Advances in Agronomy, Volume 11

ADVANCES IN AGRONOMY VOLUME XI

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ADVANCES IN

AGRONOMY Prepared under the Auspices of the AMERICANSOCIETYOF AGRONOMY

V O L U M E XI Edited by A. G. NORMAN University of Michigan, Ann Arbor, Michigan

ADVISORY BOARD D. G. ALDRICH, JR. J. E. DAWSON W. H. FOOTE J. E. GIESEKING

W. P. MARTIN R. W. PEARSON G. F. SPRAGUE H. M. TYSDAL

1959 ACADEMIC PRESS

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NEW YORK and LONDON

Copyright 0, 1 9 5 9 , by Academic Press Inc. ALL RIGHTS RESERVED NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRI'ITEN PERMISSION FROM THE PUBLISHERS.

ACADEMIC PRESS INC. 111 FIFTHAVENUE NEWYORK 3, N. Y.

United Kingdom Edition Published by ACADEMIC PRESS INC. (LONDON)Lm. 40 PALLMALL, LONDON SW 1

Library of Congress Catalog Card Number 50-5598

PRINTED IN

THE UNITED STATES OF AMERICA

CONTRIBUTORS TO VOLUMEXI

DAVIDE. ANGUS,Department of Irrigation, University of California, Davis, California.* G. W. BURTON, Research Geneticist, Forage and Range Research Branch, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, and the University of Georgia, College of Agriculture Coastal Plain Experiment Station, Tifton, Georgia.

P. DOLL,Assistant Professor of Agriculturul Economics, University of Missouri, Columbia, Missouri.

JOHN

T . W. EDMINSTER, Assistant Chief, Eastern Soil and Water Management Research Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland. D. L. GRUNES,Soil Scientist, Western Soil and Water Management Research Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Mandan, North Dakota.

R. M. HAGAN,Chairman, Department of Irrigation, University of California, Davis, California. D. W. HENDERSON, Associate Professor of Irrigation, University of Calif ornia, Davis, California, and Associate Irrigationist, Agricultural Experiment Station, United States Department of Agriculture, Davis, California .

L. W. HURLBUT,Chairman, Department of Agricultural Engineering, University of Nebraska, Lincoln, Nebrska. K. D. JACOB, Chief, Fertilizer Investigations Research Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland. P. J. GAMER, Professor of Botany, Duke University, Durham, North Carolina. P. G. M E IJERS, Agronomist, Groningen, Holland.

* On leave from the Division of

Meteorological Physics, C.S.I.R.O., Australia. V

vi

CONTRIBUTORS TO VOLUME XI

I-I. F. MILLER, JR., Chief, Harvesting and Farm Processing Resecrrch Branch, Agricultural Engineering Research Division, Agriculturul Research Service, United States Department of Agriculture, Beltsville, Maryland.

ROBERTD. MUNSON,Agronomist, American Potash Institute, St. Paul, Minnesota. M . B. RUSSEU,Head, Department of Agronomy, University of Illinois, Urbana, Illinois. Y. VAADIA,Assistant Professor of Zrrigation and Assistant Zrrigationist, University of California, Davis, California. D. WWSMA, Assistant Professor of Agronomy, Purdue University, Lafayette, Indiana.

PREFACE To serve as editor of this series is a rewarding experience on several grounds. In the past decade the editor has learned a good deal about agronomy and the ways of agronomists. Above all, however, he has had impressed on him a realization of the vigor of agronomic research, and of its accelerated pace. Investigators are abandoning empiricism and tackling head-on many of the tougher problems of soil science and crop science, frequently using the knowledge and skills developed in more basic sciences, or adapting them ingeniously to their needs. Many examples of this are to be found in the lengthy article in this volume which deals with the complexities of water in relation to the growth of plants in soils. This chapter, which occupies more than a fourth of the book, marks a new departure, in that it was prepared by an impressive group of co-authors under the sponsorship of a committee of the Agricultural Board of the National Academy of Sciences. Under the leadership of M. B. Russell, the committee sought to prepare a definitive and critical statement of the knowledge in this field, so that investigators in contiguous areas of agronomic science would be informed as to the present understanding of the many problems of water in relation to plant growth and crop productivity. The practice of including a regional survey dealing with soil resources and changing crop patterns of a selected area has been continued in this volume. P. G. Meijers discusses land use in the Netherlands, an area not generously endowed with productive soils, but raised to a high level of productivity by the development and adoption of intensive agronomic practices. The higher crop yields of the last two decades have come in part from the availability of new machinery which performs old operations more efficiently, more rapidly and more promptly, and in part from greater and more efficient use of fertilizers. It was therefore thought to be of interest to deal in this volume with some of these matters which, though perhaps not strictly a part of soil or crop science, are vital in modem agriculture. T. W. Edminster and H. F. Miller review the remarkable developments in agricultural machinery, K. D. Jacob the realm of chemical technology on which fertilizer production rests, while R. D. Munson and J. P. Doll discuss economic aspects of fertilizer use and raise issues not always considered by those concerned only with maximum yields. A. G . NORMAN Ann Arbor, Michignn August, 1959 vii

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CONTENTS

Contributors to Volume XI .

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Preface

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Page v . vii .

WATER AND ITS REL. TlON TO SOILS ..ND CROPS COORDINATED BY M. B. RUSSELL I. Introduction . . . . . . . . . . . . . . . . . 11. Water and the Hydrologic Cycle by M. B. RUSSELL,L. W. HURLBUT and D. E. ANGUS . . . . . . . . . . . . . . . . , 111. Interactions of Water and Soil by M. B. RUSSELL. . . . . , , IV. The Soil Environment and Root Development by D. WIERSMA. . . V. Plant-Water Relations by P. J. KRAMER and M. B. RUSSELL. . . , VI. Soil-Plant-Water Interrelations by R. M. HAGAN,Y. VAADIA, M. B. RUSSELL, D. W. HENDERSON and G. W. BURTON. . . . , , . VII. Summary and Conclusions . . . , . . . . . . . . , References . . . , . . . . . , . . . . . , , ,

1 4 35 43 51 77 118 122

THE ECONOMICS OF FERTILIZER USE I N CROP PRODUCTION BY ROBERTD. MIJNSON AND JOHNP. DOLL I. Introduction . . . . . . . . . . . . . . . . . 133 11. Concepts and Principles Involved in the Economics of Fertilizer Use . 134 111. Current Research on Economics of Fertilizer Use . . . . . . , 158 IV. Conclusions . . . . . . . . . . . . . . . . . 166 References . . . . . . . . . . . . . . . . . . 167 RECENT DEVELOPMENTS I N AGRICULTURAL MACHINERY BY T. W. EDMINSTER AND H. F. MILLER, JR. I. Introduction . . . . . . . . . . . . 11. Developments in Tillage and Seedbed Preparation . . 111. Developments in Planting Equipment . . . . . IV. Developments in Cultivating Equipment . . . . V. Developments in Spraying and Dusting Equipment . VI. Developments in Harvesting Equipment . . . . VII. Conclusions . . . . . . . . . . . . References . . . . . . . . . . . . .

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171 173 . 182 , 190 . 193 . 196 , 226 . 226

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FERTILIZER PRODUCTION AND TECHNOLOGY BY K . I3. JACOB I. Introduction . . . . . . . . . . . . I1. Consumption of Fertilizers and Plant Nutrients . . I11. Nitrogen . . . . . . . . . . . . . IV. Phosphorus . . . . . . . . . . . . V. Potassium . . . . . . . . . . . . . VI. Secondary Nutrient Elements . . . . . VII . Trace Nutrient Elements . . . . . . . . . VIII. Mixed Fertilizers . . . . . . . . . . . IX . Mixtures of Fertilizers and Other Agricultural Chemicals X. Future Prospects . . . . . . . . . . . References . . . . . . . . . . . .

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Page 234 235 242 262 287 294 299 303 309 311 312

SOILS AND LAND USE IN THE NETHERLANDS P . G. MEIJERS

I. General Situation: Land and People . . . . . I1. Climate . . . . . . . . . . . . 111. Soils and Cropping Systems . . . . . . IV. Plant Nutrient Requirements and Fertilizer Use . V Land Use and Productivity . . . . . . . References . . . . . . . . . . .

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EFFECT OF NITROGEN ON THE AVAILABILITY OF SOIL AND FERTILIZER PHOSPHORUS TO PLANTS BY D . L . GRLJNES

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I Introduction . . I1. Effects of Nitrogen on 1II.Summary . . . References . . .

. . . . . . . . . . . . . . . 369 the Availability of Phosphorus to Plants . . . 370 . . . . . . . . . . . . . . . 393 . . . . . . . . . . . . . . . 393

Author Index-Volume XI . . . . . Subject Index-Volume XI . . . . . Cumulative Author Index-Volumes VI-X . Cumulative Subject Index-Volumes VI-X

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WATER AND ITS RELATION TO SOILS AND CROPS Coordinated by M. 6. Russell Department of Agronomy, University of Illinois, Urbano, Illinois

Preface , . , . , , , , . , , , , . . . . . . . I. Introduction . . , , . , . , , , , . , . . . 11. Water and the Hydrologic Cycle . . . . . . , . . . . . A. The Physical Nature of Water by M. B. RUSSELL , . . . . B. The Agricultural Water Supply by M. B. RUSSELL and L. W. HURLBUT , , . . . , , . . . . . , . . C. Agricultural Water Use by D. E. ANGUS . . . . . , . . 111. Interactions of Water and Soil by M. B. RUSSELL . . . . . . . A. Water as a Factor Affecting Soil Properties . . . . . . . B. The Intake and Storage of Water by Soil . . . . . IV. The Soil Environment and Root Development by D. WIERSMA . . . V. Plant-Water Relations , . . . . , , . . . . . . . . A. The Role of Water in the Physiology of Plants by P. J. KRAMER . B. Drought Tolerance of Plants by M. B. RUSSELL . . . . . C . Crop Responses to Excess Water by M. B. RUSSELL . . . VI. Soil-Plant-Water Interrelations . . . . . . . . . . . . . A. Interpretation of Plant Responses to Soil Moisture Regimes by R. hl. HAGAN, Y. VAADIA, and M. B. RUSSELL . . . . . . . B. Factors Affecting Irrigation Practice and Water-Use Efficiency by D. W. HENDERSON . . , . . . . . . , . . . C. Crop Management for Improved Water-Use Efficiency by G. W. , . . . . . . . . . , . . . BURTON D. Moisture Conservation in Subhumid Areas by M. B. RUSSELL . . E. Management Practices Affecting Runoff and Water Yield by M. B. RUSSELL . . . . , , , , , . . , . . , . VII. Summary and Conclusions . . . . . . . . . . , , . . References . . . . . . . . . . . , , . , , , ,

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Preface

This review has been written as part of the work of the Committee on Soil-Crop-Water Relationships, appointed early in 1957 by the Agricultural Board of the National Academy of Science-National Research Council. In its consideration of present knowledge and research 1

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M. B. RUSSELL

needs in the broad field described in its name, the Committee has recognized that much work has been done on many facets of the total subject, Since several disciplinary fields are involved, it is difficult to obtain an integrated picture of the many interrelations that exist in the soil-plant-water system. This review is an attempt to develop such an over-all picture. The focus of discussion is on the part of the hydrologic cycle that begins when the raindrop strikes the soil surface and ends when the water molecule returns to the atmosphere or moves out of the range of plant roots. In determining the relevance of material, many subjective decisions were necessary. Not all possible topics are included, nor are those presented all discussed in equal detail. Such variations reflect both the authors' evaluation of the need for detail and the degree to which the subject seems to diverge from the central theme of the review. Even in the more abridged discussions, however, an attempt has been made to call attention to existing reviews or references through which the reader can obtain more detailed treatments. Several members of the Committee actively participated in the preparation of the review. Others not on the Committee also assisted in the writing of certain sections. The authorship of each section is indicated in the Table of Contents and in the text, The membership of the Soil-CropWater Relationships Committee is: G . W. Burton, A. S . Crafts, R. M. Hagan, L. W. Hurlbut, P. J. Kramer, Dan Wiersma, and M. B. Russell, Chairman. I. Introduction

Water, the earth's most abundant compound, is a vital constituent in all living matter. Because of its unique properties and ubiquitous nature, water affects in innumerable ways all aspects of human activity. It continues to reshape the landscape, is a dominant factor governing all aspects of the environment on the earth's surface, and since the beginning has been intimately involved in the rise and fall of civilizations. The use and control of water is therefore of vital concern to every human being and to every nation. High mobility is one of the distinguishing characteristics of water. Since it is the only compound that exists naturally in substantial quantities in the three physical states-solid, liquid, and gas-and since substantial quantities of heat are involved in transformations between ice, liquid water, and water vapor, this compound also plays a major role in the thermal economy of the earth and its atmosphere. The high mobility and thermal behavior of water are well illustrated in the series of inter-

WATER AND ITS RELATION TO SOILS AND CROPS

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connected dynamic events that are collectively called the hydrologic cycle. This review is concerned with biologic phenomena representing only a small sector of the hydrologic cycle: those involving water’s interrelations with soils and crops. Water may be considered as a renewable natural resource. From the geologic point of view it is indestructible, though for man’s purposes it can be used up through important changes that modify its suitability for other uses. Such incompatibility of alternative uses is a fundamental factor affecting man’s attempt to achieve maximum benefits from water use. The fact that water use itself undergoes continuous changes as a consequence of population changes and technologic advances further complicates the problem of achieving maximum benefits. History records that man has long recognized a need for developing procedures that will reconcile the conflicting demands for water. Recent reports by Ackerman and Lof ( 1959), The President’s Water Resources Policy Commission ( 1950), and The Presidential Advisory Committee on Water Resources Policy (1955) emphasize that a need still exists for improved water policies in the United States. No attempt will be made in this article to discuss the broad problem of resource development or the political and economic aspects of alternative use of water, although it is recognized that, in the final analysis, the use of water in the production of crops is inextricably linked with the broader economic, social, and political aspects of total water resource development. Although many aspects of the relation of water to soils and to crops have been discussed in recent comprehensive reviews, the authors of this article feel that insufficient attention has been given the interrelations between the properties and processes that characterize the soil-plant-water system. Therefore the main purpose of this review is to focus attention on the nature and importance of such interrelations and on the dynamic and interconnected nature of water in the soil-plant-atmosphere system. Although the review places major emphasis on conditions and problems encountered in the United States, it is believed that the principles discussed have wider applications and can serve as a basis for analyzing similar phenomena under different soil, crop, and climatic conditions in other countries. The discussion opens with a brief review of the physical nature of water, since its behavior in soils and plants is a direct consequence of the unique properties of the water molecule. This is followed by a discussion of the several components of the agricultural water supply and of the principal factors affecting water use by plants. The broad effects of water on soil properties and a brief discussion of the intake and storage of water lead to a more detailed consideration of soil factors affecting

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M. B. RUSSELL

the development of roots. Attention then turns to the physiologic role of water in plants and to the response of crops to excessive water and to drought. Interactions of the total soil-plant-water system are then considered, together with certain management practices that affect it. The review concludes with a brief summary and a statement concerning broad areas of research that merit increased attention. II. Water and the Hydrologic Cycle

To understand the role of water in crop production it is first necessary to examine the properties of the compound itself and to appreciate the over-all physical aspects of the hydrologic cycle of which agricultural water usage is a component part. Such are the objectives of this section.

A. THEPHYSICAL NATUREOF WATER M. 8. Russell University of Illinois, Urbono, Illinois

The water molecule is one of the simplest known, but its properties and characteristics are unique, which explains why this compound OCcupies such a vital role in all biological and most of the physical and chemical phenomena known to man (Hutchinson, 1957; Dorsey, 1940; Hendricks, 1955;Crafts et al., 1949).The two small hydrogen atoms and the much larger oxygen atom are held together by chemical bonds formed by pairs of electrons. Each pair consists of the orbital electron of the hydrogen atom and one of the outer orbital electrons of the oxygen atom. The remaining four outer orbital electrons of the oxygen atom also tend to form two pairs, which, as a consequence of mutual repulsion, tend to arrange themselves as far apart as possible from each other and from the two pairs formed with the hydrogen atoms. Thus the water molecule can be considered as an oxygen atom around which, and attracted to it, are four pairs of electrons forming the points of a tetrahedron. Since the hydrogen atoms are located at two corners of the tetrahedral arrangement of electron pairs, there results an asymmetric distribution of charge in the water molecule, which is reflected in its highly dipolar character. Another important consequence of the structure of the water molecule arises from the asymmetric distribution of electrons around the hydrogen nucleus. This gives rise to an attraction, called hydrogen bonding, between the hydrogen of the water molecule and unsatisfied electron pairs of other molecules. Since two such unsatisfied pairs are present in the water molecule itself, this type of bonding, although much weaker than

WATER AND ITS RELATION TO SOILS AND CROPS

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the 0-H chemical bond, is a factor of prime importance in determining the physical properties of water. The high heat of vaporization, a property of water that is of great significance in relation to the hydrologic cycle, is a manifestation of the high degree of hydrogen bonding of water. Such bonds, which have to be broken in transforming water from the liquid to vapor state, also account for the fact that this transformation takes place at a temperature 260” C. above that of another simple molecule, methane, which has nearly the same molecular weight but is free of hydrogen bonding between its molecules. Hydrogen bonding and the tetrahedral distribution of electron pairs around the oxygen atom also serve to explain the unusual increase in volume that occurs when water freezes. The open nature of the spatial arrangement of the water molecules arising from the bonding between the water molecules gives ice a lower specific gravity than water. The ice structure, upon melting, partially collapses, with water molecules OCCUPYing the “open spaces” in the ice structure. The facts that ice is less dense than water and that water has maximum density at a temperature slightly above the freezing point are both properties of great significance in the role of water in the thermal and hydrologic phenomena of the earth and its atmosphere. Hydrogen bonding is also responsible for the viscous nature of water and for the rapid decrease in this property as temperature increases. The intermolecular hydrogen bonds are disrupted by heat. Other important consequences of hydrogen bonding are the properties of adhesion, cohesion, and surface tension, properties that largely determine the retention and movement of water through porous media, such as soil and plant tissues. A final illustration of the unique properties arising from water’s molecular structure is the solvent action that is so intimately related to the role of water in biological systems. Water acts as a solvent for organic and some inorganic compounds by the mechanism of hydrogen bonding. In the case of saltlike compounds, water acts as a solvent by means of charge interaction as a consequence of the separation of charge between the hydrogen and oxygen atoms in the water molecule. In addition to the physical phenomena discussed above, stemming largely from the unique ability of the water molecules to associate through hydrogen bonding, the molecular structure of water has profound effects on its chemical properties. These properties depend on breaking the strong hydrogen-to-oxygen bond, resulting in the formation of the positive hydrogen ion and negatively charged hydroxyl ion. Through this mechanism, water becomes an active participant in chemical reactions and, as

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M. B. RUSSELL AND L. W. HURLBUT

such, is involved in most of the important chemical processes occurring in nature. Throughout the remaining sections of this article, water is considered in terms of its more macroscopic and familiar properties and in its behavior in soils and plants. The reader is asked to remember, however, that the observed behavior of this truly unique compound is, in the final analysis, traceable back to the structure and electronic configuration of the water molecule itself.

B. THEAGRICULTURAL WATERSUPPLY M. B. Russell and L. W. Hurlbut University of Illinois, Urbana, Illinois, and University of Nebraska, Lincoln, Nebraska

Water may be considered as the lifeblood of the earth. Its mobility, energy transformations, and physical and chemical behavior impinge on every facet of organic life. We live in and are part of the unending flux of water known as the hydrologic cycle. This complex series of interconnected flows and phase changes is shown in part in the schematic diagram in Fig. 1. The water that is agriculturally useful during any one year is an extremely small part of the world's total water supply. Including ground water to a depth of 12,500 feet, total supply is estimated to be about 165 trillion acre-feet. Roughly 93 per cent of this amount is found in the oceans and seas, and 7 per cent in fresh-water forms. The latter consists primarily of ground water (about 5 per cent), and polar ice and glaciers (about 2 per cent). The total amount of water in lakes, rivers, and soil moisture is about 1 per cent of the total fresh-water supply, or only about 0.08 per cent of the world's total water supply. A summary of estimated quantities of water in the several parts of the earth's hydrosphere is shown in Table I. Interchange of water is continuous, at varying speeds, among the several parts of the hydrosphere. In some instances the transit time is of the order of thousands of years, as in the case of deep ground-water movement or the cyclic movement of water through the polar ice caps and glaciers. Short-term cycles of only a few hours are also common, as in the case of the return of water to the atmosphere by evaporation from the wet soil surface immediately following a rain. The part of the hydrologic cycle of greatest general agricultural concern is the annual precipitation cycle. Each year about 89 billion acre-feet of water fall on the land surfaces of the world. This amounts to 7%times the moisture content of the earth's atmosphere, and 13%times the estimated amount of water stored in the soil. Roughly four-fifths of annual precipitation returns directly to

WATER AND ITS RELATION TO SOILS AND CROPS

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M. B. RUSSELL AND L. W. HURLBUT

TABLE I Estimated and Relative Quantities of Water in the Earth's Hydrospherea Acre-feet Total water Total fresh water Ground water to 12,500-ft. depth Lakes and streams Atmosphere Soil moisture Plants and animals Annual precipitation Annual runoff

165,000 X 11,000 x 8,200 X 118 x 12 x 6.5 x 0.9 x 89 x 17 x

Ratio to annual precipitation

lo9 109

lo9 109 109 109 109 109 109

1850 124 92 1.3 0.14 0.07 0.01 1 .o 0.2

Adapted from Ackerman and Lijf (1959).

the atmosphere, as evapotranspiration, with the remaining one-fifth accounted for in stream flow. Except for the relatively small amounts of water used from the ground-water reserves, whose cycle of depletion and recharge is much longer, practically all agricultural water use is identified with the annual precipitation cycle and involves the use of relatively short-term, low-capacity storage media. The water resources of continental United States are tabulated in summary in Table 11. These data indicate that average annual precipitaTABLE I1 Water Resources of Continental United States" Annual precipitation 4.75 X 109 acre-feet Annual runoff 1.3 X 100 acre-feet Estimated total usable ground water 47.5 X 109 acre-feet Soil moisture 0.6 X lo9 acre-feet Lake storage 13.0 X lo9 acre-feet Average annual precipitation 30 inches Average annual runoff 8 inches Average soil moisture storage 3.7 inches a

Adapted from Ackerman and LSf (1959).

tion is about 30 inches and average annual runoff is about 8 inches. Usable ground-water reserves are estimated to be equal to ten years of precipitation, and the total storage in lakes is 3%times the yearly precipitation. The average amount of available water stored in the soil for the area of the United States, however, is only about 3%inches of water. If the water supplies discussed in the preceding paragraphs were uniformly distributed over the United States, and if seasonal distribution

WATER AND ITS RELATION TO SOILS AND CROPS

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of the precipitation were matclied to crop needs, there woulcl be fcw areas of agricultural water shortage in this country. Neither of the two foregoing conditions exist, however, with the result that many areas are characterized by a marked imbalance between available water and agricultural needs. Geographic distribution of precipitation and runoff is shown in Figs. 2 and 3. Figure 4 shows the manner in which agricultural water use, as measured by potential evaporation, varies throughout the United States. The preceding figures indicated that, on the average, the eastern part of the United States and parts of the Pacific Northwest are regions of water surplus. The area west of the 95th meridian is, except for some of the mountain areas, a region of moisture deficiency if potential evaporation is taken as an index of agricultural water need. Even in the regions of average annual water surplus, water deficiencies are common in specific localities, because of (1) failure of seasonal distribution of rainfall to match seasonal water needs, ( 2 ) deviations of annual rainfall from average values, ( 3 ) excessive runoff resulting from high intensity of precipitation, steep topography, or low infiltration rate, as with frozen soil, and (4) low soil-moisture storage capacity for supplying crop needs between rains. Current rainfall and soil moisture constitute the “working water supply” for crop production. Because of its agricultural significance, water storage by the soil is of great importance, even though it averages only about 12 per cent of annual rainfall and 0.01 per cent of the world’s freshwater supply. Even so, the soil plays an important role in the hydrologic cycle. As a water storage medium it reduces runoff peaks, supplies moisture for growing plants, and retains a significant portion of precipitation in a manner permitting its early evaporation back to the atmosphere. The water storage capacity of soil is a function of its depth and physical composition, The volume fraction of voids multiplied by the soil depth is a measure of the gross water storage capacity of a unit area of soil. In many soils the volume fraction of voids varies with depth, making necessary an integration over each of the soil horizons to obtain the total profile storage capacity. In well-drained soils, and in dry regions where the subsoil is perennially dry, not all of the soil pores remain filled with water. Therefore the effective storage capacity of a soil is determined by the volume fraction of pores that remain water-filled after water essentially ceases to move downward. The volume fraction of water retained under such conditions is affected by soil texture, ranging from 0.08, for sands, to 0.30, for clays. For soils of intermediate textures such as loams and silt loams, 0.25 is a good approximation of the gross field water storage capacity. Using this figure, we find that 3 feet of a silt loam soil will store 9 inches

10 M. B. RUSSELL AND L. W. IIURLBUT

FIG.2. Average annual precipitation for United States (Evans and Lemon, 1957).

WATER AND ITS RELATION TO SOILS AND CROPS

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Y

wo

FIG.3. Average annual runoff for United States (Langbein and Wells, 1955).

12 M. B. RUSSELL AND L. W. HURLBUT

m

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a

30-36

81

36-42

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42-40 over 4 0

inches 19

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Y

G

FIG. 4. Average annual potential evapotranspiration for United States (Thornthwaite, 1948).

13 of water. However, not all of this water is available to plants. The volume fraction iinavailable to plants is also a function of soil texture, increasing from about 0.04, for sands, to 0.18, for clays, with 0.10 being a good approximation for soils of medium texture. As shown in Fig. 5, about 60 per cent of the effective storage capacity of well-drained soils may be considered available to plants. Factors affecting the retention of water by soils, the laws governing its movement, and its availability to plants are discussed in later sections of this review. WATER AND ITS RELATION TO SOILS AND CROPS

FIG.5. The effect of soil texture on water retention ( U . S . Dept. Agr. Yearbook Agr. 1955, p. 120).

In localities where rainfall and soil-moisture storage are inadequate to meet crop needs, other components of the hydrologic cycle must be drawn on to correct the deficiency. Surface water from streams and lakes and ground water are the sources that can be used. It can be seen from Tables I and I1 that each of these sources of water is much larger than the annual rainfall, and each has an order of magnitude larger than the soil moisture supply. However, as with annual precipitation, surface and ground-water supplies, as shown in Figs. 3 and 6, are not uniformly distributed and, in fact, are largely concentrated in those areas where current rainfall and soil storage are most adequate. Thus, in the humid region east of the 95th meridian, all streams of any size are permanent, and annual runoff exceeds 10 inches in most areas. Even there, surplus stream flow undergoes a pronounced seasonal variation. Except in Florida and the southeastern coastal plains, half or more of the annual runoff occurs in three months of the year. Since the period of peak stream flow

GROUND-WATER

AREAS IN THE UNITED STATES i I A

Y

!+

P

UNCONSOLIDATED AND SEMICONSOLIDATED AQUIFERS CONSOLIDATED- ROCK AOUIFERS

\.

BOTH CONSOLIDATED AND UNCONSOLIDATEDROCK AQUIFERS NOT KNOWN TO BE UNDERLAIN BY AQUIFERS THAT WILL GENERALLY YIELD AS MUCH AS 50g.p.m. TO WELLS

FIG. 6. Ground-water areas in the United States (Thomas, 1955).

WATER AND ITS RELATION TO SOILS AM) CROPS

15

is in the late winter or early spring, it does not coincide with the period of maximum agricultural need. Therefore, to achieve maximum use, runoff must be impounded for periods of about six months. Average annual runoff is much less in the Great Plains region than in the more humid Eastern States, and seasonal concentration is more pronounced-50 to 70 per cent in a three-month period over most of the region. Flow in major streams in this region is stabilized to some degree by runoff from the bordering mountains to the west. Runoff patterns in the western third of the United States reflect the mountainous nature of much of this area. Rainfall and soil storage are generally insufficient for intensive agricultural production. Irrigation, based on impounded mountain streams and on ground-water supplies, is widely practiced throughout this region. Runoff from the lowland areas is slight except in the Puget Sound area. As might be expected, the ground-water supplies depicted in Fig. 6 reflect in a general way the precipitation and evapotranspiration patterns and the geologic structures of the country. Three major types of groundwater areas are shown in Fig. 6: (1) the channels and associated alluvial deposits along water courses, ( 2 ) loose sands and gravels in glacial drift and outwash, and ( 3 ) such consolidated rocks as limestones, basalt, and sandstones. In 1950, ground-water withdrawals accounted for about 20 per cent of all the water withdrawn for municipal, rural, industrial, and irrigation use. The last accounts for more than 60 per cent of all ground water used in the United States. Nearly all of rural use, 25 per cent of municipal use, 7 per cent of industrial use, and 25 per cent of irrigation use are supplied from ground-water sources. The usefulness of a groundwater source is determined by capacity, depth and recharge rate of the aquifer, and, in some instances, the chemical quality of the water. Large quantities of ground water are held in clays and fine-textured materials of such low permeability that the discharge rate from wells is too low for practical utilization. It is estimated that 80 per cent of all water obtained from wells in the United States comes from unconsolidated sand and gravels, 5 per cent from limestone, 3 per cent from sandstone, and 2 per cent from basalt. For ground-water aquifers to serve as a continuing source of water, recharge rate must equal withdrawal rate. In some areas in the southwestern United States the underground water reserves are being steadily depleted (Fig. 7) and remedial measures are being employed to increase the recharge rate (Mitchelson and Muckel, 1937; Muckel and Schiff, 1955). The chemical nature, or quality, of surface and ground-water supplies has an important bearing on their agricultural usefulness. Water quality for irrigation is determined by total concentration of soluble salts, con-

16

M. B. RUSSELL AND L. W. HURLBUT

FIG.7. Areas in the western United States having ground-water reservoirs with perennial overdraft (Thomas, 1955).

centration of sodium and proportion of sodium to divalent ions, concentration of bicarbonate, and the content of toxic qualities of certain minor elements ( Richards, 1954; Thorne and Peterson, 1949). In humid regions, the water quality of streams is not a factor affecting their use for irrigation unless industrial pollution is heavy at the point of water withdrawal. In subhumid and arid regions, however, variations in water quality are great among different streams and different parts of a given stream (Fireman and Hayward, 1955). Ground water also varies widely in quality, reflecting both the nature of the source of recharge and the chemical properties of the aquifer. Salt water may invade fresh-water aquifers if excessive pumping shifts the interface between fresh water and salt water. The concentration of soluble salts in the soil solution is increased by the loss of water from the soil by evaporation and transpiration. To prevent a continued build-up of salt concentration, the net downward flow of water through the soil must be great enough to carry the soluble salts out of the plant root zone. Thus the salt balance of the soil is determined by the amount and quality of irrigation water applied and the effectiveness of the leaching and drainage processes. The relation of water

WATER AND ITS RELATION TO SOILS Ah?) CROPS

17

quality to soil behavior and to plant growth and irrigation practice is discussed in later sections of this review. As mentioned above, roughly three-fourths of the precipitation reaching the soil surface returns to the atmosphere by evaporation and transpiration. Since stream flow and the recharge of surface and underground supplies depend on the residual one-fourth of the precipitation, it is apparent that factors affecting evapotranspiration have a major influence on the yield of watersheds and river basins. Evaporative loss from lakes, ponds, and water-storage reservoirs becomes a major factor when the surface-to-volume ratio of the water body is large, as in shallow farm ponds, where seasonal evaporation loss will frequently be 25 per cent or more of the storage capacity of the pond. Surface-applied monomolecular organic films substantially reduce such losses and may offer a practical solution to this problem (Mansfield, 1955; Moran and Garstka, 1957). Water intercepted by plants or absorbed by the upper 2 inches of the soil is readily available for evaporation. It normally returns to the atmosphere within a few hours of the end of rain. For this reason, small showers (less than 0.10 inch) are of limited effectiveness, except as they change transpiration losses by shifting the energy balance and watervapor pressure of the leaf environment, thereby reducing plant use of soil moisture. Interception losses are affected by the nature of the vegetation and the type of precipitation, but are estimated to amount to 5 to 15 per cent of annual precipitation. Water that has penetrated more deeply into the soil is less susceptible to quick return to the atmosphere by evaporation, but may be returned through plant transpiration. Such return by nonbeneficial vegetation is a major factor affecting ground-water supplies in certain arid regions. In the seventeen Western States, an estimated 20 to 25 million acre-feet of water are wasted annually by nonbeneficial plant use (Robinson, 1952). This is equivalent to about two-thirds of the storage capacity of Lake Mead. The effects of use and management practices on consumptive use of water and yield of watersheds are discussed further in later sections of this review. In summary, it can be said that the agricultural water supply of the United States is generally good. The rapid growth of use for municipal, industrial, and irrigation purposes is shown in Table 111. Since the economic return per unit of water used is lower for crop production than for use by industries or municipalities, the long-range situation indicates a need for greatly increasing agricultural water-use efficiency. Figure 8 illustrates the major causes of the present low efficiency of water currently used for irrigation. East of the 95th meridian, where annual precipitation equals or exceeds potential evaporation, soil moisture and surface

1s

M. B. RUSSELL AND L. W. HURLBUT

TABLE I11 Preseiit and Estimated Water Use for t,he IJiiiLed SOates.’

Use Municipal and rural Direct industrial Irrigation

Estimated use, 1950

Estimated need, 1975

Billion yd./day

Billion gal./day

Per cent

Increase (per cent) 1950-75

25 215 110 350

7 62 31 100

50 170 25 90

Per cent

17

9

80

-

88

43 48 -

185

100

The Presidential Advisory Committee on Water Resources Policy (1955).

and ground-water supplies of high-quality water are available to meet crop needs during periods of moisture deficiency, which are normally short. In the Great Plains area, moisture deficits occur regularly. These may be alleviated by more complete development and use of surface and ground-water supplies and by management practices that reduce the short-term recycling of moisture back to the atmosphere. Agricultural water needs in the Mountain and Pacific Coast States can be met by transporting water from higher-elevation, nonagricultural areas to the productive valleys. Improved technology in storage, transport, and utilization of irrigation water will permit further development of ir-

10.2 TAKEN

5.3 DELIVERED

6.0 RECEIVED ON LAND

1.6 USED BY CROPS

L

c

; REGULATION WASTE

(EST.)

(EST.)

VALUES IN FEET-DEPTH OF WATER

FIG.8. A schematic summary of water conveyance (U.S. Dept. Agr. Yearbook

Agr. 1955, p. 120).

WATER AND ITS RELATION TO SOILS AND CROPS

19

rigated agriculture in this region coincident with the expansion of industrial and municipal water use. Long-term increases in the total freshwater supply for the United States may come from techniques for weather modification, economical recovery of sea water, and the development of techniques for sustained use of low-yield aquifers.

C. AGRICULTURAL WATERUSE D.

E.

Angus

University of California, Davis, California

Having considered various aspects of the agricultural water supply, particularly in the United States, attention now turns to the physical aspects of water use by crops. This is primarily an energy-controlled process, but it is modified by plant, soil, and atmospheric factors that govern the absorption and distribution of energy at evaporating surfaces, and by the flux of liquid water to, and water vapor from, such surfaces. Biologic processes that affect water use are discussed in later sections of the review. For further discussion of the topics presented here, the reader is referred to several recent publications (Deacon et al., 1958; Lettau and Davidson, 1957; Kramer, 1950; McIlroy, 1957; Penman, 1948a, 1956).

1. Crop Characteristics The main effect that type of crop has on consumptive use of water is the number of months the crop is in leaf. In designing or operating an irrigation system one is interested not only in the peak rate of water use but also in total seasonal use. A perennial crop, such as alfalfa, may have a low transpiration rate during the winter, but its use of water will increase and continue fairly high right through the spring, summer, and fall. On the other hand, with a crop such as sugar beets, which is not planted until spring and does not completely cover the ground for some time, the rate of water use will be low until complete ground cover is achieved; further, water use ceases when the crop is harvested. Thus, total water use is appreciably lower in sugar beets than in alfalfa. Percentage of ground cover also has an important effect on water use. A wet, bare soil surface will evaporate initially at quite a high rate, but evaporation is considerably reduced as soon as a thin layer of the surface soil dries out. If a crop in its young stages consists of small plants covering, say, 25 per cent of the soil surface, virtually no water will be lost from the bare soil in between the plants, provided, of course, that the soil surface is not rewetted by rain or irrigation. It might be expected, therefore, that

20

D. E. ANGUS

the use of water per acre by a crop would be more once the plants have grown sufficiently to cover the ground surface completely. Figure 9, a plot of the accumulated evapotranspiration against the various months of the year for a number of crops, shows that, though the curves start at different times of the year, they have essentially equal slopes during the summer months. This suggests that, given adequate moisture and complete plant cover, evapotranspiration is independent of crop type. Thornthwaite, the first to separate the climatic factors from the crop and soil factors, introduced the concept of potential evapotranspiration ( Thornthwaite, 1954). This maximum evaporation that can occur from crops under given climatic conditions was defined more pre-

a

a

c o n

___WINTER WHEAT 4 HARVEST a PLANTING

W

0

w

c

FIG.9. The seasonal distribution of water use by three crops.

cisely, by Penman, as “the amount of water transpired in unit time by a short, green crop completely shading the ground, of uniform height and never short of water.” Starting with this definition, Penman made the following two generalizations: First, for complete crop covers of different plants having about the same color, i.e., the same reflectivity, the potential evapotranspiration rate is the same, irrespective of plant or soil type. Second, this potential evapotranspiration rate is determined by the prevailing weather. The curves already referred to support such a concept. Since potential evapotranspiration is extremely difficult to measure, experimental data (Lemon et aZ., 1957; Mather, 1954; Rider, 1957; Penman, 1949; Makkink and van Heemst, 1956; Mendel, 1945) put forward in support of the concept, or otherwise, is of doubtful value. In some experiments there is evidence that soil and plant factors cannot be neglected, but in others there have been poor experimental techniques or inadvertent oversight of complicating factors. Nevertheless, it has not been demonstrated by

WATER AND ITS RELATION TO SOILS A N D CROPS

21

reliable experiments that there is any marked difference in the potential evapotranspiration rates of various crops. Penman ( 1956) and Neumann (1953) have attempted to show on theoretical grounds that the concept is valid, but the assumptions in their approaches do not permit an accuracy of better than about 210%.The reasons why different crops cannot be expected to have exactly the same evapotranspiration will be apparent in the following discussion of the physics of evaporation. 2. Physics of Evaporation Evaporation from natural surfaces, such as open water, bare soil, or vegetative cover, is a diffusive process, by which water in the form of vapor is transferred from the underlying surface to the atmosphere. Because the atmosphere is in a continuous state of turbulence, the process is overwhelmingly turbulent rather than molecular. There are two necessary physical requirements in the evaporation process. First, a source of heat is needed, to cause the liquid water to vaporize. This source may be in solar energy, in the air blowing over the surface, or in the underlying surface itself. Second, diffusion of matter can proceed only in the presence of a gradient of concentration of the substance in question. Thus, evaporation can occur only when the vapor concentration at the evaporating surface exceeds that in the overlying air. The first of these considerations provides the basis of the energybalance approach to the study of evaporation. To use it, it is not necessary to know the details of the process, but simply to be able to measure or to estimate all the other factors contributing to the thermal balance at the evaporating surface. Solar energy arrives at the upper limit of the earth's atmosphere at the rate of about 2 calories per square centimeter per minute. A considerable fraction of this is reflected or scattered by the atmosphere back into space, and does not affect the energy balance at the earth's surface. A schematic picture of the energy balances obtaining at the earth's surface at midday and at night is shown in Fig. 10. The thermal balance can be written in the following equation form: where R, is the incoming solar radiation, R,. the reflected solar radiation, R,, the net outgoing longwave radiation, H a the sensible heat flow into the air, H , the heat flow into the soil, and H , the evaporation heat, or latent heat flow into the air. Since the main source of energy for evaporation or transpiration is solar radiation, it is obvious that evapotranspiration must be less in regions where this intensity is low. The effect of the radiation is to in-

22

D. E. ANGUS

UNIVERSAL SPACE

LEGEND HEAT TRANSPORTED BY:

y SHORT

WAVE RAOIATION

LONG WAVE RADIATION

a OF

CHANGES OF THE PHYSICAL STATE THE WATER

TO THE S U R F A C E EVAPORATION

1 I

THE GROUND HEAT EXCHANGE AT NOON FOR A SUMMER DAY SUPPLIEO TO

I SUPPLIED FROM THE OROUNO

HEAT EXCHANGE AT NIGHT

THE WIDTH OF ARROWS CORRESPONDS TO THE TRANSFERRED HEAT AMOUNTS

FIG.10. A schematic summary of energy relations at the soil surface (adapted from Geiger, 1950). crease the temperature of objects that absorb it. During the day, therefore, plant foliage would always be warmer than the adjacent air if other factors did not enter in. As the leaf temperature is increased, however, so is the vapor pressure of the air in the substomatal cavity, since the saturation vapor pressure of air in contact with a wet surface is a function of the surface temperature. The vapor pressure gradient from the leaf outward into the adjacent air, which causes the water loss from the leaves, is also increased. The evaporation of liquid water requires energy in the form of latent heat; transpiration therefore has a cooling effect on leaves. In direct sunlight, leaves probably will still be at a higher temperature than will the nearby air, because of the large quantities of solar energy absorbed, but leaves in the shade may well be at a lower temperature than air because of the cooling effect of transpiration. If the air around the leaves is perfectly still, the flow of vapor will gradually increase the vapor pressure there, reducing the vapor-pressure gradient and thus reducing the transpiration rate. In the presence of wind, however, the moistened air is moved away from the leaves and replaced by drier air from some distance away. The vapor-pressure gradient con-

WATER AND ITS RELATION TO SOILS AND CROPS

23

tinucs to be maintained, and, within limits, an increase in wind speed will cause an increase in transpiration rate. Again, if the humidity of the air in the region is low, the transpiration rate will be higher than if humidity is high. The effect of increasing the air temperature is not quite so obvious. If the amount of water held in the air remains constant, increasing the temperature will of course decrease the relative humidity, but the actual pressure of water vapor will remain the same. However, increased air temperature will result in an increased leaf temperature of the plants, which in turn will result in an increased saturation vapor pressure at the leaf surface. Again, the net effect is to increase the vapor-pressure gradient from the leaf to the air, thus increasing the transpiration rate. Since the solar energy absorbed by a leaf is used, not only in evaporating water but also in heating the air and the plant, it might be expected that there would be an upper limit, given by the solar energy available, to the rate of evaporation or transpiration. This is not always true (Halstead and Covey, 1957). For example, consider an isolated plant in dry surroundings; none of the radiation absorbed by the surroundings goes into evaporating water, so that the temperature of the surroundings is increased over the temperature of the plant. When this heated air blows over the cooler plant, some of the extra heat of the air is given up to the plant and can be used to increase the transpiration rate. Under such conditions of advective heating, plant transpiration can exceed the maximum rate possible if radiation were the only source of energy. The thermal-balance equation gives the conditions that need to be met if potential evapotranspiration is to be independent of the crop and not exceed evaporation from a free water surface. Considering the first factor, the incoming solar radiation will, of course, be independent of crop type, but the amount of reflected shortwave radiation may not be (Geiger, 1950). This will depend on crop reflectivity. The loss of heat by longwave radiation will depend on the temperature, but, as a first approximation, can probably be considered independent of crop type. Thus, the energy available for evaporation will be the same only for crops of similar color. The amount of heat flowing into the soil will depend on the insulating properties of the crop above the soil surface. These may well differ between a short turf lawn and alfalfa 2 feet high. The sensible and latent heat flowing into the air will depend on the turbulence of the overlying air layers, and this will differ with the crops. A crop such as sugar beets, which has considerable variations in the height of the upper layer of vegetation, will cause greater turbulence in the overlying air than will a short-grass surface. A crop such as wheat, which bends in the

24

D. E. ANGUS

wind, also will have a different effect on the energy transferred by turbulence. These all suggest that the concept of potential evapotranspiration is only an approximate one, with certain crops using either more or less water than the average. In comparing a crop to a water surface, the interval over which the evaporation is considered is of considerable importance. Vegetation virtually ceases to transpire at night, but a water surface continues to evaporate. In fact, the considerable quantity of heat absorbed by a water body during the day becomes a source of heat at night. Thus, over a period of, say, a month, evaporation from water will probably exceed transpiration from a crop. Over a few hours during the day, however, a crop may evaporate more than a water surface.

3. Methods of Determining Evapotranspiration a. Water-balance methock, All these methods make use of some form of the hydrologic equation

P=E'+O+D+W where P is precipitation, E is evaporation, 0 is surface runoff, D is subsurface drainage, and W is the change in water content of the block of soil being considered. By measuring or eliminating all but one of these variables, the remaining one can be found. A well-known application of this principle on a large scale is the catchment area balance sheet, where, over long periods or from one state of wetness or dryness to the next similar one, W can be neglected, P and 0 are generally comparatively easy to measure, and D is small enough to require only an estimate. Where lakes and reservoirs are known to be free from seepage, D can be neglected, stream gaging will give 0, and W can be obtained volumetrically. On a smaller scale, enclosing a block of soil eliminates 0 and D, and W can be obtained by periodically weighing soil and container. This is the principle of lysimeters (Harrold and Dreibelbis, 1951, 1953; King et al., 1956; Kohnke et al., 1940). A lysimeter is a soil mass, including vegetation, isolated from its surroundings. By use of precise weighing mechanisms, evapotranspiration over short periods can be determined with a high degree of precision. Difficulties in isolating a soil block and yet maintaining its representative nature have the consequence that no single standard type of lysimeter suits all problems and soil types. To ensure representative behavior, a number of precautions must be taken. In particular, containers should be large enough to reduce the importance of boundary effects and to avoid restricting root development. To ensure proper drainage, the bottom of an isolated soil column will

WATER AND ITS RELATION TO SOILS AniD CROPS

25

often require artificial application of a moisture suction equivalent to that present at the same depth in the natural soil. Finally, because of possible advection effects, the influence of the surroundings must be reduced. To do this, each lysimeter should have around it a guard area maintained under the same crop and moisture conditions. For the same reason the container wall and the surrounding retaining wall should be as thin as possible, though not made of metal, which will increase heat flow to the deeper layers of the soil. Because of the very high cost of adequate lysimeter installations, a somewhat simpler device has been developed by Thornthwaite and others (Garnier, 1952; Gilbert and van Bavel, 1954; Mather, 1950, 1951). These potential evapotranspirometers, as they are called, are really a specialized form of lysimeter, limited in use to soils kept permanently moist. Instrumentally, they can be very simple. The basic requirements are a simple container, such as a large oil drum, with provision for irrigation and collecting and measuring percolation. In operation, a measured amount of water, always sufficient to bring about some drainage, is applied at regular intervals. With the soil never far from field capacity, W in the hydrologic equation can generally be neglected by comparison with D,particularly over long periods. The water consumption is then obtained as the difference between the amount of water supplied and that collected as percolate. The least expensive, but often also the least accurate, method of arriving at W is by sampling changes in soil moisture content throughout the volume of soil considered. Evapotranspiration from soil areas of any size can be determined in this way if there is no water table and if deep percolation is absent or measurable. Various means are available for measuring the soil moisture content, but the most precise yield evapotranspiration values accurate to only about 5' per cent in uniform soil thoroughly permeated by roots, and then only when the period of measurement is relatively long, e.g., a week or more. The recent development of a neutron-scattering method promises greater accuracy over shorter intervals. Apart from the poor resolution of the sampling method, there are two further sources of error. In humid regions an unknown percentage of measured rainfall may be lost by runoff or deep percolation instead of increasing soil moisture storage in the sampling zone. Further, any additions of moisture from dew are not included. In some regions an appreciable percentage of total annual precipitation may come in this form. Soil moisture sampling would indicate lower evapotranspiration from a crop under these conditions than under a similar climate with no dew. b. Standard deuices. Because of their ease of operation, standardized

26

D. E. ANGUS

tanks of water have become the most widely adopted evaporation instruments throughout the world, considerable effort has been devoted to empirical studies using pans of arbitrary dimensions, construction, and exposure (Kohler, 1952; Kohler et al., 1955; Bonython, 1950; Pruitt, 1959; Ramdas, 1957; Rohwer, 1931, 1934; Young, 1947, 1948). What is sought, ideally, is a single conversion factor applicable at any time under all conditions. In actual fact there are large variations in the conversion factors, depending in part on the size and type of pans used, the local environment, and the season. Most of this work has been applied directly to large water surfaces, and the assumption made that transpiration from an irrigated area will be similar to evaporation from such a water area. Not all of this variation in coefficients is due to the pans themselves. The evaporation rate from a large water surface is itself affected by such local characteristics as the depth and turbidity of the water and its rate of mixing. These determine the vertical extent of the water that shares in the incoming energy supply during the day or summer and supplies energy to the surface at night or in winter. In other words, they determine the effective heat-storage capacity of the lake and, hence, the degree of “lagging” of its daily and annual cycle of temperature and vapor pressure. Thus, the peak temperature and vapor pressure of a lake of sufficient depth will occur appreciably later than that of the surrounding countryside or a nearby pan. Its peak rate of evaporation will do likewise. An extreme example is afforded by the contrast between Lake Superior and a smaller, neighboring lake. The peak evaporation of the former occurs in winter, six months after that of the latter. The reason is that the large heat capacity of Lake Superior causes its surface vapor pressure to exceed the vapor pressure of the cold, overlying air in winter, whereas in summer it will not warm sufficientlyfor the surface vapor pressure to exceed that of the overlying air, moistened by strong evaporation over the land. Such phase differences, combined with appreciable amplitude differences, put out of question a universal pan coefficient. They also make it dangerous to apply existing empirical coefficients outside the ranges for which they have been determined. Although tank readings may have some use for indicating rates of evapotranspiration from thoroughly moist soil and vegetation, they again become inapplicable when water stress occurs. Under arid conditions, natural evapotranspiration may be virtually zero while pan evaporation, because of advection of heat from the surroundings, might increase rather than decrease. Other simple devices are being used in various parts of the world. One is a very shallow, black-metal evaporation pan. Virtually no solar radiation is reflected, and the half-inch layer of water contained in the pan has a very small heat capacity. Rate of evaporation and rate of transpiration

WATER AND ITS RELATION TO SOILS AND CROPS

27

might be more closely related with this type of pan than from the normal depth of water used in standard tanks. The Piche evaporimeter (Prescott and Stirk, 1951; de Vries and Venema, 1954) consists of a piece of blotting paper clamped across the mouth of a water-filled glass tube. It is supposed to simulate the behavior of transpiring plants. However, it gives little more than an indication of the water demand on a single leaf having the same exposure as the paper disk. When compared with the demands of vegetation as a whole, this device overestimates the effect of wind and underestimates that of radiation. Yet another device is the porous porcelain atmometer (Livingston, 1935). Strictly speaking, an atmometer is defined as any instrument of whatever form for measuring evaporation rates. However, the term is now more commonly applied to these porcelain spheres. Various workers have shown that such atmometers are sensitive to solar radiation, air movement, and the dryness of the surrounding air. Livingston originally used only white atmometers, but, realizing that plants absorbed more energy than these, he used darkened atmometers to make his measurements more representative of plant transpiration. Later, black and white atmometers were paired and the difference in their evaporation rates was found to be very well correlated with crop use of water (Halkias et al., 1955; Robertson and Holmes, 1956). Other workers have found a high degree of correlation between the difference in black and white atmometer evaporation and the intensity of solar radiation (O’Conner, 1955). However, using a difference in evaporation rates between two atmometers involves certain difficulties. Since a fairly small difference between two large quantities is involved, a small error in either quantity gives a very large percentage error in their difference. Further, dust and other types of soiling under field conditions make it difficult to maintain the reflectivity of either atmometer within a few per cent of its nominal value. Thus, the difference between evaporation rates can vary up to 25 per cent or more. c. Empirical equations. Because the meteorological factors on which evaporation depends are hard to measure over a large area with sufficient accuracy, several workers have combined more easily measured climatic elements into simple empirical formulas to give the water loss. The first empirical formula to be discussed is that of Blaney and Criddle (1950). This formula is based on the mean monthly temperature and the percentage of total annual daytime hours that occur in that month. The formula U = K F is used, where U is the consumptive use over the period, K is a coefficient depending on the crop, and F is the sum of the monthly consumptive use factors for the period considered. The monthly

28

D. E. ANGUS

consumptive use factor is equal to the mean monthly temperature in degrees Fahrenheit multiplied by the monthly percentage of daytime hours occurring in the year divided by 100. One disadvantage is that the coefficient K must be determined empirically for each different crop in the area in which it is desired to use the formula. Although this formula provides a reasonably good estimate of evapotranspiration in some moist regions, it fails badly in dry regions or in areas, such as that of Hawaii, where temperature variations are small. It is at first difficult to see why this formula should fit experimental data at all, since evapotranspiration does not depend on monthly temperature, and only indirectly on the number of daytime hours in the month. A consideration of the physical bases of evapotranspiration, however, shows that the underlying physics suffice for this to be a reasonable approximation. As mentioned previously, evapotranspiration depends in part on the vapor-pressure gradient from the evaporating surface to the air, The surface vapor pressure is a function of the surface temperature, whereas the vapor pressure in the air is relatively constant during the day. A simplifying approximation is thus to relate evaporation to surface temperature. A further approximation is to replace surface temperature with air temperature, which reveals the basis for the Blaney-Criddle formula. The term involving the daylight hours takes into account the fact that transpiration is predominantly a daytime process. Another formula has been developed by Prescott (1952), who uses the quantity “saturation deficit” raised to a power that is empirically determined. Saturation deficit is an estimate of the drying power of the air, on which evaporation is supposed to depend. It is equal to saturation vapor pressure at air temperature minus actual vapor pressure at the same temperature. Actually, evaporation does not depend on the saturation deficit, but the difference between the saturation vapor pressure at the surface temperature and the actual vapor pressure in the overlying air can be approximated by the saturation deficit. A third formula, which is well known, was developed by Thornthwaite (1948). His formula is based on latitude and mean monthly temperature. The actual formula and method of working are somewhat complicated, but their basis can be justsed in the same way as the Blaney-Criddle formula. Thornthwaite’s formula gives reasonably good estimates in climates similar to the humid climate in which it was developed, but the values it gives are considerably too low for semiarid climates. Also, it gives values which are out of phase with solar radiation (van Wijk and de Vries, 1954; van Wijk et al., 1953). Since the various empirical formulas show greater or smaller errors in different climatic regions, it is worth considering which climatic ele-

WATER A N D ITS RELATION TO SOILS AND CROPS

29

ments are the most appropriate to use. This depends in part on the type of problem for which the formula is required. In considering the requirements of proposed irrigation systems in new areas, it must be kept in mind that introducing irrigation will change the local climate. Any formula based on existing climatic elements will therefore not hold when the climate is changed. In such cases the formulas should be mainly in terms of the more conservative weather elements. Precipitation and incoming solar radiation are among the most conservative elements; net radiation, temperature, and wind speed are less so; and atmospheric humidity least. For this reason, Thornthwaite takes the somewhat extreme view that humidity factors should be excluded, but this obviously leads to difficulties between humid and arid climates. Since evapotranspiration is predominantly a daytime process, the factors in any empirical formula should be weighted heavily toward daytime values. Thus, mean maximum air temperatures or the saturation deficit at the time of maximum temperature might be expected to give greater reliability than the mean values employed at present. From the foregoing it is seen how simple empirical formulas can be useful over limited climatic ranges, but will require more factors for extended use. In view of the drastic simplifications involved, accuracy should not be expected for such formulas. If used, they should be developed for specific purposes and not expected to apply over a great range of latitudes or climates. Two other formulas should be mentioned at this time. They might be called semiempirical and have been developed independently by Penman (1948a, b, c, 1952) and Ferguson (1952). Both of these workers derived approximate solutions to evaporation formulas based on physical considerations. The more readily measured meteorological quantities are then used in these approximate solutions. Both formulas apply strictly to water surfaces. Penman’s formula, which includes net radiation, saturation deficit, and wind speed, appears to fit both humid and semiarid conditions reasonably well (Pearl, 1954; Penman, 1953). Its main drawback is in the tedious computations that are often required, but this is likely to apply to any formula that includes radiation. d. Vapor-flow methods. There are several approaches to finding the upward flow of water vapor into the atmosphere. The scientific study of evaporation began with Dalton, who designed experiments to investigate the factors controlling it and showed his results to be consistent with the formula that now bears his name, although there is no evidence that he expressed his results in this form. The rate of evaporation is given by the product of the vapor-pressure gradient from the evaporating surface into the overlying air and a function of the wind speed.

30

D. E. ANGUS

Many important evaporation studies have been undertaken, usually confined to evaporation from small water surfaces, and the constants in the Dalton equation determined (Kennedy, 1933;Marciano and Harbeck, 1952; Pasquill, 1949). Part of the variation of the experimental values of the constants is due to the different types and sizes of tank employed. Some is due to the arbitrary measuring heights used for both wind speed and atmospheric vapor pressure. The remainder arises from omission of other important factors in the Dalton formula. As previously discussed, it is the turbulent conductivity of the air, rather than actual wind speed, that conducts water vapor along the vapor-pressure gradient. Determination of this turbulent eddy conductivity under various meteorological conditions provides one of the most challenging problems in agricultural meteorology today. The second vapor-flow method involves direct determination of the eddy flow of water vapor in the atmosphere. At any point in the atmosphere, turbulent air movement will normally be present, giving rise to a fluctuating vertical component of wind speed. In the presence of a vertical moisture gradient these fluctuations will be associated with simultaneous fluctuations in the moisture content of the air. On the average, rising air will tend to be moister than descending air when the vapor-pressure gradient is directed away from the surface. This will give rise to a net transfer of water vapor. The mean rate of this transfer per unit of time and area is known as the vertical eddy flux of water vapor. Up to several meters above the surface, storage and side-flow effects in the intervening air layers are negligible, so that the eddy flux anywhere within this layer is virtually equal to evapotranspiration at the surface. The tremendous instrumental difliculties involved in measuring turbulent fluctuations of vertical wind speed and the moisture content of the air at a point have been surmounted by a group in Australia, and appropriate equipment has been in use there for some years ( McIlroy, 1955; Swinbank, 1951, 1955). Recently additional instrumentation has enabled automatic averaging, multiplication, and integration of the quantities involved as the eddy fluctuations are detected, so that evapotranspiration over an interval is given as a reading on a counter (Taylor, 1958; Taylor and Dyer, 1958). This method is completely independent of the nature of the underlying surface, and in no way disturbs it. It is, therefore, the best method currently available for measuring evapotranspiration from natural surfaces. The only limitation is in the height of the sensing’elements. The closer they are to the surface, the more nearly does the eddy flux become a measure of surface evapotranspiration, though the greater is the influence of inhomogeneities in the underlying surface. As the height of the

WATER AND ITS RELATION TO SOILS A N D CROPS

31

measurements increases, the readings become more representative of a11 extensive area. The final method to be discussed is that known as the aerodynamic approach. Like the Dalton equation, this method makes use of the general flow law, which states that the rate of flow of any quantity is equal to the product of the concentration gradient and the conductivity, but it is based on a better physical understanding of the principles involved. It considers the turbulent transfer of water vapor at a small distance above the ground, rather than the over-all process of ground-to-air transfer. In its basic form the empirical wind coefficient is replaced by the eddy conductivity. The moisture gradient can be measured directly between two heights, so the problem becomes one of finding a suitable expression for eddy conductivity in terms of other measurable quantities. Under most conditions near the ground, the eddy conductivity of water vapor can be assumed to be equal to that for momentum. The latter is in principle determinable from measurements of changes in wind speed with height. Considerable precision in the measurements is necessary when this approach is used, since even small errors in individual measurements can alter the final result considerably. However, the physics underlying the method, together with the limitations and precautions that must be observed in its use, are now fairly well understood (Halstead, 1951; Osborn, 1951; Pasquill, 1949a, b, 1950; Rider and Robinson, 1951; Rider, 1954a, b; Thornthwaite and Holzman, 1942). It has been shown to be workable with water surfaces, pastures, and short crops, but is not likely to be practical with tall crops, such as orchards, or with flexible crops, such as wheat, which bend to varying degrees with changes in wind speed. e. Surface-energy balance. The basic equation used above in discussing the physics of evaporation was an energy equation. It is possible to measure a sufficient number of terms in this equation to calculate the evaporation ( Cummings, 1940; Penman, 1951; Sumner, 1947; Anderson, 1952; Holzman, 1941; Suomi and Tanner, 1958). The over-all radiation balance can be found from separate measurements of the incoming and outgoing components of both shortwave and longwave radiation. However, it is far better to obtain net radiation directly, using one of several designs of net radiometers now available. Heat flow into the soil is more difficult to determine, but, except over short periods, it is only a small component of the balance, and is often neglected. It can be measured directly with heat-flow meters buried in the ground, or can be found from changes in the temperature profile of the soil, combined with measurements or estimates of its heat capacity. The partition of the remaining energy between evaporation and heating of the

32

D. E. ANGUS

air has to be approached indirectly, since the heating of the air cannot be measured any more easily than evaporation itself. This partition is usually carried out by means of the Bowen ratio. Bowen derived an expression for the ratio of sensible heat flow to latent heat flow as being proportional to the ratio of the temperature difference between two heights and the vapor-pressure digerence between the same heights (Bowen, 1926). Strictly speaking, this holds only under neutral conditions, i.e., when the rate of temperature fall with increasing height from the earth‘s surface is small. Whenever this temperature fall becomes large, as in the middle of the day, buoyancy effects begin to play a part, and there is some evidence that, under these conditions, sensible heat is transferred from the surface more readily than water vapor. However, by working close enough to the ground, such effects can be reduced, and it becomes reasonable to assume equal eddy conductivities for heat and water vapor. As in the aerodynamic method, accurate measurement of humidity is a problem. Nevertheless, this method appears attractive. f. Advantages and disadvantages of the above methods. Of the waterbalance methods, application of the hydrologic equation to large areas is useful only over long periods and can tell us nothing about the process of evaporation. Similarly, the measurement of changes in soil moisture content will never be accurate enough for short periods. Large weighing lysimeters with adequate moisture control and guard-ring areas are capable of high accuracy over short intervals. The main disadvantages here are immobility and the high cost of installation. Where only moist conditions are being considered, the simple form of potential evapotranspirometer is worthy of consideration. Where great accuracy is not required, evapotranspiration from crops can be correlated with evaporation from standard devices. For certain purposes, evaporation pans can still be useful, and, for reasons of continuity and comparability of records, these devices cannot yet be abandoned. Empirical formulas, although of some help where only scanty data are obtainable, should not be expected to apply outside the range of conditions for which they were derived. Since such formulas have relatively poor physical bases for giving reasonably good approximations to actual evapotranspiration, one suspects that this phenomenon is fairly insensitive to appreciable changes in many of the factors on which it depends. There is, therefore, some reason to hope for the development of simpler formulas, such as the Dalton equation, that will have sufficient accuracy and yet require only simple meteorological observations. Of all the formulas adequately tested so far, Dalton’s formula and Penman’s formula are the only ones that appear to have sufficient reliability.

WATER A N D ITS RELATION TO SOILS AND CROPS

33

Of the other vapor-flow methods, the eddy-flux determination should, with further development, have considerable use. The mobility of the equipment is an advantage, as is the considerable range of natural and crop surfaces that can be investigated without disturbance. The aerodynamic method has been shown to be workable with some crops, but computation of the results is laborious. It is likely to be more useful and practical in short-period studies than in protracted investigations. The energy-balance method has the advantage of requiring only relatively simple instrumentation and is capable of fair accuracy when temperature and humidity observations are made at low levels. A disadvantage is the considerable labor involved in analyzing the records, as in the aerodynamic method, 4. Amount and Importance of Dew Many conflicting opinions have been expressed about the importance of dew for vegetation, but very few of them have been substantiated by sufficiently accurate experimental measurements. The atmospheric processes involved in the deposition of dew are just the reverse of those in evaporation and can be studied through the application of the same principles (Neuman, 1956; Yamamoto, 1937). Basically, over an extensive area, there is some upper limit to the energy that can be released from the latent heat of dew condensation. Secondly, dew formation requires a vapor-pressure gradient directed from the atmosphere to the condensing surface. These gradients are smaller than those of the opposite sign, observed during the day, and the vertical-eddy conductivities are at least a magnitude smaller. The quantities involved in dew fall are, therefore, correspondingly less than in evaporation. For a large area the net outgoing radiation per unit area of projected horizontal surface is controlled by temperature and humidity. The latent heat of the total dew released will, in general, fall far short of this radiated heat, since heat is also brought to the surface by eddy conduction. This leads to the assignment of an upper limit of about 1mm. per night for the depth of the film of dew released (Monteith, 1957). Even in quite moist air this would be the total amount of vapor held in the lowest several hundred feet of air over the surface. It is therefore clear that a much deeper layer contributes to dew fall, since this layer of air does not dry out appreciably during the night. It also follows that there can be no general law of proportionality between dew deposition and plant leaf area, as has sometimes been assumed. This assumption has, in fact, led on occasions to estimates in excess of the physical limits mentioned above. These principles must be modiiied before being applied to isolated

34

D. E. ANGUS

plants, because the pattern of radiation and moisture flow may then be distorted from the previous one-dimensional form. A single tall plant surrounded by bare soil, or a shorter crop, will radiate more than in proportion to its projected area (Angus, 1958). The leafier plants, with poorer thermal diffusivity, will cool more than others, thus attracting dew toward them. The process of dew attraction brought about in this way, however, is one in which some parts gain at the expense of others, and the limitations on the average deposit over a large area are in no way affected, There may thus be an optimum spacing at which the highest possible fraction of the dew is attracted to the plants rather than to the intervening barer areas, and it may well be that the natural habit of some desert species to grow in clumps separated by clear spaces represents to some extent a natural regression toward this optimum. When the thermal diffusivity of the surface is lowered-for example, by tillage, mulching, or crop growth-the surface temperature will be lowered. When such a surface is of considerable extent, the application of energy conservation indicates that the rate of dew fall is unlikely to be affected, i.e., the dew on a large area of tall grass will be no greater than on a large area of short grass; but if such areas occur in patches, random advection from warmer and moister areas will enhance the rate of dew formation. In the same way, dew will normally condense on the top of the plant cover, which is the coldest part, and the lower parts may be entirely dew free. These factors depend entirely on the nature of the crop, and studies purporting to deal with variations of dew with height above ground are almost meaningless as such, Since the vapor-pressure gradient is small, the difficulties inherent in making reliable measurements of dew can be seen from the foregoing discussion. Small variations in the temperature, and hence the saturation vapor pressure, of the condensing surface will have a large effect on the gradient, and hence a large effect on the rate of condensation. It is therefore virtually impossible to obtain on a standard, artificial surface, measurements of dew that are generally representative of natural vegetation. If the condensed moisture is absorbed in some way-for example, by filter papers-and then weighed, the moisture will probably have come not only from the atmosphere but also from the soil. Any moisture from the soil will, of course, be of no additional benefit to plants in arid regions. The only reliable way of determining condensation from atmospheric sources is to weigh an isolated representative portion of the natural surface (Craddock, 1951; Jennings and Monteith, 1954), although an optical method has been attempted (Duvdevani, 1947). Lysimeters of sufficient sensitivity could be used, but the dead mass of soil that must

WATER AND ITS RELATION TO SOILS AND CROPS

35

be included usually makes sensitivity insufficient to detect the small amount of dew condensation. Since the maximum amount of dew condensation on a uniform extensive area may be only 10 per cent of the water normally transpired during the following day, and even for isolated plants is unlikely to exceed 50 per cent of daytime transpiration, it is difficult to find any physical explanation for the importance that is attached to dew in many arid regions (Ashbel, 1936). It appears that, for dew to have any appreciable benefit, a plant must be able to exercise some physiological control in reducing rate of transpiration appreciably. There are two possible roles that dew may play. The passive role is to remain on the surface (Drummond, 1945) and delay the rise in temperature and the onset of transpiration stress the following day. There is good evidence that the evaporation of liquid water from plant foliage conserves the same amount of water in the soil that would otherwise have been removed by the plant. The possible active role of dew is to be taken up by the plant and enter into the liquid cycle within (Stone d al., 1950; Stone and Shachori, 1954). However, the physiological details of such a role are still matters of controversy, requiring more research. 111. Interactions of Water and Soil

M. B. Russell University of Illinois, Urbona, Illinois

As already mentioned, the soil plays an important stabilizing role in the part of the hydrologic cycle that is of agricultural significance. Water, in turn, plays a major part in determining the properties of the soil. These interactions are briefly discussed in this section.

A. WATERAS

FACTOR AFFECTINGSOIL PROPERTIES Water plays a major role in determining the nature of soils and the properties and processes that govern their agricultural usefulness. In the geologic processes leading to soil formation, water has participated in the two major processes of weathering and erosion. The nature and extent of weathering is a major factor governing the inorganic nutrient status of a soil. Such weathering, as well as the biological processes that determine the nitrogen supply in the soil, are both determined primarily by temperature and the moisture supply. However, this article is limited to other ways in which water affects soils and crop production. A

M. B. RUSSEXL

36

Water, either in liquid or solid form, has been the major vehicle for the movement, segregation, and deposition of soil parent materials. When soil detachment and movement are accelerated by water movement resulting from man-induced changes in land use, a soil-management problem is created. Major problems have resulted in many agricultural areas. No discussion of the many important agricultural, social, and economic aspects of erosion control will be attempted here, although it must be recognized that such phenomena are an important facet of the over-all hydrologic cycle and have many far-reaching effects on crop production. Within the more restricted framework of current crop production practices, water has important effects on the amount and kinds of nutrients found in the soil and on their availability to plants. In the permeable soils of humid regions where there is an annual flux of water through the solum, planning a fertility program involves consideration of the loss of soluble soil constitutents by leaching. The supply of soluble ions such as nitrate must be matched to uptake by roots if excessive losses are to be avoided. On soil having an appreciable base exchange capacity, leaching of other nutrients is less critical; but, in very sandy soils, these also must be applied at relatively frequent intervals in small amounts matched to crop needs if efficient fertilizer use is to be obtained. In subhumid and arid regions the soluble salts are not leached from the soil but tend to be concentrated at the soil surface as the result of movement of water to the surface and its loss by evaporation, The concentrations of total soluble salts or of sodium are sufficient in some soils to prevent their use in agriculture. In subhumid and arid soils the possibility that toxic salt concentrations will develop in the surface soil must always be considered when irrigation water is supplied. In all irrigation systems in such soils, water applied as rainfall or irrigation must be sufficient to provide a net downward flux of water through the soil profile. Waterlogging, caused by excessive water and inadequate drainage, TABLE IV State of Oxidation of Certain Soil Constituentso

Element Carbon Carbon Nitrogen Sulfur Iron Manganese a

Russell (1952).

Normal form in well-oxidized soils

coz NOa-

so,- Fe+ + + (ferric) Mn++ + (manganic)

Reduced form in waterlogged soils CHI Complex aldehydes, etc. NI and NHa HIS Fe+ (ferrous) Mn+ (manganous) +

+

WATER AND ITS RELATION TO SOILS A N D CROPS

37

results in significant changes in the level of oxidation and solubility of several important nutrient ions and soil constitutents (Robinson, 1930). A summary of such changes is given in Table IV. Because of the altered solubilities and ionic forms associated with anaerobic soil conditions, plants subjected to waterlogged conditions may show either toxicity or deficiency symptoms. A more detailed discussion of plant responses to excessive water will be found in a later section of this article. On the purely physical side, water modifies the thermal properties of the soil largely through an increase in heat capacity accompanying the increase in volume fraction of water in the soil. For this reason, both the magnitude and rate of diurnal and seasonal changes in temperature are smaller in wet soils than in dry soils. The movement of air into and out of the soil also is highly dependent on the amount of water present in the soil voids. Finally the effects of water on mechanical behavior should be mentioned. Dry soils are able to support heavy loads, because of the rigidity of the individual soil particles and the high internal friction of the soil mass. But added water reduces internal friction, making the soil susceptible to plastic deformation and consolidation. As water is further increased, the soil takes on the properties of a fluid and loses its loadbearing properties entirely. This extreme change in the rheologic properties is of agricultural importance particularly as it affects the susceptibility of soils to compaction when subjected to traffic, as in tillage operations. The effects of moisture content and organic matter on the compaction that will result from a given amount of impact are shown in Fig. 11.

FIG.11. The effect of moisture content on soil compaction. Soils A and B contain 2.8 and 4.1 per cent organic matter, respectively (Free et al., 1947).

38

M. B. RUSSELL

B. THEINTAKE AND STORAGE OF WATERBY SOIL Water arriving at the soil surface moves laterally over the surface, infiltrates the soil, or is impounded on the surface until it returns to the atmosphere by evaporation. The division of incident water into these three portions is determined by the amount and intensity of the precipitation and by the slope and water-intake characteristics of the soil surface. Water impounded in small irregularities on the soil surface evaporates rather quickly, contributing little to crop production, and can be considered as a loss of water from the agriculturally important part of the hydrologic cycle. Standing surface water may contribute to the incidence and spread of crop diseases, and, if present for extended periods, will cause serious crop damage by restricting the movement of oxygen into, and carbon dioxide out of, the soil. Water that moves over the soil surface concentrates in drainage ways and reaches stream channels. Such water, accounting for about onefourth of total precipitation of the United States, is a major source of water for irrigation. Erosion, one of the important consequences, of runoff, reduces the productive capacity of agricultural lands and the capacity of water-storage structures. Although both of these phenomena are of great significance as factors affecting crop production, they are outside the scope of this article, and are not discussed herein. The entry of water into soil and its retention in the rooting zone for use by crop plants are processes of paramount agricultural importance in the hydrologic cycle. The capacity of a soil to store water is described above as a function, primarily, of its depth and porosity. The amount of water that actually enters the soil also depends on porosity, as well as on previous moisture content and the intensity and duration of precipitation. Infiltration tends to be higher in warm months than in cool months, and is greatly affected by the type of vegetative cover on the soil. It is correlated positively with organic matter content, the percentage of large pores, and state of aggregation of the surface soil. The rate of water intake decreases with time during a given infiltration period, partly because of the plugging of larger pores in the soil surface that accompanies destruction of the surface aggregates. A protective cover of living or dead vegetation dissipates the kinetic energy of the falling raindrops, greatly reducing such aggregate destruction and slowing the rate of decrease of infiltration. The effects of soil texture and type of crop on the rate of water infiltration are shown in Fig. 12. Water that enters the soil is held by capillary and surface forces. The affinity with which it is held is a reciprocal function of moisture content. Thus, at high moisture contents, water will move out of the soil by

39

WATER AND ITS RELATION TO SOILS AND CROPS

SHALLOW, SANDY, SILTY SOILS CLAYEY SUBSOIL

I

HIGH CLAY CONTENT HIGH SWELLING PERCENT

MINUTES

120

140

240

300

FIG.12. The effects of texture and vegetative cover on infiltration (Musgrave, 1955).

gravity, whereas at lower moisture it is retained even against extracting forces several thousand times that of gravity. Moisture desorption curves representing the functional relation between moisture content and the security with which the water is held by the soil have proved to be a very useful way of describing soil-water relations. Figure 13 shows typical desorption curves for three soils of widely differing textures. The curves illustrate the fact that soils of similar gross moisture-storage ca-

FIG.13. Idealized moisture-retention curves for three soils.

40

M. B. RUSSELL

pacities may differ greatly in the amounts of water retained at a given energy of retention. The energy of retention of soil moisture is expressed in terms of an equivalent negative pressure, called soil moisture tension. This important moisture parameter can be measured in situ by soil moisture tensiometers. Such measurements are useful in analyzing soil moisture movement and water usage by plants. Although tensiometers are limited to tensions of 1 atmosphere or less, this covers the range in which most moisture-flow occurs and in which 50 per cent or more of the water use by plants is found, Soluble salts in the soil also affect water uptake by plants. The effects of such salts on the energy status of soil water is described by the osmotic pressure of the soil solution. Therefore, in soils containing significant quantities of soluble salts, soil moisture tension and osmotic pressure are summed to describe soil moisture-plant relations. This sum is called the soil moisture stress. When water enters the surface of a dry soil, it moves downward, under the influence of attractive forces and gravity, in the form of a more or less well-defined wetting front. If the amount of water added is insufficient to cause the wetting front to reach the lower end of the soil mass or to contact a zone of moisture saturation, the downward movement will stop within a few hours after water entry at the surface is discontinued. The moisture content of the wetted portion of the soil column, in quasi-equilibrium with drier soil below, is known as field capacity ( FC ) This parameter represents, for most practical purposes in well-drained soils, the upper limit of soil moisture used by plants. For many soils of medium texture it is approximately equal to the water retained by a soil at a soil moisture tension of % atmosphere. The lower limit of plant-available soil moisture is known as the permanent-wilting percentage ( PWP ) , This biologically determined parameter is measured by growing dwarf sunflowers in a standardized quantity of soil. When the plants have attained a given size, no further water is added to the soil and the plants are allowed to wilt. When the wilting reaches a predetermined severity, the plants are removed and the moisture content of the soil is determined. The moisture percentage so obtained is called the PWP and represents for most practical purposes the lower limit of plant-available water in a soil. For many soils it is closely approximated by the moisture content held at a soil moisture tension of 15 atmospheres (FAP ). The movement of water into and through soil is the result of a potential gradient. In saturated soils the soil moisture tension is at or near zero throughout the entire soil mass. Under such conditions the driving force causing flow is the gradient of the gravitational potential, and

.

WATER AND ITS RELATION TO SOILS AND CROPS

41

water thus flowing is commonly known as gravitational water. This is the water that flows downward through the soil into the drainage channels. Water flow in saturated soils is described by Darcy’s Law, u = ki, which states that the velocity of flow, u, is proportional to the hydraulic gradient, i. Hydraulic conductivity, k, a parameter dependent on the properties of the fluid and the porosity of the soil, is widely used to characterize drainage and water-transmitting behavior of soils. The flow of water in unsaturated soils is more difficult to analyze. In such situations, flow occurs as a consequence of gradients in soil moisture tensions and/or the gravitational potential. The vector sum of these two gradients is termed the net driving force, and may result in flow in any direction. The proportionality between the velocity of flow and the net driving force is a function of the properties of the liquid and of the volume and configuration of the liquid-filled pores. This proportionality is expressed as hydraulic conductivity and is highly dependent on the volume fraction of water in the soil. The effects on moisturetransmitting ability of changes in degree of saturation of soils of different texture are shown schematically in Fig. 14. It is seen that the relative

MOISTURE CONTENT .-c

SATURATION -

FIG.14. Curves summarizing the effects of texture and moisture content on the water-transmitting ability of soils.

water transmissibilities of coarse and fine-grained soils undergo a reversal at relatively low degrees of unsaturation. In view of recent investigations emphasizing the importance of moisture transmission as a factor affecting water uptake by plant roots, the curves in Fig. 14 give added significance to the effects of soil texture on soil-water-plant relationships. Water also moves through unsaturated soils by vapor transfer. Such movement is a consequence of gradients in the aqueous vapor pressure in the soil atmosphere. Since the vapor pressure of water is strongly

42

M. B. RUSSELt

temperature dependent and insensitive to soil moisture changes in the plant-available moisture range, the flow of water vapor in soil is largely the result of temperature differences, with vapor moving toward areas of lower temperature. Because vapor transport occurs through the airfilled voids, its importance as a moisture-moving process increases as the degree of unsaturation of the soil increases. Vapor transport is probably responsible for much of the seasonal changes in the profile distribution of water in soils in which the moisture content is well below field capacity. Although the gross water-storage capacity of a given soil is essentially constant, the amount of water present usually undergoes rather wide seasonal variations. In humid regions of the United States, the soil profile is normally recharged to capacity at the beginning of the growing season. Depletion of subsoil water reserves occurs during the summer months, when evapotranspiration exceeds precipitation. During the fall, winter, and early spring, precipitation normally exceeds evapotranspiration, and the subsoil is recharged. In regions where total annual precipitation is less than potential evapotranspiration, the subsoil is seldom recharged to capacity, and efficient water management becomes the prime consideration in land use. The annual depletion and recharge cycle that characterizes soil moisture storage emphasizes the importance of the process of water entry into the soil. For rainfall to be fully effective in using the soil storage capacity, its rate of fall must not exceed the rate at which water can infiltrate the soil. Where topography is uneven and annual precipitation is less than annual potential evaporation, or only slightly greater, major attention should be placed on maintaining the highest possible infiltration rate for efficient use of water. This can be accomplished by providing a protective canopy of living or dead vegetation over the soil surface as much of the time as possible, and by developing a stable well-aggregated surface soil. On sloping land, water-retention practices that increase the time available for water infiltration are also effective in increasing profile recharge. In summary, it may be said that the soil acts as an important waterstorage device that smooths out month-to-month fluctuations in water supply available for plant use. The capacity of the soil for storage of available water is determined by its texture and porosity and the rooting volume of the crop. The rates of movement of water into and through the soil are important in determining the effectiveness of a soil as a storage medium of water for plants.

WATER AND ITS RELATION TO SOILS AND CROPS

43

IV. The Soil Environment and Root Development D. Wiersma Purdue University, Lafayette, Indiana

The amount of soil water that is available to a plant is determined by the moisture characteristics of the soil, the depth to which the plant roots extend, and the proliferation or density of the roots. The moisture characteristics, such as field capacity and wilting percentage (commonly used as limits of plant-available water) are peculiar to a soil, and are a function of texture, structure, and organic matter. Little can be done to alter these limits; greater possibilities lie in changing the characteristics of the plant, enabling it to extend its rooting system deeper into the soil, thereby enlarging its reservoir of water. The density of root proliferation in the soil is also an important consideration. Water in unsaturated soil moves very slowly, and only a distance of a few centimeters. It is necessary for the plant to have roots that completely ramify in its rooting zone in order to utilize all water available within the reservoir. Plants vary genetically in their rooting characteristics. Some, such as onions and potatoes, have a sparse rooting system and are unable to use all the soil water within the root zone; others, such as the forage grasses and sorghums, have very fibrous, dense roots. Lettuce has a single tap root, whereas corn extends out a distance from its base. Alfalfa has a deep root; that of Ladino clover is very shallow. Whether a plant is an annual or perennial is another factor affecting its water relations. An annual plant must extend its roots down into the soil to make available all the water it can potentially use. A perennial rooting system is already established as to depth, and needs only to extend its small roots and root hairs to be able to utilize the entire amount of soil water. Rooting characteristics of plants are difficult to observe. However, techniques have been developed for studying root distribution and activity (Weaver, 1926; Bloodworth et al., 1958; Lipps et al., 1957). Plants may be limited in their rooting by other factors than genetics. Any factor that will affect the vigor or condition of a plant may be expected to influence its extraction of moisture from the soil. Physical conditions of the soil, such as moisture, aeration, compaction (bulk density), and temperature can limit or enhance the growth of roots. Chemically, the pH, fertility, and salinity of the soil have been shown to influence the rooting of plants. Crop-management practices, such as cutting the

44

D. WIERSMA

top growth at different physiological stages, and the cultivation and cutting of surface roots alter rooting habits and the ultimate water economy of the crop. The presence or absence of beneficial or harmful soil organisms and diseases and insects, as brought about by any of these physical or chemical soil conditions, may limit root growth and the ability to absorb soil moisture. A number of recent reviews (Hagan, 1952; Richards and Wadleigh, 1952; Lutz, 1952; Russell, 1952) consider over-all plant growth as affected by soil conditions, Root development is discussed as part of the presentation. The intent here is not to repeat the reports of these reviewers, but to use them as a basis for discussing additional, recently acquired knowledge of the effects of soil environment and root development. Soil moisture below the wilting point or at saturation is detrimental to root development. Roots of some plants have been observed to extend into a dry soil layer if a portion of the root is in a moist area, but they were unable to absorb radioactive phosphorus from the dry soil (Hunter and Kelley, 1946b). The absorption of nutrients from a dry soil may be of importance in humid regions where the major portion of the fertility lies in the surface soil. If roots can obtain moisture from deeper, infertile soil, but are not able to utilize the essential plant nutrients in the dry surface layer, it may be necessary to keep the entire soil profile moist in order to maintain a proper moisture-fertility balance. In the main, this would not be as serious a problem in arid sections where soil is younger and less differentiated. This may account for some of the differences in thinking on the importance of irrigation frequency to maximum yields. It is generally agreed that plants are able to extract very little, if any, moisture from soil, the major portion of which is below the wilting point. The importance of root extension into dry soil layers could be threefold: (1)roots penetrating from a moist surface soil through a dry layer down into a moist subsoil region would have an additional water supply; (2) roots in a dry area would be available for water absorption at a subsequent rain or irrigation; or (3) roots may be able to take up plant nutrients from soil deficient in water. Cell elongation in roots and hypocotyls have different demands on the water supply to maintain growth (Ronnike, 1957). Studies of lupine seedlings grown in sphagnum media of various water contents have shown that hypocotyl elongation is totally inhibited at diffusion pressure deficits (DPD) of 8 to 10 atmospheres, while root growth continued at DPD values far beyond 15 atmospheres, Winter wheat grown in Nebraska has been found to penetrate into soil that is below the 15-atmosphere percentage (Kmock et al., 1957).

WATER AND ITS RELATION TO SOILS AND CROPS

45

Eight weeks after planting on plots preirrigated to depths 0, 2, 4, and 6 feet, roots attained a depth in excess of 3 feet in the deeper wetted plots, whereas they grew 22 feet with wetting depths of 0 and 2 feet. Roots from plots with no supplemental moisture formed a dense network, with long branches, even though soil moisture was below the wilting percentage. An inverse relationship exists between soil moisture content and aeration (Raney, 1949; Taylor, 1949). Oxygen is essential for root growth and aerobic respiration. There is no general agreement as to whether the limiting factor for optimum root function is insufficient oxygen or excess carbon dioxide. The rate of gaseous diffusion to and from the root surface may be of greater physiologic importance than the actual concentration of oxygen or carbon dioxide in the soil voids since it is necessary that the soil atmosphere be continually replaced and renewed, largely by diffusion, which is linearly related to the volume of air-filled pores in the soil. It is clear that variations in moisture content will greatly affect soil aeration. A marked interaction between soil moisture tension and aeration has been observed on the growth of corn seedlings (Gingrich and Russell, 1956). When oxygen was not limiting, radicle elongation became progressively less as soil moisture tension increased from 1 through 12 atmospheres, being most sensitive to tension in the range of 1 to 3 atmospheres. There was an interdependence of oxygen and moisture tension on radicle elongation, and at low soil moisture tensions, an oxygen concentration of 10.5 per cent or higher was needed for maximum growth. Plants vary in their ability to withstand saturated or poorly aerated conditions. Rice, on the one hand, grows in standing water, apparently possessing some mechanism whereby oxygen is supplied to the roots. Other plants, such as tobacco, are highly sensitive to lack of good aeration (Harris and van Bavel, 1957). Studies of the 0xygen:carbon dioxide ratio of the tobacco root atmosphere have shown a decrease in root weights as oxygen percentage decreased from 21 to 0 per cent, but the decline was not drastic until carbon dioxide concentration was greater than oxygen concentration. A water table definitely limits the volume in which good soil aeration exists. Plants can use the moisture from the water table if it is near enough to the surface for the roots to reach the capillary fringe. However, a water table too close to the surface can be detrimental, because of the restriction it places on the rooting volume. A fluctuating water table is especially deleterious to plants. As the water table rises, roots are killed; then, as it recedes, the plant is left with insufficient root surface to absorb enough water or nutrients to meet its needs. Such fluc-

46

D. WIERSMA

tuation may occur with a tile drainage system that removes excess water too slowly to prevent loss of a large portion of the lower roots, but which eventually lowers the water table below the still-active root zone. The effects of a fluctuating water table have been observed most frequently in orchards, especially citrus (Jamison, 1956; Ford, 1954). Growth of apple-tree roots has been observed to be limited by a water table fluctuating between depths a few feet below the surface. When water table depths were lowered and stabilized between depths of 30 and 70 inches, the size of the trees increased, and the quantity of feeder roots doubled in four years (Greenham, 1956). Grasses vary in their ability to withstand a high water table. In a study of forty-two species grown in pots with water levels maintained at 38 cm. below the surface, three main types of root development, related to difference in susceptibility to carbon dioxide, were observed: ( 1)vigorous growth down to layers of high moisture saturation (Loliumtype); (2) some roots penetrating moisture-saturated layers (Poa-type), and ( 3 ) roots dying at high moisture contents (Dactylis-type) (Baumann and Klauss, 1955). Soil aeration is also related to structure or aggregation. Generally, the larger the aggregates, the more rapid the exchange of atmospheric oxygen and soil carbon dioxide. Where synthetic conditioners have aggregated the soil, improved root development has followed (Hely et al., 1954). After 2 weeks of growth in coarse, medium, and fine aggregated soils, the rooting of carnation cuttings had progressed farthest in coarse aggregated soil and least in the medium. The fine aggregated soil showed intermediate results, probably because cracks formed at the insertion of cuttings, improving aeration. It was concluded that rooting was correlated with oxygen diffusion (Monselise and Hagin, 1955). Tillage operations with large, heavy machinery tend to compact soils. Compacted soil shows reduced oxygen diffusion and provides a mechanical impedance to the growth of roots. Root penetration studies with sunflowers and grapes showed no penetration of sands having bulk densities greater than 1.75 g./cc. The critical values for clays ranged from 1.46 to 1,63 g./cc. (Veihmeyer and Hendrickson, 1948). Failure of roots to penetrate soils of higher bulk density was attributed to the small pore size rather than the lack of oxygen. The rigidity of the pore structure also affects root penetration (Wiersum, 1957). Other work has shown a pronounced interaction of mechanical impedance and aeration on the root growth of seedlings (Gill and Miller, 1956). As reported by other investigators, they noted that in the absence of mechanical impedance, a 50 per cent reduction of normal oxygen supply did not impair seedling elongation, and growth did not cease at concentrations as low as 1 per

WATER AND ITS RELATION TO SOILS AND CROPS

47

cent oxygen. As soon as mechanical restraint was applied, however, growth reductions occurred at a relatively modest decrease in oxygen content, and the rate of growth fell to zero at small levels of impedance when oxygen contents were low, Horizons within some soil types possess high bulk densities naturally. Corn roots in four soil types derived from Wisconsin Glacial Till, under similar fertility, pH, and weather conditions, have shown root penetration of about 3 feet in the Elliott and Clarence soils, with subsoil bulk densities of 1.70 g./cc. The lower bulk densities of Ringbrook and Saybrook subsoils permitted deeper rooting ( Fehrenbacher and Rust, 1956). Low aeration associated with the high bulk density was thought to limit root development. Even though a soil has good structure and aeration, it does not necessarily follow that roots will penetrate deeply if fertility is inadequate (Fehrenbacher and Snider, 1954). Corn roots have been observed to penetrate to a depth of 6 feet in a fertile permeable Muscatine silt loam, but to be limited to 3 feet by a compact layer in Elliott silt loam, and to 3 feet in Cisne silt loam-not because of structure but because of low p H and poor fertility. At the Al and Az horizons of the Cisne, the fertility level was so low that corn roots did not develop vigorously enough to penetrate into the more compact soil of the B horizon. On limed and fertilized plots, corn roots were able to grow into the more dense subsoil and obtain additional moisture from it. Soil moisture at the B and D horizons averaged 5 per cent lower in fertilized plots than in untreated plots. To some extent, plants may be able to overcome compact layers if there is adequate fertility in the subsoil. Phosphorus and calcium appear to be of major importance for root development. Grass roots were compared in two Crete soils in Nebraska, one deficient in soluble phosphorus at the B horizon, the other with a relatively high level. In the deficient soil, roots were restricted to the surface horizon, while development in the latter was good in the subsoil. The limited root development in the surface layer of the Butler soil has been associated with a low content of nitrogen and exchangeable calcium. The Judson soil, with a good supply of plant nutrients in all depths of its profile, supports deeply rooted bluegrass, whereas the same grass had a shallow root system in Carrington, which is deficient in available phosphorus in the subsoil (Fox et al., 1953). A favorable chemical environment has also been shown to be more essential than a favorable physical environment for deep, fibrous, and well-nodulated rooting of alfalfa (Fox and Lipps, 195513). It is generally observed that increasing the nitrogen content of the soil will decrease the root:top ratio. With high nitrogen, more of the

48

D. WIERSMA

carbohydrates are used for the synthesis of top tissue, and less translocated to the roots. Studies of wheat grown in nutrient solutions of varying nitrogen content have shown that, with a nitrogen deficiency, the roots were longer and more slender, owing to an increase in cell length (Bosemark, 1954). On examination it was noted that inhibition from a high nitrogen supply was the result of the combined action of reduced cell multiplication and cell elongation, and perhaps of some relation between nitrogen supply and the natural auxins that affect root growth. The placement and composition of the fertilizer band have been shown to affect the rooting habit of corn (Duncan and Ohlrogge, 1958). A 1:s ratio of nitrogen and phosphorus seems to favor the most profuse root system. The total weight of corn roots is practically the same, whether the fertilizer is placed in a band or is mixed in equal amount throughout the plow layer. With band placement, a mass of fine, wellbranched roots is located in the fertilized area, whereas with broadcast application the roots are more evenly distributed through the soil. Although band fertilization may stimulate growth earlier, it would seem that the corn plant would have a larger reservoir of water available in the case of the broadcast application. Nitrogen does not always depress root development, as evidenced by work with winter wheat grown at four moisture levels and three nitrogen fertilizer rates (Kmock et al., 1957). Added nitrogen increased root weights at all moisture levels and at nearly all soil depths and permitted more complete utilization of subsoil moisture. When ample nitrogen was supplied, moisture was depleted to a depth of 8 feet. Similar results have been reported for nitrogen in increasing the rate of root penetration of several species of grasses (Burton et al., 1954; Haas, 1958). Minor elements, though they have not received much attention, may also play a part in root growth. A zinc deficiency has been shown to reduce the meristematic activity of the tip tissue and cambium of the tomato root (Carlton, 1954). In the absence of zinc, small tumors were observed, similar to those formed on roots exposed to low concentrations of certain growth-regulating substances. This suggests an interrelationship between plant hormones and zinc. A number of workers have tried subsoiling and deep placement of fertilizer as a means of improving root development. Younts and York (1956) reported that deeper root penetration of corn and crimson clover was stimulated in the early season by concentrating fertilizer in the surface 12 inches. Toward the end of the season, deep placement stimulated greater root activity in the 24-26-inch layer. Nitrogen depressed root activity at all depths throughout the entire growing season. Corn roots were found not to penetrate deeply into a compacted silty

WATER AND ITS RELATION TO SOILS A N D CROPS

49

clay loam soil with a bulk density of 1.5 g./cc., mainly because of lack of oxygen. Subsoiling alone encouraged better root development in such soil, but fertilization of the subsoil promoted even greater growth (Bertrand and Kohnke, 1957; Kohnke and Bertrand, 1956). In Plainfield fine sand, corn roots penetrated to 6 feet in fertilized subsoil, and to 4 feet in check plots, with intermediate penetration in a subsoiled plot. Improved root development on Leon fine sand, Ona fine sand, and Norfolk loamy fine sand has been reported from deep placement of fertilizer (Robertson et al., 1957). Salinity and alkalinity are major problems of crop production in some areas of the world. A high salt content not only increases osmotic tension enough to alter plant metabolism, but also affects the amount of roots developed for water and nutrient absorption. From earlier work ( Breazeale and McGeorge, 1932), the low content of carbon dioxide found in alkaline-calcareous soils is believed to lower the availability of phosphorus. Since this element is effective in stimulating root growth, this may be a partial explanation of reduced root development in alkaline soils. Studies with radish seedlings grown in soil systems of various ca1cium:sodium ratios showed maximum elongation in the soil that was 10 per cent saturated with sodium. Elongation rate decreased as the sodium:calcium ratio increased (Schreiber et al., 1957). Root length of peas decreased as the sodium: calcium ratio increased, and root growth was greatly reduced at 60 per cent sodium saturation (Elgabaly and Ghani, 1958). Work reported from Russia (Strogonov, 1956) shows that cotton roots will continue vertically downward in weakly saline soils, but, in highly saline conditions, growth is decreased and root tip may be killed. Saline layers stimulated lateral root growth. Studies of red kidney beans, corn, cotton, and alfalfa grown in layered soils with concentrations of NaCl ranging from 0.00 to 0.25 per cent showed that few bean roots penetrated layers containing 0.1 per cent salt, whereas corn was not seriously limited until the 0.2 per cent concentration was encountered. Alfalfa roots penetrated all the layers of saline soil, as did cotton roots (Wadleigh et al., 1947). Others have reported that alfalfa roots penetrated a horizon at which there was a high sodium content, though fibrousness and abundance were reduced (Fox and Lipps, 1955a). Excessive sodium accumulation by roots may affect root functions, particularly water absorption (Bernstein and Pearson, 1956). With the fertilizer band placed near the corn row, the primary roots died and shriveled up as soon as they came in contact with the high salt content of the fertilizer band. However, a dense mass of active secondary roots developed in this highly osmotic area (Duncan and Ohlrogge, 1958).

50

D. WIERSMA

Discases and insects hosting on plant roots are subject to the same physical limitation as the root itself (Hagan, 1952). Frequently, mobture and oxygen supply have an influence on their virulence, growth, and multiplication. Heavy irrigation has been shown to increase the amount of nematode infection on tomatoes (Oteifa and El-Gindi, 1957). This effect is attributed to the fact that the organism, dormant under dry soil conditions, is stimulated as soon as moisture is added. Also, films of irrigation water tend to carry nematode larvae from one plant to the other. In addition to moisture content, large pore size and adequate aeration seem to affect the infection and rate of emergence of the nematode larvae (Wallace, 1956). Verticillium wilt infection takes place mainly from the soil through the root system, and in the case of cotton is restricted to highly alkaline soils. Contlicting views on the influence of soil conditions on the Verticillium organism are discussed in a review (Isaac, 1956). It is generally agreed that increasing soil nitrogen increases the incidence of the disease. The incidence in pot cultures was decreased by adding sulfate of potash or ammonium sulfate, or by decreasing the soil moisture. The frequency of irrigation of cotton, especially in early season, increased both prevalence of the disease and the severity of lygus bug infestation ( Stockton and Doneen, 1957). Certain crop-management practices also can influence root development; this is especially true with grass and forage crops, where grazing and clipping occur during the summer months. Using moisture extraction as a measure of root activity, with pasture mixtures clipped at intervals of 2, 3, 4, and 5 weeks, it was found that, if the botanical composition remained unchanged, the distribution of root absorption for Ladino clover-grass and broadleaf trefoil-grass mixtures remained unchanged by clipping frequency (Hagan and Peterson, 1953). Other studies have shown that the roots of some grass species are affected by frequent clipping (Weaver and Zink, 1946). Bromus inermis lost 15 per cent of its roots during a growing season if clipped at 10-day intervals, and Agropyron cristatum lost 73 per cent. Another management practice influencing the roots is cultivation. Serious root damage in orchards has resulted from two or three summer cultivations, especially if the depths were aIlowed to vary (Coker, 1955). Surface moisture and nutrients thus become unavailable to the plant, and light precipitations are lost by evaporation. Also, the surface layer has the greatest concentration of available nutrients. Much of the research reported to date on root development and the physical and chemical conditions of the soil has been of a qualitative nature and was frequently based on observations incidental to an experi-

WATER AND ITS RELATION TO SOILS AND CROPS

51

ment directed toward another objective. Work of Weaver and others, who have made extensive studies on the rooting habits of many cultivated and native plants, has been of real value in understanding variations between species. In view of the heterogeneous nature of soils and the many other plant and climatic factors that affect crop growth and root development, the need for fundamental studies becomes apparent. The use of highly controlled experimental conditions involving a limited number of variables would seem to be a fruitful approach. More extensive research on the physiology and morphology of roots grown under known physical and chemical conditions would assist in giving a better understanding of plant root behavior. V. Plant-Water Relations

Following examination of the water-soil and soil-root systems, attention is now turned to the remaining two-component combination: the functions of water in the plant and the responses of crops to excessive water and to drought. Admittedly, these responses are conditioned by the soil and the atmospheric environment, but the following discussion centers on the binary system of plant and water.

A. THEROLE OF WATERIN P.

J.

