attributed to the extra energy required to break hydrogen bonds (Bernal and Fowler, 1933; Pauling, 1945). The unusually high viscosity of water has the same basis (Ewe11 and Eyring, 1937), as does the elevated dielectric constant ( Pauling, 1945). Evidence of the kind cited here has led to four different concepts of the structure of water. The first is that proposed by Bernal and Fowler

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

273

(1933) and modified by Morgan and Warren (1938). It is probably the most commonly accepted concept. Water is regarded as having a tendency to bond itself tetrahedrally to four neighbors because of the aforementioned electron distribution in the molecule. The bonds are continually breaking and re-forming so that, on the average, each molecule has slightly more than four neighbors but is bonded to fewer than four of them. As the hydrogen bonds are broken the resulting fragments tend to pack together as closely as possible. This tendency toward close packing as bonds are broken explains the volume decrease of about 9 per cent on melting even though the intermolecular distance increases. As the temperature increases, the increased thermal agitation results in the rupture of additional hydrogen bonds. But the increased agitation also results in an increase in intermolecular distances. The former effect of thermal agitation predominates below 4" C.; whereas, the latter effect predominates above 4" C. The result is that water has a maximum density at 4" C. The second concept was proposed by Lennard-Jones and Pople (1951) and expanded by Pople (1951). According to them, the molar heat of fusion (1.44 kcal.) is small compared to the hydrogen bond energy per mole of water in ice (9.0 kcal.). Therefore, few bonds could be ruptured on melting, especially in view of the fact that much of the available energy would be used for distortion or bending of bonds. Further, the value of RT at 0" C. (0.5 kcal. per mole), which is a measure of the kinetic energy of the molecules, is small relative to the bond (potential) energy. Therefore, Pople (1951) states "as the temperature of ice rises, the hydrogen bonds will become increasingly bent, until at a certain stage this leads to a breakdown of the long-range order, corresponding to fusion. After fusion the four hydrogen bonds from one molecule may be regarded to a good approximation as being able to bend independently, whereas before they could only bend in such a way that the lattice order was maintained. This is the essential difference between ice and water in this theory. In water, individual bonds will have increased freedom, as a result of which some of the molecules will move into the formerly unoccupied regions of the tridymite-like ice lattice, leading to the observed volume diminution. This process will lead to a gradual smoothing out of the radial distribution function with rising temperature." Thus, water is regarded as being a giant polymer of hydrogenbonded water molecules. A third concept is that advanced by Forslind (1952). He proposed a type of hydrogen bonding between water molecules similar to that of Lennard-Jones and Pople ( 1951 ) but involving bond hybridization. Not all of these bonds are supposed to have equal strength in ice. As the tem-

274

PHILIP F. LOW

perature is increased, the thermal vibrations of the molecules result in the breaking of some of the weaker bonds. If the vibrations are of sufficient amplitude, the vibrating molecule will pass through the face of the surrounding tetrahedron to occupy an interstitial position. The lattice defects thus produced are of the so-called Frenkel type. Just below the melting point, a number of Frenkel defects will appear in the ice lattice. The vacant lattice sites or “holes” and the interstitial molecules associated with them can diffuse independently. When the “holes” reach the boundaries of the system they are annihilated. The annihilation of the “holes” will produce a volume decrease and a corresponding increase in density due to the lone interstitial molecules. Melting, according to this concept, corresponds to the annihilation of “holes” and the latent heat of fusion is determined by the energy of formation of the Frenkel defect. Presumably, the fluidity of water results from the existence of numerous defects in the lattice, the number of defects increasing with the temperature. Frank and Wen (1957) and Frank (1958) have another concept of water structure. It is based on a different idea of the hydrogen bond. They believe that the hydrogen bond is covalent in character. The covalency arises from the displacement of the lone-pair electrons of one bonded molecule toward the proton of the other as a result of their mutual attraction. This displacement of electrons increases the polarity of the molecule and enhances the possibility of bond formation with a second molecule, and so forth. Thus hydrogen bond formation is considered to be a cooperative phenomenon. According to these authors, liquid water consists of flickering clusters of hydrogen-bonded molecules enclosed in a fluid of nonbonded molecules. The clusters have rigidity because the covalent hydrogen bonds are capable of relatively little “bending.” When the bonds are ruptured there remains the electrostatic interaction of the Bernal and Fowler (1933) and Lennard-Jones and Pople (1951) models so that the nonbonded molecules of the fluid are by no means random in their orientation. Therefore, the energy change involved when a cluster forms or “melts” is not large. These clusters appear or disappear as a result of energy fluctuations in the medium. When an energy fluctuation creates a suitably cold region, a cluster will form. A moment later another energy fluctuation in the same region provides the energy for the cluster to “melt.” The bonded cluster is limited in size by the competition at its boundaries between two kinds of orienting influences, one exerted by the ordered array of molecules in the cluster and the other by the relatively disordered molecules in the surrounding fluid. The torques and displacements of the latter are transmitted to the former and provide the necessary energy for “melting.”

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

275

Combining this picture of water with the “premelting” of ice indicated by the work of R. S. Bradley (1957) and Glen (1958), cited earlier, Frank (1958) states: “It may be suggested that if water contains flickering clusters of ice-like material and ice contains flickering droplets of waterlike nature, then the essential difference between the two may be one of connectivity, so that when solid surrounds liquid we have ice, and when liquid surrounds solid we have water.” At the present time an unequivocal preference cannot be made for any one of the above concepts. Each of them is capable of explaining the available data. In fact, the concepts are quite similar. Included in each is the basic idea that there is a high degree of hydrogen bonding in water that orders the molecules in a loose icelike arrangement. Consequently, we shall adopt the terminology of Morgan and Warren (1938) and say that water has a “broken down ice structure.” In view of the preceding discussion it might be expected that the kinetic unit in such processes as dipole rotation in an alternating electric field, self-diffusion, and viscous flow should be a cluster or domain of bonded water molecules. In fact, according to classic theory, the latter process involves the relative movement of whole layers of water molecules. However, the evidence is to the contrary. From the theory of absolute reaction rates (Glasstone et al., 1941), we obtain the equations

h 0 1 --rl h

e A S t / R e-AHS/RT

(3)

where: t

k

= the relaxation time, i.e., the time required for the dipolar

=

T h AS$ AHt R D h

=

7

= =

o

= = = =

= =

molecules to revert to a random distribution after the removal of an impressed electric field. It is also considered as the time required for the molecule to rotate through 180”. Boltzmann constant absolute temperature Planck‘s constant entropy of activation heat of activation molar gas constant diffusion coefficient of the molecule distance between successive equilibrium positions along the diffusion path coefficient of viscosity volume of the molecule

276

PHILIP F. LOW

For these processes AHt probably equals E, the activation energy, because volume changes should be negligible. Now if the entropy and heat of activation are the same for each process it follows that

D = h 2 / t = kTh2/qv

(4) Another relationship between z and q was obtained by Debye (1945) by quite a different method. His relationship is

Here T is the radius of the rotating molecule. A familiar relationship between D and q is the Stokes-Einstein equation, viz.,

kT

D = 6xry

The similarity between the latter two equations and those of Eq. ( 4 ) is apparent. Wang et al. (1953) determined the activation energy for the selfdiffusion of water in water and compared the value obtained with the activation energies for the dielectric relaxation (dipole rotation) and viscous flow of water. The activation energies had the same value. Saxton (1952) also found the activation energies for dielectric relaxation and viscous flow to be the same. One would expect the activation mechanism for dielectric relaxation to be the breaking of hydrogen bonds so that the molecule can rotate. The activation mechanism for both self-diffusion and viscous flow should be the breaking of hydrogen bonds and the pushing aside of surrounding water molecules so that the molecule can move forward. However, the identity of the activation energies for these processes suggests that the major activation mechanism is the breaking of hydrogen bonds and that the pushing aside of the surrounding water molecules requires little energy. Possibly this is because vacant spaces in the open water structure are already available for the molecule to move into. From the magnitude of the activation energy Ewe11 and Eyring (1937) and Wang (1951a) calculated that about two bonds had to be broken per molecule for movement to occur. This number agrees with the estimated number of hydrogen bonds remaining in water at room temperature (Cross et al., 1937; Grjotheim and Krogh-Moe, 1954; Haggis et al., 1952). Of course, there is a decrease in the activation energy with an increase in temperature in keeping with the fact that raising the temperature breaks hydrogen bonds. According to Eqs. (4)and (5) the value of q / t T should be essentially constant at all temperatures. And, further, one should be able to cal-

PHYSICAL (3HEMISTRY OF CLAY-WATER INTERACTION

277

culate the volume of the kinetic unit from the magnitude of this constant. Collie and co-workers (1948) showed that r/tT was constant and, using Eq. (5), showed that the calculated value of z agreed with the experimental value if the radius of the water molecule was used for T . Robinson and Stokes (1955) have summarized the results of Collie, Hasted, and Ritson and have shown that the calculated molecular radius varies only from 1.44 A. to 1.48 A. between temperatures of 0" C. and 75" C., respectively. Saxton (1952) also showed the constancy of y/zT. If Eq. ( 4 ) is used with the value of his constant then 0,the molecular volume, can be shown to be 0.5 x 10-22 cc. which is of the same order of magnitude the known molecular volume. Recently, Grant (1957) as 0.3 X presented additional evidence for this relationship and reported that the kinetic unit is of the order of one molecule instead of a cluster or domain of water molecules. Wang (1951a, b) and Wang et al. (1953) obtained a constant value of Dq/T in the temperature range 10" to 55" C. Hence, their evidence supports the validity of Eqs. ( 4 ) and (6). Wang (1951a) also reported that the value of the molecular volume, calculated from Eq. (6),suggests that the diffusing unit is a single molecule. Robinson and Stokes (1955) have summarized these results as well. It is apparent from the preceding discussion that the processes of dielectric relaxation, self-diffusion, and viscous flow in water all involve the individual molecule rather than a cluster or domain of molecules. Evidently, less energy is expended in dissociating the molecule from the cluster or domain, so that it can move independently, than in moving the intact cluster or domain. Or perhaps the active molecules are between these clusters or at their edges. 111. Nature of Ionic Solutions

In view of the charge distribution in the water molecule, one would expect charged ions to attract water molecules electrostatically. In other words, one would expect the ion to hydrate by the formation of ion-dipole bonds. Hydration will occur if the potential energy of the water molecule is less in the hydration shell of the ion than it is in the hydrogen-bonded water structure. Bernal and Fowler (1933) made calculations to show that this is the case, especially for small or multiply charged ions. However, the water molecules around the ion can exchange with other water molecules in the medium, the frequency of exchange depending on the intensity of the ion-dipole bonds ( Samoilov, 1957). Now, in normal water each molecule is surrounded by four others in tetrahedral fashion. Two of the neighboring water molecules are oriented with their protonic

278

PHILIP F. LOW

corners toward the central molecule; the other two are oriented with their lone-pair electron corners toward it and their protonic corners away from it. When an ion is introduced into the water structure, the situation is different. All the water molecules around a cation have their resultant electronic centers directed inward. Around an anion, all the water molecules have their protonic corners directed inward. Therefore, even if the ion is of the right size to fit into the space normally occupied by a water molecule, the water of hydration cannot "match" or coordinate with the surrounding water. The result is a disruption of the quasi-crystalline water structure. The disruption will be enhanced if the ion differs in size from the water molecule. In general, the larger the ion, the greater the disruptive effect. In the work of Morgan and Warren (1938) it was established that the second or minor peak in the X-ray diffraction curve of pure water diminishes in height with increasing temperature until, at 83" C., the peak is gone. This diminution in peak height has been regarded as being due to a breakdown of the water structure and an increase in the coordination number of the molecules. Ions are supposed to break down water structure. Therefore, Stewart (1939, 1943, 1944) determined the change in height of the minor diffraction peak with increasing salt concentration and found that it decreased linearly with the mole fraction of the salt. His conclusion was that ions, like an increase in temperature, break down the water structure. This conclusion was substantiated by the fact that the decrease in peak height with salt concentration was related to the change in apparent molal volume of the salt for a number of electrolytes. Evidently, the change in the apparent molal volume of the salt is to be construed as a change in the molal volume of the water. Stewart also observed that the apparent molal volume of a salt changed most rapidly with salt concentration in dilute solution. Had the disruptive effect of the ions been local so that the ionic spheres of influence did not overlap, the apparent molal volume should have remained constant until relatively high concentrations were attained. Consequently, he interpreted this observation to mean that the structural disruption produced in the water by the ions is rather extensive. The work of Corey (1943) lent support to Stewart's conclusions. He determined the adiabatic compressibility of aqueous electrolyte solutions and showed that the rate of decrease of this quantity with the mole fraction of salt was correlated with the rate of change of peak height for twenty-six different electrolytes. The application of pressure to openstructured water breaks hydrogen bonds and leads to a closer packing of molecules with a resulting decrease in compressibility. Apparently ions produce the same effect as added pressure.

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

279

In their classic paper Bernal and Fowler (1933) compared the apparent volumes of salts in solutions and observed that the volumes of the component ions were additive. Therefore, to obtain ionic volumes in solution, they divided the volume contribution of CsCl to a solution in proportion to the volumes of the component ions in the solid. Then, using the latter volumes as a basis, they were able to obtain the apparent volumes in solution for several ionic species. As an example, they could obtain the volume of K+ in a solution of KC1 by subtracting the assumed volume of the C1- ion from the apparent volume of the salt, and so on. They compared the ionic volumes so obtained with the volumes of the same ions in solids. The comparisons revealed that the apparent volumes of cations in solution were the same as those in solids only for Rb+ and Cs+. The apparent volumes of anions in solution were the same as those in solids except for OH- and F-. All other cases showed apparent volumes either much smaller than in solids or actually negative. Their conclusion was that the presence of ions in solution either contracts the water structure or breaks it down to cause a closer packing of water molecules. An analogous approach was used by Gurney (1953) with similar results. The dielectric constant data on ionic solutions are interesting but d a c u l t to interpret. However, their interpretation will be facilitated by referring to equations (16) to (19) and the discussion at the beginning of Section VII. Hasted and associates (1948) reported that ions make a negative contribution to the dielectric constant. The magnitude of the negative contribution was linearly related to the salt concentration. In general, at a given concentration, the smaller or more highly charged the ion, either cation or anion, the greater the dielectric decrement. This decrement was assumed to be due to the inability of the water molecules around the ion to rotate in the alternating field. In essence, the hydrated ions were regarded as spherical inclusions of very low dielectric constant in a continuous water medium of uniform dielectric constant. A more significant feature of their study was the depression of the relaxation time produced by the addition of salt. The depression of the relaxation time was also linearly related to the salt concentration. Recall that the relaxation time is the time required for a dipolar molecule to make a single rotation through 180 degrees in an alternating electric field. The authors proposed that molecular reorientation takes place at the boundaries of broken pieces of water lattice; hence, they considered the depression of the relaxation time to be a reasonable index of the structure-breaking effect of the ions. Larger ions, both cations and anions, produced a larger depression of the relaxation time than small ones. There was also a tendency for this depression to increase with the charge on the ion. In a

280

PHILIP F. LOW

subsequent paper, Haggis et al. (1952) showed that the depression of the relaxation time was related to the difference (negative) between the apparent and calculated volumes of the added salt. Thus credence was given to the interpretations of Hasted et al. (1948). But, in concentrated solutions the relaxation time increases again (Harris and O’Konski, 1957). Apparently, this increase in relaxation time is due to the increased orientation of water molecules in the strong electric field produced by cations and anions in proximity. A valuable index of the effect of ions on the structure of water is provided by viscosity measurements. Viscosity is a structure-sensitive property of a fluid. The relationship between the viscosity of an aqueous salt solution and the salt concentration is reported in Gurney’s (1953) book. It is q

= yo ( 1

+ A dF+ Bc)

(7) where Q is the viscosity of the pure solvent, c is the salt concentration and A and B are constants characteristic of the solute. The Ads term is supposedly related to interionic attractive forces that retard motion when oppositely charged ions move relative to each other. The B coefficients are related to the structure of the water in solution. In dilute solutions these coefficients are additive for ions. As a result, it is possible to obtain values for single ions by a method analogous to that for determining apparent ionic volumes. The B coefficients reported by Gurney (1953) and Kaminsky (1957) become more negative as the ions get larger. In fact, the coefficients for K+ and larger alkali metal cations are negative, as are those for most anions. However, as the temperature increases these coefficients become more positive. Kaminsky ( 1957) ascribes this increase to the temperature-induced breakdown of the water structure with a concomitant decrease in the contribution of the ions to this breakdown. In a revealing graph Gurney showed that the B coefficients decrease linearly with an increase in the partial molal entropy of the ions. Theoretically, the partial mold entropy of an ionic species is the increase in entropy (disorder) of an infinitely large solution when a mole of ions is added to it at constant pressure and temperature. Apparently then, the ions which have negative B coefficients produce disorder in the solution. The fluidity of a solution is the reciprocal of its viscosity. The changes in fluidity induced by ions in aqueous solutions are additive, as might be expected. Bingham (1941) has tabulated the ionic elevations of fluidity. His tables reveal that, in the alkali metal cation and halide anion series, the fluidity elevation decreases with decreasing ionic size. In these series only Li+ and F- have negative fluidity elevations. An interesting point is that multiply charged ions have negative fluidity elevations, the neg-

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

281

ative character of the elevation increasing with charge. The indication is that a small multiply charged ion reduces the fluidity of the water molecules for a considerable distance from it. Recent X-ray evidence (Brady, 1980) suggests that the water may even have the structure of ice for some distance from a small highly charged ion. Another structure sensitive quantity is the digusion coefficient of a diffusing species. Wang ( 1954 ) has measured the self-diffusion coefficient of H2018in solutions of NaCl, KC1, and KI. In the former solution the self-diffusion coefficient decreased with concentration, but in the latter two solutions it increased. Now the presence of the ions should increase the tortuosity of the diffusion path of the water molecules and thereby decrease the observed diffusion coefficient. But in the KCl and KI solutions the presence of the ions increased this coefficient. Therefore, it seems evident that the ions of these salts disrupted the quasi-crystalline structure of water to make the water more fluid. In the case of the NaCl solutions it is possible that the smaller Na+ actually tightened the structure. Or the decrease in diffusion coefficient with salt concentration may have been due to the tortuosity factor mentioned above. The entropy of a system is a quantitative measure of its disorder: the greater the entropy, the greater the disorder. The partial molal entropy of a component of the system is the change in entropy of the system per mole of added component at constant composition, pressure, and temperature; hence, it is the added disorder per mole of component. With this in mind, Frank and Robinson (1940) determined the partial molal entropies of water in different salt solutions relative to the entropy per mole for pure water at the same pressure and temperature. Their work showed that the relative partial molal entropies of the water in dilute solutions were negative and became more negative as the salt concentration increased until a minimum value was reached. Then the relative partial molal entropies of the water increased with salt concentration to rather large positive values. They explained the initial decrease in the disorder of the water molecules in solution as being due to the polarizing effect of the increasing field strength between oppositely charged ions in accordance with Debye-Huckel theory. The later increase in disorder was attributed to the disruptive effect of the ions, which became predominant at the higher salt concentrations. As would be expected, the minimum in the partial molal entropy versus salt concentration curve occurred at higher concentrations for the smaller ions than for the larger. And the final partial molal entropy values were less positive for the smaller ions than for the larger ones. Later, Frank and Evans (1945) studied the entropies of vaporization of ions in solution. The entropy of vaporization is the increase in disorder

282

PHILIP F. LOW

of the ions in going from the liquid phase to the gas phase. The entropies of vaporization were obtained by subtracting the partial molal entropies in solution from the statistically calculated molal entropies of the ions as perfect gases. These values were compared with the theoretical entropy losses arising from ( 1 ) restriction of the ions in “free volume” cells in the liquid phase, ( 2 ) immobilization of the water in the hydration shells of the ions, and ( 3 ) dielectric polarization of more distant water. The results showed that for all the alkali and halide ions except Li+ and F-, too little entropy was lost when the ions were dissolved from the gaseous state. In other words, there was more disorder in the liquid state than the theory would predict. Their conclusion was that this disorder arose from the breakdown of the water structure by the ions. Again, the structural breakdown increased with increasing ionic size. The values for that part of the ionic entropy of vaporization attributed to breakdown of water structure were compared with the ionic fluidity elevations of Bingham (1941). In general, the ionic disruption of structure was correlated with the ionic fluidity elevation. Further, the total entropy of vaporization of the ions was correlated with the ionic fluidity elevation. For the small highly charged ions the entropies of vaporization were high and the fluidity elevations had large negative values. The authors proposed that these ions owed both their low entropy and high viscosity in solution to the existence around each ion of a large patch of “frozen” water. Frank and Evans (1945) also listed the partial molal heat capacities of the alkali halides. The partial molal heat capacity is the change in heat capacity of a system per mole of added component at constant pressure, temperature, and composition. The partial molal heat capacities were all negative. This was expected because the water molecules in the hydration shell of an ion are not free to rotate. As a result, they do not absorb heat to increase their rotational energy and thereby contribute to the heat capacity. However, on this basis alone, one would expect the partial molal heat capacity to become more negative as the ionic size decreased. The reverse was true. Therefore, the authors concluded that, in addition to the reduction in heat used for molecular rotation, there was a reduction in the heat used to “melt” hydrogen-bonded clusters because these were already partially destroyed by the structure-breaking effect of the ions. As usual, the structure-breaking effect of the ions increased with ionic size. Recently, Frank and Wen (1957) discussed the structure of water in ionic solutions. On the basis of evidence of the kind presented in this section, they proposed a reasonable model for ion-water interaction, According to them each ion is surrounded by three regions. The innermost region ( A ) is one of immobilization. In this region the water molecules

PHYSICAL (3HEMISTRY OF CLAY-WATER INTERACTION

283

have little kinetic energy. They are strongly oriented in the intense electric field of the ion. In the second region ( B ) the water structure is broken down and is more random or less ice-like than normal. The outermost region (C) contains normal water which is polarized in the ordinary way by the ionic field which, at this distance, is relatively weak. The cause of the structural breakdown in region B is presumably the competition between the normal structural orienting influence of the neighboring water molecules and the orienting influence of the spherically symmetrical ionic field. The latter influence predominates in region A and the former in region C. The outward orientation of like poles of the water dipoles around the ion should always produce some disorder in region B, the large univalent ions producing the most. Small ions, especially the multivalent ones, should have region B, but region A should be great enough for the entropy of vaporization to be greater than the predicted value. In other words, region A should be more extensive than region B so that the net effect of these ions is to produce order even though the arrangement of the molecules is different from that in normal water. Before concluding this section something should be said about the effect of nonpolar solutes on the water structure. Frank and Evans (1945) noted that the entropies of vaporization of these solutes from water were much greater than their entropies of vaporization from organic solvents when compared at common heats of vaporization. The entropies of vaporization of nonpolar solutes from water also far exceeded the entropies of vaporization of ions from water. Further, the entropies of vaporization decreased much more rapidly with temperature when the nonpolar solutes were dissolved in water than when they were dissolved in an organic solvent. And the partial molal heat capacities of these solutes in water were abnormally high. These observations led the authors to conclude that, when a nonpolar molecule dissolves in water, it modifies the water structure in the direction of greater “crystallinity.” In their descriptive language, the water builds a tiny “iceberg” around the solute particle. However, it is not implied by them that the quasi-crystalline structure about the nonpolar solute molecule is exactly like ice. The rapid decrease in entropy of vaporization of the nonpolar solutes with temperature was ascribed to the “melting” of the icebergs. The energy consumed in the “melting process was supposed to account for the abnormally high partial molal heat capacities. Additional support was given to this picture of nonpolar solutes in water by the observation of Frank and Wen (1957) that the apparent molal heat capacity of tetra-n-butyl ammonium chloride was about 120 cal. per degree per mole more than the theoretical value calculated from the additivity rules for hydrocar-

284

PHILIP F. LOW

bons. And Frank (1958) has pointed to the fact, reported by Haggis et al. (1952), that nonpolar solutes or solutes with nonpolar groups increase the dielectric relaxation time of the solution. The formation of the “icebergs” is considered to arise from the lack of attraction between the nonpolar molecules and the surrounding water. This water would not be subjected to the normal torques and displacements; hence, it would have a greater opportunity to develop the tetrahedral structure. Contrary to the postulate of Frank and his co-workers, Claussen (1951) postulated that nonpolar solutes form definite hydrates in water and proposed probable structures for them. Later Stackelberg and Muller (1951) confirmed one of these structures by X-ray analysis. The structure is extensive, having 136 water molecules per unit cell. However, it is by no means certain that definite hydrates form with all nonpolar solutes. Rather, it may be that the larger nonpolar solutes induce the formation of the “icebergs” described earlier. Frank and Wen (1957) point out that the hydrates or cages of Claussen (1951) and Claussen and Polglase (1952) should have specificity in the size and shape of solutes they accommodate; whereas, there seems to be a smooth proportionality between the degree of “ice-like-ness” and the size of the nonpolar region of the molecule. IV. Mechanisms of Clay-Water Interaction

There are several possible mechanisms by which water may interact with clay surfaces. These mechanisms of interaction may operate separately or unitedly. All we can observe is their net effect on the water. At this time we shall give a brief discussion of each of them. Then, when the experimental data are presented, we shall try to evaluate their relative importance. We have seen that hydrogen bonds normally form between 0-H groups and oxygen atoms and that there is a tendency for water molecules to be hydrogen bonded in a tetrahedral arrangement. We know that the surface of clay minerals is made up of either oxygen atoms or hydroxyl groups arranged in a hexagonal pattern which, according to Hendricks and Jefferson (1938), Macey ( 1942), and Forslind ( 1952), can coincide at points with a similar pattern in a hydrogen-bonded water structure. Further, we know that the crystal lattice of most clay minerals contains excess electrons which arise from the isomorphous substitution of cations in the lattice. And, from the work of Lennard-Jones and Pople (1951) and Frank (1958), we have reason to believe that covalency may occur in hydrogen bond formation if one of the systems involved is capable of having its lone-pair electrons distorted by the proton or positive element of the other. Such distortion is conducive to the formation of additional

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACI'ION

285

hydrogen bonds in a cooperative manner (Frank, 1958). Now, the lonepair electrons of the oxygen atoms in the surface of a clay mineral should be easily distorted because of the excess electrons in the lattice. Therefore it is reasonable to believe that water molecules adjacent to a claymineral surface are bonded to the oxygen atoms of the surface by covalent hydrogen bonds. The existence of the covalent bonds should alter the electron distribution in these molecules and make it easier for them to form additional covalent bonds with other molecules in the same and next layer. Those in the next layer, in turn, may be expected to form hydrogen bonds of partially covalent character with their neighbors, and so on. The bonded water molecules should be arranged in a tetrahedral fashion because of the directional properties of the bonds. However, the degree of covalency in the bonds should decrease with distance from the surface and, for this reason, the tetrahedral arrangement should become less rigid in the same direction. Thus, it is possible for a tetrahedral structure of water molecules to be attached to and propagated, with decreasing rigidity, away from the oxygen surface of a clay mineral. It is not unlikely that a hydrogen-bonded water structure builds up also on the hydroxylic surface of a clay mineral. Here the excess of electrons in the mineral lattice should help to screen the protons of the hydroxyl groups and render them less electropositive. Consequently, the lone-pair electrons of the oxygen atoms in the bonded water molecules should experience little distortion and the degree of covalency in the hydrogen bonds should be slight. For this reason the water structure on an hydroxylic surface may be expected to be less stable than that on an oxygen surface. But, as we have seen, the balance between order and disorder in water is delicate. Therefore, even the hydroxylic surface, by fixing the positions of a layer of bonded molecules, should tip the balance in favor of order for considerable distances. Probably, the water structure that develops on either mineral surface is not that of ice. One reason is that the exchangeable cations would disrupt the water structure (Mackenzie, 1950). Another is that the surface atoms of the mineral may not coincide exactly with protons or oxygens of the ice lattice so that the latter would be distorted (Mathieson and Walker, 1954). Or surface irregularities, which must exist, may produce distortions in the ice lattice. Further, there may be other hydrogen-bonded structures with tetrahedral coordination which are more stable in such an environment (Hendricks and Jefferson, 1938; Claussen, 1951). A second mechanism by which water may be attracted to a clay surface is hydration of the exchangeable cations. There is little doubt that cations hydrate, especially if they are small or multiply charged. Since the cations cannot escape from the negatively charged surface, neither

286

PHJLIP F. LOW

can the water of hydration. This mechanism of attracting water should be most important at low water contents. At higher water contents the exchangeable cations should still play a role in clay-water interaction. Those exchangeable ions that are dissociated from the surface may be regarded as being in solution. Undoubtedly, they lower the activity of the water in the vicinity of the clay surface in the same manner as ions lower the activity of water in solution. Consequently, water should tend to move into the surface region. In short, clays may be expected to attract water by osmosis. As mentioned earlier, the crystal lattice of a clay has an excess of electrons. For this reason, the flat clay particle may be regarded as a negatively charged condenser plate. Double-layer theory predicts that the electric field of this plate decreases with distance from the surface. Water molecules, having a dipolar nature, should tend to orient with their axes parallel to the field and their positive poles directed toward the surface. Their degree of orientation should decrease as the electric field intensity decreases. And midway between the clay plates, where the field intensity is zero, there should be a region of structural disorder in the water because the negative poles of the water molecules on either side are all directed inward. The situation should be analogous to that in the vicinity of an ion. Only the clay, being much larger than the ion, should disrupt the water more extensively. This arrangement of water molecules would occur only if their orientation energy in the electric field of the clay is less than their energy in a tetrahedral arrangement involving hydrogen bonds. The two arrangements are incompatible. However, it should be noted that the presence of the electric field does not preclude the latter arrangement. In fact, the field may aid in distorting the lone-pair electrons of the water molecules and thereby enhance the covalent character of the hydrogen bonds. Finally, it is possible that London disperson forces are responsible for clay-water interaction. These forces are the ones that exist between neutral molecules and are nondirectional in nature. They arise from the instantaneous in-phase fluctuations of the electronic atmospheres around the oxygen nuclei. As the electronic atmosphere of a given oxygen atom is displaced relative to its positive nucleus, a temporary dipole is formed. This dipole induces corresponding displacements in the electronic atmospheres of neighboring molecules so that dipole-dipole attraction occurs. An instant later the atmospheres are displaced in phase in another direction, but always so that a net electrostatic attraction between molecules exists. According to this picture, the electronic atmospheres of the oxygen atoms in the water would fluctuate in phase with the electronic atmospheres of the surface oxygens. The resulting water structure, being

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

287

held together by nondirectional bonds, should be close packed. In addition, it should be more fluid and less extensive than the hydrogen-bonded one. In the preceding discussion of the possible mechanisms of clay-water interaction it was assumed that the clay surface was clean and uncontaminated. Frequently this may not be the case. In most natural clay systems there will be hydrous oxide impurities, especially those of aluminum. And free silica will usually be present. These impurities also contain surfaces composed of oxygen atoms and hydroxyl groups. Further, their surfaces have electric double layers. Therefore, the mechanisms of water interaction with them will be similar to those for water interaction with the clays. Unfortunately, the experiments which we will now discuss were conducted on clays which may or may not have had hydrous oxides and silica present. We will assume that the results apply to the clays. But we will recognize the possibility that they apply instead to the afore-mentioned impurities. Although it would be desirable to know which substance is the one affecting the water, these substances are so intimately related in the clay fraction of the soil that, for most purposes, the distinction is relatively unimportant. Our major concern is the nature and properties of the water in such a system. V. Specific Volume of Clay-Adsorbed Water

One property which is very useful in determining whether or not water has a simple close-packed structure is the specific volume or its reciprocal-the density. Attempts have been made to determine the specific volume of clay-adsorbed water by a pycnometer technique. The volume, V, of a pycnometer can be determined accurately at a given temperature. In addition the weight of clay, m, and of the water, w, in the pycnometer are measurable. Hence, using the equation

V = wv,

+ mv,

(8)

and assigning a value to the specific volume of the clay, vm,it is possible to calculate the apparent specific volume of the water, .0, However, the difficulty lies in assigning a correct specific volume to the clay. This fact is illustrated in Table I, which was prepared from unpublished data of Anderson ( 1958). The clay was Na-saturated Wyoming bentonite. The specific volumes of 0.3703,0.3571, and 0.3448 correspond to clay densities of 2.70, 2.80, and 2.90, respectively. Evidently the apparent specific volume of the water appears to be less than, equal to, or greater than that of pure water (sp. vol. = 1.0029) depending on the value of the specific volume assigned to the clay. And the apparent spec& volume of the

TABLE I The Specific Volumes of Water at 25" C. in Suspensions of Different Na-Clay Concentration, Assuming Different Specific Volumes for the C l a p Pycnometer Weight of Pycnometer volume clay number (e.) (g.) 1 49.986 1.1101 4 49.966 2.1392 5 49.978 2.6696 6 49.980 3.5883 7 49.873 4.4417 8 49.995 6.3986 10 49.896 6.7895 5 Data from D. M. Anderson (unpublished).

