la&nosis an

Improvement of

United States Salinity Laboratory Staff

Agriculture Handbook No. 60

UNITED STATES DEPARTMENT OF AGRICULTURE
Document D€>líV9ry Services Branch

ÜSOA, National Agricuitural Library

N^t Bidg.

10201 Baltimore Blvd.

Böftsvili^. MD 20705-2351
Diagnosis and

Improvement of "^^WJ^///> Vr/,

'y

I 4'^W^ ,M^^ fi

^^^^

U.S. Department of Agriculture

National Agricultural Library

Division of Lending

Beltsviile, Maryland 20705

X«IM8^

United States Salinity Laboratory StafiF

Contributing Authors:

L. E. Allison L. Bernstein C. A. Bower

J. W. Brown M. Fireman J. T. Hatcher

H. E. Hayward G. A. Pearson R. C. Reeve

L. A. Richards L. V. Wilcox

L. A. Richards, Editor

Soil and Water Conservation Research Branch

Agricultural Research Service

Agriculture Handbook No. 60 Issued February 1954

Keviewed and approved for reprinting August I969

UNITED STATES DEPARTMENT OF AGRICULTURE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402 - Price $1.25

Preface

The Bankhead-Jones Act adopted by Congres« în 1935 made funds available for agricultural research

on a regional basis. At a meeting of representatives of the United States Department of Agriculture and the

directors of the Agricultural Experiment Stations of 11 Western States, the decision was made to establish a

sahnity laboratory to conduct research on problems connected with the success and permanence of agriculture

on saline and alkali soils. In 1937 the United States Regional Salinity Laboratory was established by the

then Bureau of Plant Industry on grounds adjacent to its Rubidoux Laboratory in Riverside, Calif. A memorandum of understanding, providing for official collaborators, was entered into with these 11 Western States and

Hawaii.

The Rubidoux Laboratory had been established by the Bureau's Division of Western Irrigation Agriculture

in 1928 primarily to conduct research relating to the quality of water, with special emphasis on the toxicity

of boron to plants. It was combined with the United States Regional Salinity Laboratory in 1948.

In 1951 official cooperation and collaborator representation was extended to include the 17 Western States,

and the name of the Laboratory modified to United States Salinity Laboratory.

Close cooperative relations are maintained with the State Agricultural Experiment Stations and Hawaii

through the official collaborators who meet annually to review the Laboratory's research program.

The United States Salinity Laboratory is administered in the Agricultural Research Service.

'">^*. Mar,ta4 20705
Introduction

Saline and alkali soil conditions reduce the value and productivity of considerable areas of land in the United

States. The problem is an old one, and there is much information on this subject in the technical literature.

It is the purpose of this handbook to bring together and summarize information that will be useful, particularly

to professional agricultural workers, for the diagnosis and improvement of saline and alkali soils.

The nomenclature for these problem soils is still in a formative stage. This is illustrated by the diversity

of usage of such prominent investigators as Gedroiz (1917), Hilgard (1906), Hissink (1933), Kelley (1948,

1951), and De Sigmond (1938). Ultimate agreement on nomenclature will depend on the role of exchangeable

potassium. The facts now available on this subject are meager, but they suggest that the undesirable physical

properties that are characteristic of alkali soils are caused by excessive exchangeable sodium. Other elements

of the alkali metal group either do not occur in significant quantities or do not appear to have similar action in

soils.

It is not the purpose of the writers to emphasize the definition of terms or to influence the usage of others;

but, for clarity in the presentation of the subjects treated in this handbook, it was necessary to consider

terminology, and a glossary of special terms has been included. In deference to past usage, the term "alkali

soil" is employed to refer to soils that have a high exchangeable-sodium-percentage; and "saline soil" is used

in connection with soils having a high value for the electrical conductivity of the saturation extract.

This handbook was first issued in multilithed form in 1947, and it has been widely distributed in this

country and abroad.

No attempt is made to present a comprehensive review of the literature, because the handbook is intended

primarily as a practical guide for those who are confronted with soil, plant, and water problems involving

salinity and alkali. The first five chapters provide a basis for the evaluation and interpretation of measurements. The procedures and measuring methods given in chapters 6, 7, and 8 are those with which the Laboratory

has had experience, and they are believed to have general applicability in the diagnosis and improvement of

saline and alkali soils.

There are other measuring methods in current use in various localities that have not been included, but no

particular significance should be attached to this omission. It is not possible to cover all special methods, and

it is always advisable to consult with the State agricultural experiment stations for detailed information on

local problems.

There is need for continued research on problems of saline and alkali soils and the many complicated interrelations to crop production on these soils. The close cooperative relations of the Salinity Laboratory and the

agricultural experiment stations of the 17 Western States and Hawaii have provided an efficient arrangement

for conducting investigational work with a minimum of duplication of effort and for exchanging and

disseminating research information.

This handbook is the result of the combined efforts of the entire staff of the Salinity Laboratory. Those

listed as authors have carried responsibility for writing various sections. Former staff members C. H. Wadleigh and A. D. Ayers were among the authors of the earlier draft and assisted in reviewing the present one.

The illustrations were prepared by Miles S. Mayhugh and R. H. Brooks.

The writers are indebted to many reviewers, not all of whom are mentioned, who have offered helpful

criticisms and suggestions. The sections relating to leaching and drainage in chapter 3 were reviewed by

F. M. Eaton, Vaughn E. Hansen, 0. W. Israelsen, and Dean F. Peterson, Jr. W. C. Cooper, W. P. Cottam,

F. M. Eaton, W. G. Harper, and W. J. Leighty reviewed chapter 4 and contributed suggestions relating to salt

tolerance and indicator plants. Chapter 5 on quality of irrigation water was given special consideration by the

collaborators, and this chapter was also reviewed by C. S. Scofield. Chapters 6, 7, and 8, dealing with methods,

were reviewed by L. T. Alexander, B. J. Cooil, E. E. Frahm, J. C. Hide, A. J. MacKenzie, C. D. Moodie,

A. H. Post, R. F. Reitemeier, and others.

Special acknowledgment is made to the official collaborators of the Salinity Laboratory for their many helpful

suggestions and for their cooperation and encouragement. The preliminary drafts of all sections of the handbook

were made available to all collaborators, and the great majority of them responded with constructive criticisms

and comments.

H. E. HAYWARD

Director

United States Salinity Laboratory

Riverside, Calif.

May 1953.
Contents

Page

Chapter 1. Origin and nature of saline and alkali soils. 1

Sources of soluble salts 3

Salinization of soils • • • 3

Alkalization or accumulation of exchangeable sodium

in soils 4

Characteristics of saline and alkali soils 4

Saline soils 4

Saline-alkali soils 5

Nonsaline-alkali soils 5

Chapter 2. Determination of the properties of saline

and alkali soils '^

Soil sampling 7

Estimation of soluble salts from electrical conductivity 7

Conductivity of the saturation extract and the

saturation percentage 8

Relation of conductivity to salt content and

osmotic pressure 9

Conductivity of 1:1 and 1:5 extracts 13

Salinity appraisal from the electrical resistance of

soil paste 13

Conversion of conductivity data to a standard reference temperature 16

Comparison of percent salt in soil and extract

measurements lo

Chemical determinations 17

Soil reaction—pH 17

Soluble cations and anions 18

Soluble boron 19

Exchangeable cations 19

Gypsum 20

Alkaline-earth carbonates (lime) 21

Phvsical determinations 21

Infiltration rate 21

Permeability and hydraulic conductivity 22

Moisture retention by soil 23

Density and porosity 23

Aggregation and stability of structure 23

Crust formation 25

Choice of determinations and interpretation of data. 25

Equilibrium relations between soluble and exchangeable cations 25

Chemical analyses of representative soil samples. 26

TSonsaline-nonalkali soils 26

Saline soils 26

Nonsaline-alkali soils 26

Saline-alkali soils 30

Cross-checking chemical analyses for consistency

and reliability 30

Factors that modify the effect of exchangeable

sodium on soils 30

Texture 30

Surface area and type of clay mineral 31

Potassium status and soluble silicate 31

Organic matter 31

Sequence of determinations for soil diagnosis .... 33

Chapter 3. Improvement and management of soils in

arid and semiarid regions in relation to salinity

and alkaH 34

Basic principles , 34

Irrigation and leaching in relation to salinity control. 34

Irrigation 35

Leaching 36

Leaching requirement 37

Leaching methods 38

Field leaching trials 39

Special practices for salinity control 40

Page

Chapter 3—Continued

Drainage of irrigated lands in relation to salinity

control 41

Drainage requirements 42

Water-transmission properties of soils 44

Boundary conditions 44

Layout and placement of drains 44

Techniques for drainage investigations 45

Measurements of hydraulic head 45

Determination of subsoil stratigraphy . 46

Determination of water-transmitting properties

of soils 47

Chemical amendments for replacement of exchangeable sodiimi 48

Suitability of various amendments under different

soil conditions • 48

Chemical reactions of various amendments in

alkali soils 48

Class 1. Soils containing alkaline-earth carbonates 48

Class 2. Soils containing no alkaline-earth carbonates; pH 7.5 or higher 49

Class 3. Soils containing no alkaline-earth carbonates; pll less than 7.5 49

Estimation of amounts of various amendments

needed for exchangeable-sodium replacement. . 49

Speed of reaction of amendments and economic

considerations *^"

Applicati(m of amendments • 50

Laboratory and greenhouse tests as aids to diagnosis. 51

Reclamation tests in the field • • Réclamât ion ofsaline and alkali soils in humid regions . 54

Chapter 4. Plant response and crop selection for saline

and alkali soils • ^^

Significance of indicator plants for saline soils 55

indicator plants ^6

Crop response on saline soils ^8

Salinity and water availability ^9

Specific ion effects "1

Sodium 62

Calcium 62

Magnesium 62

Potassium 62

Chloride 62

Sulfate 63

Bicarbonate 63

Boron 63

Plant analysis 63

Crop selection for saline soils 65

Germination 65

Relative salt tolerance of crop jilants 65

Relative boron tolerance of crop plants 67

Chapter 5. Quality of irrigation water 69

Methods of analysis 69

Characteristics that determine quality 69

Electrical conductivity 69

Sodium-adsorption-ratio 72

Boron 75

Bicarbonate 75

Typical waters 75

Classification of irrigation waters 75

Salinity hazard 76

Sodium hazard 76

Diagram for classifying irrigation waters 79

Conductivity 79

Sodium ^1

Effect of boron concentration on quality 81

Effect of bicarbonate ion concentration on quality 81

V
VI CONTENTS

Page

Chapter 6. Methods for soil characterization 83

Sampling, soil extracts, and salinity appraisal. . 83

(1) Soil sample collecting, handling, and subsampling. 83

\¿) Saturated sou paste 84

(3) Soil-water extracts 84

(3a) Saturation extract 84

(3b) Twice-saturation extract for coarse-textured soils (tentative) 88

(3c) Soil-water extracts at 1:1 and 1:5. .... ..] 88

iA\^è} ^*^-' extract in the field-moisture range. . 88

(4) Electrical conductivity of solutions 89

(4a) Standard Wheatstone bridge 89

(4b) Direct indicating bridge 89

(5) Resistance of soil paste and percent salt in

soil g^

(6) Freezing-point depression 91

(6a) Freezing-point depression of solutions. . 91

(6b) Freezing-point depression of water in soil

cores Q3

Soluble cations and anions 94

(7) Calcium and magnesium by titration with

fQ\ n **^y'«"«<^'^'"i"etetraacetate (Versenate) . . 94

/m i"/'^*"'" ^y precipitation as calcium oxalate. 95

(9) Magnesium by precipitation as magnesium

ammonium phosphate 95

(10) Sodium 05

(10a) Sodium by flame photometer. . 96

(10b) Sodium by precipitation as sodium uranyl

zinc acetate 9-7

(11) Potassium nn

9i\i}\ ^"*^^^*"'» by flame photometer. .....' 97

(lib) Potassium by precipitation as potassimn

dipicrylaminate 98

(12) Carbonate and bicarbonate by titration with

acid Q8

/î?^ Ç^^f^"*^® by titration with silver nitrate 98

(14) bulfate go

(14a) Sulfate by precipitation as barium sulfate gg

(14b) Sulfate by precipitation as calcium sulfate gg

(|^) ^¡f«^« by phenoldisulfonic acid . . 100

(16) Silicate as silicomolybdate 100

(17) Boron JQ^

Exchangeable cations IQQ

(18) Exchangeable cations IQQ

(19) Cation-exchange-capacity 101

(20) Exchangeable-cation percentages. .. 101

(20a) Exchangeable-cation percentages by direct determination 101

(20b) Estimation of exchangeable-sodium-per.

centage and exchangeable-potassiumpercentage from soluble cations . 102

Supplementary measurements IO2

(21) pH determinations 109

%\u\ ^îî ''^^*^*"S «^ saturated soil paste. . 102

}i\ I P;; »"^««i'^g of soil suspension 102

(^Ic) pH reading of waters, solutions, and soil

extracts ino

(22) Gypsum {H

(22a) Gypsum b;^ precipita I ¡on with acetone

(qualitative) 1Q2

(22b) Gypsum by precipitalion with acetone

(22o\ p ^^"^"»'f^^^'^«) 104

UJc; Gypsum by increase in soluble calcium

f00A\ n ^ "lagnesium content upon dilution. 104

r^qf All, VP^'"" requirement IO4

^ /¿A A " ^i"^^'' ^^'•bonates (lime).... 105

Uáa; Alkahne-earth carbonates by effervescence

with acid ' jQc

(23b) Alkaline-earth carbonates by gravimetric

K^6c) Alkahne-earth carbonates from acid neu-

,^ . tralization i/jc

(^4) Organic matter {^^

(25) Total and external ethyle.ie glycu>l retention. 106

Chapter 6—Continued

Soil water

(26) Soil-moisture content

(27) Saturation percentage

(27a) Saturation percentage from oven-drying. '.

(27b) Saturation percentage from volume of

water added

(27c) Saturation percentage from the weight

of a known volume of paste

(28) Infiltration rate.

(28a) Basin

(28b) Cylinder

(29) 1/10-atmosphere percentage

(30) 1/3-atmosphere i>ercentage

(31) 15-atmosphere percentage

(32) Moisture-retention curve

(33) Field-moisture range

(34) Hydraulic conductivity

(34a) Hydraulic conductivity of soil cores' .

/o \ ^^y^''^"bc conductivity of disturbed soil .

(34c) Hydraulic conductivity from piezometer

measurements

(34d) Hydraulic conductivity from auger-hole

measurements

(35) Hydraulic-head measurements in saturated

soil

(35a) Piezometers installed by driving

(35b) Piezometers installed by jetting W

(35c) Observation wells uncased or with p<;rforated casing

(36) Ground-water graphical methods

(36a) Water-table contour maps

(36b) Water-table isobath maps

/f^*i\ w!^^^'^ ^^^ patterns for ground water. . .

(36d) Water-table isopleths for showing time

variations in the elevation of the water

table

Physical measurements

(37) intrinsic permeability

(37a) Permeability of soil to air

(37b) Permeability of soil to water

(38) Bulk density '.'/.'.'.'.'.'.'..'

(39) Particle density

(40) Porosity

(41) Particle-size distribution

(42) Aggregate-size distribution

(42a) Wet sieving ' * '

(42b) Aggregation of particles less than 50

microns

(43) Modulus of rupture

Chapter 7. Methods of plant culture and plant

analysis

Plant-culture techniques adapted to sall-loierance

investigations

(50) Artificially salinized field plots

(51) Drum cultures

(52) Sand and water <Mdtures

Methods of plant analysis

(53) Sampling an<l pre¡>aration of plant samples

(54) Ashing

(54a) Wet digestion

(54b) Magnesium nitrato ijznition

(55) Calcium

(55a) Calriunj by fiatnr photorntMcr

(55b) ('alciuni bv oxaJatt- nu'thoij

(56) Magnesium . .

(57) Sodium

(57a) So<liiifn by flarne }»li<»t<»fn<'i<*r

(57b) So<liinn by nrariyi /iin ;ir<>(a(<

(58) Pi>tassinm . . '

(58a) Potassium by flanie pliotoin«'t*r

(58b) IV>laftsium by iobai(initri(»'

(59) Chloride

(60) Sulfur '..'.'....

(61) Phosphorus. ...

(62j Boron

Page

107

107

107

107

107

107

108

108

108

109

109

109

110

111

111

111

112

113

114

116

116

117

117

118

118

118

118

119

120

120

120

121

121

122

122

122

124

124

125

126

127

127

127

128

128

128

128

129

129

129

129

129

130

130

131

13 Î

13i

132

132

132

133

131

134

134
CONTENTS VII

Page

Chapter 8. Methods of analysis of irrigation waters. . 136

(70) Collection of irrigation water samples 136

(71) Records, reports, and expression of results. . 136

(72) Electrical conductivity 136

(73) Boron 140

(73a) Boron, electrometric titration 140

(73b) Boron, colorimetric, using carmine 142

(74) Dissolved solids 142

(75) pH of waters 142

(76) Silica 142

(76a) Silica, gravimetric 142

(76b) Silica, colorimetric 143

(77) Calcium 143

(78) Magnesium 144

(79) Calcium and magnesium by the Versenate

method 144

(80) Sodium 144

(80a) Sodium by uranyl zinc acetate, gravimetric. . 144

(80b) Sodium by flame photometer 144

Page

Chapter 8—Continued

(81) Potassium 144

(81a) Potassium by cobaltinitrite, gravimetric.. 145

(81b) Potassium by cobaltinitrite, volumetric. . 145

(81c) Potassium by flame photometer 145

(82) Carbonate and bicarbonate 145

(83) Sulfate 146

(84) Chloride 146

(85) Fluoride 147

(86) Nitrate 147

(86a) Nitrate, phenoldisulfonic acid 147

(86b) Nitrate, Devarda 147

Literature cited 148

Glossary 154

Appendix 157

Symbols and abbreviations 157

Conversion formulas and factors 157

Chemical symbols, equivalent weights, and common names 158

Soil sampler and core retainer 159

Modulus of rupture apparatus 160

Chapter 1

Origin and Nature of Saline

and Alkali Soils

The soils under consideration in this handbook owe

their distinctive character to the fact that they contain

excessive concentrations of either soluble salts or exchangeable sodium, or both. For agricultural purposes, such soils are regarded as a class of problem

soils that requires special remedial measures and management practices. Soluble salts produce harmful

effects to plants by increasing the salt content of the soil

solution and by increasing the degree of saturation of

the exchange materials in the soil with exchangeable

sodium. The latter effect occurs when the soluble constituents consist largely of sodium salts and is of a more

permanent nature than the salt content of the soil solution, since exchangeable sodium usually persists after

the soluble salts are removed.

In discussing these problem soils it is convenient to

use terms that refer specifically to the two principal

causes of the problem. "Saline soil," as used in this

handbook, refers to a soil that contains sufficient soluble

salts to impair its productivity. Similarly, alkali soils

can be defined in terms of productivity as influenced by

exchangeable sodium. In accordance with this usage,

alkali soils may or may not contain excess soluble salts.

Probably the most common problem involves soils that

contain an excess of both soluble salts and exchangeable sodium, and, in agreement with the terminology of

De Sigmond (1938),^ these soils will be referred to as

saline-alkali soils.

The salt content of soils above which plant growth is

afTected depends upon several factors, among which are

the texture of soil, the distribution of salt in the profile,

the composition of the salt, and the species of plant.

Several arbitrary limits for salinity have been suggested

for distinguishing saline from nonsaline soils. Kearney and Scofield (1936), in discussing the choice of

crops for saline lands, considered that plants begin to

be adversely afTected as the salt content of the soil exceeds 0.1 percent. De Sigmond (1938) was in agreement with this limit. In the report of the United States

National Resources Planning Board (1942, pp. 263-

334) relative to the Pecos River investigation, Scofield

considered a soil to be saline if the solution extracted

' References to Literature Cited (p. 148) are herein indicated

by the name of the author (or authors) followed by the year of

publication.

from a saturated soil paste had an electrical conductivity value of 4 mmhos/cm. or more. The electrical conductivity of the saturation extract was adopted by the

Salinity Laboratory as the preferred scale for general

use in estimating soil salinity. The Soil Survey Staff

( 1951) of the United States Department of Agriculture

now uses either this method or the earlier method based

on the electrical resistance of a sample of soil paste,

the latter reading being converted to the dry-weight

percentage of soluble salt in the soil.

The decision regarding what level of exchangeable

sodium in the soil constitutes an excessive degree of

saturation is complicated by the fact that there is no

sharp change in the properties of the soil as the degree

of saturation with exchangeable sodium is increased.

In the past an exchangeable-sodium-percentage of 15

has been used at the Laboratory as a boundary limit

between nonalkali and alkali soils. Insufficient data

and experience are available to justify a change, but this

limit must be regarded as somewhat arbitrary and tentative. In soiue cases, for example, 2 or 3 milliequivalents of exchangeable sodium per 100 gm. of soil has

equal or even greater usefulness as a critical limit.

There has been uncertainty in the past regarding the

effect of exchangeable potassium on the physical properties of soils and if, as De Sigmond (1928) and Magistad (1945) have proposed, exchangeable sodium and

potassium should be considered as additive in defining

alkali soils. It has been observed in several instances

that alkali soils high in exchangeable potassium have

better physical properties and are more readily reclaimable than other alkali soils containing similar amounts

of exchangeable sodium but low amounts of exchangeable potassium. The view that exchangeable potassium

has only a slight or no adverse eíTect upon the physical

properties of soils is supported by the results of measurements made recently at the Laboratory '^ on samples

of seven soils adjusted to various levels of exchangeable

sodium and exchangeable potassium (fig. 1).

The magnitude of the air: water permeability ratio

is a measure of the extent to which soil structure

deteriorates when water is applied, a high ratio indicating a high degree of deterioration. The data for

- Unpublished data by R. C. Reeve, C. A. Bower, R. H. Brooks,

and F. B. Gschwend.
AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

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FIGURE 1.—Relative effect of exchangeable sodium and exchangeable potassium on the ratio of the air permeability to the water

permeability of soils.
SALINE AND ALKALI SOILS

two soils are not plotted in the graph showing the

effect of the exchangeable-potassium-percentage, because they nearly coincide with the lower curve. In

general, the increase in ratio with increase in exchangeable sodium is directly related to the total specific

surface of the soils.

Improvements are being made in methods of appraising both the susceptibility and the status of soils with

respect to the injurious effects of exchangeable sodium.

For these reasons, both the terminology and the classification limits for alkali soils must be regarded as being

in a transitional stage.

Sources of Soluble Salts

The soluble salts that occur in soils consist mostly of

various proportions of the cations sodium, calcium, and

magnesium, and the anions chloride and sulfate. Constituents that ordinarily occur only in minor amounts

are the cation potassium and the aiiions bicarbonate,

carbonate, and nitrate. The original and, to some extent, the direct source of all the salt constituents are the

primary minerals found in soils and in the exposed

rocks of the earth's crust. Clarke ( 1924) has estimated

that the average chlorine and sulfur content of the

earth's crust is 0.05 and 0.06 percent, respectively, while

sodium, calcium, and magnesium each occur to the extent of 2 or 3 percent. During the process of chemical

weathering, which involves hydrolysis, hydration, solution, oxidation, and carbonation, these constituents are

gradually released and made soluble.

Bicarbonate ions form as a result of the solution of

carbon dioxide in water. The carbon dioxide may be

of atmospheric or biological origin. Water containing carbon dioxide is a particularly active chemical

weathering agent that releases appreciable quantities

of the cation constituents as the bicarbonates. Carbonate and bicarbonate ions are interrelated, the relative

amounts of each present being a function of the pH

value of the solution. Appreciable amounts of carbonate ions can be present only at pH values of 9.5 or

higher.

While the above-mentioned salt constituents are of

most importance in saline soils, there are places, as in

parts of Colorado, Utah, and Washington, where high

concentrations of nitrate are found. Various theories

(Kelley, 1951) have been proposed to explain the origin

of excessive nitrate salts in soils. Boron, owing to its

marked toxicity to plants when present even in low concentrations, also deserves mention (Eaton and Wilcox,

1939). The principal source of this element is the

mineral tourmaline, which is a rather widespread but

minor constituent of primary rocks.

Although weathering of primary minerals is the indirect source of nearly all soluble salts, there are probably

few instances where sufficient salts have accumulated in

place from this source alone to form a saline soil.

Saline soils usually occur in areas that receive salts

from other locations, and water is the primary carrier.

The ocean may be the source of salts as in soils where

the parent material consists of marine deposits that were

laid down during earlier geologic periods and have

since been uplifted. The Mancos shales occurring in

Colorado, Wyoming, and Utah are typical examples of

saline marine deposits. The ocean is also the source

of the salts in low-lying soils along the margin of seacoasts. Sometimes salt is moved inland through the

transportation of spray by winds and is called cyclic

salt (Teakle, 1937). More commonly, however, the

direct source of salts is surface and ground waters. All

of these waters contain dissolved salts, the concentration

depending upon the salt content of the soil and geologic

materials with which the water has been^in contact.

Waters act as sources of salts when used for irrigation.

They may also add salts to soils under natural conditions, as when they flood low-lying land or when

ground water rises close to the soil surface.

Salinization of Soils

Saline soils occur for the most part in regions of arid

or semiarid climate. Under humid conditions the soluble salts originally present in soil materials and those

fornfed by the weathering of minerals generally are carried downward into the ground water and are transported ultimately by streams to the oceans. Saline soils

are, therefore, practically nonexistent in humid regions,

except when the soil has been subjected to sea water in

river deltas and other low-lying lands near the sea. In

arid regions leaching and transportation of soluble salts

to the ocean is not so complete as in humid regions.

Leaching is usually local in nature, and soluble salts

may not be transported far. This occurs not only because there is less rainfall available to leach and transport the salts but also because of the high evaporation

rates characteristic of arid climates, which tend further

to concentrate the salts in soils and in surface waters.

Restricted drainage is a factor that usually contributes to the salinization of soils and may involve the presence of a high ground-water table or low permeability

of the soil. The high ground-water table is often related to topography. Owing to the low rainfall in

arid regions, surface drainageways may be poorly developed. As a consequence, there are drainage basins

that have no outlet to permanent streams. The drainage of salt-bearing waters away from the higher lands

of the basin may raise the ground-water level to the soil

surface on the lower lands, may cause temporary flooding, or may foim permanent salty lakes. Under such

conditions upward movement of saline ground water or

evaporation of surface water results in the formation

of saline soil. The extent of saline areas thus formed

may vary from a few acres to hundreds of square miles.

Many of the saline soils in the Great Basin were formed

in this manner. Similar areas occur throughout the

Western States. They are often referred to as playas

or dry lakes.

Low permeability of the soil causes poor drainage

by impeding the downward movement of water.

Low permeability may be the result of an unfavorable

soil texture or structure or the presence of indurated

layers. The latter may consist of a claypan, a caliche
AGRICULTURE HANDBOOK 60, U. S, DEPT. OF AGRICULTURE

layer, or a silica hardpan. De Sigmond (1924) considered the presence of an impermeable soil layer essential for the formation of the saline soils found in

Hungary.

The salinity problem of principal economic importance arises when previously nonsaline soil becomes

saline as the result of irrigation. Such soils are often

located in valleys adjacent to streams, and, because of

the ease with which they can be irrigated, the more

level areas are usually selected for cultivation. While

such soils may be well drained and nonsaline under

natural conditions, the drainage may not be adequate

for irrigation. When bringing new lands under irrigation, farmers have frequently failed to recognize the

need for establishing artificial drains to care for the

additional water and soluble salts. As a result, the

ground-water table may rise from a considerable depth

to within a few feet of the soil surface in a few years.

During the early development of irrigation projects,

water is frequently plentiful and there is a tendency to

use it in excess. This hastens the rise of the water table.

Waters used for irrigation may contain from 0.1 to as

much as 5 tons of salt per acre-foot of water, and the

annual application of water may amount to 5 feet or

more. Thus, considerable quantities of soluble salts

may be added to irrigated soils over relatively short

periods of time. When the water table rises to within

5 or 6 feet of the soil surface, ground water moves

upward into the root zone and to the soil surface.

Under such conditions, ground water, as well as irrigation water, contributes to the salinization of the soil.

Alkalization or Accumulation of Exchangeable Sodium in Soils

Soil particles adsorb and retain cations on their surfaces. Cation adsorption occurs as a consequence of

the electrical charges at the surface of the soil particles.

While adsorbed cations are combined chemically with

the soil particles, they may be replaced by other cations

that occur in the soil solution. The reaction whereby a

cation in solution replaces an adsorbed cation is called

cation exchange. Sodium, calcium, and magnesium

cations are always readily exchangeable. Other cations, like potassium and ammonium, may be held at

certain positions on the particles in some soils so that

they are exchanged with great difficulty and, hence,

are said to be fixed.

Calion adsorption, being a surface phenomenon, is

identified mainly with the fine silt, clay, and organic

matter fractions of soils. Many different kinds of minerals and organic materials occurring in soils have

exchange properties and together are referred to as the

exchange complex. The capacity of a soil to adsorb

and exchange cations can be measured and expressed

in chemical equivalents and is called the cation-exchange-capacity. It is commonly expressed in milliequivalents per 100 gm. of soil. Various chemical and

physical factors interact to make the measured value

depend somewhat on the method of determination, but,

nevertheless, the cation-exchange-capacity is a reasonably definite soil property that has considerable practical significance. In view of the fact that the adsorbed

cations can interchange freely with adjacent cations

in the soil solution, it is to be expected that the proportion of the various cations on the exchange complex

will be related to their concentrations in the soil

solution.

Calcium and magnesium are the principal cations

found in the soil solution and on the exchange complex

of normal soils in arid regions. When excess soluble

salts accumulate in these soils, sodium frequently becomes the dominant cation in the soil solution. Thus,

sodium may be the predominant cation to which the soil

has been subjected, or it may become dominant in the

soil solution, owing to the precipitation of calcium and

magnesium compounds. As the soil solution becomes

concentrated through evaporation or water absorption

by plants, the solubility limits of calcium sulfate, calcium carbonate, and magnesium carbonate are often

exceeded, in which case they are precipitated with a

corresponding increase in the relative proportion of

sodium. Under such conditions, a part of the original

exchangeable calcium and magnesium is replaced by

sodium.

FVom a practical viewpoint, it is fortunate that the

calcium and magnesium cations in the soil solution are

more strongly adsorbed by the exchange complex than

sodium. At equivalent solution concentrations, the

amounts of calcium and magnesium adsorbed are several times that of sodium. In general, half or more of

the soluble cations must be sodium before significant

amounts are adsorbed by the exchange complex. In

some saline soil solutions, however, practically all of

the cations are sodium, and in these sodium is the predominant adsorbed cation.

Characteristics of Saline and Alkali Soils

The term "soiK' is used in several senses by agriculturists. In one sense a soil is considered to be a threedimensional piece of landscape having shape, area, and

depth (Soil Survey, 1951). The concept of a soil as a

profile having depth but not necessarily shape or area

is also a common use of the term. In another sense,

often used in this handbook, the term is applied to

samples representing layers or points in the profile.

Saline and alkali soils are defined and diagnosed on the

basis of determinations made on soil samples, and the

significance of information thus obtained contributes

substantially to scientific agriculture. The extension

and harmonization of these definitions to the problems

and purposes of soil survey and soil classification have'

not been attempted, because it lies somewhat beyond

the scope of the present work.

To facilitate and clarify this discussion, the problem

soils under consideration have been separated into three

groups: Saline, saline-alkali, and nonsaline-alkali

soils.

Saline Soils

Saline is used in connection with soils for which the

conductivity of the saturation extract is more than 4
SALINE AND ALKALI SOILS

mmhos/cm. at 25° C. and the exchangeable-sodium-percentage is less than 15. Ordinarily, the pH is less than

8.5. These soils correspond to Hilgard's ( 1906) "white

alkali" soils and to the "Solonchaks" of the Russian soil

scientists. When adequate drainage is established, the

excessive soluble salts may be removed by leaching and

they again become normal soils.

Saline soils are often recognized by the presence of

white crusts of salts on the surface. Soil salinity may

occur in soils having distinctly developed profile characteristics or in undifferentiated soil material such as

alluvium.

The chemical characteristics of soils classed as saline

are mainly determined by the kinds and amounts of

salts present. The amount of soluble salts present controls the osmotic pressure of the soil solution. Sodium

seldom comprises more than half of the soluble cations

and hence is not adsorbed to any significant extent. The

relative amounts of calcium and magnesium present in

the soil solution and on the exchange complex may

vary considerably. Soluble and exchangeable potassium are ordinarily minor constituents, but occasionally

they may be major constituents. The chief anions are

chloride, sulfate, and sometimes nitrate. Small

amounts of bicarbonate may occur, but soluble carbonates are almost invariably absent. In addition to the

readily soluble salts, saline soils may contain salts of

low solubility, such as calcium sulfate (gypsum) and

calcium and magnesium carbonates (lime).

Owing to the presence of excess salts and the absence

of significant amounts of exchangeable sodium, saline

soils generally are flocculated; and, as a consequence,

the permeability is equal to or higher than that of

similar nonsaline soils.

Saline-Alkali Soils

Saline-alkali is applied to soils for which the conductivity of the saturation extract is greater than 4

mmhos/cm. at 25° C. and the exchangeable-sodiumpercentage is greater than 15. These soils form as a

result of the combined processes of salinization and

alkalization. As long as excess salts are present, the

appearance and properties of these soils are generally

similar to those of saline soils. Under conditions of

excess salts, the pH readings are seldom higher than 8.5

and the particles remain flocculated. If the excess soluble salts are leached downward, the properties of these

soils may change markedly and become similar to those

of nonsaline-alkali soils. As the concentration of the

salts in the soil solution is lowered, some of the exchangeable sodium hydrolyzes and forms sodium hydroxide. This may change to sodium carbonate upon

reaction with carbon dioxide absorbed from the atmosphere. In any event, upon leaching, the soil may

become strongly alkaline (pH readings above 8.5), the

particles disperse, and the soil becomes unfavorable for

the entry and movement of water and for tillage. Although the return of the soluble salts may lower the

pH reading and restore the particles to a flocculated

condition, the management of saline-alkali soils continues to be a problem until the excess salts and exchangeable sodium are removed from the root zone and a

favorable physical condition of the soil is reestablished.

Saline-alkali soils sometimes contain gypsum. When

such soils are leached, calcium dissolves and the replacement of exchangeable sodium by calcium takes

place concurrently with the removal of excess salts.

Nonsaline-Alkali Soils

Nonsaline-alkali is applied to soils for which the exchangeable-sodium-percentage is greater than 15 and

the conductivity of the saturation extract is less than 4

mmhos/cm. at 25° C. The pH readings usually range

between 8.5 and 10. These soils correspond to Hilgard's "black alkali" soils and in some cases to "Solonetz," as the latter term is used by the Russians. They

frequently occur in semiarid and arid regions in small

irregular areas, which are often referred to as "slick

spots." Except when gypsum is present in the soil or

the irrigation water, the drainage and leaching of salinealkali soils leads to the formation of nonsaline-alkali

soils. As mentioned in the discussion of saline-alkali

soils, the removal of excess salts in such soils tends to

increase the rate of hydrolysis of the exchangeable

sodium and often causes a rise of the pH reading of

the soil. Dispersed and dissolved organic matter present in the soil solution of highly alkaline soils may be

deposited on the soil surface by evaporation, thus causing darkening and giving rise to the term "black alkali."

If allowed sufficient time, nonsaline-alkali soils develop characteristic morphological features. Because

partially sodium-saturated clay is highly dispersed, it

may be transported downward through the soil and accumulate at lower levels. As a result, a few inches of

the surface soil may be relatively coarse in texture and

friable; but below, where the clay accumulates, the soil

may develop a dense layer of low permeability that

may have a columnar or prismatic structure. Commonly, however, alkali conditions develop in such soils

as a result of irrigation. In such cases, sufficient time

usually has not elapsed for the development of the

typical columnar structure, but the soil has low permeability and is difficult to till.

The exchangeable sodium present in nonsaline-alkali

soil may have a marked influence on the physical and

chemical properties. As the proportion of exchangeable sodium increases, the soil tends to become more

dispersed. The pH reading may increase, sometimes

becoming as high as 10. The soil solution of nonsaline-alkali soils, although relatively low in soluble

salts, has a composition that diñ"ers considerably from

that of normal and saline soils. While the anions

present consist mostly of chloride, sulfate, and bicarbonate, small amounts of carbonate often occur. At

high pH readings and in the presence of carbonate ions,

calcium and magnesium are precipitated; hence, the

soil solutions of nonsaline-alkali soils usually contain

only small amounts of these cations, sodium being the

predominant one. Large quantities of exchangeable

and soluble potassium may occur in some of these soils.
6 AGRICULTURE HANDBOOK 60, U. S, DEPT. OF AGRICULTURE

The effect of excessive exchangeable potassium on soil

properties has not been sufficiently studied.

Nonsaline-alkali soils in some areas of western

United States have exchangeable-sodium-percentages

considerably above 15, and yet the pH reading, especially in the surface soil, may be as low as 6. These soils

have been referred to by De Sigmond (1938) as degraded alkali soils. They occur only in the absence

of lime, and the low pH reading is the result of exchangeable hydrogen. The physical properties, however, are dominated by the exchangeable sodium and

are typically those of a nonsaline-alkali soil.
Chapter 2

Determination of the Properties

of Saline and Alkali Soils

This chapter discusses determinations that give information on the chemical and physical properties of

saline and alkali soils and thus serve as a basis for their

diagnosis, treatment, and management. The status of

knowledge on this subject is such that it is not yet possible to prepare a brief handbook containing a few

simple measurements that will give all the necessary information. A number of different types of measurements are presented. Some of these must be regarded

as tentative and subject to change and improvement.

In some cases alternate procedures are proposed, and

the individual worker will need to decide what kind

and how many measurements will be required for the

problem at hand. The purpose, application, and interpretation of the various determinations are discussed in

this chapter. Detailed directions for making the

measurements are given in chapter 6.

Soil Sampling

There is no standard procedure for obtaining soil

samples for appraising salinity and alkali. Usually the

details of procedure will depend upon the purpose for

which the sample is taken. If the objective is to obtain

a general evaiuation of salinity in a given area* the

average salt content of a number of samples provides

an index for the over-all appraisal. The variation

among samples gives an index of the variation in salt

content fhat may be encountered in the field. The

larger the number of samples, the more accurate the

appraisal will be. Too few samples may give a completely erroneous index of the salinity status. The

deviation between the actual conditions existing in an

area and the evaluation of the situation from the

sampling procedure is designated as the "sampling

error." It is evident that the larger the number of

samples and the more carefully they are selected, the

smaller the sampling error will be.

Salt concentration in soils may vary greatly with

horizontal or vertical distance and with time. The nature of the soil, microrelief, and the cause and source

of salinity should be considered. Factors that cause

migration of salt, such as seasonal precipitation, irrigation, and phase in the crop cycle, should be taken into

account in relation to the time of sampling. In cultivated areas, soil management history may be the most

important single factor in determining salinity status,

and field boundaries may enter the problem of where

to sample and how to composite the samples.

The interpretation and use of salinity and alkali

measurements necessarily depend on the completeness

and accuracy of observational data recorded at the time

of sampling. A record of the species and condition of

the plant cover is of particular importance. When attempting to correlate crop conditions in the field with

soil-salinity measurements, it is necessary to take

samples from the active root zone of the plants.

The following suggestions are offered on where and

how to sample :

(a) Visible or suspected salt crusts on the soil

surface should be sampled separately and the

approximate depth of sample recorded.

(b) If the soil shows evidence of profile development or distinct stratification, samples should

be taken by horizons or layers.

(c) In the absence of profile development or distinct stratification, the surface samples (excluding the surface crust) should be taken to

the plow depth, usually to a depth of 6 or 7

inches.

(d) Succeeding samples may be taken at intervals

of 6 to 18, 18 to 36, and 36 to 72 inches, or

other convenient depths, depending on the

depth of the root zone, the nature of the problem, and the detail required.

(e) Sometimes soil samples taken for salinity

and alkali determinations may be composited

to reduce analytical work.

(/) The size of samples will depend on the measurements that are to be made.

Detailed suggestions on taking and handling soil

samples along with a sample of the field data sheet used

at the Salinity Laboratory are given in Method 1.

Estimation of Soluble Salts From Electrical

Conductivity

The choice of a method for measuring salinity depends on such things as the reason for making the measurements, the number of samples to be handled, and the

time and effort available for doing the work. Accurate

7
8

AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

methods usually require more time and, therefore,

limit the number of determinations.

Electrical-resistance measurements can be made

quickly and accurately and have long been used for estimating soluble salts in soil (Whitney and Means, 1897) ;

however, electrical conductance, which is the reciprocal of resistance, is more suitable for salinity measurements, because it increases with salt content, thus

simplifying the interpretation of readings. Moreover,

expressing results in terms of specific conductance or

conductivity makes the determination independent of

the size and shape of the sample.

Electrical conductance is expressed in mhos, i. e.,

reciprocal ohms, while electrical conductivity has the

dimensions ofmhos per centimeter. In this handbook,

the symbol "£C" is used to represent electrical conductivity.^

The salt content of the soil can be estimated roughly

from an electrical-conductivity measurement on a saturated soil paste or a more dilute suspension of soil in

water. A better estimate of soluble salts can be obtained from the conductivity of a water extract of the

soil. In general the higher the moisture content, the

easier it will be to obtain the extract, but the less representative the extracted solution will be of the solution

to which plant roots are exposed in the soil.

Soil solutions in the field-moisture range can be extracted for study and analysis by the displacement

method (White and Ross, 1937) or with the pressuremembrane apparatus (Method 3d). These methods are

used mainly for research and special chemical studies.

Plants in saline soil are responsive to the concentration of the soil solution, and the relation of concentration to the normal field-moisture range is sometimes

overlooked. There is more than a tenfold range in the

wilting percentage of various soils. Consequently, the

(leld-moisture range may vary greatly from one soil to

another. For example, a sand and a clay could have

the same soluble-salt content expressed as percent, dry

weight basis, but the soil-solution concentration when

near the wilting percentage could be 10 times as high

for the sand as for the clay.

' The standard unit for conductivity (mho/cm.) is a large unit,

so that most solutions have a conductivity that is much less than

one unit. For instance, a measurement on one sample of water

from the Rio Grande at the Elephant Butte Dam gave EC=

0.000694 mho/cm. For such cases, with physical and chemical

measurements, it is customary to choose a small subunit that

gives a more convenient location of the decimal point when

recording or expressing data. For example, the unit ECxW

is called the millimho per centimeter. This is a convenient,

practical conductivity unit for most soil salinity work. Until

recently ECXW (or KxW) has been in common use.

^CxlO" designates conductivity expressed in micromhos per

centimeter. This is the unit most generally used for expressing

the conductivity of waters. The conductivity of the Rio Grande

sample mentioned above, when expressed in these various units,

is:

£:C=0.000694 mho/cm.

£CX10'=0.694 millimho/cm.

ECXl0'=69A (=KXiœ)

ECXl0^—694> micromhos/cm.

Conductivity of the Saturation Extract and the

Saturation Percentage

The conductivity of the saturation extract is recommended as a general method for appraising soil salinity

in relation to plant growth. The method is somewhat

less rapid than a resistance measurement of the soil

paste, but the result is easier to relate to plant response.

The procedure involves preparing a saturated soil paste

by stirring, during the addition of distilled water, until

a characteristic endpoint is reached. A suction filter

is then used to obtain a sufficient amount of the extract

for making the conductivity measurement.

The special advantage of the saturation-extract

method of measuring salinity lies in the fact that the

saturation percentage is directly related to the fieldmoisture range. In the field, the moisture content of

the soil fluctuates between a lower limit represented

by the permanent-wilting percentage and the upper,

wet end of the available range, which is approximately two times the wilting percentage. Measurements on soils indicate that over a considerable

textural range the saturation percentage (SP) is approximately equal to four times the 15-atmosphere

percentage (FAP), which, in turn, closely approximates

the wilting percentage. The soluble-salt concentration

in the saturation extract, therefore, tends to be about

one-half of the concentration of the soil solution at the

upper end of the field-moisture range and about onefourth the concentration that the soil solution would

have at the lower, dry end of the field-moisture range.

The salt-dilution effect that occurs in fine-textured soils,

because of their higher moisture retention, is thus automatically taken into account. For this reason, the

conductivity of the saturation extract (ECe) can be

used directly for appraising the effect of soil salinity on

plant growth.

Table 1 gives some of the experimental data supporting the foregoing statements. Since the 15-atmosphere percentage appears to be the most significant

moisture property that can be readily measured, this

retentivity value was used to separate soil samples into

three textural groups: Coarse, medium, and fine (table

1). The FAP ranges arbitrarily selected to designate

these textural groups were: Coarse, 2.0-6.5; medium,

6.6-15.0; and fine, greater than 15.1. The numbers in

the FAP column of table 1 are the actual F^P values

for the available samples in the various textural groups.

The SP/FAP ratio of the medium-textured group,

which is largest in number, is approximately 4 and the

standard deviation is small; whereas the ratios for

the fine-textured and high organic matter groups are

somewhat lower (Campbell and Richards, 1950).

The saturation percentage for sands, when determined

by the standard procedure, gives values that, relative

to the field-moisture range, are higher than for other

soils. This occurs because in sands the large pores that

are filled with water at the saturation-paste condition

do not correspondingly retain water under field conditions. Consequently, £CeX10'^ for sands, when referred to the regular saturation-extract scale, gives an
SALINE AND ALKALI SOILS 9

TABLE 1.—Relation of saturation percentage (SP) fo 15-atmosphere percentage (FAP) as influenced by soil

texture

Soil

samples

FAP SP SP/FAP

Soil group

Minimum

Maximum

Average

MinimumMaximum

AverageMinimum

MaxinuunAverage

Standard deviation

Coarse

Number

10

23

11

18

3.4

6.6

16.1

27.6

6.5

14.2

21.0

51.3

5.0

10.8

18.5

37.9

16.0

26.4

41.8

81.0

43.1

60.0

78.5

255

31.8

42.5

59.5

142

4,68

3.15

2.03

2.53

8.45

5.15

4.26

4.97

6.37

3.95

3.20

3.66

1.15

Medium .48

Eine .60

Oreranic .75

optimistic index of salinity, i. e., underrates the salinity

condition. Method 3b gives a tentative procedure for

estimating the upper limit of the field-moisture range.

From this, a moisture content for extraction is determined and a procedure for obtaining a conductivity

value that can be used on the regular saturation-extract

scale is suggested. This new procedure is tentative

because it has not been subjected to extensive testing,

but it has given good results for soils with SP values

of approximately 25 or less.

It would be more reliable to appraise salinity by using

measurements of extracts of the soil solution in the

field-moisture range. However, difficulty of obtaining

such extracts would make them prohibitive for routine

use. The next higher feasible moisture content appears

to be the saturation percentage. The following scale

is recommended for general use in appraising the

effect of soluble saUs on crops. It shows the relation

of crop response to soil salinity expressed in terms

of the conductivity of the saturation extract.

Use of the conductivity of the saturation extract as

an index of soil salinity was introduced at the Rubidoux

Laboratory in 1939 for the Pecos River Joint Investigation. The salinity scale given in the earlier draft of

this handbook was substantially the same as the scale

originally proposed by Scofield in his report on the

Pecos River Joint Investigation ( United States National

Resources Planning Board, 1942, pp. 263-334). The

scale given here has been modified somewhat from

those previously used.

It is often desirable, because of the extra information provided on soil texture and moisture retention,

to determine the soil-moisture content at saturation,

i. e., the saturation percentage {SP) when saturated

soil paste is prepared for salinity measurements. A

rapid procedure for SP determination based on the

weight of a known volume of saturated paste has been

described by Wilcox (1951) and is included as

Method 27c.

The endpoint for mixing a saturated soil paste is

reasonably definite; and, with a little training, good

agreement can be obtained among various operators.

Slight variations in technique, such as adding practically all the water to the soil sample before stirring

or adding the air-dry soil to a known amount of water,

do not appreciably affect the saturation percentage of

most soils. Special precautions, however, must be

taken with very fine and very coarse textured soils.

For example, in some clay soils the amount of water

that must be added to bring about saturation can be

varied 10 percent or more, depending upon the rate of

adding water and the amount of stirring. The more

rapid the rate of water addition in relation to stirring,

the lower the saturation percentage may be. The lower

value is desirable to reduce the time and effort during

mixing and also to minimize puddling of the soil.

Campbell and Richards (1950) found that the conductivity of the saturation-extract method is applicable

also for the measurement of salinity in peat soils. With

air-dried peats, an overnight wetting period is necessary

to obtain a definite endpoint for the saturated paste.

Relation of Conductivity to Salt Content and

Osmotic Pressure

The relation between the electrical conductivity and

the salt content of various solutions is shown graphically

in several figures. The curves (fig. 2) for the chloride

salts and Na2S04 almost coincide, but MgS04, CaS04,

and NaHCOs have lower conductivities than the other

salts at equivalent concentrations. When the concentration is given in percent salt or parts per million,

the curves (fig. 3) are more widely separated.

Salinity effects mostly

negligible

Yields of very sensitive

crops may be restricted

Yields of many crops

restricted

Only tolerant crops

yield satisfactorily

Only a few very tolerant

crops yield satisfactorily

16

Scale of conductivity (millimhos per centimeter at .25° C.)
10 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

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MILLlMH0S/cM.(ECxl03) AT 25» C.

FIGURE 2.—Concentration of single-salt solutions in milliequivalents per liter as related to electrical conductivity.

With soils from widely separated areas in western

United States, the concentration range was higher

(fig. 4) than that shown in figures 2 and 3; consequently, the electrical conductivity is expressed in millimhos per centimeter. This is a convenient unit to use

for extracts from saline soils. Soils represented by

points that are considerably above the average line

usually contain a relatively high amount of calcium or

magnesium sulfate. Information on the salt content

of irrigation water in relation to electrical conductivity

is given in chapter 5.

Experimental work conducted at the Salinity Laboratory by Hayward and Spurr (1944), Wadleigh and

Ayers (1945), and workers elsewhere indicates that the

osmotic pressure of the soil solution is closely related

to the rate of water uptake and growth of plants in

saline soils. The osmotic pressure (OP) of solutions

expressed in atmospheres is usually calculated from

the""^freezing-point depression, in degrees C, AT, in

accordance with the relation, OP =12.06 AT —

0.021 A T\ given in the International Critical Tables.

The relation between osmotic pressure and electrical
SALINE AND ALKALI SOILS 11

.6 .8 I

CONDUCTIVITY

FIGURE 3.—Concentration of single-salt solutions in percent as related to electrical conductivity.

4 6 8 10

MILLIMHOS /CM.(ECxl03) AT 25» C.
12 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

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o _____z:^:

4 -... ^ 1 - ^ t»1

EE = E;E:;ííE::^- :i -:ñ^Ul Ê;-;;:-; ;; ''• :; :-." - - ^ —

ffîfiWlIM ... : ~=~:-:r r- _ - u _

= ~ = = = = ="= ..;:"-::.-" z'^.zl :" t\ - -

^4:^^;: 4;; ;; ,---,-EE- —

::

= = ;EïE = - E = _-: -

4/f -^t: _=^~

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2 —

iSSftfMMilllî

— r* a' fe^

_„_. ______ ___ Il , U

|||||B • i- 4

;rV 4 f 1 l}-- 1 1--^-"- . ,, —.

,^ ^_...... 1 III 1,

I 2 4 6 8 10 2 4 6 8 100

CONDUCTIVITY Ml LLIMHOS/CM. (ECxlO^) AT 25» C.

FIGURE 4.—Concentration of saturation extracts of soils in milliequivalents per liter as related to electrical conductivity.
SALINE AND ALKALI SOILS 13

conductivity (fig. 5) is useful for some agricultural

purposes. This measurement is in general use and can

be more readily measured than freezing-point depression. The relation between OP and EC for salt mixtures

found in saline soils is indicated in figure 6 from data

reported by Campbell and coworkers (1949). The OP

values were calculated from freezing-point measurements. In the range of EC that will permit plant

growth, the i^elation OP=-0.36X£CeX 1(P can be used

for estimating the osmotic pressure of soil solutions

from conductivity measurements.

Conductivity of 1:1 and 1: 5 Extracts

For soil : water ratios of 1:1 and 1: 5, the extract

is obtained by filtering without the use of vacuum or

pressure. The conductivity of these extracts is sometimes used for estimating salinity from the line in

figure 4 or, preferably, from special curves that apply

for the salts and soil in question.

Salinity estimates based on the conductivity of 1:1

and 1:5 extracts are convenient for rapid determinations, particularly if the amount of soil sample is limited, or when repeated samplings are to be made in the

same soil to determine the change in salinity with time

or treatment. The reliability of such estimates depends

upon the kind of salts present. For chloride salts, the

results will be only slightly affected by moisture content, but, if sulfate or carbonate salts, which have

relatively low solubility, are present in appreciable

quantities, the apparent amount of soluble salt will depend on the soil : water ratio (table 2). In an experiment conducted by Wadleigh, Gauch, and Kolisch

(1951) to determine'the salt tolerance of orchardgrass,

the salts shown in the table were individually added

to a loam soil. During the course of the experiment,

many samples were taken to check distribution of the

salt in the soil and conductivity measurements were

made of the saturated soil (ECs), the saturation extract

(ECe), the 1:1 extract {ECi), and the 1:2 extract

[ECo] ' The regression coefficients, which are the slopes

of the best fit straight lines, were calculated for various

comparisons among the data (table 2).

The theoretical values given in the table are based

on the saturation percentage of 30 for the soil used.