THE

PHYSIOLOGY OF PLANTS

Kramer

Duke University, Durham, North Corolina

Everyone knows that plant growth and crop yields are often reduced by water deficits, but too little is known about the mechanism of such reductions. Insufficient attention has been given to the role of water in the physiology of plants. Much is known about the factors affecting the availability of soil water and its absorption by plants, and about the factors affecting the rate of loss of water by transpiration. There has been little attempt, however, to correlate the numerous studies of soil, atmospheric, and plant-water relations and to use them in explaining plant growth behavior. This is unfortunate, because concentration on any single phase of water relations, such as soil moisture or evapotranspiration, cannot fully explain the variations in the quantity and quality of plant growth that are caused by variations in water supply. The most important aspect of plant-water relations is the internal water balance, because internal water balance and turgidity are closely related to the rates of various physiological processes that control the quantity and quality of plant growth. The internal water balance is not

52

P. J. KRAMER

an independent condition, but is controlled by the relative rates of water absorption and water loss. It seems clear that we not only need more information about plant-water relations, but that we also need better correlation of data already available. The physiological significance of water deficits is herein discussed under three general headings: (1) How internal water deficits affect plant growth. ( 2) Why internal water deficits develop. (3) Methods of measuring internal water deficits. 1. Water in Relation to Growth

It is a basic biological principle that the quantity and quality of growth made by a plant is controlled by its hereditary potentialities and its environment, acting through its internal physiological and biochemical processes and conditions. The only way in which environmental factors such as water, temperature, or mineral nutrients can affect growth is by affecting internal processes and conditions. Thus, the effects of water deficits on physiological and biochemical processes must be studied to understand why they reduce plant yields. a. Functions of water in plants. Consideration will first be given to the role of water in plants, where it serves the following four general functions: (1)Water is an important constituent of protoplasm. It makes up 85 to 90 per cent of the fresh weight of actively growing plant parts, and even trees are more than half water. As water content decreases, physiological activity usually decreases, and extreme dehydration kills most pIants. The reIation of water content to physiologicaI processes is shown very strikingly in seeds, where respiration and other physiological activity increase manyfold as water content increases. ( 2 ) In photosynthesis, water is as essential a reagent as carbon dioxide. It is also an essential reagent in hydrolytic processes such as digestion of starch to sugar. (3) Water is the solvent in which salts and gases enter plants and in which solutes move from cell to cell and tissue to tissue within the plant. (4)Water is essential to maintain sufficient turgidity for growth of cells and maintenance of the form and position of leaves, new shoots, and other slightly lignified structures. Turgidity also is important in connection with the opening of stomata and the movement of flower parts and leaves. Lack of turgidity results in immediate reduction or cessation of growth. The total quantity of water required for these essential functions is relatively small, usually less than 5 per cent of all the water absorbed, Most of the water entering a plant is lost in transpiration, directly contributing little or nothing to its growth. The data for corn in Table V show that in this instance, only 1

WATER AND ITS RELATION TO SOILS AND CROPS

53

TABLE V Ail

Estiinated Water Budget for a Corn Plant"

Water occurring as constituent, BoIvcnt, 1,872 grams and in maintenance of turgidity 250 grams Water used as a reagent Water lost in transpiration 202,106 grams 204,228 grams or 54 gallons ~