Weight of water (g.)

Apparent specific volume of water ( U, = 0.3703) (cc./g. 1

Apparent specific volume of water ( U, = 0.3571) ( cc./g.

Apparent specific volume of water (urn= 0.3448) (cc./g. 1

49.4477 49.0755 48.8910 48.5656 48.1549 47.5741 47.3380

1.0025 1.0020 1.0020 1.0018 1.0015 1.0011 Lo009

1.0028 1.0026 1.0027 1.0027 1.0027 1.0029 1.0028

1.0031 1.0031 1.0034 1.0036 1.0039 1.0045 1.0046

r

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

289

water appears to decrease, remain constant, or increase with increasing clay concentration depending on this value. It is obvious that the correct interpretation of pycnometer data is contingent upon obtaining a correct value for the specific volume of the clay. To obtain the correct specific volume of the clay, it has been customary to use pycnometer liquids other than water in the hope that these will not interact with the clay and thereby have their speci6c volumes altered. The normal specific volume is assigned to the liquid being used. However, many years ago Russell (1934) showed conclusively that clay interacted differently with every liquid that he investigated. The liquids were: tetralin (tetrahydronaphthalene), decalin (decahydronaphthalene) xylene ( dimethylbenzene), benzene, carbon tetrachloride, nitrobenzene, aniline, amyl alcohol, benzyl alcohol, and water. The first five liquids are nonpolar, and the remaining liquids are polar. Using the tetralin as the reference liquid he plotted T-Lr against corresponding values of T-L, for several clays. Here T represents the apparent specific volume of a clay in tetralin and L represents the apparent specific volume of the same clay in another liquid, the subscripts i and j denoting different liquids. In this manner the real volumes and weights of the clays and ions were eliminated and only the relative effects of the clays on the pycnometer liquids were observed. The plots were straight lines with different slopes, which depended on Li and LI. These results are consistent with the well-documented fact (Henniker, 1949; Adamson, 1960) that all surfaces influence the structure and properties of a vicinal liquid to a depth of many molecular diameters. DeWit and h e n s (1950) used a petrol fraction as pycnometer liquid and determined the density of oven-dried Wyoming bentonite to be 2.348 ( 0 , = 0.426). Then they suspended the bentonite at different moisture contents in this liquid to obtain the densities after hydration. The data permitted them to calculate the apparent specific volumes of the adsorbed water which, for percentage moisture contents (dry weight basis) of 28.4, 16.6, and 11.6 were 0.76, 0.73, and 0.71 cc. per gram, respectively. Two faults in this work make the results unacceptable. One is that the true density of Wyoming bentonite is closer to 2.80, as will be shown later. The second is that water confined in a thin layer between a clay surface and a nonpolar liquid may not have the specific volume it would have in the absence of the nonpolar liquid. In the previous discussion of ionic solutions it was pointed out that nonpolar solutes markedly affect the water structure. Since DeWit and Arens used too large a value for the specific volume of the clay, their calculated apparent specific volumes for the adsorbed water would appear to decrease with decreasing hydration, i.e., increasing clay concentration (see Table I).

290

PHILIP F. LOW

Oakes (1958) used benzene as pycnometer fluid to determine the density of oven-dried Wyoming bentonite and obtained a value of 2.5196 ( v , = 0.397). If this value is used with his density-concentration data for bentonite-water systems, it is found that the apparent specific volume of the adsorbed water appears to decrease with increasing clay concentration. However, the objection to the use of such a high value for v, applies here also. In order to avoid the difficulties inherent in the conventional pycnometer - technique, Low and Anderson (1958a) derived an expression for ow, the partial specific volume of water in a clay-water suspension in terms of e, the density of the suspension and c, the clay concentration. The expression is not based on any assumptions. It is

in which the subscript m indicates that the weight of clay is constant. A similar expression can be used to determine the partial specific volume of the clay. The partial specific volume of a component is the change in volume of the system per gram of added component at constant pressure, temperature, and composition. It is a differential quantity, whereas the apparent specific volume, if properly measured, is an average quantity. In the clay-water system the volume of the water may not be the same at all points of the system; hence, the partial specific volume of the water will be regarded as the volume per gram of water midway between the particles. Now when Low and Anderson plotted the suspension density against the clay concentration for three homoionic Wyoming bentonites, straight lines were obtained with intercepts on the ordinates at density values corresponding to that of pure water. Therefore, the necessary conclusion was that the partial specific volume of the water was the same at all concentrations of clay ( u p to 13 per cent by weight) as that of pure water. At the greatest clay concentration the average maximum distance of water from the particles was 84 A. In an earlier paper Hauser and LeBeau (1938) reported that the apparent density of Wyoming bentonite clay in aqueous suspensions increased with clay concentration. They used the pycnometer technique and assigned the normal density to water. Their maximum clay concentration was 2 per cent by weight. Their conclusion was that the water must be compressed on the surface of the bentonite. But Martin (1960) re-examined their data in terms of Eq. ( 9 ) and found that their results agreed with those of Low and Anderson (1958a). From their pycnometer evidence Low and Anderson (1958a) could conclude only that water in clay suspensions had the same partial specific

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

291

volume as normal water beyond about 84 A. from the clay surface. To study the partial specific volume of the water closer to the clay surface, they compressed a homoionic Wyoming bentonite gaste between a mercury piston and a stainless steel filter and observed the corresponding changes in volume of the paste and expressed water. The volumes of expressed water were converted to grams of expressed water. From the latter quantities and the initial water content of the paste, the grams of water in the paste at the different paste volumes were calculated. Then a plot was made of paste volume against grams of water in the paste. The slope of the resulting line at any water content equals the partial specific volume of the water at that water content. Again no assumptions are involved. For the Li, Na, and K bentonites that they worked with, the following conclusions could be drawn: (1) the partial specific volume of water is different from normal water out to distances in excess of 60 A.; ( 2 ) the partial specific volume increases continuously as the clay surface is approached; (3) within 10 A. from the clay surface the partial specific volume is as much as 3 per cent greater than that of normal water (ice has a specific volume only about 8 per cent greater); ( 4 ) as the temperature is lowered the partial specific volume of the water increases; ( 5 ) the exchangeable ions affect the partial specific volume of the water. Confirmation for the partial specific volume results of Anderson and Low (1958) has been provided by the calculations of W. F. Bradley ( 1959). He used the unit cell dimensions, based on X-ray measurements, and the structural formula of montmorillonite in analyzed Wyoming bentonite to obtain a density for the clay laminas alone. The calculated density was 2.83. This value is slightly higher than the density of 2.78 which was calculated for another sample of Wyoming bentonite by Mackenzie (1959). It is also slightly higher than the value obtained by Low and Anderson (1958a) using Eq. ( 9 ) . Their value was 2.80 for Wyoming bentonite saturated with Na+, the predominant ion on the natural clay. Using his density value, and the data of DeWit and Arens (1950) and of Oakes (1958), Bradley calculated apparent specific volumes of the water by subtracting the volumes of the montmorillonite laminas present from the total volume. He found the apparent specific volumes of the water in the clay-water systems to be larger than the specific volume of pure water. At a clay concentration of 68.5 per cent the calculated apparent specific volume of water was 1.023, in good agreement with the partial speciiic volumes of 1.02 to 1.03 reported by Anderson and Low (1958) for comparable clay concentrations. At higher clay concentrations his calculated values rose as high as 1.33 cc. per gram.

292

PHILIP F. LOW

Using an air pycnometer, Nitzsch (1940) also found high apparent specific volumes for water adsorbed on a montmorillonite-rich clay. Initially the apparent specific volume of the water was as high as 5.00 cc. per gram. As the water content increased to 8 per cent the apparent specific volume decreased to 1. However, as Martin (1960) points out, water can penetrate many spaces in a clay that exclude air; hence, it is probable that his values are too high. Another method for determining the apparent specific volume of adsorbed water is to measure the C-axis spacing of an expanding mineral by X-ray analysis and multiply the value obtained by one-half the experimentally determined surface area. The product is presumed to be the interlayer volume. This volume is divided by the weight of water present to determine its apparent specific volume. The author considers this method to be unreliable because all the water in the system, whether it is between the clay layers or in the interstices between the particles, is considered to be interlayer water. If any interstitial water is present, the calculated apparent specific volume will be too low. An additional complication is the difficulty in accurately measuring the surface area of the clay. And at water contents corresponding to a few monolayers, water “islands” may exist between clay layers. In this case the calculated apparent specific volume would be too high. Nevertheless, Martin (1960) used the X-ray and water content data of Mooney et al. (195213) and of Norrish (1954) to calculate the apparent densities of adsorbed water on montmorillonite. The values calculated from the data of Mooney et al., for water contents below 0.28 g. per gram of clay, were higher than that of pure water and increased with increasing clay concentration; whereas, the values calculated from the data of Norrish, for water contents above 0.37 g. per gram of clay, were lower than that of pure water and decreased slightly with increasing clay concentration. In terms of the apparent specific volume the converse would be true. Since the results obtained from the data of Mooney et al., agreed with those of DeWit and Arens (1950), and since the results obtained from the data of Norrish agreed with those of Anderson and Low (1958), Martin concluded that all the results were acceptable. Consequently he proposed, in effect, that the specific volume of water is less than that of normal water at water contents below about 0.3 g. per gram of clay and is greater than that of normal water at water contents above this value. Now it is not unreasonable to believe that, at low moisture contents, the exchangeable ions, being concentrated near the clay surface, would disrupt the water structure and lead to a closer packing of molecules. Indeed, this would be expected. But in view of the previous criticism of the work of DeWit and h e n s (1950) and the possible complicsstions in the X-ray method for

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

293

determining apparent specific volumes, Martin's conclusion cannot be accepted without serious reservations. In fact, the only evidence not based on questionable assumptions, namely, that provided by Anderson and Low (1958) and by W. F. Bradley (1959), argues against it. Therefore, in the following discussion we will regard the afore-stated conclusions of Anderson and Low as acceptable. However, we will not discount the possibility that for certain relatively dry systems the specific volume of the adsorbed water may be less than that of normal water, especially if the exchangeable ions are small and multivalent. At this point let us consider the possible mechanisms of clay-water interaction in the light of the specific volume evidence cited thus far. Undoubtedly, water in the vicinity of clay surfaces is not a simple closepacked liquid. It must have a relatively open structure, a fact that suggests the presence of bonds with directional properties. In consequence, the London dispersion forces can be discounted as being of primary importance. Even though the ions infiuence the existing water structure, they are not responsible for it because the evidence is overwhelming that they decrease the specific volume of water, either by contracting the water structure or by breaking it down. Therefore, the only mechanisms that must still be considered as possibilities are the orientation of water molecules in the electric field of the clay plate and hydrogen bond formation between surface atoms and water with the consequent development of a coordinated water structure. The former mechanism will be considered first. The molar potential energy, 8, of dipoles of polarizability, a, and dipole moment, p, in an electric field of intensity, E, was given by Low and Deming (1953). It is

in which N is Avogadro's number, E is the dielectric constant, k is the Boltzmann constant and T is the temperature. This equation is strictly applicable to nonassociated dipoles, but we will use it as an approximation. For water, the coefficient of E2 has the magnitude at 25" C. of 0.219 cc. per mole, assuming the normal dielectric constant. Therefore, for the molar energy in the electric field to equal the energy required to break a mole of hydrogen bonds (4.5 kcal.) the field intensity would have to be close to 2.8 volts per angstrom. Recall that the zeta potential, i.e., the change in voltage between the plane of shear and a point an infinite distance away, is usually only about 0.05 volt for the clay minerals. Despite the approximate nature of our calculations, it appears that the field strength in the diffuse part of the double layer would not be suf-

294

PHILIP F. LOW

ficient to break enough hydrogen bonds to orient the water molecules. This conclusion becomes more reasonable if one realizes that water is not oriented about an ion for more than a few molecular layers; the field intensity near an ion is much larger than near a clay particle. As a result, it is highly unlikely that the observed elevations of the partial specific volume can be attributed to molecular orientation in the electric field. Additional evidence which dictates against the electric field being the causative factor is the observed trend (Anderson and Low, 1958) in the partial specific volume. Even at average interlayer distances as small as 20 A., this property was being augmented at a constantly increasing rate. There was no indication of disordered water midway between the layers, which is a necessary consequence of the electric field mechanism. It will be shown later that other evidence also argues against this mechanism. Now let us evaluate the hydrogen bond mechanism in the light of the specific volume data. If this mechanism obtains, one would expect the adsorbed water to have a very open structure with the degree of openness increasing toward the surface where the degree of covalency of the bonds would be greatest. Such is the case. Moreover, one would expect the degree of openness to increase with a decrease in temperature, since fewer bonds would be disrupted at lower temperatures. Here also the expectation is realized. Further, as might be expected, the exchangeable ions influence the water differently. It appears, therefore, that a hydrogen-bonded water structure is attached to and propagated away from a clay mineral surface. Since there is no indication of a disordered region in the interlayer water, there must be no incompatibility between the molecular configurations induced by adjacent surfaces. Recently Fripiat and associates ( 1960) presented evidence, obtained by infrared spectroscopy, that the 0-H stretching frequency of free water adsorbed on montmorillonite and vermiculite was decreased below that in normal water. The formation of hydrogen bonds is reported to stretching frequency. Frohnsdorff and Kington decrease the O-H (1958), also using infrared spectroscopy, showed that water is hydrogen bonded to zeolitic surfaces. And Mathieson and Walker (1954) determined from X-ray data that the water molecules are attracted toward the surface oxygens. These investigations lend credence to the hydrogen bond mechanism of clay-water interaction. The point has been made by Bolt (1960) that the partial specific volume data of Anderson and Low (1958) can be explained by the compression of a relatively thin ( 0 to 10 A.) water layer near the clay surface. An expanded water layer of the proposed magnitude is unacceptable to him. Let us examine his explanation by considering AVe, the excess

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

295

volume change of the clay paste over that which would have occurred had the water possessed its normal specific volume. The appropriate equation is

I

i

where & is the partial specific volume of the adsorbed water, u is the specific volume of pure water, w is the weight of water per gram of paste and the limits i and f refer to the initial and final water content, respectively. From the data of Anderson and Low (1958) for Na-bentonite, the first term on the right was obtained by graphical integration between the limits of 4 g. of water and 1 g. of water per gram of clay. Then the value of the second term on the right was subtracted from the value of the first. The result was 0.045 cc. At a water content of 1 g. of water per gram of clay, the observed partial specifk volume of the water was 1.029 cc. per gram. If the excess volume change had been confined to the gram of water closest to the surface, then the initial specific volume of this water must have been at least 1.029 0.045 = 1.074 cc. per gram, notwithstanding the high ionic concentration. The specific volume of ice is only 1.092. Therefore, if Bolt's explanation is correct, there is an adsorbed water layer about 13.5 A. thick (assuming surface area of 800 m.2 per gram) with a specific volume approximating that of ice; beyond this there is a sudden transition to normal water. It is more reasonable to believe that the change of specific volume is gradual and that near the surface the water is not so icelike. Finally, let us estimate the degree of hydrogen bonding in the adsorbed water for a specific volume of 1.029 cc. per gram at 25" C. We shall assume that the adsorbed water has a structural arrangement similar to that in ice and ignore the effect of the ions. Using the equations of Grjotheim and Krogh-Moe (1954), the degree of hydrogen bonding turns out to be 55 per cent of the maximum possible. Their calculated value for pure water at 25" C. was about 36 per cent, and at 0" C. about 44 per cent. Thus, according to this calculation, there are more intact hydrogen bonds in the first gram of clay-adsorbed water at 25" C. than in pure water at 0" C.

+

VI. Viscosity of Clay-Adsorbed Water

Viscosity is a structure-sensitive property of a fluid. It depends on the bond type and coordination of the molecules. This fact is well illustrated by the work of Andrade (1934), Ward (1937), and Ewe11 and Eyring (1937), among others. Therefore, accepting the specific volume

296

PHILIP F. LOW

data of Anderson and Low (1958) and of W. F. Bradley (1959), it can be stated unequivocally that the water on the surface of Wyoming bentonite clay has a viscosity different from normal water out to distances in excess of 60 A. The same should be true for other clay minerals with comparable atomic arrangements. Further, if the proposed quasi-crystalline structure exists, the adsorbed water should have a greater viscosity than normal water. How great the viscosity elevation should be is still uncertain, but the experimental evidence suggests that it is significant. Before discussing the experimental evidence, attention is directed to the electroviscous effect. The electroviscous effect is a retarding force on water moving through a narrow capillary. It arises because the counterions in the electric double layer are swept downstream relative to the fixed charges on the surface. The resulting electric potential gradient acts on the ions in a direction opposite to the direction of water flow, retarding their movement and creating an electroviscous drag on the water (Elton, 1948a, b; Elton and Hirschler, 1949; Kemper, 1960). Alternatively, the electric potential gradient may cause electroosmotic flow in a direction opposite to the forward flow (Michaels and Lin, 1955). As will be seen, the electroviscous effect is very difficult to separate from the true viscous effect due to structural rearrangements in the water. It is possible also that the electric field of the particle may have an effect on the viscosity, either by partially orienting the water molecules or by polarizing them so that the intermolecular bonds are strengthened. Following the work of Andrade and his colleagues ( Andrade and Dodd, 1946; Andrade, 1952; Andrade and Hart, 1954) we write AT/T = f E 2

where Aq is the increase in viscosity due to an electric field of intensity, E, and f is the viscoelectric constant. The viscoelectric constant is of the order of 10-7 when E is expressed in electrostatic units. Therefore, for Aq/y to equal 10 per cent, E would have to be about 0.003 volts per angstrom. This voltage gradient may occur near the clay surface. However, Andrade and his colleagues have shown that the viscoelectric effect in pure liquids is very small compared to the effect when dissolved ions are present. Therefore, it may be assumed that the viscoelectric effect is much smaller than the electroviscous effect. Different investigators ( Macey, 1942; Winterkorn, 1955; Schmid, 1957) have observed that a plot of the water permeability of clay versus clay porosity yields a straight line until very low permeability values are reached. Then the plot curves toward the origin. If the straight portion of the plot is extrapolated, it intercepts the abscissa at relatively large porosity values. This behavior, and the inapplicability of conventional

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

297

flow equations to clays, was taken by these investigators as an indication of an immobile or highly viscous water layer on the particle surfaces which makes the effective porosity much less than the measured porosity. It should be noted that, if an immobile water layer of a given thickness is present, the permeability versus porosity plot should remain linear until it intersects the abscissa; whereas, if the viscosity of the water increases toward the surface, this plot should curve, as it does. This viscous layer, if present, could be due to a combination of structural, electroviscous, and viscoelectric effects. In a paper by Olsen (1960), the concept is presented that the clay matrix is made up of clusters of clay particles with relatively large intercluster voids through which most of the water flow occurs. As the porosity is decreased by compression, the larger intercluster voids are supposed to collapse first. Then, as the dimensions of the intercluster voids approach those of the intracluster voids, the void space decreases more uniformly. Using this concept, Olsen predicts qualitatively the observed deviations from the Kozeny-Carman equation. The idea of clay clusters is similar to the domain idea of Aylmore and Quirk (1959, 1960). Now, an initially rapid decrease followed by a more gradual decrease in permeability with porosity is qualitatively consistent with Olsen’s concept. Consequently, the observed permeability versus porosity relationship cannot, by itself, be regarded as a reliable criterion for the presence of a surface water layer of high viscosity. There is another aspect of water flow through clays which points more directly to the existence of the proposed layer of abnormal viscosity. At low hydraulic gradients the flow does not obey Darcy’s law, viz.,

q r - iK

r

where q is the flow rate per unit cross-sectional area, K is the permeability of the medium, q is the viscosity of the fluid, and i is the hydraulic gradient. Instead, there is a disproportionate increase in flow rate with increasing hydraulic gradient, i.e., the flow rate increases more rapidly with increasing gradient than the expected linear relationship predicts. This phenomenon was observed for water flow in sandstones contaminated with clay by von Engelhardt and Tunn (1955). It was observed for water flow in pure and natural clays by Lutz and Kemper (1959). And it was observed by Hansbo (1960) for water flow in natural clays. Each of the above authors referred to other investigators who had made similar observations. The reports of the latter investigators were not available to the author. Apparently then, it is not uncommon for Darcy’s law to be violated in fine-grained material. Whether or not the law is

298

PHILIP F. LOW

obeyed depends upon the porous medium, its packing, and the hydraulic gradient. Neither von Engelhardt and Tunn (1955) nor Lutz and Kemper (1959) found obedience to Darcy's law at hydraulic gradients up to about 168 and 900, respectively, for sodium-saturated systems. However, Hansbo (1960) found that the law was obeyed above hydraulic gradients of about 5; Low (1959) found the same to be true above hydraulic gradients of 5000. Kemper (1960) explained the disproportionate increase in flow rate with hydraulic gradient as being due to the electroviscous effect. As the hydraulic gradient increases, the counterions are supposedly swept out of the smaller constrictions in the medium with the result that their retarding effect is diminished. Martin (1960) explained the nonlinearity of flow with hydraulic gradient in terms of the unplugging of voids. But the work of von Engelhardt and Tunn (1955) dictates against both these explanations because their investigations showed that the nonlinearity obtained even for the flow of a 3.5 molal sodium chloride solution through sandstone. In such a concentrated solution the electroviscous effect would be absent. Also, the clay particles would be flocculated and, therefore, unable to move in the small pores. In addition, these investigators found the flow rate-hydraulic gradient relationship to be reversible. Reversibility would not be expected in the unplugging of voids. Note that, had the increase in hydraulic gradient caused a compression of the clay, the flow rate would have decreased with increasing gradient. It appears, therefore, that the most tenable explanation is the one offered by von Engelhardt and Tunn (1955). They regard the water near the mineral surfaces as a non-Newtonian liquid whose viscosity is dependent on the shearing force. As the shearing force is increased, the water structure breaks down; as a result, the viscosity of the water decreases. Hansbo (1960) also subscribes to this explanation. There is a crucial test of the elevated viscosity concept. If a threshold hydraulic gradient exists which must be exceeded before flow is initiated, then the water must have a yield point and structural resistance to flow. Before flow begins there is no movement of ions relative to the clay surface and, hence, no electroviscous effect; secondly, there is no particle movement with subsequent plugging or unplugging of the voids. Fortunately, this test has already been performed. Hansbo (1960) reported that Derjaguin and Krylov (1944), working with rigid ceramic filters, observed a threshold value of the hydraulic gradient below which flow did not occur. And Derjaguin and Melnikova (1958) referred to the same work in stating their case for the elevated viscosity concept. Dr. G. A. Leonards (private communication, December 20, 1960) has informed the author that the concept of an initial or threshold gradient for water

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

299

flow in clays is quite commonly accepted in Russia. Recently, Oakes (1959) discussed an experiment wherein a 6 per cent suspension of Wyoming bentonite about 30 cm. long was subjected to a head of 50 cm. of water for 6 weeks without any detectable flow. Other experiments that he performed gave similar results. On the basis of studies such as those in the preceding paragraphs the author recommends, following Hansbo (1960), that for water flow in close-packed clays at hydraulic gradients above a certain limit the flow equation be written in the form

q = -K

r

( i - io)

where i, is the intercept of the extrapolated linear portion of the q versus i plot with the abscissa. At gradients below the afore-mentioned limit, q would be a more complex function of i. Hansbo suggested a simple exponential function, but, in the opinion of the author, the exact form of this function must await further investigation. It is probable that flow in films in dry soils would also obey these equations. Michaels and Lin (1954) conducted experiments on the permeability of kaolinite. Kaolinite, initially packed in water, was desolvated with as little particle rearrangement as possible by displacing the water with dioxane, acetone, and dry nitrogen in sequence. Then the permeability of the clay to nitrogen was measured and compared with its original permeability to water. The permeability to nitrogen always exceeded the water permeability by at least 40 to 60 per cent. When the kaolinite was initially packed in an organic liquid and was subsequently desolvated with dry nitrogen, its permeability to the gas was essentially the same as its original permeability to the organic liquid. It is interesting to observe that von Engelhardt and Tunn (1955) reported the water permeability of their sandstone to be much less than the air permeability, but the air permeability and the permeability to organic solvents were nearly the same. Michaels and Lin attributed the relatively small permeability in water to the electroviscous effect. However, in a subsequent paper (Michaels and Lin, 1955) they calculated the permeability reduction due to this effect and found it to be only about 5 per cent. Further, they eliminated the electric double layers of the particles, and thereby the electroviscous effect, by successively increasing the salt concentration of the permeant water. The maximum permeability increase was only about 10 per cent. Consequently the relatively low water permeability of the clay cannot be attributed entirely to the electroviscous effect. Increased structural viscosity must also have been a contributing factor. Yet, neither the electroviscous effect nor the “struc-

300

PHILIP F. LOW

tural" effect seemed to account for the wide differences in kaolinite permeability observed when the clay was packed in different liquids and then permeated by them. The authors attributed these differences to differences in particle arrangement. Low and his co-workers have utilized the activation energy concept to elucidate the viscous nature of water in clays. Low (1959) replaced q in Eq. (13) by its equivalent from Eq. (3) to obtain

q = -

B

.ie - E / R T

in which B is a constant and E, the activation energy, has replaced AH' because there should be no volume change involved in the process of flow. Then, to obtain the activation energy for the viscous flow of water through a paste of Wyoming bentonite (containing 55 per cent solids), he plotted the logarithms of observed values of q at constant i against the corresponding values of 1/T. As is evident from Eq. (15), the slope of the resulting line equals -EJR. The activation energy value so obtained was equal to 4350 cal. per mole after 3 weeks of clay-water contact. The activation energy for the viscous flow of normal water in the same temperature range is 3850. These values are approximations. They could be off by as much as 100 cal. per mole. Nevertheless, it is evident that the activation energy for flow in the clay is about 500 cal. per mole greater than that in pure water. And since, in the experiment, the clay was confined in a stainless steel cylinder between stainless steel filters, it is unlikely that any streaming potential developed. According to the equations of electroviscosity (e.g., Elton, 1948a; Michaels and Lin, 1955), the electroviscous effect is absent when the streaming potential is zero. Therefore, the increase in activation energy for water flow in the clay can be ascribed to enhanced structural viscosity. The conclusion just stated is consistent with the following observations: ( 1) the activation energies for exchangeable ion movement (Low, 1958a) are directly correlated with the partial specific volumes of the adsorbed water (Anderson and Low, 1958) on the same clays; (2) the activation energies for exchangeable ion movement are directly correlated with the unfrozen water at -5" C. (Kolaian, 1960) on the same clays; and (3) the activation energies for exchangeable ion movement have essentially the same values as the activation energies for deuterium oxide diffusion (Dutt, 1960) in the same clays. The interrelationships of these observations have been discussed in two recent articles (Low, 1960a, b). All the ionic activation energies for movement in the clay were greater than those for movement in pure water. Evidently, the adsorbed water properties, as manifested by measurements of partial specific

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

301

volume, resistance to freezing, and self-diffusion, govern the movement of the adsorbed cations. This is in keeping with the concept that a coherent, hydrogen-bonded water structure exists in the neighborhood of clay surfaces. The ease of ionic movement through this structure should depend on the viscous resistance it offers. Martin (1960) has criticized the foregoing activation energy data on the basis that K , the permeability of the clay, and A S , the entropy of activation, may change with temperature. He maintains that these changes would be included in E , the activation energy term. But if these quantities change with temperature and are included in E, K must be an exponential function of 1/T and A S must be a linear function of 1/T (see Eqs. 3 and 15); otherwise a straight line would not be obtained on plotting log 4 against 1/T. Similar considerations would hold for the ionic activation energy data. There is no evidence for the necessary functional relationships. Further, if such functional relationships did exist, the correlation between the experimental values of E and the other water properties would remain to be explained. Therefore, the author contends that, although both K and A S may change slightly with temperature, they do not change suffciently to invalidate the results. Accepting Low’s (1959) data on the activation energy for viscous flow of water in Na-bentonite as being reliable, we can estimate the degree of hydrogen bonding in this water. The assumption is made, in keeping with the previously mentioned ideas of Ewe11 and Eyring (1937) and of Wang (1951a), that the activation energy equals the energy required to break hydrogen bonds. The number of hydrogen bonds per molecule is, therefore, 4.35/4.5 = 0.97. Recall that each hydrogen bond is shared by two molecules so that, energywise, the number of hydrogen bonds formed by a 4-coordinated molecule is 2. As a result, we may say that 100 x 0.97/2 = 48 per cent of the bonds are unbroken. Using the same method of calculation, we arrive at a figure of 43 per cent for the fraction of unbroken bonds in pure water. Earlier, on the basis of specific volume calculations, we estimated that 55 per cent of the hydrogen bonds were unbroken in the adsorbed water. This figure represented an increase of 19 per cent over that for pure water. The activation energy value is for a water content of 45 per cent; whereas the specific volume value is for a water content of 50 per cent. Both water contents are expressed on a wet weight basis. Although these values are to be regarded as approximations at best, they do suggest an increased degree of hydrogen bonding in the water adjacent to the mineral surfaces. Rosenqvist (1955) studied the diffusion of deuterium oxide through clay pastes. To correct the observed diffusion coefficients for the path tortuosity, he assumed that all particles were at an angle of 45 degrees

