Advances in agronomy volume 01

Four major segments of the plant aspects of the problem will be reviewed: a the physiological basis of salt and alkali tolerance, b how saline and alkali soils affect plant growth, c sal

VOLUME I

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ADVANCES IN

AGRONOMY

Prepared under the Auspices of the

Copyright 1949, by

ACADEMIC PRESS INC

125 EAST ,23m STREET

NEW YORK 10, N Y

All Rights Reserved

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

K C BERGER, Associate Professor of Soils, University of Wisconsin, Madison, Wisconsin

FRANCIS E CLARK, Bacteriologist, U S Department of Agriculture, De- partment of Agronomy, Ames, Iowa

A S CRAFTS, Professor of Botany, University of California, Davis, Cali- fornia

L A DEAN, Senior Soil Scientist, Division of Soil Management and Irri- gation, Bureau of Plant Industry, Beltsville, Maryland

J E GIESEKING, Professor of Soil Physics, University of Illinois, Ur- barn, Illinois

W A HARVEY, Associate in Botany, University of California, Davis, California

H E HAYWARD, Director, U S Regional Salinity and Rubidoux Labora- tories, Riverside, California

RANDALL J JONES, Chief, Soils and Fertilizer Research Section, Division

of Agricultural Relations, Tennessee Valley Authority, Knoxville, Tennessee

HovAm T ROGERS, Agronomist, Soils and Fertilizer Research Sectiom, Division of Agricultural Relations, Tennessee Valley Authority, Knoxville, Tennessee

ORA SMITH, Professor of Vegetable Crops, Cornell University, Ithaca,

WILLIAM J WHITE, Oficer-in-Charge, Dominion Forage Crops Labora-

tory, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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Many sciences and skills contribute to the subject of agronomy; many persons with widely different duties can properly call themselves agron- omists Not all of these agronomists would agree as to the precise defini- tion of the word agronomy, yet all, in some way or another, have interests that relate directly or indirectly to the growth of plants in soil The kind of professional training required of those studying the genesis and classification of soils has few points in common with that required of men engaged in genetical studies of a particular crop Yet their fields of activity are linked by their colleagues who must develop the proper fertilizer recommendation for profitable production of adapted varieties

of that crop on various soil types

The great body of knowledge about plants-their nutritive require- ments and growth, their management and improvement, their adaptation and utilization-is continually expanding It is becoming increasingly difficult for many of those involved in one way or another in the theory

or practice of soil management and crop production to keep themselves even reasonably well informed of the newer developments in all but their immediate fields of activity Progress is to a degree centrifugal in it.s

effects and through specialization tends to throw us apart

This volume, Advances in Agronomy, has as its objective the survey

and review of progress in agronomic research and practice The articles are written by specialists They are critical and reasonably compre- hensive in treatment They are written primarily for fellow agronomists across the hall and across the continents who also find i t difficult to keep well informed in all phases of agronomy The authors of this volume all live on the North American continent, and it is primarily North Ameri- can agronomy that is reviewed It is not intended that this shall always

be the case, and contributions to latcr volumes will be sought from workers in other countries overseas

I n the selection of topics for these volumes a n attempt will be made

to include material that will be helpful to workers with diverse subject matter and regional interests The edit,ors’ definition of what constitutes agronomy is catholic; they will be guided in their choice more by what information may be of use to agronomists than by what constitutes agronomy The central theme must be soil-crop relationships, for soils without crops are barren and field crops cannot be considered without

vii

viii PREFACE

reference to the soil on which they are produced From time to time articles may be included that deal with related fields of horticulture and forestry The editors will take cognizance of other publishing plans, in

so far as they are known to them, in order to avoid duplication of treatment For example, such considerat,ions led them to omit from this volume the subject of pastures, which was comprehensively surveyed in the 1948 Yearbook of Agriculture, and the subject of soil classification which was reviewed in a recent issue of Soil Science

The editors wish to acknowledge the co-operation of the several con- tributors t o this volume, whose articles have been prepared as a service

to the profession of agronomy

Frederick, M d

October, 1949

A G NORMAN

Page

Contributors t o Volume I v

P r e f a c e , vii

Plant Growth on Saline and Alkali Soils BY H E HAYWARD A N D C H WADLEIGH U S Regional Salinity and Rubidoux Laboratories Riverside California I Introduction

I1 Characteristics of Saline and Alkali Soils

IV Physiological Basis of Alkali Tolerance

VI Salt Tolerance as Related to the Life Cycle of the Plant References

I11 Physiological Basis of Salt Tolerance

V How Saline and Alkali Soils Affect Plant Growth VII Specificity in Salt Tolerance

1

2

5

9

10

20

29

35

New Fertilizers and Fertilizer Practices BY RANDALL J JONES AND HOWARD T ROGERS Tennessee Valley Authority Knoxville Tennessee I Introduction 39

I11 Recent Developments in Fertilizer Use 53

I1 New and Improved Fertilizer Materials 41

References 72

Soybeans BY MARTIN G WEISS Iowa State College Ames Iowa I Introduction

I1 Production and Distribution

I V Physiology of the Soybean Plant

V Effect of Climate and Location

VI Effect of Cultural Practices

I11 Disposition and Utilization

VII Genetics and Cytology VIII Variety Improvement

I X Effect on Soils

X Disease and Insect Pests

XI The Regional Approach to Soybean Research

References

ix 78

80

83

85

97

101

115

123

136

143

150

I 152

X CONTENTS

The Clay Minerals in Soils

HY J E GIESEKING Universil// n j Illinois lirbnrra Il1iiini.s

Pagc

I Introduction 159

I1 Historical Developiiicnt ol Clay Mineralogy 160

I11 Crystal Structure of the Clay Minerals in Soil Clays 162

IV Qualitative Identification and Quantitative Estimation of the Clay Minerals 171

177

Properties 180

VII The Physicochemical Reactions of the Clay Minerals 184

VIII Functions of the Clay Minerals 196

IX Conclusions 199

References 200

V Distribution of the Clay Minerals in Soils VI The Configuration of the Clay Mineral Crystals as Related to their Alfalfa Improvement BY WILLIAM J WHITE Dominion Forage Crops Laboratory Univeraily of Saskatchewan Saskatoon ,S%skalchewan Canada I Introduction 205

I1 Seed Setting and Production 206

I11 Progress in Methods of Breeding 225

IV Conquering Some Diseases 232

V Summary and Conclusions 237

References 238

Soil Microorganisms and Plant Roots Experiment Station Ames Zowa BY FRANCIS E CLARK U S Department of Agriculture and Iowa Agricultural I Introduction

I1 Types of Relationships between Microorganisms and Plant Roots I11 The Rhiaosphere Microflora in Relation to the Growth of Higher Plants IV The Numbers of Microorganisms Associated with Plant Roots V The Kinds of Microorganisms Found on Plant Roots

VI Modification of the Root Surface Microflora

VII Influences of the Rhizosphere Flora on Succeeding or Associated Plants References

242 247 249 264 270 274 278 282 Weed Uontrol BY A S CRAFTS A N D W A HARVEY University o f California Davis California I Introduction 289

I1 Tillage Cropping and Competition in the Control of Weeds 290

I11 Chemical Weed Control 293

IV Principles of Chemical Weed Control 293

I’agc

V Herbicidal Action 295

VI Molecular Properties of Herbicides 296

VII Emulsions and Emulsion Stabilizers 298

VIII Selectivity of Herbicides 299

I X The 2, 4-D Herbicides 300

X Uses of 2 4-D 303

XI Nitro- and Chloro-substituted Phenols 307

X I I 0 i l s 308

XI11 Other Organic and Inorganic Chemicals 310

XIV Water Weed Control 312

XV Herbicide Application Equipment 312

XVI Drift, Volatilization, Blowing of Herbicides Secondary and Residual Effects 312

S V I I Flame Cultivation 314

S V I I I Thc New Agronomy 314

References 315

Boron in Soils and Crops BY K C BERGER University of Wisconsin Madison Wisconsin I Introduction 321

I1 Boron Determination 323

111 Boron Availability in Soils 327

IV Boron Requirement of Plants 336

V Summayy 347

References 348

Potato Production BY OM SMITH Cornell University Illmca New Yolk I Introduction 363

I1 Breeding and Improving Potato Varieties 355

111 Chemical Weed Control 357

IV Fertilizer Practices 360

V Rotations and Green Manures 363

VI Response to Nitrogen Fertilization 365

VII Response to Phosphorus Fertilization 366

VIII Response to Potassium Fertilization 367

IX Effects of Magnesium Liming and Soil Reaction 369

X Minor Elements 371

X I Time and Method of Application of Fertilizers 372

XI1 Relation of Yield and Tuber Composition t o Plant and Soil Analyses 374 XIII Killing Potato Vines 377