a

~

~~~~

Miller (1938).

per cent of the water passing through the plants was used in them. However, failure to replace water lost by transpiration results in loss of turgidity, cessation of growth, and eventual death from dehydration. b. Eflects of water deficits on certain physiological processes. It is probable that every process in plants is more or less affected by water deficit. The effects of water deficits on only a few processes have been studied in sufficient detail to deserve special mention, and the relation of water supply to physiological processes has been concisely reviewed by Richards and Wadleigh (1952). Unfortunately, in most of these studies, physiological processes and conditions were merely correlated with soil moisture content; no attempt was made to measure soil moisture tension or the internal water deficit of the plants. It seems probable that, in studying the effects of water deficits on plant processes, internal water balance ought to be determined in order to have some quantitative measure of the extent of the water deficit existing in the plants. Stomata1 opening seems to be one of the most sensitive plant processes with respect to internal water deficits. A slight decrease in turgidity sometimes is accompanied by increased opening of stomata (Stllfelt, 1955), but further reduction is nearly always accompanied by a decrease in stornatal aperture. According to Magness and associates (1935), stomata of apple trees begin to close prematurely long before soil water falls to the permanent-wilting percentage. Decreasing soil moisture also causes premature closure in citrus ( Oppenheimer, 1953; Oppenheimer and Elze, 1941). Stomata usually close earlier each day as soil water becomes less available, until finally they may remain open for only a short time each morning ( Aldrich and Work, 1934, in pear; Jones, 1931, in peach; Maximov and Zernova, 1936, in wheat). Figure 15 shows the effects of water deficit on stornatal closure. Premature closure of stomata is undesirable because, in at least some species, it cuts off the supply of carbon dioxide for photosynthesis (Nutman, 1937), although, in others, considerable carbon dioxide appears

54

P. J. KRAMER

100

00 0

2 4: 60 1

0

t

b 40 I-

z

W 0

20

0 0

S

10

II

IZN

I

Z

3

FIG. 15. Effects of moisture deficit on daily closure of stomata of pear. From Kramer (1949), after Aldrich and Work (1934).

to enter through the epidermis (Dugger, 1952; Freeland, 1948; Mitchell, 1936) . One effect of stornatal closure is to reduce transpiration, because by far the larger fraction of water loss occurs through the stomata. This reduction would be desirable in itself, for there is little doubt that very responsive stomata that close early in the development of an internal water deficit must materially increase drought resistance and survival (Pisek, 1958; Stocker, 1956); but, unfortunately, they also reduce photosynthesis by reducing the supply of carbon dioxide. For this reason it is doubtful if very responsive stomata are desirable in crop plants, except, possibly, in plants such as tomato, which seems to manufacture most of its food before noon. The relation of stornatal behavior to control of water loss and photosynthesis of crop plants deserves further study. When the stomata are closed, water loss is controlled by the characteristics of the cuticle or by the waxy layer covering the leaf epidermis. This suggests the possibility of reducing cuticular transpiration by applying some sort of waterproof film to the leaves, and coatings of wax and latex have proved moderately successful in protecting nursery stock and other plants after transplanting (Allen, 1955; Comar and Barr, 1944). Various practical difficulties limit their usefulness at present, but they show promise for some purposes where water conservation is more important than reduction in photosynthesis. There has been much argument as to the time when transpiration begins to decrease in plants in drying soil. Veihmeyer and Hendrickson

55

WATER AND ITS RELATION TO SOILS Ah?) CROPS

( 1950) long contended that transpiration does not decreasc materially iintil soil moistiire falls almost to thc permanent-wilting percentage, hiit it now seems clear that transpiration and other physiological processes are usually affected considerably earlier (Gates, 1955; Richards and Wadleigh, 1952; Slatyer, 1955, 1957). Figure 16 shows the changes in TOMATO

>

COTTON 0

100-

c

4

12

9

20

16

24

-

980-

5

I-

w

-I-

60

-- a

DRY WEIGHT

-I ..

:-

w 40 a v

-

1

1

1

1

1

l

1

1

1

1

~

-100

E

SOIL MOISTURE STRESS

4

SOIL WATER

DAYS

0

4

-

s

FIG.16. Effects of soil moisture on transpiration, relative turgidity, and growth of tomato and cotton (Slatyer, 1957). .

transpiration, relative turgidity, and growth of tomato and cotton plants subjected to increasing soil moisture stress. Most of the controversy on this subject could have been avoided had it been realized more clearly that plant processes are controlled directly by the water content of the plant, and only indirectly by the water content of the soil. If the diffusion pressure deficit or the relative turgidity of the leaves had been measured, it would have been possible to correlate physiological processes with the water condition inside the plant. If, in addition, the moisture tension of the soil at various stages of drying had been known, it would have been possible to correlate soil and plant water conditions with the course of transpiration and other processes. It seems that, in all studies of the effects of water on plant growth,

56

P. J. KRAMER

we need an accurate characterization of plant water conditions as well as of soil water conditions. This is essential to indicate when water actually becomes a limiting factor within the plant. Plant scientists have long been interested in differences in the efficiency with which plants of different species use water. This is usually expressed in terms of pounds of water used per pound of dry matter produced, and is often termed the “water requirement.” This is an unfortunate term, because there is no specific water requirement, but only a highly variable ratio of water used to dry matter produced. Since this ratio is largely controlled by transpiration, it might more accurately be termed the transpiration ratio, Miller (1938) summarized much of the early work on transpiration ratio. In measurements at Akron, Colorado, extending over several years and involving plants of about 150 species, the transpiration ratio varied from 216, for Kursk millet, to 1131, for Franseria, a native weed. Polster (1950) estimated that the amount of water required to produce a gram of dry matter in a German forest ranged from 170 g. for beech and Douglas fir, to 317 g. for birch, and 344 g. for oak. Although it was obvious that wide variations exist among species in the transpiration ratio, interest in this aspect of water relations has lagged. Probably this is chiefly because the concept of evapotranspiration advanced by Penman and others indicated that the type of plant cover would make little difference in total water loss from an area of land. Recently, interest has again developed in possible differences in efficiency of water use and their relation to crop yields when subjected to drought. An example is a recent German paper by Koch ( 1957), in which he describes daily changes in the ratio of photosynthesis to transpiration and discusses the possibility of modifying this relationship. Other examples of renewed interest in efficiency of water use are a study of water use by cotton, peanuts, and sorghum, by Slatyer (1955) in Australia, and one on grasses, by Burton and associates (1957) in Georgia. It has been known for many years that fertilization and other cultural practices that increase yields usually increase the efficiency of water use by crops. This is supported by extensive investigations by Arland and colleagues at Leipzig (Arland, 1955; Zwicker, 1954), whose work on the physiology of crop plants should be better known. Burton, Prine, and Jackson’s experiments summarized in Table VI also show that, in four of five grasses tested, increasing the nitrogen supply greatly increased the amount of dry matter produced per unit of water used. Another result was that Coastal and Suwanee Bermudagrasses, the most drought-resistant of the five grasses studied, produced more dry matter per unit of water under drought conditions than when well watered,

57

WATER AND ITS RELATION TO SOILS A N D CROPS

TABLE VI Effect of Rainfall and Nitrogen Supply on the Water Use of Five Grasses" Pounds of water used per pound of dry matter produced 1953

1954

Nitrogen added (lb./A.)

Nitrogen added (1bJA.)

Grass

50

100

200

50

100

200

Coastal Bermudagrass Suwanee Bermudagrass Common Bermudagrass Pensacola Bahiagrass Pangolagrass

2478 1923 6812 2200 2249

1431 1105 2896 1321 1585

803 692 1546 870 2240

1547 1107 9738 3103 2843

982 725 5028 1945 2513

641 452 4336 1239 3016

Burton el al. (1957). Rainfall April 1 through October 31 was 39.66 inches in 1953 and 13.68 inches in 1954. a

b

whereas the reverse was true for the less drought-resistant species. When the physiological causes for these differences are learned, we will be on our way toward a physiological explanation of drought resistance. Ballard ( 1933) claimed that increasing nitrogen supply reduced transpiration ratio, by changing the pattern of growth rather than by decreasing transpiration per unit of leaf surface. Williams (1935) found that increasing the supply of phosphorus decreased the transpiration ratio of oats (i.e., increased the amount of dry matter produced per unit of water used) by decreasing transpiration rate per unit of plant weight in two ways: (1) by decreasing transpiration per unit of leaf weight, and ( 2 ) by decreasing ratio of leaf weight to total plant weight during the latter part of the growth cycle. More studies of this type are needed to add to our understanding of the various factors that affect the efficiency of water use by plants. As Burton has pointed out, it is difficult to breed plants for drought resistance and efficient use of water until we know what plant factors are responsible for these desired characteristics. It seems, as Parker (1956) suggests, that drought resistance may depend on various characteristics occurring throughout plants. These may range from depth of rooting, thickness of cutin, and responsiveness of stomata, to permeability, structure of protoplasm, and reaction of enzyme systems to dehydration. Rate of photosynthesis is rarely limited by lack of water as a reagent, but is often limited because dehydration of protoplasm reduces its photosynthetic capacity (Brilliant, 1924; Dastur, 1925; Ensgraber, 1954), and

58

P, J. KRAMER

because stornatal closure reduces the supply of carbon dioxide. Many investigators have reported large decreases in photosynthesis in wilting leaves ( Ashton, 1956; Brilliant, 1924; Dastur, 1925; Schneider and Childers, 1941; Thomas and Hill, 1949; Upchurch et d.,1955; Verduin and Loomis, 1944; and others). There has been much discussion as to whether photosynthesis is materially reduced before soil water approaches the permanent-wilting percentage. Schneider and Childers ( 1941) reported that photosynthesis of apple leaves was reduced 50 per cent before wilting was visible, and in wilted leaves fell to only 15 per cent of the expected rate. The original rate was regained rather slowly after rewatering. Similar results were reported for several forest trees (Bourdeau, 1954; Kozlowski, 1949). On the other hand, no reduction in photosynthesis was observed in Ladino clover until the readily available soil moisture was almost entirely depleted, although the rate fell rapidly when wilting occurred ( Upchurch et al., 1955), and similar results were observed for apple, by Allmendinger et al. (1943), and for pecan, by Loustalot (1945). Unfortunately, neither the soil moisture tension nor the water deficit in the leaves was reported in any of these studies. Hence, it is impossible to determine what relation existed between leaf turgor or leaf water deficit and photosynthesis before wilting was visible. For really conclusive studies of the relation of water supply to photosynthesis, measurement should be made of changes in leaf water conditions at the time that photosynthesis is measured. There is some evidence that rate of photosynthesis is maximum in some leaves at slightly less than maximum leaf turgor (Brilliant, 1924; Schneider and Childers, 1941); this has been attributed to the wider stornatal apertures at slightly less than maximum turgidity ( StiFilfelt, 1955). But, opposed to this view is the claim that, in at least some plants, the stomata exercise less control over photosynthesis than over water loss ( Heinicke and Childers, 1936; Mitchell, 1936; Schneider and Childers, 1941). Verduin and Loomis (1944) reported that the porosity of corn leaves to gases was greatly reduced by wilting, but lack of a high correlation between porosity (measured with a porometer ) and photosynthesis indicated that factors in addition to stornatal closure are responsible for the reduction in photosynthesis. As shown in Fig. 17 midday reduction in photosynthesis occurs in plants of many species on sunny days. This reduction is usually attributed to midday closure of stomata (Nutman, 1937; Polster, 1950). It does not occur on cloudy days. Stocker and associates (1954) regard midday sprinkling of crops in hot weather as very beneficial by keeping leaves turgid and stomata open, preventing a midday decrease in photosynthesis.

WATER AND ITS RELATIQN TO SOILS AND CROPS

''

I I

6M !

NOON

I

6 PM

-2

59

6AM

NOON

6 PM

FIG. 17. The diurnal variations of photosynthesis and transpiration for birch, beech, and oak ( P o k e r , 1950).

The cooling elfects of sprinkling might also reduce respiration, thereby increasing net photosynthesis. Bloodworth et al. ( 1956) concluded that showers too light to increase soil moisture materially may be quite beneficial in decreasing internal water deficits. Irrigation by sprinkling during periods of high evapotranspiration might produce unexpectedly good results in proportion to the amount of water applied if wetting the leaves increases photosynthesis and, by its cooling effect, decreases respiration. Dew and atmospheric moisture may also increase growth far beyond the same amount of water added to the soil, because it produces direct rehydration of tissues. The importance of dew may be much greater than is generally believed, and it certainly deserves more study (Gessner 1956; Slatyer, 1956, 1957; Stone et al., 1956; Stone, 1957).

60

P. J. KRAMER

Ordinarily, a decrease in water content is accompanied by a decrease in respiration, but respiration occasionally increases during dehydration. Schneider and Childers (1941) found that respiration was increasing in apple trees in drying soil while photosynthesis was decreasing, and Upchurch et al. (1955) found small increases in respiration of wilting Ladino clover. Increased respiration, combined with a decreased photosynthesis, can seriously reduce the amount of food available for other processes. The effects of water supply on respiration are particularly noticeable in dry seeds, where a small increase in water content above a certain critical content results in a very marked increase in respiration (Bakke and Noecker, 1933; Appleman and Brown, 1946). The nature and course of various biochemical reactions are often changed by water deficits, resulting in changes in chemical composition. Best known is the decrease in proportion of starch to sugar in plants subjected to water deficits, because of increased hydrolysis of starch. This has been observed in many plants, including apple (Magness et al., 1933), beans ( Wadleigh and Ayers, 1945), cotton (Eaton and Ergle, 1948), and beans and tomato (Woodhams and Kozlowski, 1954). In most instances, total carbohydrate reserves are decreased. In general, water deficits seem to increase nitrogen content, but this probably depends on the stage of growth at which the deficit develops and on the organ sampled. It is said that water stress during maturation increases the protein content of wheat (Miller, 1938), and Table VII TABLE VII Effccta of Soil Moisture Stress on Tobacco" Average soil moisture stress (cm. HzO)

Yield (lt>./A.)

Value (dollars/ 100 lb.)

Nicotine

(%)

( 76)

100 242 1470

1649 1622 1334

23.74 23.21 18.35

1.76 2.10 4.02

1.65 1.67 2.40

Total N

Sugars

(%)

Burn test

21.18 22.56 14.85

1.47 1.40 1.25

van Bavel (1953).

shows that the concentration of total nitrogen and nicotine is significantly increased in tobacco plants subjected to drought, but the sugar content and burning quality are decreased (Darkis et al., 1937; van Bavel, 1953). Water deficits are said to reduce the concentration of quinine sulfate and other alkaloids in cinchona (Loustalot et al., 1947). Gates ( 1955, 1957) concluded that even small reductions in water content have marked effects on metabolism. During moderate wilting, uptake of nitrogen and

WATER AND ITS RELATION TO SOILS A N D CROPS

61

phosphorus by tomato plants was reduced, and both elements tended to move out of leaves into stems. Even moderate wilting caused leaf changes resembling senescence, but during recovery they tended to return to a more juvenile condition. Wilting is known to disturb the normal pattern of translocation in plants and probably also affects composition by affecting enzyme activity. Some information on enzyme activity was recently summarized by Mothes ( 1956). Sometimes a moderate water deficit produces desirable changes in composition. For example, the rubber content of guayule plants is increased by a moderate water deficit ( Wadleigh et nl., 1946), and the quality of apples (Overley et al., 1932) and of pears (Ryall and Aldrich, 1944) is improved by moderate moisture stress during the latter part of the season. Clark and Levitt (1956) found an increase in lipids in soybean leaves subjected to wilting, which was accompanied by decreased transpiration and increased resistance to dehydration. Tobacco subjected to a moderate water deficit produces leaves with more body and aroma than does tobacco not so treated (Garner, 1946). Protoplasmic properties such as permeability and viscosity are modified by water deficits (Levitt, 1951, 1956), but there is no general agreement as to the nature or causes of these changes. We are almost entirely ignorant of the protoplasmic factors that give some plants greater resistance to dehydration than others. Some of these differences are related to cell size and shape (Iljin, 1953), but there are also basic differences in protoplasmic characteristics that have not yet been adequately explained. There is obviously a great need for more investigation of the effects of dehydration on the properties of protoplasm itself, as well as on the processes of entire plants. The effect of water deficits on growth as expressed in terms of yield is dealt with elsewhere, but is discussed here in terms of cell processes. In terms of cells, growth consists of cell division, enlargement, differentiation, and maturation. Although the relations between turgor and growth are not fully understood, all of these processes are affected by water deficits and dehydration of the protoplasm. Cell division is reduced, but cell enlargement is reduced even more, because some degree of turgor is essential to the expansion of cells (Broyer, 1950; Burstrom, 1956; Cleland and Bonner, 1956; Heyn, 1940). One of the first effects of water deficit is a decrease in or cessation of elongation of stems and enlargement of leaves and fruits, because these processes are dependent on a turgid condition of the cells. Loomis (1934) and Thut and Loomis (1944) concluded that the supply of water to the growing point is the most important factor affecting the rate of growth of corn. Their work indicated that water supply is

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limited and growth reduced by ( 1 ) bright light, ( 2 ) deficient soil moisture, and ( 3 ) low humidity, important in the order named. Thus, atmospheric factors were even more important than soil moisture in reducing the growth of corn. As a result, in hot, dry weather, when transpiration is rapid, corn and other plants often grow more at night than during the day. Nightingale and Mitchell (1934) found that tomatoes grown in moist soil at a constant temperature were much smaller and more woody if kept at a relative humidity of 35 per cent than when grown at a relative humidity of 70 per cent. This demonstrates the importance of atmospheric conditions on the internal water balance of plants, even when growing in moist soil. A decrease in leaf size from lack of turgidity is especially serious because it decreases the photosynthetic surface and production of food. According to Watson ( 1947, 1956), yield of crops is more closely related to leaf area than to variations in photosynthetic efficiency per unit of leaf area. Hence, the first requirement for high yields is a large leaf area. The effect of increased humidity on the leaf size of tobacco grown under artificial shades is well known. Wadleigh and Gauch (1948) found that enlargement of cotton leaves ceased by the time the soil moisture stress reached 15 atmospheres, and Slatyer also has data showing a decrease in stem elongation and a slowing of increase in dry weight with increasing soil moisture stress. The effects of water deficit and dehydration on differentiation are equally important ( Meyer and Anderson, 1952; Richards and Wadleigh, 1952). In general, water deficit hastens maturation of cells and tissues, increases the thickness of cell walls, and decreases succulence. This not only decreases the size of plants and their yield, but may also decrease quality, especially in the case of pasture and hay crops (Willoughby, 1944). On the other hand, the increased amount of dry matter and supporting tissue produced in plants subjected to a moderate water deficit may be beneficial by increasing stem strength or resistance of fruit to injury by bruising. The increased thickness of cutin often found on plant organs matured in dry air is beneficial because it reduces cuticular transpiration. The Australians are making particularly valuable studies on the effect of water deficits on growth. Gates (1955) found that even moderate wilting affects plant growth and claimed that it is erroneous to suppose that plant growth is affected only after permanent wilting is reached. He found, relative to tomato plant weight, that leaf weight increase was slowed down while stem weight increase was increased, and that, after recovery from wilting, growth rates of moderately to slightly wilted plants soon increased above the growth rate of controls. Perhaps this was be-

WATER AND ITS RELATION TO SOILS AND CROPS

63

cause growth was checked more than photosynthesis by moderate wilting. Plants wilted more severely probably would not have recovered so rapidly (Ashby and May, 1941). Williams and Shapter (1955) studied the effects of moderate wilting on the distribution of dry matter in barley and rye. They found that the effects varied with the stage of growth at which the plants were allowed to wilt, and suggested that failure to consider the possibility of differences in behavior at different stages of growth has resulted in much confusion regarding the effects of water deficits on growth. The plant parts growing most actively at the time a water deficit occurs are most affected, and this causes many of the differential effects of wilting that have been reported for various organs. Williams and Shapter found that, in addition to reduction in growth, the rate of photosynthesis per unit of leaf area was reduced, the uptake of phosphorus was reduced and nitrogen content of leaves was decreased, whereas that of stems increased. As Gates (1955) said, the value of these studies would be greatly increased if the water content, or preferably the diffusion pressure deficit, of the plant tissues were known. Only from careful studies of changes in composition of various organs and in rates of processes can we hope to learn how water deficits affect plant growth. In studies of the effects of water on growth it must be remembered that the full effects of water supply on growth will be expressed only if the plants are well supplied with nutrients, especially nitrogen. This was pointed out long ago by Crowther (1934), and shows up nicely in a recent paper by Burton and associates ( 1957). Nitrogen deficiency sometimes checks growth as much as lack of water if the surface soil containing most of the nitrogen becomes so dry that salt as well as water absorption is hindered. 2. Why Water Deficits Develop in Plants In view of the importance of internal water deficits it is desirable to discuss their causes. As mentioned previously, plant-water relations consist of a group of interrelated and interdependent processes. Thus the internal water balance or degree of turgidity of a plant depends on the relative rates of water absorption and water loss, and is affected by the complex of atmospheric, soil, and plant factors that modify the rates of absorption and transpiration. The transpiration rate of well-watered plants is controlled by such plant factors as leaf area, internal leaf structure, thickness of cutin, and extent of stomata1 opening, and by such environmental factors as solar radiation, humidity, temperature, and wind. The rate of water absorption depends on the rate of water loss, the extent and efficiency of root systems,

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P. J. KRAMER

and the availability of soil moisture. The rate of water absorption is affected by aeration, concentration of the soil solution, and soil temperature, as well as by the soil moisture tension. It is not surprising that two processes controlled by quite different sets of factors do not always keep in step. As shown in Fig. 18 the rate of absorption even in moist soil tends to lag behind the rate of transpiration, chiefly because of resistance to the movement of water into roots

6AM

8

I0

12PM 2

4

6

8

10

12

2AM 4

8

FIG.18. Diurnal variations in transpiration and water absorption during a bright, hot summer day (Kramer, 1949).

(Kramer, 1949). On hot, sunny days the rate of transpiration so far exceeds absorption that severe midday water deficits often develop, even in plants growing in moist soil. Such deficits usually are eliminated by absorption during the night, but, as soil moisture is depleted, absorption becomes slower and slower, and the midday deficit persists later and later, until permanent wilting finally occurs and growth ceases. Thus, plant water deficits can be caused either by excessive loss of

WATER AND ITS RELATION TO SOILS AND CROPS

65

water, by slow absorption of water, or by a combination of the two. Deficits caused by excessive transpiration are usually shorter and less severe than those caused by inadequate absorption, but periods of hot, dry, windy weather can cause severe damage, even to plants in moist soil, by causing excessive transpiration. Conversely, during foggy, showery, humid weather, even plants in dry soil may be subjected to relatively small water deficits. Thus, the effects of soil-moisture supply may be greatly modified by atmospheric conditions that affect the rate of transpiration, as noted by Hagan (1955) and Letey and Peters ( 1957). This situation explains why the behavior of plants cannot be explained satisfactorily in terms of either soil or atmospheric conditions alone (Hagan, 1955). Plant growth is affected by the turgor, or internal water balance, which depends on the relative rates of absorption and transpiration, which are affected by both atmospheric and soil conditions. The internal water balance, or turgidity, of the plant represents the integration of all the factors affecting plant water relations. Thus, we need to give more attention to the internal water balance as a measure of whether or not plants are adequately supplied with water. This approach seems to have been applied with considerable success to sugar cane in Hawaii (Clements and Kubota, 1942) and to various crops in Israel (Oppenheimer, 1953). In this connection it seems possible that more attention ought to be given to plant factors that reduce water loss, such as thickness of cutin and responsiveness of stomata. Emphasis on the idea that evapotranspiration should be potentially equal from all types of vegetation cover has led to neglect of the probability that differences in rate of water loss may exist between different kinds of plants. It seems very probable that, under identical environmental conditions, plants with thick layers of cutin and stomata that close promptly as soon as a water deficit begins to develop are likely to survive drought better than those with less responsive stomata (Pisek, 1956; Stocker, 1956). Rider ( 1957) recently warned that there is reason to doubt if evapotranspiration is the same from all types of plant cover. It seems probable that more attention should be given to the importance of anatomical and physiological factors in controlling water loss,

3. Measurement of Water Deficits There has been considerable uncertainty concerning the best method of measuring or characterizing the condition of plants and plant tissues with respect to water. Ecologists have concerned themselves with this problem for many years, but have not settled on a single method. a. Water content. The oldest method is to express the water content

66

P. J. KRAMER

as percentage of fresh or dry weight. The water content of plant tissue varies with species, organs, tissues, and age. It also varies with the time of day, and in perennial plants with the season of the year. In general, young tissue has a high water content, but as cells mature the walls thicken and the proportion of dry matter increases, causing R decrease in percentage of water. For example, Ackley (1954) found that the water content of pear leaves decreased from 73 to 59 per cent of their fresh weight from May to August, although water content per leaf increased somewhat. Practically all herbaceous species tend to become more woody and lower in water content as they mature, and this trend is particularly notable in seeds (Shirk, 1942). A noticeable decrease in water content at about the time of flower bud initiation is said to occur in some annual plants (Loehwing, 1942; Burns, 1951; Dennison, 1945; Hall, 1949). In addition to the seasonal changes caused by maturation, there are also well-defined seasonal changes in trees caused by seasonal digerences in the balance between absorption and transpiration. In general, maximum water content of tree trunks seems to be attained in the spring, just before the leaves open, and decreases to a minimum in late summer, shortly before leaf fall (Clark and Gibbs, 1957; Gibbs, 1939). The tissues of most transpiring plants show diurnal variations in water content, with the maximum at night and the minimum near midday (Wilson et al., 1953). These midday deficits are important physiologically because they often cause stomata1closure and reduction in photosynthesis, as well as less obvious disturbances of other physiological processes. One important effect of reduction in water content is the increase in osmotic pressure of cell sap that often accompanies it. Also, there is often an undesirable increase in diffusion pressure deficit, It is obvious that percentage water content is too variable to be very useful as an indicator for evaluating water conditions in plants or plant tissues, because it is affected by age and kind of tissue as well as by environmental factors. One refinement is to make moisture content measurements on leaves or other parts of the same age, as is done on growing leaf sheaths of sugar cane in Hawaii (Clements and Kubota, 1942). This seems to have proved to be a satisfactory indicator for irrigation of sugar cane, and might be adapted to some other plants. b. Relative turgidity. Many years ago, Stocker (1929) suggested that a good measure of internal water balance is the amount of water taken up by leaves or twigs that are allowed to absorb water until saturated. The saturation deficit is calculated as follows: Saturated fresh weight Saturated fresh weight

- field fresh weight - oven-dry weight

loo

=

saturation deficit

WATEX AND ITS RELA1’tON TO SOILS AND CROPS

67

This procedure has been used in Eiiropc, h i t c.onsideral>lctinw is rcquired for a twig or large leaf to attain equilibrium. A variation of this method was introduced by Weatherley (1950) for measuring the “relative turgidity” of plant tissue. Disks of leaf tissue about 1 cm. in diameter are cut out of leaves, weighed, floated on water until they attain equilibrium, and their fresh weight and oven-dry weight obtained. The relative turgidity is then calculated as follows: Field fresh weight - oven-dry weight Saturated fresh weight - oven-dry weight

x

100 = relative turgidity

This method requires smaller samples of tissue and less time than the Stocker method, and the method of calculation is more satisfactory because the smaller the uptake of water, the larger the number representing relative turgidity. This method has been used extensively by Slatyer ( 1955, 1957), who decided that it gave a good measure of internal water balance in plants of several species. Werner (1954) also used it to measure changes in water balance in several varieties of potatoes, and Oppenheimer (1954) reported favorably on its use. It seems that relative turgidity may be a useful measure of water balance in plants, and might even be used as an indicator for irrigation. Its use is based on the assumption that the fully turgid condition is normal and desirable, but this assumption perhaps needs more study. The time of day when plants are sampled will make considerable difference in the relative turgidity observed, and earlymorning sampling seems best for most purposes. Unfortunately, the effect of a given relative turgidity is not the same in all species; hence, the critical level probably must be determined for each species, and perhaps even for each variety (Slatyer, 1957). c. Osmotic pressure. At one time, osmotic pressure of sap expressed from leaves or other plant organs was used extensively as an indicator of the water conditions in plants, and this is still favored by Walter (1955). Much of the work in this field is summarized by Miller (1938), and methods and results are discussed by Crafts and associates (1949). In general, dehydration is accompanied by increase in osmotic pressure, and increase in water content by decrease in osmotic pressure, but osmotic pressure is not sufficiently sensitive to be used as an indicator of small changes in water balance. d. Diflusion pressure deficit. In recent years attention has tended to shift from measurements of osmotic pressure to measurements of diffusion pressure deficit, because the latter is a more sensitive indicator of the degree of turgidity. The diffusion pressure deficit ( Meyer, 1945; Crafts, et al., 1949; Meyer and Anderson, 1952) is a measure of the pressure with

68

P. J. KRAMER

which water tends to move into cells and tissues, and is comparable to the total soil-moisture stress (moisture tension plus osmotic pressure) of a soil. Fully turgid tissue has a zero diffusion pressure deficit, and the diffusion pressure deficit increases as the water deficit increases until it equals the osmotic pressure, except that, when, as shown in Fig. 19, nega-

TOMATO

E

COTTON

-110

T2

- d! I

P

TOTAL SOIL MOISTURE STRESS (ATMI

TOTAL SOIL MOISTURE STRESS (ATMI

FIG. 19. Effect of soil moisture stress on diffusion pressure deficit, osmotic pressure, and relative turgidity ( Slatyer, 1957).

tive wall pressure or tension develops, it may even exceed the osmotic pressure (Slatyer, 1957). This value is particularly useful because it is expressed in atmospheres and can therefore be compared with soil moisture tension and osmotic pressure of the soil solution. The chief obstacle to extensive use of the diffusion pressure deficit is the difficulty of measuring it, One method is to determine the conkentration of sucrose or mannitol in which no change in length occurs in strips of tissue, but this is applicable only to thin tissues containing little supportive tissue. Another method is to determine the concentration of solutions in which no change in weight of pieces of tissue occurs, but this is applicable only to tissue, such as beet or potato, from which a number of uniform pieces of tissue can be cut. Lem6e and Laisn6 (1951) immersed pieces of leaf tissue in sugar solutions of various concentrations and determined the changes in concentration of the solution by measuring the change in refractive index with a refractometer. This method was described by Ashby and Wolf (1947). Slatyer ( 1957) determined the diffusion pressure deficit of leaf tissue by placing disks over solutions of various vapor pressures to find at what solution diffusion pressure deficit no gain or loss of water occurred. This method requires careful control of temperature. Weatherley and Slatyer (1957) suggest that if the rela-

WATER A N D ITS RELATION TO SOILS AND CROPS

69

tion of the diffusion pressure deficit to the relative turgidity were established for a species, the relative turgidity might be used to estimate the diffusion pressure deficit. e. Stomata1 opening. As mentioned earlier, in many plants premature closure of stomata is one of the most sensitive indicators of water deficit. More or less successful attempts have been made, especially in Israel (see Oppenheimer, 1953; Oppenheimer and Elze, 1941), to use the closure of stomata as an indicator of developing water deficit before visible wilting occurs. For this purpose the extent of infiltration by a series of liquids of different surface tensions has been used to estimate the degree of stomata1 opening. If stomata are wide open, even a substance of relatively high surface tension, such as kerosene, penetrates readily; if nearly closed, only one of low surface tension will enter. Alvim and Havis (1954) found that a graded series of Nujol and n-dodecane gave a good range of surface tensions. In Israel, kerosene seems to have been used most often, and this method is described by Oppenheimer and Elze ( 1941), Oppenheimer (1953), and others. It is, of course, only an indirect indicator of water deficit, not a quantitative measure. f . Other possibilities. One of the most useful tools would be some kind of instrument that indicates changes in the water balance of tissues of growing plants. One possibility being investigated is measurement of changes in electrical resistance by inserting electrodes into the plant tissue. Another possibility might be to measure changes in electrical capacitance. The difficulties of calibrating such methods seem quite formidable, but this field deserves more study. Bloodworth and associates (1956) used the rate of flow of the transpiration stream as an indicator of transpiration rate and internal water conditions. Although this permitted an analysis of the effects of irrigation practices and defoliants on transpiration, it provides no direct evidence concerning internal water balance.

4. Summary 1. Plant growth is affected by internal water deficits because they affect numerous internal processes and conditions, such as turgidity, diffusion pressure deficit, photosynthesis, respiration, and cell enlargement. Changes in these processes and conditions modify both the quantity and quality of plant growth. We therefore need more information concerning the relationship between internal water balance and plant processes. 2. Water deficits occur in plants because the rate of water loss often exceeds the rate of absorption. Although the two processes are partly interdependent, absorption is controlled chiefly by soil factors, and water loss chiefly by atmospheric factors, and they often occur at different rates.

70

M. B. RUSSELL

Water deficits can occur because of rapid water loss or slow absorption, or a combination of the two. Thus, it is impossible to explain plant growth solely in terms of either water supply or the rate of evapotranspiration. 3. The most important factor in plant-water relations is internal water balance, because this is the result of the interaction of all plant and environmental factors as integrated by the plant itself. Furthermore, it is internal water content that affects the processes controlling growth. A reliable method is therefore needed to evaluate internal water balance. Water content has been used for sugar cane, and relative turgidity appears promising enough to deserve further investigation, but neither measures the energy status of the water. Osmotic pressure is not sufficiently sensitive, nor does it measure the free energy. Diffusion pressure deficit seems to be the best measure in theory, because it is a measure of the free-energy status and is expressed in the same units as soil moisture stress, but it is difficult to measure. 4. Some simple method is needed to measure the water balance of crop plants in the field in order to detect internal water deficits before wilting occurs.

B. DROUGHT TOLERANCE OF PLANTS M. B. Russell Universify of Illinois, Urbana, Illinois

Even in humid regions, crop plants may develop water deficits severe enough to be described as drought conditions. Such deficits may arise from augmented transpiration caused by the high desiccating capacity of the aerial environment, or by a reduction in the absorption of water from the soil. The term drought resistance as used here refers to the overall suitability of plants for cultivation under dry conditions. As will be apparent from the following discussion, such resistance may arise from many sources, one of which is the capacity of the plant tissue to survive desiccation. Drought hardiness is the term used to describe the latter phenomenon. Drought resistance was first reviewed in a comprehensive manner by Maximov (1935). More recent reviews (Levitt, 1951, 1956; Iljin, 1957; Kursanov, 1956; Richards and Wadleigh, 1952) have been used to prepare this discussion. The above reviews contain extensive bibliographies, to which the reader is referred for a more detailed discussion of the various aspects of drought resistance. Since this subject has apparently received continuing research attention in the U.S.S.R., it is hoped that translations of the numerous articles cited by Kursanov will become available soon.

WATER AND ITS RELATION TO SOILS AND CROPS

71

The complexity of drought tolerance is illustrated by the following schematic summary of the processes and properties that contribute to it (Newton and Martin, 1930). Drought Resistance A. Absorption 1. Soil factors a. Available moisture b. Concentration of soil solution c. Toxic substances in solution d. Temperature e. Aeration 2. Root development a. Spread and depth of penetration b. Intensiveness of branching c. Number and persistence of root hairs 3. Physiological adaptations a. Osmotic pressure of cell sap of root hairs b. Imbibition pressure of hydrophilic colloids in cells c. Mucilaginous secretions in region of root hairs B. Transpiration 1. Atmospheric factors a. Temperature b. Humidity c. Air movements d. Light intensity e. Atmospheric pressure 2. Structural features a. Ratio of root to leaf b. Conducting tissue c. Reduction of leaf surface d. Rolling, folding, or thickening of leaves e. Deciduous leaves f. Epidermal coverings g. Diminution of intercellular spaces h. Sunken stomata i. Size and number of stomata j. Stornatal regulation k. Surface hairs 3. Physiological adaptations a. Osmotic pressure of cell sap b. Imbibition pressure of hydrophilic colloids in cells C. Wilt endurance

From the foregoing summary it is seen that plants exhibit drought tolerance either because of factors affecting their intake or loss of water or because the plants are able to survive tissue desiccation. Since the physical and physiological processes involved in the movement of water into, through, and out of the plant have been discussed in previous sec-

72

M. B. RUSSELL

tions of this article, they are not repeated here; attention will be restricted to the more specific question of drought hardiness. Appropriate literature citations will be found in Levitt ( 1956), Iljin ( 1957), or Kursanov ( 19%) for each of the statements made in the remainder of this section. Reproductive bodies, such as seed and spores, are noted for their ability to remain viable under rather severe desiccation. To a certain degree, seedlings in their earliest stages also show drought hardiness considerably above that which they possess at later stages of growth. This is in agreement with the fact that, within a given plant, the younger tissues are the most hardy. Such hardiness is due, in part at least, to the greater ease with which water can move to the younger tissues. Drought injury is believed to result both from metabolic and mechanical effects that accompany tissue dehydration and overheating. Since the effects of moisture deficiency on plant metabolism have been discussed in the preceding section, it will suffice here to recall that net photosynthesis may be reduced as much as 90 per cent when leaves are badly wilted. Also, cellular water depletion may result in irreversible reduction of the protein-synthesizing capacity of the chloroplasts and in a loss of the ability of protoplasmic structures to bind certain enzymes. Structural changes in the protoplasm, resulting from mechanical stress induced by the loss of water from the cells, are believed to be a major cause of drought injury. Cell size and the speed of water removal are both positively correlated with the severity of the protoplasmic disruption. Increased resistance to such injury is effected by pretreatments that produce plasmolysis in the cells. The severity of the protoplasmic injury is reduced if subsequent rehydration of the tissues is slow. Drought-hardy plants usually have smaller cells than those living in moist habitats. When desiccated, small cells undergo a much smaller proportionate reduction in volume than do large cells and therefore do not suffer as large disturbances as the latter. Although there are variations between species, it is generally accepted that increased osmotic values are characteristic of plans having superior drought hardiness, Cellular water deficit often is accompanied by an increase in the hydrolysis of starch to sugars. The higher osmotic values not only increase the ability of the cells to retain water, but also may have an additional effect by increasing the resistance of the protoplasm to dehydration. In a general way, drought hardiness varies inversely with the natural water content of the tissues; there are, however, many exceptions to this rule. For many years it has been believed that resistance to tissue desiccation was related to the bound water content of the protoplasm. However, widespread unequivocal demonstration of such a relation has been hampered by the considerable difficulties encountered in measuring the

WATER AND ITS RELATION TO SOILS AND CROPS

73

amount of protoplasmic-bound water independent of water adsorbed by nonprotoplasmic plant constituents and of the water in the cellular solution itself. Significant increases in permeability to polar substances and in viscosity of the protoplasm have been observed in plants subjected to drought conditions. Such changes in the physical properties of the protoplasm are considered to result from concurrent changes in solute concentration and from the protoplasm dehydration that result from drought conditions. Some evidence has been published of the importance of the ca1cium:potassium ratio to observed changes in protoplasmic permeability and viscosity. However, the analyses were based on the total foliar content of the ions, which is not considered to indicate the ionic constitution of the protoplasm itself; therefore, no definite statement on the role of ionic ratios to the physical properties of the protoplasm seems justified at this time. The hypothesis also has been advanced that hydration of the colloids in wheat leaves (and, presumably, their drought hardiness) depends on the hydrophilic nature of the adsorbed cations. The effects of anions in such reactions were found to be more complex and to depend on the age of the plant. If this hypothesis can be substantiated, it raises the possibility of influencing drought hardiness by nutritional means. Of all of the theories advanced to explain drought injury, the completely mechanical explanation advanced by Iljin seems to be the most adequate. Further detailed biochemical and biophysical studies at the cellular and subcellular levels may lead to a more complete understanding of plant response to drought conditions, but the phenomenon may presently be described as follows, based on Iljin’s theory (Levitt, 1956) : “. . . it is not the water loss itself that kills, but the mechanical injuries that accompany drying and remoistening. When a plant part dies, the cells collapse. The protoplasm is pulled inward by the shrinking vacuole, outward by the resisting wall to which it adheres. If the cell wall is sufficiently rigid it opposes the collapse and thereby subjects the protoplasm to a strong tension that may lead to destruction. If the wall is thin and soft, it is pulled together with the vacuole and forms folds and wrinkles. Before regaining their normal size and shape, cells that survive drying are subjected to new mechanical stresses and remoistening, which may lead to death.”

74

M. B. RUSSELL

C. CROPRESPONSES TO EXCESS WATER M. B. Russell University of Illinois, Urbana, Illinois

The effects of excess water on soil properties, discussed in Section 111, A, explain in part the reductions in crop growth associated with poor drainage, In addition, such conditions also directly affect the normal processes of the plants themselves, largely because of the deficient soil aeration that is a corollary of inadequate drainage. Since the effects of soil aeration on plant growth and physiologic processes have been the subject of recent reviews (Russell, 1952; Wesseling et al., 1957), only the general nature of the phenomena rather than a detailed discussion of specific research findings is presented here. Aquatic plants and those indigenous to poorly drained soils frequently adapt themselves by a superficial rooting system or by morphological root modfications that serve to supply oxygen for the normal aerobic respiration of the submerged tissues. When roots grown in well-aerated soil are deprived of oxygen by soil flooding, the roots die, except that certain species develop an adventitious root system that enables the plant to continue growth. Roots of some plants develop large intercellular spaces when grown in poorly aerated media. The differences exhibited by plants in their tendency to develop adventitious roots or intercellular spaces are believed to explain in part the differences between crops in tolerance to submergence. Poor aeration arising from inadequate soil drainage affects both the growth and functioning of roots. The uptake of water is influenced in two ways. Of minor importance is the effect of insufficient oxygen on the respiration-dependent or “active” water absorption. The effects of low oxygen or high carbon dioxide on root permeability are responsible for the great reduction in water intake that follows root submergence. Aeration is a major factor affecting the uptake and accumulation of nutrient ions by roots. The role of aerobic metabolism, both as a source of energy and as a factor governing root permeability, is well established. Since this process is oxygen-dependent, it is apparent why restricted aeration is usually followed by the appearance of serious nutrient-deficiency symptoms in plants. Toxicity symptoms also may accompany poor soil aeration, because of its effects on the solubility and level of oxidation of both inorganic and organic compounds found in soils. Lack of aeration is a major factor causing damage to flooded plants,

WATER AND ITS RELATION TO SOILS AND CROPS

75

but some available evidence indicates that deleterious effects are also produced by microbiological activity in an anaerobic environment. The extent of damage caused by the flooding of standing crops depends on the plant species, the duration of the flooding, the prevalent temperatures at the time of flooding, and the organs in the process of being formed. Most plants are quite tolerant to flooding during the dormant period. During the growing season, some plants show moderate tolerance and others die soon after the onset of the flooding. Cereals are severely damaged when flooded at the time of flowering and initial ear formation, but are quite tolerant when the grain matures. The manner in which some plants, notably pear trees, certain forest trees, and grasses, can withstand long periods of submersion of their root system during the growing period is not understood. Injury to crops is particularly severe when they are flooded on hot days. The damage thus done is generally referred to as scalding. Although the physiological reasons for this behavior are uncertain, certain measures can be taken to reduce scalding damage in irrigated crops. Silt deposits are often left behind by irrigation or flood waters. They may have a beneficial effect because of their nutrient content or by helping to improve the texture of a coarse soil. Silt deposits are harmful when they reduce the infiltration rate of fine-textured soils or when they coat plant surfaces, thereby making the plants more sensitive to injury by heat. The contribution made by the water supply from a relatively shallow water table is often largely overlooked, but practical experience and field observations have shown that crop production can be substantially increased by careful management of this source of water. Even a relatively deep water table may serve as a primary source of water in alfalfa, meadow, and forest tree production under certain conditions. A high water table can be either harmful or beneficial to crop production. An optimum level of water table seems to exist for each particular crop under given conditions, but attainment of this optimum level under field conditions is not easily achieved. A crop may be damaged by submersion of the deeper roots or by a lack of aeration in the overlying soil resulting from a high water table. Roots forced near the surface of the soil may suffer from nitrogen or other nutritional deficiency. If the water table falls rapidly in the spring, roots may remain near the surface, with the result that the plants become more susceptible to drought injury during periods of lower rainfall. Under field conditions, the optimum level for the water table may frequently be exceeded because of rains during the growing season. The permeability of the soil and the water-table-control mechanisms em-

76

M. B. RUSSELL

ployed determine the ease with which the water table can be controlled at any predetermined level under given conditions. Opportunity for effective control is greatest in highly permeable sandy soils, and least in clays. The difficulty encountered in maintaining an optimum level of water table also arises out of the variable rate of consumptive water use by the crop, so that, theoretically, different levels of water table should be maintained at different times during the growing season. However, raising the water table at times of high consumptive water use may kill the deeper roots, induce weed infestation and diseases, and make land operation difficult. In certain cases, high bulk yields obtained at a high water table may be offset by a low protein content of the crop. The success of subirrigation will depend on whether the moisture removed by the roots is replenished by capillary rise with sufficient rapidity. The magnitude of the rate of rise is such that, under most growing conditions, the two factors can be matched in sandy soils and in peat. The rate of capillary rise in a clay soil is often too slow to make up for moisture removal in the root zone; therefore, this soil tends to dry out, even where a water table is close to the surface, unless the crop uses moisture at a low rate or recurrent rain assists moisture supply. However, even under conditions of high consumptive use and in the absence of rain, crops can still benefit from a relatively high water table level in a clay soil, if it is stable and if the roots can penetrate into the capillary fringe, just above the water table. The effects of water table elevation on the yield of several crops have been studied at a few locations and summarized by Wesseling et aZ. (1957). The yield responses were found to vary widely from year to year, largely because of differences in the amount and seasonal distribution of rainfall. Major differences in response were found between crops and between locations, indicating the importance of the rooting habits of the crop and the physical characteristics of the soil. Because of the complex nature of soil-crop-weather interactions, the limited data now available do not permit any generalizations on optimum water table depths for specific crops. Similar difficulties are encountered in summarizing the results of many experiments in which yields are compared on drained and undrained land. The small amount of information available on the optimum depth of the water table during the off-growing season indicates that a high water table during that period may lower crop production in the subsequent season, perhaps because of deterioration of soil structure under water-logged conditions. For a more extended discussion of crop re-

WATER AND ITS RELATION TO SOILS AND CROPS

77

spiises to excessively high soil moisture levels see van’t Woudt and Hagan ( 1957). VI. Soil-Plant-Water Interrelations

In preceding sections the hydrologic cycle and the three possible binary combinations of soil, plant, and water have been discussed. Most of the phenomena encountered in the field, however, represent the interaction of all three components. Therefore the remainder of this review considers the behavior of plants as affected simultaneously by soils and water, the effects on water of the combined action of soils and plants, and the joint influence of water and plants on soil behavior. The role of certain management practices in modifying such responses is also considered. A. INTERPRETATION OF PLANTRESPONSES TO SOIL MOISTURE REGIMES R.

M. Hogan, Y.

Vaadia, and M. 6. Russell

University of California, Davis, California, and University of Illinois, Urbano, Illinois

The response of crops to various soil moisture regimes is a matter of great agricultural importance, but little general agreement. One school of thought maintains that soil moisture is available for plant growth equally over the range from FC (field capacity) to PWP (permanentwilting percentage ). Much experimental evidence supporting this view has been cited by Veihmeyer and his colleagues, in their own papers and in reviews citing the work of many others. Another group of investigators maintains that plant growth shows differential response as soil moisture varies between FC and PWP. Several review articles and many research papers have presented experimental evidence supporting this view. The two views are presented schematically in Fig. 20. The voluminous literature ( Stanhill, 1957) on the opposing positions is not recapitulated here; rather, the experimental results presented by both sides are examined to discover areas of general agreement and to pinpoint the crucial conflicts. The hope is to lay bare the reasons for the difference and perhaps to suggest what kinds of information are needed to resolve the conflict. Overgeneralization appears to have been a major factor in creating and maintaining the controversy. Many soil, plant, and environmental factors are known to affect the dynamic processes of water in the soilplant-atmosphere continuum, perhaps in unknown ways. In many ex-

78

R. M. HAGAN, Y. VAADIA, AND M. B. RUSSELL

periments the observed plant response is the integrated effect of all such growth factors, and many of these factors may have varied considerably during the course of the experiment. Thus the response of a mature prune tree growing on a deep, permeable soil in the Sacramento Valley cannot be predicted from the response of a corn plant in Iowa. Neither is it possible to predict the behavior of the Iowa corn plant from studies I-

Ba

Y loo i

D

I 0& W

c W

>2

00

25

50

'15

I )O

AVAILABLE MOlSTURE DEPLETION, PER CENT

FIG.20. Schematic representation of the view that soil moisture is equally available for plant growti between field capacity (FC) and permanent-wilting percentage (PWP) and the view that plant growth shows differential response to soil moisture between FC and PWP.

on the amount of root elongation occurring in 24 hours in a highly controlled root-culture experiment. Experience has shown that altering certain environmental factors within known limits has little effect on plant responses to differences in soil moisture. However, until the nature of the response curve of all such factors and their interactions are more completely studied, it is very doubtful if generalizations can be made about plant responses to various soil moisture conditions in the field.

1. Factors Involved in Interpretating Experimental Data Let us examine the difficulties encountered in arriving at a general relationship between soil moisture availability and plant growth, and some of the factors that complicate the interpretation of available experimental data. This discussion is by no means complete. The examples chosen simply illustrate some of the problems involved. a. Soil moisture movement in unsaturated soils. It is obvious that plant growth may be slowed by lack of water. However, there is no reason to expect any symptoms of damage as long as water uptake does not lag appreciably behind water loss from the leaves. In essence, the theory of equal availability of soil moisture between FC and PWP im-

WATER A N D ITS RELATION TO SOILS AND CROPS

79

plies that the rate of moisture uptake is not limiting in that range. The opposing theory-that plant growth shows differential response as soil moisture is varied from FC to PWP-assumes that increasing moisture stresses decrease water uptake to a harmful degree. Both theories have received thermodynamic justifications ( Veihmeyer, 1956; Wadleigh, 1946). Interest in the kinetic aspects of moisture movement to roots has developed recently, and several papers (Taylor and Haddock, 1956; Biggar, 1956; Gingrich and Russell, 1957; Phillip, 1957; Peters, 1957) have stressed the necessity of giving the dynamic aspects of soil moisture much more emphasis than in the past. 100 r

00

-E

5

48-Hour

Growth 100~/0

60

c

0 ._ c 0

0 -

40

w

20

0

I

I

I

I

2

4

6

8

Soil Moisture Tension (ATM.)

FIG. 21. Elongation of corn roots as a function of soil moisture tension. The numbers 25, 50, 75, 100 refer to the percentage of clay soil in the sand-soil mixture (Peters, 1957).

Peters (1957) devised an experiment that illustrates the difference in effect on plant growth of soil moisture stress and soil moisture content. Germinated corn seeds in controlled-growth chambers were covered with soils (soil and sand mixed in various proportions) that had been brought to definite soil-moisture-tension values. The results are presented in Figs. 21 and 22. The number on each curve in Fig. 21 indicates the per cent of soil in the mixture. A t the same tension, plants grew better in fine soil than in coarse soil. Thus, soil moisture tension

R. M. HAGAN, Y.

80

VAADIA,

AND M. B. RUSSELL

was not the only factor affecting water availability; moisture content was also important, probably because of the greater unsaturated hydraulic conductivity in fine soil. Another experiment seems to point the same way. Gingrich and Russell (1957) used a series of soil samples and mannitol solutions of corresponding stresses. The results, presented in Figs. 23 and 24, show that corn seedlings grew better in osmotic solutions than in soil samples having the same stress. The fresh weight and elongation of the seedlings decreased with increasing stresses, both in soil and in osmotic solutions. Dry weight, however, decreased only in the soil, remaining relatively unaffected in osmotic solutions. The greatest

40

E 30 E c

.-0

'0 20 c1,

c

w 0

10

II

0

5

10

15

Per cent

20

25

30

Water

FIG.22. Elongation of mrn roots in %-hour periods as a function of soil moisture content. Numbers refer to respective soil moisture tension (Peters, 1957).

stress tested was 12 atmospheres. The authors concluded that the watertransmission characteristics of the unsaturated soil, a factor not involved in osmotic solutions, were responsible for the differences. The effect is most pronounced at stresses of 1to 3 atmospheres. Because young seedlings were used, one should be cautious in drawing conclusions about well-established plants. The results nevertheless illustrate the importance of considering rate of moisture movement to the roots in addition to moisture stress values. b. Fertility considerations. The relation of fertility to water availability is very complex. A recent review (Hawthorne, 19%) listed nine papers reporting that P uptake is unaffected by soil moisture within the

81

WATER A N D ITS RELATION TO SOILS AND CROPS

32Lf frn

24

0 0 Soil

Moisture Tension

+ Osmotic

-

Stress

:20 0 16 .-I

Q

U

2

12-

0'

0

I

I

I

I

2

3

I

I

I

I

I

1

I

I

t

4

5

6

7

8

9

10

II

12

Moisture

Stress

(Atm.)

FIG. 23. The effect of soil moisture tension and osmotic stress on corn radicle elongation (Gingrich and Russell, 1957).

?

-E" In

150 140

0

c

130

0

+

al

v, 120

2 ;110 c

.c

0 100 .-

s

ZI

90

6

80

0 Soil moisture tension

I'

0

+ Osmotic I

2

Stress

3

4

5

6

7

8

9

10

II

12

Moisture Stress (Atm.)

FIG. 24. Effect of soil moisture tension and osmotic stress on the dry weight of corn seedlings (Gingrich and Russell, 1957).

82

R. M.

HAGAN,

Y.

VAADIA,

AND M. B. RUSSELL

availablc range, twelve papers reporting that P uptake is dccrcasctl by increased soil moisture stress, and some papers reporting that P uptake is increased by decreased soil moisture content. Such codict is only to be expected, for the lack of agreement on water availability should lead to inconsistent results in a more complex relationship that depends at least partially on moisture movement. From an experiment with corn seedlings growing both in osmotic solutions and in soil samples, Danielson and Russell (1957) concluded that the accumulation rate of Rbsa is reduced by increasing soil moisture tension, but not directly affected by increasing osmotic concentrations in solution. They proposed that the results might reflect slower water I

ISOYIELD CURVES

I

J

MOIST

MOO MOIST MOD.DRY IRRIQATION TREATMENT

DRY

FIG. 25. Isoyield curves illustrating the possible interaction between nitrogen application, irrigation, and yield.

movement in the soil, and hence slower water uptake by the plant. Other evidence confirms these conclusions for various cations and anions (Dean and Gledhill, 1956; Wadleigh and Richards, 1951). Over the growing season, plant growth may be influenced as much by nutrient uptake as by water uptake. Since nutrient and water uptakes may be closely linked, fertility considerations are of great importance in analyzing the results of irrigation experiments. Where some essential element is confined to the upper soil, a dryingout of the root zone in that layer may seriously retard plant growth even when the plant has adequate water. F. J. Hills and co-workers (unpublished data, University of California, Davis) showed that sugar beets in a phosphate-fertilized soil suffered phosphorus deficiency whenever

WATER AND ITS RELATION TO SOILS AND CROPS

83

the surface soil dried out, even though deeper roots were obtaining SUEcient water to maintain plant turgor. This sort of situation may occur often in studies of the relations between soil moisture availability and plant growth. Then increased yield from increased irrigation is not entirely-if at all-attributable to a correction of inadequate moisture. In Fig. 25 a series of curves shows the interaction between soil moisture and the level of nitrogen fertilization. The figure is idealized, but can be plotted from data obtained in many experiments. Isoyield contour lines are plotted for various irrigation and fertilization levels. Ni-

?!

0

25

-

2 4 0 Ib. /acre Nitrogen

0 \

u?

c

2 20-

120 Ib. /acre Nitrogen

C

B

A

irrigation

Treatment

FIG.28. The influence of excessive irrigations on leaching of nutrients and on crop yield (adapted from Robins et al., 1956).

trogen status is clearly a limiting factor, for yield increases greatly with fertilizer applications. Such data support both views on soil moisture availability: at low nitrogen levels, water is equally available between FC and PWP for plant growth; at higher nitrogen levels, yield is materially reduced by drier irrigation treatments within the available range. The importance of the relation of fertility to irrigation practice is again illustrated in Fig. 26. Plants were irrigated every 2 weeks, with group C receiving enough water to wet the complete root zone, group B receiving twice as much, and group A three times as much. At each level of fertilization yield decreased with increased water. Nitrogen was evidently

84

R. M.

HAGAN,

Y.

VAADIA,

AND M. B. RUSSELL

leached from the soil by heavy water applications. These results indicate the importance of regulating irrigation water carefully. c. Stage of growth. Some data (Hagan et al., 1957; Petinov, 1954; Shardakov, 1957) have indicated that optimal soil moisture for plant growth may vary with a plant’s stage of growth: germination, vegetative growth, flowering, or maturity, for example. If we assume the various stages of growth to be a function of their respective moisture supplies, we obtain the following: GI = fl (soil moisture) germination G2 = f 2 (soil moisture) emergence G3 = f3 (soil moisture) vegetative growth flower and bud initiation G4 = f 4 (soil moisture) Ga = fs (soil moisture) fruit development To continue this idea further, functional relationships within each of these stages are not independent of the previous stages. The relationships become :

-f

soil moisture) ( Gg, G,, soil moisture) Gilowerlng (Gg, G,, Ge, soil moisture) If these growing periods become infinitesimally small, and, hence, infinite in number, we can (given sufficient mathematical knowledge and Ggermination Gelongation

-f -f

(Gseed,

TIME

FIG.27. A schematic representation of the idea that the cumulative growth curves of crops may differ owing to variations of soil moisture stress at various stages of growth. Curve A illustrates the growth curve under one series of soil moisture values. Curve B illustrates the relation for another.

WATER AND ITS RELATION TO SOILS AND CROPS

85

data) calculate growth curves such as those in Fig. 27. The total growth of a crop is a summation of the growth at each growth stage as affected by the particular relationships to the prevailing moisture regime for a given set of conditions described by known values for other soil factors, plant factors, and climatic factors. An irreversible influence of soil moisture on the growth rate at any particular stage may affect subsequent growth and total yield at maturity. This consideration is of great importance. Some data indicate that moisture availability or other factors are so critical at certain stages that growth can be restricted at later stages, whether or not growing conditions are then optimal. An established example is the importance of irrigating corn just before tasseling (Robins and Domingo, 1953). A deficiency in soil moisture then will permanently restrict growth, regardless of later irrigation. The preceding discussion illustrates two points: (1) many variables are involved, and (2) the changing relations between growth stage and soil moisture availability make extremely difficult the formulation of a widely applicable general relationship between plant growth and soil moisture. d. Growth and yield. What is meant by plant growth is often insufficiently specsed, thereby complicating the interpretation of results in irrigation experiments. We easily recognize several different manifestations of growth: elongation of plant organs, increase in fresh or dry

-YIELD

OF SHRVB

YIELD OF RUBBER

00

L

25 L

.

50

75

loo

IDRII 0

8

J

AVAILABLE MOISTURE DEPLETION, PER CENT

FIG.28. Difference in effects of moisture depletion on dry weight yield of shrub and on yield of rubber per acre (adapted from Hunter and Kelley, 1946a).

86

R. M.

HAGAN,

Y.

VAADIA,

AND M. B. RUSSELL

weight, development of reproductive rather than vegetative organs, ctc. They result from intricate combinations of many physiological processes; probably not all are equally affected by changes in the internal water balance of cells and tissues when soil moisture stress is increased. Many examples support this idea, though the results sometimes contradict each other. Following are some of the findings reported, including some conflicting results: In guayule the dry-weight yield of vegetative parts was decreased with increasing soil moisture stress, but the yield of rubber increased (Hunter and Kelley, 1946a). This is shown in Fig. 28. The per cent of

:- -

a

1200

200

W 0

nFORAGE

J

- 150 8

150-

W

z W W

%

- 100

100-

lA

=

2s 88 on

0 0 -I

J

wa-

50-

tJ

o

$ 1

, - 50 ,

.

,

25

50

75

,- -

100

w

).

- ---1 O %d

W

a

& W

SEED

4

d W

a

AVAILABLE MOISTURE DEPLETION, PER CENT

FIG.29. Effect of irrigation treatment on clover green forage and seed yield (Hagan et al., 1957).

sugar in cane and beets was raised by increasing the moisture stress (Raheja, 1948; Wiersma, 1955). As shown in Table VII increasing soil moisture stress is reported to decrease sugar content and increase nicotine and nitrogen in tobacco (van Bavel, 1953). Tests on beans and tomatoes indicate that soil moisture stress depleted the reduced starch and sugars in the roots, stems, and leaves before the soil reached the wilting percentage (Woodhams and Kozlowski, 1954). Moisture deficiency in the soil was reported to increase oxidase and catalase activity and reduce ascorbic acid content in tobacco leaves (Tombesi, 1951). The fact that various measurable aspects of growth do not respond

87

WATER AND ITS RELATION TO SOILS AND CROPS

in the sainc manner to moisture stress must be recognized in analyzing plant-soil-water relations and proposing efficient irrigation practices. Furthermore, from a practical standpoint, harvestable yield is more important than potential yield or “growth.” Increase in vegetative growth does not necessarily indicate an increase in potential yield, and an increase in potential yield does not always mean an increase in harvestable yield. In addition, considerations of market quality and nutrient content may be involved. Therefore the irrigation practice to be applied to one

U

2.5

-.-

1 50

25

Available

Moisture

75

too

Depletion,

Per cent

Wilted

FIG.30. Influence of date of harvest on observed relation between soil moisture depletion and harvestable yield of Ladino clover seed. Midseason harvest made on July 23, final harvest August 17. Reduced yield at time of final harvest on B treatment (wet) was caused by germination of seeds from heads produced earlier in the season (Hagan et al., 1957).

crop may not be adequate for another depending on the type of organ harvested. A study of the influence of soil moisture stress on various aspects of growth in Ladino clover (Hagan et al., 1957) has shown that increasing soil moisture stress decreases forage production and potential seed production, but increases the yield of harvestable seed; it holds preharvest germination of newly produced seed to a minimum by keeping dry the atmosphere around the plants. This is shown in Figs. 29 and 30.

88

R. M.

HAGAN,

Y.

VAADIA,

AND M. B. RUSSELL

e. Importance of the root system. The nature of the root system is important in determining the relation between measurable soil moisture stress and plant growth. Different interpretations of root development and moisture conditions within root-penetrated soil contribute to contradictory views on plant-soil-water relations. When soil and growing conditions are favorable, many perennial crops develop well-branched root systems that thoroughly permeate the soil to depths characteristic of the plants. Below these depths the density of the root systems diminishes until moisture extraction cannot be detected. It is desirable to distinguish what may be called the “complete extraction” zone from the “partial extraction,” or “transition,” zone. The complete extraction zone may be defined as the volume-or, with closely spaced plants, the depth -from which the plant can remove all available water. In this zone the soil is thoroughly permeated with fine roots. The transition zone contains a varying concentration of roots, usually decreasing with depth. The existence of this transition zone often seriously complicates the interpretation of soil moisture experiments. With the expanding root systems of annual crops, analysis of moisture conditions is even more complicated. In the seedling stage, only a tap root or a few branched roots penetrate the soil. Later some annuals rapidly develop well-branched root systems that continually permeate ever-greater soil volumes. At the same time some roots grow out ahead into a correspondingly enlarging transition zone. Thus, if the soil has been wet to field capacity to a considerable depth, these growing roots continuously come into contact with additional supplies of available water at low tensions. If the roots are well branched and grow rapidly enough, they may contact new supplies of readily available water fast enough to meet the needs of transpiration. On a pre-irrigated deep alluvial soil at Davis, California, a crop with a rapidly extending root system, such as watermelon, does not respond to later irrigations, although a relatively high soil moisture stress may develop within an ever-increasing soil volume. Other annuals send out but a few widely spaced roots, leaving large volumes of unexplored soil between roots, particularly in the early stages of growth:In such cases, soil moisture samples, or even moisture-indicating devices, may give quite a false picture of moisture conditions at the root surface. Even when the measured soil moisture stress may be quite low, crops with sparse roots will respond to irrigations. As shown in Fig. 31 the sparser the roots, the greater the likelihood that growth will be retarded if irrigation is delayed. Were it possible to sample only the soil adjacent to absorbing roots, one might find that actual moisture stress conditions are considerably higher than is indicated by present methods. Until methods are devel-

WATER AND ITS RELATION TO SOILS AND CROPS

89

oped to measure the moisture stress experienced by sparsely rooted crops, it apparently will be necessary to establish for them some rather arbitrary moisture depletion limits. Such limits will depend on other soil factors, stage of growth, and climatic conditions. It is not uncommon to find reports that yields decline unless crops are irrigated at some given soil moisture stress. But the depth at which the stress was measured is often not stated, and its relation to rooting depth and spatial density is seldom indicated. Where spatial density diminishes rapidly with depth, plant response at conventionally measured moisture stresses will vary markedly over a short depth interval. c

=al too 0 L 0)

a

f

3 0

L

c3 0

al

c

0 Q)

> c 0

‘ L o

0

50

25

Available

Moisture

Depletion,

75

100

Per cent

FIG.31. Schematic representation of the effects of spatial density of roots on the relation of growth to the apparent depletion of available moisture ( Hagan, 1955).

The suggestion has been made (Wadleigh, 1946) that roots in the sparsely developed part of the root zone, presumably absorbing water against low soil moisture stresses, may mask the effects of relatively high soil moisture stresses developed in denser parts of the root system. Thus, it may be argued that, whenever widely spaced roots extend through a considerable volume of soil, depletion of most of the available moisture from the dense portion of the root zone may have relatively little influence on growth. This idea is represented in Fig. 32. Whether or not the crop will respond to increasing stress within the complete extraction zone will depend on whether the roots in the transition zone can supply water fast enough to maintain the optimum water balance in the plant.

e

c

0 1 $ L

-

d r c %

2 (3

subjected to stress

'c

0

W + 0

a 0)

.-c>

0 W

0,

I

FIG.32. Schematic representation of the relation between the portion of the roots subjected to stress and rate of growth (Hagan, 1955).

Evaluation of effective soil moisture is complicated particularly in annual crops by the unknown fraction of roots in the transition zone and their continued extension into moist soil. Data on grain sorghum presented in Table VIII are interesting in TABLE VIII Effects of Irrigation Treatment on Yield of Grain Sorghum" Irrigation treatment

A. No irrigation B. One irrigation a t early boot stage C. Two irrigations; one a t first stress symptoms; one at second boot stage D.One irrigation when plants 6 inches high E. One irrigation when plants 6 inches high; six additional irrigations at 11-day intervals a Unpublished

Yield (lb./A.)

3805 4744 5537 5747 5725

data, D. W. Henderson, University of California.

this connection. There was no significant difference in yield between a rather wet irrigation treatment (treatment E) and a single irrigation applied early in the season (treatment 0 )although other treatments, such as late irrigation (treatments B and C),receiving as much water or

WATER AND ITS RELATION TO SOILS AND CROPS

91

more, decreased yield significantly. This may indicate that considerable available moisture near the soil surface is essential for rapid adequate root growth early in the season; otherwise, yield may be depressed. f . Plant diseases. Plant diseases, nematodes, and insects may also complicate the interpretation of soil moisture studies. By reducing root surface, soil-borne plant diseases and nematodes may cause crops to respond favorably to irrigation at very low moisture stress levels. Because plant diseases and insects often do well in moist environments, their development (and resulting damage) may be increased under wet irrigation regimes. Disease and insect infestations favored by frequent irrigation may reduce yields more than moisture deficiencies involved in drier irrigation regimes that are unfavorable to such infestations. The data in Table IX illustrate the effect of irrigation frequency on the TABLE IX Effect of Irrigation Timing on Yield of Cotton and Severity of Verticillium Wilt Infectiona Treatment

A

B

Number of irrigations Date of first irrigation Yield (bales/acre) Per cent plants infected

7 6/15 1.74 71

5 7/9 2.16 38

0

Stockton and Doneen (1957).

severity of wilt infection on cotton. The early June irrigations were probably responsible for the greater infestation and smaller yield of treatment A. g. Climatic considerations. The aerial environment influences the relation between soil moisture availability and growth or potential yield. As a first example, let us take rate of photosynthesis in a controlled environment. Figure 33 illustrates the relation between soil moisture and rate of photosynthesis of Ladino clover under constant temperature, humidity, and continuous light ( Upchurch et al., 1955). These results agree well with the theory of equal soil moisture availability. However, several investigators ( Richards and Wadleigh, 1952) have pointed out that the rate of photosynthesis depends on leaf turgidity rather than soil moisture per se. This suggests that the relation for Ladino clover may depend on atmospheric environmental conditions. One might predict that, at lower humidity and higher light intensity, the relation would fit the second viewpoint on moisture availability. Japanese investigators (Nagisi and Satoo, 1954) who worked on pine and Cryptomeria, reported

92

R. M. HAGAN, Y. VAADIA, AND M. B. RUSSELL

Rvoi/ub/e moLdure Dep/ehon, per cent

FIG.33. Effect of moisture depletion on photosynthesis in Ladino clover grown in containers under continuous light and at constant temperature and humidity (Upchurch et al., 1955).

that “the higher the level of light intensity, the more effect of decreasing of soil moisture could be observed.” Thus Fig. 34 indicates that, under conditions favoring greater water losses from the leaf surface, the rate of photosynthesis diminishes as soil moisture diminishes. Figure 35 illustrates sugar beet yield as a function of irrigation treat-

Available

Moisture

Depletion,

Per cent

FIG.34. A schematic representation of the possible effects of soil moisture stress on growth under different conditions of light intensity and relative humidity,

93 ment-in Logan, Utah (Kelley and Haddock, 1954) and in Davis, California (Doneen, 1942; Marcum et al., 1942). Although the irrigation treatments were not identical in the two locations, they were similar enough to warrant comparison. The widely divergent resuIts lead to opposing conclusions. Both dry treatments closely approached the wilting point, but yield was unaffected at Davis and markedly decreased at Logan. At Logan, yield was highest with the wettest treatment; at Davis, yield was not significantIy affected by the various moisture treatments. It is probable that the differences between these two experiments priWATER AND ITS RELATION TO SOILS AND CROPS

UOlST

DRY

MOD MOIST

I S

w* HOIST

w3 MOD MOIST IRRIO4TION

WE

MOO. DRV

Wl DRY

TREITMLNT

FIG.35. Yield and per cent sugar of sugar beets in Davis, California, and in Logan, Utah (adapted from Doneen, 1942; Kelley and Haddock, 1954).

marily reflect differences in root development and climate. The date of planting sugar beets is February or early March in California, and May in Utah. Sugar beets in Davis have about four months to develop a deep and well-branched root system before the hot dry weather and highevaporation conditions of summer. This is not the case in Utah. Measurements of soil moisture stress, which were used as indicators of the various irrigation treatments, do not approximate the situation near the root surface under these conditions. The indicated stress is much smaller than that actually experienced by the roots. As shown in Fig. 35 effects

94

R. M. HAGAN, Y.

VAADIA,

AND M. B. RUSSELL

on sugar percentages were also diametrically opposed. The divergence may have resulted from differences in fertility levels and differences in moisture stress that developed within the plant after the last irrigation. 2. Present Status and Future Outlook It is generally agreed that the growth and physiologic well-being of living plants is partly determined by the water status of the plant tissues. A full expression of the effects of varying the water status cannot be obtained if factors other than water are simultaneously limiting the plant processes being studied. The movement of water into, through, and out of a plant is in response to an over-all free-energy difference exceeding by several orders of magnitude that in soils in the range from FC to PWP. Much of the over-all free-energy difference is in the laminar air layers immediately adjacent to the leaf surface, and in the substomatal cavities. Thus the free energy of water in the root cells has the same order of magnitude as the free energy of soil water in the FC to PWP range. If measured during a drying cycle, the energy of retention of water by soils is a continuous single-valued function of the moisture content, with functional dependence being determined by the nature and extent of the hydrated surface and, at high moisture contents, by the size distribution of the soil voids. The energy-moisture content relation of soils exhibits marked hysteresis, hence no fixed relation exists between the two variables even if the antecedent moisture variations are identical. The full significance of such hysteresis effects in relations to soil moisture-plant growth responses remains to be established. On a macroscopic, or field-scale, basis, most water use by plants occurs at continuously decreasing soil moisture percentages, interrupted by sudden returns to field capacity or above. On that basis it appears that hysteresis effects might be ignored in interpreting soil moisture-plant behavior experiments. However, at the root-soil interface and on the basis of the diurnal variations of water intake rate, it is apparent that the soil immediately adjacent to the absorbing root probably undergoes cyclic wetting and drying, and would therefore exhibit hysteresis with respect to the energy-moisture content relationship. The free energy of soil water is determined not only by the energy of retention by the soil but also by its concentration of solutes. In fertilizer bands and in many soils of arid regions the soluble-salt, or osmotic, effects on the free energy of soil water may be of the same order as those due to interfacial forces found in the moisture range from FC to PWP. Here too, the short-term changes occurring immediately adjacent to the absorbing root are not established.

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The impossibility of uniformly rewetting a given volume of soil to moisture contents lower than the field capacity without resorting to mechanical mixing is generally recognized, as is the corollary impossibility of maintaining in the presence of living roots a constant soil moisture at values below field capacity. Therefore, two principal types of experiments are used to study plant response over the soil moisture range from FC to PWP. In one type, plant behavior is studied as soil moisture is reduced by root withdrawals or by evaporation over a predetermined portion of the FC to PWP range. When the desired degree of depletion is achieved, sufficient water is added to bring the entire soil volume up to FC. Plant observations may then be repeated over another cycle of depletion. Thus the plant is subjected to one or more cycles of soil moisture depletion and recharge. Such cycles may have predetermined amplitudes, ranging essentially from zero to 100 per cent of the FC to PWP moisture range. When differential plant responses are observed in such experiments, attempts are usually made to relate the responses to a depth- and/or a time-averaged mean moisture content characteristic of each moisture regime. There is little a piori basis for such an averaging process, since it implicitly assumes that a linear soil moisture-plant response function exists. If the averaging extends over several cycles, it further assumes that the plant responses are independent of previous treatment history. There is no justification, therefore, for attempting to relate the observed plant response to such an artificial mean moisture content. Plant responses can with validity be related only to the different soil moisture regimes under which they were produced, unless it can be demonstrated that the assumptions implicit in the averaging process are justified. The second principal kind of experiment used to study soil moistureplant relations is the short-term, constant-moisture type. In such studies different soil moistures, either within or extending beyond the FC to PWP range, are prepared by distributing water mechanically through the experimental soil. Plant roots are then brought into contact with the soil and allowed to grow for a period short enough that the soil moisture changes resulting from the withdrawal of water by roots are too small to alter initial soil moisture content materially. Observed plant responses may be thus related to a particular soil moisture condition. But such studies warrant no conclusions as to the effects to be expected from continued exposure of roots to such conditions, since it is recognized that plants are capable of making gradual physiologic adjustments to slowly changing root environmental conditions. A modification of the constantmoisture type of study has also been used, in which small plants are transplanted into rather large volumes of soil of known moisture content.

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It is assumed that the roots will then grow through the soil fast enough to keep in contact with essentially constant soil moisture. This assumption may be open to serious question, but the procedure does permit longer periods of study and may enable the plant to make physiologic adjustments. Therefore, more valid conclusions can be drawn concerning the nature of the soil moisture-plant growth relation. Most investigators agree that differences exist among plants and among the several physiologic processes, both in the nature and extent of response to changes in soil moisture in the FC to PWP range. In general, vegetative growth and cell enlargement are more commonly involved than are reproductive growth and cell division. Perhaps because of differences in seasons when vegetative growth occurs, the nature and extent of their root systems, and storage within the plants, perennial crops as a class are less responsive to soil moisture changes than annuals. It is generally agreed that extended exposure of a major portion of a plant's root system to moisture contents at or below the PWP results in injury from which the plant may recover only slowly and only partially when the soil moisture content is raised. No general quantitative statement can at present be made concerning the portion of roots that can be so affected or the length of the exposure or the recovery time. Such quantities undoubtedly will vary with the crop, its stage of growth, intensity of the transpiration demand, the 1eaf:root surface ratio, and the moisture-transmitting characteristics of the 'soil. In view of the great complexity of the response of plants to differential moisture conditions, it appears that two major lines of experimentation should be further developed. One type would be field study of the response to controlled soil moisture regimes of individual crops on specific soils. The scientific usefulness of such experiments will be greatly enhanced if detailed supplementary data concerning the soil, the plant, and the aerial environment are also collected during the full course of the study. A lack of relevant supplementary data on an experiment frequently makes it impossible to rationalize treatment responses that, viewed by themselves, appear contradictory. The second type of investigation needed involves highly controlled studies in which, to the extent possible, all environmental factors other than the one under study are held constant at known levels. By systematically studying the effects of each factor separately, the range of plant tolerance and the shape of the response curve can be established for each independent variable. The interactions arising from simultaneous variation of two or more variables can then be examined. In this

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way it will be gradually possible to increase the complexity of experimental treatments, in a known and controlled fashion, to a level approaching that encountered in the field. The two main avenues of attack on the problem of plant response to soil moisture obviously apply equally well to other growth factors. The results obtained from the two kinds of experiments will in time tend to converge, and will, perhaps, lead to valid generalizations of wide applicability. It is doubtful if either of the two lines by itself would ever yield such generalizations. It is true that any studies of the response of plants to environmental variations may make a contribution to a more complete understanding of crop responses to different soil moisture conditions, but it is believed that the most rapid advance of knowledge in this important field will come from added information on the following specific topics: (1) The development of techniques for measuring, without plant injury, the energy status and dynamic behavior of water in living plants and plant parts. To date, most studies of plant response to moisture environment have used treatments defined in terms of moisture conditions external to the plant. In view of the highly dynamic nature of water in the soil-plant-atmosphere continuum, it is very doubtful that equivalence of external moisture conditions can be construed to result in equivalence of the moisture conditions within a particular plant part or organ. Consequently, differences in plant response, which frequently occur even though the externally measured moisture conditions are identical, may be fully explainable in terms of the moisture conditions obtaining in the plant at the site of the observed response. ( 2 ) A detailed study of the energy, vapor, and gaseous fluxes occurring at leaf surfaces. Large amounts of radiant energy reach the leaf surface. The manner in which this energy is dissipated is a major factor affecting the water economy of the plant. The energy status undergoes tremendous changes within a very short distance at the leaf-atmosphere interface. The changes arising in that critical region from the effects of environmental variations are only qualitatively understood, but may hold the key to the dynamics of water in plants. Associated studies of gaseous exchange also should be made, because of the importance of such exchange in photosynthesis. (3) More detailed infomtion on root growth and on the root area that is operative in water intake. The lack of more quantitative information on the root area through which water is absorbed, the longevity of such absorptive areas, and the rate of root advance through soils are serious barriers to a more quantitative analysis of water movement from the soil

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into the root. An extensive study of the effects of various treatments on these root parameters would logically follow the development of reliable techniques for the measurement of such parameters. ( 4 ) Greater attention to the short-term fluctuations in moisture within the plant. Although diurnal variations in transpiration, translocation, and metabolism are generally recognized, their significance to over-all plant behavior merits greater attention. Thus it may well be found that the amplitude of such diurnal fluctuations may be of much greater physiologic significance than the mean value obtained by averaging over one or more days. In fact, fluctuations at frequencies less than 24 hours may be of even greater significance. ( 5 ) An examination of the physiologic adjustments of plants to moisture stress. The detailed biochemical study of physiologic processes has developed to the point where investigations of the effects of moisture stress on such processes may be feasible. Until such studies can be made, it is doubtful if anything other than an empirical approach can be made to the study of the effects of moisture condition on plant composition and quality.

B. FACTORS AFFECTING IRRIGATION PRACTICE AND WATER-USE EFFICIENCY D. W. Henderson University of California, Davis, California

Irrigation practices must be adapted to many social, economic, climatic, and topographic conditions as well as to characteristics of crops, soils, and water supply. Since integration of all these factors into a workable system of irrigation management on an irrigated farm is largely the responsibility of the farmer, actual practices are frequently determined by the knowledge, skill, and temperament of the individual. Water-use efficiency is best considered in terms of the factors that influence irrigation management rather than in terms of the practices themselves. This approach not only points out the reasons for certain practices, but also is less complex. It permits some generalization in terms of more or less typical reaction to factors determining irrigation practices and water-use efficiency. These factors are segregated for discussion, but it should be remembered that they are interrelated, and that water-use efficiency in any given situation is determined by Several interdependent factors. 1. Concepts of Water-Use Eficiency The meaning of water-use efficiency depends on the viewpoint. In the broad sense, high water-use efficiency might involve maximum utiliza-

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tioii of all the water falling on areas of collection and use and would require allocation of water supplies to conflicting uses for greater public benefit. To the agriculturist, high water-use efficiency means that the maximum possible percentage of precipitation be used by plants of economic or esthetic value, The agriculturist’s viewpoint might even be based on maximizing monetary returns or food or feed value per unit volume of water available. Short-term and long-term points of view may differ, particularly in arid areas where it is essential to “waste” some water to maintain a favorable salt balance for sustained productivity of the land. Water supplies are subject to loss throughout the processes of precipitation, collection, storage, conveyance, and use, just as power is lost in each component part of a compound machine. The efficiency of the entire process or of any part may be calculated. However, interpretation of water-use efficiencies depends on a clear understanding of the bases of calculation. Water-application efficiency on the farm is the subject of most of the following discussion. Water-application efficiency is specifically defined as the ratio of the volume of water stored in the root zone of the crop to the volume of water delivered to the farm. Nevertheless, certain precautions must be observed in interpretation. It is possible to apply a shallow depth of water with poor uniformity, so that the water is all retained in the root zone. The calculated water-application efficiency would be high, but if subsequent irrigations were applied in the same manner, crop yields would be drastically reduced or evaporation and percolation losses would be excessive, depending on the frequency of irrigation. It should also be noted that low water-application efficiency arising from deep percolation is not necessarily serious from the standpoint of over-all water supply utilization, because of the possibility of re-use. The principal factors then become costs of storing and distributing “wasted water, costs of reclamation, damage caused by any water out of control, and deterioration of water quality. In some areas, re-use of runoff water on individual farms or fields is an accepted practice.

2. Economic and Organizational Factors Many general considerations influence irrigation practice and waterapplication efficiency, principally economic and social. Some are quite indirect, consisting of pressures from the general public, in the form of conflicting demands for water supplies or resistance to nuisances caused by uncontrolled water. a. Water costs. High water-application efficiency is markedly encouraged by high water costs. Where there is only sufficient water to meet crop requirements, waste is discouraged even though the unit cost of

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water is low. The basisfor levying charges for water is also important. Many farmers pay essentially an annual charge per acre, which is independent of the actual volume of water, because such a procedure is convenient for the agency delivering the water or because it is the only legal means of collecting money under the laws authorizing the agency. Water-application efficiency is higher if the farmer pays for the actual volume delivered, and is further encouraged by limiting the amount he may receive per irrigated acre. In some areas, strict limits are replaced by increasing unit cost as the quantity of water delivered per acre increases. b. Crop value. One might assume that production of crops of high monetary returns would encourage high water-application efficiency because of the high potential return per unit volume of water used, However, the opposite is true in general. Acreages of high-value crops are frequently limited, and there is a marked tendency to apply excessive depths to eliminate water as a limiting factor in yield or quality. This has proved true even where water costs are high, because of the possibility of added profits far in excess of additional water costs. In the other extreme, low-value crops are frequently neglected or given inadequate irrigation for maximum yields. While water per se is not wasted, potential production per unit volume of water may be lost. c. Water delivey schedules. High water-application efficiency is discouraged by delivery schedules that do not match crop requirements and water-storage capacities of soils, or that result in unmanageable stream sizes. Deliveries beginning or ending at inconvenient times often lead to neglect, and deliveries extending through night hours are difficult to avoid, but have obvious disadvantages. Continuous delivery usually involves allotment of an irrigation stream to each farmer, based on his irrigated acreage. This stream is delivered to him continuously throughout the irrigation season. There is no adjustment of the stream to conform to lower rates of water use by crops (and smaller acreages requiring irrigation) in spring and fall as compared to requirements in midsummer. If the stream size is adequate for midseason, earlier and later deliveries are excessive. Stream sizes are small, and may be inadequate for efficient application, especially for certain methods. Adjustment of stream size to conform more closely to changing requirements would increase efficiency if still smaller stream sizes in spring and fall were not a serious factor. From the standpoint of the agency delivering water, continuous delivery has advantages. Canal capacities required are low, and delivery, measurement, and record-keeping operations are much simplified by constant flows throughout the system.

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In rotation delivery, the streams that might be allotted to a group of farmers under continuous delivery are combined and delivered to each farmer in turn for a period based on his irrigated acreage or water right. The principal advantages are larger streams and elimination of irrigation as a constant operation. Adapting rotation schedules to crop requirements and water-storage capacities of soils is often difficult, however, especially where cropping patterns are variable and soils heterogeneous. In extreme cases, depths of water application exceed the storage capacity of the soil, yet the crop suffers from drought before the succeeding irrigation. Rotation delivery is still relatively simple for the agency delivering water. Flows in main canals are constant, and required capacities are minimized, since rotation is accomplished mainly on the laterals. Fixed schedules simplify routing of water and keeping records. However, there are many different schedules of varying degrees of complexity. For the irrigator, the best delivery system is demand delivery. He may receive water at his request at any time at the desired flow rate for the period he wishes, provided he gives reasonable advance notice. In some instances, limitations may be imposed by the capacity of the delivery and control system or by the total seasonal water supply. Over-all operation of the demand delivery system is complex. Canal capacities must be high to meet peak demands. Routing, control, and records are complicated by shifts in flows and delivery points that are difEcult to predict much in advance. Usually more water is spilled into waste ways near the ends of the canals than in other types of delivery systems, to assure steady flows and good service to farmers. Except in rare cases where water is delivered through pipes with adequate and strategically placed storage facilities, night deliveries cannot be avoided. Reservoirs on the farm capable of storing overnight deliveries not only eliminate this problem but afford greater flexibility and may further increase water-application efficiency by allowing the use of larger streams. The principal objection to reservoirs is cost, especially if they must be lined to prevent seepage losses. d. Water rights. One of the major reasons for adopting a continuous delivery schedule is that the farmer receives water in exactly the manner prescribed by his water right, since the right may state that he is entitled to a given flow of water for a stipulated period. The beneficiary of such a right has no incentive to save water for use elsewhere, and he very likely interprets any move to do so as an infringement of his right. The laws of the Western States limit exercise of water rights to reasonable beneficial use. However, the courts usually interpret losses

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incurred in storage, delivery, and application in the system currently in operation as reasonable, provided there is no obvious negligence. The exercise of water rights may be controlled either by individual water users or by the agency operating the system, depending in p& on the type of organization. Control of water rights by the agency rather than by individuals tends to encourage conservation of water, since surplus supplies may be used to irrigate additional land within the geographical boundaries, or they may be sold to an outside agency. 3 . Engineering Factors

The planning and engineering of farm irrigation systems are complicated by the number of variables involved and by diillculty in evaluating them quantitatively, This is especially apparent when one considers the choice of several types of systems and the need for flexibility in operation. The principal need is to adapt the system to existing site conditions so that the irrigator has the means of applying an adequate depth of water, uniformly and efficiently, at proper intervals, under all cropping conditions anticipated. If the range of operating conditions is great enough, temporary or portable components of the system may be desirable. Furthermore, the irrigation sy.stem must be integrated into an over-all plan that provides for surface drainage and other forms of water control on the farm. a. Distribution systems. The means of conveyance and control of water must meet several requirements to allow efficient irrigation in normal operation. There must be adequate capacity to deliver the required flow to any point on the farm. The system must minimize seepage losses, allow complete control of the water, and be convenient to operate and maintain. In practice, operational convenience and ease of maintenance frequently determine the degree of control actually exercised, and therefore affect water-application efficiency. b. Irrigation layout. With certain methods of water application, especially flooding and sprinkling, the area to be irrigated at one time is more or less fixed by the layout of the system. Since efficient irrigation requires careful adjustment of the area irrigated to the stream size, the water-intake rate of the soil, the slope of the land surface, and the depth of water to be applied, system dimensions must be carefully chosen, This consideration includes the area of basins, length and width of border strips, length and spacing of furrows, and length and spacing of sprinkler laterals. Here again, the need for flexibility may be met by making these dimensions as small as is practical for other considerations. For example, if border strips are so large that the entire irrigation stream is required for one strip for ordinary depths of water applied, it is virtually impossible

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to apply a shallow depth, since a larger stream would be necessary but would not be available. On the other hand, if dimensions are such that two or more strips are ordinarily irrigated at one time, the irrigator would also have the option of using the entire stream on one strip should it prove desirable. c. Land grading. Until recently, land grading was practiced only to the extent of removing high spots inaccessible to water or filling low spots in which water ponded. Now the trend is not only toward continuity of slope, but uniformity as well. In deep uniform soils, it is likewise fairly common practice to change the magnitude of the slope to a more desirable one for surface irrigation. Along with the greater land-grading precision that is possible with modern equipment and techniques, these trends have contributed greatly to more efficient irrigation. Grading also eliminates odd-shaped fields, thus facilitating irrigation and other operations. Slope requirements vary for different conditions and methods of irrigation. It is unfortunate that some land is being graded that should be irrigated by a method that does not require extensive grading. The result is exposure of subsoil with undesirable physical or chemical properties and loss of production. d . Collection and return systems. Where water intake rates of soils are slow, prolonged irrigation is necessary for adequate penetration of water. With ordinary precision of land grading, runoff is unavoidable if good coverage is attained. Unless runoff water is reclaimed and re-used, low efficiency of water application results. One essential feature of a system that collects runoff water is a small storage sump or reservoir, which eliminates a need for the irrigator to handle highly variable flows. The water may be pumped from the sump with a float-operated pump, either directly onto a lower field or, more commonly, through a pipe back into the distribution system. The collection and return system also has the advantage of providing for surface drainage of irrigation water. 4. Operational Factors There is a tendency for many farmers to spend large sums on land grading and other features of the irrigation system, only to defeat the purpose of these investments by improper operation. The reason is not known, but possibly the feeling is that a well-engineered system should operate more or less automatically. However, in most systems, flexibility and automation are not compatible. The scarcity and cost of skilled irrigation labor is another factor, and, especially in certain surface irrigation methods, a skilled irrigator who understands irrigation requirements and the behavior of water on the surface and in the soil is absolutely essential for efficient irrigation.

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Irrigation should be both adequate and efficient. The majority of irrigators meet the first requirement, but often do so by excessive irrigation. The tendency is to consider irrigation as an operation similar to plowing, for example, without any attempt to put it on a quantitative basis, both in regard to timing and depth of water application. a. Control of timing. Efficient irrigation must begin with proper timing and a reasonable estimate of the average depth of water required. The irrigator’s task then becomes one of applying the water uniformly without excessive runoff.Even though they are vital, the timing and depth of water to be applied are difficult to determine with facilities available to most farmers. Fortunately, however, water can be applied more frequently than is necessary, and high efficiencies will still be attained if the depth applied is not excessive. The difficulty lies in the fact that most farmers tend to put on the same depth when irrigating frequently as when irrigation intervals are longer. In many cases, this is due to limitations of the method of irrigation or the irrigation layout. With most surface irrigation systems it is more difficult to apply a shallow depth uniformly than a greater depth unless the water intake rate is very slow, and the result is low efficiency for shallow irrigation. Thus, any factor that affects depth of irrigation tends to influence surface irrigation efficiency-including water-storage capacity of the soil, depth of rooting, intensity of rooting (whether resulting from genetic factors, soil physical characteristics, root diseases, or nematodes), and soil salinity. The depth of water should be considered in relation to the water-intake rate of the soil, and perhaps the best criterion for difficultyin attaining uniform application with surface irrigation is the duration of irrigation or the depth applied divided by the intake rate. If the duration is very short, irrigation is likely to be nonuniform; if it is long, runoff losses become excessive. b. Attendance. Except in extreme cases of poor design, most of the disadvantages of irrigation systems can be offset by frequent adjustment and increased attendance, particularly by a skilled irrigator. There is a limit to the amount of labor that can be profitably expended, however, because labor costs may exceed the value of the water saved. Then the only possible incentive to conservation of water is avoidance of injury to crops or soils by surplus water. C. CROPMANAGEMENT FOR IMPROVED WATER-USE EFFICIENCY G. W. Burton Georgia Coortal P k i n Experiment Station, Tifton, Georgia

At the 6th International Grassland Congress, Schofield (1952) stated that the consumptive use, or maximum “evaporation,” of water by plants

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depends on the weather, particularly on the amount of incident solar radiation. He expressed the opinion that it was influenced scarcely at all by the nature of the vegetation so long as it was green. Van Bavel and Wilson (1952) found evidence in North Carolina for concluding that “a closed vegetation cover under equal meteorological conditions disposes of the soil water supply with equal rapidity, regardless of the botanical composition.” The work of Blaney and Morin (1942), Penman (194% 1951), and Thornthwaite and Mather ( 1951), although not completely confirmed by experimental data, points to solar radiation and its effects on weather as being largely responsible for consumptive use rates. If this generally accepted, broad relationship holds, then plant breeders and agronomists, as they have increased the yields of crops in a given region, have automatically and often unconsciously improved the efficiency of water use. Hundreds of examples exist to prove that crops specialists, through breeding and management, can make a substantial contribution to solving the world problem of dwindling water resources.

1. Selection of Crop Species vary greatly in the amount of water they require to produce a pound of dry matter. Even varieties within a species may show great differences in water-use efficiency, As shown in Table VI Burton and associates observed the respective water uses of common Bermuda, Coastal Bermuda, and Pensacola Bahia to be 1546, 803, and 870 in a wet year, and 4336, 641, and 1239 in a dry year. They observed striking differences in the rate of growth, depth, distribution and P32 absorptive powers of the roots of these grasses, with a tendency toward correlation with their efficiencies in water use. Increasing the depth, ramification, and growth rate of plant roots through breeding should result in increased water-use efficiency. Hagan (1957) reported that, because of rapid root growth, watermelons on a deep alluvial soil may not respond to irrigation even though a relatively high soil moisture stress may develop within an ever-increasing soil volume. Thus, proper choice of species or variety may materially improve the efficiency with which dwindling water resources may be used. Drought is unquestionably the most important environmental factor influencing the growth of plants in the semiarid regions of the world. Nevertheless comparatively little specific breeding for drought resistance has been carried on, probably because of the complex and poorly understood nature of plant reactions to severe moisture stress. Ashton (1948), reviewing more than a hundred papers dealing with techniques of breeding for drought resistance, concluded: “in general, physical characters such as water requirements and transpiration rate, and anatomical and morphologi-

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cal cliaracters have not been found to provide a simple and practical index of drought resistance in selection work.In the case of physico-chemical

characters, there is less general agreement as to their significance in breeding investigations." Thus, most investigators have used the direct method of testing for drought resistance in field or pot experiments and in drought chambers. Wilting tests and techniques that permit rating varieties on their resistance to artificial drought and heat have been promising. Of particular interest to breeders who must screen large populations have been methods that involve the testing of seedlings or germinating seeds. A lack of agreement concerning the value of these techniques, however, is proof that a standard method of screening all plant populations for drought resistance has not been established. The complexity of the problem and Ashton's observation that "in all the crops investigated, the capacity to endure drought varies according to the stage of growth" indicate that one standardized method may never test drought resistance in all plants. It is believed, however, that original research in this area can lead to the development of methods that will greatly facilitate the breeding of drought-resistant varieties. Although there has been comparatively little specific breeding for drought resistance, much progress has resulted as breeders have sought to improve the adaptation of species to arid environments. Not only have improved varieties from such programs been more drought resistant, but they have often shown greater salt tolerance as well. CALIFORNIAMARIOUT barley, for example, bred for the dry sections of California, is much better adapted to high-salt soils than other varieties of this species (Suneson, 1957). Since consumptive use of water is geared largely to solar radiation, moving a crop to a cooler region or a cooler part of the growing season may, within limits, increase the quantity of plant product per unit of water used. Increasing the hardiness of winter wheats allowed this principle to operate as wheat production moved northward. Breeding corn capable of germinating at lower temperatures increased its efficiency of water use as it permitted its growth in the northern United States. Investigators in California have increased the water-use efficiency of wheat by breeding the RAMONA variety, which matures 30 days earlier than old varieties (Suneson, 1957). Obviously, much of the increased efficiency derived from shortening the growing season of a crop may be lost if rainfall during the remainder of the year cannot be conserved. Controlling weeds and manipulating the soil to permit recharging the soil profile with water are generally recommended. These practices, highly desirable in some regions, may lead to both water and nutrient losses in regions of high rainfall.

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Growing crops during periods when rainfall is most apt to be available generally makes for most efficient water use. This is nature’s way. Desert plants bloom after the rain, exhaust the moisture that fell, and die. Storing water in soil or reservoirs may be costly, and involves water, and often soil, loss as well. By breeding varieties capable of growing during the rainy periods, the geneticist can often automatically increase the efficiency of water use. In India and Africa, very short-season millets (Pennisetum glaucum) have been developed for dry years when the sorghums fail. Planted after a rain, these varieties with rapidly developing root systems are able to make a small yield of grain, largely on the moisture in the soil. Extending the maturity date may result in greater efficiency of water use. In the southeastern United States the soil profile is almost always overcharged during the winter. Thus, it is impossible to carry over water from one summer to the next. To attempt to do so will merely result in accentuated leaching losses of the already limited soil-nutrient supply. Here, short-season varieties like common Pearl millet ( Pennisetum glaucum) are as efficient during their short season as the full-season STARR variety. However, they produce less forage during the frost-free period than do the full-season varieties, and hence are less efficient in using the water that must be used or lost. Heterosis, which seems to increase the efficiency of most plant processes and increases yields, may be expected to increase water-use efficiency any time the plant breeder can incorporate it into the varieties that he is creating. Deeper root systems with greater ramification will probably be one of the desirable changes associated with heterosis, as shown for corn by Kiesselbach and Weihing ( 1935). Some species and varieties, such as sorghum and Coastal Bermuda, are abIe to remain green during periods of high moisture stress, whereas others turn brown and die. There is need for much fundamental research designed to discover the characteristics that are responsible for the drought tolerance of such plants. Armed with this information, the plant breeder should be more efficient and successful in increasing the drought tolerance of other species. An analysis of the cause of crop loss during drought may assist plant breeders by pinpointing changes that must be made. During the summer of 1954, much corn in the Southeast failed to produce grain because of pollen failure during a severe drought at the time of pollination. Varieties capable of producing pollen during such periods of moisture stress could have produced some grain. Although plant breeders have made, and will continue to make, progress in breeding for drought resistance and increased efficiency in

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water use, advances can be greatly accelerated once the responsible plant characteristics are understood. The urgent need for fundamental research in this field is apparent.

2. Cultural Practices A study of successful dryland farming reveals that a wise choice of cultural practices can do much to improve the efficiency of water use. Leaving cultivated crop residues on the soil during the winter in the snow belt often conserves water in the form of snow that might otherwise be blown from the field. Such residues have been shown to reduce runoff and increase infiltration of the torrential rains that characterize the area. The mulching effect of these dead residues, by reducing evaporation losses, has made for more efficientwater use. This has been one of the benefits claimed from stubble-mulch farming, which endeavors to keep substantial portions of the dead plant refuse at the soil surface throughout the cropping season. Planting cultivated crops in rows on the contour has increased the infiltration of summer rainfall, Cloddy tillage in areas where summer fallowing is practiced has increased infiltration rates and reduced water and soil losses. Thorough weed control in fallowing operations has facilitated water storage by eliminating transpiration losses. Research conducted at the many dryland experiment stations has demonstrated the tremendous influence of seeding date and rate upon the establishment and production of crop plants. Generally, the earliest safe planting date is best, because it enables the plants to utilize accumulated winter moisture and experience lower evapotranspiration rates. Plant populations must be large enough to permit full use of the available moisture (accumulated and expected) if maximum water-use efficiency is to result. Excessive seeding rates, however, can drastically reduce grain yields. To hedge against loss from excessive plant population during droughts, most farmers usually plant somewhat less than the optimum stand where rainfall is an undependable entity. Cultural practices must take into consideration the very important root system of the plant. Soil treatments-physical, chemical, or boththat will increase the rooting volume of the soil will make for better crop yields and more efficient water use. Deep cultivation that prunes roots, particularly as maturity approaches, can materially reduce the yields of corn and similar crops. Thus, for maximum water-use efficiency, cultivation must be designed to control weeds without pruning roots. The use of pre- and postemergent herbicides may become increasingly important in achieving this end.

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Obviously, pasture and range management that will minimize runoff and maximize infiltration of water will increase efficiency in the use of available precipitation, Part of the reduction in yield experienced on overgrazed ranges may be due to a lack of sufficient vegetation to hold the rain and snow that fall. Overgrazing also reduces water-use efficiency by restricting the depth and ramification of the grass roots. Crider (1954) has shown that cutting off half or more of the top growth of several grasses, including smooth brome, causes the roots to stop growing for as long as 6 to 18 days. Obviously, such roots are unable to move into areas of available water. Interestingly enough, orchardgrass roots did not stop growing when the plant was defoliated. Removing only 40 per cent of the foliage did not restrict root growth of any of the grasses studied. Management that will increase the production of total digestible nutrients (TDN) will increase the efficiency of water use by pasture and forage plants. Prine and Burton (1956) showed that extending the period between clippings in Coastal Bermuda from 1 to 6 weeks more than doubled the annual production of dry matter, Since digestibility of the grass decreased with age, the increase in TDN was not as great as the increase in dry matter. Generally, because of digestibility differences, the TDN per acre per year was lower when the grass was cut at 8-week intervals than when it was cut at 6-week intervals, even though the yields were often higher. The water-use efficiency of plants can be increased in a number of ways without altering the growing season or changing their climatic environment. One of the most obvious of these is to control all features of the environment so that water is the limiting factor for growth. Actually, agronomists have been doing this for years, usually without fully realizing the significance of their results as far as water use was concerned. As early as 1912, Montgomery and Kiesselbach (1912) reported that applying manure to corn growing on an infertile soil reduced from 550 g. to 350 g. the water required to produce a gram of dry matter. More recently, Stansberry et al. (1955) showed that increasing from 100 to 500 pounds per acre the P206 applied to irrigated alfalfa decreased from 14.2 to 8.4 the acre-inches of water required to produce a ton of hay. Burton et al. (1957) found that the pounds of water required to produce 1 pound of dry Coastal Bermuda hay decreased from 2478 to 803 as the nitrogen application was increased from 50 to 200 pounds per acre. If consumptive use of water is reasonably constant, regardless of crop yield, then any improvement in the environment, such as disease and insect control, adequate nutrition, proper cultivation, etc., that results in increased yields will also result in increased efficiency of water use.

110

M. B. RUSSELL

D. MOISTURIC CONSERVAT~N IN SUBHUAIID AIIEAS M. B. Russell Univmnify of Illinois, Urbana, Illinois

Moisture is the primary factor limiting crop production in the Great Plains. This region, extending roughly from the 95th meridian to the Continental Divide, has an annual rainfall that varies from 40 inches in the southeast, to 12 inches in central Montana. Even so, the area suffers an average water deficit ranging from zero, along its eastern boundary, to nearly 40 per cent, at its western edges. Other areas, notably the Columbia basin and portions of the Intermountain region, have similar rainfall deficiencies. Of equal significance to the average annual deficit are the rather wide annual and seasonal variations in precipitation that characterize the subhumid areas. Such variation is illustrated in Fig. 36, which

0

z m TEMPLE, TEXAS

P

40

30 20 10

0-

S

0

NID

J

F

M I A

MIJ

J I A

FIG.36. Variations in precipitation at three locations and the relation of cotton production to soil moisture (Johnston, 1957).

shows annual precipitation values ranging between 50 and 200 per cent of normal. Moisture conservation and its efficient use by crops are therefore the first considerations in managing soils and crops in these areas (Hobbs, 1957).

WATER AND ITS RELATION TO SOILS A N D CROPS

111