302

PHILIP F. LOW

with the direction of diffusion; the correction factor for a water content of 40 per cent by volume was, therefore, d3/0.40. This correction factor should be valid for randomly arranged particles. It would be in error if particle orientation exists. When the observed diffusion coefficients were multiplied by the correction factor they were of the order of 0.1 to 0.2 cm.2 per day. But the diffusion coefficient for deuterium oxide in pure water is about 3 cm.2per day. Hence, the diffusion coefficient of the water in the clay voids was only about 5 per cent of that of normal water. The relationship between diffusion and viscosity is apparent from Eqs. ( 4 ) and ( 6 ) . Rosenqvist made the reasonable conclusion that the water in the clay was more viscous than ordinary water. In a later paper Rosenqvist (1959) presented a graph of the average viscosity of water in a clay paste against the water content of the clay on a dry weight basis. The viscosities were calculated from self-digusion measurements such as the one described in the preceding paragraph. The graph shows a gradual decrease of viscosity from above 153 centipoises at water contents less than 10 per cent to 24 centipoises at a water content of about 30 per cent. The viscosity is represented as approaching that of pure water (near one centipoise) at water contents of 50+ per cent. The latter water content corresponds to an average film thickness of about 40 A. This thickness was calculated from the water content and the clay surface area, which was reported in the earlier (1955) paper to be 130 m.2 per gram. Rosenqvist (1955) also measured the activation energy for deuterium oxide diffusion through a clay paste using an equation of the form of Eq. ( 2 ) . His measured activation energy was 11,500 cal. per mole. However, the author is inclined not to accept this value because the diffusion experiment involved the evaporation of the deuterium oxide from the moist clay surface. The activation energy for evaporation from the clay, which may have been the rate limiting step, should have approximated the heat of vaporization. In the range of his experimental temperatures, namely, 14"to 43" C., the heat of vaporization of pure water is 10,500 cal. per mole. Recall that the activation energy for the viscous flow of pure water, which should be nearly the same as the activation energy for selfdiffusion, is only about 3850 cal. per mole. If Rosenqvist's experimental value is accepted as valid, the activation energy for water diffusion in the clay is greater than the activation energy for the diffusion of water molecules in ice. Recently, Pickett and Lemcoe (1959) presented the results of a radiofrequency spectroscopy study of water adsorbed on kaolinite and homoionic Wyoming bentonites. They reported that the width of the absorption curve at half amplitude is a measure of the intensity of intermolec-

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

303

ular bonding and, hence, of viscosity. This width was called the line width. The line width was plotted against moisture content on a dry weight basis. For kaolinite the line width decreased uniformly with increasing moisture content to a nearly constant value at 10 per cent moisture; whereas, for the bentonites the line width decreased uniformly with increasing moisture content to a nearly constant value at about 50 per cent moisture but continued to decrease slightly thereafter. Pickett and Lemcoe concluded that the viscosity of water near clay surfaces is greater than that of free water and that it decreases uniformly with distance from the clay surface. If we take the surface areas of the kaolinite and bentonite to be 15 and 800 m.2 per gram, then the films of elevated viscosity were at least 66 A. and 8 A. thick, respectively. The sensitivity of the method precludes reliable interpretation of the data at higher moisture contents. Now, if water in the vicinity of clay surfaces has a quasi-crystalline structure as postulated, its flow properties should be as follows: (1) it should have a yield value leading to a threshold hydraulic gradient below which flow will not occur; ( 2 ) after flow commences there should be a range of hydraulic gradients over which non-Newtonian flow occurs, i.e., the viscosity should be dependent on the shearing force; ( 3 ) the viscosity should increase with proximity to the clay surface; and (4) near the clay surface the viscosity of the water should be greater than the viscosity of free water. The experimental observations described in this section are in complete harmony with these requirements. VI 1. Dielectric Properties of Clay-Adsorbed Water

Before considering the experimental data, a very brief summary of the pertinent equations will be presented. These equations are due largely to Debye (1945), but they have been taken from various sources. The , a static electric field is given by dielectric constant, E ~ in Es=l+-P

4x

E

where E is the field strength and P is the polarization induced by the field per unit volume of dielectric. The polarization is made up of: (1) the distortion of electronic distributions within atoms and of atoms within molecules, which is called distortion polarization and ( 2 ) the partial orientation of permanent electric dipoles, which is called orientation polarization. The distortion polarization occurs instantaneously whereas the orientation polarization requires time. If measurements are made in an alternating electric field of low frequency the dipoles will

304

PHILIP F. LOW

have time to rotate in each half cycle; but as the frequency increases the molecules will have less and less time to rotate. Eventually, depending on the viscosity and temperature, a frequency will be reached at which the molecules have insdcient time to rotate. Then the polarization will decrease and the dielectric constant will fall. The fall of dielectric constant with increasing frequency is called anomalous dispersion. The dielectric constant, E, in the region of anomalous dispersion is given by

where w equals 2xv in which Y is the frequency of the alternating field, t is the relaxation time as before and i is the operator q x. From the equation it is evident that E~ is the dielectric constant at very high frequencies. For water has a value near 80 and E~ has a value near 5.0. Alternatively, the dielectric constant in the region of anomalous dispersion is E = E’ - iE” (18) where E‘ is the real and E” is the imaginary part, respectively. The values of E’ and E” can be obtained from measured dielectric constants. In the region of normal dispersion e”, which is a measure of the “dielectric loss,” is very small; whereas, in the region of anomalous dispersion E” increases rapidly to a maximum and then falls off. The relaxation time is related to the frequency, vm, at which the masimum occurs by the equation

- _-

+

2xvm (19) &+2 Cownie and Palmer (1952) showed that the dielectric constant of a natural clay, at a frequency of 430 Mc./sec., increased with increasing water content. Later Palmer (1952), using a formula for calculating the dielectric constant of a mixture from the dielectric constants of its constituents, calculated the average dielectric constant of the water in this clay at different moisture contents. The average dielectric constant increased from a value of about 5 at near zero moisture to a value of about 80 at 80 to 100 per cent moisture (dry weight basis). If Palmer’s calculations are correct, the average relaxation time of the molecules must have decreased as successive layers of water were added. Either the ions associated with the clay or the clay surfaces or both reduced the rotational freedom of the adsorbed water. In another paper Palmer et al. (1952) separated mica plates by thin films of water. They observed that the dielectric constant, at 2 Mc./sec., decreased with the thinness of the film from more than 20 for films about t

Ea

PHYSICAL CHEMISTRY OF CLAY-WATER INTI%RACTION

305

5 p thick to less than 10 for films about 2 11 thick. They also used a frequency of 2.5 kc./sec. The dielectric constant at the lower frequency was approximately double that at the higher frequency. A check showed that the dielectric constant of the mica alone remained constant at these frequencies. They concluded that the water was acting like “liquid ice” because ice is frequency-sensitive in this range. This conclusion is plausible. The anomalous dispersion of free water occurs at much higher frequencies-of the order of 3 x lo4 Mc./sec. From the work of Hasted and associates (1948) and the expected ionic concentrations between the plates, it is safe to conclude that the mineral surfaces and not the ions were responsible for these results. The dielectric properties of water on talc, kaolinite, metahalloysite and halloysite at different relative humidities were studied by Muir (1954)in the frequency range 2.5 kc./sec. to 25 Mc./sec. This frequency range is far below that for the anomalous dispersion of free water. He plotted measured values of E” against the frequency of the alternating field. It was pointed out by Muir that the maximum in E” i.e., the maximum dielectric loss, is proportional to the number of absorbing molecules and that ,Y in keeping with Eq. ( 5 ) and (19), depends on the strength of the binding forces preventing the orientation of the dipoles in the alternating field. The following are Muir’s observations. Talc: A single maximum in dielectric loss at 10 kc./sec. which increased in height through the humidity range of 0 to 80 per cent. Kaolinite: Two maxima, one which remained at 10 kc./sec. and increased in height through the humidity range of 0 to 30 per cent and another of constant height which moved to higher frequencies with increasing relative humidity. Metahalloysite: A single maximum which came into the frequency range of the experiment at 20 per cent relative humidity and moved, with increasing height, to higher frequencies as the relative humidity was increased until, at saturation, a v, of 10 Mc./sec. was reached. The results for halloysite were similar to those for metahalloysite. Since talc has only oxygen surfaces and kaolinite has both oxygen and hydroxyl surfaces, Muir concluded that the stationary loss maximum, which occurred at the same vnl for both minerals, was due to the water on the oxygen surfaces, And we may add that the lower value for this maximum indicates, in keeping with the prediction of Section IV, that water is adsorbed with greater intensity on the oxygen surfaces. Muir attributed the moving maxima for kaolinite and the halloysites to the water on the hydroxyl surfaces. The constancy in height of the moving maximum for kaolinite was attributed to the fact that dielectric loss was occurring only in the initially adsorbed water layer; the moving of this

306

PHILIP F. LOW

maximum to higher frequencies with increasing water content was attributed to interaction of the outer layers with the first. Such interaction is indicative of cooperative bonding between layers. Following Muir’s reasoning, one would conclude that the increase in dielectric loss at constant v, for water on the oxygen surfaces of talc and kaolinite was due to successive increments of water being bonded with the same intensity; further, one would conclude that the increase in dielectric loss and in v, for water on the halloysites was due to successive increments of water being bonded with less intensity. But the water adsorbed on all the clay surfaces had a much lower value for Y,, and hence a much longer relaxation time, than does free water, In fact, the v,, for water on the oxygen surfaces and for initially adsorbed water on the hydroxyl surfaces was about the same as that for ice at -5’ C., namely, 7 kc./sec. According to Muir, the number of water layers adsorbed on the minerals at the maximum relative humidities varied from 3 for kaolinite to 10 for metahalloysite. Thus, Muir showed that water in the vicinity of these mineral surfaces has less freedom than normal water. Recently, Goldsmith and Muir (1960) extended the range of frequencies in the dielectric loss experiment to 0.09 kc./sec. They showed that the extended E” versus frequency curves were compatible with those obtained earlier for the same minerals. In addition, they also measured the dielectric properties of the adsorbed water before and after a preheating of the clays. And clays saturated with different cations were used. On the basis of the evidence obtained, they concluded that the adsorbed water is more ordered in the presence of the smaller exchangeable cations. Also, it is more ordered when the exchangeable cations are buried in the hexagonal cavities of the lattice by heat treatment than when they are implicated in the water layer. These conclusions are consistent with the ones derived from solution studies. VIII. Supercooling and Freezing of Clay-Adsorbed Water

The effect of clay particles on the supercooling of water is difficult to ascertain. The reason for this is the uncertainty regarding the extent of supercooling in pure water. It is extremely difficult to rid water completely of all foreign particles which might act as crystallization nuclei. And if water is in contact with any solid surface, particles adhering to the surface or the surface itself may influence the degree of supercooling. In order to eliminate the possible effect of solid surfaces, many investigators have studied the freezing of small water droplets suspended in air or in immiscible liquids. In the opinion of the author, the latter technique may not entirely eliminate the surface effect. The droplets are

307

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

usually of the order of 1p in diameter, and there is ample evidence (Henniker, 1949; Adamson, 1960) that surfaces can influence liquid properties over such distances. The difficulties encountered in determining the degree of supercooling of water have been reviewed recently by Mossop (1955), Mason (1957), and Langham and Mason (1958). It is conceded by these investigators that pure water will supercool to 4 0 ' C. Contrary to the popular belief, supercooled water is rather stable. Its freezing is not initiated readily by mechanical disturbance ( Dorsey, 1948). Accepting, for the present, d o o C. as the temperature of spontaneous freezing for pure water, we will now consider the effect of clay on its supercooling. It is known (Kumai, 1951; Isono, 1955) that clay particles occur frequently at the center of snow crystals. But this does not necessarily mean that the particles act as crystallization nuclei. Instead, by reason of their adsorptive properties, they may collect the water vapor into droplets which freeze subsequently when conditions are right. Nevertheless it does appear that certain clay particles do nucleate supercooled water and cause it to freeze. In an interesting study Mason and Maybank (1958) introduced mineral particles as fine dust into a cloud chamber containing a supercooled cloud and noted the number of ice crystals formed as a result. The mineral particles were tested for their ice-nucleating ability at successively higher temperatures to determine a threshold temperature above which they were not effective. At least six tests were made for each kind of mineral particle. Part of their results are shown in Table 11. Although the threshold temperatures for montmorillonite and quartz were not given, the authors noted that these minerals were ineffective as freezing nuclei even at -25' C. Apparently, TABLE I1 Substances Initially Active as Ice Nuclei at Temperatures Above -18" C.@A Substance

Crystal structure

Lattice parameters

Threshold temp. ( '(2.) ~

P-Tridymite Kaolinite Microline Gibbsite Halloysite Biotite Vermiculite Phlogopite Anorthoclase Ice

Hexagonal Triclinic

= 5.03,c = 8.22 a = 5.16, b = 8.94, c = 7.38

Monoclinic

a

= 5.16,b = 8.94,c = 10.1

Monoclinic

a

= 5.34, b = 9.20,c = 28.9

Hexagonal

a = 4.52, c = 7.37

a

-7

-9 -9.5 -11.0 -13.0 -14.0 -15.0 -15.0 -17.0

-

Data from Mason and Maybank (1958). Samples inactive above -18" C. include: monimorillonite, sepiolite, albite, muscovite, orthoclase, talc, quartz, and P-tridymite. 0

b

308

PHILIP F. LOW

particle size has little effect on the threshold temperature. Two ranges of particle sizes were used for kaolinite, namely, 0.2-0.5 p and 5-10 p. No difference in threshold temperature was detected. The reason for the difference in ice-nucleating ability of the various mineral particles is unknown. It was noted by Mason (1958) in another article that the most effective particles, including non-clay materials, are hexagonal crystals in which the lattice spacing in the a dimension, which is important for epitaxy on basal faces, differs from that of ice by less than 16 per cent. However, as he pointed out, kaolinite and montmorillonite have nearly identical lattice spacings. Yet one nucleates water at -9" C. and the other at less than -25" C. The fit between the mineral and ice lattices must not be the only important factor. In this regard it should be noted, however, that other results (Mason, 1957) showed both a Georgia kaolin and a Montana bentonite to have a threshold temperature within a degree of -22" C. If there is a difference in ice-nucleating ability between kaolinite and montmorillonite, the difference may be due to the exposed hydroxyl surface of the gibbsite layer in the former. Note that the threshold temperatures of kaolinite and gibbsite are comparable. Recall also that the dielectric properties of the adsorbed water were not the same on hydroxyl surfaces as on oxygen surfaces. An interesting feature reported by Mason and Maybank (1958) was the preactivation of clays by a preliminary freezing of their adsorbed water. If, thereafter, the temperature of the clay particles was not raised above 1" C., the threshold temperature for subsequent freezings was much higher than the original one. For instance, preactivation raised the threshold temperature from less than -25" C. to -10" C. for montmorillonite. It raised the threshold temperature from -9" C. to 4"C. for kaolinite. The reason for the preactivation is not known, but it may be the result of the retention of ice embryos in surface cracks (Turnbull, 1950). In the author's laboratory J. H. Kolaian (1960) determined the temperature of spontaneous freezing, i.e., the threshold temperature, for water in Wyoming bentonite pastes containing 65 per cent water. A thermocouple, connected to a potentiometric recorder, was inserted in the clay paste which was cooled uniformly in a refrigeration bath. A sudden rise in the thermocouple temperature indicated the initiation of freezing. The temperature immediately preceding the sudden rise was regarded as the temperature of spontaneous freezing. Regardless of the ion on the clay, this temperature was always in the range -5°C. to -7" C. More than forty determinations were made. The temperature of spontaneous freezing for deionized water was in the same range. His logical conclusion was that an active foreign freezing nucleus contained

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

309

in the water was responsible. These determinations illustrate the difficulty in obtaining reliable information on the nucleating ability of clays. Earlier J. M. Deming (1951) studied the effect of Wyoming bentonite clay on the supercooling of water. Except for one graph in a review article by Low (1958b) his results have not been published. The reason for not publishing is that his data do not allow a clear-cut separation between the effects of zeta potential and solution concentration on the degree of supercooling. To make the zeta potential of the particles in each suspension diflerent from that in every other suspension, the suspensions were made up in solutions of different salt concentration. Nevertheless, the data are pertinent to this discussion and will, therefore, be reviewed. Deming placed homoionic clay suspensions, containing about 2 per cent clay by weight, in centrifuge tubes which were immersed in a refrigeration bath at -5" C. If a suspension did not freeze in 12 hours it was diluted with the salt solution originally used in preparing it. Then it was returned to the bath for an additional 12 hours. Dilution of the clay (but not the salt) in the suspension was continued at 12-hour intervals until the suspension froze. The concentration of clay in suspension at the time of freezing was measured. The zeta potential of the particles in a duplicate suspension was also measured. In general, the higher the zeta potential of the particles, the more the clay had to be diluted before freezing occurred. This fact suggests that an increase in ion dissociation from the clay surface altered the vicinal water structure so that it became less susceptible to nucleation by foreign freezing nuclei; therefore, the particles had to be moved farther apart in order to reduce their influence on the intervening water. Very little information is available on the rate of freezing of claywater systems. Kolaian (1960), in his studies on the freezing of suspensions of Wyoming bentonite, observed that the suspensions took longer to freeze after setting for a while (gel state) than immediately after stirring (sol state). He followed the freezing process by means of a thermocouple inserted in the suspension and connected to a potentiometric recorder. The length of the plateau in the temperature versus time curve was taken as an indication of the time required for freezing. Table I11 is taken from his thesis. Evidently, the water takes longer to freeze when thixotropy obtains. Possibly the transition from water to ice is hindered by a water configuration which either develops with or is responsible for thixotropy. More will be said of this later. Not all the water in a clay-water system freezes after freezing is initiated. Buehrer and Rose (1943) determined the amount of water remaining unfrozen in Pima clay at 3" C. by the dilatometer technique. They showed that more water remained unfrozen when the clay was

310

PHILIP F. LOW

puddled than when it was unpuddled. In the former state the water would be more intimately associated with the surfaces. More water remained unfrozen after the removal of free salts from the clay than before; hence, the free salts were not responsible for the lack of freezing. Buehrer and Rose calculated that the free salt could lower the freezing point of less than half the water to the temperature of the experiment. However, the exchangeable ions did influence the amount of unfrozen water. In one experiment the Pima clay was saturated with different ions and then the unfrozen water was measured on the homoionic clays. When the moles of unfrozen water per gram-ion of cation was plotted against the surface potential of the ion, a straight line was obtained. However, reasonable estimates of the extent of ion hydration precluded the belief that the ions alone prevented freezing. Buehrer and Rose concluded that the clay surface also plays an important role in the process. TABLE I11 The Effect of Thixotropic Development on the Rate of Freezing of Bentonite Suspensions@ Clay tVpe

5.9 11.3 12.3 Data from Kolaian (1960).

Li-clay Naelay K-clay 0

Per cent clay in suspension

Freezing time in minutes Sol state

Gel state

3 3 2

12 8 6

In another paper Buehrer and Aldrich (1946) studied the unfrozen water in pastes of kaolinite and bentonite, the latter being from Otay, California. Very little water remained unfrozen in the kaolinite. For instance, at about 75 per cent moisture (dry weight basis) approximately 3 per cent of the water remained unfrozen. Assuming a surface area of 15 m.2 per gram, this weight of water corresponds to a film 15 A. thick. Approximately 97 per cent of the water remained unfrozen in the bentonite up to a water content of about 75 per cent (wet weight basis). Above this water content there was a sudden drop in the percentage remaining unfrozen, e.g., at a water content of 90 per cent only about 10 per cent of the water remained unfrozen. It is of interest to note that the sudden drop in the percentage of unfrozen water corresponded to the water content at which the gel to sol transformation occurred. Assuming a surface area of 800 m.2 per gram of clay, the thickness of the unfrozen film at a water content of 75 per cent was 36 A. Now the base exchange capacities were 3.3 and 88.5 me. per 100 g. for the kaolinite and montmorillonite, respectively. A simple calculation shows that the exchangeable cations, dissolved in the unfrozen water, were sufficiently

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

311

concentrated to prevent freezing at -3" C. in the case of the kaolinite but not in the case of the montmorillonite. In the latter case the freezing point depression due to salt would be only about 0.5" C. Here again we have a difference in the effect of the two minerals on the freezing properties of water. Hemwall and Low (1956) used the dilatometer technique of Buehrer and Rose (1943) to determine the unfrozen water at -5" C. in four different samples of Wyoming bentonite, namely: a Na-bentonite, a silaned Na-bentonite, a Na-Th-bentonite and a Th-bentonite. An X-ray analysis of these bentonites disclosed that the silane had coated only the external surfaces of the treated clay, rendering them hydrophobic and leaving the internal surfaces unaffected; the Th4+ had bonded the crystal layers together so that only external surfaces were accessible to water; the remaining two clays exhibited normal behavior. Thus, the characteristics of the silaned Na-bentonite and of the Th-bentonite permitted water interaction with internal surfaces to be distinguished from water interaction with external surfaces. The zeta potentials of the different bentonite particles in the suspending solutions were also determined. They were: -35 mv. for the Na-bentonite, -17.6 mv. for the Na-Th-bentonite, and $53.8 mv. for the Th-bentonite. The silaned clay had zero electrophoretic mobility. From the zeta potential values one can deduce that cation dissociation from the clay surfaces was in order Na+ > Na+Th4+ > Th4+.At a water content of 85 per cent (wet weight basis) as much as 22 per cent of the total water remained unfrozen in the Nabentonite. This weight of water corresponds to a film 15 A. thick. Since the exchange capacity of the clay was 1.05 me. per gram, the cation concentration in the unfrozen water was not sufficient to prevent freezing at -5" C. The unfrozen water on the Na-bentonite was from two to three times that on the silaned Na-bentonite, depending on the moisture content. Further, the unfrozen water on the Th-bentonite was about the same as that on the silaned Na-bentonite. These results indicate that external surfaces are much more efficient in preventing the freezing of water than are internal surfaces. The unfrozen water on the three nonsilaned clays decreased in the same order as the cation dissociation, suggesting that when the cations are close to the surface the water is less resistant to freezing. Possibly the reason that external surfaces prevent freezing better than internal surfaces is that, owing to the relative proximity of the latter, the ions dissociate less from them. This idea receives additional support from the observation that, as the interlayer and interparticle distances increased in reponse to the addition of water, the total amount of unfrozen water also increased. On the basis of these results and those from the supercooling study of Deming (1951) it is proposed

312

PHILIP F. LOW

that, as the ions dissociate from the mineral surface, competition between their disordering influence and the ordering influence of the surface atoms is lessened with the result that a more ordered water structure develops. The reasons why such a structure should prevent freezing are discussed in the following paragraph. It is possible that the lack of freezing could be due to curved icewater interfaces in wedges between adjacent, nonparallel surfaces. Sill and Skapsi (1956) have shown that curved solid-liquid interfaces do exist under such conditions. The freezing point depression, AT of the water in these wedges is given by the Kelvin equation, i.e.,

where To is the freezing temperature of the water under a flat surface, 6 is the interfacial tension between water and ice, e is the density of ice, T is the radius of curvature of the ice-water interface, and L is the heat of fusion. According to this equation, the ice front should penetrate farther toward the apex of the wedge as the temperature drops. However, the Kelvin equation, with the customary values for o and L, does not explain the direct corrsepondence between activation energies for ion movement in bentonite-water systems and the amounts of unfrozen water at -5" C. in the same systems. Kolaian (1960) measured the unfrozen water in homoionic pastes of Wyoming bentonite by a calorimetric technique. The amount of unfrozen water depended on the exchangeable cation and ranged from 41 to 55 per cent of the total water present. The pastes contained 65 per cent water (wet weight basis). As before, the exchangeable ions could not account for the water remaining unfrozen. When the activation energies for exchangeable ion movement were determined in the pastes, the afore-mentioned relationship between activation energies and unfrozen water was observed. Undoubtedly, the adsorbed water structure affected both ion movement and freezing. If the Kelvin equation is still to be invoked, it would be logical to ascribe the differences in the amounts of unfrozen water on the different homoionic clays to differences in (r or L. As the water became more or less ordered in response to a change in the cationic suite on the surface, 0 and L would have changed, probably in opposite directions. At this point it is important to note that the clay-adsorbed water does not have the structure of ice, If it did, this water would act as a perfect crystallization nucleus and the water in the system would not supercool; nor would it remain unfrozen at subzero temperatures. For the montmorillonite, at least, it may be supposed that an increase of order in the adsorbed water produces an increase in o. An increase in order would also lower the heat

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

313

content of the water, bringing it closer to that of ice; hence, L would decrease. But the Kelvin equation may not be applicable here at all. Babcock and Overstreet (1957) have shown that the electric field of the particle can lower the freezing point of the nearby water. In fact, it can be shown thermodynamically that the freezing point of water is lowered by any force field, long range or intermolecular, which lowers the potential energy of the water relative to that of ice. The potential energy of the water molecules in an ordered structure should be less than in a disordered structure. Elevated hydrostatic pressures existing close to the mineral surfaces, by reason of adsorptive forces, cannot account for the unfrozen water. Some of the water which remains unfrozen at -5" C. can be squeezed from the clay at pressures of 10 to 15 atmospheres. Added pressure on both phases, ice and water, lowers the equilibrium freezing temperature by only 0.0075"C. per atmosphere. Regardless of the causative factor, the freezing point of the water must become lower as the surface is approached. Love11 (1957) showed that the unfrozen water in a soil clay decreased continuously with decreasing temperature. At a temperature of -24°C. the unfrozen moisture amounted to 13 per cent on a dry weight basis. It should not be inferred that the structure of the unfrozen water is rigid. Leonards and Andersland (1960) have shown that the electrical resistance of a clay soil mixed with a 0.25 N solution of LiI increases continuously with the percentage of total moisture frozen. The frozen moisture was determined calorimetrically. When 18 per cent of the total C.) the electrical resistance was 1500 ohms; moisture was frozen (4" when 45 per cent of the total moisture was frozen (-16" C.) the electrical resistance was 14,000 ohms. An interesting feature of their data was that the electrical resistance at any temperature was different on the cooling and warming curves. The relatively low resistances observed by them indicate that the ions were quite mobile and, therefore, that the unfrozen water retained considerable fluidity. In concluding this section of the paper, attention is called to the work of Rosenqvist (1959), who studied the cooling curves of an illitic clay down to extremely low temperatures. When the moist clay was immersed in a dry ice-toluene mixture at -78" C. there were several small irregularities in the temperature versus time curve showing that exothermic reactions (probably freezing) were taking place. But when the same moist clay was immersed in liquid air at -185" C. the cooling curve was smooth. In the latter case the intense cold must have transformed the water to a glassy state which is known to occur on quick-freezing. Apparently, Kato (1959) also observed water-ice transitions in clay at

314

PHILLP F. LOW

subzero temperatures. He made a differential thermal analysis of montmorillonite and other clay minerals down to -195" C. by using dry ice and liquid nitrogen. Montmorillonite gave three peaks, one at each of the following temperatures: A", -7", and -20" to 3 0 " C. Attapulgite, nontronite, vermiculite, and endellite gave three peaks similar to those for montmorillonite; whereas, kaolinite, illite, and alumina gave only one peak at -2" to -5". Here again we observe a difference between montmorillonite and kaolinite in their effect on the freezing of water, The former is conducive to more supercooling and less eventual freezing than the latter. IX. Thermodynamic Properties of Clay-Adsorbed Water

Thermodynamics is concerned with changes in certain properties of a system such as the free energy, heat content, entropy, etc. It has nothing to do with the mechanism by which these changes are brought about. We are concerned here largely with the mechanism of clay-water interaction. Although thermodynamics cannot be expected to provide this mechanism, it can assist us in selecting between alternative mechanisms. In fact, we have already used thermodynamics for this purpose. In this section we shall consider some of the thermodynamic properties of the adsorbed water. Then we shall attempt to interpret these properties in terms of a suitable mechanism. The work of adhesion between a solid and a liquid is defined as the work necessary to separate the liquid from the solid by pulling them perpendicularly from each other against the adhesive forces between them. The work of adhesion can be determined from the surface tension of the liquid and the contact angle between liquid and solid (Adam, 1941). If the liquid attracts the solid as much as it attracts itself, the contact angle is zero. As the adhesion between liquid and solid decreases, the contact angle increases. Enright and Weyl (1950) determined the initial contact angles between water and thin dried films of homoionic Wyoming bentonites. They also determined the times required for drops placed on these films to spread uniformly over them. They observed that the contact angle increased with increasing polarizability of the adsorbed cations, being zero for the alkali metal cations and as great as 70 degrees for ions such as Pb++ and Hg++. The average time for spreading of the drop increased with the increase in contact angle. On the clays saturated with the alkali metal cations the spreading was immediate; whereas, 15 to 30 minutes were required for the water drops to spread over the clays saturated with Pb+ + and Hg+ +. The spreading times were intermediate on clays containing cations of intermediate polarizability. Enright and

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

315

Weyl ascribed these differences to the hydratability of the cations on the clays. Next to the negative clay surface the ions were assumed to be polarized, their electrons being repelled toward space and their positive nuclei being attracted toward the surface. This polarization reduced the effect of the positive charge on the ion as viewed from the space side; consequently, the attraction of the ion for water was also reduced, the reduction depending on the polarizability of the ion. Although the authors did not mention it, the probability exists that the ions affect the electron distribution in the surface and thereby alter its afEinity for water. In any event, it is apparent that the adsorbed cations have a marked effect on the work of adhesion between clay and water. Many investigators including Hendricks et al. (1940), Rios and Vivaldi (1950), Walker and Milne (1950), Keenan et al. (1951), Mooney et al. (1952b), White (1955), and Orchiston ( 1955, 1959), among others, have shown that the nature of the exchangeable cation affects the amount of water adsorbed by a clay. Since the clay will adsorb water until the partial molar free energy of the adsorbed water equals that of the water in the equilibrium solution or vapor, we can conclude that the exchangeable ion also affects the partial molar free energy of the adsorbed water, The pertinent equations are: and

F

-Fo = RTlnp/p,

P

-Fo

(21)

-

=

(22) Here f? is the partial molar free energy of the adsorbed water and F o is the partial molar free energy of pure water. In the first equation, which applies to adsorption from the vapor, p is the pressure of water vapor in equilibrium with the clay and p, is the vapor pressure of pure water. In the second equation, which applies to adsorption from solution, JC is the equilibrium swelling pressure of the clay. The other symbols were defined previously. Evidently, the partial molar free energy of clay-adsorbed water is less than that of pure water until the adsorbed films become very thick because even dilute clay suspensions exert a measureable swelling pressure (Day, 1956; Kolaian and Low, 1960). It is a commonly accepted fact that the osmotic activity of the exchangeable ions makes a significant contribution to clay swelling ( Schofield, 1946; Eriksson, 1950; Norrish, 1954; Bolt and Miller, 1955). But, if the potential energy of the adsorbed water is decreased by reason of its implication in a quasi-crystalline structure induced by either external or intermolecular force fields, this energy decrement should also contribute to swelling (Low and Deming, 1953). In other words, both the osmotic activity of the exchangeable cations and the intensified bonding in the circumambient water are capable of lower-tJD,Jc

316

PHILIP F. LOW

ing the partial molar free energy of this water. The relative importance of the two factors is still unknown. Hendricks and co-workers (1940) studied the dehydration of Mississippi bentonite by differential thermal analysis. They observed two endothermic peaks when the clay was saturated with Li+ or the alkaline earth cations but only one endothermic peak when the clay was saturated with hydrogen or the alkali metal cations. The lower temperature peak in the former case occurred at the same position as the single peak in the latter case. The lower temperature peaks were ascribed to dehydration of the surface; the higher temperature peaks were ascribed to dehydration of the cations. Hence, it was concluded that the alkali metal cations, excepting Li+, were not hydrated on the clay surface. The clay hydration that did occur when these cations were present was supposed to be due to the surface only. The bulk of the evidence from studies of electrolyte solutions indicates that Cs+, Rb+, and possibly K+ are not hydrated. Therefore, their conclusion is not unreasonable. It is interesting to note from their data that the water ascribed to hydration of the surface decreased with increasing crystallographic radius of the adsorbed ion. This is consistent with the fact that the larger ions would be detrimental to the formation of a water structure. Other interpretations have been given to the data of Hendricks et al. (1940). Forslind (1950) states that Li+ and the alkaline earth cations, because of their electronegativity, are able to form bonds of partially covalent character with water molecules. Resonance between the covalent and ionic states increases the net ion-water bond energy but produces a disturbance in the surrounding water lattice as the bonded water molecules are pulled toward the ion. Thus the water exists in two energy states, corresponding to the bonded water and the disturbed water, respectively. The other alkali metal cations, on the other hand, are supposedly unable to attain covalent states with the water molecules and so they produce a purely disturbing effect which depends on ionic size. However, Forslind does not argue with the basic conclusions of Hendricks and co-workers. Mackenzie (1950), in criticizing the concept of an ordered water configuration adjacent to a clay surface, pointed out that a linear relationship exists between the peak temperatures obtained by Hendricks et al. (1940) and the hydration energies of the ions. This relationship was regarded as evidence in favor of ion hydration as the primary factor in water adsorption by clays at low water contents. However, the fact should not be overlooked that the Cs+ ion, and possibly the K+ ion, are not hydrated in solution. The data points for these ions fell on Mackenzie’s peak temperature-ionic hydration energy line. In addition, the line intercepted