XIV Recent Developments in Insect Control 381

XV Recent Developments in Disease Control 385

References 386

xii CONTENTS

Fixation of Soil Phosphorus

BY L A DEAN U S Department of Agriculture Beltsville Maryland

Page

I Introduction 391

I1 Accumulation of Phosphorus in Soils 392

I11 Phosphorus Fixation by Soils Clay Minerals and Hydrous Oxides 393

IV Chemically Precipitated Phosphorus 397

VI Biological Fixation of Phosphorus in Soils ! 406

References 409

Author Index 413

Subject Index 436

V Fixation of Phosphorus by Surface Reactions 400

H E HAYWARD AND C H WADLEIGH

U S Regionnl Salinity and Rubidoux Laboratories, Riverside California

C 0 N T E N T S

Page

I Introduction 1

I1 Characteristics of Saline and Alkali Soils 2

I11 Physiological Basis of Salt Tolerance 5

IV Physiological Basis of Alkali Tolerance 9

V How Saline and Alkali Soils Affect Plant Growth 10

1 Saline Soils 11

a Sodium 15

b.Calcium 15

c Magnesium 16

d Potassium 16

e Chloride 16

f Sulfate 17

g Bicarbonate 18

h.Nitrate 19

2 Alkali Soils 19

VI Salt Tolerance as Related to the Life Cycle of the Plant 20

1 Germination 20

2 Vegetative Growth and Maturation 25

29 References 35

VII Specificity in Salt Tolerance

I INTRODUCTION

The yield of a given crop is the net resultant of the effects of the prevailing weather conditions, the ravages of pathogens, and the existing status of the soil, within the genetic limitations of the plant Under normal conditions, soils affect yield through three primary factors: (a) moisture availability, (b) nutrient availability, and (c) physical con- dition A fourth factor excess salt may be present due to the accumu-

lation of chemical components in the soil t h a t are inhibitive to plant growth I n the irrigated soils of arid or semi-arid regions this factor

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

2 H E HAYWARD AND C H WADLEIGH

may be a principal consideration on those soils t.hat contain accumula- tions of salts or alkali

The problems of plant growth on saline and alkali soils are related primarily to the irrigated areas west of the Mississippi River According

to the Bureau of the Census, there were 20,258,191 acres of irrigated land

in this region in 1944 This represents a substantial increase since 1939

of 2,435,228 acres, or 13.7 per cent; and additional areas are coming

under irrigation as a result of new irrigation projects For example,

about 1,000,000 acres are proposed for development in the Columbia Basin in the Northwest, approximately 500,000 acres of new land are

being developed in the Lower Colorado Basin, and the proposed develop-

ment of the Missouri Basin may involve as much as 4,500,000 acres

Although the soils of some irrigated areas are nonsaline, the accumulation

of salt is a continuing threat to crop production on much of the irrigated land The trend in irrigation agriculture is in the direction of using all the available water including the drainage water and return flow from older irrigated lands The increased salt content of such water may be expected to increase rather than diminish the salt problem

Owing t o the importance of irrigation agriculture in the Western States and the fact that salt accumulation is a major problem in many

of the irrigated soils of this region, this review is designed t o consider some aspects of plant growth on saline and alkali soils The classifica- tion and composition of saline and alkali soils and their chemical nature

have been reviewed by Magistad (1945) Therefore, t.he consideration

of these topics will be limited to a brief statement of the characteristics

of saline and alkali soils and to definitions of soil terminology as used

by the authors Four major segments of the plant aspects of the problem will be reviewed: (a) the physiological basis of salt and alkali tolerance, (b) how saline and alkali soils affect plant growth, (c) salt tolerance a s related to the life cycle of the plant, and (d) specificity in salt tolerance

11 CHARACTERISTICS OF SALINE AND ALEALI SOILS

Saline and alkali soils occur for the most part in regions of arid or semi-arid climate and the process of salinization is frequently accelerated

by injudicious irrigation and poor drainage I n arid regions, leaching and transport of soluble salts to the ocean is not as effective or complete

as in humid regions Leaching is usually local and the soluble salts may not be transported far, owing to low rainfall and the high rates of evapo- ration characteristic of arid climates On the other hand, water is plenti- ful during the early development of an irrigation system and there is a tendency to use it in excess This may accelerate the rise of the water table unless provision is made for adequate drainage, and under such

Sampling

Chemical Composition of Some River Waters Used for Irrigation in

Western United States a

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

2 8 4 7.63

3 30 0.90

0 3 2 3.49 0.34 1.05 6.78 7.50 0.39 1.19

-

Milliequivalents per liter

Na

1127 4.06

022 3.47

025 3.00 23.02

1520 0.19 1.96

3.68 2.64 0.73 3.95 0.91 2.67 1.70 4.10

126 2.18

-

“These analyses were made by the U S Regional Salinity and Rubidoux Laboratories, Riverside, California

ECxl06 = conductivity expressed in micromhos per centimeter

T = trace

c1

9.95 2.05 0.09 0.62 0.05 1.10

2333 14.00 0.07 0.76

-

-

326

639 0.15 9.80 0.32

320 12.44 8.30

021 3.17

4 H E HAYWARD AND C H WADLEIGH

conditions ground water may contribute to the salinization of the soil This is particularly true if the water applied carries appreciable amounts

of dissolved salts as is frequently the case in irrigated areas Further- more, loss of drainage water from irrigated areas upstream and the pick-up of saline ground water result in more salt downstream The range of quality in irrigation waters is shown in Table I which gives t,he parts per million, electrical conductivity, chemical composition and sodium percentage for a number of river waters used for irrigation in western United States

Although many salt problems are man-made, it should be recognized that the occurrence of saline and alkali areas is related fundamentally

to changes in climatic conditions, the chemical composition of soil-form- ing materials in the primary rocks, and to geologic changes that have taken place with time due to deposition, erosion, weathering and other

processes (Harris, 1920; Hilgard, 1906; de Sigmond, 1938)

There are numerous publications dealing with various aspects of saline and alkali soils, some of which go back before the turn of the century

(Burgess, 1928; Gardner, 1945; Goss and Griffin, 1897; Hibbard, 1937; Hilgard, 1886, 1895-1898; Kelley, 1937; Powers, 1946; Tinsley, 1902) Magistad (1945) has reviewed a number of the schemes of classification

for saline and alkali soils and has reported the terminology proposed for them I n view of the differences in the meanings of terms as used in the

literature, the U S Salinity Laboratory (1947) has published a termi-

nology and description of saline and alkali soils The terms as defined

in that publication will be followed in this review and are given below:

Alkali Soil-A soil that contains sufficient exchangeable sodium to int.er- fere with the growth of most crop plants, either with or without ap- preciable quantities of soluble salts (See Saline-Alkali and Nomaline- Alkali Soil)

Nonsaline-Alkali S o i G A soil which contains sufficient exchangeable sodium to interfere with the growth of most crop plants and does not contain appreciable quantities of soluble salts The exchangeable-

sodium-percentage is greater than 15, the conductivity of the satura- tion extract is less than 4 millimhos per centimeter (at 25°C.) and the

p H of the saturated soil usually ranges between 8.5 and 10

Saline-Alkali Soil-A soil containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreci- able quantities of soluble salts The exchangeable-sodium-percentage

is greater than 15 and the conductivity of the saturation extract is greater than 4 millimhos per centimeter ( a t 25°C.) The p H of the saturated soil is usually less than 8.5

Saline Soil-A nonalkali soil containing soluble salts in such quantities that they interefere with the growth of most crop plants The con-

ductivity of the saturation extract is greater than 4 millimhos per centimeter (at 25"C.), the exchangeable-sodium-percentage is less than

15, and the p H of the saturated soil is usually less than 8.5

Alkalization A process whereby the exchangeable sodium content of the soil is increased

Salinization-The process of accumulation of salts in the soil

Exchangeable-sodium-percentage-This term indicates the degree of saturation of the soil exchange complex with sodium and is defined as follows:

Exchangeable sodium (m.e per 100 g soil)

ESP = Cation exchange capacity (m.e per 100 g soil)

relation to the total cation concentration, defined as follows:

Soluble-sodium-percentage-The proportion of sodium ions in solution in

Soluble sodium concent.ration (m.e per liter)

Total salt concentration (m.e per liter)

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

111 PHYSIOLOGICAL BASIS OF SALT TOLERANCE

Successful agriculture on saline and alkali soils requires the use of crops capable of producing a sat.isfactory yield under moderate intensi- ties of salt or alkali accumulation The question arises immediately as

to what constitutes the physiological capacity of a plant to tolerate salt

or alkali T h a t is, what is salt tolerance and how may it be defined? The salt tolerance of a variety or a species may be evaluated in three ways Firstly, salt tolerance may be looked upon as the capacity to persist in the presence of increasing degrees of salinity A given species may make little or no growth a t the higher levels of salt accumulation, but i t does survive That is, power of survival in increasingly saline soils regardless of growth would be the measure of salt tolerance This

is largely the criterion of the ecologist in evaluating halophytic environ- ments, since the species most capable of persisting in a saline area be- comes the climax vegetation of that area