Except for small changes in the activity coefficients

of the ions with dilution, the conductivity ratios should

be inversely proportional to the moisture contents of the

soil at extraction if the total dissolved salt is independent of the moisture content at which the extraction

is made. The average measured conductivity ratios

were always greater than the theoretical. The differences were not large for the chloride salts, but when

NaHC03, NaoS04, or MgS04 were added to this soil,

in which the exchange complex was largely saturated

with calcium, some CaS04 and CaCOs were precipitated.

It is evident from the table that the regression coefficients are quite different for extracts obtained at high

moisture contents if the less soluble salts are present in

the soil. This example illustrates why the estimation

of salinity from the conductivity of the extract at 1:1

or at higher moisture contents is not recommended for

general use. These higher moisture contents may be

used to advantage in certain cases, but the limitations of

the method should be clearly understood.

Salinity Appraisal From the Electrical Resistance

of Soil Faste

Salinity determinations based on the electrical resistance of a standard sample of wet soil have been in use

for many years (Whitney and Means, 1897; Briggs,

1899). The Bureau of Soils cup and the data published by Davis and Bryan (1910) have been widely

used by various agencies in this country for estimating

the percentage of soluble salts in soils. The apparatus

is simple and rugged, the measurements can be quickly

made, and the results are reproducible.

To obtain the relation between wet-soil resistance

and percent salt, Davis and Bryan made measurements

using 4 soil samples representing the textural groups

of sand, loam, clay loam, and clay. These samples of

soil were composited from various types of nonsaline

soils. A mixture of chloride and sulfate salts was used

to obtain 5 levels of added salt ranging from 0.2 to

3 percent, and resistance values were obtained on the

saturated pastes. Making use of these 20 readings on

the synthetic soil and salt mixtures, Davis and Bryan

used graphical interpolation to obtain the relation of

soil-cup resistance to percent salt for mixed sulfates and

chlorides. The Davis and Bryan procedure for the

Bureau of Soils method of determining soluble salt in

soil is given in Method 5. The method is also described

in the Soil Survey Manual (1951, p. 343).

TABLE 2.—Regression coefficients (b) between various criteria for evaluating soil salinity by a conductance

procedure

Soils containing— hECi-ECe hEC2-ECe hEC2-ECi hEC-ECe

NaCl 0.359 ±0.0070

.356± .011

.376± .010

.379± .027

.590d= .023

.600± .068

0.185 ±0.0037

.191± .0028

.192± .0042

.227± .017

.355± .010

.471± .060

0.514 ±0.0069

.534± .0046

.507± .012

.589± .011

.600± .011

.780± .027

0.235 ±0.0066

CaCl2 .242± .0078

MgCla .237± .019

NaHCOa .222± .013

Na2S04 .217± .015

MgS04 .226± .0054

Theoretical .333 .167 .5
14 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

60

5

K 10

3 9

(0

CO 8

Ul 7

a. 6

o 5

H

O

4

(0

O

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2 3 456789 10 2 3 456 70

CONDUCTIVITY M ILLI M HOS/CM. (ECxIO^) AT 25» C.

FIGURE 5.—Osmotic pressure of single-salt solutions as related to electrical conductivity. (Data from International Critical Tables.)
SALINE AND ALKALI SOILS 15

30

<

10

m

a: 6

(O

O)

ÜJ

er

a

3

(O

o

0.6

0.3

= ==EEEzzziEz:::::::::^^m5 ff EEEEEE= E = = = = ± = Í= = "^^-A-== ±M5

~ TI ~ ttíl I " ""^""iffl till iTÍtill I îtiTTi"

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===================== S ------ IB====^='F============ hsp===^

I 3 6 10 30 60

CONDUCTIVITY MI LLI MHOS/CM.(ECxl03) AT 25^ C.

FIGURE 6.—Osmotic pressure of saturation extracts of soils as related to electrical conductivity.
16 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

TABLE 3.—Comparison of measured and calculated values of ECe after correcting for effect of SP

Soils fromLower Rio Grande Valley, Texas.

Grand Junction district, Colorado

Tucurncari district. New Mexico.

Gem County, Idaho

Four Western States

Number

of

samples

6

12

11

7

12

Average ECeXW^

at 25° C.

Measured

9.93

8.64

9.63

5.73

10.25

Calculated

11. 10

9.45

11.85

16.24

13.05

Difference

1.17

.81

2.22

10.51

2.80

Standard

deviation

of

differences

1.30

.85

1. 16

2.90

1.76

A similar procedure was used by Davis and Bryan

to obtain calibration data for "carbonate" salts, presumably sodium carbonate. Tests at the Laboratory,

however, indicate that table IV of Davis and Bryan

for carbonate salts is unreliable and should not be used.

The unreliability of the calibration data for these salts

is a result of cation-exchange reactions that were not

generally understood at the time the original work was

done.

The conductivity of the saturation extract (ECe) is

recommended in this handbook as a measurement for

general use for indicating soil salinity, but the method

based on the soil-paste resistance (/?«) is still commonly used. The electrical conductivity of the soil

paste (£Cs) is related to paste resistance by the relation

i^Cs = 0.25//?s, where 0.25 is the constant for the

Bureau of Soils electrode cup. In a study by Reitemeier

and Wilcox (1946), it was found that the relation between ECe and EC^ is markedly influenced by variations

in the saturation percentage, the salinity, and the conductivity of the soil minerals. From unpublished work

at the Laboratory, Bower concluded that there is no

easy method for simplifying the relation of EC^ (or R^)

to ECe. He equilibrated a group of western soils with

various concentrations of a 1:1 mixture of sodium and

calcium chloride and found that on the average

ECe/EC, = 5Ä--0.07{SP). Using this average relationship and SP values calculated from the weight of

the soil paste as described by Wilcox (1951), he calculated values for ECe based on R^ measurements. The

degree of correspondence between measured and calculated values is indicated by the data in table 3.

The calculated average values for ECQ are somewhat

high but are acceptable except for the soils from Gem

County, Idaho. These soils had a low salinity level but

were high in exchangeable sodium. The large discrepancy here and for some other locations apparently

is owing to conduction by the clay minerals, when they

contain exchangeable sodium. Bower found, for example, that the electrical conductivity of a 5-percent

suspension of calcium-saturated montmorillonite was

0.072 mmhos/cm., but when saturated with sodium,

the conductivity was 0.446 mmhos/cm.

No method has been found for improving the reliability of the paste-resistance method that does not destroy its simplicity. The method may be acceptable

for estimating salinity for purposes of soil classification,

but for soils like those of Gem County, Idaho, it does

not have acceptable reliability.

Conversion of Conductivity Data to a Standard

Reference Temperature

The electrical conductivity of solutions and of soils

containing moisture increases approximately 2 percent

per degree centigrade increase in temperature. To

simplify the interpretation of salinity data, it is customary either to take the measurements at a standardreference temperature or to determine the temperature

at which the measurement is made, and then, by means

of correction tables or a correction dial on the bridge, to

convert the measurement to a standard-reference temperature.

Whitney and Briggs (1897) measured the resistance of 9 soils at 13 temperatures and calculated the

average relation of resistance to temperature. Whitney

and Means (1897) used these temperature data to construct a table used in converting resistance measurements of saturated soil to the standard temperature of

60° F. Data from this table, which has been widely

used since its publication 50 years ago, are given in

table 16 in chapter 6, along with instructions for its use.

More recently a study was made by Campbell, Bower,

and Richards (1949) to determine the effect of temperature on the electrical conductivity of soil extracts.

Saturation extracts from 21 soils were measured at 5

temperatures, ranging from 0° to 50° C. The temperature coefficient of the electrical conductivity for these

representative soil extracts varied somewhat with temperature, but in the range from 15° to 35° it was verified that for each degree centigrade increase in temperature the conductivity increased very nearly 2 percent of the value at 25°. The details of the procedures

for measuring electrical conductivity and making temperature corrections are given in Method 4.

Comparison of Percent Salt in Soil and Extract

Measurements

The diagram shown in figure 7 facilitates the interpretation of salinity in relation to crop response. It is

based on the following assumptions: Psw = p. p. m./

10,000-0.064X£CX 10^; P,,= {P,^XP^)/100; OP
SALINE AND ALKALI SOILS 17

OSMOTIC PRESSURE OF SATURATION EXTRACT - ATMOSPHERES

1.44 2.88 5.76

2 4 6 8 10 12 14 16

CONDUCTIVITY OF SATURATION EXTRACT - MILLIMHOS/CM.

4-

B i

PLANT RESPONSE

FIGURE 7.—Relation of the percent salt in the soil to the osmotic pressure and electrical conductivity of the saturation extract and to

crop response in the conductivity ranges designated by letters. These ranges are related to crop response by the salinity scale

on page 9.

= 0.36X£'CX 10^. F¡,yf = percent salt in water; Pss —

percent salt in soil; Pw~percent water in soil; and

OP = osmotic pressure in atmospheres. The lower scale

gives values for the conductivity of the saturation extract. The top scale shows the osmotic pressure of the

saturation extract. The osmotic pressure of the soil

solution at the upper limit of the field-moisture range

will be approximately double these values.

The diagonal lines help correlate the conductivity

of the saturation extract with the percent salt content

for various soil textures. For example, at ECe X 10^ =4,

nearly all crops make good growth and for a soil with

a saturation percentage of 75, as seen in the diagram,

this corresponds to a salt content of about 0.2 percent.

On the other hand, 0.2 percent salt in a sandy soil for

which the saturation percentage is 25 would correspond

to £CeX 10^=12, which is too saline for good growth

of most crop plants. Partial lists of crop plants in

their order of tolerance to soil salinity are given in

chapter 4.

The diagram indicates the growth conditions of crops

to be expected for various degrees of salinity in the

active root zone of the soil, i. e., the soil volume that is

permeated by roots and in which moisture absorption

is appreciable. Obviously, the diagram does not apply

for soil in which salt has been deposited after the roots

have been established and have become nonabsorbing,

or to soil adjacent to the plant, either high or low in

salt, that has not been permeated by roots. With mature row crops, for example, salt may have accumulated

in the ridge to such an extent that the roots no longer

function as moisture absorbers and, therefore, the ridge

cannot be considered as characteristic of the active

plant-root environment.

Chemical Determinations

Soil Reaction—pH

The pH value of an aqueous solution is the negative

logarithm of the hydrogen-ion activity. The value may

be determined potentiometrically, using various electrodes (Method 21), or colorimetrically, by indicators
18 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

whose colors vary with the hydrogen-ion activity.

There is some question as to the exact property being

measured when methods for determining the pH values

of solutions are applied to soil-water systems. Apparent pH values are obtained, however, that depend on

the characteristics of the soil, the concentration of dissolved carbon dioxide, and the moisture content at

which the reading is made. Soil characteristics that

are known to influence pH readings include : the composition of the exchangeable cations, the nature of the

cation-exchange materials, the composition and concentration of soluble salts, and the presence or absence

of gypsum and alkaline-earth carbonates.

A statistical study of the relation of pH readings to

the exchangeable-sodium-percentages of soils of arid

regions has been made by Fireman and Wadleigh

( 1951 ). The effect of various factors such as moisture

content, salinity level, and presence or absence of

alkaline-earth carbonates and gypsum upon this relationship was also studied. Some of the more pertinent

statistical data obtained are presented in table 4. While

all the coefficients of correlation given in the table are

highly significant, the coefficients of determination

show that at best no more than 54 percent of the variance in exchangeable-sodium-percentage is associated

with the variance in pH reading. The data on the effect

of moisture content indicate that the reliability of prediction of the exchangeable-sodium-percentage from pH

readings decreases as the moisture content is increased.

Similarly, the data on the effect of salinity indicate that

the reliability of prediction is lowest when the salt level

is either low or very high. An increase in pH reading

of 1.0 or more, as the moisture content is changed from

a low to a high value, has been found useful in some

areas for detecting saline-alkali soils. However, the

reliability of this procedure should be tested before use

on any given group of soil samples.

Experience and the statistical study of Fireman and

Wadleigh permit the following general statements regarding the interpretation of pH readings of saturated

soil paste: (1) pH values of 8.5 or greater almost invariably indicate an exchangeable-sodium-percentage

of 15 or more and the presence of alkaline-earth carbonates; (2) the exchangeable-sodium-percentagc of soils

having pH values of less than 8.5 may or may not exceed

15; (3) soils having pH values of less than 7.5 almost

always contain no alkaline-earth carbonates and those

having values of less than 7.0 contain significant

amounts of exchangeable hydrogen.

Soluble Cations and Anions

Analyses of saline and alkali soils for soluble cations

and anions are usually made to determine the composition of the salts present. Complete analyses for soluble ions provide an accurate determination of total salt

content. Determinations of soluble cations are used to

obtain the relations between total cation concentration

and other properties of saline solutions, such as electrical conductivity and osmotic pressure. The relative

concentrations of the various cations in soil-water extracts also give information on the composition of the

exchangeable cations in the soil.

The soluble cations and anions commonly determined in saline and alkali soils are calcium, magnesium,

sodium, potassium, carbonate, bicarbonate, sulfate, and

chloride. Occasionally nitrate and soluble silicate also

are determined. In making complete analyses, a determination of nitrate is indicated if the sum of cations

expressed on an equivalent basis significantly exceeds

that of the commonly determined anions. Appreciable

amounts of soluble silicate occur only in alkali soils

having high pH values. In analyses made by the usual

me;thods, including those recommended in this handTABLE 4.—Coefficient of correlation (JY and coefficient of determination (r^) for the relation of pH reading to

exchangeable-sodium-percentage as influenced by moisture content^ salinity levels and presence or absence of

alkaline-earth carbonates and gypsum

MoiBture

content

(percent)

Salinity as

ECe X 103

at 25° C.

Alkalineearth

carbonates

Gypsum Samples r r2

Saturation Variable

do

Variable

do

Variable

Number

868

271

289

346

349

91

115

87

69

237

452

154

0.66

.65

.53

.48

.56

.72

.70

.74

.49

.72

.56

.41

Percent

44

500 do 43

1,000 do do .....do 28

6,000 do do do 24

Saturation 0-4 do . . . ..do 31

Do 4-8 do do 52

Do 8-15 do do 49

Do 15-30 ... do do 54

Do > 30 do do 24

Do Variable Present Present 52

Do .. .do do Absent 32

Do do Absent do 17

* All values are significant at the 1-percent level.
SALINE AND ALKALI SOILS 19

book, any soluble silicate present is determined as

carbonate.

As shown by Reitemeier (1946) and others, values

obtained from determinations of the soluble-cation and

soluble-anion contents of saline and alkali soils are

markedly influenced by the moisture content at which

the extraction is made. The total dissolved quantities

of some ions increase with increasing moisture content,

while concurrently those of others may decrease; almost invariably values obtained for total salt content

increase with increasing moisture content at extraction.

Processes that are responsible for the changes in the

relative and total amounts of soluble ions which occur

with increasing moisture content include cation-exchange reactions, negative adsorption of ions, hydrolysis, and the increased solution of silicate minerals,

alkaline-earth carbonates, and gypsum. Ideally, the

determination of soluble ions should be made on extracts obtained at a moisture content in the fieldmoisture range. However, the preparation of such

extracts is time-consuming and requires the use of

special extraction equipment (Method 3d). Saturation

percentage is the lowest practical moisture content for

obtaining extracts on a routine basis. Use of the

saturation extraet is, therefore, recommended for the

determination of soluble ions. Methods are available

that permit determination of the electrical conductivity

and the common soluble constituents on 10 to 50 ml.

of saturation extract. As a rule, about one-fourth of

the moisture in a saturated soil paste can be removed

by ordinary pressure or vacuum filtration.

The choice of methods for the determination of the

various cations and anions depends upon the equipment available and the personal preference of the

analyst. No attempt is made here to present all of the

methods that are suitable. The methods given were

chosen on the basis of their convenience and reliability.

Owing to the fact that the amount of extract available

for analysis is usually limited, most of the methods

selected are of the semimicro type. They generally involve the use of a centrifuge, a flame photometer, arid a

photoelectric colorimeter. Where the amount of extract is not limited, the macromethods employed for

water analysis given in chapter 8 may be used. Most of

these methods do not require the use of a centrifuge or

photoelectric colorimeter.

Soluble Boron

The importance of soluble boron from the standpoint

of soil salinity lies in its marked toxicity to plants when

present in relatively small amounts. Toxic concentrations of boron have been found in the saturation extracts of a number of saline soils. It is necessary,

therefore, to consider this constituent as a factor in the

diagnosis and reclamation of saline and alkali soils.

High levels of boron in soils can usually be reduced by

leaching. During the leaching process, boron may not

be removed in the same proportion as other salts. If the

concentration of boron is high at the outset, a considerable depth of leaching water may be necessary to reduce

the boron content to a safe value for good plant growth.

This is illustrated by a recent leaching test. At the

beginning of the test, the conductivity of the saturation

extract of the top 12 inches of soil was 64.0 mmhos/cm.

After 4 feet of irrigation water had passed through the

soil, the conductivity was reduced to 4.2 mmhos/cm.;

after 8 feet, the conductivity was 3.4 mmhos/cm. ; and

after 12 feet, it was 3.3 mmhos/cm. The concentration

of boron in the saturation extract at the start of the test

was 54 p. p. m. After the passage of 4 feet of water,

the concentration was 6.9 p. p. m.; after 8 feet, it was

2.4 p. p. m.; and after 12 feet, it was 1.8 p. p. m. Thus,

leaching with 4 feet of water reduced the salinity to a

safe level, but the boron content was still too high for

good growth of plants sensitive to boron.

Permissible limits for boron in the saturation extract

of soils can at present be given only on a tentative basis.

Concentrations below 0.7 p. p. m. boron probably are

safe for sensitive plants (ch. 4) ; from 0.7 to 1.5 p. p. m.

boron is marginal; and more than 1.5 p. p. m. boron

appears to be unsafe. The more tolerant plants can

withstand higher concentrations, but limits cannot be

set on the basis of present information. For land on

which crops are being grown, a better appraisal of

boron conditions often can be made by an analysis of

plant samples (ch. 4) than can be obtained from an

analysis of soil samples.

Exchangeable Cations

When a sample of soil is placed in a solution of a

salt, such as ammonium acetate, ammonium ions are

adsorbed by the soil and an equivalent amount of

cations is displaced from the soil into the solution.

This reaction is termed "cation exchange," and the

cations displaced from the soil are referred to as "exchangeable." The surface-active constituents of soils

that have cation-exchange properties are collectively

termed the "exchange complex" and consist for the most

part of various clay minerals and organic matter. The

total amount of exchangeable cations that a soil can

retain is designated the "cation-exchange-capacity," and

is usually expressed in milliequivalents per 100 gm. of

soil. It is often convenient to express the relative

amounts of various exchangeable cations present in a

soil as a percentage of the cation-exchange-capacity.

For example, the exchangeable-sodium-percentage

(ESF) is equal to 100 times the exchangeable-sodium

content divided by the cation-exchange-capacity, both

expressed in the same units.

Determinations of the amounts and proportions of

the various exchangeable cations present in soils are

useful, because exchangeable cations markedly influence the physical and chemical properties of soils. The

exchangeable-cation analysis of saline and alkali soils

is subject to difficulties not ordinarily encountered with

other soils, such as those from humid regions. Saline

and alkali soils commonly contain alkaline-earth carbonates and a relatively high concentration of soluble

salts. They may have low permeability to aqueous

solutions and to alcohol. Solutions capable of displac-
20 AGRICULTURE HANDBOOK 60, U. S, DEPT. OF AGRICULTURE

Ing exchangeable cations from soils dissolve most or all

of the soluble salts and significant amounts of the carbonates of calcium and magnesium if they are present.

The soluble salts should not be washed out of the soil

prior to extracting the exchangeable cations, because of

significant changes that take place as a result of dilution

and hydrolysis. The dissolving of salts, therefore,

necessitates independent determinations of solublecation contents and correction of the exchangeablecation analysis for their presence, while the occurrence

of calcium and magnesium carbonates prevents accurate

determination of exchangeable calcium and magnesium.

Furthermore, the low permeability of many alkali soils

renders the conventional leaching techniques for displacement of cations time-consuming and inconvenient.

Neutral normal ammonium acetate is the salt solution

most commonly used for the extraction of exchangeable

cations and for the saturation of the exchange complex

in the determination of cation-exchange-capacity. Although this solution has many advantages for exchangeable-cation analysis, some saline and alkali soils fix appreciable amounts of ammonium as well as potassium

ions under moist conditions. The fixation of ammonium does not interfere with the extraction of exchangeable cations, but values obtained for cation-exchangecapacity by ammonium saturation are low by amounts

equal to the quantity of ammonium fixed. The desirability of using a cation not subject to fixation for the

determination of cation-exchange-capacity is, therefore,

evident.

As discussed in a previous section, the determined

values for the soluble-ion contents of soils vary with

the moisture content at which the extraction is made.

Because equilibria exist between the soluble and exchangeable cations in soils, the changes in relative and

lotal concentrations of soluble cations with variations

in moisture content are accompanied by changes in the

relative composition of the exchangeable cations. In

a strict sense, therefore, values for exchangeable-cation

contents apply only at the moisture content used for the

extraction of soluble cations. Owing to difficulties involved in the determination of soluble cations at moisture contents in the field range, it is convenient to determine exchangeable-cation contents at the saturation

percentage.

Consideration of the various factors involved in the

determination of the exchangeable cations and the

cation-exchange-capacity of satine and alkali soils has

led to the adoption of the following scheme of

analysis:

(a) Extract a sample of the soil with an excess of

neutral normal ammonium acetate solution and determine the milliequivalents of the various cations removed

per 100 gm. of soil.^

(6) Prepare a saturation extract of the soil and determine the milliequivalents of the various soluble

cations per 100 gm. of soil.^

(c) Calculate the exchangeable-cation contents of

the soil by subtracting the amounts of the various cations dissolved in the saturation extract from the

amounts extracted by the ammonium acetate solution.

id) Determine the cation-exchange-capacity by

measuring the milliequivalents of sodium adsorbed per

100 gm. of soil upon treating a sample with an excess

of normal sodium acetate solution of pH 8.2.

The difficulties encountered in leaching soil samples

of low permeability are overcome by shaking and

centrifuging samples in centrifuge tubes with successive

portions of the extraction and wash liquids. Neutral

normal ammonium acetate solution is used for the extraction of exchangeable plus soluble cations, because

its interference in analytical procedures is easily eliminated. Of the common cations, sodium appears to be

the most suitable for determining cation-exchangecapacity. As mentioned previously, ammonium and

potassium are subject to fixation in difficultly exchangeable form and the usual presence of calcium and magnesium carbonates in saline and alkali soils precludes

the use of extractants containing calcium or magnesium.

The fact that sodium is a prominent cation in most

saline and alkali soils also favors its use in the determination of cation-exchange-capacity (Method 19).

Gypsum

Gypsum is found in many soils of arid regions, in

amounts ranging from traces to several percent. In

some soils, gypsum was present in the sedimentary deposits from which the soil was derived; whereas, in

other soils the gypsum was formed by the precipitation

of calcium and sulfate during salinization. Owing to

leaching, gypsum commonly occurs at some depth in the

former instance, while in the latter its content is

usually greatest in the surface layers of the soil.

Information regarding the gypsum content of alkali

soils is ini})ortant, because it usually determines whether

the application of^ chemical amendments will be required for reclamation. Also, the presence of considerable amounts of gypsum in the soil might permit

the use of an irrigation water having an unfavorably

high sodium content.

The precise determination of gypsum in soils is

difficult, because of inherent errors involved in the

extraction of this mineral by water. Studies by Reitemeier ( 1946) and others show that at least three factors

other than the solution of gypsum may influence the

amounts of calcium and sulfate extracted from gypsiferous soils. They are: (1) The solution of calcium

from sources other than gypsum; (2) exchange reactions in which soluble calcium replaces other cations,

such as sodium and magnesium; and (3) the solution

of sulfate from sources other than gypsum.

^ If the soil is known to contain carbonates of calcium and

magnesium, determination of these cations is omitted. Likewise, if the soil is known to contain gypsum not completely

soluble in the saturation extract, the determination of calcium

is omitted. In the absence of prior knowledge regarding the

calcium and magnesium carbonate and gypsum contents of the

soil, the calcium and magnesium determinations are disregarded

if upon completion of the exchangeable-cation analysis the sum

of the values obtained for exchangeable-cation contents is found

to exceed the cation-exchange-capacity value.
SALINE AND ALKALI SOILS 21

Three methods are given in chapter 6 for the estimation of gypsum in soils. Methods 22a and 22b are

based on the low solubility of the salt in an aqueous

solution of acetone. Method 22a is essentially qualitative, although a rough estimate of gypsum content

may be obtained by visual observation of the amount

of precipitate obtained. This method can be successfully employed under field conditions. In Method 22b

the separated and washed gypsum precipitate is determined quantitatively. The use of Method 22c is advantageous when characterization of the soil includes

the determination of calcium plus magnesium in the

saturation extract. It is based on the increase in

soluble-divalent-cation content as the moisture content

of the soil is increased from the saturation percentage

to a moisture content sufficient to dissolve the gypsum

present. It should be noted that this method can give

negative values for gypsum content as a result of the

replacement of exchangeable sodium and potassium by

calcium as the moisture content of the soil is increased.

This is likely to occur only in alkali soils containing

little or no gypsum.

Alkaline-Earth Carbonates (Lime)

The alkaline-earth carbonates that occur in significant amounts in soils consist of calcite, dolomite, and

possibly magnesite. Owing to low rainfall and limited

leaching, alkaline-earth carbonates are usually a constituent of soils of arid regions. The amounts present

vary from traces to more than 50 percent of the soil

mass. Alkaline-earth carbonates influence the texture

of the soil when present in appreciable amounts, for

the particles commonly occur in the silt-size fraction.

The presence of fine alkaline-earth carbonate particles

is thought to improve the physical condition of soils.

Conversely, when alkaline-earth carbonates occur as

caliche or as cementing agents in indurated layers, the

movement of water and the development of root systems

is impeded. Alkaline-earth carbonates are important

constituents of alkali soils, for they constitute a potential source of soluble calcium and magnesium for the

replacement of exchangeable sodium. As discussed in

another section, the choice of chemical amendments

for the replacement of exchangeable sodium is directly

related to the presence or absence of alkaline-earth

carbonates.

Efl'ervescence upon application of acid (Method 23a)

can be used to detect as little as 0.5 percent of alkalineearth carbonates in soils. This test suffices for most

purposes. When a better estimate of the alkaline-earthcarbonate content of soils is desired, Methods 23b or

23c may be used. A quantitative determination of

small amounts of alkaline-earth carbonates in soils is

sometimes desirable in connection with proposed applications of acid-forming amendments. For precise determinations, the reader is referred to the methods of

Williams (1949) and Schollenberger (1945).

Physical Determinations

The problem of evaluating soil physical conditions

has recently been separated into components by the

American Society of Agronomy (1952) ; and they are

discussed under the headings of mechanical impedance,

aeration, soil water, and soil temperature. These are

logical ultimate aspects; but, for practical work on

alkali soils, measuring methods are needed that yield

immediate results having more or less direct diagnostic

significance. Some progress is being made toward

evaluating the physical status of soil in terms of physical properties, i. e., intrinsic qualities of soil that can

be expressed in standard units and that have values

which are substantially independent of the method of

measurement. Infiltration rate, permeability, bulk

density, pore-size distribution, aggregation, and modulus of rupture appear to be such properties. Experience indicates that the physical status of any given soil

is not static. There is a range of variation of physical

status that is related to productivity, and this is reflected in corresponding ranges in the values of pertinent physical properties.

Information on the existing physical status of a

problem soil is useful for purposes of diagnosis or

improvement, but it might also be useful to know how

much better or worse the status can be made by chemical and physical treatments simulating those applicable

under field conditions. Soils can be treated to increase

the exchangeable-sodium-percentage and then puddled

to indicate how unfavorable the physical status can be

made. It should also be possible by use of soil amendments and chemical aggregants to get some indication

of how favorable the physical status can be made.

Practical use of the concept that there is a range of

physical states for any given soil may have to wait for

refinements in measuring methods, but the idea seems

to be pertinent to the improvement of alkali soils.

Infiltration Rate

Water-movement rates attainable in soil under fieW

conditions relate directly to irrigation, leaching, and

drainage of saline and alkali soils. Infiltration refers

to the downward entry of water into soils and the term

"infiltration rate" has special technical significance in

soils work. Definitions of soil-water terms adopted by

the Soil Science Society of America (1952) are followed, and are included in the Glossary.

The infiltration rate of soil is influenced by such

factors as the condition of the soil surface, the chemical

and physical status and nature of the soil profile, and

the distribution of water in the profile. All of these

factors change more or less with time during infiltration.

The infiltration rate is measured under field conditions. The principal methods used have involved flooding or impounding water on the soil surface, sprinkling

to simulate rain, and measuring water entry from rills

or furrows. In addition to the multitude of local

physical conditions that are encountered in the field,
22 AGRICULTURE HANDBOOK 60, U. S. DEFT. OF AGRICULTURE

the availability of equipment, materials, and services

will largely decide what method to use in measuring infiltration. Although many measurements have been

made, as evidenced by the extensive bibliography of

Davidson (1940), there does not seem to be a generally

accepted procedure applicable to all situations. Many

of the infiltration measurements made by this Laboratory have been in connection with basin irrigation on

test plots ranging from 10 to 20 feet square. The

water-subsidence rate in a large plot is probably the

best indication of the infiltration rate as related to

leaching operations, but this method is usually not

feasible for exploratory or diagnostic measurements in

new areas. The cylinder method of Musgrave (1935) is

probably the most versatile of the various methods

available. A guard ring is needed if lateral spreading

is excessive. Procedures for making infiltration measurements are given in Method 28.

Water having the same quality as that which will be

used for irrigation or leaching must be used for infiltration tests in the field, otherwise the results may be

misleading. The length of time the tests should be conducted and the depth of water to be applied depend

upon the purpose of the test and the kind of information

that is sought. If it is a matter of appraising an irrigation problem, the depth corresponding to one irrigation

may be sufficient; but, if information on infiltration for

planning a leaching operation is needed, it may be desirable to apply the full depth of leaching water to a

test plot. It often happens that subsurface drainage is

sufficiently restricted to cause the infiltration rate to

decrease considerably with time. It should be kept in

mind, therefore, that although small area tests will give

useful information on soil changes during leaching, the

infiltration values thus obtained will apply to large areas

only if underdrainage is not limiting.

Experience indicates that the infiltration rate of a

given soil can be high or low, depending on physical

status and management history. Infiltration rate is

often critically influenced by surface soil conditions,

but subsurface layers also are sometimes limiting.

Water distribution in the profile and depth of water

applied are modifying factors. The infiltration rate

can be undesirably high or undesirably low. It is the

low end of the range that may be a critical limiting

factor in the agricultural use of alkali soils. It is difficult to specify a boundary limit between satisfactory

and unsatisfactory infiltration rates at the low end of

the range, because so many factors are involved, including the patience and skill of the farmer. However,

if the infiltration rate is less than 0.25 cm./hr. (0.1

in./hr.) special water-management problems are involved that may make an irrigation enterprise

unprofitable for average operators.

Permeability and Hydraulic Conductivity

The permeability of soil, in a qualitative sense, refers

to the readiness with which the soil conducts or transmits fluids. In a quantitative sense, when permeability

is expressed with numbers, it seems desirable that permeability be defined as a property of the porous medium

alone and independent of the fluid used in its measurement. The term "hydraulic conductivity," on the other

hand, is used to refer to the proportionality factor in

the Darcy flow equation. These distinctions represent

increased specialization in the use of these terms as approved by the Soil Science Society of America (1952).

No change in the qualitative use of the word "permeability" is involved. In the quantitative sense, involving

numerical values, the term "intrinsic permeability"

will mostly be used and will refer to a length-squared

measurement that may be identified in a general way

to the cross-sectional area of some equivalent or effective size of pore.

An immediate consequence of this clarification of

nomenclature is a new method for evaluating pore-space

stability or structural stability of soil. For porous

media with fixed structure, such as sandstone or fired

ceramic, measurements of intrinsic permeability with

air, water, or organic liquids all give very nearly the

same numerical value. Gravity, density, and the viscosity of the liquid are taken into account in the flow

equation. However, if the intrinsic permeability for

a soil as measured with air is markedly greater than the

permeability of the same sample as subsequently measured using water, then it may be concluded that the

action of water in the soil brings about a change in

structure indicated by the change in permeability. The

ratio of air to water permeability, therefore, is a measure of the structural stability of soils, a high ratio

indicating low stability.

Intrinsic-permeability measurements are based on

the equation v= k'dgi/rj, where v is the flow velocity,

k' is the intrinsic permeability, d is the density of the

fluid, g is the acceleration of gravity, i the hydraulic

gradient, and rj is the viscosity. Procedures for measuring intrinsic permeability with gases and liquids are

given as Methods 37a and 37b. The air-water permeability ratio increases greatly as the exchangeablesodium content of the soil increases, indicating that

exchangeable sodium decreased the water stability of

the soil structure.

It is seen from the Darcy equation, v= ki, that k,

the hydraulic conductivity, is the effective flow velocity

or discharge velocity of water in soil at unit hydraulic

gradient, i. e., when the driving force is equal to 1

gravity. Methods 34a and 34b give procedures for

measuring hydraulic conductivity on undisturbed and

disturbed soil samples.

Under some circumstances, especially when the soil

surface has been subject to submergence by water for

a considerable period and when the hydraulic conductivity is nearly uniform with depth, the hydraulic

gradient beneath the soil surface may approach unity,

i. e,, the downward driving force is composed entirely

of the gravity force with no pressure gradient. Under

this condition the infiltration rate is equal to the hydraulic conductivity, but this is probably the exception

rather than the rule under field conditions. Consequently, the relation between infiltration rate and

hydraulic conductivity is not a simple one. For ex-
SALINE AND ALKALI SOILS 23

ample, at the Malheur Experimental Area in Oregon,

very low hydraulic-conductivity values were obtained

and yet infiltration was adequate to support good crops

with sprinkler irrigation. It was found by use of

tensiometers that values for the hydraulic gradient

during infiltration ranged up to 10 in some cases. This

soil was deep and silty and the suction gradient in the

soil added significantly to the rate of downward movement of water. If the downward flow is interrupted

by a layer of very low conductivity, then the hydraulic

gradient may approach zero as the soil pores become

filled and the condition of static equilibrium under

gravity is approached.

It is to be expected that if the hydraulic conductivity

of surface soil is as low as 0.1 cm./hr. (0.04 in./hr.)

leaching and irrigation may present serious difficulties.

Irrigation agriculture under average conditions of

management skill, water quality, and drainage conditions would have doubtful success unless the hydraulic

conductivity could be increased appreciably by soilimprovement measures.

Moisture Retention by Soil

The effect of soil salinity on crops is related to the

range over which the moisture content of the soil varies,

because the concentration of the soil solution depends

both on the amount of soluble salt and the amount of

water present. The permanent-wilting percentage, as

indicated in the review by Veihmeyer and Hendrickson

(1948), is generally accepted as being the lower limit

of water available for plant growth in nonsaline soil.

For all practical purposes, the 15-atmosphere percentage (Method 31) can be used as an index of the

permanent-wilting percentage and, therefore, also as

an acceptable index of the lower limit of the available

range of soil moisture. This lower limit appears to be

an intrinsic property of the soil that is largely determined by soil texture and appears to be substantially

independent of the kind of plant grown on the soil.

It is much more difficult to set an upper limit for

the range of water content available to plants in the

field. In addition to dependence upon soil texture at

the point in question, the upper limit depends also on

the variation throughout the profile of such factors as

pore-size distribution and water conductivity. The distribution of water with depth influences the hydraulic

gradient, and, therefore, also the rate of downward

movement of water. For example, with or without

active roots, the moisture content in the surface layers

of a deep permeable soil will decrease more slowly if

the profile is deeply wetted than if only a shallow depth

is wetted and the underlying soil is dry. Also, the total

amount of water actually available from any given layer

of surface soil depends on the rooting depth and transpiration rate of the crop. The hydraulic boundary conditions that characterize the field situation would be

extremely difficult to reproduce for a soil sample removed from the profile, and it is not surprising that no

generally satisfactory laboratory method has been

found for estimating the upper limit of water available

for crop growth under field conditions. A field determination under representative field conditions is the

best method for obtaining the upper limit of the fieldmoisture range.

For most medium- to fine-textured soils, the upper

limit of available water is approximately twice the

moisture percentage of the lower limit. This does not

hold true for the coarse-textured soils. It has been

found by the United States Bureau of Reclamation

(1948) that for the sandy soils occurring on the Yuma

Mesa, Arizona, the water retained in a sample of soil

at the i/io"^t"^<^sphere percentage (Method 29) satisfactorily approximates the upper limit of available

water under field conditions.

Density and Porosity

The bulk density (apparent density) of soil is the

mass of soil per unit volume, and the porosity of soil

is the fraction of the soil volume not occupied by soil

particles. Bodman (1942) has discussed soil density

in connection with water content and porosity relationships and has prepared useful nomograms (fig. 8).

The bulk density of soil can be measured by several

methods. For a certain range of moisture contents with

soils that are comparatively free of gravel and stones,

it is possible to press into the soil a thin-walled tube

having a suitable cutting edge. The soil is then

smoothed at each end of the tube and oven-dried at

105° C. The bulk density is the mass of soil contained

in the tube divided by the volume of the tube, as indicated in Method 38.

The porosity of soil (n) may be obtained directly

from air-pycnometer measurements or can be calculated from the relation n= (dp — d]y)/dp, where dp is the

average density of the soil particles and i/b is the bulk

density.

The particle density of many soils averages around

2.65 gm. cm.~^. The average particle density for peat

soils or for pumice soils is much lower. Direct measurements of particle density can be made with pycnometer bottles (Method 39).

The bulk density of most soils ranges from 1.0 gm.

cmr^ for clays, to 1.8 gm. cm."^ for sands. This corresponds to the range of 62.4 to 112 lb. ft.'^. The corresponding porosity range will be from about 0.60 to 0.30.

Bulk density may become a critical factor in the productivity of soil. Veihmeyer and Hendrickson (1946)

found that plant roots were unable to penetrate a

gravelly loam soil when the bulk density exceeded a

value of around 1.8 gm. cm.^^. Also, when the bulk

density of medium- to fine-textured subsoils exceeds

about 1.7 gm. cm.~^, hydraulic conductivity values will

be so low that drainage difficulties can be anticipated.

Aggregation and Stability of Structure

The arrangement of soil particles into crumbs or aggregates that are more or less water stable is an important aspect of soil structure. Alkali soils often

have a dense, blocky, single-grain structure, are hard to

till when dry, and have low hydraulic conductivity when
24 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

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SALINE AND ALKALI SOILS 25

wet. This is generally because the aggregates and also

the pores of such soils are not stable. The aggregates

slake down in water, and the pores become filled with

fine particles.

Several methods have been proposed for measuring

the water stability of soil aggregates, the most common

being the wet-sieving method proposed by Yoder

(1936). A modification of the Yoder procedure is

given as Method 42a. Soils that are low in organic

matter and contain appreciable amounts of exchangeable sodium seldom contain aggregates of larger sizes

and for that reason measuring procedures adapted for

the smaller aggregates are included as Method 42b.

This determination is related to Middleton's (1930)

"dispersion-ratio," but Method 42a gives the percentage

by weight of particles smaller than SOfx that are bound

into water-stable aggregates greater than 50/x. Insufficient data are available at the present time to specify

limits that will help to distinguish between problem and

nonproblem soils as far as aggregate-size distribution is

concerned.

Childs (1940) followed the change in moisture-retension curves with successive wettings to get an index of

the stability of structure, or, more precisely, the stability of the pore-space arrangement. Reeve and coworkers (fig. 1) have shown that the ratio of the air

permeability to the water permeability for soils is also

a useful index of the stability of soil structure (Method

37).

Recent studies by Allison (1952) and by Martin

and associates (1952) indicate that dispersed soils may

be rapidly and effectively improved by application of

aggregating agents of the polyelectrolyte type. Applied at the rate of 0.1 percent on the dry-soil basis,

this material has effectively improved the physical

condition of alkali soils on which it has been tried.

Salinity appears to have little or no effect on the process.

A higher degree of aggregation was obtained where

the aggregating agent in solution was sprayed on dry

soil and mixed in than when it was applied dry to a

moist soil followed by mixing. Regardless of the manner of application, large increases in infiltration rate

and hydraulic conductivity resulted from its use.

Although not yet economically feasible for general

agricultural use, aggregating agents can be an effective

research tool for investigational work with saline and

alkali soils. By their use, for instance, plant response

to different levels of exchangeable sodium or different

Ca : Na ratios may be studied on "conditioned" soils

in the absence of poor structure and accompanying

conditions of deficient aeration and low water-movement rates ordinarily present in alkali soils.

It seems likely, also, that soil-aggregating chemicals

may provide a rapid method for appraising the structural improvement potentially attainable from organicmatter additions. Organic-matter additions, while

slower to give results, have long been used in agriculture. There may be soils, such as those high in silt

and low in clay, in which coarse organic matter may

give improvements in physical condition that are unattainable with chemical aggregants.

Crust Formation

Soils that have low stability of structure disperse

and slake when they are wetted by rain or irrigation

water and may develop a hard crust as the soil surface

dries. This crust presents a serious barrier for emerging seedlings, and with some crops often is the main

cause of a poor stand. Alkali soils are a special problem in this regard, but the phenomenon is by no means

limited to these soils.

F'actors influencing development of hard surface

crusts appear to be high exchangeable sodium, low

organic matter, puddling, and wetting the soil to zero

tension, which occurs in the field with rain or irrigation. Crust prevention would, therefore, involve removal of exchangeable sodium, addition of organic

matter, and care to avoid puddling during tillage and

other operations. Where possible, the placement of

the seed line somewhat above the water level in a furrow is desirable so that the soil above the seed will be

wetted with water at appreciable tensions, thus lessening the tendency for soil aggregates at the surface to

disintegrate.

The procedure for measuring the modulus of rupture

of soil (Method 43) was developed for appraising the

hardness of soil crusts, since a satisfactory measuring

method is essential in developing and testing soil

treatments for lessening soil crusting.

Choice of Determinations and

Interpretation of Data

Equilibrium Relations Between Soluble and

Exchangeable Cations

Cation exchange can be represented by equations

similar to those employed for chemical reactions in

solutions. For example, the reaction between calciumsaturated soil and sodium chloride solution may be

written: CaA^, + 2NaCl;=:^2NaZ-hCaCl„ where X designates the soil exchange complex. As shown by the

equation, the reaction does not go to completion, because as long as soluble calcium exists in the solution

phase there will be adsorbed calcium on the exchange

complex and vice versa. Equations have been proposed by various workers for expressing the equilibrium

distribution of pairs of cations between the exchangeable and soluble forms. For metallic cation pairs of

equal valence, many of the equations assume the same

form and give satisfactory equilibrium constants, but

variable results are obtained with the different equations

when cations of unequal valence are involved. According to the work of Krishnamoorthy and Overstreet

(1950), an equation based on the statistical thermodynamics of Guggenheim (1945) is most satisfactory

for cation pairs of unequal valence. All of the equations become less satisfactory when applied to mixtures

of cation-exchange materials having different equilibrium constants.

The use of cation-exchange equations for expressing

the relationship between soluble and exchangeable
26 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

cations in soils of arid regions involves inherent difficulties. The difficulties arise from the presence of

mixtures of different kinds of cation-exchange materials in soils and from the fact that usually four cation

species must be dealt with. Moreover, there are no

accurate methods available for determining exchangeable calcium and magnesium in soils containing

alkaline-earth carbonates and gypsum. Despite these

difficulties, some degree of success has been attained

in relating the relative and total concentrations of

soluble cations in the saturation extract of soils to the

exchangeable-cation composition, using a somewhat

empirical approach. Direct determinations show that,

when soils are leached with salt solutions containing

a mixture of a monovalent cation and a divalent cation

until equilibrium between the soil and solution is

established, the proportions of exchangeable monovalent and divalent cations present on the soil-exchange

complex vary with the total-cation concentration as

well as with the monovalent : divalent cation ratio of

the salt solutions. Gapon (1933), Mattson and Wiklander (1940), Davis (1945), and Schofield (1947)

have proposed, in effect, that the influence of totalcation concentration is taken into account and a linear

relation with the exchangeable monovalent: divalent

cation ratio is obtained when the molar concentration

of the soluble monovalent cation is divided by the

square root of the molar concentration of the soluble

divalent cation.

Two ratios of the latter type, designated as the

sodium-adsorption-ratio (SAR) and potassium-adsorption-ratio (PAR), are employed for discussing the

equilibrium relation between soluble and exchangeable

cations. The sodium-adsorption-ratio and potassiumadsorption-ratio are defined as Na^/V(Ca^^-t-Mg'^^)/2

and KVV(Ca'^-f Mg'^)/2, respectively, where Na% K\

Ca^^, and Mg'^ refer to the concentrations of the designated soluble cations expressed in milliequivalents per

liter.

The relationship between the sodium-adsorptionratio and the ratio exchangeable sodium : (exchange

capacity minus exchangeable sodium) at the saturation

moisture percentage for 59 soil samples representing 12

sections in 9 Western States is shown in figure 9. A

similar relationship involving the potassium-adsorptionratio, exchange capacity, and exchangeable potassium

is given in figure 10. The correlation coefficients for

the two sets of values are sufficiently good to permit

practical use of the relations. Data for soils having

exchangeable sodium/(exchange capacity minus exchangeable sodium) and exchangeable potassium/

(exchange capacity minus exchangeable potassium)

ratios greater than 1, which correspond to exchangeable-cation-percentages of more than 50, are not included in the graphs. Limited data indicate that for

these soils the relations shown in the graphs are somewhat less precise. Using the data presented in figure 9,

the relation between the exchangeable-sodium-percentage (ESP), and the sodium-adsorption-ratio (SAR) is

given by the equation :

ESP= 100^^ -0.0126 + 0.01475 5^Ä)

1 + ( -0M26 + 0.0U15 SÄRJ

Similarly, the relation between the exchangeablepotassium-percentage (EPP) and the potassiumadsorption-ratio (PAR) is given by the equation:

EPP =

100 (0.0360 + 0.1051 PAR)

1+(0.0360 +0.105iP^/?y

The former equation was employed to obtain the average relation between exchangeable-sodium-percentage

and the sodium-adsorption-ratio, which is shown by the

nomogram given in figure 27, chapter 6.

Chemical Analyses of Representative Soil

Samples

Data of typical chemical analyses of saline, nonsaline-alkali, and saline-alkali soil samples are given

in table 5. Similar analyses of samples of normal soils

from arid regions are also given for comparative purposes. These analyses are presented to show the

differences in the chemical characteristics of the four

classes of soils and to illustrate how the analyses may be

interpreted and cross-checked for reliability.

Nonsaline-Nonalkali Soils

Samples numbered 274L 2744, and R-2867 are

classed as normal with respect to salinity and alkali,

because the electrical conductivity of their saturation

tîxtracts is less than 4 mmhos/cm. and their exchangeable-sodium-percentage is less than 15. The reaction

of the samples ranges from slightly acid to slightly

alkaline. While the composition of the soluble ions

varies somewhat, the amounts present are small, and

all of the saturation extracts have low sodium-adsorption-ratios. Alkaline-earth carbonates may or may not

be present. Also, gypsum may or may not be present,

although none of the samples selected contains this

constituent.

Saline Soils

The electrical conductivity of the saturation extracts

of these samples is in excess of 4 mmhos/cm., but the

exchangeable-sodium-percentage is less than 15. In

no case does the pH reading exceed 8.5. Chloride and

sulfate are the principal soluble anions present in these

samples, the bicarbonate content is relatively low, and

carbonate is absent. The soluble-sodium contents exceed those of calcium plus magnesium somewhat, but

the sodium-adsorption-ratios are not high. Gypsum

and alkaline-earth carbonates are common constituents

of saline soils. As shown by the values for the electrical conductivity of the saturation extracts, the salinity

levels are sufficiently high to affect adversely the growth

of most plants. Reclamation of the soils will require

leaching only, providing drainage is adequate.

Nonsaline-Alkali Soils

The exchangeable-sodium-percentages of these soil

samples exceed 15, but the soluble-salt contents are low.
SALINE AND ALKALI SOILS 27

1.00

20 30 40

SODIUM—ADSORPTION —RATIO —

FIGURE 9.--Exchangeable-sodium ratio iES/ÍCEC—ES]) as related to the sodium-adsorption-ratio iSAR) of the saturation extract.

ES, exchangeable sodium; CEC, cation-exchange-capacity.
28 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

I.OOi I:I::S::í::í:S:S::;:

2 3 4 5 6

POTASSIUM-ADSORPTION-RATIO — PAR

FIGURE 10.—Exchangeable-potassium ratio (EP/ICEC—EP]) as related to the potassium-adsorption-ratio (f/ÍA) of the saturation

extract. EP, exchangeable potassium; CEC, cation-exchange-capacity.
SALINE AND ALKALI SOILS 29

TABLE 5.—Chemical analyses of soil samples from arid regions

SOIL DETERMINATIONS

Soil and sample No.

Normal soils:

2741

2744

R-2867

Saline soils:

574

756

573 • . ; - Nonsaline-alkali soils:

2747

2738

535

Saline-alkali soils:

2739

2740

536

Satura- pH of

tion satupercent- rated

age soil

35-6 6.4

32.4 7.8

40.4 7.9

52.0 7.9

46.5 8.0

40.0 8.0

58.8 8.3

61.2 7.3

38.7 9.6

61. 5 7.3

59.7 7.8

35.8 9.3

Cationexchangecapacity

Meq./lOOgm.

20.3

29.4

17.4

14.4

17.0

18.6

33.4

34.2

21.9

35.7

40.3

26.2

Na

Exchangeable-cation-percentages

Ca Míí

10

3

13

8

10

18

24

46

26

26

63

8

1

1

3

2

17

3

2

32

2

2

8

54

31

27

29

30

35

II

7

0

0

0

0

0

0

13

0

10

0

0

Gypsum

Meq.l

100 gm.

0

0

0

7. 1

0

0

0

0

0

0

42.2

0

Alkalineearth carbonates '

4-

+

+

+

SATURATION EXTRACT DETERMINATIONS

Soil

and

Elec- Cations

tncal

sample conducNo tivity Ca++ Mg^+ Na+ K+ Total

Normal Mmhosj

soils: cm. Meq./L Meq.lL Meq.lL Meq./L Meq./L

2741 0.60 2.71 2.26 1.20 0.91 7.08

2744 1.68 3.33 1.94 12.2 .70 18. 17

R-2867.. .84 2.76 1.69 5.22 .18 9.85

Saline soils:

574 13.9 31.5 37.2 102. 0 .21 170.91

756 12.0 37.0 34.0 79.0 .40 150.40

575 8.8 28.4 22.8 53.0 1.10 105. 30

Nonsalinealkali

soils:

2747 1.74 1.10 1.42 15.6 .42 18.54

2738 2.53 1.41 1.01 21.5 .28 24.20

535 3.16 1.10 .30 29.2 4.10 34.70

Salinealkali

soils:

2739 9.19 6.73 9.85 79.5 .48 96. 56

2740 16.7 32.4 38.3 145.0 .51 216.21

536 5.6 .60 .90 58.5 1.6 61.60

Anions

C03=

Meq./L

0

0

0

0

0

0

0

0

8.40

0

0

5.00

HCO3

I Meq./L

2.60

6. 14

6.63

4.50

7.20

5.20

6.51

3.29

18.70

2.35

3.29

19.9

sor

Meq./l

2.09

4.28

2.67

90. 0

62.2

74.0

8.48

3.80

4.60

20. 1

105.0

21.5

Meq./L

0.87

4.93

.44

78.0

47. 0

29.0

2.86

16.7

7.50

72.0

105. 0

16.3

Meq./L

5.56

15.35

9.74

172. 50

2 148. 40

108.20

17.85

23.79

39.20

94.45 '

213.29 !

62.70

Sodiuraadsorptionratio

{SAR)

0.8

7.5

3.5

17.4

13.3

10. 5

13. M

19.6

35. O

27. b

24. I

67.6

* +, Present; , absent.

Usually the pH readings are greater than 8.5, but they

may be lower if the exchangeable-sodium-percentage

does not greatly exceed 15 (sample No. 2747) or if

alkaline-earth carbonates are absent (sample No.

2738). Gypsum seldom occurs in these soils. The

chief soluble cation is sodium, and appreciable amounts

of this cation may be present as the bicarbonate and

carbonate salts. The sodium-adsorption-ratio of the

saturation extract may be quite high. Sample No. 2738

2 Includes 32.0 meij./l. oí NOa.

is an example of a nonsaline-alkaii soil that is i rec

of alkaline-earth carbonates and contains some exchangeable hydrogen. Replacement of exchangeable

sodium will be required for its reclamation. Gypsum

is a suitable amendment, but the application of acid

or acid-forming amendments may cause excessive soil

acidity unless limestone is also applied. The application of limestone alone will tend to replace the exchangeable sodium. Sample Nos. 2747 and 535 will
30 AGRICULTURE HANDBOOK 60, U. S, DEPT. OF AGRICULTURE

also require replacement of exchangeable sodium for

reclamation; but, owing to the presence of alkalineearth carbonates, acid, any acid-forming amendment,

or gypsum may be applied. The application of limestone alone will obviously be of no value.

Saline-Alkali Soils

Soils of this class are characterized by their appreciable contents of soluble salts and exchangeable

sodium. The electrical conductivity of the saturation

extract is greater than 4 mmhos/cm., and the exchangeable-sodium-percentage exceeds 15. The pH reading

may vary considerably but is commonly less than 8.5.

Except that a higher proportion of the soluble cations

consists of sodium, the composition of the soluble salts

usually is similar to that of saline soils. Although

only the most salt-sensitive plants will be affected by

the salinity level of sample No. 536, the exchangeablesodium-percentage is too high to permit the growth of

most crops. Both replacement of exchangeable sodium

and leaching are required for reclamation of these soils.