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

317

the peak-temperature axis at a value that was 70 per cent of the maximum value attained. It would appear, therefore, that the mineral surface made a substantial contribution to the average adsorption energy of the water. Rios and Vivaldi (1950) observed that the amount of water adsorbed by a clay at a given relative humidity was a linear function of the polarization capacity, z/T2, of the exchangeable cation. Here z is the ionic valence and T is the ionic radius. The linear relationship was obtained for both monovalent and divalent cations on kaolinite, a Morroco bentonite and the Mississippi bentonite of Hendricks et d.(1940). For zero ionic polarization capacity the water adsorption amounted to a large fraction of that for the maximum ionic polarization capacity. The authors concluded that both the exchangeable ions and the clay surface affect water adsorption. Apparently, as the size of the exchangeable ion increases, the wettability of the surface (Enright and Weyl, 1950), the energy of water retention (Mackenzie, 1950), and the amount of water adsorbed all decrease. Working with kaolinite Keenan et al. (1951) found that the number of Li+ ions on the surface had no effect on the amount of water adsorbed from the vapor phase. This was not true for the other ions they used. They believed that the Li+ ion was buried in the hexagonal cavities in the lattice and did not affect water adsorption for this reason. Consequently they subtracted the amount of water adsorbed by the Li-clay from the amounts adsorbed by the other homoionic clays to obtain estimates of ionic hydration. The hydration of the ions decreased with increasing ionic size, as expected. The water attributed to surface adsorption alone constituted more than half of the water adsorbed in all cases. When the water vapor adsorption of Wyoming bentonite was studied by Mooney and associates (1952b), they detected little difference in the amounts of water adsorbed by the K+, Rb+, and Cs+ clays. As a result, they assumed that these ions were not hydrated and assigned all the adsorbed water to the mineral surface. By subtracting the surface-adsorbed water from the total water adsorbed by the other clays, the relative ionic hydrations were obtained. Again the expected order was realized. And again the water held by the surface amounted to more than half the total. On the basis of the evidence cited in the preceding paragraphs, one may conclude that both ions and surface hydrate, the degree of hydration being influenced, for a given ionic charge, by the ionic size. The heat of adsorption of water on clays can be determined by means of the Clausius-Clapeyron equation, namely,

318

PHILIP F. LOW

In this equation pl and p 2 are the equilibrium water vapor pressures above a clay at the temperatures T 1 and T2, the water content being held constant, and AP is the heat of adsorption. Stated differently, it is the change in the partial qolar heat content of the water on adsorption from the vapor state. All that is needed to obtain values for the variables in the equation are adsorption isotherms at two different temperatures. Since equilibrium conditions are maintained, the partial molar free energy of the water in the clay is the same as that in the vapor; hence, from we have

A 3 = AH/T where A?? is the partial molar entropy change in the water on adsorption. Thus, it is also possible to obtain the change in disorder of the water molecules in going from the vapor to the condensed phase at the surface. The heat of adsorption is greater than the heat of condensation of pure water for both kaolinitic clays (Goates and Bennett, 1957) and montmorillonitic clays ( Mooney et al., 1952a; Barshad, 1955). Further, the change in partial molar entropy is greater for water adsorption on these clay types than for condensation to pure water, the initial state being the vapor in each case. Evidence for this was given by the same authors. In fact, Goates and Bennett (1957) point out that the partial molar entropy of the water in a surface monolayer on kaolinite is less than that in ice at the same temperature. Although Mooney et al. (1952a) did not calculate entropy changes in their work, their data permit this calculation. The calculated partial molar entropy of the water on their montmorillonite is less than the partial molar entropy of pure water. Martin (1959) also used the Clausius-Clapeyron equation and, from his data, calculated negative values for the partial molar entropy of adsorbed water relative to pure water. However, he maintains that an integral entropy, which is not the customary quantity obtained from the partial molar entropy by integration, is the most significant thermodynamic quantity. Because his calculated values of the integral and partial molar entropies are of opposite sign for water adsorption on kaolinite, he is obliged to conclude that the adsorbed water is less ordered than pure water from one approach and more ordered from the other. Martin has provided no explanation for this discrepancy. Therefore, the burden of proof for accepting the integral entropy in preference to the

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

319

partial molar entropy is on him. Until such proof is forthcoming, the author will regard the conventional partial molar entropy as a reliable criterion for the degree of disorder in the adsorbed water. Accordingly, there must be greater order in the adsorbed water than in free water. Whether the increased order is due to hydration of the cations or to hydration of the surface is not known. In view of the evidence discussed thus far, it would be reasonable to assume that ions and surface are mutually responsible. The differential heat of wetting of clays has been determined calorimetrically by Robins ( 1952), Rosenqvist ( 1955), and Zettlemoyer et al. (1955). Integral heats of wetting have been determined calorimetrically by Slabaugh (1955). Additional values for the latter quantity are reported in the book by Grim (1953). Actually, we would be more correct to refer to the differential heat of swelling, but, for the present purpose, we will ignore the distinction. Edlefsen and Anderson (1943) discuss this subject. We will regard the differential heat of wetting as the heat liberated per gram of liquid water added to a clay at constant temperature, pressure, and moisture content. It differs from the heat of adsorption by the heat of condensation of pure water. All the heat of wetting studies referred to have two features in common, namely, the heat of wetting becomes very close to zero when only a few layers of water have been formed and the first layer or two are adsorbed with the release of large amounts of energy, more than that released on the freezing of water. Robins (1952) found that the differential heats of wetting for a Yo10 clay varied continuously from a value of -191 cal. per gram of water at 1per cent moisture (dry weight basis) to near zero at about 12 per cent moisture. The relative partial specific entropy varied from -1 cal. per gram per degree to near zero in the same moisture range (the relative partial specific entropy for water in ice is about -0.3). The relative partial specific free energy varied correspondingly. The partial specific quantities are per gram instead of per mole. Zettlemoyer et al. (1955) found differential heats of wetting for a bentonite clay which ranged from about -380 cal. per gram near zero moisture to practically zero calories per gram at 20 per cent moisture. Rosenqvist’s (1955) data were comparable for an Oslo clay. The exchangeable cations affect the heats of wetting and adsorption. The data presented by Grim (1953) are illustrative of this fact. Slabaugh (1959) has also presented evidence in this regard, as has Rosenqvist ( 1955). Generally speaking, the heats of wetting increase with decreasing ionic size in keeping with expectation. Rosenqvist (1955) has provided data on 14 different Norwegian and Danish clays which show that the heat of wetting increases linearly with increasing base exchange capacity.

320

PHILIP F. LOW

Grim (1953) has cited similar data. However, Mortland (1954) has shown that base exchange capacity and specific surface are also linearly related; so one cannot be sure that the heat of wetting depends on the exchange capacity instead of the surface. Throughout this paper we have observed the effect of ionic size on the various properties of free and adsorbed water. For the most part, the property in question changed regularly with the ions in normal order. Although attention has not previously been called to the fact, it should be noted here that the Li+ ion frequently is out of sequence in its effect. The results of Hendricks et al. (1940), Keenan et al. (1951), White (1955), Rosenqvist (1955), Rowland et al. (1956), Anderson and Low (1958), Low (1958a), and Kolaian (1960) are cited in this regard. Possibly the occasional peculiar behavior of the Li+ ion is explicable on the basis of covalent states which it may form with water (Forslind, 1950). Or perhaps this peculiar behavior arises from the relationship of the ion to the crystal lattice (Keenan et al., 1951). Attention has already been directed to the rapid change in the thermodynamic properties of adsorbed water with distance from the clay surface. On the basis of the vapor adsorption and heat of wetting experiments, one would be justified in assuming that the thermodynamic status of the water was entirely normal at a distance of three or four molecular diameters. Granted, most of the change in these properties occurs in a narrow region adjacent to the surface. But the point is made here that the methods used in these experiments are relatively insensitive. And very small changes in the thermodynamic properties can have significant consequences. The latter point will now be considered briefly. By existing vapor adsorption methods it is very difFicult to measure relative partial molar free energies to less than 10 to 20 cal. per mole. But at the wilting point the partial molar free energy of the water in a soil is reduced relative to that of pure water by only 6.5 cal. per mole, and most soil processes occur at higher moisture contents than the wilting point. For instance, 1 atmosphere is an appreciable swelling pressure. Yet this is the swelling pressure when the partial molar free energy of the soil water is different from that of pure water by less than 0.5 cal. per mole (see Eq. 22), and water often flows under a hydraulic gradient of < 2. A gradient of this magnitude corresponds to a partial molar free energy difference per centimeter of less than 0.001 cal. per mole. Further, if the heat of adsorption results from the formation of additional hydrogen bonds in the water, and if the heat of activation for viscous 00w is the energy required to rupture hydrogen bonds, then it is evident from Eq. (3) that significant changes in viscosity would result from relatively small changes in the heat of adsorption-changes too small to measure

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

321

by the vapor pressure or calorimetric methods. Continuing this line of thought, small entropy changes, representing minor adjustments in the adsorbed water structure, may have appreciable effects on the supercooling of the water involved. Still, such changes may not be detected by these methods. It may be said, therefore, that it is incorrect to assume that thermodynamic changes are insignificant because they are small or not readily measured. The only work known to the author in which the thermodynamic properties of water in wet clays have been studied is that of Kolaian and Low (1960). These investigators measured the change of water tension with temperature in Wyoming bentonite suspensions containing 5 to 10 per cent clay. Then they used the equation of Low and Anderson ( 1958b), i.e., (S-so)Po=~

- ( );

+

P,. N

to determine the relative partial molar entropies. In this equation is the partial molar entropy of water in suspension, So is the partial molar entropy of pure water, Po is the pressure on the pure water, N is the composition of the suspension, g is the temperature coefficient of water expansion, and the other symbols have the meanings assigned earlier. From equations (22), (24), and (26) the value of AH was also determined. Their results showed that all of the relevant thermodynamic quantities were less in the dilute clay suspensions than in pure water; hence it appears that the influence of the clay surface extends over rather large distances. An interesting feature of the work of Kolaian and Low (1960) is that the measured water tension in each suspension increased with time after stirring, i.e., as the suspension became thixotropic. Immediately after stirring the tension was close to zero but as time elapsed it increased to a maximum value which was achieved after about 50 to 100 hours, depending on the adsorbed cation. The tension-time relationship suggested that water molecules were falling into positions of minimum energy in a gradually developing water structure. The development of the water structure and of thixotropy were concomitant. Now remember that Buehrer and Aldrich (1946) found essentially all the water in a bentonitewater system remaining unfrozen as long as the water content of the system was insufficient to convert it from a thixotropic gel to a sol. As soon as enough water was added to prevent thixotropy, the percentage of unfrozen water fell suddenly to a low value. Remember also, that Kolaian (1960) found the rate of freezing of a bentonite suspension to increase remarkably with the development of thixotropy. Evidently, the

322

PHILIP F. LOW

condition of the water is quite different in a clay-water system in the gel state than it is in the same system in the sol state. This idea was commonly accepted by colloid chemists many years ago. To the water was ascribed the role of gelling agent. But with the advent of double-layer theory and the monolayer concept of adsorption, this idea was discarded in favor of particle-particle interaction as the cause of thixotropy. Probably a reconsideration of the problem is warranted. X. A Working Hypothesis

In a recent presentation Richards (1960),referring to the water films on soil particles, stated: “When the thickness of adsorbed water films is reduced to 6 or 8 monomolecular layers of water, the soil water is so tightly bound that crop growth ceases. All agriculture is conducted in a soil-water film thickness range from this value up to two or three times this thickness.” This being the case, it is essential that we understand the properties of water in these films. As an aid to understanding, and as an aid in designing future research, the author presents the following hypothesis. The reader will recognize that it is new only in its details. Several investigators have entertained similar hypotheses, notably: T. F. Buehrer, E. Forslind, R. E. Grim, S. B. Hendricks, H. H. Macey, I. Th. Rosenqvist, and H. F. Winterkorn. The author hereby acknowledges their contributions to his thinking. When a clay is exposed to water vapor, the exchangeable cations hydrate first if they are small enough to be capable of hydrating. Then the remainder of the surface hydrates by the formation of hydrogen bonds between the surface hydroxyls, or oxygens, and the water molecules. In the case of the oxygen atoms the bonding tends to be of a covalent character because the excess electrons in the lattice make it easier for the lone-pair electrons of these atoms to be distorted by the protons of the nearby water molecules. The water molecules in the initial layer are not arrayed in perfect order. The competition between adsorbed ions and surface atoms for these molecules is too great. Nevertheless, the initially adsorbed molecules have very low free energy, heat content, and entropy owing to the intensity with which they are held. As other molecules come within the force fields of those in the first layer, they are captured by them. The first-layer molecules, having their electron distributions affected somewhat by covalent bonding with the surface oxygens, form partially covalent bonds with the captured molecules. These, in turn,are induced to form partially covalent bonds with their neighbors, including third-layer molecules. As additional layers accrue by this type of cooperative action, the degree of bond covalency, and hence of rigidity,

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

323

decreases. However, the normal electron distribution within the molecules is still conducive to tetrahedral coordination and the neighboring molecules on the side toward the surface do not have the usual freedom to exert torques and promote displacements. Therefore, the degree of order decreases gradually with distance from the surface. In addition to being responsible for electroviscous and osmotic effects, as modified by the properties of the water, the adsorbed ions promote disorder. If they are small and monovalent their contribution in this direction is relatively small. But if they are large or multivalent their disordering effect is large. Further, the more they dissociate from the critical surface region where the structure is “anchored,” the less they disturb the structure. Those ions that create the least disturbance in the quasi-crystalline water “dissolve” in it most readily. When these ions are present, the water structure may extend with considerable regularity for distances of the order of 75 to 100 A. An attenuated structure may persist as far out as 200 to 300 A. At lower temperatures the structure extends farther and is more coherent than at higher temperatures. Under these conditions the water structure is connected from mineral surface to mineral surface without any intervening region of disorder. However, in the presence of large or multivalent cations there may be little or no order in the adsorbed water. The specific volume, viscosity, and freezing resistance of the water are directly related to its structural development. Consequently, the magnitudes of these properties decrease continuously with distance from the mineral surface. The same applies to the relative thermodynamic properties. It should be noted, however, that nowhere is the water structure so rigid that ions cannot diffuse through it; nor is it so rigid that it will not shear under stress. Nevertheless, it has a yield point, which depends upon the proximity of adjacent mineral surfaces, and it exhibits non-Newtonian behavior at low hydraulic gradients. Although the precise molecular arrangement of the water cannot be specified, it is not that in ice. All that has been said regarding water interaction with oxygen surfaces pertains also to hydroxyl surfaces. However, the coherence and extent of the structure which forms on the latter are not so great. REFERENCES Adam, N. K. 1941. “The Physics and Chemistry of Surfaces,” 3rd ed. Oxford Univ. Press, London and New York. Adamson, A. W. 1960. “Physical Chemistry of Surfaces.” Interscience, New York. Anderson, D. M., and Low, P. F. 1958. Soil Sci. SOC. Am. Proc. 22, 99-103. Andrade, E. N. da C. 1934. Phil. Mag. “71 17, 698-732. Andrade, E. N. da C. 1952. Proc. Roy. SOC.A215, 36-43.

324

PHILIP F. LOW

Andrade, E. N. da C., and Dodd, C. 1946. Proc. Roy. SOC. A187, 296-337. Andrade, E. N. da C., and Hart, J. 1954. Proc. Roy. SOC. A226, 463-472. Aylnmre, L. A. G., and Quirk, J. P. 1959. Nature 183, 1752-1753. Aylmore, L. A. G., and Quirk, J. P. 1960. Proc. 9th Natl. Conf. on Clays and Clay Minerals, Lafayette, Indiana (in press). Babcock, K. L., and Overstreet, R. 1957. Soil Sci. 83, 455-464. Barshad, I. 1955. Proc. 1st Natl. Conf. on Clays and Clay Technol., California, Dept. Nut. Resources, Div. Mines Bull. 169, 70-77. Bernal, J. D., and Fowler, R. H. 1933. J. Chem. Phys. 1, 515-548. Bingham, E. C. 1941. J. Phys. Chem. 46, 885-903. Bolt, G. H. 1960. Trans. Intern. Congr. Soil Sci., 7 t h Congr., Madison. Bolt, G. H., and Miller, R. D. 1955. Soil Sci. Sac. Am. Proc. 19, 285-288. Bradley, R. S. 1957. Trans. Faraday SOC. 63, 687-691. Bradley, W. F. 1959. Nature 183, 1614-1615. Brady, G.W. 1960. J. Chem. Phys. 33, 1079-1082. Buehrer, T. F., and Aldrich, D. G. 1946. Arizona Agr. Expt. Sta. Bull. 110. Buehrer, T. F., and Rose, M. S. 1943. Arizona Agr. Expt. Sta. Bull. 100. Claussen, W. F. 1951. J. Chem. Phys. 19, 259-260. Claussen, W. F., and Polglase, M. F. 1952. J. Am. Chem. SOC. 74, 4817-4819. Collie, C. H., Hasted, J. B., and Ritson, D. M. 1948. Proc. Phys. SOC. B60, 145-160. Corey, V. B. 1943. Phys. Rev. 64, 350-357. Cownie, A., and Palmer, L. S. 1952. Proc. Phys. SOC. B66, 295-301. Cross, P. C., Burnham, J., and Leighton, P. A. 1937. J. Am. Chem. SOC.69, 11341147. Day, P. R. 1956. Ann. Rept. California Agr. Expt. Sta. Project 1686. Debye, P. 1945. “Polar Molecules.” Dover, New York. Deming, J. M. 1951. Ph.D. Thesis. Purdue University, Lafayette, Indiana. Derjaguin, B. V., and Krylov, N. A. 1944. “Proceedings of the Conference on Viscosity of Liquids,” U.S.S.R. Acad. Sci. Press, Moscow. Derjaguin, B. V., and Melnikova, N. K. 1958. Highway Research Board Special Rept. 40, Natl. Acad. Sci.-Natl. Research Council Publ. 629, 43-54. DeWit, C. T., and Arens, P. L. 1950. Trans. Intern. Congr. Soil Sci., 4th Congr., Amsterdam 2, 59-62. Dorsey, N . E. 1948. Trans. Am. Phil. Sac. 98, 247-328. Dutt, G. R. 1960. Ph.D. Thesis. Purdue University, Lafayette, Indiana. Edlefsen, N. E., and Anderson, A. B. C. 1943. Hilgardia 16, No. 2, 1-298. Elton, G. A. H. 1948a. Proc. Roy. Sac. A194, 259-274. Elton, G.A. H. 1948b. Proc. Roy. SOC. A194, 275-287. Elton, G.A. H., and Hirschler, F. G. 1949. Proc. Roy. SOC.A198, 581-589. Enright, D. P., and Weyl, W. A. 1950. 1. Appl. Phys. 21, 338-344. Eriksson, E. 1950. Ann. Roy. Agr. Coll. Sweden 17, 17-23. Ewell, R. H., and Eyring, H. 1937. J. Chem. Phys. 6, 726-736. Forslind, E. 1950. Trans. Intern. Congr. Soil Sci., 4th Congr., Amsterdam 1, 110113. Forslind, E. 1952. Acta Polytech. 3, No. 5. Frank, H. S. 1958. Proc. Roy. SOC. A247, 481-492. Frank, H.S., and Evans, M. W. 1945. J. Chem. Phys. 13, 507-532. Frank, H. S., and Robinson, A. L. 1940. J. Chem. Phys. 8, 933-938. Frank, H. S., and Wen, W. 1957. Discussions Faraday Sac. No. 24, 133-140. Fripiat, J. J., Chaussidon, J., and Touillaux, R. 1960. J. Phys. Chem. 64, 1234-1241.

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

325

Frohnsdorff, G. J. C., and Kington, G. L. 1958. Proc. Roy. SOC. A247, 469-472. Glasstone, S., Laidler, K. J., and Eyring, H. 1941. “The Theory of Rate Processes.” McGraw-Hill, New York. Glen, J. W. 1958. Advances in Phys. 7, 254-265. Goates, J. R., and Bennett, J. S. 1957. Soil Sci. 83, 325-330. Goldsmith, B. J., and Muir, J. 1960. Trans. Faraday SOC. 66, 1656-1661. Grant, E. H. 1957. J. Chem. Phys. 26, 1575-1577. Grim, R. E. 1953. “Clay Mineralogy.” hlcGraw-Hill, New York. Gjotheim, K., and Krogh-Moe, J. 1954. Acta Chem. Scand. 8, 1193-1202. Gumev. R. W. 1953. “Ionic Processes in Solution.” McGraw-Hill. New York. Haggis, G. H., Hasted, J. B., and Buchanan, T. J. 1952. J. Chem. Phys. 20, 14521465. Hansbo, S. 1960. Swedish Geotech. Znst. Proc. No. 18. Harris, F. E., and O’Konski, C. T. 1957. J. Phys. Chem. 61, 310-319. Hasted, J. B., Ritson, D. M., and Collie, C. H. 1948. J. Chern. Phys. 16, 1-11. Hauser, E. A., and LeBeau, D. S. 1938. J. Phys. Chem. 42, 1031-1049. Hemwall, J. B., and Low, P. F. 1956. Soil Sci. 82, 135-145. Hendricks, S. B., and Jefferson, M. E. 1938. Am. Mineralogist 23, 863-875. Hendricks, S. B., Nelson, R. A., and Alexander, L. T. 1940. J . Am. Chem. SOC. 62, 1457-1464. Henniker, J. C. 1949. Reus. Modern Phys. 21, 322-341. Isono, K. 1955. J. Meteorol. 12, 456-462. Kaminsky, M. 1957. Discussions Faraday SOC. No. 24, 171-179. Kato, C. 1959. Yogyo Kyokai Shi 67, 243-246; see Chem. Abstr. 64, 14000 (1960). Katzoff, S. 1934. J . Chem. Phys. 2, 841-851. Keenan, A. G., Mooney, R. W., and Wood, L. A. 1951. J . Phys. Chem. 66, 14621474. Kemper, W. D. 1960. Soil Sci. SOC. Am. Proc. 24, 10-16. Kolaian, J. H. 1960. Ph.D. Thesis. Purdue University, Lafayette, Indiana. Kolaian, J. H., and Low, P. F. 1960. €‘roc. 9th Natl. Conf. on Clays and Clay Minerals, Lafayette, Indiana (in press). Kumai, M. 1951. J . Meteorol. 8, 151-156. Langham, E. J., and Mason, B. J. 1958. Proc. Roy. SOC. A247, 493-504. Lennard-Jones, J,, and Pople, J. A. 1951. Proc. Roy. SOC. A206, 155-162. Leonards, G. A., and Andersland, 0. B. 1960. Presented at Am. SOC. Civil Eng. Research Conf. on Shearing Strength of Clays. Lovell, C. W. 1957. Highway Research Board Bull. 168, Natl. Acad. Sci. Natl. Research Council Publ. 628, 74-95. Low, P. F. 195th. Soil Sci. SOC. Am. Proc. 22, 395-398. Low, P. F. 1958b. Highway Research Board Special Rept. 40, Natl. Acad. Sci.Natl. Research Council Publ. 629, 55-64. Low, P. F . 1959. Proc. 8th Natl. Conf. on Clays and Clay Minerals, Norman, Oklahoma ( in press ) . Low, P. F . 1960a. Trans. Intern. Congr. Soil Sci., 7th Congr., Madison (in press). Low, P. F. 1960b. Proc. 9th Natl. Conf. on Clays and Clay Minerals, Lafayette, Indiana (in press ) . Low, P. F., and Anderson, D. M. 195th. Soil Sci. SOC. Am. Proc. 22, 22-24. Low, P. F., and Anderson, D. M. 195813. Soil Sci. 86, 251-253. Low, P. F., and Deming, J. M. 1953. Soil Sci. 76, 187-202. Lutz, J. F., and Kemper, W. D. 1959. Soil Sci. 88, 83-90. , I

326

PHILIP F. LOW

Macey, H. H. 1942. Trans. Brit. Ceram. SOC. 41, 73-121. Mackenzie, R. C. 1950. Clay Minerals Bull. 1, 115-119. Mackenzie, R. C. 1959. Nature 185, 1615. Martin, R. T. 1959. Proc. 8th Natl. Conf. on Clays and Clay Minerals, Norman, Oklahoma (in press). Martin, R. T. 1960. Proc. 9th Natl. Conf. on Clays and Clay Minerals, Lafayette, Indiana (in press). Mason, B. J, 1957. “The Physics of Clouds.” Clarendon Press, Oxford. Mason, B. J. 1958. Advances in Phys. 7, 221-234. Mason, B. J., and Maybank, J. 1958. Quart. J. Roy. Meterol. SOC. 84, 235241. Mathieson, A. M., and Walker, G. F. 1954. Am. Mineralogist 39, 231-255. Michaels, A. S., and Lin, C. S. 1954. Ind. Eng. Chem. 46, 1239-1246. Michaels, A. S., and Lin, C. S. 1955. Ind. Eng. Chem. 47, 1249-1253. Mooney, R. W., Keenan, A. G., and Wood, L. A. 1952a. J. Am. Chem. SOC. 74, 1367-1371. Mooney, R. W., Keenan, A. G., and Wood, L. A. 1952b. J. Am. Chem. SOC. 74, 1371-1374. Morgan, J., and Warren, B. E. 1938. J. Chem. Phys. 6, 666-673. Mortland, M. M. 1954. Soil Sci. 78, 343-347. Mossop, S. C. 1955. Proc. Phys. SOC. B68, 193-208. Muir, J. 1954. Trans. Faraday SOC. 60, 249-254. Nitzsch, W. v. 1940. Kolloid-2. 93, 110-115. Norrish, K. 1954. Discussions Faraday SOC. No. 18, 120-134. Oakes, D.T. 1958. Proc. 5th Natl. Conf. on Clays and Clay Minerals. Natl. Acad. ScLNatl. Research Council Publ. No. 666, 46-60. Oakes, D. T. 1959. Proc. 8th Natl. Conf. on Clays and Clay Minerals, Norman, Oklahoma (in press). Ockman, N . 1958. Advances in Phys. 7, 199-220. Olsen, H. W. 1960. Proc. 9th Natl. Conf. on Clays and Clay Minerals, Lafayette, Indiana (in press). Orchiston, H. D. 1955. Soil Sci. 79, 71-78. Orchiston, H.D. 1959. Soil Sci. 87, 276-282. Owston, P. G. 1951. Quart. Revs. (London) 6, 344-363. Owston, P. G. 1958. Advances Phys., Phil. Mag. Suppl. 7, 171-188. Palmer, L. S. 1952. Proc. Phys. SOC. B66, 674-678. Palmer, L. S., Cunliffe, A., and Hough, J. M. 1952. Nature 170, 796. Pauling, L. 1945. “The Nature of the Chemical Bond.” Cornell Univ. Press, Ithaca, New York. Pickett, A. G., and Lemcoe, M. M. 1959. J. Geophys. Research 64, 1579-1586. Pople, J. A. 1951. PTOC.Roy. SOC. A206, 163-178. Richards, L. A. 1960. Presented a t 7th Intern. Congr. Soil Sci., Madison, Wisconsin. Rios, E. G., and Vivaldi, J. L. M. 1950. Trans. Intern. Congr. Soil Sci., 4th Congr., Amsterdam 2, 67-71. Robins, J. S. 1952. Soil Sci. 74, 127-139. Robinson, R. A., and Stokes, R. H. 1955. “Electrolyte Solutions.” Academic Press, New York. Rosenqvist, I. Th. 1955. Norweg. Geotech. Inst. Publ. No. 9. Rosenqvist, I. Th. 1959. Proc. Am. SOC. Civil Eng., Soil Mech. and Foundations Diu. 86, 31-53. Rowland, R. A., Weiss, E. J., and Bradley, W. F. 1956. Proc. 4th Natl. Conf. on Clays and Clay Minerals, pp. 85-95.

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION

327

Russell, E. W. 1934. Phil. Trans. Roy. SOC. A233, 361-389. Samoilov, 0. Ya. 1957. Discussions Faraday SOC. No. 24, 141-146. Saxton, J. A. 1952. Proc. Roy. SOC. A213, 473-491. Schmid, W. E. 1957. Am. SOC. Testing Materials PTOC. 57, 1195-1218. Schofield, R. K. 1946. Trans. Faraday SOC. 42B, 219-225. Sill, R. C., and Skapski, A. S. 1956. J. Chem. Phys. 24, 644-651. Slabaugh, W. H. 1955. J. Phys. Chem. 59, 1022-1024. Slabaugh, W. H. 1959. J. Phys. Chem. 63, 436-438. Smyth, C. P., and Hitchcock, C. S. 1932. J. Am. Chem. SOC. 54, 4631-4647. Stackelberg, M. v., and Miiller, H. R. 1951. J. Chem. Phys. 19, 1319-1320. Stewart, G. W. 1939. J. Chem. Phys. 7, 869-877. Stewart, G. W. 1943. J. Chem. Phys. 11, 72-74. Stewart, G. W. 1944. Am. J. Phys. 12, 321-323. Tumbull, D. 1950. J. Chem. Phys. 18, 198-203. von Engelhardt, W., and Tunn, W. L. M. 1955. Illinois State Geol. Suruey Circ. 194.

Walker, G. F., and Milne, A. 1950. Trans. Intern. Congr. Soil Sci., 4th Congr., Amsterdam 2, 62-67. Wang, J. H. 1951a. J. Am. Chem. SOC. 73, 510-513. Wang, J. H. 1951b. J. Am. Chem. SOC. 73, 4181-4183. Wang, J. H. 1954. J. Phys. Chem. 58, 686-692. Wang, J. H., Robinson, C. V., and Edelman, I. S. 1953. J. Am. Chern. SOC. 75, 466470. Ward, A. G. 1937. Trans. Faraday SOC. 33, 88-97. White, W. A. 1955. Proc. 3rd Natl. Conf. on Clays and Clay Minerals. Natl. Acad. Sci.-h'atl. Research Council Publ. 395, 186-204. FVinterkom, H. F. 1955. Am. SOC. Testing Materials Spec. Tech. Publ. No. 163, 27-35. Zettlemoyer, A. C., Young, G. J., and Chessick, J. J. 1955. 1. Phys. Chem. 59, 962966.

This Page Intentionally Left Blank

IRON CHLOROSIS IN PLANTS John C. Brown Mineral Nutrition laboratory, United States Department of Agriculture, Beltsville, Maryland

I. Introduction ................................................ 11. Earlier Findings on Iron Chlorosis . ...................... 111. Factors Involved in Iron Chlorosis . ...................... A. Iron Supply ............................................. B. Characteristics of the Root Stock . . . . . . . . . . . . . . . . . . . C. Characteristics of the Soil or Growth Medium . . . . . . . . . . . . . . . . IV. Chelating Agents as Iron Carriers .............................. A. Synthetic Chelating Agents ................................ B. Apparent Chelating Activity of Roots . . . . . . . . . . . . . . . . . . . C. Absorption of Chelating Agents ... ................. V. Chelating and Other Mechanisms in Plants ...................... A. Plant Species and Variety ................................. B. Reductive Capacity at the Root . . . . . . . . . . . . . . . . . C. Kind and Concentration of Chelatins Compounds . . VI. The Future of the Iron Chlorosis Problem . . . . . . . . . . . . References ..................................................