Secondly, salt tolerance may be regarded from the standpoint of productive capacity a t a given level of salinity For example, a number

of varieties of a given crop may be tested in a soil having a certain degree

of salinization and the highest yielding variety may be designated as the most salt tolerant This method of interpretation may give a differen& evaluation of salt tolerance from the previous one, since experience has

6 H E HAYWARD AND C H WADLEIGH

shown that the capacity to produce well a t moderate levels of salinity does not necessarily imply the ability to persist a t higher levels of salt accumulation This second criterion is especially useful to the agronomist

in comparing the performance of strains and varieties of a given crop Thirdly, the relative performance of a crop a t a given level of soil salinity as compared to its performance on a comparable nonsaline soil may be used as a criterion of salt tolerance This method has certain advantages over the previously mentioned concepts in that comparisons between species are more readily evaluated For example, although pre- ference as t o salt tolerance should be given t o t h a t variety of alfalfa having the highest production on saline soil regardless of performance in the absence of salinity, one could hardly compare salt tolerance in alfalfa with that in cotton without taking into account the yielding power of these respective crops when growing on comparable nonsaline soils Evaluating salt tolerance on the basis of relative yield will not neces- sarily result in the same order of classification as power of survival a t high levels of salinity, but it will provide a more useful basis of apprais- ing agronomic crops to be grown on moderately saline soil I n variety and strain testing, tshe data on relative yield should be supplemented by data on absolute yield; ie., a strain may have a comparably poor relative yield because of unusual vigor of growth on the nonsaline soil, and yet yield the best of any of the strains a t the given level of salinity Every- thing considered, defining salt tolerance on the basis of relative yield t o that of the nonsaline condition is to be preferred for general agronomic use

I n discussing the physiological basis for the various degrees of salt tolerance which prevail among crop plants, it may be helpful t o consider the characteristics of the natural halophytes I n a review of this group

of plants, Uphof (1941) discusses the physiological characteristics of

halophytes, but i t is apparent that the specific physiology of these plants

is not well known The early investigators concluded that halophytism was essentially xerophytism, since both halophytes and xerophytes are adapted physiologically or anatomically t o a scarcity of water Ana-

tomical studies, such as those of Chermezon (1910), later revealed that

the two groups of plants must be regarded as distinct physiologically Halophytes tend to have relatively high values for the osmotic pressure

of the tissue fluids Fitting (1911) used an indirect method t o measure

the osmotic pressure of the cell contents of various species of plants on

the North African Desert The highest osmotic pressures, 100 atmos-

pheres or above, were found in plants growing on dry or highly saline soils Those growing on moist nonsaline soils had osmotic pressures of

10-20 atm The osmotic pressure of the various species tended t o vary

with the physiological scarcity of water in the environment in which the plants were growing This generalization has been verified by Harris

et al (1916, 1924), Keller (1920) and others There may be a wide

variation in the osmotic pressure of the tissue fluids depending on the environmental stress under which i t is growing Harris et al (1924)

found variations in the osmotic pressure of the tissue fluids of leaves of

Atriplex confertifolia from 31.2 to 153 atm ; in Allenrolfeu occidentalis

from 22.5 to 61.8 atm.; in Sarcobatus vermiculatus from 22.7 t o 39.8 atm.;

and in Salicornia utahensis from 36.8 to 51.9 atm

'Much of the variation in osmotic pressure of the tissue fluids was found to be associated with variations in chloride content, but not all of

it Keller (1925) observed that some halophytes may regulate the salt

content of their tissue fluids somewhat independently of the salinity of the environment- Salicornia may contain a lower concentration of sodium chloride than exists in the soil, or i t may accumulate NaCl far above the concentration of the soil, depending on the degree of soil

salinity Iljin (1922, 1932) states that only those plants should be con-

sidered halophytes whose protoplasm is resistant t o relatively high ac- cumulations of sodium ions in the cell sap Thus, halophytes may be described as having a t least three attributes which are important to

their survival on saline soil; (a) the capacity to develop rather high osmotic pressures of the tissue fluids in counteraction to the increased osmotic pressure of the substrate; (b) the capacity to accumulate con- siderable quantities of salts in the tissue fluids and to regulate that accumulation; and (c) a protoplasm which is characteristically resistant

to the deleterious effects of accumulations of sodium salts in the cell sap Application of the above criteria to an evaluation of the relative salt tolerance of economic crops is not sharply defined, and the varying physiological responses of different crop plants to saline soils prevent any generalization Brown and Cooil a t the U.S Regional Salinity Labora-

tory found in 1947 t h a t the osmotic pressures of the tissue fluids of alfalfa tops were 12.3, 14.5, 17.9, and 19.9 atm when grown on artificially

salinized soils in which the average osmotic pressures of the soil solutions

were 0.9, 4.2, 6.6, and 8.2 atm respectively Thus, even though there

was but little variation in the net osmotic gradient between soil and plant tops, there were marked reductions in yield If the yield on the

control plot that had 0.9 atm osmotic pressure in the soil solution be taken as 100 per cent, the yields on the other plots were 62.5, 32.4, and

21.5 per cent respectively T h a t is, the marked reduction in yield did not reflect the relative constancy in osmotic gradient The increase in osmotic pressure of the tissue fluids of the tops of these alfalfa plants could be largely accounted for by the increase in chloride salts in the

8 H E HAYWARD AND C H WADLEIGH

tissue fluids Alfalfa is regarded as one of thc more salt tolerant crops, and the theory could he advanced that its salt tolerance is related to thc intake of salt and the resiiltant increase in osmotic pressure of the tissue fluids as the salinity of the soil is increased Such a theory could not

be applied to certain other forage crops

Ayers and Kolisch * determined the osmotic pressure of the expressed sap of seven different leguminous forage plants grown on soil irrigated with water containing 0, 2500, 5000, and 7500 p.p.m of added salts Observations on red clover, Trifolium pratense, harvested in July showed

osmotic pressures of the expressed sap of 11.5, 20.6, and 23.7 atm re- spectively, for the first t.hree treatments The most saline irrigation water, 7500 p.p.m., killed the plants By August, the plants irrigated with water containing 5000 p.p.m of salt were killed, and by September only one or two plants survived t h a t were irrigated with water containing

2500 p.p.m added salts All control plants survived but they did not thrive during the hottest part of the summer Thus, red clover showed very poor salt tolerance, yet the increase in the osmotic pressure of the tissue fluids for a given increase in salinity of the substrate was greater than that observed for alfalfa This suggests that capacity to adjust internal osmotic pressure with respect to the substrate may be a poor criterion of salt tolerance It is pertinent to note t h a t for comparable levels of salinization, the expressed sap of red clover contained nearly three times as much chloride as that of alfalfa It, appears t h a t red clover plants were capable of effecting internal osmotic adjustments to com- pensate for the external increase in salinity, but the protoplasm of these plants was not sufficiently resistant to the deleterious effects of the ions

so accumulated

I n this connection, the observations of Ayers and Kolisch * on two species of trefoil are of interest The osmotic pressure of the expressed sap of the herbage of birdsfoot trefoil, Lotus corniculatus var TENNUI-

FOLIUS, which is a very salt tolerant legume (Ayers, 1948) was 12.0, 16.6, 17.3, and 19.1 atm respectively for the same qualities of irrigation water used on red clover Comparable values for big trefoil, Lotus uliginosus,

were 10.6, 16.9, 18.4, and 21.9 atm osmotic pressure There was a greater internal adjustment in osmotic pressure over a range of soil salinization

in big trefoil than in birdsfoot t.refoil, yet the big trefoil showed relatively poor salt tolerance At a given level of salinity, however, the expressed sap of the herbage of big trefoil contained nearly twice as much chloride

as did the birdsfoot trefoil

*This, and subsequent references in which the author’s name is followed by a n asterisk, relate to unpublished data obtained at the US Regional Salinity Labora- tory

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

IV PHYSIOLOGICAL BASIS OF ALKALI TOLERANCE

Very little is known concerning the physiological basis for the toler- ance of plants to alkali soils There appears to be considerable variat*ion among halophytes as to their tolerance to alkali as contrasted with salin-

ity Hilgard (1906) points out that Allenrolfea occidentalis and Sali- cornia subterminalis are two of the most salt tolerant halophtes, but their tolerance to “black alkali” (alkali) is relatively poor On the other hand, Sarcobatus vermiculatus and Sporobolus airoides are also highly salt tolerant, and have a remarkably high tolerance of “black alkali.”