Storage of moisture in the soil to meet peak crop demands is the major practice used to overcome moisture deficits. Numerous long-time studies have shown a close relation between crop yields and the amount of available soil moisture in the root zone. Results from fifteen field stations have shown that the average yield of spring wheat increased from 6.9 to 12.8 to 19.9 bushels per acre when the soil at planting time was wetted to depths of 1, 2, and 3 feet, respectively. Under continuous wheat, such depths of wetting were observed in about 30,44, and 26 per cent of the years, respectively. In the Great Plains a minimum of 8 inches of precipitation is required for wheat to produce grain, and each additional inch on the average increases wheat yields 2.2 bushels per acre. In southern Canada, 5 or 6 inches of stored moisture plus crop-season rainfall was found to be required to produce a minimum spring wheat yield of 1 or 2 bushels per acre. Each inch of water above the 5-inch minimum gave an average increase of 3.4 bushels per acre (Norum et al., 1957). Summer fallowing is the most widespread practice used to overcome the moisture shortage in dry-farming areas. This essentially consists of using for one crop the moisture received in two years. As presently practiced, however, it is very inefficient. Even in the cooler parts of the region, the efficiency of moisture storage by summer fallow seldom exceeds 30 per cent, and at Fort Hays, Kansas, the 40-year average efficiency was only 15 per cent (Evans and Lemon, 1957). Low efficiency is caused by evaporation of moisture from the soil surface and its loss through runoff or transpiration by weeds. Storage efficiency is highest in the fall, winter, and spring, except where runoff is excessive from frozen soil. Low storage efficiency in the Great Plains arises in part from the high concentration of rainfall in the summer months, when evaporative losses are high. In Utah, a 15-year study gave an average storage efficiency of 31 per cent on a yearly basis, but a 60 per cent efficiency during the winter, when undisturbed stubble effectively held the snow. The summer period following spring plowing showed a net loss of 8 per cent, even though 4.7 inches of rain fell during that period. An efficiency of 26 per cent was recorded during the winter, following the seeding of winter wheat. Failure to control weeds or volunteer crop plants further reduces the efficiency of summer fallowing. Although some weed growth occurs in grain stubble following harvest, the moisture losses induced thereby are more than offset by the increased moisture stored by the snow-trapping action of the stubble. Therefore, unless weed growth is very heavy, cultivation is usually deferred until the spring or summer following harvest. Such control should begin as early as possible to prevent losses of stored moisture by transpiration. Figure 37 summarizes a three-year

M. B. RUSSELL

112

WATER STORED IN SOIL (INCHES) 0 I 2 3 4 5 r

-

v

-

r

d

RELATIVE WHEAT YIELDS 020406080100 1

1

1

d

NO WEEDS FIRST CULTIVATION

MAY 15

FIRST CULTIVATION

JUNE 15

FIRST CULT IVAT ION

JULY I5

I

I

U

b

J

U

0

FIG.37. Effects of summer fallow practices on soil moisture storage and on wheat yields (Evans and Lemon, 1957).

study in Saskatchewan on the effect of date at which cultivation was started. Subsoiling or deep plowing have been widely studied as means of improving soil moisture storage. In general, such practices have not proved effective in increasing water entry or penetration; however, in special situations, reductions in runoff and increased subsoil moisture storage have been reported. Because of the increased power required, deep tillage is more expensive than conventional methods, and should be limited to those special soil situations where definite yield increases can be obtained. Contour cultivation is another moisture-conserving practice that is widely used on sloping land in both humid and subhumid regions. By reducing the velocity of water movement over the soil, this practice gives a longer time for water entry into the soil. A summary of results from thirty-seven locations in twelve states indicated an increase of 11per cent in corn yields attributable to contouring. A similar comparison showed a 20 per cent yield increase for wheat. Although the amount of water saved by contouring is usually small in terms of increasing the supply in the root zone, the small increments frequently come at critical stages in the growth period, thereby giving substantial yield increases. Mechanical barriers or terraces are useful in moisture conservation. In low-rainfall areas on permeable soils of moderate slopes, level terraces without outlets are used to catch runoff water and permit it to enter the soil. The effects of terracing and contouring on soil moisture supply and cotton yields are shown in Table X. The redistribution of runoff water from adjacent sloping areas onto a smaller cropped area is another water conservation practice used in subhumid areas. In a nine-year study, increases in moisture of about 15 per cent of average annual rainfall were obtained by spreading the runoff

TABLE X Summary of Experiments Comparing Runoff and Soil Losses under Dfierent Cropping Systems

Location Clarinda, Iowa Lacrosse, Wis. Z a n e d e , Ohio Pullman, Wash. Pullman, Wash. Temple, Texas Temple, Texas Bethany, Mo. S t a t e d e , N. C. Guthrie, Okla. Tyler, Texas Watkinwille, Ga.

Annual precipitation Cinches) 28.3 32.6 38.0 20.7 18.9 33.8 32.7 29.5 47.8 30.2 40.7

Continuous row crops

Rotation

Meadow

Bare

Slope

(%I

(1)"

9 16 12 30 30 2 4

18.7 38.3 29.2 111.7 40.3 99.3 2.3b 1.3b 0.3b 0.03b 10.8 7.8 13.0 20.2 27.1 50.9 12.4 31.2 12.5 18.9 17.8 22.4 23.6 26.9

8

10 7.7 8.8

7

(2)-

(1)

(2)

(1)

(2)

8.8

11.3 27.0 13.4 3.6 0.5 7.0 5.6 9.1 14.4 4.2c 17.2 3.2

1.2 5.5 4.4 1.7 0.2 1.2 0.05 8.1 1.9 1.0 1.0

0.03 0.1 0.02 0.15 0.00

17.0 19.8 6.6 0.8 10.8 5.7 16.2 10.0 10.2c 17.3 10.5

(1)

(2)

29.0

191.0

22.0 20.2

20.5 23.6

28.9 29.5

81.2 66.2

18.1

21.0

0.08

0.02 0.16 0.3 0.02 0.08

(1) Average annual runoff aa percentage of precipitation; (2) average annual soil loss in tons per acre. b Continuous spring wheat, no continuous row crop reported. c Data from the cotton plots of the rotation only.

a

*'

Reference

3

Browning et ul., 1948 Haysetd., 1949 Borst et d.,1945 Horner etal., 1944 Horner etal., 1914 2 Hill et d.,1944 +I 0 Hill et d.,1941 Smithetd.,1945 cn Copleyetal., 1914 Daniel et al., 1943 Popeetal., 1946 Hendrickson, 1949

2

5

9 8cn

114

M. B. RUSSELL

from a waterslied over an area one-tenth the size of the watershed. Pn studies in Oklahoma, the extra water obtained by spreading has increased alfalfa yields by 20 to 100 per cent. Residue management is important in moisture conservation in the Great Plains. The role of stubble in holding snow has been reported repeatedly. Runoff is reduced and soil moisture storage is increased by tillage practices that leave crop residues on the surface rather than incorporating them into the soil. Residue mulches reduce somewhat the rate of evaporative loss from the soil, but the total loss by evaporation is unaffected. Since surface mulches may increase the amount of intercepted water that is quickly lost by evaporation, it is apparent that they are of little value in reducing evaporative losses of moisture from the soil. The selection of crops and their management are other ways in which increased efficiency of moisture can be achieved in subhumid regions. The most obvious example is that of such crops as winter wheat, which make their growth during the cool season, when the transpiration demand is relatively low. The sequence in which crops are grown also is determined in part by their relative moisture demands. Thus, the yield of grain in the first year after a crop such as alfalfa is grown, is usually depressed because of a deep exhaustion of subsoil moisture by the deeprooted perennial predecessor. Row crops, such as corn, usually do not utilize available soil moisture as completely as do grains; crops that follow, therefore, are favored by the greater amount of residual soil moisture. Deep-rooted crops, such as alfalfa, and crops that can resume growth after a drought interruption, such as cotton and sorghum, are more tolerant of periods of moisture deficit. In all crops, however, adequate fertilization and proper planting rates are important factors affecting water-use efficiency. Numerous experiments have been reported in which proper fertilization has given substantial increases in yield with little or no change in the amount of water used. As pointed out in an earlier section of this article, adequate fertilization is essential for extensive and vigorous root development, which is necessary for effective exploitation of subsoil moisture. Large areas of the subhumid regions of the United States are used for grazing. Conservation and efficient use of water are the prime problems on rangeland, and their practical solution depends largely on reducing runoff losses and increasing infiltration. The rate of water intake is greatly affected by soil characteristics, vegetation, and the intensity of grazing. Soils of intermediate texture show the greatest improvement in infiltration in response to good management. Making pits in short-grass range has been shown to increase water intake, increasing grazing capacity 33 per cent. Vegetative barriers of unpalatable plants to increase snow trapping,

WATER AND ITS RELATION TO S O U AND CROPS

115

and water spreading to replenish ground water, give promise as means of improving the supply of moisture in some range lands. In summary it can be said that the central problem of moisture conservation in subhumid regions is that of increasing subsoil water storage. Water that penetrates below the surface soil layer is safe from evaporative loss, and, given suitable soil and crop management practices, can be used for crop production. The fact that 70 to 80 per cent of the rainfall is lost during a summer-fallow year, and perhaps as much as 50 per cent is lost by evaporation during a crop year, indicates the tremendous potential that still exists for increasing the efficiency of agricultural water use in subhumid areas.

E. MANAGEMENT PRACTICES AFFECTING RUNOFFAND WATERYlEm M. B. Russell Univorrify of Illinois, Urbana, lllinob

As mentioned in an earlier section of this review, roughly one-fifth of total annual precipitation in the United States appears as stream flow. Since surface runoff is the major path taken by water in moving from the intercepting surface to the channels of concentration, study of the factors affecting the process is of considerable hydrologic importance. The amount and intensity distribution of the precipitation, the storage-capacity and water-intake characteristics of the soil, and the topography are the fundamental factors governing the runoff process. Certain of these are susceptible to modification by cultural or other management practices. To discuss the effects of such practices on surface runoff, rather than the primary factors themselves, is the purpose of this presentation. For a more complete discussion of runoff hydrology, see Wisler and Brater ( 1949), Harrold and Dreibelbis ( 1951),and Meinzer (1942). Interest in the effects of management on watersheds centers on somewhat different problems in humid and in subhumid and arid regions. In the former, major attention is given to erosion control, with water yield usually a secondary consideration. In drier regions, however, major emphasis is given to the yield of water for off-site storage and use. Many studies conducted on agricultural land show the effect of vegetative cover on runoff and erosion (Colman, 1953). The data summarized in Table X, although representing a rather wide range of soils, slopes, and climatic conditions, all indicate that the average annual runoff from clean-tilled row crops is frequently 50 to 100 per cent greater than from a three- or four-year rotation containing small grains and meadow crops, and around ten times that from permanent sod. Soil losses show

116

M. B. RUSSELL

even greater responses to cropping practices than do runoff volumes. In both phenomena, however, the seasonal occurrence and intensity of the precipitation greatly modify the effects of vegetation. A high percentage of both runoff and erosion result from only a relatively small number of the storms that themselves make up a small part of the total precipitation. The reader is referred to the references listed in Table X for additional details concerning the data summarized, and for discussions of other factors, such as length of slope, row direction, fertilization, crop sequence, residue management, etc., that modify the main effects of vegetative cover on soil and water losses. In the humid regions, extensive studies have also been conducted on contouring, strip cropping, and terracing as means of controlling runoff and erosion. A summary of the results of experiments with contouring shows that it reduced runoff more than 30 per cent in 15 of 21 cases. However, field experience did not show as great an effect, because of the practical difficulties of maintaining true contours. The weighted average crop yield increase from contouring was 11 per cent (Stallings, 1945). Strip cropping has not shown a consistent effect on runoff, but in controlled experiments in six states gave an average reduction of soil loss of 49 per cent (Stallings, 1945). Soil movement occurs within the strips, but, because of shorter slopes, the losses are reduced. Field terraces also effectively reduce slope length, reduce runoff, and decrease erosion. Runoff peaks were reduced 54 per cent in watershed comparisons at Bethany, Missouri ( Smith et aZ., 1945). The development of good physical conditions in the surface soil through fertilization, residue management, and cropping practices is of greater over-all benefit in increasing infiltration and reducing soil and water losses than are the supplemental practices discussed above. Despite its local effects on runoff and erosion, land management is considered of little importance in downstream flood prevention. Leopold and Maddock (1954) pointed out that 20 to 50 per cent reductions in peak runoff rates apply to moderate storms, and that the quantities of runoff so affected are in tenths of inches. Major floods are usually associated with high rainfall coupled with a saturated or frozen soil having little or no residual moisture storage. In the absence of such storage capacity, runoff occurs from sloping land largely independent of the crop cover. In contrast with the foregoing, watershed management at the headwaters of mountain streams in Utah has proved to be a major factor in determining flood damage (Bailey et d.,1947). The management of rangeland has important effects on runoff and erosion. Studies in Idaho (Craddock and Pearse, 1938) showed that runoff

WATER A N D ITS RELATION TO SOILS AND CROPS

117

increased 0.4 to 60.8 per cent when overgrazing created a shift from wheat grass to annual weed cover. Soil losses increased from 0.003 to 7.6 tons per acre per year. More recent studies (Packer, 1951) indicated the importance of both total ground cover and the maximum size of unvegetated spaces in determining both runoff and soil loss. Results from 97 sites typical of the mountain ranges of Utah, summarized by Bailey et al. (1947), indicated a runoff of about 5 per cent when the normal plant cover and litter were present. On deteriorated areas, runoff increased to values ranging from 20 to 80 per cent as the density of cover decreased. Brush removal by burning is widely practiced for range improvement in the Western States. Replacement of deep-rooted woody species that draw on soil moisture throughout the entire summer by shallower-rooted grasses and forbs that mature and stop growth in the early summer increases water yield by reducing the amount of water required for subsoil recharge (Adams et al., 1947). Such effects are not observed on soils having little deep-storage capacity. Where a suitable grass cover is established after burning, infiltration is not impaired nor is erosion increased, and water losses due to interception are reduced. A summary and discussion of the hydrologic consequences of brush burning appears in the article by Adams et al. (1947). Forests occupy large parts of the areas that yield water for irrigation and other off-site uses. Therefore the hydrologic effects of forest management are of importance (Kittredge, 1948). Water yield is increased by watershed deforestation, but maximum discharge rates may rise to destructive levels during periods of excess rainfall or snow melt, and fall to zero during seasons of greatest off-site demands. Thus, forests provide flood protection and stability of yield at the expense of increased evapotranspiration. Snow is an important source of water for off-site use in the western United States. There is evidence that water yield from the snow pack in forests can be increased significantly by proper cutting and forest-management practices ( Colman, 1953). A final example of the role of management on water yield is that of the control of riparian vegetation. The water used by phreatophytes growing along streams and on river flood plains having high water tables has been estimated to amount to over 20 million acre-feet per year in the seventeen Western States (Robinson, 1952). Clearing a 2000-foot channel on a 70-mile section of the Salt and Gila rivers of Arizona was estimated to give a 75 per cent net reduction of evapotranspiration, amounting to a saving of 16,000 to 22,000 acre-feet of water per year (Turner and Skibitzke, 1952). Croft (1948) reported diurnal fluctuations of 25 per cent in stream flow caused by evapotranspiration of riparian vegetation along the stream courses. Mechanical removal, chemical treatment, and

118

M. B. RUSSELL

lowering of the flood-plain water table are the techniques used to control undesirable phreatophytes. To date, however, the cost of such control limits this water-conservation practice to situations where a high value is placed on the water. VII. Summary a n d Conclusions

The asymmetric distribution of the charge in the water molecule is the fundamental cause of the highly significant properties-high dipole moment and hydrogen bonding-that are primarily responsible for such unique properties of water as its high heat of vaporization, viscosity, surface tension, and solvent properties. These and other physical properties, all arising from the tendency of water molecules to associate with one another and with other molecules, are responsible for the observed behavior of the unique and ubiquitous compound-water. The average annual precipitation received in the United States is about 30 inches, of which about 8 inches returns to the ocean as runoff. Neither of these quantities is uniformly distributed, either geographically or seasonally. Annual precipitation exceeds potential evapotranspiration in areas east of the 95th meridian, and in some of the mountain areas of the West. In much of the humid region, and especially in the subhumid and arid regions, the seasonal distribution of precipitation is not in phase with seasonal water use by crops. Soil storage and irrigation are two principal ways of bringing water supply and crop demands into balance. Total ground-water reserves and lakes are equivalent to about thirteen years of precipitation. Aquifer recharge, usually by water-spreading, is used in some areas to correct depletion of ground water by excessive pumping. Water use by crops is primarily an energy-dependent process. The rate of vapor transfer away from the evaporating surface, and, in certain cases, the rate at which liquid water is supplied to such a surface, serve to modify the energy dependence of evapotranspiration from the soil and leaf surfaces. Water use can be estimated from losses from standardized evaporimeters, from calculations based on the energy balance of vegetated surfaces, from aerodynamic measurements of energy and vapor fluxes, and from “inventory” calculations based on precipitation and soil moisture records. Soil properties reflect current and antecedent moisture regimes in many important ways. Weathering, leaching, profile differentiation, and erosion are water-induced processes that greatly affect the agricultural potentialities of soils. The capacity of soils to store water for plant use is primarily a function of their porosity and depth. The affinity with which

WATER AND ITS RELATION TO SOILS AND CROPS

119

soil moisturc is held and tlie ease with which it moves through the unsaturated soil are both determined b y tlir moisture content and texture of the soil. These properties largely determine the availability to plants of water in the rooting zone. The nature and extent of root development are major factors governing plant responses to moisture conditions. Plants vary genetically in rooting characteristics, but also are highly responsive to the physical and chemical properties of the soil. Aeration, temperature, nutrient supply, and soil compaction strongly influence root growth. Insects, nematodes, and diseases often seriously reduce the extent and effectiveness of root systems, thereby altering plant responses to moisture conditions. Water makes up 85 to 90 per cent of the fresh weight of actively growing plants, is an essential ingredient in many of the biochemical reactions, serves as a medium of transport within the plant, and by means of turgor pressure imparts mechanical strength to plant tissues. When the rate of transpiration exceeds the rate of water intake, plants develop internal water deficits, and characteristic metabolic changes and anatomical responses result. Cell enlargement, photosynthesis, respiration, transpiration, and biochemical composition are altered in plants subjected to water deficit. Water content, relative turgidity, diffusion-pressure deficit, and stomatal opening are measures that have been used to determine water deficit in plants. Plants show large differences in drought tolerance. Such differences reflect the abilii of the plant (1) to avoid internal water stress by effectively balancing water intake and water loss or (2) to adjust physiologically to such stress. Drought injury is believed to be caused by mechanical disruption of the protoplasm rather than by the loss of water per se. Inadequate aeration is the principal explanation of crop response to excessive water. Plants that tolerate water-logged conditions have morphologic and physiologic adaptations enabling them to function in an anaerobic root environment. Roots of other plants cease growth and are unable to absorb water or nutrient ions in a normal manner in the absence of oxygen sufficient for aerobic respiration. Crop responses to different soil moisture regimes have been extensively studied, but, because of the many interactions between the soil, plant, and climatic factors involved, few generalizations concerning field response have been established. Vegetative production is more responsive to moisture differences than is the production of reproductive parts or storage products. Crop yields are invariably reduced if the plant undergoes wilting for extended periods, the responses of certain crops being conditioned by the stage of growth at which the moisture deficiency

WATER AND ITS RELATION TO SOILS AND CROPS 120 occurs. Some plants, particularly perennials growing on deep permeable soils, show no significant growth response to changes in soil moisture content between field capacity and the permanent-wilting percentage. By contrast, responses are expected with annual crops, crops having poorly developed root systems, and crops subjected to high transpiration demands. Under highly controlled conditions, various growth and metabolic processes have shown differential response to moisture differences throughout the range from field capacity to the permanent-wilting percentage. A basic difficulty encountered in interpreting most experiments involving crop response to moisture conditions is the fact that the observed effects represent an integration, over the entire growth period and throughout the entire rooting volume, of a moisture condition that is variable in both time and space. The rate at which water moves through the soil-plant-atmospherecontinuum, and short-term hysteresis effects in the soil adjacent to the absorbing root, are also believed to be important in explaining some of the differences observed in plant responses to soil moisture treatments. The over-all efficiency of water use in crop production is low. In irrigated regions, low use efficiency reflects the water losses incurred in collection, storage, conveyance, and field application. Economic factors, administrative practices, and on-farm water-management methods, as well as soil and crop characteristics, have an influence on the efficiency with which water is used. In subhumid areas, surface evaporation and use by nonagricultural vegetation are the chief causes of low water-use efficiency. Such losses account for 70 to 80 per cent of the precipitation in fallow years, and perhaps 50 per cent in crop years. The central problem of such areas is to increase subsoil water storage at depths where it will be safe from evaporative loss. The water-use efficiencyof crops can be increased by breeding varieties the period of growth of which nearly coincides with the period of greatest precipitation or lowest potential evapotranspiration. Varieties or strains having more extensive rooting are more effective in meeting their water needs. Since water-use efficiency is normally improved by increasing acre yields, all breeding and management practices producing such increases indirectly lead to greater efficiency of water use. Breeding for specific morphologic or physiologic traits depends on estabIishing both the significance of such traits and the nature of their genetic dependence. Runoff and erosion, although largely determined by soil, climatic, vegetative, and topographic factors, also respond to management practices. Maintaining high infiltration capacity and increasing the infiltra-

WATER A N D ITS RELATION TO SOILS AND CROPS

121

tion period are key objectives for the control of runoff and erosion. These are achieved by practices that increase the supply of readily decomposable organic matter, provide a protective canopy over the soil surface, and reduce the number of tillage operations, particularly during periods of high soil moisture. Grazing practices on rangeland, and forest-management practices in timbered areas, have important effects on both water yield and distribution of the runoff intensities of watersheds. In conclusion it may be repeated that water, because of its unique properties, intimately affects nearly all of the physical and biological processes of plants and animals. The water that enters the soil, is absorbed by plant roots, and is returned to the atmosphere, has been the main focus of this review. This quantity, huge though it is for an area such as the United States, represents but a small component of the total hydrologic cycle, which encompasses both long- and short-term cycles, involving the interchange and movement of quantities of water and energy so enormous as to defy comprehension. The supply of water and the demand for it imposed by vegetation vary widely, both geographically and seasonally. Acting as a reservoir, the soil serves to bring supply and demand into phase, and irrigation serves to correct both geographic and seasonal inequalities. In both cases, however, serious losses of water, due to evaporation, runoff to the sea, deep percolation, or use by nonagricultural vegetation, place strict limits on the efficiency with which present-day practices are able to effect a balance between water supply and crop demands. To understand the complex behavior of plants under different water regimes and environmental conditions, it is essential to recognize the continuity and dynamics of water through the entire soil-plant-atmosphere system. Only confusion results from neglecting the complete system in attempting to explain the field.behavior of plants. Proper consideration must be given not only to the intensity and capacity, but also to the rate factors of each of the component parts of the soil-plant-atmospheresystem before useful generalizations concerning plant response to the hydrologic environment can be developed. Except for specific situations, it is unlikely that any meaningful and consistent relation can be developed between a particular soil moisture parameter and plant growth. As the total fresh-water resources of the United States are more completely developed, agricultural use will face increasing competition from industrial and other uses that have a considerable economic advantage. In irrigated regions particularly, this represents an important long-range agricultural problem the solution of which will require a major increase in the over-all efficiency with which water is used in crop production. In

122

WATER AND ITS RELATION TO SOILS AND CROPS

humid and subhumid regions, solving the problem will probably involve increasing the efficiency of natural precipitation through improved soilmoisture storage and utilization. Long-range improvement of the competitive position of agriculture for water would seem to center on the two major areas of water supplyincluding collection, storage, distribution, application-and plant-use efficiency. In each area, more specific and quantitative information concerning the basic physical and biological processes is needed, together with the development of practical methods of modifying the phenomenon in the desired manner. More specific examples of major research areas include: runoff and ground-water hydrology; the physics of, and methods for, modifying evaporation from water, soil, and plant surfaces; improvement of the water-intake and storage properties of soil; the dynamics of water in living plants; and the effects of water status on specific metabolic processes. Since water is an active participant in all experiments involving growing plants, it is also important that greater attention be given to a more complete description of the moisture regime followed in all plant experiments. There usually are strong interactions between moisture and the treatments being studied. Therefore, the absence of relevant information on the moisture regime often prevents a full understanding of results. REFERENCES Ackerman, E. A., and Lof, G. 0. G. 1959. “Technology in American Water Resources Development.” Johns Hopkins Press, Baltimore, Maryland. Ackley, W. B. 1954. Pkmt Physbl. 29, 445-448. Adams, F., Ewing, P. A,, and Huberty, M. R. 1947. “Hydrologic Aspects of Burning Brush and Woodland-Grass Ranges in California.” California State Board of Forestry, Sacramento, California. Aldrich, W. W., and Work, R. A. 1934. Proc. Am. SOC. Hort. Sci. 32, 115-123. Allen, R. M. 1955. J. Forestry 53, 724-727. Allmendinger, D. F., Kenworthy, A. L., and Overholser, E. L. 1943.Proc. Am. SOC. Hort. Sci. 42, 132-140. Alvim, P. de T., and Havis, J. R. 1954. PZunt Physiol. 29, 97-98. Anderson, E. R. 1952. U. S. Geol. Survey Cfrc. 229. Angus, D. E. 1958. UNESCO Arid Zone Research, Climatology and Microclimatology, Proc. Canberra Symposium 11, 3014303. Appleman, C. O., and Brown, R. G. 1946. Am. J . Botany 33, 170-181. Arland, A. (ed. ). 1955. “Die Anwelkmethode im Dienste des Landbaues.” Deutscher Verlag der Wissenschaften, Berlin. Ashbel, D. 1936.J. Palestine Oriental SOC. 16, 316-321. Ashby, E.,and May, V. 1941. Proc. Linnean SOC. N. S. Wales 66, 107-112. Ashby, E., and Wolf, R. 1947. Ann. Botany (London) [N. S.] 11, 261-268. Ashton, F. M. 1956. Plant Physiol. 31, 266-274.

WATER AND

rrs

RELATION TO SOILS AND CROPS

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Robinson, W. 0.1930. Soil Sci. 30,197-217. Rohwer, C. 1931. U.S. Dept. Agr. Tech. Bull. No. 271. Rohwer, C. 1934. Trans. Am. SOC. Ciuil Engrs. 99,673-703. Ronnike, F. 1957. Physiol. Plantarum 10,440444. Russell, M. B. 1952. In “Soil Physical Conditions and Plant Growth” (B. T. Shaw, ed.), pp. 253-291. Academic Press, New York. Ryall, A. L., and Aldrich, W. W. 1944. Agr. Research 68, 121-123. Schneider, G. W., and Childers, N. F. 1941. Plant Physwl. 16,565583. Schofield, R. K. 1952. Proc. Intern. Grassland Congr. 6th Congr. Univ. Park, Penna. I , 757-762. Schreiber, H. A., Davis, L. E., and Overstreet, R. 1957. Soil Sci. 83.91-99. Shardakov, V. S. (ed.). 1957. “Akademiya Nauk Uzbekskoii SSR., Tashkent,” Vol. 1. Shirk, H. G. 1942. Am. I. Botany 29,105-109. Slatyer, R. 0. 1955. Australian 1. Agr. Research 6,365377. Slatyer, R. 0. 1956. Australian 1. Bwl. Sci. 9,552558. Slatyer, R. 0. 1957. Australian 1. Bbl. Sci. 10,320336. Smith, D. D., Whitt, D. M., Zingg, A. W., McCall, A. G., and Bell, F. G. 1945. U.S. Dept. Agr. Tech. Bull. 883. Smelt, M. G. 1955. Physiol. Plantarum 8, 572593. Stallings, J. H. 1945. U. S. Soil Conservation Service (Multilith). Stanhill, G. 1957. Soil Sci. 84,205-214. Stansberry, C. O., Converse, C. D., Hake, H. R., and Kelley, 0. J. 1955. Soil Sci. SOC.Am. Proc. 19,3034310, Stocker, 0. 1929. Plantu 7,382387. Stocker, 0. 1956. In “Encyclopedia of Plant Physiology” (0. Stocker, ed.), Vol. 3, pp. 43f3-488. Springer, Berlin. Stocker, O., Leyerer, G., and Vieweg, G. H. 1954. Kuratorium Kulturbauwesen 3, 45-77. Stockton, J. R., and Doneen, L. D. 1957. California Agr. 1 I ( 4 ) , 16-18. Stone, E. C. 1957. Ecology 38.414422. Stone, E. C., and Shachori, A. Y. 1954. California Agr. 8,7-10. Stone, E. C., Shachori, A. Y., and Stanley, R. G. 1956. Plant Physiol. 31, 120-126. Stone, E. C., Went, F. W., and Young, C. L. 1950. Science 1 1 1,540-548. Strogonov, B. B. 1956. Doklady Akad. Nauk. S.S.S.R.98,285-288. Sumner, E. J. 1947. Meteorol. Mug.76,151. Suneson, C. A. 1957. Repts. 3rd Intern. Conf. Irrigation and Drainage, San Francisco. Suomi, V. E., and Tanner, C. B. 1958. Trans. Am. Geophys. Union 39,298-304. Swinbank, W. C. 1951.1. Meteorol. 8,135-145. Swinbank, W. C. 1955. Australia, Commonwealth Sci. Ind. Research Organization, D k . Meteorol. Phys., Tech. Paper No. 2. Taylor, R. J. 1958.1. Sci. Instr. 35,4752. Taylor, R. J., and Dyer, A. J. 1958. Nature 181,408-409. Taylor, S.A. 1949. Soil Sci. SOC. Am. Proc. 14,5541. Taylor, S. A., and Haddock, J. L. 1956. Soil Sci. SOC. Am. Proc. 10,284-288. Thomas, H. E. 1955. In “Water the Yearbook of Agriculture” (A. Steffered, ed.), pp. 62-78. U. S. Gov’t. Printing O5ce, Washington, D. C. Thomas, M. D., and Hill,G. R. 1949. In “Photosynthesis in Plants” (J. Franck and W.E. Loomis, eds.), pp. 19-52. Iowa State Coll. Press, Ames, Iowa.

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THE ECONOMICS OF FERTILIZER USE IN CROP PRODUCTION Robert D. Munson and John P. Doll American Potash Instifute, St. Paul, Minnesota, and the University of Missouri, Columbia, Missouri

I. Introduction . . . . . . . . . . . . . . . . 11. Concepts and Principles Involved in the Economics of Fertilizer Use A. Early Concepts concerning Crop-Yield Response to Fertilizers B. The Application of the Principles of Production Economics to tilizer Use . . . . . . . . . . . . . . . 111. Current Research on Economics of Fertilizer Use . . . . . A. Planning and Designing Experiments . . . . . . . B. Economic Analyses of Yield Data . . . . . . . . C. Initial Soil Fertility Considerations . . . . . . . . IV. Conclusions . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . .

. . .

. .

.

Page 133 134 134

Fer-

. .

140 158 . . 158 . . 159 . . 164 . . 166 . . 167 . . 167

. .

1. Introduction

The need for commercial fertilizers to supplement the soil supply of available nutrients for crops has been amply demonstrated by agronomic research. Recognition of this need, along with many other factors, has caused a tremendous increase in fertilizer use in the last two decades. In 1937, 8 million tons of fertilizer were used in the United States. By 1957, consumption was over 22 million tons, an increase of 175 per cent. Demand for agricultural crops, reflected in high prices during and following World War 11, also contributed to increased fertilizer use. Improved technology and fertilizer practices adopted during that period resulted in crop surpluses when demand declined. research workers in agronomy and agricultural In the early 1950’~~ economics, recognizing the importance of economics of fertilizer use for crop production, began cooperating on research projects designed to estimate the most profitable rates of plant nutrients for different soils. The purpose of this research was to provide information that would 133

134

ROBERT D. MUNSON AND JOHN P. DOLL

enable farmers to make more efficient use of fertilizer, thereby leading to an over-all increase of efficiency in agricultural production. Because of the interest in the economics of fertilizer use, this research has undergone rapid development in recent years. Therefore, the objectives, procedures, and terminology of this research may seem obscure to those not familiar with it. This paper attempts to define concepts and to review past and present research on the economics of fertilizer use, often referred to as “agronomic-economic” research. In detail, the objectives of this paper are to: 1. Present the development of concepts involving plant nutrient-yield response relationships and some of their implications. 2. Explain and discuss the economic concepts and terminology used in economic analyses of crop yield response to fertilizers. 3. Discuss and interpret the current status of research on economics of fertilizer use in crop production. 4. Suggest future needs in considering research on economics of fertilizer use. II. Concepts and Principles Involved in the Economics of Fertilizer Use

A. EARLYCONCEPTS CONCERNING CROP-YIELD RESPONSE TO FERTILIZERS Early research concerning plant nutrients was devoted largely to determining elements essential to the growth, development, and maturation of plants. After establishing the need for certain elements, fertilizer materials containing these elements or nutrients were developed and added to soil, and resulting crop response relationships were observed. Theories or ‘laws” were formulated to explain these input-output relationships. Initially, only statements about the nature of the relationships were made. But later, mathematical yield equations were used in an attempt to formulate Yaws” that would express the observed yield responses. 1. The Law of the Minimum One of the first explanations of the relationship between crop yields and plant nutrients was the ‘law of the minimum.” Th. de Saussure, a Swiss chemist, in his Recherches chimiques sur la vkgktation, alluded to t h i s law in 1804 (Browne, 1942). During the late 1830’s, Carl Sprengel, a German agricultural chemist, made specih statements to the effect that a soil could be unproductive because of the deficiency of a single nutrient ( Browne, 1942; Lang, 1924). Sprengel also stressed that deleterious effects on plants could occur if excess amounts of a plant nutrient

135

ECONOMICS OF FERTILIZER USE

were present. However, formal statement and elaboration of the law of the minimum has been attributed to Liebig, the famous German chemist. Initially, Liebig, describing the relationship between crop yields and applied plant nutrients, stated that “The crops on a field diminish or increase in exact proportion to the diminution or increase of the mineral substances conveyed to it in manure,” and also, that “by the deficiency or absence of one necessary constituent, all others being present, the soil is rendered barren for all those crops to the life of which that one constituent is indispensable” (Russell, 1950). These two quotations provide the basis of the law of the minimum as propounded by Liebig, and later by P. Wagner of Germany. Few would take issue with the latter quotation, but many have disputed the former statement. Examination of the disputed statement shows that if only one nutrient were limiting crop yields the resulting yield equation for a given soil would be

P = a!+ px,

(1)

where P is the estimated crop yield, a! reflects the combined effects of all soil nutrients present in the soil and other growth factors, and p indicates the yield increase produced by additions of the deficient nutrient XI. The relationship described by this equation is shown in Fig. 1.

01

0

I

I

I

I

I

40

80

120

160

200

POUNDS OF NUTRIENT APPLIED PER ACRE

FIG.1. Theoretical yield curve if crop responses were increased in direct proportion to the amount of applied nutrient.

Under these conditions, the yield increases produced by applications of the nutrient, XI, would be constant regardless of the total quantity added. This situation, of course, is unrealistic, implying that unlimited production can be obtained from a plot of land. If a linear relation is

136

ROBERT D. MUNSON AND JOHN P. DOLL

found to exist in experimental results, it probably implies that the treatment combinations and rates used have not “bracketed” the response range, thus limiting the usefulness of the information. Further references to the “law of the minimum” are presented in “Liebig and After Liebig,” edited by Moulton ( 1942). 2. The Law of Diminishing Soil Yield or the Law of Diminishing Increments The basis of this concept conforms to the theory of diminishing returns proposed by Ricardo about 1817 (Ricardo, 1951). Liebig, in some of his writings on soil tillage and manuring operations, subscribed to this theory. However, formal development of the concept concerning soil-plant-fertilizer relationships fell to men such as Mitscherlich, Baule, and Spillman. A description of the mathematical expressions proposed by these workers is discussed below. a. MitscherZich. In 1909, Mitscherlich ( 1909) mathematically expressed the law of physiological relationship or the law of diminishing soil yield concerning crop-yield response to applied plant nutrients. He stated that this law “was known before Liebig’s time.” In developing his equation, Mitscherlich discarded Liebig’s assumption that the yield increase produced by fertilization was directly proportional to the quantity of nutrient or nutrients applied. Noting that yield increases from fertilization usually increased at a decreasing rate, Mitscherlich made two assumptions: ( 1 ) A maximum yield, A, exists for a crop grown on a given soil. (2) The yield increment from fertilizer additions is proportional to the decrement from the maximum yield, A. From these assumptions he developed the following equation:

&!

= (A - y)k dx where dy/dx is the rate of increase in yield, Y, produced by additions of a deficient nutrient, x; k is a proportionality constant which Mitscherlich later proposed as being constant, different, and independent for each growth factor. By integrating equation (2)and converting to logarithms to the base 10, the expression becomes

log (A

- Y)

= log A

- cx

where c = 0.4343k. This equation may also be written

Y

=

A(l

- 10-cs)

or, to the base e Y = A(1 - e--k*)

(3)

ECONOMICS OF FERTILIZER USE

137

when none of the deficient nutrient is contained in the growth medium. However, the expression becomes

y = A(1 - ~ O - - C ( L . + ~ ) )

(5)

where b is the quantity of the nutrient supplied by the soil and seed. In actuality, b has a varying value if estimated in different years. Dean and Fried (1953) discussed the use of b in predicting the amount of a nutrient (phosphorus) in the soil and indicated that it may sometimes give erroneous estimates of the amount of nutrient in the soil. This discussion will not include the controversial issue of the constancies of the “effect factors,” c, for the various nutrients (Mitscherlich, 1947; Van Der Pauuw, 1952-1953; Willcox, 1955), but will deal only with its agronomic and economic utility in analyzing experimental data. Mitscherlich recognized the importance of the economics of crop fertilization and included sections in his original paper showing how his equation could be used to determine the most profitable rate of fertilization. He discussed the following costs involved in crop production: 1. “Fixed costs” or those costs that remain constant regardless of the yield level, e.g., taxes and interest on investment. 2. “Variable costs” or those costs that vary with the magnitude of the total yield, e.g., the quantity of fertilizer applied and the harvesting and handling cost. The method outlined for determining the most profitable rate of fertilizer application is easily applied once the parameters of the equation have been estimated. These parameters can be estimated algebraically, graphically, or by the method of least squares. The method of least squares involves an iterative process which can be time consuming if electronic computers are not used. Mitscherlich ( 1909), Rauterberg (1939), Pimentel-Comes (1953), and Eid et al. (1954) have described methods for determining these parameters. Black (1955), writing on the evaluation of nutrient availability and predicted yield response from fertilization, described Mitscherlich‘s method for determining the most profitable rate of fertilizer application, assuming unlimited amounts of capital are available. This rate can be determined by the equation

x,= log [2.3cA(M - C,)/Cz]

- cb

C

where X, is the number of units of fertilizer that produce the highest profit, Czis the cost per unit of fertilizer, M is the value per yield unit of crop, C, is the cost per yield unit, and the “constants,” A, b, and c

138

IlOBEnT D. MUNSON AND JOHN P. W L L

are the determined values described above. Net profit, P, for any ratc of application can be found by solving the equation

+

P = A ( M - CS)(l - lo--”* ) - (Cl C2x) (7) where C1is the fixed cost per unit area of land associated with production and the meaning of the other symbols remains the same. b. Buule. Baule (1918),a German mathematician, became interested in Mitscherlich’s law of physiological relationship and proposed a modification of it. According to Baule, it is not the absolute increase in yield that reflects the effect of fertilizer, but the percentage increase in yield or the percentage of the maximum possible yield produced by fertilization. He showed that when one uses this concept the percentage increase in yield is independent of the quantity of nutrient present in the soil. Baule used an “effect quantity,” h, inversely proportional to Mitscherlich‘s c value. This quantity, the “Baule unit,” is defined as the amount of fertilizer nutrient required to increase the yield one-half of the maximum possible. In other words, each additional quantity, h, will increase yield by half of the decrement from the maximum yield. One Baule unit will increase yield within 50 per cent of the maximum; 2 Baules, 75 per cent of the maximum, etc. Baule also suggested that all nutrients required for plant growth should be included in the yield equation. This leads to the following generalization of the Mitscherlich equation Y = A(l

...

- lO-cl~)(l - 1O-c~~). * . (1 - lo-&)

(8)

..

where c1, cz, cn are the “effect factors” for the nutrients xl, xz, , Xn. Y and A are the values previously defined. This equation provides for yields of zero when a given essential nutrient is not present in a soil nor added in a fertilizer application. For example, considering an equabl = 0, and q and bz are some positive tion for two nutrients, if XI= quantities y

=

A(1

becomes y = A(1

- 10-~(n+bl))(l- ~ O - C ( Z Z + ~ Z ) ) - lo-~CO))(l- ~ O - C ( ~ S + ~ Z ) )

and Y=O because 10-c@) = 1. This function does not allow for yield decreases above the maximum yield. However, as discussed later, it can be used to determine most profitable nutrient combinations. Willcox (1947), who has long been a proponent of the Mitscherlich-

139 Bade method in the analysis of yield response to fertilization, described a method for estimating yield equations using a standard yield diagram. The diagram is used to estimate the parameters of the equation. After the equation is obtained, the same techniques of economic analysis outlined previously apply in determining the most profitable rates of fertilizer application. One of the major drawbacks of this method is in the analysis of year-to-year yield variations and the difficulty of estimating equations for more than one nutrient. c. Spillman. In 1924 Spillman edited a book dealing with the law of diminishing returns and its relation to biological production relationships. In that publication ( Spillman, 1924), and a subsequent research bulletin ( Spillman, 1933), Spillman developed a mathematical expression of crop yield response to plant nutrients. This equation is ECONOMICS OF FERTILIZER USE

Y=M-ARo (9) where Y is the yield expected from x amount of applied nutrient, M is the theoretical maximum yield attainable, A is the theoretical maximum increase in yield obtainable by increasing x , and R is the ratio of successive yield increments. R is defined as R = -AYz - - =- -AYs AY, - ... A Yn AY1 AY2 AYa A Yn-1 where AY1 equals the yield increase produced by the first increment of fertilizer, A Y is ~ the yield increase produced by the second increment, etc. Another expression of Spillman's equation is Y = M(1- Rz) (10) Spillman's equation is similar to Mitscherlich's with the primary exception that the ratio ( R ) of successive yield increments is not assumed to be constant for a given nutrient but may vary with soil and climatic conditions. The M of the Spillman equation corresponds to Mitscherlich's A value, while Spillman's A and Rm correspond to Mitscherlich's and respectively. Spillman (1933) outlined algebraic and graphic methods for evaluating the parameters of the equation for a given set of yield data. Magistad et al. (1932) presented the least-squares solution for determining the parameters of the equation from experimental data. Farden and Magistad (1932) applied Spillman's equation in an economic analysis of the most profitable rates of fertilizer for pineapple cultures. For three nutrient variables Spillman (1933) proposed the equation Y = Ma(1 - R"*)(l - RP+*)(l - RHc) (11) where the exponential terms n + a, p b, k + c are quantities of ni-

+

140

ROBERT D. MUNSON AND JOHN P. DOLL

trogen, P205,and KzO,respectively, available from the soil, plus that applied in the fertilizer. The other symbols are as previously defined except that Ma specifies the maximum yield for three nutrients. The above discussion has outlined some of the important early developments concerning production relationships and the profitability of fertilizer rates. Recently, more emphasis has been placed on the economic analysis of yield-response data. The following section outlines the economic analyses now being applied to crop-yield data.

B. THEAPPLICATIONOF THE PRINCIPLES PRODUCTION ECONOMICS TO FERTILIZER USE The fundamental purpose of production economics is the maximization of net farm income, It provides criteria that can lead to the most profitable and efficient use of agricultural resources. In general, a farmer has certain resources available for his use in production operations. Usually, these are owned, but they may also be rented or purchased on credit. Production economic principles consider the costs and returns of each production alternative and provide criteria that aid the farmer in using his resources in the most profitable manner. Yield curves or production functions, such as those developed by Mitscherlich and Spillman, provide a basis for production economics analyses. The purpose of these curves is to establish a continuous cause and effect relationship among variables such as fertilizers and crop yields. Although Spillman and Mitscherlich specified a certain mathematical form for the production function, other types of equations may also be used. Regardless of the form of equation used, the relationship must display diminishing returns if it is to be consistent with known biological phenomena. For plant nutrient-yield relationships, the theory of diminishing ret u r n s can be stated as follows: As the quantity of one variable input or factor of production necessary for crop growth, in which the soil is deficient, is applied in increasing amounts while other inputs or factors of production are held constant, the added product or yield increase caused by the variable factor will eventually decrease. The economic analysis which follows requires that the production function display diminishing returns. OF

1. Single-Nutrient Analysis The purpose of the production function or yield curve is to predict yield response to fertilizer applications. The predicted curve, along with crop and fertilizer prices, is used to determine profits that can result from using different rates of fertilizer. This is done by considering the

ECONOMICS O F FERTILIZER USE

141

value of the added yield and the cost of the fertilizer required to produce that added yield. As an example, suppose an increment of nitrogen ( A N ) produces an increase in corn yield ( AY) . Then, if P, is the price of nitrogen per pound and P, is the price of corn per bushel, (P, x AN) is the cost of the added nitrogen and ( P , x AY) is the value of the added yield. As long as the cost of the added nitrogen is less than the value of the added corn yield, profits can be increased by additional nitrogen. There will be a particular nitrogen application and resultant yield at which the profit from nitrogen fertilization will be a maximum. Using either more or less nitrogen will reduce profits. This optimum exists when P,(AN) = P,(AY) (124 or, A Y P, AN - P, With a yield equation, the expression on the left side of (12b) becomes an approximation of the slope of the yield curve. When the increments of nitrogen (AN) become infinitely small, the approximate slope (AY/AN) is replaced by an exact expression for the slope-the first derivative of the production function (dY/dN). Therefore, (12b) can be replaced by dY - P _., dN - P, The above analysis is called a “marginal” analysis. The yield increase or additional yield caused by an additional nutrient input is called the marginal physical product (MPP) of that nutrient and expresses the rate at which the yield curve is increasing or decreasing. An example of an economic analysis of a single-nutrient yield curve has been selected from Paschal and French (1956). The experiment used in this analysis, experiment 10 in their bulletin, deals with the response of irrigated corn to nitrogen on Greenleaf silt loam. It was conducted at Ontario, Oregon, in 1952. The experimental site had received a 20pound application of nitrogen the year before the experiment was conducted, but had not been fertilized for seven previous years. Fifty pounds of Pz05were applied to all plots in 1952. The experiment was designed to estimate optimum or most profitable fertilizer recommendations. Twelve rates of nitrogen, ranging from 0 to 320 pounds per acre, were used. An arithmetic marginal analysis of the observed corn yields is presented in Table I. A corn price of $1.40 per bushel and a nitrogen cost of $0.15 per pound were used to calculate the costs’and returns pre-

142

ROBERT D. MUNSON AND JOHN P. DOLL

TABLE I An Arithmetic Marginal Analysis of the Response of Irrigated Corn to Nitrogen, Greenleaf Silt Loam, Ontario, Oregon, 1952“ ~~

~

~~

Pounds of nitrogen applied per acre

Bushels of corn per acre (Y)

Added yield ( W

Added nitrogen (AN)

0 40

64.6 90.4 118.2 132.4 140.7 141.0 146.8 141.2 147.1 145.8 147.4 143.6

25.8 27.8 14.2 8.3 0.3 5.8 -5.6 5.9 -1.3 1.6 -3.8

40 40 20 20 20 20 20 20 40 40 40

80 100 120 140 160 180 200 240 280 320

Cost of Value of added yield nitrogen (AY X PY), (AN X pn)c $36.12 38.92 19.88 11.62 0.42 8.12 -7.84 8.26 -1.82 2.24 -5.32

$6.00 6.00 3.00 3.00 3.00 3.00 3.00 3.00 6.00 6.00 6.00

” Source: Paschal and French (1956). c

Mean of four replications. Price of corn ( P J is $1.40 per bushel and cost of nitrogen (P,) is $0.15 per pound.

sented in this table. Other costs could have been included if relevant. For the prices used, the data indicate that it would be profitable to apply at least 120 pounds of nitrogen per acre. Although yield responses resulting from higher nitrogen rates are too erratic to establish a trend, it appears that net profit might still be increasing at 160 pounds per acre. A Spillman yield equation was fitted to the experimental data by an iterative least-squares procedure which requires an initial estimate of R, the ratio previously defined. The estimated equation is

P, = 150.17 - 89.40 (0.75)cf

(13)

where Y, is the mean corn yield of the jth fertilizer treatment and xj is the quantity of fertilizer measured in 20-pound units of nitrogen. In analysis of variance presented in Table 11, the treatment sums of squares is divided into two components, that due to regression and that due to deviations from regression. Because the treatment sums of squares represent the yield variation which the regression attempts to “explain,” it is appropriate to fit the equation to the treatment means or totals rather than the individual observations. The authors (Paschal and French) emphasized the fact that the regression analysis is a supplement to the analysis of variance rather than a substitute for it.

143

ECONOMICS OF FERTILIZER USE

TABLE I1 Analysis of Variance for Corn Yields on Irrigated Greenleaf Silt Loam, Ontario, Oregon, 1952" Source of variation

Degrees of freedom

Replications Fertilizers Due to regression Deviations from regression Error

Sums of squares

3 11

315.10 2,783.85

945.29 30,622.22 29,567.41 1,063.53

2 9

T o hl

Mean squares

33

2,191.95

47

33.759.46

14,783.71 118.17 (36.66

Source: Paschal and French (1956).

The yield-response curve calculated from equation (13) is shown in

Fig. 2. Little or no yield response to nitrogen applications larger than 160 pounds per acre were observed. However, 320 pounds did not appear to cause a marked decrease in total yields. The most profitable rates of nitrogen can be found by equating the derivative of (13) to the nitrogen:com price ratios and solving for the amount of nitrogen. If nitrogen costs $0.15 per pound, the most profitable rate would be 157 pounds if corn were $1.12 per bushel, 172 pounds if corn were $1.40 per bushel, and 185 pounds if corn were $1.68 per bushel. The dashed curves in Fig. 2 represent the 67 per cent confidence limits derived for equation (13). The circles on the dashed curves denote the most profitable amounts of nitrogen for these limits. When nitrogen costs $0.15 per pound and the price of corn is $1.40 per bushel, the amounts of nitrogen are 155 and 192 pounds, respectively, or a range of 37 pounds between the upper and lower limits. BU. PER ACRE

1

160 120 a0 0

0

40

I

0

80

1.40 1.68

I

I

160

240

16. N APPLIED PER ACRE

320

FIG.2. Yield curve, 67 per cent confidence limit curves, and most profitable rates of nitrogen application (at $0.15 per pound) in irrigated corn experiment on green-

leaf silt loam, Ontario, Oregon, 1952 (Paschal and French, 1956).

TABLE I11 Predicted Total Costs and Returns, Marginal (Additional) Costs and Returns, and Returns per Dollar for Nitrogen Applications on Irrigated Corn, Greenleaf S i t Loam, Ontario, Oregon, 1952" Pounds of Bushels nitrogen of applied yield per acre increase

20 40 60 80 100 120 140 160 180 220 240 260

22.35 16.76 12.57 9.43 7.07 5.30 3.98 2.98 2.24 1.68 1.26 0.94 0.71

280

0.53

200

o

b

Value of Costof yield added increaseb nitrogen

$31.29 23.46 17.60 13.20 9.90 7.42 5.57 4.17 3.14 2.35 1.76 1.32 0.99 0.74

$3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00

Returns Returns per dollar above onlast nitrogen 20 lb. of costsb nitrogen

$28.29 20.46 14.60 10.20 6.90 4.42 2.57 1.17 0.14 -0.65 -1.24 -1.68 -2.01 -2.26

$10.43 7.82 5.87 4.40 3.30 2.47 1.86 1.39 1.05 0.78 0.59 0.44 0.33 0.25

Total nitrogen application

Total yield increaaeb

Pounds

Cost

Bushels

Value

20 40 60 80 100 120 140 160 180

$ 3.00

22.35 39.11 51.68 61.11 68.18 73.48 77.46 80.44 82.68 84.36 85.62 86.56 87.27 87.80

S 31.29 54.75 72.35 85.55 95.45 102.87 108.44 112.62 115.75 118.10 119.87 121.18 122.18 122.92

200

220 240 260 280

Source: Paschal and French (1956). Corn price is $1.40per bushel and nitrogen price is $0.15per pound.

6.00 9.00 12.00 15.00 18.00 21.00 24.00 27.00 30.00 33.00 36.00 39.00 42.00

Average Total return per return dollar to spenton nitrogenb nitrogen

$28.29 48.75 63.35 73.55 80.45 84.87 87.44 88.62 88.75 88.10 86.87 85.18 83.18 80.92

$10.43 9.12 8.04 7.13 6.36 5.72 5.16 4.69 4.29 3.94 3.63 3.37 3.13 2.93

id 0

kl p

5

2 Z

8

8

L(

'd

g

145

ECONOMICS OF FERTILIZER USE

Total costs and returns, additional or marginal costs and returns, and returns per dollar spent are presented in Table I11 and Fig. 3. These data were calculated using a corn price of $1.40 per bushel and a nitrogen price of $0.15 per pound. The curves in Fig. 3 demonstrate economic principles which should be considered when making fertilizer recommendations. The optimum rate of nitrogen application is the same, 172 pounds, for either the criterion of total return above total cost (value of total yield increase minus total cost of nitrogen application) or the marginal criterion (additional or marginal costs should equal the value

80 40

-[

o.----

Total return above cost of

I-___+--

N

--

Cost a1 total application

ADDITIONAL RETURNS

-

^^I.

..'

Value of additional

16. N APPLIED PER ACRE

FIG.3. Costs and returns from rates of nitrogen on irrigated corn Greenleaf silt loam, Ontario, Oregon, 1952 (nitrogen at $0.15 per pound, corn at $1.40 per bushel) (Paschal and French, 1956).

of the additional or marginal yield). The returns per dollar spent, however, do not determine the most profitable rate of fertilization. Because the yield curve exhibits diminishing returns, returns per dollar spent decrease as nitrogen applications increase. Therefore, returns per dollar spent is not a satisfactory criterion for determining the most profitable rate of fertilizer. However, returns per dollar spent or net returns per dollar can be used to determine the minimum amount of fertilizer that is profitable. The optimum rates of fertilization discussed above were calculated assuming that all costs other than the cost of fertilizer are insignificant

14G

ROBERT

1).

MUNSON AND JOIIN P. DOLL

and that the farmer has stifficient capital to purchase optimiim amounts of fertilizer. Very seldom, if ever, will both of these assumptions be valid. Usually, there is a fixed cost involved in applying fertilizer. This fixed cost might include depreciation and interest charges on capital invested in machinery used to apply fertilizer. And, because they vary only slightly with variations in application rates, labor, fuel, and grease costs might also be considered fixed costs in fertilizer application. When fixed costs are considered, the value of the yield increment must be large enough to cover fixed as well as variable costs. Low rates of application may not return enough to cover &ed costs even when they pay for the variable costs. Therefore, there is a minimum below which the rate of fertilization should not fall. Pesek and Heady (1958) have defined this minimum rate to be the rate at which net returns per dollar are at a maximum. Pesek and Heady define the net return per dollar (NR) as

where Y represents the yield equation and C represents the cost equation

C=F+PZ (15) where F is the fixed cost of application and P, is the cost per unit of X. Y and C must be for the same unit area and must be expressed in the same units of measure, If C is expressed in dollar units as above, Y can be expressed in dollar units by multiplying the yield equation by the unit price of the product ( Pu).Or, if physical units are used, C can be expressed in physical units by dividing F and P, by Pu.Either method gives similar results. The minimum rate of fertilization is found by determining the application rate at which equation (14) is a maximum. This is done by using maximization procedures from calculus or by plotting ( 14) . The significance of the minimum rate is: When the farmer does not have enough nitrogen to fertilize an entire field at the minimum rate or above, he should apply the minimum rate until the fertilizer supply is exhausted and leave the remaining acres unfertilized. Any other rate of application, such as spreading the fertilizer at rates greater or less than the minimum to different acreages, will reduce returns from the given quantity of fertilizer. Therefore, the minimum and the optimum rates define the range within which fertilizer recommendations should be made. Although the optimum rates of fertilizer maximize profits from the particular crop enterprise involved, they do not necessarily maximize

ECONOMICS OF FERTILIZER USE

147

profits from the entire farm. Only when the farmer has large amounts of capital, enough to extend the use of every input to its optimum amount, is the optimum use of one input to produce one crop, such as the use of nitrogen to produce corn, consistent with the maximization of profits for the entire farm. When the amounts of capital or credit are limited, the farmer must decide which production alternatives are the most profitable and allocate his capital to them. Therefore, the farmer may purchase breeding stock or machinery rather than nitrogen because they return more than the last few pounds of nitrogen needed to attain the optimum rate. Another reason why farmers may not use the optimum amounts of fertilizer, other than capital limitations or inadequate knowledge of fertilizer use, is uncertainty. Conceptually, if all factors of production were known and measurable, a general yield equation expressing yield as a function of those factors could be determined. In practice, however, this general equation is not known and can only be approximated. Many factors in yield analysis which are supposed constant are actually variable, and, because the effects of other variables are not independent or random, the yield curve is not a “single line” but a “wide band.” Therefore, farmers may not use optimum fertilizer rates because they do not know the effects uncontrolled variables might have on yield responses. This uncertainty causes them to forego profits when they use more or less than optimum amounts. However, recommended practices, such as proper fertilizer placement, are known with certainty to increase fertilizer efficiency and should be readily adopted when possible. 2. Analysis for Two or More Nutrients

The economic analysis of yields resulting from experiments with two or more fertilizer nutrients is an extension of the marginal principles of the single-variable analysis. However, different terminology and concepts are involved in multinutrient or multivariate analyses in agronomiceconomic literature. Using the single-nutrient economic analysis as a foundation, an analysis of experiments with two or more nutrients will be discussed. Data presented by Heady ct al. (1955) dealing with corn-yield response to nitrogen and P 2 0 j in calcareous Ida silt loam are selected for illustrative purposes. The experiment was conducted in western Iowa in 1952, a favorable year for fertilizer response (see Table IV). The authors indicated that the plant population of 18,000 plants per acre may have limited responses at high fertilization rates. A 9 x 9 incomplete NP factorial was replicated twice in a completely randomized design. This type of experiment was used to include a wide

148

ROBERT D. MUNSON AND JOHN P. DOLL

TABLE IV Bushels of Corn per Acre Resulting from Applications of Nitrogen and PZOSto Ida Silt Loam, Iowa, 1952" Pounds of PZOS per acre

0 40 80 120 160 200 240 280 320 4

b

Pounds of nitrogen per acre ~~

0 15.4* 28.2 26.4 33.1 23.0 36.0 36.5 29.9 11.6

40

80

120

160

200

240

280

320

17.9 71.4

17.6

24.8

10.8 101.5 94.3 107.9 123.0 119.0 126.7 131.0 122.8

8.7 95.4

11.5

17.3

22.1 79.2 105.5 103.8 129.2 127.1 137.9 119.5 123.4

107.5 88.5 66.1

105.4

108.4 128.9

102.5 59.6

100.3

127.4 128.6

119.0 110.6 141.3

127.4

122.1 133.1

117.7 132.4

133.4

136.0 121.9

Source: Heady et al. (1955). Yields are mean of two replications.

range of nutrient inputs without making the experiment unduly large. As in the Oregon experiment cited above, nutrient applications ranged from 0 to 320 pounds per acre in the Iowa experiment. This range was used to insure that the most profitable rates derived by marginal analysis would fall within the limits of the experiment. However, the range of application can be increased or decreased depending upon the crop to be grown and existing soil and climatic conditions. In general, it is desirable to plan the application rates so that the maximum yield occurs within the experimental limits. While economic recommendations do not include the maximum yield except when plant nutrients are free, they may approach the maximum depending upon the nutrient:yield price ratio. Also, wide input ranges have desirable characteristics from a statistical standpoint (Ostle, 1954). Heady and associates (1955) presented several types of regression equations which were estimated by least-squares procedures. A quadratic equation, containing squared ( N2, P2) and cross-product ( NP) terms was used for an economic analysis of the experiment by J. P. Doll (1958). This equation is P = -7.51 0.58N 0.66P - 0.0016N2- 0.0018P2 0.0008NP (16) where P is the predicted bushels of corn per acre; N and P are the pounds of nitrogen and PzOa applied per acre. An analysis of variance for this equation is presented in Table V. The equation has an R2 or coef€icient of determination of 86 per cent based upon the treatment sums of squares. Yields predicted by equation (16) are presented in Table VI. They

+

+

+

149

ECONOMICS OF FERTILIZER USE

TABLE V Analysis of Variance for Corn Yiclda on Ida Silt, Loam, 1952" Degrees of freedom

Source of variation

113 Total Treatments 56 Due to regression Deviations from regression Among plots treated alike 57 a

Sums of squares

Mean squares 4,175

242,707 233,811 5 51

201,943 31,868

40,389 625

8,896

156

Source: Heady et al. (1955).

can be visualized either as a series of yield curves caused by varying one nutrient while the other is held constant or as a number of individualresponse curves resulting from varying the two nutrients in different ratios. All these yield curves can be displayed by a three-dimensional production or yield surface. The corn-yield surface shown in Fig. 4A was drawn from the data in Table VI. In this figure, rates of nitrogen and PzOs are pictured as the width and depth, respectively, of the diagram and predicted corn yields are represented by the height of the surface above the nutrient plane. The lines over the surface trace its general shape and represent the single-nutrient yield curves described above. A yield-response curve to any fixed ratio of nutrients could be represented by a curve passing through the origin ( N = P = 0), ascending the yield surface, and tracing a linear path on the nutrient plane. TABLE VI Bushels of Corn per Acre Predicted for Specified Combinations of Applied Nitrogen and PZOS, Ida Silt Loam, Iowa, 1952" Pounds of Pa06 applied per acre

0

40

80

120

160

200

240

280

330

0 40 80 120 160 200 240 280 320

-b

16.2 34.1 46.3 53.0 53.4 48.3 37.5 20.9

13.3 38.3 57.5 71.0 78.7 80.7 76.9 67.4 52.1

29.2 55.4 75.9 90.7 99.8 103.0 100.5 92.3 78.3

39.8 67.4 89.2 105.3 115.6 120.2 119.0 112.0 99.4

45.5 74.4 97.5 114.9 126.6 132.2 132.5 126.8 115.4

46.1 76.3 100.7 119.4 132.3 139.4 140.8 136.5 126.4

41.7 73.1 98.8 118.8 133.0 141.5 144.1 141.1 132.3

32.2 64.9 91.9 113.2 128.7 138.4 142.4 140.7 133.2

17.6 51.6 79.9 102.5 119.3 130.4 135.7 135.2 129.0

a

b

Pounds of nitrogen applied per acre

Source: J. P. Doll (1958). Predicted yield has a negative value.

I I

(B) FIG. 4. Production surfaces depicting a three-dimensional view of corn yield response to applications of nitrogen and PaOs, Ida silt loam, Iowa, 1952. A. Yield surface represented by interlaced single-nutrient curves. B. Yield surface represented by yield contours defining yield levels. 150

ECONOMICS OF FERTILIZER USE

151

The corn yield production surface in Fig. 4A can also be pictured as in Fig. 4B. The latter is a three-dimensional view of the yields presented in Table VI. In Fig. 4B, however, the shape of the surface is characterized by yield contours similar to contour lines on a topographic map. The curve labeled 50 connects all points on the surface at which the corn yield is 50 bushels per acre. Ascending the surface, the curves labeled 90 and 130 connect the points on the surface representing yields of 90 and 130 bushels per acre, respectively. If the assumptions of the analysis are correct, a maximum yield of 144 bushels can be produced only by a single combination of nitrogen and P205 and is, therefore, represented by a dot. Just as contour lines on a topographic map can be drawn for any altitude level, curves could be drawn in Fig. 4B for any yield level. If the angle of perception of Fig. 4B were altered and observed directly from above, Fig. 5 would result. In Fig. 5, the yield contours defining nutrient combinations that produce a given yield are called “isoquants” (“iso” for equal and “quant” for quantity of yield) and the figure is called an isoquant map. The numbers SO, 70, 90, 110, and 130 indicate isoquant yield levels. For example, 90 bushels of corn could be produced with 60 and 161 pounds of nitrogen and P205, respectively, or 120 and 82 pounds of nitrogen and Pz05, respectively. The 90-bushel isoquant in Fig. 5 is also labeled AB to show that it corresponds to the curve AB on the nutrient plane in Fig. 4B.

POUNDS OF NITROGEN APPLIED PER ACRE

FIG.5. Corn yield isoquants and ridge lines predicted for Ida silt loam, Iowa, 1952.

152

ROBERT D. MUNSON AND JOHN P. DOLL

The dashed lines in Fig. 5 are called ridge lines. At any point within the ridge lines, yields can be increased by the addition of either or both nutrients. Outside the ridge lines, additional application of nitrogen, P206, or both, may decrease yields. If ridge lines were drawn on Fig. 4A, they would intersect each of the single nutrient yield curves at its point of maximum yield. The ridge lines intersect at the point of maximum yield, where the marginal physical products or yield increases from both nutrients are zero. Ridge lines define the area on the isoquant map within which all economic recommendations fall. While the ridge lines are not known beforehand, an estimate of the nutrient levels at which they may occur could enable the research worker to make more efficient use of his resources. Isoquants are used to estimate combinations of nutrients which minimize the cost of producing a given yield. Suppose a farmer's field is now yielding 76 bushels of corn per acre and the farmer wishes to increase the yield to 90 bushels per acre. If, as shown in Fig. 5, the fertility level of the field is equivalent to 80 pounds each of nitrogen and P206, the farmer could increase yield to 90 bushels by adding either 42 pounds of nitrogen or 36 pounds of PzOa, neglecting the possibility of adding some combination of the two nutrients. Thus, the predicted amount of nitrogen needed to increase yield 14 bushels relative to the amount of PzO6 needed to increase yield 14 bushels is 36/42 or, in general, ( AP/AN). But, ( AP/AN) is also the average slope of the isoquant between points C and D. Because the yield increase ( A Y ) is the same (14 bushels) for the change in nitrogen (AN) or in Pz06 (AP), the ratio (AP/AN) can also be written AY AP AY/AN D ' H = W P

Thus, the slope of the isoquant is determined and can be represented by the ratio of the marginal physical products of the nutrients (MPP,/ MPPp). By reducing the size of the nutrient increments, the marginal products and the slope of the isoquant can be estimated as precisely as desired. Thus, the slope can also be derived as

dP dN

6Y -

E

6N = -MPP, --

6Y 6P

MPP,

(18)

Usually, this expression is the most convenient method of obtaining the isoquant slope.

ECONOMICS OF FERTILIZER USE

153

The slope of an isoquant denotes the “marginal rate of substitution” between the nutrients. In this example, the marginal rate of substitution of nitrogen for P2Ob may be defined as the quantity of PZOS which must be added to the production process to maintain yield at a constant level when the quantity of nitrogen is decreased by one unit, This concept has caused considerable discussion among those familiar with plant nutrition. The purpose of estimating the production function is to predict rates of fertilizer application which will minimize costs of producing a given yield or maximize profits from the use of fertilizer. In this sense, even though the function is estimated from experimental yields obtained in past cropping seasons, the production surface represents yields which might be expected in future years and can therefore be used in planning future action. As stated previously, isoquants define various combinations of nutrients that can be expected to produce given yield levels. Thus, the isoquant and isocline map can be regarded as a guide for making future decisions; it is in this framework that one becomes involved with the concept of substitution. When an economist refers to two nutrients as substitutes, he means that the addition of either nutrient to the soil will have the same effect on yield. Thus, if the fertility level of a soil is such that it will release 80 pounds of available nitrogen and P20s per acre (Fig. S), a yield level of 90 bushels can be attained by adding 36 pounds of PzOaor 42 pounds of nitrogen, then nitrogen and phosphorus are economic substitutes, i.e., they both increase yield. This concept of substitution does not imply physiological substitution of nutrients, but only that either nutrient will increase yields when applied to the soil. This is probably due to the effect that the addition of one nutrient can have on the utilization of other soil nutrients. One example of this would be the increased efficiency of water utilization by a crop in the presence of adequate fertility. Also, Olson and Dreier (1953) have shown that fertilizer phosphorus is utilized more efficiently in the presence of ammonium nitrogen. Lang (1924), when discussing Liebig’s law of the minimum with regard to interrelationships among factors affecting growth, stated: “There can now be no doubt that the various reciprocal relations between growth conditions, which have thus far in fact been little investigated, constitute at least somewhat of a limitation on the minimum law. An increase in yield can be obtained not only by improvement in the one factor present at a relative minimum, but by improvement in all factors which are removed from the optimum to a greater or less extent -and most of them will be that.”

154

ROBERT D. MUNSON AND JOHN P. DOLL

( Lang’s reference to “optimum” means, not the profit-maximizing, but the yield-maximizing, quantity of nutrients. ) And that: “From all these results, Mitscherlich draws the conclusion that the old Liebig Law of the Minimum, according to which yield is dependent on the vegetative factor which is at the minimum, is not justified by the facts, at least not in this strict statement of it. In fact, ‘the magnitude of the yield is governed by all the growth factors together.’ But since each of the many factors exerts the greater influence on the yield increasethe yield increase is all the more rapid-the less there is present of the requisite quantity of the factor in question, we are often all not so far from the Liebig Law. Apparently this law might be justified; but in reality it is inconsistent with the new knowledge; not only may yield be materially increased by the minimum factor, but also by every other factor present in insufficient quantity.” Equations for isoquants can be obtained from the yield curve by expressing one nutrient as a function of yield and the other nutrient or nutrients. For the equation ( 16), the isoquant equation expressing P20j as a function of yield and nitrogen is

P = 184.70

+ 0.23N f 278.24 (0.3867 + 0.0053N - 0.00001N2- 0.0072P)’/2

(19)

where all terms are defined as before. When a desired yield, 2, is inserted in (19), the result is an isoquant equation for that yield level. Combinations of nutrients predicted to produce that yield can be found by substituting Merent values of nitrogen into the equation and solving for P2Oa. Combinations of nitrogen and PzOBderived by this method for a yield level of 90 bushels are presented in Table VII. The slopes of the 90-bushel isoquant at these points, representing the marginal rate of TABLE VII Combinations of Nitrogen and P~OSand Corresponding Marginal Rates of Substitution Predicted for a 90-Buehel Corn Yield Contour on Ida Silt Loam, Iowa, 19520 ~~~

~

Pounds of nitrogen Pounds of PzO~ Marginal rate of per acre per acre substitution ( d P / d N ) 60

80

100 120

161 118 96 82

* Source: J. P. Doll (1968).

-3.94

- 1.40 -0.86 -0.68

ECONOMICS OF FERTILIZER USE

155

substitution of nitrogen for PzOn,are also presented in Table VII. These slopes were predicted by the equation

+ +

0.0008P = -0.58 - 0.003N 0.66 - 0.004P 0.0008N MPP, In Table VII, as the amount of nitrogen available to produce the 90-bushel yield increases, the amount of P205 needed decreases. Also, the marginal rate of substitution varies widely. For 60 and 161 pounds of nitrogen and P2O5, respectively, 1 pound of nitrogen “replaces” almost 4 pounds of Pz05. However, when nitrogen is 120 pounds and P&, is 82 pounds, one pound of nitrogen will “replace” only 0.6 pounds of PzOs. This is consistent with the law of diminishing returns and with the quotations of Lang. The importance of the marginal rate of substitution is associated with profit maximization and cost minimization. If a yield is to be produced at a minimum of cost, the marginal rate of substitution or ratio of the marginal products of the nutrients should be equal to the ratio of prices of the nutrients. Referring back to equation (18), this can be expressed as

--MPP,

P, = -MPP, P, where P, and P , are the price of PZO5and nitrogen per pound, respectively. The cost-minimizing condition means that if nitrogen costs twice as much as Pz05, the cost of producing any yield is minimized when the combination of nutrients used is such that the marginal product of nitrogen is twice as large as the marginal product of PZO5. Equation (21) represents a condition which will hold for all yield levels. There will be a point on every isoquant where the slope is equal to a given price ratio. A curve connecting these points of equal slope is called an “isocline” ( a line of equal gradient or inclination). In Fig. 6, isoclines for the Iowa experiment are shown imposed upon an isoquant map. The isocline labeled 1.4 connects the points on the isoquants which have the slope -1.4. Thus, the only nutrient combinations which should be used when nitrogen and P2O5 are $0.13 and $0.09 per pound, respectively, are combinations occurring along this isocline. Isoclines for other price ratios are also shown. For the quadratic equation, all isoclines converge on the point of maximum yield. These isoclines were calculated from the equation

--MPP,

P=

0.66a

- 0.58 + (0.0008a+ 0.003N) (0.0008

+ 0.004a)

where a equals ( P , / P , ) . This equation was derived by substituting the marginal product equations derived from quadratic equation (16) into

156

ROBERT D. MUNSON AND JOHN P. DOLL 280

P V

240

a

200

W

a

fi

160

i

B a I20 0" C

b

80

v)

i

40

0

0

40

80

120

160

200

240

280

POUNDS OF NITROGEN APPLIED PER ACRE

FIG.6. Corn yield isoclines, isoquants, and ridge lines predicted for Ida silt loam, Iowa, 1952.

the general isocline equation (21) and solving for PzOs as a function of nitrogen and the price ratio, Q. The most profitable rate of nutrient application is found by moving out an isocline until the combined costs of the added nutrients equals the value of the added yield. This can be done by computing costs and returns for discrete intervals along an isocline or by solving the following equations

Equations (23) are analogous to (12c), used to find the optimum rate for one variable nutrient. The partial derivatives of the yield equation are set equal to the nutrientmop price ratios, and the equations are solved. This method of solution may be extended to consider more than two nutrients by including the appropriate equations. This solution predicts a combination of nutrients that represents both a least-cost and a profit-maximizing combination. It represents a point on an isocline where maximum profits occur. For the quadratic yield equation (lS), the profitmaximizing equations are

ECONOMICS OF FERTILIZER USE

+ 0.0008P PJP, 0.66 - 0.004P + 0.0008N = Pp/P,

0.58 - 0.003N

157

=

(24)

Because the quadratic yield curve (16) contains an interaction term, equations (24) must be solved simultaneously. For nitrogen and P205 prices of $0.13 and $0.09 per pound, respectively, the predicted amounts of nitrogen and P2OS which maximize profits per acre are: 215 and 218 pounds, respectively, when corn is $1.60 per bushel; 208 and 212 pounds, respectively, when corn is $1.30 per bushel; and 196 and 204 pounds, respectively, when corn is $1.00 per bushel. For these price situations the predicted profits are $180, $138, and $96 per acre, in the same order. In practice, as mentioned before, a farmer may not apply optimum amounts of fertilizer because of capital limitations or uncertainty. However, when these conditions are present, he should try to use the leastcost combinations because they represent the most efficient use of his limited resources. When a specific amount of capital per acre, K,is available to purchase a fertilizer, Tramel (1957a) has shown that the following equation P,N P,P = K (25)

+

should be solved simultaneously with equation (23). This solution is readily extended to include more nutrients. When K is less than the amount needed to purchase the optimum amount of fertilizer, this procedure will indicate the combination of nitrogen and P205 which will maximize profit, given the capital restriction. It is analogous to moving out an isocline until all capital is used; the nutrient combinations derived are least-cost combinations. For the Iowa example, if the prices of nitrogen and Pa05 are $0.13 and $0.09 per pound, respectively, equation (25) becomes

0.13N

+ 0.09P = K

(26)

By solving (26) simultaneously with (24), the estimated recommended amounts of nitrogen and P20a per acre are: 74 and 116 pounds, respectively, when $20 is available to purchase fertilizer for each acre; 125 pounds and 153 pounds, respectively, when $30 is available; and 176 and 190 pounds, respectively, when $40 is available. The predicted profits from the use of these three combinations are, in order, $93, $121, and $135 when the price of corn is $1.30 per bushel. These results indicate that fertilizer ratios which minimize production costs vary for different yield levels. In the above example, for $20, $30, and $40 amounts of capital, the N:Pa05 ratio are 0.6, 0.8, and 0.9, respectively.

158

ROBERT D. MUNSON AND JOHN P. DOLL

111. Current Research on Economics of Fertilizer Use

A. PLANNING AND DESIGNING EXPERIMENTS

As might be expected, most fertilizer experiments are in a sense evaluating the economics of fertilizer use. However, recently more emphasis has been placed on economic analyses and on the use of more and higher rates of plant nutrients. This aids in establishing the “response range” that is of importance to both agronomists and economists. In many cases experiments are being jointly planned by agronomists, economists, and statisticians, In this respect, Johnson ( 1956), Pesek (1956), Robertson et al. (1957), and Mason (1957) have presented discussions of factors that “joint research” planners should consider. In general, most workers seem to agree that certain requirements should be met in conducting experiments that will be used as a basis for making fertilizer recommendations to farmers. Most often listed among these requirements are: 1. The range of plant nutrients applied should be wide enough to attain the “maximum” physical yield, under conditions that consider as many “limiting factors” as possible. 2. A sufficient number of rates and combinations of fertilizer treatments should be used to define the response curve or surface, and to provide reliable statistical estimates. 3. As many variables as feasible should be measured and included in the analysis. These should include initial fertility of the soil, climate, soil moisture, variety, plant population, fertilizer placement, and others. 4. Measurement of the residual effects of fertilizer to determine the range of application within which the “long-time most profitable rates” will occur when fertilizers are used year after year. Consideration of these requirements will aid in providing experimental data that will be of lasting use. As more and higher rates of plant nutrients are being used, the problem of experimental design becomes important. The 3” factorial treatment arrangement usually is not adequate to establish a response curve or surface. Recently, five or more levels of each nutrient have been used in experiments. These experiments require a large number of plots and have caused research workers to become more cognizant of designs. Initially, in experiments conducted with many rates and nutrient combinations, complete or incomplete factorials were used with various modifications. Heady et al. (1955) used an 9 x 9 incomplete factorial to evaluate corn and forage responses. Knetsch et al. (1956) and Sundquist and Robertson ( 1959) also utilized incomplete factorials. Hutton

ECONOMICS OF FERTILIZER USE

159

nl. (1956) conducted a 5 x 5 x 5 x 2 complete factorial with corn. For estimation of the yield curve, it is very desirable to have the yield estimates that these experiments produce, but the work and cost involved in conducting them are often excessive. This caused research workers to attempt to reduce the number of treatment combinations and yet obtain reliable estimates of the regression coefficients. The composite designs proposed by Box (1954) appear to be applicable in this respect. These designs require fewer plots per replication, and the results are readily analyzed by regression techniques. Composite designs were initially developed for evaluating multifactorresponse surfaces in industrial research. Cochran and Cox (1957)have presented a discussion of the application and analysis of composite designs for studying response surfaces. The basis of these designs is a 2” factorial treatment arrangement, with additional treatments included. A composite design requires a minimum of 15 treatment combinations per replication for a three-factor experiment, while a S3 complete factorial requires 125 treatment combinations. Hader d al. (1957)have discussed the use and analysis of these designs in agronomic research. Baird and Fitts (1957)discussed their application in field experiments. Also, Tramel (195%) developed the triplecube design, a modifkation of the composite design, which requires a minimum of 31 treatment combinations for a three-variable experiment. Another design being used is the interlaced factorial developed by C. G. Hildreth of Michigan State University. To the authors’ knowledge, a description of this design has not been presented in the literature. Composite designs may not be as readily adaptable to exploration of response surfaces in agricultural research as they are in industrial research. The use of these designs in agronomic research is still in the trial phase. Whether or not they will receive widespread acceptance remains to be seen, but indications are that their use is increasing. Where multifactor experimentation is necessary, composite designs seem to hold definite promise. However, complete factorials are needed to compare estimates found when all of the treatment combinations are used with those found when only the treatment combinations of the composite design are used. et

ANALYSESOF B. ECONOMIC

YIELDDATA

Currently, the fitting of yield curves and calculation of fertilizer recommendations is being done primarily by agricultural economists. As with earlier work, much of the current research is concerned with the selection of equations that approximate the “true” yield function. Although the selection of an adequate equation is undoubtedly impor-

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tant, other factors such as cropping sequence, weather, residual fertility should also be considered, This section will review recent research on these phases of the economics of fertilizer use and present some references for the differing approaches. 1. The Spillman Equation

This type of equation is widely used, both in the form developed by Mitscherlich and the form developed by Spillman. Paschal (1953) presented a graphic method of approximating the Spillman equation and applied an analysis to alfalfa yields resulting from P2Oa applications. Ibach and Mendum (1953) described an extension of the graphic method permitting the estimation of equations containing more than one nutrient variable and applied it to pasture-yields data from a 4 X 3 X 3 factorial experiment with nitrogen, P20a, and K20. Ibach (1953) compared the use of a Spillman equation, a quadratic equation, and a power equation ( Y = axb),sometimes called the Cobb-Douglas function, for predicting fertilizer recommendations. Paschal and Evans (1954) presented further applications of the Spillman equation determining the profitability of nitrogen fertilization of irrigated sorghum. Paschal and French (1956) used the Spillman equation in the economic evaluation of nitrogen fertilization in several irrigated-corn experiments. The Oregon experiment described in Section 11, B, 1 was among these. They used an iterative method of least squares developed by Stevens (1951), which requires an initial estimate of R, to estimate parameters of the equations for experiments containing enough application rates to characterize the response curve. French (1956), in a further analysis of data presented by Paschal and French ( 1956), compared the Spillman equation, the quadratic equation, the “square root” equation ( Y = a bx1I2 c x ) , and the Gompertz curve ( Y = e(M-ARO)) . He also compared equations estimated from fewer than the twelve fertilizer rates included in some of the experiments and concluded that at least five or six adequately spaced fertilizer applications should be used. Ibach (1958) presented further information on graphic estimation of the Spillman equation, and Ibach and Mendum (1958) further developed graphic and least-squares analyses to estimate parameters for this equation. Having estimated the yield curve, the above workers calculated fertilizer recommendations in accordance with the procedures described in Sections 11, B, 1 and 11, B, 2. It should be noted, however, that most of the analyses using the Spillman equation have dealt with yield response to only one nutrient.

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2. Other Types of Equations Many workers have used equations other than the Spillman or Mitscherlich function. The quadratic and square-root equations are frequently used. These equations are linear in the coefficients and may be readily estimated by least-squares regression procedures. Heady and Pesek (1954) and Heady et al. (1955) presented an application of production economics techniques to yield data from experiments conducted in Iowa. These experiments were specscally designed for economic interpretation. Isoquants, isoclines, and optimum recommendations were derived using quadratic and square-root yield curves for corn, red clover, and alfalfa. The Iowa corn experiment discussed in Section 11, B, 2 is an example of this research, An important finding of these workers was that the ratio of nutrients in least-cost nutrient combinations varied with yield levels. This suggests that different nutrient ratios should be used, depending upon the yield desired. Pesek et al. (1959) applied an economic analysis of corn yields resulting from varying rates of nitrogen and different plant populations. They found that, in order to make high rates of nitrogen profitable, the plant population had to be high. The optimum number of plants per acre was dependent upon the price of nitrogen and corn rather than the cost of seed corn required to increase plant population. Knetsch (1956) and Knetsch et al. (1956) analyzed corn-yield response to nitrogen on a Kalamazoo sandy loam soil in Michigan. These workers utilized the equation (Y = aXbc'") which has been described by Halter et al. (1957). Sundquist (1957) and Sundquist and Robertson (1959) presented the results of an economic analysis of yield curves for oats, wheat, and beans. In this work, the equation used by Knetsch (1956) was compared with square-root and quadratic equations. It was found that, although the former equation characterized the observed yields equally as well as the latter equations, it required lengthy calculations to derive fertilizer recommendations. Other research using quadratic and square-root equations has been reported by Brown et al. (1956), Woodworth et al. (1957), Orazem and Smith (1958), and Doll et al. (1958). 3. A Discrete Analysis

To avoid the problem of selecting an appropriate equation to represent the yield-response curve, Hildreth (1954) developed a method of estimating discrete points along the yield curve or surface. This method estimates the yields to be expected from each fertilizer application used in the experiment. By using the discrete analysis, an investigator 'need

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not select a continuous equation and may avoid possible biases that could result from selecting an inappropriate equation. The estimates are obtained by the method of maximum likelihood subject to the restriction of diminishing returns. Therefore, the yield increase caused by any fertilizer increment must be equal to or smaller than the yield increase caused by the preceding fertilizer increment. A curve drawn through the yields estimated by this method would be concave to the abscissa. Estimates of yields resulting from nutrient inputs falling between any two application rates included in the experiment must be obtained by interpolation. Stemberger (1956, 1957) applied Hildreth's discrete analysis and a continuous production function analysis to the same set of experimental data. A study of these data suggests that differences among fertilizer recommendations derived by the two methods are slight. Also, Stemberger indicated that computations involved in the discrete analysis were long and tedious.

4. A Rotation Analysis Stritzel (1958) presented an analysis of a corn, oats, meadow, meadow rotation. The experiment was a 5 x 4 x 3 NPK factorial, in a randomized block design replicated twice. Some plots were top-dressed with nitrogen and PzOa during the rotation to compare initial and residual responses. Using quadratic, square-root, and cubic equations, Stritzel analyzed yield response for each year of the rotation. An economic analysis of the corn yields was conducted. Nitrogen, Pz05,and KzO yield of nutrient curves were used to evaluate nutrient uptake for some of the crops in individual years. The results for the complete rotation were analyzed by converting crop yields into total digestible nutrients and also by converting them into total value products, a measure of gross income. For both methods, crop yields in the second, third, and fourth years were discounted or reduced to account for uncertainty and time considerations. Equations were then estimated from the four-year totals and an economic analysis was conducted. For relevant price combinations, yield isoclines calculated from the four-year total digestible nutrient surface were almost perpendicular to the Pa06 axis of the isoquant map. This differs from the isoclines estimated for individual years and indicates that P206should be applied at approximately the same rate regardless of nitrogen applications, or that minimum cost combinations would include the same amount of P205 regardless of yield levels. This seems consistent with the concept that PzOa, due to its role in plant nutrition, should be maintained at a high level regardless of application rates of other fertilizers ( E. C.Doll, 1958).

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5. Weather Variability

One important cause of year-to-year variations in yield response to fertilizer applications is weather, Brown and Oveson (1957, 1958) have discussed variations in the response of spring wheat to nitrogen applications over a ten-year period. They demonstrated the use of a yield curve representing the average response for the ten years. Climate and rainfall variations are undoubtedly major causes of the differences in the yearly response functions they present. Orazem and Herring (1958) have analyzed the effects of soil moisture at seeding time, rainfall during the growing season, and nitrogen on grain sorghum yields in southwestern Kansas. Six years’ data were reported. Soil moisture at seeding time was found to have the largest effect on yields, and the effectiveness of rainfall during the year increased as initial soil moisture increased. Knetsch and Smallshaw (1958), using a drought criterion developed by Van Bavel (1953) which considers the daily precipitation, the daily moisture loss, and the water-holding capacity of the soil, computed the drought incidence in the Tennessee Valley for a thirty-year period. Using the same drought criterion, Knetsch and Parks (1958) analyzed an experiment with varying rates of nitrogen and irrigation water. A continuous yield curve, expressing Starr Millet yield as a function of nitrogen and drought, was calculated. Using the yield-response data and weather records for the area, they predicted the probability of the occurrence of different drought intensities and the anticipated response to nitrogen. An analysis which enabled probability estimates to be placed upon response to fertilizers was applied to the results. The nitrogen application that returned the most profit when averaged over all drought conditions was considered to be the most profitable application in the long run. Optimum rates of nitrogen and irrigation were also derived. Although Knetsch and Parks state that their data are not conclusive, their results indicate that average profits over time varied only slightly for nitrogen rates between 120 and 210 pounds per acre. The analysis, however, did not adequately account for nutrient accumulation or depletion over time. 6. Discussion

The mathematical approximation of the yield curve should be as representative as possible of the “true” yield relationship. Also, these curves should be, within limits, easily estimated and analyzed. Quadratic, square-root, or other equations linear in the coefficients seem most nearly to meet these objectives. These equations can be easily estimated by

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least-squares regression procedures and readily lend themselves to economic analysis. J. P. Doll (1958) has described various types of equations, linear in the coefficients, which might be used. Kenneth W. Meinken of Rutgers University, in an unpublished manuscript reviewed by the authors, has used a family of curves of the form (Y = a bx'" &). In the analysis of twenty-eight sets of experimental data, t h i s type of function appeared to be better than those previously used. Meinken also proposed a different approach to the concept of interaction. At the present time, very little of the reported research on the economics of fertilizer use has dealt with the variations of yield curves over time. One problem in this regard is that completely adequate research methods are not available for analyzing this type of data, even when it is available. And, as suggested by Kempthorne ( 1957), problems in the statistical analysis of such data are not small. Recommendations derived from yield curves for a single year are point estimates of unknown reliability. One of the goals of research in the economics of fertilizer use should be to estimate the probable returns from a given fertilizer application for an individual year and to estimate the returns from that application rate if it were used over time. This is not to say that problems related to depicting and analyzing yield responses within individual years are not important, because the whole can only be as accurate as its individual parts.

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C. INITIALSOIL FERTILITY CONSIDERATIONS Soil tests, along with past-management information, are generally being used to provide guidance in recommending rates of fertilization farmers should use. These tests are effective for making such recommendations only after ample experiments have been conducted in the field and the resulting yields of different crops to rates of fertilization have been related to soil-test values. In other words, as Bray (1948) has stated, ''. , . the test must measure the total amount of the available nutrient . , the crop must be allowed to indicate the significance of that amount in terms of growth and response to the added nutrient." Various methods have been used to relate soil-test values to crop-yield response to fertilization. Few, however, have considered economic levels of fertilization in conjunction with soil tests or the simultaneous effect of more than one soil-test nutrient in analyzing experimental results. Both of these are important considerations if recommendations are going to be made to farmers on the basis of soil tests and supplemental information. With regard to relating soil-test information to economic levels of fertilization, Hanway and Dumenil (1955) used nitrogen soil-test results and corn response to nitrogen fertilization for predicting the most profit-

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able rate of nitrogen. These workers used a modification of the Mitscherlich equation to relate yield increases from nitrogen applications to the soil-test values. Utilizing corn:nitrogen price ratios, they devised a graph to estimate the most profitable rate of nitrogen application, given the soil-test value. Other functional models could be used in analyses of t h i s kind and, if adequate data were available, extended to include several nutrients simultaneously. If the latter were done, perhaps variations in results now attributed to error could be attributed to the appropriate variables. Pesek (195s) has proposed a functional analysis that considers both the soil and fertilizer nutrient components. He suggests that the soil-test values be used for the soil components in the yield equation. Hildreth (1957) has proposed a somewhat similar approach, If a generalized function were found to exist, these proposed methods would predict the most profitable rates of nutrient application, if the soil-test values and price relationships were known. It is possible that an experimental model such as this could be further improved if weather data were incorporated into the analysis. The possible existence of a generalized production function, a continuous response curve for different initial fertility levels of a soil type, is currently being investigated in greenhouse research at Iowa State College. The theoretical considerations and results of preliminary research have been presented by Jensen and Pesek (1959a, b). Among the findings thus far, indications are that for certain fertility levels on a given soil type, a generalized function may exist. Also, as might be expected, the results indicate that interactions probably occur between the soil and fertilizer nutrients. By regression analysis, crop yields from unfertilized soils, as well as fertilized, may be analyzed as dependent variables, with the different soil-test values as independent variables. Such analysis can aid in determining nutrients that should be evaluated by soil tests. They can also be used to reflect the contribution nutrients make toward yield, and they indicate possible improvements in soil tests (Eid et al., 1954; Black, 1955). D. D. Mason and others at North Carolina in unpublished results have used four regression equations to relate corn yields from fertilized plots to soil-test values. Different combinations of' soil-test values were included as independent variables. The soil-test variables initially included were pH, Ca, Mg, P, K, and organic matter. Of the four regression models studied, little difference existed in the amount of variation that could be accounted for by regression analysis. Care should be exercised in the interpretation of such analyses because, in some cases, soil-test values of certain nutrients are highly correlated. Unless this is taken into con-

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sideration the results of such analyses can be biased and lead to erroneous conclusions. IV. Conclusions

The authors believe that certain aspects of crop production research need greater emphasis. Among these are: 1. Additional variables, such as climatic factors and soil properties, should be measured and included in the analyses. The purpose of their inclusion should be to estimate average returns over time, as well as probable returns in a given year, resulting from varying amounts of fertilizer. 2. Analyses should be conducted on the physical, chemical, and biological changes that occur in soils under continuous and rotational systems of cropping. The introduction and widespread use of commercial fertilizers has apparently affected current thinking on rotations; that is, crops that add nutrients to the soil are no longer as important as they once were. Therefore, a quantification of the nutrient additions and depletions caused by different cropping sequences under various levels of fertilization appears to be useful. 3. The “fertilize the soil” and “fertilize the crop” concept should be compared and analyzed. The conditions under which each of these concepts is the most useful should be explored further. 4. Soil-test values should be utilized along with other measurable variables in an attempt to determine generalized yield equations for soils. 5. The economics of various recommended practices should be studied in conjunction with fertilization rates. Many of these practices would increase yield response to fertilizers and would usually involve little risk to farmers. Consideration of the above would add a time dimension to the economics of fertilizer use and base the yield curve on a dynamic foundation rather than a static, single-year foundation. Because yield curves constitute one of the basic elements of farm planning, casting them in a dynamic setting would, in turn, more adequately permit the consideration of time in farm-planning studies. The use of yield curves in determining the profitability of fertilizer applications has been outlined and discussed. Needless to say, all facets of this subject have not been considered. A general review of the economics of fertilizer use has been presented by Nelson and Ibach (1957). Also, the readers are referred to North Central Regional Publication No. 54 (1954),“Profitable Use of Fertilizer in the Midwest,” and to two books based on seminars sponsored by TVA, “Methodological Procedures in the

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Analysis of Fertilizer Use Data” (Baum et aZ., 1956) and “Economic and Technical Analysis of Fertilizer Innovations and Resource Use” ( Baum et al., 1957) for more extensive coverage of the subject. Finally, a comprehensive bibliography is scheduled to be prepared soon by the Agricultural Research Institute’s Committee on the Economics of Fertilizer Use for the Agricultural Board of the National Research Council. The outstanding membership of this Committee insures the preparation of an excellent publication concerning the various aspects of the economics of fertilizer use.

ACKNOWLEDGMENTS The major portion of this paper was prepared while the authors were on the staff of the Division of Agricultural Relations, Tennessee Valley Authority, Knoxville, Tennessee. We wish to thank Dr. L. G. Allbaugh, Director of the Division, Dr. E. L. Baum, Chief of the Agricultiual Economics Branch, and Dr. George Stanford, Chief of the Soils and Fertilizer Research Branch, for their aid and encouragement during the time this paper was being prepared.

REFERENCES Baird, B. L., and Fitts, J. W. 1957. In “Economic and Technical Analysis of Fertilizer Innovations and Resource Use” (E. L. Baum, E. 0. Heady, J. T. Pesek, and C. G. Hildreth, eds.), pp. 135-143. Iowa State College Press, Ames, Iowa. Baum, E. L., Heady, E. O., and Blackmore, J., eds. 1956. “Methodological Procedures in the Analysis of Fertilizer Use Data.” Iowa State College Press, Ames, Iowa. Baum, E. L., Heady, E. O., Pesek, J. T., and Hildreth, C. G., eds. 1957. “Economic and Technical Analysis of Fertilizer Innovations and Resource Use.” Iowa State College Press, Ames, Iowa. Bade, B. 1918.Landwirtsch. Jahrb. 51,363-385. Black, C. A. 1955.Iowa State Coll. J . Sci. 30, 1-11. Box, G.E. P. 1954.Biometdcs 10,16-60. Bray, R. H. 1948. In “Diagnostic Techniques for Soils and Crops” (H. B. Kitchen, ed.), pp. 59.American Potash Institute, Washington, D. C. Brown, W. G., and Oveson, M. M. 1957. Oregon State Coll. Agr. Expt. Sta. Circ. No. 583. Brown, W. G., and Oveson, M. M. 1958.J. Farm Econ. 40,451457. Brown, W. G., Heady, E. O., Pesek, J. T., and Stritzel, J. A. 1956. louu Stute Coll. Agr. Expt. Sta. Bull. 441. Browne, C. A . 1942. I n “Liebig and After Liebig” (F. R. Moulton, ed. ), pp. 71-82. Science Press Printing Co., Lancaster, Pennsylvania. Cochran, W. G.,and Cox, G. M. 1957.“Experimental Designs,” 2nd ed. Wiley, New York. Dean, L. A., and Fried, M. 1953.Agronomy 4,43-58. Doll, E. C. 1958.Better Crops with Plant Food 42, 26-30. Doll, J. P. 1958.Ph.D. thesis, Iowa State College, h e s , Iowa.

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Doll, J. P., Heady, E. O., and Pesek, J. T. 1958.Iowa State Coll. Agr. Expt. Sta. Bull. 469.

Eid, M. T., Black. C. A., Kempthorne, O., and Zoellner, J. A. 1954. Iowa State Coll. Agr. Expt. Sta. Bull. 406. Farden, C. A., and Magistad, 0. C. 1932.Agron. I. 24, 984-974. French, B. L. 1958.I. Farm Econ. 38,736-747. Hader, R. J., Harward, M. E., Mason, D. D., and Moore, D. P. 1957. Soil Sci. SOC. Am. Proc. 21,59-74. Halter, A. N.,Carter, H. O., and Hocking, J. G. 1957.J. Farm Econ. 39,986-974. Hanway, J,, and Dumenil, L. C. 1955.Soil Sct. SOC. Am. Proc. 19,7740. Heady, E. O.,and Pesek, J. T.1954.J. Farm Econ. 36,486482. Heady, E. O.,Pesek, J. T., and Brown, W. G. 1955. Iowa State Coll. Agr. Ezpt. Sta. Bull. 424. Hildreth, C. G. 1954.J. Am. Statist. Assoc. 49,598-819. Hildreth, C. G. 1957. In “Economic and Technical Analysis of Fertilizer Innovations and Resource Use” (E. L. Baum, E. 0. Heady, J. T. Pesek, and C. G. Hildreth, eds.), pp. 176-188. Iowa State College Press, Ames, Iowa. Hutton, C. E., Robertson, W. K., and Hanson, W. D. 1958.Soil Sci. SOC. Am. Proc. 20,531537. Ibach, D.B. 1953.J. Farm Econ. 35,938-952. Ibach, D.B. 1958. U.S. Dept. Agr. ARS 43-69. Ibach, D.B., and Mendum, S. W. 1953. U.S. Dept. Agr. Farm Management Ser. 105. Ibach, D.B., and Mendum, S. W. 1958.J. Fawn Econ. 40,489-478. Jensen, D., and Pesek, J. 1959a.Agron. J. 51,255-259. Jensen, D., and Pesek, J. 1959b.Agron. J. 51,259-283. Johnson, G. L. 1958. In “Methodological Procedures in the Analysis of Fertilizer Use Data” (E. L. Baum, E. 0. Heady, and J. Blackmore, eds.), pp. 2238. Iowa State College Press, Ames, Iowa. Kempthorne, 0. 1957.Advances in Agron. 9,177-204. Knetsch, J. L. 1958.M.S. thesis, Michigan State Univ., East Lansing, Michigan. Knetsch, J. L., and Parks, W. L. 1958. Report T 59-1 AE, Tennessee Valley Authority. Knetsch, J. L., and Smallshaw, J. 1958. Report T 58-2 AE, Tennessee Valley Authority. Knetsch, J. L., Robertson, L. S., Jr., and Sundquist, W. B. 1958. Quart. Bull. Michigan Agr. Expt. Sta. 39, 10-18. Lang, E. 1924. In “The Law of Diminishing Returns” (W. J. Spillman, ed.), pp. 81-175. World Book, New York. Magistad, 0. C., Farden, C. A., and Lambert, C. B. 1932.Agron. J. 24, 810-822. Mason, D. C. 1957. J. Farm Econ. 39, 370-381. Mitscherlich, E. A. 1909. Landwirtsch. Jahrb. 38, 537552. Mitscherlich, E. A. 1947. 2.Pfinzenerniihr. Diing. Bodenk. 38, 22-35. Moulton, F. R., ed. 1942. “Liebig and After Liebig.” Science Press Printing Co., Lancaster, Pennsylvania. Nelson, L. B., and Ibach, D. B. 1957. In “Soil-The 1957 Yearbook of Agriculture” (A. Stefferud, ed.), pp, 287-278. U. S. Government Printing Office, Washington, D. C. North Central Farm Management Research Committee’s Subcommittee on Economics of Fertilizer Use. 1954.Wisconsin Unto. Agr. Expt. Sta. Bull. 108. Olson, R. A., and Dreier, A. F. 1958. Soil Sci. SOC. Am. Proc. 20, 509-514. Orazem, F., and Herring, R. B. 1958.J . Famz Econ. 40, 847-708.

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Orazcm, F., and Smith, F. W. 1958.Kunsas Agr. Expt. Sta. Bull. 94. Ostle, B. 1954.“Statistics in Research,” pp, 149-151. Iowa State College Press, Ames, Iowa. Paschal, J. L. 1953. U.S. Dept. Agr. F . M . 104. Paschal, J. L., and Evans, C. E. 1954.Soil Sci. SOC. Am. Proc. 18,454-458. Paschal, J.L.,and French, B. L. 1956.U.S. Dept. Agr. Tech. Bull. 1141. Pesek, J. T. 1956. In “Methodological Procedures in the Analysis of Fertilizer Use Data” (E. L. Baum, E. 0. Heady, and J. Blackmore, eds.), pp. 101-112. Iowa State College Press, Ames, Iowa. Pesek, J. T., and Heady, E. 0. 1958. Soil Sci. SOC. Am. Proc. 22, 419-423. Pesek, J. T., Heady, E. O., Doll, J. P., and Nicholson, R. P. 1959. Iowa State Coll. Agr. Ex@. Sta. Bull. (in press). Pimentel-Comes, P. 1953.Bwmetrics 9,498-516. Rauterberg, E. 1939.Bodenk. Pflanzenerniihr. 14, 10-28. Ricardo, D. 1951. “The Works and Correspondence of David Ricardo” (P. Scaffa, ed.), Vol. I. Cambridge Univ. Press, London and New York. Robertson, L. S., Johnson, G. L., and Davis, J. F. 1957. In “Economic and Technical Analysis of Fertilizer Innovations and Resource Use” (E. L. Baum, E. 0. Heady, J. T. Pesek, and C. G. Hildreth, eds.), pp. 226-240. Iowa State College Press, Ames, Iowa. Russell, E. J. 1950. “Soil Conditions and Plant Growth,” 8th ed., p. 13. Longmans, Green, New York. Spillman, W. J. 1924. In “The Law of Diminishing Returns” ( W. J. Spillman, ed. ) , pp. 1-77. World Book, New York. Spillman, W. J. 1933. U.S. Dept. Agr. Tech. Bull. 348. Stemberger, A. P. 1956. Ph.D. thesis, North Carolina State College, Raleigh, North Carolina. Stemberger, A. P. 1957.North Carolina State CoZZ. Agr. Expt. Sta. BuU. 126. Stevens, W.L. 1951.Biometrics 7,247-267. Stritzel, J. A. 1958.Ph.D. thesis, Iowa State College, Ames, Iowa. Sundquist, W. B. 1957. Ph.D. thesis, Michigan State University, East Lansing, Michigan. Sundquist, W. B., and Robertson, L. S., Jr. 1959. Michigan State Univ. Agr. Expt. Sta. Tech. Bull. 269. Tramel, T. E. 1957a.1. Farm Econ. 39,790-793. Tramel, T. E. 1957b. In “Economic and Technical Analysis of Fertilizer Innovations and Resource Use” (E. L. Baum, E. 0. Heady, J. T. Pesek, and C. G. Hildreth, eds.), pp. 168-175. Iowa State College Press, Ames, Iowa. Van Bavel, C. H. M. 1953.Agron. ].45,167-172. Van Der Pauuw, F. 19521953.Plant and Soil 4,97-106. Willcox, 0.W.1947.Agron. I. 39.7477. Willcox, 0. W. 1955.Soil Sci. 79,467473. Woodworth, R. C., Proctor, R. E.. Burton G. W., and Mackie, A. B. 1957. Georgia Agr. Expt. Stus. Bull. 13.

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RECENT DEVELOPMENTS IN AGRlC ULTURAL MACHINERY

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United Stater Deparfmenf of Agriculture. Belfrville. Maryland

I . Introduction . . . . . . . . . . . . . . I1. Developments in Tillage and Seedbed Preparation . . . A. Conventional Equipment and Practices . . . . B . Special Tillage Equipment and Practices . . . . C. Seedbed Finishing Tools . . . . . . . . I11. Developments in Planting Equipment . . . . . . A. Row-Crop Equipment . . . . . . . . . B. Grain and Seed Drills . . . . . . . . . C Aerial Seeding and Fertilizing . . . . . . . IV Developments in Cultivating Equipment . . . . . . A . Mechanical Cultivation . . . . . . . . . B Flame Cultivation . . . . . . . . . . C. Equipment for Applying Chemical Herbicides . . D Thinning Equipment . . . . . . . . . V . Developments in Spraying and Dusting Equipment . . . A . General Developments . . . . . . . . . B . Spraying Equipment . . . . . . . . . . C . Dusting Equipment . . . . . . . . . . VI . Developments in Harvesting Equipment . . . . . . A. Forage Harvesting Equipment . . . . . . . B . Corn Harvesting Equipment . . . . . . . C. Grain, Legume. and Grass Seed Harvesting Equipment D . Cotton Harvesting Equipment . . . . . . . E . Dry Bean and Pea Harvesting Equipment . . . . F Vegetable Harvesting Equipment . . . . . . G. Root Crop Harvesting Equipment . . . . . . H . Tree Nut and Fruit Harvesting Equipment . . . I. Miscellaneous Crop Harvesting Equipment . . . VII . Conclusions . . . . . . . . . . . . . . References . . . . . . . . . . . . . .

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manufacturers. Most rapid progress is made when all disciplines combine their efforts into well-planned and executed programs. As new crop management ideas are developed through the research process, machines must be developed to meet the specific need. One new management practice may require only a minor modification of an existing machine, i.e., a different type of moldboard, a means of-placingfertilizer closer or farther away from the seed, or a gage wheel to give greater precision in the height of cut in a harvesting operation, Another new management practice may necessitate the development of an entirely new machine specifically designed to do a specific job. The constant-volume pump for liquid fertilizer application, the intercrop drill, and the castor bean harvester are examples of this type of evolution. Sometimes the machinery development process takes an opposite pattern. An improved mechanical linkage or a more rapid hydraulic control mechanism may make it feasible to step up drastically the operating speed of a certain machine. But, before the new speed can be safely utilized, new crop management practices must be developed-perhaps a simple land smoothing before planting, or use of a modified bed shape that will facilitate the use of this potential speed increase. Thus, progress in agricultural machinery development is closely interwoven with progress in the soil and plant management fields. When new scientific break-throughs are made in one field, new evidence of progress is certain to be seen in the related fields. There are few phases of agriculture that have seen such rapid growth and development as that of agricultural machinery. Tractor and machine development during the last fifty years has been the major key to the present high level of American agricultural production per unit of farm labor input. The history and romance of this development have been the subject of many excellent summaries. The works of Gray (1954, 1958) and Gray and Dieffenbach (1957) trace the history and development of the tractor. Fifty years of progress in farm machinery was reported by McColly (1957) and has also been summarized in a series of U.S. Department of Agriculture publications 1935a,b,c,d, 1949a,b). McKibben ( 1953) significantly points out that the absence of a serf or peasant class in American agriculture provided a major tribute to this technological advance. Much of the impetus to mechanization has been provided by certain economic and sociological trends in the course of the Nation’s development. Nutt (1950b) points out the factors guiding the mechanization of the Southeast, while Carreker (1950)illustrates the relation between the development of new conservation practices, such as terracing, strip cropping, mulch tillage, and grass-based rotations, and the parallel shifts in

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the design and application of farm machines to meet this new trend. The over-all value of this technological development to the farmer was summed up by Walker (1952) in terms of increased workers efficiency, increased yields, and higher standards of living. Another important phase of early machinery development has been the splendid effort of engineers, industrial and public agencies alike, to foster a common goal for standardizing equipment. Our present-day ability to widely interchange parts, components, and whole machines, has been of tremendous importance. The intricate, but effective, network of committees and commissions that have made this possible are outlined by Tanquary ( 1957). These extensive publications reviewing early agricultural machine development have led the authors to limit their discussion to the more significant developments since 1950. Where there has been sufficient evidence of successful research progress on equipment not yet in production, the authors have suggested possible trends that might be anticipated through 1960 or shortly beyond. II. Developments in Tillage and Seedbed Preparation

A. CONVENTIONAL EQUIPMENTAND PRACTICES

The plow has been considered the basic implement for seedbed preparation for many centuries. The review of McColly (1957) stresses the drastic changes that took place as the walking plow was replaced by the sulky plow, and then progressed through various changes in general design until today’s modern lift-type tractor-mounted plows were developed. Another early and important break-through was the development of soft-centered steel for shares and moldboards providing a smooth effectively scouring surface on a tough shock-resistant base. Metallurgical developments continue to play an important part in the improvement of tillage equipment. Reed and Gordon (1951) and Mohsenin et al. (1956) have made important studies on the relative wear resistance of various metals for use in plows, disks, and other tillage tools. A number of workers are currently exploring the possibilities of coating tillage tools with various materials such as Teflon to improve scouring action and to reduce draft. Although there have been few startling changes in the basic design of tillage tools, there has been a growing comprehension of the soil physical and dynamic factors that must be considered in all attempted improvements of these basic tools. Agricultural engineers and soil physicists have teamed up in the research that has shown the relation between soil structure and consistency on implement design as reported by Nichols

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and Reaves (1955).Soil reaction to plowshare design is described by Nichols et al. ( 1958),and its reaction to subsoiling equipment is shown by Nichols and Reaves (1958).The improved means of measuring this compaction factor have been made possible by the development of the straingage cell by Cooper et nl. (1957).More general concepts of the role of soil physics have been illustrated by Browning (1950). The effectiveness of various tillage implement forms and practices in achieving good mixing of soil was successfully evaluated through use of tracer techniques by Hulburt and Menzel ( 1953). Individually, none of these findings has resulted in radically new plow design; collectively, however, they have resulted in a new awareness of the specific criteria of design. With new understanding of the effects of adhesion, shear, and pressure translocation, designers have new means for improving the efficiency of plow design. The way a plow is mounted, adjusted, and controlled also contributes to its over-all effectiveness. In recent years there have been many advances in hitch and mounting design that give more effective results. Heitshu (1952) presents an exhaustive analysis of the kinematics of tractor hitches as they relate to mounted plows, disks, and subsoilers. The ability to provide weight transfer either to the tool for improved penetration or to the tractor for increased traction is a major advance. Collins (1951) and Tanquary and Clyde (1957) further describe the factors in hitch design that affect weight balance, side thrust, and suction control. The hydraulic capacity requirements for controlling these implements have been further analyzed by Worthington and Seiple ( 1952). These major developments in hitches and implement mounting and control systems are the cause for the growing popularity of mounted tillage tools. Integrally mounted on the tractor, these plows can be adjusted and controlled more effectively. This is particularly important in conservation farming where plows must be lifted to protect grassed waterways and terrace outlets and where implement position control is important in following the contour layouts in terraced and strip-cropped fields. With the added precision in control, plus the built-in safety devices, higher operating speeds, particularly in turning on headlands, is possible. The use of two-way or reversible plows that permit turning furrows in one direction, without leaving dead furrows or back furrows, has become increasingly important. They are particularly valuable in maintaining a uniform surface in irrigation borders and in maintaining uniform surface configurations under strip cropping and terracing practices. The higher cost of these plows is offset by the reduction in special smoothing and leveling steps that must be used to remove dead furrows made by

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conventional equipment. The upslope plowing with this equipment compensates for the natural downslope soil movement, particularly on the steeper slopes. Where this has been practiced, more intensively cropped rotations can be more safely used than when downslope plowing is practiced. Similar advances have been made in the design of disk plows and harrows. Again metallurgy has played a significant role. Studies by Reed and McCreery (1954) showed that disk life was closely related not only to the type and hardness of steel used, but also to the directions that the steel blank was rolled during its fabrication. By cross rolling, its resistance to cracking and chipping could be greatly increased. The type of edge and the method of mounting on the hub also affected disk life. A review of recent commercial advertisements indicates that these findings have been quickly incorporated in nearly all industrial designs. Further improvements have been the result of new understanding of the relationship between soil reaction and disk geometry as reported by McCreery and Nichols (1956) and Thompson and Kemp ( 1958). These workers have shown the relation between disk penetration and weight, of pressure and forward motion on the shear forces acting on the soil, and of disk angle as it affects the speed of rotation. A very significant relationship between the design of the bevel on the disk edge and its effects on soil compaction was discovered. Each of these factors when incorporated into a new design will result in cleaner, more rapid, and more effective cutting by disk implements. Both Kramer (1955) and Clyde (1956) have made important contributions to a basic understanding of offset and conventional disk harrow design. Through an analysis of the dynamic forces involved in the operation of these implements, they have proposed means of providing for simpler construction and improved durability. Added flexibility and greater range in adjustment to meet soil variables have also been proposed. Another important aspect of harrow and disk plow design has been the tremendous improvements in bearing design. Howe and Raley (1958) stress the impact that prelubricated and “lifetime” lubricated bearings have on the life of equipment that must operate under severe dust and shock conditions. For example, a triple-sealed prelubricated bearing system on a harrow that had recently completed disking 2200 acres of Arizona sand was without appreciable wear. Other changes in design of the so-called conventional tillage equipment are imminent. For example, Brown (1957) reports on a design for individual spring release beams for tractor-mounted moldboard plows. Tests show that it takes at least 1 minute to rehitch a conventional breakaway plow as opposed to only 8 seconds to relatch a spring release

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beam. Plows equipped with this type of beam are already commercially available. With the continuing trend to increased power and speed for tillage operations, this has an important economic aspect. Draft reduction is an objective of most mechanical designers. Gunn and Tramontini (1955) in preliminary studies have shown the possibility of reducing draft by oscillating the tillage implement. EQUIPMENT AND PRACTICES B. SPECIALTILLAGE In recent years there has been a growing interest and awareness, on the part of agronomists, engineers, and soil scientists, of the question of what is the optimum in tillage and seedbed preparation. The objectives of conventional tillage have been basically to stir and loosen the soil and to control the weeds. While this has generally been effective in creating a satisfactory medium for plant growth, it has also, in many instances, resulted in destruction of soil structure, reduced infiltration capacity, increased susceptibility to erosion, accelerated reduction of organic matter, and other evidences of soil decline. It is with this in mind that Melsted (1954) asks the question: “How should cultivated crops, especially row crops, be tilled and managed so that they will become soil conserving?” He goes on to establish the importance of studying new concepts in tillage practices that will (1) achieve erosion control, ( 2 ) maintain organic matter, (3) control weeds, and ( 4 ) provide optimum soil tilth for plant growth. During the period following World War I1 literally hundreds of studies have been conducted that compared various forms of mulch tillage, trash plowing, balk tillage, ridge planting, minimum tillage, plowplant techniques, and many others with conventional seedbed preparation methods. Summaries of many of these studies are to be found in such references as Cook and Peikert ( 1950), Jacks et al. (1955), Aldrich ( 1956), Baugh et aZ. ( 1950), Schaller and Evans (1954), Buchele et al. (1955a,b), McCalla (1958), U. S. Dept. Agr. (1958a), Moody et aZ. ( 1952), and Willard et al. (1956). These reports stress the many variables and problems that have been encountered in attempting to develop new and modified tillage practices. Further discussion of these practices will be limited to specific factors in implement design and development. Some of the early mulch tillage work in the Southeast is reported by Nutt (1950a), who used a tractor-mounted tool bar to carry a set of cut-away middlebusters with disk hillers attached to throw vegetation away from the planting furrow. This operation was preceded by heavy disking or ripping 2 or 3 weeks in advance to kill the vegetation. Regrowth in the row middle was further controlled at lay-by through use of broad, flat sweeps.