Page 329 331 334 334 337 351 351 360 360

366

1. Introduction

A continuing supply of iron is essential to the welfare of the green plant. Any factor that interferes with absorption or utilization of iron may cause the plant to become iron deficient and chlorosis to develop. Iron chlorosis refers to the yellowing of plants which can be alleviated by suitable iron compounds. Several reviews of iron metabolism, including iron chlorosis, have been published within the past six years (Spiers, 1954; Brown, 1956; Granick, 1958; Evans, 1959). The most recent review by Wallace and Lunt (1960) “Iron Chlorosis in Horticultural Plants” stresses the following causative factors: “ ( a ) Low iron supply, ( b ) calcium carbonate in soil, ( c ) bicarbonate in soil or irrigation water, ( d ) over-irrigation or high-water condition, ( e ) high phosphate, ( f ) high levels of heavy metals such as manganese, copper, zinc, (g) low or high temperatures, ( h ) high light intensities, ( i ) high levels of nitrate nitrogen, ( j ) unbalanced cation ratios, ( k ) poor aeration, (1) certain organic 329

330

JOHN C. BROWN

matter additions to the soil, ( m ) viruses, and ( n ) root damage by nematodes or other organisms.” Iron chlorosis is potentially a problem on most calcareous soils. Possibly a third of the worlds land is calcareous at the surface horizon. Kuykendall (1956) estimated that about 44 million acres of cropland in the United States are of this type. An example of the severity of the limitation of iron supply is shown by the failure of sorghum to grow on a calcareous soil (Fig. 1).

FIG.1. Response of WHEATLAND milo to 2.66% ferrous sulfate spray at the rate of 30 gal. per acre: (left) plants sprayed three times, (center) no spray, and (right) plants sprayed once. (Photograph by courtesy of C. W. Carlson, Garden City Branch Station, Kansas Agricultural Experiment Station.)

Any factor that decreases the availability of iron in a soil or competes in the absorption process contributes to iron chlorosis. Thus, phosphate, bicarbonate, and calcium salts in growth media have often been mentioned as contributing to some type of chlorosis. The terms copper-, manganese-, nickel-, and zinc-induced iron chlorosis are common in the literature. Wallace and Lunt (1960) concluded their review on iron chlorosis by giving a list of problems to be understood or solved before iron chlorosis can be controlled. Some of these problems are: (1) how do plants

IRON CHLOROSIS IN PLANTS

331

ordinarily obtain iron that is very insoluble in soil? ( 2 ) how is iron translocated in plants? (3) why are some plants susceptible to lime-induced chlorosis while others are not? ( 4 ) what happens to iron in plants to make it nonfunctional? (5) why is the micronutrient balance so intimately related to the development of iron chlorosis? ( 6 ) how are bicarbonates and carbon dioxide related to iron chlorosis? ( 7 ) how does phosphorus metabolism in roots affect iron chlorosis? (8) how do the factors that induce iron chlorosis relate to chlorophyll synthesis? and ( 9 ) why is iron chlorosis so nearly irreversible? Evans (1959) stressed the biochemical role of iron in plant metabolism, and Wallace and Lunt (1960) emphasized the causative factors of iron chlorosis in horticultural plants. To avoid unnecessary duplication this review will emphasize particular phases of iron chlorosis that seem pertinent at the present time. Reductive and chelating capacities of roots have developed as factors related to iron chlorosis and will be stressed with illustrations from this laboratory. Other phases of the problem, developed by other scientists, could be discussed with equal emphasis. A limited approach is taken in this paper to give greater meaning and clarity to a particular phase of this complex subject “iron chlorosis in plants.” It. Earlier Findings on iron Chlorosis

Gris (1843) showed that plants which are deprived of an adequate supply of iron failed to develop chlorophyll and became chlorotic. Sachs (1860) is credited as having been the first to establish that iron is an essential element for the growth of higher plants. In looking for the causative factors of chlorosis, Juritz (1912) analyzed soils and related iron chlorosis to an excessive amount of calcium carbonate in the soil. The conditions that favored chlorosis were intensified by unsatisfactory moisture conditions in the soil and by the existence of a fairly impermeable substratum of marl. “Lime-induced chlorosis is the term often used to denote the iron chlorosis that develops in plants grown on naturally calcareous soils. The term is misleading in view of the marked differences that exist between plant responses on naturally calcareous soils and artificially calcareous soils (acid soils made calcareous by addition of calcium carbonate). The iron compounds in soils of humid regions come from the weathering of many iron-containing minerals of the parent material. Two extremes are represented by podzolic and lateritic soils. In regions of podzolic soils rainfall is high and temperatures are usually low, with a resultant accumulation of organic matter at the A. horizon. Under such conditions, which prevail in much of the northeastern part of the United

332

JOHN C. BROWN

States, the top strata are acidic. A great deal of the iron present in the parent minerals becomes reduced and dissolved and is transported to the lower, less acid stratum, where it is precipitated and forms new iron compounds such as iron hydrates, oxides, and some organic iron complexes. The more hydrated iron compounds are the most reactive. The organic iron in acid soils is somewhat soluble in water, and the iron in this form tends to be protected from precipitation. In the region of lateritic soils rainfall is abundant and temperatures are high such that organic matter is rapidly decomposed. Under such conditions the base elements are leached away. The iron is converted largely to hydrated oxides and eventually to goethite (FeO-OH) and hematite ( Fe203).These iron compounds form coatings on clay, silt, and sand particles in many of the soils, imparting various shades of red, reddish brown, yellow, and yellowish brown colors. Soils of the arid and semiarid regions are less weathered and leached than those formed under heavier rainfall. Many of these soils are calcareous and alkaline in reaction. The primary iron minerals are less altered and are more uniformly distributed in the soil profile. Soluble salts are sometimes present at the surface, and caliche often develops in lower strata. Extensive areas of the arable soils of the intermountain areas of the United States are on alluvial fans, lake terraces, stream terraces, and river bottoms. Many of these soils show some weathering and profile development, but much of the soil material had been partially disintegrated by frost, wind, and water erosion before its deposition. Most of these soils are well supplied with nutrient elements, but because of their alkaline nature the soluble iron is often low. The “soil iron” available to plants is affected markedly by reactions in the soil. The organic matter in an acid soil normally increases the content of available iron. The bicarbonates formed from the carbon dioxide of decomposing organic material in an acid soil enhance the solubility of iron compounds. The reverse is true in an alkaline-calcareous soil. Green manure crops disked into a calcareous soil, followed by irrigation, have often caused severe iron chlorosis to develop in deciduous fruit trees. Wadleigh et al. (1936) relate the chemical nature of the growth medium to the degree of chlorosis. Iron chlorosis developed in corn and cotton upon the addition of copper to a nutrient solution (Willis and Piland, 1936). Walsh and Clarke (1948) found iron chlorosis produced on acid peat soils containing large quantities of soluble zinc. Hopkins et al. (1944) report that severe manganese toxicity in pineapples was corrected by spraying the plants with iron sulfate. Iron chlorosis of pineapples on the black manganous soils of Hawaii was corrected by

IRON CHLOROSIS IN PLANTS

333

Johnson (1916) by spraying the plants with 8 per cent iron sulfate solution. Johnson (1924) drew attention to the difference between limeinduced chlorosis and that caused by manganese, the latter usually occurring under acidic conditions. Willis and Carrero ( 1921), Allyn ( 1927), Bennett ( 1931), and Aston (1934) report data which show that lime-induced chlorosis is caused not only by the inability of the plant to obtain iron from the soil, but also by inactivation of the iron after it has been absorbed by the plant. Biddulph (1948), Olsen (1935), and Franco and Loomis (1947) found that iron chlorosis developed in plants grown in solution cultures containing relatively high concentrations of phosphate. They concluded that iron is precipitated as iron phosphate in the leaf tissue. The amount of irrigation water applied to a crop influenced iron chlorosis. Reuther and Crawford (1946) found that grapefruit developed iron chlorosis in the winter in plantings receiving 2 to 3 acre-inches of water per week in summer and 2 to 3 acre-inches of water every 2 or 3 weeks in winter. Trees receiving 2 to 3 acre-inches of water every 4 to 6 weeks in summer and every 6 to 8 weeks in winter developed normally. Harley and Lindner (1945) found that apple and pear trees developed varying degrees of iron chlorosis when they were irrigated with water containing 200 parts per million (p.p.m.) or more of bicarbonates. Blodgett ( 1946) regarded “lime-induced chlorosis in the arid regions of the United States as the most important single nutrient disorder affecting orchard and small fruit production. Chlorosis of citrus is a major problem in Arizona (McGeorge, 1948) and is also evident in deciduous fruit trees and on some field crops. Lindner and Harley (1944) suggest that there is competition between various organic compounds in the plant for the available iron. When the nutrition of the plant is upset in any way, there is probably a shift in the equilibrium between the various iron compounds which could affect metabolism. Schander ( 1945) considers the formation of organic acids, especially citric, as playing a role in the development of iron chlorosis. Iljin (1945) analyzed chlorotic apple leaves and found 28.7 mg. of citric acid per gram of dry matter and 1.9 mg. of citric acid in healthy green leaves. Other scientists (T. Wallace, 1928; Thorne and Wallace, 1944; Lindner and Harley, 1944) have found that chlorotic leaves contain a much higher percentage of potassium than calcium. The percentage of nitrogenous compounds has also been found to be higher in chlorotic plant material than in nonchlorotic (Pettinger et al., 1932; Bennett, 1945; Buslova, 1948). Rogers and Shive (1932) have suggested that the transport of iron within the plant might be affected by the pH of the conducting tissue.

334

JOHN C. BROWN

They found that iron accumulations usually occurred in regions where the pH gradient is high. It was concluded from the chemical analysis that iron in these accumulations is in a precipitated form not available for plant processes. Iron accumulations were absent in plants with low pH tissues throughout. In such plants, the iron content was low and uniformly distributed in practically all the tissues. Noack and Liebich (1941) reported that 82 per cent of the total iron in spinach leaves is associated with the chloroplasts. They separated this iron into various fractions and concluded that five-sixths of the firmly bound iron was absorbed on phosphorus-containing proteins. Other substances are listed by Smythe and Schmidt (1930) which possess a particular grouping within their molecule to hold iron as a compound which is undissociated: ( 1) hydroxymonocarboxylic acids (lactic, gluconic) ; (2) dicarboxylic acids (oxalic, malonic); (3) hydroxydicarboxylic acids (tartaric, citric); ( 4 ) amino acids which are also hydroxy or dicarboxylic acids ( aspartic acid, serine). The failure of iron to be translocated or exchanged in a site of physiological activity may be caused by competition from other iron-binding compounds. Keilin and Mann (1944) placed iron, when it is present in a physiological role, as occurring as an iron-porphyrin-protein complex acting as an oxygen carrier, oxidizing catalyst, or enzyme. Iron plays an important role (Evans, 1959) in a whole series of respiratory enzymes. Iron, as a constituent of cytochromes, not only functions in respiratory enzymes, but very likely also is involved in photosynthetic reactions. With a limited supply of iron available, competition between iron-requiring metabolic systems will possibly result in a deficiency of the least competitive of the iron-binding compounds. 111. Factors Involved in Iron Chlorosis

A. IRON SUPPLY

It is quite apparent from many observations over the past fifty years that both the growth medium and the plant species must be considered as factors in the development of iron chlorosis. The germinating seed usually contains sufficient iron to supply the requirements of a plant in the early stages of growth. In soybeans, enough iron is supplied from the cotyledons to maintain a green plant up to the first trifoliate leaf ( Brown and Holmes, 1955a). Figure 2 shows the distribution of radioiron from the cotyledons of a soybean plant growing on a calcareous Millville soil. Both of the trifoliate leaves were beginning to develop iron chlorosis. If sufficient iron is available to meet the needs of the plant, the iron continues to move up the stem with growth. In contrast, if the supply

IRON CHLOROSIS I N PLANTS

335

FIG.2. The distribution of radioiron from the seed of a chlorosis-susceptible SOYbean to other parts of the plant. The seed used was harvested from plants previously treated with radioiron. The lighter the area in the autoradiograph, the more radioiron it contains. Iron chlorcsis was developing in the two trifoliate leaves.

336

JOHN C. BROWN

FIG. 3. The distribution of radioiron in soybean from an approach graft with one set of roots in a sand culture containing radioiron and the other set of roots in a Millville soil (no radioiron). Iron chlorosis developed in the first two trifoliate leaves (lower left) before the graft became effective. The third, fourth, and fifth trifoliate leaves became green (upper left) when the graft became effective. The sixth trifoliate leaf (upper left) developed chlorosis when the available source of iron was stopped by cutting the stem from the root system contained in the sand culture. Upper and lower right: Autoradiographs showing the distribution of radioiron in the green and chlorotic leaves.

IRON CHLOROSIS IN PLANTS

337

is suddenly withheld, mobility in the plant ceases (Brown and Holmes, 1955b) (Fig. 3 ) and the new growth which develops is chlorotic. It is known that plant species and varieties differ in their capacity to absorb iron from a given growth medium after the iron supply in the seed is exhausted. PI-54619-5-1 (PI) soybeans are inefficient and HAWKEYE (HA) soybeans efficient (Weiss, 1943) in their capacity to absorb and translocate iron from the same nutrient medium, the difference in efficiency of iron utilization being conditioned by a single gene. PI-soybeans developed an iron-deficiency chlorosis when grown on several naturally calcareous soils and on nutrient solutions containing less than 5 p.p.m. inorganic iron (Brown and Holmes, 195513). Iron chlorosis has been induced in PI soybeans by increasing the phosphorus and copper concentration in the nutrient solutions (Brown et a[., 1955). In contrast, HA soybeans did not develop iron chlorosis when grown on nutrient solutions containing 2 p.p.m. inorganic iron (Holmes and Brown, 1955), on naturally calcareous soils, or on phosphorus and copper treatments which induce chlorosis in PI soybeans. B. CHARACTERISTICS OF THE ROOTSTOCK Grafting PI soybean tops on HA soybean rootstocks showed that both PI and HA tops were nonchlorotic on HA rootstock, but both PI and HA tops developed chlorosis (Fig. 4) on PI rootstock when grown on R naturally calcareous soil containing radioiron (Brown et al., 1 9 5 9 ~ ) . Figure 5 shows the distribution of radioiron in the tops of these plants where the relationship between chlorosis and the content of radioiron in the tops of the plants is particularly evident. A fundamental difference between these two soybean varieties is that HA roots greatly exceed those of the PI variety in absorptive capacity for iron from a relatively low iron supply.

C. CHARACTERISTICS OF THE SOILOR GROWTH MEDIUM The growth of plant species and varieties on different soils has led to some knowledge of the causative factors of iron chlorosis. An example is afforded by WHEATLAND milo and PI and HA soybeans when grown on Quinlan, Tripp, and Millville soils (Fig. 6 ) . Quinlan is a reddish chestnut soil developed from calcareous, very weakly consolidated, silty red beds and old sedimentary beds of sandy clay of Tertiary and Quaternary Ages. It is located in the warm temperate, subhumid grasslands of western Oklahoma and adjacent areas where the average annual rainfall varies from 20 to 30 inches. The soil used for the growth of plants shown in Fig. 6 is a red silt loam soil with

338

JOHN C. BROWN

FIG.4. Rootstocks of PI and HA soybeans affect the development of iron chlorosis and the absorption and translocation of radioiron from Quinlan soil. PI top on PI root (top left), HA top on HA root (bottom left), PI top on HA root (top right), HA top on PI root (bottom right). Compare with Fig. 5 and note the relationship between iron and chlorophyll in the leaves. The first two trifoliate leaves had developed before the graft was effective.

moN CHLOROSIS IN PLANTS

339

a pH of 7.5, contains 2 per cent calcium carbonate, and has montmorillonite as the predominant clay mineral. Tripp is a chestnut soil which has developed from light-colored cal-

FIG.5. Autoradiograph corresponding to Fig. 4; lighter areas show an accumulation of radioiron.

careous sandstones and shales of Tertiary Age. The soils are located on stream terraces and are nearly level or gently undulating with a slight slope downvalley toward the stream. The soil used for the growth of plants shown in Fig. 6 is an alluvial silt loam with a pH of 7.3, contains

340

JOHN C. BROWN

0.5 per cent calcium carbonate, and has illite as the predominant clay mineral. Millville, a chestnut soil located in the intermountain West, developed from alluvial outwash consisting of limestone, quartzite, sandstone, shale, and granitic materials deposited over old lake clays. The high parts of the areas are generally steeply sloping and rolling and are largely treeless, dry-farmed, or range lands. Lower parts are flatter and are in many places rather highly developed irrigated lands. Parent soil materials are

FIG. 6. Plant species differ in their susceptibility to iron chlorosis. PI soybeans (left), WHEATLAND milo (center), and HAWKEYE soybeans (right) grown on Quinlan soil ( bottom row ), Tripp soil ( center row ) , and Millville soil ( top row ) .

extremely varied and in many places extremely calcareous. The Millville soil used for the growth of plants shown in Fig. 6 is a silty clay loam containing 9 per cent calcium carbonate in the surface soil which increased to 35 per cent at a 2.5-foot depth. The pH of the soil is 7.8. It contains an appreciable quantity of amorphous aluminosilicate material in the clay fraction. HA soybeans remained green and grew well on all three soils. PI soybeans, on the other hand, developed chlorosis on all three soils. WHEATLAND milo developed chlorosis on Quinlan and Tripp soils but remained green on the Millville soil. Assuming that a chelating agent may have some of the properties of

341

IRON CHLOROSIS IN PLANTS

a root in its capacity to extract iron from a soil, two different rates of diethylenetriaminepentaacetic acid (DTPA) (97 and 237 p.p.m. ) were added to the Tripp, Quinlan, and Millville soils (Brown et al., 1959a). A crop of soybeans was grown and harvested on each soil within a period of 40 days, after which water extracts were made from each soil. The extracts and dried plant materials were analyzed for Cu, Mn, Fe, P, Mg, Ca, and K with results shown in Table I. The results for P, Ca, and K are included because these elements are often discussed in relation to chlorosis.

FIG.7. Response of WHEATLAND milo to chelating agent and iron chelate treatments added to 3.6 kg. of Quinlan soil: (left to right) no chelate; 1 g. DTPA; 1 g. FeDTPA; 1 g. EDDHA; 1 g. FeEDDHA (10 per cent iron).

If the extract in the presence of the chelating agent is an index of the relative Fe-, Cu-, and Mn-supplying capacities, the soils are seen to differ Mn ratios. The considerably both in the iron levels and in the Fe:Cu Mn Quinlan soil extract has the lowest Fe level and the smallest Fe:Cu ratio. The growth of 'WHEATLAND milo was markedly limited both on the Quinlan and the Tripp Soils. The iron chlorosis developing in milo on the Quinlan soil could be alleviated by addition of iron chelates which served to raise the level of Fe supply and shift the Fe:Cu Mn ratio (Fig. 7), (Brown et al., 1959a). The results suggest that growth of WHEATLAND milo on Quinlan soil is limited chiefly by an improper balance of Fe and Cu and that the iron supply is adequate in the Millville soil even though it is calcareous.

+

+

+

342

JOHN C. BROWN

TABLE I Contents of Elements in the Tops of PI and HA Soybeans and Milo Crown on NaDTPA-Treated Soils and in a Subsequent Aqueous Extract from These Soilsa PI Tops Elements

Millville

Quinlan

HA Tops Tripp

Millville

Quinlan

Tripp

Soils containing 97 p.p.m. Na-DTPA chelate Cu (p.p.m.) Mn (p.p.m.) Fe (p.p.m.) P x 102 (p.p.m.) Ca x 102 (p.p.m.) K x 102 (p.p.m.) Fe:Cu + Mn ratio Yield ( 3 plants, dry wt. g.)

15.7 78 50 34 200 290

9.6 97 67 24 160 300

12.3 102 63 34 176 346

9.3 100 110 24 210 320

14 122 80 25 230 320

9.7 103 102 28 170 350

0.62

0.53

0.56

1.02

0.59

0.90

1.72

1.91

2.18

1.91

1.74

2.44

Soils containing 237 p.p.m. Na-DTPA chelate Cu (p.p.m.) Mn ( p.p.m. ) Fe (p.p.m.) P x 102 (p.p.m.) Ca x 102 ( p.p.m. ) K x 102 ( p.p.m. ) Fe:Cu + Mn ratio Yield ( 3 plants, d'Y wt. g.) a

10.7 83 70 25 176 336

15.3 108 58 30 203 296

9.4 98 67 38 160 346

9.8 87 93 24 180 350

14.3 140 92 26 230 330

9.9 120 85 33 180 400

0.72

0.47

0.60

0.97

0.59

0.66

1.78

1.92

2.34

2.00

2.04

2.75

Brown et al. ( 1959a ) .

These results might be compared to those reported by Ramirez-Silva (1946), who found that iron up to 5 p.p.m. was without toxic effects on pineapple plants when added as the only micronutrient element to a nutrient solution. If manganese and copper were also present in the solution, higher concentrations of iron were required for normal plant growth. 1. Copper Supply Copper may be much more effective as a competitor with iron in the absorption process of WHEATLAND milo than PI and HA soybeans. The copper uptake of WHEATLAND milo from all three soils and both chelate treatments was more than double that of PI and HA soybeans (Table I ) . This suggests a digerential absorption of ions by plant species and invites further investigations as to the mechanism involved.

IRON CHLOROSIS IN PLANTS

343

TABLE I (Continued)

Milo tops Elements

Quinlan

Millville

Soil extract Tripp

Millville

Quinlan

Tripp

Soils containing 97 p.p.m. Na-DTPA chelate Cu (p.p.m.) Mn ( p.p.m. ) Fe (p.p.m.) P x 102 (p.p.m.) Ca x 102 (p.p.m.) K x 102 (p.p.m.) Fe:Cu + Mn ratio Yield ( 3 plants, dry wt. g . )

19.3 155 72 23 63 350

32.7 135 40 22 137 310

22.0 175 40 39 100 340

0.41

0.24

0.21

2.74

0.370

0.730

1.80 0.32 4.10 0.31 4.07 0.82

3.30 0.39 2.10 0.19 5.51 1.33

1.90 0.23 3.00 0.29 4.60 1.70

2.00

0.68

1.31

Soils containing 237 p.p.m. Na-DTPA chelate Cu ( p.p.m.) Mn (p.p.m.) Fe (p.p.m. ) P x 102 (p.p.m. ) Ca x 102 (p.p.m. ) K x 102 (p.p.m. ) Fe:Cu + Mn ratio Yield ( 3 plants, dry wt. g.) b

18.7 125 77 20 70 330

34.3 115 39 26 130 280

21.3 125 41 45 113 340

0.54

0.26

0.28

2.64

0.600

1.000

2.3 0.75 9.70 0.48 6.77 2.57

3.4 0.97 3.40 0.31 5.89 1.40

2.7 1.08 7.40 0.51 5.63 1.77

3.20

0.79

1.94

Plants chlorotic.

2. Phosphorus and Calcium Supply PI soybeans were the only plants to develop iron chlorosis when grown on the Millville soil (Fig. 6 ) . Millville soil contains much more phosphorus and calcium than do the Quinlan and Tripp soils and has a higher pH. When extracted with a chelating agent Millville soil also yields more available iron than the other two soils. The high pH, phosphorous and calcium concentrations appear to be the causative factors of iron chlorosis in PI soybeans. Some plant species are much more susceptible to this type of chlorosis than others. DeKock (1955) refers to the P:Fe ratio as a useful index to the status of iron in plants. Biddulph (1951) found that bean plants grown in a nutrient solution at pH 4.0 with PO, at lo-, M were normal; at pH 7.0, the iron was still absorbed by the roots but precipitated out in the vein

344

JOHN C. BROWN

system. The plants were chlorotic when grown at pH 7 with M PO1 and iron precipitated both on the surface and within the root. Using a split-medium technique (Fig. 8) (Brown et al., 1958) demonstrated an internal inactivation of iron in PI soybeans (Fig. 9 ) . In these experiments, the iron was separated from any precipitating or competitive effects caused by the several nutrient elements in the nutrient solution. It was necessary for the phosphorus to be absorbed and translocated up the root before it could affect the iron. Radioiron taken up from the soil

SPLIT

- MEDIUM

SPLIT -,ROOT

FIG.8. Sketch of the split-medium and split-root culture methods used for differential treatment of soybean root systems.

by PI soybeans was immobilized in the lower parts of the plants by the phosphorus absorbed from the nutrient solution, whereas the variety HA did not develop iron chlorosis and contained radioiron well distributed throughout the plants (Fig. 10). Calcium has an additive effect on the development of chlorosis in that more phosphorus and calcium are absorbed by the plant when the two elements are in solution together. Iron chlorosis did not develop where the two elements were added separately (Fig. 9). Olsen (1958) and Taper and Leach (1957) have both reported that increasing the concentrations of calcium in nutrient solution will decrease the absorption of iron by plants.

IRON CHLOROSIS IN PLANTS

345

It is of special interest that the soybean varieties differ in their capacity to absorb and transport iron in an environment conducive to the inactivation of iron (Table 11).Again, these data show that HA soybeans must have a more efficient mechanism in their roots for the absorption of iron than do PI soybeans. This same conclusion is supported in the work with bicarbonate as a causative factor of iron chlorosis.

FIG.9. PI and HA soybeans respond differently to nutrient solutions containing phosphorus and calcium. PI (left) and HA soybean roots (right) growing from a soil mixture (source of iron) into variable nutrient solutions: (left to right) demineralized water; 10 ml. solution A ( ( 3 x 7 3 , Mg=9, N=71, Kd3p.p.m.); 5 ml. solution A and 5 ml. solution B; 10 ml. solution B (P=19, K=48, N=14 p.p.m.); roots cut off-not allowed to grow into nutrient solution.

3. Presence of Bicarbonate Considerable evidence has been published in recent years in support of the view that the bicarbonate ion is a factor contributing to the development of iron chlorosis. Impetus to bicarbonate investigations was provided by the observations of Harley and Linder (1945).They found

346

JOHN C . BROWN

that the application of irrigation waters relatively high in bicarbonate ion induced iron chlorosis in apple and pear trees. Subsequent irrigation with water low in bicarbonate ion tended to alleviate the iron chlorosis.

FIG. 10. Phosphorus has the capacity to inactivate iron more easily in PI soybeans than in HA soybeans. PI soybean top and roots (upper) and HA soybean top and roots (lower) grown on a split-medium culture of roots growing from a soil mixture (source of radioiron) into a nutrient solution containing 8.5 p.p.m. P. First trifoliate leaf of PI soybeans developed chlorosis (top left), and very little radioiron was translocated into this leaf (top right). In contrast, HA soybeans remained green (bottom left) and absorbed radioiron (bottom right) from the soil.

TABLE I1 Contents of Elements in Roots and Tops of PI and HA Soybeans Using a Split-Medium Technique of Roots in Unfertilized Soil and Various Nutrient Solutionsa Element content of soybeans with various treatments ( p.p.m. ) ~~

Elements

lOAc

5A+5B

P x 102 Mg x 102 Ca x 102 K x 102 Fe

17 66 180 220 70

83 70 210 460 50

PI rootsb

HA tops

PI tops

10Bc

10A

5A+5B

42 72 130 310 70

14 54 160 210 75

89 61 230 520 66

10B

35 67 170 290 90

1OA

5A+5B

HA rootsb 10B

1OA

5A+5B

10B

60

93 120 35 630 50

38 7 7 380 50

15 96 34 300 55

130 150 34 670 65

59 9 15 340 100

12 58 20

18 70 186

54

9 62 20

17

23

52 200

g 59

12 91 35 420

Ratio

K:Ca Ca:Fe P:Fe

1.2 257 24 0.80

Yield

2.2 420 166 1.03

2.4 186 60 0.85

1.3 213 19 0.65

2.3 348 135

1.7 189 38

Dry weight (g.) 1.13 0.70

13 69

b

+

2 cl

QB

g v ; 3

Ln

0.13

0.11

0.11

Data from Brown et al. ( 1 9 5 9 ~ ) . Roots harvested from nutrient portion of split-medium experiment. c Nutrient solutions: 10A = 10 ml. stock solution A added to 7 liters water (Ca=73, Mg=9, N=71, and K=6 p.p.m.); 10B = 10 ml. stock solution B added to 7 liters water (P=19, Kr48, N=14 p.p.m.); 5A 5B = 5 ml. stock solution A and 5 ml. stock solution B added to 7 liters water.

0

g

0.17

0.18

0.08

348

JOHN C. BROWN

During the following decade a number of workers reported reduced Fe accumulation in the shoots of plants when grown in nutrient solutions containing bicarbonate (Baxter and Belcher, 1955; J. W. Brown and Wadleigh, 1955; DeKock, 1955; Gauch and Wadleigh, 1951; Lindsay and Thorne, 1954; Porter and Thorne, 1955). Miller and Thorne (1956) found that in the presence of bicarbonate the respiration rate of root tips of plants known to be susceptible to “lime-induced chlorosis was much reduced, but the rate in root tips of plants not susceptible to chlorosis was but slightly affected. Miller and Evans (1956b) showed that cytochrome oxidase activity of root preparations was consistently less in the presence of HC03 than in the presence of C1, SO4, NO3, HP04, or H2P04 ions. When bicarbonate is added to nutrient solution containing iron salts, most of the iron precipitates and its nutrient status is uncertain. This difficulty can be avoided by supplying the iron to a part of the roots using the split-medium technique (Brown et al., 1959b) (see Fig. 8 ) . In this case, Fe was furnished to PI soybeans through a soil mixture contained in an inverted Erlenmeyer flask. The roots passed through the soil mixture, through an air gap, and into a nutrient solution containins various levels of bicarbonate. The addition of 10 meq. per liter of NaHC03 to the nutrient solution did not induce iron chlorosis or reduce the absorption of Fe from the soil mixture at low phosphorus concentration (Table 111). As the phosphorus concentration was increased in the nutrient solution, the plant absorbed and translocated more phosphorus and calcium to the tops of the plants. And the plants developed iron chlorosis (Table IV) , Sodium bicarbonate affected the solubility of phosphorus. More soluble phosphorus was found in nutrient solutions containing NaHC03 than in nutrient solutions containing equivalent amounts of NaCl or no NaHC03 (Fig. 11). Miller et al. (1960) showed that cytochrome oxidase activity in extracts prepared from PI and HA roots was as great in roots grown in 10 meq. per liter of NaHC03 solution as in roots grown in 10 meq. per liter of NaCl if the split-medium technique was used. All the experiments involving the split-medium technique showed conclusively that bicarbonate had very little direct effect upon inducing iron chlorosis. Iron chlorosis did develop in PI soybeans grown in nutrient solutions (no split medium) containing HC03-, P, Fe, and Ca (pH 7.8). In these experiments iron was not separated from the precipitating effects of phosphorus and pH. The effect of bicarbonate on iron chlorosis is, in part, the interrelationship between HC03-, P, Ca, and Fe in the growth medium. Bicarbonate per se does not appear to be a direct cause of iron chlorosis or

TABLE Ill Effect of the Bicarbonate Ion, in a Split-Medium Experiment Involving Nutrient Solution and Soil, on Yield and Content of Iron, Phosphorus, and Calcium in PI and HA Soybeans with Roots Split between Soil and Nutrient Solutiona.b NaHCO, or

Yield of topsd

NaClC (meq./l. )

NaHCO, (g.)