I n evaluating tolerance of plants to alkali soils distinction must be made as to whether the soil is (a) high in exchangeable sodium but hav- ing a moderate pH, (b) high in exchangeable sodium, but with a p H

of 8.5 or above, and (c) high in exchangeable sodium but with a consider-

able accumulation of titrat,able carbonate The latter condition repre-

sents the status in “black alkali” soils as described by Hilgard (1906)

Although concrete evidence is very meager, it may be inferred that tolerance of a species to high percentages of adsorbed sodium is modified

by the p H of the soil and the accumulation of soluble carbonate

Breazeale (1927) concluded from his studies, however, that sodium car-

bonate occurs in “black alkali” soils in insufficient concentration to be toxic Thus, the infertility of most of these soils must be sought in their poor permeability to water and to other nutritional disturbances

Ratner (1935, 1944) presents evidence t h a t plant growth is inhibited

on high-sodium soils owing to availability of calcium Hence, tolerance

to soil alkali may involve the capacity by the plant to secure an adequate supply of calcium under conditions of relatively low availability Bower

and Wadleigh (1948) studied the influence of various levels of exchange- able sodium upon growt,h and cationic accumulation by dwarf red kidney beans, garden beets and Rhodes and Dallis grasses under controlled cul-

10 H E HAYWARD AND C 13 WADLEIGH

tural conditions in the greenhouse The culture media consisted of a

mixture of sand and synthetic cation- and anion-exchange resins (“Am- berlites”) containing the desired amounts of various cations and anions

in adsorbed form Adsorbed K, H2P04, NOs and SOr were supplied in constant amounts to all cultures, the potassium making up 10 per cent

of the cation exchange capacity Six levels of exchangeable sodium, wiz.,

0, 15, 30, 45, 60, and 75 per cent of the cation exchange capacity, con- stituted the treatments The remainder of the cation exchange capacity

was satisfied by calcium and magnesium, the Ca:Mg ratio being 3 : l

The p H value of all cultures was approximately 6.5

The tolerance of the different species to the presence of exchangeable sodium in the substrate varied greatly Beans were found to be especi- ally sodium-sensitive Growth of this species was markedly decreased a t exchangeable-sodium-percentages as low as 15 and almost completely inhibited a t the three highest levels of sodium employed I n sharp con- trast with the data for beans, Rhodes grass and garden beets were found

to be very sodium-tolerant Significant reductions in the growth of these species occurred only a t the highest level of sodium The growth of Dallis grass was not significantly lowered a t exchangeable-sodium-per-

centages of 30 or less but a t the higher sodium levels practically no

growth was obtained

The Ca, Mg, K, and Na contents of the roots and tops of each species were determined after harvest Accumulation of Ca, Mg, and K by the plants as a whole tended t o decrease and t h a t of sodium to increase progressively as higher proportions of exchangeable sodium were supplied The magnitude of the decreases in Ca, Mg, and K accumulation and the extent of sodium accumulations varied greatly among the species studied and between the roots and top parts of the plant These observations suggest the possibility that the species that are more tolerant t o high levels of exchangeable sodium are the ones which normally take in con- siderable amounts of sodium, whereas the more sensitive species are the ones which normally tend to exclude sodium

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

the availability of calcium and magnesium; (b) the activity of thc

hydroxyl ion may be sufficiently high to be toxic per se to the plant; and

(c) an accumulation of adsorbed Na on the exchange complex may have

a dispersive effect on the soil, and thereby bring about a “puddled” condition which may seriously curtail permeability to water and air

1 Saline Soils

Most evidence indicates t h a t accumulations of neutral salts in the substrate inhibit plant growth primarily as a consequence of the increase

in osmotic pressure of the soil solution and the accompanying decrease

in the physiological availability of water Magistad e t al (1943)

studied the growth response of numerous crops in sand cultures in which relatively large quantities of chloride and sulfate salts were added to a control nutrient solution Growth inhibition accompanying increasing concentrations of added salts was virtually linear with increase in osmotic pressure, and was largely independent of whether the added salts were chlorides or sulfates The slope of the negative regressions of yield on osmotic pressure of the substrate varied with the salt tolerance of a given crop The experiment was carried on under three different climatic condi- tions, and it was found that the slope of the regressions of yield on osmotic pressure for a given crop varied with climate Gauch and Wadleigh

(1944) studied the growth response of beans to increasing concentrations

of NaC1, CaC12, Na2S04, MgC12, and MgS04 added t o a control nutrient solution Growth depression was linear with respect to the osmotic pres- sure of the substrate and independent of whether a given level of osmotic pressure was developed by NaCl, CaC12, or Na2S04 Magnesium salts had a toxic effect in addition t o that which might be attributed to osmotic pressure

Hayward and Spurr (1943) attached potometers to corn roots and

measured the rate of entry of water into the roots as conditioned by the osmotic pressure of the substrate They found that for a given location

on the root, the rate of entry was inversely proportional to the osmotic pressure of the substrate and virtually independent of whether the in- creased osmotic pressure was developed by NaCl, CaC12, Na2S04, sucrose,

or mannitol Entry of water ceased when the osmotic pressure of the

substrate was maintained a t 6.8 atm.; in fact, a small outward movement

of water was recorded Significantly, an osmotic pressure of the tissue

fluids of 5.7 atm was recorded for roots comparable t o the ones studied

potometrically Roots which were permitted to become a t least partially adjusted to a given saline substrate had a higher rate of entry of water than comparable roots which were not subjected t o a preconditioning

treatment prior to the observation period (Hayward and Spurr, 1943)

12 H E HAYWARD AND C H WADLEIGH

Eaton (1941) and Long (1943) using divided root systems have also

shown that the rate of entry of water into roots is inversely proportional

to the physiological availability of the water as mensiired by the osmotic pressure of the nutrient solution

The evaluation of plant response to salinized sand or water cultures

is relatively simple as compared to the appraisal of growth behavior on

PERCENT SOIL MOISTURE (DRY BASIS)

-

saliniaed soils The os-

motic pressure of the artificial substrate may

be controlled r a t h e r precisely, but such con- trol is not possible in a

salinized soil The os- motic pressure of the soil solution a t a given salt content of the soil will vary inversely with changes in the moisture

c o n t e n t of the soil That is, the normal fluctuation in soil mois- ture content between rains or irrigations is

a c c o m p a n i e d by in- verse fluctuations in osmotic pressure of the soil solution Also, wa- ter cannot move into

or through a soil with-

o u t c a r r y i n g s o l u t e with it Consequently, Fig 1 Relationship between soil moisture stress and marked variations in

moisture percentage with the salt content a t different Q the salt content of the

soil may occur within values in a sample of Panoche loam

the root zone as a result of water movement (Wadleigh and Fireman,

1948) Furt,her, the withholding of water from the plant through sur-

face force action by the soil varies with the moisture content of the soil, and the effect of this retentive force is theoretically additive t o that of physiological unavailability of water induced by the osmotic pressure of

the soil solution Wadleigh (1946) has discussed the complexities con-

t.ributed by these variables in determining the relationship between salt content of soil and plant response

This problem may be illustrated by reference to Fig 1 showing the

moisture tension curve of a sample of Panoche loam together with the effect of increasing degrees of salinization upon the total soil moisture

stress as conditioned by t.he moisture content of the soil (Wadleigh, 1946)

The level of salinity in this instance is measured by a n arbitrarily chosen “Q value” which specifies the osmotic pressure of the soil solution The “total soil moisture stress” is defined as the summation of the osmotic pressure of t,he soil solution and the soil moisture tension ex- pressed in atmospheres

The most useful concept of soil moisture from the agronomic stand- point, is the “available range” as delimited by “field capacity” and “per-

manent wilting percentage” (Veihmeyer and Hendrickson, 1927) The

soil moisture tension a t field capacity is evident.ly somewhere in the

neighborhood of 1/10 to 1/3 atm The moisture retained by a soil in equilibrium with a displacing force of 15 atm has been found to approxi-

mate the permanent wilting percentage for many soils (Richards and

Weaver, 1943) Thus, the curve on the left in Fig 1 shows the change

in moisture tension between the field capacity and the permanent wilting percentage of this sample of Panoche loam