With respect to the suitability of various amendments

for the replacement of exchangeable sodium, sample

No. 2739, like No. 2738, will require the application of

soluble calcium, whereas sample No. 536, like samples

2747 and 535, can be treated with soluble calcium,

acid, or acid-forming amendments. Owing to its high

content of gypsum, sample No. 2740 will not require the

application of amendments for the replacement of

exchangeable sodium.

Cross-Checking Chemical Analyses for

Consistency and Reliability

A means of locating gross errors in the chemical

analyses of soils is provided by the considerable number of interrelations that exist among the values obtained for various determinations. An understanding

of the principles involved in these interrelations aids

in the interpretation of the analyses.

ELECTRICAL CONDUCTIVITY AND TOTAL CATION CONCENTRATION.—The EC of soil solutions and saturation

extracts when expressed in millimhos per centimeter at

25° C. and multiplied by 10 is approximately equal to

the total soluble-cation concentration in milliequivalents per liter.

CATION AND ANION CONCENTRATION.—The total soluble-anion concentration or content and the total solublecation concentration or content, expressed on an

equivalent basis, are nearly equal.

PH AND CARBONATE AND BICARBONATE CONCENTRATIONS.—If carbonate ions are present in a soil extract

in titratable quantities, the pH reading of the extract

must exceed 9. The bicarbonate concentration seldom

exceeds 10 meq./l. in the absence of carbonate ions,

and at pH readings of about 7 or less seldom exceeds

3 or 4 meq./l.

PH AND CALCIUM AND MAGNESIUM CONCENTRATIONS.—

The concentration of calcium and magnesium in a saturation extract seldom exceeds 2 meq./l. at pH readings

above 9. Therefore, calcium plus magnesium is low

if carbonate ions are present in titratable amounts, and

calcium plus magnesium is never high in the presence

of a high concentration of bicarbonate ions.

CALCIUM AND SULFATE IN A SOIL-W^ATER EXTRACT AND

GYPSUM CONTENT OF THE SOIL.—The solubility of gypsumi at ordinary temperatures is approximately 30

meq./l. in distilled water and 50 meq./l. or more in

highly saline solutions. However, owing to the common ion effect, an excess of either calcium or sulfate

may depress the solubility of gypsum to a value as low

as 20 meq./l. Hence, the saturation extract of a nongypsiferous soil may contain more than 30 meq./l. of

both calcium and sulfate (i. e. saline soil No. 756), and

that of a gypsiferous soil may have a calcium concentration as low as 20 meq./l. As a general rule, soils

with saturation extracts that have a calcium concentration of more than 20 meq./l. should be checked for

the presence of gypsum.

PH AND ALKALINE-EARTH CARBONATES.—The pH

reading of a calcareous soil at the saturation percentage

is invariably in excess of 7.0 and generally in excess of

7.5; a noncalcareous soil may have a pH reading as

high as 7.3 or 7.4.

PH AND GYPSUM.—The pH reading of gypsiferous

soils at the saturation percentage is seldom in excess of

8.2 regardless of the ESP.

PH AND ESP.—A pH reading at the saturation percentage in excess of 8.5 almost invariably indicates an

ESP of 15 or more.

ESP AND SAR.—In general, ESP increases with SAR.

There are occasional deviations, but generally low SAR

values of the saturation extract are associated with low

ESP values in the soil, and high SAR values denote high

ESP values.

CEC AND SP.—Because both cation-exchange-capacity and moisture-retention properties are related to

the texture of soils, there generally exists a fair correlation among these properties, particularly in soils with

similar parent materials and mode of origin.

Factors That Modify the EflFect of Exchangeable

Sodium on Soils

As might be expected, alkali soils having similar exchangeable-sodium-percentages may vary considerably

with respect to their physical properties, their ability to

produce crops, and their response to management practices, including the application of amendments. Although the reasons for the variable behavior of alkali

soils are imperfectly understood, experience and limited

data indicate that the effect of exchangeable sodium

may be modified by several soil characteristics. Determinations of some or all of these characteristics are

often of value in the investigation of alkali soils.

Texture

It is well known that the distribution of particle sizes

influences the moisture retention and transmission

properties of soils. Particle-size analysis may be made,

using Method 41. As a rule, coarse-textured soils have

low-moisture retention and high permeability, whereas
SALINE AND ALKALI SOILS 31

fine-textured soils have high-moisture retention and

generally have lower permeability. However, owing

to a high degree of aggregation of the particles, there

are notable examples of fine-textured soils that are

moderately permeable. The presence of a high percentage (50 or more) of silt-size particles (effective

diameter 2/x to 50/x) often causes soils to have relatively

low permeability. There is also evidence that some

silt-size particles, presumably those having a platy

shape, are more effective in reducing permeability than

others. In general, the physical properties of finetextured soils are affected more adversely at a given

exchangeable-sodium-percentage than coarse-textured

soils. For example, the hydraulic conductivity of a

coarse-textured soil having an exchangeable-sodiumpercentage of 50 may be as great as that of a finetextured soil having an exchangeable-sodium-percentage of only 15 or 20. Inasmuch as fine-textured soils

generally have higher cation-exchange-capacities than

coarse-textured soils, expressing the critical levels of

sodium in milliequivalents per 100 gm. tends to eliminate the texture factor in evaluating the effect of

exchangeable sodium.

Surface Area and Type of Clay Mineral

Soil particles may be considered to have two types

of surfaces: external and internal. Primary minerals

such as quartz and feldspars and the clay minerals kaoiinite and illite have external surfaces only. Clay minerals of the expanding lattice type such as montmorillonite, which exhibits interlayer swelling, have internal as

well as external surfaces. The external surface area

of soils is directly related to texture, whereas internal

surface area is related to the content of minerals that

exhibit interlayer swelling. Determinations of the

amounts of ethylene glycol retained as a monomolecular layer by heated and unheated samples of soil

(Method 25) permit estimation of the external and the

internal surface areas, provided appreciable amounts

of vermiculite and endellite minerals are not present.

In any case, the ethylene glycol retained by unheated

soil in excess of that retained by a corresponding heated

sample is an index of interlayer swelling.

As determined by Method 25, the external surface

areas of most soils lie in the range 10 to 50 m.Vgm.

(square meters per gram), whereas the internal surface

area varies to a greater extent, being nil in soils that

contain no interlayer swelling minerals and as high

as 150 m.Vgm. or more in soils with a high content of

expanding lattice-type minerals. X-ray diffraction

patterns indicate that the clay fraction (particles <2/i

effective diam.) of many soils of arid regions are predominantly interstratified mixtures of various proportions of montmorillonite and illite, although sometimes

individual crystals of these minerals occur. The

amount of kaolinite present is usually small.

It is generally recognized that soils containing clay

of the expanding lattice (montmorillonitic) type exhibit

such properties as swelling, plasticity, and dispersion

to a greater extent than soils containing equivalent

amounts of nonexpanding lattice (illitic and kaolinitic)

clays, especially when appreciable amounts of exchangeable sodium are present. Whether the more adverse physical properties imparted by the former type

of clays are caused by their greater total surface area

or to the fact that they exhibit interlayer swelling is

not definitely known. Further studies may show that

the susceptibility of soil to injury by exchangeable

sodium is related to total surface area as measured by

ethylene glycol retention.

Potassium Status and Soluble Silicate

Several medium- to fine-textured alkali soils have

been examined at the Laboratory and have been found to

be much more permeable than would ordinarily be expected on the basis of their high exchangeable-sodiumpercentages. In some cases, the permeability is such

that the soils can be leached readily with large quantities of irrigation water and the excess exchangeable

sodium removed without the use of chemical amendments. The soils have several characteristics in common, which include a high pH value (9.0 or higher),

a high exchangeable-potassium-percentage (25 to 40),

and an appreciable content of soluble silicate. The

silicate concentration of the saturation extracts of these

soils has been found to vary from 5 to 40 meq./L, and

additional quantities of this anion as well as sodium

are removed upon leaching. As shown by ethylene

glycol retention, Dyal and Hendricks (1952) and Bower

and Gschwend (1952), saturation of montmorillonite

clays and soils with potassium followed by drying decreases interlayer swelling. Moreover, Mortland and

Gieseking (1951) have shown by means of X-ray

diffraction studies that montmorillonite clays, when

dried in the presence of potassium silicate, are changed

to micalike clays that would have less tendency to svell

and disperse under the influence of exchangeable

sodium. Ethylene glycol retention determinations

made on some of the alkali soils having high exchangeable-potassium-percentages and containing appreciable

soluble silicate give relatively low values for interlayer

swelling. While further research is needed to clarify

the role of exchangeable potassium and soluble silicate,

there is a distinct indication that alkali soils containing

unusually high amounts of these constituents are less

susceptible to the development of adverse physical

conditions.

Organic Matter

While the organic-matter content of soils of arid

regions is usually low under virgin conditions, it commonly increases with the application of irrigation water

and cultivation, especially when crop management is

good. Aside from its value as a source of plant

nutrients, organic matter has a favorable effect upon

soil physical properties.

There is considerable evidence that organic matter

tends to counteract the unfavorable effects of exchangeable sodium on soils. Campbell and Richards (1950)
32 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

£

SOIL SAMPLE

HYDRAULIC

CONDUCTIVITY

BAR

H L

1

SATURATED PASTE

H

NOT A

PERMEABILITY

PROBLEM

/POSSIBLE \

' TOXICITY \

I

NOT A

SALINITY

PROBLEM

J-

\

TOTAL

EXTRACTABLE

SODIUM

FROM r

HIGH ESP/ /POSSIBLE^

^ . ^y [ TOXIC )

I

EXCH.

POTASSIUM

íTE

IONS

\

I

PH, SP

SATURATION

EXTRACT

CONDUCTIVITY

OF

SATURATION

EXTRACT

EXCHANGEABLE

SOOI

ESP CEC

I

UM

1

NOT AN

ALKALI

PROBLEM :ii.

N

/ POSSIBLE\

/UNFAVORABLE\

GYPSUM ALKALI

PROBLEM

I PHYSICAL '

\CONDITION /

\ /

A

ALKALINE

EARTH

CARBONATES

LEACHING

PROBLEM

FIGURE 11.—Sequence of deterniinalions for the diagnosis and treatment of saline and alkali soils: //, High; L, low; /?s,

electrical resistance of soil paste; 5^/?, sodium-adsorption-ratio; ESP, exchangeable-sodium-percentage; CEC, cationexchange-capacity.
SALINE AND ALKALI SOILS 33

and Fireman and Blair ^ found that peat and muck soils

containing appreciable quantities of exchangeable

sodium had good physical properties, and numerous

investigators have demonstrated a beneficial effect of

organic matter additions upon alkali soils. For example, Bower and associates (1951) found that the

application of manure at the rate of 50 tons per acre

to an alkali soil of the "slick spot" type increased the

degree of aggregation of the surface soil significantly

and the infiltration rate approximately threefold. The

available data indicate that organic matter improves

and prevents deterioration of the physical condition

of the soil by its interaction with the inorganic cationexchange material, by serving as energy material for

micro-organisms which promote the stable aggregation

of soil particles, and by decreasing the bulk density of

soils.

The organic-matter content of soils is ordinarily obtained by multiplying the organic-carbon content by

1.72. The dry-combustion method is most accurate

for the determination of organic carbon, but it is timeconsuming and cannot be applied to soils containing

carbonates. Wet-combustion methods such as the one

given in Method 24 are suitable for use on soil containing carbonates, but the application of a correction factor

is required to compensate for the incomplete oxidation

of the organic matter.

Sequence of Determinations for Soil Diagnosis

The salinity status and the hydraulic conductivity

are measured for all samples. The sequence of further

determinations depends on whether the result obtained

from a previous determination (fig. 11) is considered

to be high or low. Criteria for distinguishing high and

low values are discussed in chapter 6.

The determinations are ordinarily discontinued when

the guide lines of the two main branches of the diagram

lead to a heavy-walled box, except in the case of an

^ FIREMAN, M., and BLAIR, G. Y. CHEMICAL AND PHYSICAL

ANALYSES OF SOILS FROM THE HUMBOLDT PROJECT, NEVADA.

[Unpublished.] January 1949.

alkali problem where alkaline-earth carbonates should

also be determined if the use of acid or acid-forming

amendments is contemplated. At two places in the diagram, dotted lines indicate where optional alternate determinations can be made. The alternate determinations cost somewhat less but have lower reliability.

Hydraulic-conductivity measurements on disturbed

samples provide an indication of the moisture-transmission rate of the soil. It has been found for most soils

that exchangeable sodium is not excessive if this rate is

high. However, coarse soils such as sands and peats

may contain sufficient amounts of exchangeable sodium

to be toxic to plants and yet have high permeability.

If the hydraulic conductivity is low, the total extractable sodium or the sodium-adsorption-ratio (SAR)

should be determined. If either of these is low, the

low hydraulic-conductivity value previously obtained

may be the result of an inherently unfavorable physical

condition related to texture, low content of organic

matter, or high-swelling type clay rather than the presence of exchangeable sodium. For these samples, organic matter, ethylene glycol retention, and particle-size

analyses may yield useful information.

If the total extractable-sodium content or the SAR

value is high, the exchangeable sodium should be determined or, alternatively, the exchangeable-sodiumpercentage can be estimated from the SAR value. If

the exchangeable-sodium content or exchangeablesodium-percentage is high, a gypsum determination

should be made. A high-gypsum value indicates that

leaching only is required, while a low-gypsum value

indicates need for amendments. When there is a lowgypsum value, the presence or absence of alkaline-earth

carbonates is ascertained to indicate the type of chemical amendment that can be used for the replacement of

exchangeable sodium. The addition of amendments

should be followed by leaching. Other determinations,

such as pH, saturation percentage, cation-exchangecapacity, exchangeable potassium, toxic ions, and texture, provide additional information and are made if

circumstances warrant.
Chapter 3

Improvement and Management of Soils in Arid

and Semiarid Regions in Relation

to Salinity and Alkali

The development and maintenance of successful irrigation projects involve not only the supplying of irrigation water to the land but also the control of salinity

and alkali. The quality of irrigation water, irrigation

practices, and drainage conditions are involved in salinity and alkali control. In establishing an irrigation

project, soils that are initially saline require the removal

of the excess salts and may require chemical amendments in addition to an adequate supply of irrigation

water. On the other hand, soils that initially are nonsaline may become unproductive if excess soluble salts

or exchangeable sodium are allowed to accumulate

because of improper irrigation and soil management

practices or inadequate drainage.

Basic Principles

Although farming practices may vary from one irrigated area to another, the following general principles

related to salinity and alkali have universal application.

Plant growth is a function of the total soil-moisture

stress, which is the sum of the soil-moisture tension and

the osmotic pressure of the soil solution. Through

controlled leaching, the osmotic pressure of the soil

solution should be maintained at the lowest feasible

level; and, by a practical system of irrigation, the soilmoisture tension in the root zone should be maintained

in a range that will give the greatest net return for the

crop being grown.

Water flows in both saturated and unsaturated soil

in accordance with Darcy's law, which states that the

flow velocity is proportional to the hydraulic gradient

and the direction of flow is in the direction of the

greatest rate of decrease of hydraulic head. This principle makes it possible to determine the direction of

flow of ground water by simple methods. A knowledge of the source and direction of flow of ground

water is especially useful in solving drainage problems.

Soluble salts in soil are transported by water. This

is an obvious but basic principle pertaining to the control of salinity. Salinity, therefore, can be controlled

if the quality of the irrigation water is satisfactory and

if the flow of water through the soil can be controlled.

34

The concentration of soluble salts in the soil solution

is increased as water is removed from the soil by evaporation and transpiration. Desiccation of surface soil

by transpiration and by evaporation creates a suction

gradient that will produce an appreciable upward movement of water and salt. This upward flow, especially

if the water table is near the soil surface, is a process

by which many soils become salinized.

Soluble salts increase or decrease in the root zone, depending on whether the net downward movement of

salt is less or greater than the net salt input from irrigation water and other sources. The salt balance in soil,

as affected by the quantity and quality of irrigation

water and the effectiveness of leaching and drainage, is

of paramount importance. If irrigation agriculture is

to remain successful, soil salinity must be controlled

(Scofield, 1940).

Elquilibrium reactions occur between the cations in

the soil solution and those adsorbed on the exchange

complex of the soil. The use of amendments for changing the exchangeable-cation status of soil depends upon

these equilibrium reactions. Adsorption of excessive

amounts of sodium is detrimental to the physical status

of the soil and may be toxic to plants. When the exchangeable-sodium content of soil is excessive or tends

to become so, special amendment, leaching, and management practices are required to improve and maintain favorable soil conditions for plant growth.

Whether soil particles are flocculated or dispersed

depends to some extent upon the exchangeable-cation

status of the soil and, also, upon the ionic concentration

of the soil solution. Soils that are flocculated and

permeable when saline may become deflocculated when

leached.

Irrigation and Leaching in Relation to

Salinity Control

Irrigation is the application of water to soil for the

purpose of providing a favorable environment for

plants. Leaching, in agriculture, is the process of dissolving and transporting soluble salts by the downward

movement of water through the soil. Because salts
SALINE AND ALKALI SOILS 35

move with water, salinity depends directly on water

management, i. e., irrigation, leaching, and drainage.

These three aspects of water management should be

considered collectively in the over-all plan for an irrigated area if maximum efficiency is to be obtained.

Irrigation

In subhumid regions, when irrigation is provided on

a standby or supplemental basis, salinity is usually of

little concern, because rainfall is sufficient to leach out

any accumulated salts. But in semiarid or arid regions

salinity is usually an ever-present hazard and must be

taken into account at all stages of planning and

operation.

The subject of water quality in relation to irrigation is discussed at length in chapter 5 and is mentioned

here only to emphasize the fact that water quality must

be considered in determining the suitability of soils for

irrigation. In general, waters with high salt contents

should not be used for irrigation on soils having low

infiltration and drainage rates. The higher the salt

content of the water, the greater the amount of water

that must be passed through the soil to keep the solublesalt content at or below a critical level. Experience

indicates that there are soils in which low water-movement rates make the cost of drainage so high that

irrigation agriculture is not feasible under present

economic conditions.

Pumping from ground water for irrigation has several advantages. It often affords direct local control

of the water table when water is pumped from unconfined or partially confined aquifers. This has been

demonstrated in the Salt River Valley, Arizona, the San

Joaquin Valley, California, and elsewhere. Wells can

often be located on the farm, thereby eliminating the

need for elaborate distribution systems. Water is available for use at all times, which provides maximum flexibility in irrigation. If it is possible to obtain irrigation water from both ground-water and surface supplies, a balance between the two sources can often be

established to insure favorable drainage of the irrigated

soils. Another indirect advantage of pumping water

for irrigation comes from the fact that the direct visible

cost of operating pumps causes the farmer to avoid the

wasteful overuse of water which often is the cause of

the need for drainage improvement.

Excessive losses from water conveyance and distribution systems must be prevented, otherwise drainage

problems will be aggravated with attendant salinity

hazards. Distribution systems and irrigation schedules

should be designed so that water is available at times

and in amounts needed to replenish the soil moisture

without unnecessary use on irrigated fields and without

regulatory waste of water which may directly or indirectly contribute to unfavorable drainage conditions.

In some cases, water is used under continuous free-flow

systems to maintain water rights rather than on a basis

of consumptive use. Salinity and drainage problems

could undoubtedly be alleviated in some areas by

changing to a system of direct charge for the volume

of water used.

The quantity of water available for irrigation may

have a marked effect upon the control of salinity. In

areas where water is cheap and large volumes are used,

irrigation practices are often inefficient. Overuse and

waste of irrigation water contribute to drainage difficulties and salinity problems. Efficient irrigation practices can be developed more readily in the planning of

irrigation systems than by applying corrective measures

on the farm. Limited quantities of water should be

supplied, based upon consumptive use and leaching requirements, for the area in question. Where an abundant supply of water is available for irrigation, restrictions may become necessary if drainage problems arise.

Water requirements for leaching are discussed in a

following section.

Lining canals to reduce seepage losses and the distribution of water by underground pipe systems should

receive careful consideration. Much can be done in the

layout of distribution systems to reduce seepage losses

by locating canals and laterals properly. In some

areas, earth and asphalt linings for irrigation canals

have been used successfully. The buried asphalt membrane lining used by the United States Bureau of Reclamation on a number of projects has been shown to be

effective in reducing seepage losses. In the Coachella

Valley, California, an underground concrete-pipe distribution system, and a concrete-lined main canal, serve

approximately 70,000 acres of land. Reduction of

seepage losses and improvement in drainage conditions

were major factors in the selection of these facilities.

Automatic control of distribution systems, combined

with lined canals and laterals, is being used successfully in Algeria and elsewhere to eliminate regulatory

waste and to reduce the cost of operation. Automatic

control makes water available at the farm at all times

and allows water to be taken out or shut off from the

main distribution system at laterals or at farm outlets

at any time. All regulatory changes to maintain

proper flow from the point of diversion to the farm are

performed automatically. This eliminates waste on the

farm and throughout the system. Older irrigation districts with drainage and salinity problems might well

consider some of the advantages of the newly developed

automatic distribution systems. A modernization of

the distribution system in some cases may be the most

economical way to solve a drainage problem.

The selection of an irrigation method for applying

water to the soil is related to salinity. The method

that is best adapted in any particular case depends upon

a number of conditions: The crop to be grown, topography, soil characteristics, availability of water,

soluble-salt content of the water, and salinity status of

the soil. The primary objective of any irrigation

method is to supply water to the soil so that moisture

will be readily available at all times for crop growth,

but soil salinity is definitely an influencing factor.

It is desirable, both for plant use and for leaching,

to apply the water uniformly over the irrigated area.
36 AGRICULTURP: HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

The four principal methods used for the application of

water are flooding, furrow, sprinkling, and subirrigation.

The flooding method should be favored if salinity

is a serious problem. Wild flooding, border-strip or

border-check flooding, and basin flooding are used.

Wild flooding is not practiced extensively, except for

pastures, alfalfa, and small grains. This method can

be used only in relatively level areas where water can be

flooded over the surface without the use of levees or

borders for control. The border-strip or border-check

method of irrigation utilizes levees or borders for control of the water. The water is not impounded by this

method, except perhaps at the lower end of the strip,

but is flooded over the surface and down the slope in

the direction of the borders. It is adapted for use with

alfalfa and grains and in orchards; but excessive water

penetration near the head ditch and at the ends of the

strips usually results. There is a tendency for insufficient penetration to occur midway or two-thirds of the

way down the strip which generally causes salt to

accumulate in this location.

The basin method of flooding is often used for

orchards and various other crops in areas where water

can be impounded in a rectangular basin. A variation

of this method is the contour-basin method. Borders

are constructed along the contours at intervals of about

0.1 to 0.2 foot. This allows larger basins to be made

where there is appreciable slope. The basin methods

of irrigation provide better control of the depths of

water applied and greater uniformity in application

than border or furrow methods.

Furrow irrigation is well adapted to row crops and

is suitable for use where the topography is too rough or

steep for other methods. With this method there is a

tendency for salts to accumulate in the ridges, because

the leaching occurs only in the furrows. Wide-bottomed furrows that resemble narrow border strips have

certain advantages for wetting the soil surface uniformly and thereby controlling salt accumulation in a

larger fraction of the root zone. Where the area is

plowed and the surface soil is mixed occasionally, the

increase in salt over a period of time may not be serious.

If excess salt does accumulate, rotation of crops accompanied by a change in method of irrigation to flooding

or ponding is often possible as a salinity-control

measure. In the furrow and border-check methods

the length of run, size of stream, slope of the land, and

time of application are factors that govern the depth

and uniformity of application. Proper balance among

these factors, therefore, is directly related to leaching

and salinity control.

Irrigation by sprinkling is generally more costly

than by other methods and has not been used extensively until recent years. Originally this method was

used primarily for orchards, truck crops, and nurseries;

but its use has been extended to include sugar beets,

peas, beans, and many other crops. This method allows

a close control of the depth of water applied and when

properly used results in uniform distribution. It is

often used in areas where the slope is too great for other

methods. There is a tendency to apply too little water

by this method; and, unless a special efl'ort is made,

leaching to maintain the proper salt balance will not be

accomplished.

Subirrigation is the least common of the various

methods of irrigation and is not suitable for use where

salinity is a problem. Even under the most favorable

circumstances, this method does not appear to be suitable for long-time use unless periodic leaching is accomplished by rainfall or surface irrigations.

Leaching

The leaching of soluble salts from the root zone is

essential in irrigated soils. The need for leaching can

be illustrated by considering the eff'ect that salts in

irrigation water have upon the salinity of soil if no

leaching occurs. Without leaching, salts accumulate

in direct proportion to the salt content of the irrigation

water and the depth of water applied. The concentration of the salts in the soil solution results principally

from the extraction of moisture from the soil by the

processes of evaporation and transpiration. Assuming

no precipitation of soluble constituents during the

salinization process, the depth of irrigation water

(Djw) of known electrical conductivity (ECiy,) that

will contain sufficient salt to increase the electrical conductivity of the saturation extract of a depth of soil

(Z)Ji by an amount {AEC^) can be calculated from

the (equation:

D,JD,= {d,/d^) (SP/lOO) (AECe/EC,^) (1)

where djdy, is the ratio of the densities of the soil and

the water, and SP is the saturation percentage.^

As an example, let: fCj^ X 10^= 1,000, d^=-\.2 gm.

cm."^ dyf=^\ gm. cm. "\ and SP=40. Make the calculation for a change in electrical conductivity of the saturation extract of 4 mmhos/cm., or Ai^CeXlO^' =

4,000. Substituting these values in the equation we

find Diw//)s'=1.9. Thus less than 2 feet of reasonably

*^ For the purposes of this problem, electrical conductivity of

water is a satisfactory measure of salt concentration. If Z)iw

represents the depth of irrigation water applied and Dsw

represents the equivalent free depth of this water after entering

the soil and being concentrated by transpiration and evaporation, then Dl^v/D^w=^ECsw/EC^w, where the right-hand side

of the equation is the ratio of the electrical conductivities of

the soil water and the irrigation water. The conductivity of

the saturation extract EC>- provides a convenient scale for appraising soil salinity; therefore, consider the condition where the

content of moisture in the soil is the saturation percentage and

AECe is the increase in soil salinity produced by the water

application under consideration. For this case, the depth of

soil water (Z)sw) contained in a depth of soil (DH) is given by

the relation

"" d, 100 "•

Substituting these values in the above equation and rearranging

gives :

D^^d^ SP AECe

D« Jw ' 100 ' ECi^ (1)

The equation makes it possible to calculate the depth of irrigation water per unit depth of soil required to produce any

specified increase in soil salinity expressed in terms of AEC^y

for any given conductivity of the irrigation water (ECi^).
SALINE AND ALKALI SOILS 37

good quality irrigation water contains sufficient salt to

change a 1-foot depth of a salt-free loam soil to a saline

condition, if there is no leaching or precipitation of salt

in the soil.

Hundreds of thousands of acres of land in western

United States have been profitably irrigated for many

years with water having an electrical conductivity value

approximating 1,000 micromhos/cm. It is apparent

that considerable leaching has been provided, since almost enough salt is added to the soil each season to

make the soil saline. With this quality of water,

salinity troubles have occurred if the water table has

approached to within 3 or 4 feet of the surface of the

soil. In such cases extensive drainage and leaching

operations have been necessary. Some areas have been

abandoned, because it was not economically feasible to

provide soil drainage sufficient to take care of required

leaching.

Leaching Requirement

The leaching requirement may be defined as the

fraction of the irrigation water that must be leached

through the root zone to control soil salinity at any

specified level.^ This concept has greatest usefulness

when applied to steady-state water-flow rates or to total

depths of water used for irrigation and leaching over

a long period of time. Obtaining calculated or experimentally determined values of the leaching requirement

is complicated by many factors, but it is profitable to

consider some simplified theoretical cases. The leaching requirement will depend upon the salt concentration of the irrigation water and upon the maximum

concentration permissible in the soil solution. The

maximum concentration, except for salt crusts formed

by surface evaporation, will occur at the bottom of the

root zone and will be the same as the concentration of

the drainage water from a soil where irrigation water

is applied with areal uniformity and with no excess

leaching. Increase of the concentration of salts from

the value existing in the irrigation water to the value

occurring in the drainage water is related directly to

consumptive use. On cropped areas this will consist

mostly of water extracted from the soil by roots and

so will depend on the salt tolerance of the crop. Ex-

'In the report of the U. S. National Resources Committee

(1938), C. S. Scofield with the cooperation of R. A. Hill, proposed a formula for what was called "service equivalence,"

in which the concentration of the drainage water and the concentration of the irrigation water are taken into account. In

addition to the salt removed through drainage, it is inherent

with this formula that soluble salt is removed from the soil

at a rate equal to the consumptive use of water times half the

concentration of the irrigation water.

A further contribution to this subject was made at the

Irrigation Conference sponsored by the Texas Agricultural Experiment Station at Ysleta, Texas, in July 1951. At this conference, F. M. Eaton proposed what he called a "drainage

formula" for calculating the fraction of the irrigation water to

be used for leaching. A private communication to the Laboratory from F. M. Eaton, under date of August 1952, contained a

mimeographed paper entitled "Formulas for estimating drainage and gypsum requirements for irrigation waters," in which

the bases for the Ysleta formula are presented.

pressed in terms of electrical conductivity, the maximum concentration of the soil solution should probably be kept below 4 mmhos/cm. for sensitive crops.

Tolerant crops like beets, alfalfa, and cotton may give

good yields at values up to 8 mmhos/cm., while a very

tolerant crop like barley may give good yields at values

of 12 mmhos/cm. or higher.

To illustrate the significance of the leaching requirement, consider first the simplest possible case with the

following assumed conditions: Uniform areal application of irrigation water; no rainfall; no removal of

salt in the harvested crop ; and no precipitation of soluble constituents in the soil. Also, the calculation will

be based on steady-state water-flow rates or the total

equivalent depths of irrigation and drainage waters

used over a period of time. With these assumptions,

moisture and salt storage in the soil, depth of root zone,

cation-exchange reactions, and drainage conditions of

the soil do not need to be considered, providing that

drainage will permit the specified leaching. The leaching requirement (LR) as defined above, is simply the

ratio of the equivalent depth of the drainage water to

the depth of irrigation water (D^w/i^iw) and may be

expressed as a fraction or as percent. Under the foregoing assumed conditions, this ratio is equal to the

inverse ratio of the corresponding electrical conductivities, that is :

LR-- - (2)

For field crops where a value of £'Cdw =8 mmhos/cm.

can be tolerated, the formula would be Z)dw/Öiw =

ECiy,/8. For irrigation waters with conductivities of

1, 2, and 3 mmhos/cm., respectively, the leaching requirements will be 13, 25, and 38 percent. These are

maximum values, since rainfall, removal of salt by the

crop, and precipitation of salts such as calcium carbonate or gypsum in the soil are seldom zero; and, if

properly taken into account, these factors all would

enter in such a way as to reduce the predicted value of

the leaching requirement.

Some care must be exercised in using equation 2, to

make sure that the condition of steady-state or longtime

average is understood. The equation does not apply

if leaching is automatically taken care of by rainfall.

Depending on soil texture and depth to water table,

this may be the case even in semiarid regions, if the

precipitation is confined to a small fraction of the .year.

Under these conditions, equation 1, which gives the

buildup of salinity with depth of irrigation water

applied, is useful for predicting salinity increases

during an irrigation season or over a period of several

seasons when rainfall may be abnormally low.

As an average over a long time, the conductivity

of the irrigation water used in equation 2 should be a

weighted average for the conductivities of the rainwater

(Ä'Crw), and the irrigation water (¿"Ciw), i. e.:

TP/^ -^rw^' rw 1 -^iw-^'^lw /Q\
38 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

where D,w and />iw are the depths, respectively, of the

rainwater and irrigation water entering the soil. Longtime averages may deviate markedly from actual conditions at any one time, as, for example, if the entire

root zone is leached through during a short period of

extra high rainfall.

Information on the consumptive use of water by the

crop is necessary if the leaching-requirement concept

is to be used for determining either the depth of irrigation water that must be applied or the minimum

depth of water to be drained, in order to keep the soil

salinity from exceeding a specified value. The depth

of irrigation water (Diw) is related to consumptive use,

(Dew) and the equivalent depth of drainage water

(Ddw) by the equation:

Aw=Aw+ödw (4)

Using equation 2 to eliminate Ddw from equation 4

gives :

Aw=öcw/(l-iÄ) (5)

Expressing the leaching requirement (LR) in this equation in terms of the conductivity ratio in equation 2

gives:

The depth of irrigation water (Diw) is thus expressed

in terms of the electrical conductivity of the irrigation

water and other conditions determined by crop and

climate; namely, consumptive use and salt tolerance of

the crop. The salt tolerance of the crop is taken into

account in the selection of permissible values of ECav^

Equations 5 and 6 are subject to the assumptions made

in deriving equation 2.

Under actual farming conditions, the depth of water

applied per irrigation and the areal uniformity of

application are certainly not precisely controlled.

Measured water apJDlication efficiencies often run as low

as 25 percent and seldom exceed 80 percent. Under

these conditions, high precision in the determination

of the leaching requirement has little significance. A

formula like equation 2 would appear to have greatest

usefulness in connection with the more saline irrigation

waters, and for this case it appears to be justifiable to

disregard the salt removed from the soil in the harvested

crops. Consider alfalfa growing in the Imperial Valley, California, where 6 tons per acre of sun-cured hay

is a common annual yield. The salt added to the soil in

the irrigation water consumed by this crop would be

about 4 tons. Of this salt, not more than 0.4 ton would

be removed in the harvested crop. Under these conditions, therefore, neglecting the salt removed in the crop

overrates the salt input to the soil by a factor of about

one-tenth. Taking JE^C^W —8 and ECiyf=l, the calculated steady-state leaching requirement for salt-tolerant

crops of the Imperial Valley is 13 percent, A fractional

error of one-tenth in this value would not be serious,

in view of other uncertainties involved in the practical

use of the figure.

The relative significance of the salt removed in the

harvested crop will increase as the salt input from irrigation water decreases, but for soils with normal

drainage the practical usefulness of a calculated value

of the leaching requirement decreases as the salinity

of the irrigation water decreases. A special case exists

where leaching is severely restricted by low soil

permeability and the salt content of the water is also

very low. Under these conditions, salt removed from

the soil in the harvested crop might conceivably become

an important factor determining the permanence of

irrigation agriculture.

The steady-state leaching requirement (equation 2),

expressed in terms of electrical conductivity, is convenient where soil moisture availability to plants and

osmotic pressure relations are the principal concern.

Cation exchange is known to effect a change in the

relative composition of irrigation and drainage waters,

but this process is stoichiometric and does not enter

explicitly in the equation. It may happen, however,

that with a particular irrigation water and a particular

crop, some specific toxic constituent as, for example,

the chloride ion or boron, might comprise the most

critical problem. A leaching requirement for this constituent could then be calculated, provided some

maximum permissible concentration of the toxic ion

Caw in the water draining from the soil can be specified

and provided also that the other assumptions previously made are tenable. The leaching requirement

equation then becomes:

D P

JY~~7T~ \* )

where Ciw is the concentration of the toxic ion in the

irrigation water.

There will be instances, of course, where precipitation of soluble constituents in the soil cannot be neglected when calculating the leaching requirement. Gypsum is deposited in soils from some irrigation waters.

Data are being accumulated on the precipitation of calcium and magnesium with bicarbonate in the irrigation

water. This latter reaction is considered in chapter 5

on irrigation water quality. Taking precipitation

effects into account complicates a leaching requirement

equation and will not be included in the present discussion. It should be recalled again that the foregoing

equations are based on the assumptions: uniform water

application to the soil, no precipitation of soluble salt

in the soil, negligible salt removal in the harvested

crop, and soil permeability and drainage adequate to

permit the required leaching.

Quantitative consideration of the leaching requirement is important when drainage is restricted or when

the available irrigation water is efficiently used. If a

large fraction of the water diverted for irrigation is

wasted in various conveyance, regulatory, and, especially, application losses, then estimates of leaching requirement have little practical significance.

Leaching Methods

Leaching can be accomplished by ponding an appreciable depth of water on the soil surface by means of

dikes or ridges and thus establishing downward water
SALINE AND ALKALI SOILS 39

movement through the soil. This is the most effective

procedure that can be used for removing excess soluble

salts from soil. Contour checks can be used for ponding water on the soil where there is considerable slope.

Contour borders ranging from 1.5 to 4 ft. or more high

are constructed at elevation intervals ranging from 0.2

to 0.5 ft. Overflow gates, placed in the borders connecting adjacent plots, facilitate the control of water

and allow a number of contour checks to be kept full

simultaneously. Frequent applications of excess irrigation water applied by flooding between border strips

while a crop is being grown are sometimes used for

leaching. The effectiveness depends upon how uniformly the water is applied and how much water passes

through the soil. Either continuous flooding or periodic water applications may be used for leaching. If

the soil transmits water slowly, periodic drying may

improve infiltration rates.

In cold climates, leaching operations can often be

conducted in the fall after the crops mature and before

the soil freezes. In warmer climates, leaching operations can be conducted during winter when the land

would otherwise be idle. At this time, also, water may

be more plentiful and the water table and drainage conditions more favorable than during the regular irrigation season. Unless drainage is adequate, attempts at

leaching may not be successful, because leaching requires the free passage of water through and away from

the root zone. Where drainage is inadequate, water

applied for leaching may cause the water table to rise

so that soluble salts can quickly return to the root zone.

Visible crusts of salt on the surface of saline soils

have sometimes led to the use of surface flushing for

salt removal, i. e., the passing of water over the soil

surface and the wasting of the runoff water at the

bottom of the field. This method does not appear to

be sufficiently effective to be worth while for most field

situations. All known tests of the flushing method

under controlled conditions confirm this conclusion.

Turbulence in the flowing water causes some mixing,

but mostly the water at the soil surface that contacts

and dissolves the salt moves directly into the dry soil

during the initial wetting process when the infiltration

rate is highest. In one test the salt added to the soil

in the water used for flushing exceeded the amount of

salt removed in the waste water.

The depth of water required for irrigation and leaching and the effect of leaching on the depth to water

table can be estimated with the aid of the nomograms

given in figure 8, chapter 2. The following examples

will serve to illustrate the use of the nomograms in

connection with irrigation, leaching, and drainage.

(a) For a uniform soil with an initial moisture percentage of 10, an upper limit of field moisture of 20

percent, and a bulk density of 1.6 gm. cm. ^ how

deeply will a 6-in. irrigation wet the soil? In the left

nomogram of figure 8, place a straightedge on 1.6 of

the scale Bi and on 10 of scale A. Scale Ci then indicates that 1.94 in. of water are required to raise the

moisture content of 1 foot of this soil by 10 percent.

Therefore, 6/1.94 = 3.09 ft. = 37 in. is the depth of

wetting.

(6) For a uniform soil with an initial moisture content of 12.5 percent, an upper limit of field moisture of

25 percent, and a bulk density of 1.3 gm. cm.""\ what

depth of water must be applied to make 3 in. of water

pass through the soil at the 4-ft. depth? Evidently the

moisture content of the surface 4 ft. of soil must be

increased by 12.5 percent before leaching will occur.

Place a straightedge on 1.3 of the left nomogram of

scale Bj (fig. 8), and on 12.5 of scale A. Scale Ci

then indicates that 2 in. of water per foot of soil are

required to change the moisture percentage of this soil

from 12.5 to 25. Eight inches of water would be required to bring the top 4 ft. of soil to the upper limit

of moisture retention, and therefore 11 in. of irrigation

water should be applied in order to cause 3 in. of water

to pass below the 4-ft. depth.

(c) For a uniform soil with a bulk density of 1.5

gm. cm.^ and an average moisture content of 20 percent over a depth interval of 1 foot above the water

table, what depth of water in surface inches, when added

to the ground water, will make the water table rise 1

foot? Assume the particle density (real density) of

the soil is 2.65 gm. cm.~^. In the right-hand nomogram

of figure 8, place a straightedge on 1.5 of scale Ai and

on 2.65 of scale Bj. Scale Ci then indicates a porosity

of 0.44. Consequently, this soil when completely saturated will hold 0.44 ft. of water per foot of soil. In the

left nomogram place a straightedge on 20 of scale Ai

and 1.5 of scale Bi. Scale Co then indicates that a

moisture content of 20 percent corresponds to 0.3 ft. of

water per foot of soil. Subtracting this from 0.44 indicates that 0.14 ft. of water per foot of soil, or (from

scales Ci and C.) 1.7 surface inches of water is sufficient to bring 1-ft. depth of this soil to saturation and

hence to cause a rise of approximately 1 foot in the

ground-water level.

Field Leaching Trials

Numerous field trials have demonstrated the effectiveness of leaching for salt removal. For example, Reeve

and coworkers (1948) found that gypsiferous, salinealkali soils in the Delta Area, Utah, are reclaimable by

leaching with 4 ft. of water. The right-hand curve in

figure 12 shows the salt distribution with depth at the

beginning of leaching tests. This soil had been idle for

many years, with the water table fluctuating between

2 and 5 feet below the soil surface. Leaching treatments of 0, 1, 2, and 4 ft. of water were applied to test

plots. The curves in the figure show the resulting

change in salt content with depth. Wheat was planted

and subsequently irrigated with 18 to 24 in. of water

in 3 applications of 6 to 8 in. each. In addition, approximately 12 in. of rain fell during the winter

months, making a total of 30 to 36 in. of water applied

in addition to the initial differential leaching treatments. The increase in yield of wheat was approximately linear in relation to the depth of water used for

leaching (fig. 13).
40 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

SOIL SURFACE

2 FT. WATER FT WATER

IUJ

CL

UJ

O

BEFORE LEACHING

4 FT. WATER

ELECTRICAL CONDUCTIVITY OF i:i EXTRACTS-MILLIMHOS/CM.

5 10 15 20 25 30 35 40

I ' I I ' 1 1—I I I . . » . . t ,

0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

PERCENT SALT

FIGURE 12.—Distribution of salt content with depth as related to depth of water applied for leaching in the Delta Area, Utah (Reeve

and others, 1948).

Leaching practices, although basically the same, may

vary from one region to another. In the Delta Area

tests, the ponding method was used, and water was

added in successive increments until the total amount

for leaching had been applied. About 10 days were

required to leach the plots with 4 ft. of water. In some

parts of the Imperial and Central Valleys of California,

where infiltration rates are low, water is ponded on the

surface by the contour-check method for periods up

to 120 days. In such instances, rice is sometimes grown

to aid in the reclamation process and also to provide

income during leaching. In other areas, rice is included

regularly in the crop rotation as an aid in salinity

control.

In addition to the removal of excess salts and exchangeable sodium, other practices are usually required

for complete reclamation. Plant nutrients that are

leached from the soil must be replaced, and fertilizer

practices following leaching should compensate for

plant nutrient losses. Nitrogen is the principal nutrient subject to leaching loss. Soil structure that may

have deteriorated during the salinization or alkalization process must be restored. Unfavorable soil structure after leaching is sometimes a special problem and

may be improved by adding manure or other forms of

organic matter, by growing crops that are beneficial to

structure, or by alternate wetting and drying, as indicated by the field tests of Reitemeier and associates

(1948) and Bower and coworkers (1951).

Special Practices for Salinity Control

The failure to recognize that saline and alkali soils

require special management practices can result in low

production or in complete crop failure. These special

practices can be followed over a period of time to

improve lands that are partially affected or to prevent

reclaimed lands from again becoming unproductive.

Where only irrigation water of poor quality is available or where drainage and full-scale reclamation are

not economically feasible, it may be possible to carry

on successfully what might be referred to as "saline

agriculture.'' Irrigation, leaching, and tillage practices

can all be directed toward salinity control. Salt-tolerant crops can be selected and chemical amendments

used when necessary.

Many crop failures result from growing crops that

have low salt tolerance. Alfalfa, barley, sugar beets,

and cotton are tolerant crops that can often be grown

where salinity is a problem. Lists of salt-tolerant

fruits, vegetables, field, and forage crops are given in

chapter 4.

In general, irrigation methods and practices that

provide uniformity of application and downward movement of water through soils favor salinity control.

Methods that pond or flood water over the soil surface,

such as border, check, and basin methods of irrigation,

give greater uniformity of application than furrow or

corrugation methods. Only part of the surface is cov-
SALINE AND ALKALI SOILS 41

ered by water with the furrow and corrugation methods

so that movement of water is downward and outward

from the furrow and is upward into the ridges. Wadleigh and Fireman (1949) have shown that by furrow

irrigation excessive amounts of salts concentrate in the

ridges. Salt distribution resulting from furrow irrigation in a test plot that was salinized initially to 0.2

percent is shown in figure 14. They further showed that

cotton plants in the ridges extracted moisture mainly

from beneath the furrows where leaching occurred and

that there was little root activity in the ridges.

Germination and emergence of plants is often a critical factor in over-all production. Ayers (1951) has

shown that the germination of seeds is greatly retarded

and that the number of seeds germinating may be materially decreased by salinity. If favorable conditions

can be maintained during the germination and seedling

stages, certain crops may make fair growth even under

moderately high salinity conditions. Heald and others

(1950) conducted experiments in Washington on the

preemergence irrigation of beets. They showed that

irrigation next to the seed row caused movement of

salts away from the seeds and into the ridges. This

allowed the seeds to germinate and to become established in essentially nonsaline conditions, thereby increasing yield by increasing stand (fig. 15). Further

over-all leaching increased sugar beet yields.

Careful leveling of land makes possible more uniform application of water and better salinity control.

Barren spots that appear in otherwise productive fields

are often the result of high spots that do not get sufficient

water for good crop growth and likewise do not get

sufficient water for leaching purposes. Lands that have

been irrigated 1 or 2 years after leveling can often be

improved by replaning. This removes the surface unevenness caused by the settling of fill material. Annual

crops should be grown following land leveling, so that

replaning after 1 or 2 years of irrigation can be accomplished without crop disturbance.

Crusting of the soil and failure of seedlings to emerge

may indicate an alkali condition that might be corrected

by amendments. Irrigating more frequently, especially

during the germination and seedling stage, will tend to

soften hard crusts and help to get a better stand.

Drainage of Irrigated Lands in Relation to

Salinity Control

Drainage in agriculture is the process of removal of

excess water from soil. Excess water discharged by

flow over the soil surface is referred to as surface

drainage, and flow through the soil is termed internal

or subsurface drainage. The terms "artificial drainsJ\J

UJ 40

^

(T. ^ '^^'"^"^

O ^^_^,.^^^*''''^ ^^^

<

^.^ -'^^'^^Z^^^^^^^^^^^^ ^^"^^^^

cd ^—'—'^^^^^^— ^---^^^^

ÜJ ^„„''^'''''^ ^— ^^^^"^

û30 </)

UJ

---___

X ^ ^^^ (/) ^^--^ ^y'^

3 /

^^f^

CD /

^y^

20 /

/ e\35>^^^

û / ^.^^

-j / ^y^

UJ

>-

10 / /

yy

^ 1 1 1 1

LEACHING FEET OF WATER

FIGURE 13.—Grain yields as related to the depth of water used for leaching in the Delta Area, Utah (Reeve and coworkers, 1948).
42 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

PLANT STALK

30-«

CONDUCTIVITY OF EXTRACT FROM SATURATED SO«L - MILLIMHOS/CM.

D Less than l.O

1.0 to 2.0

2.0 to 5.0

5.0 to 6.0

6.0 to 10.0

Above 50

>Xv 10.0 to 50 I

FIGURE 14.—Salt distribution under furrow-irrigated cotton for soil initially salinized to 0.2 percent salt and irrigated with water of

medium salinity (Wadleigh and Fireman, 1949).

age" and "natural drainage" indicate whether or not

man has changed or influenced the drainage process.

Irrigated land is drained primarily to increase agricultural productivity, but there are other beneficial

eíTects. Areas that are poorly drained require the

expenditure of large sums of money annually for construction of highway subgrades and for safeguarding

public health, since mosquito control and other disease

problems are related to drainage conditions. Drainage

improvements serve many public and private interests,

and the justification for drainage improvements should

be based upon all benefits that may be derived

therefrom.

The drainage program for irrigated land should be

initiated and continuously integrated with the development of the irrigation system in order to attain an efficient over-all water and salinity control program. The

removal of excess water and salts must be considered in

every irrigation enterprise. Excess water may be

partially discharged or removed from the soil by natural

means, but often supplementary drainage facilities are

required. Irrigation practices, together with methods

of distributing water, are related to drainage, and sometimes the need for artificial drainage facilities may be

lessened or avoided altogether by efficient management

of irrigation water.

The design of drainage systems is influenced by many

factors, and there are no simple rules or formulas by

which all of these factors can be taken into consideration. However, the principal factors can be grouped

under drainage requirements, water-transmission properties of soil, and boundary conditions.

Drainage Requirements

The permissible depth and mode of variation of the

water table with respect to the soil surface and the

quantity of water that a drainage system must convey,

both surface and subsurface, relate to drainage design

and may be referred to as the drainage requirements.

The climate, the quality of the irrigation water, the

characteristics of the soil, the crops, and the cropping

system must all be considered in the determination of

drainage requirements for any given locality.

The adequacy of drainage for agricultural purposes

depends upon whether or not there is an excess of water

on or in the soil for periods of time that are detrimental

to crops. Inadequate aeration of the soil may be a

direct consequence of inadequate drainage and may

result in a limitation of growth of plants or severe

damage to root systems through pathological, physiological, or nutritional disturbances, or through limitation of the effective depth of the root zone. The optimum moisture content of the soil for tillage and other

farming practices is also involved because farm operations can be seriously delayed by wet soil.
SAUNE AND ALKALI SOILS 43

In irrigated regions the adequacy of drainage is related to salinity. Salts in the irrigation water, in the

soil, or in shallow ground waters increase the drainage

requirements. In addition to aeration effects and soilmoisture requirements for tillage, a minimum allowable water-table depth that will permit adequate leaching and that will prevent concentration of salts in the

root zone by upward flow must be established. The

depth to the water table must be such that upward flow

of saline ground water into the root zone is reduced or

eliminated. Thus, irrigation, leaching, and soilmanagement practices that are involved in the control

of salinity are important in establishing drainage

requirements.

As a minimum requirement, a drainage system must

be adequate to remove from the soil the equivalent

depth of water that must be passed through the root

zone in order to maintain a favorable salt balance.

With a knowledge of the consumptive use, the minimum

amount of water required to be drained can be estimated by the use of equations 2 and 4:

LR= Uñyf ¿LCU

"Aw (2)

(4)

Equation 2 gives the fraction of the water applied as

irrigation that must pass through and beyond the root

zone to maintain the electrical conductivity of the drainage water below a specified value (i^Cdw) for the steadystate or long-time average salt-balance conditions.

Equation 4 gives the depth of irrigation water (Z)iw)

as a function of consumptive use (Z>ow) and the equivalent depth of drainage water ißa^). Solving equation

2 for Dyy,, substituting in equation 4, and rearranging

gives :

Z>. A,

l-LRLR (8)

Expressing LR in this equation in terms of the conductivity ratio of equation 2 gives:

ECly,

UñMO

EC¿yf—ECi^

D. (9)

The depth of the water to be drained (/>dw) is thus

expressed in terms of the electrical conductivity of the

irrigation water and other conditions determined by the

crop and climate; namely, consumptive use and salt

tolerance of the crop. The salt tolerance of the crop

is taken into account in the selection of permissible

values of EC^y,. Equations 8 and 9 are subject to the

assumptions made in deriving equation 2.

AT THINNING TIME

FOLLOWING

PRE-EMERGENGE IRRIGATION

AT MATURITY

FOLLOWING

NORMAL IRRIGATION

RELATIVE SALT CONCENTRATIONS

Very Low V///////A Moderately High

Lew High

Moderate Very High

FIGURE 15.—Salt concentration in the vicinity of growing beets as related to position in the furrow (redrawn from Heald and other

1950).
44 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

The term Ddw in the equation does not include drainage water that moves in laterally from adjacent areas

and that must pass into and through the drainage

system, but represents only the depth by which irrigation water, assumed to be applied uniformly at the soil

surface, exceeds the consumptive use. For any specified ¿"Cdw, which depends upon the salt tolerance of the

crop, the depth of drainage water (Ddw) is the minimum depth of water that is required to be drained.

This condition is satisfied when the previously defined

leaching requirement is just met. For a value of

ECdvf — 8, which applies for moderately tolerant crops,

and for irrigation waters of ECiy^ — O.S, 1, 2, and 4

mmhos/cm., the depths of drainage water that must

pass through the soil are 7, 14, 33, and 100 percent of

the consumptive use (/)cw), respectively.

The passage of excess water through the root zone is

accompanied by a decrease in the electrical conductivity

of the drainage water. The equivalent depth of drainage water that is required to be drained (Ddw) from soil

where irrigation water is applied inefficiently but uniformly may be estimated by substituting in equation 9

the electrical conductivity of the drainage water (EC^w)

as sampled and measured from the bottom of the root

zone.

The depth of water that is drained beyond the root

zone may also be expressed in terms of the waterapplication efficiency and the total depth of water

applied or the consumptive use. The equation

£= Dew/Diw is based on the definition of water-application efficiency (Israelson, 1950), where E represents

water-application efficiency and the other symbols are

as previously defined. Solving this equation for Dew

in one case and for Aiw in the other, substituting in

equation 4 and solving for Ddw, we obtain:

Ddw =Diw(l~^) (10)

Ddw =Dew(^-l) (11)

Measured application efficiencies often run as low as

25 percent and seldom exceed 80 percent. Correspondingly, the water to be drained that comes directly from

irrigation will range from 20 to 75 percent of the

irrigation water applied and from 25 to 300 percent

of the consumptive use. The total quantity or equivalent depth of water to be drained will be equal to that

given by these equations plus that from other sources,

such as seepage from canals and artesian aquifers.

Seepage from canals is a major source of excess ground

water in many areas, and seepage losses of 30 to 50

percent of the water diverted often occur.