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These same general principles were later incorporated in a commercially available Mulch-Till planter described by Poynor ( 1950). This heavily constructed machine utilized a series of sweeps and rotary hoe sections to prepare planting furrows in which the crop was planted by a set of rear-mounted planters. High power requirements and problems of maintaining adjustment were frequently encountered with this device. In many areas the preceding crop that furnished the desired mulch residue was made up largely of perennials. The conventional undercutting by sweeps that had been effective with annual grain stubbles, as reported by Duley (1948, 1954), did not adequately kill perennial crops when they were substituted in the rotation in place of annual stubble crops. The resulting regrowth of the sods and other perennials, together with the other nutritional, temperature, and weed-control problems, caused, in many cases, sharp declines in crop yield-too sharp to offset the advantages of increased infiltration capacity and reduced surface runoff and erosion. This problem of managing perennial residues led to the work of Lillard et al. (1950) in which the double-cut plow principle was developed. Through use of the commercially available Oliver T-N-T plow, adjustments were made in the plow to slice free and invert the top 2% to 3 inches of sod while rather thoroughly tilling the 3- to 4-inch depth zone of soil immediately below with the plow’s extra subbase. After drying out for a period of 10 days to 2 weeks, the ribbon of inverted sod could be broken up with a field cultivator, disk, or other implement that would not too deeply incorporate it in the soil. Essentially a 100 per cent kill of the perennial residue was thus achieved. This double-cut principle formed the basis for other work by Free (1953) in which standard plows were modified by attachments. In concurrent work in Ohio, Harrold and Dreibelbis (1950) found that disking alone, or the use of field cultivator alone, would not provide the necessary kill of the vegetation. Disking in combination with herbicides showed some promise. Preplowing followed by the field cultivator after a 2-week period also gave better weed control but resulted in less surface mulch. Hays and Taylor (1958) report on similar studies in the Upper Mississippi Valley. These are but a few examples of the great number of studies made throughout the Humid Region in which attempts have been made to utilize crop residues for mulches under cultivated crops. Nearly all of the studies involved the use of standard plows, disks, or field-type cultivators, in either a modified form or in new patterns of sequence or timing. Such studies prompted many important side studies regarding the effects of these desired mulches upon soil structure, soil temperature, nutritional

T. W. EDMINSTER AND H. F. MILLER, JR. 178 balance, moisture relations and on runoff and soil loss. None of them materially contributed to development or advancement in machinery design until some more radical or drastic approaches were taken in the mid-1950’s. With the advent of the “minimum tillage” concept new machine developments have rapidly taken place. The minimum tillage approach to seedbed preparation for cultivated crops has several objectives. The first, and most obvious, is to reduce the soil compaction caused by the extra implement traffic. This results in improved infiltration through the loose surface layers and a higher level of hydraulic conductivity through the soil layers immediately beneath the surface, Both of these factors contribute to general reduction in the soil erosion and runoff hazards that occur when the field becomes too firmly packed and smoothed. In some instances a reduction in the weed population is a by-product of the minimum tillage approach. Some of the earliest work on minimum tillage was conducted at the Ohio Agricultural Experiment Station in 1935, where seedbed preparation was limited to use of a light smoothing harrow on plowed ground prior to planting. Over a fourteen-year period crop yields were essentially the same under this practice as under conventional preparation. Cook et al. (1953) reported similar results when the plow was followed by various types of packers that would smooth and firm the surface enough to permit accurate planting. Out of preliminary studies of this type came the practice of tractor-track planting, as described by Peterson et al. (1958). In this practice the tractor wheels are set to the same spacing as the planter, thus crushing down and firming the plowed field just ahead of the conventional planter. This practice has been further modified to put the planting and plowing all into one operation. It is commonly referred to as the “plow-plant” method. To accomplish this, researchers developed several machine modifications ranging from a trailing-type planter towed behind the plow, to planting units mounted on the plow frames, as described by Musgrave et al. (1955) and Aldrich and Musgrave (1955). A further development in which the planter unit is mounted on the forward cultivator bar is illustrated by Winkelblech (no date) and by Hansen et al. (1958). With this approach, one row can be planted with each pass of a 3-bottom 14 inch plow, or two rows with a 5- or &bottom plow (Fig. 1). In the development of plow-plant devices it is important to mount the planter in such a way as to assure accurate tracking of the planter shoe so that the corn row will be placed directly in the middle of the furrow slice, thus giving greater uniformity to depth-of-seed placement and seed cover. Aldrich (1956) also points out that the degree of packing

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FIG.1. Experimental “plow-plant” equipment. ( Courtesy of the Agricultural Engineering Department, Cornell University.)

that precedes the actual opening of the seed furrow can be adjusted by mounting an extra press wheel in front of the planter shoes or by the use of a specially designed planter shoe that will firm the seed bed (Fig. 2 ) . Minimum tillage can also be adapted to the use of 2-, 4-, or 6-row

FIG.2. Detail of special soil-firming shoe on furrow opener. (Courtesy of the Agricultural Engineering Department, Cornell, University.)

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planting equipment by trailing some type of compaction tool behind the plow, i.e., a culti-packer, rotary hoe (pulled backward as a treader), a spiral roller, or other seedbed finishing tools that will do a minimum of smoothing and compaction. Conventional planting equipment can then follow as a separate operation. Adapting the conventional ridge planting long used in the Southeast, Buchele et al. (1955a,b) and Lovely (1956) have proposed the ridgeplanting of corn as a further modification of minimum tillage. In this practice two 14-inch furrows are turned to each other on top of a 28-inch unplowed strip. A disk furrow opener replaces the conventional runnertype opener to provide for better trash cutting and to stabilize the position of the planter on top of the ridge. A disk cultivator is used in cultivating and maintaining the high ridge upon which the crop is planted. Each of these minimum tillage practices, while still in the research and development stage, shows considerable promise of meeting the objectives of ( 1) lower-cost seedbed preparation, ( 2 ) improved infiltration and, with it, better erosion control, and ( 3 ) reduced cultivation needs. While present studies are dependent upon shop-built equipment to provide the desired test conditions €or detailed study by soil scientists and agronomists, these practices are providing the farm machinery industry with a challenge to develop “line” models for more extensive use. The development of special tillage practices has not been limited to those for Humid Region conditions. Throughout the more arid western states modified tillage practices have long been under development and used to provide a means of protecting the soil, both under crops and during fallow periods, from the ravages of wind erosion and, in some areas, water erosion caused by rapid spring snow melt and accompanying rains. Duckfoot cultivators, “stubby” moldboard plows, and early versions of the modem sweep cultivators paved the way for the present-day equipment. Zingg and Whitfield (1957) have summarized the research on stubblemulch practices in the West and provided the early history and data showing the effect of various practices on erosion control, wheat production, soil properties, and the problems of production management. A critical analysis of the machinery requirements for stubble-mulch tillage, particularly for the Pacific Northwest, was reported by Ryerson ( 1950). Analysis of the operating characteristics and requirements of implements for wind erosion control has been made by Woodruff and Chepil (1956); and Chepil and Woodruff (1955). Krall et al. ( 1958) and Aasheim (1949) have summarized the results of various tillage practices from the standpoint of soil and water conservation and crop production under summer fallow conditions.

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The machinery-development problems for the western area are severe; i.e., equipment must be able to handle straw residue from a few hundred

to over 12,000 pounds per acre, or residue standing over 2 feet in height, under hard dry soil conditions. The tillage job is essentially one of undercutting so as to provide (1) minimum of mixing of the soil and mulch except as needed to anchor the mulch in place, ( 2 ) minimum breakage of the mulch, and (3) minimum pulverization of the soil. Tools that are in the process of development include various types of mulch pulverizers that will beat the straw into 8- to 12-inch lengths that can be handled by the tillage implements’ rotary cutters, and hammer-mill type beaters of extra heavy design, all of which have high clearance. Sweep plows measuring 5 and 6 feet with 90- to 120-degree blade angles are being developed for undercutting. Rod weeders have also been effective, particularly when they are designed with a center drive which off sets clogging problems of conventional end-drive machines. Various types of field cultivators, particularly those equipped with coil shanks that will provide added vibration to assist in clearing the shanks, are finding adaptation. In areas where it is essential to break up hard soils to permit

FIG.3. Skew treader operating in heavy wheat stubble. Note straw-chopping effect and partial incorporation of straw to provide good wind and water erosion control. (Courtesy of the Agricultural Research Service, USDA, by T. R. Horning.)

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improved infiltration of the limited moisture, the rotary subsoiler has been used. It creates a series of pits 8 to 10 inches deep that will serve as reservoirs and points of entry for moisture. Various types of treaders, frequently consisting of heavy-duty rotary hoes pulled backward, either straight or on a skew, are also used to break up surface crusts, cut and anchor mulch into the soil, and improve conditions for moisture penetration (Fig. 3 ) . Many of these specialized tillage implements have been produced by small, local, machinery companies, Each has added certain modifications to meet local real or assumed needs. This section discusses only some of the major tillage equipment. Of equal interest would be a review of developments in the special equipment lines such as the giant moldboard and disk plows capable of plowing 3 and 4 feet deep, the various developments in rotary tillage equipment, and recent trends in subsoiler design to reduce draft and increase effective soil shatter. Since each of these lines of equipment has a more limited area of application, their development will not be described here. C. SEEDBED FINISHING TOOLS The previous discussion has been chiefly devoted to the primary tillage operations of plowing and disking. Under conventional tillage management practices these operations are generally followed by various seedbed finishing operations, the nature and extent of which are dependent upon the fineness of seedbed desired. The precision seeding of fine, high-priced vegetable seed may dictate an extremely smooth, well-pulverized soil, while wheat might be drilled into a rough, cloddy, wind erosion-resistant soil on the High Plains. In recent years such conventional finishing tools as the disk and spiketooth harrows, spring-tooth harrows, floats, and drags have been supplemented by a series of specialty tools. These range from a Germandeveloped, flexible knitted-steel rod-spring-tooth harrow which, as described by Sack (1951), will conform to all surface irregularities, such as beds and furrows, to the more functional harrows, knives, packers, and rollers, each intended as a tool to break up clods and thus leave a smooth, uniform soil to accommodate the planter. 111. Developments in Planting Equipment

Recent trends in planting equipment are cumulative resultants of many separate advances in materials of construction, refinement of power controls, and improved metering-system design. Some of the trends seen in tillage equipment are also occurring in the planting lines, viz., the shift

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from pull-behind units to flexible, high-speed, tractor-mounted units, greater interchangeability of parts, and improved adjustments and controls that permit more precise operation at higher speeds.

A. ROW-CROP EQUIPMENT The factor of precision is becoming increasingly important. Uniformity in the depth of seed placement is essential if uniform germination is to be achieved. Uniformity of plant spacing, while not a critical factor from the standpoint of crop yield, is important in mechanical harvesting to assure an even flow of crop material into the harvester. The effectiveness of mechanical thinners is also dependent upon uniform planting, just as high-speed mechanical, flame and chemical weed-control practices depend on such uniformity of plant position. There has been a significant trend toward the use of bigger multiple planting units. Four- and six-row units (Fig. 4 ) are rapidly replacing

FIG.4. A six-row planter equipped to apply liquid fertilizer and granular insect and weed-control chemicals. (Courtesy of the International Harvester Co. )

two-row equipment. Where land smoothing and conditioning is practiced this trend has been most rapid. Similar trends are occurring in grain drills, both through use of additional furrow openers and through the use of multiple units operating on one hitch. This increased use of multiple planting units has created several related machine-design problems. When planting is done in multiple units the cultivation equipment must

T. W. EDMINSTER AND H. F. MILLER, JR. 184 be designed on the same basis to insure proper alignment. Equipment for side dressing, weed and insect control sprays, and other follow-up management practices must be adapted to the same multiples as the planters. With the increased length to which the tool carriers must be extended to accommodate these multiple units a problem in suspension develops. Provision must be made to carry heavy seed and fertilizer hoppers without causing changes in the depth adjustment of the planter as the weight changes with dispersal of their contents. In four- and particularly six-row equipment, provision should be made for individual suspension of the planting units to permit them to follow irregularities in the contours without affecting the planting depths. Provision for folding or otherwise retracting these long tool carriers is essential to provide for roadability and to facilitate passing through gates and into storage sheds. Precision placement of seed in the planting furrow has been the goal of agronomists and engineers alike. During recent years the intensive cotton mechanization research program has focused major attention on cotton-planting equipment. Studies to determine the relative importance of various planting techniques and spacings have been reported by Corley et al. ( 1955),Hudspeth and Jones ( 1954), Miller ( 1955a,b), and Colwick (1955). The importance of using hill-drop cotton planters on heavy soils is stressed by Miller (1955a,b). His studies showed that, where several seeds germinated close together, they could, by combined effort, emerge under more serious crusting conditions than would be possible under single-drop plantings. Closer spacing, three to five plants per foot, gives better vegetative growth control, grass and weed control through shading and improves harvester efficiency. Autry and Schroeder (1953) have made detailed studies of the design factors for hill-drop planters resulting in new concepts on cell shape, plate speed, plate-to-ground speed ratio and in tube design. The simple matter of shortening the tube length has resulted in more accurate seed drop and placement, according to Porterfield et aZ. (1954). There has been a complete redesign of the planter shoe and seed-covering devices to achieve precise control of seed depth and depth of seed cover. The development of the seed press wheel has been traced by Miller (1955a). While this device was first used as early as 1925, it did not come into prominent use until the steel rim was replaced by the zero-pressure, hollow rubber tire in the early 1950's. Its use and application is described by Tavernetti and Miller ( 1954) ( see Fig. 5). The seed press wheel principle is becoming more important on a number of other planters where seed germination is critically dependent upon intimate seed-soil contact. This contact permits quicker moisture

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pick-up giving earlier germination and emergence. This permits shallower planting and helps to combat crusting problems. Interchangeable hoppers, agitators, plates, and speed-control gears have made the modern planter extremely versatile; one basic planter meets all of the farmer’s needs. Each design improvement-sometimes insi@cant when viewed alone-has contributed to the present-day planter capable of accurate and reliable seed placement at speeds of over 4 miles per hour. Taken in total, this is a major break-through in improving farm operation efficiency.

FIG.5. Detail of planter-shoe equipped with soft rubber-tired seed press wheel. ( Courtesy of the International Harvester Co. )

The application of fertilizer as a part of the planting operation has become a universal practice. Here, as with planters, the progress in development has been the result of many minor steps. The engineering problems in fertilizer placement have been summarized by Walker (1957). He pointed out that on early models, placement of the fertilizer has been haphazard; the split boot roughly divided the application into two bands that were approximately level and theoretically to the side of the seed. As agronomists unraveled the feeding habits of seedlings and plants, the importance of precise fertilizer placement was established. Special fertilizer-disk openers, equipped with tapered prelubricated roller bearings, now give a positive furrow for fertilizer placement at any depth or distance relative to the seed that may be desired. Nearly all planters can be modified to use either dry or liquid fertilizer. Positive valves are designed to open and close automatically as the planter is raised and lowered from the carrying and planting positions. The fertilizer hoppers have changed. On early planters the fertilizer

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hopper’s capacity was about equal to the capacity of the seed hopper. Higher rates of application, longer rows and higher speeds gradually forced redesign; hoppers of one and two hundred-pound capacity thus becoming common. Their sharp angular designs are being replaced by smooth one-piece construction. Glass-fiber reinforced plastic hoppers are rapidly replacing metal units. This is a major development in meeting the corrosion problem, Plastic coating of other portions of the fertilizerplacement equipment and use of reinforced cast plastic components, while still in the research and development stage, promise further advances. (Fig. 6 ) .

FIG.6. Multiple row planter equipped with large plastic hoppers for dry-fertilizer application and special hoppers for adding other soil treatment materials. (Courtesy of the International Harvester Co.)

The introduction of many new forms of fertilizer has forced other developments. Merrill (1956) and Guelle (1954) have provided a general review of these developments. The safe and accurate transfer and metering of materials such as anhydrous ammonia has necessitated the development of a whole array of transfer systems, metering devices, and injection equipment. Hedman and Turner (1954) reported on the early developments in direct-injection metering systems, variable orifices, and on the use of the ratometer to indicate rate of flow for anhydrous ammonia. After several additional years of research, Hansen (1958) made an exhaustive report on the engineering principles involved in handling liquid materials. The multiple-discharge hose-type fertilizer pump, described by Gantt (1956) and Gantt et al. (1956), was made possible by the introduction of improved bearing designs and the development of plastic hose material that could stand up against constant flexing. Devices and schemes for actual placement of fertilizer in the soil with minimum waste and loss continues to be a challenge, A unique suggestion has been made by Arya and Pickard (1958). They suggest

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direct injection in which the kinetic energy of the material is substituted for the tractor energy now used to force the injection knives through the soil. This is an example of how far reaching the application of sound engineering principles may be in the design of agricultural machinery. In the continuing trend toward unitized operations the applications of insecticides, fungicides, nematicides, and herbicides are rapidly being combined with the planting operation. An array of tanks, pumps, valves, sprays, and injectors are being added to planting units. While each item added requires additional power, it does reduce the number of trips over a field and thus reduces labor output.

B. GRAINAND SEEDDRILLS Equipment for planting and fertilizing those crops commonly grown in narrow-spaced rows or drills has been modernized in the same way that row planters have been changed, Seed-metering devices have been improved to give positive seed spacing. Both single and double-disk openers, equipped with dust-tight prelubricated bearings, cut through trash and clods to give precise depth placement. Fertilizer, formerly loosely broadcast on the surface, is now drilled into the soil at an exact distance to the side and at a predetermined depth below the seed. Carefully designed covering blades have replaced the chain-type covers, and press wheels firm the soil around the seed. Both the grain and fertilizer hoppers have been enlarged, treated to prevent corrosion, and equipped with positive-action agitators. There is a strong trend toward the use of tractor-mounted drill equipment. Buhr (1955) notes that a mounted drill can plant a daily acreage equivalent to that of a 25 per cent larger drill towed on wheels. This increased production is due to faster field travel, more rapid turns on headlands, easier loading since the drill can be readily backed up to a truck, and smoother fields due to self-elimination of tractor wheel tracks. Each small change has had a significant bearing on the improvement of farm efficiency. More rapid and timely planting with improved percentages of germination is vital in modern farm operations. Aside from these general machine changes there have been some significant changes in the uses being made of drill equipment. The interseeding of legumes and grasses in corn at the last cultivation, as reported by Van Doren and Hays (1958) has been an important conservation and crop-management development. When interseeding is practiced the corn rows are generally spaced 60 to 80 inches apart to reduce shading and moisture competition for the interseeded crop. Since moisture may be limited at time of seeding, it is important that the soil be firmly packed around the seed. Two machines that have been found

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successful are a conventional grain drill with packer wheels attached and the cultipacker seeder. Drills have been modified by removing one or two openers to permit straddling the corn rows or by cutting off part of the machine to reduce the drill width. Specifically designed drills for interseeding are appearing on the market. Johnson (1955) and Peterson ( 1955) describe many of the machine modifications in detail (Fig. 7).

FIG.7. Interseeding alfalfa in corn up to 40 inches in height. Fenders protect corn plants. Fertilizer is applied in bands directly in front of tubes delivering legume seeds to the soil surface. Legume seed is broadcast directly in the corn row. Pressure on packer wheels may be adjusted to suit soil conditions. A mounted grain drill may be modified to provide a similar unit. (Courtesy of the Allis Chalmers Manufacturing Company.)

The seeding of small grains in permanent pasture to provide supplementary winter grazing has become another important new management practice, particularly in the South. This practice resulted in the development of a number of multiple-use drills designed to operate in heavy pasture sods where conventional drills would have been unable to provide an adequate seed furrow and proper fertilization and seed coverage. Early studies on the design of the machines are reported by

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Jones et al. (1951), Dudley and Wise (1953), and Howell and Jones (1954). Particular attention has been given to the development of this machine by Hulburt (1956), Wagner and Hulburt (1953), and U. S. Dept. Agr. ( 1956a). With growing use of rough, cloddy, and trashy fallows for wind and water-erosion protection throughout the West, new drills have been developed to operate under these severe conditions. Many of these machines are essentially standard drills that have been made heavier and equipped with larger disk openers, One unique machine that has been developed at several western locations is the blade-type drill. A hollow horizontal blade (similar to a blade weeder) is drawn through the soil at planting depth, a moving chain conveys the seed down a tube and meters it out along the trailing edge of the blade. Krall (1951) reports that while many refinements are needed to improve uniformity of seed metering and to reduce seed damage, this drill operates very well under extreme trash conditions (over 2000 pounds per acre) where conventional drills will clog. C. AERIAL SEEDINGAND FERTILIZING The use of aircraft for seeding and fertilizing rough hilly areas and wet areas has advanced tremendously in recent years. The first rice seeding by plane was reported in California in 1929, when it was necessary to replant areas that had already been flooded. Most of the rice acreage in this country is now seeded by plane, Southwell (1951) describes the use of planes for seeding burned and cut-over pasture and range land that is too rough for conventional drills. Pelleted seed has been found effective in this work. Much attention has been given to the development of special planes for this work. High payload capacity with maximum maneuverability is a major objective. Weick (1952) and Anonymous (1956a) trace the development of specialized airplane equipment both here and in England. With further refinements in seed-pelleting techniques and in improved equipment, this type of seeding and fertilization will gain in importance. The spreading of over 450,000 tons of fertilizer on approximately 4 million acres by plane during 1957-1958 in New Zealand (Anonymous, 1958) is an important indication of this trend. A comprehensive discussion of planting and fertilizer-placement equipment would be too extensive for this report. Each specialty crop has certain planter requirements that result in the development of special equipment. For example, Futral and Allen (1951) describe a special high-speed peanut planter that uses a perforated belt with sized holes to pick up, convey, and then “throw” the seeds to the ground. The

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development of graded and later monosperm sugar beet seeds required the design of special planting equipment in this industry. Burgesser (1950)points out that the use of coated and pelleted seeds will require a complete new study of planter design to assure proper metering, placement, and covering. Descriptions of these special planter developments can best be found in the literature pertaining to the individual crop under study. IV. Developments in Cultivating Equipment

Cultivation of crops generally refers to those tillage operations that are carried out after the seed has been planted. Weed control is the major objective of cultivation but in some areas, particularly where irrigation is practiced, it is intended to loosen the soil, thus improving infiltration. While the actual cultivators themselves have changed little, the ways in which they are mounted, powered, adjusted, and managed have been modified. The chemical and flame control of weeds have further modified the entire cultivation program.

A. MECHANICAL CULTIVATION It is pointed out by Bainer et al. (1955) that nearly all present-day cultivators are tractor mounted. The tricycle-type tractor with high clearance and adjustable rear-wheel tread has become the primary base for cultivating equipment. Small tractors with rear-mounted engines to provide maximum visibility have been adopted to carry cultivators for vegetable crops where extremely close and precise tillage is required. Some improvements have been made in the methods of mounting cultivators on the tractor frame. While front-mounted units are preferred because of better visibility and more responsive control from steering, rear-mounted units still have certain advantages. Quick coupling on the drawbar links and the fact that they may be set to partially remove the compaction effects of the tractor wheels, are important considerations. There is still serious need for simplification in mountings and in adjustment. This becomes more important as the number of row units increases to four-, six-, and even eight-row outfits. Quick interchange is desired for the many different cultivator tooth designs. The advances in metallurgy and in heat treatment have made possible the development of cultivator teeth that have much longer life through resistance to shock and abrasion. Williamson (1955)points out that sweeps of thin, broad-angle, low-crown design provide excellent weed cutting with a minimum of soil throwing even at high speeds. Only

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3/16 inch thick, they are self-sharpening and can be used without other care until worn out. Hydraulic controls have replaced nearly all of the mechanical lift equipment. Double-acting cylinders permit positive penetration under hard-ground conditions. In some instances the units on each side of the tractor can be lifted or lowered independently. This provides a great advantage when cultivating along grass waterways, variable-width contour strips, and terraces. In tractors equipped with both front- and rearmounted units, delayed-action valves are being used to permit the rear gang to stay in the soil until they have moved forward to the field edge, where the front gang was raised. There have been many modifications in plant-shield design. Rotary shields and floating self-adjusting stationary shields permit much closer operation to the row. To improve this further, one company has developed an electronic control mechanism that automatically senses the position of the cultivator in relation to the plant and then transmits this to an automatic power-steering mechanism. This system virtually eliminates the hazards of human error in guiding the tractor in close-cultivation work. The rotary hoe, and a number of its modifications, has been in use for many years, Recent improvements in design and construction has placed new emphasis upon its use. A major design change has been the sectional and individual suspension of the spiders in place of the single, rigid axle mounting. This has resulted in maximum flexibility, allowing the unit to conform to surface irregularities, and thus doing away with the problem of excessive depth of penetration on high spots and weeds left in depressions. Rea (1954) has shown that excellent results can be obtained with such equipment at speeds up to 18 miles per hour. R. W. Wilson (1956b) has also shown that this equipment has considerable promise in early cultivation of tobacco. Sectional units consisting of three or four rotary-hoe wheels have been mounted on conventional planters directly over the crop row in order to break crusts and dislodge small weeds that could not be reached with conventional cultivating attachments. Rotary cultivating units, in which the row middles are stirred by units driven from the power takeoff that have various-shaped knives and blades mounted parallel to the axis of rotation, have been developed for cultivating special crops. The degree of soil pulverization can be controlled by adjusting the relative speed of rotation to the rate of forward travel. There is continued effort to find better ways of controlling weed growth in the row where it is difficult to reach without injuring the crop.

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Liljedahl et n2. (1956) proposes the use of heat, ultrasonic and impact energy to damage weed seeds in the soil before planting. They developed experimental equipment for picking up the soil in the immediate area in which the seeds are to be planted, treating it, and then replacing it immediately in front of the planter. Initial results indicate that such an approach should be given thorough study, particularly where special high-value crops are to be seeded.

B. FLAMECULTIVATION Flame control of weeds on ditch banks and rights-of-way has become a standard practice. Its application to weed control between plants under cultivation was first considered in the early 1940’s. Major attention has been given to the use of flame cultivators in cotton growing. Stanton ( 1954) and Stanton and Tavernetti (1956) have outlined in detail the development of more precise and uniform seedbeds, This, together with uniform seed depth, results in a plant uniformity that permits close adjustment of the flame with minimum plant damage. Several burner designs involving various shapes, slopes, and types of flame deflectors have been developed. Staggered mounting prevents the flames from striking each other and thus being deflected up the plant. In some areas, particularly California, there is growing interest in this equipment. FOR APPLYINGCHEMICAL HERBICIDES C. EQUIPMENT Weed control specialists have made tremendous strides in the development of chemicals for the control of weeds in cultivated crops. Equipment for the application of these chemicals has passed through many stages of development. Basically, it consists of standard assemblies of tanks, pumps, pressure-control regulators, and application nozzles. Engineers have been challenged to develop units that can be used in conjunction with planting and mechanical cultivation equipment. Nozzle designs that will give a uniform, precisely placed application with a minimum gallonage have been the major objective. Most of this equipment is rather standard in design and application, and hence will not be discussed in detail. Excellent discussions of its development, design, and use are to be found in the reports of Akesson and Harvey (1948), Fairbanks (1951), Page (1952), and Yeo (1955).

D. THINNING EQUIPMENT Many crops such as sugar beets, cotton, and vegetables have low and unreliable emergence rates. To assure a good stand, these crops are planted thick and then thinned to desired spacing and stand. AS

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hand-labor costs have increased, many types of mechanical blockers and thinners have been developed. The simplest device is the cross-blocker with a series of rotating knives or sweeps of selected length that is driven across the row to cut out unwanted plants. Down-the-row thinners follow the direction of the row but have rotary knives that are geared to the rate of forward movement in such a way as to remove the unneeded plants. This latter type has received much attention from design engineers. Blade speed, design, and orientation have been carefully worked out to provide a high degree of precision and uniformity in the thinning operation. A detailed analysis of the design factors included is given by Richardson (1958). British studies are reviewed by Maughan et al. (1959). Flame and chemical thinners have been developed but have not been extensively used. Baggette ( 1949) describes equipment that suspends a cover over one or more plants at the selected plant-spacing interval; the remaining plants are then burned out by a flame or sprayed with a direct contact-type herbicide. The thinners discussed above are all of the random selection type. One selective-type thinner uses an “electric eye” to locate and examine the plants. A windowed box containing the phototube is passed over the row. Light reflected from the first plant actuates the tube. This, in turn, puts a thinning knife into operation until the tube has located the next plant at the required distance. The knife then moves aside until it has passed this “selected” plant. Holmes (1950) indicates that by adjusting the phototube it can be made to select only the larger crop plants, thus duplicating, to a’degree, the selective ability of a human thinner. This is a costly device; however, it indicates the unlimited possibilities that exist for further automation in the field of farm equipment, V. Developments in Spraying and Dusting Equipment

A. GENERAL DEVELOPMENTS The increased use of both ground machines and aircraft for the application of agricultural chemicals has been phemonenal in the past decade, with an estimated use of approximately 260 million dollars worth of these chemicals on United States farms annually. Major uses are for insect, disease, weed and brush control, and defoliation. The type of chemical used has changed in the past ten years from predominantly dust to spray. An example of the phenomenal growth in spray equipment is shown by the fact that in 1947 approximately 3000 tractor power-take-off sprayers were manufactured, whereas in 1957 about 60,000 were manufactured. An average of approximately 50,000

T. W. EDMINSTER AND H. F. MILLER, JR. 194 were manufactured during each of the intervening years. In 1947 one buyers’ guide listed only five manufacturers of tractor-mounted powertake-off sprayers, but a hundred manufacturers were listed in 1957. The airplane is considered an agricultural machine for applying spray and dust materials, since an estimated 5000 planes treat over 60 million acres annually for pest control, A U. S. Department of Agriculture (1958~)report on the pesticide situation for 1957-1958 stated: “The acreage treated by aircraft for pest control in California rose from 296,059 in 1946 to 5,611,000 in 1956, with the area in 1956 almost twice that in 1951.” The U. S. Department of Commerce (1957) gives a breakdown of aviation application uses for agriculture. The U. S. Department of Agriculture (1958d) also gives a selected list of references on aircraft in agriculture. Much progress has been made in the manufacture of spray equipment for both ground machines and aircraft by the use of better materials and manufacturing techniques. Improved nozzles, pumps, valves, as well as longer-lasting tanks and lines, have come about by the use of higher-grade metals or newly developed synthetic materials. For instance, as many as six different types of stainless steel are used in the manufacture of present-day spray equipment. Detailed discussion of the use and development of spray and dusting equipment is given by Smith (1955) and Bainer et al. (1955). Considerable work has been done on the effect of particle and droplet size when using different chemicals for various purposes. However, researchers are still working for a method to control droplet size and to produce sprays with a large percentage of droplets in a narrow range of sizes. Other problems concern methods of increasing the percentage of material which actually sticks to the plant stem and leaf surfaces, and ways of measuring these amounts quickly and accurately. A new photographic and electronic counting method of measuring spray droplet size has been reported by Farnham (1958) to be a hundred times faster than presently used methods. Brittain et al. (1955) discuss a relatively simple method of evaluating the deposit on plants, and Kromer (1949) relates the engineering challenge of spray application. Black (1956) reports on the corrosion and abrasion effects of pesticides on application equipment.

B. SPRAYINGEQUIPMENT Sprayers for field crops are primarily of three types-tractor-mounted, tractor-trailed, and high-clearance self-propelled. Orchard sprayers are generally classified as high pressure or blower (mist) types. Recent developments have been the increased use of self-propelled sprayers for

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ficld crops and blower (mist) typc sprayers for orchards. The introduction of blower (mist) sprayers for use on field crops, primarily vegetable crops, has been for disease control. Williamson ( 1958) discusses recent increased use of self-propelled high-clearance sprayers in cotton, Black et al. (1954) describe the de-

FIG. 8. New m i s t blower sprayer maneuverable for spraying in any direction. (Courtesy Food Machinery and Chemical COT.)

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velopment of a high-clearance, self-propelled sprayer for sweet corn. These sprayers have recently been equipped with attachments such a s topping devices for cutting tops from crops, flame cultivators for use in flaming rank-growing cotton for weed control, and granular insecticide distributors for corn borer control. The new development of blower-type sprayers, sometimes referred to as air-blast or mist-concentrate sprayers, is significant and their use is rapidly expanding. These sprayers use less water, thereby applying more concentrated spray while also obtaining equal or better coverage than hydraulic sprayers using large volumes of water. The use of this type sprayer for row crops and vegetables is discussed by J. D. Wilson ( 1956) (Fig. 8 ) . Recent developments and methodology in the use of airplanes for forest and row-crop spraying are discussed by Isler and Thornton (1955), Young et al. (1957), Chamberlin et aZ. (1955), U. S . Department of Agriculture ( 1954), and Anonymous ( 1956~). Helicopters are being used to a small but increasing extent. Their use is limited to spraying of high-value specialty crops, such as cranberries, which are difficult to get to with either ground equipment or winged-type aircraft. C. DUSTING EQUIPMENT Although the use of dusting equipment has rapidly declined owing to increased use of sprays in the past decade, there has been some improvement in application equipment. This is particularly true with respect to modification of dusting equipment for use of granules. Improved hopper and metering equipment design has resulted in more uniform distribution of dust across the swath for both ground machines and aircraft. For aircraft, an additional small airfoil has been closely coupled to numerous discharge points to aid in promoting rapid spreading of the materials. This new equipment dispenses liquid, dust, or granules with only minor adjustments being necessary for dispensing the different types of materials. VI. Developments in Harvesting Equipment

Quality changes through improved design and manufacturing processes have produced harvesting machines which do a better job in less time, last longer, and require less labor for operation. The number of machines has increased, although the number of farms has decreased. During the period 1950 through 1958, the number of grain combines increased by 46 per cent, corn pickers by 63 per cent, pickup balers by

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201 per cent, and field forage harvesters by 215 per cent. Percentage increase of machines on specialized crops is even greater, depending upon the number needed and the degree of success in perfecting the equipment. The harvesting of specialized crops is rapidly changing from hand to mechanical methods. Scarcity of labor and tightening of the economic situation is expediting this change-over. Increase of mechanized harvesting contributed primarily to the 21 per cent reduction in man-hours used on farms in the past ten years. There has been very little reduction in man-hours used for those crops in which the harvesting has not been mechanized. The general trend has been toward harvesting equipment that can be operated by one man with the least expenditure of his energy. The trend in design is toward more automatic operation, increased use of hydraulic systems, V-belt drives, self-aligning prepacked bearings, and lighter materials for construction where possible. Less vibration is experienced owing to better balancing of moving parts. The use of large harvester-mounted bulk bins unloaded by gravity dumping or auger conveyors is becoming standard practice. There is a trend toward larger self-propelled machines for the bigger farms. Smaller machines are being designed to mount on tractors or other power units which can accommodate several types of equipment. Harvesting machines are being designed to operate under a wider range of crops and cropping conditions. HARVESTING EQUIPMENT A. FORAGE Forage crop production of over 100 million tons (excluding that used for silage) constitutes approximately one-fifth of all harvested crop acreage in the United States. S t r i d e r and Phillips (1956) report that while only 29 per cent of all hay was baled in 1944, 73 per cent was baled in 1954. This trend was due primarily to the introduction of automatic-tie pickup balers, reduction of storage space requirements, and ease of handling as compared with loose hay. Chopped hay for curing and dehydration increased from approximately 2 to 7 per cent during the same period. Long, loose hay has steadily declined during these same years to a low of 20 per cent in 1954. Although hay crops in general have a relatively low cash value per acre, much progress has been made in equipment for mechanizing the crop. Improvements in hay crushers have decreased the hazard of crop loss under changing weather conditions. Automatic one-man-operated balers with a second man loading the trailer has been a common practice for the past ten years. Recently, one-man hay balers have been designed to kick or throw the bale into the trailer, thereby eliminating one man

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FIG.9. One-man baler throws bales from machine to trailer. (Courtesy Deere and C o . )

(Fig. 9). It appears that hay pelleting or wafering by field machines may further reduce the handling costs in the field, in storage, and in feeding. 1. Mowing With over 75 per cent of the mowers equipped with power drive in lieu of ground drive, 6- and 7-foot cutter bars have become standard. Elfes (1954) reports on the design and development of a new highspeed mower having a reduced stroke length and a dynamically counterbalanced reciprocating blade so designed that its operation is not affected by the raising and lowering of the bar. It is equipped with special sealed antifriction bearings throughout, and a V-belt replaces the pitman rod.

2. Hay C w h e r s The general use of field hay crushers has increased from only a few hundred to several thousand between 1955 and 1958. Consisting primarily of a pair of steel rollers held together under pressure by adjustable springs, the crusher is driven by the power-takeoff of the tractor. A pickup unit lifts the hay from the swath and feeds it between the rolls. The crushed hay is dropped back onto the stubble in a swath. Crushers are made as separate units or in combination with mowers.

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Ramser and Kleis (1952) and Butt et al. (1956) report that crushed hay generally dries to a safe storage moisture content in one-third to two-thirds of the time required for uncrushed hay. Although the reduction in drying time is seldom sufficient to allow cutting and storing in the same day, it often allows the hay to be picked up on the second day when normally it would take 3 or 4 days for it to reach a safe moisture content. Earlier models of the hay crushers generally had smooth crushing rolls, whereas some recent models are equipped with fluted rolls which tend to crimp the hay. 3. Raking

Agricultural statistics indicate that side-delivery rakes on farms have become the predominant type, with approximately 1.3 million in use in 1957. Bainer et al. (1955) classify side-delivery rakes as ( 1 ) cylindricalreel, ( 2 ) oblique reel-head, ( 3 ) finger wheel, and (4)drag-type. Each of these rakes, owing to differences in design, imparts a different velocity and movement to the hay in the raking process. Giles and Routh (1951) in comparative tests on the three side-delivery-type rakes found that the leaf loss with the finger-wheel rake was considerably less than with the other two types. The increased mobility of tractor-mounted rakes facilitates their use on grassed waterways, terraced outlets, and in odd-shaped land areas that may occur in conservation layouts. 4. Baling

Improvements in equipment for baling have increased the popularity of this method of harvesting and handling hay. Outstanding changes have been the development of the automatic baler and the use of twine for tying. There has been a threefold increase in the number of automatic pickup balers since 1950. Of the approximately 600,OOO in 1958, approximately 80 per cent used twine. In addition to the shift toward more twine-tied balers, there has been a trend toward smaller bales. These bales are easier to handle and are particularly adapted to the one-man balers equipped to eject the bale into the trailer. At least one company is proposing a control which permits the tractor driver to change the direction in which the bale is thrown. The two main types of balers are the plunger type (rectangular bale) and the round-bale type. Hay buyers have generally demanded denser bales. Burrough and Graham (1954) developed a method employing strain gages and a sensing unit for measuring the power input to various drives of plunger-type forage balers which show the effects of varying moisture content, bale density, baling rate, and plunger speed. The maximum

T. W. EDMINSTER A N D H. F. MILLER, JR. 200 force required for an increase in bale density of 8 to 10 pounds per cubic foot increased from 4500 to 10,OOO pounds, and the baling energy increased from 1.2 to 2.2 horsepower-hour per ton. For only a 14-pomd increase in a 7-cubic foot bale, almost twice as much energy is required. Automatic twine-tying devices have been refined and improved. Baler twine is heavier than binder twine, having a tensile strength of approximately 275 pounds. The normal rectangular twine-type bales use slightly over 3 pounds of baler twine per ton, while round bales require slightly over 2%pounds of binder twine per ton. Bainer et al. (1955) describe the need and present-day use of safety devices or shear pins in such places as (1)between the flywheel and the plunger, ( 2) in the drive ahead of the baler flywheel on power-take-off driven balers, (3) in needle drive should it strike an obstruction, (4) in the drive to the tying mechanism, ( 5 ) in the pickup and conveying drive, and (6) in the feed-mechanism drive to prevent overloading.

5, Chopping With different attachments, field forage harvesters can be used to harvest row crops for silage, grass from a standing crop or windrow for silage, straw and other kinds of forage, and green chopped hay for direct feeding or dehydration. Field forage harvesters, developed first about 1936, have increased rapidly in recent years from approximately 80,000 in 1950 to 240,000 in 1957. These machines took the place of the row-crop binder. Field forage harvesters are of two major types, cIassified as to placement of cutting knives into a flywheel or cylinder arrangement. The flail-type forage harvester developed from the swinging-blade stalk shredders since World War I1 has recently increased in importance Fig. 10) Bockhop and Barnes (1955) ran tests on power distribution and requirements of a flail-type forage harvester, reporting the power requirements as relatively high when compared with conventional-type forage harvesters but having comparable capacity. The flail-type machine is of simple design with relatively few working parts and can be utilized as an ensilage harvester, hay and straw chopper, stalk shredder, weed cutter, beet topper, and for other purposes. The authors further state that it can probably be used economically by a farmer who uses an ensilage harvester an average number of hours per year and who already has a three- or four-plow tractor. The economy-model flail harvester results in a product with somewhat longer length of cut, and when this is not objectable, farmers should find it acceptable, especially where green-feeding practices are used. I

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King and Elliott (1955) report on the development of a semi-selfpropelled baler and forage harvester, both pickup and direct cut. The tractor is used as a prime mover, with mounting hitches so designed and arranged that the tractor may be hitched to the harvester in a matter of 2 or 3 minutes. A separate power unit when needed may be mounted on the three-point hitch behind the tractor and beside the harvester. Cykler (1950) tells of harvesting Napier grass in Hawaii with a harvester developed and used for harvesting green feed the year around.

FIG. 10. Flail-type harvester cuts, chops, and loads forage by direct-cut or from windrow. (Courtesy Lundell Manufacturing Co., Inc. )

It was mounted on a track-type tractor for use on small, rough, rocky, and irregularly shaped fields. A commercial machine for field hay pelleting was announced during 1958. While not yet in wide commercial use, it should have great potential if present experimental harvesting and feeding trials continue to be successful. It will greatly reduce the cost of handling forage both from the field to storage and from storage to feeding. Dobie (1959) reports that by far the greatest activity is being exerted in the field of producing large wafer-type pellets. He states that numerous manufacturers are

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making pellets 3 to 4?iinches in diameter and % to 1%inches thick. Most of these machines are plunger-type machines with the application of the high pressure necessary to make the wafer of suitable density resulting in terrific impact pressure at the forward end of the stroke. A heavy frame plus the heavy flywheel needed to move the pulsating load, results in a fairly heavy machine per ton per hour output. The principal advantage of the plunger machine for wafering is that it will handle either long or chopped hay; this reduces the cost of processing prior to pelleting and places field pelleting in a more favorable position because less auxiliary equipment is needed. Wafers made from chopped hay are usually more uniform in shape and thickness than long hay wafers. The present field machines for making wafers have the capacity of about one-half that of a conventional baler and make wafers best from hay containing from 12 to 20 per cent moisture. 6. Mbcelluneous Equipment

Windrowers, hay tedders, and various types of buck rakes and hay loaders are passing through a period of redesign and modernization. Hydraulic-operated, tractor-mounted units of many types are available for putting up loose hay. B. CORNHARVESTING EQUIPMENT Corn is the largest acreage field crop in the United States and the most important source of feed, with 90 per cent of the total corn acreage harvested for grain. Ninety per cent of the total annual production of corn, 3%billion bushels, is from the twelve North Central States. There are approximately 745,000 corn pickers in the United States, an increase of about 42 per cent since 1950. According to Scoville (1956) over onehalf of these are in the Corn Belt, but the most rapid rates of increase in the past few years have been in the South, Northeast, and Far West. A new type of picker-sheller unit, introduced since 1953, is a modified grain combine for picking and field shelling of corn. According to the best estimates, there were slightly over 5000 picker-sheller units in use in 1957 and approximately 9OOO grain combine attachments for picking and field shelling. Corn harvesters may be classified as snappers, picker-huskers, or picker-shellers. Bainer et al. (1955) report a fourth type, introduced commercially in 1954 as a combination picker-chopper. This machine picks the ears and delivers them into a wagon while the stalks are cut and fed into a conventional forage-chopper and either discharged on the ground or delivered into a truck or moving wagon beside the harvester.

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Safety is one of the greatest problems to consider in the use of a mechanical corn picker. Scranton (1952) reports that in a survey it was found that nearly all accidents with corn pickers were caused by carelessness. There is a major educational task in teaching users of such machines to be careful. There are some twenty-one rules listed for normal and safe operation of corn pickers by the Farm Division of the National Safety Council. 1. Snappers and Picker-Huskers A snapper-type corn harvester is essentially the same as a pickerhusker except that there is no provision for removing the husks. The snapped ears are conveyed directly into a wagon. The snapping rolls are generally operated at about 500 to 600 f.p.m. (peripheral speeds). The rolls are ordinarily made of cast iron or cast steel, with spiral ribs or lugs on their surfaces. On a picker-husker, the husking rolls may be on a separate bed, incorporated in the snapping-roll elevator, or may be a direct extension of the snapping rolls. Most husking units have one roll of each set made of rubber; the other roll may be of steel, wood, rubber, or cast iron. Field losses generally run in the range of 5 to 10 per cent. Field losses may be kept to a minimum by proper adjustment of the machine, careful driving, and avoiding high speeds. Under most conditions, speed should not exceed 3 to 3%m.p.h. Richey et al. (1956) report a new design for corn-snapping rolls. The new principle varies from the conventional in that the stalk is bent sideways and passes through the snapping rolls at about a 45-degree angle. This action clears the ears from the rolls and reduces the shelling losses. The side-snapping action tends to spread the rolls in a 45-degree plane and is referred to as the side-snapping principle. 2. Picker-Shellers and Combine Attachments

Picker-sheller corn harvesters may be of the same general design as the picker-huskers, except for the addition of the shelling unit and a grain bin or elevator to elevate the shelled corn into a trailing wagon or one pulled beside the harvester. Corn may be harvested with pickershellers with a moisture content ranging from 14 per cent (dry enough to store) up to 30 per cent or higher. When the early-harvest method is practiced, the corn will be too high in moisture content to store and must be artificially dried down to approximately 14 per cent moisture. Ear corn will keep in storage up to moisture contents of around 20 per cent. Hurlbut (1955) and Pickard (1955) tell of experimental work between 1950 and 1954 on an experimental ear-corn harvesting attachment which mounts on the front of a combine, and also compare differ-

T. W. EDMINSTER AND H. F. MILLER, JR. 204 ent types of cylinders and concaves for efficiency in shelling of corn. Morrison (1955) reports on the commercial adaptation in 1953 of a com-harvesting attachment on a self-propelled combine. He lists fourteen different advantages of combining corn, including more economical and efficient harvest, an earlier harvest, crop residues left in the field, and less storage space required. Goss et al. (1955) reported on tests, conducted in California during the 1954 harvest season with corn-harvesting attachments, which indicated that the ordinary grain combine equipped with a rasp-bar cylinder is well suited for shelling corn under California conditions. The major problem in harvesting corn with adapted combines was the delivery of the unshelled corn to the cylinder without loss of corn, primarily ear corn on down or lodged stalks. The combine attachments for corn are relatively simple, and fast change-overs from grain to corn are possible on most machines because both front ends attach to the same bearing points and use common drives and the same lifting system. There is no wagon to pull and the machines operate better in soggy fields.

C. GRAIN,LEGUME,AND GRASSSEED HARVESTING EQUIPMENT Although grain combines are used primarily to harvest small grains and soybeans, they are used for many other crops, such as rice and various legume and grass seeds. In the United States, combines have almost completely taken over the job of threshing grain, either by direct cutting or combining from a windrow. Brodell et al. (1952) reported that 85 to 95 per cent of the barley, wheat, and soybean crops in the United States were harvested with combines in 1950. There were approximately 1,040,OOO combines in the United States in 1957, compared to approximately 700,000 in 1950. Approximately 20 per cent of the combines on farms today are the self-propelled type, and this trend is rapidly increasing, since approximately one-half of the combines manufactured during 1956 and 1957 were of the self-propelled type. The main advantages of self-propelled combines over the trailingtype machines are that they have greater flexibility and maneuverability, save more grain in opening up fields, and permit the driver to have better control of his machine and a better view. Gray (1955) gives the function, operation, and care of the combine, as well as details with respect to adjustments for twenty different crops, Witzel and Vogelaar (1955) report that the first self-propelled combines on rolling ground did not gain widespread popularity, probably because during the period from 1910 to 1930 the track-type tractor came into being as the primary answer to pulling the large hillside combines.

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During the period from 1910 to 1949, hillside combine development was marked by general design improvements, such as universal adoption of steel frame and body, antifriction bearings, V-belt drives, rubber tires, and weight reduction. Another advance during this period which helped to reduce manpower requirements was the adoption of bulk handling of threshed grain, thereby eliminating the sack sewer and sack “jigger.” In addition, several ingenious automatic leveling-control devices had been developed, making possible a two-man crew in place of the original five-man crew. They further report that the relatively recent development and widespread adoption of level-land, self-propelled, pusher-type combines was watched with a great deal of interest by the hillside area farmers. At least two hillside self-propelled pusher combines went into the field in 1949. A limited number of hillside self-propelled combines were put in the field by one manufacturer in 19%. Inventors converted several hillside combines to the self-propelled type. Several makes of hillside combines were available by 1954. This year also marked the introduction of the first hillside combines with factoryinstalled automatic leveling controls. The main difference between a hillside combine and a conventional combine is that the separator body is kept level in the lateral direction regardless of the ground slope. Some combines have a part of, or the entire, separator kept level to some degree in the longitudinal direction. The need for and advantage of longitudinal leveling depends somewhat on the separator design. The two main types of sensing devices for automatic controls are those in which the force of gravity acts on a solid mass and those in which it acts on a liquid. The first of these is primarily the pendulum type which controls hydraulic valves directly and is also used to control electric switches which energize hydraulic-valve solenoids. The second type depends on the force of gravity acting on a liquid, such as a mercury switch. Safety is important in the use of hillside combines. They are designed with a wide wheel base so that the center of gravity falls well within the wheel base at extreme hillside conditions. Each individual wheel also has a brake. Power steering used on models produced during the last few years is itself a good safety device. Automatic controls also assist in safety since they guard against the operator leveling the wrong way. Heitshu (1956a) describes the chain of events leading to the development of one manufacturer’s self-propelled hillside combine. The complete drive-axle assembly and a schematic drawing of the automatic leveling circuit is given. Many of the operating characteristics and safety features of the hillside combine are described in detail.

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Pool (1958) describes controls for full-leveling of hillside combines.

This article is the partial history and gives details of engineering developments leading to the manufacture of a hillside combine by another

major company. The pendulum system with direct-acting valves was used to activate the mechanism for leveling this particular combine. Bigsby (1958) describes power requirements of a combine cylinder when threshing solid and hollow-stemmed varieties of wheat. By the use of strain gage equipment, he determined that 20 to 25 per cent more power is required on the cylinder when threshing solid-stemmed wheat than is required to thresh hollow-stemmed wheat at the same rate. This problem became one of major interest when severe losses in wheat yield in western Canada caused by the wheat stem sawfly resulted in the development of solid-stem varieties of hard spring wheat. Self-propelled combines are now being offered with 18-foot-wide cutter bars, both for the level and the hillside types. Many features are now being built in the combines, such as lifetime lubricated bearings, improved V-belt and pulley drives with mechanisms for easy adjustment or changes in speed, as the case may be. Many combines are also offered with windrow pickup attachments for combining grain and other crops from the windrow if desired. Many companies are offering attachments for corn harvesting, as previously discussed. Air-conditioned cabs are now available for self-propelled combines which give year-round heating or cooling. The compressor for cooling is belt-driven from the combine engine. Full-view windows with tinted safety glass are featured for greater comfort for the operators, Although soybeans are harvested primarily by conventional-type combines, problems of germination in connection with cracking of seed during harvest are common, as discussed by Moore ( 1957). Heitshu ( 1956b) discusses the problems of ridging and low pods as being serious to contend with in combining and the necessity of keeping the ridges low so that the height of beans on the ridge and those in the middles will not be too different. He also points out the important contribution which could be made by plant breeders and agronomists with respect to soybeans, as has been the case with many other crops. He feels that, through the cooperation of plant breeders and engineers, soybeans which have higher fruiting characteristics and a more desirable row profile can be grown, and that machines can be made which will do a more efficient job of harvesting and reduce cracking. Bunnelle et al. (1954) say that successful harvesting of small-seeded legumes depends as much on the cultural practices used to produce the crop as on the operation of the harvester. Combines, when properly adjusted, are capable of doing a good job of harvesting these crops,

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although most machines require some modification for the best performance. They further conclude that at normal load rates, the factors affecting combine performance in small-seed legume harvesting are cylinder speed, loss of free seed over the straw walkers, and the cleaning shoe adjustment. Park and Webb (1958), reporting on southeastern seed harvest studies, give several conclusions for different crops. They conclude that angle-bar cylinders threshed more crimson clover seed with less seed damage than rasp-bar cylinders. Supplemental angle bars are available for most combines with rasp-bar cylinders for use on hard-to-thresh crops. Rubber on angle bars significantly reduced seed damage. Low ground speed was necessary to minimize seed losses in crimson clover. In small grains tests, when comparing the angle-bar and rasp-bar cylinders and comparing open and closed grates, Park and Webb found no appreciable differences in threshing performance. The most serious losses in rescue, fescue, and lespedeza seed were due to weather and cutter-bar shattering, often amounting to over 50 per cent. Cutter-bar loss was reduced and cutting performance was improved by use of a tined pickup reel. Klein and Harmond (1959) give the development of a suction-type field reclaimer for shattered seed which was mounted on a combine and used as a once-over operation. The attachment for picking up shattered seed used revolving swinging chains to loosen the seed at the ground surface within the suction head mounted behind the combine. The seed, straw, and soil are conveyed back into the combine separating mechanism where the seed is saved. Tests show an increase in recovery of pure live seed from 46 per cent to 68 per cent in crimson clover and from 24 per cent to 62 per cent in sub clover when compared with combining without the benefit of the suction seed reclaimer.

D. COTTONHARVESTING EQUIPMENT Mechanical cotton harvesters available today are of two basic types, commonly referred to as pickers and strippers. Picking machines remove seed cotton from the open bolls, the unopen bolls being left on the plant. Stripper-type harvesters strip the entire plant of the cotton, including the open or closed bolls and many leaves and stems. It was 1946 before mechanical cotton harvesters were manufactured in any appreciable number; by 1957 about 20,400 pickers harvested 19 per cent of the crop and 26,500 stripper-type machines were used on farms to harvest 13 per cent of the crop, making a total of one-third or less of the cotton crop in the United States machine-harvested. Colwick and Regional Technical Committee members (1953) describe the Beltwide harvesting results from several phases of regional

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work. A U. S. Department of Agriculture Special Report (1956b) describes the impact of mechanization on cotton production. Less than 2 per cent of the cotton in the Southeast is mechanically harvested, while 70 per cent or more of the crop in the irrigated West is harvested with mechanical pickers. Smith (1955) gives a detailed description of both picking and stripping machines as to types, construction, and use.

1. Picking Machines Fairbank and Smith (1950) divided the operating cost of mechanical picking as follows: operating, 25 per cent; overhead, 20 per cent; field losses, 20 per cent; and grade reduction, 35 per cent. It was concluded that even with field losses and losses due to grade reduction, mechanical cotton harvesting was definitely profitable under the existing price and labor conditions. These conditions have continued to exist up to the present time with a gradual increase in the percentage of the crop harvested, Harrison (1951) states that conformation of the boll largely determines how well spindles will perform the task of harvesting cotton. Good recovery was from bolls having relatively smooth inner surfaces when dry,the burs straight, approximately equidistant one from the other with the cotton protruding far enough to afford good contact by the spindles but set deeply enough into the carpels to provide reasonable storm resistance. Unfavorable environment, particularly lack of moisture, causing a gnarled opening of burs of any variety tends to produce difEicult machine- or hand-picking conditions. Harrison has been successful in breeding in California a variety of cotton well adapted to spindle picking. Tavernetti and Miller (1954) report on the importance of properly managing all growing practices for most efficient harvesting. With good management, machine-picking efficiencies in California run 95 per cent or better. Williamson et al. (1954) describe factors affecting the efficiency of mechanical pickers for picking quality cotton in the Yazoo-Mississippi Delta as varieties, field layout and water control, stalk disposal and seedbed preparation, seed preparation and planting, weed and grass control, insect control, defoliation, and machine performance. Wooten and Montgomery (1956) give the effects of relative humidity and spindle moisture on machine-picked cotton, showing that cotton should not be machine picked early in the morning when the humidity is high. Relative humidity has more effect on moisture content of machine-picked cotton than the water which is added to the picking spindles. Powers (1949) concludes that weeds, lack of good defoliation, lack

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of good gins in the area for machine-picked cotton, small farms, and no particular shortage of hand labor are the main reasons why inechanical pickers are not used more in the Southeast. Important recent developments in spindle pickers include improved wearing quality of spindles, improved spindle grease seals, reduction in spindle twist left in cotton through improved spindle design and adjustment, and introduction of low-drum, single-row pickers for smaller acreages.

2. Stripping Machines Harvesting of cotton with stripping machines is limited primarily to the High Plains of Texas and Oklahoma, but it is beginning to expand into other areas as varieties and gins are made available for this type of harvest. Oates et nl. (1952) report on the development of a new, brush stripper-type cotton harvester. While this machine proved to give good efficiency of gathering most varieties, difficulty was encountered with problems of quality because of the mixing of excessive soil and trash with the lint. Both the brush gatherer and air-conveying system are reported to be major contributors to quality difficulties. There have also been numerous trials in the use of rubber paddles for stripping, according to Smith (1955), but neither the brush nor rubber paddles have come into any extensive use up to the present time. A recent development in the stripper-type harvester has been the use of extremely long brush-type stripping rolls to harvest taller, highyielding cotton of the open-boll type. After being stripped from the plants, the cotton with burs and trash is conveyed into a combination stick-and-bur-remover-type cleaner, mounted on the tractor, which takes most of the burs and sticks out of the cotton before putting the partially cleaned seed cotton into the harvester basket. With further improvements this type of operation may prove to be economically feasible. It no doubt can be successfully used for the second picking operation. One advantage of this type of stripper is that it leaves the burs and most of the trash in the field rather than carrying them to the gin. It can be expected that stripper-type machines of conventional or improved types will be used more and more as cotton is mechanized, primarily because of their low cost of operation.

E. DRYBEANAND PEA HARVESTING EQUIPMENT On almost 1%million acres, dry edible beans and peas are grown in the United States each year. Most of the production is in the states of California, Michigan, Idaho, Colorado, New Yak, Nebraska, and Wyoming. During the past ten years, the older method of cutting, windrow-

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ing, stacking or baling, and stationary threshing of beans has declined sharply. McColly (1958) reports that a number of threshing machines were converted to a combined harvester-thresher, either pull type or self-propelled, and fitted with a special pickup device and feeder. The most common method employed in harvesting navy beans in Michigan is the combine pickup from a windrow. Many growers have special bean combines which usually have two cylinders designed especially for this crop; however, the trend in edible bean harvesting is to utilize the grain combine with attachments which include cylinder speed-reduction drives, screens, and other parts varying somewhat with manufacturers ( McColly, 1958). A few manufacturers have rubber-covered bars available to replace grain-threshing bars on the cylinder, and a bean grate can be placed under or back of the cylinder. There have been recent developments of a torsion spring-tooth cylinder and concaves for combines which have proved very effective in threshing edible beans. Cracking of beans can be eliminated and there is not as much delay waiting for the heavy bean root to dry out enough to go through the cylinder. This spring-tooth equipment is not as effective in threshing small grains, however, as are other types of cylinders and concaves. A blade-type bean harvesting cultivator attachment fitted with vine turners, row dividers, and windrowing rods is very commonly used to push dry beans from the soil and to form them into windrows. A rotary crop cutter is under development in Michigan and has proved successful in harvesting tests. The machine consists of hydraulic motor-driven notched disks horizontally rotating toward each other so that each, in cutting a row, forms a windrow from two rows. McColly further states that farmers would prefer direct-combining beans in order to avoid the possible loss in the windrowing in case of inclement weather. Therefore, crop scientists are working on a new variety of navy bean which grows as an upright plant rather than a vine. In direct combining, a power-driven six-bat finger-type reel is employed with an eccentric mechanism to vary the pitch of the tines during reel rotation. The finger-type reel is more efficient than the standard bat-type reel because the tines can be adjusted so that they lift the bean plant toward the cutter bar. This action saves many bean pods from being severed by the sickle and results in decreasing cutter-bar shatter loss by as much as 50 per cent. When a grain combine is used, the cylinder speed should be reduced to about one-half to one-third that required for threshing wheat, according to McColly. Defoliation of the crop when the beans are harvested standing is necessary for best harvesting and to help reduce quicMy the moisture content in the bean. Harvesting efficiency depends very much

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upon correct combine adjustment. The machine should be so designed that these adjustments can be made easily and in a reasonable time. Unfortunately, in many cases this is not done and there is a need for improvement in combine design so that these adjustments can be made more easily. Combine adjustments required for harvesting peas are essentially the same as described for soybeans. Gray (1955) describes these adjustments in detail. Peas may be combined direct as well as from the windrow. F. VEGETABLE HARVESTING EQUIPMENT Mechanizing the harvest of many vegetable crops is proving to be one of the most difficult equipment problems to solve. However, tremendous progress has been made in the past five years primarily because of the necessity of reducing the labor requirements for harvesting these crops. For instance, the man-hours required for harvesting asparagus, carrots, cherries, celery, and strawberries by hand are two to eight times greater than for all other growing operations. This contributes to a critical peak seasonal labor demand. Mechanized harvest can largely eliminate this problem. Because of the generally smaller acreage of vegetable crops as compared with other field crops, development work on harvesting equipment for vegetables has been done primarily by individual growers, smallequipment manufacturers, and in many cases, public research.

I. Asparagus Several different types of harvest-aid machines of the type which carry the person at a slow rate of speed through the field so that his handcutting efficiency is increased have been developed by asparagus growers. Miller (1957) describes a machine developed by one grower at a cost of approximately $750. By utilizing foot-actuated controls, both hands of the operator are free to cut the asparagus passing directly in front of and below him. With three harvesters, this one grower has reduced the number of hired hands from 24 to 3. Several years of work on a completely mechanical asparagus harvester is described by Kepner (1957). The machine has a bandsaw type of blade which cuts all the spears from a band 30 to 36 inches wide at or just below the ground surface immediately after they have been caught by the gripping units. This machine cuts all spears regardless of their length; thus, spears that have just emerged are not long enough to save. Spears shorter than 3%inches are below the normal gripping level and are not recovered. Kepner states that the machine-harvested yield was considered to be 55 per cent of hand-cut yields due to loss of the short spears. Under certain assumed conditions and estimated cost, mechanical

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harvesting and hand cutting would yield the same net return per acre if the machine-harvested yield were 65 per cent of hand-cut yield. By further modification of cultural and management practices, such as bed height and shape and harvest timing, further improvement in the percentage of harvested yield can be expected. 2. Green (snap) Beans

Outstanding progress has been made recently in the development and use of mechanical harvesting equipment for snap beans. Work (1958) reports that some 200 mechanical bean harvesters were used during that year, The two-row machine takes the leaves and stems between rollers, pulls the beans, and drops the refuse on the ground. The field is picked only once. Some growers pick their beans first by hand and then follow with the machine for the second and last harvest, Breeders are working on strains of beans which are well adapted for machine harvest. The stage of maturity and other factors make a great difference; however, growers are willing to sacrifice total yield in favor of low harvesting costs. 3. Pole and Lima Beans Kubik (1957) reports that an experimental pole bean harvester is under development. Operated by one man, it picks up to 4 acres per day. Straddling the row, a pair of picker heads work simultaneously on both sides, snapping beans from the vine and dropping them into a container. Kubik also reports that an experimental machine picks lima bean pods from the vines with less than 5 per cent loss of beans. By mounting a regular stationary lima bean huller onto a trailer and equipping it with a pickup attachment, lima beans can be hulled directly from windrows, 4. Lettuce, Cabbage, and Celery Several different types of labor-saving devices have been developed for aiding in the harvesting and packing of vegetable crops, such as lettuce, cabbage, and celery. The most common type harvest-aid, laborsaving device is one mounted on a large trailer or tractor chassis, incorporating a packing unit and conveyor belts extending out on either side to bring the hand-cut vegetables into the packing line on the machine. There have been several attempts to develop a mechanical harvester for celery. The main difficulty has been the proper gaging of a cutting device for removing the roots from the stalk. Been (1957) reports on a conveyor belt harvester-loader for cabbage. This machine is towed by a row crop tractor and loads the cabbage into a trailer.

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5. Sweet Corn During the past ten years, the mechanical harvesting of sweet corn for processing has come into widespread use. The tractor-mounted, tworow harvester which is produced commercially, automatically strips the ears from the stalk and conveys them to trucks or wagons in the field. This type harvester reduces the picking costs approximately 60 per cent and replaces ten to fifteen hand pickers.

6. Cucumbers Hall (1956) and Farrow ( 1956) describe self-propelled, home-built cucumber harvester-type mechanical aids that utilize the principle of a moving platform upon which pickers are carried in the prone position. A conveyor belt takes the cucumbers to containers. A completely mechanical experimental cucumber harvester is described by Chisholm (1955). This machine will harvest 1 to 1%acres per hour, replacing forty harvest hands. The machine leaves the vine practically undamaged. This machine is still under development.

7. Spinach and Peas Spinach and green pea harvesters are in general use. A sickle-bar cutting device cuts the entire plant which is then hauled to the processing or shelling plant. These machines are generally referred to as “greencrop harvesters.” 8. Tomatoes

Tomatoes harvest has been expedited through the use of self-propelled field conveyors. Hofmeister (1955) reports the development of a field conveyor for harvesting tomatoes and other vegetables which increases harvest rate 30 to 100 per cent. This particular conveyor is propelled by a 4-horsepower gasoline engine which also drives the conveyor belt. Lorenzen ( 1956) describes an experimental tomato harvester designed primarily for use with pear-type processing tomatoes. The harvester cuts the vines and then separates the fruit from the vine. The development of a successful tomato harvester depends in part upon the plant breeder developing a variety which ripens uniformly. 9. Other Vegetables

Many crops, such as broccoli, melons, and peppers may also be more efficiently harvested by the use of field conveyors and mechanical aids. Kubik (1957) discusses several types of field conveyors ranging from small self-propelled types to large tractor-mounted units which have

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conveyor belts as long as 70 feet. This type of equipment will continue to assist in the harvesting of vegetable crops until mechanical harvesters can be developed to do the complete job. G . ROOTCROPHARVESTING EQUIPMENT Much progress has been made in the development and use of root harvesting equipment for the approximately 5 million acres of root crops grown in the United States, The principal root crops include peanuts, potatoes, sugar beets, sweet potatoes, and onions. Crops such as sugar beets are completely mechanized insofar as harvesting, while some edible crops such as sweet potatoes are dug mechanically, but virtually the entire crop is still picked up by hand.

1. Peanuts Mechanization of producing and harvesting the Spanish-type peanuts grown in the Southwest is practically 100 per cent complete. Anonymous (1950) describes many of these early developments. The harvesting trend in the Southeast has been toward the use of commercially available or modified peanut combines designed specifically for combining peanuts from the windrow. Approximately one-half the crop in the Alabama-Georgia-Florida area was mechanically harvested in 1958. Machines designed principally on the raddle or carding principle have been most popular, but indications at present point toward the possible use of a combination cylinder and carding principle on the same machine to increase capacity and efficiency. Combination equipment incorporating tractor-mounted digger blades with a pulled-behind shaker-windrower has been developed in recent years. This has largely replaced the need for side-delivery rakes for windrowing peanuts. Mechanical harvesting in the Virginia-Carolina area has been nonexistent up to the present time. However, peanut combines were used by a few growers in 1957 with considerable increase in the number used in the 1958 harvest season. Duke (1957) describes the development of an experimental peanut digger-shaker-windrower for use on Virginia-type peanuts. For minimum shelling damage and foreign material during combining from the windrow, careful adjustments must be made in rate of forward speeds, picking cylinder and raddle clearances, and in the air baffles. Mills and Dickens (1958) explain that windrow harvesting method for peanuts offer many peanut growers the opportunity to eliminate the need for keeping a large labor force for peak harvest seasons. For instance, over 22 man-hours of labor per acre can be saved with the windrow method. They further state that the savings on labor costs alone can

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justify the cost of the equipment needed for the windrow method if over 30 acres of peanuts are grown. Windrow combined peanuts in the Virginia-Carolina area are seldom dry enough for storage and must be artificially dried. Teter and Givens (1957) discuss curing and mechanical drying of peanuts and give recommendations for these operations in connection with mechanical harvesting and windrowing in the VirginiaCarolina area.

2. Irish Potatoes Approximately 5000 mechanical potato harvesters were used on farms during 1958. Martin and Humphrey (1951) report on the early development of Idaho potato harvesters. Most of these first harvesters were equipped with sacking attachments and have since been converted to load directly into a truck with a bulk box conveying or dumping attachment for unloading. Norton et al. (1956) tell of modern equipment types that were first used to any extent in Florida in 1953 and 1954. Bartlett and Huntington (1956) tell of early concepts and present status of potato harvesting in Maine where stones in the soil are the greatest limitation to the satisfactory performance of mechanical harvesters. However, they state that during 1955 several commercial machinery manufacturers had harvesters working successfully in Maine and many growers plan to use the new-type harvester on their crops as soon as their farming operations can be adapted to bulk handling methods. The potato-growing areas of northern California, southern Oregon, and Washington are using commercially manufactured harvesters to good advantage. However, mechanical harvesting has developed slowly in the central California San Joaquin Valley area, primarily owing to the difficulty of clod separation. Glaves and French (1958) describe certain developments with respect to mechanical potato harvesting, including general harvester design and its evolution, de-vining accessories, clod and stone separation, and transfer into the hauling container. Hopkins (1956) gives the results of an investigation of potato injury during digging and the influence of digger blade and elevator chain design on these injuries. 3. Sweet Potatoes

At present sweet potatoes are harvested with some type of plow or digging blade that lifts them from the ground. This is followed by hand picking. Park et aZ. (1953) give an extensive account of machinery for growing and harvesting sweet potatoes. Although equipment has been improved in recent years, complete mechanical harvesting without excessive bruising is not possible at present.

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4. Sugar Beets A resume of sixteen years of research in sugar beet mechanization is given by Walker (1948). He explains that in 1942 there were four companies with experimental machines in the field and in 1947 approximately 4000 harvesters were used to harvest 30 per cent of the United States crop. In 1958 approximately 20,000 harvesters gathered essentially 100 per cent of the crop. Barmington and McBirney (1952) describe sugar beet mechanization in Colorado, while Armer (1953) describes the development of a harvester for Ireland's sugar beets, Bainer et al. (1955) describe the principles used in topping, gaging, cutting, lifting, and loading sugar beets with presentday machines. Several different principles of harvesting are in use. One is in-place topping followed by two rotating converging wheels that loosen and lift the beets; another method lifts the untopped beets by means of a spiked wheel and the tops are cut by two rotating disks just before they are placed in the carrying container. 5. Table Beets, Carrots, and Turnips Mahoney (1953) and Boswell (1953) discuss the necessity and progress of mechanizing such crops as red beets, carrots, and other vegetable crops. The first mechanical harvesters for these crops were developed for vegetables used in commercial processing and canning where bruising and cutting is not nearly as serious as when the crop is used for the fresh market. However, in many cases these machines are now being used for harvesting fresh market products. The latest machines dig, top, and load in one operation. Specially designed plow points lift the beets or carrots as guide rods lift the leaves which are firmly engaged between two rubber belts. After the topping mechanism has removed tops which fall onto the ground, the vegetables are conveyed into a side-delivery elevator and thence to a truck or trailer alongside the machine. 6. Onions

Lorenzen (1950) describes the development of an experimental mechanical onion harvester capable of digging, lifting, topping, and sacking the onions in one operation. Under normal field operations the machine handled about 2 acres in a 10-hour day. Since 1950, at least four individuals or small manufacturers have developed and offered onion harvesters for sale. Johnson (1957) describes one harvester which picks up onions after a machine has cut underneath the bulbs so that the tops will dry. The harvester picks the onions up, then a fan blows the tops up

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while the cutter bar clips them from the bulbs. An air blast blows the cut tops onto the field while the bulbs are caught in a box. Onion harvester development has been encouraging, and it can be expected that within the next few years a large percentage of the commercially grown crop will be mechanically harvested.

7. Radishes Small one-row, as well as three- and five-rowmounted radish harvesters have recently been put into use in several of the radish-growing areas. The multiple-row mounted harvester is capable of pulling and topping 10 acres of radishes in a 10-hour day, while traveling at a speed of about 3 miles per hour. It is most commonly adjusted to harvest five rows planted 9 inches apart, although it can be adjusted to different bed widths and numbers of rows per bed. The radishes are pulled by the tops between pairs of moving V-belts and are carried to revolving knives for topping. Conveyors take them to a trailer or truck which moves along-

FIG.11. Radish harvesting is completely mechanized with small one-row or large tractor-mounted machines as shown in this photo. (Courtesy Tawco Pruducts, Inc.)

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side the harvester. The small one-row harvesters are available for operation in small fields and are operated by a person walking behind them. A small gasoline motor powers the unit and a platform is carried on the machine for placing boxes or crates to receive the harvested radishes (Fig. 11 ).

H. TREENUT AND FRUITHARVESTING EQUIPMENT The harvesting of fruit and nuts by hand is a costly operation and much work is being done in an effort to provide harvest aids which will partially mechanize the harvesting operation. For both fruit and nut removal various mechanical shaking methods are being used. In some cases, the product is allowed to fall on the ground where it is picked up mechanically or by hand; in other cases, catching frames are provided for receiving it and conveying into some type of container.

1. Walnut, Pecan, Filbert, and Almond Walnuts are the most completely mechanized of the tree nut crops. Growers of a large percentage of the crop use mechanical shakers to remove the crop from the trees. Special tractor-mounted pickup machines windrow and pick up the walnuts and put them into trailers pulled either beside or behind the harvester. Closely spaced steel fingers act as a brush, raking and throwing the nuts into a drag elevator which in turn dumps them into a screen conveyor. Soil falls through the screen while leaves and trash are blown upward and away by a fan. It is necessary for fields to be prepared to a very smooth surface so that this type of equipment may be used for picking up the nuts. Mechanical shaking devices are becoming more popular each year for pecans although the crop is still practically all picked up by hand. Approximately 35 per cent of the 1958 filbert crop was harvested with some type of mechanical harvesting device. The nuts are allowed to fall when ripe so they will be free of the husks, they are picked up with either a brush or sweeper device, or they are raked into piles by hand and then scooped into a cleaning or sacking machine which is pulled through the orchard. It is estimated that 40 to 45 per cent of the 1958 almond crop was harvested mechanically with new harvesters similar to those used for picking up walnuts. Mechanical shakers are not used extensively for almonds at present because they do not clean the tree without additional poling or knocking. It is estimated that some type of mechanical shaker was used for 5 per cent or less of the almonds harvested in 1958, but shakers will likely be accepted rapidly once they have been sufficiently developed to clean the trees.

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2. Primes niul Figs Prunes are harvested by hand before they fall or from the ground after they fall. A machine which efficiently picks prunes up off the ground has recently been developed by Adrian and associates, U. S. Dept. of Agriculture ( 1958b) (Fig. 13).The machine is self-propelled similar to a lawn mower and is operated by one man. It is approximately 20 inches

FIG.12. Experimental prune harvester picks prunes from ground after they have been shaken from the tree. Adaptations of this principle are being used for machines to pick up other tree fruit and nut crops. (Courtesy of Agricultural Research Service, USDA.)

wide and will pick up approximately 1000 pounds of prunes per hour. Two small rollers rotate to pass objects between them as the machine moves over the ground. The fallen fruit is pulled into the space between the two rollers and passed back between two conveyor belts that carry the prunes to a box at the rear of the machine. For most efficient operation, the orchard ground must be level and free of stones, broken branches, and other debris. It is expected that a considerable number of machines

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using this principle will be in commercial manufacture and available for prune growers during 1959. Experimental results in picking figs up off the ground by use of the same type of equipment as described above for prunes are encouraging. It is quite possible that the same type of machine can be adapted for efficiently picking up figs. 3. Grapes Lamouria et al. (1958)report on the development of an experimental grape-harvesting machine that not only reduces hand-labor requirements but approaches hand-picking efficiency, provided the vines are trained so that the clusters of grapes hang uniformly underneath the wires holding them. The machine, mounted on a four-wheel tractor, clips the bunches of grapes with a moving knife and loads them into a trailer pulled by another tractor between the adjoining grape rows. The performance of the harvester during 1957 trials proved that it was capable of commercial harvests provided the vines are properly trained; however, it is expected that several years will be required for properly training vines. 4. Fruit Harvest Aids Lamouria et al. (1957) describe exploratory trials to evaluate the feasibility of shaking peaches and Bartlett pears onto a catching frame. Over 75 per cent of the peaches shaken onto a frame were free of visible injury, while at two other locations approximately 79 and 59 per cent of mechanically harvested pears were free from visible injury. Measurements show that more fruit was damaged in the fall through the tree than was injured in falling onto and then over the catching-handling apparatus. Tests showed that the taller the trees, the more the injury that can be expected from the fruit falling through the various branches and onto the catching container. Adrian and Fridley (1958) report on the effects of frequency and stroke of mechanical shakers on fruit removal and power requirements, as analyzed in a study of reciprocating-type shakers. The force and power requirement tests were carried out with the use of an oscilloscope and strain gages. Fruit removal was found to be affected primarily by four variables: ( 1) the frequency of the shake; (2 ) the length of stroke; (3) the force ( F ) required to remove the fruit divided by the weight ( W ) of the fruit ( F / W ) ; and ( 4 ) the number of fruit-bearing branches in any given tree. A number of years of observation will be needed before final judgment on possible tree damage caused by shaking can be made. Adrian and Fridley further states that visual observations made in these studies

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indicate that boom shakers may cause less tree damage than cable shakers. Hill and Brazelton (1955)have developed a machine called the “steel squirrel” which makes it possible for one man to do one and one-half to two times as much work in the orchard operations which require ladders. The machine is operated with an air-cooled, gasoline engine which also runs an air compressor. The compressed air is used to operate pneumatic tools and a lift cylinder. All controls are mounted to enable the operator to maneuver the machine by hand- or foot-operated controls so that both hands are left free to work. Other machines somewhat similar to the one described have recently been developed and placed on the market. This type of machine is becoming quite popular for pruning and thinning work, but has not been used to any large extent for picking. The cost of such machines at present is too high for use by one person in the picking operation. It is also difEcult to maneuver such machines in apple orchards and for similar crops where props are needed to hold up the limbs until the fruit is removed. Gaston and Levin (1953) made time and motion studies of apple picking to determine the possibilities of mechanizing harvesting operations. They studied existing equipment, such as mobile platform, mobile ladders, hydraulic booms, and picking tubes. They concluded that it is unlikely that apple picking will be completely mechanized in the near future, but that apple picking could be made easier and per-man production could be increased by the use of mechanical aids which would carry the weight of the harvested fruit and transfer it to the ground automatically. Gaston and Levin (1956)further studied and helped develop equipment for handling apples in bulk boxes, Their studies indicated that if an extensive amount of bruising is to be avoided, considerable care must be taken in filling and emptying bulk boxes. However, actual counts showed that the amount of bruising that occurred when bulk boxes were filled does not exceed that which occurred when field crates were used. Levin and Gaston (1958)describe in detail the equipment used by deciduousfruit growers in handling bulk boxes. They estimate that in 1957 over 4 million bushels of fruit were moved in bulk boxes in the United States. This was a considerable increase over previous years and indications are that this trend will continue. McBirney (1957)reports that the use for bulk harvesting and handling of apples for the fresh-fruit market is a new development in the Pacific Northwest. He stated that approximately 750,000 bushels of apples and 2000 bushels of pears were harvested in the

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bulk bins in the Pacific Northwest in 1957. Considerable research work is being carried on in the Northwest by private and public research agencies in studying suitable bin sizes, height, capacities, types, designs, and materials, and the merits of each. Levin and Gaston (1956) developed a method of handling red cherries in water from the orchard to the processing plant. Their studies show that handling red cherries in water from the time they are harvested in the orchard until they reached the processing plant helps to maintain the quality, provides a means of improving grade by orchard sorting, reduces cost of handling, eliminates lugs and lug storage (as well as maintenance, distribution, and accounting problems connected with lugs ) , simplifies management, and is commercially feasible. In 1958, almost all the red cherry crop in Michigan was handled and transported from the orchard to the processing plant in water. Gaston and Levin (1957) developed a ventilated picking lug for strawberries. The new lug is used both for picking and transportation to the processing plant. The term “ventilated is used because the sides are lower than the ends thereby permitting air to move freely through the stack of lugs. The container is fitted with a metal handle which is easily removed and makes it possible for the picker to use the container in much the same way as the older type. Gaston et al. (1958) also developed a detachable lug carrier for harvesting raspberries to be processed. The lug is similar to that described above for use in picking strawberries, but the carrier is different in that it consists of a metal frame made from %-inch iron rod that holds the lug approximately 10 inches off the ground and at the same time provides a means of carrying it. Unpublished data further indicate that Levin, Gaston, and Hedden have made progress in blueberry harvesting by the development of a self-propelled catching frame and the use of a specially designed shaking device attached to an electric drill for removal of the blueberries which fall onto the catching frame before being further delivered into boxes at the end of the frame. This development may soon further revolutionize the harvesting of this crop so that considerable reduction in harvesting costs is realized. U.S. Department of Agriculture ( 1959) report that Adrian and Fridley in cooperation with a commercial manufacturer have developed a new self-propelled catching conveyor which will soon be available for the harvesting of such fruit as prunes, plums, and other small tree fruits where it is not desirable to shake the fruit onto the ground. With three men operating a pair of the self-propelled catcher-conveyors and a tractormounted tree shaker, it will be possible to harvest 30 to 50 trees in an

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hour. Each catcher-conveyor is 18 feet long, with a 6-foot wide conveyor belt the length of the machine. In addition, it has a 4-foot tilted but flexible flap on one side and a 1%-foot-wideflap on the other cut in the center so as to fit halfway around a tree, With two catcher-conveyors in use, the short treeside flaps would be adjusted so as to lap over the treeside flaps of the other catcher-conveyor. As the fruit falls, the 6-foot belt carries the fruit to one end of the machine into bulk boxes. Additional personnel are required to remove full containers and place empty ones on the rack as the units move from tree to tree.

I. MISCELLANEOUS CROPHARVESTING EQUIPMENT

1. Sugar cane Although the sugar cane harvesting method used at present in Louisiana is mechanized, it is not accomplished in a once-over cutting and loading operation. The present harvesters cut the cane at the ground, remove the tops and lay the cane back on the ground in windrows. The dry leaves are burned from the cane in the windrow, after which it is picked up by tractor-mounted grab forks and loaded into trailers for delivery either to the mill or a central loading point. Ramp (1956) reports on the development of an experimental sugar cane harvester for Louisiana. The machine is a once-over type harvester consisting of gathering, cutting, topping, stripping, and loading attachments mounted on a four-wheel chassis which pulls the trailer behind for receiving the harvested cane. The machine has both a lower and upper stripping device made of revolving cylinders with rubber stripping fingers which remove all of the dry leaves and most of the green leaves from the cane. The machine is provided with a cable and winch-type trailer hitch which makes the unhitching and hitching of trailers to the harvester simple and quick. Unpublished 1958 test results from the experimental harvest show that the ground loss is less than 5 per cent, depending upon the condition of the cane and the extent of cane borer damage. The average trash content of the cane after harvesting varies between 2 and 6 per cent, depending upon such factors as variety and condition of the cane and the speed at which the machine and stripper finger mechanism are operated. Field machinery for cutting, bulk loading, and hauling of sugar cane in Hawaii is described pictorially in Anonymous ( 1956b). Duncan ( 1950) describes the development of an experimental harvester for Hawaii consisting of a machine for cutting the entire cane stalks at the ground level and loading them into transport equipment.

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2. Tobacco

Production of flue-cured tobacco is just over 1 million acres. Most recent harvest-aid machine developments for tobacco vary all the way from very small one-man operated machines to large machines which carry several workers. The tobacco is primed by workers riding on the lower part of machines and is elevated by endless conveyors to the upper part of the carrier, where it is speared and placed in racks until unloaded at the end of the rows or curing shed. Winn and Burkhardt ( 1054) describe a two-man harvesting and spearing machine, operated by a foot control, with one man priming and a second man operating the spearing machine. Wilson (1056a)reports on studies comparing different types of mechanical priming devices. A careful analysis was made of the operations involved in leaf selection and removal, damage to the leaves, and the handling of leaves after priming, Wilson concludes that the design of a machine should be based upon its weekly capacity for each acre, as each acre must be harvested weekly for up to 6 weeks. A machine capable of harvesting 3 acres per day (one barn of tobacco) could handle approximately 18 acres per year. It is conceivable that a machine of this capacity could be used cooperatively by more than one grower, according to Wilson. Development work on a completely mechanical tobacco harvester is known to be in progress by at least one university.

3. Castor Beans A r m s and Hurlbut (1052) and Schroeder and Reed (1952)describe early models of experimental castor bean harvesters. Schoenleber et aZ. ( 1057) describe the development of a two-row, tractor-mounted, complete combine-type castor bean harvester which gathers, hulls, and places the beans in a bin provided for dumping them into truck transport at the end of the rows. Field tests during 1956 in dwarf castor beans yielding 2700 pounds per acre and ranging in height from 30 to 48 inches resulted in the machine having less than 5 per cent harvesting loss when traveling at a speed at 25 miles per hour. Coppock and Schoenleber (1057) describe the development of a castor bean harvester for California in cooperation with growers and industry, using basic principles described above but designed to harvest beans 10 to 15 feet tall, a height often attained with present varieties under Western irrigated conditions. The machine harvested beans yielding 3000 pounds per acre at a speed of 2% miles per hour. The main new

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features of the Schoenleber and Coppock machines are oscillating brush seals at the lower part of the gathering device which prevent seed loss, rotating knockers which hit the plant and knock the beans from the stalks, and the huller which removes the hulls from beans before placing them into the bin. Commercial machines patterned after the SchoenleberCoppock design were used to harvest approximately 25,000 acres of castor beans in the United States in 1958. 4. Tung

Kilby and Jezek (1957) describe the development of harvesting principles used experimentally to harvest tung nuts. The machine being tested at present consists of a rotating brush-type windrower which places the nuts in a windrow which is straddled by the tractor. A pickup device, pulled behind the tractor, using steel fingers operating on an endless flexible metal belt, picks the nuts up from the windrow, a fan separates some of the foreign matter, and conveyors deposit the nuts into the trailer pulled behind the harvester. One of the main obstacles encountered in mechanizing the harvest of this crop has been the difficulty of smoothing the ground in presently established orchards and the large amount of dead stems, leaves, and trash which are gathered with the nuts. Nuts harvested in this manner generally contain too much moisture, particularly during a wet season, and must be dried, which further complicates the harvest problem. 5. Kenaf Whittemore and Cocke (1954) tell of the mechanization of kenaf, a plant which produces fiber that can be used as a substitute for jute. Although there is no commercial kenaf production in the United States at this time, considerable work has been conducted toward mechanization of this crop. Byrom (1958) describes the development of a kenaf harvesting machine known as a harvester-ribboner which cuts the tall kenaf plants and passes them through a decorticating device mounted on the harvester. The decorticator removes all of the extraneous matter from the stalks, leaving the long kenaf ribbons which are loaded onto a trailer and taken to the processing plant.

6. Fiber Flax Pulling machines for harvesting fiber flax are described by Harmond and Klein (1955). Several different foreign makes are described as well as the self-propelled push-type puller which was developed on the U. S. Department of Agriculture project in cooperation with Oregon.

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VII. Conclusions

With the tremendous improvement, modernization, and development of agricultural machinery in recent years, it is obviously impossible to explore all of the changes that have taken place. It is equally impossible to present more than an introduction to the major trends that are discussed. The details of each new development must be found in the literature citations and through more intensive study of specific phases of agricultural progress. It should be recognized, however, that each new development and improvement in modern farm equipment is the result of joint contributions from many disciplines, This progress could not have been achieved without the rapid advances in metallurgy and bearing design, or the many improvements in hydraulic controls and electrical devices. New concepts in the fields of basic plant science from the standpoint of plant breeding, plant physiology, fertilization requirements, and over-all management needs have provided other clues and guides in the machine development process. The future holds much promise! The continued need for increased farm efficiency provides the stimulus for new developments. Automation, now reflected in new tractor torque converters and transmissions, the bale ejector-loader, the photoelectric thinners, and the electronic cultivator guides, will provide a whole new realm for machine development. The combining or “unitizing” of equipment and operations, well exemplified by planters that prepare the final seedbed, apply insecticides, fungicides, herbicides, fertilizers, and plant seeds all in one pass over the field, will become increasingly important. The reduction of seasonal peak labor loads through use of larger and more efficient tillage, planting, and harvesting equipment is inevitable. Machines will be developed to replace the high hand-labor requirements of many specialty crops just as the mechanical cucumber harvester replaced forty pickers on one farm. Farmers, engineers, and agricultural scientists alike have an unlimited challenge in the field of continued agricultural mechanization.

REFERENCES Aasheim, T. S. 1949.Montana Agr. Expt. Sta. Bull. 468. Adrian, P. A., and Fridley, R. B. 1958. California Agr. 12, 3, 15. Akesson, N. B., and Harvey, W. A. 1948.California Agr. Expt. Sta. C ~ T C 389. . Aldrich, S . R. 1956. What’s New in Crops and Soils 9, 3.

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Aldrich, S. R., and Musgrave, R. B. 1955. New York Agr. Expt. Sta. Agron, Mimeo. 951.

Anonymous. 1950. Texas Agr. Expt. Sta. Bull.

727;

Oklahoma Agr. Expt. Sta. Bull.

6-361.

Anonymous. 1956a. Engineering 182,316. Anonymous. 1956b. Hawaiian Sugar Planters’ Assoc. Engr. Bull. 73. Anonymous. 1956~.“Texas A & M College System Handbook on Aerial Application in Agriculture.” College Station, Texas. Anonymous. 1958. Ann. Rept. The Aviation Industry of N e w Zealand, Inc., Wellington, N e w Zealand. Armer, A. A. 1953. Agr. Eng. 34,312,314. Arms, M. F., and Hurlbut, L. W. 1952. Agr. Eng. 33,784-786. Arya, S. V., and Pickard, G.E. 1958. Agr. Eng. 39, 16, 19,23. Autry, J. W., and Schroeder, E. W. 1953. Agr. Eng. 34,525,527,531. Baggette, T. L. 1949. Agr. Eng. 30,489,495. Bainer, R., Kepner, R. A., and Barger, E. L. 1955. “Principles of Farm Machinery.” Wiley, New York and Chapman and Hall, London. Barmington, R. D., and McBirney, S. W. 1952. Colorado Agr. Expt. Sta. Bull. 420-A. Bartlett, H. D., and Huntington, D. H. 1956. Maine Agr. Expt. Sta. Bull. 549. Baugh, E. R., Hickock, R. B., Kohnke, H., and Mayer, I. D. 1950. Agr. Eng. 31, 399. Been, W. G. 1957. Market Growers J. 06, 17. Bigsby, F. W. 1958. Am. SOC. Agr. Engr. Mimeo. Paper. 58-61. Black, D. T. 1956. U. S. Dept. Agr. ARS 42-6. Black, D. T., Ditman, L. P., and Burkhardt, G. J. 1954. U. S. Dept. Agr. Circ. 946. Bockhop, C. W., and Barnes, K. K. 1955. Agr. Eng. 36, 453-457. Boswell, V. R. 1953. Am. Vegetable Grower 1, 7, 20. Brittain, R. W., Brazee, R. D., and Carleton, W. M. 1955. Agr. Eng. 36, 319320, 323. Brodell, A. P., Strickler, P. W., and Pittman, D. D. 1952. U. S. Dept. Agr. Bur. Agr. Econ. Pub. FM91. Brown, R. T. 1957. Agr. Eng. 38, 804-805. Browning, G.M. 1950. Agr. Eng. 31, 341344. Buchele, W. F., Collins, E. V., and Lovely, W. G. 1955a. Agr. Eng. 36, 324-329, 331. Buchele, W. F., Collins. E. V., and Lovely, W. G. 195%. Iowa Farm Sci. 9, 3-5. Buhr, A. G. 1955. Agr. Eng. 36, 649, 650, 653. Bunnelle, P. R., Jones, L. G., and Goss, J. R. 1954. Agr. Eng. 35, 554-558. Burgesser, F. W. 1950. Proc. 6th Meeting Am. SOC. Sugar Beet Technologists pp. 79-84.

Burrough, D. E., and Graham, J. A. 1954. Agr. Eng. 35, 221-229, 232. Butt, J. L., Kelley, W. B., Martin, C. M., and Smith, L. A. 1956. Alabama Agr. Expt. Sta. L e a f i t 49. Byrom, M. H. 1958. Proc. Kenaf Conf. Hauana, Cuba. Carreker, J. R. 1950. Agr. Eng. 31, 445-447. Chamberlin, J. C., Getzendaner, C. W., Hessig. H. H., and Young, V. D. 1955. U.S. Dept. Agr. Tech. Bull. 1110. Chepil, W. S., and Woodruff, N. P. 1955. Kansas Agr. Expt. Sta. Circ. 318. Chisholm, J. A. 1955. Market Growers 1. 84, 8. Clyde, A. W. 1956. Agr. Eng. 37, 173-176. Collins, E. V. 1951. Agr. Eng. 32, 216-217. Colwick, R. F. 1955. Cotton Gin 6 Oil Mill Press 56, 11-12. Colwick, R. F., and Regional Technical Committee Members. 1953. Southern Coop. Ser. Bull. 33.

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Cook, R. L., and Peikert, F. W. 1950. Agr. Eng. 31, 211-214. Cook, R. L., Turk, L. M., and McColly, H. F. 1953. Soil Sci. SOC.Am. Proc. 17, 410414.

Cooper, A. W., Vanden Berg, G. E., McCoUy, H. F., and Erickson, A. E. 1957. Agr. Eng. 30,232-235. Coppock, G. E., and Schoenleber, L. G. 1957. U.S. Dept. Agr. ARS 42-8. Corley, T. E., Stokes, C. M., and Kummer, F. A. 1955. Highlights Agr. Research Alabama Polytechnic Inst. 2, 1. Cykler, J. F. 1950. Agr. Eng. 31,78-77,79. Dobie, J. B. 1959. Agr. Eng. 40,78-77,92-93. Dudley, R. F., and Wise, L. N. 1953. Mlsstssippi Agr. Expt. Sta. Bull. 505. Duke, G. B. 1957. U.S . Dept. Agr. ARS 42-1 1. Duley, F. L. 1948. U.S. Dept. Agr. Farmers’ Bull. 1997. Duley, F. L. 1954. Nebraska Agr. Ext. Seru. Ext. Circ. EC 54-100. Duncan, R. A. 1950. Agr. Eng. 31, 85-88, 70. Elfes, L. E. 1954. Agr. Eng. 35, 147-153. Fairbank, J. P., and Smith, K. 0. 1950. Agr. Eng. 31, 219-222. Fairbanks, G. E. 1951. Karwm Engr. Expi?. Sta. Bull. 66. Farnham, S. E. 1958. Ind. Lab. 9, 5758, 80. Farrow, R. 1958. Market Growers 1. 05, 33. Free, G. R. 1953. Soil Sd.SOC.Am. Proc. 17,185-170. Futral, J. G., and Allen, R. L. 1951. Agr. Eng. 32, 215-218. Gantt, C. W., Jr. 1958. Agr. Eng. 37, 537439, 542. Gantt, C. W., Jr., Hulburt, W. C., and Bowen, H. D. 1958. U.S. Dept. Agr. Farmers’ Bull. 2096. Gaston, H. P., and Levin, J. H. 1953. Michigun Agr. Expt. Sta. Quart. Bull. 36, 18-23.

Gaston, H. P., and Levin, J. H. 1958. Michigan Agr. Expt. Sta. Bull. 409. Gaston, H. P., and Levin, J. H. 1957. Michigan Agr. Expt. Sta. Quart. Bull.

39, 548-

558.

Gaston, H. P., Levin, J. H., and Hedden, S . L. 1958. Michigan Agr. Expt. Sta. Quart. Bull. 41,118-121. Giles, G. W., and Routh, C. A. 1951. Agr. Eng. 32, 537440, 544. Glaves, A. H., and French, G. W. 1958. Potato Handbook 3,5348. Goss, J. R., Bainer, R., Curley, R. G., and Smeltzer, D. G. 1955. Agr. Eng. 36, 794-796.

Gray, R. B. 1954. “Development of the Agricultural Tractor in the U. S.,” Pt. I. Am. SOC. Agr. Engr., St. Joseph, Michigan. Gray, R. B. 1955. U.S. Dept. Agr. Farmers’ Bull. 1761. Gray, R. B. 1958. “Development of the Agricultural Tractor in the U. S.,” Pt. 11. Am. SOC.Agr. Engr., St. Joseph, Michigan. Gray, R. B., and Dieffenbach, E. M. 1957. Agr. Eng. 30, 388-397. Guelle, C. E. 1954. Agr. Eng. 35, 185-187. Gunn, J. T., and Tramontini, V. N. 1955. Agr. Eng. 36, 725-729. Hall, B. J. 1958. Am. Vegetable Grower 4,24-25. Hansen, C. M., 1958. Agr. Eng. 39, 548551. Hansen, C. M., Robertson, L. S., and Grigsby, B. H. 1958. Michigan Agr. Erpt. Sta. Quart. Bull. 40,549-554. Harmond, J. E., and Klein, L. M. 1955. U.S. Dept. Agr. C ~ T C955. . Harrison, G. J. 1951. Agr. Eng. 32, 488-488, 492.

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Harrold, L. L., and Dreibelbis, F. R. 1950. Agr. Eng. 31,393-397. Hays, 0.E., and Taylor, R. E. 1958. U. S. Dept. Agr. Farmers’ Bull. 2116. Hedman, C. L., and Turner, J. R. 1954. Agr. Eng. 35,801-803. Heitshu, D. C. 1952. Agr. Eng. 33,343-346. Heitshu, D. C. 1956a. Agr. Eng. 37,182183,187. Heitshu, D. C. 1956b. Soybean Dig. 16,5042. Hill, F. L., and Brazelton, R. W. 1955. Agr. Eng. 36, 17-19. Hofmeister, H. J. 1955. Mayland Agr. Expt. Sta. Misc. Publ. 224. Holmes, L. J. 1950. Proc. 6th Meeting Am. SOC. Sugar Beet Technologists pp. 249254. Hopkins, R. B. 1956. Agr. Eng. 37,109-111. Howe, R. S., Jr., and Raley, G. H. 1958. Agr. Eng. 39, 152-155,171. Howell, W. C., and Jones, T. N. 1954. Mississippi Agr. Expt. Sta. Inform. Sheet 501. Hudspeth, E. B., and Jones, D. L. 1954. Texas Agr. Expt. Sta. PrOgT. Rept. 1673. Hulburt, W . C. 1956. PTOC.Am. Assoc. Aduance. Sci. Sect. 0. Hulburt, W. C., and Menzel, R. G. 1953. Agr. Eng. 34,702-704,706,708. Hurlbut, L. W. 1955. Agr. Eng. 36,791-792. Isler, D. A., and Thornton, D. G. 1955. Agr. Eng. 36, 600-601,604. Jacks, G. V., Brind, W. D.,and Smith, P. 1955. Commonwealth BUT. Soil S C ~ . (Gt. Brit. ) Tech. Commun. 49. Johnson, W. B. 1957. Market Growers I. 86,28-29. Johnson, W. H. 1955. Ohio Farm and Home Research 40,40-42,49. Jones, J. N., Jr,, Lillard, J. H., and Hines, R. C., Jr. 1951. Agr. Eng. 32,417419. Kepner, R. A. 1957. California Agr. 11,4-6, 13-14,20. Kilby, W. W., and Jezek, R. E. 1957. Mksissfppi Agr. Expt. Sta. Bull. 548. King, R. W., and Elliott, B. G. 1955. Agr. Eng. 36,235-238,241. Klein, L. M., and Harmond, J. E. 1959. U. S . Dept Agr. ARS 42-24. hall, J. L. 1951. Montana Agr. Expt. Sta. Circ. 194. hall, J. L., Power, J. F., and Massee, T. W. 1958. Montana Agr. Ezpt. Sta. BuU. 540. Kramer, R. W. 1955. Agr. Eng. 36,587590. Kromer, 0.W. 1949. Agr. Eng. 30,524527. Kubik, B. M. 1957. Am. Vegetable Grower 12,27-29. Lamouria, L. H., Harris, R. W., Abernathy, H. H., and Leonard, S. H. 1957. CaZif~rraiaAgr. 1 1,ll-12, 14. Lamouria, L. H., Winkler, A. J., Abernathy, G. H., and Kaupke, C. E. 1958. Agr. Eng. 39,218-221,286. Levin, J. H., and Gaston, H. P. 1956. U. S . Dept. Agr. C ~ T C 981. . Levin, J. H., and Gaston, H. P. 1958. U. S. Dept. Agr. ARS 42-20. Liljedahl, J. B., Carleton, W. M., and Kinch, D. M. 1956. Agr. Eng. 37, 550-552, 554. Lillard, J. H., Moody, J. E., and Edminster, T. W. 1950. Agr. Eng. 31,395. Lorenzen, C.. Jr. 1950. Agr. Eng. 31, 13-15. Lorenzen, C., Jr. 1956. Am. Vegetable Grower 4,20. Lovely, W . G. 1956. PTOC.loth Hybrid Corn I d . Research Conf. Iowa, pp. 59-67; Iowa Agr. Expt. Sta. 1. Paper 2055. McBirney, S. W. 1957. Proc. Washington State Hot?. Assoc. 53,139-143. McCalla, T. M. 1958.1. Soil and Water Comeru. 13,255-258. McColly, H. F. 1957. Agr. Eng. 38,398404. McColly, H. F. 1958. Tram. Agr. Eng. 1,68-71, 75. McCreery, W. F., and Nichols, M. L. 1956. Agr. Eng. 37,808-812.

230

T. W. EDMINSTER AND H. F. MILLER, JR.

McKibben, E. G. 1953.Agr. Eng. 34,91-93. Mahoney, C.H.1953.Am. Vegetabk Grower 1,15. Martin, J. W., and Humphrey, E. N. 1951.Idaho Agr. Expt. Sta. Bull. 283. Maughan, G. L., Wood, G. M., and Chittey, E. T. 1959. J. Agr. Eng. Research 4, 30-35. Melsted, S. W. 1954.Aduances in Agron. 6, 121-142. Merrill, R. M. 1956.Plant Food Reu. 2,4-7. Miller, H. F.,Jr. 1955a.Cotton Trade J. ( 1955-56) Intern. Yearbook 36,5657. Miller, H. F.,Jr. 195513.Cotton Gin G Oil Mill Press 56,56-59. Miller, J. B. 1957.Market Growers J. 86, 16. Mills, W. T.,and Dickens, J. W. 1958. North Carolina Agr. Expt. Sta. Bull. 405. Mohsenin, H., Womochel, H. L., Harvey, D. J., and Carleton, W. M. 1956.Agr. Eng. 37,815820. Moody, J. E., Lillard, J. H., and Edniinster, T. W. 1952. Soil. Sci. SOC.Am. Proc. 16, 190-194. Moore, R. P. 1957.Soybean Dig. 17,1616. Morrison, C. S. 1955.Agr. Eng. 36,796-799. Musgrave, R. B., Zwerman, P. J., and Aldrich, S. R. 1955. Agr. Eng. 36, 593-594. Nichols, M. L., and Reaves, C. A. 1955.Agr. Eng. 36,517-520. Nichols, M. L., and Reaves, C. A. 1958.Agr. Eng. 39,340443. Nichols, M. L., Reed, I. F., and Reaves, C. A. 1958.Agr. Eng. 39,336-339. Norton, J. S., Greene, R. E. L., and Kushman, L. J. 1956. Florida Agr. Ex@. Sta. Bull. 579. Nutt, G. B. 1950a.Agr. Eng. 31,391-392. Nutt, G. B. 1950b.Agr. Eng. 31,443-444. Oates, W. J., Witt, R. H., and Wood, W. S. 1952.Agr. Eng. 33, 135-136,142. Page, G.E.1952.Oregon Agr. Ezpt. Sta. Bull. 493. Park, J. K., and Webb, B. K. 1958.South Carolina Agr. Expt. Sta. Bull. 461. Park, J. K., Powers, M. R., and Garrison, 0. B. 1953. South Carolina Agr. Expt. Sta. Bull. 404. Peterson, A. E. 1955.Plant Food Reu. 1,1617,30,31. Peterson, A. E.,Gerge, 0. K., Murdock, S T., Peterson, D. R. 1958. Wisconsin Ext. Seru. Circ. 559. Pickard, G. E. 1955.Agr. Eng. 36,792-794. Pool, S. D.1956.Agr. Eng. 37,245-248. Porterfield, J. G., Schroeder, E. W., and Smith, E. M. 1954. Oklahoma Agr. Expt. Sta. Tech. Bull. TbO. Powers, M. R. 1949.Agr. Eng. 30,496. Poynor, R. R. 1950.Agr. Eng. 31,509510. Ramp, R. M. 1956.Agr. Eng. 37,821-824. Ramser, J. H.,and Kleis, R. W. 1952.Illinois Ext. Sera Circ. 693. Rea, H. E.1954.Texas Agr. Erpt. Sta. Progr. Rep. 1691. Reed, I. F.,and Gordon, E. D. 1951.Agr. Eng. 32,98-100. Reed, I. F.,and McCreery, W.F. 1954.Agr. Eng. 35,91-94,97. Richardson, R. D. 1958.J. Agr. Eng. Research 3,299323. Richey, C. B., O’Donnell, J. F., Ashton, J. T., and Groves, R. J. 1956. Agr. Eng. 37, 93-97. Ryerson, G. E. 1950.Agr. Eng. 31,506-508,510. Sack, W. 1951.Agr. Eng. 32,159-160. Schaller, F. W.,and Evans, D. D. 1954.Agr. Eng. 35,731-734,736.

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231

Schoenleber, L. G., Bouse, F., and Coppock, G. E. 1957. Oklahoma Agr. Expt. Sta. Bull. B-489. Schroeder, E. W., and Reed, I. F. 1952. Agr. Eng. 33,775-776,779. Scoville, 0. J. 1956. Proc. U.S. Dept. Agr. Conf. Field Shelling and Drying of Corn, pp. Rl-R2. Scranton, C. J . 1952. Agr. Eng. 33, 140-142, Smith, H. P. 1955. “Farm Machinery and Equipment,” 4th ed. McGraw-Hill, New York. Southwell, P. H. 1951. World Crops 3,464-468. Stanton, H. S. 1954. Arkansas Farm Research 3. [8]. Stanton, H. S., and Tavernetti, J. R. 1956. Proc. Western and California W e e d Control Conf. pp. 140-145. Strickler, P. W., and Phillips, H. C. 1956. U.S. Dept. Agr. ARS 43-27. Tanquary, E. W. 1957. Agr. Eng. 38,606-609. Tanquary, E. W., and Clyde, A. W. 1957. Agr. Eng. 38,88-93. Tavernetti, J. R., and Miller, H. F., Jr. 1954. California Agr. Expt. Sta. Bull. 747. Teter, N . C., and Givens, R. L. 1957. U . S. Dept. Agr. ARS 42-12. Thompson, J. L., and Kemp, J. G. 1958. Agr. Eng. 39,285-287. U . S . Dept. Agr. 1935a. U.S . Dept. Agr. Inform. Bur. Agr. Eng. Ser. 48. U. S . Dept. Agr. 1935b. U . S. Dept. Agr. Inform. Bur. Agr. Eng. Ser. 52. U . S . Dept. Agr. 1935c. U . S. Dept. Agr. Inform. Bur. Agr. Eng. Ser. 53. U . S . Dept. Agr. 1935d. U . S. Dept. Agr. Inform. Bur. Agr. Eng. Ser. 70. U. S.Dept. Agr. 1949a. U.S . Dept. Agr. Inform. Bur. Agr. Eng. Ser. 73. U . S. Dept. Agr. 1949b. U.S. Dept. Agr. Inform. Bur. Agr. Eng. Ser. 74. U. S . Dept. Agr. 1954. U.S. Dept. Agr. Farmers’ BuU. 2062. U . S. Dept. Agr. 1956a. U . S. Dept. Agr. ARS Spec. Rept. 22-21. U . S. Dept. Agr. 1956b. U. S. Dept. Agr. ARS 22-34. U . S. Dept. Agr. 1958a. U.S. Dept. Agr. Farmers’ Bull. 2118. U . S . Dept. Agr. 1958b. U S. Dept. Agr. Agr. Research 6,14. U. S. Dept. Agr. 1958c. U.S. Dept. Agr. T h e Pesticide Situation fw 195748. U . S. Dept. Agr. 1958d. U . S. Dept. Agr. Library List No. 65. U. S. Dept. Agr. 1959. U . S. Dept. Agr. Agr. Research 7. U . S. Dept. Commerce. 1957. U . S. Dept. Commerce C A A Statistical Handbook of Civil Aviation 42,47-52. Van Doren, C. A., and Hays, 0. E. 1958. U . S. Dept. Agr. Leafkt 435. Wagner, R. E., and Hulburt, W. C. 1953. What’s New I n Crops and Soils 6,8-9. Walker, H . B. 1948. Agr. Eng. 29,425-430. Walker, H. B. 1952. Agr. Eng. 33,698,701,704. Walker, H. B. 1957. Agr. Eng. 38,6584361,676. Weick, F. E. 1952. Agr. Eng. 33.361364. Whittemore, H . C., and Cocke. J. B. 1954. Agr. Eng. 35,488-491. Willard, C. J., Taylor, G. S., and Johnson, W. H. 1956. Ohio Agr. E r p t . Sta. Research Circ. 30. Williamson, E. B. 1955. Cotton Trade 1. (1955-56) Intern. Yearbook 36, 58, 61-62. Williamson, E. B. 19.58. Proc. 12th Ann. Cotton Mechanization Conf., Brownsoilk,

Texas. Willimisoii, E. B., Wooten, 0. B., and Fulgliain, F. E. 1954. Mississippi Agr. Exlit. Sta. Bull. 515. Wilson, J. D. 1956. Am. Vegetable Grower 4,9, 32-33.

232

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Wilson, R. W . 1956a. Agr. Eng. 97,407410. Wilson, R. W. 1956b. North Carolina Agr. Expt. Sta. Bull. 397. Winkelblech, C. S. (no date) New York Agr. Engr. Elct. Bull. 331. Winn, P. N., and Burkhardt, G. J. 1954. Mayland Agr. Expt. Sta. Bull. 542. Witzel, H. D., and Vogelaar, B. F. 1955. Agr. Eng. 36,525,528. Woodruff, N. P., and Chepil, W. S. 1956. Agr. Eng. 37,751-754,758. Wooten, 0. B., and Montgomery, R. A. 1956. Mkdssippl Agr. Expt. Sta. Clrc. 204. Work, P. 1958. Am. Vegetubk Grower 6,13-15. Worthington, W. H., and Seiple, J. W. 1952. Agr. Eng. 33,273-276,278. Yeo, R. R. 1955. N e w Mexico Agr. Expt. Stu. BuU. 391. Young, V. D., Chamberlin, J. C., Getzendaner, C. W., and Doenier, C. E. 1957. U.S. Dept. Agr. ARS 42-10. Zingg, A. W., and Whitfield, C. J. 1957. U.S. Dept. Agr. Tech. Bull. 1166.

FERTILIZER PRODUCTION AND TECHNOLOGY

. .

K . D. Jacob

U S Department of Agriculture. Beltrville. Maryland

. .