The hyperbolic nature of this curve is a prime consideration in the evaluation of plant responses to variations in soil moisture in terms of the energy status of the moisture At the higher levels of soil moisture within the “available range,” there is little change in the energy Bf retention over a considerable range in moist.ure content, whereas a t moisture levels just above the wilting percentage, there is a marked change in surface force action with little change in moisture content This hyperbolic relationship is common to most soils and part.ially ex- plains the observation that for all practical purposes under field condi- tions in the nonsaline soil, the soil moisture between field capacity and permanent wilting percentage is “equally available” to the plant (Con-

rad and Veihmeyer, 1929; Hendrickson and Veihmeyer, 1929, 1942; and Veihmeyer, 1927) Certainly, moisture withheld from the plant b y a force of 15 atm is not as readily available as that retained by a forcc

of only 1/3 atm.; but, the hyperbolic relationship found for most soils

indicates that most of the available water is absorbed from the soil

before the moisture tension reaches 2 or 3 atm

The remaining curves in Fig 1 show how increasing concentrations of

salt in the soil affect the relationship between soil moisture stress and moisture content T h a t is, the soil moisture stress may approach or even

exceecl a value of 15 atm a t tlie moisture content of field capacity Richards and Weaver (1944) indicated that growth of most plants ceases

14 H E HAYWARD AND C H WADLEIGH

when the moisture tension reaches about 15 atm., and Wadleigh and Gauch (1948) found t h a t leaf elongation of cotton stopped when the total soil moisture stress in a saline soil reached about 15 atm

There is evidence, however, that different species of plants vary con- siderably as to the level of soil moisture stsress a t which symptoms of marked water deficit will be in evidence, Wadleigh et al (1947) grew bean, corn, alfalfa, and cotton plants in containers of soil, 1 foot square and 36 inches deep, varying in added salt content from none in the surface 6 inches to 0.25 per cent a t the bottom Observations on these soil columns when the plants of each species were showing marked moisture stress revealed that as the salt content of the soil strata in- creased, the roots of the various species showed a corresponding decrease

in their ability t o remove water Comparable cultures of nonsaline soil showed that roots of all species were normally capable of penetrating the deepest layer in the culture and removing all available water I n the salinized cultures, water was removed from each layer to such n degree that final osmotic pressures of all layers in the soil column were nearly uniform These critical osmotic pressures of the soil solution were found

to be 7 to 8 atm for beans; 10.5 to 11.5 atm for corn; 12 t o 13 atm for alfalfa; and 16 to 17 atm for cotton Wadleigh and Fireman (1948)

found a comparable inverse relationship in the patterns of salt distribu- tion and water removal in the root zone of furrow-irrigated cotton

On the basis of the preceding st.atements, it is evident that plant growth on saline soil, as conditioned by water relations, involves an in- tegration of the following variables affecting moisture availability in the root zone: ( a ) variation in salt distribution within the soil mass and its consequent effect on the variation in the osmotic pressure of the soil solution a t a given moisture content; (b) variation in osmotic pressure in relation to change in moisture content; (c) variation in moisture tension

in relation to moisture content; (d) variation in moisture content within the soil mass a t a given time; and (e) variation in total water content

of the soil in the root zone with time A mathematical method (Wad- leigh, 1946) has been developed to integrate these variables and permit the derivation of the average moisture stress affecting the plant over an extended period of time It has been found that vegetative growth of beans (Wadleigh and Ayers, 1945) and guayule (Wadleigh et al., 1946)

is rather closely related to the average moisture stress if other factors are not limiting t o growth The daily rate a t which cotton leaves enlarge has also been found to be correlated with the intensity of the soil mois- ture stress (Wadleigh and Gaucli, 1948)

I n summary, one of the main effects of moderate levels of soil salinity

is that of limiting water supply to the plant and thereby inducing those modifications in plant behavior normally associated with water deficits

in the tissues Obviously, a soil may become sufficiently saline to prevent even the growth of halophytes, just as it may become too dry to support growth of xerophytes

Depending on the species, each of the various components that may

be present in saline solutions may have some specific toxic effect on the plant over and above that which may be accounted for on the basis of the osmotic pressure of the soil solution

The ions which may accumulate in saline soils are: N a + , C a + + , Mg++, K + , C1-, SOa=, HC03-, and NOs-

a Sodium There is relatively little evidence that indicates positively

the specific toxicity of the sodium ion to plants growing in saline soils

Many species tend to exclude sodium (Collander, 1941; Gauch and Wad- leigh, 1945; Hayward e t al., 1946; Wallace et al., 1948); and specific

toxic effects may arise from such exclusion of sodium along with accumu-

lation of accompanying anions from the substrate (Hayward, 1946)

Such instances should not be classed as sodium toxicity

Lilleland e t al (1945) found that a tip-burn condition on almond

leaves in California was directly related to the sodium content of the leaf Neither the salinity of the soil nor the sodium content of the soil solution was high, and the condition may have been more indicative of

an alkali soil condition rather than salinity It is possible that accumula- tion of sodium within the plant may be associated with a depression in the accumulation of the other cations to the extent that their content may be below adequate levels, or a n unfavorable cationic balance may

be induced Whether or not such a condition should be designated as sodium toxicity is merely a question of definition At present, there exists little clearcut evidence that strictly saline soils may induce sodium toxicity per se

b Calcium The calcium ion may accumulate to high concentrations

in saline soil solutions, and this concentration may be specifically toxic The specific effect of high concentration of calcium varies with the species For example, guayule was found to be relatively more tolerant

of a saline substrate induced by CaClz than to those induced by other

neutral salts (Wadleigh and Gauch, 1944) Masaewa (1936) found t h a t

applications of CaClz t o soil cultures of flax were more highly toxic than applications of NaC1 The chloride ion accumulated to high levels in the plants on CaClz cultures, so she ascribed the difficulty to chloride toxicity and to an unfavorable Ca/K ratio since Ca was also found to accumulate t o rather high levels Wadleigh and Gauch * observed the same effect on orchard grass, but they also' noted that salinization of the

16 H E HAYYWARD AND C H WADLnIGH

soil with C a ( N 0 3 ) a had the samc effect as CaC12 There was a high accumulation of Ca in orchard grass on the C a ( N 0 3 ) 2 treatments but only a small amount of chloride, hence chloride toxicity was not involved Ayers * has secured comparable data for tall fescue grass Lehr (1942)

attributes the stimulative effect of sodium on sugar beets to the fact that the sodium effectively counteracts absorption of calcium, thereby pre- venting the development of what hc calls a “calcium-type-plant.” Such

a plant has a bluish-green east and appears to be stunted in growth

c Magnesium High accumulations of magnesium in the substrate have been found t o be especially toxic to plants over and above any inhibition in growth that might be associated wit.h osmotic pressure (de

Sigmond, 1938; Trelease and Trelease, 1931 ; Wadleigh and Gauch, 1944)

Magnesium injury may be associated with an inadequate supply of calcium

within the tissue (Gauch, 1940) Both Ayers * and Wadleigh and Gauch *

have obtained evidence that plants may not show specific symptoms of magnesium toxicity under conditions of accumulated magnesium in the soil solution when calcium ions are also present, a t a relatively high level

d Potassium Accumulations of potassium in the soil solut.ian are rather rare, but they may occur If such an accumulation is partially balanced by calcium no specific inhibitive effects of potassium on plant response are noticeable (Ayers *; Wadleigh and Gauch *) Cases have been reported in which relatively high levels of potassium have induced

characteristic symptoms of iron chlorosis (Walsh and Clarke, 1942) and magnesium deficiency (Boynton and Burrell, 1944)

Cations differ markedly in their effect upon the physical properties

of the colloidal constituents of protoplasm; and there are pronounced antagonistic effects exhibited between various cation pairs in counteract- ing the adverse effect of one or both cations upon protoplasmic activity (Chambers et al., 1937; Heilbrunn and Daugherty, 1932; Moyer and Bull,

1935) As Lundegbrdh (1940) points out, the monovalent cations have

such a dispersive effect upon protoplasmic colloids t h a t they may induce complete disorganization and deat.h unless balanced by a divalent cation, especially calcium On the other hand, the divalent ions tend t o have a coagulative eff eet and may seriously inhibit permeability of the mem- branes There are instances, however, in which the effect of M g + + on protoplasm is more nearly comparable to K + and N a + than to C a + +

(Heilbrunn and Daugherty, 1932) Hence the frequently noted mutual

antagonism between Ca++ and Mg++ It is apparent that, species differ widely as to the extent to which they may be susceptible to the adverse effect of abnormally wide ratios between various cations

e Chloride There has been some tendency to regard chloride toxicity and the adverse effect of soil salinity as synonymous For many species of

plants, chloride salts are no more inhibitive to growth than isosmotic con-

centrations of sulfate salts (Eaton, 1942; Hayward and Long, 1941; Mag-

istad, et al 1943) Eaton (1942) found that lemon cuttings, navy beans,

and dwarf milo were more sensitive t o chloride than to isosmotic concen- trations of sulfate when grown in sand cultures Hayward et al (1946)

noted that peach trees were especially sensitive t o chloride salts, and

Harper (1946) that pecans were quite sensitive to chloride The work of Garner et al (1930) on tobacco provides a good insight into the mecha-

nism of chloride toxicity They found that the high level of chloride ac- cumulation in tobacco leaves resulting from heavy fertilization with potas- sium chloride was associated with a pronounced dissipation of the malic acid content of t.he leaves Since the organic acids are the major com- ponents of the buffer mechanism of plant cells, conditions effecting dis-

sipation of organic acids could have a significant effect on the p H control

within the cell and the associated activity of the protoplasm These in- vestigators also noted that if the tobacco plant receives a n excess of chloride the normal amylolytic activity is disturbed and the leaves be-

come gorged with starch Baslavskaja (1936) observed that accumulation

of the chloride ion in potato leaves interferes with the photosynthetic mechanism, i.e., causes a reduction in chlorophyll content; and, conse- quently, a reduction in total carbohydrate content, even though there was

a definite increase in the starch/sugar ratio Schuphan (1940) concluded

from his data that it is not possible to make sweeping conclusions as to the effect of chloride on carbohydrate metabolism since species vary so greatly in their response to the chloride ion