Water-Transmission Properties of Soils

The principles and background theory for fluid flow

in porous media are well known and are adequately

treated in the literature. A discussion of the forces and

properties determining the flow and distribution of

water in soil, both saturated and unsaturated, and a

description of measuring methods are given by Richards

(1952). An important part of this background theory

is embodied in the well-known Darcy equation, which

in its generalized form states that for isotropic media

the flow velocity, or specific discharge, is proportional

to the hydraulic gradient and is in the direction of the

greatest rate of decrease of hydraulic head.

The water-transmission properties of subsoils that

cannot be controlled or changed appreciably have a

direct bearing upon the design and layout of drainage

systems. Soils, generally, are highly variable with

respect to water-transmission properties, and it is necessary to assess the nonhomogeneity and to appraise the

influence of soil variations on the direction and rate

of flow of ground water.

Boundary Conditions

This concept is commonly used in the solution of

flow problems and involves a geometric surface defining the boundaries of the problem along with hydraulic

conditions over this surface, i. e., hydraulic head,

hydraulic gradient, and flow. In other words, the external influences and constraints characterizing any

givem flow problem are included in the boundary conditions. While the root zone is the region of primary

concern for agricultural drainage, a drainage problem

may involve a considerably larger and deeper region.

The upper and lateral bounding surfaces may be reasonably definite, but the lower boundary will depend on

stratigraphy and hydraulic conditions. Many irrigated

areas of the West are in alluvial valleys where topography and stratigraphy vary widely and where there

may be diverse sources of ground water. The identification and delineation of these sources is especially

important in establishing and defining boundary

conditions.

Surface drains function mostly to eliminate water

from the soil surface that may otherwise contribute to

u-.derground flow. Deep gravity drains, tile, and open

ditches provide outflow points below the ground surface for controlling water-table depths and hence are

a part of the boundary conditions. They are mostly less

than 15 ft. deep because of construction limitations.

Where conditions are favorable for pumping, water

tables can usually be maintained at greater depths and

thereby be controlled more effectively by pumping than

by any other method. Most wells are installed to obtain water for irrigation, but often they also function

to improve drainage conditions.

Layout and Placement of Drains

Drainage systems may consist of intercepting drains

or relief-type drains, depending upK)n their location and

function. Intercepting drains collect and divert water

before it reaches the land under consideration, and

relief drains are placed to remove water from the land

being drained. Pumped wells, tile, or open drains may

serve either of these purposes. Relief-type drains are

used in broad valleys where the land has little slope^

whereas intercepting drains more often are used in

areas where topography is irregular. In areas of roll-
SALINE AND ALKALI SOILS 45

ing or irregular topography, where lands of appreciable

slope are irrigated, water that percolates downward

through the surface soil often flows laterally through

subsoil materials in the direction of the land slope.

In these areas, seeps may be caused by a decrease in

grade, a decrease in soil permeability, a thinning out

of permeable underlying layers, the occurrence of dikes

or water barriers, or the outcropping of relatively impermeable layers or hardpans. If the seepage water

cannot be eliminated at its source, the placement of tile

or open drains immediately above the seep to intercept

such flows is usually the most effective procedure for

solving this type of drainage problem.

Proper placement of drains is of considerable importance in the design of a drainage system. In nonuniform soils drainage systems may best be designed

by considering the nature and extent of subsoil layers

and by locating the drains with respect to these subsoil

materials. Generally drains should be oriented perpendicular to the direction of ground-water flow and,

where possible, should connect with sand and gravel

layers or deposits. In soils of alluvial origin, the

orientation of both permeable and impermeable deposits may be such that a few well-placed drains may

control ground water over a much larger area than the

same length of drain installed with uniform spacing

in accordance with some arbitrary pattern. This has

been demonstrated in a number of irrigated areas.

For example, in the Grand Valley, Colorado, open

drains that cut across and intercept sand and gravel

deposits provide much more effective drainage than

drains dug parallel to these deposits.

In areas where artesian conditions occur, drainage

by tile and open drains is often impractical. Although

the quantity of upward flow from an artesian source

may be small, it usually exerts an important controlling

effect on the height of the water table between drains.

Artesian aquifers in many cases may be highly permeable and ideally located for drainage purposes, but

they may be unavailable for receiving and discharging

excess water applied at the soil surface because of the

artesian pressure condition. Reduction of the water

pressure in these aquifers by pumping or other means

should be a first consideration.

The problem of flow into drains under falling watertable conditions has not been solved analytically.

However, solutions have been developed for the ponded

condition where drains are installed in saturated

isotropic soil with a layer of water covering the surface.

The falling water-table case typifies the drainage conditions in irrigated soils where it is desired to maintain

adequate depth of water table between drains, whereas

the ponded area more nearly represents conditions in

humid regions where it is desired to remove excess

water in short time periods following precipitation.

Although the falling water-table condition differs appreciably from the ponded case, some of the important

findings with the ponded area may have useful application for the falling water-table condition. For the

ponded case, assuming isotropic soil, Kirkham (1949)

concluded that "The most important single geometrical

factor governing rate of seepage of water from soil

into drains is the drain depth. Doubling the depth of

drains will nearly double the rate of flow." For the

falling water-table case, which is the usual condition

in arid regions, the depth to the water table midway

between drains is directly dependent upon the depth

of the drains. For a given spacing, assuming soil conditions do not change with depth and other conditions

remain constant, the depth to water table midway between drains increases directly with drain depth.

Proximity of drains to relatively impermeable layers

is also an important consideration. Kirkham (1948,

p. 59) states: "Drains should not be placed too near,

on, or in an impervious layer. ... It is found that

lowering the drain onto or into an impervious layer,

although increasing the hydraulic head, decreases the

flow rate. . . ." He further states that "Drain shape

(as well as size) appears to be unimportant in governing seepage rate into drains." From this, it is apparent

that drain size should be determined primarily upon

the basis of flow-velocity requirements. A gravel pack

around tile drains is commonly used as a filter to allow

free flow of water and at the same time to prevent sediment from entering the tile line.

Techniques for Drainage Investigations

A drainage investigation should provide information

regarding the occurrence, flow, and disposition of excess

water within a given basin or area. Information regarding hydrology, geology, meteorology, topography,

and soils is needed and for some areas is already published and available. Reports of earlier drainage surveys should not be overlooked.

Measurements of Hydraulic Head

Inadequate drainage may be manifest by the presence of ponded water, marshy lands, and the growth

of hydrophytic plants; but, in the absence of these

obvious signs, depth to ground water is the most common index of the adequacy of drainage. Uncased

observation wells are commonly used for determining

the depth of the water table. Sometimes ground-water

observation wells are lined with perforated casing.

If there is a vertical component of flow, the true elevation of the water table is difficult to determine unless

piezometers are used.

The water table is the elevation in the profile at

which the soil water is at atmospheric pressure. This

elevation corresponds to the bottom of the shallowest

hole in which free water will collect. In a deeper hole

or an observation well with perforated casing, the

equilibrium elevation at which the water stands represents a balance between inflow and outflow for all the

soil layers penetrated by the hole and may not be a

useful hydraulic-head value.

The hydraulic head of ground water at each point in

the soil is the elevation at which water stands in a riser

connected to the point in question. There should be

no leakage externally along such a riser or piezometer

in order to insure that the elevation at which water
46 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

stands in the piezometer is determined by the pressure

in the ground water at the bottom end of the tube.

This condition of external sealing is readily met under

most field conditions for piezometers installed in accordance with Methods 35a and 35b. Measurements

of hydraulic head and hydraulic gradient provide basic

information on drainage conditions and the source

and flow of ground water.

The number and arrangement of sites at which

ground-water measurements should be made will depend upon the nature of the area in question and the

purpose for which the measurements are made. In

typical irrigated valleys information on both the adequacy of drainage and direction of ground-water flow

is usually desired. Wells may be located to serve both

purposes. Observation wells are often placed in a grid

pattern for which spacing is selected to coincide with

the land-survey system. In gently sloping areas, points

of measurement can be farther apart than in areas of

irregular topography. For determining the direction

of the horizontal component of flow, water-table readings may be made at any desired spacing. More measurement sites are required in localities where there are

abrupt changes in the slope of the water table.

Water-table contour maps and water-table isobath

maps are useful in interpreting water-table data

(Methods 36a and 36b). Profile flow patterns

(Method 36c) may be used to show the nature of flow

in cases where vertical as well as horizontal components

of flow occur, such as sidehill seeps, seepage from

canals, flow into drains, and upward flow from artesian

aquifers. Water-table isopleths, which are described

in Method Sóá^ can be used to show time fluctuations

of the water table on a profile section.

Convenient methods for installing small-diameter

piezometers have been described by Christiansen

(1943), Pillsbury and Christiansen (1947), and Reger

and others (1950). Piezometers may be installed by

either driving or jetting as outlined in Methods 35a

and 35b. The jetting technique provides a log of the

nature and arrangement of subsoil materials in addition to the installation of a pipe for hydraulic-head

readings. Piezometers 150 feet deep have been installed by this method.

Water levels in irrigation and domestic wells are

often used for ground-water study. Water levels in

such wells may or may not represent the water-table

level. Deep-well readings should not be used as a

measure of water table unless it can be definitely established by independent water-table measurements that

the well reflects the true water-table level. Information regarding wells, such as total depth of well and

depth of screens or perforations, is necessary in order

to interpret well readings correctly.

Determination of Subsoil Stratigraphy

Hand augers, power augers, driven tubes, standard

well-drilling equipment, and jetted piezometers can be

used for studying subsoil materials and for locating

and characterizing subsurface layers. The development of the jetting method of installing piezometers

has made it possible to make subsoil investigations at

only a fraction of the cost of augering or the use of

well-drilling methods. Piezometers may be jetted for

the sole purpose of determining subsoil stratigraphy,

or the pipe may be left in place after the soil log is obtained as a permanent installation for hydraulic-head

measurements.

Subsoil logs from jetted piezometers are usually

made on the basis of texture, since information on texture provides an indication of the water-transmission

properties of soils. Depths of strata changes may

sometimes be obtained to within ±0.1 ft. by this

method, and soil layers can be distinguished that are

too thin to be logged by well-drilling methods. An

estimate of the texture and consolidation of the material is made from the vibration or feel of the pipe to

the hands during the downward motion, from the rate

of downward progress, from the examination of sediments carried by the effluent, and from the observation

of color changes that occur in the effluent. (See

Method 35b.)

Standard well-drilling equipment may be used for

obtaining samples of subsurface materials and for

logging underground strata. Logs of irrigation, domestic, or municipal water-supply wells that have been

drilled in an area may usually be found in either county

or State governmental offices. Some States require

well drillers to file with the State engineer a log of

each well drilled. Such logs provide useful information regarding the major clay layers and principal

water-bearing aquifers. They are often deficient in

pertinent details, however, especially concerning subsoil changes at shallow depths. In interpreting well

logs the method of drilling should be taken into consideration. Logs of wells drilled by bailing methods,

where sediments are actually obtained and examined

from within a limited depth range, are usually more

reliable than logs obtained by other drilling methods.

Hand augers and driven tubes are generally limited

to depths less than 20 ft. They are used mainly for

appraising stratigraphy near the surface. Power

augers of various types are commercially available

that can be used to depths of 60 ft. or more. In sandy

soils it is sometimes necessary to case the hole with pipe

or tubing as augering progresses in order to get a hole

drilled to the desired depth and to obtain samples.

Undisturbed cores, 4 in. in diameter and from depths

up to 10 ft., can be obtained by use of the power-driven

core-sampling machine, an earlier model of which has

been described by Kelley and coworkers (1948).

This machine is trailer mounted and is usable over

terrain passable to trucks. Soil cores are useful for

the observation of structure and for making various

physical measurements on undisturbed subsoil materials. Cracks, root holes, and fine sand lenticles may

be overlooked with augering and other sampling

methods, but these are preserved for examination in an

undisturbed core.
SALINE AND ALKALI SOILS 47

Determination of Water-Transmitting Properties

of Soils

In addition to determining the position and extent

of subsoil materials as outlined above, information on

the rates at which soils transmit water is required in

planning and designing drainage systems. Soils are

extremely variable with regard to water transmission.

The heterogeneous nature in which most alluvial soils

are deposited adds materially to the problem of assessing their water-transmitting properties. Soils formed

both in place and by alluvial deposition may be extremely variable not only in a lateral direction but with

depth as well. The problem of appraising the watertransmitting properties of soils involves measurements

by suitable methods at representative sites or on representative samples.

The ratio of the waterflow velocity to the hydraulic

gradient is called the hydraulic conductivity. This is

the proportionality factor in the Darcy equation. This

quantity varies over a range, as much as 100,000

to 1, in earth materials in which drainage operations

are conducted. Hydraulic conductivity is often related to texture, coarse soils having high conductivity.

Particle-size distribution may also be an important

factor. Porous media with uniform particle sizes tend

to be more permeable than materials having a more or

less continuous range of sizes.

The hydraulic conductivity of soils, although related

in a general way to texture, depends also upon soil

structure. Soils near the surface that may be dry

much of the time and are subject to alternate wetting

and drying, freezing and thawing, plant root action,

and alteration by other biological processes may exhibit entirely different water-transmitting properties

than soils of similar texture below a water table. From

the standpoint of drainage the latter are of greater

importance, since subsurface drainage is concerned

largely with water movement below the water table.

Hydraulic conductivity can be measured for disturbed samples or undisturbed cores in the laboratory

or for undisturbed soil in the field. Measurements on

disturbed samples of aquifer materials may be satisfactory for drainage investigation purposes, if the

samples are packed to field density. Methods for

making such measurements are summarized by Wenzel

(1942).

Several methods have been developed for measuring

the hydraulic conductivity of soil in place in the field

below a water table. A procedure developed by

Diserens (1934) and Hooghoudt (1936) in Holland

makes use of the rate of water seepage into an auger

hole below the water table and is described in Method

34d. The mathematical treatment developed by Kirkham and Van Bavel (1949) for this method assumes

homogeneous isotropic soil, but hydraulic-conductivity

determinations by this method in nonuniform soils may

be taken as average or effective values. The auger-hole

method is limited to soils below a water table in which

the walls of the auger hole are stable. With the use of

suitable screens it may also be used in sands or other

noncohesive soils.

The piezometer method, based on the analysis by

Kirkham (1946), has been adapted for large diameter

tubes by Frevert and Kirkham (1949) and for small

diameter pipes by Luthin and Kirkham (1949). The

latter procedure is particularly suitable for determining

the hydraulic conductivity of individual layers of soil.

It is essentially a cased auger hole in which an opening

or cavity is placed at any desired depth in the soil,

following the procedure outlined in Method 34c.

Drainage design may be influenced by the fact that

both uniform and nonuniform soils may be anisotropic

with respect to hydraulic conductivity, i. e., the conductivity may vary with direction in the soil. Alternate

lenses of coarse and fine sediments are commonly found

in alluvial soils and usually conduct water more readily

in a horizontal than a vertical direction. The above

field methods may be useful in obtaining information

on the degree to which soils are anisotropic. Reeve

and Kirkham (1951) point out that field methods in

which long cavities with respect to the diameter are

used, such as is usually the case with both the augerhole and the small-pipe piezometer methods, measure

essentially the hydraulic conductivity in a horizontal

direction, whereas the large-diameter tube method,

which has a horizontal inflow surface, essentially

measures conductivity for vertical flow. Hydraulic

conductivity in any desired direction can be measured

with undisturbed cores.

Since most soils are not uniform, the problem of

appraising the water-transmitting properties, as related

to depth and spacing of drains, involves not only the

method of measurement but also a statistical problem

of sampling as well. The number of samples required

for soil appraisal is increased if the soil is highly

variable or if the samples are small in size. Reeve and

Kirkham (1951) showed that the effective sizes of

sample associated with a small core (2-in. diam. X2 in.

long), a piezometer (1-in. diam. X4-in. cavity), a tube

(8-in. diam. with a cavity length equal to zero), and

an auger hole (4-in. diam. X 30 in. deep), are in the

ratio of 1, 35, 270, and 1,400, respectively; the latter

three values being based on the region in which 80

percent of the hydraulic-head difference is dissipated.

It is apparent that field methods for appraising conductivity on large undisturbed volumes of soil have

distinct advantages over laboratory methods.

Information on the water conductance of subsurface

aquifers often has application to drainage appraisal

and can be obtained from well tests. High specific

yield, i. e., high rate of flow per unit drawdown, indicates high aquifer permeability and vice versa. Data

from existing wells can be used or new wells can be

drilled. Wenzel (1942) has summarized and discussed

the equations and methods used by a number of investigators of pumped wells. Theis (1935) presented equations for flow into wells for nonequilibrium conditions,

and Jacob (1940, 1947) reviewed the principles of flow

in artesian aquifers. Peterson and coworkers (1952)

have developed equations and procedures for study of
48 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

ground-water flow to wells for the steady-state or

equilibrium condition.

Chemical Amendments for Replacement of

Exchangeable Sodium

The kind and amount of chemical amendment to be

used for the replacement of exchangeable sodium in

soils depend upon the soil characteristics, the desired

rate of replacement, and economic considerations.

Suitability of Various Amendments Under

DiflTerent Soil Conditions

Chemical amendments that are applied to alkali

soils are of three types:

Amendments for alkali soils: Chemicals

Soluble calcium salts Calcium chloride

Gypsum

Acids or acid-formers Sulfur

Sulfuric acid

Iron sulfate

Aluminum sulfate

Lime-sulfur

Calcium salts of low solubility Ground limestone

(May also contain magnesium) Byproduct lime from

sugar factories

While each type of amendment has a place in reclamation, effectiveness under different soil conditions is

governed by several factors, the principal ones being

the alkaline-earth carbonate content and the pH reading. From the standpoint of their response to the

various types of amendments, alkali soils may be

divided into three classes : ( 1 ) Soils containing alkalineearth carbonates, (2) soils having a pH reading greater

than 7.5 but practically free of alkaline-earth carbonates, and (3) soils having a pH reading of less than

7.5 and containing no alkaline-earth carbonates.

Any of the soluble calcium salts, acids, or acidformers may be used on soils in class 1, but limestone

will be of no value. The addition of acid or acidforming amendments to soils in classes 2 and 3 tends to

make them acid in reaction. When the amount of acid

or acid-forming amendment needed for reclamation is

sufficient to make the soil excessively acid, the choice

of amendment is limited to soluble calcium salts, unless

limestone also is applied. In general the acidification

of soils of arid regions to a pH reading as low as 6 to

6.5 is usually beneficial to plant growth. To determine

if the amount of acid or acid-former needed for reclamation is sufficient to cause excess acidity, the amendment can be applied at the desired rate to a sample of

the soil and a pH reading can be obtained after the

reaction is complete. If the addition of sulfur, which

reacts slowly in the soil, is contemplated, the addition

of a chemically equivalent amount of sulfuric acid may

be useful to predict the pH reading that may eventually

be obtained upon complete oxidation of the sulfur.

While the application of limestone alone to soils of

classes 2 and 3 will tend to be beneficial, the effectiveness of lime on different soils varies markedly, inasmuch as the solubility of CaCOa decreases with increasing pH reading. Data on CaCOa solubility in

relation to pH reading are given by De Sigmond (1938)

as follows :

Solubility of CaCOa

pH value of CaCOa saturated solution : {Meq./l.)

6.21 19. 3

6.50 14.4

7.12 7.1

7.85 2.7

8.60 1.1

9.20 . 82

10.12 .36

Sodium carbonate or carbon dioxide was used to obtain pH readings above or below 7. On the basis of

these data it is apparent that the effectiveness of limestone as an amendment is markedly decreased at pH

readings above 7.5, whereas it may be quite effective at

pH readings below 7. Hence, limestone may be used

to advantage on class 3 soils, but its value on class 2

soils is questionable. Some soils that contain excess

exchangeable sodium also contain appreciable exchangeable hydrogen and, therefore, have an acid

reaction. In Hungary large areas of such soils have

been quickly and effectively reclaimed by the addition

of chalk (CaCOa).

Chemical Reactions of Various Amendments in

Alkali Soils

The following chemical equations illustrate the manner in which various amendments react in the different

classes of alkali soils. In these equations the letter X

represents the soil exchange complex.

Class 1. Soils Containing Alkaline-Earth Carbonates

GYPSUM.—2NaZ + CaS04 ^CaZ^ -f Na.SO,

SULFUR.—

(1) 2SH-302:^2SO3 (microbiological oxidation)

(2) SO,-fH20^H.>S04

(3) H,S04 + CaCO,^CaSO,-hCO. + H..O «

(4) 2NaZ + CaS04;=±CaZ,4-Na2SÖ4

LIME-SULFUR (CALCIUM POLYSULFIDE).—

(1) CaS5 + 802^-4H20í=iCaS04^-4H2S04

(2) H.S04 + CaC0.^CaS04 + C02Í-H>0«

(3) 2ÑaZ-hCaS04^CaX,Na2S04

IRON SULFATE.—

(1) FeS04 4-HoO;=iiH>S04 + FeO

(2) H2SO4 + CaCOa^CaS04-f CO.,+ H.0«

(3) 2Na.Y+CaS04^CaZ.>-fNa.SÓ4

^ The reaction of H2SO4 and CaCOa may also be written as

follows: H.S04+2CaCO..;=±CaS04+Ca(HCO..).. Under these

conditions the Ca(HC03)2 as well as the CaS04 would be

available for reaction with exchangeable sodium and 1 atom of

sulfur when oxidized to H2SO4 could theoretically result in the

replacement of 4 sodium ions by calcium. Kelley (1951, p. 135)

found under field conditions that approximately 3 exchangeable

sodium ions per atom of sulfur were replaced, whereas a greenhouse-pot experiment conducted at this Laboratory indicated

that the reaction takes place without the formation of appre-
SALINE AND ALKALI SOILS 49

Class 2. Soils Containing No Alkaline-Earth Carbonates; pH 7.5 or Higher

GYPSUM.—Same as in class 1.

SULFUR.—Steps (1) and (2) as in class 1.

(3) 2NaA: + H,S04^2HZ ^Na2S04

LiME-suLFUR.—Step (1) as in class 1.

(2) 10NaZ + 4H2SO4 + CaSO4^8HX + CaX,+

SNa^SO^

IRON SULFATE.—Step (1) as in class 1.

(2) 2NaZ + H,SO,^2HZ + Na2S04

LIMESTONE.—Two possibilities suggested by Kelley

and Brown (1934) are:

(1) 2NaX + CaC03F±CaZ, + NaXO;,

(1) NaZ-fHOH^:

(2) 2HJ + CaC0,

íNaOH + HZ

=±CaZ-hC02 + H,0

Class 3. Soils Containing ISo Alkaline-Earth

Carbonates; pH Less Than 7.5

GYPSUM.—Same as in class 1 and 2.

SULFUR.—Same as in class 2.

LIME-SULFUR.—Same as in class 2.

IRON SULFATE.—Same as in class 2.

LIMESTONE.—Same as in class 2, and if exchangeable hydrogen is present:

(1) 2HZ + CaC03;=^CaZ2 + CO,-hH,0

Estimation of Amounts of Various Amendments

Needed for Exchangeable-Sodium Replacement

Exchangeable sodium and cation-exchange-capacity

determinations serve as valuable guides for estimating

the amounts of chemical amendments needed to reduce

the exchangeable-sodium-percentages of alkali soils to

given levels. The procedure for estimating the amount

of amendment needed for a given set of conditions can

be illustrated by an example. Suppose the 0 to 12-in.

layer of an alkali soil contains 4 meq. of exchangeable

sodium per 100 gm. and has a cation-exchange-capacity

of 10 meq. per 100 gm. The exchangeable-sodiumpercentage is therefore 40. It is desired to reduce the

exchangeable-sodium-percentage to about 10. This will

necessitate the replacement of 3 meq. of exchangeable

sodium per 100 gm. Assuming quantitative replacement, it will be necessary to apply the amendment at

the rate of 3 meq. per 100 gm. of soil. By referring to

table 6, which relates tons of gypsum and sulfur per

acre-foot of soil to milliequivalents of sodium per 100

gm. of soil, it is found that 5.2 tons of gypsum or 0.96

ton of sulfur are required. If it is desired to use

amendments other than gypsum or sulfur, the suppleciable amounts of Ca (HC03)2. A high soil-moisture level, low

soil temperatures, and the release of CO2 by plant roots would

favor the formation of Ca (HC03)2 as a product of the reaction.

mentary data given below will be helpful in converting

the tons of sulfur found to be needed in table 6 to tons

of other amendments.

Tons equivalent to 1

Amendment:. ton of sulfur

Sulfur _•-_.. 1.00

Lime-sulfur solution, 24 percent sulfur 4. 17

Sulfuric acid 3.06

Gypsum (CaS04-2H20) 5.38

Iron sulfate (FeSOvTH^O) 8.69

Aluminum sulfate (Al2(S04)3l8H20) 6.94

Limestone (CaCOs) 3.13

TABLE 6.—Amounts of gypsum and sulfur required

to replace indicated amounts of exchangeable sodium

Exchangeable sodium

(Meq. per

100 gm. of

soil)

Gypsum ^

(CaSOi2H2O)

Gypsum ^

(CaS04-

2H2O)

Sulfur

(S)

Sulfur

(S)

1

2

Tonslacrefoot^

1.7

3.4

5.2

6.9

8.6

10.3

12.0

13.7

15.5

17.2

Tons¡acre6 inches^

0.9

1.7

2.6

3.4

4.3

5.2

6.0

6.9

7.7

8.6

Tons¡acrefoot^

0.32

.64

.96

1.28

L60

1.92

2.24

2.56

2.88

3.20

Tons/acre6 inches^

0.16

.32

3 .48

4 .64

5 .80

6 .96

7 1.12

8 1.28

9

10

1.44

1.60

^ The amounts of gypsum are given to the nearest 0.1 ton.

2 1 acre-foot of soil weighs approximately 4,000,000 pounds.

^ 1 acre-6 inches of soil weighs approximately 2,000,000

pounds.

The reaction between an amendment such as gypsum

and exchangeable sodium is an equilibrium reaction

and, therefore, does not go entirely to completion.

The extent to which the reaction goes to completion is

determined by the interaction of several factors, among

which are the differences in the replacement energies

of calcium and sodium, the exchangeable-sodiumpercentage, and the total cation concentration of the

soil solution. For the usual case where a quantity of

gypsum equivalent to the amount of exchangeable

sodium present in the surface 6- or 12-in. layer of soil

is applied, some progress has been made in determining

the percentage of the applied calcium that reacts with

exchangeable sodium. The available data indicate that

when the exchangeable-sodium-percentage of the soil

exceeds 25, 90 percent or more of the calcium supplied

bv the amendment replaces exchangeable sodium as the

soil is leached. The percentage of added calcium that

replaces exchangeable sodium does not become less

than 50 until the exchangeable-sodium-percentage becomes less than 10. It should be pointed out that under

the above conditions not all of the replacement of exchangeable sodium takes place in the depth of soil upon

which the application is based, although the greater

part of it does. As a general rule, it is suggested that
50 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

the rates of gypsum and sulfur application indicated by

table 6 be multiplied by the factor 1.25 to compensate

for the lack of quantitative replacement.

A simple test based on the work of McGeorge and

Breazeale (1951) has been proposed by Schoonover for

determining the gypsum requirement^ of alkali soils.

The test, which is given as Method 22d, involves an

arbitrary procedure and does not measure a distinct

chemical property of the soil. The relation between

the exchangeable-sodium content and the gypsum requirement, as determined by Method 22d, of 29 nongypsiferous soil samples has been studied at the

Laboratory. The ranges in various characteristics of

the samples were as follows: electrical conductivity

of the saturation extract, 0.2 to 30 mmhos/cm.; exchangeable-sodium-percentage, 6.3 to 65.5; and exchangeable-potassium-percentage, 2.1 to 27.3. As indicated by a correlation coefficient of 0.96, a good

relation was found between exchangeable-sodium content and gypsum requirement. For soil samples having exchangeable-sodium contents ranging from 0.1 to

12 meq./lOO gm., the relation between the two variables is expressed by the equation: Exchangeable

sodium, milliequivalents/100 gm. = 0.96 4-0.99 X gypsum requirement, milliequivalents/100 gm.^ Inasmuch

as Method 22d gives a good estimate of the exchangeable-sodium content of these alkali soils, it would appear to be useful for estimating the amount of gypsum

needed when information on the exchangeable-sodium

content and the cation-exchange-capacity is not otherwise available. Amounts of gypsum can be converted

to quantities of other chemical amendments by the use

of table 6 and data on page 49.

Speed of Reaction of Amendments and Economic

Considerations

The choice of a chemical amendment may be influenced by the time required for its reaction in the soil.

In general, the cheaper amendments are slower to react.

Consequently, if immediate replacement of exchangeable sodium is desired, one of the quicker acting but

more expensive amendments will be needed.

Owing to its high solubility in water, calcium

chloride is probably the most readily available source of

soluble calcium, but it is seldom used because of its

cost. Sulfuric acid and iron and aluminum sulfates

that hydrolyze readily in the soil to form sulfuric acid

* SCHOONOVER, W. R. EXAMINATION OF SOILS FOR ALKALI.

University of California Extension Service, Berkeley, California. 1952. [Mimeographed.]

In a private communication, C. D. Moodie of the Washington

Agricultural Experiment Station has reported a study of the

relation between the gypsum requirement and the exchangeablesodium contents of soils from the Yakima Valley, Washington.

A relation similar to that obtained by Schoonover was obtained

for soils containing low amounts of exchangeable potassium, but

for soils containing high amounts of exchangeable potassium

the slope of the regression line was considerably lower. Thus,

estimates of the exchangeable-sodium content based on the

gypsum requirement and the equation given in this handbook

may be high if the soil contains large amounts of exchangeable

potassium.

are also quick-acting amendments. Sulfuric acid is

often cheap enough for field application, but the use of

iron and aluminum sulfates usually is not economically

feasible. Because of their relatively low cost, gypsum

and sulfur are the most common amendments used for

reclamation. The rate of reaction of gypsum in replacing sodium is limited only by its solubility in water;

its solubility is about 0.25 percent at ordinary temperatures. The presence of sodium and chloride ions in the

water increases the solubility of gypsum, whereas calcium and sulfate ions tend to decrease its solubility.

Limited data indicate that the application of 3 to 4 ft.

of irrigation water is sufficient to dissolve 4 or 5

tons/acre of agricultural gypsum having a degree of

fineness such that 85 percent will pass a 100-mesh

sieve.

As sulfur must first be oxidized by microbial action

to the sulfate form to be available for reaction, it is

usually classed as a slow-acting amendment. McGeorge

and Greene (1935) have shown in laboratory studies

of Arizona soils that sulfur applications of about 1

ton/acre are rapidly and usually completely oxidized

in 2 or 3 weeks under favorable moisture and temperature conditions. Larger applications required more

time for complete oxidation. They also found that

within the usual particle-size limits of agricultural

sulfur, the coarse-grade material was practically as

effective as the finer and more expensive grades. In

spite of these findings, various agriculturists frequently

report incomplete oxidation of sulfur in soils a year

or more after application. Often this appears to be

caused by the presence of lumps of the sulfur and insufficient mixing of the amendment with the soil following application.

As previously mentioned, the solubility of limestone

when applied to alkali soils is markedly influenced by

the pH reading and by the presence of exchangeable

hydrogen. Unless the soil is decidedly acid, the chemical reaction of limestone is slow. Particle size is also

an important factor affecting the rate at which limestone, gypsum, and sulfur react in soils. The finer the

particle size the more rapid the reaction.

There is considerable interest at present in the use

of lime-sulfur as an amendment. Lime-sulfur is a

brown, highly alkaline liquid containing calcium

polysulfides and some calcium thiosulfate. The calcium content is ordinarily about one-fourth that of the

sulfur content, and its action depends mostly on the

sulfur content. Usually the material is applied in irrigation water. Like elemental sulfur, it must first be

oxidized to sulfuric acid and then react with alkalineearth carbonates to produce a soluble form of calcium.

Application of Amendments

From the standpoint of efficiency in replacing exchangeable sodium, it is advantageous to leach most of

the soluble salts out of the soil before applying chemical amendments. As a result of the removal of soluble

salts, a higher proportion of the calcium supplied by

the addition of amendments is adsorbed by the soil-
SALINE AND ALKALI SOILS 51

exchange complex. The advantage gained through increased efficiency in exchangeable-sodium replacement

by leaching prior to the application of amendments

may be more than offset by the decrease in soil

permeability that usually accompanies the leaching of

saline-alkali soil. Whether amendments should be applied before or after removal of soluble salts, therefore,

will depend upon permeability relationships.

Such chemical amendments as gypsum, sulfur, and

limestone are normally applied broadcast and then incorporated with the soil by means of a disk or plow.

Thorough incorporation is especially important when

sulfur is used to insure rapid oxidation to the sulfate

forra. Because of hazards in handling, the application

of sulturic acid is difficult under ordinary field conditions. However, special equipment is now available

that sprays the concentrated acid on the soil surface.

Although chemical amendments are ordinarily applied

to the surface, deeper placement may be advantageous

if the exchangeable-sodium accumulation occurs uniformly in the subsoil, or B horizon. While there appears to be no information on the subject, it is possible

to obtain deep placement by distributing the amendment

behind a plow or subsoiler.

Amendments are sometimes applied in the irrigation

water. Special equipment for treating irrigation waters

with gypsum has been described by Fullmer (1950).

A simple method of treatment consists in placing a bag

of gypsum with the side slit open in the irrigation ditch,

preferably at a weir where the water has considerable

turbulence.

Except where sulfur is used, saline-alkali soils

should be leached immediately following the application of amendments. Leaching dissolves and carries

the amendment downward, and it also removes the

soluble sodium salts that form as a result of cation exchange. Soils receiving sulfur ordinarily should not be

leached until sufficient time has been allowed for most

of the sulfur to oxidize and form gypsum, but the soils

should be kept moist, as moisture is essential to the

process of microbial oxidation.

Improvement of the physical condition of alkali

soils involves the rearrangement and aggregation of

soil particles as well as the replacement of exchangeable

sodium. This has been demonstrated and emphasized

by Gardner (1945). The rearrangement of soil particles so as to improve physical condition is facilitated

by alternate wetting and drying, by alternate freezing

and thawing, and by the action of plant roots.

Laboratory and Greenhouse Tests as Aids

to Diagnosis

While physical and chemical analyses made on saline

and alkali soil samples provide basic data that may be

needed to ascertain the cause of low productivity and

the treatments required for reclamation, supplementary

tests conducted on soil columns or in greenhouse pots

are often helpful in obtaining satisfactory answers to

soil problems. Such tests may be used to verify conclusions reached on the basis of physical and chemical

tests or to check on how the soil responds to indicated

treatments for improvement. It should be recognized,

however, that plant growth on saline and alkali soils

contained in small pots may be at variance with growth

obtained under field conditions. Laboratory and

greenhouse tests are less costly, less laborious, and less

time-consuming than field tests and often provide valuable clues as to the behavior of the soil in the field.

Generally, all but the more promising procedures for

improving saline and alkali soils can be eliminated by

laboratory and greenhouse studies.

Laboratory tests on soil columns may be used to estimate the amount of leaching needed for removal of excess soluble salts; to determine the response of soils

to the addition of various kinds and amounts of amendments; and to determine the changes in such soil properties as permeability, pH reading, and exchangeablesodium-percentage that take place upon leaching. Determinations on soil columns are especially useful in

the diagnosis of saline-alkali soils, as the characteristics

of these soils usually change markedly upon being

leached.

It would be best to conduct tests on undisturbed soil

cores. A power-driven soil sampler capable of .taking

4-inch diameter cores to a depth of 10 feet has been

developed by Kelley and associates (1948). In the

absence of a core sampler, disturbed samples representing the various soil layers may be packed in tubes

of convenient diameter and length. A technique similar

to that used for making hydraulic-conductivity measurements on disturbed soil samples can be used in

setting up these soil columns. Leaching and amendment treatments may then be applied to the soil

columns, and the effects upon water-movement rates

noted. Changes in soluble-salt content, pH reading,

and exchangeable-sodium status obtained by various

treatments may be determined by removing the treated

soil from the tube and making the appropriate

analyses.

Greenhouse tests are useful when it is desired to obtain information on plant-growth responses. They

may be used for various purposes such as to determine

whether the soil contains sufficient soluble salt or exchangeable sodium to affect plant growth adversely, to

determine plant response to leaching and the addition

of chemical amendments, and to estimate the fertilizer

needs of saline and alkali soils (Bower and Turk, 1946).

Greenhouse pot tests may be conducted under various conditions. The procedure to be followed will

depend upon the facilities available, the kind of plant

to be grown, and the purpose of the tests. A few suggestions for conducting greenhouse tests are:

(a) If possible, use the crop or crops to be grown

in the field.

(6) Use containers of soil as large as feasible. If

leaching treatments are to be employed, provision should be made for measuring the

volume and salt content of the leachate.

(c) An attempt should be made to grow the crop

during its normal season and to avoid excès-
52 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

sive temperatures that are often obtained

under greenhouse conditions.

(d) Replicate each treatment at least twice and

arrange each set of treatments in randomized

blocks.

{e) If possible, irrigate with water having the

same composition as that to be used in the

field.

(/) If the soil has been leached or amendments

applied, it may be desirable to analyze the

soil at the conclusion of the test to determine

the changes in the soil properties that have

taken place.

Although this handbook is not primarily concerned

with soil fertility, it should be recognized that saline

and alkali soils, like other soils of arid regions, usually

respond markedly to nitrogen and phosphorus fertilization. Adequate fertilization after the removal of excess

soluble salts and exchangeable sodium is usually required to obtain maximum productivity. The greenhouse technique devised by Jenny and coworkers

(1950) for determining nutrient level and fertilizer response is suggested as a possible method for determining the fertilizer requirements of saline and alkali

soils.

Reclamation Tests in the Field

Leaching operations and the application of amendments in the field usually entail considerable expense.

Therefore, before attempting the improvement of saline

and alkali soils on a large scale, it is frequently desirable to determine whether a proposed treatment will

be successful. Often this can be ascertained on an experimental basis by the use of field plots. It is not the

purpose of this section to give methods for conducting

field-plot experiments of the research type. However,

procedures are given that are considered adequate for

testing treatments involving leaching, cultural practices, and the application of amendments. Tests in

which drainage is a treatment are difficult to conduct on

a plot basis and, hence, will not be considered.

Saline and alkali soils usually are extremely variable

in nature, their characteristics often changing markedly

over relatively short distances. Therefore, considerable

care should be taken to select a test area that is as uniform as possible and yet representative of the soils to be

considered. Examination and tests of soil samples

from various locations over the proposed test area are

valuable in determining soil uniformity. Sometimes it

is difficult to locate a single area of sufficient size and

uniformity to conduct the test. Then it is advisable to

place individual replications on separate areas within

the field.

Selection of the size and shape of plots is influenced

by the kinds of treatments to be used, the crop to be

grown, the method of applying water, and the amount

of space needed for the operation of equipment. Ordinarily, the plots should be as small as possible, as this

tends to reduce soil variability within the test area. If

at all feasible, a border or dike should be constructed

around each plot to control the application of water.

This permits the impounding of water for leaching and

the estimation of infiltration rates. Tests that involve

only the application of amendments such as gypsum or

manure may be conducted on plots as small as 15 ft.

by 15 ft. On such plots, the amendments can be applied by hand. When leaching is a differential treatment, plots of somewhat larger size are needed, as

border effects may be of considerable magnitude in

small plots. Leaching tests have been satisfactorily

conducted on y^Q-acre plots. Cultural treatments, such

as subsoiling and deep plowing, may require the use of

fairly large plots to permit operation of the machinery.

From the standpoint of minimizing border effects, plots

should be as nearly square as possible. Square plots

are usually convenient to handle when the land is

flood-irrigated, but when the slope of the land is such

that water must be applied in furrows or corrugations

a long narrow plot must be used. Cropping procedure

and tillage operations must also be considered in selecting the shape of the plot.

The design of field-plot tests is governed primarily

by the treatments to be used (fig. 16). The simplest

design is that in which the various treatments are arranged in blocks and located at random, each treatment

occurring only once in each block. Individual blocks

serve as replications. This design is satisfactory for

comparing various amendments or cultural practices or

for testing the effect of leaching. If the test involves a

combination of amendments and leaching or cultural

treatments, it is advantageous to employ a split-plot

design in which leaching or cultural treatments constitute main plots and the amendment treatments consist

of subplots. Owing to the marked variability of saline

and alkali soils, it is recommended that treatmentá be

replicated at least four times. All treatments within

each replicate block should be located at random.

The improvement of saline and alkali soils may be

evaluated by means of plant-growth responses, soil

analyses, and determinations such as infiltration rate.

When the problem is one of excess salinity only, determinations of crop yields on the various plots often

will suffice for the evaluation of the treatfnents. If

facilities are available, it is also advisable to determine

by analysis the soluble-salt content of the soil before

and after treatment. In alkali soils where poor physical condition is a problem, the effect of the treatments

upon the soil as well as upon plant growth should be

determined. Changes in the exchangeable-sodium content of the soil upon treatment may be determined by

soil analyses, whereas improvement in water-transmission properties may be estimated by means of infiltration measurements. Estimates of infiltration rates are

readily obtained when individual plots are flood-irrigated. Infiltration rates on furrow-irrigated plots may

be estimated by measuring the amount of water applied

to the plot and the amount that runs off.

Applications of chemical amendments influence both

the physical and chemical properties of alkali soils.

In studying the response of plants on alkali soils to the

application of chemical amendments, it may be desir-
SALINE AND ALKALI SOILS 53

LEACHING OR CULTURAL TREATMENTS

( BASIN IRRIGATION )

AMENDMENT TREATMENTS

( BASIN IRRIGATION )

^ 30'^

Lg or Cg L3 0r C3

il

'0

ro

L4 or 0^ L| or C|

15' —

1. A5 A7 ^ A3

'•0 Ag A4 ^6 As

COMBINATION OF AMENDMENT AND

LEACHING OR CULTURAL TREATMENTS

( BASIN IRRIGATION )

CULTURAL OR AMENDMENT

TREATMENTS

(FURROW IRRIGATION)

4

or

Cc

1 ^ o or

•-4

or

30'

-*\ 15'

'— ^1

— or

T

r^'^n

C| C5 C3 C2 C4

or or or or or

A, S A3 A2 A4

FIGURE 16.—Example showing individual replicates of plot layouts for conducting field tests: C, Cultural treatments; L, leaching

treatments; A^ amendment treatments; main plot boundary; subplot boundary. The subscripts refer to treatment levels, for example: Li, control; L2, 12 surface inches; ¿8,36 swface inches.
54 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

able to separate the strictly chemical aspects of the

response from the physical aspects. Preliminary tests

indicate that treatment of alkali soils with the recently

developed commercial aggregating agents will largely

eliminate poor physical condition without altering the

chemical characteristics appreciably. Therefore, reclamation tests that include applications of chemical

amendments and commercial aggregating agents singly

as well as in combination are suggested as a means for

determining the nature of the response.

Reclamation of Saline and Alkali Soils in

Humid Regions

This chapter deals primarily with the improvement

and management of saline and alkali soils as they occur

in the arid and semiarid regions of western United

States. Any treatment of the subject would be incomplete, however, without reference to the pioneer research work and the extensive practical experience vv ith

the reclamation of saline and alkali soils in the Netherlands and other low countries in humid regions. Underlying principles relating to soil properties and plant

responses apply equally well to both cases. The main

difference is that in humid climates precipitation exceeds consumptive use, so that if drainage is adequate,

i. e., if the water table is maintained at a sufficient

depth, excess soluble salts are leached out of the soil

by rain water.

It often happens that the rainfall pattern in humid

climates during the crop growing season is not ideal

and it is profitable to maintain the water table at some

elevation that is in or near the root zone. Subirrigation

is hazardous in arid regions, but it is a relatively common practice in humid climates. In any climate this

practice requires close attention to the concentration of

soluble salts in the root zone, and careful coordination

between subirrigation, leaching, and drainage requirements. Hooghoudt (1952) has recently reviewed the

methods and practices used in the Netherlands for tile

drainage and subirrigation.

A special case of salinity in humid as well as arid

climates occurs in greenhouse soils. This type of agriculture has considerable economic importance in many

countries. Since crop production is directly dependent

on irrigation and the leaching action of rainfall is

absent, water management to control salinity and exchangeable sodium in the soil is the same as for irrigation agriculture in an arid climate.

Economically, in humid climates the most important

consideration of soil salinity and exchangeable sodium

has been in connection with the drainage and reclamation of soils underlying salty lakes and shallow coastal

waters. In the Netherlands, experience with this

process extends over many centuries, and the large

areas of fertile agricultural land that have been gained

by this means have become a major factor in the national economy. Zuur (1952) has sketched historical

and technical aspects and has given an introduction to

the extensive literature of the Netherlands on this subject. He states that, to start with, soils reclaimed from

the sea contain about 2 percent sodium chloride. In

2 years after ditching, this content is reduced "in the

wet Dutch climate" to 0.1 percent or less in the surface

80 cm. of sandy soils. Clay soils require a longer time

to leach to this depth, but crops can be grown fairly

soon after artificial drainage is established.

Most of the polder soils of the Netherlands, coming

both from recent marine deposits and from old sea

clays, contain sufficient sulfur and calcium carbonate

so that with the oxidation processes which accompany

drainage, the soil solution is kept saturated with gypsum for several years. This is a most fortunate circumstance because the removal of exchangeable sodium

takes place simultaneously with the reduction of salinity, without the need for the addition of chemical

amendments. Zuur (1952) has given the data in table

7 as being typical of changes in the exchangeablecation status of a polder soil following drainage.

TABLE 7.—Exchangeable cations in the topsail of a

polder reclaimed from salt water (Zuur, 1952)

Time Ca Mg K Na

Just after drainage

4 years after ditching.

7 years after ditching

Percent

17

73

82

87

Percent

35

17

10

8

Per.

cent

9

5

6

4

Percent

39

5

2

Final situation 1

The reclamation of soils that have been subjected to

sea-water inundation is an agricultural problem that

has assumed considerable economic importance and

has been given a great deal of attention by soil and

plant scientists. This is particularly serious when it

occurs on older cultivated soils in humid regions, because of the lack of soluble calcium for replacing exchangeable sodium concurrently with the leaching out

of the soluble salts. Leaching by rain water changes

the soil from the saline-alkali to the nonsaline-alkali

condition, with the attendant deterioration of structure.

Reclamation then requires soluble calcium for replacing exchangeable sodium and careful management and

cultural practices for some time to reestablish a favorable physical status of the soil. Van den Berg (1952)

provides an introduction to the literature on this

subject.
Chapter 4

Plant Response and Crop Selection

for Saline and Alkali Soils

Significance of Indicator Plants for

Saline Soils

Hilgard (1906) was among the first to recognize the

significance of certain native plants as indicators of the

characteristics of soils, and to make use of them in

determining the availability of saline and alkali soils

to agriculture. More recently, Sampson (1939, p. 200)

has stated :

In the future a broader use of indicator communities and species is likely, but such use is sure

to be backed by sounder evidences than it has at

this time. Preceding this possible broadened use

there must first be more critical study of the

growth requirements of both the indicator plant

and the economic species; only then will the indicator concept reach its maximum reliance.

Some progress has been made in developing quantitative methods for the study of the indicator plant

concept; and, in some areas, data have been obtained

that relate the growth performance of indicator plants

and their ability to survive to the physical and chemical measurements of the soils in which they grow.

Kearney and associates (1914) made a quantitative

study of plant communities as indicators of salinity

and soil moisture in the Tooele Valley, Utah, and determined the moisture equivalent, wilting coefficient,

and salt content for six characteristic plant communities. For example, they concluded that land characterized by a sagebrush association is capable of crop

production with irrigation, and that a greasewoodshadscale type of vegetation indicates land that is suitable for crop production under irrigation only after

the excess salts are removed by leaching. Harris and

coworkers (1924, p. 922), working in the same valley,

found "a close parallelism between physiochemical

properties of tissue fluids of native species on the one

hand and the characteristics of the soil and the capacity

of the land for crop production on the other."

In connection with investigations of grazing in

western Utah, Stewart, Cottam, and Hutchings (1940)

investigated the root penetration of several desert

plants as influenced by soil salinity and the nature of

the root system of the plant. They found that roots of

shadscale readily penetrated soil having 1,000 to 10,000

p. p. m. salt, but those of sagebrush did not. Billings

( 1945 ) studied the soil characteristics of several plant

communities in western Nevada, including greasewood

and greasewood-shadscale associations, and reported

data on soil type, texture, pH, and electrical conductivity of the soil solution. He found rather high

alkalinity (pH 8.5 to 9.5) throughout the profile in

the greasewood association, and conductances of 1: 5

soil .-water extracts ranging from 1.6 to 8.4 mmhos/cm.

in the 2- to 50-cm. depths.

Roberts (1950) has investigated the chemical effects

of salt-tolerant shrubs on soils in the semiarid regions

of western United States, and found that such shrubs

as greasewood and shadscale were responsible for significant changes in some of the chemical characteristics

of the soil profile. Data from several hundred field pH

tests and some laboratory analyses showed striking differences among the pH, exchangeable sodium, and

total salt content of soils under some species of shrubs

as compared to those under other species and to the

soils in intervening barren areas. Soils in a mixed

shadscale-greasewood association in the Antelope

Springs silty clay loam in southwestern Utah had a

higher pH value under greasewood than under shadscale, and both values were higher than the pH of

barren soil. A similar relationship was found with

respect to ECe and sodium status.

Fireman and Hayward (1952) made a quantitative

study of several indicator plants growing in mixed and

pure associations in the Escalante Desert, Utah, to

determine the relation of vigor, age, and distribution

of indicator plants to the physical and chemical characteristics of the soils of their habitats, and to compare

soils occupied by the root systems of indicator plants

and the soils in the adjacent interspaces. The pH values of saturated soil pastes and 1: 10 soil: water suspensions, particularly of the surface soil, generally were

higher under shadscale and invariably higher under

greasewood than under sagebrush or in the adjacent

bare areas. The ESP of the soil was somewhat higher

under shadscale and very much higher under greasewood than under sagebrush or in the barren areas, and

the soluble-salt content was appreciably higher under

shadscale and greasewood than in adjacent bare soil

or under other shrubs.

These and other studies by Flowers (1934), Harris

55
56 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

(1920), and Shantz and Piemeisal (1924) indicate that

a vegetational survey can be useful in appraising an

area if quantitative data are available regarding the

soils and the ecology and physiology of the indicator

plants. However, certain precautions should be taken

in the use of indicator plants as a basis for the diagnosis of saline and alkali soils. In the first place, it

would be unwise to appraise land on the basis of a

single species unless it is a reliable indicator. Some

species of plants growing in semiarid regions are poor

indicators, even though they may tolerate large quantities of salt, because they will also grow very well

in the absence of salinity or alkali. Tussockgrass, saltgrass, and shadscale, especially, tolerate an appreciable

degree of salinity but will grow well in the absence of

salt. Even greasewood is not an infallible indicator,

since it has been found making thrifty growth on

nonsaline sand dunes.

A second point to emphasize is the need for very careful sampling. The studies by Roberts and by Fireman

and Hayward, cited above, indicate that large differences may occur in soil samples from sites only a few

feet apart, especially when the plant association is a

mixed one. If a vegetational survey and related soil

sampling are to serve as a basis for determining the

suitability of soils for irrigated agriculture, the analysis

of the plant population and the collection of soil samples must take into account the possible effects of the

indicator plants on the chemical and physical characteristics of the soil.

A third consideration relates to the purity and

density of stand and the vegetative vigor of the various

species present in the area to be evaluated. In the following paragraphs, the statements regarding the significance of various indicator plants are based on the

assumption that they occur in a relatively pure stand

and that they are growing in a normal manner. In the

case of mixed associations, the appraisal should take

into account the indications of all of the dominant

species in the plant community.

Several species of plants native to western United

States are regarded as good indicator plants if the precautions noted above are observed. Some of the best

known indicator plants are listed below, and pertinent

available data are given regarding their ranges, characteristics of the soils on which they grow (texture,

soil moisture, salinity, etc.), and the conditions which

they may indicate with respect to reclamation or soilmanagement practices needed for irrigation agriculture. The order in which the indicator plants are listed

is based on the approximate level of soil salinity associated with the occurren^.^ ^i the species in pure stand

or as one of the dominant species. The information

given was compiled from the data and field observations of the authors cited in this chapter.

Indicator Plants ^^

MESQUITE (Prosopis jaliflora) .—Range: Southern

Kansas to southeastern California, Baja California and

Sonora, Mexico, to eastern Texas. Occurs on a variety

of textural soil classes that are very permeable and

well-drained, with a low water table and an intermediate moisture-holding capacity (SP 25 to 50).^^ The

soils are usually nonsaline throughout the 4-foot profile,

but salt may accumulate at the surface under some conditions. Indications: Suitable for agriculture if water

is available.

CREOSOTEBUSH {Larrea tridentata).—Range: Southern Colorado and southern Utah to west Texas, west to

California and Mexico. On dry plains and slopes.

Occurs on soils of coarse and moderately coarse texture

that are very permeable and well-drained, with low

water table and low to intermediate moisture-holding

capacity {SP 15 to 40). The soils are nonsaline to a

depth of 4 feet (<0.03 percent) ^^ and nonalkali. Indications: Where stands are good, the soils are nonsaline and of sufficient depth to support a specialized

agriculture provided water is available. If stands are

poor, the soils may be shallow, underlain with layers

of rock or hardpan, and unsuitable for crop production.