I Introduction . . . . . . . . . . . I1 Consumption of FertiIizers and Plant Nutrients . . A. World Consumption . . . . . . . B Consumption in the United States . . . . I11. Nitrogen . . . . . . . . . . . . A Fixation Methods . . . . . . . . B. Nitrogen Materials . . . . . . . . IV Phosphorus . . . . . . . . . . . . A Phosphate Rock . . . . . . . . . B. Elemental Phosphorus . . . . . . . C Phosphoric Acid . . . . . . . . D Superphosphates . . . . . . . . E . Ammonium Orthophosphates . . . . . F Dicalcium Phosphate . . . . . . . G Treatment of Phosphate Rock with Nitric Acid H . Metaphosphates . . . . . . . . . I. Basic Slag . . . . . . . . . . J . Defluorinated Phosphate Rock . . . . . K . Phosphate Rock-Magnesium Silicate Glass . L. Rhenania-Type Phosphates . . . . . . M . Aluminum and Iron Phosphates . . . . N Radioactive Phosphates . . . . . . . 0. Other Phosphate Materials and Processes . . V Potassium . . . . . . . . . . . . A.Production . . . . . . . . . . B. Resources . . . . . . . . . . C. Mining and Processing of Raw Materials . . D . Potassium Fertilizer Materials . . . . . VI Secondary Nutrient Elements . . . . . . . A Calcium . . . . . . . . . . . B Magnesium . . . . . . . . . . C.Sulfur . . . . . . . . . . . VII. Trace Nutrient Elements . . . . . . . . A. Multinutrient Glasses . . . . . . . B Chelates . . . . . . . . . . . C Boron . . . . . . . . . . . 233

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D. Copper, A4angancsc, and Zinc . . . E. Cobalt, Iron, and Molybdeiiririi . . VIII. Mixed Fertilizers . . . . . . . . A. Ammoniation . . . . . . . B. Conditioning Agents . . . . . C. Granular Mixtures . . . . . . D. Liquid Mixtures . . . . . . . IX. Mixtures of Fertilizers and Other Agricultural A. Mixtures with Pesticides . . . . B. Mixtures with Gibberellic Acid . . X. Future Prospects . . . . . . . . References . . . . . . . . . .

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302 302 303 304 305 308 307 309 309 310 311 312

I. Introduction

Developments in fertilizer production and technology were discussed by Jones and Rogers (1949) in the first volume of Advances in Agronomy. The present paper reviews the progress in the later years. This progress was marked by a doubling of the world consumption of the primary plant nutrients-nitrogen ( N ) , phosphoric oxide ( PzOS), and potash ( K20)-supplied by commercial fertilizers and an increase of over 75 per cent in their use in the United States. However, the American farmers’ expenditure for such nutrients in 1957 ($1171 million) was only 60 per cent higher than in 1948, a saving which was made possible, at least in part, by continued advances in fertilizer manufacture and processing. Through the great expansion in facilities for winning the raw materials and for their conversion into fertilizers, the United States is now in the position of being entirely self-sufficient in supplies of plant nutrients. Further conservation of raw-material resources has been accomplished by the development of better methods of mining and beneficiation, and new resources have been discovered. Manufacturing techniques have been steadily improved and new processes that offer much promise have been developed. New products of far-reaching importance have attained large-scale production and use, and others are in the offing. The older products have generally undergone much improvement. Research in the chemistry, technology, and use of fertilizers has far outstripped that in other similar periods of the industry’s history, and the progress has been aided greatly by an increasing degree of cooperation between workers in the several fields. Many companies have installed modern laboratory and pilot-plant facilities, and activities by government agencies and other organizations have been expanded. The literature has grown at an unprecedented rate.

235

FERTLLIZER PRODUCTION AND TECHNOLOGY

It. Consumption of Fertilizers and Plant Nutrients

A. WORLDCONSUMPTION Excluding the Union of Soviet Socialist Republics, the Chinese People's Republic, and North Korea, the world consumption of primary nutrients in commercial fertilizers (excluding ground phosphate rock I 8.0

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7.8

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I

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70

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65-

v)

2 6.00

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45

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40

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3D-

I I940

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I980

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1952

YEAR

ENDED

I 1014

I 'I986

I

IS87

JUNE 30

FIG. 1. World consumption of nitrogen, phosphoric oxide, and potash, years ended June 30, 1948-1957 ( 1957, preliminary). (Food and Agriculture Organization of the United Nations. Excludes Union of Soviet Socialist Republics, Chinese People's Republic, and North Korea; excludes ground phosphate rock used directly as fertilizer. )

236

K. D. JACOB

applied directly to the soil) totaled 11,420,876 short tons in the year ended June 30, 1948 (Food and Agriculture Organization, 1949), an increase of 27.5 per cent over the comparable average for 1936-1938 (Clark and Sherman, 1946). This increase reflected chiefly the remarkable rise (2,566,000 tons, 136.5 per cent) in the use of such nutrients in the United States and the United Kingdom during and after World War 11. In many countries, however, the consumption decreased greatly during the war (Jacob and Parker, 1946), and in some-notably Italy, Germany, Japan, and Spain-the use in 1947-1948 was still much below the prewar average. The world consumption of primary nutrients has continued to rise steadily since 1947-1948, with the setting of new records each year for each of the nutrients (Fig. 1). Thus, according to a preliminary estimate (Page, 1957), the use in 1956-1957 totaled 22,162,216 short tons, or 94 per cent more than in 1947-1948. Likewise, the consumption of nitrogen increased 126 per cent, as compared with 61.5 per cent for phosphoric oxide and 113 per cent for potash. During this period all of the continents (Table I ) and most of the individual countries showed large percentage increases in use of the nutrients. TABLE I World Consumption of Primary Nutrients, in Short Ton by Continents, Year Ended June 30, 19570 Total Increase over 1947-48

Continent

N

Africa Asia Europe North and Central America Oceania South America

237,378 1,278,271 3,381,164 2,209,631 39,606 129,197 _

Total

_

K20

Quantity

(%)

316,306 544,352 3,862,685

62,388 660,688 3,918,649

616,072 2,483,311 11,162,498

187.3 228.6 87.6

2,637,633 719,902 108,347

2,038,658 44,432 73,029

6,785,822 803,940 310,573

74.3 76.1 97.2

PSOS

_

7,275,147

_

_

_

.

8,089,225

_

_

_

6,797,844

22,162,216

93.9

Page (1957); excludes Soviet Union, Chinese People’s Republic, and North Korea.

* Excludes ground phosphate rock used directly as fertilizer.

B. CONSUMPTION IN THE Urmm STATES In the United States and Territories the total consumption of primary nutrients in commercial fertilizers set a new record in 1954-1955 for the

237 sixteenth consecutive year. Thus, the use rose from 1,584,900 short tons in 1938-1939 (Mehring et al., 1957), to 6,119,139 tons in 1954-1955 (Scholl et al., 1958a), an increase of 291 per cent. Another record (6,377,541 tons) was set in 1956-1957, corresponding to increases of 308 FERTILIZER PRODUCTION AND TECHNOLOGY

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3,631,083 I

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-3.5 I

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and 76 per cent over the use in 1938-1939 and 1947-1948, respectively (Fig. 2). With a few exceptions, new records for the consumption of each of the primary nutrients were set in each year of the period 1947-1948 to

238

K. D. JACOB

1956-1957 (Fig. 3). In 1956-1957 the use of nitrogen was 149.2 per cent higher than in 1947-1948, as compared with increases of 110.5 and 24.3 per cent for K20 and available P206,respectively. The smaller increase 2.3

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1956 1957

YEAR ENDED JUNE 30

FIG.3. Consumption of nitrogen, available phosphoric oxide, and potash in the United States and Territories, years ended June 30, 1948-1957. ( U . S. Department of Agriculture.)

in P206relative to nitrogen and K20reflected the trend toward the 1 : l : l ratio in the consumption of these nutrients. Thus, the ratio changed in 1947-1948 to 0.93:l.OO: gradually from 0.46:1.00:0.50( N:P20a:K20) 0.84 in 1956-1957.

1948 1937 1948 I957

NEW ENGLAND

MIDDLE ATLANTIC

1948 1967 1948 1967

WEST NORTH

3

1948 1957

20

w e

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CENTRAL

EAST SOUTH CENTRAL

l9S7

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0: 1948

WEST

SOUTH CENTRAL

I957

MOUNTAIN 1948

I967 1948 I967 1908 I967

PACIFIC

TERRITORIES

FIG.4. Regional consumption of primary nutrients in the United States and Territories, years ended June 30, 1948 and 1957. ( U. S. Department of Agriculture. )

240

K. D. JACOB

1. Regdona1 Consumption Continuing the previous trends, the consumption of total primary nutrients showed increases in each of the regions' except the New England States (Fig. 4). Especially noteworthy were the very large gains in the newer areas of fertilizer use. For example, the annual consumption in 1947-1948 to 1956-1957 rose 243 per cent in the West North Central region, and increases of 192, 115, 112, and 111 per cent were recorded for the Mountain, West South Central, Pacific, and East North Central regions, respectively. Likewise, remarkable gains in the use of the individual nutrients, especially nitrogen, usually occurred in each of these regions. 2. Consumption in Mixed Fertilizers Since about 1925, the proportion of the annual consumption of total primary nutrients furnished by mixed fertilizers has ranged from 60 to 75 per cent. The trend was definitely upward before 1947-1948, but it has tended to reverse in later years. Among the individual nutrients, mixed fertilizers have supplied consistently the smallest proportion of the nitrogen and the largest of the KzO.In 1947-1948 to 1956-1957 the proportions of the nitrogen and KzO used in this way decreased almost continuously, while the proportion of Pz06increased substantially (Table 11). The considerable decline in the proportion of nitrogen-from 57.6 per cent in 1947-1948 to 39.5 per cent in 1956-1957-reflectedY among other things, the great increase in the direct use of ammonium nitrate and anhydrous ammonia, especially the latter, as fertilizers in most parts of the country (see Section 111, B).

3. Consumption by Crops Relatively few studies have been made of the fertilizer, and even fewer of the plant nutrients, used on individual crops in the United States (Mehring et al., 1957), and they usually have dealt only with selected crops. However, comprehensive data on such use in 1947, 1950, 'The units comprising the regions are: New E n g l a A M a i n e , New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut; Mfddle Atlantic--New York, New Jersey, Pennsylvania, Delaware, District of Columbia, Maryland, West Virginia; South Atlanttc-Virginia, North Carolina, South Carolina, Georgia, Florida; East North CentruLOhio, Indiana, Illinois, Michigan, Wisconsin; West North CentraL Minnesota, Iowa, Missouri, North Dakota, South Dakota, Nebraska, Kansas; East South Centrul-Kentucky, Tennessee, Alabama, Mississippi; West South CentraL Arkansas, Louisiana, Oklahoma, Texas; Mountatn-Montana, Idaho, Wyoming, Colorado, New Mexico, Arizona, Utah, Nevada; Pactfic-Washington, Oregon, California; Tedtories-Alaska, Hawaiian Islands, Puerto Rico.

FERTILIZER PRODUCTION AND TECHNOLOGY

241

TABLE I1 Percentages of Primary Nutrients Consumed BA Mixed Fertilizer8 in the United States and Tcrritories, 1948-1957" Year ended June 30

N

Available PZOS

Kz0

Total

1948 1949 1950 1951 1952 1953 1954 1955 1956 1957

57.6 55.7 49.3 47.2 45.6 44.5 42.1 41.0 41.2 39.5

70.5 71.3 68.9 73.1 76.4 78.5 80.3 79.7 79.4 78.8

94.8 93.1 92.3 90.9 89.8 89.3 88.4 88.4 88.3 86.8

73.6 73.6 70.4 71.5 72.1 72.0 70.8 70.0 70.0 68.1

Q

U. S. Department of Agriculture. The quantities of N, available P~06,and KaO consumed as mixed fertilizers in

1947-1948 were 493,300, 1,307,700, and 872,900 short tons, respectively. The comparable quantities in 1956-1957 were 843,600, 1,816,600, and 1,682,500.

and 1954 were reported recently (Farm Economics Research Division, 1957). The proportionate distribution of the total consumption of nutrients among the principal crops in 1947 and 1954 is shown in Table 111. The use of the primary nutrients on corn and on hay and pasture ranked first and second, respectively, in both years, and the same relative positions were held also by wheat (5),tobacco ( 8 ) , and fruits (9).The rank of cotton, oats and barley, and nonfarm use was higher in 1954, while that of vegetables and of potatoes and sweetpotatoes was lower. Compared with 1947, higher proportions of the nutrients were used on corn, hay and pasture, and oats and barley and for nonfarm purposes. The proportions were considerably lower, however, for crops, such as vegetables, tobacco, and potatoes and sweetpotatoes, which historically have received high applications of nutrients per acre. Because of the great diversity in the use of fertilizers for nonfarm purposes and the large number of dissimilar consumers, estimates of such use are more difficult to make and are less accurate than those for farm crops. The data of Table I11 clearly show, however, that the nonfarm consumption has increased remarkably in recent years, reflecting the generally high level of national prosperity, the increase in leisure of large segments of the population, and the trend to suburban living, among other things. Further evidence of the magnitude of this phase of fertilizer use is given in a survey by Mehring (1957) which indicates that in 1956 the nonfarm consumption of fertilizers supplying the primary

2442

K. D. JACOB

nutrients totaled 2,124,000 short tons in the continental United States, or around 10 per cent of the total use of such fertilizers. The tonnage was about six times that reported by Scholl and Wallace (1950) for the nonfarm me of fertilizers in the year ended June 30, 1948. TABLE I11 Percentage Distribution of Total Primary Nutrients Supplied by Fertilizers Used on Principal Crops in the Continental United States, Calendar Years 1947 and 1954" Crop

Corn Hay and pasture Vegetables Cotton Wheat Potatoes and sweetpotatoes Oats and barley Tobacco Fruits Nonfarm use

Total

1947

1954

25.4 11.2 11.0 9.9 8.2 7.9 7.2 6.2 5.4 1.4 93.8

32.1 12.7 6.5 9.7 7.7 2.7 8.2 3.8 3.1 4.3 90.8

Based on data reported by Farm Economics Research Division (1957). Total consumption of primary nutrients (N, available P~OS,K10) in the continental United States was 3,396,864 short tons in the calendar year 1947 and 5,873,086 tons in"1954.

Among the opportunities for great expansion in the profitable use of fertilizer the potential is very large for grass- and rangelands. Forest fertilization, as yet scarcely done in the United States, appears also to have great possibilities. More than 100,000tons of fertilizer ,are now applied annually to fish ponds in this country (Mehring, 1957), and it is expected that such use will rise to far higher levels in the future. 111. Nitrogen

A. FIXATION METHODS Long-time commercial operation has been achieved by only three of the numerous methods that have been proposed for chemical fixation of atmospheric nitrogen. These are the electric arc, cyanamide, and synthetic ammonia processes.

FERTILIZER PRODUCTION AND TECHNOLOGY

243

1. Nitrogen Oxide Processes The first commercial application of the arc process-involving the direct union of nitrogen and oxygen at high temperature to form nitrogen oxides-was in 1905 in Norway, where it found its principal use. The process contributed important quantities of fertilizer nitrogen until the Norwegian plant ceased operation, for economic reasons, in 1939. Closure of this plant apparently marked the end of the arc process as a factor in the nitrogen industry. Study has continued, however, of methods for directly combining nitrogen and oxygen, especially as regards replacement of the electric arc by other sources of heat. Based on the method developed by Royster (1933) and Cottrell (1938) for the regenerative heating and cooling of gases, research on fixation of nitrogen in a gas-fired furnace was started in 1939 at the University of Wisconsin (Daniels, 1955; Foster and Daniels, 1951; Gilbert and Daniels, 1948). The basic patent (U. S . 2,422,081) was issued in 1947 to Frederick G. Cottrell, who suggested the study. A double bed of magnesia pebbles served to preheat the air and to lower the temperature (about 2200' C.) fast enough to prevent excessive dissociation of the nitric oxide (NO) into nitrogen and oxygen. Removal of the dilute nitric oxide (less than 2 per cent) from the furnace gas was effected by oxidation to nitrogen dioxide ( NOz) and adsorption of the latter on silica gel from which it was recovered by heating and absorbing in water to form nitric acid. Further development of the process was undertaken by the Food Machinery and Chemical Corporation at San Jose, California, in 1946, and in 1951 the Department of the Army authorized construction of a plant to produce 40 tons of nitric acid (basis 100 per cent HNOs) per day at Sunflower, Kansas. The plant was completed in May 1953 and was closed in the fall of 1954 because of the impossibility of maintaining continuous operation of the furnace at 2200" C. with the magnesia pebbles that were then available. In addition to papers by Ermenc ( 1956), excellent discussions of the plant and the preceding developments are given in the record of the nitrogen symposium held at Rye, New York, in November 1955 under the sponsorship of the Food Machinery and Chemical Corporation and the Wisconsin Alumni Research Foundation. Renewed interest in the utilization of solar energy has resulted in the recent development of research-type furnaces capable of yielding temperatures up to 3500" C. Daniels (1957) pointed out, however, that the practical use of solar energy is still in its infancy, and he believed

244

K. D. JACOB

that, for some time to come, such energy would not compete with cheap petroleum and electricity. It is of interest to note, nevertheless, that research on fixation of nitrogen by reaction with oxygen under the influence of solar heat has been done recently in Algeria (Grindrod, 1955). Harteck and Dondes (1956) reported the use of nuclear energy in laboratory experiments whereby substantial yields of nitrogen dioxide were obtained by irradiating air with uranium-235 at about 200" C. and 25 atmospheres pressure.

2. Cyanamide Process Commercial operation of the cyanamide process-whereby nitrogen is fixed by reaction with calcium carbide at 1100" C. in the presence of calcium chloride or fluoride to form crude calcium cyanamide-was started about 1906 and has been confined chiefly to Europe and Japan. Kastens and McBurney (1951) have given an excellent description of cyanamide manufacture at the plant of the American Cyanamid Company, Niagara Falls, Canada, which began production in 1909. Besides the U. S. Government plant completed at Muscle Shoals, Alabama, near the end of World War I but not operated after its break-in runs, manufacture of cyanamide in the Western Hemisphere has been confined to the Canadian plant. 3. Synthetic Ammonia Process The establishment in 1913 at Oppau, Germany, of the first large-scale plant for manufacture of ammonia by union of nitrogen directly with hydrogen at elevated temperature and high pressure in the presence of a catalyst marked the commercial beginning of a process which now furnishes most of the world's supply of fertilizer nitrogen (Mittasch, 1951). Countries that had no synthetic ammonia industry in 1945 but which had plants in operation or under construction in 1958 included Brazil, Colombia, Egypt, Finland, Iceland, India, Israel, Mexico, Pakistan, Peru, Philippines, Portugal, South Korea, Taiwan, Trinidad, Turkey, Venezuela, and Yugoslavia. For more than twenty-five years Canada has supplied the United States with important quantities of nitrogen fertilizers based on synthetic ammonia. The Canadian industry-initiated in 1930 with a small plant at Windsor, Ontario, and a much larger one at Trail, British Columbiawas expanded greatly during World War 11, and several plants have been added since 1950. The ammonia and other fertilizer operations of the principal Canadian producer have been described by the Consolidated Mining and Smelting Company of Canada Limited (1954).

245 a. Facilities in the United States. Following an abortive attempt by the Government at Sheffield, Alabama, during World War I, the first commercial unit for direct synthesis of ammonia in the United States was placed in successful operation in August 1921 by the Atmospheric Nitrogen Corporation at Syracuse, New York. From that time the process has been an increasingly important factor in the domestic nitrogen industry. The progress in synthetic ammonia before 1950 has been sketched by Anonymous (1950), Cope (1949), Jones and Baber (1947), Skeen (1948b), Taylor (1953), and others. The data, partially summarized in Table IV, reflect the great impact of World War I1 on the demand for nitrogen. FERTILIZER PRODUCTION A N D TECHNOLOGY

TABLE IV Synthetic Ammonia Plants in the United States, 1934-1947"

Year

Number

1939 1941 1943 1945 1947

7 9 17 19 19

Annual capacity, short tons N ~~~~

370,000 469,000

1,oO0,000 1,127,000 1,258,000

Skeen (1948b).

Under the impetus of continued shortages of fertilizer nitrogen, the Korean hostilities, and the incentives of the Government's fertilizer expansion program ( Commodity Stabilization Service, 1954; Minor, 1951), a further tremendous increase in the facilities for making synthetic ammonia has occurred since 1950, with the result that in 1958 the capacity greatly exceeded the total domestic and export demand for nitrogen for all purposes. The plants and their capacities in various years of this period have been discussed by Anonymous (1954f; 1957c7n), Adams and Scholl (1956) Andrews (1956), Bradley et al. (1957), Homer (1955), Matthew et al. (1955), Matthew and Perkins (1954), Taylor (1953), and others. The data for 1950 and 1957 are shown by regions in Table V. In 1950 to 1957 the number of plants increased from 22 to 56 (155 per cent) and the capacity rose from 1,655,000 to 4,104,000 tons of nitrogen (148 per cent). Besides the New England and Mountain regions and the Territories, which had no plants in 1950, the regional increases in capacity ranged from 162,000 tons in the Middle Atlantic region to

246

K. D. JACOB

645,000 tons in the West South Central region; the percentage increases ranged from 44 (Middle Atlantic) to 329 (Pacific). The high capacity in the West South Central region (28 per cent of the total in 1950 and 27 per cent in 1957) reflects chiefly the ready availability of abundant supplies of natural gas in that area. Synthetic ammonia plants were located in 14 States in 1950 and 28 States in 1957. The Puerto Rican plant, opened in 1957, was the first in the Territories and in the West Indies. TABLE V Synthetic Ammonia Plants in the United States and Territories, by Regions, 1950 and 1957 Number of plants Regiona New England Middle Atlantic South Atlantic East North Central West North Central East South Central West South Central Mountain Pacific Territories

August 1950

4 1 3 Id 3 6 4

-

Annual capacity, short tons N

December 1957" 1 8 3 7 6 6 12 2 10 1"

August 1950

-

December 1957

113,000 -

35,000 533,000 459,000 585,000 415,000 386,000 1,107,000 66,000 485,000 33,000

1,655,000

4,104,000

371,000 240,000 184,000 140,Wd 145,000 462,000

__

Total

22

56

See Section 11, B, 1 for the units comprising the regions. Taylor (1953) ; includes plants under construction. 0 The data are principally from Anonymous (1957c,n); includes plants under construction. d Excludes the facilities at Louisiana, Missouri, which were being used by the U. S. Bureau of Mines for research on synthetic fuels. e Puerto Rico. (1

b

Construction of two new plants and expansion of several existing plants was undertaken in 1958. b. Technological deuelopments. An economical source of hydrogen is a major consideration in the location of a synthetic ammonia plant. The hydrogen may be obtained: (1) by processing carbonaceous materials such as coal, coke, lignite, wood, charcoal, petroleum, and natural gas; (2) by electrolysis of water; or (3) as a by-product of the electrolytic process for caustic soda and chlorine. Also, gases resulting as by-

247

FERTILIZER PRODUCTION AND TECHNOLOGY

products of certain industries, especially coke manufacture and petroleum processing, are important materials for hydrogen production. A plant in Travancore, India, completed in 1947, makes hydrogen by reacting steam with iron and using producer gas from wood to regenerate the iron (Van Denburg, 1947; Van Ness, 1950). By far the greater portion of the United States capacity for synthetic ammonia before World War I1 was based on hydrogen produced with coke by the water-gas reaction (Cope, 1949; Taylor, 1953). In 1950, however, about 52 per cent of the capacity was based on coke, 43 per cent on natural gas, and 5 per cent on by-product hydrogen from caustic soda-chlorine cells. Further great changes have occurred in the hydrogen TABLE VI Sources of Hydrogen for Ammonia Synthesis in the United States and Territories, 1950 and 1957 Number of plants Hydrogen source Natural gas By-product, oil refining Coke By-product, N a O H C l cells Other sourcesc Total

August 1950 9 6b

7 22

Annual capacity, short tons N

December 1957 32 7a 2 b

10 5 56

August 1950

December 1957

705,000 864,000 86,000

-

2,812,000 512,000 355,000 216,000 209,000

1,655,000

4,104,000

Includes 2 plants also using natural gas. Includes 1 plant also using natural gas. c Fuel oil, 2 plants; coke-oven gas, 2 plants; by-product gas from manufacture of chemicals from natural gas, 1 plant. a

sources since 1950. Several large plants switched from coke to natural gas, and many new plants employed the latter material; by-product gas from oil refining became an important factor; and coke-oven gas, fuel oil, and by-product gas from the chemical processing of natural gas came into use. Thus in 1957, about 69 per cent of the capacity was based on natural gas, 12 per cent on oil refinery gas, and only 9 per cent on coke. The data for 1950 and 1957 are summarized in Table VI. Preparation of ammonia-synthesis gas with carbonaceous materials commonly involves reactions with steam and air to produce properly proportioned mixtures of pure hydrogen and nitrogen. The methods of using natural gas for that purpose are described by Eickmeyer and Mar-

248

K. D. JACOB

shall ( 1955), Mayland et al. ( 1954a), Reidel ( 1954b,d), Taylor ( 1953), Tuttle (1952), Yeandle and Klein (1952), and others. Partial oxidation of natural gas with commercially pure oxygen under high pressure is a recent innovation (Anonymous, 19%; Reidel, 1954a,c). Partial oxidation is applied also to fuel oil (Mayland et al., 1954b) at ammonia plants recently built in Maine and Puerto Rico. The use of by-product gases from oil refineries as a source of hydrogen for ammonia synthesis is discussed by PfeBer and Sandler (1955). Considerable attention has been given to the preparation of ammoniasynthesis gas directly from coal by partial oxidation of the latter with oxygen (Anonymous, 1954a; Grossman and Curtis, 1954; Martin, 1950). Studies by the U. S. Bureau of Mines indicate that in some areas of the United States it is economically feasible to produce synthesis gas by direct gasification of coal and lignite (Heising, 1954; Ongstad et al., 1956; Oppelt et al., 1957; Skinner et al., 1952). Coke-oven gas is an important source of hydrogen for ammonia synthesis in Europe (Guillaumeron, 1949). It was not until 1956, however, that the first of the two domestic plants using such hydrogen was placed in operation ( Norden, 195613). Recent descriptions of ammonia-synthesis processes and plants include those by Anonymous (1951a, 1 9 5 4 ~ )Hein ~ (1952), Jones and Baber (1947), McCullough et al. (1951), Norden ( 1956b), Reidel (1954a,b,c,d), Resen (1951, 1954), Shearon and Thompson ( 1952), Taylor (1953), and Thompson et al. ( 1952). Burt (1954) has discussed the conversion from coke to natural gas in ammonia production by the Tennessee Valley Authority, Wilson Dam, Alabama, and Slack et al. (1953) have outlined the operating problems at that plant. Recent research includes studies of the action of hydrogen-nitrogen mixtures on steels (Ihrig, 1!%9), stabilization of ammonia synthesis catalysts (Burnett et al., 1953), and reaction rates in ammonia synthesis ( Adams and Comings, 1953).

B. NITROGEN MATEFULS Natural organic materials supplied about 90 per cent of the domestic consumption of commercial fertilizer nitrogen in 1900, but less than 2 per cent in 1956. After 1940, a year in which by-product ammonium sulfate and Chilean nitrate together furnished about 60 per cent of the nitrogen, the synthetic ammonia products rapidly gained dominance; they accounted for about 87 per cent (1,685,000 short tons) of the fertilizer nitrogen used in 19561956, Synthetic ammonia is also the source of large quantities of nitrogen for industrial purposes (European Productivity Agency, 1957)-595,000 tons in 1953 and 518,000 tons in 1954, ac-

249 cording to Horner (1955), and probably more than 700,000 tons in 1958. The domestic production of synthetic ammonia for all purposes totaled 3,071,203 tons of nitrogen in 1957. A characteristic of the chemical nitrogen industry is the large number of fertilizer materials, most of which are used both in mixed fertilizers and for direct application. The quantities of nitrogen supplied by the chemical materials used for direct application in 1947-1948 to 19561957 are shown in Fig. 5. Especially noteworthy is the great increase in consumption of fertilizer-grade ammonium nitrate (33.5 per cent nitrogen) and anhydrous ammonia (82 per cent nitrogen). Ammonium nitrate FERTILIZER PRODUCTION A N D TECHNOLOGY

400

-

U O

-

3EO

-

910

-

u)

z

-

0 I -

-

I

I

1

I

-

I

240-

-

0 200-

-

0

In

p I

-

z

8

110K 0 -

t 2

-

lee-

80

--o-----..

40

-0

-

-

and Aqua Ammonia

0-

-

I

I941

I

I950

I

1962

I

1964

I

I956

1987

YEAR ENDED JUNE 30

FIG.5. Nitrogen materials used directly as fertilizers in the United States and Territories, years ended June 30, 1948-1957. (U. S. Department of Agriculture.)

t

250

K. D. JACOB

furnished 31.9 per cent (115,910 tons) of the direct-application nitrogen in 1947-1948 and 28.8 per cent (371,972 tons) in 1956-1957. The corresponding data for anhydrous ammonia are 9.8 per cent (35,556 tons) and 28.8 per cent (371,668 tons). Before World War I1 fertilizer-grade ammonium nitrate was not produced in the United States or elsewhere, and the world use of anhydrous ammonia for direct application was confined to limited quantities in the Far Western States. The recent trend in direct use of nitrogen solutions, together with aqua ammonia, and most of the other chemical nitrogen materials has also been markedly upward, and much larger quantities of ammonium sulfate and of ammonium nitrate-limestone mixtures were used in 1956-1957 than in 1947-1948. The trend in sodium nitrate, however, has been downward since 1951, reflecting in some degree its relatively low nitrogen content (16 per cent) and high unit cost. Ammonium orthophosphates, nitric acid-phosphate rock products, and ammonium metaphosphate are discussed, respectively, in Section IV, E, G, and H. 1. Anhydrous Ammonia As previously mentioned, about 87 per cent of the fertilizer nitrogen used in 1955-1956 was supplied by synthetic ammonia and its derivatives. About 20 per cent of the nitrogen from these sources was in the form of anhydrous ammonia applied directly as fertilizer. The use of ammonia in the latter way provides the cheapest nitrogen to many farmers in large areas of the country (Gaines and Crowe, 1950; Heady and Baum, 1957). Direct use of anhydrous ammonia-initiated on a commercial basis in California in 1934 by the Shell Chemical Corporation (Anonymous, 1957m; Leavitt, 1953)-was mostly confined, at first, to its application in irrigation water. Toward the end of World War 11, however, interest in the injection of anhydrous ammonia directly into the soil was greatly stimulated by the research investigations at the Mississippi Agricultural Experiment Station under the leadership of W. B. Andrews (1954, 1956). The latter practice soon spread rapidly in the Mississippi Delta States and then to other parts of the United States. As yet, direct use of anhydrous ammonia has made little headway in other countries. Adams (1957) and Adams and Scholl (1956) have discussed the progress in the direct application of anhydrous ammonia, and its regional use in alternate years of the period 1947-1948 to 1958-1957 is shown in Table VII. Three regions each used more than one-fifth of the total consumption in the latter year-West South Central 23.4, Pacific 22.2, and West North Central 21.4 per cent. Anhydrous ammonia was used in 41 States and Puerto Rico in 1956-1957, but only 4 States together 86-

251

FERTILIZER PRODUCTION AND TECHNOLOGY

countcd for 59.5 per cent of the total-California 19.2, Texas 16&, Nebraska 12.2, and Mississippi 11.5 per cent. The high pressure of liquid ammonia (197 psi. gage), the form in which the anhydrous material is commonly handled, poses many problems in its direct use, especially by injection into the soil. Lack of suitable equipment for its local storage, transportation, and application in the field was a serious handicap in the early stages of the development, and resort was had to numerous improvisions-including the use of equipment employed in distributing the low-pressure types of liquefied petroTABLE: VII Anhydrous Ammonia Used Directly as Fertilizer in the United Statcs tliid Territories, Years Ended June 30 Nitrogen, short tons ~

Region0 New England Middle Atlantic South Atlantic East North Central West North Central East South Central West South Central Mountain Pacific Territories Total

1948

10,598 2,588

(dl 22,370e 35,556

1950

1,002 63 1,746 28,044 12,086 ( dl 27,176e 6 70,123

1952

1954

1956c

1957"

315 2,750 5,401 9,982 29,049 37,161

2 1,128 9,052 35,835 69,592 45,818 58,028

2 1,378 13,783 30,317 70,498 50,636 75,333 24,528 76,199 1,643

0 1,724 17,914 28,359 79,575 47,167 86,891 26,917 82,379 742

344,317

37 1,668

(*I

51,245" 2,080 137,983

( dl

67,120" 580 287,155

See Section 11, B, 1 for the units comprising the regions. Adams and Scholl (1956). From annual fertilizer consumption surveys made by U. S. Department of Agriculture. d Included with Pacific region. 8 Includes Mountain region.

leum gas, a particularly hazardous practice (Bartz, 1947) which was soon abandoned. The subsequent development of efficient and safe equipment designed specifically for handling and applying anhydrous ammonia is discussed by Andrews ( 1954, 1956), Barr ( 1952), and others. Additional publications dealing with equipment specifications and safety precautions for anhydrous ammonia include those issued by the Agricultural Ammonia Institute ( 1957), the Compressed Gas Association ( 1953), the Manufacturing Chemists' Association ( 1952), and the National Safety Council ( 1954). Rules and regulations concerning anhydrous ammonia

252

K. D. JACOB

equipment and storage have been issued by most, if not all, of the States in which large quantities of this material are used.

2. Aqua Ammonia Trials of aqueous ammonia solutions applied directly as fertilizer were conducted in Scotland as early as 1843 (Bishop, 1843), and such use of ammoniacal liquors, chiefly from gas works, has been made from time to time in Europe. So far, however, large use of ammonia in this way, principally since 1950, has been confined to the United States. Thus the domestic consumption in 1956-1957 totaled 381,432 short tons (76,844 tons N)-74.9 per cent in the Pacific States and 18 per cent in Hawaiiof material prepared mostly, if not entirely, from synthetic ammonia. Much of the aqua ammonia is prepared locally from the anhydrous material with the aid of efficient, compact equipment designed especially for this purpose (Tielrooy, 1949). Extensive preparation of the aqueous product is done also at large-scale conversion stations and at some synthetic ammonia plants, from which it is shipped in various ways, for example, by tanker from California to Hawaii. One large manufacturer of synthetic ammonia finds it economical to store much of the production in the form of aqua ammonia, from which the anhydrous material is regenerated for shipment during rush seasons (Anonymous, 1956f). 3. Nitrogen Solutions Besides aqua ammonia, large use is made of other aqueous solutions that contain nitrogen as the only primary nutrient. In the year ended June 30, 1957, such solutions furnished around 40 per cent of the domestic consumption of nitrogen in mixed fertilizers and 6 per cent of the nitrogen in materials used directly as fertilizer. Commercial utilization of nitrogen solutions in the preparation of mixed fertilizers has been confined principally to the United States where the use of ammonia-urea solutions began in 1932 (Jensen, 1953) and of ammonia-ammonium nitrate solutions in 1934 (Rogers, 1952). Little practiced before World War 11, direct application of nitrogen solutions-also limited mostly to the United States-has expanded greatly since 1950. Nitrogen solutions can be handled in simpler equipment and usually more safely than anhydrous ammonia (Andrews, 1954; Chronister, 1954; Franklin, 1957; Gantt et al., 1956; Krueger, 1951; Lathrope, 1956; Perrine, 1954); their nitrogen content generally is much higher than that of aqua ammonia; and their cost per unit of nitrogen applied to the crop may be lower than that of nitrogen in solid forms (Bowen et al., 1956; Heady and Baum, 1951). The composition and properties of nitrogen solutions have been discussed by Adams (1957), Crittenden (1953), Jensen (1953), Shultz and

FERTILIZER PRODUCTION A N D TECHNOLOGY

253

Elmore (1947), and Worthington et al. (1952). As reported by Adams (1957) and summarized in Table VIII, the solutions (excluding aqua ammonia) on the domestic market as of August 1, 1957, comprised fiftythree formulations of nine kinds of low-pressure and nonpressure products having wide ranges of nitrogen content (8.5 to 53 per cent), vapor pressure (0 to 140 p.s.i.g. at 104"F.), and salting-out temperature (-96 TABLE VIII Aqueous Nitrogen Solutions Marketed for Fertilizer Purposes in the United States, as of August 1, 1957" Number of Constituents

Formu- Prolations ducersb

Nc

(%)

Low-pressure solutions Ammonia, ammonium nitrate 17 18" 32.0-53.0 Ammonia, ureaf 9 7 21.0-46.0 Ammonia, urea, formsldehydeo 1 1 37.1 Ammonia, ammonium nitrate, urea 11 10 41.0-49.5 Nonpressure Ammonium bisulfite * 1 Ammonium nitrate 9 Ammonium nitrate, calcium nitrate 1 Ammonium nitrate, sodium nitrate 1 3 Ammonium nitrate, urea

solutions 1 8.5 8" 16.0-29.0 1 17.0 1 20.0 3 28.0-32.0

Vapor Salting-out pressure tem(p.8.i. a t perature 104" F.)d 1-104 0-71 24 10-60 -

-

-

-96 to +85 1-58 34 720 to +35

($1 11-154 32 58 1-32

Adams (1957) ; excludes aqua ammonia. Producers that also make synthetic ammonia. c Approximate. d Approximate gage pressure. e Includes one producer in Canada. The solutions often contain COz salts. Also contains COa salts. h Produced only in Canada. i No data. a

b

to 154"F.). With one exception (ammonium bisulfite) all the solutions contained either ammonium nitrate or urea or both. The low-pressure solutions-characterized by the presence of free ammonia, 4.3 to 49 per cent by weight-are marketed principally for manufacture of mixed fertilizers, but they are finding increasing use for direct application. The nonpressure solutions are employed chiefly for the latter purpose; exceptions are the straight ammonium nitrate solutions containing more than 60 per cent NH4N03 (21 per cent N), which

254

K. D. JACOB

usually are subjected to further processing in one way or another. Choice of a solution for a particular purpose and area of use involves, among other things, consideration of its vapor pressure and salting-out temperature, properties which generally vary inversely among the different formulations of a specific kind of product. For example, solutions having low salting-out temperatures are preferable for direct application in the colder regions. 4. Nitric Acid Use of nitric acid is predominantly for production of fertilizers and industrial explosives. It is estimated that nitric acid manufactured by oxidation of anhydrous synthetic ammonia and converted into ammonium nitrate products, sodium nitrate, and other materials supplied 20 to 25 per cent of the domestic consumption of fertilizer nitrogen in 1957. Although nitric acid is not applied directly as fertilizer, Aldrich et al. ( 1953) have investigated its plant-nutrient value, and Aldrich and Buchanan ( 1955) have studied the reactions between injected liquid nitrogen dioxide ( NOz), the precursor of nitric acid, and various soils with refqrence to its possible use in this way. According to Horner (1957), the United States annual capacity for production of nitric acid by oxidation of ammonia was expected to reach 6,036,500 short tons (basis 100 per cent HN03) by 1958, including 2,263,000 tons in Government facilities. Of the 49 commercial plants, 29 were adjacent to anhydrous ammonia facilities. Recent progress in nitric acid production and facilities has been reviewed also by Skeen (1949a) and others (Anonymous, 1954h, 1957g), and Crittenden ( 1952) has discussed the economics of nitric acid in the fertilizer industry. Strelzoff (1956) stated that practically all nitric acid plants built in the United States in recent years have been designed for oxidation of the ammonia under pressure with the aid of a platinum-rhodium catalyst, followed by absorption of the nitric oxides under practically the same pressure conditions. Plants using various modifications of this scheme, as well as those using other processes, have been described by Anonymous (1953a, 1956a, 1958a), Cronan (1958), Inskeep and Henry (1953), and Spratt ( 1958). Commercial, aqueous solutions are the source of ammonia for a small-capacity nitric acid plant described by Knecht ( 1950).

5. Fertilizer-Grade Ammonium Nitrate Before World War I1 the world fertilizer use of solid ammonium nitrate materials was confined mostly to mixtures of ammonium nitrate with calcium carbonate, limestone, or dolomite and to combinations of ammonium nitrate with ammonium sulfate or potassium chloride. In the

FERTILIZER PRODUCTION AND TECHNOLOGY

255

spring of 1943, however, straight ammonium nitrate from ordnance and industrial plants in the United States and Canada became available for use as fertilizer (Ross et al., 1946; Scholl et al., 1948; Whittaker et aZ., 1948). Although the product first marketed was unsatisfactory, because of its marked tendency to cake and to absorb moisture from humid atmospheres, the difficulties soon were overcome, with the result that fertilizer-grade ammonium nitrate rapidly gained the dominant position among the nitrogen materials for direct application in the United States (Fig. 5 ) . Other countries also use large quantities of ammonium nitrate in this way. Much smaller quantities-about 90,000 short tons in the United States in 1954, for example-are used in making mixed fertilizers. Preparation of the fertilizer-grade material, usually containing 33 to 33.5 per cent nitrogen, involves conversion of the ammonium nitrate into granular form and coating the particles with diatomaceous earth, clay, or other inorganic anticaking agent. Protection of the product from atmospheric moisture usually is done by packaging in double-asphaltlaminated, polyethylene-lined, or polyethylene bags. Large-particle ammonium nitrate is now produced in the United States by prilling, by vacuum crystallization, and by the Stengel process. Manufacture of the fertilizer grade of grained, ordnance-type material (Miller et al., 1946; Ross et al., 1946)-treated with a petrolatum-rosinparaffin mixture and an anticaking agent-which was involved in the explosion at Texas City, Texas, on April 16, 1947 (Kintz et al., 1948), ceased in August 1953. Burns et al. (1953) and Cook and Talbot (1951) have shown that the petrolatum-rosin-paraffin treatment increases the explosion hazard of ammonium nitrate. The prilling or spray-granulation process, developed in Canada in 1943 (Anonymous, 1945; Ross et al., 1946; Scholl et al., 1948), is the most widely used method of preparing fertilizer-grade ammonium nitrate. The process consists essentially of spraying a hot, 95 per cent solution of ammonium nitrate into a tower against a stream of conditioned air, whereby the droplets are congealed into small pellets which are then dried, cooled, screened, and coated ( Anonymous, 1955c; Consolidated Mining and Smelting Company, 1954; Resen, 1951; Shearon and Dunwoody, 1953). In the vacuum crystallization process-as operated by the Tennessee Valley Authority (Anonymous, 1955~;Miller and Saeman, 1947, 1948; Saeman et al., 1952) and in Iceland (Felio and Brown, 1954), for example-the vacuum-evaporated solution ( 79 per cent ammonium nitrate) passes to vacuum crystallizers designed and operated to produce large, rounded crystals which are further treated as in the prilling process. The Stengel process, first placed in operation by the Commercial

256

K. D. JACOB

Solvents Corporation at Sterlington, Louisiana, in 1953 (Anonymous, 195%; Dorsey, 1955; Hester et aZ,, 1954; Stengel, 1951), involves reaction of preheated ammonia and nitric acid in a packed, tubular reactor to produce ammonium nitrate and steam, which are separated to yield a relatively dry product, The molten nitrate is further dried, solidified on continuous water-cooled, stainless steel belts, crushed, classified, and coated. Ross et al. (1945, 1946) and Whetstone (1949b) have discussed the factors infiuencing the caking of ammonium nitrate, one of which-according to Whetstone (1952)-is the normal crystal habit of ammonium nitrate IV, the stable form of the salt at -18 to 32" C. Whetstone (1949a, 1952) reported that addition of 0.03 per cent of Acid Magenta (among other dyes), by spraying on the hot ammonium nitrate granules as they enter the cooler, changes the crystal habit of the nitrate from the normal stout needles to thin, fragile plates which, being essentially devoid of mechanical strength, cause little or no bonding of the granules under conditions favoring deposition of the salt from the solution phase. Kadey (1958) studied the characteristics of various diatomites used as anticaking agents. Campbell and Campbell (1946) found that ammonium nitrate 111, normally stable at 32 to 83" C., can be kept indefinitely in this form by adding 5 per cent of potassium nitrate to the solution or melt before crystallizing. Especially since the Texas City explosion, many publications have been issued on the fire and explosion hazards of fertilizer-grade ammonium nitrate and on the safety precautions in its handling, storage, and transportation, including those by Burns et al. (1953), Cook and Talbot (1951), Davis (1945), Elliott (1948), Feick (1954), Feick and Hainer (1954), Manufacturing Chemists' Association (1950), and Scott and Grant (1948). Shortly after the Texas City disaster, the U. S. Coast Guard organized the Interagency Committee on the Hazards in Transportation of Ammonium Nitrate to study ammonium nitrate characteristics, hazards, and safety precautions and to recommend a national policy in conformity with these objectives. The first two reports of this committee were issued on August 20 and November 10, 1947, respectively, and the third (final) on April 7, 1954. 6. Other Nitrates Natural sodium nitrate from Chile has been an important fertilizer ,inthe United States for more than eighty years. Domestic production of the synthetic material by neutralization of nitric acid with sodium carbonate was started in 1929 at Hopewell, Virginia. Its manufacture from nitric acid and sodium chloride, also at Hopewell, with chlorine as

FERTILIZER PRODUCTION AND TECHNOLOGY

257

a coproduct, began about 1935 (Gordon, 1953). Crittenden ( 1951) has briefly described the process. There is no domestic manufacture of fertilizer-grade potassium nitrate as such, and the small consumption of this material is largely in the form of nitrate of soda-potash ( N 15, KzO 14 per cent), a mixture of natural sodium and potassium nitrates imported from Chile. However, potassium nitrate is generally present in mixed fertilizers as a result of reaction between potassium chloride and ammonium nitrate ( Silverberg et d., 1958). Potassium nitrate is usually made by base exchange between potassium chloride and ammonium nitrate, Though technically feasible, its production from potassium chloride by reaction with nitric acid-in the way that sodium nitrate is made from sodium chloride-is not yet done commercially in the United States or, apparently, in foreign countries. The first domestic manufacture of fertilizer-grade calcium n i t r a t e marketed as an aqueous solution (17 per cent N) containing 36.2 per cent calcium nitrate and 30.9 per cent ammonium nitrate (Adams, 1957)was in California around the end of 1956. The previous domestic use of calcium nitrate, chiefly in California, was entirely in the form of solid material (15.5 per cent N)-some 50,000 tons annually-imported mostly from Norway.

7 . Ammonium Sulfate Material made with ammonia from by-product sources, chiefly coke ovens, usually comprised more than 90 per cent of the annual domestic production of ammonium sulfate before 1945. In later years, however, the proportion of by-product sulfate has decreased markedly-it was 46.6 per cent (908,903 short tons) in 1957-while that of material produced with synthetic ammonia has increased correspondingly. Excluding the Territories, the domestic consumption of ammonium sulfate as fertilizer in 1954 totaled some 1,400,000tons, of which about 67 per cent was used in making mixed fertilizers-as compared with 828,000 tons and 83 per cent in 1944. The various processes and plants for recovering by-product ammonia at coke ovens and gas works and converting it into ammonium sulfate have been discussed in numerous papers, including those by Bell ( 1950), Fisher (1950), Klempt (1952), Lewis (1955), H. Otto (1949), A. Otto (1956), and Williams (1954). Manufacture of the sulfate from synthetic ammonia and virgin sulfuric acid, extensively done in the United States and other countries, is described by Consolidated Mining and Smelting Company (1954), McCullough et al. (1951), Resen (1951), Taylor

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K. D. JACOB

(1946), and others. Much ammonium sulfate is made also with synthetic ammonia and spent, waste, and reclaimed acids from various industrial operations, such as petroleum refining (Bragg, 1952; Fox, 1952; Ruys, 1941) and organic syntheses ( Majewski, 1954). Recovery of metals from certain sulfide ores by pressure leaching with ammonia solutions is a new source of by-product ammonium sulfate that promises increasing importance. A plant using this process on a nickelcopper-cobalt concentrate recently was placed in operation at Fort Saskatchewan, Canada (Anonymous, 1951b; Forward, 1953). A captive synthesis plant supplies the ammonia, and the annual output of ammonium sulfate is reported to be 70,000 tons. Another metallurgical process involves a treatment of ferrochrome whereby ferrous ammonium sulfate is obtained as a by-product and is marketed as fertilizer ( Norden, 1956a). A process for recovering ammonium sulfate as a by-product of the leaching of manganese ores with sulfurous and sulfuric acids has been described by Ketzlach (1950). Pioneered in Germany just before World War I, manufacture of ammonium sulfate by reaction of ammonia and carbon dioxide with anhydrite or gypsum is now done in several other countries, including England, France, Belgium, India, and Pakistan, but not in the United States. According to Higson (1951b) the world capacity for making ammonium sulfate in this way totaled about 2,100,000 short tons in 1951. Although synthetic ammonia is commonly used in the process-as described by Anonymous ( 1952a), Higson ( 1951a,b), Van Denburg ( 1947), and Van Ness (1950)-studies by Klempt (1950) indicated the suitability of byproduct ammonia for this purpose. Recent research has shown the technical feasibility of making ammonium sulfate as an end product of certain processes proposed for the removal of sulfur dioxide from the flue gases of coal-fired steam plants (Craxford et al., 1952; Francis and Lepper, 1951; Rees, 1953; Tarbutton et al., 1957). As with other fertilizer materials, resistance to caking in storage and ease of application in the field are important requisites of ammonium sulfate, both of which may be influenced markedly by the size and shape of the crystals. Crystal size-and, to some degree, shape-can be controlled largely by proper design and operation of the saturator or the crystallizer (Anonymous, 1948; Bamforth, 1949; Klempt, 1952; Mantel and Hansen, 1952; Seifert, 1955). Production of “stubby” crystals, the desirable form, is favored by the presence of small amounts of ferric iron and, sometimes, aluminum in the solution (Ettle, 1949; Klempt, 1952; Mantel and Hansen, 1952). According to Whetstone (1949a) and Butchart and Whetstone (1949) caking of the sulfate can be prevented

259

FERTlLIZER PRODUCTION AND TECHNOLOGY

also by spraying the hot crystals with very dilute solutions of certain dyes, especially Amaranth. A process for producing granular coke-oven sulfate by compacting the crystals, drying, crushing, and screening was adopted recently by several plants in the United States (Anonymous, 19S7a).

8. Urea Fertilizer urea is marketed as the nearly pure solid-containing about 45 per cent nitrogen-and as solutions with ammonia and (or) ammonium nitrate (see Table VIII) , both forms of which are used in mixed fertilizers and for direct application. Much urea is used also for other purposes, especially as a protein supplement in feeds for ruminants and in the manufacture of plastics. Data on the domestic production of urea and its consumption as fertilizer in certain of the years 1936 to 1957 are given in Table IX. The figures for 1936 to 1948 are estimates, though TABLE I X Product,ion and Consumption of Urea in the United States in Certain Years from 1936 to 1957, in Short Tons" Production Fertilizer materials Calendar year

Solid

Solutions

1936 1938 1940 1942 1944 1946 1948 1956 1957

1,000 12,000 45,000 21,000 17,000 15,000 20,000 171,690 175,804

15,000 16,000 20,000 14,000 25,000 23,000 27,000 75,547 166,274

Total 19,900 33,000 80,000 59,300 77,550 82,000 > 107,000 420,901 480,185

Consumption as fertilizer * 4,000 16,000 34,000 20,000 20,000 14,000 20,000 92,373" 108,916"

0 1 n terms of urea. 1936-1948, Mehring et al. (1957, p. 34); 1956-1957 (except consumption), U. S. Tariff Commission. * Includes Territories; excludes Calurea and solutions. Year ended June 30, annual fertilizer consumption surveys by U. S. Department of Agriculture; excludes urea used in making mixed fertilizers.

probably of the right order of magnitude, and complete data on the consumption of urea as fertilizer are still lacking. As compared with 1948, the greatly increased output of urea in 1956-the first year for which such data were released by the U. S. Tariff Commission, the Federal agency

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responsible for their collectios-reflected chiefly the activities of several new producers. The production in 1951 is reported to have been 230,OOO short tons (Anonymous, 1957f). The use of solid urea directly as fertilizer in 1956-1957 (108,916 tons) was nearly 59 per cent higher than in 19541955. Progress in urea production and use to 1949 has been reviewed by Skeen ( 1949b). Formerly made also by hydrolysis of calcium cyanamide, the world production of urea now is manufactured entirely, or nearly so, by reaction of synthetic ammonia with carbon dioxide under pressure. Before World War I1 the domestic production in the latter way was confined to a single plant which began operation about 1933. Since 1950, when the second plant was built, the number of urea facilities has increased greatly. In January 1958, according to Horner ( 1958), urea was manufactured by 9 plants having a total annual capacity of 623,100 short tons, and 3 new plants and expansions of 3 existing facilities (total capacity 172,700 tons) were being built. Later in 1958,3 additional plants (total capacity about 106,000 tons) were announced for construction. The first Canadian plant (capacity 66,000 tons) began operation in 1959. Japan is second to the United States in urea production, with a plant capacity totaling 605,000 tons in 1959 (Anonymous, 1958b). Among the other countries, urea is made in Austria, England, France, Germany, Italy, Netherlands, Norway, South Korea, Soviet Union, Switzerland, and Taiwan. Plants were being built in India, Trinidad, and the Union of South Africa in 1959. Recent papers on urea processes and plants include those by Anonymous ( 1952c,d; 1954i; 1955a,b), Brien ( 1954), Cook ( 1954), Grindrod (1957b) McCullough d al. ( 1951), Pastonesi (1953), Reidel (1954d), Rooseboom (1951), and Tonn (1955). Cambon (1955) has discussed the chemistry and technology of urea manufacture. Egan et al. (1946) investigated the dissociation pressure of ammonium carbamate, an intermediate in urea synthesis, and Seiberlich and Campbell (1952) studied the effect of heat on the properties of urea at its melting point. Merz and Brown ( 1943) and Hardesty (1955a) have summarized the properties and use of fertilizer-grade urea. With the object of decreasing the hygroscopicity of urea, research has been done on the formation of complexes between urea and other materials of fertilizer interest, such as nitric, phosphoric, and sulfuric acids and calcium sulfate (Nagai and Kanazawa, 1950, 1951; Nagai and Kuroi, 1950; Nagai and Toshima, 1950). Present practice in the processing of solid urea for fertilizer use commonly involves its conversion into pellets by spraying a concentrated solution at elevated temperature ( Grindrod, 195%; Reidel, 1954d).

FERTILIZEX PRODUCTION AND TECHNOLOGY

261

These conditions favor the formation of biuret (Redemann et al., 1958; Seiberlich and Campbell, 1952), a compound which has toxic effects on some plants under certain conditions, especially when present in foliage sprays of urea for citrus and pineapple (Haas and Brusca, 1954; Jones and Embleton, 1954; Oberbacher, 1954; Sanford et al., 1954). Starostka and Clark (1955) found, however, that soil applications of a urea-biuret mixture (10 per cent biuret) at the rate of 100 pounds of nitrogen per acre were not harmful to corn, cotton, tomatoes, and oats in greenhouse cultures. As reported by Starostka and Clark (1955) and by M. D. Thorne (Pineapple Research Institute of Hawaii, private communication), several samples of pelleted urea marketed about 1953 from foreign and domestic sources contained 1.3 to 7.0 per cent biuret. Although the biuret content of crystal urea did not exceed 0.3 per cent in three samples of domestic material, two samples of foreign products contained 2.9 and 8.4 per cent. In comparison, the biuret content of urea marketed in California in 1957 was less than 1.0 per cent in 37 samples (less than 0.6 per cent in 29 samples), while one sample contained 1.88 per cent (R. Z. Rollins, Bureau of Chemistry, California Department of Agriculture, private communication).

9. Urea-Formaldehyde Materials Apparently confined as yet to the United States, commercial production of solid urea-formaldehyde fertilizer materials, containing about 38 per cent nitrogen, was started in 1955 and was done by at least three companies in 1958: Designated by the generic term “urea-form,yysuch materials, when properly made, have low solubility in water and are capable of releasing nitrogen to crops over extended periods of time (Armiger et al., 1948, 1951; Clark, 1952; Fuller and Clark, 1947; Kralovec and Morgan, 1954; Mruk et at., 1957; Musser and Duich, 1958; Musser et al., 1951; Wisniewski et al., 1958; Yee and Love, 1946). Marketed under trade names such as Borden’s 38, Nitroform, and Uramite, ureaform is presently used principally on greenhouse crops and turf and as a constituent of mixed-fertilizer specialties. As shown by Armiger et al. (1951), Clark et al. (1956), and Kralovec and Morgan (1954), properly prepared materials having urea to formaldehyde mole ratios ranging from 1.2 to 1.4 usually exhibit the desired characteristics of low solubility, slow release of nitrogen, and high nutrient value. Products having higher ratios behave much like urea itself, whereas the urea-formaldehyde resins (mole ratios of 1.0 or less) are of little or no value as fertilizers. The processing conditions necessary to obtain suitable materials have been defined by the researches of

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K. D. JACOB

Clark et al. (1948, 1951), who, with others (Clark et al., 1956; Kralovec and Morgan, 1954; Owen et al., 1952; Yee and Love, 1946), have discussed the chemistry of the reaction between urea and formaldehyde and have outlined the characteristics and properties of the products. A laboratory method has been developed for determining the activity of the nitrogen in straight urea-form materials (Morgan and Kralovec, 1953; Kralovec and Morgan, 1954), a modification of which promises applicability to mixed fertilizers containing such materials (Clark et al., 1959). Inclusion of urea-formaldehyde compounds in mixed fertilizers may be done in other ways than by addition of straight urea-form products. Formation of such compounds in mixtures by ammoniating with an ammonia-urea solution containing 8 per cent of formaldehyde was put into commercial practice in the late 1930's (Jensen, 1953; McCool, 1941). Another way, initiated in 1956 (Anonymous, 1956g), is to formulate the mixture with solid urea and a liquid concentrate containing about 25 per cent of urea and 60 per cent of formaldehyde. A third commercially practiced method involves treating mixtures containing solid urea with a solution supplying formaldehyde and free ammonia (Davenport, 1952). Products having properties and nutrient characteristics similar to those of the urea-form materials can be prepared by reaction of urea with acetaldehyde (Scheffer et al., 1956). IV. Phosphorus

The world's phosphorus fertilizer industry is based essentially on phosphate rock, large deposits of which occur in many places. Apatite, bone, plant and animal products, high-phosphorus iron ores, and bird guano are also important sources of phosphate in some countries. A major segment of the fertilizer industry is concerned with mining, beneficiating, and processing phosphate rock and apatite, operations that are discussed in much detail by Waggaman (1952) and collaborating authorities. A. PHOSPHATE ROCK

1. Production As shown in Table X, the world production of phosphate rock in 1957-including apatite and guano-totaled 32,350,000 long tons, an increase of 65.9 per cent over the output in 1948. For the United States, North Africa (Algeria, French Morocco, and Tunisia), and the Soviet Union-the principal producing areas-the increases were 48.9, 43.1,

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FERTILIZER PRODUCTION AND TECHNOLOGY

TABLE X World Production of Phosphate Rock, 1948-1957, in Long Tons" Calendar year

United States

North Africab

Soviet Unionc

Worldc

1948 1949 1950 1951 1952 1953 1954 1955 1956 1957

9,388,160 8,877,474 11,114,159 10,775,032 12,064,892 12,503,830 13,821,100 12,265,248 15,746,781 13,976,000

5,670,000 5,692,000 5,986,000 7,059,000 6,811,000 6,390,000 7,496,000 8,052,000 8,075,000 8,111,000

2,300,000 2,500,000 3,005,000 3,295,000 3,590,000 3,965,000 4,430,000 4,870,000 5,265,000 5,660,000

19,500,000 19,800,000 23,300,000 24,500,000 26,000,000 26,750,000 29,950,000 30,050,000 33,750,000 32,350,000

U. S. Bureau of Mines; includes apatite and guano. Algeria, French Morocco, and Tunisia. c Estimate.

a

and 146.1 per cent, respectively. Together, the three areas produced 89.0 and 85.8 per cent of the respective outputs in 1948 and 1957, as compared with 48.1 and 43.2 per cent for the United States alone. Other areas in which the production of phosphate rock exceeded 200,000 long tons in 1957 were Nauru Island ( 1,105,000), Egypt (590,O00), Christmas Island (336,oOo), China (3OO,OoO), Makatea Island (300,000), Ocean Island (292,000), Peru (guano, 280,000), and Jordan (258,000). Florida continued a longtime dominance in domestic production by contributing 75.4 and 72.9 per cent of the phosphate rock sold or used by producers in 1948 and 1957, respectively (Fig. 6 ), as compared with 15.1 and 12.2 per cent for Tennessee and 9.5 and 14.9 per cent for the Western States (Idaho, Montana, Utah, and Wyoming). The disposition of the total output in 1948 was 75.3 per cent for domestic agricultural use, 13.0 per cent for industrial purposes, and 11.7 per cent for export; the comparable figures for 1957 were 55.8, 23.6, and 20.6 per cent.

2. Resources From data reported by Jacob (1951b) and Le Cornec (1951), the world reserve of phosphate rock and apatite is estimated to have been more than 45,959 million long tons in 1949 (Jacob, 1953), about 45 per cent of which was in French Morocco, 29 per cent in the United States, and the rest in some thirty countries. Besides considerable reserves in known resources for which no estimates were available, additional large quantities are in newly discovered deposits.

264

K. D. JACOB

I0

z

-

0

-I

5-

0 0

4-

9 0

0

-

9

-

8-

-

-

-

3-

-

-

2,175,000

-

1822,004

-

0-

I

I

I

I

I

I

I

I

I

FIG.8. Phosphate rock sold or used by producers in the United States, by States of origin, calendar years 1948-1957. (U. S. Bureau of Mines.)

Of the afore-mentioned domestic reserve (some 13,300 million long tons) about. 60 per cent is in the Western States and 38 per cent in Florida. This estimate includes only deposits for which reliable data are available and, for the most part, only rock containing 40 per cent or more of tricalcium phosphate (18.3 per cent PzOs). It does not distinguish, however, between rock which may or may not be economically mined under present conditions. The total minable reserve has been estimated at 5100 million tons of marketable product, with an additional

FERTILIZER PRODUCTION AND TECHNOLOGY

265

49,000 million tons minable under changed conditions (McKelvey et nl., 1953; Waggaman and Ruhlman, 1956). Besides the numerous investigations of the United States phosphate resources mentioned by Jacob ( 1953) and McKelvey et al. ( 1953),recent studies include those of the Florida deposits by Cathcart (1950), Feeley (1949), Moxham (1954), and Pirkle (1957); the Tennessee deposits by Burwell (1950); the Idaho deposits by Long (1949), Lowell (1952), McDivitt ( 1956), and Mansfield (1952); the Montana deposits by Cressman (1955); and the Arkansas, Kansas, Oklahoma, and Texas deposits by Barnes (1954), Runnels et al. ( 1953), and Ryan (1956). Harris et nl. ( 1954) have compiled an extensive bibliography of the western deposits, and recent investigations of these deposits have been discussed by Swanson et al. (1953). The newly discovered deposits in Beaufort County, North Carolina, add a large, though as yet undetermined, tonnage to the known domestic reserves. According to Brown (1958) the deposits, covered by 45 to 250 feet of overburden, are several feet to nearly 90 feet thick and extend over an area of about 450 square miles. The phosphate column consists of phosphatic sands and intercalated shell limestone. The PtOa content of the raw sand-a mixture of phosphate and quartz grains with some silt, clay, and organic matter-ranges from 8 to 31 per cent. Discovered in 1951, the deposits at Olinda near Recife, State of Pernambuco, Brazil, are the most promising source of phosphate rock yet found in South America (Fine and Frommer, 1954; Gossett, 1954; Kegel, 1955). Other Brazilian phosphate resources include the large apatite deposits in the AraxL area, State of Minas Gerais (Anonymous, 1953b; Araujo, 1947). At the Pacific Southwest Mineral Industry Conference, Reno, Nevada, April 5-6, 1957, W. D.Smiley (Stanford Research Institute, Menlo Park, California) and A. K. Schellinger reported the discovery of large phosphate deposits in the Santo Doming0 area off the southwest shore of Baja California, Mexico. Consisting of beach sands-comprising a mixture of such minerals as collophane, quartz, feldspar, zircon, and ilmenite and averaging about 3 per cent P205-the deposits extend for some 45 miles as barlike islands. They are estimated to contain approximately lo00 million tons of commercial-grade phosphate concentrates recoverable by flotation and magnetic separation. Rogers et al. (1956) have discussed the phosphate deposits in the Concepih del Oro district of Zacatecas and in some other Mexican localities. Of considerable interest is the, reputedly, world's largest deposit of calcium aluminum phosphate at Pallo near Thiks, Senegal, French West

266

K. D. JACOB

Africa (Bouyer, 1954; Capdecomme, 1952; Moriceau, 1953; Prevot and Ollagnier, 1956; Visse, 1952), commercial mining of which was started in 1948. Shipments, mostly to France, are made as the crude ore (25 per cent PzOa) and as a fertilizer material prepared by calcining the ore at 1000" C. (Martin, 1957). Other recently found, or developed, phosphate deposits in foreign countries include those in French Togoland (Martin, 1957; Visse, 1953), Israel (Anonymous, 1954e; Brumby et al., 1957), Jordan (Duncan, 1955), Rhodesia ( Dougherty, 1950), Uganda ( Bevilacqua and Hooper, 1957; Davies, 1947, 1949; Johnson, 1947), Union of South Africa (Holz, 1955; Van Wyk, 1953; von Lossberg, 1957), and Venezuela (Ochoa and Istok, 1949; Rodriguez, 1951; Rutherford, 1954). 3. Mining and Beneficiation Only brief mention can be made of the recent developments in mining and preparing phosphate rock for the market. Descriptions of facilities in the Florida pebble field include those by Avery (1952), Crago (1950), and Lenhart (1951). Among the individual phases of the operations, dragline problems have been discussed by Dunlap and Jacobs (1955); pipeline transportation of the matrix from the mines to the beneficiation plants, by Pascoe (1951), Tillotson (1953), and Tillotson et al. ( 1952); hydroseparation and thickening, by Dyrenforth ( 1954); drying and grinding the concentrate, by Hughes (1954,1956) ; and storing and handling the wet and the dry rock, by Lenhart (1954) and Lyle (1954). Open-cut mining in the western field, formerly done there to a limited extent, is now widely practiced (Emigh, 1954; Sweetwood, 1952; Utley, 1951) , and ore-dressing procedures-such as washing, flotation, calcination, and dry attrition and separation-are being used. Studies of methods for mining and beneficiating western phosphate have been reported by Howard (1956), Howard and Burnet (1956), and Stickney and Wells (1955). As with the Tennessee ore, large tonnages of the higher grade phosphatic shales from the western deposits are sintered or nodulized and used directly for electric-furnace manufacture of elemental phosphorus. As pointed out by Waggaman and Ruhlman (1956), application of flotation to the recovery of mineral phosphates-first done commercially in January 1929 in the Florida pebble district (Martin, 1934)-was a major development in the direct conservation of the phosphorus resources of the United States. Flotation and kindred agglomeration processes of one kind or another-installations of which are described by Anonymous (1957k), Adam and McGarry (1954), Crago (1950), Dayton ( 1958),

267 and Lenhart (1951)-now account for the greater portion of the production of Florida pebble phosphate, and at some operations they contribute 75 per cent or more of the output. The other commercial types of domestic phosphate rock are usually less susceptible to beneficiation by flotation. Among foreign countries, flotation is used in recovering apatite in the Soviet Union (Delgass, 1931) and South Africa (Holz, 1955), and its application to phosphate rock in Brazil (Fine and Frommer, 1954) and North Africa (Miche, 1954) has been studied. Fox and Jackson (1955, 1957) have shown that certain flotation reagents, especially the amine cationic type (Lentz et al., 1955), adsorbed on the surface of phosphate rock may influence favorably the behavior of the rock in manufacture of superphosphates. Phosphatic slime resulting from the washing and flotation of phosphate rock poses important problems of disposal and of conservation of phosphorus resources, especially in the Florida pebble district ( Specht, 1950; Tyler and Waggaman, 1954). It is estimated that more than 125,000,000 long tons of such slime-a typical analysis of which is Pz05 20, SiOz 21, CaO 18, and A1203 20 per cent-have accumulated in the waste ponds of the pebble district since the advent of flotation in 1929, and that the quantity is being increased at the rate of about 10,0o0,000 tons per year (Tyler and Waggaman, 1954; Waggaman and Ruhlman, 1956). Studies have been made of ways for dewatering the slime to permit its ready handling for further processing (Houston et al., 1949; Thompson, 1950; Thompson and Vilbrandt, 1954; Tyler and Waggaman, 1954), but none has proved to be economically practicable. Another phosphorus carrier lacking economical use is the so-called leached-zone ore which overlies much of the Florida pebble matrix and contains around 12 per cent of P205largely as calcium aluminum phosphates (Davenport et al., 1954, Hignett et al., 1957; Hill et al., 1950). An estimated 14,000,000 tons of leached-zone material are moved and discarded each year (Barr, 1953; Barr et al., 1955). Studies of methods for concentrating and processing this material have been made by Davenport et al. ( 1954), Hignett et al. ( 1957), and Sun et al. ( 1957), among others. FERTILIZER PRODUCTION AND TECHNOLOGY

4. Source of Uranium, Vunudium, and Fluorine According to McKelvey and Nelson (1950), uranium in phosphate rock was first discovered in 1924 in material from Algeria. Spurred by the need for uranium as a source of atomic energy, recent studies have shown the presence of this element in phosphate rock from deposits in the United States (Barr, 1953; Barr et al., 1955; Moxham, 1954; Thompson, 1953, 1954; U. S. Atomic Energy Commission, 1954), and many

268

K. D. JACOB

other countries (Capdecomme and Pulou, 1954; Davidson and Atkin, 1952; Delaney et al., 1953; Lenoble et al., 1952; McKelvey and Nelson, 1950). The phosphate rock produced annually in the United States contains about 1500 tons of uranium ( Waggaman and Ruhlman, 1956). The Florida pebble and leached-zone ore and the western phosphates carry about 0.01 to 0.02 per cent uranium oxide ( U808),while the Tennessee phosphates contain much smaller quantities. Development of methods for uranium recovery in phosphate rock processing has been encouraged by the U. S. Atomic Energy Commission (Barr, 1954; Barr et al., 1955; Flaschenberg et al., 1958; Greek et al., 1957; Long et al., 1956; Persons, 1956). The recovery methods used at most of the five plants-three in Florida and one each in Illinois and Texas-reported to have been built by early 1957 (Greek et al., 1957), all of which are adjuncts of other facilities treating Florida pebble phosphate, involve solvent extraction of the uranium from phosphoric acid made by treating phosphate rock with sulfuric acid. Studies by Young and Altschuler ( 1958) have indicated possibilities for selective concentration of the uranium in slag from phosphorus furnaces. Some of the phosphate rocks of the western United States carry significant percentages of vanadium. Beginning in 1941 this element has been recovered in the form of vanadium pentoxide ( VzOa) from phosphoric acid made by the sulfuric acid process from Conda, Idaho, phosphate rock averaging about 0.30 per cent VzOa (Anonymous, 1942; Argall, 1943; Caro, 1949; Waggaman and Ruhlman, 1956). Methods of recovering vanadium from the ferrophosphorus in which it accumulates during the manufacture of elemental phosphorus have been studied by Banning and Rasmussen ( 1951) The 13,101,000 short tons of phosphate rock comprising the apparent domestic consumption of this material in 1957 carried about 475,000 tons of fluorine, of which-based on the data of Hill and Jacob (1954) for 1 9 5 k b o u t 114,000 tons were evolved in the various processes for treating the rock. The proportionate recovery of the evolved fluorine in the form of useful compounds was probably higher than that (about 16 per cent) estimated for 1950. For many years the domestic supply of fluosilicates has been obtained principally, if not entirely, as by-products of the manufacture of superphosphates, wet-process phosphoric acid, and conversion products of the latter (Adams et al., 1952; Hill and Jacob, 1954; Jacob et d.,1942; Pettit, 1951; Shoeld et al., 1949). Other operations in which fluorine is evolved as either silicon tetrafluoride or hydrogen fluoride, but usually is not recovered extensively, include the manufacture of elemental phosphorus (Burtand Barber, 1952), calcium metaphosphate (Brosheer, 1953), de-

.

269 fluorinated phosphate rock (Brosheer and Hignett, 1953; Gilbert et al., 1953; Hignett and Siegel, 1949; Whitney and Hollingsworth, 1949, 1952), and defluorinated superphosphate (Fox et nl., 1946; MacIntire et al., 1941; Rockwood, 1944) and the nodulization of phosphate rock (Tarbutton et al., 1958). Many plants make no effort to recover the evolved fluorine, but its health and nuisance hazards usually necessitate treatment of the effluent gases in one way or another (Davenport and Morgis, 1954; Pettit, 1951; Poppe, 1954). As pointed out by Waggaman and Ruhlman (1956), a major problem in the utilization of the fluorine appears to be the development of simpler and more economical methods for its recovery in the form of hydrofluoric acid and high-grade calcium fluoride. FERTILIZER PRODUCTION AND TECHNOLOGY

5. Direct Use as Fertilizer Amounting to less than 100,000 short tons annually before World War 11, the direct use of phosphate rock as a fertilizer in the United States increased to a peak of 1,176,962 tons in 195s1953 (Table XI). TABLE XI Phosphate Rock Used Directly as Fertilizer in the United States and Territories, 1948-1957, in Short TonsG Year ended June 30

Illinois

Missouri

Other are-

Total

1948 1949 1950 1951 1952 1953 1954 1955 1956 1957

583,574 509,371 452,608 601,219 646,576 700,835 563,876 370,775 570,294 517,508

18,792 44,192 75,372 170,255 227,872 253,656 158,644 105,307 236,112 200,773

175,061 189,137 221,273 268,150 285,572 222,471 190,156 128,571 124,508 117,902

777,427 742,700 749,253 1,039,624 1,160,020 1,176,962 912,676 604,653 930,914 836,183

U. S. Department of Agriculture; includes colloidal phosphate.

Expressed as total P206, the domestic use was 56 per cent of the world consumption of phosphate rock for direct application in 19561957 (Page, 1957). Illinois has long accounted for the greater portion (56 to 75 per cent in 1947-1948 to 19561957) of the domestic use of phosphate rock for direct application. Beginning in 1949-1950 Missouri has been second, with proportions of 10 to 25 per cent. Before 1945, the direct-application rock came chiefly from the Tennessee deposits, but later from Florida.

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Florida supplied about 87 per cent of the tonnage in 1956-1957, as compared with 12 per cent from Tennessee and 1 per cent from thc Western States. It has long been known that phosphate rocks from different deposits vary in the solubility of the phosphorus in ammonium citrate and citric acid solutions and other solvents (Fleury, 1953; Jacob et al., 1933, 1947; Martens, 1943) and in their efficiency as direct sources of phosphorus for plant nutrition, Greenhouse experiments by Brown and Jacob (1945) with several phosphate rocks indicated a positive correlation between the nutritive efficiency of the phosphorus and its solubility in neutral ammonium citrate and 2 per cent citric acid solutions. Studies by Armiger and Fried (1957) and Caro and Hill ( 1956b), involving measurements of solubility, surface properties, physical properties of particles, and chemical composition of phosphate rocks, showed that phosphate-bound carbonate content and citric acid solubility were the criteria giving best agreement with the nutritive value of the phosphorus in greenhouse tests. According to Carbona (1956) the solubility of natural phosphates is related to the specific total surface, as well as to mesh size and chemical composition. Other studies of the surface properties of various phosphates have been reported by Caro and Hill (1956a), Hill (1956), Hill et al. (1954), Norland et al. (1957), and Olsen ( 1952).

B. ELEMENTAL PHOSPHORUS Although the domestic output of elemental phosphorus, made by the electric-furnace process, goes chiefly for nonagricultural purposes a substantial portion moves into fertilizers. Of the total distribution (262,500 short tons) of phosphorus in 1954, for example, fertilizer use accounted for 45,512 tons, including 17,000 tons supplied by private producers (Homer, 1956). For use as fertilizer the phosphorus is converted principally into phosphoric acid which in turn is processed mostly into triple superphosphate, ammonium phosphates, and liquid and solid mixed fertilizers. The Tennessee Valley Authority uses considerable phosphorus in making calcium metaphosphate which does not involve the intermediate production of phosphoric acid. Experiments by MacIntire et al. (1950) showed that yellow phosphorus is toxic to plants and that red phosphorus behaves as an inert material in the soil. The growth of the phosphorus industry in recent years is indicated by the data of Table XI. The 12 facilities in 1958 comprised 3 plants each in Florida and Tennessee, 2 in Idaho, and 1 each in Alabama, Montana, New York, and South Carolina. At the close of the year another plant was under construction in Idaho. The elemental phosphorus industry has shifted markedly to the West, beginning with the first operation in 1949.

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TABLEJXII Elemental Phosphorus Plants, Capacity, and Production in the United States, 1940-1954 Calendar year

Plants

Annual capacity, short tons

Production, short tons

~~

1940 1945 1950 1952 1954"

6 6 9 12 13'

63,9000 88,100" 161,300b 270,000d 320,0000

48,500" 8O,00Oa

153,233" 195,157' 266,887

Skeen (1948a). *As of January 1, 1951 (Horner, 1956). Bixler et al. (1956). *As of December 31,1952 (Gordon, 1952). a As reported by the U.S. Department of Commerce, the production was 292,954tons in 1955, 312,213 in 1956, and 339,164in 1957. One plant ceased operation in October 1954. # A s of January 1, 1955 (Horner, 1956). a

Thus at the end of 1952 the 3 plants in Idaho and Montana had nearly 35 per cent of the phosphorus capacity (Gordon, 1952). Waggaman and Bell (1950) have discussed the factors affecting the development of a phosphate fertilizer industry based on phosphorus from western rock. Progress in the production and use of phosphorus is outlined in papers by Aall (1952), Bixler et al. (1956), Gordon (1952), Hill ( 1952), Homer ( 1956), Riley ( 1956), and Skeen ( 1948a), and descriptions of phosphorus plants, processes, and equipment are given by Bixler et a2. (1956), Burt and Barber (1952), Callaham ( 1951), Curtis (1952), Shepherd (1956), Striplin et al. (1951, 1953), Waggaman (1952), and Wright (1951). Threlfall (1951) has written a very interesting history of phosphorus making by the firm of Albright 8.1 Wilson established at Oldbury, England, in 1851. Information on the chemistry and properties of phosphorus and its compounds has been compiled by Farr (1950) and Van Wazer (1958), and developments in the agglomeration of phosphate fines for furnace use were reported by Stout ( 1950). Processes for preparing fertilizers and animal-feed supplements from ferrophosphorus, a by-product of phosphorus manufacture, have been studied by Bridger et al. (1949), Burt and Barber (1952), and Potts et al. (195813). Calcium silicate slag (Burt and Barber, 1952; MacIntire and Sterges, 1953) and precipitator dust containing phosphorus, potassium, and other nutrient elements (Burt and Barber, 1952; Caro et al., 1958) are phosphorus-furnace by-products which find agricultural use.

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C. PHOSPHORIC ACID Phosphoric acid is made in the United States by the wet process (treatment of phosphate rock with sulfuric acid) and from elemental phosphorus. Its manufacture by the blast-furnace process-initiated by the Victor Chemical Works at Nashville, Tennessee, in 1929 (Easterwood, 1933)-ceased in 1940. Data on the production and the use pattern of phosphoric acid and the number and annual capacity of the plants are given in Tables XI11 and XIV. Much of the acid from elemental phosphorus is manufactured at locations apart from those where the elemental phosphorus is produced, TABLE XI11 Production of Phosphoric Acid in the United States, 1948-1957, in Short Tons of Equivalent 50% HsPOla Calendar year

Acid from phosphorus

Webprocess acid

Total

1948 1949 1950 1961 1952 1953 1954 1955 1956 1957

584,147 716,944 813,158 911,364 987,361 1,277,139 1,399,155 1,490,957 1,574,708 1,747,907

609,686 677,600 799,511 934,363 1,073,738 1,369,828 1,742,828 2,034,428 2,241,709 2,585,128

1,193,833 1,394,544 1,612,669 1,845,727 2,061,099 2,646,967 3,141,983 3,525,385 3,816,417 4,333,035

a

U. S. Department of Commerce.

TABLE XIV Phosphoric Acid: Plants, Capacity, and Distribution of Use in the United States, Calendar Year 19550 Item

Acid from phosphorus

Webprocess acid

Total

~~

Plants, numberb Capacity bee Fertiliiers Other uses Total

25 2,131,280

19 2,743,674

44 4,874,954

186,705 1,350,042 1,536,747

1,716,932 410,754 2,127,686

1,903,637 1,760,796 3,664,433

Horner (1956). As of January 1, 1955. Short tons of equivalent 50% H8P04.

273 while the wet-process acid is made chiefly in plants coexistent with facilities for its conversion into other products. One wet-process plant markets its entire output of acid as such, mostly for fertilizer manufacture (Anonymous, 1957j ) . According to Horner (1956), fertilizers accounted for 51.9 per cent of the total distribution of phosphoric acid in 1955. The proportion of the acid from phosphorus used in this way was 12.1 per cent, as compared with 80.7 per cent for the wet-process acid. The fertilizer use of the first type of acid is largely as triple superphosphate and ammonium phosphates, and such acid is favored for production of liquid mixed fertilizers (Slack, 1955, 1957) because of its freedom from solid material. The principal fertilizer use of wet-process acid is in the manufacture of triple superphosphate, and large quantities are processed into ammonium phosphates. Increasing use is being made of both types of acid in the manufacture of granular mixed fertilizers (Hignett and Slack, 1957), and limited quantities are applied directly as fertilizer (Hill, 1952; Jacob and Scholl, 1955; Jones, 1950; MacIntire et al., 1947), principally in irrigation water in the West. Processes and equipment for making phosphoric acid from elemental phosphorus have been described by Almond and Steinbiss (1948) , Striplin ( 1948), and Waggaman (1952), and further studies of oxidation of phosphorus with steam to produce phosphoric acid and hydrogennot yet applied commercially-have been reported (Hein et al., 1950; Shultz et al., 1950). A process has been developed for making from phosphorus a superphosphoric acid (76 per cent Pz05, equivalent to 105 per cent H3POa) which does not crystallize at ordinary temperatures (Striplin et al., 1958) and can be used in the manufacture of high-analysis fertilizers ( McKnight and Striplin, 1958; Phillips et al., 1958b). According to Huhti and Gartaganis (1956), such acid contains about 49 per cent of its phosphate as ortho-, 42 per cent as pyro-, 8 per cent as tri-, and 1per cent as tetraphosphoric acid. Recent papers on the manufacture of wet-process phosphoric acid include descriptions of the anhydrite (Nordengren et al., 1955), Giorgini (Anonymous, 1956b; Inskeep et al., 1956), Prayon (Anonymous, 1954d, 195%; McNally, 1956), and other processes (Atwell, 1949; Porter and Frisken, 1953; Weber, 1952). A novel method of making phosphoric acid by aqueous extraction of a clinker prepared by heating mixtures of phosphate rock and sulfuric acid was developed on the pilot-plant scale by Legal et al. (1957); it yields strong, relatively pure phosphoric acid without evaporation. Among the recent researches on the properties of phosphoric acid are studies of its density (Christensen et al., 1955), vapor pressure FERTILIZER PRODUCTION AND TECHNOLOGY

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(Brown aiid Wliitt, 19S2; Handlos and Nixon, 1956), heat capacity (Egan et al., 1958; Egan and Wakefield, 1957), viscosity (Edwards and Huffman, 1958), structure of the crystalline acid (Smith et al., 1955), azeotropy in the system phosphoric oxide-water (Tarbutton and Deming, 1950)) and action of the acid on metals (Yates, 1951). D. SUPERPHOSPHATJS For about a hundred years the fertilizer phosphorus used in the United States has been chiefly in the form of normal superphosphate made by treating phosphate rock with sulfuric acid. Domestic manufacture of triple superphosphate (also called double, treble, or concentrated superphosphate), made by treating phosphate rock with phosphoric acid, gained permanence in 1907 (Mehring, 1944), and substantial quantities of enriched superphosphate, prepared with mixtures of sulfuric and phosphoric acids, have been produced. In some parts of the country there has also been considerable production of wet-base goods, made by treating mixtures of phosphate rock and nitrogenous organic materials with sulfuric acid. TABLE XV Production of Superphosphates and Wet-base Goods in the Continental United States, 1948-1957, in Short Tons of Available P20~' Superphosphate Calendar year

Normal and enrichedb

Triple

Wet-base goods

Total

1948 1949 1950 1951 1952 1953 1954 1955 1956 1957

1,677,545 1,633,663 1,673,289 1,708,825 1,765,000 1,678,459 1,644,515 1,558,111 1,481,248 1,380,009

210,920 246,827 309,084 322,420 388,055 457,235 561,870 706,584 753,417 831,510

11,069 10,921 11,141 13,659 12,405 11,768 8,325 7,800 6,729 ( "1

1,899,534 1,891,411 1,993,514 2,044,904 2,165,460 2,147,462 2,214,710 2,272,495 2,241,394 2,211,519

U. S. Bureau of the Census. Includes 40,360 tons of P2OSas enriched superphosphate in 1954, 39,042 in 1955, and 12,888 tons in 1956. c Included with other phosphatic fertilizers not shown in this table. 0

Table XV shows the production of available PnOaas superphosphates and wet-base goods in the United States in 1948 to 1957. During this time the annual output as normal and enriched superphosphates decreased nearly 18 per cent, while that as triple superphosphate increased

275 294 per cent. The proportion furnished by triple superphosphate rose from 11.1to 37.6 per cent. Mounting transportation costs and the trend to higher-analysis mixed fertilizers were among the factors contributing to the rapid expansion in triple superphosphate production. FERTILIZER PRODUCTION A N D TECHNOLOGY

1. Normal Superphosphate As reported by Adams et al. (1952), the total capacity for production of normal superphosphate in the continental United States on September 1, 1951, was 2,976,800 short tons of available PzOBin 202 plants located in, thirty-two States. Although plant-capacity data are not available for later years, the U. S. Bureau of the Census (1958) reports that in 1957 normal superphosphate was produced by 202 plants in thirty-three States. The principal steps in the manufacture of normal superphosphate comprise mixing ground phosphate rock with sulfuric acid, solidification of the semifluid mass in an enclosed den, and transfer of the fresh product to open piles for curing and further manipulation as may be necessary or desired. Formerly done mostly as batch operations, these steps are now performed in many plants by continuous methods of one kind or another (Anonymous, 1957i, 1958c; Jacob, 1951a; Parrish and Ogilvie, 1946; Waggaman, 1952). A recent development is the use of a simple funneltype continuous mixer for the rock and acid (Bourdier, 1956; Jackson, 1958; Yates and Williams, 1952), which was applied first by the Tennessee Valley Authority in production of triple superphosphate (Bridger et al., 1947). Reaction of phosphate rock dust with atomized sulfuric acid in a tower is a principal feature of the Sackett continuous process for normal superphosphate (Demmerle and Sackett, 1949). Production of normal superphosphate in granular form for direct application as fertilizer is practiced extensively ( Demmerle and Sackett, 1949; Mackall and Shoeld, 1940; Nordengren, 1947; Parrish and Ogilvie, 1946; Proctor, 1949). In some plants the granular product is obtained by screening the regular run-of-pile superphosphate, while in others it is prepared by processes involving agglomeration and drying. Studies of the use of different types of phosphate rock in the production of normal and triple superphosphates have been made by Dee et uZ. (1957).Nunn and Dee (1954) and Shoeld et al. (1949) investigated the factors influencing the acid-rock reaction in normal superphosphate manufacture. Bridger and co-workers ( Bridger and Drobot, 1956; Bridger and Kapusta, 1952; Bridger and Kearns, 1956) studied the processing conditions favoring rapid curing of normal superphosphate. Arvan et d. (1955) reported that in the manufacture of superphosphate the rate and extent of phosphorus conversion to available forms are favored by the presence of small concentrations of surface-active agents.

276

K. D. JACOB

No pronounced effects of this kind were observed, however, by Bridger and Kearns (1956), Fox et al. (1954a,b), Kumagai and Hardesty (1955), and Retzke et al. (1955). It appears that the beneficial action of surfaceactive agents is limited very largely to small improvements in the physical character of the products. Fox and Jackson (1955, 1957) found that phosphate rock which has been beneficiated by flotation may retain a suf6cient quantity of surface-active agents to produce these effects. Since most of the domestic consumption of all types of superphosphates is in the form of ammoniated mixed fertilizers, considerable study has been made of the characteristics of superphosphates that influence their reaction with ammonia (Harvey and Rohner, 1942; Kumagai et al., 1954; Perrine, 1956a; Waters et al., 1955). The rate and degree of absorption of anhydrous ammonia, for example, are favored by high moisture content, fine particle size, and low apparent density of the superphosphate, among other factors. Young and Heil (1957) have described a method of producing normal superphosphate for immediate use in ammoniation and granulation processes. A recently developed, continuous rotary ammoniator for superphosphates and mixed fertilizers has found wide commercial use (Yates et al., 1954). Much superphosphate is made with spent sulfuric acid from various chemical and industrial operations, including the refining of metals and petroleum and the manufacture of alcohol, detergents, dyestuffs, explosives, insecticides, and other organic products. About 800,000 short tons of normal superphosphate were manufactured with such acid in 1950 (Adams et al., 1952). 2. Triple Superphosphate Data on plants and capacities for triple superphosphate in the United States in 1930 to 1957 are given in Table XVI. The figures for 1957 are revised from those reported by Anonymous (1957e) which included 2 plants that have ceased operation (1in Tennessee in 1952 and 1in South Carolina in 1956) and omitted a plant (in Missouri) that commenced operation in 1953. Thus at the close of 1957 the facilities comprised 8 plants in Florida having a total annual capacity of 1,765,000 short tons of triple superphosphate, or 79 per cent of the national capacity (2,225,000 tons), and 1plant each in Alabama, Idaho, Missouri, Montana, New Jersey, Texas, and Utah. Another plant, in Mississippi, was reported to be under construction in 1958. Phosphoric acid was produced at each plant, at 14 by the wet process and at 1 (Tennessee Valley Authority) by oxidation of elemental phosphorus. Some triple superphosphate is made also with purchased acid, produced largely from phosphorus, at plants operated primarily for manufacture of normal superphosphate;

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TABLE XVI Triple Superphotiphatc Plant,s arid (hpacit?, in thc United States, 1930-1957" Year

Plants

1930 1940 1945 1951 1957

5 8 9 9 15

Annual capacity, short tonsb 97,800 400,000 496,500 793,000 2,225,000

a 1930-1945 (Jacob, 1951a); 1951 (Adams et al., 1952); 1957, revision of data reported by Anonymous (1957e). The data relate only to plants having coexisting facilities for making phosphoric acid. * 1930-1951, tons of material basis 45% avail1957, tonr of material as produced. able P20s;

triple superphosphate was made at 11 such plants in 1957 (U. S. Bureau of the Census, 1958). Phosphoric acid is produced from elemental phosphorus in such concentration that the triple superphosphate made therewith does not require artificial drying. This is true also of the wet-process acid used at some plants, while at others the acid concentration is such that the product must be dried. One plant makes triple superphosphate by concentrating a slurry of phosphate rock and dilute phosphoric acid in a rotary kiln (Waggaman, 1952). Most of the triple superphosphate used directly as fertilizer is in granular form. Bridger (1949), Bridger et al. (1947), Inskeep et al. (1956), McNally (1956), Porter and Frisken (1953), Waggaman (1952), and others have described commercial processes, plants, and equipment for making triple superphosphate. Phillips et al. ( 1958c) reported the pilot-plant development of a one-step, continuous process for making granular triple superphosphate. The effect of various factors on the rate and degree of the reaction between phosphoric acid and phosphate rock to form triple superphosphate has been studied by Bridger and Markey (1953) with the aid of radioactive techniques. Hill et al. (1955) found that commercial triple superphosphate commonly contains some PzO5 (usually less than 0.5 per cent) in nonorthophosphate forms. Phillips et al. (195813) investigated the production of high-analysis superphosphate (54 to 56 per cent Pz05)from phosphate rock and superphosphoric acid (74 per cent PzOa). About 65 per cent of the fluorine in the rock was evolved during the process, and the product, chiefly

278

K. D. JACOB

anhydrous monocalcium phosphate, usually contained 1 per cent or less of moisture. 3. Enriched Superphosphate With the aid of normal superphosphate equipment, enriched superphosphate has been made in the United States for more than fifteen years by treating phosphate rock with mixtures of sulfuric and phosphoric acids, Thus, with proper choice of acid concentrations and proportions, all grades of products between normal superphosphate and triple superphosphate can be manufactured without resort to artificial drying. As practiced by one plant, for example, enriched superphosphate containing about 26 to 28 per cent of available PZO5 is made by using dilute phosphoric acid, the initial product of the wet process, in conjunction with concentrated sulfuric acid. In 1957 enriched superphosphate was produced by two plants, in Illinois and Pennsylvania. Studies of the characteristics of enriched superphosphate and of the factors involved in its preparation have been reported by Bridger ( 1949), Fox and Hill ( 1952), and Yates et al. (1953).

E. AMMONIUMORTHOPHOSPHATES Hill (1957) estimated that somewhat more than a third of the domestic consumption of phosphorus as fertilizer in 1953 reached the soil as mono- and diammonium phosphates. These compounds originate ( 1 ) as individual materials used for direct application and for formulating mixtures and ( 2 ) chiefly as reaction products of ammonia and its salts with phosphoric acid and monocalcium phosphate in the processing and storage of mixed fertilizers. The domestic production of all kinds and grades of the individual materials totaled 171,351 short tons of available P z O ~in 1957 (U. S. Bureau of the Census, 1958). Fertilizer-grade ammonium phosphates, consisting chiefly of the monobasic compound, have been manufactured with wet-process phosphoric acid almost continuously in this country since the first large-scale plant (Landis, 1936) was opened at Warners, New Jersey, in 1917. Production of such material in Canada was started in 1931 at Trail, British Columbia (Atwell, 1949; Consolidated Mining and Smelting Co., 1954). In October 1958, facilities for making ammonium phosphates with captive wet-process acid comprised at least 9 plants in the United States and 3 in Canada, all of which used slurry agglomeration processes. The products of these operations included crude monoammonium phosphate (1148-0) and its processed mixtures with ammonium sulfate (13-39-0, 16-20-0, and other grades) and with ammonium nitrate (27-144, for example). Products ( 16-48-0, for example) containing

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phosphate chiefly as the diammonium salt were also manufactured by several of the plants, one of which has been described by Horn (1956). Burnet ( 1957) has discussed commercial methods for making ammonium phosphate fertilizers with wet-process acid. A continuous vacuum-crystallization process for making diammonium phosphate with this acid was developed on the pilot-plant scale (Houston et al., 1955). Although manufacture of diammonium phosphate products for fertilizer use was started in Germany some thirty years ago, only negligible quantities were made for this purpose in the United States until 1955. In addition to the previously mentioned production with wet-process acid, considerable quantities of such materials-chiefly the practically pure compound (21-53-0) and aqueous solutions of the mono and di salts (8-24-0, for example)-are now made in this country with acid prepared from elemental phosphorus. Such acid was used in 1957 by at least 6 plants, 3 of which utilized by-product ammonia from coke ovens. With one exception the elemental phosphorus was from noncaptive productions. Pilot-plant investigations of the use of acid from elemental phosphorus in continuous production of crystal diammonium phosphate by saturator and vacuum crystallizer processes have been reported by Thompson et al. (1950) and Getsinger et al. (1957), respectively. The latter process is adaptable to the preparation of large crystals the shape of which is changed from plates (the normal habit) to near cubes ( a more desirable form) by the presence of a small amount of ammonium sulfate in the mother liquor. The properties of diammonium phosphate and its use in production of mixed fertilizers have been studied by Hignett et al. (1956), Silverberg and Heil (1957), Silverberg and H o h e i s t e r (1958), Smith et nl. (1957), and Thompson et nl. (1949). McKnight and Striplin (1958) have discussed the preparation of ammonium phosphate fertilizers with superphosphoric acid.

F. DICALCIUM PHOSPHATE Although straight dicalcium phosphate is a minor article of manufacture and trade in the fertilizer industry, it is a highly important constituent of superphosphate-containingproducts that have been treated with ammonia or other basic materials. Thus, Hill (1957) estimated that in 1953 about one-third of the phosphorus in fertilizers consumed in the United States reached the soil in the form of dicalcium phosphate. Large quantities of this phosphate are produced for use as a phosphorus supplement in animal feeds, but, for various reasons, attempts at domestic manufacture of the fertilizer-grade material have been sporadic and generally of limited duration. The world production of fertilizer-grade

280

K. D. JACOB

dicalcium phosphate is mostly in Europe, where it is made by processes involving treatment of hydrochloric acid extracts of phosphate rock with lime. The output in Belgium, France, and Sweden totaled about 150,000 tons in 1950-1951 ( Organisation for Europen Economic Co-operation, 1953). Methods for making dicalcium phosphate have been described by Gadre and Gupta (1953) ,Leroy ( 1947),Pate1 ( 1953),Waggaman ( 1952), Walthall (1953), and others. An objective of much work, largely unsuccessful, has been the production of the material directly by reaction of phosphate rock with mineral acids. According to Bridger et aZ. ( 1956), a fertilizer product containing phosphorus principally in the form of dicalcium phosphate can be obtained in this way be refluxing a mixture of phosphate rock, water, and sulfuric or phosphoric acid and then heating in an open container to 130" to 185" C. Boull6 and Dupont (1955) studied the preparation of anhydrous dicalcium phosphate from the dihydrate. As a source of phosphorus for plant nutrition the dihydrate is said to be more effective than the anhydrous compound ( Terman et aZ., 1958). Car0 and Hill (1956a) reported values of 0.4 to 15.3 square meters per gram for the surface area of various dicalcium phosphate preparations, as determined by gas-adsorption and isotope-exchange techniques. Norland et aZ. (1957) found, however, that the surface area of the material, at least within the limits 0.7 to 5.5 square meters per gram, is not a criterion of its plant-nutrient value. OF PHOSPHATE ROCK WITH NITRIC A m G. TREATMENT Treatment of phosphate rock with nitric acid has been done commercially in Europe for at least twenty-five years. In Germany, for example, manufacture of one type of Nitrophoska in this way started in 1933 (Buchner, 1952). The first two plants in the United States for making phosphate fertilizers with nitric acid were opened in 1954 (Huang, 1955) and the third in 1956; their annual capacity totals about 360,000 short tons of products. Other countries producing nitric phosphates include Finland, France, Hungary, Italy, Japan, Netherlands, Norway, Sweden, Switzerland, and Taiwan. Plusj6 (1948, 1951) made fundamental studies of the nitric acid-phosphate rock reaction. Hignett ( 1951), the Organisation for European Economic Co-operation ( 1953), and Walthall ( 1953) have reviewed the processes based on this reaction, and Stanfield (1953) has investigated their economics. Descriptions have been published of commercial plants in Europe (Anonymous, 1953c; Clark and Hardesty, 1951) and the United States ( Norden, 1956~).

FERTILIZER PRODUCTION AND TECHNOLOGY

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The principal processes, studied on pilot-plant scales by the Tennessee Valley Authority, involve acidulation with nitric acid and sulfuric acid (Striplin et al., 1952) or phosphoric acid (Houston et al., 1951) or mixtures of sulfuric and phosphoric acids, and addition of ammonia and potassium chloride to make various grades of granular products, for example, 12.-1212, 14-14-14, and 16-16-8. Nitrogen-phosphorus grades are obtained by omitting the potassium chloride. The sulfuric acid and ammonia may be replaced by ammonium sulfate and (or) potassium sulfate ( D’Leny, 1953; Nielsson and Yates, 1953). In another commercial process, studied in the pilot plant by Nielsson et al. (1953), the nitric acidulate is treated with ammonia and carbon dioxide, with or without addition of potassium chloride. A modification of the nitric-sulfuric acid process has been applied on the pilot-plant scale to production of fertilizers from the low-grade, high-aluminum phosphate ore occurring in the leached zone of the Florida pebble district (Hignett et al., 1957). Use of the continuous ammoniator (Yates et al., 1954) in production of nitric phosphates was investigated in the pilot plant by Hignett et al. ( 1958). Brosheer and Lenfesty ( 1958) studied the factors influencing the ammoniation of nitric acid extracts of phosphate rock. Work by McKnight et al. (1953) indicated possibilities for the partial replacement of sulfuric acid by nitric acid in superphosphate manufacture.

H. METAPHOSPHATES Calcium metaphosphate fertilizer, containing approximately 62 per cent of available PzOaand made by reacting phosphoric oxide vapor with phosphate rock at high temperature, has been produced by the Tennessee Valley Authority in demonstration and full-scale plants for about twenty years. The TVA’s output was 73,296 short tons in the year ended June 30, 1958. The world production of this material is believed to have been entirely by the TVA until its manufacture and conversion to dicalcium phosphate began recently in South Africa (Anonymous, 1957d). The manufacture and technology of calcium metaphosphate have been discussed by Brosheer (1953), Walthall (1953), and Yates et al. (1951). Studies have been made of its thermodynamic properties (Egan and Wakefield, 1956) and hydrolytic degradation products (Brown et al., 1951). For use in the preparation of mixed fertilizer, calcium metaphosphate has the disadvantage that, unlike the superphosphates, it contains no water-soluble phosphorus and it does not react with anhydrous ammonia or ammoniating solutions. The work of Phillips et al. (195%) and Seymour (1955) indicates, however, that these hindrances can be overcome by hydrolyzing the metaphosphate with a mineral acid and

282

K. D. JACOB

then processing with ammonia and other nutrient materials to produce granular, high-analysis mixtures. Stinson et al. (1956) reported the pilot-plant preparation of a material (N 17, PzO5 73 per cent) containing 80 to 86 per cent ammonium metaphosphate by a process involving reaction of ammonia with phosphorus pentoxide vapor at 600" to 1000" F. and steam hydrolysis of the intermediate compound to form a relatively nonhygroscopic, granular product having high solubility of the nitrogen and phosphorus in water (70 to 80 per cent) and of the latter in neutral ammonium citrate solution (96 to 99 per cent), Nitrification and greenhouse tests indicated that the material is an excellent fertilizer, Potassium metaphosphate fertilizers have not been made commercially in the United States or, apparently, in foreign countries. Potts et al. ( 1958a) prepared fused potassium phosphates, including potassium metaphosphate ( KzO 31, PzO5 55 per cent) and potassium calcium pyrophosphate ( K 2 0 25, PzO6 42 per cent), in which the initial water solubility of the potassium ranged from 5 to 100 per cent.

I. BASICSLAG The only domestic source of basic slag is the low-analysis material ( 8 to 10 per cent total PZO5)obtained as a by-product of the manufacture of open-hearth steel from phosphorus-bearing iron ores in the Birmingham, Alabama, district. The consumption totaled 162,662 short tons in the year ended June 30, 1957. Except for this source and small quantities in Canada and Japan, the European production (about 1,225,000 short tons of P205 in 1956-1957, mostly in France, Germany, Belgium, Luxembourg, and the United Kingdom) accounts for practically all of the world output of basic slag. Among the recent papers dealing with the chemistry and technology of basic slag are those by Gericke (1953a,b, 1954a, 1955), Kurmies (1953), and Tromel (1952). Other research includes studies of the system Ca0-Pz05-SiOz (Tromel et al., 1948), dephosphorization of iron with lime (Tromel and Oelsen, 1955), optical and X-ray characteristics of the basic-slag mineral silicocarnotite (Riley and Segnit, 1949), and occurrence and solubility of trace elements in basic slag (Chichilo and Whittaker, 1953; Dickens and Radmacher, 1955).

J. DEFLUORINATED PHOSPHATE ROCK Initiated commercially in 1944 by the Coronet Phosphate Company at West Conshohocken, Pennsylvania, production of defluorinated phosphate in rotary kilns-involving essentially the heating of phosphate rock

FERTILIZER PRODUCTION AND TECHNOLOGY

283

in the presence of water vapor-has been continued by that company at Plant City, Florida, since September 1945 (Walthall, 1953; Whitney and containing Hollingsworth, 1949, 1952). Although the product-now about 37.5 per cent available P205 and 0.1 per cent fluorine, as a result of modifications and improvements in the process-is an excellent fertilizer material, it finds a more lucrative market as a phosphorus supplement in animal feeds. A material manufactured in Japan is understood to be similar to the Coronet product. After nine years of operation, the Tennessee Valley Authority’s demonstration plant near Columbia, Tennessee, for production of fused tricalcium phosphate by defluorinating phosphate rock in a shaft furnace (Almond and Albrecht, 1955; Brosheer and Hignett, 1953; Walthall, 1953; Whitney and Hollingsworth, 1952) was closed in 1954. Production of the material during this time totaled 190,755 short tons (Tennessee Valley Authority, 1954). The typical product contained about 28 per cent total P2O5, 22 per cent available P2OS,and 0.3 per cent fluorine. Hall and Banning (1958) reported laboratory and pilot-plant studies of the removal and recovery of fluorine from Idaho phosphate rock. Ando (1958a) found that the impurities in phosphate rock influence markedly the thermal stability of the alpha and beta forms of tricalcium phosphate in the defluorinated product.

K. PHOSPHATE ROCK-MAGNESIUM SILICATEGLASS Beginning in 1946, phosphate rock-magnesium silicate glass ( also known as Thermo-Phos, MP Phosphate, fused calcium-magnesium phosphate, and serpentine-fused phosphate) containing about 19 per cent available P205 was manufactured for several years in California and Washington by quickly cooling an electric-furnace melt of phosphate rock and olivine or serpentine (Crossman, 1949; Granberg, 1948; Hill et ul., 1948; Moulton, 1947, 1949; Walthall, 1953; Walthall and Bridger, 1943). A similar material, known as Uniwapo-Phosphat, has been produced on a semicommercial scale in Germany (Tromel, 1949, 1952). Pilot-plant work at the Iowa State College has shown that the process can be operated also in gas-fired furnaces ( Boylan, 1957). According to Page ( 1957), production of this material (designated as fused phosphate) in the year ended June 30,1957, was confined to Japan and Taiwan44,000 and 2000 metric tons of P205, respectively. Its manufacture is reported, however, to have been started recently in Poland. The technology and properties of phosphate rock-magnesium silicate glass have been studied recently in England (Reynolds, 1951), India

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(Mariakulandai ct al., 1955), Japan (Ando, 1958b; and many others), New Zealand ( Billinghurst and Nicholson, 1956; Karlovsky, 1957), and Taiwan (Huang, 1953, 1954; Huang and Yang, 1951).

L. RHENANIA-TYPEPHOSPHATES The world production of phosphate fertilizers by processes involving the heating of phosphate rock with alkali salts appears to be comprised largely of West Germany’s output of Rhenania phosphate-about 200,000 short tons in 1950-1951 (Organisation for European Economic Co-operation, 1953)-a product made with the aid of sodium carbonate (Tromel, 1952; Whitney and Hollingsworth, 1952; Walthall, 1953). Under the names of silicophosphate and soda phosphate, small quantities of a material similar to Rhenania phosphate have been manufactured for several years in British East Africa by heating Uganda phosphate with sodium carbonate (Jones, 1949). Comprehensive laboratory and plant-scale studies of the Rhenania process and various modifications thereof were made in England in 1940 to 1946 (British Ministry of Supply, 1951), but the work was not extended to commercial production. The few efforts to manufacture Rhenania-type phosphate in the United States have not been commercially successful. Ando ( 1958b) has studied the system Ca3(P04)&aNaP04. A citrate-soluble fertilizer called Palatia phosphate has been produced in West Germany by heating phosphate rock with sodium sulfate, silica, and coal (Tromel, 1952). Similar products (about 90,000 tons in 1948) are made in Chile with the use of either sodium sulfate or sodium nitrate (Dion and Callister, 1953). Following World War 11, a product called Rochling phosphate was manufactured for a time in West Germany by heating phosphate rock with soda slag, a waste material from the desulfurization of iron with sodium carbonate (Hennenberger, 1939; Hofmann and Amberger, 1953; Maclennan et al., 1946; Tromel, 1952; Whitney and Hollingsworth, 1952). M. ALUMINUMAND IRON PHOSPHATES The fertilizer use of aluminum and iron phosphates is chiefly as incidental occurrences in products, principally superphosphates, made by acid-treatment processes, where they result from the presence of iron and aluminum in the phosphate rock. Although some aluminum and iron phosphates, especially the first, are good sources of phosphorus for plant nutrition (DeMent and Seatz, 1956; Hill et al., 1950; Moriceau, 1953; Starostka et al., 1955), high proportions of these compounds in superphosphates are undesirable because of their adverse effect on the physical condition of the products and their well-known tendency to favor rever-

285 sion of water-soluble phosphorus to insoluble forms (Lehrecke, 1947; Marshall and Hill,1952). The latter behavior is an important consideration in countries where solubility in water is the criterion for evaluating phosphate fertilizers. Aside from their incidental presence in other fertilizer products the agricultural use of aluminum and iron phosphates in the United States appears to be limited to relatively small quantities of a by-product material precipitated during the manufacture of sodium phosphates from wet-process phosphoric acid. This material contains about 35 to 55 per cent total PzO5, of which 90 per cent or more is in available forms ( chiefly citrate soluble ) , and considerable percentages of aluminum, iron, calcium, sodium, and fluorine. Much barium and sulfur are also present when barium carbonate is added as a precipitant for the sulfate ion commonly present in the phosphoric acid. Only a few attempts have been made to manufacture fertilizer directly from iron phosphate minerals. In fact, deposits of such minerals that might have commercial interest seem to be rare. On the other hand, deposits of aluminum phosphate are known in several countries, and some of them have been worked at one time or another. The present production of aluminum phosphate fertilizer appears to be chiefly from ore mined near "hies, Senegal (see Section IV, A, 2), which is processed by calcining at 1O00" C. (Martin, 1957). The dehydrated product, called Phosphal, contains 34 per cent total Pz05, of which about 75 per cent is soluble in ammonium citrate solution (Organisation for European Economic Co-operation, 1953;Prevot and Ollagnier, 1958). Hill et al. (1950) studied the physical properties, behavior under thermal treatment, and fertilizer value of fluorine-containing pseudowavellite ( hydrous calcium aluminum phosphate) which occurs in an extensive deposit near Bartow, Florida. About 80 per cent of the phosphorus was rendered soluble in neutral ammonium citrate solution by heating the mineral at 500" C., but the quality of the product deteriorated rapidly under moist conditions such as prevail in the soil. As mentioned in Section IV, A, 3, huge quantities of leached-zone ore, consisting largely of aluminum phosphate minerals, are moved and discarded in the mining of Florida phosphate rock. Treatment of the calcined material with a mixture of nitric and sulfuric acids, with further processing to obtain various grades of granular, high-analysis mixtures, appears to be the most promising of the proposed methods for making fertilizers from this ore (Hignett et al., 1957). The fertilizers produced in this way are reported to have excellent storage and drilling characteristics (Rapp and Hardesty, 1955) and to be good sources of phosphorus for plant nutrition ( DeMent and Seatz, 1956;Starostka et al., 195s). FERTILIZER PRODUCTION AND TECHNOLOGY

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Other research on aluminum and iron phosphates includes studies of their composition, properties, and characteristics ( Chang and Jackson, 1957; Fujiwara, 1950; Haseman et al., 1951; Lehrecke, 1947), solubility and phase diagrams in the system aluminum phosphate-phosphoric acidwater (Brosheer et al., 1954; Jameson and Salmon, 1954), and phase relations in the system Ca0-Al2O3-P2O6 (St. Pierre, 1956; Stone et al., 1956). N. RADIOACTIVEPHQSPHATES Radioactive phosphates have been used extensively in fertilizer evaluation and plant nutrition studies and other agronomic and technological investigations since World War 11. In the United States, these phosphates have been prepared mostly with the aid of radioactive phosphorus ( P32) in the form of neutron-irradiated potassium dihydrogen phosphate produced at the Oak Ridge National Laboratory, Oak Ridge, Tennessee. For example, during the period 1948 to 1958 this material was used by the U. S. Department of Agriculture in labeling about 10,000 pounds of fertilizers-including normal and triple superphosphates, ammoniated superphosphates, dicalcium phosphate, ammonium phosphates, calcium metaphosphate, alpha-tricalcium phosphate, fused tricalcium phosphate. nitric phosphates, and a wide variety of multinutrient mixtures-for research studies in many of the States and several foreign countries. Hill et al. (1949) and Wieczorek and Caro (1958) have discussed the methods, equipment, and safety precautions for labeling these fertilizers and the composition and characteristics of the products. Procedures for labeling basic slag and macrocrystalline fluorapatite with P3*have been described by Gericke (1954b) and Elmore and H u b a n ( 1952), respectively. Introduction of Pa2by direct neutron-irradiation of orthophosphates -a necessary procedure with natural materials, such as phosphate rock, which cannot be synthesized and with other materials which do not lend themselves to processing with radioactive potassium dihydrogen phosphate-may be accompanied by considerable conversion of the phosphorus to nonortho forms. Borland et al. (1952)and MacKenzie and Borland (1952) determined the proportions of these forms-among which pyrophosphate appeared to be dominant-in several irradiated orthophosphates and developed methods for their substantially complete elimination from labeled phosphate rock and potassium dihydrogen phosphate. Aside from agronomic investigations, Paa has been used in studies of the surface characteristics of various phosphates, for example, phosphate rock ( Caro and Hill, 1956b; Olsen, 1952),hydroxylapatite (Olsen, 1952),

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and dicalcium phosphate (Caro and Hill, 1956a). Radioactive techniques have been applied in studies of the reaction between phosphoric acid and phosphate rock (Bridger and Markey, 1953) and of the performance of mixing equipment in the preparation of superphosphate (U. S. Department of Agriculture, unpublished work). The performance of processes and equipment in the manufacture of wet-process phosphoric acid and granular mixed fertilizers was studied by Craven (1957) with the aid of radioactive sodium carbonate and potassium chloride, respectively,

0. OTHERPHOSPHATE MATERIALSAND PROCESSES According to Boylan and Larson ( 1957), fertilizers containing about 10.5 per cent total P205(completely soluble in neutral ammonium citrate solution) can be prepared by fusing phosphate rock with calcium sulfate. Products containing 10 to 16 per cent PzOs and 11 to 13 per cent KzO were obtained by Bridger and Boylan (1953) with the use of magnesium sulfate and potassium sulfate. Gadre (1954) obtained high conversion of the phosphorus to citrate-soluble forms by heating phosphate rock with either magnesium chloride or sea-water bittern. In other research, fertilizers largely soluble in 2 per cent citric acid solution were produced by sintering mixtures of phosphate rock, silica or potash feldspar, gypsum, and iron-manganese ore (Jayaraman and Krishnaswami, 1953) or by fusing mixtures of phosphate rock, greensand, and limestone or dolomite (Cornes, 1948). Hunter and Thornton (1956) found that phosphine (PHB) was an excellent source of phosphorus for radishes and wheat in greenhouse culture. V. Potassium

Publications dealing with general developments in the potassium fertilizer industry of the United States and other countries include those by Cowie (1951), Lamer (1957), Marin ( 1950), Reed (1953), Ruhlman ( 1956), Smith ( 1949), and Turrentine ( 1950). Progress in the consumption of potassium as fertilizer is discussed in Section 11.

A. PRODUCTTON The world production of marketable potassium compounds in 1957 was 8,700,000 short tons of equivalent potash (K20),an increase of 129 per cent over the average for 1946-1950 (Table XVII). Germany produced 40 per cent of the total in 1957 (East Germany, 19; West Germany, 21); United States, 26; France, 18; and the Soviet Union, 12. Other pro-

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TABLE XVII World Production of Marketable Potash, 1946-1950 (Avcrage) and 1951-1957, in Short Tons of Equivalent KaOa Germany Calendar year

Eastb

West

Total

United States

1946-50 1951 1952 1953 1954 1955 1956 1957

985,400 1,510,000 1,440,000 1,488,000 1,488,000 1,580,000 1,598,000 1,650,000

585,887 1,211,439 1,445,128 1,459,309 1,783,394 1,870,848 1,823,000 1,862,000

1,571,287 2,721,439 2,885,128 2,947,309 3,271,394 3,450,848 3,421,000 3,512,000

1,101,537 1,420,323 1,665,113 1,911,891 1,948,721 2,066,706 2,171,584 2,266,481

a

France

Soviet Unionb

791,833 960,730 1,022,539 414,900 996,575 480,700 1,192,083 593,700 1,310,961 870,500 1,462,722 983,600 1,529,000* 1,040,OOO

Worldb 3,800,000 5,600,000 6,200,000 6,500,000 7,300,000 7,900,000 8,300,000 8,700,000

U. 8. Bureau of Mines.

* Estimate.

ducing countries were Spain (3 per cent), Israel (0.6 per cent), Chile, Japan, China, Ethiopia, Italy, and Korea. Chloride salts account for more than 90 per cent of the world's primary production of potash. About 95 per cent of the total output of potash is used as fertilizer. The annual capacity for producing potash in the United States was 2,480,000 short tons of KzO in July 1957 (Anonymous, 1957h), or about 140 and 27 per cent more than the respective capacities in 1947 and 1954. Nearly 90 per cent of the 1957 capacity was in the six plants processing the deposits of water-soluble minerals near Carlsbad, New Mexico, and the remainder was almost entirely in the three brine operations at Salduro Marsh, Utah; Midland, Michigan; and, chiefly, Searles Lake, California. Production from a seventh plant in the Carlsbad area was expected to begin in 1959.

B. RESOURCES Ruhlman (1956) considered 37,000 million metric tons of KzO to be a reasonable figure for the reserve of soluble potassium salts in the principal producing countries, of which Germany and the Soviet Union had 65 per cent; Israel-Jordan (Dead Sea), 23; France, 6; Spain, 5; and the United States, 1. The figure for the United States reserve apparently relates chiefly, if not solely, to the potash in the Searles Lake and Salduro Marsh brines and in the soluble minerals of New Mexico. Deposits of soluble potassium salts in Texas, undeveloped as yet, have also been known for some time. Renewed attention has been given recently to the soluble potassium

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minerals in Grand County, Utah. The deposits, the first of which was discovered in 1924, extend for about 35 miles southward from Thompsons on the Denver and Rio Grande Western Railroad (Dyer, 1945; Severy et al., 1949). They are in a complicated geological structure-generally at depths of 3000 to 4000 feet, as compared with an average of 1000 feet in the Carlsbad field-and the potassium content, chiefly in the forms of sylvite ( KCl) and carnallite ( KCl.MgC12.6H20), varies considerably from one location to another. Substantial reserves have been reported (Anonymous, 1956e), and plans for mining operations have been indicated (Anonymous, 1958e). Noteworthy among the recent discoveries of deposits of soluble potassium minerals in foreign countries are those in Canada, England, and Sicily. The Canadian deposits, by far the most important of the recent finds, were discovered in 1946 during explorations for oil in Saskatchewan. As described by Cole ( 1948), Tomkins (1955), Williams (1947, 1952), and others, the deposits, consisting of beds of sylvite and smaller quantities of carnallite, range in thickness to more than 100 feet and in K20 content to some 35 per cent. They lie at depths of 3000 to 8000 feet and extend from The Battlefords County southeastward about 450 miles to Assiniboia County on the United States border. The first production was in June 1958 near Saskatoon by the Potash Company of America, a major operator in the Carlsbad, New Mexico, area. Another Carlsbad operator, the International Minerals & Chemical Corporation, expects to be in production at Esterhazy, Melville County, near the end of 1959. Concerning the Canadian reserve, Tomkins (1955) writes as follows: “Considering only those areas having a five-foot bed of 25 per cent K2O shallower than 4,000 feet, a reserve of 5 billion tons K2O is estimated. Adding the remaining area where the potash is found at depths less than 4,000 feet, and taking into account the multiple beds, the reserve should reach 30 billion tons. If the potash at greater depths is also considered, there is little doubt that the Saskatchewan potash deposits exceed the known world reserve and may reach the 100-billion-ton mark.” The petrology and geology of the English deposits-discovered in 1938-1939 during oil explorations in the northeastern part of the North Riding district, Yorkshire, near Whitby on the North Sea-have been discussed by Armstrong et al. (1951), Lees and Taitt (1945), Raymond (1953), Stewart (1949), and Stewart and Vincent (1951). According to Fleck (1950), the deposits consist of two beds of sylvite at depths around 4000 feet-averaging, respectively, 20 and 25 feet in thickness and about 11 and 20 per cent in K20-and a deeper bed of polyhalite (KzS04. MgS04.2CaS04.2Hz0) . Based on extensive drilling operations initi-

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ated in 1948 (Anonymous, 1949), the sylvite beds are estimated to contain about 220 million tons of KzO (Anonymous, 1955f). Work on the deposits was discontinued in 1955 because their exploitation appeared to be uneconomical. Deposits of soluble potassium salts in Sicily have been known at Casazze-Sambuco, Capra Schifano, and Colle Pezzente for more than ten years ( Cultrera, 1947). Later discoveries include beds of carnallite near Enna (Castrogiovanni) and Caltanisetta and of kainite (MgS04.KCl. 3HzO) in the area of San Cataldo, Serradifalco, and Racalmuto (Anonymous, 1956d); plans for the early development of these deposits include production of potassium sulfate from kainite. Sea water contains an average of about 0.04 per cent of potassium and constitutes a potentially inexhaustible source of this element. A process for recovery of the potassium in the form of potassium nitrate by means of a regenerable organic precipitant ( dipicrylamine) had reached the pilot-plant stage in Norway by 1940 (Anonymous, 1940; Armstrong and Miall, 1946), Further work-at Ijmuiden, Holland, after World War II-demonstrated the technical feasibility of the process, but plans for its commercial operation were shelved for economic reasons ( Anonymous, 1954g, 19558). OF RAW MATERIALS C. MINING AND PROCESSING

1. Soluble Minerals and Brines Recent descriptions of the facilities, methods, and processes for mining and beneficiating soluble potassium minerals in the Carlsbad, New Mexico, field include those by Atwood and Bourne (1953), Bruhn and Miller ( 1954), Chafetz and SeedorfE (1955), Chafetz and Skinner (1954) , Harley ( 1953), Miller ( 1956), and Nordyke ( 1954). Other publications deal with such developments in France (Gar&, 1953), Germany (Kiihnel, 1953), and Spain (Marin, 1950). Potassium chloride is the principal primary product of these operations. The plant at Searles Lake, California, established on a permanent basis in 1916 (Teeple, 1929, p. 24), is the worlds largest operation for the recovery of potassium salts from natural brines. Besides potassium chloride, the principal commodity, the products include potassium sulfate, borax, sodium carbonate, and sodium sulfate. The processes have been described by Harley ( 1953), Hightower ( 1951), Leonardi ( 1954), and Ryan ( 1951). Recovery of potassium chloride from the brines of Salduro Marsh, Utah, by a solar evaporation process (Anonymous, 19553’; Harley, 1953), first done during World War I, was developed on a commercial basis in