0 1.5 5 10 20

0.87 0.89 0.93 0.74

Content of element in tops

Fe

NaCl (g.)

NaHC0, (p.p.m.)

0.76 0.93 0.88 0.78

60 60 60 65

0.75

NaCl (p.p.m.) PI soybeans

P x 102 NaHC03 NaCl (p.p.m.) (p.p.m.)

C a x 103 NaHCO, NaCl (p.p.m.) (p.p.m.)

16

16

60 62 57 65 65

17

15

15 15 21

15 15 17

16 15 14 13

n 14 13 13 13

HA soybeans 0.72

0

1.5 5 10 20 a b

c d

0.69 0.77 0.76 0.74

78 70 80 90

18

70 80 75 75

Data from Brown et al. ( 1939b). Chlorosis did not develop in the plants of any of these treatments. In nutrient solution. Dry weight.

! E 5 v

82 0.68 0.65 0.83 0.78

8Z

18 16 16 18

20

15 15 16 16

19 17 16 17

16 16 14 15

5 3

vl

w cn 0

Effect of HC0,-

TABLE IV and P on the Content of P, Ca, and Fe in PI and HA Soybeans with the Split-Medium Techniqueajb PI-54619-5-1

Treatment P (p.p.m. 1 0.00 0.09 0.19 0.38 0.57 1.14

10 meq. NaHCO, Fe

PX10’

80 85 80 75 80c 48c

24 23 27 28 36 49

Cax103

b

Data from Brown et al. (1959b). Values = parts per million.

c

Chlorosis.

a

19 16 20 22 24 24

HAWKEYE

No NaHC0, Fe 65 70 70c 750 48c

No NaHCO,

10 meq. NaHCO,

PX102 CaX103 Fe 120 105 25 24 28 26 110 34 24 110 47 27 110 90 66 28

Px102 CaX103 20 22 27 39 28 46

16 18 23 27 27 30

L(

Fe

Px102

CaXIOJ

-

-

-

115 95

20 24 32 36 52

23 25 30 31 35

105 110 90

0

4

p U

3

351

IRON CHLOROSIS IN PLANTS

an inactivator of cytochrome oxidase at the concentrations used in the above experiments. Development of iron chlorosis was more related to phosphorus concentration in the nutrient solution or to that absorbed by the plant than to the bicarbonate concentration in solution. Greenwald (1945) and Olsen ct al. (1960) have both observed that bicarbonate increases the solubility of phosphorus in solution. Quality of irrigation water, use of green manures, fertilizer, and cultural practices all become factors which can aggravate the iron chlorosis

I

c 0

.c

010(

a

0

v)

t

-a c

0

.-c n 0.I

-

& *4 +

*:sT

4 1.1 +

P ppm! FIG. 11. P and Ca concentrations in solution cultures as influenced by NaHCO, and increasing additions of P. Treatments

problem. For example, a moist soil containing decomposing organic matter (Table V ) provides a condition for maximum HC03- accumulation. If a calcareous soil contains or has been fertilized rather heavily with phosphate, bicarbonate may increase phosphate availability and decrease the available iron supply. IV. Chelating Agents as Iron Carriers

A. SYNTHETIC CHELATING AGENTS Iron chelates are water-soluble forms of iron which find use in absorption studies because the usual precipitation of iron from solution is prevented. They have been used as sources of iron for plants growing in

352

JOHN C. BROWN

nutrient solutions ever since Heck and Bailey (1950) and Jacobson (1951) reported their satisfactory use for this purpose. Leonard and Stewart (1952, 1953b), Wallace et al. (1953), and Holmes and Brown (1955) corrected an iron chlorosis in plants grown in soils by adding iron chelate to the soil. The introduction of metal chelates into agriculture has been helpful not only as a remedy for iron chlorosis, but as an excellent tool to help learn more about the iron absorption process in plants. TABLE V The Effect of a Green Manure Crop on the Development of Iron Chlorosis and the Absorption of 7 Elements by Peach Trees Grown on Two Calcareous Soils in the Fielda Elements in leaves ( p.p.m. ) Soils

cu

Mn

Fe

A1

PX102 CaXlOZ KXlO2

Green manure crop Chlorosis Soil AA Soil BA

18 15

55 37

90 70

128 80

43 32

160 140

390 330

19 24

265 265

305 325

No ereen manure croD

No chlorosis Soil AB Soil BB a

11 12

34 53

235 135

315 153

Data from Brown et al. (1955).

Plant species are known to differ in their capacity to absorb iron from a metal chelate, and the effectiveness of metal chelates on specific soils is known to differ (Wallace et al., 1955; Hill-Cottingham and LloydJones, 1958). There is the suggestion (Brown and Tiffin, 1960) that chelating agents may compete with the root for the iron in a nutrient solution and that roots themselves act much like chelating agents in the absorption of iron. Four synthetic chelating agents have been used rather extensively for experimental purposes in plant nutrition. They are ethylenediaminedihydroxyphenylacetic acid ( EDDHA ), DTPA, ethylenediaminetetraacetic acid ( EDTA), and cyclohexanediaminetetraacetic acid (CDTA). Solutions of FeEDTA, FeDTPA, FeCDTA, and FeEDDHA have different stability constants-loglo K = 24.8, 27.9, 29.3 (Bond and Jones, 1959), and greater than 30.0 (Frost et al., 1958), respectively. To characterize the four chelating agents further, the capacity of EDDHA to chelate Fe has been compared in nutrient solution and in water over the pH range 3 to 8 (Brown et al., 1960). Fe in solution as FeEDDHA was determined by measuring the absorbance of the solutions at 480 mp. It was found

353

IRON CHLOROSIS IN PLANTS

that EDDHA chelated Fe much better in nutrient solution than in water, The capacity of EDDHA to chelate Fe was measured with EDTA, DTPA, and CDTA in the nutrient solution as competing chelating agents. EDDHA had the capacity to form more FeEDDHA with EDTA as the competing chelating agent than with either DTPA or CDTA as competitors. They ranked in the order of their relative stability in competing with EDDHA for Fe: EDTA < DTPA < CDTA (Brown et al., 1960). Keeping Fe and EDDHA concentrations constant and adding EDTA, a

A = CHELATING AGENT * x B = Fe x c = EDDHA x

2.0

I 0 O

w

1.5-

10-5

M

1 0 - 5 ~ 10-5

M

d In 0

EDTA"

.-.

1.0-

.'

lL 0)

,

I y 0.5-

. . '

0

.

2

0.01 A= 0 8.4 c=2 1.51

2

0.5

.

.I6 4 2

, .5

4 2

. I

4 2

.

2 4 2

. . .

..-.-.CDTA" -.-.

4 4

2

,\..

DTPA"

6

4 2

1.

12 4 2

18 4 2

36 4 2

I 0 0.0

A= 0 8.2 c = 2

.I6

2 2

FIG. 12. The capacity of EDDHA to form FeEDDHA was affected by the concentration of the competing chelating agent in solution at: (top) 4 x 10-5 M Fe and (bottom) 2 x 10-5 M Fe. Competition for Fe did not occur until the molar concentration of chelating agents exceeded the molar concentration of Fe.

CDTA, and DTPA at increasing concentrations, EDDHA competed successfully with EDTA, DTPA, and CDTA for Fe if chelating agents and Fe were in equal molar concentrations (Fig. 12). But with a further increase in the concentration of EDTA, DTPA, and CDTA, these chelating agents competed with EDDHA for Fe in the order: EDTA < DTPA < CDTA (Fig. 12). A higher concentration of EDTA, DTPA, or CDTA was required to be competitive if the Fe concentration in the nutrient solution was increased. Thus, the chelating capacity of EDDHA is dependent upon both the concentration of the Fe and the concentration of the competitive chelating agent in solution.

354

JOHN C. BROWN

B. APPARENTCHELATINC ACTIVITYOF ROOTS In vitro experiments, using synthetic chelating agents, demonstrated competition between chelating agents for iron in accordance with their relative stabilities. In vivo experiments have demonstrated that roots have chelating properties in their capacity to compete for the iron in a growth medium (Brown et aZ., 1960). Similar to the chelating agent EDDHA, roots from different plant species differ in their capacity to

FIG.13. Iron concentration must exceed EDDHA concentration in nutrient solution in order to prevent iron chlorosis in milo plants. Milo plants (left) in nutrient solution which contained 1 x 10-5 M Fe; (right) 6 x 10-5 M Fe. EDDHA concentration was increased for each treatment (left to right): 0.16, 1, 2, 6, and 18 x 10-5 M .

absorb iron from metal chelates. WHEATLAND milo was unable to absorb and utilize Fe from FeEDDHA unless the Fe concentration greatly exA4 Fe and ceeded the EDDHA concentration (Fig. 13). At 2 X 0.16 x 10W5 M EDDHA, WHEATLAND milo absorbed 120 pg. Fe. At the same Fe concentration ( 2 x l o w 5Fe), but with the EDDHA concentration increased to 1 X loF5 M, WHEATLAND milo absorbed only 13 pg. Fe, PI soybeans 123 pg. Fe, and HA soybeans 128 pg. Fe. Increasing the Fe concentration from 2 x lov5M Fe to 6 x 1 O - j M Fe did not appreciably

355

IRON CHLOROSIS IN PLANTS

affect the total iron absorbed by PI and HA soybeans, but it did increase the iron absorbed by milo tops and roots (Brown et al., 1961b). Out of eleven plants grown in a complete nutrient solution containing M Fe and 0.16, 1, 2, 6, and 18 X M DTPA, total weight 1X was reduced sharply in okra, wheat, rye, corn, PI soybeans, and red kidney beans when the molar concentration of DTPA (2 x M) exceeded that of Fe (Table VI ) . Yields of HA soybeans and yellow, blue, and white lupine did not decrease markedly until the DTPA concentration had been increased to 6 x M. TABLE VI Effect of Concentration of Chelating Agent (DTPA) in Nutrient Solution Containing 1 x 10-5 M Fe on the Yield of Tops of Eleven Different Plantsa Yield, dry weight (grams) Treatments, DTPA x 10-6 M Plants Okra Wheat Rye Corn Milo Yellow lupine Blue lupine White lupine PI soybeans HA soybeans Red kidney beans a

0.16

1

2

6

18

4.95 1.09 2.48 10.54 0.23 2.93 4.02 8.88 1.04 1.06 3.02

5.11 1.03 1.05 9.05 0.26 3.01 3.81 6.85 0.95 1.04 4.46

2.14 0.29 0.55 1.13 0.17 2.05 3.61 5.45 0.44 0.87 2.13

1.29 0.24 0.41 0.69 0.15 2.88 2.62 3.46 0.46 0.67 1.40

0.74 0.21 0.20 0.56 0.13 0.66 0.81 1.73 0.47 0.60 0.88

Data from Brown et al. (1961b).

Corn, okra, and wheat developed iron chlorosis when the molar concentration of DTPA exceeded the molar concentration of Fe (Fig. 14). Red kidney beans did not develop iron chlorosis until the DTPA concentration was 18 X M Fe, but internode elongation was stopped at 2 x M DTPA (Fig. 15). In the latter treatment, DTPA concentration exceeded the Fe concentration in the nutrient solution which was reflected in the nutrient element content of the plants (Table VII). M DTPA, Fe concentration was not decreased in the top of At 2 X the plant, but Mn, Ca, Mg, and P showed a marked increase in concentration. The altered nutrition of the plant caused the failure of the internodes to elongate, possibly owing to a change in the metabolism of the plant. With a further increase in the concentration of the chelating agent, the amount of Fe absorbed by red kidney beans decreased. It is important to remember how nutrient element balance can be changed merely by limiting the supply of Fe or by increasing the con-

356

JOHN C. BROWN

centration of a chelating agent. Also of importance is the effect of stability of the iron chelate upon the absorption of Fe by plant roots. WHEATLAND milo, PI, and HA plants each give a different growth response in nutrient solution containing 1 x M Fe and 0.16, 1, 2, 6, and 18 X &I DTPA (Fig. 16). A probable explanation is that in some way the stability of the metal chelate is changed at the root by the soybeans. This could be done at the root by changing F e + + + + F e + + .

FIG.14. When the molar concentration of DTPA exceeded the molar concentration of Fe, DTPA competed with the roots of corn (left), okra (bottom right), and wheat (top right) for Fe. The nutrient solution contained 1 x 10-5 M Fe and (left to right) 0.16, 1, 2, 6, and 18 x 10-5 M DTPA.

Bond and Jones (1959) have shown that the stability constants of some ferrochelates are less than the ferrichelates. Early investigators (Stewart and Leonard, 1952; Heck and Bailey, 1950; Hutner et al., 1950; Schatz and Hutner, 1949) have suggested that the metal chelates deliver iron to the roots or absorbing surfaces, but that the chelating agent is not absorbed. Later research (Leonard and Stewart, 1953a; Stewart and Leonard, 1954; Wallace and North, 1953) led to the view that both the iron and the chelating agent were absorbed. Some emphasis has been placed on equivalent uptake of metal chelate components by plants (Wallace et al., 1955) but later reports (Wallace

IRON CHLOROSIS IN PLANTS

357

et d.,1957) suggest a nonequivalent uptake. Tiffin and Brown (1959) concluded that Fe and EDDHA are not absorbed by sunflower plants in equivalent quantities, but that iron is released to the roots and most of the EDDHA remains in the nutrient solution. They showed the presence of considerable iron-free EDDHA in a nutrient solution following the

FIG.15. Internodes of red kidney beans failed to elongate when the molar concentration of DTPA exceeded the molar concentration of iron. The nutrient solution contained increasing concentrations of DTPA: (left to right) 0.16, 1, 2, 6, and 18 x 10-5 M . Plants (top row) received 1 x 10-5 A 1 Fe and plants (bottom row) received 2 x 10-5 M Fe in each nutrient solution.

absorption of Fe from FeEDDHA by sunflower roots. After 42.5 hours the sunflower plants had absorbed 1.8 mg. of Fe from a liter of nutrient solution. If this quantity of iron had been accompanied by equivalent chelating agent, the nutrient solution should have lost 11 mg. of EDDHA. It lost only 1.6 mg. of EDDHA, indicating a differential absorption of

TABLE VII Element Concentration in Red Kidney Beans as Affected by Concentration of Chelating Agent in Nutrient Solution Containing 1 x 10-5 M Fea Element concentration ( p.p.m. )

Treatments, chelating agent

x10-5~

DTPA DTPA DTPA DTPA DTPA 0

b c

0.16 1.00 2.00 6.00 18.00

L(

In nutrient solutionb

In plant top CaX104 MgxlO4 K x l W

PxlO4

B

Fe

Mu

Cu

Ca

Fe

Mn

CuXlO

0.27 0.26 0.22 0.22 0.42

0.21 0.22 0.36 0.36 0.52

31

88 78 83 43 23

22 22 70 39 33

4.3 3.1 2.0 2.1 3.8

1.1 2.3 2.0 4.3 13.5

0.02 0.18 0.11 0.32 0.55

NDC ND 0.01 0.03 0.07

0.01 0.02 0.01 0.06 0.08

1.5

1.7 3.2 2.8 3.1

1.4 1.5 1.6 1.5 3.4

Data from Brown et al. (1960). After containing red kidney beans 10 days. Not detectable spectrochemically.

35 46 51 66

H ? O1

g

3

IRON CHLOROSIS IN PLANTS

359

Fe and EDDHA by the root (Tiffin et al., 1960a). Brown and Tiffin (1960) came to the same conclusion using soybeans as the test plant. They suggested that susceptibility to chlorosis in some plant species may be related to a metabolic process, such as reduction, involved in the separation of iron from the metal chelate.

FIG. 16. Plant species differ in their susceptibility to iron chlorosis caused by increasing chelate concentration in nutrient solution. WHEATLAND milo (bottom), PI soybean (center), and HA soybean (top) grown on a nutrient solution containing 1 x 10-5 M Fe and (left to right) 0.16, 1, 2, 6, and 18 x 10-5 M DTPA.

C. A~SORPTION OF CHELATING AGENTS In most of the experiments cited, there was acceptable evidence of absorption of some of the chelating agent by the plant. It is of interest to know what effect this small amount of absorbed chelating agent might have on the mobility of iron within the plant. Tiffin et al. (1960b) in-

360

JOHN C. BROWN

vestigated this problem using the split-root and split-medium techniques explained in Fig. 8. Absorption of chelating agent (45 p.p.m.) by roots, under the conditions of these experiments (Fig. 17), did not correct iron chlorosis. Any Fe-activating effect the chelating agent might have had within the plant was effectively counteracted by phosphorus. Plant nutrients also affected the transfer of radioiron applied to the primary leaves of soybeans. Split-medium roots in this case were growing from Millville soil into a complete nutrient solution, with and without chelating agent, The presence of the chelating agent (30 p.p.m.) did not have much effect on the movement of iron within the plant. Iron was moved into the leaves of the plants, the roots of which were in water or chelating agent, and, in contrast, iron was concentrated in the stem of plants with the roots in nutrient solution with and without chelating agent. Tiffin at al. (1960b) concluded that very little emphasis can be placed on absorbed chelating agent as an effective activator of iron within soybean plants. This conclusion is not in agreement with the findings of Weinstein et al. (1954; Weinstein and Robbins, 1955), who used split-root experiments to show that 5.0 p.p.m. of NaEDTA corrected iron chlorosis in sunflower plants. V. Chelating and Other Mechanisms in Plants

A. PLANT

SPECIES OR VARIETY

The HAWKEYE soybean (HA) has been compared with 16 plant species and has always stood out as one of the plants most resistant to iron chlorosis. This soybean has grown well without chlorosis symptoms on calcareous Quinlan, Tripp, and Millville soils and has tolerated P, Cu, EDTA, DTPA, CDTA, or EDDHA concentrations in nutrient solutions which have induced chlorosis in other plant species. HA soybeans have always had the capacity to absorb, translocate, and utilize the iron from such solutions. Thus, it was concluded that HA soybean roots must have a metabolic process which affects the solubility or availability of iron at the roots. It was reasoned that this could be a chelation type reaction or a reducing action at the root, or possibly both.

FIG.17. In PI soybeans grown with split roots, phosphorus concentration affected the development of iron chlorosis more than chelating agent: (top) 45 p.p.m. EDDHA, (center) 45 p.p.m. EDTA, and (bottom) no chelating agent. The tube on the right in each pair of tubes contained 10 p.p.m. Ca and the chelating agent. The tube on the left contained a complete nutrient solution with 2 p.p.m. Fe and (left to right) increasing P concentration 1, 2, 3, 4, 5, and 0 p.p.m. respectively,

IRON CHLOROSIS IN PLANTS

361

362

JOHN C. B R O W

B. REDUCTIVE CAPACITY AT THE ROOT Reductive capacity at the root could be a factor related to Fe absorption and may be analogous to the observations which have been made concerning Fe absorption in animals. Loewus and Fineberg (1957) suggest that the storage and mobilization of Fe in animals are controlled by the oxidation and reduction of iron. In ferritin, the normal mammalian iron storage protein, the iron is present in the ferric state. Granick (1958) found that plasma iron in transport to storage depots also occurs in the ferric state. Its storage presumably requires an intermediate reduction to separate the Fe from its carrier compound. As with the F e + + + in the animal, Fe++ + at the root may be a rather immobile or nonexchangeable form. The ferric chelate, FeEDDHA, has a stability constant greater than 30 (Frost et al., 1958), whereas the ferrous chelate has an apparent stability constant of 15 (Bond and Jones, 1959). A reduction process at the root could make iron more available from FeEDDHA by reducing iron from the ferric to ferrous form. Reductive capacity at the root was determined for HA and PI soybeans using a ferricyanide-ferrichloride solution as a source of iron (Brown et al., 1961a). Ferricyanide reduction was determined by spectrophotometric measurements of the reaction mixture at 400 mp (Avron and Jagendorf, 1959). Prussian blue formation was also observed. Iron-deficient HA roots did reduce Fe+ + + to Fe+ + and in doing so absorbed much more Fe from the ferricyanide-ferrichloride solution than iron-sufficient HA roots or iron-deficient PI roots (Fig. 18). A ferricferrocyanide “Prussian blue” formed in the reaction mixture when the iron was reduced. Prussian blue was visually present on the fine root hairs of HA soybeans after 6 hours, and after 8 hours the solution turned from a yellow color to a blue color. Greater reduction occurred if the tops of the plants were not removed (Brown et al., 1961a). There is an apparent relationship between the reductive capacity of HA and PI roots, their capacity to absorb iron, and the susceptibility of these plants to iron chlorosis. Similar differences in reductive capacity may explain the results of Bell et al. (1958), who found that corn plants of genotypes ysl and *ysl responded alike when grown in nutrient solutions with a ferrous source of iron, but responded differently to ferric iron. Kliman (1957) found considerable ferrous iron in the epidermis of the roots and in the vascular bundles of pea and corn stems and postulated that ferric iron is reduced to ferrous iron by the roots before it is absorbed and transported into the plant. Dawson (1950) found that copper in “resting” (nonfunctioning ) ascorbic acid oxidase does not undergo exchange with

363

IRON CHLOROSIS IN PLANTS

C d 4 . An exchange does take place between the copper of the enzyme and the C U in~ the ~ presence of ascorbic acid and oxygen. Dawson postulated that exchange occurs only when the copper of the enzyme appears in the monovalent state, which has a higher dissociation constant than the divalent state.

a

L

0

pH * 6.0 at s t a r t

0 t

-

.

c

0

A”,

c

z E 0

/

\

-

I

I

u)

L

e ti-

.’

\

r

0

\

u

P c

-PI-

No F e - - H A - No Fe

V P

0

PH 4.0 8

ot start

30

Time interval taps removed -hr,

FIG. 18. Ferricyanide-ferrichloride reduction, measured as an optical density (0.0.) change at 400 mw by PI and HA soybean roots at starting pH of 6.0 and 4.0. Note greater reduction by HA roots coincident with the absorption and translocation of radioiron to the tops of the plants from the ferrichloride solution,

C. KIND AND CONCENTRATION OF CHELATINC COMPOUNDS In addition to the iron supply as affected by the reductive capacity of the roots, the type and concentration of the chelating compounds are important factors which may affect the absorption of iron by roots. Just

364

JOHN C. BROWN

as concentration of a competing metal ion may affect the available or active iron in a plant, plant species may have the capacity to alter the activity of a metal ion by increasing or decreasing the concentration of a specific chelating agent inside the plant or in the root exudate. Neilands ( 1957) has isolated a crystalline iron-containing substance from Ustilago sphaerogena with a stability constant much greater than that of FeEDTA. U . sphaerogena did not produce this substance when grown in a culture medium containing 1 p.p.m. Fe. Jacobson and Oertli (1956) found that iron-deficient plants absorbed abnormally large quantities of iron when placed in a nutrient solution containing iron. Partially chlorotic HA SOYbeans absorbed much more iron than nonchlorotic, green plants and produced a root exudate that fluoresced under ultraviolet light. All these data indicate an adaptation to an iron stress with resultant greater capacity of plants or fungi to absorb Fe. Intermediate metabolites with chelating properties may be produced in plants that are deficient in iron. Iljin (1951, 1952) was one of the first to study the intermediate metabolites of chlorotic and nonchlorotic plants. He found that chlorotic plants usually contained more citric and malic acids than normal green plants. Rhoads and Wallace (1960) and DeKock and Morrison (1958) have confirmed these findings. Some of the amino acids have been reported (Brown et al., 1958; Holley and Cain, 1954) to be in greater concentration in chlorotic than in nonchlorotic plants. Nicholas and Goodman (1958) report that iron deficiency in Neurospora reduced catalase, peroxidase, cytochrome c reductase, and oxidase activity in that order. Catalase was the first enzyme affected. Intermediate metabolites may affect the pH of the tissue fluids. Ingalls and Shive (1931) found that the soluble (filterable) iron content in tissue fluids fluctuates and in most cases is directly proportional to the H-ion concentration. Potato tissue fluids have a higher pH than those of corn and have less soluble or filterable iron per gram of dry tissue than corn (Rogers and Shive, 1932). Yet corn shows a greater tendency to develop iron chlorosis than potatoes when grown in nutrient solution. Rogers and Shive (1932) concluded that there is an inherent difference between potato plants and corn plants with respect to the transfer of iron from channels of translocation to places of utilization. They suggested as an explanation that in the plant certain organic iron compounds are formed which resist the precipitation action of alkaline media and that such iron is retained in soluble form. Precipitated iron may serve as a reserve iron supply. Thus, the maintenance of a supply of active or functional iron may require a mechanism for making precipitated iron soluble and keeping it in the soluble form. Two factors seem to be of primary importance in this regard: ( 1 ) re-

IRON CHLOROSIS I N PLANTS

365

ductive capacity at the root and within the plant, reducing F e + + + to Fe++, thus making iron more available to metabolic systems in the plant; ( 2 ) the kind and concentration of iron-chelating compounds at the root and within the plant. Both phases of this problem may be of equal importance to an understanding of how iron is absorbed by plants, and no single explanation of iron chlorosis in plants seems apparent. It is not sufficient to conclude that the cause of iron chlorosis is related solely to a high phophorus:iron ratio in plants ( DeKock, 1958), or to imply that such a ratio expresses satisfactorily the probability of a sound chemical basis from which to explain iron chlorosis ( DeKock, 1958). Phosphorus concentration may be the causative factor of iron chlorosis in one plant species, under particular conditions, but of equal significance is the fact that a plant of another species may continue to absorb iron under these same conditions. The P:Fe ratio between two plant species may differ not because of the high phosphorus in the growth medium, but because the plant species differ in their capacity to absorb iron. Total phosphorus absorbed by the two plant species may be quite similar. VI. The Future of the Iron Chlorosis Problem

Wallace and Lunt (1960) state that perhaps the most practical solution to "lime-induced" chlorosis is learning to live with it. Knowing that most naturally calcareous soils are inherently soils which will induce iron chlorosis in plants, there is not much alternative but to learn to live with it. This means that (1) the causative factors of iron chlorosis must be recognized and alleviated-not aggravated, and ( 2 ) plant species must be selected and developed which are resistant to iron chlorosis. A causative factor that induces iron chlorosis may not easily be removed once it is established in a soil. For example, Smith and Specht (1953) suggest that the main cause of iron chlorosis in Florida is soil accumulation of copper applied over several decades in sprays and fertilizers. Some of these soils now contain approximately 700 pounds of copper and manganese per acre. Similar virgin soils contain 5 to 10 pounds of copper and 30 to 40 pounds of manganese per acre. Today, iron chelates are being used in Florida to correct a copper-induced iron chlorosis in plants. This involves an additional expense in the management of these soils. Nutrient element balance as discussed by Cook and Davis (1957) and by DeKock (1958) should be of primary concern to all agronomists interested in iron chlorosis. Fertilizer is being applied to farm soils in ever increasing quantities. Changes in technology and competition for

366

JOHN C. BROWN

business have resulted in greatly different formulations. Each plant requires its own optimum balance of nutrients in the leaves for maximum growth, and any element supplied in too small or too great amount might eventually cause retardation in growth. As fertilizer applications are increased in expectation of high yields, the chances of imbalance and other injurious effects increase. For example, the continuous application of high phosphate fertilizer to calcareous soil is a practice that will aggravate the iron chlorosis problem. The selection of plant species that are not susceptible to iron chlorosis and the use of iron chelates have been two of the most promising ways of dealing with iron chlorosis. Other practices used where iron chlorosis has developed include adding amendments that make the soil iron more available, spraying the plants with solutions of iron salts, injecting iron salts into the trunks or limbs of trees, or grafting and budding susceptible varieties on chlorosis-resistant rootstocks ( Wann, 1941) . Improper management practices involving irrigation, fertilizer additions, and green manure crops are very important factors which can aggravate the chlorosis problem. Progress in the future depends on our ability to recognize and alleviate the causative factors of iron chlorosis in soils and on our ability to develop new crop varieties that are resistant to iron chlorosis. Iron chlorosis can become a major problem in agriculture if we fail to recognize it as an inherent problem in most calcareous soils. REFERENCES Allyn, W. P. 1927. Proc. Indiana Acad. Sci. 37, 405-409. Aston, B. C. 1934. New Zealand J. Agr. 49, 31-36. Avron, M., and Jagendorf, A. T. 1959. J. Biol. Chem. 234, 967-972. Baxter, P., and Belcher, R. 1955. J . Australian Inst. Agr. Sci. 21, 32-34. Bell, W. D., Bogarad, L., and McGrath, W. J. 1958. Botan. Gaz. 120, 36-39. Bennett, J. P. 1931. California Agr. Expt. Sta. Circ. 321. Bennett, J. P. 1945. Soil Sci. 60, 91-105. Biddulph, 0. 1948. “Proceedings of the Auburn Conference on Use of Radioisotopes in Agriculture,” December 18-20, 1947, pp. 90-102. Alabama Polytechnic Institute, Auburn, Alabama. Biddulph, 0. 1951. In “Mineral Nutrition in Plants” (T. Truog, ed.), pp. 261-278. Univ. Wisconsin Press, Madison, Wisconsin. Blodgett, E. C. 1946. Idaho Univ., Agr. Expt. Stu. Circ. 110. Bond, J., and Jones, T. I. 1959. Trans. Furaday SOC. 66, 1310-1318. Brown, J. C. 1956. Ann. Rev. Plant Physiol. 7, 171-190. Brown, J. C., and Holmes, R. S. 1955a. Soil Conserv. Mag. 20, 259-262. Brown, J. C., and Holmes, R. S. 195513. Plant Physbl. SO, 451-457. Brown, J. C., and Holmes, R. S. 1956. Soil Sci. 82, 507-519. Brown, J. C., and Tiffin, L. 0. 1960. Soil Sci. 89, 8-15. Brown, J. C., Holmes, R. S., and Specht, A. W. 1955. Plant Physiol. SO, 457-462.