SAGEBRUSH {Artemisia tridentata).—Range: South

Dakota to British Columbia, south to northern New

Mexico and northern Arizona; rare in southern California. Occurs on loamy soils (loamy sand, gravelly

loam, sandy loam, loams, sih loam, and clay loam)

that are more or less permeable and well-drained, and

the soil moisture may vary from low to high {SP

15 to 70). The soils are nonsaline (<0.05 percent)

and nonalkali in the zone occupied by the roots. Indications: The soils are suited for irrigation agriculture

or dryland farming, provided they are in an area where

precipitation is adequate and the growing season is

favorable. No reclamation practices are required.

Sagebrush is not a good indicator of soil texture, because it occurs on a wide range of textural classes. It

may grow well on soils that are too stony for farming.

WiNTERFAT, or W^HITESAGE {Eurotia lanata).—

Range: Saskatchewan, Canada, to Washington, south

to Texas, Arizona, and California. May'be in pure

stands, but frequently occurs in mixed associations

with shadscale, rabbitbrush, and greasewood. Occurs

on loamy soils that are permeable and well-drained,

with a low water table and low to intermediate moisture-holding capacity {SP 20 to 45). Soils usually

nonsaline in the first foot or two (<0.03 percent), but

roots may penetrate soil layers having salt in excess

of 1,000 p. p. m. (0.1 to 0.6 percent). Indications:

Where winterfat is dominant, the soils are usually nonsaline, but this plant can tolerate some salt; therefore,

leaching may be required.

DESERT SALTBUSH {Atriplex polycarpa).—Range:

Arizona, Nevada, Utah, central California to northwestern Mexico. Occurs on moderately coarse-textured

soils (sandy loam, fine sandy loam) that are moist in

winter and dry in summer and fall. The water table

'"The authors acknowledge the assistance of W. G. Harper,

Division of Soil Survey, in the preparation of this section.

" See Method 3b for estimating saturation percentage (SP)

in coarse-textured soils.

Values for salinity in this and following statements are

given as percent salt (dry-weight basis).
SALINE AND ALKALI SOILS 57

is usually low, and the moisture-holding capacity is

intermediate {SP 25 to 50). The soils may be nonsaline in the first foot, but they usually contain some

salt in the subsoil (0.04 to 0.5 percent). Indications:

Where stands are pure and growth is good, the soils

are nonsaline or slightly saline and are suitable for

irrigation agriculture. Where growth is poor, there

may be a limy hardpan or salt in the subsoil. Leaching and drainage may be necessary.

ARROWWEED {Pluchea sericea).—Range: Texas,

southern Utah, southern California, and northern

Mexico. Occurs on loamy soils which are usually

permeable, with an intermediate moisture-holding

capacity (SP 30 to 50). There is usually a high water

table or available moisture below the first foot

throughout the year. It frequently occurs with the

saltbush {Atriplex lentiformis), but it is less salt tolerant than that plant. The soils may be strongly saline

in the surface foot (0.6 to 2.0 percent), the salinity

decreasing with depth (O.I to 0.5 percent in the fourth

foot). Indications: The soils are usually saline or

strongly saline; but, where the subsoil is permeable, the

land is suitable for agriculture after drainage and

leaching.

SHADSCALE {Atriplex confertifolia).—Range: North

Dakota to Oregon, south to New Mexico, northern

Arizona, and California. Plains and valleys in mountainous areas. Usually occurs on medium to moderately fine-textured soils. The soils have an intermediate to high moisture-holding capacity (SP 25 to

60), may have restricted permeability, and a high

water table may develop, depending upon subsoil conditions. The soils are nonsaline to slightly saline in

the first foot (0.02 to 0.1 percent), the salinity increasing with depth (0.3 to 1.0 percent). The soils may

contain exchangeable sodium, and the pH of the surface soil may exceed a value of 9.0. Indications: Shadscale is salt and alkali tolerant, but it has a wide range

of tolerance and may grow well on soils that are nonsaline or slightly saline. It usually indicates a soil

with harmful amounts of salt or exchangeable sodium

in the subsoil. The soils may be farmed after leaching,

but drainage may be required.

GREENMOLLY (Kochia americana).—Range: Wyoming to northeastern California, south to northern

Arizona and New Mexico. Occurs on medium- to finetextured soils that are usually homogeneous to a depth

of several feet. They may puddle easily, and the permeability is lower than that on sagebrush lands, which

frequently adjoin Kochia associations. Soil moisture

is intermediate to high [SP 40 to 70), and there may

be a high water table. The salinity is moderately low

in the first foot (0.12 to 0.3 percent), but it increases

with depth so that the second to fourth feet may be

strongly saline (0.55 to 1.5 percent). Since Kochia

tends to have a shallow^ root system which does not

penetrate the more saline deeper portions of the profile, it should not be regarded as especially salt tolerant.

Indications: Pure stands of Kochia occur in soils that

are low in salt in the first foot but have a saline subsoil.

Leaching and drainage are required, and suitability of

such land for irrigation agriculture is doubtful.

ALKALI-HEATH (Frankenia grandi folia var. campestris).—Range: Central and southern California and

Nevada. On low-lying lands and alkali flats. Occurs

on soils of various textures (sandy loams to fine-textured loams) with soil-moisture conditions which vary

from well-drained to wet with a high water table. The

salinity is also variable, ranging from low to very high

(0.02 to 2.0 percent), and exchangeable sodium is frequently present. Indications: Where alkali-heath is

growing luxuriantly in a uniform stand, the soils are

generally highly saline and the lands are unsuitable

for agriculture unless they are drained and leached.

Where growth is sparse, the soil may be much less saline

and easier to reclaim.

GREASEWOOD (Sarcobatus vermiculatus).—Range:

North Dakota to Alberta, Canada, south to California,

Arizona, and northern Mexico; rare in California and

southern Arizona. Usually occurs on fine-textured

soils (clay, clay loam) but occasionally on soils of

coarser texture. The moisture content of the soil is

intermediate to high [SP 45 to 70), especially below

the second foot, permeability may be restricted, and

frequently the water table is high. The soils are generally saline-alkali; the range of salinity is wide (0.05

to 1.6 percent) and varies with depth; exchangeable

sodium is present in most areas and the values are

moderate to high. Indications: Greasewood is very

salt and alkali tolerant, and usually indicates a finetextured, relatively impervious soil with high salinity

and exchangeable sodium. Drainage and leaching are

required, and amendments may be necessary.

CRESSA {Cressa truxillensis),—Range: Texas to

southern Utah and southern California and Mexico.

Occurs on saline flats where the soils are fine-textured,

usually moist, with restricted permeability. The

salinity is very high (1.0 to 2.0 percent). Indications:

Cressa is a good indicator of saline soil and is more

reliable than alkali-heath, because the range of salinity

under which it grows is less variable. The soils require

drainage and leaching.

SALTCRASS {Distichlis stricta).—Range: Saskatchewan to Washington, south to Texas, Arizona, and

California. On salt flats and wet meadows. Occurs on

soils of various textures, but it is most commonly found

on loamy soils. The moisture-holding capacity is

usually high {SP 45 to 90), and the soils are moist or

wet throughout much of the year with a high water

table. The salt content of the 4-foot profile is usually

high (0.8 to 2.0 percent), with the highest content in

the first foot. However, good stands may occur on soils

containing very small amounts of salt (0.05 percent).

Exchangeable sodium may or may not be present. Indications: Usually indicates wet, strongly saline soils

with high water tables, but the plant may occur in

areas low in salinity. Drainage and leaching are

essential.

SALTW^ORT, or SEEPWEED [Suacda spp.).—Range:

Alberta to Oregon, south to northern Mexico. Salt

flats and marshes. Occurs on loamy soils of varying
58 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

textures which may be puddled and underlain with

hardpan. Usually found on moist seep lands with

high water tables but may occur on better drained land.

Moisture-holding capacity is intermediate to high

{SP 30 to 60). The soils are saline or saline-alkali,

with high concentrations of salt in the first foot (0.6

to 3.2 percent) and decreasing amounts with depth,

but the average salinity for a 4-foot profile may exceed

1 percent where the growth is luxuriant. The soils may

contain exchangeable sodium. Indications: Where

virgin growth is vigorous, seepweed is a good indicator

of highly saline or saline-alkali soil. Drainage and

leaching are essential, and amendments may be

required.

ALKALI SACATON, or TUSSOCKGRASS {Sporobolus

airoides).—Range: South Dakota to Washington,

south to Texas, Arizona, and southern California. In

low, wet areas, and river valleys. Occurs on loamy

and clayey soils that have an intermediate to high

moisture-holding capacity [SP 45 to 75). The soil surface is moist a great part of the year, and the water

table is usually high. The salinity of the soil may vary

within wide limits (0.3 to >3.0 percent), the higher

values being in the first foot; but the plant grows best

in the lower range (0.3 to 0.5 percent). Exchangeable

sodium may be present, and this grass is very tolerant

to it. Indications: In pure, vigorous stands, this plant

is a good indicator of wet, very saline or saline-alkali

soils, with a high water table. It may occur on soils

without a high moisture content in the subsoil on sites

receiving runoff water. The land requires drainage

and leaching, and soil amendments may be needed

unless gypsum is present.

SAMPHIRE, or GLASSWORT {Salicornia spp.).—

Range: Saskatchewan to British Columbia, south

through Colorado and Nevada. On salt flats and along

shores of saline ponds and lakes. Occurs on finetextured clayey soils that are very wet throughout the

profile, with high water tables. The salinity is very

high, and this plant grows well where salt may average

1 to 4 percent in the 4-foot profile. Exchangeable

sodium may be present in varying amounts. Indications: Soils are usually very wet, with excessive

salinity. Useless for agriculture without drainage and

prolonged leaching.

PICKLEWEED, or IODINEBUSH (Allenrolfea occidentalis).—Range: Oregon to Baja California, Mexico,

east through Arizona and New Mexico to western Texas.

On saline flats. Occurs on a wide range of soil textures

(loamy and clayey soils), but usually on fine-textured

soils. The soils are moist or wet throughout the year,

with high water tables that may be close to the surface."

The soils are excessively saline in the first foot (1.0 to

>2.5 percent) and are very saline throughout the 4-foot

profile (average 1.0 to 1.5 percent), but the salinity

decreases somewhat with depth. Exchangeable sodium

may be present in varying amounts. Indications: Soils

are usually fine-textured, very wet, and excessively

saline. If the stand is good, the land is not suited for

agriculture without drainage and prolonged leaching.

Crop Response on Saline Soils

A field of crop plants growing on saline soil usually

has barren spots, stunted growth of the plants with considerable variability in size, and a deep blue-green

foliage; but these features are not invariable indications of salinity. For example, barren spots may occur

in nonsaline fields because of faulty leveling and the

resultant inadequacy of irrigation; and retarded growth

and abnormal color may result from nutrient

deficiencies.

The extent and frequency of bare spots in many areas

may be taken as an index of the concentration of salt

in the soil. Inasmuch as most plants are more sensitive

to salinity during germination than in later stages of

growth, barren spots are more indicative of salinity

around the seed during germination than they are of

the general salinity status of the soil profile. Frequently, cultural practices contribute to an accumulation of salt around the germinating seed with resultant

failure in germination. The vigor of the plants adjacent to barren spots may indicate the distribution of

salt in the soil. Full-sized vigorous planes immediately

adjacent to a bare spot suggest a local concentration of

salt, while stunted plants in this position indicate a more

general distribution of salinity in the area. If the

level of salinity is not sufficiently high to result in

barren spots, the major characteristic in the appearance

of the crop may be a marked irregularity in vegetative

vigor.

Caution should be exercised to avoid confusion between effects of low soil fertility and those caused by

salinity. Plants that are stunted because of low fertility are usually yellowish green, whereas those stunted

owing to salinity are characteristically blue green. The

bluish appearance is the result of an unusually heavy

waxy coating on the surface of the leaves, and the

darker color to an increase in the chlorophyll content

on a surface-area or fresh-weight basis. Sugar beets,

crucifers (cabbage, mustards, and related species),

alfalfa, some clovers, grasses, and other crops generally

develop a noticeable blue-green coloration when grown

on saline soils.

There are many regions where plants may develop

an intense chlorosis because of certain soil conditions.

The causes of chlorosis are not fully understood, but

this condition is frequently associated with calcareous

soils or, in some cases, with the use of irrigation waters

of high bicarbonate content (Harley and Lindner,

1945). Although calcium carbonate is relatively insoluble, much crop injury is associated with its presence. Since this soil condition frequently occurs in

the absence of an accumulation of soluble salts, chlorosis cannot be regarded as a definite symptom of salinity.

Some species of plants develop characteristic necrotic

areas, tipburn, and firing of the margins of the leaves

when grown on saline soil. Many stone fruits, avocado, grapefruit, and some of the less salt-tolerant

varieties of cotton belong in this category.

The cupping or rolling of leaves is a common manifestation of moisture deficiency in plants, but these
SALINE AND ALKALI SOILS 59

symptoms may be indicative of salinity when they

occur in the presence of apparently adequate soil moisture; however, other factors that cause malfunction of

the root system, such as root diseases and high water

tables, may produce similar leaf symptoms. While the

appearance of the crop may, therefore, be indicative of

saline conditions, a reliable diagnosis of salinity usually

requires additional evidence derived from appropriate

soil and plant tests.

Salinity and Water Availability

Numerous laboratory experiments with sand and

water cultures have demonstrated the close relationship

between plant growth and the osmotic pressure of the

culture solution. On a weight or equivalent basis,

chloride salts are generally more inhibitory to the

growth of plants than sulfate salts, but this difference

tends to disappear when concentrations are expressed

on an osmotic basis. These relationships indicate that

it is the total concentration of solute particles in the

solution rather than their chemical nature which is

mainly responsible for the inhibitory effects of saline

solutions on the growth of crop plants. Direct experimental evidence of the influence of osmotic concentration on water uptake by plant roots has been reported

by Hayward and Spurr (1944). In addition to the

osmotic pressure of the solution, the nature of the

salts present may exert an important influence on plant

growth. Such specific ion efliects are discussed in a

subsequent section.

There is much evidence to indicate that an increase

in the osmotic pressure of the soil solution may result

in a decrease in the water uptake by plant roots, but

an additional factor must be taken into account in

dealing with the soil system; that is, soil-moisture tension, or the molecular attraction of the surface of the

soil particles for water. Soil-moisture tension increases

as the soil becomes drier and the water films around

the soil particles become thinner. This equivalent

negative pressure is apparently additive to the osmotic

pressure of the soil solution in limiting the availability

of water to plant roots. The sum of soil-moisture tension and the osmotic pressure of the soil solution is

termed "total soil-moisture stress." Studies on the

effects on growth of several moisture treatments and

0)

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60

50

40

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INTEGRATED MOISTURE STRESS

IN ATMOSPHERES

FIGURE 17.—Growth of bean plants as influenced by total soil-moislure stress. The salinity level for each treatment is indicated as

percentage on a dry-soil basis (Wadleigh and Ayers, 1945).
60 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

salinity levels indicate that plant growth is a function

of total soil-moisture stress, regardless of whether this

stress arises primarily from salinity or moisture tension

(fig. 17).

It is possible to extract the soil solution and determine its osmotic pressure, but this procedure is seldom

used because it is simpler to estimate salt concentration by determining the electrical conductivity of the

saturation extract (ECe). Since saturation percentage

is related to the field-moisture range, ECe bears a close

relationship to the EC of the soil solution. The relationship between EC and the osmotic pressure of saturation extracts is given in figure 6. The ECe, therefore,

provides information on the concentration of salt in

the soil solution and its osmotic properties. The yield

of orchardgrass when grown on soil to which various

single salts had been added indicated that growth was

simply related to salinity, expressed in terms of ECe

for various neutral salts (fig. 18). The response to

sodium bicarbonate was, however, exceptional. In this

case, calcium and magnesium ions from the soil

exchange complex were precipitated as carbonates,

thereby greatly increasing the exchangeable-sodiumpercentage and producing an alkali soil.

The Scofield scale, in which crop response to

salinity under average conditions is expressed in terms

of the conductivity of the saturation extract, was discussed in chapter 2. This salinity scale has been widely

used for a number of years and has been found to be

satisfactory for salinity appraisal. To facilitate the

discussion of plant response on saline soils, this salinity

scale in its latest modified form is given again.

Salinity effects mostly

negligible

Yields of very sensitive

crops may be restricted

Yields of many crops

restricted

Only tolerant- crops

yield satisfactorily

Only a few very tolerant

crops yield satisfactorily

4 8

Scale of conductivity (millimhos per centimeter at 25° C.)

16

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X CONTROL

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^ CaClg

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MgSO^

^ NdHC03

NOgCO;^

1 1 1 1 1

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-e1

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CONDUCTIVITY OF THE SATURATION EXTRACT—MM HO s/CM.

FIGURE 18.—Growth of orchardgrass, as influenced by various salts added to a sandy loam soil (Wadleigh and others, 1951).
SALINE AND ALKALI SOILg 61

It should be emphasized that this classification of

plant growth in relation to various salinity levels refers

to the salt status of the soil in the active root zone. It

is possible to obtain samples from the surface soil

around the base of row crops that may contain 5 percent salt or more with ECe values of 50 mmhos/cm. or

higher. This high concentration of salt represents an

accumulation in the bed during the growth of the plants

and not the salt concentration in the active root zone.

Therefore, in correlating crop growth with salinity,

care should be exercised to take soil samples from the

active root zone that are uncontaminated by surface incrustations of salt. With row crops, the mass of soil

making up the bed is frequently more saline than the

soil below the furrow, and studies of root distribution

and water uptake by plants indicate that under such

conditions the major root activity occurs in the less

saline parts of the soil, as shown in figure 14. These

considerations should be borne in mind in determining

the salt status of a soil with reference to plant response.

A technique for measuring the freezing point of soil

moisture has been developed that provides a rapid, useful method for obtaining, by a single determination,

the total moisture stress in a soil sample at fieldmoisture conditions (Method 6b). This eliminates

errors caused by dilution of the soil solution and the

resultant dissolving of moderately soluble salts, such

as gypsum. Total soil-moisture-stress values obtained

by freezing-point measurements are in good agreement

with previously used methods involving determination

of ECe and moisture tension for the soil studied (Wadleigh, 1946, and Ayers and Campbell, 1951).

The experimental evidence cited above supports the

concept that decreased growth on saline substrates is

related to decreased water availability, but certain relationships between plant and substrate are still not

fully understood. Despite marked decreases in growth

with increasing concentration of the substrate, osmotic

gradients between tops of plants and substrate are

sometimes unaffected by increased osmotic pressure or

total soil-moisture stress of the substrate. This is

caused by increases in osmotic pressure of aerial parts

of the plant that parallel increases in osmotic pressure

of the substrate (Eaton, 1942). In addition, the

osmotic pressure of expressed tissue fluids from the

tops of plants does not appear to be correlated with

the salt tolerance of some species. It is possible, however, that such measurements of osmotic gradient between plant tops and substrate may not represent the

effective osmotic force which limits water absorption by

the roots.

Specific Ion Effects

The previous discussion has dealt primarily with

the effect of soluble salts in limiting the availability

of moisture to plants. Other effects of salt may be

equally important in restricting the growth of certain

species. Injury or growth depression of plants, which

cannot be accounted for on the basis of the osmotic

pressure of the solution, will be referred to as a toxic

effect of the salt in question. It should be recognized

that toxicity so defined need not involve a direct effect

of the salt or ions, either on surface membranes of plant

roots or in the plant tissues. Frequently, toxicity may

be caused, in part, at least, through effects on the uptake

or metabolism of essential nutrients. As it is not always

possible to distinguish clearly the mechanism underlying specific ion effects, it is convenient to refer to such

phenomena as toxicities in contrast to the general osmotic effect of salt on plant growth.

The influence of excessive concentrations of specific

salts on plant growth is an extremely complex subject

involving many fundamental principles of plant nutrition. It is beyond the scope of this handbook to review

the voluminous and diversified literature bearing on

this subject. Much of the pertinent literature is cited

in a review by Hayward and Wadleigh ( 1949). Literature citations in the following discussion are restricted

mainly to papers of special significance in connection

with certain topics not considered in the review cited

above.

Ions that are frequently found in excess in saline

soils include chloride, sulfate, bicarbonate, sodium,

calcium, and magnesium. Less frequently encountered

in excessive amounts are potassium and nitrate. The

effects of all these ions on plant growth are being investigated by comparing plant response to isosmotic solutions of different salts. Species and even varietal differences among plants make it difficult to generalize

regarding the toxicity of various salts or ions. It appears, however, that differences in plant tolerance to

excessive concentrations of ions in the substrate are

related, in some degree, to specific selectivity in ion

absorption and nutrient requirements of the plants. In

addition to these factors, there is also a marked difference among species in the amounts of such ions as

sodium and chloride that can be accumulated without

toxic effects.

Before considering specific toxic effects caused by

excessive concentrations of soluble salts, other effects

of certain ions deserve some mention. Although not

considered essential plant nutrients, sodium and chloride, when present in relatively small concentrations,

may stimulate the productivity of certain crops. Thus,

Harmer and Benne (1941) have attributed increased

yields of beets, celery, Swiss chard, and turnips to

sodium. These authors consider sodium to be "nearly

as much needed as a nutrient for these crops as is the

potassium ion." Other investigators believe the effect

of sodium to be more indirect, either substituting to

some degree where potassium is deficient (Lehr, 1949;

Dorph-Petersen and Steenbjerg, 1950) or limiting excessive accumulation of calcium, which with beets

results in the development of a "calcium-type plant"

characterized by a blue-green color and stunted growth

(Lehr, 1942). Chloride, like sodium, has been observed to increase yields of some crops, notably beets,

spinach, and tomato (Hayward and Wadleigh, 1949).

On the other hand, chloride salts have long been known

to affect adversely the quality of such crops as potatoes

and tobacco. However, on saline soils, chloride and

sodium ions occur in much higher concentrations than
62 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

customarily employed in fertilizer studies. Under such

conditions the high osmotic pressure of the soil solution tends to obscure specific effects of sodium or chloride on crop yields and quality (Bernstein and others,

1951).

Sodium

Plant species vary greatly in the amounts of sodium

that they may accumulate, and many species tend to

exclude sodium from their leaves, although they may

accumulate it in their stems or roots. Notwithstanding this extreme selectivity in accumulation of sodium

by plants, few well-defined instances of sodium toxicity

have been reported. Lilleland and coworkers (1945)

described a tipburn of almond leaves that is related

to sodium content, and Ayers and associates (1951)

have described a sodium-scorch of avocado leaves. In

both studies, the soils on which affected trees grew were

sufiSciently low in soluble salts and exchangeable sodium to be regarded as nonsaline and nonalkali.

Although sodium salts in water cultures rarely cause

toxic plant reactions, stone-fruit trees (Brown and

others, 1953) and avocados (Ayers, 1950) evidenced

the same types of leaf injury in sand or water cultures

containing added sodium salts as were observed in the

field, thus confirming the relationship of sodium to

leaf injury in these species. Unpublished data by

Wadleigh and Gauch indicate that leaf burn in saltsensitive cotton varieties is closely correlated with the

sodium content of leaves.

Sodium in the soil may exert important secondary

effects on plant growth through adverse structural modifications of the soil. Thus, if the exchange complex contains appreciable amounts of sodium, the soil may become dispersed and puddled, thereby causing poor

aeration and low water availability (McGeorge and

Breazeale, 1938). This is especially true in fine-textured

soils. Also, if the exchange complex becomes more

than 40 to 50 percent saturated with sodium, nutritional disturbances may result (Ratner, 1935; Thorne,

1945). Ratner (1944) stated that under such conditions the exchange complex actually removes calcium

from the root tissues of the plant and that death may

ensue because of calcium deficiéWcy. Laboratory experiments have shown that the addition of calcium, and

sometimes magnesium, to alkali soils can improve plant

growth very markedly with an associated increase in

the uptake of these added elements by the plants (Bower

and Turk, 1946).

Bower and Wadleigh (1949), using amberlite resins,

determined the effects of various levels of exchangeable

sodium on cationic accumulation and growth of four

species of plants. The effect of increasing levels of exchangeable sodium on cationic accumulation varied

among the species and between tops and roots of a given

species and was related to inherent ^specificity of the

species in accumulating the several cations. In general,

increasing the exchangeable-sodium-percentage of the

substrate resulted in a decreased accumulation of calcium, magnesium, and potassium in the plants.

Calcium

The effect of high concentrations of calcium ions in

saline soil solutions varies with the species. Some

species, such as guayule, are more tolerant of added

calcium salts than of other neutral salts (Wadleigh and

Gauch, 1944). Masaewa (1936), however, found

added calcium chloride to be more toxic to soil cultures of flax than added sodium chloride. Wadleigh

and COworkers (1951) have reported specific toxicity

of calcium salts added to soil cultures of orchardgrass,

and unpublished data by Ayers indicate a similar relation for tall fescue. Both the calcium and chloride

contents of the grasses from the calcium chloride treatments increased markedly; but since calcium nitrate

produced a toxic effect similar to that of calcium

chloride, the toxicity was attributed to calcium accumulation rather than to chloride (Wadleigh and coworkers,

1951). Moderate concentrations of calcium chloride

are highly toxic to stone fruits in sand culture, and it

appears that this toxicity is associated with an accumulation of chloride in the leaves. This chloride accumulation is more pronounced in the presence of excess

calcium ions than when sodium occurs in excess (Brown

and others, 1953).

Magnesium.

High concentrations of magnesium in the substrate

are frequently more toxic to plants than isosmotic concentrations of other neutral salts. This toxicity of

magnesium may be alleviated by the presence of relatively high concentrations of calcium ions in the

substrate.

Potassium.

Although the occurrence of high concentrations of

potassium in the soil solution is rare, toxic effects of

high potassium have been reported. There is evidence

to indicate that toxicity of high potassium, like that of

high magnesium, may be lessened when balanced by

high calcium concentrations. High concentrations of

potassium may also induce magnesium deficiency

(Boynton and Burrell, 1944) and iron chlorosis

(Walsh and Clarke, 1942).

Chloride

As indicated under the discussion of calcium toxicity,

the accumulation of chloride ion in plant tissues manifesting toxic symptoms is not an infallible indication

of the specific toxicity of chloride. Many plant species

are no more sensitive to chloride salts than they are to

isosmotic concentrations of sulfate salts. There is good

evidence, however, for the specific toxicity of chloride

to some tree and vine crops. Hayward and associates

(1946) and Brown and coworkers (1953) have found

chloride salts to be toxic to peaches and other stone

fruits, and Harper (1946) has reported chloride burn

of pecan and native tree species of Oklahoma. Chloride

burn has also been reported for citrus (Reed and Haas,

1924; Cooper and Gorton, 1951), avocados (Ayers,
SALINE AND ALKALI SOILS 63

1950; Ayers and others, 1951; Cooper, 1951), and

grapevines (Thomas, 1934; Ravikovitch and Bidner,

1937).

Reference has been made in the discussion on toxic

effects of high concentrations of potassium and magnesium to the ameliorative effects of increased concentrations of calcium. In such cases, high concentrations of

potassium or magnesium result in increased absorption

of these ions and decreased absorption of calcium;

hence, the beneficial effect of increasing the calcium

concentration in the substrate. It is pertinent, at this

point, to consider whether such effects occur in the

anion nutrition of plants; specifically, whether high

levels of chloride (or sulfate) may interfere with nitrogen, phosphorus, or sulfur nutrition. Available evidence indicates that such interference in absorption of

essential anions from saline substrates is of relatively

minor importance and that decreased growth on saline

media is not related in any appreciable degree to decreased availability of essential anions. However,

Breazeale and McGeorge (1932) have emphasized the

importance of decreased availability of phosphorus and

nitrogen in calcareous alkali soils.

Sulfate

Specific sensitivity of plants to high sulfate concentrations has been noted for a number of crops, and it

appears that such sensitivity is related to the tendency of

high sulfate concentrations to limit the uptake of calcium by plants. Associated with this decrease in calcium are increases in the absorption of sodium and

potassium, so that harmful effects of high sulfate in the

substrate may be related to a disturbance of optimum

cationic balance within the plant.

Bicarbonate

Plant species differ markedly in their tolerance to the

bicarbonate ion, which sometimes exerts specific toxic

effects, resulting in serious injury even at low osmotic

concentrations. Beans and Dallis grass are very sensitive, while Rhodes grass and beets are relatively tolerant

(Wadleigh and Brown, 1952; Gauch and Wadleigh,

1951 ). Studies in sand culture indicate that the bicarbonate ion affects the uptake and metabolism of nutrients by plants and that the nature of these effects

varies with the plant species. For example, bean plants

in the presence of the bicarbonate ion contain less

calcium and more potassium than control plants, while

the main effects in beets are a decrease in magnesium

and an increase in sodium content. The pattern of

effects is obviously related to the inherent selectivity

of species in relation to mineral nutrition.

The studies by Wadleigh and coworkers cited above

are of interest in connection with the problem of limeinduced chlorosis. Chlorotic symptoms and associated

divergences in metabolism, involving contents of active

iron, organic acid fractions, and essential cations,

are very similar for typical cases of lime-induced chlorosis (Iljin, 1951, 1952; McGeorge, 1949) and bicarbonate-induced chlorosis. Since the basic causes of

these chloroses are not understood, it would be speculative to suggest any closer relationship of the two disorders than the common features indicate. Thorne

and others (1951) have shown that chloroses owing to

such diverse causes as development in darkness, zinc

deficiency, virus infection, and lime-induced chlorosis

may be accompanied by very similar changes in potassium accumulation, water-soluble nitrogen fraction, and

other features frequently considered characteristic of

lime-induced chlorosis.

Boron

In addition to the elements that frequently occur in

relatively high concentrations, boron may cause injury

to plants even when present in very low concentrations

in the soil solution. Boron is essential to the normal

growth of all plants, but the concentration required is

very small and if exceeded may cause injury. Plant

species vary both in boron requirements and in tolerance to excess boron, so that concentrations necessary

for the growth of plants having high boron requirements may be toxic for plants sensitive to boron.

Symptoms of boron injury may include characteristic

burning, chlorosis, and necrosis, although some boronsensitive species do not develop perceptible symptoms.

Citrus, avocados, persimmons, and many other species

develop a tipburn or marginal burn of mature leaves,

accompanied by chlorosis of interveinal tissue. Boron

injury to walnut leaves is characterized by marginal

burn and brown necrotic areas between the veins. Stonefruit trees, apples, and pears are sensitive to boron, but

they do not accumulate it in high concentration in"their

leaves nor do they develop typical leaf symptoms. Cotton, grapes, potatoes, beans, peas, and several other

plants show marginal burning and a cupping of the

leaf that results from a restriction of the growth of

the margin.

Boron toxicity occurs in limited, scattered areas in

arid or semiarid regions. While its incidence is not

restricted to saline or alkali soils, excess boron is

frequently present in saline soils.

Plant Analysis

The normal mineral composition of plant parts is

frequently altered under saline- or alkali-soil conditions, and analysis of appropriate plant organs may

serve for diagnosing mineral excesses as well as for

mineral deficiencies of soils. In addition, plant analysis

may indicate salt injury in cases where the soil is regarded as nonsaline. This condition may occur with

plants that are very sensitive to salt, such as beans and

stone fruits, or in cases where the soil salinity is

transitory.

Under some conditions, as in the presence of the

bicarbonate ion, the entire complex of factors in the

mineral composition of plants may be altered, and caution should be exercised in relating malfunction of

these plants to a specific ion. Frequently, excessive

accumulation of an ion in the plant may be the result

of conditions other than high concentration of that
64 AGRICULTURAL HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

ion in the soil solution. Any factors that inhibit plant

growth, such as mineral deficiencies and high moisture

stress, may result in abnormal accumulation of ions in

plant tissues. For example, plants deficient in potassium will often show greater accumulation of calcium,

magnesium, or sodium than normal plants (Cooil,

1948). Owing to the high degree of variability in the

composition of "normal" plants under diverse growing

conditions, the chemical composition of plant parts

should usually be considered as only one line of evidence in the diagnosis of crop injury on saline or alkali

soils. Appropriate soil tests, as described elsewhere in

this handbook, may be used to furnish corroborative

evidence.

Eaton (1942) has pointed out that no particular

range of salt concentration in the substrate is critical

in retarding plant growth. Growth depression is usually

progressive as salt concentration increases. This type

of relationship is to be expected in cases characterized

by a predominance of the osmotic factor in limiting

growth on saline substrates. Correspondingly, there

is usually a progressive increase in salt concentration in

the plant tissues as salinity in the substrate increases,

although frequently the curve relating concentration of

a specific ion in the leaves to that in the substrate may

be exponential rather than linear. Considering the

progressive nature of growth depression and ionic accumulation, it becomes apparent that for such cases no

critical level of salt concentration in the tissues can be

established with reference to the onset of "salt injury."

In some instances, however, specific toxic eifects of an

ion may be of predominant importance in limiting

plant growth. In extreme cases, death may ensue,

whereas isosmotic concentrations of salts not specifically toxic to the species may cause only minor

growth depression. Under such conditions, fairly definite limits of accumulation of ions, such as sodium or

chloride, have been observed to be associated with the

development of toxic symptoms in certain plant species.

Foliar analysis is commonly used in studying salt

accumulation. The relationships between foliar composition and the principal ions that occur in excess in

saline soils can be summarized as follows: (1) Chloride concentration in leaves usually bears a close relationship to the chloride concentration of the substrate;

(2) excessive sulfate in the substrate causes small relative increases in total sulfur of the leaf tissue; (3) increases in calcium concentration in leaves are frequently

associated with excess calcium in the substrate; (4)

excess soluble sodium may or may not be reflected in

the sodium content of leaf tissues. The influence of

high exchangeable-sodium-percentage in depressing the

calcium concentration of plant tissues has been mentioned in an earlier section.

Recent studies have furnished information on levels

of chloride and sodium accumulation in leaf tissues

associated with leaf injury. Kootstock studies by

Cooper and Gorton (1951) and Cooper and associates

(1951 and 1952) indicate that grapefruit and Valencia

orange may develop leaf burn when chloride accumulation reaches about 1.0 to 1.5 percent on a dry-weight

basis, whereas bronzing may occur with even lower

chloride accumulation. Avocados appear to be more

sensitive than citrus, since leaf tipburn symptoms were

reported at chloride concentrations of 0.5 to 0.9 percent

(Avers, 1950; Haas, 1950; Ayers and coworkers, 1951;

Cooper, 1951). Cooper and Gorton (1951) have observed tipburn symptoms when chloride was only 0.2

percent. Burning of peach leaves has been noted when

chloride content reached 1.0 percent (Hayward and

others, 1946) ; and, in a study of six varieties of stone

fruits, leaf burn was not observed until chloride levels

reached values of 0.6 to 1.8 percent of the dry weight of

the leaves, depending on the variety (Brown and others,

1953). Plum and prune showed leaf-burn symptoms

with about 0.6 percent chloride in the leaves, while

burning occurred in peach and apricot at 1.0 percent

chloride. The leaves of the two almond varieties. Nonpareil and Texas, developed burn at 1.2 and 1.8 percent,

respectively. In a study of salt injury to pecans and

native trees of Oklahoma, Harper (1946) found approximately 0.6 percent chloride to be associated with

the development of leaf-burn symptoms. Thomas

(1934) observed leaf burn of grapes having a chloride

content of 0.5 percent, and Ravikovitch and Bidner

(1937) found 1.2 percent. The latter reported that the

variety Chasselas accumulated as much as 3.0 percent

chloride in severely burned leaves, while the variety

Muscat Hamburg accumulated a maximum of 1.5 percent. Such varietal difl'erences in levels of chloride accumulation doubtless will be found for other crops.

Other factors that may affect the level of accumulated

chloride include age of leaf, season, and climatic conditions. Brown and coworkers (1953) and Thomas

(1934) reported increasing levels of chloride in leaves

of stone fruits and grapes, respectively, as the season

progressed. Hot, windy weather may result in very

rapid chloride accumulation in leaves in a very short

time (Thomas, 1934) ; and, under such conditions,

higher chloride levels may appear to be critical in the

development of leaf injury. Although chloride contents of 0.5 to 1.0 percent may be associated with foliar

injury of some crops, it should be pointed out that many

species of plants, including some possessing no outstanding salt tolerance, such as potatoes, may accumulate as much as 5.0 percent chloride on a dry-weight

basis without showing foliar symptoms (Bernstein and

associates. 1951 ).

While the causal relationship between chloride accumulation and leaf symptoms has been demonstrated

by means of carefully controlled experiments for some

of the crops mentioned in the above discussion, such as

avocado and stone fruits, the data for other crops are

based only on a close concomitance between chloride

accumulation and observed leaf injury.

Few instances of injury related to excessive sodium

accumulation have been noted. With some crops

sodium injury may he obscured by simultaneous chloride injur\. as Avers and others (1951) have pointed

out for HNuado. h a water-culture study. Ayers

( 1 950 < r;h>r "ved leaf iiurn oí avocado when leaves contained 0.5 iH rcent sodium on a drv-w^eieht basis. Lille-
SALINE AND ALKALI SOILS 65

land and associates (1945) have indicated that sodium

accumulation of 0.3 percent in almond leaves is associated with incipient leaf-burn symptoms. In sand

cuhures, Brown and coworkers (1953) have observed

tipburn of Texas almond leaves containing 0.4 percent

sodium and of plum leaves containing 0.3 percent.

Unpublished data by Wadleigh and Gauch have indicated that leaf burn of salt-sensitive cotton varieties

may occur in leaves containing 0.2 percent sodium.

Chapman (1949) has indicated that chloride or

sodium accumulations of 0.25 percent or higher in

citrus leaves should be regarded as excessive. While

these values are lower than those at which definite

foliar injury may appear, they do emphasize the fact

that under some conditions even lower values than those

cited in the above discussion may indicate a definite

tendency toward excessive accumulations of harmful

ions in the plant.

Foliar analysis is useful in the diagnosis of boron

injury of many plant species. The boron content of

normal, mature leaves of such plants as citrus, avocados, walnuts, figs, grapes, cotton, and of alfalfa

tops is about 50 p. p. m. Boron contents of 20 p. p. m.

or less indicate deficiency, while values above 250

p. p. m. are usually associated with boron toxicity.

Stone-fruit trees, apples, and pears do not accumulate

high concentrations of boron in their leaves, although

these species are sensitive to excess boron. If due allowance is made for varietal specificity in boron accumulation, foliar analysis may provide a readier basis for

diagnosis than analysis of soil or water.

Crop Selection for Saline Soils

Because of saline irrigation water, high water table,

or low permeability of the soil, it may not be economically feasible to maintain low salinity. In such instances, the judicious selection of crops that can produce satisfactory yields under saline conditions and the

use of special management practices to minimize salinity may make the différence between success or

failure.

As has already been pointed out, the availability of

water to plants is always a factor under saline conditions. For example, suppose alfalfa is being grown

on a loam having a salt content of 0.2 percent sodium

chloride and a wilting percentage of 6 when the latter

is determined on a nonsaline sample. Under such conditions, because the osmotic effect is additive with soilmoisture tension, alfalfa will stop growing when the

soil dries to a moisture content of only 13 percent. In

other words, if the soil Contains 0.2 percent salt, the

alfalfa plant cannot use a large part of the soil moisture

that is normally available under nonsaline conditions.

The presence of even smaller quantities of salt in this

soil would cause a fraction of the soil moisture above

the wilting percentage to be unavailable to the plant.

More frequent irrigation would be required to decrease

the inhibitory effect of the salt on the growth of alfalfa.

Although it has been shown that crop growth on

saline soils is definitely benefited by more frequent

irrigation, the need for this irrigation may not be indicated by the appearance of the crop (Richards and

Wadleigh, 1952 ). In nonsaline soils, there is usually a

relatively abrupt transition from low moisture stress to

high moisture stress conditions, and the wilting of the

plant indicates the need for irrigation. In saline soils,

changes in moisture stress are more gradual and,

although the plants may be subjected to high stress,

there is no abrupt transition in the turgor condition of

the plant and, hence, no sign of the need for irrigation.

Nevertheless, experiments have shown that crop growth

is greatly improved by more frequent irrigation under

such conditions. Careful leveling of the fields to insure

more uniform moisture distribution during irrigation

will also improve chances for successful crops on saline

soils.

Germination

In selecting crops for saline soils, particular attention should be given to the salt tolerance of the crop

during germination because poor crops frequently result from a failure to obtain a satisfactory stand. This

problem is complicated by the fact that some crop

species which are very salt tolerant during later stages

of growth may be quite sensitive to salinity during

germination (fig. 19). Sugar beets, for example,

which are very salt tolerant during later stages of

growth, are extremely sensitive during germination.

On the other hand, barley has very good salt tolerance

during all stages of growth, although it is more sensitive

during germination than at later stages (Ayers and

others, 1952). Under field conditions, it is possible by

modification of planting practices to minimize the tendency for salt to accumulate around the seed and to improve the stand of crops that are sensitive to salt during germination (Heald and coworkers, 1950).

Relative Salt Tolerance of Crop Plants

The salt tolerance of many species and varieties of

crop plants has been investigated at the Laboratory.

Previously published lists (Magistad and Christiansen,

1944, and Hayward and Magistad, 1946) have been

modified on the basis of recent findings and are presented in table 8.

The salt tolerance of a crop may be appraised according to three criteria: (1) The ability of the crop

to survive on saline soils, (2) the yield of the crop on

saline soils, and (3) the relative yield of the crop on a

saline soil as compared with its yield on a nonsaline

soil under similar growing conditions. Many previous

observations on salt tolerance have been based mainly

on the first criterion, ability to survive; but this method

of appraisal has very limited practical significance in

irrigation agriculture. Although it is recognized that

the second criterion is perhaps of greater agronomic

importance, the third criterion was used in compiling

the present salt-tolerance lists because it provides a

better basis of comparison among diverse crops.

The salt-tolerance lists are arranged according to

major crop divisions; and, in each division, crops are
66 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

100

15 9 13 17

CONDUCTIVITY OF SATURATION EXTRACT - MILLIMHOS/CM.

FIGURE 19.—Percent germination of four crops, as related to the conductivity of the saturation extract of the soil, under laboratory

conditions (Ayers and Hayward, 1949).

listed in three groups. Within each group, the crops

are listed in the order of decreasing salt tolerance,

but a difference of 2 or 3 places in a column may not

be significant. ECe values given at the top of a column

represent the salinity level at which a 50-percent decrease in yield may be expected as compared to yields

on nonsaline soil under comparable growing conditions. For example, for crops with high salt tolerance

in the division of field crops, ECe values of 16 mmhos/

cm. occur at the top of the column and 10 mmhos/cm. at

the bottom. This indicates that crops near the top of

this column will produce about 50 percent as well on a

soil having an ECe of 16 mmhos/cm. as on a nonsaline

soil under similar conditions, and crops near the bottom

of this column will produce about 50 percent as well

on soils having an ECe of 10 mmhos/cm. as on a nonsaline soil. ECe values having similar significance have

been shown for each group of plants for which such

data are available.

In most instances, these data are based on a field-plot

technique in which crops are grown on soils that are

artificially adjusted to various salinity levels after the

seedlings are established. By this method, crop yields

were related to ECe values for comparable saline and

nonsaline soils, and the salinity level associated with a

50-percent decrement of yield was determined graphically. In many of these studies, a number of varieties

of a given crop were compared. Significant varietal

differences were found for cotton, barley, and smooth

brome, while for truck crops such as green beans,

lettuce, onions, and carrots varietal différences were

not of practical significance.

In applying the information in the following table,

it is important to remember that climatic conditions

may influence profoundly the reaction of plants to

salinity. The choice of suitable salt-tolerant varieties

and strains will depend on local climatic factors; and,

consequently, information on salt-tolerant varieties

should be evaluated with reference to the conditions

under which the crops are to be grown. The position

of each crop in this table reflects its relative sah tolerance under management practices that are customarily

employed when this crop is grown under irrigation agriculture and not the inherent physiological ability of

the crop to withstand salinity under some given set of

conditions that is uniform for all crops.

A salt-tolerance list for some important crops of

Holland has recently been prepared by Van den Berg
SALINE AND ALKALI SOILS 67

TABLE 8.—Relative tolerance of crop plants to salt ^

FRUIT CROPS

FIELD CROPS

High salt tolerance Medium salt tolerance Low salt tolerance

Date palm Pomegranate Pear

Fig Apple

Olive Orange

Grape Grapefruit

Cantaloup Prune

Plum

Almond

Apricot

Peach

Strawberry

Lemon

Avocado

VEGETABLE CROPS

£;c,xio'=i2 ECeXlO^=10 ECeXlO^=4:

Garden beets Tomato Radish

Kale Broccoli Celery

Asparagus Cabbage Green beans

Spinach Bell pepper

Cauliflower

Lettuce

Sweet corn

Potatoes (White

Rose)

Carrot

Onion

Peas

Squash

Cucumber

ECeX10^=10 ECeXlO^ =4: EC,X10^=S

FORAGE CROPS

ECeXlO^=lS ECeX10^^12 EC,X10^=4:

Alkali sacaton White sweetclover White Dutch

Saltgrass Yellow sweetclover clover

Nuttall alkaligrass Perennial ryegrass Meadow foxtail

Bermuda grass Mountain brome Alsike clover

Rhodes grass Strawberry clover Red clover

Rescue grass DalUs grass Ladino clover

Canada wildrye Sudan grass Burnet

Western wheat- Hiibam clover

grass Alfalfa (California

Barley (hay) common)

Bridsfoot trefoil Tall fescue

Rye (hay)

Wheat (hay)

Oats (hay)

Orchardgrass

Blue grama

Meadow fescue

Reed canary

Big trefoil

Smooth brome

Tall meadow oatgrass

Cicer milkvetch

Sourclover

Sickle milkvetch

EC,X10^=12 ECeX10^ =4: fi:c,xi0 3=2

ECeX10^== 16 ECeX10^=10 £:CeX10»=4

Barley (grain) Rye (grain)

Wheat (grain)

Field beans

Sugar beet

Rape Oats (grain)

Cotton Rice

Sorghum (grain)

Com (field)

Flax

Sunflower

Castorbeans

ECeXlO^ == 10 £?C,X10 3=6

* The numbers following ECe X10 ^ are the electrical conductivity values of the saturation extract in millimhos per

centimeter at 25° C. associated with 50-percent decrease in

yield.

( 1950). Based on field-plot studies in areas which had

been inundated by salt or brackish water in 1944-45,

the salinity values (**salt index," expressed as grams

NaCl per liter of soil water) associated with 75 percent

of normal yields for 14 crops were determined. Despite obvious differences in climate and cultural practices, Van den Berg's results for relative salt tolerance

are in good agreement with those in table 8.

Relative Boron Tolerance of Crop Plants

Plant species differ markedly in their tolerance to excessive concentrations of boron. In sections where

boron tends to occur in excess in the soil or irrigation

water, the boron-tolerant crops may grow satisfactorily,

whereas sensitive crops may fail. The relative boron

tolerance of a number of crops was determined by Eaton

(1935), and his results are reported in table 9 with

TABLE 9.—Relative tolerance of plants to boron

[In each group, the plants first named are considered as being more tolerant

and the last named more sensitive]

Tolerant Semitolerant Sensitive

Athel {Tamarix Sunflower (native) Pecan

aphylla) Potato Black walnut

Asparagus Acala cotton Persian (English)

Palm (Phoenix Pima cotton walnut

canariensis) Tomato Jerusalem artiDate palm (P. dac- Sweetpea

Radish

choke

tylifera) Navy bean

Sugar beet Field pea American elm

Mangel Ragged Robin rose Plum

Garden beet Olive Pear

Alfalfa Barley Apple

Gladiolus Wheat Grape (Sultanina

Broadbean Com and Malaga)

Onion Milo Kadota fig

Turnip Oat Persimmon

Cabbage Zinnia Cherry

Lettuce Pumpkin Peach

Carrot Bell pepper Apricot

Sweetpotato Thornless blackLima bean berry

Orange

Avocado

Grapefruit

Lemon
68 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGiyCULTURE

minor modifications based on field observations. The

boron-tolerance lists are analogous to the salt-tolerance

lists and subject to much the same limitations in interpretation. Differences in position of a few places may

or may not be significant, and there is no sharp division between successive classes. Climate and variety

may also be factors in altering the indicated tolerance

of a given species under specific conditions.

Available information on boron tolerance does not

permit the establishment of definite permissible limits

of boron concentration in the soil solution. Irrigation

waters are classified on the basis of boron content in

table 14, chajpter 5, with reference to sensitive, semitolerant, and tolerant crops. The effect of a given concentration of boron in the irrigation water on the boron

content of the soil solution will be conditioned by soil

characteristics and management practices that influence

the degree of boron accumulation in the soil. In the

discussion of saturation extracts of soils (ch. 2), 0.7

p. p. m. boron in the saturation extract was indicated

as the approximate safe limit for sensitive crops.
Chapter 5

Quality of Irrigation Water

The concentration and composition of dissolved constituents in a water determine its quality for irrigation

use. Quality of water is an important consideration in

any appraisal of salinity or alkali conditions in an

irrigated area. Much work has been done on quality

of irrigation water. The United States Geological Survey is very active in general quality-of-water studies,

and the analyses made by this agency are published at

irregular intervals in the USGS Water-Supply Papers.

In addition to current programs, analyses dating back

to the beginning of irrigation in the western United

States are recorded in this series. The Geological Survey took the leadership in preparing an index of water

analyses that has proved to be very useful.^^ Agricultural experiment stations in the Western States have

also been active in quality-of-water studies and have

published a number of bulletins on this subject (Smith,

1949; Smith and others, 1949; Miller, 1950; Jensen

and others, 1951; Thorne and Thorne, 1951).

The Rubidoux Laboratory since 1928 has analyzed

more than 22,000 samples of irrigation water. Much

of the information has been published, and all of it is

available in the records- of the Laboratory. This work

shows that poor quality of both surface and ground

waters is a limiting factor in the irrigation of many

areas in this country and abroad.

There are many places in western United States, particularly in the desert areas of California, Arizona,

Texas, and New Mexico, and also in the other parts

of the world, where ground water is available but the

quality is questionable or unsatisfactory. Similarly,

where surface waters are used, the present rate of increase of irrigation development and changes in management practices are resulting in serious quality-ofwater problems. There is the tendency to divert for

irrigation all of the available water. This means that

over a period of years the downstream diversions may

change from uncontaminated river water to a substantial proportion of drainage return-flow of poor quality.

To cope with such problems, it is necessary to have detailed information concerning the quality of irrigation

water and a background of experience relating to the

effect of irrigation waters on soils and crops.

" U. S. DEPARTMENT OF THE INTERIOR, GEOLOGICAL SURVEY.

INVENTORY OF PUBLISHED AND UNPUBLISHED CHEMICAL ANALYSES

OF SURFACE WATERS IN THE WESTERN UNITED STATES, NOTES ON

HYDROLOCic ACTIVITIES. Bul. No. 2, October 1948. [Processed.]

Methods of Analysis

The methods used by this Laboratory for the analysis

of irrigation waters are given in chapter 8, Methods 70

to 86. The Versenate titration (Method 79) for calcium plus magnesium, the flame photometer method for

sodium and potassium (Methods 80b and 81c), and the

colorimetric method for boron (Method 73b), make

possible rapid determinations without sacrificing

accuracy. Also, the volume of water required for an

analysis is less, thus simplifying the collection and

transportation of samples.

If all of the principal constituents have been determined and expressed in chemical equivalents, the sum

of the cations should equal the sum of the anions, and

a lack of balance indicates an error. There are a number of ways in which a water analysis can be checked.

The numerical value of the ratio—electrical conductivity in micromhos per centimeter divided by cations

in milliequivalents per liter—should be about 100 for

most waters (fig. 20). This ratio may be as low as 80

for bicarbonate or sulfate waters in which calcium and

magnesium are high, but for chloride waters that are

high in sodium the ratio may be as high as 110. The

numerical value of the ratio—dissolved solids in parts

per million divided by conductivity in micromhos per

centimeter—should be approximately 0.64 (fig. 21).

A third ratio—dissolved solids in parts per million

divided by cations in milliequivalents per liter—has a

value of approximately 64. These values are averages

based on a large number of determinations for natural

waters.

Characteristics That Determine Quality

The characteristics of an irrigation water that appear

to be most important in determining its quality are:

(1) Total concentration of soluble salts; (2) relative

proportion of sodium to other cations; (3) concentration of boron or other elements that may be toxic; and

(4) under some conditions, the bicarbonate concentration as related to the concentration of calcium plus

magnesium.

Electrical Conductivity

The total concentration of soluble salts in irrigation waters can be adequately expressed for purposes of

diagnosis and classification in terms of electrical con69
70 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

O

ÜJ

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<

oc

Z

bJ

O

O

O

60

6

0-5

8 100 2

CONDUCTIVITY

6 8 1000 2 4

MICR0MH0S/CM.(ECxl06) AT 25<>C,

FIGURE 20.—Concentration of irrigation waters in milliequivalents per liter of cations as related to conductivity.

ductivity. The conductivity is useful because it can be

readily and precisely determined.

Nearly all irrigation waters that have been used

successfully for a considerable time have conductivity

values less than 2,250 micromhos/cm. Waters of

higher conductivity are used occasionally, but crop production, except in unusual situations, has not been

satisfactory.

Saline soils are those in which the conductivity of

the saturation extract is greater than 4 millimhos/cm.,

or 4,000 micromhos/cm. It has been found that the

conductivity of the saturation extract of a soil, in the

absence of salt accumulation from ground water, usually

ranges from 2 to 10 times as high as the conductivity

of the applied irrigation water. This increase in the

salt concentration is the result of continual moisture

extraction by plant roots and evaporation. Therefore,

the use of waters of moderate to high salt content may
SALINE AND ALKALI SOILS 71

result in saline conditions, even where drainage is satisfactory. In general, waters with conductivity values

below 750 micromhos/cm. are satisfactory for irrigation insofar as salt content is concerned, although saltsensitive crops may be adversely affected by the use of

irrigation waters having conductivity values in the

range 250 to 750 micromhos/cm.