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1938. Beginning in 1946 a small tonnage of the chloride has been produced annually from the natural brines of wclls at Midland, Michigan (Harley, 1953). The principal source of potassium chloride from natural brines in foreign countries is the Dead Sea, from which it is recovered with the aid of solar evaporation (Armstrong and Miall, 1946). Bloch (1951) reported that an addition of naphthol green considerably hastens evaporation of the brine and improves recovery of the potassium. Potassium chloride was produced from the Dead Sea in 1930 to 1948. The plant at the north end of the Sea (in Jordan) was destroyed during the hostilities between Israel and Jordan. After its rehabilitation and modernization, the plant at the south end of the Sea (in Israel) resumed operation in 1953. Novomeysky (1958) has recounted the developments leading to the original concession for the Dead Sea operations. Perhaps the most significant of the newer achievements in potash technology was the application of flotation techniques to the recovery of potassium chloride from soluble ores, which was introduced on a large scale at Carlsbad, New Mexico, in 1935 in what is believed to have been the first commercial plant to concentrate a soluble salt by flotation in a saturated brine (Pierce and Anderson, 1942). The process is now used in France and Germany and by most of the plants in New Mexico, as well as for upgrading the potassium chloride obtained from the Utah brines. Recent publications on the process include those by Anonymous (1956c), Barr ( 1947), Beerwald (1949), Burr (1954), Cowie (1951), Gaudin (1956), Horst and Morris (1956), and Riisberg ( 1955). 2. Insoluble Minerals and Rocks

The United States has huge resources of potassium in insoluble minerals and rocks, including alunite, greensand, leucite, and certain shales and feldspars. Chiefly for economic reasons, however, their processing into fertilizers has received little attention since the New Mexico deposits of soluble potassium minerals were opened in 1931. Nevertheless, considerable research on the utilization of such materials has continued in several countries which do not have resources of potassium in readily soluble forms. at Marysvale, Utah, The deposits of alunite [K2A16(OH)lz( have been discussed by Hild (1946) and Willard and Proctor (1946), and experimental work on processing the ore has been done by the Bureau of Mines (Baroch et al., 1946) and elsewhere. Operation of the Government-financed experimental plant at Salt Lake City for production of alumina and potassium sulfate from alunite by the Kalunite process (Fleischer, 1944) was discontinued in 1945. No further work on the

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process appears to have been done in the United States, but limited quantities of ground, raw alunite have recently been included in a mixed fertilizer sold under the name of Calunite. Other research on the preparation of fertilizers from alunite has been conducted in Australia ( Wienert, 1950), Japan ( Inoue and Nagai, 1949), the Soviet Union ( Shargorodskii et ul., 1956), and Taiwan (Ting, 1952). Methods for making potassium fertilizers from jarosite [K2Fea(OH) 12 (SO,),] have been studied in Japan (Katayafna et al., 1952; Kawamura and Enomoto, 1950; Nagai and Miyabe, 1950; Nagai and Okada, 1948; Nagai and Yamanouchi, 1949; Suzuki and Kodama, 1953; Suzuki et al., 1953). Research in India and Japan on the preparation of fertilizers from potassium feldspars has included the processing of the minerals with calcium oxide, carbonate, and sulfate (Asaoka and Ando, 1948; Saxena and Datar, 1951; Suzuki and Sato, 1954; Venkatesham, 1949); sodium carbonate and sulfate (Saxena and Datar, 1954); potassium hydroxide (Suzuki and Sato, 1953); and ferric oxide (Das Gupta and Chatterjee, 1952). According to S a d et al. (1953), some 95 per cent of the potassium in granites (usually containing about 2.5 to 7.5 per cent K 2 0 ) can be extracted by hydrothermal reaction with calcium hydroxide under pressure. Swanback (1950) reported that a ground, raw granite-containing about 8 per cent K 2 0 and applied at the rate of 2000 pounds per acrewas a good source of potassium for tobacco, but Jackson and Burton (1958) found a commercial granite meal (5.2 per cent K2O) to be a poor source of potassium for Coastal Bermudagrass. Other studies of the nutrient value of raw, water-insoluble potassium-bearing minerals and rocks -including granite, alunite, basalt, biotite, greensand, illite, microcline, muscovite, orendite, rhyolite, and wyomingite-have been made by Eno and Reuszer ( 1951, 1955), and Graham and Albrecht ( 1952). D. POTASSIUM FERTILIZER MATERIALS

1. Potassium Chloride Potassium chloride containing 60 per cent or more of K 2 0 accounts for about 93 per cent of the primary production of fertilizer-grade potassium salts in the United States. The remainder is almost completely in the forms of potassium sulfate (50 to 52 per cent K2O) and sulfate of potash-magnesia (K2O 22, MgO 18 per cent). The extremely small primary production of chloride materials containing less than 60 per cent K 2 0 is mostly, if not entirely, as manure salts (25 per cent KzO). Potassium chloride is the principal primary product of the beneficiation of the

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Searles Lake, Salduro Marsh, and Dead Sea brines and of deposits of soluble potassium minerals throughout the world. Great progress has been made in improving the physical condition of potassium chloride. Hardening of the small-crystal material during transportation and storage, formerly a very troublesome matter, has been virtually eliminated by the use of anticaking agents. The aliphatic amines widely employed in the flotation of the Carlsbad ores (Barr, 1947; Cowie, 1951) appear to be especially effective for this purpose. The rapidly increasing production of granular mixed fertilizers has been a major factor in spurring the demand for coarse-particle potassium chloride. Such material comprised some 40 per cent of the fertilizergrade chloride marketed in the United States in the first half of 1958. The ways of producing the coarse products include high-pressure compaction (Anonymous, 1957a; Pierce and Anderson, 1942) or fusion of the fine material, followed by crushing and screening; recovery of large crystals directly from the ore; and slow crystallization from a saturated solution.

2. Potassium Sulfate Fertilizer-grade potassium sulfate is produced in the United States in several ways. At Carlsbad it is obtained by base exchange between langbeinite ore ( KZS0,.2MgSO4) and potassium chloride (Harley, 1953; Harley and Atwood, 1947; White and Arend, 1950). The process used at Searles Lake is based on reactions involving potassium chloride, burkeite ( Na2Co3.2Na2SO4), glaserite [K,Na( S04)z], and sodium chloride (Harley, 1953). Potassium sulfate is manufactured in Texas by the Hargreaves process, comprising treatment of hot, solid potassium chloride with sulfur dioxide and water vapor (Anonymous, 1954b), and the coproduct hydrochloric acid is used chiefly for acidizing oil wells. A small quantity of crude potassium sulfate (about 40 per cent KzO) is recovered in Colorado as a by-product of the preparation of monosodium glutamate from the waste liquors of the beet sugar industry (Anonymous, 1955d). Manufacture of potassium sulfate from potassium chloride and ammonium sulfate has been proposed by Dolique and Pauc (1948). The domestic production of sulfate of potash-magnesia, confined to one company in the Carlsbad area and marketed under the name of SulPo-Mag, consists essentially of washed langbeinite ore ( Harley, 1953; Harley and Atwood, 1947; White and Arend, 1950). Products supplying potassium and magnesium sulfates are sold in Germany under such names as Reformkali (KzO 26, MgO 9 per cent), Patentkali or Kalimagnesia ( K z 0 26-30, MgO 9 per cent), and Emgekali (KzO 33-37, MgO 5 per cent ) .

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3. Other Potassium Matetd.uk Potassium nitrate and potassium metaphosphate are discussed, respectively, in Section 111, B, 6 and Section IV, H. According to Lunt and Kwate (1956), a potassium-glass frit ( Duna-K), which contains about 36 per cent KzO and releases potassium slowly to plants, is manufactured in California by fusing orthoclase feldspar with potassium carbonate or potassium nitrate at about 2200" F. The release rate of the potassium depends on the rate of quenching the melt and the particle size of the final product. The material appears to be capable of supplying adequate potassium to plants for prolonged periods, even under severe conditions of leaching (Lunt and Kwate, 1956; Lunt et al., 1956; Woltz, 1957). Recent work with several by-product dusts (2.7 to 6.5 per cent KzO) from portland cement manufacture indicate that the material is a good source of potassium for crops (Lityliski et al., 1955) and is an excellent soil-liming agent (Whittaker et al., 1959). Andrzejewski et al. ( 1955), Grzymek ( 1954), and Oleksynowa ( 1955) studied the characteristics of such dusts and their processing for recovery of potassium salts. VI. Secondary Nutrient Elements

As regards commercial sources, the so-called secondary nutrient elements (calcium, magnesium, and sulfur) are applied to United States crops almost entirely as constituents of fertilizers that supply one or more of the primary nutrient elements (nitrogen, phosphorus, potassium) and of soil amendments such as liming materials, gypsum, and elemental sulfur. Although they are usually of secondary interest to fertilizer manufacturers, they are essential to plant life and their percentages in vegetation often exceed those of phosphorus, for example. Huschke (1953) has discussed the resources of the secondary nutrient elements and the developments in the technology of their carriers to 1950. The average percentages of such elements in some fertilizers and soil amendments are given in Table XVIII. A. CALCIUM According to Mehring (1948a) the calcium content of fertilizers used in the United States in 1946 averaged close to 12 per cent and totaled about 1,958,000 short tons, as compared with approximately 8,726,000 tons consumed as liming materials. The calcium in the fertilizersamounting in tonnage to some 90 per cent of the total quantity of nitrogen, phosphorus, and potassium therein-was supplied chiefly by normal

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TABLE XVIII Average Percentages of Calcium, Magnesium, and Sulfur in Some Fertilizer Materials and Soil Amendments Used in the United Statesa Material Aluminum sulfate Ammonium nitrate-limestone c Ammonium nitrate-sulfate Ammonium phosphate (1 1-48-0)’ Ammonium phosphate-sulfate (16-20-0)’ Ammonium sulfate Basic slag, open hearth Bone meal, raw Calcium cyanamide Calcium metaphosphate Calcium nitrate Copper sulfate, CURO4.51<20 Dolomite Epsom salt Ferrous sulfate, FeSOl .7Hz0 Gypsum Kieserite, calcined Lime, hydrated Limestone Magnesia Magnesite Manganese sulfate, 65% MnSOd Phosphate rock Potassium sulfate Sulfate of potash-magnesia Sulfur Superphosphate, normal Superphosphate, triple’ Zinc sulfate, ZnS04.H20

Calcium*

Magnesium c

Sulfurd

7.3

4.4

14.9

-

15.1 2.2 15.4 23.8

1.1 -

29.0 22.4 38.5 19.0 19.4

-

21.8 22.3

-

44.7 31.7 -

-

11.8 9.7 18.4 5.9 3.4 55.5 27.1

33.0

12.9 12.7 11.5 17.6 25.9

14.5 17.7 22.7 95.0 11.9 1.5 17.9

Omission of a figure indicates that the quantity is lem than I yo. Mehring (1948a) and other sources. c Mehring (1948b) and other sources. d Mehring and Bennett (1950) and other sources. ,. Dolomite. Made with wet-process phosphoric acid.

0

b

and triple superphosphates and ground phosphate rock and by the limestone used as a neutralizing agent and make-weight filler in mixed fertilizers, These materials have continued to be the principal sources of calcium. More recent estimates of the calcium furnished by fertilizers have not been made. It appears, however, that the total quantity-though prob-

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ably larger now than in 1946-has not kept pace with the fertilizer tonnage. This is partly because the use of phosphate materials has shifted markedly from normal superphosphate (Ca:P ratio 2.3:l) to triple superphosphate (Ca:P ratio 0.7:l) (Table XV) and to ammonium phosphates. Another factor is the great increase in the average concentration of primary nutrients in mixed fertilizers, from 21.90 per cent in 1947-1948 to 29.54 per cent in 1957-1958 (Table XX) , which has been accompanied by a substantial reduction in their percentage content of limestone. Reflecting the decrease in domestic consumption of agricultural liming materials, the total quantity of calcium supplied by such materials in 1957-apart from their inclusions in primary nutrient fertilizers-was about 75 per cent of that in 1946. Calcium sulfate used as such, chiefly for soil-amendment purposes, furnished about 115,000 tons of calcium in 1946 and 200,000 tons in 1957. B. MAGNESIUM Apart from farm manures, magnesium, like calcium, is applied to United States crops chiefly as a constituent of primary nutrient fertilizers and of liming materials. The quantities furnished in these ways are much smaller, however, than those of calcium. Thus, Mehring (1948b) estimated that the fertilizers and liming materials used in 1946 contained about 120,000 and 1,450,000 short tons of magnesium, respectively. Then, as now, the magnesium in fertilizers was supplied by a number of materials, but the principal source has continued to be the dolomite used in formulating mixed fertilizers. The material used for this purpose probably contained around 65,000 tons of magnesium in 1946 and perhaps also in 1957. Agricultural liming materials used as such in 1957 supplied about 1,100,000 tons of magnesium. In the United States, the commercial fertilizer materials that carry both a primary nutrient and an important percentage of magnesium are ammonium nitratdimestone mixtures, basic slag, and sulfate of potashmagnesia (Table XVIII ). Materials containing minor percentages of magnesium include phosphate rock, superphosphates, ammonium phosphates made with wet-process phosphoric acid, bone products, and natural organics. Kainite, manure salts, potassium chloride, and several kinds of potassium sulfate products (Section V, D, 3) are among the German potassium materials that carry considerable magnesium. According to Jacob ( 1958, p. 138) two nitrogen-magnesium fertilizers-respectively containing N 10, MgO 8 and N 15, MgO 4 per cent-are manufactured in West Germany by treating solutions of magnesium sulfate with am-

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monia. The products are marketed under the names of Ruhr-nitrogenmagnesia 10/8 and Ruhr-nitrogen-magnesia 15/4. Among the phosphate fertilizers other than basic slag, phosphate rock-magnesium silicate glass (Section IV, K ) appears to be the principal commercial product containing substantial proportions of magnesium (about 15 to 20 per cent MgO). This material, formerly manufactured in the United States, is made mostly in Japan and Taiwan. Fertilizer materials presently marketed in the United States primarily for their magnesium content include chiefly, though in relatively small quantities, calcined brucite [Mg(OH)z] and magnesite ( MgC03), several grades of magnesium sulfate, and magnesium oxide. Forbath (1958) and Thorp and Gilpin (1949) have described the manufacture of magnesium oxide from sea water. MacIntire and Stansel ( 1953 ) found that substantially complete conversion of the magnesium carbonate to magnesium oxide, without decomposition of the calcium carbonate, can be effected by calcining dolomite at 700" C. in the presence of steam. The product is said to have some advantages over dolomite as a fertilizer and liming material. Davis (1957) has discussed the magnesium resources of the United States.

c. SULFUR The fertilizers used in the United States in 1947-1948 contained about 1,390,000 short tons of sulfur (Mehring and Bennett, 1950). This was 57 per cent of the total content of nitrogen, phosphorus, and potassium and was considerably more than the respective quantities of either of these elements-for nitrogen 162, phosphorus 172, and potassium 182 per cent. About 85 per cent of the total sulfur was in primary nutrient carriers, principally normal superphosphate and ammonium sulfate, made with sulfuric acid, and the remainder was mostly in natural sulfates such as the potassium, calcium, and magnesium salts. Besides the sulfur in commercial fertilizers, large quantities are supplied to the soil by farm manures, liming materials, pesticides, and other substances used in crop production. Such substances furnished about 500,000 tons of sulfur in 1948, for example (Mehring and Bennett, 1950). Much sulfur, from smoke and industrial fumes, also reaches the soil through atmospheric precipitations. Historically, the fertilizer interest in sulfur has centered on supplies of the element in forms suitable for making sulfuric acid to process phosphate rock and manufacture ammonium sulfate. The total quantity of this acid used for domestic fertilizer production in 1957 (6,150,OOO

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short tons, basis 100 per cent HzS04) was 77 per cent higher than in 1948, but a smaller proportion of the sulfur was retained in the fertilizers. This was chiefly because of the marked increase in production of triple superphosphate and ammonium phosphates as compared with normal superphosphate. Thus, from 1948 to 1957 the total tonnage of sulfur carried by all types of superphosphate decreased about 15 per cent while the quantity of available phosphorus therein rose 17 per cent. Although the smaller sulfur content of phosphate fertilizers in 1957 was probably compensated largely by greater quantities from other sources, continuation of the trend to higher concentrations of the primary nutrients may well result in progressively smaller tonnages of sulfur in fertilizers. For many years the domestic production of sulfur has been mostly in the form of the native element mined from the salt-dome deposits of Texas and Louisiana by the Frasch process (Price, 1952; Shearon and Pollard, 1950; Todd, 1950). This material accounted for 88.1 per cent of the production in 1949 and 78.4 per cent in 1957 (Table XIX). The TABLE XIX Production of Sulfur and Sulfur-containing Raw Materials in the United States, Calendar Years 1949 and 1957, in Long Tons of Equivalent Sulfur" 1949

Material Native sulfur or sulfur ore: From Frasch-process mines From other mines Recovered elemental sulfur Pyrites (including coal brasses) By-product sulfuric acid produced at Cu, Zn, and Pb plants Other by-product sulfur compounds Total

1957

Tons

Per cent

Tons

Per cent

4,745,014 2,092 56,781 378,456

88.1 <0.1 1.1 7.0

5,491,212 87,313 510,511 436,012

78.4 1.2 7.3 6.2

167,000 37,935

3.1 0.7

390,394 88,446

5.6 1.3

5,387,278

100.0

7,003,888

100.0

_ _ _ _ _ _ _ ~ (1

U. S. Bureau of Mines.

United States was the world's only source of Frasch sulfur until the opening in 1954 of the deposits in the State of Vera Cruz, Mexico (Kearney, 1955), from which the production was about l,OoO,OoO long tons in 1957. Economical processes have been developed for producing elemental sulfur from the hydrogen sulfide in natural gas (Sawyer et al., 1950) and oil-refinery gases (Anonymous, 1952b). Such gases, now the second largest source of sulfur in the United States, supplied about 1 per cent of

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the output in 1949 and 7 per cent in 1957. Other countries producing important quantities of sulfur from these gases include Canada and France. Elemental sulfur is produced from smelter gases in Canada, Norway (Kiaer, 1954), and elsewhere, Researches in England ( Butlin et aZ., 1956; Grindrod, 1957a) have indicated commercial possibilities for producing hydrogen sulfide and sulfur by anaerobic digestion of calcium sulfate with raw sewage sludge and inoculants of sulfate-reducing bacteria. Technically feasible processes have been developed for the recovery and utilization of sulfur compounds from the flue gases of fuel-fired power plants (Craxford et al., 1952; Francis and Lepper, 1951; Rees, 1953; Tarbutton et d.,1957). Bienstock et al. (1958) have discussed tbe chemistry of these and other processes for removing sulfur dioxide from industrial waste gases. In foreign countries, considerable use is made of calcium sulfate as a raw material in manufacturing ammonium sulfate (Section 111, B, 7) and sulfuric acid. Production of sulfuric acid in this way-apparently done only in Europe, chiefly in England-involves heating calcium sulfate, usually anhydrite, with coke and clay or shale to obtain cement and sulfur dioxide, with subsequent conversion of the latter into sulfuric acid (Bedwell, 1952; Higson, 1951a,b; Hull et al., 1957; Manning, 1951). VII. Trace Nutrient Elements

The trace nutrient elements of chief interest to the fertilizer industry are boron, copper, iron, manganese, molybdenum, and zinc. Cobalt is also of interest where crop applications are needed to improve its level in animal feeds. Nikitin (1954) has discussed some of the technological aspects of trace element usage and has pointed out that metallurgical operations are the principal primary sources of these elements except boron. Relatively little information is available on the domestic consumption of the trace elements, the fertilizer trade in which has developed almost entirely since 1920. The most complete data are those compiled by Lowe (1953) for 1950-1952, who reported that the use of boron, copper, manganese, and zinc in the continental United States in 1952 totaled 11,400 short tons (elemental basis)-B 1268, Cu 4474, Mn 3650, Zn 2008. Among the individual States, Georgia used the most boron in 1952 (21 per cent of the total), while Florida used S9, SO, and 56 per cent of the copper, manganese, and zinc, respectively. Fungicides furnish much copper to crops, about 2200 to 8200 tons in

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certain years of the period 1939-1952 (Mehring et al., 1957). Further quantities of the trace elements are supplied as incidental occurrences in fertilizers and liming materials, for example, in phosphate rock and its products (Caro et al., 1958; Clark and Hill, 1958), sewage sludge (Anderson, 1955), slags (Chichilo and Whittaker, 1953), and limestone ( Chichilo and Whittaker, 1958). Trace elements are applied as the individual carriers, often as foliage sprays, and as constituents of mixed fertilizers. In the interest of improved nutrient efficiency and lower toxicity hazard, increasing attention is being given to the possibilities in replacing the highly soluble carriers, now commonly used, by less-soluble materials and other products that offer better performance under field conditions. The chemistry and technology of mixed fertilizers containing trace elements are scarcely touched fields for research (Nikitin and Rainey, 1952). A. MULTINUTRIENT GLASSES Incorporation of the trace elements in silicate glasses is a recently developed way of decreasing their solubility and rate of release to crops, characteristics which are markedly influenced by the composition of the glass and its fineness. Multinutrient materials of this kind are now on the domestic market under the name of Fritted Trace Elements, FTE (McIntyre, 1954; Schaal, 1956). One such product is said to contain boron 2.8, copper 2.0, iron 3.9, manganese 9.7, molybdenum 0.13, and zinc 4.0 per cent. Holden and Engel ( 1954) and Naftel and Fajans (1954) have studied some of these glasses with respect to the solubility of the elements in water, neutral ammonium citrate solution, and citric and hydrochloric acids.

B. CHELATES Since 1950, synthetic chelates have gained importance as carriers of certain trace elements (Anonymous, 1955h; Haertl and Martell, 1956; U. S. Dept. Agr., 1958). These organic compounds have the property of maintaining the element in the water-soluble nonionized condition so that it often may be absorbed readily by plants under situations where its inorganic forms are less effective in alleviating deficiencies. The commercial trace-element chelates are chiefly complexes of polyaminoacetic acids such as ethylenediaminetetraacetic acid ( EDTA ) , hydroxyethylethylenediaminetriacetic acid ( HEDTA ) , and diethylenetriaminepentaacetic acid (DTPA). They are applied as direct soil treatments or foliage sprays or as constituents of solid and liquid mixed

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fcrtilizcrs. When the trace element is not initially present within the complex itself certain chelates also have the property of improving its absorption by plants from occurrences in soils and fertilizers. Haertl (1958) reported that additions of chelating agents favor the solubility of iron, copper, manganese, and zinc in phosphorus-bearing, liquid fertilizers. The domestic consumption of chelates is believed to total well over a million pounds annually, chiefly as the iron complex. A considerable quantity of zinc chelate is used, but the consumption of the cobalt, copper, and manganese compounds is small. Stable chelates of boron and molybdenum appear to be lacking.

C. BORON The world's principal supply of boron compounds comes from the brines of Searles Lake, California, and the mineral deposits of borax ( Na2B407.10H20), colemanite ( Ca2BB011.5H20), kernite ( Na2B407. 4Hz0), and ulexite ( CaNaB50s.8H20) elsewhere in that State. The marketed domestic production of these compounds totaled 63,222 short tons of equivalent boron in 1957. Recent papers have dealt with the history of the borax industry in the United States (Ver Planck, 1956) and the recovery of boron products from Searles Lake brines (Bixler and Sawyer, 1957; Leonardi, 1954) and other domestic sources (Anonymous, 1958d). The chief source of boron, the most widely used trace element, is borax and its dehydrates, highly soluble materials containing about 11.0 to 20.5 per cent boron. Recently, however, a grade of colemanite, containing about 10 per cent boron and 24 per cent calcium has been marketed. Being less soluble than the borax products, this material appears to have lower hazards of toxicity to crops and of loss by leaching from soils (Page and Cooper, 1955; Wear and Wilson, 1954; Winsor, 1950). Studies of borosilicate glasses have shown that chemical reactivitywhich depends on composition-and fineness are important factors jointly influencing the rate of solution of the boron and its nutritive effectiveness (Holden and Engel, 1957; Holden and Hill, 1958; Hortenstine et al., 1958; Page and Cooper, 1955). With proper choice of these factors the performance of the glass can be superior to that of borax, for example. Boron is applied to crops principally as a constituent of mixed fertilizers. In certain parts of the country, notably the east coast States, it is the practice of some companies to add boron to practically all mixtures at the rate of at least 5 pounds of equivalent borax per ton. According to

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Fisher and McIntyre (1957), a partially dehydrated combination of sodium tetra- and pentaborates performs better than borax in the preparation of liquid mixed fertilizers.

D. COPPER,MANGANESE, AND ZINC Although the normal sulfates are the principal carriers of copper, manganese, and zinc for fertilizer purposes, increasing attention is being given to the use of other materials, besides the previously mentioned multinutrient glasses. Much progress has been made in the development of slightly soluble compounds which may be safely used for nonsoil applications of these elements, including their incorporation in pesticidal sprays and dusts (Nikitin, 1954). Copper reaches the fertilizer industry mostly as readily soluble cupric sulfate, chiefly the pentahydrate. Cupric oxide (Kretschmer and Forsee, 1954), oxidized ores (Dunne, 1948), and basic copper sulfates having the general formula CuS04SCu(OH)z are among the less-soluble sources of nutrient copper. Various grades of normal manganous sulfate supply most of the fertilizer manganese. Other reported sources include manganous oxide and basic manganous sulfate of the type 2MnS04.Mn0.3H20. Fertilizer zinc is used mostly in the form of the normal sulfate, largely the monohydrate. Other carriers comprise principally zinc oxide and basic zinc sulfate, ZnS04.4Zn(OH)2. Boawn et al. (1957) reported that ZnS04, ZnO, ZnCOs, and Zns( P 0 4 ) 2 were equally effective sources of zinc for grain sorghum. E. COBALT,IRON, AND MOLYBDENUM Inclusion of cobaltous sulfate in the fertilizer, or its direct application to the soil, is often done in areas where animals show cobalt deficiency. The quantity of the sulfate required may be only a few ounces per acre. Ferrous sulfate is the principal inorganic source of fertilizer iron. Use is also made of ferrous ammonium sulfate obtained as a by-product of a metallurgical operation (Norden, 1956a). Anderson (1956) has reviewed the use of molybdenum as a fertilizer. Applications of no more than 2 ounces per acre are often sufficient to correct soil deficiencies in this element. While molybdenum trioxide (Moos) and sodium molybdate ( Na2Mo04.2H20) appear to be equally effective carriers the latter is chiefly used. The world consumption of the fertilizer products is said to have been about 85 short tons of contained molybdenum in 1956, including 30 tons in New Zealand and 15 tons in the United States (Anonymous, 1951).

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VIII. Mixed Fertilizers

Mixed fertilizers accounted for some 70 per cent of the annual total consumption of the three primary nutrients in the United States in 19471948 to 1956-1957 (Table 11). In the latter year, 14,702,807 short tons of such fertilizers supplied 4,342,700 tons of the nutrients, or 20.4 and 62.4 per cent more than the respective quantities in 1947-1948. During this period the average nutrient content of the mixtures rose steadily from 21.90 to 29.54 per cent, while the average nutrient content of the straight materials used directly as fertilizer showed an even larger increase, 19.21 to 28.81 per cent (Table XX). In 1956-1957 the average nutrient content TAULIC X S Avertye Percentages of Primary Il'utrients in Fertilizer Mixtures and Materials Consumed in the United States and Territories, 1948-1957" Mixtures Year ended June 30

N

Available pzo5

IizO

Total

Materials, total

1948 1949 1950 1951 1!)52 1953 1954 1955 1956 1957

4.04 3.99 4.02 4.18 4.30 4.G3 5.01 5.24 5.39 5.74

10.71 10.78 10.93 11.03 11.14 11.34 11.54 11.86 12.08 12.36

7.15 7.78 8.29 8.98 9.42 9.87 10.32 10.80 11.20 11.44

21.90 22.55 23.24 24.19 24.86 25.84 26.87 27.90 28.67 29.54

19.21 20.00 21.41 21.14 22.16 23.24 25.99 27.88 27.44 28.81

U. S. Department of Agriculture; excludes products not guaranteed to contain N, PzO~,or KzO. * Excludes materials used in making mixed fertilizers.

of mixed fertilizers ranged from 23.85 per cent in the South Atlantic region to 38.86 per cent in the West North Central region. Among the individual States the range was from 21.33 per cent in Florida to 44.32 per cent in North Dakota. The mixed fertilizers consumed in the continental United States comprised 613 identified grades in 1947-1948 and 1510 in 1956-1957. The grades that were used in quantities of at least 50,000 tons (35 in 1947-1948 and 46 in 1956-1957) accounted for 89 and 84 per cent of

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K, D. JACOB

the respective totals of all mixed fertilizers in the area. Only one grade was used in quantities of a million tons or more in each year-2-12-6 (1,702,OOO tons) in 1947-1948, 5-10-10 (1,408,000 tons) in 1956-1957. In comparison, 122,000 tons of 5-10-10 were used in 1947-1948 and only 16,216 tons of 2-12-6 in 1956-1957. For its first 75 years the mixed-fertilizer industry was confined almost entirely to the blending of the individual, solid materials by relatively simple methods which involved no chemical processing. Around 1930, however, large-scale development of the practice of adding anhydrous ammonia, or its aqueous solutions with ammonium nitrate or urea, directly to mixed fertilizers containing superphosphate marked the beginning of revolutionary technological advances. These advances, especially since 1950, have substantially transformed mixed-fertilizer manufacture into a major chemical industry in its own right.

A. AMMONIATION About 60 per cent of the total nitrogen used in mixed fertilizers in the continental United States in 1954 entered the mixtures as anhydrous ammonia and its aqueous solutions ( Adams and Scholl, 1958). Harvey and Frear (1952), Sharp and Crowe (1951), and others have reviewed the earlier progress in the chemistry and technology of ammoniation, and Angus (1953) has discussed English research on the process and its products. A major recent advance in plant practice is the continuous rotary ammoniator developed by the Tennessee Valley Authority (Yates et al., 1954). Kumagai et al. (1954) and Waters et al. (1955) found that the rate and efficiency of ammonia absorption are influenced by the initial moisture content of the superphosphate and the size, density, and hardness of its particles. Bridger and Burzlaff ( 1954) reported that substantial loss of nitrogen may occur when ammoniated superphosphate and its mixtures are dried under certain conditions, for example, 15 to 20 per cent loss when the product temperature is 250" F. It has been known for some time that reversion of available phosphorus to citrate-insoluble forms in ammoniated superphosphate products is favored by the fluorine present in the original superphosphate. Datin et al. (1952) showed that such reversion is a linear function of the fluorine content in the range of 0.47 to 1.27 per cent fluorine. Hecht et al. (1952) found that the reverted phosphorus is present chiefly as fluorapatite. According to Way and Nelson (1954) the reversion is inhibited by small amounts of inorganic sodium salts, and magnesium salts are said to have a similar effect ( AndrBs, 1955).

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The water solubility of the available phosphorus in domestic mixed fertilizers has decreased appreciably in recent years. Thus, the averagc for N-P-K mixtures was about 59 to 64 per cent in 1900 to 1930, as compared with 46 per cent in 1935, 48 per cent in 1949-1950 (Clark and Hoffman, 1952), and 43 per cent in 1954-1955 (Clark et al., 1958). The widespread adoption of ammoniation, a practice which results in reduced water solubility of the phosphorus in mixtures containing superphosphates, was undoubtedly an important factor in this change,

B. CONDITIONING AGENTS It has long been the practice to add various materials to solid mixed fertilizers for the purpose of reducing caking, facilitating application in the field, and otherwise improving their physical condition. The properties of numerous such materials, organic and inorganic, and their relative efficiency in conditioning powdered mixtures have been studied by Davies et al. (1949), Hardesty and Kumagai (1949, 1952), Sherwin (1949), and others. The behavior of a conditioning agent was found to be influenced by interrelated factors residing in the agent and in the fertilizer itself. Diatomaceous earth, magnesium oxide, and vegetable meals were among the most effective materials tested. A recent proposal to condition powdered mixtures with small quantities of surface-active agents (as little as 0.025 per cent) aroused great interest in the fertilizer industry. Although such agents rapidly gained considerable use, doubts as to their practical value soon arose. Kumagai and Hardesty (1955), Retzke et al. (1955), and Tucker (1955) found that additions of various types and kinds of surfactants during the preparation of the mixture had little or no influence on its caking tendency. The physical condition of some mixed fertilizers was slightly improved when the surfactant was added during the manufacture of the superphosphate component of the mixture. Granular mixed fertilizers usually are less subject to caking than powdered mixtures of the same formulation. For satisfactory physical condition, however, they may require coatings of finely divided materials. Kumagai and Hardesty (1956) reported that the effectiveness of such materials depends on the kind, fineness, and quantity used, as well as on the initial moisture content of the fertilizer and the size and shape of its granules. The most efficient materials were those having low bulk densities, such as hydrated silica, synthetic calcium and magnesium silicates, and diatomaceous earth. The several salts (potassium nitrate, ammonium chloride, monoammonium phosphate, ammonium nitrate, and a urea-ammonium chloride complex) involved in the bonding of fer-

306

K. D. JACOB

tilizer granules have been identified by Silverberg et nl. (1958), who studied the mechanism of the action of coiiditioning agents in modifying their effects. C. GRANULAR MIXTURES Granulation of fertilizer mixtures, as a means for improving their storage and handling qualities, is the most important of the recent technological advances in the mixed-fertilizer industry. When properly done, granulation reduces the caking tendency of the mixtures, minimizes segregation of the constituents, decreases losses by dusting, and facilitates their application in the field. It, together with packaging in moisture-resistant containers, also permits the use of much higher proportions of materials having poor physical characteristics, such as ammonium nitrate and urea, in the manufacture of more highly concentrated products than otherwise would be feasible. Granulation appears to have made its greatest progress in western Europe. In Great Britain, for example, granular products supplied over 90 per cent of the total output of mixtures in 1954 (Hopkins, 1955). British practices and plants have been discussed by Angus and Hudson ( 1947), Brook ( 1957), Hardesty and Clark ( 1951), Procter ( 1949) , Raistrick ( 1956), and Williams ( 1955). Granular mixtures did not become an important factor in the domestic industry until after 1950. In the year ended June 30, 1955, the consumption of such products in the continental United States, including simple blends of granular materials, was 1,331,524 short tons, or 8.8 per cent of the total for all mixed fertilizers (Scholl et al., 1957). The proportion ranged from 0.25 per cent in the South Atlantic region to 36.7 per cent in the West North Central region. N-P-K grades accounted for 84.2 per cent of the tonnage of granular mixtures, as compared with 13.9, 0.2, and 1.7 per cent for the N-P, N-K, and P-K grades, respectively. Hignett and Slack (1957) estimated that in 1957 the production of granular mixtures was at the rate of 3 to 4 million tons per year. Granular mixtures may be made by the simple blending of the individual, granular materials or by the chemical processing of phosphate rock in combination with other nutrient carriers as, for example, in the manufacture of nitric phosphate fertilizers (Section IV, G). Their preparation in the United States, however, usually involves treatment of combinations of superphosphates and other solid, previously processed materials with anhydrous ammonia and (or) ammoniacal solutions of ammonium nitrate or urea and with sulfuric and (or) phosphoric acid to produce slurries or, generally, damp masses for conversion into multi-

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nutrient granules which are then dried, cooled, and screened to the desired range of particle size. The processes, practices, and equipment concerned in the latter technique have been reviewed by Hignett and Slack (1957). Individual plants have been described by Haines and Lange (1956), Iliff (1956), Madigan ( 1954), Martenet (1954), Reynolds (1957), Sharp (1956), and Spillman (1956). Papers by Hardesty (1955b), Hein et aZ. (1956), Hignett (1956), and Rapp and Hardesty ( 1957) have each dealt with the several technological factors in mixed-fertilizer granulation. Studies of the individual factors have included the effects of formulation (Hardesty et al., 1956) and of particle size of the ingredients (Phillips et aZ., 1958a) and the use of calcium metaphosphate (Phillips et al., 195%) and ammoniaurea solutions (Jensen, 1956; Phillips & al., 1957a). Poor absorption of ammonia, formation of ammonium chloride fume, and decomposition of ammonium nitrate and urea are among the causes of the serious losses of nitrogen that often occur in the production of high-nitrogen, granulated mixtures such as the 10-10-10 and 12-12-12 grades. As discussed by Hignett (1958), the factors influencing the losses in these ways include the design and maintenance of the ammoniator, the degree and temperature of ammoniation, and the temperature at which the granules are dried. Improper use of sulfuric acid in the preparation of granulated products with ammoniating solutions containing ammonium nitrate has caused flash fires in the mixer (Perrine, 1956b). Harris (1958) has discussed the nature and treatment of gaseous effluents from granulation plants. The factors influencing the degree of caking of granulated mixtures during storage have been studied by Kumagai and Hardesty (1956), Mitchell ( 1954), Raistrick ( 1956), Silverberg et al. ( 195S), and Whynes and Dee (1957). Included in these factors are the formulation of the mixture, size and moisture content of the granules, curing period, use of coating agents, storage temperature and pressure, and degree of protection from atmospheric humidity. For example, drying to a moisture content of less than 2 per cent, with packaging in moisture-resistant containers, is often s&cient to maintain even some of the hygroscopic high-nitrogen grades in good physical condition under practical storage circumstances. D. LIQUID MIXTURES Until recently the domestic use of liquid mixed fertilizers containing two or more of the primary nutrients was confined mostly to California. In the year ended June 30, 1954, for example, the consump-

308

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tion in that State was 20,654 short tons or 75 per cent of the estimated total for the continental United States (Anonymous, 1955i; Jacob and Scholl, 1955). The East and West North Central regions together accounted for 8.3 per cent (2291 tons) of the consumption. Preliminary data compiled by the U. S. Department of Agriculture indicate, however, that by 1956-1957 the total use of such mixtures had risen to about 245,000 tons, of which some 27 per cent was in California and 61 per cent in the North Central area. Compared with solid products, liquid mixtures appear to have the advantage of lower capital investment and labor costs for their production; the problems of physical condition and uniformity of composition are less difficult; and they are more convenient to handle and to distribute in the field. The requirement of a high degree of solubility in water restricts considerably, however, the choice and availability of the nutrient materials, especially those supplying phosphorus, and it may increase the cost of some nutrients. Other problems include the low solubility of most of the secondary and trace nutrient carriers, crystallization of salts from solution, corrosion of equipment, and the storage of materials and products. Progress, problems, and general considerations in the production and use of liquid mixtures have been reviewed in several papers (Anonymous, 1955i; Jacob and Scholl, 1955; Lesser, 1953; Luckhardt, 1953; Pizer, 1957; Slack, 1955, 1957). Operations at individual plants have been described in other articles (Anonymous, 1955e, 1956h; Franklin, 1956). Studies of the economics of the manufacture of liquid mixtures have been made by Bone (1955) and Stanfield (1956). Arvan and Mowry (1954), Langguth et al. ( 1955), and Slack (1955, 1957) have discussed the factors involved in formulating liquid mixtures. The principal source of nitrogen is ammonia, with supplements of ammonium nitrate and (or) urea as may be desired. Although the phosphorus is derived mostly from thermal-process phosphoric acid, because of its freedom from sediment and dissolved impurities, increasing attention is being given to the utility of wet-process acid. As with solid mixtures, potassium chloride is the chief source of potassium. Owing to solubility limitations, the primary nutrients in the liquids usually do not total more than 30 per cent. McKnight and Striplin (1958) and Silverberg and Hoffmeister (1958) investigated the preparation of liquid mixtures based on highly concentrated phosphoric acid (76 per cent Pz05) and diammonium phosphate, respectively. Fisher and McIntyre (1957) reported that products containing 0.5 per cent boron can readily be made with the use of a partially dehydrated combination of sodium tetra- and pentaborates. Haertl ( 1958)

FERTILIZER PRODUCTION AND TECHNOLOGY

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found that in the range of pH 3.7 to 7.0 the solubility of iron, copper, manganese, and zinc in phosphorus-containing solutions is benefited by addition of chelating agents. Studies by Vreeland and Kalin (1956) indicated that chromiumnickel stainless steels should be suitable for tanks for N-P-K liquid mixtures. IX. Mixtures of Fertilizers and Other Agricultural Chemicals

Incorporation of other agricultural chemicals in fertilizer materials and mixtures, a practice which has developed mostly since World War 11, is now done extensively in some parts of the United States. The use of such combinations has the advantage of convenience and often of economy, as compared with separately timed applications of the individual products. The interest, thus far, has been principally in combinations for pest control.

A. MIXTURES WITH PESTICIDES The use of fertilizer-pesticide mixtures is chiefly against soil-borne insects but they are also applied for control of fungi, nematodes (Benedict, 1958), and weeds. The first comprehensive survey of their consumption in the United States was reported by Scholl et al. (1958b) for the year ended June 30, 1956. Previous estimates by Jacob (1954, 1955) for 1952-1953 and 1953-1954 were based on fragmentary data gleaned from various sources. The consumption of such mixtures in 1955-1956 totaled 120,868 short tons, of which 38.9, 27.7, and 12.9 per cent was in the South Atlantic, West North Central, and East North Central regions, respectively. The States showing the largest use were Iowa, 19.2 per cent; Florida, 18.5 per cent; South Carolina, 11.9 per cent; and Illinois, 6.2 per cent. Combinations with mixed fertilizers accounted for 91 per cent of the tonnage. Products containing aldrin supplied 57.9 per cent of the total tonnage, as compared with 30.8 and 5.3 per cent for those containing chlordane and heptachlor, respectively. Pesticides used in smaller quantities included BHC, DDT, dieldrin, IPC, lead arsenate, toxaphene, and 2,4-D. Mehring (1951, 1953) has reviewed the earlier developments in fertilizer-pesticide mixtures. Jacob ( 1954) and Folckemer and Loeffler (1954) have summarized some of the practices and problems in their production and use, and Floyd ( 1957) has discussed manufacturing procedures and experiences. Conner ( 1954) has outlined the legal aspects of the marketing of the mixtures.

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The quantity of pesticide added to a fertilizer depends on the nature of the pesticide, the kind of pest to be controlled, the degree of its infestation, and other factors. Examples are 10 pounds of chlordane per ton, 10 to 20 pounds of aldrin, and up to 40 pounds of DDT. The commercial forms of the pesticides include granules and dustless powders formulated with inert carriers, solutions in low-viscosity solvents, and emulsion concentrates. The pesticide usually is added to the prepared fertilizer just before the mixture is shipped. Arne11 (1956), Folckemer et al. (1955), and Smith (1956) have described the various forms in which pesticides are supplied to the trade and have discussed the methods for their blending with fertilizers. Incorporation of the chlorinated hydrocarbon insecticides in liquid fertilizers requires the use of special emulsions or liquid-suspendible powders and precaution to maintain homogeneity of the mixture. Several investigations have been made of the stability of pesticides in mixtures with fertilizers. Marth et al. (1949) found no serious loss in the herbicidal potency of 2,4-D when its mixtures with solid fertilizers were stored in air-tight containers for 10 months at ordinary temperatures. Accelerated, laboratory tests by Fleck and Haller (1945) indicated considerable reaction of DDT with dolomitic limestone but not with numerous fertilizer materials and mixtures. According to Yost et al. ( 1955), malathion has good compatibility with potassium chloride, superphosphate, and urea but rapidly decomposes in the presence of phosphate rock. Folckemer et al. (1955) reported loss of 10 to 15 per cent of aldrin from its inclusions in several mixed fertilizers stored for 12 months in paper bags at laboratory temperature. The loss was reduced considerably by storage in tight containers. Studies by Creamer and Lamont (1959) showed high stability of aldrin mixtures with a wide variety of fertilizers kept in closed containers at 30" C. The decomposition was usually less than 6 per cent in 8 months. Under the same conditions, the stability of several other chlorinated hydrocarbons decreased in the approximate order: DDT, dieldrin, heptachlor, chlordane. 13. MIXTURESWITH GIBBERELLIC Acrr,

The remarkable results obtained with gibberellic acid as a plantgrowth stimulant have aroused interest in the possibilities for its inclusion in fertilizers. Norland and Erickson (1959) found that the effectiveness of this compound in promoting growth of pinto beans was not influenced greatly by mixing its potassium salt with each of several nitrogen, phosphorus, and potassium fertilizer salts and storing the mixtures for 16 weeks at atmospheric temperature. Similar effects were produced whether

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the solid mixtures were added to the soil or their aqueous solutions were applied directly to the plant itself. X. Future Prospects

Fertilizers will undoubtedly play an increasingly important part in the world agricultural economy, and there is every indication that the trend in their production and use will continue markedly upward. The potentiality for the United States is indicated by the fact that in this country the total use of the three primary plant nutrients per acre of agricultural land still averages less than 25 per cent of that in western Europe. Substitutions for or among the several nutrient elements are not possible because each of them performs specific functions in the growth and fruiting of plants which permit no alternates. Thus, the essence of fertilizers is not subject to change, though its forms may differ greatly. The United States has very large resources of fertilizer raw materials and of their requisites for processing, which are economically usable under present conditions, and far greater resources reside in less favorable situations. The country has extensive facilities for winning and processing the raw materials, which constantly are undergoing expansion and improvement. The future will see important advances in the technology of fertilizers and in the economy and efficiency of their use. New kinds of products will be developed, and the average nutrient content of the materials and mixtures will rise to even higher levels. Further improvement will be made in the physical condition of fertilizers and in the methods and means for distributing them in the field. Both atomic and solar energy may eventually be applied in fertilizer manufacture. Continued dominance of synthetic ammonia as the primary nitrogen product is expected. Anhydrous ammonia, nitrogen solutions, ammonium nitrate, and urea will gain further ground as the principal nitrogen materials. Increasing attention will be given to less-soluble forms of nitrogen, such as the urea-formaldehyde products. The paramount position of normal superphosphate in the phosphate industry will be threatened increasingly by more concentrated materials. Much greater use of triple superphosphate and of ammonium phosphate products can be expected. Phosphoric acid, especially the wet-process material, will gain progressive prominence as an article of trade in the fertilizer industry, and elemental phosphorus may serve increasingly for fertilizer manufacture. Potassium chloride is unlikely to suffer serious competition from

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other potash carriers in the foreseeable future, but production of nonchloride forms of potassium will increase. Potassium nitrate, as yet scarcely used as a fertilizer, has considerable promise, and there is a need for materials which slowly release potassium to crops. The production and use of trace element materials having improved nutrient efficiency and lower toxicity hazard are scarcely touched fields for research. Further progress will be made in the manufacture of granular mixed fertilizers, and the potentialities of liquid mixtures are just beginning to be realized. Wider adoption of soil-testing practices and ever-mounting transportation costs could well result in major shifting of mixed-fertilizer manufacture from the larger centralized plants to smaller local units, with increasing emphasis on bulk distribution of the products to the farmer. Agronomic research has been a vital factor in shaping the destiny of the fertilizer industry. Still greater emphasis, however, needs to be placed on basic studies in plant nutrition and all of its interrelationships, so that, together with technological researches, the industry may be guided even more intelligently in providing the nutrients in the most efficient and economical forms.

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Thorp, H. W., and Gilpin, W. C. 1949. Intern. Chem. Eng. 6 Process Inds. 30, 553555. Threlfall, R. E. 1951. “The Story of 100 Years of Phosphorus Making 1851-1951.” Albright & Wilson, Oldbury, England. Tielrooy, J. 1949. Chem. Eng. 56 ( 3 ) , 127-128. Tillotson, I. S. 1953. Mining Congr. J. 139 ( l ) , 41-44. Tillotson, I. S., Burt, R. B., and Barr, J. A. 1952. Mining Eng. 4,273-282. Ting, C. W. 1952. Research Bull. Taiwan Fertilizer Co. 8,148. Todd, J. M. 1950. I d . Eng. Chem. 42,2210-2211. Tomkins, R. V. 1955. Can. Mining Met. Bull. 48,70-73. Tonn, W. H., Jr. 1955. Chem. Eng. 62 ( l o ) , 186-190. Tromel, G. 1949. Angew. Chem. 61,245-251. Tromel, G. 1952. “Die chemischen und technischen Grundlagen der Herstellung von Phosphat-Diingemitteln.” Landwirtsch. Versuchsanstalt Thomasphosphat-Erzeuger, Essen-Bredeney. Tromel, G., and Oelsen, W. 1955. Arch. Eisenhuttenw. 26,497506. Tromel, G., Harkort, H. J., and Hotop, W. 1948. Z. anorg. Chem. 256,253-272. Tucker, W. J. 1955.1. Agr. Food Chem. 3,669-672. Turrentine, J. W. 1950. Better Crops with Plant Food 34 ( 4 ) , 12-16, 4042; 35 (5), 6-14,4143 (1951); 37 (6), 6-16,4041 (1953). Tuttle, H. A. 1952. Chem. Eng. Progr. 48,272-275. Tyler, P. M., and Waggaman, W. H. 1954. Ind. Eng. Chem. 46,1049-1056. U. S. Atomic Energy Commission. 1954. U . S. Atomic Energy Comm. TEI-440. U. S . Bureau of the Census. 1958. Facts for Ind. Ser. M28D-07. U. S. Department of Agriculture. 1958. U . S. Dept. Agr. ARS 22-45. Utley, H. F. 1951. Pit 6 Quarry 43 ( l l ) , 83-85. Van Denburg, J. V., Jr. 1947. Chem. Eng. 54 ( B ) , 92-95. Van Ness, H. W. 1950. Chem. Eng. 57 ( 6 ) , 102-105. Van Wazer, J. R. 1958. “Phosphorus and Its Compounds,” Vol. I. Interscience, New York. Van Wyk, D. J. R. 1953. S. African Ind. Chemist 7,5558. Venkatesham, Y. 1949. J. Sci. Ind. Research (India) 8B, 184-186. Ver Planck, W. E. 1956. California J. Mines Geol. 52, 273-291. Visse, L. D. 1952. Compt. rend. 234,1377-1378. Visse, L. D. 1953. Compt. rend. 237,1171-1179. von Lossberg, V. 1957. Mineral Trade Notes ( U . S . Bur. Mines) 44 ( l ) , 27-30. Vreeland, D. C., and Kalin, S . H. 1956. Corrosion 12,569t575t. Waggaman, W. H., ed. 1952. “Phosphoric Acid, Phosphates, and Phosphatic Fertilizers,” 2d ed. Reinhold, New York. Waggaman, W. H., and Bell, R. E. 1950. Ind. Eng. Chem. 42,264-292. Waggaman, W. H., and Ruhlman, E. R. 1956. Ind. Eng. Chem. 48,360-369. Walthall, J. H. 1953. In “Fertilizer Technology and Resources in the United States” (K. D. Jacob, ed.),pp. 205-255. Academic Press, New York. Walthall, J. H., and Bridger, G. L. 1943. Ind. Eng. Chem. 35,774-777. Waters, C. E., Arnold, W. W., and Payne, W. H. 1955. J. Agr. Food Chem. 3, 218222. Way, R. W., and Nelson, E. L. 1954.1. Agr. Food Chem. 2, 624-628. Wear, J. I., and Wilson, C. M. 1954. Soil Sci. SOC.Am. Proc. 18,425-428. Weber, W. C. 1952. In “Phosphoric Acid, Phosphates, and Phosphatic Fertilizers” (W. H. Waggaman, ed.), 2d ed., pp. 179-204. Reinhold, New York.

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Whetstone, J, 1949a. Discussions Faraday Soc. 1949 ( 5 ) ,261-266. Whetstone, J. 1949b. Ind. Chemist 25,401-407. Whetstone, J. 1952. Ind. Eng. Chem. 44,2663-2687. White, N. C., and Arend, C. A., Jr. 1950. Chem. Eng. Progr. 46,523-530. Whitney, W. T., and Hollingsworth, C. A. 1949. Ind. Eng. Chem. 41,1325-1327. Whitney, W. T., and Hollingsworth, C. A. 1952. In “Phosphoric Acid, Phosphates, and Phosphatic Fertilizers” (W. H. Waggaman, ed.), 2d ed., pp. 378-406. Reinhold, New York. Whittaker, C. W., Brown, B. E., and Adanis, J. R. 1948. U.S. Dept. Agr. Circ. 771. Whittaker, C. W., Erickson, C. J., Love, K. S., and Carroll, D. M. 1959. Agron. J. 51,280-282. Whynes, A. L., and Dee, T. P. 1957. J. Sci. Food Agr. 8,577591. Wieczorek, G. A., and Caro, J. H. 1958. J. Agr. Food Chem. 6, 34-39. Wienert, F. 0. 1950. Chem. Eng. Mining Rev. 42,441-445. Willard, M. E., and Proctor, P. D. 1946. Econ. Geol. 41,619-643. Williams, A. J. 1947. Can. Chem. Process Inas. 31,410-412. Williams, A. J. 1952. Trans. Can. Inst. Mining Met. 55, 170-171. Williams, D. 1955. Fertiliser SOC. (Engl.) Proc. 35. Williams, T . H. 1954. Coke and Gas 16,6147. Winsor, H. W. 1950. Soil Sci. 69, 321-332; 71,99-103 ( 1951). Wisniewski, A. J., DeFrance, J. A,, and Kollett, J. R. 1958. Agron. J. 50,575-576. Woltz, S. S. 1957. Proc. Florida State Hort. SOC. 70,346-350. Worthington, E. A., Datin, R. C., and Schutz, D. P. 1952. Ind. Eng. Chem. 44, 910913. Wright, R. E. 1951. Ind. Eng. Chem. 43 ( 2 ) , 89A-90A, 96A. Yates, L. D. 1951. Tennessee Valley Authority Chem. Eng. Rept. 9. Yates, L. D., and Williams, W. B. 1952. Farm Chem. 115 (9), 2940,32, 69. Yates, L. D., Roy, L. F., and Meline, R. S. 1951. Chem. Eng. 58 ( 6 ) , 135-137; ( 9 ) , 254-255. Yates, L. D., Nielsson, F. T., Fox, E. J., and Magness, R. M. 1953. Ind. Eng. Chem. 45,681490. Yates, L. D., Nielsson, F. T., and Hicks, G. C. 1954. F a n Chem. 117 ( 7 ) , 38, 41, 43,45,4748; ( 8 ) , 34,36-38,40-41. Yeandle, W. W., and Klein, C. F. 1952. Chem. Eng. Progr. 48,349-352. Yee, J. Y., and Love, K. S. 1946. Soil Sci. SOC. Am. Proc. 11,389492. Yost, J. F., Frederick, I. B., and Migrdichian, V. 1955. Agr. Chem. 10 ( 9 ) , 43-45, 137, 139; (lo), 4244,105,107, Young, E., and Altschuler, Z. S. 1958. Ind. Eng. Chem. 50,793-796. Young, R. D., and Heil, F. G. 1957.1. Agr. Food Chem. 5,682487.

SOILS AND LAND USE IN THE NETHERLANDS P. G. Meijers Groningen, Holland

I. General Situation: Land and People . . . . 11. Climate . . . . . . , , , , , . 111. Soils and Cropping Systems . . . . . . A. Introduction . . . , . . . . . B. Sandy Soils: . . . . . . . . . C . Loess and River ‘Clay Soils . . . . . D. The Peat Soils . . . . . . . . . E. The Marine Clay Soils . . . , . . IV. Plant Nutrient Requirkments and Fertilizer Use . A. Nitrogen . . , , . . , , . , B. Phosphate . . . . , , . , . , C. Potassium . . . , , . . . . . D.Lime , . , . . . , , . , . V. Land Use and Productivity . . . . . . . A. History and Development of Arable Farming B. Current Use , . . . . , . . . C. Educational and Advisory Services . . . References . , , , . , . . . . .

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Page 533 337 339 339 340 345 347 352 359 360 360 361 362 362 362 364 367 368

1. General Situation: Land and People

By way of introduction we propose to give a sketchy outline of the acreage of cultivated land in the Netherlands and the density of the population. The acreage of the country may be compared to that of Belgium or Denmark, Belgium being a little smaller, Denmark slightly larger. Table I shows in broad outline-in thousands of hectares (1ha. = 2.47 acres)-the use that is made of the land. This gives us a picture of a densely populated and intensively cultivated country. A striking feature is the enormous growth of towns, villages, etc., since 1900 and the sharp reduction of wasteland as a result of reclamation. It should be borne in mind that this “waste” land also includes that used for recreational purposes. Extension of towns, villages, ports, roads, airports, etc., annually con333

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TABLE I Land Use in the Netherlandcl 1900 (1000 ha.)

Arable land Grassland Horticulture Forests Towns, canals, roads Wasteland

0

850 1200 100 250 270 590 3260

1956 Percentage (1000 ha.) in 1957"

900 1300 130 250 550 220 3350

27 39 4 7 16 6

100

Round numbers.

sumes an area of 4000 ha., mostly at the expense of the farmer and market gardener. On the other hand, there is also land reclamation, but in the long run this gain is smaller than the loss. The acreage in forest is small and remains stationary. It is surprising

FIG.1. Areas occupied by permanent grassland in the Netherlands.

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335

that horticulture, which is of major economic importance, covers only 4 per cent of the total area. The large area of grassland (Fig. 1) is characteristic of a country of cattle breeding and dairy farming, industries to which the country naturally lends itself owing to its moist climate and its low elevation.

FIG.2. Provinces of the Netherlands and area of greatest population density (enclosed in curve). A = Amsterdam, R = Rotterdam, H = The Hague, U = Utrecht.

The Netherlands are the most densely populated country in Europe. A hundred years ago there were only 100 inhabitants per square kilometer, now this figure has risen to 325 and is still on the increase; there is a large excess of births over deaths. It is to be expected that such a dense population (11million) creates many social and economic problems. Neither the population nor their means of subsistence are uniform; indeed there are great differences between the west part of the country and the other provinces. In the west, the so-called “Randstad Holland,” all big towns and a great deal of the trade and industry are concentrated in a comparatively small area (Fig. 2 ) . Almost half the total population is to be found in an area of no more than 20 per cent of the country with a

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population density of 700 per square kilometer. The north, the east, and the south are predominantly agrarian districts and are less densely populated, though the number of inhabitants per square kilometer still amounts to 200. Public bodies who have to deal with this aspect of social life are occupied nowadays with the knotty problem of securing better population distribution. Now it is a fact that the total acreage of arable land is hardly capable of any expansion and certainly not of providing more employment, so that for its subsistence the population surplus is dependent on trade and industry. With regard to the number of people employed in agriculture and horticulture, it is true that there is a slight increase, but this lags far behind the total increase in population, so that in reality there is a great decline, from 44 per cent in 1850 to 31 per cent in 1900 and only 19 per cent in 1957. The prediction for 1980 is that no more than 15 per cent of the population will be engaged in agriculture (Fig. 3 ) . Development of the professional population i n the N e t h e r l a n d s since 1850

5 0 r o / o or population

FIG.3. Occupations of the population of the Netherlands since 1850.

The Netherlands have always been a country of small farmers, frugal and hard-working. Originally conservative and of strong principles, during the last half century they have, under the influence of science and education, become much more advanced, especially in the clay regions and areas of reclaimed peaty soils, “Veenkolonien.”

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Table I1 gives a classification of Dutch farms arranged in six classes according to sizc and number. From this it appears that nearly threefourths of the farms are smaller than 10 ha. (25 acres). Farms in these six size groups now cover 32 per cent of the total area of the country. TABLE I1 Size Distribution of Dutch Farms ~~

Percentage of total farm area of the country

Thousands of farms Size (hectares)

1910

1930

1955

1910

1930

1955

1- 3 3- 5 5-10 10-20 20-50 50

76 33.5 41.5 31 24 3.5

i2

38.5 55.5 41.5 24 2.5

59.5 34.5 66 50 24.5 2

7 7 15 22 37 12

6 7 18 27 33 9

5 6 21 31 31 7

209.5

234

236.5

Farms smaller than 5 ha. show a tendency to fall off both in number and in total acreage. Medium-sized farms are on the increase. Large farms again tend to fall off, in number as well as in total acreage, owing to partition into smaller holdings. This naturally creates a small-holdings problem, a problem that has been much under discussion of recent years. From an economic point of view the small farmer is not in a strong position. What policy has the government to pursue here? The policy of laissez faire, leaving it to the small farmer to decide whether he will remain a small farmer or not? Or has the process of elimination now in progress to be speeded up? And what is to be done with the victims of this process? These and other questions are not easy to answer. It is to be regretted that the number of farms in the category of 20 and more hectares is declining, since in this group we find the more vigorous and progressive farmers, who are influential and enterprising. II. Climate

The Netherlands have an oceanic climate, moist and temperate, the west coast being even milder and moister than the rest of the country. The winters are not severe and as a rule, the summers are not hot. The rainfall averages 718 mm. per year and is rather evenly dis-

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tributed over the various seasons. This, in part, accounts for the good pastures and the heavy agricultural crops. The figures for temperature and precipitation in Table I11 indicate the climate for the whole year. TABLE 111 Temperature and Rainfall in the Netherlands Temperature in "C., monthly averages Locality N. E. Groningen S. W. Flushing

J

F

1.1 1.4 3.2 3.1

M 4.0 5.3

A

M

J

J

A

S

O

N

7.3 11.5 14.2 16.4 16.0 13.6 9.1 5.0 8.4 12.0 15.0 17.2 17.3 15.6 11.3 7.0

D 1.8 3.9

Precipitation, monthly averages (mm.) 58

45

38

48

51

54

72

75

71

73

74

59

Although the variations are considerable, winter temperatures of -20" C. and summer figures of more than 30" C. occur infrequently. Night frosts may occur till the middle of June. It is interesting to compare precipitation with evaporation, both expressed in millimeters. Table IV gives an example which, however, applies only to one particular year (1956). These figures show that in summer there is more evaporation than precipitation, the more so since evaporation is considerably increased by transpiration from plants. This makes it quite possible that in this TABLE I V Precipitation and Evaporation in 1956

Month January February March April May June July August September October November December

Evaporation (mm.)

Precipitation bm.1

Surplus bm.1

Deficiency bm.1

4 14 31 59 83 95 92 77 49 23

58 44 43 47 49 57 72 76 69 72 70 65

54 30 12 -

12 34 38 20 1

8

3

-

20 49 62 62

-

-

SOILS AND LAND USE IN THE NETHERLANDS

339

season little soil moisture is available, so that there may be a water deficiency on the more elevated sandy soils and on some heavy clay soils. This is most evident with spring-sown crops and grasslands. Although the temperature figures in Table I11 show comparatively little difference between the north and the south of the country, yet the difference in growth and production of some crops is very conspicuous. In the south, for instance, field work in the spring can generally be started 1 or 2 weeks earlier than in the north; the harvest of cereals is likewise 1or 2 weeks earlier. Beans (Phnseoltis vulgaris) and maize (Zen mays) grow a great deal better in the south. Another striking point is that on similar soils sugar beets give a yield in roots of 50 to 55 tons per hectare in the south and of only about 40 tons in the north. This is to be attributed not only to the milder climate, but also to a growing season estimated to be over 3 weeks longer than in the north. The yields of cereals show little difference although the quality in the south is often better. 111. Soils and Cropping Systems

A. INTRODUCTION

.

We now give a brief survey of the chief Dutch soils and the types of farming practiced on them. First, we must distinguish many kinds of soil; these can be subdivided into types which, however, will be mentioned here only sporadically, For details the reader may consult the Provisional Soil Map of the Netherlands (Soil Survey Institute, Wageningen) (Edelman, 1950). Formation of the Dutch soils started in the Pleistocene period. During that time the rivers Rhine and Meuse, principally, transported and deposited masses of sand and gravel, preglacial materials, which not only formed the present Dutch area, but also filled up a large part of the North Sea between the Netherlands and England. Although in the south of the country these layers still occur at the surface, by far the greater part of the deposited material was so exposed to the forces of ice, wind, and water that considerable changes set in and new material was brought down in the course of years. The loess territory came into existence, and the river beds were partly filled up with river clay. In the coastal areas the sea created old and young sea clay. After the glacial period natural vegetation (forest, or bog and heathland) also played an important part in the formation of various kinds of soil now recognizably different. Here follows a survey of the extent (in thousands of hectares) of the groups of soils which will be discussed hereinafter:

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Sandy soils Loess loam River clay soils

1459 67

324

High-peat soils (“dal” soil) Low-peat soil Old sea clay Young sea clay

110 145 91 957

The figures in each case represent the total area, i.e., the cultivated area plus that of land which is used in other ways. The distribution of these soils is shown in Fig. 4.

FIG.4. Distribution of the principal soils in the Netherlands: 1, low peat; 2, high peat ( “dal” soils ) ; 3, loess loam; 4, reclaimed pools; 5. Zuider Zee bottom soils.

B. SANDY SOILS 1. Pleistocene Sandy Soils During the third North European or Risz glaciation the fluviatile material in the central and northern Netherlands was ice-capped. In places (for instance on the “Veluwe”) the ice pushed up the sand and

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SOILS AND LAND USE IN THE NETHERLANDS

gravel layers to form rows of hills (Fig. 5 ) . These push moraines left behind fluvioglacial sediments with only a slight content of fine colloids. This accounts for the fact that these soils have such a low moisture-holding capacity. L

ICE

CAP

OF T H l

G L A C lATl ON

FIG.5. Effects of the third (Risz) glaciation on the Netherlands.

In the north, especially in the province of Drente, the original sands are covered with glacial deposits (ground moraine). Here there are vast expanses of boulder clay which in the last phase of the Pleistocene period was covered by a layer of younger sand. These cover sands are found over a great part of the diluvial Netherlands. In many cases the water has displaced this sand again so that the depression in the landscape ( e.g., the “Gelderse Vallei” ) became filled. Thus arose a tundralike area, in which reindeer hunters left their traces (Paleolithic age). In all probability there was yet no question of permanent settlement or of agriculture. The tundra was superseded by extensive birch and fir forests, later changing into quercetum mixturn. In places where the forest perished,

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either by climatic changes or, in more recent times, through the activity of man, it was succeeded by heathland. It is generally supposed that the inhabitants in the Neolithic age burned down forests in order to obtaih space for their primitive agriculture. These inhabitants, still few in number, undoubtedly made extensive use of the soil. After exhaustion and degeneration, within a few years, the land was abandoned as there were plenty of forests to move to. The heathlands which then developed came to occupy a large part of the sandy soils. Little is left of the original vegetation, but the soil profiles indicate its nature. In areas that once were wooded we find the brown podsolic forest soil, whereas on the moors the heathland podsol profile was formed (Fig. 6 ) . Both profiles are found on the light-textured elevated sandy soils.

FIG.6. Podsol soil developed in heathland.

The sandy soils with a higher water table, where unreclaimed, are generally divided into medium-high and low heathlands. They are generally less strongly podsolized. The low-lying sandy soils especially are rich in humus, sometimes peaty, and often occur as natural grassland. Such a soil sometimes contains an iron pan, which may be used as iron ore. Many of the sandy soils were not brought into agricultural use until the nineteenth century. Apart from the early agricultural uses of the land in prehistoric times, the oldest arable soils are the “essen” or “engen” found on the more highly elevated heathlands. The profile contains a thick dark layer, rich in humus developed from heathersod manure from cowsheds and sheepfolds. The heathersod darkens the soil, and the abundant use of forest litter gives it a brown color. These “essen,” many of which have been used for more than a thousand years, frequently consist of a layer of good soil a meter or more in depth. They are found in the neighborhood of old settlements or villages situated on the higher areas bordering

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343

river basins or swamps. The subsoil not infrequently has a podsol profile. The high humus content of thesc soils confers on them substantial moisture-holding properties, but the addition of heathersods has made them acid. They possess a considerable reserve of plant nutrients. In earlier times, even as late as the mid-nineteenth century, farming was a family business around which a dependent village community developed. Mixed faming involved livestock such as cattle, sheep, and hogs. Substantial areas of heathland were necessary for the sheep, and low-lying grassland for the cattle. The crops of the arable land, on the so-called essen, were originally rye, oats, and buckwheat together with small quantities of such other crops as flax, spurry, and oilseed. The original three-course rotation: rye, rye, fallow, was later replaced by one of rye, oats, potatoes, sometimes with the substitution of rye plus turnips as the second crop. In the western part of the province of North Brabant there are sandy soils which, owing to the presence of loess, retain moisture more readily and have a higher nutrient status. On these soils wheat has been grown for a long time in addition to oats and rye, and more recently, crops of potatoes, sugar beets and pulse have been successful. Almost half of this land is in grass. The rate of application of fertilizers in this area is not abnormally high. Reclamation of the large areas of high and low heathland was mainly accomplished during the last century. In general, the low heathland, which mostly had a grassy cover, was relatively easily converted into permanent grassland by the farmers themselves, little by little. Grassland means manure, and the farmers wanted it in large quantities. Reclamation of the medium-high heathlands followed later, step by step, and was a more difficult task. A zone of bleached sand and underlying hardpan had to be dealt with. The equipment available to the ordinary farmer was not adequate for this purpose. Land leveling, where necessary, presented some difficulties. This reclamation was carried out to a large extent by the Nederlandse Heide Maatschappij, an organization which reclaims barren soils, converting them into forest, arable land, or grassland at the request of a farmer and afterward in some cases exploiting the land for him. Generally the method adopted was to turn under the bleached sand and to cover it with pulverized material from the humus pan. When the water level is not too low and the moistureholding capacity is satisfactory, these soils can be converted into good arable land. They are, it is true, poor in plant nutrients and often not rich in humus, but with the addition of fertilizers they can produce good crops. As is well known, organic manures are of great importance in improving sandy soils. The chief crops grown are rye, oats, and potatoes

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and recently, mixed cropping with oats and barley has been introduced. In the better areas mangolds and spring wheat are grown and turnips often follow the rye. Of all the sandy soils those at the higher elevations are the most difficult to convert for arable use because of the pronounced podsol profile, low humus content, and low water table. They are often quite acid and retain little water, Conversion into grassland is almost out of the question, That in spite of these difficultiesmany of these soils have been reclaimed is attributable to the great shortage of arable land and unemployment in the Netherlands in the years preceding World War 11. As in the case of the low and medium-high heathland, the work was done by reclamation companies, well equipped and well directed. It is a fact, however, that these are initially very poor soils with inadequate moisture-holding capacity. Manuring with organic residues is most important. Compost made from town refuse, which is available in some areas in the Netherlands, is not always satisfactory. The supply of farmyard manure is often quite inadequate on these farms because of inability to establish enough grassland to support cattle. The application of fertilizers and green manure crops in small areas is the best recourse. The usual crops are rye (followed by turnips), oats, and potatoes. Nutrient deficiencies of copper ( Mulder, 1938) , manganese ( Henkens, 1958a, b; Hudig and Meijer, 1919), and magnesium may occur on these soils, though they are not limited to them. Copper deficiency in crops such as rye or oats was formerly called reclamation disease; the symptoms of “likzucht” (licking disease) in cattle are ascribed to the same shortage. Recent research, however, has shown that “likzucht” is more probably due to cobalt deficiency. Thus, thanks to modern agricultural methods and to the addition of plant nutrients, the yields on these soils are not disappointing in wet years. Supplementary irrigation by sprinkling has given good results; however, this method is still expensive. On the sandy soils in the eastern part of North Brabant, groups of farms have cooperatively practiced supplemental irrigation. In this way it is possible to raise substantially the productivity of grasslands, and under some circumstances, arable land may profit from this practice. Farming on these sandy soils, often a family business limited to 3 to 25 ha., is based on cattle. The greater part of the production of the arable land is converted into animal products (cattle, dairy products, pigs, chickens, and eggs). Indeed, wherever practicable, the acreage of grassland is being increased at the expense of the arable land. This grassland is mostly permanent; only in some districts are leys of any importance. Ley farming, so common in Denmark, is of rare occurrence here.

SOILS AND LAND USE IN THE NETHERLANDS

345

2. Dune Soils The dune sand soils are situated along a narrow strip on the landward side of the dunes on the west coast, The most important areas are found in the provinces of North and South Holland. The dunes are excavated on the landward side, and the sand removed is transported to the periphery of rapidly growing Dutch towns to raise the general level and provide additional areas for the construction of houses. The subsoil left behind after the removal of the dune sand is also sandy and has proved to be very suitable for growing tulips and other bulbs. The bulb district of Holland is composed to considerable extent of these excavated, and thereby reclaimed, dune soils. These dune sand soils are low in humus and, generally, in lime. The essential feature of them is a well-regulated water table about 55 an. below the surface. Good bulb soils are scarce and high prices are paid for them. Bulb growing alternates with the growing of other crops such as lupines, but gradual infestation with disease organisms often takes place. This makes subsequent deep digging essential. Recently this has sometimes been done by means of a sand dredge, a piece of equipment that sucks up deep sand and places it on the surface. The horticultural soils of the “Westland,” the so-called “glass district,” situated in the southwestern part of the Province of South Holland, have also been developed from dune sands. These horticultural soils have been formed by applying a thick layer of slush from drainage ditches and canals to the top of a permeable dune sand soil. Fertility is supplied by application of various fertilizers. The products of the “‘Westland” area-grapes, tomatoes, and early vegetables grown in the greenhouses-are almost as famous as the bulbs from the vicinity of Haarlem and the cut flowers and ornamentals from Aalsmeer and Boskoop.

C. LOESSAND RIVERCLAYSom

1. Loess Loam In the last period of the Pleistocene, aeolian sediments from the glacial north were deposited in a thick coating over the hills and valleys of South Limburg. This area is part of the large loess area extending into the adjacent countries. The loess deposit consists mainly of minute particles ranging in size between 2 and 50 microns. Originally the loess was calcareous, but the lime has mostly been removed down to a depth of 2 to 3 meters. The loess layers were later partly shifted by water along the slopes of the hills and in some areas were mixed with sand

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and gravel. At one time this loess landscape had a forest cover, so that even now at the depth of the furrow a forest soil profile varying from light to dark brown can often be seen. Because of the high content of fine material, these soils are frequently heavy but, though deficient in humus, have an adequate moisture-holding capacity and are fairly permeable. In general, these soils lie high and drainage presents no problems. Because calcium has been removed, these soils are frequently acid and liming is desirable. They are, in general, well endowed with plant nutrients, as are the original river clay soils. Farming on the loess is of mixed types, The statistics show that half the area is in grassland but a considerable part of the grassland may be planted with fruit trees. The products of these orchards, cherry, apple, pear, and plum, contribute substantially to the farmer’s income, as they do also on some river clay soils. Cereals occupy more than 60 per cent of the arable land. Wheat, rye, and oats are grown in about equal proportions. Potatoes, sugar beets, and mangolds also are important and, among the fodder crops, red clover should be mentioned. 2. River Clay Soils

Sediments of a diverse nature were deposited in the broad Pleistocene river basins dug out by the Rhine and the Meuse in the center of the Netherlands. Along the frequently changing channels of the rivers in the east and south ( Achterhoek and Limburg ), ridges of sand and gravel were formed, Between these ridges, and especially in the older river beds, fine material was deposited. This is the river loam, often scarcely distinguishable from the river clay which was deposited farther to the west. The loam is more strongly weathered; its structure is bad, with the result that the river loam soils are impermeable and intractable. The humus content is ordinarily low and plant nutrients are scanty. The hprovement of these soils is very difficult and they are generally used as grassland. Farther downstream nearer the center of the country, the rivers did not so frequently change their course. Floodwaters, which were not then controlled by strong dikes, spread over large areas in winter, and this resulted in the deposition near the river beds of materials varying in texture from sand to clay. These so-called “river ridge” soils, which occupied a large part of the area, were originally rich in lime in the m i n e basin and poor in lime in the Meuse basin. Low parts became filled with a thick layer of heavy clay deficient in lime (basin clay or ‘%om”clay), Until recently these naturally marshy and almost impermeable soils have generally served as hayfields, isolated and neglected. Recently, however,

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there lias been much interest in this “komgrondenvraagstuk (basin soil problem). With the aid of new reclamation procedures attempts arc being made to make these soils usable. The quality of the river ridge soils mentioned above varies a great deal. If a river clay layer of proper thickness rests on a sublayer of fine sand, water movement is generally favorable. Thin clay layers resting on coarse sand, on the other hand, may present serious daculties because such soils quickly dry up in summer and become swampy in winter when the water level in the adjacent region is high. In general, the river clay soils, both the lighter and the heavy, are difficult to work and apt to become hard when dry. Although sometimes calcareous, they are, as a rule, poor in lime and, therefore, acid. The humus content of the arable land is low, and because the phosphate and potash status of these soils is only moderate the use of fertilizers is essential. It should also be mentioned that potash is rapidly fixed in these soils (Schuffelen and vander Marel, 1955). Mixed farming is usual with little arable land and much grassland, perhaps in the proportion of 1 to 3. Orchard enterprises and cattle breeding predominate. In many places broad strips of land lie between the river and a dike. These are called “washes” (uiterwaarden) because in winter when the water level is high they are regularly flooded. Because they now have a thick coating of silt, they are very fertile and make excellent grassland. Cereals occupy 60 per cent of the arable land; these are wheat, oats, and spring barley. Potatoes, too, are grown and, in appropriate areas, sugar beets also. It is not to be denied, however, that the farmers concentrate their attention on the orchards and neglect their fields, which are often full of weeds. Chemical weed control is becoming more widely adopted in this area. The patterns of land holding and drainage both still leave much to be desired. Attempts have been made to improve this situation, but progress has been slow. D. THEPEATSOILS

We must distinguish two types-high peat (moor peat) and low peat (bog peat). The former high-peat district covers a large area in the borderland of the north of the Netherlands and the north of Germany, and a smaller area in the provinces of Drente, Overijssel, and North Brabant (Fig. 7). The lower-lying low-peat soils in the west of the country are of an entirely different nature.

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w A pe,,FIbo;w,, m;;:: FIG.7. Land form of the Netherlands in the Atlantic period.

1. High Peat, “Dal” Soil In the dips and pools of the sand area low peat began to develop and gradually changed into moss peat. During the Atlantic period and later, peat layers more than 4 meters thick were formed in this way. The profiles show their changing history. On the low peat (reed peat) we find a thick forest-peat layer with tree stumps. On the top of this lies the older moss peat, shut off by the border horizon of Weber, marking a long period of drought during the sub-Boreal. Afterward the younger moss resumed its growth. The top layer of the soil is weathered surface peat, also called “bolster” or “bonkaarde” (Fig. 8). Except in some small areas and some special reserves, all these highpeat soils have been removed and used as fuel in the course of years. This peat digging was systematically started as early as the seventeenth century and has continued for more than three hundred years, When the peat was removed, the sandy subsoil was exposed as the surface, On

349

SOILS AND LAND USE IN’ THE NETHERLANDS High moor profil

moss

older

moss peal

Son’d

+ Bolater

mined

......... :..: ................... . . . . . . . . . . . . . ............................. ...... *

.

FIG.8. Reclamation of high-peat (dal) soil.

this surface gradually, very gradually, arose the miracle of the “Veenkolonien” (peat colonies), which may show the world that the word colony may have widely divergent meanings. The soil thus made by man is called “dalgrond” (dd= valley, dale), because the digging made high land into low land. A network of canals was dug to cany off the peat, but afterward they served as waterways for transport of agricultural produce and manure and fertilizers. Every farm-the size varies from 20 to 30 ha.-lies on a canal. All the farms are built in the same style. When the peat was being dug, the diggers had to leave a layer of about half a meter of “bolster” (young moss peat) on the subsoil. Afterward, this “bolster” was leveled and covered with about 10 cm. of sand. Plowing mixed this sand with several centimeters of the moss peat (Fig. 8). If this operation is carried out correctly, and if no sticky layer of peat is left behind on the sandy subsoil to check free drainage of water, we have a physically ideal arable soil, spongy, permeable, with good waterholding capacity, easy to work. There is only one drawback. The soil contains practically no plant nutrients and is fairly acid. This makes heavy fertilizing essential, especially in the early years. Before 1880 farmers exclusively used town refuse (compost), at first from 70 to 100 tons per hectare, but afterward at lower rates. At present manuring with compost is expensive; furthermore, the amount of the various nutrients contained therein does not always fulfill the exact requirements of the crops. An advantage is, however, that it contains lime and microelements (copper, manganese, magnesium, etc.), which are very important for these acid and poor soils. At first it was not possible to keep many cattle

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on these “dal” soils, owing to the scarcity of grass. Later cattle breeding was increased until about a third of the acreage was grassland. About 1880, fertilizers made their appearance, and this brought about an enormous change. They are cheaper, easy to apply, and, most important of all, the rates required can be much more accurately ascertained. Not only during the reclamation, but also afterward, when cropping the land, large amounts are needed to ensure high yields. It was gradually discovered that fertilizers composed exclusively of nitrogen, phosphorus, and potassium were not sufficient, Various symptoms of disease proved that calcium, copper, manganese, and magnesium were necessary as well. It took many years of careful investigation, however, to solve these problems. Before 1850, rye, buckwheat, and potatoes were grown-potatoes mostly for human consumption. Later, potatoes were grown for industrial purposes and then potato culture assumed enormous proportions. It was about 1850 that the potato-starch industry was founded; as a result of this, potato culture increased to such an extent that 50 and sometimes even 60 per cent of the arable land was planted to potatoes. One of the problems that then arose was infestation of the soil with the eelworm ( Heterodera rostochii?’nsis).This had to be vigorously fought by cutting down the growing of potatoes to only once in three years on the same field. This measure had favorable results. Another development of importance was that about 1870 the manufacture of strawboard was taken up in these same districts. The strawboard factories preferred ryestraw and thus made rye growing more profitable. Finally, in 1910 the sugar beet also was introduced in this region. The chief crops at present are potatoes, rye, oats, spring wheat, and sugar beets. Cattle are relatively unimportant; more than 90 per cent of the land is arable. We are, therefore, considering an almost purely arable farming system based on fertilizers and supported by agricultural industries, which are for the greater part cooperative. The yields are just as high as, and for potatoes decidedly higher than, those on soils naturally much more fertile. One of the disadvantages of these soils is that they are very susceptible to wind erosion. Has fertilizing been carried too far? Would it be better to apply more organic manure and to use more leys? This is a point for further investigation. 2. Low Peat On the whole north and west coast of the country, in earlier times, behind the ridge of dunes was an area composed of sand and clay banks

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covered by shallow water. In the Atlantic period low peat (reed peat) began to grow here in the fresh water. It later was succeeded by moss peat, and in places where the peat soil was mixed with clay there were oak forests. This marshland gradually sank away, either as the result of a general lowering of the land or because of a change in the sea level. Owing to these events a large part of the Netherlands now lies below sea level. When man started to use the peat soil as grassland, the land had to be sufficiently drained, and this drainage caused a further lowering of the surface by the shrinking of the peat itself. (Peat soil contains more than 80 per cent water.) The better the drainage, the greater the subsidence. In the sixteenth century windmills came into use for drainage; most of them are now replaced by steam, Diesel, or electric pumping stations, As a result, this peat land at the moment lies far below sea level and high dikes are needed to protect the polders against the outside water. In the polder itself the water should be only a few decimeters below the land surface. This is necessary to ensure a good growth of grass, for peat soil too thoroughly drained is apt to dry up. Once dried, peat does not regain its former physical properties on rewetting. But, even if the drainage is correct, grassland often becomes swampy and presents difficulties for the grazing of cattle. This is why the land is often raised with mud dredged out of the drainage ditches. Claycontaining mud is best suited to this purpose. Inevitably the frequent dredging of ditches makes them wider and wider and the strips of land narrower and narrower. Indeed, there are places where the ditches occupy as much area as the land itself, and it is a surprising sight here to see farmers go out milking, hay making, or taking cattle to fresh pastures-in row boats (Fig. 9). We do not find much arable land in these areas, and they are not benefited by the newer developments in agronomic practice. The population is conservative and modern methods are only slowly introduced, but the permanently high moisture content of the soil renders it highly productive. Cheese making is still generally performed on the farm itself, whereas nearly everywhere else factories have taken this over. This is the district of the unsurpassed Gouda cheese. Here and there horticulture is practiced, just as on the dune soils. For instance, the “Langendijk,” in the province of North Holland, is an area well known for its culture of cabbage. Aalsmeer and Boskoop likewise are world-famous centers for the culture of cut flowers and ornamental plants. For centuries a large part of the peat soil in the west of the country has been used for fuel. Eventually what was left were pools of water.

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FIG.9. Aerial photograph of grass district (Vinkeveen) near Amsterdam: low peat, many canals, awkward-shaped holdings.

Erosion made these pools larger and increasingly dangerous to the boggy fields in the neighborhood, owing to the wash of the waves. In the seventeenth century these pools or lakes were quite a feature of the country, occupying as they did an enormous area. Afterward they were drained and reclaimed, as will be detailed in Section E, 1 below.

E. THEMARINE CLAYSons 1. The Old Sea Clny (Reclaimed Pools) There are some indications that during the first phase of the Holocene period the western and the northern parts of the then existing Netherlands were flooded by the sea, while in the following period the Straits of Dover came into existence. This entirely changed the situation of the districts bordering on the North Sea. In front of the low coast, sand banks were formed on which

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sand dunes developed, protecting from the sea the lagoon lying behind them. Many large rivers flowed through this lagoon and the clay was deposited in saline conditions. Thus was born the “Wadden” landscape (Wudden = shoals)-in many places having a sea clay bottom; this is now called “old clay” and is also found in the reclaimed pools. The reclamation of these pools was started as early as the sixteenth century, at first on a small scale. Larger undertakings followed, e.g., the Schermer, the Beemster, the Wormer, and at last the largest of all, the Haarlemmermeer of 20,000 ha. The Wieringermeerpolder, too, situated in the region of the Zuider Zee (now called Ijsselmeer; mew = English mere, lake) is partly old clay. All these polders (reclaimed pools) are now some meters below the level of the sea (Fig. 10). The clay varies in heaviness, is generally calcareous and very fertile. Great care has been

F l o o d e d by rRa

F l o o d e d by r i v e r s

FIG.10. Areas of the Netherlands which would be flooded if the dikes were not present.

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taken to make the size of holdings and the drainage favorable, at any rate a good deal better than on many other soils dating from earlier times. Some of these reclaimed pools are now chiefly grassland; arable farming, however, is predominant on this clay, which we shall discuss together with the younger clay in the southwest of the country. 2. The Young Sea Clay a. Zeeland and South Holland. About 2000 years ago the sea began to penetrate into the wide estuaries of the rivers and to ruin the low-peat landscapes. The formerly almost continuous line of dunes was broken and much peat soil was swept away or covered here and there with a layer of younger clay. That was the course of the formation of the archipelago of the province of Zeeland, The now low-lying centers of the islands often consist of peat soil covered with a clay layer. The peat shrank, the clay was swampy, the lime was washed away, and today the structure of the soil is mostly too bad for use as arable land. Only reclamation practices and liming can make these soils productive. In this delta-district destruction, erosion, and accretion have alternated up to the present time. About the year lO00, dikes were constructed to protect the land. This was a great improvement, but it was not sufficient because every now and then the floods played havoc with the work of man. Furthermore, the dikes had often to be raised and strengthened because of the rising sea level. After the latest calamity in 1953 plans to eliminate the floods began to assume more definite shape. The so-called “delta plan” is now being carried out with this goal (Fig. 11). The Zeeland and South Holland islands will be connected with one another on the seaward side, so that the old estuaries will be closed from the sea. This has various advantages: In the first place fresh-water basins will be created between the islands; these inland basins will not be subject to the tide so that it may be expected that gales will have less effect. The length of the sea dikes will be greatly reduced; inner dikes are easier to maintain. The clay of which this southwest island group consists varies greatly in age and heaviness. The level of the water table and the rapidity of drainage, too, varies from bad to good. The shape of the farms in the older parts often leaves much to be desired, In 1944 the island of Walcheren fell a victim to the liberation of the country by the Allied Forces, so that the sea had free play for a whole year. When it was again reclaimed it was realized that this old and badly planned island had to be fundamentally reshaped and provided

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with roads and waterways. The same course was adopted after the disastrous floods of 1953 for the reclamation of areas then flooded. Taken as a whole, however, this island group has for centuries been one of the best arable areas of the Netherlands.

FIG. 11. Zeeland reclamation plan. Hatched lines indicate very strong dikes; solid lines, other dikes.

b. North Holland. Another territory from which the sea swept away large peat areas is the northern part of the province of North Holland. In later times a light clay was deposited in place of the peat. Along the borders we find a sticky clay soil, hard, poor in lime, resting on peat soil. c. Friesland and Groningen. Considering finally the sea clay district of the two northern provinces of the country, we must leave it undecided whether this landscape has had the same history as that of Zeeland. The clay in this area has properties similar to the Zeeland clay; the sea clay soils vary in texture from light to very heavy. Starting from the interior, i.e., from the sandy soils in the province of Drente, we find as we go toward the coast, successively: (1) A low peat district, with many pools (the Frisian lakes). Over large expanses the peat is covered with clay. In several respects this

?J

n

FIG.12. Aerial photograph of portion of the West polder (farm of plant breeder, Dr. Mansholt). The soil is light marine clay.

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landscape resembles the low-peat area in the west of the Netherlands. It is a grassland district. ( 2 ) Farther on we find a strip of clay of an older formation, often heavy, poor in lime and difficult to use as arable land. In the east part of the province of Groningen this soil is called “rodoorn.” These brownish clay soils, rich in humus, are mostly used as arable land. ( 3 ) A territory formerly belonging to the “Wadden” landscape, perhaps dating from about 1000 B.C. We find here the old islands or centers with heavy clay on the landward side and light or sandy clay on the seaward side. Between these old areas the younger clay was deposited, here heavy, there light. On the seaward side this landscape merges into the latest polders, (Fig. 12) which are sandy clay again. In only one area these polders consist of very heavy clay (60 to 80 per cent colloids). This is the Dollard area in the eastern part of the province of Groningen. ( 4 ) The ‘Wadden” sea, a broad, shallow stretch of water all along the north coast, with a sand bottom, protected from the North Sea by a range of “Wadden” islands on which dunes have formed. Land reclamation along the north coast of Friesland and Groningen is still going on, and accretion is still being artificially promoted. Like the Zeeland clay, this northern clay and sand makes good arable soil.

3. The “Zuider Zee” Bottom Clay Soils The most courageous and important reclamation undertaken so far in the Netherlands has been the enclosure of the Zuider Zee and the reclamation of large parts of it (Table V ) . TABLE V Zuider Zee Reclamation Reclaimed polders Wieringermeerpolder Noordoostpolder Oostelijk (East) Flevoland Westelijk (West) Flevoland Markerwaard

Size (1000 ha.) 20 48

54 42 56

Remarks Under cultivation Under cultivation Reclaimed in 1957 Projected Projected

Apart from the new land gained are the added advantages that a large fresh-water lake is created, the water level in the whole of the north of the country is better controlled, and the long sea dikes become inner dikes and these may even become cheaper to maintain. The “Wieringermeerpolder” was the first to be reclaimed, As stated

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before, this polder is for the most part old sea clay of a good quality; part of it, however, is Pleistocene sand, with a podsol profile. The “Noordoostpolder” came next. This polder has been under cultivation since 1943.At the moment, East Flevoland is enclosed and dry, and operations preparatory to cultivation are in full swing. Reclamation of West Flevoland and Markerwaard (see Table V ) will be achieved in the course of the next decade. These polders together will yield 220,000 ha., for the greater part good fertile land. This is equivalent to 10 per cent of the cultivated area of the Netherlands. Here follows a brief survey of the origin of these soils, particularly those in the southern part of the Ijsselmeer (the former Zuider Zee). In earlier times the bottom of the Zuider Zee was composed of peat soil resting on Pleistocene sand. In the western part there was also old clay under the peat soil, and sometimes boulder clay. When the sea flooded this whole area, the peat soil was almost completely eroded and only the sandy subsoil was left. Part of the mud, however, remained in this inland sea and, in contact with salt water, was in places deposited again as an acid substance, rich in iron sulfide called by agrogeologists “detritus.’’ Detritus is acid, infertile, shinks considerably when desiccated, and has a bad structure. After reclamation the iron is leached down and becomes deposited again in the drainage tile, which may become choked by it. Another peculiar soil element is “sloef,” a fine sand with few clay particles. This sand, originating from the northern part of the Zuider Zee, drifted to the south and was deposited there in almost fresh water. “Sloef” is much like sandy clay. The most important sediment for agriculture in the “Noordoostpolder,” however, is the young marine clay, light or moderately heavy. It is supposed that this material was not precipitated before the fifteenth century because only then did the water attain a sufficiently high concentration of salt. This fertile clay, rich in lime, is found in layers on and between the two other components. 4. Clay Soils in General

The variety of the physical and chemical properties of the sea clay soils is so great that any detailed discussion of it would be beyond the scope of this article. Clay is naturally rich in plant nutrients and adsorbing colloids, and requires a smaller amount of fertilizers than do other soils. Heavy clay is best in this respect but has the disadvantage of being hard to manage and is consequently suited to only a limited group of crops. The content of lime may vary a great deal. Young polder soils for instance are usually rich in lime, say 10 per cent CaCOs. Older clay

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is often practically devoid of lime and its reaction may even be acid. The structure is then often bad and lime is generally applied. As a rule, the humus content of all clay soils is low, varying from 1.5 to 4 per cent. Only the soil that has long been used as grassland contains 6 or even 8 per cent organic matter. Such soils and the newly reclaimed outer marshes are rich in plant nutrients, so that for many years they require little or no manuring. On those clay soils that have been under cultivation for centuries, the two features which make difficult the economic production of arable crops are inadequate drainage and the unsuitable size and shape of many farm holdings. In the last few decades they have indeed been greatly improved, but these improvements are expensive and require much time. IV. Plant Nutrient Requirements and Fertilizer Use

It is well known that in the Netherlands, high rates of fertilizer application are common. The average rates are much higher than in most other Western European countries including even Belgium and Denmark, where similar systems of intensive agriculture are encountered. In Table VI are given average rates of fertilizer application for the three TABLE VI Average Rates of Fertilizer Application in the Netherlands

Wartime rationing terminated.

* Rate for 1947; wartime rationing terminated. years 1939, 1949, 1955. During the war the supply of fertilizers was limited. When rationing was terminated, fertilizer consumption rose sharply. In the case of nitrogen, this increase is continuing. The use of phosphate has leveled off, but that of potassium is probably still increasing slightly. The Laboratory of Soil and Crop Testing has now had several decades of experience in determining the requirements of Dutch soils for plant nutrients. The methods adopted in this routine testing were worked out by the Institute for Soil Fertility. The recommendations are based

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on the results of many field trials. Items covered by routine testing are percentage of clay, lime content, organic matter content, pH, and contents of available phosphorus and potassium. Where necessary, magnesium and certain micronutrients can also be determined. A. NITROGEN

No routine procedure for the determination of available nitrogen has been adopted, but many field data are accessible, In making recommendations as to nitrogen, the kind of soils, previous crop, and intended crop are all borne in mind. In general, these amounts have been determined in field experimentation. For example, the following figures (kilograms N per hectare) are generally used: Sugar beets and potatoes Colza, caraway Cereals Pulse

100-150 100-130 40-100 0-20

In certain circumstances, these figures are exceeded. For example, in the case of cereals a demand for straw may make this desirable. Sugar beets too are sometimes given extra nitrogen although this may reduce the sugar content. Grassland stands in a special position with respect to nitrogen fertilization. Very intensive production of grass is only a decade or so old and is not yet generally practiced. Under intensive systems of farming, amounts of 300 kg. N per hectare, and sometimes more, may be applied in split applications directly after mowing or grazing. When frequent mowing is practiced 100 kg. N per hectare is applied each time. When the grassland is grazed, about 50 kg. may be applied each time. Taken as a whole, there is no doubt that the use of nitrogen may still increase substantially, particularly in the livestock areas.

B. PHOSPHATE In some districts farmers have made abundant use of phosphate for many years. Serious deficiencies of phosphate are rather rare. There are, however, some soils where phosphate fixation is a problem. As a result of the large numbers of soil tests that have now been carried out, it has been possible to generalize as to the phosphate needs of soils of different types. The phosphorus status of most sea clay soils was found to be good to quite good, the estimated annual requirement being from 0 to 60 kg. P B 0 5 per hectare. Many sandy and high-peat soils which have been under cultivation for a long time have a similar phosphate status with an estimated annual requirement of 40 to 60 kg. PzOa per hectare. The

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river clay and loess soils appear to be poorest in available phosphate, as are also newly reclaimed sandy soils. The annual rcquiremcnts here are estimated at 50 to 80 kg. per hectare or more. In general, the average for the whole country would seem to be about 54 kg. PaOa per hectare. Such an average figure cannot be depended upon in special cases or unusual situations. Soil testing is widely practiced but is still capable of expansion. The Extension Service uses locally derived figures as the basis for advice to the farmers.

C. POTASSIUM Potassium fertilizers have been widely used on sand and high-peat soils since 1900. For various reasons, however, the correct fertilizer rate is more difficult to arrive at with potassium than with phosphate. In the first place, potassium is more readily leached. Secondly, the quantity of indigenous potassium may vary greatly, as does the clay or organic matter content. In some soils potassium fixation is encountered and, finally, the crop requirements for potassium may differ greatly. Sugar beets, for example, have a much higher potassium requirement than cereals. Too high a rate of application of potassium may be disadvantageous. For example, the starch content of potatoes is depressed under such circumstances. In Table VII are given average figures for the requirements of some crops on various Dutch soils. TABLE VII Potassium Requirements for Various Soils in the Netherlands

KzO(kg./ha.) Heavy and medium-heavy sea clay Light and very light sea clay For root crops For cereals River clay For root crops For cereals Sandy and “dal” soils For root crops For cereals Reclaimed sandy and “dal” soils during the early years For root crops For cereals Grassland on light soils I n case of much mowing I n case of much grazing

Small amounts 100-160 0-120 120-200 60-140 120-240 80-200 200-300 140-200 120-200 60-100

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1). LIME

Sandy and peat soils infrequently contain lime; acidity generally varies from pH 3.8 to 5.5. It is, therefore, necessary to apply lime, or fertilizers which effect some neutralization, if the optimum pH for the crop is to be provided. This requires some care because too high a pH on these light soils may cause difficulties. Some crops, especially cereals, may develop manganese deficiency, and potatoes are more susceptible to scab. The desirable quantity of lime can, however, be calculated from soil test data. It usually runs from 500 to 2000 kg. CaC03 per hectare and may be supplied in two or three applications. Grassland, which should be about pH 5 may not have as high a requirement for lime as arable land. The situation with respect to the clay soils is quite different. Most sea clay was originally rich in lime and may have a CaCOs content of 10 per cent. In the course of centuries, the lime is leached out so that these soils progressively become deficient and acid. This greatly diminishes their agricultural value. Lime, therefore, is applied to bring about an improvement of the structure, to add nutrients, and to improve the microbiological environment. Large amounts may have to be applied, especially to heavy clay where good results are obtained with 15 to 25 tons CaC03 per hectare. Experience shows that on lighter clay smaller amounts are inadequate. From many data collected over a wide area the following conclusions can be drawn: ( 1) Old and young sea clay is usually rich in lime. Some of the older soils may have become deficient, but these, in general, have been limed adequately. ( 2 ) Loess and river clays are poor in lime. The use of liming is on the increase in these areas but is still not adequate. (3) Sandy soils in the north and central parts of the Netherlands are mostly too acid, but farmers in these areas have been somewhat slow in making the necessary applications of lime. The sandy soils in the south are less frequently too acid than in the north, yet adjustment of pH through liming is often desirable. (4) High-peat soils are frequently acid, but inasmuch as they are used for potatoes, the practice is not to raise the pH more than is strictly necessary. V. Land Use and Productivity

A. HISTORY AND DEVELOPMENT OF ARABLEFARMING

A century ago the indigenous fertility of the soil was the most important factor. The second factor was the amount of organic manure available (cattle and grassland). The rotation adopted was greatly in-

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fluenced by the interplay of these two factors. A rotation often applied in clay districts was: ( 1 ) fallow, with manuring for structure improvement; ( 2 ) colza or winter rape (the chief crop of that time); ( 3 ) barley or wheat; ( 4 ) field beans (Vicin fnbn) for improvement of fertility; ( 5 ) wheat; ( 6 ) oats. The fallow was gradually replaced by clover, and later such other crops as potatoes, fiber flax, and sugar beets were introduced into the rotation. A long rotation of this type has several advantages, the chief of which perhaps are prevention of the establishment of diseases through alternation of crops, and economy in the use of manure. After 1880 the farmers made considerable use of fertilizers, and liming, too, became the vogue. At present, little is left of the old rotation discussed above. Fallow land is a thing of the past, Market prospects now largely decide the farmer’s choice of crops. Animal manure has given way to fertilizers and the number of cattle has been greatly reduced. Soils are tested at the Laboratory for Soil and Crop Testing, and the Extension Service is prepared to give information about various diseases, proper fertilization, and management of crops. It would seem almost as if the soil itself now plays only a minor role in the production of many crops. This is largely due to the progress made through agronomic research and the wide diffusion of the results through educational programs for the farmers. With proper management quite different kinds of soils may give similar yields. In Table VIII are given yields on heavy sea clay in the Oldambt in TABLE VIII Representative Yields of Crops on Two Very Different Types of Soil in 1955 “Dollard” heavy clay (yield in tons/ha.)

Reclaimed high peat “dal” soil (yield in tons/ha:)

4.1 3.1 4.3 23 39

3.7 3.1 3.B 28 39

~

Spring wheat Rye Oats Potatoes Sugar beets

the province of Groningen, which is a naturally rich soil, and on “dalgrond,” a naturally poor soil derived from sand and moss peat. These yields were representative of crops in 1955, which was a favorable season. In Table IX are given average yields of crops in other areas in the

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P.

G. MEIJERS

TABLE IX Representative Yields (tons per hectare) of Crops in Several Clay Areas in 1955

Winter wheat Spring wheat Spring barley Oats Green peas Potatoes Sugar beets

Zeeland

Northeastern polder

Groningen light sea clay

Groningen heavy “Dollard” clay

3.8 3.7 3.9 3.5 3.8 29.2 48.9

4.0 4.0 3.8 4.5 3.6 28.5 48.0

4.1 4.1 3.8 4.4 2.8 25.0 41.0

3.6 4.1 3.7 4.3 2.8 23.0 39.0

same year. Where there are significant differences, these cannot be entirely explained. Some soils are particularly suitable for special crops. Examples are: caraway (Curium curoi) and colza (Brassicu nupus var. oleifem), which grow best on rich and heavy clay. Sugar beets make a good crop on all clay soils, but when grown on heavy clay, irregular germination and difficult harvesting are drawbacks, Potatoes are best suited to light clay because of the planting and digging operations. Fiber flax (Linum usitutissimum) is best on clay that is light or only moderately heavy, where it comes up more uniformly and where the fertility of the soils is not so high that rank growth occurs.

B. CURRENT USE The general characteristics of the major soils of the Netherlands are summarized in Table X. It will be recognized that the use made of these soils depends to a considerable degree on local economic considerations. In Table XI the distribution of various crops is given as percentage of the total land of each group of soils in cultivation. It is evident that there is more crop diversification on sea clay than on soils of the other groups. The acreage of cereals on sea clay is smaller but this, however, varies greatly from district to district. For example, in Zeeland, only 35 per cent of the sea clay soils are in cereals whereas in Groningen, the figure amounts at present to 60 per cent because of the proximity of strawboard factories. Sugar beets are grown mainly on clay, mangolds on river clay and sandy soils where they are used in cattle feeding. Potatoes are grown everywhere but especially on high-peat soils for the starch industry. One may alternatively examine the contribution made by the soils of

365

SOILS AND LAND USE IN THE NETHERLANDS

TABLE X Summary of the Characteristics of the Major Soils in the Netherlands Soil characteristics Inorganic colloids

Sea clay

River clay

Sandy soils

High-peat soils

Rich

Rich

Poor

Poor

Organir mlat,trr Poor

Poor

Slight to moderate

Rich

Reserve of nutrients

Large

Moderate (often Small potaRh fixation)

Water-holdiiig oapaci ty

Good, not sensitivc to drought

Partly good, partly sensitive to drought

Low soils good; high soils sensitive to drought

Favorable, not sensitive to to drought

Workability

Variable

Not as good us sea clay

Eally

Easy

Productivity

Good, stable, Moderate to High, stable, ample choice of ample choice of good, less stable, crops with mod- crops with mod- limited choice of crops with erate amounts erate amounts regular heavy of fertilizers of fertilizers fertilization

Small

Good, fairly stable, limited choice of crops with regular heavy fertilization

different origins to the total production of the major crops (Table XII). It will be readily seen that the production of many crops on the sea clay soils outweighs that on all others. Seasonal climatic differences are larger on some soils than on others. In Table XI11 are given average yields of some major crops in 1955 and 1956. The year 1955 was a favorable one with a dry period at harvest time, whereas 1956 was wet and cold, especially at harvest. It will be seen that in these two years cereals were the crops least affected, and TABLE XI Area of Crops Planted as Per Cent of Cultivated Land of Each Soil Group Crop Cereals Green peas Potatoes Sugar beets Mangolds

Sea clay

42 10 13 .5 12. 3

River clay

Sandy soil

60 3

70 1

10

15

8 12

3

7

High-peat soil 60 0.3 30 6 1.5

366

P. G. MEIJERS

TABLE XI1 Distribution of Total Production among Soil Groups" Crop

Sea clay

Wheat Rye Barley Oats Pulse Fiber flax Potatoes for consumption Industrial potatoes Sugar beets 0

River clay

Sandy soil

High-peat soil

6 79 11 50 4 2 38 36 14

8 9 2

71 3 70 24 88 95 47 9 70

14

0 0 2 55 5

In per cent of weight (mass) per crop. TABLE XI11

Comparison of Average Yields in Tons per Hectare of Some Crops, 1955 and 1956 Sea clay

River clay

Sandy soil

High-peat soils

Crop

1955

1956

1955

1956

1955

1956

1955

1956

Cereals Green peas Potatoes Sugar beets

4.0 3.2 28 47

3.8 2.2 27 39

3.5 3.0 26 41

3.2 1.3 23 35

3.1 2.5 26 37

2.9 1.5 22 31

3.3

2.9

-

28 38

-

22 30

peas showed the greatest differences. Sugar beets also are responsive to wet and cold weather. Of the various soils those from sea clay are less affected by fluctuations in the weather. There has been a steady increase in the yields of all crops in the last half century (Table XIV). Statistics are not available for comparison of TABLE XIV Increase in Crop Yields in the Netherlands from 1890 to 1957 (Verslag over de Landbouw) Years

Winter wheat (kg./ha.)

Rye (kg./ha.)

Oats (kg./ha.)

Potatoes (tonslha.)

1891-1900 1901-1910 1911-1920 1921-1930 1931-1940 1946-1950 1951-1957

1870 2200 2470 2910 3060 3460 3940

1510 1690 1760 2050 2260 2220 2840

1940 2200 2050 2070 2370 2500 3220

12.3 15.4 16.7 19.3 23.4 25.4

-

Sugarbeets (tons/ha.) 30.1 30.4 30.5 33.0 37.6 39.6 41.0

367

SOILS AND LAND USE IN THE NETHERLANDS

these figures on the different groups of soils, but it is quite apparent that yields everywhere have increased as a result of progress in agronomic research and of educational programs reaching farmers. In parallel with the increased yields of harvested crops, grassland is similarly becoming much more productive. This can best be expressed in terms of the number of cattle supported. In recent years the figure for the number of milk cows was 110 to 115 per 100 ha. If one adds, in addition, the young cattle and other livestock, such as horses and sheep, one arrives at the equivalent of 200 milk cows per 100 ha. In areas of intensive grassland farming this may even reach 250.

C. EDUCATIONAL AND ADVISORY SERVICES After the great agricultural depression of 1880 to 1895, it was realized that, for the welfare of the country, agricultural science must support and enrich farm practice and that farmers must be informed as to the requirements of good husbandry. A government extension service was set up and agricultural colleges were founded, at first only in a few places but later in almost every center of agricultural importance. In this way efficient agronomic methods are made known to the farmers. The agricultural research and extension activities in the Netherlands are not inferior to those of other Western countries. The scale of agricultural education in the Netherlands may be measured by the average number (per year) of pupils receiving instruction in recent years : Undergraduates at the Agricultural University Pupils of agricultural and horticultural schools Students attending various organized classes

800 26,000

34,000

The government extension service also brings economic and technical information to farmers by word of mouth, by published papers, and by demonstrations. Extension service personnel play an important part in essential research work. Thousands of field plots are set out annually to test varieties, fertilizer rates, disease control, etc. This has the great advantage that the extension service staff can speak from direct experience and can understand and evaluate the more basic work carried out at a higher level. The organization of the extension service recognizes subdivisions such as agriculture, animal husbandry, horticulture and forestry. As a rule, for the sector agriculture, each province of the country is divided into two districts, each under an advisory agricultural expert assisted by a staff of specialists. One of the specialists is generally an agronomist. Each of the two districts is further subdivided into 10 to 20 areas; in each area

368

P. G. MEIJERS

is an assistant previously trained at a secondary agricultural college. This man works directly with the farmers and depends on his own experience in normal circumstances. When problems arise, he may invoke the help of the specialist, who can in turn, if necessary, consult with experts in the research institutes. Every area assistant is responsible for 300 to 600 farmers. The advice given is gratis. It is not possible to pay each and every farmer an annual visit, and it is still a matter of dispute whether the extension service should be expanded so as to make this possible. An intensive service of this type on a national basis entails great expense, and there is, therefore, some reluctance to embark on a larger program.

REFERENCES Edelman, C. H. 1950. “Soils of the Netherlands.” North Holland Publ., Amsterdam. Directie van de Landbouw. 1913. “De Nederlandse Landbouw in het tijdvak 18131913.” Gebr. v. Langenhuysen, ’S Gravenhage. Henkens, Ch. H. 1958a. Neth. J . Agr. Sci. 6 ( 3 ) , 183-190. Henkens, Ch. H. 195813. Neth. J. Agr. Sci. 6 ( 3 ) , 191-203. Hudig, J,, and Meijer, C. 1919. Verslag. Landbouwk. Onderzoek Rijkslandbouwproefsta. 23, 1-39. Meijers, P. G. 1958. “Bijzondere Plantenteelt.” J. B. Wolters, Groningen. Ministerie van Landbouw, Visserij en Voedselvoorziening. 1958. “Verslag over de Landbouw in Nederland over 1956.” (Annual reports with a summary in English. ) Staatsdrukkerij en Uitgeversbedrijf, ’s Gravenhage. Mulder, E. G. 1938. “Over de betekenis van koper voor de groei van planten en microorganismen.” Thesis, Wageningen. Schuffelen, A. C., and vander Marel, H. W. 1955. Potassium Symposium, Rome, 1955, pp. 157-201. Sneller, Z. W. 1951. “Geschiedenis van de Nederlandse Landbouw 1795-1940.” J. B. Wolters, Groningen.

EFFECT O F NITROGEN ON THE AVAILABILITY O F SOIL A N D FERTILIZER PHOSPHORUS T O PLANTS D. L. Grunes U. S. Northern Great Plains Field Station, United Stater Deparfment

of Agriculture, Mandon, North Dakota

I. Introduction . . . . . . . . . . . . . . . 11. Effects of Nitrogen on the Availability of Phosphorus to Plants . A. Biological Effects . . . . . . . . . . . . 1. Root Area and Absorbing Capacity . . . . . . 2. Root Efficiency 3. Ammonium Ion Effect . . . . . . . . . 4. Stage of Growth-Plant Nutrient Uptake Functions . 5. Effect of Nitrogen on Plant Metabolism and on Ability to Absorb Phosphorus . . . . . . . . . . B. Chemical Effects . . . . . . . . . . . . 1. Salt Effects 2. pH Effects 1II.Summary. . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . .

. . . . . . . . . . .

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

.

.

.

.

.

.

Page . 369

. 370

. 370 370 . . . 372 . . . 377 . . . 378 of Roots . . . 381 . . . 385 . . . 385 . . . 389 . . . 393 . . . 393

. . .

1. introduction

Various workers have found that the addition of nitrogen has affected the uptake of soil and fertilizer phosphorus by plants. The effect of nitrogen on phosphorus availability has been studied by various techniques and from various standpoints such as: effect on harvest yields (Chapman, 1936; Davis, 1938; Lorenz and Johnson, 1953); phosphorus concentration and total phosphorus absorbed by plants (Arnon, 1939; Breon et al., 1944; Bennet et al., 1953; Domby et al., 1951; Glover, 1953a,b; Goodall and Gregory, 1947; Grunes and Krantz, 1958; Krantz and Chandler, 1951; Lorenz, 1944; Luders, 1955; Prince, 1954; J. S . Russell et al., 1954; Smith et al., 1950, 1951; Viets et al., 1954; Volk, 1944); relative amounts of soil and fertilizer phosphorus absorbed by plants (Dion et al., 1949a,b; Fine et al., 1955; Grunes et al., 1958a,b; Haddock et al., 1957; Miller and Ohlrogge, 1958; Mitchell et al., 1952; Ohlrogge et al., 1957; Olson and Dreier, 1956a,b; Olson d al., 1956; Rennie and Mitchell, 369

370

D. L. GRUNES

1954; Rennie and Soper, 1958; Robertson ct al., 1954; Smith et aZ., 1951); uptake of radioactive phosphorus (Yatazawa et al., 1953); and studies of solubility effects on soil and fertilizer phosphorus (Bouldin and Sample, 1958; Buehrer, 1932; Olsen, 1953; Starostka and Hill, 1955). In this paper an attempt will be made to review the literature and to analyze some of the causes of the effect of nitrogen on the availability of soil and fertilizer phosphorus to plants. II. Effects of Nitrogen on the Availability of Phosphorus to Plants

The effects of nitrogen on the availability of phosphorus to plants may be divided into biological effects and chemical effects. Biological effects are those caused indirectly by the effects of nitrogen on the form and functions of the plant, independently of any direct chemical effects the applied nitrogen may have on the availability of the phosphorus sources in the soil.

A. BIOLOGICAL EFFECTS 1. Root Area and Absorbing Capacity Since both nitrogen and phosphorus are elements essential for plant growth, it would be expected that a deficiency of either element would limit growth of the above- and below-ground portions of the plants. If all other essential elements are supplied it would be expected that the addition of nitrogen would stimulate root growth. Increased root growth in the vicinity of a phosphorus fertilizer band should preferentially increase the absorption of fertilizer phosphorus, and stimulation of root growth by placement of nitrogen away from a band of phosphorus fertilizer should preferentially increase the absorption of soil phosphorus. Working with split-root techniques in culture solutions, Gile and Carrero (1917) reported that the weight of corn and rice roots was greater in the solutions containing nitrogen. Grunes et al. (1958b) found in a growth chamber that the addition of ammonium sulfate generally increased the relative amount of barley roots in the vicinity of a band of concentrated superphosphate. They believed that this was one of the reasons why banding nitrogen and phosphorus fertilizers together increased the percentage of the plant phosphorus absorbed from the fertilizer. Although placement of ammonium sulfate fertilizer in a band on the opposite side of the barley plants from the phosphorus fertilizer band increased the total weight of barley roots produced by the plants, the proportion of roots in the phosphorus fertilizer band was generally similar to that when no nitro-

371

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

gen was added to the banded phosphorus treatment. As a result of studies in Indiana, Miller and Ohlrogge (1958) and Ohlrogge et al. ( 1957) also believed that banding nitrogen with phosphorus preferentially increased root growth in the phosphorus bands, and as a result, preferentially increased absorption of fertilizer phosphorus. An example of a greenhouse study showing the marked effect of nitrogen additions on the absorption of fertilizer phosphorus is shown in Fig. 1 (from

r

0

PHOSPHORUS

0-

U

I

1

4

I

A--

PHOSPHORUS + NITROGEN MIXED

[3-----

PHOSPHORUS t NITROGEN SEPARATE

1

8

I

I

I

I

12 16 SOIL PHOSPHATE LEVEL P205 ADDED- CWWACRE

I

20

FIG. 1. The influence of soil phosphate level and nitrogen additions on the phosphorus absorption by corn from band-applied phosphorus fertilizer. ( From Miller and Ohlrogge, 1958.)