Beneficial effects from added chloride salts have been noted for table

beets (Raleigh, 1948), sugar beets (Eaton, 1942; Tottingham, 1919) tomato (Eaton, 1942), and spinach (Schuphan, 1940)

f Sulfate There are numerous observations on several species of crop plants indicating specific toxicity of high concentrations of the

sulfate ion This has been reported for flax (Hayward and Spurr, 1944), tomato (Eaton, 1942), cotton and orchard grass (Wadleigh and Gauch"), and leek (Schuphan, 1940) But there is a dearth of information to

explain why the sulfate ion has an inhibitive effect on the growth of certain species Harris et al (1925) found t h a t Egyptian cotton varieties

tend t o accumulate considerably more chloride in their tissues than do American upland varieties, and that the converse tendency was expressed with respect t o the sulfate ion Cotton variety tests conducted in large outdoor sand cultures by Wadleigh and Gauch * failed to show any clear- cut distinction between these two types of cotton in their respective tolerance of high accumulations of sulfate It is obvious, however, that high concentrations of sulfate in the substrate definitely limit the activity

18 H E HAYWARD AND C H WADLEIGH

of the calcium ion and thereby condition cationic intake by plants Analyses of leaves of beans (Gauch and Wadleigh, 1945), peach trees (Hayward et al., 1946), orchard grass and cotton (Wadleigh and Gauch ")

showed that the tissues contained an appreciably lower content of cal- cium and higher contents of sodium and potassium when sulfate was the predominant anion in the substrate as compared to similar cultural condi- tions in which chloride was the predominant anion It may be presumed that specific adverse effects of sulfate are related to a disturbance in the optimum cationic balance within the plant, but the evidence is too limited t o warrant a broad generalization

g Bicarbonate As pointed out by Heller et al (1940), the bicarbon-

ate ion is quite toxic to plants They noted t h a t the presence of accumu- lations of bicarbonate in the substrate markedly inhibited the intake of calcium by plants Harley and Lindner (1945) reported t h a t apple orchards in Washington irrigated with water relatively high in bicarbon- a.te tended to become chlorotic, and that the condition could be partially alleviated by subsequent irrigation with low bicarbonate water They found also a heavy incrustation of calcium and magnesium carbonates upon the roots of the apple trees in orchards which had been irrigated for some time with water relatively high in calcium and magnesium bicarbonates They suggested t h a t such a condition could seriously affect the mineral nutrition of the tree, and as a consequence induce the symp- toms of chlorosis so frequently observed Gauch" found evidence of

marked specificity in tolerance of the bicarbonate ion His data showed that the addition of 12 m.e./l of bicarbonate to a nutrient solution had virtually no effect on the growth of Rhodes grass, whereas the same con- centration caused Dallis grass to become seriously chlorotic or to bc killed Increasing concentrations of bicarbonate have caused pronounced chlorosis and inhibition of growth in beans (Wadleigh and Brown") whereas the same treatments effected a comparably small decrease in growth and little visual evidence of chlorosis on garden beets (Brown and Wadleigh ")

Steward and Preston (1941) studied the effect of bicarbonate on ionic absorption and metabolism by potato disks At a constant pH, increas- ing the external concentration of potassium bicarbonate depressed botli protein synthesis and bromide accumulation Indirect evidence indicated that KHC03 also depressed respiration and carbohydrate metabolism The study with bean plants (Wadleigh and Brown") showed tshat in- creasing concentrations of bicarbonate induced a pronounced depression

in the intake of calcium, and an increase in intake of potassium I n beets, with their normally low calcium content, intake of calcium was not affected, that of magnesium was depressed markedly, that of potas-

sium depressed slightly, and intake of sodium increased These data serve to illustrate that the adverse effect of bicarbonate upon plant response is intimately associated with the specificity of the plant with respect to ionic intake and metabolism

h Nitrate The nitrate ion may accumulate to rather high levels in certain naturally saline soils, the condition being characterized as the

development of “niter spots” (Stewart and Peterson, 1915) There are

several instances known in which high levels of nitrate supply inhibited

growth (Chapman and Liebig, 1940; Eaton and Rigler, 1945; Leonard

et al., 1948), but i t is usually difficult t o draw a olearcut distinction between any specific effect of the nitrate ion and concomitant effects in- duced by the higher osmotic pressure of the substrate or the effect of the

complementary cations Howcver, Headden’s early observation (1912)

that nitrate accumulations in the soil contribute to the production of inferior quality sugar beets because of low sugar content has been verified many times

6 Alkali Soils

Kelley (1928) suggested 20 years ago that the presence of a relatively

high proportion of sodium on the exchange complex of soils may prevent the plant roots from obtaining an adequate supply of calcium because

of “the pronounced avidity of the sodium-exchange complex for calcium.” This effect of adsorbed sodium on the availability of calcium has been

observed many times (Gedroix, 1931; Ratner, 1935, 1944; Thorne, 1944; Van Itallie, 1938) For most crop plants, the calcium becomes unavailable

when the exchangeable-sodium-percentage approaches 50 Bower and

Turk (1946) found that high percentages of exchangeable potassium were

just as effective as those of sodium in preventing calcium and magnesium availability to plants

Although alkali soils may actually have a n acid reaction, many are

found that have a p H of 9 or even 10 Few if any crop plants can thrive

under such alkalinity Arnon and Johnson (1942) grew tomatoes, lettuce

and Bermuda grass in nutrient solution in which a range from p H 3 to

pH 9 was maintained Although all three species were tolerant of a wide range in p H value of the subshate, a marked decline in growth

was observed a t p H 9 Breazeale and McGeorge (1932) found that the

carbon dioxide content of alkaline-calcareous soils was extremely low

They concluded that the low level of COz in such soils was a major factor

in the lack of availability of phosphate which also was observed for these soils That is, their observations indicated that phosphorus unavailabil- ity was the major limiting factor in these alkaline-calcareous soils, and

20 13 E HAYWARD AND C H WADLEIGH

that this condition was brought about by thc low level of CO, occurring

in a substrate high in alkalinity

Sodium soils readily become dispersed and a dispersed soil is not

conducive t o vigorous growth of plants McGeorge and Breazeale (1938)

studied the effect of puddled soils on plant growth, and found that plants growing under normal conditions will wilt after the soil is puddled, even though an abundant supply of moisture may be present The puddled soil has a lowered capacity for gaseous interchange which may result, in oxygen deficiency a t the absorbing surfaces of the roots, and most plants are unable to take in adequate quantities of water a t low oxygen tensions These investigators also presented evidence that nutrient availability as well as water availability is lowered in a puddled soil Fireman and

Reeve (1948) made a study of alkali soils in Gem County, Idaho I n this area, barren islands of alkali soil are interspersed among soil areas supporting fairly good crop growth They made a study of various soil attributes which might show a wide differential between soil supporting good growth and adjacent barren areas They found that rate of infil- tration was the most consistent criterion, the poor infiltration on the barren areas causing the soils to be deficient in moisture most of the time That is, the very poor structural status of the alkali soil in the barren areas prevented a replenishment of the soil moisture reservoir that is

essential for plant growth

VI SALT TOLERANCE AS RELATED TO THE LIFE CYCLE OF THE PLANT

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

1 Germination

Under saline soil conditions, the first phase, germination and seedling growth, is critical, since the ability of a given variety to germinate and est*ablish the seedling is frequently the limiting factor in crop producton There are two ways in which saline soils may effect germination: ( a ) there may be enough soluble salt in the seed bed t o build up the osmot.ic pressure of the soil solution to a point which will retard or prevent intake

of necessary water, and (b) certain constituent salts or ions may be toxic

to the embryo and seedling

The effect of high osmotic pressure of the soil solution was investi-

gated in early work by Buffum (1896, 1899) who concluded that “the

retarding effect of a salt solution on the germination of seeds is in direct proportion t o its osmotic pressure when the solutions are strong.’’ Similar

conclusions were reached by Slosson and Buffum (1898) and Stewart

(1898) They found that if the osmotic pressure was high enough, no germination occurred ; but it was noted that a t a given salt concentration various species of agricultural plants exhibited differential salt tolerance

with respect to germination Stewart (1898) found that the cereals as

a group were more tolerant of salt than the legumes, and listed their relative salt tolerance in the following descending order, barley, rye, wheat, oats His order of tolerance for legumes was peas, red clover, alfalfa, and white clover

Early investigators tested a large number of agricultural crops to determine the limits within which seeds would germinate; but, in many instances, the methods used were not standardized and comparison of

data is impossible Harris (1915) has reviewed the early literature on

seed germination which was done chiefly in solution cultures, in many cases using single salts He points out that conclusions drawn from such studies “should not be too definitely applied to the action of alkali as

it is found in the soil,” citing as an example that “the salts of magnesium when present alone are very toxic, while if added to a normal soil they are no more toxic than a number of other salts.”