Waters in the range of 750 to 2,250 micromhos/cm.

are widely used, and satisfactory crop growth is obtained under good management and favorable drainage

conditions, but saline conditions will develop if leaching

and drainage are inadequate. Use of waters with conductivity values above 2,250 micromhos/cm. is the exception, and very few instances can be cited where such

waters have been used successfully. Only the more salttolerant crops can be grown with such waters and then

only when the water is used copiously and the subsoil

drainage is good.

As discussed in chapter 3, the steady-state leaching

requirement for soils where no precipitation of salts

occurs is directly related to the electrical conductivity

of the irrigation water and the permissible conductivity

Q:

1000

8

<

Io

z

o

o

100

8

6

5 O

^^j^íMtMÍMtiikliiiMMM

SURFACE STREAM

WELL WATERS

APPROX. AVERAGE

wHf

m^Siï

-I -* u

8 100 8 1000

CONDUCTIVITY MICROMHOS/CM. (ECxlO^) AT

6

25» C.

FIGURE 21.—Concentration of irrigation waters in parts per million as related to conductivity.
72 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

of the water draining from the root zone. The leaching requirements for specified electrical conductivity

values of the irrigation and drainage waters, as determined from equation 2, chapter 3, are given in table 10.

TABLE 10.—Leaching requirement ^ as related to the

electrical conductivities of the irrigation and drainage waters

Electrical

conductivity

of irrigation

Leaching requirement for the indicated maximum values of the conductivity of the drainage water at the bottom of the root zone

(micromhos/

cm.) 4 mmhos/

cm.

8 mmhos/

cm.

12 mmhos/

cm.

16 mmhos/

cm.

100

Percent

2.5

6.2

18.8

56.2

Percent

L2

3.1

9.4

28.1

62.5

Percent

0.8

2.1

6.2

18.8

4L 7

Percent

0.6

250 1 6

750

2,250

5.000

4.7

14.1

31.2

^ Fraction of the applied irrigation water that must be

leached through the root zone expressed as percent.

Although, for reasons stated in chapter 3, these

leaching requirement values are probably somewhat

high, they illustrate the manner in which the electrical

conductivity of irrigation waters influences the leaching

requirement under various levels of soil salinity, expressed in terms of electrical conductivity of the soil

solution at the bottom of the root zone. It is apparent that the water-transmission and drainage properties of the soil and the salt tolerance of the crop to be

grown are important factors in appraising irrigation

waters from the standpoint of total salt concentration.

Sodium-Adsorption-Ratio

The soluble inorganic constituents of irrigation

waters react with soils as ions rather than as molecules.

The principal cations are calcium, magnesium, and

sodium, with small quantities of potassium ordinarily

present. The principal anions are carbonate, bicarbonate, sulfate, and chloride, with fluoride and nitrate

occurring in low concentrations. The alkali hazard

involved in the use of a water for irrigation is determined by the absolute and relative concentrations of

the cations. If the proportion of sodium is high, the

alkali hazard is high; and, conversely, if calcium and

magnesium predominate, the hazard is low. The importance of the cationic constituents of an irrigation

water in relation to the chemical and physical properties of the soil was recognized even before cation exchange reactions were widely understood. Scofield and

Headley (1921) summarized the results of a series of

alkali reclamation experiments with the statement:

"Hard water makes soft land and soft water makes

hard land." Alkali soils are formed by accumulation

of exchangeable sodium and are often characterized by

poor tilth and low permeability.

In the past the relative proportion of sodium to other

cations in an irrigation water usually has been expressed in terms of the soluble-sodium percentage.

However, as was shown in chapter 2, the sodium-adsorption-ratio of a soil solution is simply related to

the adsorption of sodium by the soil; consequently,

this ratio has certain advantages for use as an index of

the sodium or alkali hazard of the water. This ratio

is defined by the equation :

SAR = NaV V{Ca- + Mg^-)/2

where Na^, Ca^^, and Mg^^ represent the concentrations

in milliequivalents per liter of the respective ions. A

nomogram for estimating the SAR value of an irrigation water is shown in figure 22. This nomogram is

similar to figure 27, but figure 22 has scales more

suitable for the cationic concentrations encountered in

irrigation waters.

An ESP scale is included in the nomogram opposite

the SAR scale. This ESP scale is based on the regression line shown in figure 9, chapter 2, in which the

relation between SAR and ESP was given as

FSP= IQQ (-.0126+ .01475 SAR)

1-f (-.0126+ .01475 S^Ä)

This empirical equation was used to relate the ESP

scale to the SAR scale in figure 22. After the SAR

value of an irrigation water is determined by use of the

nomogram, it is possible from the central scale to estimate the ESP value of a soil that is at equilibrium with

the irrigation water. It is to be expected, however, that

this condition would not often occur in the field, because the soil solution is nearly always appreciably

more concentrated than the irrigation water.

The concentration of the soil solution is increased

by the extraction of water from the soil by roots and by

evaporation. As the quantity of salt absorbed by

plants is relatively small, the solution remaining in the

soil is more concentrated than the applied irrigation

water. At the next irrigation this more concentrated

solution may be displaced downward or diluted, and

so the concentration of the solution in contact with

the soil varies with time and location in the profile. It

is not unusual to find shallow ground water or drainage water that is from 2 to 10 times as concentrated as

the irfigation water. It is reasonable to assume, however, that for a limited depth of soil, such as the top

12 inches, the concentration of the soil solution is not,

on the average, more than 2 or 3 times the concentration

of the irrigation water.

Under conditions in soil where it is permissible to

neglect precipitation and absorption of soluble salts

by roots, it is clear that irrigation water, after entering

the soil, becomes more concentrated without change in

relative composition, i. e., the soluble-sodium percentage does not change. The SAR value, however, increases in proportion to the square root of the total

concentration, i. e., if the concentration is doubled the

SAR value increases by a factor of 1.41. If the concentration is quadrupled the SAR value will be doubled.
SALINE AND ALKALI SOILS 73

Ca"^^+ Mg"^"^

FIGURE 22.—Nomogram for determining the SAR value of irrigation water and for estimating the corresponding ESP value of

a soil that is at equilibrium with the water.
74 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

It has been observed that where an irrigation water of

relatively constant composition is used and drainage

conditions are good, the ESP value of soil varies only

slightly from season to season or from year to year.

This implies that the cation-exchange material of the

soil has reached a steady state relative to the cations

in the soil solution which are derived from the irrigation water. All suitable data bearing on the relationship between the soluble cations of the irrigation water

and the exchangeable cations of the soil have been assembled from the records of the Laboratory. Only

24

those instances were selected in which the drainage was

known to be good and only the surface sample of soil

from each site was considered. It was further required

that the composition of the irrigation water be relatively

invariant with time and that the water must have been

used for many seasons in the field experiments or for

many irrigations in experiments conducted at the

Laboratory.

The relation between the SAR value of the irrigation

waters and the ESP values of the soil samples is shown

by the points on the graph in figure 23. The solid

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SODIUM-ADSORPTION--RATIO OF IRRIGATION WATER

24

FIGURE 23.--The exchangeable-sodium-percentage volues of samples of surface soil as related to the sodium-adsorption-ratio values

of the irrigation waters: O, small lysimeters after 42 irrigation cycles; -f-, large lysimeters after prolonged leaching; •, field

observations.
SAUNE AND ALKALI SOILS 75

curve represents the relation between SAR and ESP

given by the equation shown in the figure, and also indicated by scales C and D in the nomogram of figure

22. It is apparent that, under the conditions existing

in the field, the ESP values of the soil samples are

generally higher than the estimated values. The deviations from the solid curve are undoubtedly owing to the

fact that the concentrations of the soil solutions are

somewhat higher than the concentrations of the irrigation waters.

The dotted curve in figure 23 shows the ESP values

that would be attained by the soils, assuming a threefold increase in the concentration of the irrigation

waters. In other words, if the soluble-sodium percentages of the irrigation waters after entering the soils

remain unchanged but the total concentrations increase

by a factor of 3, the SAR values would increase by a

factor equal to the square root of 3 and the resulting

predicted ESP values for the samples would lie along

the dotted curve in the figure. The distribution of the

points on the graph that represent the field samples indicates that the saturation extracts were 1 to 3 times as

concentrated as the irrigation waters applied.

More data are needed to explain the relation of exchangeable sodium to water quality and irrigation practices. On the basis of the relationship shown in figure

23, SAR appears to be a useful index for designating the

sodium hazard of waters used for irrigation.

Boron

Boron is a constituent of practically all natural

waters, the concentration varying from traces to several

parts per million. It is essential to plant growth, but is

exceedingly toxic at concentrations only slightly above

optimum. Eaton (1944) found that many plants made

normal growth in sand cultures with a trace of boron

(0.03 to 0.04 p. p. m.), and that injury often occurred

in cultures containing 1 p. p. m.

Bicarbonate

In waters containing high concentrations of bicarbonate ion, there is a tendency for calcium and magnesium to precipitate as carbonates as the soil solution becomes more concentrated. This reaction does not go to

completion under ordinary circumstances, but insofar

as it does proceed, the concentrations of calcium and

magnesium are reduced and the relative proportion of

sodium is increased. Eaton (1950) uses three terms in

connection with this reaction:

(1) Soluble-sodium percentage "found"=(Na^ X 100)/

(Ca'^'^ + Mg^'^-I-Na*) ; (2) Soluble-sodium percentage

"possible" = (Na*X100)/((Ca-^ + Mg^^-f Na^) -

(COa'-f-HCOa-) ), where the CO3- + HCOg" deduction

does not exceed Ca*^ + Mg^^ ; ( 3 ) "Residual NagCOa" =

(CO3--4-HCO3-) -(Ca--f Mg-).

In these relations the ionic constituents are expressed

as milliequivalents per liter.

The influence of the bicarbonate ion concentration

of irrigation waters upon the exchangeable-sodiumpercentage has been studied at the Laboratory. One

experiment involved the growth of Rhodes grass in pots

of Hanford loam soil. The soils were irrigated with the

waters under test, then allowed to dry to a soil-moisture

tension of about 700 to 800 cm. of water between irrigations. There were low- and high-leaching treatments.

The high-leaching treatment provided for the application of excess irrigation water so that about 25 percent

of the applied irrigation water was collected as percolate after each irrigation. The low-leaching regime

provided for the same proportion of leachate every

fourth irrigation. Exchangeable-sodium-percentages

were somewhat greater with the low-leaching treatments than with the high. Table 11 describes the irrigation waters that were tested and reports the results of

the analyses of soil samples from the low-leaching

treatments. The soil samples were collected after the

42d and 86th irrigations.

The use of two of the bicarbonate waters, 20b and

10b, gave rise to substantially higher ESP values than

the corresponding chloride waters, 20a and 10a (table

11). At the end of 42 irrigations, there was no appreciable difference between the 5a and 5b waters, but a

significant difference was found after the 86th irrigation. With the remaining waters, there appears to be

no difference between the chloride and bicarbonate

treatments.

Typical Waters

The analyses of a group of surface waters from

western United States that are typical of the waters that

are being used for irrigation purposes are presented in

table 12. The composition of a surface water may vary

considerably, but the analyses shown were selected to

represent average conditions.

Ground waters are much more variable in composition than surface waters. With few exceptions, it is not

possible to select ground waters that are typical of an

area or to generalize about the ground waters of a given

basin. Analyses of samples from a large number of

wells in the Coachella Valley in Riverside County, California, illustrate this point. Electrical conductivity

varies from 208 to 13,200 micromhos/cm ; boron from

a trace to 3.15 p. p. m., and soluble-sodium percentage

(SSP) from 21 to 97. Even where wells are only a

short distance apart or are pumping from different

strata, great variation is sometimes noted. Two wells

within a half mile of each other had conductivities of

13,200 and 604 micromhos/cm., and 3.03 and 0.38

p. p. m. boron, respectively. The first of these wells is

565 feet deep and the second 180 feet deep. The quality

of the water from different strata tapped by the same

well may vary, or the quality may change with length of

time of pumping. This change with time is usually

associated with overpumping but it does not often

occur. The quality of water from a new well should

be determined prior to its use for irrigation.

Classification of Irrigation Waters

In classification of irrigation waters, it is assumed

that the water will be used under average conditions
76 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

TABLE 11.—The composition of the irrigation umters used and the analyses of soil samples from a bicarbonate

experiment at the Laboratory; low-leaching noncalcareous treatments

Irrigat ion water Analyses ofsoil samp es collected after—

Composition Irrigation No. 42 Irrigation No. 86

Irrig. Concentration

Residual

water Na2COj

No.

Ca+^^ Na+ HCO- Cl- EC, X10«

at 25° C. pH. ES ESP EC. X 10»

at 25° C. pH, ES ESP

20a

20b

Meq./I.

20.0

20.0

Percent

25

25

Percent

75

75

Percent

0

50

Percent

100

50

Meq./l.

0

5.0

3.73

5.94

6.8

8.6

Meq.l

100 am,

L16

4.52

12

52

5.30

16.0

6.8

9.4

Meq.l

100 gm.

1.40

6.45

16

72

10a

10b

10.0

10.0

25

25

75

75

0

50

100

50

0

2.5

2.22

2.03

7.2

8.6

.80

L72

8.4

20

3.70

7.30

7.3

9.0

L40

3.80

15

42

5a ...

5b

5.0

5.0

25

25

75

75

0

50

100

50

0

1.25

L28

1.18

6.8

8.4

.84

L02

9.0

10

L78

2.42

6.7

7.7

L12

L98

11

20

la ...

lb

1.0

LO

25

25

75

75

0

50

100

50

0

.25

.40

.36

7.1

7.0

.22

.24

2.4

2.6

.32

.34

6.4

6.4

.25

.22

2.4

2.1

5aL....

5bL. ...

5.0

5.0

75

75

25

25

0

85

100

15

0

.50

L02

.69

6.9

8.1

.22

.29

2.2

3.1

L50

L05

6.1

7.4

.23

.36

2.2

3.5

laL....

IbL. ...

LO

LO

75

75

25

25

0

85

100

15

0

.10

.34

.33

7.0

7.2

.14

.14

L5

L4

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6.4

6.4

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.13

LI

L3

with respect to soil texture, infiltration rate, drainage,

quantity of water used, climate, and salt tolerance of

crop. Large deviations from the average for one or

more of these variables may make it unsafe to use what,

under average conditions, would be a good water; or

may make it safe to use what, under average conditions,

would be a water of doubtful quality. This relationship

to average conditions must be kept in mind in connection with the use of any general method for the classification of irrigation waters.

A diagram for classifying irrigation waters was suggested by Wilcox (1948), and this was subsequently

modified by Thorne and Thorne ( 1951 ) for the classification of the irrigation waters of Utah. Both diagrams

have been widely used. In the classification presented

below, certain features of both diagrams are used.

However, the SAR value rather than the soluble-sodium percentage is taken as the index of sodium status

or sodium hazard.

Salinity Hazard

Waters are divided into four classes with respect to

conductivity, the dividing points between classes being

at 250, 750, and 2,250 micromhos/cm. (See figure

25). These class limits were selected in accordance

with the relationship between the electrical conductivity

of irrigation waters and the electrical conductivity of

saturation extracts of soil as discussed previously in this

chapter under the heading. Electrical Conductivity.

The frequency distribution of the electrical conductivity

of three groups of irrigation waters with respect to

these four classes has been determined on the basis of

number of water sources and acreage irrigated. These

three groups of data were compiled from the following

sources :

Group 1. Data from Laboratory files for 1,142 irrigation water sources, both surface and ground

water.

Group 2. Data estimated from figure 1, page 10, of

Utah Agr. Expt. Sta. Bui. 346; Irrigation Waters

of Utah, by Thorne and Thorne (1951).

Group 3. All conveniently available data from projects irrigated with surface waters of known and

reasonably constant composition.

The frequency distribution of the first two groups is

with respect to the number of sources, while distribution of the third group is with respect to acres irrigated.

These data are presented in table 13. The frequencydistribution curves for the first two groups of waters are

shown in figure 24.

It is apparent that more than half of the waters considered in table 13 have conductivity values below 750

micromhos/cm., the lowest limit used in the earlier

schemes of classification. The establishment of a class

limit at 250 micromhos/cm. further divides this large

group. Considering the first group of data, 11 percent

of thé sources had conductivity values below 250

micromhos/cm. and are in the low-salinity class. The

waters of the medium-salinity class have conductivities

of 251 to 750 micromhos/cm. and comprise 47 percent

of the sources. The remaining 42 percent represent

irrigation waters of high or very high salinity. Distribution of waters in group 2 is similar to those in

group 1.

Sodium Hazard

The establishment of water-quality classes from the

standpoint of the sodium hazard is more complicated

than for the salinity hazard. The problem can be appreciched from the point of view of the probable ex-
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78 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

200

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U

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PERCENT IN EACH CONDUCTIVITY CLASS

47 33 9

NUMBERS 1142

MEDIAN s 613

MEAN s 934

250

Töüö ^ïm'^'-^^^o^

750 2250

CONDUCTIVITY — MICROMHOS/ CM.

DATA FROM LABORATORY FILES

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48

PERCENT IN EACH CONDUCTIVITY CLASS

33 12

NUMBER« 217

MEDIAN s 683

MEAN ~ 1051

250

3000

I

1000 2000 4000

750 2250

CONDUCTIVITY — MICROMHOS/CM.

DATA FROM THORNE AND THORNE (1951)

5000

FIGURE 24.—Frequency distribution of two groups of irrigation waters with respect to electrical conductivity.
SALINE AND ALKALI SOILS 79

TABLE 13.—Distribution of 3 groups of irrigation

uxiters among 4 concentration classes

Conductivity

ranse (micromnos/cm.

at 25** C.)

Group 1 Group 2 Group 3

Sam.

pies

Percent

Samples

Percent Acres Percent

<250

Number

124

541

378

99

11

47

33

9

Number

15

105

71

26

7

48

33

12

Thousands

453

977

671

22

21

251-750

751-2,250

2,251-5,000

46

32

1

Total 1,142 100 217 100 2,123 100

tent to which soil will adsorb sodium from the water

and the rate at which adsorption will occur as the water

is applied. Consider the simple case where a nonalkali

soil is leached continuously with a high-sodium irrigation water and an increase in concentration of the salts

in the solution is prevented by the absence of plant

growth and of surface evaporation. Under these conditions the ESP which the soil will attain when it and

the water are in equilibrium can be predicted approximately from the SAR value of the water; the rate at

which the equilibrium condition will be attained will

depend on the total cation concentration or electrical

conductivity of the water. Thus, for this situation, application of waters having the same sodium-adsorptionratio and variable electrical conductivities would ultimately result in about the same exchangeable-sodiumpercentages, but the amount of water required to bring

the soil to this ultimate exchangeable-sodium-percentage would vary inversely with the electrical conductivity. In actual practice, the SAR value of the

water increases in the soil, owing to the increase in concentration of all salts and the possible precipitation

of calcium and magnesium salts as the moisture content

is decreased by plant extraction and surface evaporation. This results in a somewhat higher ESP than

would be predicted directly from the SAR value of the

water (fig. 23). Although the SAR value is the best

available index of the equilibrium ESP of soil in relation to irrigation water, total cation concentration or

conductivity is an additional factor and is taken into

account in the following classification of sodium

hazard.

Diagram for Classifying Irrigation Waters

The diagram for the classification of irrigation waters

is shown in figure 25 and is based on the electrical conductivity in micromhos per centimeter and the sodiumadsorption-ratio.

In earlier diagrams curves representing mass-action

equations between soluble and exchangeable cations delimited the several sodium classes. The curves in figure

25 can be constructed by the use of the following

empirical equations:

Upper curve: 5= 43.75-8.87 (log C) ;

Middle curve: 5 = 31.31-6.66 log C) ;

Lower curve: 5=18.87-4.44 (logC) ;

Where 5= sodium-adsorption-ratio; C = electrical conductivity in micromhos per centimeter; log = logarithm to base 10.

These equations plot as straight lines on rectangular

coordinate paper when log C is used.

The curves are given a negative slope to take into

account the dependence of the sodium hazard on the

total concentration. Thus, a water with a SAR value

of 9 and a conductivity less than 168 is classed, so far

as sodium hazard is concerned, as an 51 water. With

the same SAR value and a conductivity from 168 to

2,250, it becomes an 52 water; with a conductivity

greater than 2,250, the water is rated 53. This system

by which waters at a constant SAR value are given a

higher sodium-hazard rating with an increase in total

concentrations is arbitrary and tentative, but it seems to

be supported by field and laboratory observations.

To use the diagram, the electrical conductivity and

the concentrations of sodium and calcium plus magnesium for the irrigation water are required. The determination of conductivity is described in Method 72;

sodium in Methods 80a and 80b; and calcium plus

magnesium in Method 79. If only the value for calcium

plus magnesium is known, sodium can be estimated as

follows :

Na^=(£CX10VlOO)-(Ca^^-hMg^^)

Conversely, if only sodium is known, calcium plus magnesium can be estimated by the equation:

(Ca^^ + Mg-) = (£CX 107100) -NaThe ionic concentrations are expressed in milliequivalents per liter. The sodium-adsorption-ratio may be

calculated from the equation defining the value or estimated from the nomogram of figure 22. Using the

SAR and the EC values as coordinates, locate the corresponding point on the diagram. The position of the

point determines the quality classification of the water.

This is illustrated by the analysis of the water of the

Sevier River at Delta, Utah (table 12), in which calcium

plus magnesium equals 10.04 meq./l.; sodium, 15.31

meq./l.; and electrical conductivity, 2,400 micromhos/cm. The 5^Ä value from the nomogram (fig.

22) is found to be 6.8. The point on the diagram corresponding to these coordinates (SAR = 6,S, EC X10^=

2,400) classifies the water as C4-52.

The significance and interpretation of the qualityclass ratings on the diagram are summarized below.

Conductivity

Lowr-SALINITY WATER (Cl) can be used for irrigation with most crops on most soils with little likelihood

that soil salinity will develop. Some leaching is required, but this occurs under normal irrigation practices except in soils of extremely low permeability.

MEDIUM-SALINITY WATER (C2) can be used if a

moderate amount of leaching occurs. Plants with
80 AGRICULTURE HANDBOOK 60, U. S., DEPT. OF AGRICULTURE

100 6 6 7 8 1000 4 5000

o

ce

<

N

<

a

o

CO

si 30 -

1 1 1 1 1 1 III 1 1 1

^\

28 -

CI-S4 —

X

o

X

lO

26

24

^

C2-S4

C3-S4

:

\

22

or

§20

0I-S3

C4-S4

s

3

5

IÜ

CM

I'"

ac

Q.

§ 14

(A

O

<

—

CI-S2

C2-S3

C3-S3

—

^

-

SODIUMC2-S2

C4-S3 -

8 — C3-S2 \^

o

-1

— 6

4

:

Gl-Sl

C2-SI

C4-S2

2 —

C3-SI

C4-SI

0 1 1 1 1 1 1 III 1 1 1

100

I I 1

250 750 22 50

CONDUCTIVITY - ÜICROMHOS/CM. (ECxlO^) AT 25' C.

1 2 3 4

\ LOW MEDIUM HIGH VERY HIGH

SALINITY HAZARD

FIGURE 25.—Diagram for the classification of irrigation waters.
SALINE AND ALKALI SOILS 81

moderate salt tolerance can be grown in most cases

without special practices for salinity control.

HiGH-SALiNiTY WATER (C3) cannot be used on soils

with restricted drainage. Even with adequate drainage, special management for salinity control may be

required and plants with good salt tolerance should be

selected.

VERY HIGH SALINITY WATER (C4) is not suitable for

irrigation under ordinary conditions, but may be used

occasionally under very special circumstances. The

soils must.be permeable, drainage must be adequate,

irrigation water must be applied in excess to provide

considerable leaching, and very salt-tolerant crops

should be selected.

Sodiufn

The classification of irrigation waters with respect to

SAR is based primarily on the effect of exchangeable

sodium on the physical condition of the soil. Sodiumsensitive plants may, however, suffer injury as a result

of sodium accumulation in plant tissues when exchangeable sodium values are lower than those effective in

causing deterioration of the physical condition of the

soil.

LOW-SODIUM WATER (SI) can be used for irrigation

on almost all soils with little danger of the development

of harmful levels of exchangeable sodium. However,

sodium-sensitive crops such as stone-fruit trees and avocados may accumulate injurious concentrations of

¿odium.

MEDIUM-SODIUM WATER (52) will presentan appreciable sodium hazard in fine-textured soils having high

cation-exchange-capacity, especially under low-leaching conditions, unless gypsum is present in the soil.

This water may be used on coarse-textured or organic

soils with good permeability.

HIGH-SODIUM WATER (.S3) may produce harmful

levels of exchangeable sodium in most soils and will

require special soil management—good drainage, high

leaching, and organic matter additions. Gypsiferous

soils may not develop harmful levels of exchangeable

sodium from such waters. Chemical amendments may

be required for replacement of exchangeable sodium,

except that amendments may not be feasible with waters

of very high salinity.

VERY HIGH SODIUM WATER (54) is generally unsatisfactory for irrigation purposes except at low and perhaps medium salinity, where the solution of calcium

from the soil or use of gypsum or other amendments

may make the use of these waters feasible.

Sometimes the irrigation water may dissolve sufficient calcium from calcareous soils to decrease the

sodium hazard appreciably, and this should be taken

into account in the use of Cl-53 and Cl-54 waters.

For calcareous soils with high pH values or for noncalcareous soils, the sodium status of waters in classes

Cl-53, Cl-54, and C2-54 may be improved by the

addition of gypsum to the water. Similarly, it may

be beneficial to add gypsum to the soil periodically

when C2-53 and C3-52 waters are used.

Effect of Boron Concentration on Quality

Boron is essential to the normal growth of all plants,

but the quantity required is very small. A deficiency

of boron produces striking symptoms in many plant

species. Boron is very toxic to certain plant species

and the concentration that will injure these sensitive

plants is often approximately that required for normal

gro%ith of very tolerant plants. For instance, lemons

show definite and, at times, economically important

injury when irrigated with water containing 1 p. p. m.

of boron, while alfalfa will make maximum growth with

1 to 2 p. p. m. of boron.

The occurrence of boron in toxic concentrations in

certain irrigation waters makes it necessary to consider

this element in assessing the water quality. Scofield

(1936) proposed the limits shown in table 14.

TABLE 14.—Permissible limits of boron for several

classes of irrigation uxUers

Boron clasB Sensitive crops

Semitolerant

crops

Tolerant

crops

1

2

P. p. m.

<0.33

0. 33 to . 67

. 67 to 1. 00

1. 00 to 1. 25

>1.25

P. p. m.

<0.67

0. 67 to 1. 33

1. 33 to 2. 00

2. 00 to 2. 50

>2.50

P. p. m.

<1.00

1. 00 to 2. 00

3 2. 00 to 3. 00

4 3. 00 to 3. 75

5 >3.75

The tolerance of crops to boron is discussed in

chapter 4 and a boron tolerance list is given in table

9. Boron frequently occurs in toxic concentrations

along with the other salts that are present in saline soils.

It can be leached from the soil but, if concentrations

are high initially, a quantity of boron sufficient to

cause trouble may remain after the concentration of

other salts is reduced to a safe level. The boron status

of saline soils should be determined as a part of a

salinity appraisal following Method 17.

Effect of Bicarbonate Ion Concentration on

Quality

On the basis of the data given in table 11 and using

the "residual sodium carbonate" concept of Eaton

(1950), it is concluded that waters with more than 2.5

meq./l. "residual sodium carbonate" are not suitable

for irrigation purposes. Waters containing 1.25 to 2.5

meq./l. are marginal, and those containing less than

1.25 meq./^. "residual sodium carbonate" are probably safe. It is believed that good management prac-
82 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

tices and proper use of amendments might make it possible to use successfully some of the marginal waters

for irrigation. These conclusions are based on limited

data and are, therefore, tentative.

In appraising the quality of an irrigation water, first

consideration should be given to salinity and alkali

hazards by reference to figure 25. Then consideration should be given to the independent characteristics,

boron or other toxic elements, and bicarbonate, any

one of which may change the quality rating. Recommendations as to the use of a water of a given quality

must take into account such factors as drainage and

management practices.
Chapter 6

Methods for Soil Characterization

Unless otherwise specified, all chemicals referred to

in this chapter, as well as in chapters 7 and 8, are

"reagent" grade and conform to standards established

by the American Chemical Society.

The following concentrated reagents are used.

Specific

Reagents: Percent Normality gravity

Acetic acid 99.5 18

Hydrochloric acid 35-38 12 1.19

Nitric acid 70 16 1.42

Sulfuric acid 95-96 36 1.84

Ammonium hydroxide 28 (NHa) 15 .90

Dilutions are indicated by (1+2), (1 +10), and other

proportions. The first figure indicates the volume of

concentrated reagent and the second the volume of

water.

Several methods involve centrifugation processes

that are specified in terms of time and relative centrifugal force (RCF), which is the ratio of the acceleration in the centrifuge to the acceleration of gravity, i. e.,

ÄCf=0.0000112XrX(r. p. m.)^ where r. p. m. is

centrifuge speed in revolutions per minute and r is the

radius in centimeters from the axis of the centrifuge to

the bottom of the centrifuge vessel when in the rotating

position.

Sampling, Soil Extracts, and Salinity

Appraisal

( 1 ) Soil Sample Collecting, Handling, and

Subsampling

A round-nose trenching spade is a convenient tool for

sampling surface soil. A soil tube is useful for small

subsurface samples, whereas a barrel-type auger can

be used when larger subsurface samples are required.

Canvas bags are generally used as containers for soil

samples, especially for samples of 100 to 200 pounds.

For small samples, metal boxes or cardboard cartons

can be used. Samples for salinity measurements require special handling, because at field-moisture content the salt in the soil is relatively mobile and moves

with the soil water. It has been found that kraft paper

nail bags are satisfactory for handling samples of

saline soil, providing the bags are first waterproofed by

soaking in a 5 or 10 percent solution of paraffin in

gasoline or other wax solvent.

Soil should be air-dried before shipping or storing

for any extended length of time. Air-dry soils that contain deliquescent salts may accumulate enough moisture during a short shipping or storage period to decompose a canvas bag. A container impervious to

water vapor should be used for such soils. Wax-treated

bags, as mentioned above, or various types of waterproofed bags used for merchandising foodstuff or other

hygroscopic material can be used. Samples in paper

bags will withstand usual transportation handling if

they are tightly packed in wooden boxes. To guard

against accidental confusion of samples, it is desirable

to place an identification tag inside the bag, in addition to using an external marking or tag.

The following recommendations will aid in determining the size of sample required :

Soil required

Measurements to be made : ** grams

1. Electrical conductivity of the saturation extract,

saturation percentage, and pH of soil paste— 250

2. Soluble ion analysis (semimicro methods) for—

Low salinity 500

High salinity 250

3. Exchangeable-cation analysis 15

4. Hydraulic conductivity (disturbed) 400

5. Gypsum and alkaline-earth carbonates 50

The total amount of soil to be obtained for the sample

can be determined by adding up the amounts indicated

for the individual tests to be made. If measurement

2 is to be made, then no extra soil will be required for

measurement 1. Samples twice as large as those indicated above are desirable, if handling facilities permit.

Care must be taken to obtain representative subsamples of a granular material such as soil. Bulk

samples at the Laboratory are air-dried before or after

passing through a screen with 6-mm. square openings,

are mixed, and are stored in galvanized iron containers.

An attempt is made to maintain a level surface of soil

in a container so that a minimum of segregation of

particles or aggregates occurs from rolling. A subsample of the main sample is taken by means of several partial loadings of a hand scoop from different

locations on the surface of the soil. The subsample

is then screened to the desired size. For exchangeablecation analysis and other determinations requiring

samples of about 5 gm. or less, the soil is ground to

pass a 0.5-mm. sieve. For a number of tests relating

to moisture retention and moisture transmission, the

soil is passed through a 2-mm. round-hole sieve with

the aid of a rubber stopper. One purpose of such sieving is to remove rocks larger than 2 mm.; another is to

reduce all aggregates to less than 2 mm. In the removal

of rocks between 2 mm. and 6 mm., they may be

83
84 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

returned to the screened sample if desired. The entire

subsample is then placed on a mixing cloth and pulled

in such a way as to produce mixing. Some pulling

operations will produce segregation instead of mixing,

and special care must be exercised to obtain a wellmixed sample. The soil sample is then flattened until

the pile is 2 to 4 cm. deep.

For moisture retentivity, hydraulic conductivity, and

modulus of rupture tests, 2 to 6 subsamples, each having a fairly definite volume, are required. Use paper

cups to hold the individual subsamples. Mark with

a pencil line around the inside of the cup the height to

which the cup is to be filled to give the correct amount

of subsample. Then, using a thin teaspoon or a small

scoop, lift small amounts of soil from the pile, placing

each in successive cups and progressing around the pile

until the cups are filled to the desired level. It is difficuh with some soils, especially if they have been passed

through a 2-mm. round-hole sieve, to take samples from

the pile without allowing the larger particles to roll off

the spoon or scoop. This rollback should be avoided

because it makes the extracted subsample nonrepresentative. The rollback problem is practically absent

from some soils, especially if all the sample has been

passed through an 0.5-mm. sieve.

Three data forms, or work sheets, used at the Laboratory are shown herewith. The field data sheet should

be at hand during sampling as an aid in recording

pertinent information. The other two forms serve as

work sheets for recording and calculating laboratory

determinations.

(2) Saturated Soil Paste

Apparatus

Container of 250-ml. capacity or greater, such as a

cup or moisture box.

Procedure

Prepare the saturated soil paste by adding distilled

water to a sample of soil while stirring with a spatula.

The soil-water mixture is consolidated from time to

time during the stirring process by tapping the container on the workbench. At saturation the soil paste

glistens as it reflects light, flows slightly when the container is tipped, and the paste slides freely and cleanly

off the spatula for all soils but those with a high clay

content. After mixing, the sample should be allowed

to stand for an hour or more, and then the criteria for

saturation should be rechecked. Free water should not

collect on the soil surface nor should the paste stiff"en

markedly or lose its glistening appearance on standing.

If the paste does stifl"en or lose its glisten, remix with

more water.

Because soils puddle most readily when worked at

moisture contents near field capacity, sufficient v/ater

should be added immediately to bring the sample nearly

to saturation, if the paste is too wet, additional dry

soil may be added.

The amount of soil required depends on the measurements to be made, i. e., on the volume of extract desired. A 250-gm. sample is convenient to handle and

provides sufficient extract for most purposes. Initially,

the sample can be air-dry or at the field-moisture content, but the mixing process is generally easier if the soil

is first air-dried and passed through a 2-mm. sieve.

If saturation pastes are to be made from a group of

samples of uniform texture, considerable time can be

saved by carefully determining the saturation percentage of a representative sample in the usual way. Subsequent samples can be brought to saturation by adding

appropriate volumes of water to known weights of soil.

Special precautions must be taken with peat and

muck soils and with soils of very fine and very coarse

texture.

PEAT AND MUCK SOILS.—Dry peat and muck soils,

especially if coarse or woody in texture, require an

overnight wetting period to obtain a definite endpoint

for the saturated paste. After the first wetting, pastes

of these soils usually stiff"en and lose the glisten on

standing. Adding water and remixing then gives a mixture that usually retains the characteristics of a saturated paste.

FINE-TEXTURED SOILS.—To minimize puddling and

thus obtain a more definite endpoint with fine-textured

soils, the water should be added to the soils with a minimum of stirring, especially in the earlier stages of

wetting.

COARSE-TEXTURED SOILS.—The saturated paste for

coarse-textured soils can be prepared in the same manner as for fine-textured soils; however, a diff'erent moisture content is recommended for the salinity appraisal

of such soils (Method 3b).

Method 27 gives procedures for determining the

moisture content of saturated paste, i. e., the saturation

percentage.

(3) Soil-Water Extracts

(3a) Saturation Extract

Apparatus

Richards or Buechner funnek, filter rack or flask,

filter paper, vacuum pump, extract containers such as

test tubes or 1-oz. bottles.

Procedure

Transfer the saturated soil paste, Method 2, to the

filter funnel with a filter paper in place and apply

vacuum. Collect the extract in a bottle or test tube.

Pyrex should not be used if boron is to be determined.

If the initial filtrate is turbid, it can be refiltered

through the soil or discarded. Vacuum extraction

should be terminated when air begins to pass through

the filter. If carbonate and bicarbonate determinations

are to be made on the extract, a solution containing

1,000 p. p. m. of sodium hexametaphosphate should be

added at the rate of one drop per 25 ml. of extract prior
SALINE AND ALKALI SOILS 85

UNITED STATES SALINITY LABORATORY

FIELD DATA FOR SOIL SAMPLES

Soil Ace. No.

Temporary No.

Sampled by

State

Mail address

County

Nearest

settlement

Site location )^i, K, Sec ; T -;R

Station or farm District or valley .

Date

Directions for finding site: (Use reverse side for a sketch of roads showing nearest settlement and distance from local landmarks.)

References (Soil Survey Bui., other publications, or correspondence):

Profile description (color, texture, structure, horizons, hardpan, origin, parent material, water table, drainage, and soil series if

known):

Topography Surface slope Percent topsoil erosion

Microrelief at the sampling site, furrow, ridge, etc. _

Disturbance from land preparation, leveling, filling, etc.

Sample: Depth No. sacks Approx. total wt. (lb.) __

Composite sample: Depth No, holes Sampling method and pattern.

Approx. total wt. (lb.)

Undisturbed structure sample: Depth No. of replicates __

Yrs. of cultivation Yrs. of irrigation Source of water _

Crop data (rotation, yield history, detailed description of plant condition at time of sampling):

Management practices:

(It is expected that not all the above blanks can be filled for every sample but the usefulness of laboratory determinations depends

on the completeness and accuracy of the field data.)
86 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Soil SÉunpls No. _• Description:

Moisture in air^dry Soil

Can No#

Air»dry Oven-dry

Gross

Tare _

Net

CD

AD

(Oven-dry basis)

pH of Saturated Soil Paste

Prom Water Added

Saturation Percentage

Can No«.

Air-dry soil» gm»—

Oven-dry soil, gm«-

HgO.ml.r )_

Can No«

By Drying

Wet Oven-dry

'7 at sat«

Oven-dry basis)

pH of Saturation Extract

pH of Suspension

Soil

Water,

Calcium plus Magnesium

(Versenate titration)

Electrical Conductivity

Saturation Extract

Gross

Tare _

Net

Pw at sat«

(Oven-dry basis)

Alkaline-earth Carbonates

T^C^

Millimhos/cm«

at 25^ C«

(Lime)

(Scale: low, medium, high]

Lime

Boron

?2r« .

ml,

.p*p«m« B

Ca+Mg, sat« ext«

meq«/l«

Ca+Mg, dry soil

meq«/lOO gm«

Sodium

Standard meq «/l

r ». . meq«/l#

Na, sat« ext«

meq«/l«

Na, dry soil

meq«/100 gm«

B

p«p«m«

Potassium

Standard meq «/l «

meq«/l«

K, sat « ext «

meq«/l«

K, dry soil

meq«/l00 gm.
SALINE AND ALKALI SOILS 87

Soil Sample No.^

10

11

12

13

Centrifuge tube nuTnber

Sample, air-dry weight gni<

Sample, oven-dry weight gn^«

CATION-EXCHAIÍGE'CAPACITY

(Saturated with NaAc; Na by flame photometer)

Extracting solution diluted to 5^1 •

Dilution: Solution 4

dilution ratio

Flame photometer standard Na meq./l»

Flame photometer reading

Sodium, fï'om graph meq«/l«

Cation-exohange-capaoity (OD basis) meq>/lOO gm.

EXCHANGEABLE SODIUM

Extracting solution (NH4^Ac) diluted to ml«

Dilution: Solution 10 dilution ratio

Flame photometer standard Na meq»/l

14

15

16

17

18

19

20

21

22

23

24

25

26

27

Flame photometer reading

Sodium» trom graph meq#/l<

Total sodium (.OD basis) meq#/lOO gm.

Sodium in sat« extract (OD basis) meq>/lOO gm«

Exchangeable sodium (OD basis) meq,/lOO gm»

Exclmngeable-sodium-percentage

EXCHANGEABLE POTASSIUM

Extracting solution (NH^Ac) diluted to ml

Dilution: Solution 19 dilution ratio

Flame photometer standard K meq#/l«

Fleime photometer reading

Potassium» from graph meq#/l

Total potassium (OD basis) meq«/lQO gm<

Potassium in sat, extract (OD basis) meq,/lOO gm

Exchangeable potassium (OD basis) meq«/lQO gm,

Exchangeable-potassium-percentage
88 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

to Stoppering and storing. This prevents the precipitation of calcium carbonate on standing.

For appraising soil salinity for most purposes, the

extraction can be made a few minutes after preparing

the saturated paste. If the soil contains gypsum, the

conductivity of the saturation extract can increase as

much as 1 or 2 mmhos/cm. upon standing. Therefore, if gypsum is present, allow the saturated paste to

stand several hours before extracting the solution.

If the solution is to be analyzed for its chemical

constituents, the saturated paste should stand 4 to 16

hours before extraction.

References

Richards (1949a), Reitemeier and Fireman (1944).

(36) TwiceSaturation Extract for CoarseTextured Soils (Tentative)

The following procedure gives a moisture content

that is approximately 8 times the 15-atmosphere percentage instead of 4 times, which is a usual factor for

the saturation percentage of finer textured soils. The

conductivity of the "twice-saturation" extract, therefore, is doubled before using the standard saturationextract scale for salinity evaluation.

Apparatus

Soil container of 10 to 12 cm. diam. (i. e., 1-lb. coffee can) with a loosely fitting basket formed from

galvanized screen with openings approximately 6 mm.

square.

Pipet, 2-ml. capacity. Other items are the same as

for Method 3a.

Procedure

Place the wire basket in the can, fill the basket with

soil to a depth of 2 or 3 cm. Level the soil and by use

of a pipet add 2 ml. of water dropwise to noncontiguous

spots on the soil surface, cover, and allow to stand for

15 min. Gently sift the dry soil through the wire basket

and weigh the moist pellets of soil retained thereon.

Calculate the moisture content of the pellets as follows:

Pw= (2X100)/(wet weight in grams —2)

Weigh 250 gm. of air-dry soil and add sufficient water

to make the moisture content up to 4 times the value

found in the pellets. Use a vacuum filter to obtain the

soil extract. For salinity appraisal of coarse-textured

soil from which this extract was obtained, determine the

electrical conductivity of the extract at 25° C. Multiply this conductivity value by 2 before using the standard saturation-extract salinity scale for interpretation

(chs. 2and4).

(3c) Soil-Water Extracts at 1:1 and 1:5

Apparatus

Filter funnels, fluted filter paper, and bottles for soil

suspensions and filtrates.

Procedure

Place a soil sample of convenient size in a bottle, add

the required amount of distilled water, stopper, and

agitate in a mechanical shaker for 15 min. Allow the

contents to stand at least an hour, agitate again for

5 min., and filter. If shaken by hand, invert and shake

bottle vigorously for 30 sec. at least 4 times at 30-min.

intervals before filtering.

At a 1: 1 soil-water ratio, it may be desirable to correct for hygroscopic moisture. Unless high precision

is required, this is done by grouping the air-dried and

screened soils roughly according to texture, and determining the percent moisture in 2 or 3 samples from each

textural group. It is then possible to weigh out soil

samples from the various groups and add sufficient

water to bring the samples to approximately 100 percent moisture by weight. For example, an air-dry soil

containing 3 percent moisture on an oven-dry basis can

be brought to a 1: 1 soil-water ratio by adding 97 ml.

water to 103 gm. of air-dried soil.

At a soil-water ratio of 1: 5 or greater, no allowance

is ordinarily made for moisture in the air-dried sample.

(3d) Soil Extract in the Field-Moisture

Range

A displacement method such as used by White and

Ross (1937) does not require complicated apparatus;

however, the pressure-membrane method described

here can be used for a wider range of soil textures and

a wider range of moisture contents.

Apparatus

Pressuré-membrane cell with a cylinder 5 or 10 cm.

high, tank of commercial water-pumped nitrogen, cans

with watertight lids, plain transparent cellophane No.

600. ^

Procedure

Prior to use, the sheets of No. 600 cellophane are

soaked in distilled water with daily changes of water in

order to reduce the electrolyte content of the membrane.

Electrical conductivity measurements on the water will

indicate when the bulk of these impurities has been removed. Since washed and dried membranes may be

somewhat brittle, they are stored wet until ready for

use. They should be partially dried before mounting in

the pressure-membrane apparatus.

The soil should be brought from the field at the

moisture condition desired for the extraction and immediately packed in the pressure-membrane apparatus.

If the soil has been air-dried, it may be passed through

a 6-mm. screen and wetted to the desired water content

with a fine spray of distilled water while tumbling in a

mixing can or on a waterproofed mixing cloth. This

wetted soil is stored in an airtight container, preferably

in a constant-temperature room for 2 weeks and is

mixed occasionally during this time. The pressuremembrane apparatus is then assembled, using No. 600
SALINE AND ALKALI SOILS 89

plain transparent cellophane for the membrane. The

soil is firmly packed by hand on the membrane in the

extraction chamber to a depth of 2 or 4 in., depending

upon the height of cylinder available. The chamber is

then closed and the extraction process started at 225

lb. per sq. in. (15 atm.) of nitrogen gas.

The extract should be collected in fractions of approximately equal volume. The first fraction is usually

discarded to avoid contamination from the membrane.

Electrical conductivity measurements can be made on

subsequent fractions to determine the degree of uniformity of the extract. The extraction process may

require 1 to 4 days.

References

Reitemeier (1946), Reitemeier and Richards (1944),

Richards (1947), and White and Ross (1937).

(4) Electrical Conductivity of Solutions

(4a) Standard Wheatstone Bridge

Remarks

Electrical conductivity is commonly used for indicating the total concentration of the ionized constituents

of solutions. It is closely related to the sum of the

cations (or anions) as determined chemically and

usually correlates closely with the total dissolved solids.

It is a rapid and reasonably precise determination that

does not alter or consume any of the sample.

Apparatus

Wheatstone bridge, alternating current, suitable for

conductivity measurements. This may be a 1,000-cycle

a. c. bridge with telephone receivers, a 60-cycle a. c.

bridge with an a. c. galvanometer, or one of the newer

bridges employing a cathode ray tube as the null

indicator.

Conductivity cell, either pipet or immersion type,

with platinized platinum electrodes. The cell constant

should be approximately 1.0 reciprocal centimeter.

New cells should be cleaned with chromic-sulfuric acid

cleaning solution, and the electrodes platinized before

use. Subsequently, they should be cleaned and replatinized whenever the readings become erratic or when an

inspection shows that any of the platinum black has

flaked off. The platinizing solution contains platinum

chloride, 1 gm., lead acetate, 0.012 gm., in 100 ml.

water. To platinize, immerse the electrodes in the

above solution and pass a current from a 1.5-volt dry

battery through the cell. The current should be such

that only a small quantity of gas is evolved, and the direction of current flow should be reversed occasionally.

A thermostat is required for precise measurements,

but for many purposes it is satisfactory to measure the

temperature of the solution and make appropriate

temperature corrections.

Reagents

Potassium chloride solution, 0.01 N. Dissolve 0.7456

gm. of dry potassium chloride in water and make to 1

liter at 25° C. This is the standard reference solution

and at 25° C. has an electrical conductivity of 1411.8

XIO« (0.0014118) mhos/cm.

Procedure

Fill the conductivity cell with the reagent, having

known conductivity EC20' Most cells carry a mark

indicating the level to which they should be filled or

immersed. Follow the manufacturers' instructions in

balancing the bridge. Read the cell resistance, R25 at

25° C. and calculate the cell constant (A;), from the

relation,

k=EC25^ R25

The cell constant will change if the platinization fails,

but it is determined mainly by the geometry of the

cell, and so is substantially independent of temperature.

Rinse the cell with the solution to be measured. The

adequacy of rinsing is indicated by the absence of

resistance change with successive rinsings. If only a

small amount of the sample is available, the cell may

be rinsed with acetone and ventilated until it is dry.

Record the resistance of the cell (/?t) and the temperature of the solution (t) at which the bridge is balanced. Keep the cell filled with distilled water when

not in use.

Calculations

The electrical conductivity (ECt) of the solution at

the temperature of measurement (t) is calculated from

the relation

ECt = k/Rt

where

k=EC25^ R25

For soil extracts and solutions, a temperature conversion factor (/t), obtained from table 15, can be

used for converting conductivity values to 25° C. Thus,

EC2,=ECtXf, =kft/Rt

References

Campbell and others (1948), National Research

Council International Critical Tables (1929).

(4b) Direct Indicating Bridge

Apparatus

Conductivity sets are available that have a bridge

scale and cell design features suggested by the Laboratory especially for use with saturation extracts (fig. 26).

This set is convenient to use and has sufficient accuracy

for diagnostic purposes. The conductivity cell supplied with this bridge has a constant of 0.5 cm."^ and a

capacity of 2 to 3 ml. of solution. With this cell the
90 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

TABLE 15.—Temperature factors (ft) for correcting resistance and conductivity data on soil extracts to the

standard temperature of 25° C.

EC25=ECt X/t;EC25=(k/Rt) Xft;R25=Rt//t

°c. °F. ft °C. °F. ft °C. °F. ft

3.0 37.4 1.709 22.0 71.6 1.064 29.0 84.2 0.925

4.0 39.2 L660 22.2 72.0 1.060 29.2 84.6 .921

5.0 4L0 1.613 22.4 72.3 L055 29.4 84.9 .918

6.0 42.8 1. 569 22.6 72.7 1.051 29.6 85.3 .914

7.0 44.6 L528 22.8 73.0 1.047 29.8 85.6 .911

8.0 46.4 1.488 23.0 73.4 1.043 30.0 86.0 .907

9.0 48.2 1.448 23.2 73.8 L038 30.2 86.4 .904

10.0 50.0 1.411 23.4 74.1 1.034 30.4 86.7 .901

n.o 5L8 1.375 23.6 74.5 1.029 30.6 87.1 .897

12.0 53.6 1.341 23.8 74.8 L025 30.8 87.4 .894

13.0 55.4 L309 24.0 75.2 L020 31.0 87.8 .890

14.0 57.2 1.277 24.2 75.6 1.016 31.2 88.2 .887

15.0 59.0 L247 24.4 75.9 1.012 31.4 88.5 .884

16.0 60.8 L218 24.6 76.3 1.008 3L6 88.9 .880

17.0 62.6 1.189 24.8 76.6 1.004 3L8 89.2 .877

18.0 64.4 1.163 25.0 77.0 1.000 32.0 89.6 .873

18.2 64.8 L157 25.2 77.4 .996 32.2 90.0 .870

18.4 65.1 1.152 25.4 77.7 .992 32.4 90.3 .867

18.6 65.5 1.147 25.6 78.1 .988 32.6 90.7 .864

18.8 65.8 1.142 25.8 78.5 .983 32.8 9L0 .861

19.0 66.2 1. 136 26.0 78.8 .979 33.0 91.4 .858

19.2 66.6 1.131 26.2 79.2 .975 34.0 93.2 .843

19.4 66.9 1.127 26.4 79.5 .971 35.0 95.0 .829

19.6 67.3 1.122 26.6 79.9 .967 36.0 96.8 .815

19.8 67.6 1.117 26.8 80.2 .964 37.0 98.6 .801

20.0 68.0 1.112 27.0 80.6 .960 38.0 100.2 .788

20.2 68.4 1.107 27.2 81.0 .956 39.0 102.2 .775

20.4 68.7 1.102 27.4 81.3 .953 40.0 104.0 .763

20.6 69.1 L097 27.6 81.7 .950 41.0 105.8 .750

20.8 69.4 1.092 27.8 82.0 .947 42.0 107.6 .739

21.0 69.8 1.087 28.0 82.4 .943 43.0 109. 4 .727

21.2 70.2 1. 082 28.2 82.8 .940 44.0 111.2 .716

21.4 70.5 1.078 28,4 83. 1 .936 45.0 113.0 .705

2L6 70.9 1.073 28.6 83.5 .932 46.0 114.8 .694

21.8 71.2 1.068 28.8 83.8 .929 47.0 116.6 .683

bridge scale reads directly from 0.15 to 15 mmhos/cm.

The bridge is operated by alternating current and makes

use of a cathode ray tube null indicator. When the

temperature of the solution is set on the temperaturecompensating dial, the main dial, at balance, indicates

electrical conductivity at 25^ C.

The accuracy of calibration of the bridge scale should

be checked with a saturated solution of calcium sulfate

dihydrate. With the temperature-compensation dial

correctly set, the bridge should read 2.2 mmhos/cm.

with this solution.

Procedure

Obtain the saturation extract in accordance with

Method 3a. Read the temperature of the extract. Rinse

and fill the conductivity cell. Set the temperature compensation dial. Close the contact switch on the cell

briefly while balancing the bridge with the main dial.

Read and record the electrical conductivity in millimihos per centimeter at 25° C.

If the bridge will not balance, the conductivity of the

extract may be below 0.15 or above 15 mmhos/cm. If

above, estimate conductivity by adding 9 parts of distilled water to 1 part of extract, by volume, and balancing the bridge with the diluted extract in the cell. The

conductivity of the undiluted extract will be approximately 10 times the conductivity reading obtained on

the diluted extract.

Alternatively, for concentrated extracts, a cell with a

constant higher than 0.5 may be used. If, for example,

the value of the cell constant is 5.0, then the scale reading of the bridge must be multiplied by 10.
SALINE AND ALKALI SOILS 91

FicURR 26.—Bridge and cell for measuring the conductivity of saturation extracts and irrigation waters.

(5) Resistance of Soil Paste and Percent

Salt in Soil

Apparatus

Bureau of Soils electrode cup, alternating current

Wheatstone bridge, and thermometer.

Procedure

Fill the electrode cup with saturated soil paste prepared in accordance with Method 2. Tap the soil cup

on the workbench to remove air bubbles and strike off

the soil paste level with the upper surface of the cup.