Miller and Ohlrogge, 1958). In this case the effect of nitrogen on fertilizer phosphorus absorption was greater when the two fertilizer bands were placed together. For both nitrogen placements, it was at the low level of soil phosphorus that nitrogen was most effective in increasing fertilizer phosphorus absorption. Jacques (1943) found, with perennial ryegrass in New Zealand, that superphosphate increased root weights in the highly fertilized areas. Haas (1958) demonstrated that nitrogen fertilizer increased yields of grass roots, as did the higher rates of phosphorus when added in the presence of nitrogen. De Wit (1953, pp. 4748) has discussed the data of German and

372

D. L. GRUNES

American workers who found that the addition of nitrogen or phosphorus fertilizers increased root growth in both the fertilized and unfertilized portions of the soil. More fibrous roots were developed in areas where the fertilizers were placed. In England, Cooke (1954) demonstrated that banding nitrogen or potassium fertilizers increased the root growth of peas through large volumes of soil around and below the position of the fertilizer band. The extra root growth was coarse and tough. When superphosphate was placed in a band, root development was stimulated throughout a smaller volume of soil, corresponding to the small distance that water-soluble phosphorus diffuses before it is precipitated. Mainly fine roots were developed in the superphosphate bands. Nightingale (1937) has mentioned that the fibrous roots of lownitrogen plants are of small diameter, but are usually very extensive and branch and rebranch with many fine sublateral rootlets. In a later review Nightingale (1948) stated that limited nitrogen supply increased the length and carbohydrate content of roots of Lemm minor.In lettuce, peach trees, apple trees, and pine seedlings, low nitrogen supply favored the development of relatively extensive root systems. Bosemarck ( 1954) has indicated that roots of high-nitrogen EROICA wheat plants tend to be short and thick compared with roots of low-nitrogen plants. However, as Black pointed out (1957, p. ZZS), although nitrogen does tend preferentially to increase top growth, it is an important fertilizer for root crops grown on soils of low nitrogen availability. It is possible that the addition of nitrogen fertilizer to a nitrogendeficient soil increases root absorbing surface and capacity far beyond that expected from simple increase in weight. This could be accounted for by the production of new small roots and root hairs which might have a very high absorbing capacity per unit of weight. On nitrogendeficient soils the addition of a band of nitrogen fertilizer together with a band of phosphorus fertilizer would be expected to increase the relative absorption of fertilizer phosphorus by stimulation of root growth in, and phosphorus absorption from, the phosphorus fertilizer band. Therefore the percentage of the total plant phosphorus absorbed by the plant from the fertilizer would increase. 2. Root Eficiency

Grunes et al. (1958b) studied the relationship between the effect of nitrogen on the growth of barley, in a controlled light-temperature plant growth chamber, and the relative absorption of soil and fertilizer phosphorus by the plants (Fig. 2). Grunes et al (1958a) similarly studied this relationship in field experiments with sugar beets and potatoes in

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

373

North and South Dakota. It was found that the relative absorption of fertilizer phosphoriis increased with plant growth and total phosphorus absorbed. Nitrogen additions increased thc relative absorption of banded fertilizer phosphorus even when the nitrogen was not banded with the phosphorus. CL

w

L.S.D. ( 0 . 0 5 ) s

> n

Y=2.60X r = 0.930'.

I-

+ 11.5

TOTAL P ABSORBED-

MGMS./CAN

FIG.2. Relation between per cent plant P derived from fertilizer bands and total phosphorus absorbed by barley from Gardena loam soil. (A, NH4NOs band with P; B , NH,NOs band opposite P; C, ( NH4)&04band with P; D, (NH4)SO4 band opposite P; E, NaNOs band with P; F, NaNOa band opposite P; G, NasS04 band with P; H, NaSO. band opposite P; A l , NH4NOs mixed, P band; N, (NH,)&O4 mixed, P band; 0, NaNOa mixed, P band; X, no N, P band). (From Grunes et al., 1958b.)

The explanations given for the increase in relative fertilizer phosphorus absorption, concomitant with increased growth and total phosphorus absorption resulting from nitrogen fertilization, are further developed here. Because of the higher concentration of phosphorus per unit of soil mass, a band of soluble phosphorus fertilizer should be better able than the soil to maintain the concentration of solution phosphorus following phosphorus absorption by plants. In other words, the phosphorus-supplying power of the fertilizer band should be greater than that of the soil. This could affect the relative absorption of fertilizer and soil phosphorus by the following means : ( a ) By affecting the fertilizer and soil phosphorus concentration gradients and the diffusion of phosphorus to the absorbing root surfaces. This explanation will be discussed in detail.

374

D. L. GRUNES

( b ) By affecting the amounts of fertilizer and soil phosphorus which inight move to the root surface with water being absorbed by the roots. ( c ) By affecting the amounts of fertilizer and soil phosphorus in solution, and thus available to roots, moving into areas adjacent to those areas in which some absorption of phosphorus by plants had already taken place. As a result, the percentage of the total plant phosphorus coming from the fertilizer would be expected to increase following increased plant growth and absorption of total phosphorus. The effect presented under ( a ) will be further elucidated in the following. Generally, nitrogen increases the growth of tops more than roots (Black, 1957, pp. 225-227), thus increasing the requirements of plants for phosphorus more than the phosphorus-absorbing surface. A unit area of root surface would be expected to absorb more phosphorus in the case of nitrogen-fertilized than unfertilized plants because of the greater phosphorus stress in the former. That is to say, nitrogen fertilization serves to increase the efficiency of the root system in absorbing phosphorus. If the foregoing presumptions are correct, the Fick law of diffusion offers a possible explanation for the observed increase in proportionate uptake of phosphorus from the fertilizer bands accompanying nitrogen fertilization. Application of the diffusion principle to the general problem was suggested originally by Hammond et al. (1951), whose hypothesis may be elaborated as follows: Fick's law of diffusion (Nernst, 1923) states that

where dq/dt = time rate of diffusion of material through unit cross section; D = diffusion coefficient; -dc/dx = concentration gradient. The concentration gradient is negative in sign since the direction of movement is from regions of high concentration to regions of lower concentration. If the diffusion coefficient remains unchanged, the time rate of diffusion is governed by the concentration gradient. Consider the case of square-millimeter sources of soil or fertilizer phosphorus, each 1mm. distant from identical square-millimeter sections of root surface. For the purpose of this discussion, the square-millimeter sections of root surface will be assumed to be flat. If the rate of phosphorus absorption by the root is zero, the concentration gradient between the phosphorus sources and the root will likewise be zero, since the phosphorus in solution at the root surface is in equilibrium with the soil or fertilizer phosphorus in the respective locations. As the rate of

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

375

phosphorus absorption increases, the phosphorus concentration at the root surface decreases, and concentration gradients are developed between the phosphorus sources and the root. Continued transfer of phosphorus from sources to root is a consequence of these gradients. The concentration gradients and the rates of transfer will have their maximum values when absorption is rapid enough to maintain concentrations approaching zero at the root surfaces. But since the phosphorus concentration at the fertilizer source is greater than that at the soil source, the maximum concentration gradients and rates of transfer obtained will be greater between fertilizer source and root than those between soil source and root. The proportionate absorption of phosphorus from the fertilizer source thus increases with the efficiency of the root as a whole in absorbing phosphorus, i.e., with the rate of phosphorus absorption per unit area and per unit time. A schematic representation of the postulated changes in phosphorus concentration gradients for the soil and fertilizer sources is shown in Fig. 3. The ordinate represents the concentration gradients developed

SOURCE

I %,SOIL

-

SOURCE

SUM O F P DIFFUSED TO ROOT FROM SOIL AND F E R T I L I Z E R SOURCES FIG.3. Effect of phosphorus diffusion to the plant root on concentration gradients of soil and fertilizer phosphorus.

for each source. The abscissa represents the sum of the phosphorus diffused to the root from both sources. Let line ABC represent conditions for plants receiving phosphorus only, and line DEF represent plants receiving both nitrogen and phosphorus. Let us assume that the latter plants will grow more rapidly and absorb more phosphorus. As a result the ratio of fertilizer-phosphorus concentration gradient: soil-phosphorus concentration gradient will be greater for the larger plants than for the smaller plants ( D F / D E greater than A C / A B ) . Following absorption of large amounts of phosphorus, the phosphorus concentration gradients for each source will decrease since the solution concentrations at the sources decrease (Fig. 3 ) . Since the ability to renew phosphorus in solu-

376

D. L. GRUNES

tion would be expected to be less for the soil than for the fertilizer, it is expected that the solution phosphorus, and as a result the concentration gradient, would decrease more rapidly for the soil than for the fertilizer. If part of the soil phosphorus is obtained by contact exchange (Jenny, 1951), the percentage of the total plant phosphorus absorbed from the fertilizer should still be greater for the larger plants. The reason is that as phosphorus absorption proceeds, the amount of adsorbed soil phosphorus available for absorption by plants will very likely decrease more rapidly than will the concentration of phosphorus in the solution adjacent to the fertilizer particles, On some soils, plants may not exhibit growth response to added nitrogen. This may be due to large amounts of available nitrogen, limiting quantities of other essential elements, the presence of toxic materials, or to other causes. On these soils the explanations of change in relative availability of soil and fertilizer phosphorus which involve plant growth response would not be valid. Also, the degree of growth response to the added nitrogen should determine the magnitude of change in relative availability of the phosphorus sources. It would appear that additional work is needed on the effect of growth-promoting influences other than nitrogen on the relative absorption of soil and fertilizer phosphorus by plants. Other essential elements and light are two such influences. For soils high in available phosphorus, the addition of growth-stimulating materials might not increase the percentage of the total phosphorus absorbed by plants from the fertilizer as much as on soils low in available phosphorus. The concentration gradients for soil phosphorus, and also the ability of the soil to maintain its original solution concentration by solution of additional phosphorus, could be greater on soils high in available phosphorus, Grunes et al. (1958b) demonstrated that added nitrogen did not increase the percentage of the plant phosphorus absorbed from the fertilizer from systems in which the phosphorus fertilizer was mixed with the soil. This was so despite the effect of nitrogen on increasing growth and total phosphorus absorption by the plants. Their explanation is expanded below. When powdered phosphorus fertilizer is broadcast in the soil, the phosphorus particles are spread over a much larger area than in band placement. This would tend to make for a more rapid and complete reaction between soil and fertilizer. It is possible that the concentration gradients for fertilizer phosphorus would still be greater than those obtained for soil phosphorus. However, the difference in magnitude of these concentration gradients would be much less for broadcast than

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

377

for band placement. Also the ability of the fertilizer to renew the phosphorus in solution is undoubtedly less for broadcast than for band placement due to the larger amounts of fertilizer involved per unit soil mass in the latter case. The addition of a growth-promoting nutrient, such as nitrogen, would tend to deplete the soil solution of both soil and fertilizer phosphorus. The magnitude of the fertilizer phosphorus concentration gradients, as well as the capacity of the fertilizer to renew the supply of solution phosphorus following absorption by plants, might be similar for the broadcast phosphorus fertilizer and the soil. Therefore no great increase in the percentage of the phosphorus derived by plants from the fertilizer would be expected following the addition of nitrogen to soil systems wherein the phosphorus has been thoroughly mixed with the soil. Isotopic exchange between soil and fertilizer phosphorus would probably also be greater for broadcast than banded phosphorus systems. For the exchanged portions, the uptake of soil and fertilizer phosphorus would be indistinguishable. R. S. Russell et al. (1954) discuss the effect of isotopic exchange on the measurement of the relative uptake of soil and fertilizer phosphorus by plants. It has been found that on soil to which either banded or broadcast phosphorus was added, nitrogen additions generally increased total phosphorus uptake by plants (Grunes et al., 1958b). However the addition of nitrogen did not increase total phosphorus absorption from soils not fertilized with phosphorus, despite a growth response of the tops and roots to the added nitrogen. Studies of causes of this phenomenon would appear to be worth while.

3. Ammonium Ion Effect Arnon (1939) reported that the phosphorus contents were higher for barley plants growing in culture solutions with an ammonium source of nitrogen than for those supplied with a nitrate source. He believed that a rapidly absorbable cation such as the ammonium ion would favor the absorption of anions such as HzP04-. Breon et al. (1944)stated that tomato plants grown with a urea nitrogen source absorbed more phosphorus from culture solutions than those grown with a nitrate nitrogen source. It seems likely that the urea was largely hydrolyzed to ammonium before absorption by the tomato plants. Olson and Dreier (1956b) found in Nebraska that nitrogen increased uptake of fertilizer phosphorus from bands by oats (Fig. 4) and wheat. They stated that the ammonium ion exceeded the nitrate ion in this capacity, especially during early stages of plant growth. The maximum influence of either ion occurred with intimate association of fertilizer nitrogen and the super-

378

D. L. GRUNES

phosphate bands. Olson et al. (1956) reported that ammonium nitrate in contact with fertilizer phosphorus enhanced the uptake of the latter by wheat and oats. The effect was noted with both super- and metaphosphate carriers and was thought to be the cause of greater early phosphorus uptake by plants from ammonium phosphates than from other phosphate carriers. N

r@

80 80

Fert. P205

PI

0”20 -I W

F

Soil

15

P205

0 “

aN 10

5 YielId/ Pot,g.

6.2

Fert. R e covery, %

6.3

7.2

9.5

19 37

8.9

iao

26 34

9.8

I 1.0

21 23

FIG.4. Influence of fertilizer nitrogen on uptake of soil and fertilizer phosphorus

by greenhouse oats (grown on Carrington clay loam, pH 6.5, Bray’s phosphorus 0.7 p.p.m.; harvested at early dough stage). (From Olson and Dreier, 1956b.)

Rennie and Soper (1958) found in a field study in Saskatchewan, Canada, that placement of nitrogen with phosphorus-fertilized bands materially increased fertilizer-phosphorus uptake and decreased the “A” values (measures of available soil phosphorus) (see Fig. 5 ) . The effect of nitrogen on increasing fertilizer phosphorus absorption was not observed when the nitrogen was top-dressed, uniformly mixed throughout the surface soil, or added 1 inch from the phosphorus carrier. They believed that the ammonium ion materially influenced the ability of plants to absorb fertilizer phosphorus, when the nitrogen and phosphorus bands were in close association.

4. Stage of Growth-Plant Nutrient Uptake Functions Remy (1938, Table 3) summarized field experiments in Germany with small grains, potatoes, peas, and winter rape. He indicated that,

379

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

early in the growth period, absorption of nutrients occurred at a higher rate than did dry-matter production. Later in the season dry-matter production increased more rapidly than did nutrient absorption. In consequence nutrient concentrations would be high early in the growing season and decrease with time.

I

q -

FERTILIZER UPTAKE

'A" VALUES

Nltrogen Placement I.With the phosphate 2.Top- dresslng

80

2 220

70

0

<

2 r

cu 200

I.t

C

60 50

160

POUNDS OF N APPLIED AS NHqNO3

FIG.5. Effect of nitrogen placement on uptake of fertilizer phosphorus and "A" values of wheat grown on Melfort silty clay loam. (From Rennie and Soper, 1958.)

Dean and Fried (1953, pp. 44-45) have reviewed experiments by numerous workers indicating that phosphorus absorption proceeded more rapidly than plant growth early in the season. Typical data indicate that when 50 per cent of the total phosphorus has been absorbed, only 20 per cent of the total growth has occurred. Thus phosphorus concentrations would decrease later in the season. Goodall and Gregory (1947, Fig. 4a) also illustrated the decrease in phosphorus concentrations with time for barley grown in sand cultures. The above-mentioned experiments give a clue to one reason why nitrogen fertilization may increase phosphorus concentrations in plant tissues early in the growing season. If nitrogen fertilization stimulates root growth and foraging power, total phosphorus absorption should also increase. Since in the initial growth phases phosphorus absorption occurs more rapidly than plant growth, phosphorus concentrations should be higher for nitrogen-fertilized plants. This explanation should have application both as regards fertilization with nitrogen at seeding time, and also as regards the addition of nitrogen later in the season. Should the plants be grown to maturity, the concentrations of phosphorus in the plant tissue may be similar whether or not nitrogen was

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D. L. GRUNES

applied, since later in the season growth occurs more rapidly than plant nutrient absorption. In fact, where phosphorus is not limiting plant growth, concentrations of this element may frequently be lower for nitrogen-fertilized plots. This would result from a dilution of the plant phosphorus following a growth response to applied nitrogen. Grunes and Krantz (1958) reported that in field experiments in North Carolina topdressings of nitrogen (generally NaN03) in the spring, 2 to 3 weeks before winter-seeded oats were plowed under in the “boot” stage, increased the nitrogen, phosphorus, and potassium concentrations of oat tops. Although they thought that the effect on phosphorus and potassium absorption was due in part to increased root growth, they also believed that absorption of nutrients by the new roots was probably occurring more rapidly than the plant growth response to the added nitrogen. The authors believed that, had the oat plants been grown to maturity, the concentrations of phosphorus and potassium might have been similar or less for the nitrogen-fertilized plots. J. S. Russell et al. (1954) reported that in Nebraska fall and spring nitrogen fertilization increased phosphorus concentrations in bromegrass forage early in the season, but that later in the season phosphorus concentrations were similar whether or not nitrogen fertilizer had been applied. They attributed the increased phosphorus concentrations to a possible effect of nitrogen on increased root growth, and also to a possible effect of the ammonium ion. They thought that later in the season the similarity of phosphorus concentrations was due in part to the increase in dry matter production resulting from nitrogen fertilization. They also stated that later in the season a possible preponderance of the nitrate ion over the ammonium ion in the soil may have been the cause of the similarity of phosphorus concentrations, whether or not nitrogen fertilizer had been applied earlier. It appears to the author that a time lag of plant growth behind nutrient absorption may also have been a factor in the Nebraska study. Prince (1954) stated that in South Carolina winter fertilization with ammonium nitrate increased the phosphorus concentration in ryegrass and crimson clover forage early in the season. However, in the April and May cuttings, phosphorus concentrations were similar whether or not nitrogen had been applied during the winter. He believed that the ammonium ion may have had a favorable effect on phosphorus absorption. It appears to the author that in the study by Prince nitrogen fertilization probably also increased root growth and foraging capacity for phosphorus, and that there was a time lag of plant growth behind nutrient absorption.

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5. Effect of Nitrogen on Plant Metabolism and on Ability of Roots to Absorb Phosphorus Absorption of nitrogen may affect the ability of unit root absorbing surfaces to absorb phosphorus. McLean et al. (1956) have found that the addition of nitrogen has increased the cation exchange capacity of roots of oats and other crops grown in solution culture. They quote earlier work, by Drake and Steckel (1955) and McLean and Adams ( 1954), postulating that increase in the cation exchange capacity of roots of oats and other crops would generally be expected to aid in the release and uptake of additional phosphates. This would be effected by a greater solubilization of soil aluminum and iron phosphates as a result of increased production of organic anions from high cation exchange roots (Drake and Steckel, 1955). Another effect might be greater uptake of calcium by roots with a high cation exchange capacity, thus releasing phosphorus from calcium phosphate for plant absorption (Drake and Steckel, 1955; Mehlich and Drake, 1955). This effect would be most important in soils in which most of the cations were in the adsorbed form (Mehlich and Drake, 1955). McLean ( 1957) stated that, where phosphates are relatively unavailable, increased phosphate absorption would be expected with increased root cation exchange capacity, as a result of increased bonding energy for calcium, with a concomitant solubilization of the phosphate. However where the phosphate is already in solution, the negatively charged phosphate ions might be sieved out or repelled by the increased number of negative charges per unit weight of roots, i.e., greater cation exchange capacity. This would result in decreased phosphorus uptake by plants. There may be other effects of nitrogen uptake on the absorbing ability of unit areas of root surfaces. Nightingale (1937) quoting data of Hamner, and Nightingale (1948) quoting data of Alten and Gottwick has mentioned increases in respiration or carbon dioxide release following absorption of nitrogen. Hoagland (1944, pp. 142147) has also mentioned that during the absorption of nitrogen, or as a result of subsequent metabolism, a large increase in total respiration occurred in excised barley roots high in carbohydrates. Willis and Yemm (1955) reported that excised barley roots increased in oxygen uptake and carbon dioxide production following the addition of nitrogen salts. Although some respiration was observed when neutral salts, such as sodium chloride or potassium chloride, was supplied, the respiration observed was much greater when ammonium and nitrate salts were used. Nitrate was found to give a much more gradual increase in

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both carbon dioxide production and oxygen uptake, when compared with ammonium salts or nitrite. It was believed that utilization of nitrate first involves its reduction to nitrite and that this first reductive stage proceeds relatively slowly in the roots. With both nitrates and ammonium salts, the high rates of carbohydrate utilization, and the increase of cellular respiration, were attributed to metabolic changes associated with the synthesis of nitrogenous constituents in the roots. Yemm and Willis (1956) reported that although nitrate and nitrite gave rise to high rates of respiration, as measured by the evolution of carbon dioxide, and to rapid loss of carbohydrates from the tissues, the synthesis of amides is generally very much smaller than that obtained with ammonium salts. A much higher proportion of the carbohydrates metabolized appears to undergo complete oxidation to carbon dioxide during treatment of the roots with nitrate and nitrite as compared with ammonium salts. Under all conditions of rapid assimilation of inorganic nitrogen salts, a marked increase in glutamine was observed. The amount of glutamine formed was considerably greater when ammonium salts were used. With nitrate, asparagine and amino acids were also formed. Yemm and Willis (1956) stated that the enzymatic systems which bring about the synthesis of glutamic acid and its amide may be closely related to glycolysis and oxidation of carbohydrates at several important points. They believed that the reductive amination of a-ketoglutaric acid probably proceeds by means of electron transfer mediated by pyridine nucleotides, while the synthesis of the amide band is promoted by phosphorylations involving adenosine triphosphate. Thus the catalytic mechanisms of oxidation-reduction and phosphorylation involved in the synthesis of glutamine, as well as its carbon skeleton, may be directly dependent on carbohydrate breakdown. Yemm and Willis also mention that the enzymatic synthesis of glutamine is chiefly localized in the mitochondria, which appear to be the main centers of oxidation and phosphorylation in the cell. Russell (1954) suggested the following general scheme for ion accumulation. The outer layers of the cytoplasm are regarded as being relatively permeable to ions by exchange or diffusion. The inner layer of the cytoplasm, presumed to be the tonoplast, is regarded as a barrier to the free movement of ions by exchange or dfision. Such is believed to be the case because of the marked concentration gradient existing between the vacuole of the cell and the outer medium, and because of the apparently low exchangeability of ions which have penetrated sufficiently deeply into the cell. The transference of ions across the barrier zone is considered to be effected by the carrier mechanism.

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It is visualized that respiration leads to the production of a substance which is freely diffusible through the cytoplasm. This substance places ions under restraint on the outer side of the barrier, forming a complex which diffuses across the barrier to liberate the ions on the other side. Russell (1954) showed that barley plants absorbed more phosphate from solution when nitrate was added (Table I ) . He suggested that TABLE I Effect of 3 X lo-' M Nitrate 011 the Absorption of Phosphate by Barley Plants a t the Second-Leaf Stage from a Solution Containing 3 X lo-* M HsPOr" Duration of experiment

(hr)

Nitrate presentb

~

Nitrate absent6

Standard deviationb

~

3 6 24

1.63 2.01 2.42

0.96 1.42 1.52

0.27

Source: Russell (1954). Units arbitrary.

these data are compatible with the suggestion of Steward and Street (1947) that ions combine with nitrogenous materials and are released when these compounds break down. In a later paper, Steward and Millar (1954) suggest that 7-glutamyl peptides may be the carriers. Russell (1954) also offered the alternative explanation that the increase in respiration rate occasioned by nitrogen metabolism, and not the production of nitrogenous compounds, is effective in increasing absorption. Russell ( 1954) and Loughman and Russell (1957) present evidence that the first step in absorption of the phosphate ion by plants is not a purely physical process, but is dependent upon metabolic components. Ions are held by exchange reactions in the cytoplasm, the maintenance of which depends on metabolic activity. Russell and Ayland (1955) have shown that dinitrophenol can partially inhibit the initial process, thus indicating metabolic components. The results of Loughman and Russell (1957) are compatible with the view that phosphate is first incorporated into the adenosine triphosphate molecule and then transferred successively to hexose monophosphates and to nucleic acids. The absorption of phosphorus has recently been discussed in terms of binding sites on plant roots (Hagen and Hopkins, 1955; Hagen, 1956; Hagen et al., 1957). The ion absorption equations have been written as follows:

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R

+ M outside S MR kl

’k*

MR

ka

b

It’

+ M inside

where R represents a root binding site or compound; M, the ion; MR, the labile intermediate; k, the rate constant for each reaction. Breakdown of the labile intermediate results in the uptake of ions into the system. The rate constants of association and dissociation, kl and kz, of the intermediate, MR, are large with respect to the rate constant, ka, of the breakdown of the intermediate; k4 is negligible. Separate binding sites Ra+ and Rb+ are designated for H2P04- and HP04--, respectively. Hagen et al. (1957) concluded that the binding compounds involved in the rate-limiting step of orthophosphate uptake are components of the respiratory chain. As a result of a study of the effect of numerous substrates on orthophosphate uptake, they concluded that the kSRof glutamate corresponded most closely to the kBa calculated from uptake measurements of excised barley roots. They further concluded that orthophosphate uptake by barley roots is coupled to sites identical with those for oxidative phosphorylation during electron transport in mammalian mitochondria. It was mentioned earlier that the work of Willis and Yemm (1955), Yemm and Willis (1956), and Steward and co-workers does relate nitrogen absorption to the synthesis of nitrogenous constituents in the roots, respiration, and phosphorylation. It therefore appears possible that nitrogen absorption by plants affects phosphorus uptake by affecting either the formation of root binding sites or the release of phosphorus, from the labile intermediates, into the interior of the plant roots. Avery et al. (1936) reported that the production of auxin in shoot tips of sunfiower and Turkish tobacco increased with increasing nitrogen supply. Black (1957, pp. 225-226) presumed that the increase in auxin concentration probably resulted from the fact that the principal plant auxin, p-indoleacetic acid, contains nitrogen and apparently is formed by alteration of tryptophan, an amino acid. Black suggested that the concentration of tryptophan would be expected to increase with the nitrogen supply. As mentioned earlier, Bosemarck (1954) has indicated that roots of high-nitrogen EROICA wheat plants tend to be short and thick compared with roots of low-nitrogen plants. He believed that this was due at least in part to the inhibitory effects of high auxin concentration on the division and elongation of cells in the roots.

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It is not known whether there is a causative relationship between auxin concentrations and the ability of plant roots to absorb phosphorus. However Skoog (1954, p. 160) has reported that phosphate accumulates in the actively growing cells, and to some extent in accord with the tissue supply of indoleacetic acid and adenine.

B. CHEMICAL EFFECTS Chemical effects might control the absorption of soil and fertilizer phosphorus by plants by affecting phosphorus solubility. A preferential effect on the solubility of the soil or fertilizer phosphorus would be expected to affect the percentage of the phosphorus absorbed by plants from the fertilizer. This would be so regardless of whether the phosphorus diffused through the water films to the root absorbing surfaces, or whether the phosphorus moved to the plant as a result of intake of water by the plant roots.

1. Salt Effects Olsen (1953) and Wild (1950) have reviewed the literature on the effects of salts on the solubility of native and applied phosphorus in the soil. When a neutral salt having an ion in common with a slightly soluble salt, BA, is added to a saturated solution of the latter, a decrease in solubility of BA is observed. This can be explained on the basis of the solubility product principle ( Glasstone, 1946, pp. 971-972). However as the concentration of the added neutral salt is increased, the solubility of BA commences to increase. The added neutral salt increases the ionic strength of the medium, and thus decreases the activity coefficients of the ions B+ and A-. Therefore the solubility of BA increases. As expected, this effect is more easily observed with ions of high valence. When a neutral salt without a common ion is added to a saturated solution of a slightly soluble salt, BA, the ionic strength of the medium is increased. In consequence the activity coefficients of the ions B+ and A- decrease. As the solubility product remains constant, the solubility of the salt BA increases. At high ionic strengths, the activity coefficients may begin to increase, and the solubility of the salt BA consequently decreases (see Glasstone, 1946, pp. 971-972). Salts which hydrolyze to give a change in pH would have still further effects on the solubility of slightly soluble salts. While the foregoing indicates the effect of salts on the solubility of pure calcium phosphates, Olsen (1953) and Wild (1950) have indicated that the effects on soil systems were considerably more variable. In some instances salts increased the solubility of phosphorus, and in some instances decreases were noted. Buehrer (1932) pointed out that alkali

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chlorides decrease the solubility of phosphate in calcareous soils by interaction with the calcium carbonate. The salt increases the solubility and hydrolysis of the calcium carbonate to produce a higher calcium ion concentration, which in turn reduces the phosphate concentration.

I

1

0

I

I

1

1

2

1

1

3

I

4

SQUARE ROOT OF IONIC STRENGTH FIG.6. Influence of salt concentration on the solubility of dicalcium phosphate in solutions of type salts (0.4 g. of No. 3100 dicalcium phosphate agitated with 100 ml. salt solution at 30" C. for 30 minutes). (From Starostka and Hill, 1955.)

Lehr and Wesemael (1952) indicated that, in Dutch soils, the phosphate solubility decreased with increasing concentration of neutral salts. The depressing effect increased in the order Na < K < Mg < Ca, and was less marked with sulfate than with chloride or nitrate. In soil the cations in the added salts probably replaced calcium from the exchange complex, and the calcium then precipitated the phosphate from solution. Olsen (1953) reviewed studies by several workers indicating that

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

387

potassium, from added potassium chloride, may become part of a complex phosphorus precipitate involving a number of cations. He also indicated that additional studies of the nature of the interaction of salts with the soil, as it affected phosphate solubility, would appear to be highly desirable. Starostka and Hill (1955) studied the effect of salts on the solubility of, and plant response to, dicalcium phosphate. Salts were classified into three groups, according to laboratory measurements: those in which phosphorus solubility increased markedly with ionic strength; those in which phosphorus solubility increased only moderately and in which the solubility leveled off at higher ionic strengths; and salts which decreased phosphorus solubility sharply with increasing ionic strength (see Fig. 6). Greenhouse techniques, growing alfalfa as a test crop on several soils, were designed to minimize responses to nutrient constituents of the salts, changes in available soil phosphorus, and physiological effects due to the salts. The differences in growth response to tablets of binary mixtures of dicalcium phosphate and the salts were therefore indicative of differences in the solubility of dicalcium phosphate. In general, salts such as ammonium sulfate, which in laboratory tests markedly increased the solubility of dicalcium phosphate with increasing ionic strength, also gave the greatest crop responses in the greenhouse. Those salts, such as the ammonium nitrate type, which increased the solubility only moderately gave increased crop response only occasionally, while salts, such as the calcium nitrate type, which decreased the solubility sharply with increasing ionic strength, usually gave a negative yield response. The results of the study also indicated the superiority of intimate mixtures of the salts and dicalcium phosphate over separate placements. The authors emphasized the need for studies of influences of fertilizer granule size, especially reduced granule size, and salt to phosphorus ratio. Further investigations were also suggested on mixed phosphorus carriers such as ammoniated mixtures containing monoammonium phosphate and dicalcium phosphate. It was also suggested that experiments be undertaken using dicalcium phosphate and basic calcium phosphate in mixtures that do not contain appreciable quantities of water-soluble phosphorus. Bouldin and Sample (1958) studied phosphorus uptake by oats in the greenhouse from pellets of concentrated superphosphate (CSP) in soil, with and without salts incorporated in the pellets. Nitrogen and potassium were added to all pots, to minimize the nutrient effect of elements in the pellets other than phosphorus. With Hartsells fine sandy loam (pH 5.2) yields of phosphorus in plants were in the order:

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CSP
CSP+(NHI)ISOI
With Mountview silt loam (pH 6.6) yields of phosphorus were in the order : CSP
+

+

After pellets of CSP,CSP KN03, and CSP NHINOI had reacted with the moist soil for 3 weeks, soil samples were obtained at various distances from the center of the pellet residue. Both water-soluble phosphorus ( 1:15 soil-water extracts) and phosphorus extractable in 6 N hydrochloric acid were linear functions of the distance of the sample from the center of the pellet residue. The summation of the products, of water-soluble phosphorus times volume of soil containing this concentration, was calculated over the total volume of soil influenced by the fertilizer. This summation was well correlated with the uptake of phosphorus by oats, Bouldin and Sample therefore concluded that the effect of associated salts on the availability of concentrated superphosphate to plants was largely explained on the basis of chemical differences. They felt that the salts had an influence on (1) the fraction of monocalcium phosphate which reverted to dicalcium phosphate at the pellet site, ( 2) the distribution of fertilizer phosphorus in the soil, and (3) the solubility of the soil-fertilizer phosphorus reaction products. The relative magnitude of these effects depends upon soil properties. The work of Mattson, Wiklander, and co-workers, concerned with the effect of neutral salts on phosphate solubility, has been discussed by Olsen (1953) and Wiklander (1955, pp. 143-144). An electrical double layer is postulated at the insoluble phosphorus-soil solution interface. Mattson and co-workers assume that the concentration of phosphate in the outer solution ( X ) of pedalfer soils is determined by ( 1) a solubility product of the hydrous oxide-bound phosphate keeping the concentration (Y,,p,,) in the inner solution constant, (2) a Donnan distribution of the diffusible phosphate ions between the inner and outer solutions, and (3) the kind and contents of salts present. Soils that are above the isoelectric point, which is usually the case, behave like a cation exchanger and X > Y. The addition of a neutral salt tends to depress the Donnan distribution, and as a result phosphate solubility decreases. The Donnan effect increases with increasing negative charge on the soil surface. As the pH decreases, the negative charge on the soil surface also decreases and, therefore, the effect of salt on decreasing phosphate solubility also decreases. Also, as the pH decreases, anion exchange increases, thus increasing phosphorus solubility. Therefore, according to this theory, phosphate adsorption and solubility in pedalfer soils are

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

389

closely related not only to the p H and salt content, but also to the amphoteric properties of the soil. It should also be kept in mind that as the magnitude of the Donnan distribution decreases, the previously mentioned effect of neutral salts on decreasing activity coefficients, and thus increasing phosphate solubility, becomes more evident. In calcareous soils, it would be expected that the Donnan distribution would be less effective in controlling solution phosphorus. In many slightly saline soils, the soluble salts are already sufficiently high so that salt additions should not appreciably affect changes in phosphorus solubility due to a Donnan distribution. Also, as discussed by Hemwall (1957), the fixation of phosphorus in calcareous and alkaline soils is due to the formation of a whole series of insoluble calcium phosphate compounds forming heterogeneous solid solutions. In addition phosphorus can be fixed by aluminum or aluminum hydroxide in the clay fraction of alkaline soils. It does, however, appear that the magnitude of the Donnan distribution in calcareous and alkaline soils should be further investigated. As indicated by Olsen (1953), the effect of salts on root membranes must also be considered. Mattson and co-workers postulated the existence of a Donnan distribution at the soil solution-plant root membrane interface. Therefore the effect of salts on this distribution must also be dealt with in evaluating the effect of salts on the absorption of phosphorus by plants. In considering relative uptake of soil and fertilizer phosphorus by plants, it is necessary to consider the effects of salts on the soil, on the phosphorus fertilizer with which they are in contact, and on the root membranes. Nitrogen salts might tend to increase or decrease the solubility of the fertilizer or soil phosphorus. In some instances nitrogen salts might decrease the rate of reversion of soluble fertilizers to less soluble forms such as dicalcium phosphate. Effects of added salts on phosphorus solubility should be less marked on saline soils. 2. p H Effects It has been found (Amon et al., 1942) that such diverse plants as tomatoes, lettuce, and Bermudagrass were able to absorb phosphorus from culture solutions ranging in pH from 4 to 8. However, at still higher or lower pH values, phosphorus absorption decreased markedly. Black (1957, p. 271) quotes work of van den Honert indicating that plants absorbed the H2P04- ion preferentially over the HPO4-- and PO4--- ions. As the pH increased through the range suitable for the growth of plants, the proportion of the total orthophosphate present as H2P04- decreased.

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Hagen and Hopkins (1955) reported that excised barley roots generally absorbed more phosphorus at pH 5.0 than at higher pH. However, they thought that both the H2P04- and the HP04-- ions were important in phosphorus absorption. They found that hydroxyl ion competitively inhibited the absorption of both H2P04- and HP04-- ions. Various workers have considered the effect of nitrogen or phosphorus fertilizers on soil pH (Allison, 1931; Conrad, 1933; Davis, 1938; Kurtz, 1953; Overstreet and Dean, 1951; Peterson et al., 1937; Pierre, 1928,1933; Sewell et al., 1927; Sewell and Latshaw, 1931; Vittum et al., 1952; Volk, 1944; Volk and Tidmore, 1946). The effect of pH on the solubility and availability of phosphorus to plants has been discussed by Olsen ( 1953), Buehrer (1932), and Black (1957, pp. 268-271). When fertilizers containing ammonium are added to the soil, nitrification of the ammonium ion might have acidifying effects as shown by the following reactions:

+ +

+

NH~NOI 2 0 2 -+2 HNOa HzO (NH&SOa 4 0 2 Has04 2 HNOs

+

+ 2 HzO

For an equivalent amount of nitrogen, the ammonium sulfate would have a greater acidifying effect than ammonium nitrate. Preferential absorption by the plants of the rapidly absorbable ammonium ions over the slowly absorbable sulfate ions would also tend to decrease the pH. Prianishnikov ( 1951, pp. 80-82) has discussed Russian work indicating the effect of ammonium nitrate on increasing the phosphorus availability of rock phosphates to plants. It has been shown (Burd, 1948; Gardner and Kelley, 1940; Teakle, 1928) that lowering the pH of alkaline soils increases the phosphorus in solution. Lorenz and Johnson (1953) found in California that physiologically acid ammonium salts effectively released soil phosphorus for absorption by plants. Chapman (1936) had reported earlier that acid treatments increased yields on phosphorus-deficient calcareous soils in California. Grunes et al. (1958b) showed with North Dakota soils that banding residually acidic ammonium salts with the concentrated superphosphate increased the relative availability of fertilizer phosphorus to plants. As a result of studies in Saskatchewan, Canada, Rennie and Mitchell (1954) believed that lowering of the pH in the vicinity of a phosphorus fertilizer band, due to the addition of an acid-forming material such as ammonium nitrate, was the reason for the increase in availability of the phosphorus fertilizer to the plants. However, in a later publication Rennie and Soper (1958) stated that the stimulative effect of the ammonium ion on increasing fertilizer phosphorus absorption may have been more important.

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

391

The following mechanisms might be responsible for the increase in percentage of the total plant phosphorus absorbed from the fertilizer when residually acidic nitrogen fertilizers are banded with the phosphorus fertilizers in alkaline or neutral soils. The lower pH may bring more phosphorus into solution in the band of phosphorus fertilizer; the acidic fertilizer may slow down the reversion of soluble phosphorus fertilizers to a more insoluble form; the acidic fertilizer may bring more phosphorus into solution from the reaction product of the soil and the phosphorus fertilizer . If residually acidic nitrogen fertilizers were banded in an area away from the phosphorus fertilizer, the tendency might be to increase the relative absorption of soil phosphorus and thus decrease the percentage of the total phosphorus derived from the fertilizer by plants. However, unless the soil were extremely high in phosphorus which could be solubilized by acidic materials, the measured decrease in percentage of phosphorus absorbed from the fertilizer might not be very marked. Fudge (1928) found that in acid soils in Alabama, New Jersey, and Rhode Island the use of physiologically basic nitrogen fertilizers increased the soluble phosphorus and plant utilization of phosphorus, while the use of acid-forming nitrogenous fertilizers had exactly the opposite effect. He postulated that sodium nitrate and calcium nitrate leave a residue of sodium and calcium after the nitrate has been used in plant growth. As a result the pH would increase, The phosphates in the soil, combined with sodium and calcium, would be more soluble and available for plant growth than would iron or aluminum phosphates. Brown (1934) in Iowa and Davis (1938) in Louisiana demonstrated that cyanamid increased the soluble phosphorus in acid soils. Trogdon and Volk (1950) reported that, on an acid soil in Ohio, the addition of cyanamid at the rate of 48 pounds nitrogen per acre increased the solubility of phosphorus in the soil within 1 inch of a superphosphate band. However, ammonium sulfate was not as effective. The quantity of soluble phosphorus in this zone decreased with increasing rates of cyanamid, but, even when nitrogen was added at the rate of 144 pounds per acre, the soluble phosphorus remained considerably higher than in those plots receiving no nitrogen. The effect of the cyanamid was attributed to its high active-lime content. The authors believed that when cyanamid was banded with the phosphorus fertilizer the ratio of active calcium to active iron and aluminum was probably very high, in which case the fixed phosphorus would have been largely in the calcium forms. They thought that the cause of decrease in phosphorus solubility with increased amounts of cyanamid was due to fixation as tricalcium or other complex insoluble forms due to contact between larger amounts of active lime

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and the superphosphate. When ammonium sulfate was banded at the rate of 144 pounds nitrogen per acre, the soluble phosphorus was decreased below values obtained with nonnitrogen-fertilized soil. This decrease was attributed to the decrease in pH causing the active iron and aluminum in the soil to fix the phosphorus. Salter and Barnes (1935) also studied the effect of soil reaction on the efficiency of soil and fertilizer phosphorus in Ohio. Terman et al. (1958) studied the effect of liming on the availability of crystalline calcium phosphates to plants. The phosphate compounds were monocalcium monohydrate, dicalcium dihydrate, anhydrous dicalcium, octocalcium, tricalcium, and hydroxyapatite. The phosphate compounds and lime were mixed with the soil. The pH values after liming were 5.4, 6.6, 7.2, and 7.5 initially, but had fallen to 5.2, 6.2, 6.7, and 6.9, respectively, after harvest of the second crop. Ryegrass was grown first and Sudangrass was planted after the ryegrass was harvested. At all pH levels dicalcium phosphate dihydrate was most available, hydroxyapatite least available, and orthocalcium phosphate intermediate in availability. Both dicalcium phosphate dihydrate and anhydrous dicalcium phosphate were more available than was monocalcium phosphate. This indicates that much of the water-soluble phosphorus in monocalcium phosphate was fixed in the soil in forms less available than the dicalcium phosphates. Yields of ryegrass generally increased with increase in pH level, but the reverse was true for the second crop, Sudangrass. The interaction between phosphate compound and soil pH level was not significant for either yield of dry matter or total absorption of phosphorus, indicating that the relative availabilities of the various phosphates were similar at the various pH levels. It appears that on acid soils the addition of a band of residually acidic nitrogen fertilizer to a band of phosphorus fertilizer might tend to decrease the percentage of the plant phosphorus absorbed from the fertilizer. However, if the rates of nitrogen fertilizer were extremely high so that the pH was markedly lowered in the vicinity of the phosphorus band, considerable phosphorus might be brought into solution. This would tend to increase the percentage of the phosphorus absorbed by plants from the fertilizer. In acid soils the addition of a band of residually basic nitrogen fertilizer, with the fertilizer phosphorus, should increase the percentage of the plant phosphorus absorbed from the fertilizer. Large changes in pH due to the addition of residually acidic or alkaline nitrogen fertilizers would be expected on coarse-textured soils having low base-exchange capacity, Less change in soil reaction would '

EFFECT OF NITROGEN ON PHOSPHORUS AVAILABILITY

393

be expected on highly buffered soils such as those containing free calcium carbonate, soils in which the exchange complex was mainly saturated with calcium, and soils in which thc organic fraction contributed appreciably to buffering capacity. On high-calcium soils there could be a lowering of solution phosphorus due to the increase in soluble calcium following the addition of residually acidic nitrogen salts. 111. Summary

The addition of nitrogen fertilizers has marked effects on the absorption of soil and fertilizer phosphorus by plants. Nitrogen frequently increases root growth and foraging capacity for phosphorus. Some of the effects of nitrogen are related to the effect on increasing the growth of plant tops and concurrently increasing the absorption of phosphorus. The ammonium form of nitrogen frequently increases phosphorus absorption more than does the nitrate form. Nitrogen additions affect plant metabolism and may change the ability of unit areas of root surface to absorb phosphorus. Nitrogen salts may influence absorption of phosphorus by plants by altering the phosphorus solubility in the soil. The residual acidity or alkalinity of added nitrogen may very markedly change the phosphorus in solution and subsequent phosphorus absorption by plants.

REFERENCES Allison, F. E. 1931.1. Am. Soc. Agron. 23,878-908. Arnon, D.I. 1939. Soil Sd.48,295-307. Arnon, D. I., Fratzke, W. E., and Johnson, C. M. 1942. Plant Physiol. 17,515-524. Avery, G . S., Burkholder, P. R., and Creighton, H. B. 1936. Proc. Natl. Acad. Sci. U.S. 22,673478. Bennet, W. F., Stanford, G., and Dumenil, L. C. 1953. Soil Sci. SOC. Am. Proc. 17, 252-258. Black, C. A. 1957. “Soil-Plant Relationships.”Wiley, New York. Bosemarck, N. 0. 1954. Physiol. Plantarum 7,497502. Bouldin, D. R.,and Sample, E. C. 1958. Soil Scl. Soc. Am. Proc. 22,124-129. Breon, W.S.,Gillam, W. S., and Tendam, D. J. 1944. Plant Physiol. 19, 495-506. Brown, M.H.1934. I. Am. Soc. Agron. 26,442-450. Buehrer, T.F. 1932. Arizona Uniu. Agr. Expt. Sta. Tech. Bull. 42. Burd, J. S. 1948. Soil Sci. 65,227-247. Chapman. H.D.1936.1. Am. Soc. Agron. 28,135-145. Conrad, J.P. 1933. Plant Physiol. 8,509-524. Cooke, G . W.1954.1. Sci. Food Agr. 9,429440. Davis, F.L.1938. Louisiana Agr. Expt. Sta. Tech. Bull. 301. Dean, L. A., and Fried, M. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds.), pp. 4 3 5 8 . Academic Press, New York.

394

D. L. GRUNES

De Wit, C. T. 1953. “A Physical Theory On Placement of Fertilizers.” Agricultriral Univ. Wagenigen, Holland. Dion, H. G., Dehm, J. E., and Spinks, J. W. T. 1949a. Sci. Agr. 29, 512-526. Dion, H. G., Spinks, J. W. T., and Mitchell, J. 1949b. Sci. Agr. 29, 167-172. Domby, C. W., Stelly, M., and Sell, 0.E. 1951. Soil Sci. SOC. Am. PTOC.15, 213218. Drake, M., and Steckel, J. E. 1955. Soil Sci. SOC. Am. PTOC.19,449-450. Fine, L. O., Bauer, F. C., Caldwell, A. C., Grunes, D. L., Haise, H. R., Haley, L. E., Hammers, F. L., Holt, R. F., Hustrulid, A., Jackson, M. L., Kaufman, A., Koehler, F. E., Marriott, L. F., Nelson, J. L., Ohlrogge, A. J., and Yamaga, R. H. 1955. North Central Regional Publ. 67, 1955. South Dakota State Coll. Agr. Erpt. Sta. Bull. 453. Fudge, J. F. 1928.1. Am. SOC. Agron. 20,280-293. Gardner, R., and Kelley, 0. J. 1940. Soil Sci. 50, 91-102. Gile, P. L., and Carrero, J. 0. 1917. I. Agr. Research 9,73-95. Glasstone, S . G. 1946. “Textbook of Physical Chemistry,” 2nd ed. Van Nostrand, New York. Glover, J. 1953a. J . Agr. Sci. 43, 154-159. Glover, J. 195313. J. Agr. Sci. 43,160-165. Goodall, D. W., and Gregory, F. G. 1947. Imp. Bur. Hort. Plantation Crops, East Mulling, Kent, Tech. Commun. 17,49-57. Grunes, D. L., and Krantz, B. A. 1958. Agron. ].50,729-732. Grunes, D. L., Haise, H. R., and Fine, L. 0. 1958a. Soil Sci. SOC. Am. Proc. 22, 4952. Grunes, D. L., Viets, F. G., Jr., and Shih, S. H. 195813. Soil Sci. SOC. Am. Proc. 22, 4348. Haas, H. J. 1958. Agron. J. 50,5-9. Haddock, J. L., Hausenbuiller, R. L., and Stanberry, C. 0. 1957. U . S. Dept. Agr. Production Research Rept. 12. Hagen, C. E. 1956. U.S. Atomic Energy Comm. TID-7512,303-307. Hagen, C . E., and Hopkins, H. T. 1955. Plant Physiol. 30,193-199. Hagen, C . E., Leggett, J. E., and Jackson, P. C. 1957. Proc. Natl. Acad. Sci. U . S . 43,496-506. Hammond, L. C., Black, C. A., and Norman, A. G. 1951. Iowa State Coll. Agr. Expt. Sta. Research BuU. 384, 490491. Hemwall, J. B. 1957. Advances in Agron. 9,95-112. Hoagland, D. R. 1944. “Lectures On the Inorganic Nutrition of Plants.” Chronica Botanica, Waltham, Massachusetts. Jacques, W. A. 1943. New Zealand I. Sci. Technol. A25,91-117. Jenny, H. 1951. In “Mineral Nutrition of Plants” (E. Truog, ed.), pp. 107-132. Univ. Wisconsin Press, Madison, Wisconsin. Krantz, B. A., and Chandler, W. V. 1951. Agron. 1.43.547-552. Kurtz, L. T. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds.), pp. 59-88. Academic Press, New York. Lehr, J. J., and Wesemael, J. E. van. 1952. J . Soil Sci. 3, 125-135. Lorenz, 0. A. 1944. Am. Potato J. 21,179-192. Lorenz, 0. A., and Johnson, C. M. 1953. Soil Sci. 75,119-129. Loughman, B. C., and Russell, R. S . 1957. J. Ewptl. Botany 8,280-293. Luders, R. 1955. Z. PfIanzenemiihr.Diing. Bodenk. 68,56-65. McLean, E. 0. 1957. Soil Sci. SOC. Am. Proc. 21, 219-222.

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McLean, E. O., and Adams, D. 1954. Soil Sci. SOC.Am. Proc. 18,273-275. McLean, E. O., Adams, D., and Franklin, R. E. 1958. Soil Sci. SOC. Am. Proc. 20, 345-347. Mehlich, A., and Drake, M. 1955. In “Chemistry of the Soil” (F. E. Bear, ed.), pp. 286-327. Reinhold, New York. Miller, M. H., and Ohlrogge, A. J. 1958. Agron. J . 50, 95-97. Mitchell, J., Kristjanson, A. M., Dion, H. G., and Spinks, J. W. T. 1952. Sci. Agr. 32, 511-525. Nemst, W. 1923. “Theoretical Chemistry,” p. 181. Macmillan, New York. Nightingale, G. T. 1937. Botan. Reu. 3,85-174. Nightingale, G. T. 1948. Botan. Reu. 14, 185-221. Ohlrogge, A. J., Miller, M. H., and Duncan, W. G. 1957. Better Crops with Plunt Food 41 ( 8 ) , 26-30. Olsen, S. R. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds.), pp. 89-122. Academic Press, New York. Olson, R. A., and Dreier, A. F. 1958a. Soil Sci. SOC. Am. Proc. 20, 19-24. Olson, R. A., and Dreier, A. F. 1958b. Soil Sci. SOC. Am. Proc. 20,509-514. Olson, R. A., Dreier, A. F., Lowrey, G. W., and Flowerday, A. D. 1956. Agron. J. 48,111-118. Overstreet, R., and Dean, L. A. 1951. In “Mineral Nutrition of Plants” (E. Truog, ed. ), pp. 79-105. Univ. Wisconsin Press, Madison, Wisconsin. Peterson, J. B., Smith, F. B., and Brown, P. E. 1937. Iowa State CON J . Sci. 11, 293-309. Pierre, W. H. 1928. J. Am. SOC. Agron. 20, 254-269. Pierre, W. H. 1933. Am. Fertilizer 79,5-8, 24, 26-27. Prianishnikov, D. N. 1951. “Nitrogen in the Life of Plants” (translated by S. A. Wilde ) . Kramer Business Service, Madison, Wisconsin. Prince, A. B. 1954. Soil Scl. 78,445452. Remy, T. 1938. Soil Scl. 46,187-209. Rennie, D. A., and Mitchell, J. 1954. Can. J. Agr. Sci. 34, 35-83. Rennie, D. A., and Soper, R. J. 1958. J. Soil Sci. 9,155-187. Robertson, W. K., Smith, P. M., Ohlrogge, A. J., and Kinch, D. M. 1954. Soil Sci. 77,217-228. Russell, J. S., Bourg, C. W., and Rhoades, H. F. 1954. Soil Sci. SOC. Am. Proc. 18, 292-298. Russell, R. S. 1954. Symposia SOC. Exptl. Bwl. 8.343388. Russell, R. S., and Ayland, M. J. 1955. Nature 175,204-205. Russell, R. S., Rickson, J. B., and Adams, S. N. 1954. J. Soil Sci. 5, 85-105. Salter, R. M., and Barnes, E. E. 1935. Ohio Agr. Expt. Stu. Bull. 553. Sewell, M. C., and Latshaw, W. L. 1931. J. Am. SOC. Agron. 23, 799-814. Sewell, M. C., Latshaw, W. L., and Tague, E. L. 1927. Proc. 1st Intern. Congr. Soil sca. 2, 3-19. Skoog, F. 1954. In “Dynamics of Growth Processes” (E. J. Boell, ed.), pp. 148-182. Princeton Univ. Press, Princeton, New Jersey. Smith, J. C., Kapp, L. C., and Potts, R. C. 1950. Soil Sci. SOC. Am. Proc. 14, 241245. Smith, J. C., Fudge, J. F., Kapp, L. C., Adams, J. E., Pearson, R. W., and Leap, H., Jr. 1951. Soil Sci. SOC.Am. Proc. 15. 209-212. Starostka, R. W., and Hill, W. L. 1955. Soil Sci. SOC. Am. Proc. 19, 193-198. Steward, F. C., and Millar, F. K. 1954. Symposia Soc. Exptl. Biol. 8,387408.

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Steward, F. C., and Street, H. E. 1947.Ann. Rev. Biochem. 16,471-502. Teakle, L. J. H. 1928.Soil Sci. 25,143-162. Terman, G. L.,Bouldin, D. R., and Lehr, J. R. 1958. Soil Sci. SOC. Am. Proc. 22, 2529. Trogdon, W. O., and Volk, G. W. 1950. Soil Sci. SOC. Am. Proc. 14,218-220. Viets, F. G., Jr., Nelson, C. E., and Crawford, C. L. 1954. Soil Sci. SOC. Am. PTOC. 18,297401. Vittum, M . T., Lathwell, D. J., and Stanford, G. 1952. Soil Scl. SOC. Am. Proc. 16, 273-276. Volk, G. W. 1944.I. Am. SOC.Agron. 36,4856. Volk, N. J., and Tidmore, J, W. 1946.Soil Sci. 61,477-492. Wiklander, L. 1955. In “Chemistry of the Soil” (F. E. Bear, ed.), pp. 107-148. Reinhold, New York. Wild, A. 1950.J . Soil S C ~ I. , 221-238. Willis, A. J., and Yemm, E. W. 1955.New Phytologist 54,163-181. Yatazawa, M., Higashino, S., and Goto, Y. 1953. J. Sci. Soil Manure Japan 23, 121124. Yemm, E. W.,and Willis, A. J. 1956.New Phytologist 55, 229-252.

Author Index-Volume

XI

Numbers in italic indicate the pages on which the references are listed. Anderson, L. D., 291, 293, 327 A Aall, C.H., 271, 312 Anderson, M. S., 300, 312 Aasheim, T. S., 180, 226 Ando, J., 283,284,292, 312, 314 Ababi, V., 292, 328 Andrb, L. E., 304, 313 Abernathy, G. H., 220, 229 Andrews, W. B., 245, 250, 251, 252, 313 Abernathy, H. H., 220, 229 Andrzejewski, R., 294, 313 Angus, D. E., 34, 122 Achorn, F. P., 279, 281, 319, 321 Ackerman, E. A., 3, 8, 122 Angus, J., 284, 304, 306, 313, 324 Ackley, W. B., 66, 122 Appleman, C. O., 60, 122 Adam, H . W., 286, 312 Araujo, J. B. de., 285, 314 Adams, D., 381, 395 Archer, J. C., 113, 128 Adams, F., 117, 122 Arend, C. A., Jr., 293, 332 Adams, J. E., 369, 395 Argall, G. O., Jr., 268, 314 Adams, ,J. R., 237, 240, 245, 250, 251, Arland, A., 56, 122 252, 253, 255, 256, 257, 259, 268, 275, Armer, A. A., 218, 227 278, 277, 300, 304, 312, 325, 328, 332 Armiger, W. H., 261, 267, 270, 283, 284, Adams, R. M., 248, 312 285, 314, 321 Adams, S. N., 377, 395 Arms, M. F., 224,227 Adrian, P. A., 220, 226 Armstrong, E. F., 290, 291, 314 Akesson, N. B., 192, 226 Armstrong, G., 289, 314 Arnell, A. F., 310, 314 Albrecht, W. A., 292, 320 Albrecht, W. L., 283, 312 Arnold, B. P., 265, 320 Aldrich, D. G., Jr., 254, 312 Arnold, W. W., 276, 304, 331 Aldrich, S. R., 176, 178, 226, 230 Arnon, D. I., 369, 377, 389, 393 Aldrich, W . W., 53, 54, 61, 122, 129 Arvan, P. G., 275, 308, 314, 323 Allen, 0. W., 268, 320 Arya, S . V., 188, 227 Allen, R. L., 189, 228 Asaoka, K., 292, 314 Allen, R. M., 54, 122 Ashbel, D., 35, 122 Allgood, H. Y., 248, 318, 329 Ashby, E., 63, 68, 122 Allison, F. E., 390, 393 Ashley, D. A., 301, 321 Allmendinger, D. F., 58, 122 Ashton, F. M., 58, 122 Allsman, P. T., 289, 329 Ashton, J. T., 203, 230 Almond, L. H., 273, 283, 312 Ashton, T., 105, 123 Altschuler, 2. S., 265, 268, 324, 332 Atkin, D., 268, 317 Alvim, P. de T., 69,122 Atwell, J., 273, 278, 314 Amberger, A., 284, 321 Atwood, G. E., 290, 293, 314, 320 Anderson, A. J., 302, 312 AutTy, J. W., 184, 227 Anderson, D. B., 62, 67, 127 Avery, G. S., 384, 393 Anderson, E. R., 31, 122 Avery, W. M., 266, 314 Anderson, J. F., Jr., 281, 282, 286, 316, Ayers, A. D., 60, 130 324, 327 Aylsnd, M. J., 383, 395 397

398

AUTHOR INDEX-VOLUME

B Baber, R. L., 245, 248, 323 Baggette, T. L., 193, 227 Bailes, R. H., 268, 324 Bailey, R. W., 116, 117, 123 Bainer, R., 190, 194, 199, 200, 202, 204, 216, 227, 228 Baird, B. L., 159, 167 Bakke, A. L., 60, 123 Balakrishnan, M. R., 284, 325 Ballard, L. A., 57, 123 Barnforth, A. W., 258, 314 Banning, L. H., 268, 283, 314, 320 Barabanova, A. S., 292, 329 Barber, J. C., 268, 271, 316 Barger, E. L., 190, 194, 199, 200, 202, 216, 227 Barmington, R. D., 216, 227 Barnes, E. E., 392, 395 Barnes, K. K., 200, 227 Barnes, V. E., 265, 314 Baroch, C.T., 291, 314 Barr, C. G., 54, 123 Barr, H. T., 251, 314 Barr, J. A., 266, 331 Barr, J. A., Jr., 267, 268, 291, 293, 314, 315 Barry, G. L., 93, 126 Bartlett, H. D., 215, 227 Bartz, M. H., 251, 315 Batchelder, H. R., 248, 329 Batson, H. E., Jr., 271, 276, 277, 300, 316, 319, 321 Bauer, F. C., 369, 394 Baugh, E. R., 176, 227 Baule, B., 138, 167 Baum, E. L., 167, 167, 250, 252, 320 Baumann, H., 46, 123 Bedwell, W. L., 299, 315 Been, W. G., 212, 227 Beerwald, A., 291, 315 Bell, F. G., 113, 116, 123, 125, 128, 129 Bell, J., 257, 315 Bell, R. E., 271, 331 Benedict, S. H., 309, 315 Bennet, W.F., 369, 393 Bennett, G. A., 295, 297, 325 Benson, H. E., 299, 315 Bernstein, L., 49, 123 Bertrand, A. R., 49, 123, 126

XI

Bevilacqua, C. K., 266, 315 Bienstock, D., 299, 315 Biggar, J. W . , 79, 123 Bigsby, F. W., 206, 227 Billinghurst, W. M., 284, 315 Bishop, T., 252, 315 Bixler, G. H., 271, 301, 315 Black, C. A., 137, 165, 167, 372, 374, 384, 389, 390, 393, 394 Black, D. T., 194, 195, 227 Blackmore, J., 167, 167 Blair, A. E., 261, 314 Blaney, H. F., 27, 105, 123 Bloch, M. R., 291, 315 Bloodworth, M. E., 43, 59, 69, 123 Boawn, L. C., 302, 315 Bockhop, C. W., 200, 227 Boggess, W. R., 66, 131 Bone, W . R., 308, 315 Bonner, J., 61, 123 Bonnier, C., 46, 125 Bonython, C. W., 26, 123 Borland, J. W., 286, 315, 324 Borlik, R. F., 267, 268, 315 Borst, H. L., 113, 123 Bosemarck, N. O., 48, 123, 372, 384, 393 Boswell, S. B., 254, 312 Boswell, V. R., 216, 227 Bouldin, D. R., 280, 330, 370, 387, 392, 393, 306 BoullB, A., 280, 315 Bourdeau, P., 58, 123 Bourdier, J. A., 275, 315 Bourg, C. W., 369, 380, 395 Bourne, D. J., 290, 314 Bouse, F., 224, 231 Bouyer, S., 266, 315 Bowen, H. D., 186, 228, 252, 315, 319 Bowen, I . S., 32, 123 Box, G . E. P., 159, 167 Boyd, T. A., 262, 317 Boylan, D. R., 283, 287, 315 Bradley, J. W., 245, 315 Bragg, C. G., 258, 315 Brater, E. F., 115, 131 Brautigam, G. F., Jr., 308, 323 Bray, R. H., 164, 167 Brazee, R. D., 194, 227 Brazelton, R. W., 221, 229

AUTHOR INDEX-VOLUME

Breazeale, J. F., 49, 123 Breen, A. V., 276, 319 Breon, W. S., 369, 377, 393 Bridger, G. L., 271, 275, 276, 277, 278, 280, 283, 287, 304, 315, 331 Brien, E. B., 260, 315 Brilliant, B., 57, 58, 123 Brind, W. D., 176, 229 Brittain, R. W., 194, 227 Brodell, A. P., 204, 227 Brook, A. T., 306, 315 Brosheer, J. C., 268, 269, 281, 283, 286, 315, 316 Brown, B. E., 255, 260, 270, 316, 325, 332 Brown, C. O., 255, 318 Brown, E. H., 274, 281, 316 Brown, M. H., 391, 393 Brown, N. A., 282, 330 Brown, P. E., 390, 395 Brown, P. M., 265, 316 Brown, R. G., 60, 122 Brown, R. T., 175, 227 Brown, W. E., 274, 279, 281, 316, 329 Brown, W . G., 147, 148, 149, 158, 161, 163, 167, 168 Browne, C. A,, 134, 167 Browning, G. M., 113, 123, 174, 227 Broyer, T. C., 61, 123 Bruhn, H. H., 290, 316 Brumby, P. R., 266, 316 Brunn, L. W., 299, 315 Brusca, J. N., 261, 320 Buchanan, J., 254, 312 Buchanan, J. R., 254, 312 Buchele, W. F., 176, 180, 227 Buchner, A., 280, 316 Buck, K. L., 285, 324 Buehrer, T. F., 370, 385, 390, 393 Buhr, A. G., 187, 227 Bunnelle, P. R., 206, 227 Burd, J. S., 390, 393 Burgesser, F. W., 190, 227 Burkhardt, G. J., 195, 224, 227, 232 Burkholder, P. R., 384, 393 Burleson, C. A., 43, 123 Burnet, F. E., 266, 322 Bumet, G., Jr., 279, 316 Burnett, J. A., 248, 316 Burns, J. J., 255, 256, 316

XI

399

Burns, R. E., 66, 123 Burr, R., 291, 316 Burrough, D. E., 199, 227 Burstrom, H., 61, 123 Burt, R. B., 248, 266, 268, 271, 275, 277, 315, 316, 331 Burton, G. W., 48, 56, 57, 63, 109, 123, 128, 161, 169, 292, 322 Bunvell, H. B., 265, 316 Burzlaff, H. A., 304, 315 Butchart, A., 258, 316 Butlin, K. R., 299, 316 Butt, J. L., 199, 227 Byrom, M. H., 225, 227

C Caldwell, A. C., 369, 394 Callaham, J. R., 271, 316 Callister, G. J., 284, 318 Cambon, T., 260, 316 Campbell, A. J. R., 256, 316 Campbell, A. N., 256, 316 Campbell, J. U., 271, 327 Campbell, W. C., 260, 261, 328 Cantelou, M. B., 273, 329 Capdecomme, L., 266, 268, 316 Carbona, F., 270, 316 Carleton, E. A., 37, 124 Carleton, W. M., 173, 192, 194, 227, 229, 230 Carlton, W. M., 48, 123 Caro, J. H., 270, 271, 280, 286, 287, 300, 316, 321, 332 Caro, R. J., 268, 316 Carpenter, C. B., 255, 323 Carreker, J. R., 172, 227 Carrero, J. O., 370, 394 Carroll, D. M., 294, 532 Carroll, F., 267, 317 Carter, H. O., 161, 168 Carter, R. L., 48, 123 Cathcart, J. B., 265, 316, 324 Chafetz, A. B., 290, 316 Chamberlin, J. C., 196, 227, 232 Chandler, W . V., 369, 394 Chang, S. C., 286, 316 Chapman, H. D., 369, 390, 393 Chatterjee, S. K., 292, 317 Chepil, W. S., 180, 227, 232 Chetrick, M . H., 248, 326

400

AUTHOR INDEX-VOLUME

Chichilo, P. P., 282, 300, 316 Childers, N. F., 58, 60, 125, 126, 129 Chisholm, J . A., 213, 227 Chittey, E. T., 193, 230 Christensen, J. H., 273, 316 Chronister, B. S., 252, 316 Clark, J,, 66, 123 Clark, J. A., 61, 123 Clark, K. G., 236, 261, 262, 280, 305, 306, 314, 316, 317, 319, 320, 329 Clark, L. J., 271, 300, 316, 317 Cleland, R., 61, 123 Clements, H. F., 65, 66, 123 Clements, L. B., 270, 273, 324 Clyde, A. W., 174, 175, 227, 231 Cochran, W . G., 159, 167 Cocke, J. B., 225, 231 Coker, M. G., 50, 123 Cole, H . L., 289, 317 Collins, E. V., 174, 176, 180, 227 Colman, E. A., 115, 117, 123 Colwick, R. F., 184, 207, 227 Comar, C. L., 54, 123 Comings, E. W., 248, 312 Comley, E. A., 248, 325 Conner, J. D., 309, 317 Conrad, J. P., 390, 393 Converse, C. D., 109, 129 Cook, L. H., 260, 317 Cook, M. A., 255, 256, 317 Cook, R. L., 176, 178, 228 Cooke, G. W., 372, 393 Cooper, A. W., 174, 228 Cooper, H. P., 301, 326 Cope, W. C., 245, 247, 317 Copley, T. L., 113, 123 Coppock, G. E., 224, 228, 231 Corbett, C. H . J., 284, 324 Corley, T. E., 184, 228 Cornes, J. J. S., 287, 317 Cottrell, F. G., 243, 317 Covey, W., 23, 125 Cowie, G. A., 287, 291, 293, 317 Cowles, L., 266, 316 Cowley, W. R., 43, 59, 69, 123 Cox, G. M., 159, 167 Cox, M. B., 113, 124 Craddock, G. W., 116, 117,123 Craddock, J. M., 34, 123 Crafts, A. S., 4, 67, 123

XI

Crago, A., 266, 317 Crammatte, F. B., 237, 309, 328 Craven, P., 287, 31 7 Crawford, C. L., 302, 315, 369, 396 Craxford, S. R., 258, 299, 317 Creamer, R. M., 310, 317 Creighton, H. B., 384, 393 Cressman, E. R., 265, 317 Criddle, W. D., 27, 123 Crider, F. J., 109, 123 Crittenden, E. D., 252, 254, 257, 304, 317, 320 Croft, A. R., 116, 117, 123 Cronan, C. S., 254, 317 Crossman, R., 283, 317 Crowe, G. A., 304, 329 Crowe, G. B., 250, 319 Crowther, F., 63, 124 Csema, Z. de, 265, 328 Cultrera, R., 290, 317 Cumings, J. G., 307, 320 Cummings, N. W., 31, 124 Curley, R. G., 204, 228 Currier, €I. B., 4, 67, 123 Curtis, H. A,, 271, 317 Curtis, R. W., 248,320 Cykler, J. F., 201, 228

D Daniel, H. A., 113, 124 Daniels, F., 243, 317, 319 Danielson, R. E., 82, 124 Darkis, F. R., GO, 124 Das Gupta, H . N., 292, 317 Dastur, R. H., 57, 58, 124 Datar, D. S., 292, 328 Datin, R. C., 253, 304, 317, 332 Davenport, C. H., 262, 317 Davenport, J. E., 267, 317 Davenport, S. J., 269, 317 Davidson, B., 19, 126 Davidson, C. F., 268, 317 Davidson, D. F., 265, 320 Davidson, J. M., 24, 126 Davies, G. R., 305, 318 Davies, K. A,, 266, 318 Davis, F. L., 369, 390, 391, 393 Davis, J. F., 158, 169 Davis, L. E., 49, 129

AUTHOR INDEX-VOLUME

Davis, M. M., 237, 328 Davis, R. E., 297, 318 Davis, R. 0.E., 256, 318 Dayton, S. H., 266, 318 Deacon, E. L., 19, 124 Dean, L. A., 82, 124, 137, 167, 379, 390, 393,395 Dee, T. P., 275, 307, 318, 326, 332 DeFrance, J . A., 261, 325, 332 Degman, E. S., 53, 60, 126 Dehm, J. E., 369, 394 Delaney, C. F. G., 268, 318 Delgass, B. W., 267, 318 DeMent, J. D., 284, 285, 318 Deming, M. E., 273, 274, 329, 330 Demmerle, R. L., 275, 318 Dennison, R. A., 66, 124 DeVane, E. H., 48, 123 de Vries, D. A., 27, 28, 124, 130 De Wit, C. T., 371, 304 Dickens, J. W., 214, 230 Dickens, P., 282, 318 Dieffenbach, E. M., 172, 228 Dion, H. G., 284, 318, 369, 394, 395 Ditcham, J. R., 305, 318 Ditman, L. P., 195, 227 Dixon, L. F., 60, 124 D’Leny, 281, 318 Dobie, J. B., 201, 228 Doenier, C. E., 196, 232 Dokken, M. N., 255, 325 Dole, F. H., 279, 330 Dolique, R., 293, 318 Doll, E. C., 162, 167 Doll, J. P., 149, 154, 161, 164, 167, 168, 169 Domby, C. W., 369, 394 Domingo, C. E., 83, 85, 128 Dondes, S.,244, 320 Doneen, L. D., 50, 91, 93, 124, 129 Dorsey, J. J., Jr., 256, 318, 321 Dorsey, N . E., 4, 124 Dougherty, J. L., 266, 318 Drake, M., 381, 394, 395 Dreibelbis, F. R., 24, 115, 125, 126, 177, 228 Dreier, A. F., 153, 168, 369, 377, 378, 395 Driskell, J. C., 258, 299, 330 Drobot, W., 275, 315

401

XI

Drummond, A. J., 35, 124 Dudley, R. F., 189, 228 Dugger, W. M., 54, 124 Duich, J . M., 261, 325 Duke, G. B., 214, 228 Duley, F. L., 177, 228 Dumenil, L. C., 164, 168, 369, 393 Duncan, E. S., 266, 318 Duncan, R. A., 223, 228 Duncan, W. G., 48, 49, 124, 369, 371, 395 Dunham, K. C., 289, 314 Dunlap, J. W., 266, 318 Dunn, R. E., 281, 322 Dunne, T. C., 302, 318 Dunwoody, W. B., 255, 329 Dupont, M., 280, 315 Duvdevani, S., 34, 124 Dyer, A. J., 30, 129 Dyer, B. W., 289, 318 Dyrenforth, W. P., 266, 318

E Easterwood, H. W., 272, 318 Eaton, F. M., 60,124 Eckstein, R. R., 275, 314 Edelman, C. H., 339, 368 Edminster, T. W., 176, 177, 229, 230 Edwards, 0. W., 274, 318 Egan, E. P., Jr., 260, 273, 274, 281, 286, 316, 318, 330 Eickmeyer, A. G., 247, 318 Eid, M. T., 137, 165, 168 Elfes, L. E., 198, 228 Elgabaly, M. M., 49, 124 El-Gindi, D. M., 50, 127 Elliott, B. G., 201, 229 Elliott, M. A., 256, 318 Ellis, D. A., 268, 324 Elmore, G. V., 253, 286, 318, 329 Elwell, H . M., 113, 124 Elze, D. L., 53, 69, 127 Embleton, T. W., 261, 323 Emigh, G. D., 266, 318 Engel, A. J., 300, 301, 321 Eno, C. F., 292, 318 Enomoto, T., 292, 323 Ensgraber, A., 57, 124 Ergle, D. R., 60, 124

402

AUTHOR INDEX-VOLUME

Erickson, A. E., 174, 228 Erickson, C. J., 294, 310, 326, 332 Ermenc, E. D., 243, 318 Ettle, G. W., 258, 318 Evans, C. E., 10, 111, 112,124, 180,169 Evans, D. D., 178, 230 Ewing, P. A,, 117, 122

F Fairbank, J. P., 208, 228 Fairbanks, G. E., 192, 228 Fajans, E. W., 300, 326 Farden, C. A., 139, 168 Farnham, S. E., 194, 228 Farr, T. D., 289, 271, 318, 330 Farrow, R., 213, 228 Feeley, J. C., Jr., 285, 318 Fehrenbacher, J. B., 47, 124 Feick, G., 258, 318 Felio, H. G., 255, 318 Ferguson, J., 29, 124 Ferguson, J. C., 252, 315 Filbert, R. B., Jr., 278, 305, 327 Fine, L. O., 389, 372, 394 Fine, M. M., 285, 267, 319 Fireman, M., 18, 74, 78, 124, 130 Fisher, A. D., 257, 319 Fisher, T. R., 302, 308, 319 Fiskel, J. G. A., 49, 128 Fitts, J. W., 159, 167 Flaschenberg, A., 288, 319 Fleck, A., 289, 319 Fleck, E. E., 310, 319 Fleischer, A., 291, 319 Fleury, P., 270, 319 Flowerday, A. D., 389, 378, 395 Floyd, C. E., 309, 319 Folckemer, F. B., 309, 310, 319 Forbath, T. P., 297, 319 Forbes, I., Jr., 261, 314 Ford, H. W., 48, 124 Forrest, L. A., 113,123 Forsee, W. T., Jr., 302, 323 Fort, W. R., 273, 277, 322 Forward, F. A., 258, 319 Fox, E. D., 258, 319 Foster, E. G., 243, 319 Fox, E. I., 237, 306. 328 Fox, E. J.; 267; 289; 278, 278, 286, 319, 321,332

XI

Fox, R. L., 43, 44, 47, 48, 49, 124, 126 Fox, W . E., 28, 126 Francia, I., 273, 826 Francis, W., 258, 299, 319 Franklin, C. E., 252, 308, 319 Franklin, R. E., 381, 395 Fratzke, W. E., 389, 393 Frazier, A. W., 281, 816 Frear, G . L., 304, 320 Frederick, I. B., 310, 332 Free, G. R.,37, 124, 177, 228 Freeland, R. O., 54, 124 Freeman, H. P., 305, 317 French, B. L., 141, 142, 143, 144, 145, 180, 168, 169 French, G. W., 215, 228 Fridley, R. B., 220, 226 Fried, M., 137, 167, 270, 314, 379, 393 Frisken, J., 273, 277, 327 Frommer, D. W., 285, 287, 319 Fudge, J. F., 389, 370, 391, 394, 395 Fujiwara, A., 288, 319 Fulgham, F. E., 208, 231 Fuller, W. H., 281, 319 Furr, J. R., 53, 126 Futral, J. G., 189, 228 G Caddy, V. L., 281, 282, 317 Cadre, G. T., 280, 287, 319 Gaines, J. P., 250, 319 Gantt, C. W., Jr., 186, 228, 252, 319 Gardner, R., 390, 394 Gar& L., 290, 319 Garner, W. W., 81, 124 Garnier, B. J., 25, 124 Garrison, 0. B., 215, 230 Garstka, W. U., 17, 127 Gartaganis, P. A., 273, 322 Carver, H. L., 81, 127 Gaston, H. P., 221, 222, 228, 229 Gates, C. T., 55, 60, 62, 83, 124 Gauch, H. G., 49, 81, 82, 130 Gaudin, A. M., 291, 319 Geiger, R., 22, 23, 124 Gerge, 0. K., 178, 230 Gericke, S., 282, 288, 319 Gessner, F., 59, 124 Getsinger, J. G., 279, 319

AUTHOR INDEX-VOLUME

Getzendaner, C. W., 196, 227, 232 Ghani, A. M . A., 49, 124 Gibbs, R. D., 66, 123, 124 Gilbert, M . J., 25, 124 Gilbert, N., 243, 269, 319, 320 Gile, P. L., 370, 394 Giles, G. W., 199, 228 Gill, W. R., 46, 124 Gillam, W. S., 369, 377, 393 Gilpin, W. C., 297, 331 Gingrich, J. R., 45, 79, 80, 81, 124 Givens, R. L., 215, 231 Glaser, A. €I., 20, 126 Glasstone, S. G., 385, 394 Glaves, A. H., 215, 228 Gledhill, V. H., 82, 124 Glover, J., 369, 394 Gooch, S. D., 267, 284, 285, 321 Goodall, D. W., 369, 379, 394 Gordon, E. D., 173, 230 Gordon, J., 257, 271, 320 Gorlach, E., 294, 324 Goss, J. R., 204, 206, 227, 226 Gossett, J. G., 265, 320 Goto, Y., 370, 396 Goulben, B., 254, 312 Gowing, D. P., 261, 328 Graham, E. R., 292, 320 Graham, J. A., 199, 227 Granberg, W. J., 283, 320 Grant, R. L., 256, 328 Gray, F. J., 258, 299, 330 Gray, R. B., 172, 204, 211, 228 Greaves, W. S., 305, 318 Greek, B. F., 268, 320 Greene, R. E. L., 215, 230 Greenham, D. W . P., 46, 125 Gregory, F. G., 369, 379, 394 Grigsby, B. H., 178, 228 Grindrod, J., 244, 260, 299, 320 Gronhovd, C. H., 248, 326 Gross, P. M., 60, 124 Grossman, P. R., 248, 320 Groves, R. J., 203, 230 Grunes, D. L., 369, 370, 372, 373, 376, 377, 380, 990, 394 Grzymek, J., 294, 320 Grzywnowicz, J., 294, 313 Guelle, C. E., 186, 228 Gnillaumeron, P., 248, 320, 330

XI

403

Gunn, J. T., 176, 228 Cupta, J., 280, 319

H Haas, A. R. C., 261, 320 Haas, H. J., 48, 111, 125, 127, 371, 394 Hackwood, A. W., 291, 314 Haddock, J. L., 79, 93, 125, 129, 369, 394 Hader, R. J., 159, 168 Hader, R. N., 298, 328 Haertl, E. J., 300, 301, 308, 320 Hagan, R. M., 44, 50, 58, 60, 65, 74, 76, 77, 84, 86, 87, 89, 90, 91, 92, 105, 125, 130 Iiagen, C. E., 383, 384, 390, 394 Hagin, J., 46, 127 Hainer, R. M., 256, 318 Haines, H. W., Jr., 307, 320 IIaise, H. R., 109, 129, 369, 372, 394 Haley, L. E., 369, 394 Halkias, N. A., 27, 125 Hall, B. J., 213, 228 Hall, J. R., 248, 316 Hall, M. B., 283, 320 Hall, W. C., 66, 125 Haller, H. L., 310, 319 Halstead, M. H., 23, 31, 125 Halter, A. N., 161, 168 Hammers, F. L., 369, 394 Hammond, J. W., 269, 324 Hammond, L. C., 374, 394 Handlos, A. E., 274, 320 Hansen, C. M., 178, 186, 228 Hansen, H., 258, 324 Hanson, W. D., 158, 168 Hanway, J., 164,168 Harbeck, G. E., 30, 126 Hardesty, J. O., 260, 276, 280, 285, 304, 305, 306, 307, 310, 316, 319, 320, 323, 325, 327 Hardin, L. J., 269, 270, 324 Harkort, H. J., 282, 331 Harley, C. P.. 61, 127 Harle;, G. T., 290, 291, 293, 320 Harmond, J. E., 207, 225, 228, 229 Harper, J. C., II., 261, 325 Harris, D. G., 45, 125 Harris, F. J., 307, 320 Harris, R. A., 265, 320

404

AUTHOR INDEX-VOLUME

Harris, R. W., 220, 229 Harrison, G. J., 208, 228 Harrold, L. L., 24, 115, 125, 177, 228 Harteck, P., 244, 320 Harvey, C. O., 289, 314 Harvey, D. J., 173, 230 Harvey, E. W., 278, 304, 320 Harvey, W. A,, 192,226 Harward, M. E., 159, 168 Haseman, J. F., 288, 320 Haunschild, R. L., 279, 322 Hausenbuiller, R. L., 369, 394 Havis, J. R., 69, 122 Hawthorne, H. A., 80, 125 Hayward, H. E., 16, 124 Hays, 0. E., 113, 125, 177, 187, 229, 231 Heady, E. O., 146, 147, 148, 149, 158, 181, 187, 167,168,189,250, 252, 320 Hecht, W. J., Jr., 304, 320 Hedden, S. L., 222, 228 Hedman, C. L., 186, 229 Heil, F. G., 273, 276, 277, 279, 281, 326, 327, 329, 332 Hein, L. B., 248, 273, 307, 320, 321 Heinicke, A. J., 58, 125 Heising, L. F., 248, 321 Heitshu, D. C., 174, 205, 206, 229 Hely, F. W., 46, 125 Hemwall, J. B., 389, 394 Hendricks, S. B., 4,125, 258,328 Hendrickson, B. H., 27, 46, 54, 113, 125, 130 Henkens, Ch. H., 344, 368 Hennenberger, K. H., 284, 321 Henry, T. H., 254,322 Herndon, L. K., 298, 328 Herring, R. B., 183, 168 Hessig, H. H., 196, 227 Hester, A. S., 258, 321 Heyn, A. N. J., 61, 125 Hickock, R. B., 176, 227 Hicks, G. C., 276, 279, 281, 304, 307, 321, 327, 332 Hiehl, H. L., 81, 127 Higashino, S., 370, 396 Hightower, J. V., 290, 321 Hignett, T. P., 267, 289, 273, 279, 280, 281, 283, 285, 306, 307, 315, 321, 322, 324, 327, 330 Higson, G. I., 258, 299, 321

XI

Hild, J. H., 291, 321 Hildreth, C. G., 181, 165, 167, 167, 168 Hill, F. L., 221, 229 Hill, G. R., 58, 129 Hill, H. O., 113, 125 Hill, W. L., 287, 288, 289, 270, 271, 273, 277, 278, 279, 280, 283, 284, 285, 288, 287, 300, 301, 315, 316, 317, 319, 321, 322, 325, 370, 388, 387, 395 Hines, R. C., Jr,, 189, 229 Hoagland, D. R., 381,394 Hobbs, I. A., 269, 320 Hobbs, J. A., 110, 125 Hocking, J. G., 181, 168 Hoffman, W. M., 277, 305, 316, 317, 321 Hofhneister, G., 330 Hoffmeister, G., Jr., 257, 279, 306, 307, 308, 329 Hofmann, E., 284, 321 Hofmeister, H. J., 213, 229 Holden, E. R., 300, 301, 321 Hollingsworth, C. A., 269, 283, 284, 332 Holmes, L. J., 193, 229 Holmes, R. M., 27, 128 Holt, R. F., 369, 394 Hob, P., 266, 267, 321 Holzmnn, B., 31, 125, 130 Hooper, P., Jr., 268, 315 Hopkins, D. P., 306, 321 Hopkins, H. T., 383, 390, 394 Hopkins, R. B., 215, 229 Horn, W. R., 279, 321 Horner, C. K., 245, 249, 254, 260, 270, 271, 272, 273, 321 Horner, G. M., 113, 125 Horst, W. R., 291, 321 Hortenstine, C. C., 301, 321 Horzella, T. I., 280, 315 Hotop, W., 282, 331 Houston, E. C., 255, 267, 279, 281, 319, 321, 322, 328 Howard, T. E., 266, 322 Howe, R. S., Jr., 175, 229 Howell, W. C., 189, 229 Huang, T. H., 280, 284, 322 Huberty, M. R., 117, 122 Hudig, J., 344, 368 Hudson, E. P., 306, 313 Hudspeth, E. B., 184, 229 Huffman, E. O., 274, 286, 318

AUTHOR INDEX-VOLUME

Hughes, C. V. O., 266, 322 Huhti, A.-L., 273, 322 Hulburt, W. C., 174, 186, 189, 228, 229, 231, 252, 319 Hull, W. Q., 299, 322 Humphrey, E. N., 215, 230 Hunter, A. S., 44, 85, 86, 125 Hunter, F., 286, 322 Huntington, D. H., 215, 227 Hurlbut, L. W., 203, 224, 227, 229 Huschke, H. A,, 294, 322 Hustrulid, A., 369, 394 Hutchinson, G . E., 4, 125 Hutton, C. E., 49, 128, 158, 168

405

XI

J

Jenson, 0. F., 252, 262, 307, 322 Jezek, R. E., 225, 229 Johnson, B. L., 266, 322 Johnson, C. M., 369, 389, 390, 393, 394 Johnson, G. L., 158, 168, 169 Johnson, H. S.,Jr., 273, 324 Johnson, M. R., 310, 319 Johnson, P. R., 113, 128 Johnson, R. M., 279, 330 Johnson, W. B., 216, 229 Johnson, W. H., 176, 188, 229, 231 Johnston, J. R., 110, 125 Jones, D. L., 184, 229 Jones, G. W., 255, 256, 316, 323 Jones, I. D., 53, 125 Jones, G. H. G., 284, 322 Jones, J. N., Jr., 189, 229 Jones, L. G., 84, 86, 87, 125, 206, 227 Jones, R. A., 273, 322 Jones, R. J., 234, 323 Jones, R. M., 245, 248, 323 Jones, T . M., 258, 269, 273, 299, 329, 330 Jones, T. N., 189, 229 Jones, V. J., 267, 321 Jones, W. W., 261, 323 Jordan, J. E., 279, 307, 321, 327 Jurkowska, H., 294, 324

Jacks, G. V., 176, 229 Jackson, A. S., 275, 322 Jackson, J. E., 56, 57, 63, 109, 123, 292, 322 Jackson, M. L., 286, 316, 369, 394 Jackson, P. C., 383, 384, 394 Jackson, W. A., 267, 270, 276, 280, 319, 326 Jacob, A., 296, 322 Jacob, K. D., 236, 237, 240, 259, 263, 265, 268, 269, 270, 273, 275, 276, 277, 283, 300, 308, 309, 312, 316, 319, 321, 322, 325 Jacobs, H. H., 266, 318 Jacques, W. A., 371, 394 James, R. L., 245, 315, 325 Jameson, R. F., 286, 322 Jamison, V. C., 46, 125 Jayaraman, N., 287, 322 Jennings, E. G., 34, 125 Jenny, H., 376, 394 Jensen, D., 165, 168

Kadey, F. L., Jr., 256, 323 Kaplan, A., 279, 330 Kapp, L. C., 369, 370, 395 Kapusta, E. C., 275, 315 Kalin, S. H., 309, 331 Karlovsky, J., 284, 323 Kastens, M. L., 244, 323 Kamps, T. W., 248, 326 Kanazawa, T., 260, 326 Katayama, N., 292, 323 Katell, S., 248, 329 Kaufman, A., 369, 394 Kaufman, J. T., 256, 321 Kaupke, C. E., 220, 229 Kawamura, F., 292, 323 Kearney, J. H., 298, 323 Kearns, J. L., 275, 276, 315 Kegel, W., 285, 323 Kelley, 0. J., 44, 85, 86, 93, 109, 125, 129, 390, 394

I Ibach, D. B., 160, 166, 168 Ihrig, H. K., 248, 322 Iliff, J. E., 307, 322 Iljin, W. S., 61, 70, 72, 125 Ino, S., 292, 323 Inoue, G., 292, 322 Inskeep, G. C., 254, 273, 277, 322 Isaac, I., 50, 125 Isler, D. A., 196, 229 Istok, A., 266, 326

K

406

AUTHOR INDEX-VOLUME

Kelley, W. B., 199, 227 Kelso, T. M., 267, 281, 285, 321 Kemp, J. C., 175, 231 Kempthorne, O., 137, 164, 165, 168 Kennedy, R. E., 30,125 Kenworthy, A. L., 58, 122 Kepner, R. A., 190, 194, 199, 200, 202, 211, 216, 227, 229 Ketzlach, N., 258, 323 Kiaer, T., 299, 323 Kiesselbach, T. A., 107, 109, 125, 127 Kilby, W . W., 225, 229 Kinch, D. M., 192, 229, 370, 395 King, K. M., 24, 126 King, R. W., 201, 229 Kintz, C. M., 255, 323 Kittredge, J., 117, 126 Klauss, M. L., 46, 123 Klein, C. F., 248, 332 Klein, L. M . , 207, 225, 228, 229 Kleis, R. W., 199, 230 Klempt, W., 257, 258, 323 Kline, M. H., 289, 329 Kmock, H. G., 44, 48, 126 Knecht, G. B., 254, 323 Knetsch, J. L., 158, 161, 163, 168 Knickerbocker, R. C., 291, 314 Kohl, H. C., Jr., 294, 324 Koch, W., 56, 126 Kodama, K., 292, 330 Koehler, F. E., 43, 126, 369, 394 Kofranek, A. M., 294, 324 Kohler, M. A,, 26, 126 Kollett, J. R., 261, 332 Kohnke, H., 24, 49, 123, 126, 176, 227 Kothler, F. E., 44, 48, 126 Kozlowski, T. T., 58, 60, 86, 126, 131 Krall, J. L., 180, 189, 229 Kralovec, R. D., 261, 262, 323, 325 Kramer, H . P., 19, 126 Kramer, P. J., 54, 64, 66, 126, 131 Kramer, R. W., 175, 229 Krantz, B. A., 111, 127, 369, 380, 394 Kretschmer, A. E., Jr., 302, 323 Krishnaswami, K. R., 287, 322 Kristjanson, A. M., 369, 395 Kromer, 0. W., 194, 229 Krueger, H. R., 252, 323 Kube, W. R., 248, 326 Kubik, B. M., 212, 213, 229

XI

Kubota, T., 65, 66, 123 Kiihnel, W., 290, 323 Kuenen, P. H., 7, 126 Kumagai, R., 276, 304, 305, 307, 319, 320, 323 Kummer, F. A., 184, 228 Kurmies, B., 282, 323 Kuroi, T., 260, 326 Kursanov, A. L., 70, 72, 126 Kurtz, L. T., 390, 394 Kushnian, L. J., 215, 230 Kwate, B., 294, 324

L Laisn6, C., 68, 126 Lamb, J., 37, 124 Lambert, C. B., 139, 168 Lamer, M., 287, 323 Lamont, T. G., 262, 310, 317 Lamouria, L. H., 220, 229 Landis, W. S., 278, 323 Lang, E., 134, 153, 168 Langbein, W. B., 11, 126 Lange, F., 307, 320 Langguth, R. P., 275, 308, 314, 323 Larson, M . A., 287, 315 Lathrope, H. R., 252, 323 Lathwell, D. J., 390, 396 Latshaw, W. L., 390, 395 Lattig, R. M., 271, 315 Lavi, J., 268, 319 La Viola, F. S., 261, 327 Leap, H., Jr., 369, 370, 395 Leavitt, F. H., 250, 323 Le Comec, J., 263, 323 Leeper, R. W., 261, 328 Lees, C. M., 289, 323 Legal, C. C., Jr., 273, 323 Leggett, J. E., 383, 384, 394 Lehr, J. J., 386, 394 Lehr, J. R., 257, 274, 279, 280, 281, 286, 306, 307, 316,320, 329, 330, 392, 396 Lehrecke, H. W., 285, 286, 323 LemBe, C., 88, 126 Lemon, E. R., 10, 20, 111, 112,124,126 Lenaeus, G. A,, 255, 325 Lenfesty, F. A., 281, 286, 316 Lenhart, W. B., 266, 267, 323 Lenoble, A., 268, 323 Lentz, T. H., 267, 324

AUTHOR INDEX-VOLUME

Leonard, S. H., 220, 229 Leonardi, M. L., 290, 301, 324 Leopold, L. B., 116, 126 Lepper, G. H., 258, 299, 319 Leroy, A., 280, 324 Lesser, M. A., 308, 324 Letey, J., 65, 126 Lettau, H. H., 19, 126 Levin, J. H., 221, 222, 228, 229 Levitt, J., 61, 70, 72, 73, 123, 126 Lewis, B., 255, 256, 316 Lewis, H. T., Jr., 269, 330 Lewis, J. S., Jr., 277, 281, 307, 327 Lewis, J. T., 257, 324 Leyerer, G., 58, 129 Liljedahl, J. B., 192, 229 Lillard, J. H., 176, 177, 189, 229, 230 Lin, K. H., 280, 315 Lipps, R. C., 43, 47, 49, 124, 126 Lipscomb, R. W., 49, 128 Lityhski, T., 294, 324 Livingston, B. E., 27, 126 Lof, G . 0. G., 3, 8, 122 Loeffler, E. S., 309, 310, 319 Loehwing, W. F., 66, 126 Long, A. E., 265, 324 Long, M. I. E., 262, 326 Long, R. S., 268, 324 Loomis, W. E., 58, 61, 126, 130 Lorenz, 0. A., 369, 390, 394 Lorenzen, C., Jr., 213, 216, 229 Loughman, B. C., 383, 394 Loustalot, A. J., 58, 60, 126 Love, K. S., 255, 256, 261, 262, 294, 316, 328, 332 Lovely, W. G., 176, 180, 227, 229 Lowe, J. N., 299, 324 Lowell, W. R., 265, 324 Lowrey, G. W., 369, 378, 395 Luckhardt, R. L., 308, 324 Luders, R., 369, 394 Luff, B. B., 273, 274, 316, 318 Lundstrom, F. O., 261, 262, 314, 317 Lundy, H. W., 49, 128 Lunt, 0. R., 294, 324 Lutz, J. F., 44, 126 Lyle, G. L., Jr., 266, 324

M hicBirney, S. W., 216, 221, 227, 229

XI

407

MacBride, J. E., 284, 285, 329 McBurney, W. G., 244, 323 AlcCall, A. G., 113, 116, 123, 125, 128, 129 hlcCalla, T. M., 176, 229 McColly, H. F., 172, 173, 174, 178, 210, 228, 229 McCamy, I. W., 255, 279, 328,330 McCool, M. M., 262, 324 McCreery, W. F., 175, 229, 230 McCullough, C. W., 248, 257, 260, 324 McDivitt, J. F., 265, 324 McGarry, P. E., 266, 312 McGeorge, W. T., 49, 123 McIlroy, I. C., 19, 30, 126 MacIntire, W. H., 269, 270, 271, 273, 297, 324 McIntyre, C. K., Jr., 302, 308, 319 McIntyre, G. H., 300, 324 Mackall, J. N., 275, 324 McKelvey, V. E., 265,267,268,324, 330 MacKenzie, A. J., 286, 315, 324 McKibben, E. G., 172, 229 Mackie, A. B., 161, 169 McKnight, D., 271, 273, 279, 281, 308, 321, 324, 330 McLean, E. O., 381, 394 Maclennan, W. H., 284, 324 McLeod, H. M., Jr., 271, 315 McMurtrie, R., 248, 326 McNally, R. J., 273, 277, 324 Maddock, T., Jr., 116, 126 Madigan, J. E., 307, 324 Magistad, 0.C., 61, 130, 139, 168 Magness, J. R., 53, 60, 126 Magness, R. M., 278, 332 hlahoney, C. H., 216, 230 Majewski, F., 258, 324 hlakkink, G. F., 20, 126 Manil, P., 46, 125 Manning, J., 299, 324 hlansfield, G. R., 265, 324 Mansfield, W. W., 17, 126 Mantel, W., 258, 324 Manuel, G. D., 93, 126 Marciano, J. J., 30, 126 Marcum, W. B., 93, 126 hlariakulandai, A., 284, 325 Marin, A., 287, 290, 325 hlarkey, J. W., 277, 287, 315

408

AUTHOR INDEX-VOLUME

Marriott, L. F., 389, 394 Marshall, H. L., 288, 270, 285, 322, 335 Marshall, W . H., Jr., 247, 318 Martell, A. E., 300, 320 Martenet, J. S., 307, 325 Martens, P., 270, 325 Marth, P. C., 310, 325 Martin, C. M., 199, 227 Martin, D. V., 266, 285, 325 Martin, H. S., 288, 325 Martin, J. W., 215, 230 Martin, P. T., 248, 325 Martin, W. M., 71, 127 Mason, D. C., 158, 168 Mason, D. D., 159, 168 Massee, T. W., 180, 229 Mather, J. R., 20, 25, 105, 127, 130 Matthew, C. J., 245, 325 Matthews, C. M. E., 268, 318 Maughan, G. L., 193, 230 Maune, H. E., 248, 329 Maximov, N. A., 53, 70, 127 May, V., 83, 122 Mayer, I. D., 178, 227 Mayland, B. J., 248, 325 Megar, G. H.,271, 273, 320, 330 Mehlich, A., 381, 395 Mehring, A. L., 237, 240, 241, 242, 255, 259, 274, 294, 295, 298, 297, 300, 309, 325, 828 Meijer, C., 344, 368 Meijers, P. G., 368 Meinzer, 0. E., 115, 127 Meline, R. S., 287, 281, 285, 321, 332 Melsted, S. W., 178, 230 Mendel, K., 20, 127 Mendum, S. W., 180, 168 Menzel, R. G., 174, 229 Merrill, R. M., 188, 230 Men, A. R., 280, 325 Messing, R. F., 245, 315, 325 Meyer, B. S., 82, 87, 127 Miall, L. M., 290, 291, 314 Miche, R., 287, 325 Migrdichian, V., 310, 332 Mikula, K., 294, 313 Millar, F. K., 383, 395 Miller, E. C., 53, 58, 80,,87, 127 Miller, E. H., 290, 316, 325 Miller, H. F., Jr., 184, 208, 230, 231

XI

Miller, J. B., 211, 230 Miller, M. H., 389, 371, 395 Miller, P., 255, 279, 325, 330 Miller, R. D., 48, 124 Mills, W. T., 214, 230 Minor, W. A,, 245, 325 Mitchell, J., 389, 390, 394, 395 Mitchell, J. W., 54, 58, 82, 127, 310, 325 Mitchell, W. A., 307, 325 Mitchelson, A. T., 15, 127 Mitscherlich, E. A., 138, 137, 168 Mittasch, A,, 244, 325 Miyabe, T., 292, 326 Modreanu, F., 292, 328 Mohsenin, H.,173, 230 Monselise, S. P., 48, 127 Monteith, J. L., 33, 34, 125, 127 Montgomery, E. G., 109, 127 Montgomery, R. A., 208, 232 Moody, J. E., 178, 177,229, 230 Moore, D. P., 159, 168 Moore, J. W., 271, 315 Moore, R. P., 206, 230 Moran, W. T., 17, 127 Morgan, W. A., 261, 262, 323, 325 Morgis, G. G., 289, 317 Moriceau, L., 288, 284, 325 Morin, K. V., 105, 123 Morningstar, E., 298, 328 Morris, 0. M., 81, 127 Moms, T. M., 291, 321 Morrison, C. S., 204, 230 Mothes, K., 61, 127 Moulton, F. R., 136, 168 Moulton, R. W., 283, 325 Mowry, D. T., 308, 314 Moxham, R. M., 265, 287, 325 Mruk, C. K., 281, 325 Muckel, D. C., 15, 127 Mulder, E. G., 344, 368 Mullins, J. F., 288, 321 Murdock, S. T., 178, 230 Murphy, E. M., 299, 315 Musgrave, G. N., 39, 127 Musgrave, R. B., 178, 227, 230 Musser, H . B., 281, 325

N Naftel, J. A., 300, 326 Nagai, S., 280, 292, 322, 326

AUTHOR INDEX-VOLUME

Nagisi, K., 91, 127 Nakagawa, M., 292, 323, 330 Nelson, C. E.,83, 128, 369, 396 Nelson, E. L., 304, 331 Nelson, J. L., 369, 394 Nelson, J. M., 267, 268, 324 Nelson, L. B., 166, 168 Nernst, W., 374, 395 Neuman, J., 33, 127 Neumann, J., 21, 127 Newton, R., 71, 127 Nichols, M. L., 174, 175, 230 Nicholson, D. S., 284, 315 Nicholson, R. P., 161, 169 Nielsson, F. T., 276, 278, 281, 304, 326, 332 Nightingale, G. T., 62, 127, 372, 381, 395 Nikitin, A. A., 299, 300, 302, 326 Nixon, A. C., 274, 320 Noecker, N. L., 60, 123 Norden, R. B., 248, 258, 280, 302, 326 Nordengren, R., 273, 326 Nordengren, S., 273, 275, 326 Nordenson, T. J., 26, 126 Nordyke, L., 290, 326 Norland, M. A., 270, 280, 284, 285, 310, 326, 329 Norman, A. G., 374, 394 Northrup, M. A., 304, 320 Norton, J. S.,215, 230 Norton, M. M., 273, 277, 327 Norton, R. A., 113, 123 Norum, E. B., 111, 127 Novomeysky, M. A., 291, 326 Nunn, R. J., 275, 318, 326 Nutman, F. J., 53, 58, 127 Nutt, G. B., 172, 178, 230

409

XI

Okada, Y., 292, 326 Oleksynowa, K., 294, 326 Ollagnier, M., 266, 285, 327 Olsen, S. R., 270, 286, 326, 370, 385, 386, 388, 389, 390, 395 Olson, R. A,, 153, 168, 369, 377, 378, 395 Ongstad, 0. C., 248, 326 Oppelt, W. H., 248, 326 Oppenheimer, H. R., 53, 65, 67, 69, 127 Orazem, F., 161, 163, 168 Osborn, R. R., 31, 127 O d e , B., 148, 169 Oteifa, B., 50, 127 Otto, A., 257, 326 Otto, H., 257, 326 Overholser, E. L., 58, 61, 122, 127 Overley, F. L., 61, 127 Overstreet, R., 49, 129, 390, 395 Oveson, M. M., 163, 167 Owen, O., 262, 326

P

Packer, P. E., 117, 127 Page, G. E., 192, 230 Page, H. J., 236, 269, 283, 326 Page, J. B., 59, 69, 123 Page, N. R., 301, 326 Pajenkamp, H., 262, 328 Park, J. K., 207, 215, 230 Parker, F. W., 236, 322 Parker, J., 57, 127 Parks, W. L., 163, 168 Parrish, P., 275, 326 Paschal, J. L., 141, 142, 143, 144, 145, 160, 169 Pascoe, H. L., 266, 326 Pastonesi, G., 260, 326 0 Pasquill, F., 30, 31,127, 128 Patel, N. B.. 280, 327 Oates, W. J., 209, 230 Pauc, M., 293, 818 Oberbacher, M. F., 261, 326 Payne, J. H., Jr., 308, 323 Ochoa, A. E., 266, 326 Payne, W. H., 276, 304, 331 O’Connor, T. C., 27, 127 Pearl, R. T., 29, 128 O’Donnell, J. F., 203, 230 Pearse, C. K., 116,123 Oelsen, W.,282, 331 Pearson, G. A., 49, 123 Ogilvie, A., 275, 326 Pearson, R. W., 369, 370, 395 Ogilvie, R. S., 248, 257, 260, 324 Ohlrogge, A. J., 48, 49, 124, 369, 370, Peevy, W.J., 113, 125 Peikert, F. W., 176, 228 371, 394, 395

410

AUTHOR INDEX-VOLUME

Penman, H. L., 19, 20, 21, 29, 31, 105, 128 Perkins, S. E., 245, 325 Perrine, E. C., 252, 278, 307, 327 Perry, C. W., 248, 257, 280, 324 Persons, H. C., 288, 327 Pesek, J., 165, 168 Pesek, J. T., 148, 147, 148, 149, 158, 181, 185, 167, 167, 168, 169 Peters, D. B., 85, 79, 80, 126, 128 Peterson, A. E., 178, 188, 230 Peterson, D. R., 178, 230 Peterson, H. B., 18, 130 Peterson, J. B., 390, 395 Peterson, &L., I.50, 58, 60, 84, 86, 87, 91, 92, 125, 130 Petinov, N. S., 84, 128 Pettit, A. B., 268, 269, 327 Pfeiffer, C., 248, 327 Phillip, J. R., 79, 128 Phillips, A. B., 273, 277, 281, 307, 327 Phillips, H. C., 197, 231 Pickard, G. E., 186, 203, 227, 230 Pierce, R. A,, 291, 293, 327 Pierce, W. H., 252, 315 Pierre, W. H., 390, 395 Pimentel-Gomes, P., 137, 169 Pinkerton, C., 270, 322 Pirkle, E. C., 285, 327 Pisek, A,, 54, 85, 128 Pittman, D. D., 204, 227 Pizer, N. H., 308, 327 PlusjB, M. H . R. J., 280, 327 Poll, A., 258, 299, 31 7 Pollard, J. H., 298, 329 Poker, H., 58, 58, 59, 128 Pool, S. D., 208, 230 Poole, J. H . J., 268, 318 Poppe, W. H., Jr., 289, 327 Pope, J. B., 113, 128 Porter, J. J., 273, 277, 327 Porter, L. G., 268, 275, 278, 277, 312 Porterfield, J. G., 184, 230 Potts, G. D., 260, 318 Potts, J. E., Jr., 260, 318 Potts, J. M., 271, 282, 327, 330 Potts, R. C., 389, 395 Poudrier, G. L., 304, 317 Powell, R. L., 287, 321 Power, J. F., 180, 229

XI

Powers, M. R., 208, 215, 230 Poynor, R. R., 177, 230 Prescott, J. A., 27, 28, 128 Prevot, P., 268, 285, 327 Prianishnikov, D. N., 390, 395 Price, K. T., 298, 327 Priestley, C. H . B., 19, 124 Prince, A. B., 369, 380, 395 Prine, G. M., 56, 57, 83, 109, 123, 128 Proctor, J . T., 275, 306, 327 Proctor, P. D., 291, 332 Proctor, R. E., 181, 169 Pruitt, W. O., 26, 128 Pryor, J. N., 273, 323 Pulou, R., 288, 316 Purcell, V. I., 267, 330

R Radmncher, H. W., 282, 318 Raheja, P. C., 86, 128 Rainey, J. W., 300, 326 Raistrick, B., 306, 307, 327 Raley, G. H., 175, 229 Ranidas, L. A,, 28, 128 Ramig, R. E., 44, 48, 126 Ramp, R. M., 223, 230 Ramser, J. H., 199, 230 Raney, W. A., 45,128 Rapp, H . F., 278, 285, 304, 307, 323, 327 Rasniussen, R. T. C., 268, 314 Rauterberg, E., 137, 169 Raymond, L. R., 289, 327 Rea, H. E., 191, 230 Reaves, C. A., 174, 230 Hedemann, C. E., 261, 327 Reed, I. F., 173, 174, 175, 224, 230, 231 Reed, J. F., 287, 327 Reed, R. B., 273, 316 Rees, R. L., 258, 299, 327 Reeves, E. L., 81, 127 Regeimbal, L. O., 60, 126 Reidel, J. C., 248, 260, 327 Reniy, T., 378, 395 Rennie, D. A., 389, 370, 378, 379, 390, 395 Resen, F. L., 248, 255, 257, 327 Retzke, F. A., 278, 305, 327 Reuszer, H. W., 292, 318 Reynolds, D. S., 288, 289, 270, 319, 322

AUTHOR INDEX-VOLUME

XI

411

Reynolds, F. M., 283, 327 Ryan, J. E., 290, 328 Reynolds, J. C., 248, 325 Ryan, J. P., 265, 328 Ryerson, G. E., 180, 230 Reynolds, J. E., Jr., 307, 328 Rhoades, H. F., 369, 380, 395 S Ricardo, D., 136, 169 Sabine, P. A., 289, 314 Richards, L. A., 16, 44, 53, 55, 62, 70, Sachsel, G. F., 276, 305, 327 82, 91, 128, 130 Sack, W., 182, 230 Richardson, R. D., 193, 230 Richey, C. B., 203, 230 Sackett, W. J., 275, 318 Saeman, W. C., 255, 325, 328 Rickson, J. B., 377, 395 St. Pierre, P. D. S., 286, 328 Rider, N. E., 20, 31, 65, 128 Salmon, J. E., 286, 322 Riesenfeld, F. C., 261, 327 Salter, R. M., 392, 395 Riley, D. P., 282, 328 Salvan, H., 268, 323 Riley, W. J., 271, 328 Sample, E. C., 370, 387, 393 Robertson, G . W., 27, 128 Sandberg, W. D., 269, 320 Robertson, L. S., 158, 169, 178, 228 Robertson, L. S., Jr., 158, 161, 168, 169 Sandler, H. J., 248, 327 Robertson, W. K., 49, 128, 158, 168, Sanford, W. G., 261, 328 370, 395 Sato, H., 292, 330 Satoo, T., 91, 127 Robins, J. S., 83, 85, 128 Satterwhite, L. E., 20, 126 Robinson, G. D., 31, 128 Robinson, T. L., 17, 117, 128 Sauchelli, V., 268, 275, 329 Robinson, W. O., 37, 129 Savul, M., 292, 328 Sawyer, D. L., 301, 315 Rockwood, N. C., 269, 328 Rodriguez, L. P., 266, 328 Sawyer, F. G., 298, 328 Rogers, C. L., 265, 328 Saxena, E. R., 292, 328 Rogers, 11. T., 234, 323 Schaal, R. B., 300, 328 Rogers, T. C., 252, 328 Schaller, F. W., 176, 230 Rohner, L. V., 276, 320 Scheffer, F., 262, 328 Rohwer, C., 26, 129 Schiff, L., 15, 127 Ronnike, F., 44, 129 Schleicher, J. A., 265, 328 Rooseboom, A., 260, 328 Schneider, G. W., 58, 60, 129 Ross, W. H., 255, 256, 328 Schoenleber, L. G., 224, 228, 231 Routh, C. A., 199, 228 Schofield, R. K., 104, 129 Roy, L. F., 281, 326, 332 Scholl, W., 237, 242, 245, 250, 251, 255, Royster, P. H., 243, 328 273, 304, 306, 308, 309, 312, 322, 328 Ruch, J. W., 267, 268, 315 Schon, F., 299, 322 Rusberg, E., 291, 328 Schreiber, H. A., 49, 129 Ruhlman, E. R., 265, 266, 267, 268, 269, Schroeder, E. W., 184, 224, 227, 230, 231 287, 288, 328, 331 Runnels, R. T., 265, 328 Schuffelen, A. C., 347, 368 Russell, E. J., 135, 169 Schutz, D. P., 253, 332 Scott, W. C., Jr., 271, 282, 327 Russell, J. S., 369, 380, 395 Russell, M. B., 36, 44, 45, 74, 79, 80, 81, Scott, G. S., 255, 256, 316, 328 82, 124, 129 Scoville, 0. J., 202, 231 Scranton, C. J., 203, 231 Russell, R. S., 377, 382, 383, 394, 395 Seatz, L. F., 282, 284, 285, 307, 318, Rust, R. H., 47, 124 321, 330 Rutherford, M. R., 266, 328 Seedorff, W. A., 290, 316 Ruys, J. D., 258, 328 Segnit, E. R., 282, 328 Ryall, A. L., 61, 129

412

AUTHOR INDEX-VOLUME

Seiberlich, J., 260, 281, 328 Seifert, H., 258, 328 Seiple, J. W., 174, 232 Sell, 0. E., 369, 394 Selwyn, S. C., 299, 316 Severy, C. L., 289, 329 Sewell, M. C., 390, 395 Seymour, J. E., 281, 329 Shachori, A. Y.,35, 59, 129 Shapter, R. E., 83, 131 Shardakov, V. S., 84, 129 Shargorodskii, S. D., 292, 329 Sharp, J., 307, 329 Sharp, J. C., 304, 329 Sharples, K., 275, 318 Shearon, W. H., Jr., 248, 255, 298, 329 Sheldon, R. P., 265, 330 Shepherd, F. D., 271, 329 Sherman, M. S., 238, 316 Sherwin, K. A,, 305, 329 Shih, S. H., 389, 370, 372, 373, 378, 377, 390, 394 Shirk, H. G., 88, 129 Shoeld, M., 288, 275, 324, 329 Shor, 0. I., 292, 329 Shultz, J. F., 252, 273, 329 Siegel, M. R., 287, 289, 281, 285, 321 Silverberg, J., 257, 279, 281, 308, 307, 308, 321, 327, 329 Sisler, C. C., 308, 323 Skeen, J. R., 245, 254, 260, 271, 329 Skibitzke, H. E., 117, 130 Skinner, E. C., 248, 290, 316, 329 Skoog, F., 385, 395 Slack, A. V., 248, 273, 306, 307, 308, 321, 329 Slatyer, R. O., 55, 58, 59, 67, 68, 129, 130 Smallshaw, J., 183, 168 Smeltzer, D. G., 204, 228 Smith, C. M., 258, 273, 299, 329, 330 Smith, D. D., 113, 116, 129 Smith, E. M., 184, 230 Smith, F. B., 390, 395 Smith, F. W., 181, 168 Smith, H. I., 287, 329 Smith, H. P., 194, 208, 209, 231 Smith, J. C., 389, 370, 395 Smith, J. P., 274, 279, 281, 288, 316, 320, 829

XI

Smith, K. O., 208, 228 Smith, L. A., 199, 227 Smith, P., 176, 229 Smith, P. M.,870, 395 Smith, V. C., 310, 329 Sneller, Z. W., 368 Snow, R. E., 267, 330 Snider, H. J., 47, 124 Soper, R. J., 370, 378, 379, 390, 395 Southwell, P. H., 189, 231 Specht, R. C., 287, 329 Spencer, N. L., 307, 327 Spillman, A. 307, 329 Spillman, W. J., 139, 169 Spinks, J. W. T., 389, 394, 395 Spratt, D. A., 254, 329 Spuler, A., 81, 127 Stmelt, M. G., 53, 58, 129 Stallings, J. H., 116, 129 Stanberry, C. O., 389, 394 Stanfield, 2.A., 280, 308, 329 Stanford, G., 389, 390, 393, 396 Stanford, J. P., 281, 325 Stanhill, G., 77, 129 Stanley, R. G., 59, 129 Stansberry, C. O., 109, 129 Stansel, T. B., 297, 324 Stanton, H. S., 192, 231 Starostka, R. W., 281, 284, 285, 329, 370, 388, 387, 395 Steckel, J. E., 381, 394 Steinbiss, H. K., 273, 312 Stelly, M., 389, 394 Stemberger, A. P., 182, 169 Stengel, L. A., 258, 329 Sterges, A. J., 270, 271, 324 Stevens, W. L., 180, 189 Steward, F. C., 383, 395, 396 Stewart, F. H., 289, 329 Stickney, W. A,, 268, 330 Stinson, J. M., 282, 330 Stirk, G. B., 27, 128 Stocker, O., 54, 58, 65, 86, 129 Stocking, C. R., 4, 87, 123 Stockton, J. R., 50, 91, 129 Stokes, C. M.,184, 228 Stone, E. C., 35, 59, 129 Stone, P. E., 288, 330 Stout, E. L., 271, 330 Street, H. E., 383, 396

AUTHOR INDEX-VOLUME

Strelzoff, S., 254, 330 Strickler, P. E., 204, 227 Strickler, P. W., 197, 231 Striplin, M. M., Jr., 271, 273, 279, 281, 282, 308, 320, 324, 330 Stritzel, J. A,, 161, 162, 167, 169 Strogonov, B. B., 49, 129 Strong, D. G., 49, 130 Sumner, E. J., 31, 129 Sun, S. C., 267, 330 Sundquist, W. B., 158, 161, 168, 169 Suneson, C. A., 106, 129 Suomi, V. E., 24, 31, 126, 129 Suzuki, T., 292, 323, 330 Swanback, T. R., 292, 330 Swanson, R. W., 265, 324, 330 Sweetwood, C. W., 266, 330 Swinbank, W. C., 19,30,124,129 Szabo, A,, 307, 320

T Tague, E. L., 390, 395 Taitt, A. H., 289, 323 Takeda, E., 292, 330 Talbot, E. L., 255, 256, 317 Tanner, C. B., 24, 31, 126, 129 Tanquary, E. W., 173, 174, 231 Tarbutton, G., 258, 267, 269, 273, 274, 299, 317, 329, 330 Tavera, E., 265, 328 Tavernetti, J. R., 184, 192, 208, 231 Taylor, G. S., 176, 231 Taylor, G. V., 245, 246, 247, 248, 330 Taylor, R. E., 177, 229 Taylor, R. J., 30, 129 Taylor, R. L., 257, 330 Taylor, S. A., 45, 79, 129 Teakle, L. J. H., 390, 396 Teeple, J. E.,290, 330 Tendam, D. J., 369, 377, 393 Terman, G. L., 280, 330, 392, 396 Terry, D. E.,267, 324 Teter, N. C., 215, 231 Thomas, H. E., 14, 16, 129 Thomas, M. D., 58, 129 Thompson, D., 267, 330 Thompson, H. L., 248, 279, 329, 330 Thompson, J. L., 175, 231 Thompson, L. G., 49, 128

413

XI

Thompson, M. E., 267, 330 Thorne, D. W., 16, 130 Thornthwaite, C. W., 12, 20, 28, 31, 105, 130 Thornton, D. G., 196, 229 Thornton, I., 287, 322 Thorp, H. W., 297, 331 Threlfall, R. E., 271, 331 Thut, H. F., 61, 130 Tidmore, J. W., 390, 396 Tielrooy, J., 252, 331 Tillotson, I. S., 266, 331 Ting, C. W., 292, 331 Todd, J. M., 298, 331 Tombesi, L., 86, 130 Tomkins, R. V., 289, 331 Tongue, T. O., 273, 323 Tonn, W. H., Jr., 260, 331 Toshima, T., 260, 326 Tramel, T. E., 157, 159, 169 Tramontini, V. N., 176, 228 Tremearne, T. H., 268, 275, 276, 277, 312, 322 Tromel, G., 282, 283, 284, 331 Trogdon, W. O., 391, 396 Tucker, W. J., 305, 331 Tulipman, J., 268, 319 Turk, L. M., 178, 228 Turner, J. R., 186, 229 Turner, S. F., 117, 130 Turrentine, J. W., 287, 331 Tuttle, H. A., 248, 331 Tyler, P. M., 267, 331 Tynan, D. E., 268, 320

U Uexkiill, H. Von, 262, 328 Ulloa, S., 265, 328 Upchurch, R. P., 58, 60, 84, 86, 87, 91, 92, 125, 130 Updegraff, N . C., 248,330 Utley, H. F., 266, 331

V Van Bavel, C. H. M., 25, 45, 60, 86, 105, 124,125,130, 163,169 Vandecaveye, S. C., 61, 127 Vanden Berg, G. E., 174, 228

414

AUTHOR INDEX-VOLUME

Van Denburg, J. V., Jr., 247, 258, 331 vander Marel, H.W., 347, 368 Van Der Pauuw, F., 137, 169 Van Doren, C . A., 187, 231 van Duin, R. A. A., 28, 130 van Heemst, H. D. J., 20, 126 Van Ness, H. W., 247, 258, 331 Van Nortwick, H. S., 265, 528 van’t Woudt, B. D., 74, 76, 77, 130 Van Wazer, J. R., 271, 331 van Wijk, W. R., 28, 74, 76, 130 Van Wyk, D. J. R., 266, 331 Veihmeyer, F. J., 27, 46, 54, 79, 125, 130 Veltman, P. L., 273, 323 Venema, H. J., 27, 124 Venkatachalam, S., 284, 325 Venkatesham, Aj., 292, 331 Ver Planck, W. E., 301, 331 Verduin, J., 58, 130 Viets, F. G., Jr., 302, 315, 369, 370, 372, 373,376,377,390,394,396 Vieweg, G. H., 58, 129 Vilbrandt, F. C., 267, 330 Vincent, E. A., 289, 329 Visse, L. D., 266, 331 Vittum, M. T., 390, 396 Vogelaar, B. F., 204, 232 Volk, G. W., 369, 390, 391, 396 Volk, N. J., 390, 396 von Lossberg, V., 266, 331 Vreeland, D. C., 309, 331

W Wadleigh, C. H., 44, 49, 53, 54, 60,61, 62, 70, 79, 82, 89, 91, 128, 130 Waggaman, W. H., 262, 265, 266, 267, 268, 269, 271, 273, 275, 277, 280, 331 Wagner, R. E., 189, 231, 261, 314 Wakefield, Z. T., 274, 281, 318 Wakerley, D. S., 299, 316 Walker, H. B., 173, 185, 216, 231 Walker, W. J. S., 258, 299, 317 Wallace, H. M., 242, 255, 306, 309, 328 Wallace, H. R., 50, 130 Walter, H., 67, 130 Walthall, J. H., 280, 281, 283, 284, 331 Ward, F. N., 270, 283, 321, 322 Waters, C . E., 276, 304, 331 Waters, W. F., 289, 314

XI

Watson, D. J., 62, 130 Watson, J. R., Jr., 261, 325 Way, R. W., 304, 331 Waylett, W., 266, 316 Wear, J. I., 301, 321, 331 Weatherley, P. E., 67, 68, 130 Weaver, J. E., 43, 47, 50, 124, 130 Webb, B. K., 207, 230 Weber, W. C . , 273, 277, 322, 331 Weick, F. E., 189, 231 Weihing, R. M., 107, 125 Wells, J. V. B., 11, 126 Wells, R. R., 266, 330 Went, F. W., 35, 129 Werner, H. O., 67, 130 Wesemael, J. E. van, 386, 394 Wesseling, J., 74, 76, 130 Whetstone, J., 256, 258, 316, 332 White, N. C . , 293, 332 Whitfield, C. J., 180, 232 Whitney, W. T., 269, 283, 284, 332 Whitt, C. D., 274, 316 Whitt, D. M., 113, 116, 129 Whittaker, C. W., 255, 256, 282, 294, 300, 316, 328, 332 Whittemore, H. C., 225, 231 Whynes, A. L., 307, 332 Wieczorek, G. A., 270, 280, 286, 321, 326, 332 Wienert, F. O., 292, 332 Wiersma, D., 86, 130 Wiersum, L. K., 46, 130 Wiklander, L., 388, 396 Wilbanks, J. A., 271, 327 Wild, A,, 385, 396 Willard, C. J., 176, 231 Willard, M. E., 291, 332 Willcox, 0. W., 137, 138, 169 Williams, A. J., 289, 332 Williams, D., 306, 332 Williams, R. F., 57, 63, 130, 131 Williams, T. H., 257, 332 Williams, W. B., 275, 332 Wibmson, E. B., 190, 195, 208, 231 Willis, A. J., 381, 382, 384, 396 Willoughby, W. M., 62, 131 Wilson, C. C . , 66, 131 Wilson, C. M., 301, 331 Wilson, J. D., 196, 231 Wilson, R. A,, 275, 277, 315

AUTHOR INDEX-VOLUME

XI

415

Y Wilson, R. W., 191, 224, 232 Wilson, T. V., 105, 130 Yamaga, R. H., 369, 394 Winkelblech, C. S., 178, 232 Yamamoto, G., 33, 131 Winkler, A. J., 220, 229 Yamanouchi, N., 292, 3-26 Winn, P. N., 224, 232 Yang, Y. C., 284, 322 Winsor, G. W., 262, 326 Yatazawa, M., 370, 396 Winsor, H. W., 301, 332 Yates, L. D., 274, 275, 276, 278, 279, Winterberg, S. H., 270, 273, 324 281, 304, 322, 326, 332 Winters, H. F., 60,126 Yeandle, W. W., 248, 332 Wise, L. N., 189, 228 Yee, J. Y., 255, 256, 261, 262, 316, 328, Wisler, C. O., 115, 131 332 Wisniewski, A. J., 261, 325, 332 Yemm, E. W., 381, 382, 384, 396 Witt, R. H., 209, 230 Yeo, R. R., 192, 232 Wittcoff, H., 267, 324 York, E. T., 48, 131 Witzel, 1-1. D., 204, 232 Yost, J. F., 310, 332 Wolf, F. A., 60, 124 Young, A. A., 26, 131 Wolf, R., 68, 122 Young, C. L., 35, 129 Woltz, S. S., 294, 332 Young, E., 268, 332 Womochel, H. L., 173, 230 Young, H. Y., 261, 328 Wood, G. M., 193, 230 Young, R. D., 273, 276, 277, 281, 307, Wood, W. S., 209, 230 327, 332 Woodard, A. W . , 237, 328 Young, V. D., 196, 227, 232 Woodhams, D. H., 60, 86, 131 Younts, S. E., 48, 131 Woodruff, N. P., 180, 227, 232 Z Woodworth, R. C., 161, 169 Zernova, L. K., 53, 127 Wooten, 0. B., 208, 231, 232 Ziegler, V., 268, 323 Work, J., 271, 315 Zingg, A. W., 113, 116, 129, 180, 232 Work, P., 212, 232 Zink, E., 50, 130 Work, R. A,, 53, 54, 122 Worthington, E. A., 253, 304, 317, 320, Zink, F. W., 254, 312 Zirngibl, H., 299, 322 332 Zoellner, J. A., 137, 165, 168 Worthington, W. H., 174, 232 Zwerman, P. J., 178, 230 Wright, E. H., 268, 275, 329 Zwicker, R., 56, 131 Wright, R. E., 271, 332

Subject Index-Volume

A Acid soils, 391, 392 Agriculture in the Netherlands, advisory services, 387-368 educational facilities, 367-368 Agropyron crystatum, 50 Aldrin, 309, 310 Alfalfa, 19, 20, 43, 47, 49, 75, 10% 114, 161, 387 Alkaline soils, 49, 50, 390, 391 Almond, 218 Aluminum phosphate, 284-286 Amino acids, 382 Ammonia, 153, 304-305, 311 anhydrous, 250-252, 311 aqueous, 252 metering devices, 186 synthesis, 244-248 Ammonium nitrate, 254-256, 311, 373, 378, 379, 390 Ammonium phosphate, 278-279, 311 Ammonium sulfate, 257-259, 370, 373 Apple, 46, 53, 58, 60, 61, 221, 346, 372 Ash, 64 Asparagus, harvest-aid machines, 211-212 Atmometer, 27 Auxin, 384, 385

XI

in saline soils, 49 harvesting equipment, 209-211 Beech, 56, 59 Beet, 216 Bennudagrass, 57, 107, 109, 292, 389 Birch, 56, 59 Biuret, 261 Blueberry, 222 Bog peat, see Low peat Bolster, 349 Borax, 301 Boron, 299, 301-302 Bowen ratio, 32 Brassica mpus var. oleifera, 364 Broccoli, 213 Bromus Inemis, 50 Buckwheat, 343, 350 Brush burning, 117 Buffered soils, 393

C

Cabbage, harvest-aid equipment, 212 in the Netherlands, 351 Calcium, 49, 294-296 effect on root penetration, 47 Calcium phosphate, 279-280, 392 Caraway, 360, 384 Carlum c a d , 364 Carnation, 46 B Carrot, 216 BHC, (benzene hexachloride), 309 Castor bean, 224-225 Bahiagrass, 57 Celery, 212 Baler, 199-200 Cereals, 114 Barley, 204, 241, 370, 372, 373, 377, Cereals, 381, 383, 390 effect of alkalinity, 362 in the Netherlands, 344, 347, 363, flood damage, 75 364, 366 in the Netherlands, 339, 346, 347, salt tolerance, 106 364, 365, 366 Basic slag, 282, 295 nitrogen requirement, 360 Basin clay, 34-47 potassium requirement, 361 Bade unit, 138 Chelates, 300301, 309 Bean, 80, 212 Cherry, 222, 346 in Netherlands, 339, 363 Chlordane, 309, 310 effect of moisture stress, 86 Cinchona, 60 416

SUBJECT INDEX-VOLUME

Citrus, 46, 53 toxic foliage sprays, 261 Clay soils, 79, 383, 564 Climate of the Netherlands, 337-339 Clover, 86 in the Netherlands, 363 Cobalt, 302, 344 Colza, 360, 363, 364 Combines, 204-207, 210-211 Complete extraction zone, 88 Contouring, 116 Copper, 299, 301, 302, 344 Corn, 20, 45, 47, 48, 53, 58, 61, 62, 106, 107, 109, 114, 158, 159, 161, 187, 196, 241, 261, 370, 371 harvesting equipment, 202-204 in the Netherlands, 339 response to moisture regime, 79, 80, 81, 82, 85 seasonal water use, 20 Costs in crop production, 137 Cotton, 49, 50, 55, 56, 60, 62, 114, 184, 192 cost of mechanical picking, 208 harvesting equipment, 207-209 verticillum wilt yield reduction, 91 Cranberry, 196 Crimson clover, 48, 207, 380 Cryptomeria, 91 Crop rotation, 362-383 Cucumber, 213 Cultivation, 50, 108, 111, 190-195 Cuticular transpiration, 54, 62 Cyanamide, 244, 391

D 2,4-D, 309, 310 DDT, 309, 310 “Dal” soil, see High peat Darcy’s law, 41 Defluorinated phosphate rock, 282-283 Detritus, 358 Dew, 33-35, 59 Dieldrin, 310 Diffusion pressure deficit, 67-69 Dinitrophenol, 383 Diseases of crops, 38, 50, 91 Donnan distribution, 388-389 Drought injury, 75

417

XI

Drought tolerance, 5 6 5 7 , 70-73, 105106 Dune soils, 345 Dusting equipment, 196

E Eelworm, 350 Evaporation, Dalton equation, 29 factors affecting, 17 measurement, 26-27 physics, 21-24 prevention, 17 Evapotranspiration, 117 effect of crop on rate, 20 effect of wind turbulence, 23, 30 errors in measurement, 25, 26, 27 in the Netherlands, 338 methods of determining, 24-33 potential for US.,12 Evapotranspirometer, 25, 32

F Fertilizer, caking, 305-307 effect on root growth, 4 7 4 9 efficiency, 36 hopper design, 185-186 law of diminishing soil yield, 136-140 law of the minimum, 135-136, 153, 154 minimum rate of application, 146 mixed, 303-309 placement, 48, 185-187, 370, 376-377 practices in the Netherlands, 359-362 US. consumption, 133, 236-242 world consumption, 235-236 Fescue, 207 Fick‘s law of diffusion, 374 Filbert, 218 Fir, 56 in the Netherlands, 343, 363, 364, 366 Flax, 225 Forage grasses, 43 Forage harvesting equipment, 197-198 Frasch process, 298 Fruit harvest aids, 220-223 Fungicides, 299

418

SUBJECT INDEX-VOLUME

G Grain harvesting equipment, 204-207 Gibberellic acid, 310-311 Grape, 46 harvesting equipment, 220 in the Netherlands, 345 Grasses, 46, 47, 48, 50, 56, 109, 371 in the Netherlands, 343, 346 lime requirement, 362 nitrogen requirement, 360 potassium requirement, 361 Grass seed harvesting equipment, 204207 Gravitational water, 41 Green bean, 212 Ground-water, 8, 14, 15 depletion, 16, 17 Growth of plants, effect of soil moisture stress on, 79, 80, 81, 84-87, 88, 89, 90, 93 Guayule, 61, 86 Growth regulators, 48

H Hargreaves process, 293 Harvesting equipment, 196-225 Hay crushers, 198-199 Hay peIIeting machine, 201-202 Heat exchange, 22, 23 Heat injury to plants, 75 Heptachlor, 309, 310 Heterochra rostochiensis, 350 Heterosis, 107 High peat soils, 340, 348-350, 363, 364, 365, 366 liming, 362 phosphate requirement, 360 potassium requirement, 361 Humus, 359 Hydrogen source for ammonia synthesis, 240-248 Hydrologic cycle, 3, 6-35 Hysteresis, 94

XI

Ion carriers, 382, 383 Ion exchange capacity, 381 Iron, 301, 302 Iron phosphate, 284-286 Irrigation, 36, 44, 75, 82, 85, 86, 88, 90, 91, 93, 344 effect on fertility, 83-84 efficiency, 17-18 nematode infestation, 50 practices, 98-104, 104-109 water quality, 15-16 Isotopic exchange, 377

K Kenaf, 225

1 Ladino clover, 43, 50, 58, 60 effect of soil moisture regime on, 87, 91, 92 Land reclamation, delta plan, 354-355 in Zuider Zee area, 357-358 Lead arsenate, 309 Lemna minor, 372 Legume harvesting equipment, 204, 206-207 Lespedeza, 207 Lettuce, 43, 372, 389 harvest-aid equipment, 212 Licking disease of cattle, 344 Lima bean, 212 Lime, 47, 345, 346, 347, 349, 354, 357, 358-359, 362, 363, 391, 392 Llnum usitatissum, 364 Loblolly pine, 64 Livestock in the Netherlands, 367 Loess loam, 340, 345346, 362 phosphate requirement, 360 Low peat soils, 340, 350452 Lupine, 345 Lygus bug, 50 Lysimeter, 24-25, 32, 34

I

M

IPC (isopropyl N-phenyl carbamate), 309 Indoleacetic acid, see Auxin Interseeding, 187-188

Magnesium, 296-297 Magnesium content in soil amendments, 295 Magnesium deficient soils, 344

SUBJECT INDEX-VOLUME

Maize, see Corn Malathion, 310 Manganese, 299, 301, 302 Manganese deficiency, cause of, 362 in soils, 344 Mangold, in the Netherlands, 344, 346, 364, 365 Marginal physical product, 141, 152 Marginal rate of substitution, 153 Melons, 213 Metabolism, effect of nitrogen, 381385 Metaphosphate, 281-282 Micronutrients in compost, 349 Millet, 56, 107 Moisture conservation, 110-115 Molybdenum, 302 Moor peat, see High peat Moss peat, 349, 351 Mowers, 198 Multinutrient glasses, 300

N Napier grass, 201 Navy beans, 210 Nematodes, 50, 91 Nitrate fertilizers, 254-257 Nitric acid production, 254 Nitrification, effect on soil pH, 390 Nitrogen, 63, 82, 83, 109, 369393 absorption, 60-61 biological effects, 370-385 chemical effects in soil, 385-393 compounds of, used as fertilizer, 248262 effect on root growth, 4 7 4 8 on transpiration ratio, 56-57 fertilizer production, 242-262 influence on Verticillium wilt, 50 use in the Netherlands, 360 Nitrogen oxide production, 243-244 Nitrophoska, 280 Nut harvesting equipment, 218-219

0 Oak, 56, 59 Oats, 57, 241, 261, 377, 378, 380, 381, 387 in the Netherlands, 343, 344, 346, 347, 363, 364, 366

419

XI

Oilseed in the Netherlands, 343 Onion, 43 harvesting equipment, 216-217 Opuntia, 64 Orthophosphate, 278-279 Osmotic stress, 67, 80, 81, 82 Overgrazing, 109, 117 Oxidative phosphorylation, 384

P Palatia phosphate, 284 Pangolagrass, 57 Pea, 49, 213, 372, 378 harvesting equipment, 209-211 in the Netherlands, 364, 365, 366 Peach, 53, 220, 372 Peanut, 56 harvesting equipment, 214-215 high speed planter, 189 Pear, 53, 54, 61, 66, 220 flooding, 75 Peat soils, liming, 362 Pecan, 58 harvesting equipment, 218 Pennisetum glaucum, 107 Peppers, 213 Permeability of roots, effect of poor aeration, 74 Phaseolus uulgaris in the Netherlands, 339 Pesticides, 309-310 Phosphate m e in the Netherlands, 360361 Phosphate rock, 262-270, 295, 310, 390 direct application, 269-270 mining and beneficiation, 266-267 reserves, 262-266 uranium, vanadium, and fluorine in, 267-269 Phosphate rock-magnesium silicate glass, 283-284 Phosphoric acid, 311 added to irrigation water, 273 production, 272-274 Phosphorus, 49, 82-83, 109, 153, 309, 369-393 diffusion in soil, 375 effect of salts on solubility, 385-389 effect on root development, 47, 48

420

SUBJECT INDEX-VOLUME

effect on transpiration ratio, 57 elemental, production, 270-271 yellow, toxicity, 270 Phosphorus absorption, 44, 80-83 effect of ammonium, 377-378 effect of moisture, 80-82 effect of nitrogen, 371 effect of pH, 389-393 effect of plant age, 379 Phosphorus fertilizers, 282-287 Photosynthesis, 91, 92, 5 7 5 9 effect of water deficit, 5 3 5 4 Piche evaporation, 27 Pine, 91, 372 Pineapple, toxic foliage sprays, 281 Pinto beans, 310 Planting row-crops, 183-187 Planting equipment, 182-190 airplane, 189-190 grain and seed drills, 187-189 hill-drop planter, 184 seed press wheel, 184-185 Pleistocene sandy soils, 340-344 Plow-plant method, 178-180 Plum, 222, 348 Podsolic soil, 342, 344 Pole bean, harvesting equipment, 212 Potash, 257 fixation in soil, 347 resources, 288-289 Potassium, fertilizer production, 287-294 mining and processing, 290-292 resources, 288-290 use in the Netherlands, 381 Potassium chloride, 292-293, 311 Potassium nitrate, 312 Potassium sulfate, 293 Potato, 43, 87, 241, 372, 378 effect of excess potassium, 381 effect of high pH, 382 harvesting equipment, 215 in the Netherlands, 343, 344, 348, 347, 350, 383, 384, 365, 388 nitrogen requirement, 380 Potato scab, 382 Precipitation, 8-10, 110 Profits, 142, 143, 146-147, 157 Prune, 222 harvesting equipment, 218

XI

Pulse, in the Netherlands, 343, 388 nitrogen requirement, 380

R Radioactive phosphates, 286-287 Radish, 49, 217-218 Rainfall, see Precipitation Rakes, 199 Rape, 383, 378 Raspberry, 222 Red clover, 181, 348 Reed peat, 349, 351 Relative humidity, effect on growth, 82 Relative turgidity, 86-87 Respiration, 80 Rhenania-type phosphates, 284 Rice, 45, 204, 370 planting by airplane, 189 River clay, 340, 348347, 364, 385, 388 lime content, 382 phosphate requirement, 381 potassium requirement, 381 Root-crop harvesting equipment, 214218 Root efficiency, 372-377 Root growth, 4 3 5 1 effect of minor elements, 48 effect of nitrogen, 370472 influence of cultivation, 50 iiiechanical impedance, 46-47 Root-top ratio, 47 Rubber, 85 Rubidium, 82 Runoff, 8, 11, 15, 113, 115-118 Rye in the Netherlands, 343, 348, 350, 383, 386 Ryegrass, 371, 380, 392

S Saline soils, 49 Sandy soils, 79, 340345, 364, 385, 388 lime requirement, 382 phosphate requirement, 380 potassium requirement, 381 Scalding, 75 Saturation deficit, 88 Sea clay, 340, 352-359, 384, 385, 388 lime content, 382

SUBJECT INDEX-VOLUME

nitrogen content, 360 potassium requirement, 3Gl Silt deposit, 75 Seedbed finishing tools, 182 Silage chopper, 200-202 Sodium nitrate, 373,380 Soil, aeration, 45,46 aggregates, 46 effect of water, 35-37 effect on root growth, 43-51 moisture, 8,9,35-42,44 availability to plants, 13,77-94 effect on compaction, 37 effect on fertility, 44,80-84 energy of retention, 39, 94 movement, 41,78-80,82 seasonal changes in distribution, 42 vapor transfer, 41 water storage capacity, 9 pH, 362,389-393 reclamation, 343,349 testing, 164-166 texture, effect of moisture, 37 effect on water infiltration, 39 effect on water retention, 9, 13 types in the Netherlands, 339-359,

365

Soil conditioners, 305306 Soil moisture stress, 40,55,68,77-94 effect on yield, 85-87,89,91 Soil moisture tension, 40,79-80, 82 Soil-plant-water interrelations, 77-98 effect of climate, 91-94 Soil solution, concentration, 16,36 Sorghum, 43,56,90,107,114,302 Soybean, 61,204 cracking of seed, 206 Spinach, 213 Spraying equipment, 193-196 Stornatal opening, 53,69 Strawberry, 222 Strip farming, 116 Stubble-mulching, 108 Sub clover, 207 Subsidence, 351 Sudangrass, 392 Sugar beet, 19,82,192,372 harvesting equipment, 216

421

XI

in the Netherlands, 339,343,347,350,

363,364,365,366

nitrogen requirement, 360 potassium requirement, 361 special planter, 190 sugar content, 86 yield under irrigation, 93 Sugar cane, 65,66 harvesting equipment, 223 sugar content, 86 Sulfate of potash-magnesia, 293,295 Sulfur, 295,297-299 Sunflower, 46, 64,384 Superphosphate, 274-278,295,370,371,

372,377,378,387-388,391,392

Surfactants in fertilizers, 305 Sweet corn harvester, 213 Sweet potato, 215,241

f Thinning equipment, 192-193 Tillage, 173-182 in arid regions, 180-182 minimum, 17b180 mulch, 176-178, 180-182 Tobacco, 45,61,62,191,241,384 effect of soil moisture stress, 60 harvest-aid equipment, 224 Tomato, 48,50,54,55,60,62,213,261,

345,377,389

effect of soil moisture stress, 61,86 Toxaphene, 309 Tonoplast, 382 Trace nutrient elements, 299-302 in multinutrient glasses, 300 Transpiration, 22-23,54-57,59,62,64,

65

effect of soil moisture, 55 Transpiration ratio, 56-57 Transport of ions, 382-383 Tree nut and fruit harvesting equipment, 218-223 Trefoil, 50 Triple superphosphate, 276-278,311 Tulip, 345 T u g , 225 Turnip, harvesting equipment, 216 in the Netherlands, 343,344

422

SUBJECT INDEX-VOLUME

U Urea, 259-262, 311, 377 biuret formation, 261 Urea-form, 261-262

V Vapor-flow, 2 9 3 1 Verticilliuni wilt, 50, 91 Vicia faba, 363 Vegetable crop harvesting equipment, 211-214

W Walnut, 218 Water, absorption, 64, 74 as a plant constituent, 65-66 consumption, 105 cost, 99-100 excess, 36-37, 74-77 physical nature, 4-6 resources, 6-19 role in plants, 51-70 Water-application efficiency, 99-104 Water consumption, effect of crop, 19-21, 105 in US.,18 Water deficit, in the Great Plains, 110 in the Netherlands, 339 Water deficit in plants, cause, 83-85 effect on cellular growth, 61 effect on yield, 62 measurement, 85-69

XI

Watermelon, response to irrigation, 88, 105 Water requirement of plants, 56 Water table, 45, 46, 75 Water-use efficiency, 98-104, 104-109, 111 Weed control, by flaming, 192 equipment for herbicide application, 192 Wheat, 20, 48, 53, 60, 73, 106, 111, 204, 206, 241, 372, 377, 378, 379, 384 in the Netherlands, 343, 344, 346, 347, 350, 363, 364, 366 moisture requirement, 111 seasonal water use, 20 Wilting, see Water deficit in plants

Y Yield, 393 analysis for 2 or more nutrients, 147157 economic analysis, 159-164 effects of weather, 163 marginal analysis, 141-142 response to fertilizers, 134-140 response to moisture, 60, 62, 82, 83, 85-87, 89, 90, 91,93 single-nutrient analysis, 140-147

Z Zeo mays in the Netherlands, 339 Zinc, 299, 301, 302

Cumulative Author Index Volumes VI-X

.

Aberg. Ewert: Swedish Crop Production. Recent Changes in . . . Addicott. F. T., and Lynch. R . S . Defoliation and Desiccation . . Allison. F. E .. Soil Nitrogen Balance Sheets. The Enigma of . . . Anderson. A . J .. Molybdenum as a Fertilizer . . . . . . . Andrews. W . B. Anhydrous Ammonia as a Nitrogenous Fertilizer

.

Baldridge. J . D.. See Henson. P . R .

.

.

.

.

.

.

.

.

.

.

.

Volume Page VII. 39 68 IX. VII. 213 VIII. 164 VIII. 62 IX.

122

Cheney. H . B., et a1. Field Crop Production and Soil Management VIII. Chepil. W . S.. See Olson. R . V., et a1. . . . . . . . . . x. Christensen. J . J .. See Culbertson. J . O., et ul . . . . . . . . VI. Coleman. N . T., Kamprath. E . J., and Weed. S . B.. Liming . . . x. Cook. R . L .. See Lawton. K . . . . . . . . . . . . VI. Cook. R . L., and Davis. J . F . Fertilizer. The Residual Effect of . . IX. Coons. G . H., Owen. F. V., and Stewart. D . Sugar Beet Improvement VII. Cope. W . A .. See Henson. P . R . . . . . . . . . . . 1% Cowan. J . R .. Tall Fescue . . . . . . . . . . . . VIII. Culbertson. J . O., et a1.. Seed-Flax Improvement . . . . . . VI.

2 56 181 475 254 205 90 142 283 144

Davis. J . F.. See Cook. R . L . . . . . . . . . . . . 1x9 Dawson. J . E .. Organic Soils . . . . . . . . . . . VIII. DeMent. J . D .. See Martin. J . P., et a1. . . . . . . . . VII. Dexter. S . T. Winter Hardiness. Evaluation of Crop Plants for . . VIII. Donald. C . M .. See Stephens. C . G. . . . . . . . . . x. Dungan. G. H., Lang. A. L., and Pendleton. J . W . Corn Plant Population in Relation to Soil Productivity . . . . . . . . x. Dunham. R . S .. See Culbertson. J . O., et ul . . . . . . . . VI.

205 378 2 204 168

Ennis. W . B., Jr . Weed Control in the Southern United States . Ensminger. L . E .. See Jordan. H . V . . . . . . . . . Erlanson. C . 0.. See Hodge. W . H . . . . . . . . .

.

x.

VII.

252 408 189

. .

VIII. VI. VIII.

242 152 2

.

VI.

145

1.Iafenrichter. A . L.. Grasses and Legumes for Conservation . . . Haise. H . R .. See Olson. R . V., et u1. . . . . . . . . . Harlan. J . R .. See Olson. R . V., et ul . . . . . . . . . . Harmsen. G . W., and VanSchreven. D . A . Organic Nitrogen in Soil 423

X. X.

350 47 15 300

.

.

.

.

.

Fitts. J . W . and Nelson. W . L.. Soil Tests . . . . . . . Flor. H . H.. See Culbertson. J . O., et a1. . . . . . . . Foote. W . H .. See Cheney. H . B., et a1. . . . . . . . . Geddes. W . F.. See Culbertson. J . O., et a1.

.

.

.

.

.

.

.

. .

.

VII.

x.

VII.

436 168

424

CUMULATrVE AUTHOR INDEX-VOLUMES

VI-X

Harrar. J. G.. International Collaboration in Agriculture . . Hemwall. J B.. Phosphorus. The Fixation of. by Soils . . . Henson. P . R., Baldridge. J D., and Cope. W. A. Lespedezas Heyne. E . G.. See Olson. R . V., et al . . . . . . . . Hide. J. C See Olson. R V., et a1. . . . . . . . Hodge. W. H., and Erlanson. C 0.. Plant Introduction . .

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

Joffe. J . S.. Green Manuring Viewed by a Pedologist . . . Jordan. H. V., and Ensminger. L . E .. Sulfur in Soil Fertility .

.

.

.

.

.

.

.

..

.

.

Kamprath. E . J .. See Coleman. N . T. . . . . . Kelley. 0. J .. Soil Water. Requirement and Availability Kempthorne. 0.. Statistics. The Contributions of . . Knowles. P F.. Safflower . . . . . . . . Knox. E . G .. See Cheney. H . B . . . . . . .

.

Volume Page 95 VI. 95 IX. 114 IX. 8 X. 23 x. 189 VII. VII. X.

142 408

X. VI. . . . . IX. . . . . X. . . . . VIII.

475 87 177 290 2

. . . .

of .

.

.

Lang. A. L .. See Dungan. G. H . . . . . . . . . . . Lawton. K., and Cook. R L .. Potassium in Plant Nutrition . . . Lynch. R . S See Addicott. F. T. . . . . . . . . . .

X. VI. IX.

436 254 88

Martin. J . H .. See Quinby. J . R . . . . . . . . . . . Martin. J . P., et a2 .. Soil Aggregation . . . . . . . . . Martin. W P.. See Martin. J. P., et al . . . . . . . . . McIlvain. E H., and Savage. D . A .. Range Improvement . . . . Melsted. S W . Corn Belt Soils. New Concepts of Management of Mortland. M M .. Ammonia in Soils. Reactions of . . . . . .

VI. VII. VII. VI. VI. X.

305 2 2 2 121 325

. . VIII.

321 242

.

..

.

.

. .

.

Nelson. L. B.. Mineral Nutrition of Corn . . . . . . Nelson. W L See Fitts. J . W . . . . . . . . . Nelson. W L., and Stanford. G Plant Nutrient Behavior and tilizer Use . . . . . . . . . . . . . . Nikitin. A . A. Trace Element Usage. Technological Aspects of Norman. A G Microbiology in Soil Science. The Place of .

. .. .

. . ..

. .

.

.

.

.

. VIII.

Fer-

.

X.

. .

VI. VII.

. .

.

68 183 399

Olson. R V., et al . Trends and Problems in the Great Plains . Owen. F V.. See Coons. G H . . . . . . . . . .

. .

VII.

3 90

Page. J . B.. See Martin. J . P., et al . . . . . . Pearson. R W., and Yeager. J . H . Agricultural Trends Cotton Belt . . . . . . . . . . . . Pendleton. J W .. See Dungan. G H . . . . . .

.

VII.

2

. . . . . . . .

IX. X.

1 438

Quinby. J . R., and Martin. J . H .. Sorghum Improvement

. . . .

VI.

305

. VIII.

2 2 36 143

.

..

.

.

.

Rampton. H . H .. See Cheney. H B., Raney. W A.. See Martin. J . P., et Rhoades. H . F See Olson. R V., et Rodenhiser. H . A .. See Stakman. E .

.

..

.

et al . a1.

.

.

.

.

.

.

X.

in the Old

.

.

.

. . . . . . . . al . . . . . . . . . C. . . . . . . . .

VII.

X. X.

CUMULATIVE AUTHOR INDEX-VOLUMES

Savage. 1). A.. See McIlvain. E . I1 . . . . . . Shaw. R . H.. See Wilsie. C. P . . . . . . . Smith. D. C.. Grass Breeding. Progress in . . . . Smith. G . D .. See Tavernier. R. . . . . . . Stakman. E . C., and Rodenhiser. H . A. Race 15B of Rust-What It Is and What It Means . . . . Stanford. G .. See Nelson. W. L. . . . . . . Stephens. C G., and Donald. C. M .. Australian Soils . Stewart. D.. See Coons. G. H . . . . . . . .

.

.

. .

. .

. .

. .

Wheat . . . . . .

.

.

425

VI-X

.

Volunie Page 2 199 . . 128 . . 217 Stem 143 . . . . 68 . . 168 . . 90

. .

. .

Tavernier. R., and Smith. G. D . Braunerde (Brown Forest Soil) . Thorne. W Zinc Deficiency and Its Control . . . . . . .

217 31

VanSchreven. D . A.. See Harmsen. G. W . . . . Vomocil. J . A.. Soil Bulk Density and Penetrability .

..

. .

. .

. .

. .

300 159

Wadleigh. C . H .. Mineral Nutrition of Plants . . . . . . Weed. S. B .. See Coleman. N . T. . . . . . . . . . Wilsie. C . P., and Shaw. R . H . Crop Adaptation and Climate .

.

75 475 199

.

Yeager. J . H .. See Pearson. R . W .

.

.

Zimmerman. L . H .. Castorbeans

.

.

. .

. .

. .

. .

. .

. .

.

.

. .

. .

1 258

This Page Intentionally Left Blank

Cumulative Subject Index Volumes VI-X Agricultural Trends in the Old Cotton Belt . . . Agronomic Trends and Problems in the Great Plains Ammonia in Soils. Reactions of . . . . . . Anhydrous Ammonia as a Nitrogenous Fertilizer . Australian Soils and Their Responses to Fertilizers .

. . . .

. .

. .

. . .

. . .

. . .

. . .

Volume Page 1 Ix. 3 X. . 325 X. . VIII. 62 . 168 X.

Braunerde (Brown Forest Soil). The Concept of .

.

.

.

.

.

IX.

217

Castorbeans: A New Oil Crop for Mechanized Production Corn Belt Soils. New Concepts of Management of . . Corn Plant Population in Relation to Soil Productivity . Crop Adaptation and Climate . . . . . . . Crop Production. Swedish . . . . . . . .

. .

. . . . . .

258 121

. .

. .

. .

. .

X. VI.

.

.

.

.

Defoliation and Desiccation: Harvest-Aid Practices .

.

.

.

.

Fertilizer and Lime Requirements of Soils . . . . . . . Fertilizer. Anhydrous Ammonia as a . . . . . . . . . Fertilizer. Molybdenum as a . . . . . . . . . . . Fertilizer. The Residual Effect of . . . . . . . . . . Fertilizer Use and Plant Nutrient Behavior. Changing Concepts of Field Crop Production and Soil Management in the Pacific Northwest Grass Breeding. Progress in . . . . . . . . Grasses and Legumes for Soil and Water Conservation . Green Manuring Viewed by a Pedologist . . . .

. .

x.

436

VI. VII.

199

39

IX.

68

. VIII.

242 62 164 205 68 2

.

. VIII. . VIII. . 1x3 . X. . VIII.

.

.

.

.

.

.

.

.

.

.

.

International Collaboration in Agriculture. A Pattern for .

.

Legumes and New Grasses for Soil and Water Conservation . Lespedezas . . . . . . . . . . . . . . . Lime and Fertilizer Requirements of Soils. The Determination of Liming . . . . . . . . . . . . . . . . Microbiology in Soil Science. The Place of . . . . . . Mineral Nutrition of Corn as Related to Its Growth and Culture Mineral Nutrition of Plants as Related to Microbial Activities . Molybdenum as a Fertilizer . . . . . . . . . . Organic Nitrogen in Soil. Mineralization of Organic Soils . . . . . . . .

VIII.

350

.

VII.

142

.

.

VI.

95

.

.

.

.

.

.

X. IX. VIII.

350 114 242 475

. VII. . VIII. . . VII. . . VIII.

399

.

.

X.

. .

. . . . . . . . VU. . . . . . . . . VIII.

427

128

x.

32 1

75 164 300

378

428

CUMULATIVE SUBJECT INDEX-VOLUMES

VI-X

Phosphorus. The Fixation of. by Soils . . . . . . . Plant Introduction as a Federal Service to Agriculture . . . Plant Nutrient Behavior and Fertilizer Use. Changing Concepts of Potassium in Plant Nutrition . . . . . . . . . .

Volume Page 95 189 . . VII. . . 68 X. 254 . . VI. .

=.

.

X. VI.

143 2

Safflower . . . . . . . . . . . . . . . . . X. Seed-Flax Improvement . . . . . . . . . . . . . VI. Soil Aggregation . . . . . . . . . . . . . . . VII. Soil and Water Conservation. New Grasses and Legumes for . . . X. Soil Bulk Density and Penetrability. Measurement of . . . . . IX. Soil Fertility. the Role of Sulfur in . . . . . . . . . . x. Soil Management and Field Crop Production in the Pacific Northwest VIII. Soil Nitrogen Balance Sheets. The Enigma of . . . . . . . VII. Soil Water. Requirement and Availability of . . . . . . . VI. Soils. Australian. and Their Responses to Fertilizers . . . . . . X. . . . . . . . . . . . . . VI. Sorghum Improvement IX. Statistics. The Contributions of . . . . . . . . . . . Sugar Beet in the United States. Improvement of . . . . . . VII. Sulfur in Soil Fertility . . . . . . . . . . . . . . X. Swedish Crop Production. Recent Changes in . . . . . . . VII.

290 144 2 350 159 408 2 213 67 168 305 177 90 408 39

Tall Fescue . . . . . . . . . . . Trace Element Usage. Technological Aspects of .

. VIII. . VI.

283 183

. .

VII. VIII.

252 204

IX.

31

Race 15B of Wheat Stem Rust-What It Is and What It Means . . Range Improvement. Progress in . . . . . . . . . . .

. .

. .

. .

. .

Weed Control in Principal Crops of the Southern United States . Winter Hardiness. The Evaluation of Crop Plants for . . . . Zinc Deficiency and Its Control .

.

.

.

.

.

.

.

.

.

.