I n his first germination tests, Harris (1915) used glass tumblers which held about 200 g of soil The salt levels ranged from no salt to 10,000 p.p.m., or 1 per cent on a dry weight basis; various single salts and com- binations of salts were used and over 18,000 determinations were re-

ported Like earlier workers, he found that crops varied greatly in their relative resistance to alkali salts and listed crops tested in the following descending order of tolerance, barley, oats, wheat, alfalfa, sugar beets, corn, Canada field peas

Shive (1916) using a sand culture technic and single salts tested the germination of beans and corn a t osmotic pressures ranging from 0.5 to

8.0 atm His data indicate that “retarded germination is directly related

to the amount of water absorbed by the seeds, which in turn is depend-

ent upon the concentration of the soil solut.ions.” Rudolfs (1925) tested

seeds of white lupine, watermelon, Canada field peas, buckwheat, soy- beans, wheat, corn, beans, alfalfa, and dwarf rape He used presoaked seeds and subsequent germination on beds of filter paper with single salts, NaNOs, Ca NaC1, K2C03, KCl and MgS04, a t osmotic pressures

up to 7 atm Except for some of the weaker solutions, absorption, germi- nation, and root-growth decreased with increase in concentration of the salts Peas, alfalfa, lupine, buckwheat and watermelon were far less salt tolerant than corn and wheat

It is difficult to evaluate the level of salinity conditioning the germi- nation of seeds under field conditions since the amount of soil moisture

22 H E HAYWARD AND C H WADLEIGH

and the salt conccntration adjacent to the seed are continually changing, owing to evaporation, capillary transmission, and rainfall or irrigation Ayers and Hayward (1948) have reported a method for measuring the effects of soil salinity on germination which involves moistening and salinizing nonsaline soil so that a specified soil moisture percentage and salinity level are obtained The moisture content of the soil and the salt content of the extract from the saturated soil are determined on sub- samples and these data permit a calculation of the osmotic pressure of the soil solution in the germination culture Weighed amounts of the preconditioned soil are placed in large culture dishes and planted with

a definite number of seeds The covered cultures are maintained in a constant temperature room (70°F.) to eliminate temperature as a vari- able and to prevent moisture distillation in the germinators which occurs under fluctuating temperahres Several salinity levels were set up, rang- ing from 0.05 to 0.4 per cent sodium chloride on a dry soil basis The osmotic pressures of the soil solutions, calculated from the electrical conductivity of the saturation extract and the soil moisture content at

time of planting, ranged from 0.7 to 25.3 atm

Alfalfa, sugar beets, two varieties of barley, Mexican June corn and

red kidney beans were tested No seeds germinated a t the 0.4 per cent level, but barley, (California Marriout), gave 80 per cent germination a t

the 0.3 per cent salt level (20 atm osmotic pressure) Although alfalfa and sugar beets are regarded as salt tolerant crops, the data indicate

that they are relatively sensitive during germination Alfalfa gave 80

per cent germination with 0.1 per cent added salt (7.3 atm osmotic pres- sure) and the germination of sugar beets was reduced to 50 per cent a t

5.8 atm osmotic pressure Corn, which is less tolerant than sugar beets

or alfalfa during later stages of growth, gave satisfactory germination

(93 per cent) a t approximately 10 atm osmotic pressure and red kidney beans, which are very sensitive to salt, germinated slightly better than sugar beets These data indicate that there is not always a positive correlation between salt tolerance a t germination and during later phases

of growth

The differential toxic effects of salts 01: ions in the substrate on germi- nation and the development of the embryo and seedling have been studied

by a number of inxestigators Harris (1915) found the relative toxicity

of soluble salts to be in the following descending order: NaC1, CaC12,

KCl, MgCL, KN03, Mg(NOd2, NazC03, NazSO, and MgSOr With respect to antagonism, he concluded that the effect of combined salts was not so great in soils as in solution cultures Harris and Pittman (1918)

in a continuation of the above study compared the relative toxicity of

NaCl a t concentrations of 0 to 4,000 p.p.m and of NazCOs and NazSOl

a t concentrations up to 10,000 p.p.m a t moisture levels ranging from 20

to 32 per cent Up to 1,000 p.p.m., all salts were beneficial, but above 1,500 p.p.m all salts were increasingly toxic, chloride being most so, sul- fate the least, and carbonate halfway between

The alkali carbonates are usually found to be the most toxic salts Stewart (1898) found Na2S04 less injurious than N a C l and Na2C03 most toxic Kearney and Harter (1907) tested seedlings of maize, sorghum, oats, cotton and sugar beets in water cultures, using NaC1, MgC12 and MgS04 as single salts, to determine the critical concentrations

a t which half of the root tips of seedlings exposed to these concentrations

for 24 hours failed to survive when subsequently transferred to water

They found great differences in resistance to magnesium and sodium salts

in solution among the eight species tested, maize being most resistant and cotton the least The presence of CaS04 in excess greatly diminished the toxicity of magnesium and sodium salts, the neutralizing effect being greatest when added to MgS04 cultures and least in combination with Na2C03 Rudolfs (1925) found that presoaking in distilled water re- tarded germination of all seeds, and noted differential responses to vari- ous single salts All seeds were injured in K2C03 solutions and abnor- malities occurred when this salt or MgS04 was used C a ( N O d 2 had a detrimental effect on germination and root, growth with nearly all vari- eties of seeds except corn

Uhvits (1946) studied the effect of osmotic pressure on water absorp- tion and germination of alfalfa seeds using concentrations of sodium chloride and mannitol ranging from 1 to 15 atm osmotic pressure These tests were made on filter paper in Petri dishes maintained at a constant temperature of 71°F * 2" Other tests were made in sand cultures under greenhouse conditions using sodium chloride a t concentrations of 1 to 12 atm osmot,ic pressure She found that germination was virtually in- hibited when N a C l solutions of 12 to 15 atm osmotic pressure were used, and t h a t reduction and retardation of germination were greater on N a C l than on mannitol substrates The difference in response on the tewo substrates a t isomotic concentrations suggests a toxic effect of N a C l and this assumption is supported 'by data showing the accumulation of

chloride in alfalfa seeds after 4 days of treatment For example, on a dry weight basis the per cent chloride in the seeds increased from 0.04 per cent in t a p water to 1.18 and 1.79 respectively on the 3 and 15 atm substrates The data indicate that a t high concentrations, total absorp- t.ion values were greater with mannitol than with sodium chloride; con- sequently if given enough time, relatively high germination rates were obtained with mannitol a t 12 and 15 atm (71 and 57 per cent, re- spectively) That high concentrations of sodium chloride are toxic is

24 H E HAYWARD AND C H WADLBIGH

supported further by studies which showed that recovery of seeds trans- ferred from a 12 atm substrate of sodium chloride to tap water was con- siderably greater than the recovery of seeds treated for the same length

of time on a 15 atm substrate of NaCl The percentage of deformed seeds on the sodium chloride substrate a t 15 atm osmotic pressure was greater than a t 12 atm., and the number of deformed seedlings in all con- centrations of sodium chloride was much greater than in the correspond- ing concentrations of mannitol

The influence of tcmperature as related to the effect of salt on ger- mination should be mentioned Ulivits (1946) found that an increase in the mean greenhouse temperature of 5°F reduced the per cent germi- nation a t all levels of salt treatment, the differences being more pro- nounced a t the higher salt levels Ahi and Powers (1938) studied the effect of temperature and other factors affecting salt tolerance using salt grass, alfalfa, sweet clover, strawberry clover and Astmgulus rubyii as test plants The plants were grown in sand and water cultures; and sea water, fortified with n nutrient solution and adjusted to salt concentra- tions ranging from 306 to 11,200 p.p.m., was used I n one study wit.h strawberry clover and alfalfa, temperatures were controlled a t 55", 70" and 90°F There was a definite decrease in the per cent germination with increase of temperature or salt concentrat,ion At 90°F there was prac- tically no germination regardless of salt level; but a t 55"F., 47.7 per cent of the strawberry clover and 38 per cent of the alfalfa seeds germi- nated The work of Ogasa (1939) on the effect of sodium chloride solu- t,ions on soybeans a t high and low temperatures confirms the above findings He found the limit of concentration of N a C l solutions a t which germination occurred to be 200 m.e./l for high temperature (30°C.) and

300 m.e./l for low temperature (15°C.)