Measure the resistance and the temperature of the soil

paste in the cup. Use table 16 to convert the resistance

reading to the temperature of 60° F. Then, by means

of table 17, convert the paste resistance at 60° to approximate percent salt. Inasmuch as the saturation percentage varies with soil texture, it is necessary to estimate the textural class of the sample and to select the

appropriate column in the table for making the conversion from resistance to percent salt.

Reference»

Davis and Bryan (1910), Soil Survey Manual

(1951).

(6) Freezing-Point Depression

{6a) Freezing-Poirtt Depression of Solutions

Apparatus

Wheatstone bridge with approximately the following

characteristics: 1,000 ohms equal arm ratio, 10,000-

ohm decade balancing resistance adjustable to 1 ohm;

galvanometer : type E, Leeds and Northrup DM-2430-c,

or equivalent. IJse a 2-volt lead cell for the bridge

voltage supply. Thermistor: type 14B, Western Electric. Freezing bath: with either refrigerating coil or

salt-ice mixture. Freezing tube: test tube 1.5 cm. inside diameter X 15 cm. long with rubber stopper. Airjacket: test tube 2.9 cm. outside diameter X 20 cm.

long. Use cork bushings cut by means of a grinding
92 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

TABLE 16.—Bureau of Soils data for reducing soil paste resistance readings to values at 60^ F. (Whitney and

Means, 1897y

°F.

Ohms

1,000

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

74

76

78

80

82

84

86

88

90.

92.

94.

96.

98.

735

763

788

814

843

867

893

917

947

974

1,000

1,027

1,054

1,081

1,110

1,140

1,170

1,201

1,230

1,261

1,294

1,327

1,359

1,393

1,427

1,460

1, 495

1,532

1,570

1,611

2,000

1,470

1,526

1,576

1,628

1,686

1,734

1,786

1,834

1,894

1,948

2,000

2,054

2,108

2,162

2,220

2,280

2,340

2,402

2,460

2,522

2,598

2,654

2,718

2,786

2,854

2,920

2,990

3,064

3,140

3,222

3,000

2,205

2,289

2, 364

2,442

2,529

2,601

2,679

2,751

2,841

2,922

3,000

3,081

3,162

3,243

3,330

3,420

3,510

3,603

3,690

3,783

3,882

3,981

4,077

4,179

4,281

4,380

4,485

4,596

4,710

4, 833

4,000

2,940

3,052

3,152

3,256

3,372

3,468

3,572

3,668

3,780

3,896

4,000

4,108

4,216

4,324

4,440

4,560

4,680

4,804

4,920

5,044

5,176

5,308

5,436

5,572

5,708

5,840

5,980

6,128

6,280

6,444

5,000

3,675

3,815

3,940

4,070

4,215

4,335

4,465

4,585

4,735

4,870

5,000

5,135

5,270

5,405

5,550

5,700

5,850

6,005

6,150

6,305

6,470

6,635

6,795

6,965

7,135

7,300

7,475

7,660

7,850

8,055

6,000

4,410

4,578

4,728

4,884

5,058

5,202

5,358

5,502

5,682

5,844

6,000

6,162

6,324

6,486

6,660

6,840

7,020

7,206

7,380

7,566

7,764

7,962

8,154

8,358

8,562

8,760

8,970

9,192

9,420

9,666

7,000

5,145

5,341

5,516

5,698

5,901

6,069

6,251

6,419

6,629

6,818

7,000

7,189

7,378

7,567

7,770

7,980

8,190

8,407

8,610

8,827

9,058

9,289

9,513

9,751

9,989

10,220

10,465

10, 724

10,990

11,277

8,000

5,880

6,104

6,304

6,512

6,744

6,936

7,114

7,336

7,576

7,792

8,000

8,216

8,432

8,648

8,880

9,120

9,360

9,608

9,840

10,088

10,352

10,616

10,872

11,144

11,416

11,680

11,960

12,256

12,560

12,888

9,000

6,615

6,867

7,092

7,326

7,587

7,803

8,037

8,253

8,523

8,766

9,000

9,243

9,486

9,729

9,990

10,260

10,530

10, 809

11,070

11,349

11,646

11,943

12,2m

12,53X7

12, 842

13,140

13, 455

13,788

14,130

14, 499

* Example: Suppose the observed resistance is 2,568 ohms at 50° F. In the table at that temperature, we find that 2,000

ohnis is equal to 1,734 ohms at 60° F., 5,000 ohms is equal to 4,335 ohms at 60° F., hence 500 ohms would be equal to 434 ohms.

Similarly, 60 ohms would be one-hundredth of 6,000 ohms in the table and therefore equal to approximately 52 ohms at 60° F.,

while 8 ohms would be equal to about 7 ohms. These separate values are added together thus,

2,000 1, 734

500 434

60 52

8 7

2, 568 ohms at 50°=2,227 ohms at 60°

machine to center and suspend the freezing tubes in

the air-jackets. Mount the thermistor on a glass tube

with plastic spacers so as to hold the thermosensitive

bead at the center of a 5-ml. sample of the solution to

be frozen. Plot a resistance-temperature calibration

curve for the thermistor over the range from 1 to —5°

C, using a standard thermometer or other source of

reference temperature.

Procedure

Place 5-ml. samples of solutions in the freezing tubes

and mount the tubes in the air-jacket in the freezing

bath. An undercooling of approximately 2° C. has

been found convenient for soil extracts and plant saps.

Place the thermistor in one of the samples when the

sample has attained the bath temperature as indicated

by the bridge resistance reading. Induce freezing by

touching the solution with a metal probe cooled with

solid carbon dioxide. Follow the course of the freezing by keeping the bridge approximately balanced until the minimum resistance (maximum temperature)

is attained. With an undercooling of 2° C, a time of

about 2 min. is required to attain the maximum observed freezing temperature. The minimum resistance

value is recorded as the freezing resistance. The thermistor can then be transferred rapidly to the next

sample so that ice crystals carried over in the process

may initiate freezing. Include a tube of distilled
SALINE AND ALKALI SOILS 93

TABLE 17.—Bureau of Soils data for relating the

resistance of soil paste at 60^ F. fo percentage of

^^mixed neutral salts'^'' in soil (Davis and Bryan^

1910)

Resistance

at60°F.

(ohms)

18.

19.

20.

25.

30.

35.

40.

45.

50.

55.

60.

65.

70.

75.

80.

85.

90.

95.

100

105

110

115

120

125

130

135

140

145

150

155

160

165

170

Salts in—

Sand

Percent

3.00

2.40

2.20

1.50

1.24

1.04

.86

.75

.67

.60

.55

.51

.48

.45

.42

.39-

.37

.35

.33

.31

.30

.28

.27

.25

.24

.23

.22

t.21

^.21

.20

.20.

.19

.19

Loam Clay loam

Percent

3.00

2.64

2.42

1.70

1.34

1.14

.94

.78

.71

.64

.58

.54

.50

.47

.44

.42

.39

.37

.35

.33

.32

.29

.28

.26

.25

.24

.23

.22

.21

.21

.20

.20

.19

Percent

3.00

2.80

1.94

1.46

1.22

1.04

.88

.77

.69

.63

.57

.53

.50

.47

.44

.41

.39

.37

.35

.33

.31

.29

.28

.26

.25

.24

.23

.22

.21

.21

.20

.20

Clay

Percent

3.00

2.20

1.58

1.32

1.14

.98

.86

.77

.70

.63

.59

.55

.51

.48

.45

.42

.39

.37

.35

.33

.32

.30

.28

.27

.26

.25

.24

.23

.22

.21

.20

water with each batch of samples to provide a check on

the resistance thermometer.

Calculations

By means of the standard curve constructed for the

particular thermistor in use, convert the freezing resistance to degrees centigrade. Correct for undercooling, using the following relationship:

AT= ATo (l-0.0125i^)

where AT" is the corrected freezing-point depression,

AT'O is the observed freezing-point depression, and u

is the undercooling in degrees centigrade. A table

of factors for correction for undercooling is given by

Harris (1925). Calculate osmotic pressure from the

equation :

OP-12.06Ar-0.021AP

where OP is the osmotic pressure in atmospheres and

AT is the freezing-point depression in degrees centigrade. Harris and Gortner (1914) present a table

of osmotic pressures in atmospheres covering the range

of 0 to 2.999° C. freezing-point depression.

References

Richards and Campbell (1948,1949).

(66) Freezing-Point Depression of Water

in Soil Cores

Apparatus

Use the same resistance thermometer as in Method

6a, except the thermistor must be enclosed in a thinwalled metal tube sealed at the lower end and fastened

at the upper end to the glass mounting tube. The

calibration curve should be plotted for this thermistor

after mounting in the protective metal jacket.

Soil sampling tube to deliver soil cores 1.7 cm. in

diameter. Freezing tubes—glass test tube 2.0 cm. inside diameter (2.2 cm. outside diameter) X 17.0 cm.

long with rubber stoppers. Soil core holders of rigid

tubular material (hard rubber), 1.7 cm. inside diam.

(1.9 cm. outside diameter) X 5.1 cm. long. Covers

for soil core holders are disks of hard plastic material

(Lucite), 1.9 cm. diam. X 3 mm. thick. One-half of

the peripheral surface is turned to a smaller diameter

(approximately 1.7 cm.) to give a snug fit in the ends

of the soil core holders. A tapered hole large enough

to accommodate the jacketed thermistor is drilled in

one-half of the covers just described.

The Wheatstone bridge, galvanometer, freezing

bath, and air-jacket tubes are as described in Method

6a. It is convenient to construct wooden racks to hold

about 30 freezing tubes each.

Procedure

Soil cores are pushed from the sampling tube into

the soil core holders and cut to length. A solid disk

cover is placed on the bottom and a disk with a hole

is placed on the top of the soil core holder. The disks

are then pressed into position and are held there by the

shoulder machined for that purpose. The cores are

placed in the freezing tubes that are closed with rubber stoppers bearing the sample numbers. If the samples are to be stored for some time before freezing,

both ends of the core holder may be dipped into melted

parafiBn to prevent moisture loss.

Prior to freezing the sample, a hole is drilled in the

center of the soil core. The diameter of this hole

should be slightly smaller than the thermistor jacket.

The disturbance caused by insertion of the thermistor

in an undercooled sample will then initiate freezing.

The hole is drilled by hand with a twist drill mounted

in a plastic rod having a free fit in the freezing tube.

The freezing tubes containing the samples to be

frozen are centered and suspended in the air-jacket
94 AGRICULTURE HANDBOOK 60, U. S. DEFT. OF AGRICULTURE

tubes by means of a cork bushing. The thermistor is

inserted into a soil core when the freezing bath is

initially loaded so that the approach of the temperature of the cores to the bath temperature can be followed. The bath temperature should be held constant

±0.1° C. at approximately 1.5° below the expected

freezing points for the batch of cores. When the

samples attain the bath temperature, freezing of the

first core is induced by a twist of the thermistor. Succeeding samples usually start to freeze at the time the

thermistor is inserted into the sample. Frozen

samples can be replaced in the bath with unfrozen

samples, so that for a bath with capacity for 30 samples

there is no waiting for undercooling of samples after

the initial batch. An interval of about 1 hour is

usually sufficient for samples at room temperature to

come to bath temperature.

As with the solutions, the change in resistance (temperature) is followed by means of the galvanometer,

and the minimum resistance (maximum temperature)

recorded as the freezing resistance.

Calculations

The freezing resistance is converted to observed

freezing-point depression in degrees centigrade by

means of the calibration curve of the thermistor. No

convenient method seems to be available at present for

making an undercooling correction for water in soil.

There is experimental indication that the undercooling

correction is small for undercooling of 1.5° C. or less.

Freezing-point depression is related to the sum of the

tension (suction) and osmotic pressure of water in

soil. Calculate the total soil-moisture stress (SMS)

in atmospheres from the observed freezing-point depression (ATo) of water in soil cores by the relation,

SMS=l2ATo

References

Ayers and Campbell (1951), Campbell (1952),

Richards and Campbell (1949), and Schofield and

Bothelho da Costa (1938).

Soluble Cations and Anions

(7) Calcium and Magnesium by Titration

With Ethylenediaminetetraaeetate

(Versenate)

Reagents

A. Ammonium chloride-ammonium hydroxide buffer

solution. Dissolve 67.5 gm. of ammonium chloride

in 570 ml. of concentrated ammonium hydroxide and

make to 1 liter.

B. Sodium hydroxide, approximately 4 N. Dissolve 160 gm. of sodium hydroxide in 1 liter of water.

C. Standard calcium chloride solution, 0.01 N. Dissolve 0.500 gm. of pure calcium carbonate (calcite

crystals) in 10 ml. of approximately 3 N (1 + 3)

hydrochloric acid and dilute to a volume of exactly

1 liter.

D. Eriochrome black T indicator. Dissolve 0.5 gm.

of Eriochrome black T (F 241) and 4.5 gm. of

hydroxylamine hydrochloride in 100 ml. of 95 percent

ethanol. This indicator is available under several

different trade names.

E. Ammonium purpúrate indicator. Thoroughly

mix 0.5 gm. of ammonium purpúrate with 100 gm. of

powdered potassium sulfate.

F. Ethylenediaminetetraaeetate (Versenate) solution, approximately 0.01 N. Dissolve 2.00 gm. of

disodium dihydrogen ethylenediaminetetraaeetate and

0.05 gm. of magnesium chloride hexahydrate in water

and dilute to a volume of 1 liter. Standardize the

solution against reagent C, using the titration procedures given below. The solution is standardized,

using each of the indicators D and E, as the normality

with E is 3 to 5 percent higher than with D.

Procedure

PRêTREATMENT OF SOIL EXTRACTS.—Ammonium

acetate and dispersed organic matter, when present in

appreciable amounts, must be almost entirely removed

from soil extracts prior to titration with Versenate.

Evaporation of an aliquot of the soil extract to dryness

followed by treatment with aqua regia (3 parts cone,

hydrochloric acid -f 1 part cone, nitric acid), and a

second evaporation to dryness usually suffices for the

removal of ammonium acetate and organic matter.

Very dark colored soil extracts may require additional

treatment with aqua regia. Dissolve the residue in a

quantity of water equal to the original volume of the

aliquot taken for treatment.

CALCIUM.—Pipet a 5- to 25-ml. aliquot containing

not more than 0.1 meq. of calcium into a 3- or 4-inch

diameter porcelain casserole. Dilute to a- volume of

approximately 25 ml. Add 0.25 ml. (5 drops) of

reagent B and approximately 50 mg. of E. Titrate with

F, using a 10-ml. microburet. The color change is

from orange red to lavender or purple. When close

to the end point, F should be added at the rate of about

a drop every 5 to 10 seconds, as the color change is not

instantaneous. A blank containing B, E, and a drop

or two of F aids in distinguishing the end point. If the

sample is overtitrated with F, it may be back-titrated

with C.

CALCIUM PLUS MAGNESIUM.—Pipet a 5- to 25-ml.

aliquot containing not more than 0.1 meq. of calcium

plus magnesium into a 125-ml. Erlenmeyer flask.

Dilute to a volume of approximately 25 ml. Add 0.5

ml. (10 drops) of reagent A and 3 or 4 drops of D.

Titrate with F, using a 10-ml. microburet. The color

change is from wine red to blue or green. No tinge of

the wine-red color should remain at the end point.

Calculations

Milliequivalents per liter of Ca or Ca4-Mg= (ml. of

Versenate solution used X normality of Versenate solution as determined by appropriate indicator X 1,000)/

(ml. in aliquot).
SALINE AND ALKALI SOILS 95

Remarks

Iron, aluminum, and manganese, when present in

concentrations greater than 20 p. p. m., and copper,

when present in concentrations greater than several

tenths of a p. p. m., interfere with the performance of

the Eriochrome black T indicator. Usually the concentrations of these metals in water and ammonium

acetate extracts of soils of arid regions are insufficient

to cause interference. If interference is encountered, it

may be overcome as described by Cheng and Bray

(1951).

References

Cheng and Bray (1951), Diehl and coworkers

(1950).

(8) Calcium by Precipitation as Calcium

Oxalate

Apparatus

Centrifuge and 12-ml. conical tubes.

Reagents

(Keep reagents B, C, D, and E in Pyrex bottles.)

A. Methyl orange, 0.01 percent in water.

B. Hydrochloric acid, approximately 6 N (1 + 1).

C. Oxalic acid, approximately 0.2 N. Dissolve 12.6

gm. of oxalic acid dihydrate in water and make to 1

liter.

D. Ammonium hydroxide, approximately 7 N

(1+1).

E. Ammonium hydroxide in ethanol and ether. Mix

20 ml. of cone, ammonium hydroxide with 980 ml. of a

mixture of equal volumes of ethanol, ether, and water.

F. Perchloric acid, 4 N. Dilute 340 ml. of 70 percent

perchloric acid or 430 ml. of 60 percent perchloric acid

to 1 liter.

G. Nitro-ferroin indicator (5-nitro-l,10-phenanthroline ferrous sulfate solution, 0.001 M).

H. Ammonium hexanitrate cerate, 0.01 N in perchloric acid, 1 N. Dissolve 5.76 gm. of ammonium

hexanitrate cerate in 250 ml. of 4 N perchloric acid and

dilute to 1 liter. The reagent should be standardized

in the following manner: Pipet 5 or 10 ml. of fresh

standard 0.01 N sodium oxalate into a small beaker

containing 5 ml. of reagent F, add 0.2 ml. of G, and

titrate with the cerate solution to the pale-blue end

point. Determine a blank titration correction on a

similar sample minus the oxalate solution. The milliliters of oxalate used multiplied by 0.01 and divided

by the corrected milliliters of cerate provide the normality of the cerate. Do riot attempt to adjust the solution to exactly 0.01 N. Restandardize each time the

reagent is used if more than 2 days have elapsed since

the last standardization. Keep in a dark bottle away

from light.

Procedure

Pipet an aliquot containing 0.005 to 0.08 meq. of

calcium into a 12-ml. conical centrifuge tube, dilute or

evaporate ^^ to 5 ml., and add 1 drop of reagent A, 2

drops of B, and 1 ml. of C. Heat to the boiling point

in a water bath. While twirling the tube, add D dropwise until the solution just turns yellow. Replace in

the bath, and, after 30 min., cool the tube in air or in

water. If necessary, add more D to keep the solution

just yellow.

Centrifuge at RCF= 1,000 for 10 min. Carefully decant the supernatant liquid into another 12-ml. conical

centrifuge tube and save for the magnesium determination. Stir the precipitate and rinse the sides of the

tube with a stream of 5 ml. of reagent E blown from a

pipet. Centrifuge at ÄCF= 1,000 for 10 min. Decant

and drain the tube by inversion on filter paper for 10

min. Wipe the mouth of the tube with a clean towel

or lintless filter paper.

Blow into the tube 3 ml. of reagent F from a pipet.

When the precipitate is dissolved, add X).l ml. of G.

Titrate with H from a 10-ml. microburet to the paleblue end point. If more than 5 ml. of H is required,

transfer the sample to a small beaker and complete

the titration. Determine the blank correction in the

same manner; it is usually about 0.03 ml.

Calculations

Milliequivalents per liter of Ca= (corrected ml. of

cerate solutionX normality of cerateX 1,000)/(ml. in

aliquot).

Reference

Reitemeier (1943).

( 9) Magnesium by Precipitation as Magnesium Ammonium Phosphate

Apparatus

Centrifuge, 12-ml. conical tubes, and photoelectric

colorimeter.

Reagents

A. Ammonium chloride, 3 percent solution. Dissolve 3 gm. of ammonium chloride in water and dilute

to 100 ml. Filter before use.

B. Ammonium dihydrogen phosphate, 5 percent solution. Dissolve 5 gm. of ammonium dihydrogen phosphate in water and dilute to 100 ml. Filter before use.

C. Phenolphthalein, 1 percent in 60 percent ethanol.

" Evaporation operations carried on with centrifuge tubes in

a water bath may be speeded up by the use of an áir blower.

For this, a bank of glass nozzle-tubes in an array to match

positions in the centrifuge tube rack is supplied with air from

a compressed air system. A stream of air is thus introduced

into each drying tube.
96 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

D. Ammonium hydroxide, cone.

E. Ammonium hydroxide in ethanol and ether. Mix

20 ml. of cone, ammonium hydroxide with 980 ml. of

a mixture of equal volumes of ethanol, ether, and water.

F. Magnesium sulfate solution, approximately 0.01

N, standardized. This is best prepared by dilution of a

more concentrated solution of magnesium sulfate that

has been standardized by gravimetric determination of

magnesium (Method 78).

G. Sulfuric acid. Approximately 5 N (1 + 6).

H. Ammonium vanadate, 0.25 percent solution.

Dissolve 2.5 gm. of ammonium vanadate in 500 ml. of

boiling water, cool somewhat, and then add 60 ml. of

reagent G. Cool to room temperature and dilute to 1

liter. Store in a brown bottle.

I. Ammonium molybdate, 5 percent solution. Dissolve 50 gm. of ammonium molybdate in 1 liter of water.

Store in a brown bottle.

Procedure

To the 12-ml. conical centrifuge tube containing the

calcium-free sample from Method 8, add 1 ml. each of

reagents A and B and 1 drop of C. Heat to 90° C. in

a water bath and then add D until permanently pink.

After 15 min., add an additional 2 ml. of D. Stopper

and let stand overnight.

Centrifuge ai RCF=lfiOO for 10 min., decant carefully, drain on filter paper for 10 min., and wipe the

mouth of the tube with a clean towel or lintless filter

paper. Wash the precipitate and sides of the tube with

a stream of 5 ml. of reagent E from a pipet equipped

with a rubber bulb or by a similar arrangement. Centrifuge at RCF=lfiOO for 5 min., decant, drain for 5

min., and wipe the mouth of the tube. Repeat this

washing procedure once.

Pipet 10 ml. of reagent G into the tube and twirl for

a few seconds. After 5 min. wash the contents into a

100-ml. volumetric flask. Dilute to about 60 ml. and

pipet 10 ml. each of H and I into the flask while twirling rapidly. Dilute to the mark and mix. After 10

min. measure the difference in light transmission of the

sample and water, using optical cells and a 460-m/x

filter.

Starting at the beginning of the Procedure above,

prepare a photometer calibration curve on semilogarithmic graph paper, for 0, 0.5, 1, 2, 3, 4, and 5 ml. of

reagent F. One ml. of 0.2 N oxalic acid should be added

to each tube of standard before precipitating the magnesium. The amount of magnesium in the aliquot is

obtained by simple interpolation on the curve.

Calculations

Milliequivalents per liter of Mg= (meq. of Mg found

by interpolation X 1,000)/(ml. in Ca aliquotX0.98).

The factor of 0.98 corrects for magnesium lost in the

washings from the calcium precipitate.

References

Kitson and Mellon (1944), Reitemeier (1943).

(10) Sodium

(10a) Sodium by Flame Photometer

Apparatus

Perkin-Elmer model 52 flame photometer with acetylene or propane burner.

Reagents

A. Ammonium acetate, approximately 1 N. To 700

or 800 ml. of water add 57 ml. of cone, acetic acid and

them 68 ml. of cone, ammonium hydroxide. Dilute to

a volume of 1 liter and adjust to pH 7.0 by the addition

of more ammonium hydroxide or acetic acid.

B. Sodium chloride, 0.04 N. Dissolve 2.338 gm.

of dry sodium chloride in water and dilute to exactly

1 liter.

C. Sodium chloride, 0.04 N in 1 N ammonium acetate. Dissolve 2.338 gm. of dry sodium chloride in reagent A. Dilute to exactly 1 liter with additional A.

D. Lithium chloride, 0.05 N. Dissolve 2.12 gm. of

dry lithium chloride in water and dilute to exactly

1 liter.

Procedure

Using reagents B and D prepare a series of standard

sodium chloride solutions, each containing the same

concentration of lithium chloride. Prepare a similar

series of standard sodium chloride solutions, using

reagents C and D, and use A for dilution. Recommended concentrations of sodium chloride are 0, 0.2,

0.4, 0.6, 0.8, 1, 2, 3, and 4 meq./l. The optimum concentration of lithium chloride varies with individual

flame photometers but is usually 5 to 10 meq./l.

Standard solutions made up with water are employed

for the analysis of waters and water extracts of soils;

whereas, standard solutions made up in ammonium

acetate solutions are used for the analysis of ammonium

acetate extracts of soils. Calibrate the flame photometer for operation over the concentration range 0 to 1

meq./l. of sodium, using the first 6 standard solutions

of the appropriate series. Use the first and the last 4

solutions of the appropriate series to calibrate the instrument for operation over the concentration range

0 to 4 meq./l. of sodium.

Pipet an aliquot of the solution to be analyzed, containing less than 0.2 meq. of sodium, into a 50-ml.

volumetric flask. Add an amount of reagent D that,

when diluted to a volume of 50 ml., will give a concentration of lithium chloride exactly equal to that in the

standard sodium chloride solutions. Dilute to volume

with water, or with A, if ammonium acetate extracts

are being analyzed. Mix and determine the sodium

concentration by use of the flame photometer and the

appropriate calibration curve.

Calculations

Milliequivalents per liter of Na in water or extract =

(meq./l. of Na as found by interpolation on calibration curveX50)/(ml. in aliquot).
SALINE AND ALKALI SOILS 97

(10b) Sodium by Precipitation as Sodium

Uranyl Zinc Acetate

Apparatus

Centrifuge and 12-ml. conical tubes.

Reagents

A. Uranyl zinc acetate. Weigh 300 gm. of uranium

acetate dihydrate, 900 gm. of zinc acetate dihydrate,

and 10 mg. of sodium chloride into a large flask. Add

82 ml. of glacial acetic acid and 2,618 ml. of water.

Stir or shake until the salts are dissolved, leaving only

a small amount of sodium uranyl zinc acetate precipitate. Filter before use.

B. Acetic acid-ethanol. Mix 150 ml. of glacial acetic

acid with 850 ml. of 95 percent ethanol. Shake with

an excess of sodium uranyl zinc acetate crystals. Filter

before use. Sodium uranyl zinc acetate crystals may

be prepared as follows: Add 125 ml. of reagent A to

5 ml. of 2 percent sodium chloride solution, stir, and

after 15 min. collect the precipitate in a porous-bottomed porcelain crucible. Wash several times with

glacial acetic acid, then several times with ether, and

finally dry in a desiccator.

C. Ether, anhydrous.

Procedure

Pipet an aliquot containing 0.003 to 0.07 meq. of

sodium into a 12-ml. conical centrifuge tube. Evaporate on a water bath to 0.5 ml. Cool, add 8 ml. of

reagent A, and mix by stirring with an aluminum wire

bent into a loop. Let stand 1 hour. Centrifuge at

RCF=lfiOO for 10 min. Decant and drain on filter

paper for 10 min. Wipe the mouth of the tube with a

clean towel or lintless filter paper. Suspend the precipitate and wash the sides of the tube, using 5 ml. of

B blown from a pipet equipped with a rubber bulb.

Centrifuge for 10 min., decant, and drain for 1 min.

Wipe the mouth of the tube. Wash with 5 ml. of C,

but centrifuge for only 5 min. Decant carefully without draining. Repeat washing and centrifuging once.

Clean the outside of tube with chamois, dry for an

hour or more at 60° C, cool in a desiccator, and weigh.

Add 10 ml. of water, stir with the wire until the sodium

precipitate is dissolved, centrifuge for 5 min., decant

carefully, and drain for 5 min. on filter paper. Suspend

the insoluble precipitate and wash the sides of the tube

with 5 ml. of B blown from a pipet. Centrifuge for 5

min., and decant. Wash with 5 ml. of C, centrifuge for

5 min., clean tube with chamois, dry for an hour at 60°,

cool in a desiccator, and weigh. The difference between

the two weights is the weight of the sodium precipitate.

Calculations

Milliequivalents per liter of Na= igm. of Na precipitate X 650.2)/(ml. in aliquot).

Reference

Reitemeier (1943).

(11) Potassium

(11a) Potassium by Flame Photometer

Apparatus

Perkin-Elmer model 52 flame photometer with acetylene or propane burner.

Reagents

A. Ammonium acetate, approximately 1 N. To 700

or 800 ml. of water add 57 ml. of cone, acetic acid and

then 68 ml. of cone, ammonium hydroxide. Dilute to

a volume of 1 liter and adjust to pH 7.0 by the addition

of more ammonium hydroxide or acetic acid.

B. Potassium chloride, 0.02 N. Dissolve 1.491 gm.

of dry potassium chloride in water and dilute to a

volume of exactly 1 liter.

C. Potassium chloride, 0.02 N in 1 N ammonium

acetate. Dissolve 1.491 gm. of dry potassium chloride

in reagent A. Dilute to a volume of exactly 1 liter

with additional A.

D. Lithium chloride, 0.05 N. Dissolve 2.12 gm. of

dry lithium chloride in water and dilute to 1 liter.

Procedure

Using reagents B and D, prepare a series of standard potassium chloride solutions, each containing the

same concentration of lithium chloride. Prepare a

similar series of standard potassium solutions using

reagents C and D, and use A for dilution. The concentrations of potassium chloride are 0, 0.1, 0.2, 0.3, 0.4,

0.5, 1, 1.5, and 2 meq./l. The optimum concentration

of lithium chloride varies with individual flame photometers but is usually 5 to 10 meq./l. Standard solutions made up in water are employed for the analysis

of waters and water extracts of soils; whereas, those

made up in ammonium acetate solution are used for the

analysis of ammonium acetate extracts of soils. Calibrate the flame photometer for operation over the concentration range 0 to 0.5 meq./l. of potassium, using

the first 6 standard solutions of the appropriate series.

Use the first and the last 4 solutions of the appropriate

series to calibrate the instrument for operation over the

concentration range 0 to 2 meq./l. of potassium.

Pipet an aliquot of the solution to be analyzed containing less than 0.1 meq. of potassium into a 50-ml.

volumetric flask. Add an amount of reagent D which,

when diluted to a volume of 50 ml., will give a concentration of lithium chloride exactly equal to that in the

standard potassium chloride solutions. Dilute to

volume with water or with A, if ammonium acetate extracts are being analyzed, mix, and determine the potassium concentration by use of the flame photometer and

the appropriate calibration curve.

Calculations

Milliequivalents per liter of K in water or extract=

(meg./, of K as found by interpolation on calibration

curveX50)/(ml. in aliquot).
98 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

(lib) Potassium by Precipitation as Potassium Dipicrylaminate

Apparatus

Photoelectric colorimeter, centrifuge, and 12-ml.

conical tubes.

Reagents

A. Lithium dipicrylaminate solution. Dissolve 1.65

gm. of lithium carbonate in 250 ml. of water. Warm

to 50° C. and then add 9 gm. of dipicrylamine. After

the dipicrylamine has dissolved, filter and dilute 200 ml.

of this solution to 1 liter. To the remaining portion of

approximately 50 ml., add 0.25 gm. of potassium

chloride. Separate and wash the resulting potassium

dipicrylaminate precipitate with a few milliliters of

water by means of a centrifuge. Add the potassium salt

to the warm solution of lithium dipicrylaminate and

shake for 30 min. Filter the solution before use.

B. Potassium chloride, 0.010 N. Dissolve 0.7456

gm. of dry potassium chloride in water and dilute to

exactly 1 liter.

C. Phenolphthalein, 1 percent in 60 percent ethanol.

D. Sodium hydroxide, approximately 1 N. Dissolve

40 gm. of sodium hydroxide in water and dilute to 1

liter.

Procedure

Pipet an aliquot containing 0.005-0.035 meq. of

potassium into a 12-ml. conical centrifuge tube. Add 1

drop of reagent C and then D until pink. Evaporate to

dryness. This insures removal of ammonium. Cool

and then add exactly 2 ml. of A. Grind the salt residue

in the bottom of the tube by means of a glass rod and

allow 1 hour for precipitation. Centrifuge the tube at

RCF= 1,000 for 1 min. Remove a 0.2-ml. aliquot from

the supernatant liquid by means of a blood pipet and

dilute to a volume of 50 ml. Compare the light transmission in an optical cell through a 510-m/x filter with

that of water in similar cell. Prepare a calibration curve

for each set of samples by carrying a series of 0.5, 1,

1.5, 2, 2.5, 3, 3.5 ml. of B through the same operations.

The amount of potassium in the sample is found by

interpolation on this curve. When plotted on a linear

scale the curve should be slightly S-shaped. The temperature at which the calibration curve is prepared

should be within 2° C. of that at which the unknown

determinations are made.

Calculations

Milliequivalents per liter of K=(meq. of K in aliquot as found by interpolation X 1,000)/(ml. in

aliquot).

Reference

Williams (1941).

( 12) Carbonate and Bicarbonate by Titration With Acid

Reagents

A. Phenolphthalein, 1 percent in 60 percent ethanol.

B. Methyl orange, 0.01 percent in water.

C. Sulfuric acid, approximately 0.010 N, standardized.

Procedure

Pipet an aliquot containing 0.005 to 0.04 meq. of

chloride into a 15-ml. wide-mouthed porcelain crucible

or a small porcelain casserole. Chloride is specified

here because the same sample is subsequently used for

the chloride determination in Method 13. Add 1 drop

of reagent A. If the solution turns pink, add C from a

10-ml. microburet dropwise at 5-second intervals until

the color just disappears. Designate this buret reading

as y. Add 2 drops of B and titrate to the first orange

color. Designate the new buret reading as z. Save the

titrated sample for the chloride determination.

An indicator correction blank using boiled water

should be determined and applied if it is not negligible.

The lighting should be adequate for the recognition of

the various colors. The use of comparison color standards at the correct end points is helpful.

Calculations

1. Milliequivalents per liter of C03=(2yXnormality of H0SO4X 1,000)/(ml. in aliquot).

2. Milliequivalents per liter of HC03= (z-2y) X normality of H2SO4X 1,000/(ml. in aliquot).

Reference

Reitemeier (1943).

(13) Cliloride by Titration With Silver

Nitrate

Reagents

A. Potassium chromate, 5 percent solution. Dissolve 5 gm. of potassium chromate in 50 ml. of water

and add 1 N silver nitrate dropwise until a slight permanent red precipitate is produced. Filter and dilute

to 100 ml.

B. Silver nitrate, 0.005 N. Dissolve 0.8495 gm. of

silver nitrate in water and dilute to exactly 1 liter.

Keep in a brown bottle away from light.

Procedure

To the sample preserved from the carbonate-bicarbonate determination, add 4 drops of reagent A. While

stirring, titrate under a bright light with B from a 10-

ml. microburet to the first permanent reddish-brown

color. The titration blank correction varies with the

volume of the sample at the end point, and usually increases regularly from about 0.03 to 0.20 ml. as the

volume increases from 2 to 12 ml.
SALINE AND ALKALI SOILS 99

Calculations

Milliequivalents per liter of Cl = (ml. of AgNOg- ml.

of AgNOg for blank) X0.005X 1,000/(ml. in aliquot).

Reference

Reitemeier (1943).

(14) Sulfate

(14a) Sulfate by Precipitation as Barium

Sulfate

Apparatus

Centrifuge and 12-ml. conical tubes.

Reagents

A. Methyl orange, 0.01 percent in water.

B. Hydrochloric acid, approximately 1N.

C. Barium chloride, approximately 1 N. Dissolve

122 gm. of barium chloride dihydrate in water and

dilute to 1 liter.

D. Ethanol, 50 percent by volume.

Procedure

Pipet an aliquot containing 0.05 to 0.5 meq. of

sulfate into a clean 12-ml. conical centrifuge tube of

known weight. Dilute or evaporate to about 5 ml. Add

2 drops of reagent A, then B dropwise until pink, and

then 1 ml. of B in excess. Heat to boiling in water bath.

While twirling the tube add 1 ml. of C dropwise. Return to the hot water bath for 30 min. and then cool at

least an hour in air.

Centrifuge at ÄCF= 1,000 for 5 min. Carefully

decant and let drain by inversion on filter paper for 10

min. Wipe the mouth of the tube with a clean towel

or lintless filter paper.

Stir the precipitate and rinse the sides of the tube

with a stream of 5 ml. of reagent D blown from a pipet.

H necessary, loosen precipitate from bottom of tube by

means of a wire bent in appropriate shape. Centrifuge

for 5 min. and decant, but do not drain. Repeat this

washing and decanting operation once. Wipe the outside of tube carefully with chamois and do not subsequently touch with fingers. Dry overnight in an oven

at 105° C. Cool in a desiccator and weigh.

Calculations

Milliequivalents per liter of S04= (mg. of BaS04

precipitateX8.568)/(ml. in aliquot).

(Note. Care must be taken in the preparation or

concentration of the unknowns so as not to precipitate

foreign material which might be weighed as barium

sulfate. )

(14b) Sulfate by Precipitation as Calcium

Sulfate

Apparatus

Wheatstone bridge, conductivity cell, centrifuge, and

50-ml. conical tubes.

Reagents

A. Acetone.

B. Calcium chloride, approximately 1 N. Dissolve

74 gm. of calcium chloride dihydrate in water and dilute

to 1 liter.

Procedure

Pipet an aliquot containing 0.05 to 0.5 meq. of

sulfate into a 50-ml. conical centrifuge tube. Dilute or

concentrate to a volume of 20 ml. Add 1 ml. of reagent

B and 20 ml. of A. Mix the contents of the tube and

let stand until the precipitate flocculates. This usually

requires 5 to 10 min. Centrifuge at jRCF= 1,000 for

3 min., decant the supernatant liquid, invert the tube,

and drain on filter paper for 5 min. Disperse the

precipitate and rinse the wall of the tube with a stream

of 10 ml. of A blown from a pipet. Again centrifuge

at ÄCF= 1,000 for 3 min., decant the supernatant

liquid, invert the tube, and drain on filter paper for 5

min. Add exactly 40 ml. of distilled water to the tube,

stopper, and shake until the precipitate is completely

dissolved. Measure the electrical conductivity of the

solution, using Method 4b, and correct the conductivity

reading to 25° C. Determine the concentration of

CaS04 in the solution by reference to a graph showing

the relationship between the concentration and the

electrical conductivity of CaS04 solutions. This graph

may be constructed by means of the following data

from the International Critical Tables.

Electrical

conductivity

at25'*C.

CaS04 concentration (meq./l.): Mmhos/cm.

1 0.121

2 -—-. .226

5 . 500

10 .900

20 1.584

30. 5 2.205

Calculations

Milliequivalents per liter of S04= (meq./l. of CaS04

from electrical conductivity reading)|-i-(ml. in aliquot/

ml. of water used to dissolve precipitate).

Reference

Bower and Huss (1948).
100 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

(15) Nitrate by Phenoldisulfonic Acid

Apparatus

Photoelectric colorimeter.

Reagents

A. Phenoldisulfonic acid. Dissolve 25 gm. of phenol in 150 ml. of cone, sulfuric acid, add 75 ml. of fuming sulfuric acid (13 to 15 percent SO3), and, heat at

100° C. for 2 hours.

B. Potassium nitrate, 0.010 N. Dissolve 1.011 gm.

of dry potassium nitrate in water and dilute to exactly

1 liter.

C. Silver sulfate, 0.020 N. Dissolve 3.12 gm. silver

sulfate in 1 liter of water.

D. Ammonium hydroxide, approximately 7 N

(1+1).

E. Calcium oxide.

Procedure

First determine the concentration of chloride in an

aliquot as directed under Method 13. Pipet another

aliquot containing 0.004 to 0.04 meq. of nitrate into a

25-ml. volumetric flask. Add an amount of reagent

C equivalent to the amount of chloride present. Dilute

to volume and mix. Transfer most of the suspension

to a 50-ml. centrifuge tube and separate the precipitate by centrifuging. After transferring the solution to

another centrifuge tube, flocculate any suspended organic matter by adding about 0.1 gm. of E and clear by

again centrifuging. Pipet a 10-ml. aliquot representing 2/5 of the sample into an 8-cm. evaporating dish.

Evaporate the aliquot to dryness, cool, and dissolve the

residue in 2 ml. of A. After 10 min., add 10 ml. of

water and transfer to a 100-ml. volumetric flask. Make

alkaline by the addition of D, dilute to volume, and

mix. Measure the light transmission through a 460-m/A

filter in an optical cell against that of water in a

similar cell.

Prepare a calibration curve by pipeting 0, 0.2, 0.4,

0.8, 1.2, and 1.6 ml. portions of reagent B into evaporating dishes and treating as above omitting the additions of C and E, and the clarifying procedure.

Calculations

Milliequivalents per liter of NO;í= (meq.

lot as found bv internolation on NO

iviiiiict[uivciiciiis per iiLci ui i^v^.j— i^

aliquot as found by interpolation on

l,000/(ml. in aliquot).

of NO3 in

NO, curve) X

(16) Silicate as Silicomolybdate

Apparatus

Photoelectric colorimeter.

Reagents

A. Ammonium molybdate, 10 percent solution.

Dissolve 10 gm. of ammonium molybdate in water and

dilute to 100 ml.

B. Sulfuric acid, approximately 5 N ( 1 + 6).

C. Sodium silicate, 0.01 N. Dissolve 1.5 gm. of

Na2SiO;i*9H20 in 1 liter of water. Determine the silicate

(SÍO3) concentration of this solution, using a 100-ml.

aliquot and Method 76a (ch. 8). Adjust the remaining

solution to exactly 0.01 N by the addition of a calculated amount of water. Store in a plastic bottle.

Procedure

F*ipet an aliquot containing 0.005 to 0.05 meq. of

silicate into a 50-ml. volumetric flask. Dilute to" a

volume of 40 to 45 ml. with water. Add 2 ml. of

reagent A and then 1 ml. of B. Dilute to 50 ml., mix,

and after 15 min. measure the light transmission

through a 420-m/x filter in an optical cell against water

in a similar cell. Prepare a calibration curve by carrying a series of 0, 1, 2, 3, 4, and 5 ml. of C through the

same operations.

Calculations

Milliequivalents per liter of SiOg^ (meq. of SÍO3 in

aliquot as found by interpolation X 1,000)/(ml. in

aliquot).

Reference

Snell and Snell (1936).

(17) Boron

Determine boron as directed in Method 73b. If the

solution is colored, transfer an aliquot to a platinum

dish, make alkaline with NaOH, reagent A, and evaporate to dryness in an oven at 95° C. Ignite over an

open flame until the residue fuses. Cool, add 5 ml.

dilute HCl, reagent C, and complete as suggested in

Method 73b under paragraph, Boron Concentration

Too Low.

Exchangeable Cations

(18) Exchangeable Cations

Apparatus

Centrifuge, 50-ml. round-bottom, narrow-neck

centrifuge tubes, and reciprocating shaker.

Reagents

A. Ammonium acetate solution, 1.0 N. To 700 or

800 ml. of water add 57 ml. of cone, acetic acid and

then 68 ml. of cone, ammonium hydroxide. Dilute to

a volume of 1 liter and adjust to pH 7.0 by the addition

of more ammonium hydroxide or acetic acid.

Bl. Nitric acid, cone.

C. Hydrochloric acid, cone.

D, Acetic acid, approximately 0.1 N.

Procedure

Ammonium acetate extractable cations: Samples for

this determination should be approximately 4 gm. for
SALINE AND ALKALI SOILS 101

medium- and fine-textured soils and 6 gm. for coarsetextured soils. Weigh samples to an accuracy of 1

percent and correct for the air-dry moisture content.

Place the sample in a centrifuge tube. Add 33 ml. of

reagent A to the tube, stopper, and shake for 5 min.

Remove the stopper and centrifuge at RCF'^ 1,000 until

the supernatant liquid is clear. This usually requires

5 min. Decant the supernatant liquid as completely as

possible into a 100-ml. volumetric flask. Extract with

A a total of 3 times by this procedure, decanting into

the same flask. Dilute to volume, mix, and determine

the amounts of the various extracted cations by flame

photometric or chemical methods. Flame photometric

analyses may be made directly upon aliquots of the

extract. If chemical methods are to be employed for

the determination of cations, pretreat the extract in the

following manner: Transfer to a 250-ml. beaker and

evaporate to dryness on a hot plate or steam bath.

Wash down the walls of the beaker with a small quantity

of water and again evaporate to dryness. Add 1 ml.

of B and 3 ml. of C, evaporate, and dissolve the residue

in 20 ml. of D. Filter through low-ash content filter

paper into a 50-ml. volumetric flask, using water to

wash the beaker and filter paper. Dilute to volume.

Soluble cations: Prepare a saturated soil paste as

described in Method 2, using a 200- to 1,000-gm. sample of soil. The weight of soil will depend upon the

number of cations to be determined, the analytical

methods employed, and the salt content of the soil.

Determine the saturation percentage by Method 27.

Obtain the saturation extract as described under

Method 3a and determine the soluble cation concentrations by flame photometric or chemical methods.

Calculations

Ammonium acetate extractable cations in meq./lOO

gm.= (catioii cone, of extract in meq./l. X 10)/(wt. of

sample in gm. ).

Soluble cations in meq./lOO gm.= (cation cone, of

saturation extract in meq./l.) X (saturation percentage)/l,000.

Exchangeable cations in meq./lOO gm.= (extractable

cations in meq./lOO gm.) — (soluble cations in meq./

100 gm.).

Reference

Bower and others (1952).

(19) Cation-Exchange-Capacity

Apparatus

Centrifuge, 50-ml. round-bottom, narrow-neck centrifuge tubes, and reciprocating shaker.

Reagents

A. Sodium acetate solution, 1.0 N. Dissolve 136

gm. of sodium acetate trihydrate in water and dilute

to a volume of 1 liter. The pH value of the solution

should be approximately 8.2.

B. Ethanol, 95 percent.

C. Ammonium acetate solution, 1.0 N. To 700 or

800 ml. of water add 57 ml. of cone, acetic acid and then

68 ml. of cone, ammonium hydroxide. Dilute to a

volume of 1 liter and adjust to pH 7.0 by the addition

of more ammonium hydroxide or acetic acid.

Procedure

Samples for this determination should be approximately 4 gm. for medium- and fine-textured soils and 6

gm. for coarse-textured soils. Weigh samples to an

accuracy of 1 percent and correct for the air-dry

moisture content. Place the sample in a centrifuge

tube. Add 33 ml. of reagent A, stopper the tube, and

shake for 5 min. Unstopper and centrifuge at

RCF= 1,000 until the supernatant liquid is clear. This

usually requires 5 min. Decant the supernatant liquid

as completely as possible and discard. Treat the

sample in this manner with 33-ml. portions of A a total

of 4 times, discarding the supernatant liquid each time.

Add 33 ml. of B to the tube, stopper, shake for 5 min.,

unstopper, and centrifuge until the supernatant liquid

is clear. Decant and discard the supernatant liquid.

Wash the sample with 33-ml. portions of B a total of 3

times. The electrical conductivity of the supernatant

liquid from the third washing should be less than 40

micromhos/cm. Replace the adsorbed sodium from

the sample by extraction with three 33-ml. portions of

C and determine the sodium concentration of the combined extracts after dilution to 100 ml. as described

under Method 18.

Calculations

Cation-exchange-capacity in meq./lOO gm.= (Na

cone, of extract in meq./l.X 10)/(wt. of sample in

gm.).

Reference

Bower and others (1952).

(20) Exchangeable-Cation Percentages

(20a) Exchangeable - Cation Percentages

by Direct Determination

Procedure

Determine the exchangeable-cation contents and the

cation-exchange-capacity, using Methods 18 and 19.

Calculations

Exchangeable-cation percentage= (exchangeable-cation content in meq./lOO gm.X 100)/(cation-exchangecapacity in meq./lOO gm.).
102 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

(20b) Estimation of Exchangeable-Sodi'

uni'Percentage and Exchange'

able-Potassium-Percentage From

Soluble Cations

Procedure

Prepare a saturation extract of the soil as described

under Methods 2 and 3a. Determine the calcium plus

magnesium, sodium, and potassium concentrations of

the saturation extract, using Methods 7, 10, and 11,

respectively.

Calculations

Exchangeable-sodium-percentage

^100 (-0.0126+ 0.01475%)

~ 1 + ( - 0.0126 + 0.01475A; )

where x is equal to the sodium-adsorption-ratio.

Exchangeable-potassium-percentage

^100 (0.0360+0.1051^:)

1+(0.0360 +0.1051A;)

where x is equal to the potassium-adsorption-ratio.

The sodium-adsorption-ratio and the potassium-adsorption-ratio are calculated as follows:

Sodium-adsorption-ratio = NaVV ( Ca^^ + Mg"^* ) /2

and

Potassium-adsorption-ratio = K^/ V ( Ca^^ + Mg*^ ) /2

where Na"^, K+, Ca"^"^, and Mg++ refer to the concentrations of designated cations expressed in milliequivalents

per liter.

A nomogram, which relates soluble sodium and

soluble calcium plus magnesium concentrations to the

sodium-adsorption-ratio, is given in figure 27. Also

included in the nomogram is a scale for estimating the

corresponding exchangeable-sodium-percentage, based

on the linear equation given in connection with figure 9

(ch. 2). To use this nomogram, lay a straightedge

across the figure so that the line coincides with the

sodium concentration on scale A and with the calcium

plus magnesium concentration on scale B. The sodiumadsorption-ratio and the estimated exchangeable-sodium-percentage are then read on scales C and D,

respectively.

Supplementary Measurements

(21) pH Determinations

(21a) pH Reading of Saturated Soil Paste

Apparatus

pH meter with glass electrode.

Procedure

Prepare a saturated soil paste with distilled water

as directed in Method 2 and allow paste to stand at least

1 hour. Insert the electrodes into the paste and raise

and lower repeatedly until a representative pH reading

is obtained.

(21b) pH Reading of Soil Suspension

Procedure

Prepare a soil suspension, using distilled water, shake

intermittently for an hour, and determine pH reading.

(21c) pH Reading of Waters^ Solutions^

Soil Extracts

Procedure

Determine pH reading by means of a glass electrode

assembly with the solution in equilibrium with a known

CO2 atmosphere. '

Remarks

Opinion varies as to the proper method for making

pH readings. It is desirable to select a definite procedure and follow it closely, so that the readings will

be consistent and have maximum diagnostic value. The

method used should be described accurately so as to aid

others in the interpretation of results.

The CO2 status influences pH readings, and should

be controlled or specified. Ordinarily, readings are

made at the CO2 pressure of the atmosphere. A special

high-pH glass electrode should be used for pH values

appreciably above 9.0.

(22) Gypsum

(22a) Gypsum by Precipitation With Acetone (Qualitative)

Reagent

Acetone.

Procedure

Weigh 10 to 20 gm. of air-dried soil into an 8-oz.

bottle and add a measured volume of water sufficient

to dissolve the gypsum present. Stopper the bottle

and shake by hand 6 times at 15-min. intervals or agitate for 15 min. in a mechanical shaker. Filter the

extract through paper of medium porosity. Place about

5 ml. of the extract in a test tube, add an approximately

equal volume of acetone, and mix. The formation of a

precipitate indicates the presence of gypsum in the soil.

Remarks

The soil should not be oven-dried, because heating promotes the conversion of CaS04*2H20 to

CaSO40.5H2O. The latter hydrate has a higher

solubility in water for an indefinite period following

its solution.
SALINE AND ALKALI SOILS 103

Cd^-^Mg^-^

FIGURE 27.—Nomogram for determining the SAR value of a saturation extract and for estimating the corresponding

ESP value of soil at equilibrium with the extract.
104 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

(22b) Gypsum by Precipitation With Acetone (Quantitative)

Apparatus

Centrifuge, 50-ml. conical centrifuge tubes, conductivity cell, and Wheatstone bridge.

Reagent

Acetone.

Procedure

Transfer a 20-ml. aliquot of the filtered extract obtained as described in Method 22a into a 50-ml. conical

centrifuge tube. Add 20 ml. of acetone and mix. Let

stand until the precipitate flocculates. This usually requires 5 to 10 min. Centrifuge at RCF= IfiOO for 3

min., decant the supernatant liquid, invert the tube, and

drain on filter paper for 5 min. Disperse the precipitate and rinse the wall of the tube with a stream of 10

ml. of acetone blown from a pipet. Again, centrifuge

for 3 min., decant the supernatant liquid, invert the

tube, and drain oh filter paper for 5 min. Add exactly

40 ml. of distilled water to the tube, stopper, and shake

until the precipitate is completely dissolved. Measure

the electrical conductivity of the solution, using Method

4b, and correct the conductivity reading to 25° C.

Determine the concentration of gypsum in the solution

by reference to a graph showing the relationship between the concentration and the electrical conductivity

of gypsum solutions. This graph may be constructed

by means of the following data from the International

Critical Tables.

Electrical conductivity

at25°C.

CaS04 concentration (meq./l.) : Mmhos/cm.

1 0.121

2 .226

5 .500

10 .900

20 1.584

30.5 2. 205

Calculations

Milliequivalents of CaS04 in aliquot= (meq./l.

of CaS04 from conductivity reading) X (ml. of water

used to dissolve precipitate)/1,000.

Milliequivalents of gypsum per 100 gm. of soil =

100X (meq. of CaS04 in aliquot)/(soil: water ratioX

ml. of soil-water extract used).

Remarks

Sodium and potassium sulfates when present in sufficiently high concentrations are also precipitated by

acetone. The maximum concentrations of sodium sulfate and of potassium sulfate that may be tolerated are

50 and 10 meq./l., respectively.

At a 1:5 soil-water ratio, water will dissolve approximately 15 meq. of gypsum per 100 gm. of soil.

If it is found that the gypsum content of the soil approaches 15 meq./lOO gm. by use of a 1:5 soil-water

extract, the determination should be repeated, using a

more dilute extract.

Reference

Bower and Huss (1948).

(22c) Gypsum by Increase in Soluble Calcium Plus Magnesium Content

Upon Dilution

Procedure

Determine the saturation percentage and obtain a

saturation extract of the soil using Methods 27 and 3a.