IRON CHLOROSIS IN PLANTS

367

Brown, J. C., Holmes, R. S., and Tiffin, L. 0. 1958. Soil Sci. 86, 75-82. Brown, J. C., Holmes, R. S., and Tiffin, L. 0. 1959a. Soil Sci. SOC. Am. Proc. 23, 231-234. Brown, J. C., Lunt, 0. R., Holmes, R. S., and Tiffin, L. 0. 1959b. Soil Sci. 88, 260266. Brown, J. C., Tiffin, L. O., Holmes, R. S., Specht, A. W., and Resnicky, J. W. 1959c. Soil Sci. 87, 89-94. Brown, J. C., Tiffin, L. O., and Holmes, R. S. 1960. Plant Physwl. 36, 878-886. Brown, 1. C., Holmes, R. S., and Tiffin, L. 0. 1961a. Soil Sci. 91. 127-132. Brown, J . C., Tiffin, L. O., Specht, A: W., and Resnicky, J. W.’ 1961b. Agron. J . 63. 81-90. Brown, J. W., and Wadleigh, C. H. 1955. Botan. Gaz. 116, 201-209. Buslova, E. D. 1948. Chem. Abstr. 42, 4242b. Cook, R. L., and Davis, J. F. 1957. Adoances in Agron. 9, 205-216. Dawson, C. R. 1950. In “Copper Metabolism” (W. 0. McElroy and B. Glass, eds.), pp. 18-47. The John Hopkins Press, Baltimore, Maryland. DeKock, P. C. 1955. Soil Sci. 79, 167-175. DeKock, P. C. 1958. Agr. Progr. 33, 88-95. DeKock, P. C., and Morrison, R. I. 1958. Bwchem. J . 70, 272-277. Evans, H. J. 1959. Duke Unio. Bull. 16, 89-110. Franco, C. M., and Loomis, W. E. 1947. Plant Physiol. 22, 627-634. Frost, A. E., Freedman, H. H., Westerback, S. J., and Martell, A. E. 1958. J . Am. Chem. SOC.80, 530-536. Gauch, H. C., and Wadleigh, C. H. 1951. Rotan. Gaz. 112, 259-271. Granick, S. 1958. In “Trace Elements” (C. A. Lamb, 0. G. Bentley, and J. M. Beattie, eds.), pp. 337-363. Academic Press, New York. Greenwald, I. 1945. J. Biol. Chem. 161, 697-704. Gris, E. 1843. Compt. rend. w a d . sci. 17, 679. Harley, C. P., and Lindner, R. C. 1945. Proc. Am. Soc. Hort. Sci. 46, 35-44. Heck, W. W., and Bailey, L. F. 1950. Plant Physwl. 26, 573. Hill-Cottingham, D. G., and Lloyd-Jones, C. P. 1958. Plant and Soil 9, 189-201. Holley, R. W., and Cain, J. C. 1954. Science 121, 172-173. Holmes, R. S., and Brown, J. C. 1955. Soil Sci. 80, 167-179. Hopkins, E. F., Pagan, V., and Selva, F. J. R. 1944. J. Agr. Unio. Puerto Rico 28, 43-101. Hutner, S. H., Provasoli, L., Schatz, A., and Hachins, C. P. 1950. Proc. Am. Phil. SOC. 94, 152-170. Iljin, W. S. 1945. Chem. Abstr. 39, 4113. Iljin, W. S. 1951. Plant and Soil 3, 239-256. Iljin, W. S. 1952. Plant and Soil 4, 11-27. Ingalls, R. A,, and Shive, J. W. 1931. Plant Physiol. 6, 103-125. Jacobson, L. 1951. Plant Physiol. 26, 411-413. Jacobson, L., and Oertli, J. J. 1956. PZunt Physiol. 31, 199-204. Johnson, M. 0. 1916. Huwaii Agr. Expt. Sta. Press Bull. 61, 11. Johnson, M. 0. 1924. Hawaii Agr. Expt. Sta. Bull. 52, 38. Juritz, C. F. 1912. Agr. J . Unwn S . Africa 4, 854-865. Keilin, D., and Mann, T. 1944. Proc. Nutrition S O C . (Engl. and Scot.) 1, 189-192. Kliman, S. 1957. Soil Sci. SOC.Am. Proc. 2, 385-392. Kuykendall, J. R. 1956. Soil Sci. 84, 24-28. Leonard, C. D., and Stewart, I. 1952. Proc. Florida State Hort. SOC.66, 20-24.

368

JOHN C. BROWN

Leonard, C. D., and Stewart, I. 1953a. Proc. Am. SOC. Hort. Sci. 62, 103. Leonard, C. D., and Stewart, I. 1953b. Proc. Fbrida State Hort. SOC. 66, 49-54. Lindner, R. C., and Harley, C. P. 1944. Plant Physiol. 19, 420-429. Lindsay, W. L., and Thome, D. W. 1954. Soil Sci. 77, 271-279. Loewus, M. W., and Fineberg, R. A. 1957. Biochem. et Biophys. Acta 26, 441-443. McGeorge, W. T. 1948. Arizona Agr. Expt. Sta. Tech. Bull. 116. Miller, G. W., and Evans, H. J. 1956a. Nature 178, 974-976. Miller, G. W., and Evans, H. J. 1956b. Plant Physiol. 91, 357-364. Miller, G.W., and Thome, D. W. 1958. Plant Physiol. 91, 151-155. Miller, G.W., Brown, J. C., and Holmes, R. S. 1960. Plant Physiol. 96, 619-625. Nicholas, D. J. D., and Goodman, T. 1958. J. Exptl. Botany 9, 97-108. Nielands, J. B. 1957. Bacteriol. Reus. 21, 101-111. Noack, K., and Liebich, H. 1941. Naturwiss 29, 302. Olsen, C. 1935. Compt. rend. trau. lab. Carlsberg, Skr. chim. 21, 15-52. Olsen, C. 1958. Compt. rend. trau. lab. Carlsberg, 91, 41-59. Olsen, S. R., Watanabe, F. S., and Cole, C. V. 1960. Soil Sci. 89, 288-291. Pettinger, N. A., Henderson, A. G., and Wingard, S. A. 1932. Phytopathology 22, 33-51. Porter, L. K., and Thorne, D. W. 1955. Soil Sci. 79, 373-382. Ramirez-Silva, F. J. 1946. J. Agr. Uniu. Puerto Rico SO, 197-250. Reuther, W., and Crawford, C. L. 1948. Soil Sci. 62, 477. Rhoads, W. A., and Wallace, A. 1960. Soil Sci. 89, 248-256. Rogers, C. H., and Shive, J. W. 1932. Plant Physiol. 7, 227-252. Sachs, J. 1860. Landwirtsch. Vers.-Sta. 2, 22. Schander, H. 1945. Chem. Abstr. 99, 4649. Schatz, A,, and Hutner, S. H. 1949. Abstracts of Papers, Am. SOC. Bacteriologists, p. 34. Smith, P. F., and Specht, A. W. 1953. Proc. Florida State Hod. SOC. 65, 101-108. Smythe, C. V., and Schmidt, C. L. A. 1930. J. Biol. Chem. 88, 241-268. Spiers, M. 1954. Southern Coiip. Ser. Bull. 96, 155-175. Stewart, I., and Leonard, C. D. 1952. Science 116, 564-566. Stewart, I., and Leonard, C. D. 1954. In “Mineral Nutrition of Fruit Crops” (N. F. Childers, ed. ), pp. 775-809. Horticulture Publications, New Brunswick, New Jersey. Taper, C. D., and Leach, W. 1957. Can. J. Botany 96, 773-777. Thome, D. W., and Wallace, A. 1944. Soil Sci. 67, 299-312. Tiffin, L. O., and Brown, J. C. 1959. Science 190, 274-275. Tiffin, L. O., Brown, J. C., and Krauss, R. W. 1960a. Plant Physiol. 36, 362-367. Tiffin, L. O., Brown, J. C., and Holmes, R. S. 1960b. Soil Sci. SOC. Am. Proc. 24, 120-123. Wadleigh, C. H., Robbins, W. R., and Beckenbach, J. R. 1938. Soil Sci. 49, 153175. Wallace, A., and Lunt, 0. R. 1960. Proc. Am. SOC. Hort. Sci. 75, 819-841. Wallace, A., and North, C. P. 1953. California Agr. 7, 8. Wallace, A,, North, C. P., Mueller, R. T., and Hemaidan, N. 1953. Proc. Am. SOC. Hort. Sci. 62, 118-118. Wallace, A., North, C. P., Mueller, R. T., Shannon, L. M., and Hemaidan, N. 1955. Proc. Am. SOC. Hort. Sci. 65, 9. Wallace, A., Shannon, L. M., Lunt, 0. R., and Impey, R. L. 1957. Soil Sci. 84, 27. Wallace, T. 1928. J. Pomol. Hort. Sci. 7 , 172-183.

IRON CHLOROSIS IN PLANTS

369

Walsh, T., and Clarke, E. J. 1948. Chem. Abstr. 42, 5953. Wann, F. B. 1941. Utah Agr. Expt. Stu. Bull. 299, 1-27. Weinstein, L. H., and Robbins, W. R. 1955. Plant Physiol. 30, 27-32. Weinstein, L. H., Purvis, E. R., Meiss, H. N., and Uhler, B. L. 1954. J. Agr. Food Chem. 2, 421-424. Weiss, M. G . 1943. Genetics 28, 253-268. Willis, L. G., and Carrero, J. 0. 1921. Puerto Rico Univ. Agr. Expt. Stu. Rept. 8, 9. Willis, L. G., and Piland, J. R. 1936. J. Agr. Research 62, 467-476.

This Page Intentionally Left Blank

Author Index Numbers in parentheses are reference numbers and indicate that an author's work is referred to although his name is not cited in the text. Numbers in italic show the page on which the complete reference is listed.

A Aaltonen, N. T., 36, 53 Aasheim, T. S., 146, 176, 194 Abbie, A. A., 250, 266 Abreu, J. P., 115, 117 Ackermnn, F. G., 171, 19d Adam, N. K., 314, 323 Adams, A. B., 80, 117 Adams, R. S., Jr., 200, 202, 205, 206, 207, 208, 209, 211, 213, 215 Adamson, A. W., 289, 307, 323 Afanasiev, J. N., 13, 23, 31, 53 Aitchberger, R., 261, 266 Aitken, Y., 61, 64, 67, 68, 78, 79, 88, 104, 107, 117 Albrecht, W. A., 158, 159, 160, 194 Alderfer, R. B., 149, 194 Aldrich, D. G., 310, 321, 324 Alexander, G., 106, 117 Alexander, L. T., 315, 316, 317, 320, 325 Allden, W. G., 98, 117, 119 Allen, T. C., Jr., 220, 221, 226, 228, 233, 234, 245 Allyn, W. P., 333, 366 Altemuller, H.-J., 38, 53 Andersland, 0. B., 313, 325 Anderson, A. B. C., 319, 324 Anderson, A. J., 58, 74, 75, 76, 82, 117, 120, 121 Anderson, D., 138, 194 Anderson, D. M., 290, 291, 292, 293, 294, 295, 296, 300, 320, 321, 323, 325 Anderson, D. T., 188, 189, 194 Anderson, M. S., 35, 37, 53, 198, 215 Andrade, E. N. da C., 295, 296, 323, 324 Andrew, W. D., 95, 117 Andrewartha, H. C., 104, 117

Appert, J., 203, 215 Arens, P. L., 289, 291, 292, 324 Army, T. J., 130, 145, 147, 149, 158, 177, 194, 196 Amy, D. C., 224, 230, 231, 237, 239, 240, 243, 245, 246, 247 Arthur, J. M., 89, 117 Aston, B. C., 333, 366 Audas, J. w** 58, Auron, M., 362, 366 Avery, B. W., 28, 36, 44, 53 Aylmore, L. A. G., 297, 324

B Babcock, K. L., 313, 324 BahI, K. N., 251, 266 Bailey, L. F., 352, 356, 367 Baldwin, I. L., 199, 200, 215 Rallard, J. H., 102, 117 Ballard, L. A. T., 67, 117, 120 Baluev, V. K., 262, 266 Bark, L. D., 130, 152, 153, 195 Barker, H. A., 198. 200, 203, 216 Barley, K. P., 253, 254, 256, 257, 258, 266 Barnes, 0. K., 138, 194 Banie, N., 96, 99, 121 Barrus, M. F., 220, 245 Barshad, I., 318, 324 Bartels, L. C., 87, 117 Bartholomew, W. V., 161, 196 Bath, J. C.,99, 100, 117, 121 Baver, L. D., 260, 266 Baxter, P., 348, 366 Beale, 0. W., 131, 144, 154, 165, 190, 194, 196 Beavers, A. H., 48, 54 Beck, A. B., 71, 72, 117 Beckenbach, J. R., 187,194,332, 368

371

372

AUTHOR INDEX

Beerstecher, E., Jr., 200, 201, 202, 203, 215, 215 Belcher, R., 348, 366 Beliz, J. M., 115, 117 Bell, W. D., 362, 366 Bennett, J. P., 333, 366 Bennett, J. S . , 318, 325 Bennetts, H. W., 105, 106, 117, 119, 123 Bentley, C. F., 33, 44, 55 Bentley, J. R., 60, 117 Bergersen, F. J., 75, 76, 117, 120 Bemal, J. D., 270, 271, 272, 274, 277, 279, 324 Beruldsen, E. T., 87, 117 Beutner, E. L., 138, 194 Bever, W. M., 221, 233, 248 Bickoff, E. M., 106, 117, 120 Biddescombe, E. F., 71, 122 Bidclulph, O., 333, 343, 366 Biggers, J. D., 106, 1-17 Bingham, E. C., 280, 282, 324 Birch, L. C., 266, 266 Black, C. A., 213, 215 Black, J. N., 69, 70, 71, 80, 81, 82, 113, 117, 118, 119 Blackman, G. E., 69, 70, 85, 118 Blackmon, C. A., 220, 245 Blakely, B. D., 174, 196 Blank, E., 261, 266 Biodgett, E. C., 333, 366 Bogarad, L., 362, 366 Bohmont, D. W., 138, 194 Bolt, G. H., 294, 315, 324 Bond, J., 352, 356, 362, 366 Bond, J. J., 149, 177, 194, 196 Bonnett, 0. T., 221, 233, 248 Booth, A. N., 106, 117, 120 Bomebusch, C. H., 258, 266 Borst, H. L., 144, 149, 151, 157, 158, 194 Botting, G. W., 100, 118 Bower, C. A., 183, 194 Bowers, S. A,, 130, 152, 153, 195 Bowman, D. H., 225, 236, 238, 247 Bradbury, R. B., 106, 118 Bradley, K., 262, 266 Bradley, R. S., 271, 275, 324 Bradley, W. F., 291, 293, 296, 320, 324, 326 Brady, G. W., 281, 324

Braithwaite, B. M., 75, 118 Breakwell, E. J., 94, 118 Brind, W. D., 129, 138, 145, 158, 159, 163, 169, 171, 177, 178, 181, 187, 195 Brittan, N. H. L., 64, 123 Broadbent, F. E., 198, 215 Broadbent, L., 235, 236, 242, 245 Brock, R. D., 64, 109, 118, 121 Brockwell, J., 75, 76, 104, 117, 118, 120 Brougham, R. W., 97, 118 Brown, A. M., 227, 246 Brown, B. E., 36, 53 Brown, C. M., 221, 224, 225, 233, 235, 241, 242, 243, 245, 246, 248 Brown, G., 36, 53 Brown, I. C., 33, 37, 43, 53 Brown, J. C., 329, 334, 337, 341, 342, 344, 347, 348, 349, 350, 352, 353, 354, 355, 357, 358, 359, 360, 382, 364, 366, 367, 368 Brown, J. W., 348, 367 Brown, L. R., 203, 215 Brown, P. L., 147, 149, 194 Browning, G. M., 131, 144, 159, 171, 183, 194, 195 Browning, J. A., 218, 220, 224, 225, 245 Bruehl, G. W., 220, 221, 222, 223, 224, 225, 226, 229, 230, 231, 232, 233, 234, 237, 239, 240, 241, 243, 245, 248 Buchanan, T. J., 272, 276, 280, 284, 325 Buehrer, T. F., 309, 310, 311, 321, 324 Burkhardt, H., 18, 53 Bumham, J., 272, 276, 324 Burrows, W. C., 153, 178, 196 Bushnell, L. D., 201, 202, 215 Buslova, E. D., 333, 367 Butler, F. C., 100, 104, 118, 119, 220, 229, 234, 245 Byers, H. G., 35, 37, 53, 198, 215

C Cain. 1. C., 364, 367 Calclwhl, h. 220, 224, 227, 238, 243, 246, 247 Calvert, 0. H., 220, 224, 247 Cameron, D. G., 87, 99, 100, 101, 105, 118 Came, P. B., 104, 118

G.,

AUTHOR INDEX

Carpenter, J. A., 66, 109, 118 Carr, R. H., 210, 215 Carrero, J. O., 333, 369 Carter, H. B., 98, 119 Carter, L. S., 130, 194 Cartwright, W. B., 238, 247 Cashmore, A. B., 87, 121 Cernosvitov, L., 253, 266 Chadwick, L. C., 262, 266 Chapman, J. E., 130, 195 Chaussidon, I., 294, 324 Chepil, W. 132, 133, 134, 155, 189, 194, 196 Chessick, J. J., 319, 327 Chevalier, M. A., 115, 118 Clark, D. P., 266, 266 Clarke, E. J., 332, 369 Clarke, G. H., 87, 118 Claussen, W. F., 284, 285, 324 Clowes, G. J., 72, 120 Coffman, F. A., 242, 246 Cohen, S., 257, 266 Cole, C. V., 351, 368 Collie, C. H., 277, 279, 280, 305, 324, 325 Collins, E. V., 171, 194 Combault, A., 264, 267 Compton, L. E., 220, 224, 227, 246 Cook, L. J., 73, 83, 99, 118 Cook, R. L., 176, 194, 365, 367 Coon, B. F., 236, 246 Coover, J. R., 139, 194 Corey, V. B., 278, 324 Cornish, E. A., 99, 118 Cownie, A,, 304, 324 Cox, M. B., 146, 171, 176, 178, 194 Crawford, C. L., 333, 368 Crofts, F. C., 103, 118 Cross, P. C., 272, 276, 324 Crouch, E. K., 60, 118 Cruger, L. G., 240, 247 CunhfFe, A., 304, 326 Cumow, D. H., 105, 106, 117, 118 Cuthbertson, E. G., 107, 119

s.,

D Damsteegt, V. D., 225, 237, 239, 240, 243, 245 Daniel, H. A., 146, 171, 176, 178, 194 Darwin, C. R., 258, 259, 260, 261, 266

373

Daub&, A., 11, 53 Davenport, N., 105, 123 Davern, C. I., 64, 68, 69, 77, 78, 88, 89, 110, 111, 118, 121 Davey, M., 100, 118 Davidson, B. R., 67, 117 Davidson, J, L., 71, 81, 97, 99, 118, 121 Davies, J. G., 58, 59, 82, 91, 92, 96, 99, 113, 114, 118, 119 Davis, J. B., 198, 201, 202, 203, 205, 215, 216 Davis, J. F., 365, 367 Dawson, C. R., 362, 367 Dawson, R. C., 154, 166, 167, 188, 194 Dawson, V. T., 166, 194 Day, G. M., 261, 266 Day, P. R., 315, 324 Deb, B. C., 36, 53 Debye, P., 276, 303, 324 DeKock, P. C., 343, 348, 364, 365, 367 Deming, J. M., 293, 309, 311, 315, 324, 325 Derjaguin, B. V., 298, 324 DeWit, C. T., 289, 291, 292, 324 Dickason, E. A., 243, 246 Dickson, J. G., 220, 246 Dillon, J. L., 90, 114, 121 Dimo, N. A., 250, 260, 267 Dingwall, A. R., 251, 265, 267 Dodd, C., 296, 324 Doekson, J., 251, 267 Dokuchaiev, V. V., 2, 5, 6, 9, 53, 54 Donald, C. M., 58, 71, 73, 77, 78, 79, 81, 82, 83, 84, 88, 89, 90, 91, 97, 98 99, 108, 111, 114, 118, 119, 122, 123 Dorsey, N. E., 307, 324 Dowling, E. J., 74, 120 Drake, F. R., 61, 64, 78, 107, 117 Duchaufour, Ph., 30, 36, 38, 44, 54 Duggan, W. C., 101, 102, 119 Duley, F. L., 131, 135, 136, 137, 142, 151, 167, 168, 169, 171, 179, 181, 188, 190, 191, 194, 195, 196, 261, 268 Dunne, T. C., 58, 61, 93, 119 Dutt, G. R., 300, 324 Dworkin, M., 201, 202, 203, 215

E East, J., 105, 119 Eastop, V. F., 237, 246

374

AUTHOR INDEX

Ebersole, J. C., 171, 194 Edelman, I. S., 278, 277, 327 Edlefsen, N. E., 319, 324 Edminster, T. W., 159, 185, 168, 195 Elliott, H. G., 59, 107, 119 Ellis, R., Jr., 205, 208, 207, 208, 209, 211, 213, 215 Ellison, W. D., 135, 195 Elton, G. A. H., 298, 300, 324 Elwell, H. M., 146, 171, 178, 178, 194 Emeis, C., 18, 54 Empson, D. W., 227, 246 Endo, R. M. (also Takeshita), 219, 220, 221, 225, 228, 233, 234, 239, 241, 243, 246, 247, 248 Englehom, C . L., 131, 132, 134, 148, 151, 159, 178, 195 Enright, D. P., 314, 317, 324 Eriksson, E., 315, 324 Esau, K., 227, 228, 246 Evans, A. C., 250, 251, 252, 253, 254, 255, 258, 259, 280, 262, 266, 267 Evans, D. D., 149, 158, 159, 183, 187, 196 Evans, H. J., 329, 331, 334, 348, 367, 368 Evans, L. T., 88, 78, 89, 119, 121 Evans, M. W., 281, 282, 283, 324 Ewell, R. H., 272, 278, 295, 301, 324 Eyring, H., 272, 275, 278, 295, 301, 324, 325

F Fawcett, R. G., 75, 120 Fehrenbacher, J. B., 48, 54 Fels, H. E., 98, 119 Fenske, M. R., 200, 201, 202, 216 Fenster, C. R., 144, 171, 174, 189, 190, 195 Feofarova, I. I., 21, 55 Ferguson, K. A., 98, 119 Fineberg, R. A., 382, 368 Fireman, P., 34, 54 Firsova, V. P., 44, 56 Fitzpatrick, E. N., 75, 119 Foote, W. H., 225, 237, 239, 240, 248, 246, 247 Forslind, E., 273, 284, 318, 320, 324 Forster, H. C., 99, 119 Foster, J. W., 201, 202, 203, 215

Fowler, R. H., 270, 271, 272, 274, 277, 279, 324 Franco, C. M., 333, 367 Frank, H. S., 270, 274, 275, 281, 282, 283, 284, 285, 324 Frankel, 0. H., 84, 88, 77, 78, 119, 121 Franklin, M. C., 72, 119 Franz, H., 256, 257, 267 Fraser, G. K., 38, 55 Fraser, K. M., 82, 119 Free, G. R., 149, 195 Freedman, H. H., 352, 382, 367 Frey, K. J., 218, 220, 224, 225, 245 Fricke, E. F., 95, 119 Fridland, V. M., 42, 44, 54 Fripiat, J. J., 294, 324 Frohnsdorff, G. J. C., 294, 325 Frost, A. E., 352, 382, 367 Frosterus, B., 35, 54 Fujimata, C. K., 163, 196

G Galloway, B. T., 219, 246 Gallun, R. L., 238, 247 Gamble, S . J. R., 159, 166, 195 Gardner, C. A., 58, 107, 119 Gardner, J. L., 198, 215 Gardner, W. R., 145, 195 Gasparini, M., 115, 119 Gates, G. E., 250, 287 Gauch, H. C., 348, 367 Gayford, G. W., 100, 119 Gedroiz, K. K., 20, 41, 54 Gemmerling, V. V., 20, 21, 23, 43, 54 54 Georgievsky, A,, 7, 8, 9, 13, 20, 40, 41,

54 Gerasimov, I. P., 29, 44, 52, 54 Gibson, A. H., 73, 119 Giesecke, F., 281, 266 Glasstone, S., 275, 325 Glazovskaya, M. A., 52, 54 Glen, J. W., 271, 275, 325 Glinka, K. D., 2, 7, 8, 20, 28, 29, 30, 35, 44, 54 Goates, J. R., 318, 325 Godden, G. R., 107, 119 Godding, T. H., 191, 195 Goldsmith, B. J., 306, 325 Goodchild, D. J., 220, 229, 234, 245

375

AUTHOR INDEX

Goodman, T., 364, 368 Gorman, L. W., 103, 120 Gorshenin, K. P., 47, 54 Gourley, J. H., 187, 194 Granick, S., 329, 362, 367 Grant, E. H., 277, 325 Grant, W. C., 251, 252, 267 Gray, R., 11, 54 Greacen, E. L., 258, 267 Greaves, J. E., 73, 119 Green, K. R., 87, 119 Greenwald, I, 351, 367 Greenwood, E. A. N., 76, 119 Grieve, B. J., 104, 117 Grim, R. E., 319, 320, 325 Gris, E., 331, 367 Grjotheim, K., 272, 276, 295, 325 Grossman, R. B., 48, 54 Gruen, F. H., 100, 119 Grylls, N. E., 104, 119, 220, 229, 234, 245 Guild, W. J., 250, 251, 252, 253, 254, 255, 256, 257, 259, 282, 267 Gurianova, 0. Z., 261, 267 Gurney, R. W., 279, 280, 325

H Haas, H. F., 200, 201, 202, 203, 215 Haas, H. J., 190, 195 Hachins, C. P., 356, 367 Haggis, G. H., 272, 276, 280, 284, 325 Hallsworth, E. G., 76, 119 Hamblyn, C. J,, 60, 119, 251, 265, 267 Hanks, R. J., 130, 145, 152, 153, 158, 194, 195 Hannah, A. E., 225, 229, 232, 235, 247 Hansbo, S., 297, 298, 299, 325 Hardwick, N. E., 71, 120 Hardy, M. H., 98, 119 Hardy, W. D., 90, 92, 120 Harley, C. P., 333, 345, 367, 368 Harper, H. J., 200, 203, 204, 210, 215 Harris, F. E., 280, 325 Harris, J. O., 199, 201, 215 Harris, J. R., 75, 120 Harrison, J. E., 90, 120 Hart, J., 296, 324 Hasted, J. B., 272, 276, 277, 279, 280, 284, 305, 324, 325 Hauser, E. A,, 290, 325

Hayman, R. H., 99, 120 Hebert, T. T., 235, 241, 246 Heck, W. W., 352, 356, 367 Hely, F. W., 75, 120 Hemaidan, N., 352, 356, 368 Hemwell, J. B., 311, 325 Henderson, A. G., 333, 368 Hendricks, S. B., 284, 285, 315, 316, 317, 320, 325 Hendrickson, A. P., 106, 117 Henniker, J. C., 289, 307, 325 Henrick, J. O., 103, 120 Hessel, F. A,, 203, 215 Hexter, G. W., 87, 120 Heyne, E. G., 224, 247 Higgs, E., 90, 111, 120 Hill, R., 58, 101, 120 Hill-Cottingham, D. G., 352, 367 Hille Ris Lambers, D., 228, 236, 246 Hills, K. L., 67, 79, 103, 108, 120 Hirschler, F. G., 296, 324 Hitchcock, C. S., 271, 327 Holland, A. A,, 75, 118 Holley, R. W., 364, 367 Hollowell, E. A., 67, 122 Holmes, P., 72, 120 Holmes, R. S., 334, 337, 341, 342, 344, 347, 348, 349, 350, 352, 353, 354, 358, 359, 360, 362, 364, 366, 367, 368 Hopkins, E. F., 332, 367 Hopp, H., 167, 196, 255, 263, 267 Hough, J. M., 304, 326 Houston, B. R., 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 234, 236, 238, 240, 243, 245, 246, 247 Hsu, H. K., 234, 247, 248 Hubbell, D. S., 198, 215 Hudson, E. R., 90, 120 Hudspeth, E. B., Jr., 130, 194 Hutchinson, S. A., 262, 267 Hutner, S. H., 356, 367, 368 Hutton, E. M., 88, 104, 108, 120 Hutton, W. E., 201, 202, 215

I Ikaheimo, K., 220, 246 Il’enkov, -, 41, 54 Iljin, W. S., 333, 364, 367

376

AUTHOR INDEX

Impey, R. L., 356, 368 Ingalls, R. A,, 384, 367 Isono, K., 307, 325 Ivanoff, S. S., 225, 238, 247 Ivanova, E. N., 29, 54 Ivanova, K. V., 60, 120

J Jacks, G. V., 129, 138, 145, 158, 159, 183, 185, 189, 171, 177, 178, 181, 187, 195 Jackson, M. L., 36, 53 Jacobson, H. G., 257, 267 Jacobson, L., 352, 364, 367 Jagendorf, A. T., 382, 366 Jane, A., 75, 118 Jardine, R., 99, 122 Jedlinski, H., 218, 224, 235, 241, 242, 243, 245, 246 Jefferson, M . E., 284, 285, 325 Jefferson, P., 253, 267 Jenkins, H. V., 89, 94, 117, 118, 120 Jennings, A. C., 257, 258, 266 Jensen, H. L., 73, 120 Johnson, B. E., 235, 247 Johnson, M. O., 333, 367 Johnson, T., 227, 248 Johnson, W. C., 157, 158, 180, 163, 185, 195 Jones, A. C., 203, 216 Jones, B. J., 80, 120 Jones, J. H., 80, 118 Jones, L. W., 73, 119 Jones, T. I., 352, 358, 362, 366 Joshi, N. V., 262, 267 Juritz, C. F., 331, 367

K Kahsnitz, H. G., 283, 267 Kaleski, L. J., 99, 100, 120 Kallio, R. E., 203, 216 Kamel, M., 282, 267 Kaminsky, M., 280, 325 Kanwar, J. S., 257, 260, 267 Karpachevsky, L. O., 42, 45, 48, 54 Kaserer, H., 200, 215 Kato, C., 313, 325 Katznelson, J., 84, 123 Katzoff, S., 271, 325 Kay, F. F., 38, 37, 54

Keenan, A. G., 292, 315, 317, 318, 320, 325, 326 Keilin, D., 334, 367 Kelkar, B. V., 262, 267 Kelly, L. L., 135, 136, 137, 194, 195 Kemper, W . D., 298, 297, 298, 325 Kennedy, J. S., 218, 235, 236, 246 Kester, A. S., 203, 215 Kidder, E. H., 137, 139, 195 Kiesling, R. L., 243, 246 Kileen, N. C., 99, 121 Kindler, A., 11, 54 King, C. L., 224, 247 Kington, G. L., 294, 325 Kipps, E. H., 98, 99, 121 Kirkham, D., 153, 196 Kirsanov, A. T., 51, 54 Klages, K. H., 143, 144, 147, 196 Kliman, S., 382, 367 K h e , D. M., 235, 241, 246 Kohler, G. O., 106, 117 Kohnke, H., 149, 151, 158, 196 Kolaian, J. H., 300, 308, 309, 310, 312, 315, 320, 321, 325 Kononova, M. M., 30, 54 Krall, J. L., 147, 177, 195 Krauss, R. W., 359, 368 Kreitlow, K. W., 221, 246 Krogh-Moe, J., 272, 278, 295, 325 Krylov, N. A., 298, 324 Kubiena, W. L., 38, 48, 54, 55 Kiihnelt, W., 258, 287 Kumai, M., 307, 325 Kundler, P., 48, 55 Kuykendall, J. R., 330, 367

L Laidler, K. J., 275, 325 Lamb, J., Jr., 130, 195 Langham, E. J., 307, 325 Larson, W. E., 153, 181 196 Latter, B. D. H., 103, 109, 118, 120 Laverack, M . S., 253, 267 Lawson, E. H., 77, 80, 81, 120 Lawton, K., 183, 195 Leach, W., 344, 368 Leadbetter, E. R., 203, 215 LeBeau, D. S., 290, 325 Lee, K. E., 250, 252, 282, 267 Lehane, J. J., 148, 196