T o summarize, it is evident that germination is retarded or inhibited

by the presence of soluble salts in the soil and that this effect is related primarily to the osmotic pressure of the soil solution As osmotic pres- sure increases, rate and per cent germination decrease There is evidence that certain salts or ions may be toxic to the embryo or seedling if oc- curring in sufficiently high concentrations This toxicity may be reflected

in reduced germination and is frequently accompanied by abnormalities

in the growth and development of the seedling High temperature is an important consideration ; and, a t isosmotic concentrations of salt, per cent, germination decreases with increase of temperature above optimum levels The studies reported indicate that species and varieties of plants exhibit varying degrees of salt tolerance with respect to germination and seedling growth, and they emphasize the importance of crop selection on the basis of salt tolerance in areas where salinity is a problem

2 Vegetative Growth and Maturation

Vegetative growth is retarded as the osmotic pressure of the substrate

is increased Buffum (1896) noted t h a t growth is in proportion t o the

amount of salts present in the substrate and similar conclusions were reached by Harris (1915), Harris and Pitt.man (1918), Hayward and

Spurr (1944), and others Eaton (1942) in studies on the toxicity of

chloride and sulfate salts has pointed out t h a t the growth depression curves showed no evidence of an abrupt point a t which the effect of increasing osmotic pressure became pronounced Magistad et al (1943)

reported that growth reduction was in most cases linear with increasing osmotic concentration of the substrate, and Gauch and Magistad (1943)

in a study of the effect of salt on legumes, found no evidence that there

is a given concentration of solution which may be regarded as critical, but, rather there tended to be a linear relationship between growth reduc- tion and increase in salt concentration of the solutions as expressed in atmospheres.”

The first effect of increasing concentration of salt on vegetative de- velopment is usually a reduction in rate of growth which may not be accompanied by any visible symptoms of injury As Eaton (1942) has

pointed out, this absence of leaf symptoms of diagnostic significance or other pronounced outward abnormalities suggests “that a substantial proportion of the curtailed production of crops in irrigated areas that was attributed to nutritional deficiencies or unfavorable water relations was in fact due to saline conditions customarily regarded as insufficiently high to be a cause of reduced yields.”

Under marginal conditions of salinity, and in t.he absence of de- tectable symptoms of salt injury, it is difficult to recognize salt effects under field conditions Controlled studies, however, have shown that there may be morphological changes before other symptoms are evident

I n general, the first physiological reaction to increased salt concentra- tion is reduced entry of water into the roots (Hayward and Spurr, 1944;

Long, 1943; Rosene, 1941; and Tagawa, 1934) This tends to inhibit

meristemat,ic activity and elongation of the root (Hayward and Spurr,

1943) Hayward and Long (1941) have shown that the growth of tomato

stems as measured by height, diameter and dry weight was less a t high salt concentrations than at control levels The smaller diameter of stems was correlated with significant differential reductions in the tissue sys- tems I n general, the reduction in thc vascular system on the basis of percentage of total area was greater than t h a t of the parenchymatous tissues of the cortex and pith Cambial act.ivity was inhibited and sec- ondary xylem vessels and fibers were smaller in diameter and propor-

26 H E HAYWARD AND C H WADLEIGH

tionately thicker walled Somewhat similar results were observed for

flax (Hayward and Spurr, 1944) grown under high concentrations of salt

The cambium was less active, the cells of the secondary xylem were smaller, and the number and diameter of the phloem fibers was less than

in the control plants

Various effects of increased salt concentrations on the growth and

structure of leaves have been reported, Harter (1908) working with

wheat, oats, and barley, found that increasing the salinity of a nonsaline

soil to 0.5 per cent soluble salts on a dry weight basis caused significant

modifications in leaf structure The leaves developed a pronounced waxy bloom, a thickened cuticle, and the size of the epidermal cells was de-

creased Uphof (1941) in his review on halophytes, points out t h a t such

plants show a tendency towards succulence by having thicker leaves and stems, more pronounced palisade parenchyma and smaller intercellular

spaces Lesage (1890) working with three nonhalophytes Pisum sativum, Linum grandiflorum and Lepidium sativum, found t h a t sodium chloride produced thicker leaves, st.rengthened the palisade parenchyma, and re-

duced the intercellular spaces Hayward and Long (1941), using osmotic concentrations ranging from 0.5 to 6.0 atm., noted increases in the thick-

ness of tomato leaves of from 9 to 30 per cent a t the 4.5 and 6.0 atm

levels The increased succulence of leaves was in agreement with results

reported by Wuhrmann (1935) who found that the thickness and degree

of succulence of leaves of Lepidium sativum and Nicotiana could be modified by the addition of sodium chloride to nutrient solutions Eaton

(1942), on the other hand, found no increase in the succulence of leaves

of milo, cotton, tomato, and sugar beets, or in alfalfa plants when the osmotic pressure of the substrate was increased I n barley, he found succulence decreased with the additions of salt

Recent studies by Bernstein and Ayers * have provided additional information which indicates that with increasing levels of salinity succu- lence of leaf tissues may be either decreased or increased Decrease in succulence has been obtained with some cucurbits and with alfalfa and grasses With some crops, however, succulence increases with salinity Bean leaves have shown this response to salinity in both field plot and water culture studies I n other cases, there is little effect of salinity on succulence Tomato leaves in a field plot experiment showed increased succulence a t low and medium salt levels, but a t high salt levels there was no change in succulence as compared with leaves of the control plants grown in nonsaline plots

Tomatoes have been used in several studies to illustrate the effect of

salt on vegetative growth and yield Eaton (1942) tested the growth and yield of Stone tomatoes on substrates adjusted t o 72 (control), 2.5 and

6.0 atm with sodium chloride as the added salt The relative dry weights

of t,he vines excluding fruit were 100, 77 and 27 per cent respectively and those of the fruits were 100, 81, and 4 per cent Hayward and Long

(1943) obtained comparable results with Marglobe tomatoes using N a C l

and NazSOl a t osmotic pressures ranging from 1.6 to 7.7 atm Their work indicated that the osmotic pressure of t.he substrate was more significant than the specific effect of the C1- and SO4= ions in relation

to vegetative responses and production of fruit Other crops where tshis

generalization appears to hold are flax (Hayward and Spurr, 1944), beans

(Ayers et al., 1943) and peaches (Hayward e t al., 1946)

Visible symptoms of salt injury may occur if the salt concentration

of t,he substrate is high When chlorides are present, characteristic symp- toms are incipient chlorosis accompanied by a drying and browning of the apex of the leaf blade The initial tip burn is usually followed by progressive involvement of additional tissue extending along the margins

of the blade until one-half to two-thirds, or in some cases the entire surface, becomes brown and necrotic I n severe cases, abscission of the leaves occurs, dieback of the terminal axis or small branches is evident, and death may ensue

These symptoms have been described by Hayward et al (1946) for

peaches, and Harper (1946) reports chloride injury for a number of trees

including pecan, elm, and ash, the tip burn and marginal browning being most pronounced on the former With one exception, scorched leaves contained in excess of 88 per cent chloride in the ash content- Hayward

and Blair (1942) observed moderate to severe chlorosis and tip burn on leaves of Valencia orange seedlings on a substrate containing 50 m.e./l chloride and very severe symptoms when 100 m.e./l of mixed chlorides

were added Hayward, Cooil and Brown * studied the effects of NaC1, CaCla and mixed chlorides on Marsh grapefruit grown in sand cultures,

with solutions adjusted to 0.5 (control), 2.5 and 3.5 atm osmotic pres-

sure Incipient chlorosis and marginal and tip burn were evident after two months, abscission of leaves was severe at 3.5 atm osmotic pressure, and at the end of 10 months the trees had lost approximately one-third

of their leaves A t the highest level of salt concentration, vegetative growth was reduced t o 45 per cent of the controls with mixed chlorides,

34 per cent with N a C l and to 22 per cent with CaCl2 Kelley and Thomas (1920) studied the effects of excessive concentrations of salt in

irrigation water on citrus trees grown under orchard conditions They report that a n excess of chlorides causes yellowing of the margins and tip burn followed by heavy shedding of leaves on lemon trees With orange trees, mottle leaf was one of the first symptoms, sometimes accompanied

by browning and curling of leaves and dieback of young, tender shoots