AUTHOR INDEX

377

McCuen, G. W., 156, 157, 196 McDole, G. R., 130, 194 MacEwan, D. M. C., 36, 55 Macey, H. H., 284, 296, 326 284, McGeorge, W. T., 333, 368 McGrath, W. J., 362, 366 McKay, H. C., 131, 143, 144, 147, 159, 170, 195, 196 Mackenzie, R. C., 285, 291, 316, 317, 326 McKinney, H. H., 220, 224, 226, 228, 237, 243, 245, 246 McLachlan, K. D., 74, 82, 117 McMillan, J. R. A,, 103, 108, 118, 121 Maiden, J. H., 58, 121 Mann, T., 334, 367 Manns, T. F., 220, 224, 239, 246 Marbut, C. F., 34, 55 Margaropoulos, P., 115, 121 Martell, A. E., 352, 362, 367 Martin, R. T., 290, 292, 298, 301, 318, 326 Mason, B. J., 307, 308, 325, 326 Massee, T. W., 147, 177, 195 Mathieson, A. M., 285, 294, 326 Matthews, A., 199, 201, 215 May, L. H., 71, 121 Maybank, J., 307, 308, 326 Meadley, G. R. W., 80, 82, 87, 101, 121 294, Mederski, H. J., 144, 149, 151, 157, 158, 194 311, Meiss, H. N., 380, 369 Mellanby, K., 258, 267 Melnikova, N. K., 298, 324 Merkle, F. G., 149, 194 Mestre, L., 60, 115, 121 Metcalf, C. R., 253, 267 349, Meyer, D. R., 75, 76, 120, 121 Meyer, L., 261, 267 Michaels, A. S., 298, 299, 300, 326 Michaelson, W., 250, 267 Mihara, Y., 141, 195 M Miller, G. W., 348, 368 Miller, M. D., 243, 247 McArthur, A. M., 72, 121 Miller, R. B., 250, 267 McCdrey, L. A. H., 100, 121 Miller, R. D., 315, 324 McCaleb, S . B., 36, 55 McCalla, T. M., 138, 150, 151, 152, Millikan, C. R., 70, 95, 121 154, 155, 156, 159, 160, 161, 182, Millington, A. J., 78, 107, 108, 110, 121 163, 164, 166, 167, 168, 169, 185, Milne, A., 315, 327 188, 189, 190, 191, 194, 195, 196, Milner, H. B., 198, 215 Minashina, N. G., 42, 46, 55 261, 268

Leighton, P. A., 272, 276, 324 Leitenberger, L., 256, 257, 267 Lemcoe, M. M., 302, 326 Lemon, E. R., 147, 151, 152, 194 Lennard-Jones, J., 270, 273, 274, 325 Lenz, J. V., 93, 123 Leonard, C. D., 352, 356, 367, 368 Leonards, G. A., 313, 325 Levy, E. B., 103, 120 Lewis, H. B., 257, 266 Liebich, H., 334, 368 Lightfoot, L. C., 59, 101, 119, 120 Lillard, J. H., 165, 195 Lin, C. S . , 296, 299, 300, 326 Lindner, R. C., 333, 345, 367, 368 Lindquist, B., 256, 267 Lindsay, W. L., 348, 368 Lindsten, K., 220, 246 Lipp, A. E. G., 67, 120 Lipsett, J., 84, 123 Livingston, A. L., 106, 117, 120 Lloyd-Jones, C. P., 352, 367 Loewus, M. W., 362, 368 Logsdon, C. E., 220, 246 Lonergan, J. F., 74, 75, 76, 120 Loomis, W. E., 333, 367 Love, R. M., 60, 120 Lovell, C. W., 313, 325 Low, P. F., 290, 291, 292, 293, 295, 296, 298, 300, 301, 309, 315, 320, 321, 323, 325 Ludbrook, W. V., 104, 120 Luebs, R. E., 161, 196 Lugg, J. W. H., 72, 120 Lundblad, K., 36, 55 Lunt, H. A., 257, 267 Lunt, 0. R., 329, 330, 331, 348, 350, 356, 365, 367, 368 Lutz, J. F., 297, 298, 325 Lyman, R. L., 106, 120

378

AUTHOR INDEX

Mitchell, K. G., 70, 121 Miyoshi, M., 200, 215 Moir, R. J., 72, 98, 119, 123 Moody, J. E., 165, 195 Mooers, C. A., 180, 195 Mooney, R. W., 292, 315, 317, 318, 320, 325, 326 Moore, R. M., 87, 98, 99, 121 Morgan, A., 87, 117 Morgan, J., 271, 273, 275, 278, 326 Morley, F. H. W., 84, 88, 67, 88, 89, 70, 77, 78, 79, 81, 88, 89, 110, 111, 113, 118, 121 Morozov, S. S., 45, 55 Morris, H. D., 213, 216 Morrison, R. I., 364, 367 Morrow, J. A., 99, 121 Mortland, M. M., 320, 326 Moss, W. A., 131, 144, 147, 159, 170, 195 Mossop, S. C., 307, 326 Moye, D. V., 74, 75, 117 Muckenhausen, E., 44, 55 Mueller, A., 203, 216 Miiller, H. R., 284, 327 Miiller, P., 13, 16, 17, 55 Mueller, R. T., 352, 356, 368 Mueller, W. C., 234, 246 Muir, A., 38, 37, 55 Muir, J., 305, 306, 325, 326 Mullett, H. A., 59, 121 Mulligan, T., 220, 221, 222, 223, 225, 229, 230, 231, 232, 233, 234, 248 Murphy, A. H., 93, 123 Murphy, H. C., 220, 224, 246, 248

N Nadezhdin, B. V., 33, 56 Neal-Smith, C. A., see also Smith, C. A. N., 95, 107, 117, 123 Needham, A. E., 258, 267 Nelson, J. M., 250, 267 Nelson, R. A., 315, 316, 317, 320, 325 Neustruev, S. S., 25, 55 Newton, J. D., 33, 44,55 Nicholas, D. J. D., 364, 368 Nicholas, H. T., 100, 121 Nielands, J. B., 364, 368 Nielson, R. L.,262, 267 Nighawan, S. D., 257, 280, 267

Nitzsch, W. v., 292, 326 Noack, K., 334, 368 Nogina, N., 32, 35, 43, 45, 55 Norris, K. R., 105, 121 Norrish, K., 292, 315, 326 Norstadt, F. A., 184, 195 North, C. P., 352, 356, 368 Norton, R. A., 131, 144, 159, 171, 183, 194 Nutman, P. S., 73, 74, 78, 119, 121, 122 Nutt, G. B., 131, 144, 154, 185, 171, 190, 194, 196

0 Oakes, D. T., 290, 291, 299, 326 Ockman, N., 270, 326 Oertli, J. J., 384, 367 O’Konski, C. T., 280, 325 O’Loughlin, G. T., 104, 121 Olsen, C., 333, 344, 351, 368 Olsen, H. W., 297, 326 Orchard, H. E., 87, 121 Orchiston, H. D., 315, 326 Orcutt, F. S., 159, 166, 195 Orlob, G. B., 222, 223, 228, 229, 231, 235, 237, 243, 245, 246 Osnitskaya, L. K., 203, 216 Oswald, J. W., 218, 219, 220, 221, 223, 224, 225, 226, 227, 228, 230, 234, 236, 238, 240, 246 Overgaard Nielsen, C., 250, 252, 256, 267 Overstreet, R., 313, 324 Owston, P. G., 270, 326 Ozanne, P. G., 69, 121

P Pack, R. J., 88, 87, 122 Pagan, V., 332, 367 Page, J. B., 158, 157, 196 Palmer, L. S., 304, 324, 326 Parfenova, E. I., 23, 42, 43, 48, 56 Parish, R., 90, 114, 121 Parker, D. T., 181, 178, 196 Parker, G. H., 253, 267 Pascoe, W. B., 100, 121 Paton, D. F., 75, 76, 121 Patterson, F. L.,220, 224, 227, 246 Pauling, L.,270, 271, 272, 326 Pavlinon, N., 19, 55

230, 222, 229, 255,

AUTHOR INDEX

Peak, J. W., 69, 77, 88, 104, 108, 111, 118, 119, 120, 121 Pearson, A., 87, 121 Peck, C. H., 219, 246 Peele, T. C., 131, 144, 154, 165, 190, 194, 196 Pei, M. Y., 234, 247, 248 Peikert, F. W., 176, 194 Penzhorn, K. E. W., 60, 122 Peters, D. C., 218, 245 Peterson, S. E., 105, 123 Pettinger, N. A., 333, 368 Philip, J., 71, 81, 97, 99, 118 Pickett, A. G., 302, 326 Pierre, W. H., 213, 216 Piland, J. R., 332, 369 Pine, M. J,, 198, 200, 203, 216 Pizarro, A. C., 231, 247, 247 Plice, M. J., 198, 199, 205, 208, 216 Poehlman, J. M., 220, 224, 247 Polglase, M. F., 284, 324 Ponomareva, S. I., 254, 267 Pope, G. S., 106, 122 Pople, J. A., 270, 272, 273, 274, 325, 326 Popov, T. I., 38, 55 Porter, J. R., 201, 216 Porter, L. K., 348, 368 Power, J. F., 147, 177, 195 Powning, R. F., 72, 119 Poyner, R. R., 174, 196 Pratt, G. L., 171, 196 Promersberger, W. J., 171, 196 Provasoli, L., 356, 367 Purchase, H. F., 74, 84, 122 Purvis, E. R., 360, 369

110,

171,

213,

284,

Q Quinlivan, B. J., 107, 122 Quirk, J. P., 297, 324

R Radel, L. H., 103, 122 Rademacher, B., 220, 226, 238, 243, 247 Ramage, R. T., 225, 247 Ramann, E., 34, 55 Ramirez-Silva, F. J., 342, 368 Rampton, H. H., 60, 122

379

Rasmusson, D. C., 242, 247 Raw, F., 250, 267 Raymer, W. B., 225, 237, 239, 240, 243, 246, 247 Raymond, R. L., 203, 216 Resnicky, J . W., 337, 347, 355, 367 Reuther, W., 333, 368 Rhoades, W. A., 364, 368 Ribaudcourt, E., 264, 267 Riceman, D. S., 104, 120 Richards, J. G., 265, 267 Richards, L. A., 322, 326 Richards, W. R., 221, 225, 228, 229, 232, 235, 236, 247 Rios, E. G., 315, 317, 326 Ritson, D. M., 277, 279, 280, 305, 324, 325 Robbins, W. R., 332,360,368,369 Robertson, J. A., 33, 44,55 Robertson, J . D., 257, 267 Robins, J. S., 174, 196, 319, 326 Robinson, A. L., 281, 324 Robinson, C. V., 276, 277, 327 Robinson, G. W., 8, 35, 36, 55 Robinson, R. A., 277, 326 Robinson, T. J., 105, 118 Rochow, W. F., 218, 220, 221, 222, 223, 228, 230, 231, 232, 234, 235, 238, 246, 247 Rodk, A. A., 21, 22, 32, 33, 35, 41, 42, 43, 45, 46,48, 55 Rogers, C. H., 333, 364, 368 Rogers, V. E., 77, 88, 110, 111, 121 Rogoff, M. H., 203, 216 Roots, B. I., 251, 267 Rose, Dona dalle, 115, 122 Rose, M. S., 309, 311, 324 Rosenqvist, I. Th., 301, 302, 313, 319, 320, 326 Rossiter, R. C., 59, 77, 80, 81, 86, 87, 98, 99, 106, 107, 117, 118, 119, 120, 122 Rothman, P. G., 225, 236, 247 Rovira, A,, 258, 267 Rowland, R. A., 320, 326 Russel, J. C., 142, 145, 146, 148, 159, 160, 171, 195, 196 Russell, E. J., 73, 122, 259, 262, 253, 267

380

AUTHOR INDEX

Russell, E. W., 289, 327 Ryerson, G. E., 171, 196

5 Sachs, J., 331, 368 Saeki, T., 81, 122 Salisbury, E. J., 81, 122, 257, 267 Samoilov, 0. Ya., 277, 327 Satchell, J. E., 28, 55, 250, 251, 253, 254, 256, 267, 268 Saxby, S. H., 60, 122 Saxton, J.A., 276, 277, 327 Schafer, J. F., 220, 224, 227, 238, 243, 246, 247 Schaller, C. W., 240, 242, 247 Schaller, F. W., 149, 158, 159, 163, 167, 196 Schander, H., 333, 368 Schatz, A., 356, 367, 368 Schinckel, P. G., 105, 122 Schissler, D. O., 203, 216 Schmid, W. E., 296, 327 Schmidt, C. L. A., 334, 368 Schofield, R. K., 315, 327 Schollenberger, C. J., 205, 208, 210, 212, 216 Schuster, C. E., 154, 163, 165, 196 Schwartz, W., 203, 216 Schwarz, R., 220, 226, 238, 243, 247 Scott, A. E., 82, 119 Scott, H., 159, 196 Scudield, G., 71, 122 Sears, P. D., 73, 122 Sechler, D. T., 220, 224, 247 Sedletsky, I. D., 43, 45, 55 Selva, F. J. R., 332, 367 Senft, F., 11, 55 Shands, H. L., 224, 240, 245, 247 Shannon, L. M., 352, 356, 368 Shapter, R. E., 72, 113, 122, 123 Shedd, C. K., 131, 144, 159, 194 Sherman, D. G., 1863, 196 Shier, F. L., 59, 105, 107, 117, 119, 122, 123 Shindo, B., 255, 268 Shive, J. W., 333, 364, 367, 368 Sibirtzev, N. M., 4, 5, 10, 23, 24, 34, 38, 39, 55 Siddoway, F. H., 143, 144, 147, 196 Sieveking, W. E., 239, 240, 246, 247

Sill, R. C., 312, 327 Sill, W. H., Jr., 224, 247 Simpson, G. W., 235, 247 Sims, H. J., 99, 122 Skapski, A. S., 312, 327 Skrynnikova, I. N., 43, 55 Slabaugh, W. H., 319, 327 Slater, C. S., 167, 196, 263, 267 Slykhuis, J. T., 218, 220, 221, 225, 229, 232, 235, 236, 247 Smit, N. L., 60, 122 Smith, C. A. N., see also Neal-Smith, C. A., 77, 78, 88, 91, 108, 111, 119 Smith, H. C., 227, 240, 247 Smith, K. M., 235, 236, 247 Smith, P. F., 365, 368 Smith, R., 129, 138, 145, 158, 159, 163, 169, 171, 177, 178, 181, 187, 195 Smith, W. P. Cass, 75, 118 Smyth, C. P., 271, 327 Smythe, C. V., 334, 368 Sohngen, N. L., 200, 216 Southworth, E. A., 219, 246 Spawn, G. B., 177, 178, 196 Specht, A. W., 228, 246, 337, 347, 355, 365, 366, 367, 368 Spencer, D., 74, 117 Spiers, M., 329, 352, 368 Sprague, R., 228, 247 . Sprengel, C., 11, 55 Stackelberg, M. v., 284, 327 Stallings, J. H., 140, 196 Stanton, T. R., 228, 239, 246, 247 Staple, W . J., 148, 196 StaufFer, R. S., 137, 138, 139, 154, 195, 196 Stening, H. C., 87, 122 Stephen, I., 36, 48, 53, 54 Stephens, C. G., 58, 122 Stephenson, J., 250, 252, 262, 268 Stephenson, R. E., 154, 163, 165, 196 Stetson, B. J., 235, 247 Stevenson, D. P., 203, 216 Stewart, G. W., 278, 327 Stewart, I., 352, 356, 367, 368 Stewart, J. E., 203, 216 Stockdill, S. M . J., 265, 268 Stockli, A., 260, 268 Stokes, R. H., 277, 326 Stone, R. W., 200, 201, 202, 216

381

AUTHOR INDEX

Stout, J. D., 250, 267 Strawinski, R. J., 201, 202, 203, 215, 216 Strong, T. H., 72, 73, 113, 122, 123 Sukachev, V. N., 25, 56 Sullivan, G. D., 103, 118 Summers, T. E., 238, 247 Suneson, C. A., 225, 242, 243, 247 Svendsen, J. A., 250, 268 Swain, F. G., 75, 118 Swan, D. C., 105, 122 Swinbank, J. C., 171, 190, 195 Sylvester, E. S., 244, 247 Symon, D., 65, 122

Trumble, H. C., 59, 72, 78, 82, 88, 90, 92, 100, 113, 122, 123 Tumin, G., 20, 31, 56 Tunn, W. L. M., 297, 298, 299, 327 Turelle, J. W., 156, 196 Tumbull, D., 308, 327

U Ufimtseva, K. A., 33, 43, 51, 56 Uhland, R. E., 158, 159, 160, 194 Uhler, B. L., 360, 369 Underwood, E. J., 59, 105, 106, 117, 118, 119, 123 Updegraf€, D. M., 201, 203, 215

T

V

Taggart, M. S., 205, 216 Takeshita, R. M. (see Endo, R. M.) Tamm, O., 35, 36, 56 Taper, C. D., 344, 368 Tausson, W. O., 203, 216 Taylor, R. L., 220, 246 Tedron, J. C . F., 36, 56 Teotia, S. P., 167, 168, 169, 196, 261, 268 Thaine, R., 102, 122 Thompson, J. A., 75, 117 Thome, D. W., 333, 348, 368 Thorp, J., 33, 37, 43, 53 Thung, T. H., 220, 238, 246 T a n , L. O., 337, 341, 342, 344, 347, 348, 349, 350, 352, 353, 354, 355, 357, 358, 359, 360, 362, 364, 366, 367, 368 Timonin, M. I., 163, 196 Tindale, E., 87, 122 Tiurin, I. V., 26, 28, 30, 35, 47, 48, 56 Tiver, N. S., 91, 93, 122 Toko, H. V., 220, 221, 222, 223, 224, 226, 229, 230, 231, 232, 233, 234, 237, 241, 243, 245, 248 Toler, R. W., 235, 240, 241, 246 Toms, J., 90, 122 Toogood, J. A., 33, 44, 55 Toole, E. H., 67, 122 Tortorich, J. A., 201, 216 Touillaux, R., 294, 324 Tracey, M. V., 257, 268

Vadkovskaya, 0. A., 33, 45, 56 Van der Drift, J., 252, 256, 268 Van Doren, C . A., 137, 138, 139, 195, 196 Van Doren, C. E., 139, 149, 194 van Wijk, W. R., 153, 196 Verma, A. B. S., 149, 151, 158, 196 Vincent, J. M., 74, 75, 76, 84, 122, 123 Vivaldi, J. L. M., 315, 317, 326 von Engelhardt, W., 297, 298, 299, 327 von Fin'ck, A., 260, 268 von GraE, O., 253, 254, 268 von Hensen, R., 259, 268 Vorster, P. W., 60, 123 Vysotzky, G. N., 19, 25, 56

W Wadleigh, C. H., 332, 348, 367, 368 Walker, A. J. K., 107, 123 Walker, G. F., 285, 294, 315, 326, 327 Walker, J., 104, 123 Wallace, A., 329, 330, 331, 333, 352, 356, 364, 365, 368 Wallace, T., 333, 368 Walsh, T., 332, 369 Wang, J. H., 276, 277, 281, 301, 327 Wann, F. B., 366, 369 Ward, A. G., 295, 327 Ward, L. M., 84, 122 Warren, B. E., 271, 273, 275, 278, 326 Washko, J. B., 160, 195 Watanabe, F. S., 351, 368 Waters, L. M., 76, 123

382

AUTHOR INDEX

Waters, R. A. S., 252, 253, 254, 255, 262, 263, 268 Watkin, B. R., 252, 268 Watson, M. A., 104, 123, 220, 221, 222, 225, 229, 230, 231, 232, 233, 234, 248 Watson, R. H., 106, 117 Weinstein, L. H., 360, 369 Weiss, E. J., 320, 326 Weiss, M. G., 337, 369 Wen, W., 274, 282, 283, 284, 324 Wenhardt, A., 148, 196 Wessely, J., 18, 56 Westerback, S. J., 352, 362, 367 Wexelsen, H., 84,123 Weyl, W. A., 314, 317, 324 Wheat, J. G., 220, 224, 225, 245 White, A. G . C., 200, 201, 202, 216 White, D. E., 106, 118 White, W. A., 315, 320, 327 Whitehead, M. D., 220, 224, 247 Whitfield, C. J., 131, 132, 139, 140, 141, 142, 155, 163, 165, 177, 178, 180, 182, 183, 184, 186, 194, 196 Wiese, A. F., 145, 147, 158, 177, 194, 196 Wilcke, D. E., 250, 268 Wikie, D. R., 102, 123 Willard, C. J., 156, 157, 196 Williams, C. H., 73, 82, 83, 84, 119, 123 Williams, J. D., 86, 119 Williams, V. J., 72, 123 Williams, V. R., 25, 56 Williams, W. A., 93, 123 Willis, L. G., 332, 333, 369 Willis, W. O., 153, 196 Willoughby, W. M . , 82, 86, 96, 99, 110, 123 Wilson, G. L., 70, 85, 118 Wilson, H. A,, 171, 194 Wilson, M. C., 243, 246

Wilson, S. B., 76, 119 Wilson, V. E., 220, 224, 248 Wingard, S. A., 333, 368 Winterkom, H. F., 296, 327 Winterlin, W. L., 161, 196 Wit, F., 221, 248 Withee, L. V., 162, 163, 196 Wolf, A. V., 251, 268 Wollny, E., 258, 259, 262, 268 Wood, L. A,, 292, 315, 317, 318, 320, 325, 326 Woodforde, A. H., 67, 123 Woodruff, N. P., 132, 133, 134, 189, 194, 196 Wright, H. G., 106, 122 Wright, S., 66, 123

Y Yaalon, D. H., 36, 53 Yantzi, M. F., 201, 215 Yarilova, E. A., 23, 42, 43, 46, 56 Yarkov, S. P., 29, 48, 56 Yates, J., 61, 64, 88, 123 Yocum, K. W., 187, 196 Young, G. J., 319, 327 Young, J. B., 160, 195 Young, M., 221, 236, 247 Yu, T. F., 234, 248

Z Zakharov, S. S., 20, 24, 31, 56 Zavalishin, A. A., 29, 32, 33, 44, 54, 56 Zettlemoyer, A. C., 319, 327 Zicsi, A., 262, 268 Zillinsky, F. J., 221, 225, 229, 232, 235, 236, 247 Zingg, A. W., 131, 132, 133, 134, 139, 140, 141, 142, 155, 163, 165, 177, 178, 180, 182, 183, 184, 186, 196 ZoBell, C. E., 198, 200, 201, 202, 203, 215, 216 Zohary, D., 64, 123

Subject Index A Achromobacter, 200 Aerobacillus macerans, 200 Aerobacillus polymyxa, 200 Agropyron intermedium, 221 Agropyrum obtus-iusculum, 95 Agrostis sp., 27 Aira flexuosa, 12 Alcaligenes radiobacter, 200 Algae, 169 AUolobophora caliginosa, 250, 251, 252, 256, 263, 264 Allolobophora chlorotka, 253 Alternaria, 188 Ammonia, 159, 202, 257, 258 Aphids, 104,219, 225,227,228-230,231, 230-238, 243 Aphodius howitti, 104 Apple, 187, 333, 346 Aspergdlus, 188, 200 Australia, subterranean clover, 57-117 Avenu strigosa, 239

B Bacillus amylolyticus, 200 Bacillus brevis, 200 Bacillus circulans, 200 Barley, 224, 225, 226, 227, 230, 240, 241, 242, 243 Barley yellow dwarf virus, 217-248 aphid vectors, 228-230 control, 238-244 crops affected, 220-226 history, 218-220 nature of virus, 231-234 symptoms, 226-228 Beans, 176 Bean mosaic, 104 Bentonite, 287, 289, 290, 291, 302, 308312, 316, 317, 321 Bermuda grass, 210 Bicarbonate, 345-351 Bilberry, 27 Blmastus spp., 256 Biochanin A, 106

Bleisand, 11-19, 34 Bluegrass, 221 Bromus coloratus, 95 Bromus inemis, 95, 221 Bromus rigidus, 221 Bromus tectorum, 172, 175

C Calcium, 67, 76, 163, 184, 257, 333, 343, 355 Callunu vulgaris, 11 Cape weed, 86 Carex jilifola, 190 Cbnchrus sp., 176 Chaetomium, 188 Cheatgrass, 172, 175 Chelating agents, iron chlorosis, 351-365 Chernozem, 9 Chondrilla iuncea, 87 Clay-adsorbed water, dielectric properties, 303-306 specific volume, 287-295 supercooling and freezing, 306-314 thermodynamic properties, 314-322 viscosity, 295-303 Clay-water interaction, 269-327 hypothesis, 322-323 mechanisms of, 284-287 Clostridium bifermentans, 200 Clostridium sporogenes, 200 Cockchafer scarab, 104 Colchicine, 108 Convolvulus amensis, 87 Copper, 332, 337, 342, 362 Corn, 161, 169, 174, 183, 188, 332, 355, 362 Cornstalks, 138, 152 Corticum praticolu, 104 Cotton, 332 Coumarin, 169 Coumestrol, 106 Cowberry, 27 Crabgrass, 176 Crimson clover, 154

383

384

SUBJECr INDEX

Crude oil, soil contamination, 197-216 Cryptostemma calendula, 86,87 Cyclohexanediaminetetraacetic acid, 352

D Dactylis glomerata, 25, 95 Dalapon, 177 Dendrobaena spp., 256 Deschampsia caespitosa, 27 Deschampsia flexosa, 27 2,4-Dichlorophenoxyacetic acid, 177 2,2-Dichloropropionic acid, 177 Diethylenetriaminepentaacetic acid, 341, 352 Digitaria sanguinalis, 176 Downy bromegrass, 175

E Earthworms, 167, 249-268 effect on crop yield, 262-266 on soil fertility, 256-262 population studies, 250-256 Eisenia rosea, 256 Erodium botrys, 87 Estrogen, 105-107 Ethylenediaminedihydroxyphenylacetic acid, 352 Ethylenediaminetetraacetic acid, 352

F Fenuron, 177 Festuca arundinaceae, 95 Festuca rubra, 221 Formononetin, 106 Foxtail millet, 234 Fusarium, 188

G

Genistein, 106 Gopher plowing, 131 Grapefruit, 333 Grasshoppers, 177 Great Plains, stubble mulch, 127-129,170, 175, 179 Greenbugs, 177, 224, 243 Green foxtail, 176

H Halotydeus destructor, 105 Hessian fly, 177

Hordeum leporinum, 85 Hormodendrum, 188 Hypericum perforatum, 87

I Intermediate wheatgrass, 221 Iron, 205, 213 Iron chlorosis, 329-369 chelating agents, 351-365 earlier findings, 331-334 factors involved, 334-351

K Kabatiellu cauliuora, 104 Kaolinite, 299, 302, 305, 310, 317, 318 Kidney beans, 355

1 Lateritic soils, 331-332 Leafhoppers, 231 Legume, 179 Lespedeza, 144 Lime, 264, 331 Lolium perenne, 95,221 Lolium rigidum, 82,86, 95 Lotus major, 70 Lumbricids, 250, 262, 265 Lumbricus rubellus, 253, 256 Lumbricus terrestris, 256 Lupine, 355

M Macrosiphum dirhodum, 232 Macrosiphum granurium, 230, 231, 232, 233, 234, 235, 236, 237, 238 Magnesium, 67, 184, 355 Malathion, 243 Manganese, 163, 205, 213, 332, 355 Medicago hispida, 88 Medicago lucinuta, 88 Medicago minima, 88 Melilotus alba, 106 Meloidogyne spp., 104 Metahalloysite, 305-306 Methane, 198, 200 Methanomonas, 201 Methanon-bacterium omelianskii, 198 Millet red leaf virus, 234 Milo, 354, 356 Moldboard plowing, 139

385

SUBJECT INDEX

Molybdenum, 76 Montmorillonite, 292,312 Mosses, 169 Mutation, induced, 109 Mycobacterium, 200

N Naphthelene, 202 Nardus stricta, 11 Nasellu trichotoma, 87 Natural gas, soil contamination, 197-216 Nematodes, 104,167 Nitrate, 159-161,202,257,263 Nitrite, 159-161 Nitrogen, 72-73,84,164,184,198,203,

Poa sterilis, 25 Podzolic soils, 1-56, 331 characteristics of, 47-51 geographical variants, 30-33 micromorphology, 46-47,49-51 mineral analysis, 38-46,49-51 sod-forming, 24-30 soil formation, 2-11 subtypes, 23-24 Polygonum conuoluulus, 176 Potassium, 163,183,184,333 Potato, 364 Potato leaf roll virus, 231 Pseudomonas, 202 Pseudomonas aeruginosa, 200

210,227,257,258, 259,263,299, Red fescue, 221 333 Red leaf, 219 Nitrogen fixation, 198

R

Red-legged earth mite, 105 Rhizobia, 74-76 Rhizobium trifolii, 72 Oats, 144,161,163,179,184,210,211 Rhizoctonia soluni, 104 barley yellow dwarf virus, 219, 220, Rhizopus, 188 224,225,226,227,239,242,243 Rhopalosiphum fitchii, 228 Octolusium lacteum, 256 Rhopalosiphum maidis, 232,235 Okra, 355 Rhopalosiphum padi, 228,230,231,232, Organic matter, 163-165 233,234,235,236,238 Ortstein, 8,11-19,26 Rhopalosiphum prunifoliae, 228 Ripgut grass, 221 P Root rot of wheat, 177 Rose clover, 60 Paraffin dirt, 198,205 Russian thistle, 176 Parathion, 243 Rye, 154,221,226,254,355 Pea, 362 Ryegrass, 87,95,221,263 Pear, 346 Penicillium, 188,200 S Petroleum hydrocarbons, 197-216 effect on plant growth, 210-214 Salsola pestifer, 176 on soil microbes, 198-203 Sandburs, 176 on soil properties, 203-210 Seed germination, 66-67 Phalaris tuberosa, 82,87,88,95 Senecio jacobea, 87 3-Phenyl-l,l-dimethylurea, 177 Setaria italica, 234 Pheretima hupeiensis, 252 Setaria viridis, 176 Phkum pratense, 221 Smooth brome, 221 Phosphate, 73,85,106 Soil, bleisand, 11-19 Phosphorus, 95,163,208,337,343,348, illuvial, 19-23 355 microorganisms, 165-169 Pinetum oxalidosum, 21 moisture, 144-150 Poa nemoralis, 25 ortstein, 11-19 Poa pratensis, 221 petroleum contamination, 197-216

0

386

SUBJECT INDEX

p0dz01, 1-58 temperature, 150-154 Sorghum, 134, 174 Soybean, 138, 210, 334, 337, 343, 355 Spinach, 334 Stubble mulch, 125-193 chemical effects, 159-165 crop yield, 179-187 definition, 128-127 infiltration, 135-138 insects, 177 microbial activity, 185-169 moisture conservation, 144-150 mulch maintenance, 187-191 plant diseases, 177-178 soil structure, 154-159 soil temperature, 150-154 tillage equipment, 170-175, 188-190 water erosion, 138-144 weed problems, 175-177 wind erosion, 132-135 Subterranean clover, 57-123 agrotechnology, 90-103 diseases and pests, 103-105 ecology, 77-90 estrogen problem, 105-107 history, 58-81 nitrogen fixation, 72-77 physiological variation, 88-72 potential usage, 113-117 taxonomy, 61-88 varieties, 107-113 Subterranean clover stunt, 104 Sunflower, 357 Sweetclover, 169, 184

T Talc, 305 Termites, 189 Thread leaf sedge, 190 Timothy, 221 Toxoptera graminum, 177

Trichoderma, 188 Trientalb europea, 12 Trifolium alexandrinum, 74 Trifolium campestre, 108 Trifollum globosum, 108 Trifolium glomeratum, 74 Trifolium hirtum, 80 Trifolhm lncarnatum, 74 Trifolium israeliticum, 64, 106 Trifolium ornithopodoides, 74 Trifolium pratetwe, 73, 74, 106 Trifolium repetw, 80, 70, 73, 74, 108 Trifolium subterranean, 58, 81

U Uromyces trifolii, 103 Ustilago sphaerogena, 304

V Vetch, 154

W Water, ionic solutions, 277-284 nature of, 289-277 see clay-adsorbed Water erosion, 138-144 Weed control, 172 Wheat, 134, 161, 183, 175, 179, 184, 210, 254, 355 barley yellow dwarf virus, 219, 221, 224, 228, 227, 240 Wheat mosaic, 178 Wheat straw, 138, 151 White clover, 80 Wild buckwheat, 178 Wind erosion, 132-135

Y Yellow dwarf of oats, 219

Z Zinc, 69, 332