Prepare another water extract of the soil, using a moisture content sufficient to dissolve the gypsum present

as described under Method 22a. Determine the calcium plus magnesium concentrations of the two extracts by Method 7.

Calculations

Soluble Ca + Mg at the saturation percentage in

meq./lOO gm.= (Ca-f-Mg cone, of saturation extract

in meq./l.) X (saturation percentage)/1,000.

Soluble Ca + Mg at the high moisture percentage in

meq./lOO gm.= (Ca-hMg cone, of dilute extract in

meq./l.) X (moisture percentage)/1,000.

Gypsum in meq./lOO gm. of soil= (soluble Ca-hMg

at the high moisture percentage in meq./lOO gm.) —

(soluble Ca + Mg at the saturation percentage in

meq./lOO gm.).

(22d) Gypsum Requirement '^

Reagenl

A. Approximately saturated gypsum solution of

known calcium concentration. Place about 5 gm. of

CaS04-2H20 and 1 liter of water in a flask, stopper, and

shake by hand several times during a period of 1 hr.,

or for 10 min. in a mechanical shaker. Filter and determine the calcium concentration of a 5-ml. aliquot

of the solution by Method 7. The calcium concentration should be at least 28 meq./l.

Procedure

Weigh 5 gm. of air-dried soil into a 4-oz. bottle.

Add 100 ml. of reagent A by means of a pipet.

Stopper the bottle and shake by hand several times during a period of 30 min. or for 5 min. in a mechanical

shaker. Filter part of the suspension and determine the

calcium plus magnesium concentration of a suitable

volume of the clear filtrate using Method 7.

^"^ SCHOONOVER, W. R. EXAMINATION OF SOILS FOR ALKALI.

University of California Extension Service, Berkeley, California.

1952. ( Mimeographed. )
SALINE AND ALKALI SOILS 105

Calculations

Gypsum requirement, meq./lOO gm.= (Ca cone, of

added gypsum solution in meq./l. — Ca + Mg cone, of

filtrate in meq./l.) X2.

( 23 ) Alkaline-Earth Carbonates (Lime )

( 23a) AIkaline-Earth Carbonates by Effervescence With Acid

Reagent

A. Hydrochloric acid, 3N (1 + 3).

Procedure

Place several grams of soil on a small watchglass.

By means of a pipet add sufficient water to saturate the

soil. This displaces most of the soil air so that its loss

upon the addition of acid will not be confused with

effervescence of lime. Add a few drops of reagent A to

the soil and note any effervescence that occurs. The

soil may be termed slightly, moderately, or highly

calcareous in accordance with the degree of effervescence obtained.

(236) Alkaline-Earth Carbonates by Gravimetric Loss of Carbon Dioxide

Reagent

A. Hydrochloric acid, 3 N (1 + 3).

Procedure

Pipet 10 ml. of reagent A into a 50-ml. Erlenmeyer

flask, stopper with a cork, and weigh. Transfer a 1-

to 10-gm. sample of soil containing 0.1 to 0.3 gm. of

calcium carbonate to the flask, a little at a time, so as

to prevent excessive frothing. After effervescence has

largely subsided, replace the stopper loosely and swirl

the flask. Let stand with occasional swirling until the

weight of the flask and contents does not change more

than 2 or 3 mg. during a 30-min. period. The reaction

is usually complete within 2 hours. Prior to weighing,

displace any accumulated carbon dioxide gas in the

flask with air. This is important and may be done by

swirling with the stopper removed for 10 to 20 sec.

Calculations

Weight of CO2 lost= (initial wt. of flask + acid +

soil) — (final wt. of flask + acid + soil).

CaCOg equivalent in percent= (wt. of CO2 lostX

227.4) /wt. of soil sample.

Remarks

The accuracy of this method depends to a large extent upon the sensitivity of the balance used for weighing. Using a torsion-type balance capable of detecting

weight differences of 2 to 3 mg., the relative error is

about ± 10 percent.

(23c) Alkaline-Earth Carbonates From

Acid Neutralization

Reagents

A. Hydrochloric acid, 0.5 N, standardized.

B. Sodium hydroxide, 0.25 N, standardized.

C. Phenolphthalein, 1 percent in 60 percent ethanol.

Procedure

Place 5 to 25 gm. of soil in a 150-ml. beaker, add 50

ml. of reagent A by means of a pipet, cover with a

watchglass, and boil gently for 5 min. Cool, filter,

and wash all the acid from the soil with water. Determine the amount of unused acid by adding 2 drops of C

and back-titrating with B.

Calculations

CaCOg equivalent in percent= (meq. HCl added —

meq. NaOH used) X5/weight of sample in gm.

Remarks

The calculation gives the CaCOa equivalent. This

is the amount of CàC03 required to react with the acid.

This value usually is somewhat high, because soil constituents other than lime may react with the acid.

(24) Organic Matter

Apparatus

Erlenmeyer flasks, 500-ml., thermometer, 200° C.

Reagents

A. Potassium dichromate, 1 N. Dissolve 49.04 gm.

of potassium dichromate in water and dilute to 1 liter.

B. Sulfuric acid, cone, containing silver sulfate.

Dissolve 25 gm. silver sulfate in a liter of acid.

C. Ferroin indicator (ortho-phenanthroline ferrous

sulfate, 0.025 M). Dissolve 14.85 gm. o-phenanthroline monohydrate and 6.95 gm. ferrous sulfate in water

and dilute to 1 liter.

D. Ferrous sulfate, 0.5 N. Dissolve 140 gm. of

FeS04-7H20 in water, add 15 ml. of cone, sulfuric acid,

cool, and dilute to 1 liter. Standardize this solution

daily against 10 ml. of reagent A, as directed in the

procedure below.

Procedure

Grind the soil to pass 0.5-mm. screen, avoiding contact with iron or steel. Transfer a weighed sample, not

exceeding 10 gm. and containing from 10 to 25 mg. of

organic carbon, to a 500-ml. Erlenmeyer flask. Add 10

ml. of reagent A followed by 20 ml. of B. Swirl the

flask, insert thermometer, and heat gently so as to

attain a temperature of 150° C. in a heating period of

about 1 min. Keep contents of flask in motion in order
106 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

to prevent local overheating, which results in error

caused by thermal decomposition of dichromate. After

the 150*^ temperature is reached, place the flask on an

asbestos pad, and allow to cool. Add 200 ml. of water

and 4 or 5 drops of C. Titrate with D until the color

changes from green to red.

Since some soils adsorb o-phenanthroline indicator,

the titration may be improved by a prior filtration,

using a rapid filter paper on a Buechner funnel. If

more than 80 percent of the dichromate solution is

reduced, the determination should be repeated with

less soil.

Calculations

Organic carbon in percent= (meq. of KgCrgOx

added —meq. of FeS04 used) X0.336/wt. of sample

in gm.

Organic matter in percent = organic carbon in percent XI.72.

Remarks

This modification of Walkley's rapid method (1935,

1947) for the determination of organic carbon in soils

has been found to give approximately 89 percent recovery of carbon, as compared to the dry-combustion

method. The conversion factor 0.336 was obtained by

dividing 0.003, the milliequivalent weight of carbon, by

89 and multiplying by 100 to convert to percent.

Chloride interference is eliminated by the addition of

the silver sulfate to the digesting acid as indicated.

Nitrates up to 5 percent and carbonates up to 50 percent

do not interfere.

References

Walkley (1935,1947).

(25) Total and External Ethylene Glycol

Retention

Apparatus

Vacuum pump, Central Scientific Company Hyvac

or equivalent.

Vacuum desiccators, inside diameter 250 mm., with

external sleeve or glass stopcock and porcelain plates.

Muffle furnace with automatic temperature control.

Aluminum moisture boxes, 21/2 in. in diameter and

% in. high, with lids.

Reagents

A. Hydrogen peroxide, 10 percent solution.

B. Anhydrous calcium chloride, 8 or 12 mesh,

technical.

C. Phosphorus pentoxide.

D. Ethylene glycol (Eastman). Redistill under reduced pressure, discarding the first and last 10 percent

of the distillate.

Procedure

Soil preparation. Grind the soil sample to pass a

60-mesh sieve. The increase in surface area brought

about by this degree of grinding is negligible. Treat

approximately 10 gm. of the sieved soil with reagent A

for the removal of organic matter (see Method 41).

Transfer the treated soil to a 5- to 8-cm. diameter Buechner funnel fitted with filter paper and leach with several

small portions of distilled water, using suction. Allow

the soil to air-dry, then pass through a 60-me&h sieve.

Total ethylene glycol retention. Weigh 2.10 gm. of

the 60-mesh soil into an aluminum moisture box. The

tare weight of the box and its lid should be known.

Spread the soil evenly over the bottom of the box.

Place the box in vacuum desiccator over about 250 gm.

of reagent C, apply vacuum by means of a Hyvac or

equivalent pump, and dry the soil to constant weight.

This usually requires 5 to 6 hours. Determine the

weight of vacuum-dried soil. By means of a pipet,

having a tip drawn to a fine point, distribute 1 ml. of D

dropwise over the soil surface. Place the box in a

second vacuum desiccator over 250 gm. of B and allow

to stand overnight to obtain uniform wetting of the soil.

Connect the desiccator to a Hyvac pump and evacuate

at a temperature of 25±2° C. until excess ethylene

glycol is removed from the soil. Depending upon the

temperature and vacuum conditions attained, this

usually requires from 5 to 7 hours when 8 samples are

present in a desiccator. In practice, the box is weighed

after 5 hours in the vacuum desiccator and at intervals

of 1 hour thereafter until the loss of weight per hour

interval is less than 3 or 4 percent of the weight of

ethylene glycol remaining on the soil. The next to the

last weight taken is used to calculate ethylene glycol

retained.

External ethylene glycol retention. Weigh exactly

2.10 gm. of the 60-mesh soil into an aluminum moisture

box. Spread the soil evenly over the bottom of the

box and heat at a temperature of 600±15° C. for 2

hours in a muffle furnace having automatic temperature

control. Remove the box, cover, cool in a desiccator

containing reagent B, and weigh. Apply 1 ml. of D

to the soil, let stand overnight, and remove the excess

ethylene glycol by evaporation in vacuum as described

previously for the determination of total ethylene

glycol retention.

Calculations

Assuming that 3.1 X 10~^ gm. of ethylene glycol are

required for the formation of a monolayer on 1 sq. m.

of surface, as indicated by Dyal and Hendricks (1950),

the formulas for calculation of total, external, and

internal surface areas are as follows:

Total surface area, m.^/gm. = wt. of ethylene glycol

retained by unheated soil, gm./(wt. of vacuum-dried

unheated soil, gm. X 0.00031 ).

External surface area, m.^/gm. = wt. of ethylene

glycol retained by heated soil, gm./(wt. of vacuumdried unheated soil, gm. X 0.00031).
SALINE AND ALKALI SOILS 107

Internal surface area = (total surface area)—.(external surface area).

Remarks

The Hyvac pump and desiccators are connected by

means of tight-fitting vacuum rubber tubing. A glass

tube filled with reagent B is inserted in the vacuum line

to prevent undesirable vapors from entering the

pump. The tube also permits the introduction of dry

air into the desiccators to release the vacuum. Highvacuum stopcock lubricant should be used to seal the

glass joints.

The adequacy of the vacuum system for removing

excess ethylene glycol can be checked by determining

the rate of evaporation of this liquid from a free surface. The average rate of evaporation over a 5-hour

period from an aluminum moisture box of the size

specified above and containing ethylene glycol should

be at least 1 gm. per hour.

For greatest accuracy in the determination of internal

surface area, removal of excess ethylene glycol from

heated and unheated soil should be performed concurrently. Four unheated and the corresponding 4 heated

samples are ordinarily placed together in a desiccator.

The occasional inclusion of a standard sample having

a known retention value serves as a useful control on

procedure.

The anhydrous calcium chloride placed in the desiccator to absorb ethylene glycol should be renewed after

each set of 8 determinations. The phosphorus pentoxide used for drying under vacuum may be used until

it absorbs sufficient water to develop a syrupy

consistency.

References

Bower and Gschwend (1952), Dyal and Hendricks

(1950,1952).

Soil Water

(26) Soil-Moisture Content

Procedure

Transfer a representative subsample of the soil to a

tared can with lid. For accuracy, it is desirable where

possible to use at least a 25-gm. sample. Weigh, dry

to constant weight at 105° C, and weigh again.

Calculations

Moisture content in percent, P^= (loss in weight on

drying) X 100/(weight of the oven-dry soil).

(27) Saturation Percentage

(27a) Saturation Percentage From OvenDrying

Procedure

Transfer a portion of the saturated soil paste, prepared according to Method 2, to a tared soil can with

lid. Determine the moisture content by Method 26.

Calculations

Saturation percentage (SP) = (loss in weight on drying) X 100/(weight of the oven-dry soil).

(27b) Saturation Percentage From Volume of Water Added

Remarks

When the air-dry moisture content of the sample is

known, as it usually is when exchangeable-cation analyses are made, the saturation percentage can be determined as follows:

Procedure

Transfer a known weight of air-dry soil to a mixing

cup. Add distilled water from a buret or graduated

cylinder with stirring until the soil is saturated as

described in Method 2. Record the volume of water

added.

Calculations

Weight of oven-dry soil= (weight of air-dry soil) X

100/( 100 + air-dry moisture percentage).

Total water = (water added) 4- (water in air-dry soil)

= (weight of water added) + (weight of air-dry soil) —

(weight of oven-dry soil).

5P=100X (total weight of water)/(weight of ovendry soil).

(27c) Saturation Percentage From the

Weight of a Known Volume of

Paste

Remarks

By this method, the saturation percentage is calculated from the weight of a known volume of saturated

soil paste. It is assumed that the soil particles have a

density of 2.65 gm./cm.^, and that the liquid phase has

a density of 1.00 gm./cm.^.

Apparatus

Balance, accurate to 0.1 gm.

A cup of known volume. This measurement can be

combined with the soil-paste resistance measurement
108 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

using the same loading of the Bureau of Soils electrode

cup.

Procedure

Determine the volume and weight of the cup. Fill

the cup with saturated soil paste, jarring it during filling

to exclude air, and strike off level with the top. Weigh

and subtract the cup weight to get the net weight of the

paste.

Calculations

SP=

lQQ(2.65F-r)

2.65{W-V)

where 5P=saturation percentage; F=volume of

saturated soil paste, in cm.^, = a constant; and W=net

weight of V cm.^ of saturated soil paste, gm. Calculations are simplified by the use of a table or graph relating values of W and SP for a given value of V,

Reference

Wilcox(1951).

(28) Infiltration Rate

Remarks

Infiltration rate (infiltration capacity) is the rate

of water entry into the soil where water covers the

surface at a shallow depth and downward flow into

and through the soil is nondivergent. The latter condition is satisfied by rainfall or if the ponded area is

infinitely large. For practical purposes, the subsidence

rate of the free-water surface in a large basin is taken

as a measure of the infiltration rate. The effect of

divergent flow increases as the ponded area decreases.

If small basins or cylinders are used, it is difficult to

determine the true infiltration rate. For soils in which

permeability increases with depth, errors from flow

dive#gence may be negligible; but, if the permeability

decreases with depth, the effect of flow divergence may

be considerable. Flow divergence that occurs with

small plots or cylinders may be minimized by ponding

water in a guard ring or border area around the plot or

cylinder.

If infiltration measurements are made under conditions where divergent flow may not be negligible, the

water-intake rate should be reported as infiltration

velocity and accompanied by a description of the

measuring method.

Under some conditions the evaporation rate may not

be negligible and must be taken into account in infiltration measurements. In small basins or where cylinders

are used, evaporation may be minimized by covering

the water surface with a film of oil.

There is no single method best suited to all field conditions. Experience and judgment are required in obtaining and evaluating infiltration measurements.

(See discussion in chapter 2.)

(28a) Basin

Apparatus

Gage for measuring water elevation, and watch.

Procedure

Pond water on an area of soil enclosed by dikes or

ridges. Measure the rate of subsidence of the water

surface with a staff gage (a linear scale standing in the

water), hook gage, or water-stage recorder.

The infiltration rate will depend on the time and

depth of water that has entered the soil.

Calculations

A curve showing the depth of water that has entered

the soil as a function of time can be plotted from the

water elevation and time readings or taken from a

water-stage recorder. Average or instantaneous values

of the infiltration rate can be taken from this curve,

depending on the purpose of the measurement. Express infiltration rate in centimeters per hour or in

inches per hour.

(28b) Cylinder

Apparatus

Cylinders 11 to 14 in. in diameter and 16 in. long.

The cylinders can be rolled from 16-gage sheet iron.

Butt-weld and grind the weld smooth. Reinforce the

upper end with l/g-in. by 1-in. iron strip, welded to

cylinder. Galvanize cylinders after fabrication. For

ease in transportation, cylinders can be made with

different diameters so that they will fit, one within

another.

Circular driving cap and hammer. Torch-cut the

driving cap from V2-i'^- steel plate and screw in a l/^-in.

central rod to serve as the hammer guide. The hammer

can consist of a 50- to 80-lb. block of iron. This should

have a central pipe to slide on the guide rod of the cap.

Attach crosshandles to the pipe.

Hook gage or staff gage, watch, thin metal tamp,

splash guard of rubber sheet or burlap, field source

of water.

Procedure

Drive the cylinders into the soil to a depth of 6 or 8

in. Alternatively, the cylinders can be jacked into the

soil, if a heavy tractor or truck is available. Care

should be exercised to keep the sides of the cylinder

vertical and to avoid disturbance of the soil column

within the cylinder. Tamp soil into the space between

the soil column and the cylinder. If this space is greater

than 1/g in., the cylinder should be reset. Cover the

soil with a splash guard and apply 4 to 6 in. of water.

Record the elevation of the water surface and the time

at convenient intervals. A staff gage is often satisfac-
SALINE AND ALKALI SOILS 109

tory, but a hook gage should be used if the subsidence

rate is low. Several adjacent cylinders are usually

installed. These need not be carefully leveled if a

mark is placed on each cylinder for locating the base of

the hook gage.

While the wetting front is in the cylinder, the watersubsidence rate corresponds to the infiltration rate.

When the wetting front passes below the cylinder, more

or less divergence of flow will occur and the subsidence

rate then should be designated as intake rate or infiltration velocity. Divergent flow is minimized by installing cylinders in plots or within larger diameter

rings in which the soil is kept flooded.

Where desired, water-entry rates into subsurface soil

layers can be measured by excavating to the desired

depth before setting the cylinders.

Calculations

Express infiltration rate and infiltration velocity in

centimeters per hour or in inches per hour, using values

averaged over time intervals appropriate to the purposes

of the measurement.

(29) 1/10-Atmosphere Percentage

Apparatus

Pressure-plate apparatus. Retainer rings approximately 1 cm. high and 6 cm. in diameter to held at least

25-gm. samples. Balance, drying oven, and moisture

boxes.

Remarks

Install the pressure plates to be used for the test in

a pressure cooker, fill the cooker with water, fasten

the lid on the cooker, and measure the rate of outflow

of water from the ceramic plates at a pressure of 15

lb. in."^. This rate should be about 1 cc. per cm.^ per

hr. per atm. pressure difference or greater for satisfactory operation of the porous plates. Next check

the pressure plates for entry value as follows: release

the air pressure, empty excess water from the cooker

pot and the plates, close the cooker pot, and apply a

pressure of i/^ atm. or other appropriate value. After

a few minutes, the outflow of water from the plate outlets will cease and there should be no bubbling of air

from these outlets, thus indicating that the entry values

for the plates are above the value of the pressure

applied to the pressure cooker. At the conclusion of

the entry-value test, submerge the pressure cooker in

water while the pressure is on or make other equivalent

tests to make sure that there are no air leaks at the

cooker gasket or attendant connections. Air leaks

from the cooker cause troubles with air-pressure control and may also cause serious errors in retentivity

determinations through direct loss of water vapor from

the soil samples.

Procedure

Prepare duplicate 25-gm. samples that have been

passed through a 2-mm. round-hole sieve, using the

subsampling procedure outlined in Method 1. Place

the sample retainer rings on the porous plate. In order

to avoid particle-size segregation, dump all of the soil

sample from each container into a ring and level.

Allow the samples to stand at least 16 hours with an

excess of water on the plate. Close the pressure cooker

and apply a pressure of 100 cm. of water. Samples

1 cm. high can be removed any time after 48 hours from

initiating the extraction or when readings on a buret

indicate that outflow has ceased from all of the samples

on each plate. Some soils will approach equilibrium

in 18 to 20 hours. Before releasing the air pressure in

the pressure cooker, put a pinch clamp on the outflow

tube for each plate. This prevents backflow of water

to the samples after the pressure is released. To avoid

changes in the moisture content of the samples, transfer the samples quickly to moisture boxes. Determine

the moisture content by drying to constant weight at

105° C. Express the moisture content as percent, dryweight basis.

References

Richards and Weaver (1944), and Richards

(1949b).

(30) %-Atmosphere Percentage

Apparatus

Same as in Method 29.

Procedure

Same as in Method 29, except that the extraction

pressure is 345 cm. of water.

(31) 15-Atinosphere Percentage

Apparatus

Pressure-membrane apparatus with sausage-casing

membrane. Rubber soil-retaining rings 1 cm. high and

approximately 6 cm. in diameter that hold about 25

gm. of soil. Balance, drying oven, and moisture boxes.

Procedure

Prepare duplicate 25-gm. samples that have been

passed through a 2-mm. round-hole sieve, using the subsampling procedure outlined in Method 1. Moisten the

cellulose membrane, install in the apparatus, and trim

the edge by running a knife around the brass cylinder.

Place the soil-retaining rings on the membrane. In

order to avoid particle-size segregation, dump all of

the soil sample from each sample container into one
lio AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

ring. Pouring out part of the sample and leaving part

in the container will give a nonrepresentative sample.

Level the sample in the ring, cover with a square of

waxed paper, and allow the samples to stand at least 16

hours with an excess of water on the membrane. Remove excess water from the membrane with a pipet or

rubber syringe, close the pressure-membrane apparatus and admit air to the soil chamber at a pressure of 15

atm. (220 lbs. inr^).

After a few hours, there is a marked decrease in the

rate of water outflow from the soil, the outflow rate

then being limited mainly by the low capillary conductivity of the soil rather than the low membrane permeability. At this time, the soil samples have suflScient

rigidity to resist plastic flow and compaction and so a

4 lb. in.-2 pressure differential may be applied to the

rubber diaphragm at the top of the soil chamber. This

diaphragm action holds the sample firmly in contact

with the membrane and considerably hastens moisture

extraction for fine-textured soils that shrink appreciably. The diaphragm is unnecessary for medium- and

coarse-textured soils.

Remove the samples any time after 48 hours from the

commencement of the extraction or when the readings

on an outflow buret indicate equilibrium has been

attained. Most soils will approach hydraulic equilibrium with the membrane in 18 to 20 hours, but some

soils may require a considerably longer time. In order

to avoid changes in the moisture content, transfer the

samples to moisture boxes as soon as possible after

releasing the extraction pressure. Determine the moisture content by drying to constant weight at 105° C.

Express moisture content as percent, dry-weight basis.

Remarks

Care must be taken to keep soil away from the lower

gasket of the cell. Otherwise, gasket flow that occurs

when the cell is closed may press sand particles into the

membrane and cause leaks.

References

Richards (1947), Richards and Weaver (1944).

(32) Moisture-Retention Curve

Each sample, with disturbed or undisturbed structure, is contained in a retainer consisting of a brass

cylinder, a plastic lid, and a porous ceramic bottom

plate, all held together with rubber bands. Various

moisture equilibria and weighings are thus made possible with a minimum of disturbance to the sample.

Apparatus

Pressure-plate apparatus, pressure-membrane apparatus, balance, drying oven, large straight-edged carving knife, and aluminum moisture boxes with lids, 3%

in. diameter by 2 in. high.

Soil sampling tube with retainer cylinders cut from

brass tubing 2Vi in. outside diameter by 19-gage wall.

The core retainer cylinders are 3 cm. high, and while

in the sampling tube have guard rings 1 cm. high at

each end. ( See drawing of apparatus in the Appendix.)

Plastic and ceramic disks serve as lids and bottoms

for the core retainer. The lids are cut from y^ in.

transparent plastic sheet and are 21/4 in- in diameter.

The ceramic disks are 21/4 in. in diameter by %Q in.

thick, with a peripheral groove to attach two hooks

formed from twisted wire at points on the disk 180°

apart. The porous ceramic body should be like that

used for tensiometer cups. The entry value should be

greater than 1 atm., and the hydraulic conductivity

should be equal to or greater than 8X 10"^ cm./hr.

A layer of cheesecloth and sieved soil make capillary

contact between the retainers and the control membranes. The cheesecloth should be treated with a bactéricide such as Dowicide No. 4. The fraction of a loam

soil that passes a 60-mesh screen makes a good capillary

contact medium.

A complete core-retainer set consists of a moisture

box with lid, a brass cylinder, a plastic lid, two strong

rubber bands, and a ceramic plate. All of the parts in

a retainer set should bear the same identifying number.

The tare weight of each retainer set with the ceramic

disk saturated with water should be determined and

recorded.

Procedure

Take the cores with the sampling tube when the soil

is moist. Remove the 1-cm. guard rings from either

end of the 3-cm. retainer cylinder. Roughly trim the

cores in the field and transport to the laboratory in

the aluminum moisture boxes. Trim the cores accurately in the laboratory with the carving knife. Fasten

the plastic lids and ceramic plates to the brass cylinders

by stretching the rubber bands across the lids and

attaching the bands to the hooks at the opposite edges

of the ceramic plates.

Place the core retainers on a porous brick with a

free water surface 1 or 2 mm. below the surface of the

brick. After 24 hours, wipe the excess water from the

retainers, place each in its moisture box, and weigh.

Replace the retainers on the brick with the water surface set for 10 cm. After 24 hours, weigh again.

These two weighings will not represent equilibrium

values, but high precision is usually not required at

0 and 10 cm. of suction. Prepare the pressure-plate

apparatus as indicated in Method 29. Spread approximately a 3-mm. layer of screened loam soil on the

pressure plate and cover with a single layer of treated

cheesecloth. Moisten the soil and cloth with water and

set the retainers firmly in contact. Close the pressure

cooker and adjust the pressure for the next suction

value.

Follow the approach to hydraulic equilibrium at

each pressure by connecting the outflow tube from each

plate or membrane to the lower end of a buret and

recording the buret readings occasionally. When
SALINE AND ALKALI SOILS 111

equilibrium is attained, clamp off the outflow tubes

and release the air pressure in the cooker or membrane

cell. Lift the core retainers from the membrane, brush

off any adhering soil, place each in its numbered moisture box, and weigh. For a retention curve, weighings

can be made at tensions of 0, 10, 30, 100, and 345 cm.

of water and 1, 3, and 15 atm. Other suction values

can be used, depending on the information desired.

The porous-ceramic retainers used at the Laboratory

have an entry value of 1 atm. and do not change appreciably in moisture content at suction values up to

3 atm. Therefore, the gross tare for a core-retainer

set is the same for all weighings at suction values up

to and including 3 atm. At the 15-atmosphere equilibrium, the ceramic retainer is removed before the

weighing, and a correspondingly different tare weight

is used. Determine the weight of the soil core when

oven-dried at 105° C.

Calculations

Determine the volume of the core retainer and calculate the bulk density of the soil in the core. From the

gross weights at each suction, the tare weights, and

the known weight of soil, calculate both the mass of

water and the volume of water (numerically the same

when c. g. s. units are used) in the core at each suction

value. From the foregoing data, calculate the grams

of water per 100 gm. of dry soil and the cubic centimeters of water per 100 cm.^ of soil at each suction

value. The latter may be taken as the depth percentage,

i. e., the depth of free water per 100 units of depth of

soil. Plot these values on linear coordinates with

moisture retention as the dependent variable, and

suction or soil-moisture tension as the independent

variable.

References

Richards (1947, 1948, 1949b, 1952).

(33) Field-Moisture Range

Remarks

Plants can grow in soil over a range of moisture

contents referred to as the available range. The practical upper boundary for this range, sometimes referred

to as field capacity, is characteristic of the field situation, and the best method for its determination is based

on field sampling. The determination should be made

after the soil has been wetted and the rate of downward drainage has decreased, but before appreciable

moisture is lost from the profile by evaporation and

root extraction. This determination loses significance

or requires special interpretation if drainage is restricted or if a water table is close to the soil surface.

Apparaius

Soil tube or soil auger, watertight moisture boxes,

balance, and drying oven.

Procedure

One to 3 days after the soil profile is thoroughly

wetted with rain or irrigation water, take samples by

horizons, by textural layers, or at 1-foot-depth intervals throughout the wetted zone. Determine the moisture content of the samples by drying to constant weight

at 105° C. Express the results as moisture percentage,

dry-weight basis, or as depth percentage if the bulk

density can be determined. The available range for

the soil at any given depth is then found by subtracting

the 15-atm. percentage from the field determination of

the upper limit of available water. The available range

can be expressed either as a dry-weight percentage or as

a depth percentage.

( 34) Hydraulic Conductivity

(34a) Hydraulic Conductivity of Soil

Cores

Thin-walled cylinders or cans may be pressed into

the soil in the field to obtain samples of soil of substantially undisturbed structure. More often, soil cores

are obtained in metal sleeves that fit into a sampling

tube, and, after the samples have been taken, the sleeves

serve as the core retainers. Power-driven machines are

available for taking undisturbed cores of 4- and 6-in.

diameter. Such cores are encased in the field for transportation and subsequent water-flow measurements.

Various casing methods have been used, such as painting the core with wax or plastic cement before and

after wrapping in cloth.

Procedure

In the laboratory, the cores are mounted vertically

and supported on a porous outflow surface such as

sand or filter paper and metal screen. A shallow depth

of water is usually maintained over the soil surface by

a siphon tube from a constant-level reservoir. Flow

tests should be conducted with water of the same quality

as that which occurs in the field. If discharge rates

are low, care must be taken to avoid errors arising

from evaporation of the percolate. If possible, flow

tests should be conducted at or near constant temperature.

Where desirable, especially for long cores, manometers can be attached at various points along the core.

These should be installed at transition zones between

horizons or at textural discontinuities.

Calculations

Water flow takes place in accordance with the equation:

Q..

t

-kA AH

AL

where Q is the volume of water passing through the

core in time (t), A is the area of the core, and k is the

average hydraulic conductivity in the soil interval
112 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

( AL), over which there is a hydraulic head difference

of A^. Solving for k gives h = Q!\L/Atù\E, Hydraulic conductivity (k) will be in centimeters per hour

if t is expressed in hours, Q in cm.^, A in cm.^, and A^

and AZ/ are both in the same units.

Kejerences

Bower and Peterson (1950), Kelley and coworkers

(1948), Marsh and Swarner (1949), and Richards

(1952).

(34b) Hydraulic Conductivity of Disturbed Soil

Apparatus

Soil containers are made from 20-gage seamless brass

tubing, 3 in. outside diameter, and 4 in. in length.

The bottoms of the containers are machined from 20-

gage brass sheet and are soldered into a recess or

counterbore in the cylinders. The central outflow

tubes are 2 in. long, are cut from % in. outside diameter by 20-gage brass tubing, and are attached with

solder.

Supports for soil and filter paper consist of circles

of 20-mesh or coarser bronze screen cut so as to fit

loosely on the inside of the soil container.

Packing block is made from a heavy wooden block

approximately 4 by 4 by 8 in. A hole is made in the

block to accommodate the outflow tube of the soil container, and guide rods are mounted in the block to keep

the cylinder vertical and to insure square impacts.

One rod is cut 2.5 cm. above the cylinder so that a

finger placed over this rod gives a convenient index of

height for the packing process.

Sharkskin filter paper, rack for supporting a number

of soil containers, constant-level water supply, siphon

tubes to connect soil containers to water supply, graduated cylinders, 2-mm. round-hole sieve, soil grinder,

and mixing cloth.

Procedure

Air-dry the soil and pass it through a 2-mm. roundhole screen. A power grinder may be used for hard

soils, but the grinding process must be standardized,

with the plates set to reduce only the larger particles.

Obtain representative 200-gm. subsamples in accordance with Method 1. Dump the entire subsample in one

motion into the soil container that has been fitted with

a screen and filter paper. This method of transferring

the soil is used to prevent particle-size segregation.

The cylinder containing the soil is dropped 20 times

through a distance of 2.5 cm. onto the packing block.

Place a filter paper on the soil surface and introduce

water into the container with a minimum of soil disturbance. Record the time of application of water and,

if possible, the time of the initial outflow. Collect the

percolate in a suitable receptacle and measure the

volume at convenient time intervals. Tests ordinarily

are run until the volume of water that has passed

through the soil corresponds to approximately 12 cm.

of depth of water on the soil surface. Calculate hydraulic conductivity and plot against accumulated

equivalent depth of percolate. With soils having extremely low percolation rates, an attempt should be

made to obtain at least one flow measurement, and

time rather than depth of water is used to determine

when to discontinue tests on such soils.

Remarks

While, according to theory, neither the diameter nor

the height of the soil column to be tested needs to be

within prescribed limits, it has been found that with

many soils satisfactory results are not obtained unless

the height is less than the diameter of the soil column.

This is particularly important if the soil swells appreciably on wetting. Experience indicates that the

cylinder should have at least a 7.5-cm. diameter for a

5-cm. depth of soil.

Hydraulic-conductivity measurements should be

made in the temperature range from 65° to 75° F. (18°

to 24° C.). For the most part, the effect of temperature on hydraulic conductivity in this range is small

compared with effects arising from such factors as

quality of the water and the base status and salinity of

the soil. The standard temperature for laboratory determination of hydraulic conductivity is usually taken

as 68° F. (20° C.). Corrections for viscosity effects

on measurements at temperatures other than 68° F.

can readily be calculated, but it has been observed that

temperature has other and not always predictable effects upon the hydraulic conductivity of soils in addition to those arising from viscosity.

The hydraulic gradient is usually set in the range

from 1 to 4, although values as high as 10 do not seem

to affect the results significantly.

In general the water that will be used on the soil in

the field should be used for the laboratory determinations, because small changes in water quality can produce large changes in rate of moisture movement.

Measurements are usually made in triplicate. The

samples are discarded and the test repeated if the range

of values is greater than 50 percent of the mean

hydraulic-conductivity value. Between soils or treatments, average differences in conductivity of less than

15 or 20 percent are not considered significant.

Calculations

Water flow takes place in accordance with the

equation:

t AL

where Q is the volume of water passing through the

material in time (t) ; A is the area of the soil column,

and k is the average hydraulic conductivity in the soil

interval (AL) over which there is a hydraulic-head

difference (AH). Solving for hydraulic conductivity:
SALINE AND ALKALI SOILS 113

It should be noted that A^ must be measured from

the surface of water standing on the soil to the elevation at which water will stand during the flow test in a

riser or manometer connected at the bottom of the soil

column. For experimental setups sometimes used, this

elevation may be quite different from the elevation of

the bottom of the soil column. The length of the soil

column AL should be measured during or after water

flow and not when the soil is dry.

References

Christiansen (1947), Fireman (1944), and Richards

(1952).

(34c) Hydraulic Conductivity From Piezometer Measurements

Equipment

The piezometer pipe may be of any convenient diameter. The length will be governed by the depth at

which measurements are to be made. The wall thickness should be as thin as practical to minimize soil

disturbance during.installation. Thin-walled electrical

conduit, 1 to 2 in. inside diameter, has been found suitable for hydraulic-conductivity measurements at depths

up to 10 ft. Other pieces of equipment needed for this

measurement are: a screw-type soil auger having a

free-fit inside the piezometer pipe ; a hammer, such as is

used for soil tubes or for steel fence posts, may be used

for driving the pipe; a pump, such as a hand-operated

pitcher pump, with a flexible hose attached to the inlet

is needed to remove water and sediment from the pipe

and the soil cavity; an electrical sounder is convenient

for measuring the depth to the water surface within

the pipe (see Method 35a) ; an ordinary watch is satisfactory for measuring time, except, if the rate of rise

is rapid, two stop watches may be required to obtain

a continuous rate-of-rise record; a soil-tube jack or

other tube puller is useful in recovering the piezometer

pipe.

Remarks

Hydraulic-conductivity measurements by this method

are limited to soils below a water table» An auger

hole is cased with a length of pipe and a cylindrical

cavity is formed at the lower end of the pipe. Ground

water flows into the cavity when water is pumped from

the pipe, and the rate at which the water level rises in

the pipe is a measure of hydraulic conductivity. Although the development of the equation is based upon

an idealized condition of homogenous isotropic soil,

this method may be used for determining the hydraulic

conductivity of nonuniform soils and of individual soil

layers. Information regarding water-table level and

nature and position of subsoil layers should be available

prior to installation of piezometer pipes to assist in

determining proper placement of pipes and construction of cavities. For most purposes, the extremities

of the cavity should not be closer than one cavity length

from either the top or bottom of the particular soil

layer for which the determination is made.

This method is applicable only where a cavity of

known shape can be maintained throughout the test.

In many fine-textured soils, cavities will stand without

support, but in sands and other noncohesive materials

a supporting porous structure may be required.

Procedure

Remove grass sod or debris from the soil surface

and install the pipe to any desired depth by alternately

augering and driving. Auger to a depth of 6 to 12 in.

below the end of the pipe from within the pipe, then

drive or push the pipe to the bottom of the drilled

hole. This is done to minimize soil disturbance as the

pipe is driven. When the pipe has been installed to the

desired depth, auger out a cavity below the pipe. Cavity

lengths of 4 to 8 in. have been found convenient,

with pipes 1 and 2 in. in diameter. The length of cavity

can be accurately controlled by use of a screw clamp

on the auger handle. The cavity should be formed

with a minimum of disturbance to the surrounding

soil.

Remove seepage water and sediment from the cavity

by pumping several times. Measure the depth to the

water in the pipe after allowing enough time for the

water to rise in the pipe to the equilibrium level. In

highly permeable soils the equilibrium level may be attained in a few minutes; in some fine-textured soils

several days may be required. Pump the water from

the pipe and measure the rate at which ground water

rises in the pipe. The rate of rise should be measured

as soon as practicable after pumping, since, it is assumed, in the development of the theory, that the drawdown of the water table is negligible. Rate of rise may

be measured at any point between the water-table level

and the lower end of the pipe, but measurements near

the equilibrium level should be avoided. The rise

increment should be selected to give convenient and

measurable time intervals.

If hydraulic-conductivity determinations are desired

at several depths, measurements can be made with the

same pipe by successively augering to a greater depth

following each determination.

Calculations

Hydraulic conductivity is calculated by use of the

equation given by Kirkham (1946) as follows:

logio {hi/hi)

A (Í2-Íl)

where k is hydraulic conductivity; R is the radius of

tube; A is a geometrical factor (the ^4-function), which

may be read from figure 28; hi is the distance from the

water table to the water level in pipe at time ii; /ig is

the distance from the water table to the water level in

pipe at time ig? f'2~fi is the time interval for water to

rise from h^ to h^. Hydraulic conductivity (k) will be
AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

2.5 5 7.5

DIAMETER OF CAVITY - CM.

2.5 5 7.5 10

LENGTH OF CAVITY - CM.

FIGURE 28.—Relation of the .«4-function to the length and diameter of the cavity for the piezometer method of measuring hydraulic

conductivity. (Redrawn from Luthin and Kirkham, 1949.)

in centimeters per hour if R and A are in centimeters ;

hi and A2 are both in the same units, and (Í2~íi) in

hours. However, any consistent system of units may be

used. For values of the ^-function not shown in the

illustrations see Luthin and Kirkham (1949).

The hydraulic conductivity can also be calculated

from an approximate equation that eliminates the use

of logarithms. The constant inflow-rate equation of

Kirkham (1946), slightly modified, is as follows:

k= ITR" Ah

A {t2-h) h^y

where Ah is the increment of rise of the water level in

the pipe in time tz — h; h^y is the average head, i. e.,

Äav= (Ai + A2)/2; and the other terms are as previously

defined.

This approximate equation is sufficiently accurate

for the usual values of Ah and ^av and may be used to

simplify calculations. The error introduced by using

this equation is small if the ratio Ah/h^ is small, but

increases as the ratio increases. The error in k is less

than 4 percent for ratios of Ah/hi< 0.5 and less than

10 percent for ratios as large as 0.7.

References

Johnson, Frevert, and Evans (1952), Kirkham

(1946), Luthin and Kirkham (1949), and Reeve and

Kirkham (1951).

(34d) Hydraulic Conductivity From Auger-Hole Measurements

Equipment

Soil auger; any convenient size may be used, but it

should permit making a hole below the water table with

a minimum of soil disturbance. Water-level sounder;

an electrical sounder mounted on a frame or tripod

is convenient for measuring depth to water in an auger

hole. In large-diameter auger holes, water levels can

be measured with a rule or tape. A rule attached to a

float provides a convenient means for measuring the

rate of rise of water in an auger hole.

A hand-operated pitcher pump with a flexible hose

attached to the inlet may be used to pump water and

sediment from the auger hole. In addition, a stop

watch is needed for time measurements.

Remarks

lilis method is limited to measurements in the soil

profile below a water table and is applicable only

where a cavity of known shape can be maintained

throughout the test.

Procedure

Drill an auger hole to the desired depth below a

water table with as little disturbance to the soil as

possible. Insert the pump intake hose to the bottom

of the auger hole and empty the cavity several times.

This is done to remove suspended sediment and to

reopen soil pores in the wall that may have been

altered by the auger. Measure the depth to water in

the hole when equilibrium with the surrounding ground

water is attained. In highly permeable soils the equilibrium level may be reached in a few minutes ; whereas,

in some clays several days may be required. Pump the

water from the hole and measure the rate at which the

water rises in the hole while the water level is near

the bottom of the auger hole or at the time that the

auger hole is half full. The rate of rise should be

determined as soon as practical after the water level is

pumped down, since it is assumed in the development

of the theory that the drawdown of the water table is

negligible. A small rise increment should be used since

the Y^-function varies as the hole fills up. The formula

given below involves this assumption.

Calculations

Hydraulic conductivity is calculated by use of the
SALINE AND ALKALI SOILS 115

equation given by Van Bavel and Kirkham (1949) as

follows:

T a' Ah

A id-h) At

where k is the hydraulic conductivity; a is the radius of

the auger hole; A is the .4-function, a geometrical factor which may be read from figure 29, for the case

where the auger hole is empty and where it is half full ;

d is the depth of auger hole below the water table;

h is the depth of water in the auger hole; AA is the

increment of rise of the water level in the hole in the

time interval A t.

Hydraulic conductivity {k) will be in centimeters

per hour if a, c?, A, and A are in centimeters and t is

in hours. However, any consistent system of units

may be used.

In selecting values of the ^-function (fig. 29), information on the depth (5) to an impermeable layer

below the bottom of the auger hole is required. When

an impermeable layer occurs at a depth in the range

from 5 = 0 to 5 =d, the y4-function should be selected

240

220

200

180

160

O

O

z

140

I 120

<

100

80

60 -

40

l\ HOLE HALF FULL (h/d^O.S)

HOLE EMPTY (h/d = 0)

s/d=0.2

s/d = 0

s/d = I.O

s/d = 0.5

.0 8 .12 .16

RATIO- ö/d

24

FIGURE 29.—Relation of the value of the yi-function to the ratio a/rf, for the auger-hole method of measuring hydraulic conductivity:

a, radius of the hole; </, depth of the hole below the water table; A, depth of water in the hole; s, depth of an impervious layer

below the bottom of the hole. The A values in the figure are for auger holes having a radius equal to 5 cm. For any other

radius (x), multiply the A value read from the figure x/^. (Redrawn from Van Bavel and Kirkham, 1949.)
116 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

from the curve for the nearest value of s/d. When

s/d > 1, use the curve for s/d=\. The depth(s) to

an impermeable layer is not a critical factor for the

usual values of a/d. See Van Bavel and Kirkham

(1949) for discussion of errors involved when, without knowledge of the depth to an impermeable layer,

arbitrary values ofs/d are used.

Kejerences

Diserens (1934), Hooghoudt (1936), Johnson, Frevert, and Evans (1952), Kirkham and Van Bavel

(1949), Reeve and Kirkham (1951), and Van Bavel

and Kirkham (1949).

(35) Hydraulic-Head Measurements

Saturated Soil

m

(35a) Piezometers Installed by Driving

Equipment

Iron pipe, % in., galvanized or black, cut in 7, 10.5,

14 ft., or other lengths as desired; hand-operated driving hammer (Christiansen, 1943) or pneumatic driving hammer (Donnan and Christiansen, 1944) ; rivets,

structural iron, % in. in diameter by 1 in.; rivet

punch-out rods (several lengths of % ii^- iri diameter

iron .pipe with male and female flush connections, or

other rods to fit) ; 25 to 50 ft. of semirigid plastic

tubing, 5/16 in. in diameter; ^^ hand-operated bucket

pump; 5-gal. water bucket; and carpenter's level.

After installation it is necessary to measure the depth

to water in the piezometer. This can be done with a

steel tape or other sounding device. Tapes with a dark

oxidized surface show the water-level mark readily, or

chalk can be used to make the water mark more visible.

If many readings are to be made, it is worth while to

construct an electrical water-level sounder. For this,

a length of flexible insulated wire is wound on a reel

that has a socket for mounting on the top of the piezometer pipe. A straight segment of the wire slightly

longer than 1 ft. should be exposed to view between

the reel and the top of the pipe. The lower end of the

wire is weighted with metal tubing to keep the wire

taut, and the upper end is grounded to the reel and pipe

through a battery and high-resistance voltmeter. The

insulated wire is marked at 1-ft. intervals. Fractions

of a foot can be read to the nearest 0.01 ft. from a

scale attached to the reel mount. Readings are taken

on the first mark on the wire appearing above the top

of the pipe when the voltmeter indicates the lower end

of the wire is at the water surface.

Procedure

With an iron rivet in lower end of pipe, drive first

length of pipe into soil. Additional lengths can be

added with standard pipe couplings as driving pro-

^^ Saran tubing, manufactured by the Dow Chemical Co., is

suitable for this use and is often locally available at hardware

stores.

gresses until pipe reaches the desired depth. Leave pipe

extending approximately 1 ft. above ground surface.

If hydraulic-head readings are desired at several depths

at a given location, drive pipes of different lengths into

the soil, spacing the pipes laterally with a separation

of about 1 ft. Use the carpenter's level to set the tops

of all pipes to the same elevation. This makes it convenient to record and interpret hydraulic-head readings. Pipe lengths up to 16 ft. long can be installed

by driving if a stepladder is used. In some soils, the

pipe can be pushed into the ground 5 or 6 ft. before

driving is required so that 21-ft. lengths can sometimes

be used. Insert punch-out rod in pipe and punch rivet

a distance of 3 to 6 in. out of the end of the pipe.

Push the plastic tubing, previously marked with paint

or tape to indicate the pipe length, to the bottom of

the pipe and by pumping water through the tube with

the hand pump, flush out a cavity 3 to 6 in. long below

the end of the pipe. Soil material and water will return to the surface in the annular space between the

tubing and pipe. After the cavity at the base of the

pipe is formed, test the piezometer for response rate

by filling with water and observing the rate at which

the water level drops. If the rate of change in the

level of the water in the pipe is very low, repeat the

flushing operation. In sands and gravels, the rate of

drop may be so rapid that no overflow can be obtained

during flushing; whereas, in clays the rate of drop may

be so slow that it is hardly noticeable. In any event,

the flushing should be repeated without unduly extending the plastic tube below the end of the pipe until

the rate of change of the water level in the pipe after

filling is perceptible. The level of the water in the

piezometer should then be allowed to come to equilibrium with the ground water. It is important to make

this test of the responsiveness of each piezometer because the reliability of readings depends directly upon

the readiness with which the water level in the pipe

responds to hydraulic-head changes in the ground water

at the bottom of the pipe. Piezometers should be retested for responsiveness periodically and reflushed, if

necessary.

In some soils, the rivet in the end may not be necessary. When the piezometer is driven, a soil plug from

3 to 12 in. in length may form in the lower end of the

pipe, which can be removed by the flushing operation.

In many soils, this soil plug can be flushed out in

much less time than is required to punch out the rivet.

After the piezometers have been installed, flashed,

and allowed to come to equilibrium with the ground

water, the depth to water surface from the top of the

pipe is measured and recorded.

Remarks

The hydraulic head of ground water at any given

point, i. e., at the bottom of the pipe, is the equilibrium

elevation of the surface of the water in the piezometer.

This elevation can be referenced to any standard datum.

All hydraulic-head readings in a single ground-water

system or locality should be referenced to the same
SALINE AND ALKALI SOILS 117

datum, mean sea level being commonly used. Water

elevations at each site can be recorded as read from

the top of the pipe. The elevations of the top of the

pipe and the adjacent soil surface are determined by

standard surveying methods.

The hydraulic gradient is the change in hydraulic

head per unit distance in the direction of the maximum

rate of decrease in head. The vertical component of

the hydraulic gradient at a site where piezometers have

been installed at several depths is equal to the difference in the equilibrium elevation of the water surface

in two pipes divided by the différence in the elevation

of the cavities at the bottoms of the pipes. This is an

average value for the vertical component of the hydraulic gradient in the depth interval. (See Method 36 for

graphical procedures that are useful in the interpretation of hydraulic-head readings.)

References

Christiansen (1943), Donnan

(1944), and Richards (1952).

and Christiansen

(35b) Piezometers Installed by Jetting

Equipment

Iron pipe, %-in. in diameter, galvanized or black,

10.5-ft. lengths, threaded at both ends; power-driven

pump, 300 to 600 Ib./in.^, 10 to 15 gal./min. capacity

with a water tank of 300-gaL capacity, truck- or trailermounted (an auxiliary 300-gal. water tank, truckmounted, is also desirable) ; 25 to 50 ft. high-pressure

hose, %-in. in diameter, with a swivel coupling for

attachment to the pipe; driller's mud; and steel measuring tape or electrical sounder.

Procedure

The installation of piezometers by the jetting technique makes use of the eroding and lubricating properties of a stream of water issuing from the end of the

pipe for opening a passage into the soil. Piezometers

may be installed by hand or with simple hoisting and

handling equipment, such as has been used in Coachella

Valley, California, and described by Reger and associates (1950). During installation, the pipe ¡s oscillated up and down from 1 to 2 ft. to facilitate the

jetting. Water and soil material in suspension return

to the surface around the outside of the pipe. The

return flow acts as a lubricant for the upward and

downward movement of the pipe and serves as a means

for logging materials penetrated. An adjustable

measuring tape used with the Coachelia jetting rig

serves to indicate depth of penetration to ±0.1 ft. If

the jetting is done without a rig, the pipe should be

marked at 1-ft. intervals to facilitate logging.

An estimate of texture and consolidation of the material is made from {a) the nature of the vibrations in

the pipe that are transmitted to the hands of the operator, (6) the rate of downward progress, (c) examination of sediments carried by the effluent, and (d)

observation of color changes of the effluent. Logging

subsurface layers by this method requires experience

that can be gained and checked by jetting in profiles

for which data on stratigraphy are available from

independent logging procedures.

Return flow may be lost and penetration may stop in

permeable sands and gravels. A commercial preparation, Aquagel, a form of driller's mud, was found by

Reger and associates (1950) to be effective for maintaining return flow in coarse materials. Approximately

10 lb. per 100 gal. of water was sufficient for jetting

conditions encountered in the Coachella Valley. It is

necessary to add this preparation to the water supply

slowly and to agitate thoroughly as it is added.

A record of the depth and nature of material penetrated is kept as the jetting progresses. Where several

hydraulic-head measurements are desired at different

depths, the deepest pipe is usually installed first. The

log from the first pipe serves for selecting depths at

which additional pipes are to be installed. It is often

desirable to terminate piezometers in sandy lenses to

increase the rate at which they respond to hydraulichead changes in the soil. Jetting is stopped immediately as each pipe reaches the desired depth, so that

excessive washing of material from around the pipe

will not occur. The material in suspension settles back

around the pipe and usually provides a satisfactory seal.

Several pipes that terminate in the soil at different

depths may be installed as close as 1 ft. apart. Experience has shown that, under most conditions, the effect

of leakage along the pipe or from one pipe to another is

negligible.

After the piezometers are installed, they are flushed,

reference elevations are set, and readings are made as

outlined in Method 35a. (For details of jetting-equipment construction, refer to the article by Reger and

others Í1950).)

References

Pillsbury and Christiansen ( 1947}, Reeve and Jensen

( 1949), and Reger and others ( 1950 ),

(35c) Observation W ells^ Uncased or With

Perforated Casing

Equipment

Soil auger; perforated tubing or pipe; steel tape, or

electrical sounder.

Procedure

It is desired to measure depth to water lable. An

uncased auger hole can often be used to measure depth

to water table. Where soils are sandy and will not

stand or where a more permanent well is desired, an

auger hole may be cased with perforated casing. Sometimes it is necessary to install the casing during the

augering process.

Water-table observation wells are usualiv installed

to a depth great enough to reach the minimum ex-
118 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

pected position of the water table. As a result, observation wells are sometimes installed to considerable

depths and perforated throughout a portion or all of

the underwater length. Under many conditions, waterlevel readings in such wells coincide with the watertable level, but if there is a vertical flow component of

water in the soil, either upward or downward, waterlevel readings in an open or perforated casing well may

not represent the true water-table level. Where verticalflow conditions occur, the water level in a perforated

cased well represents a steady-state flow condition

within the well itself, and may not give useful information. Such a condition is more likely to occur where

an observation well penetrates layers that differ greatly

in permeability. Where such conditions occur or where

there is any question about water-table readings, hydraulic-head determinations should be made at several

depths in the profile by the use of piezometers.

The elevation of the water table can be determined

by a graphical method as follows. Plot the elevation

of the terminal points of the piezometers in the soil

as a function of the corresponding pressure heads, i. e.,

the lengths of the columns of water standing in the piezometers. Extrapolate this curve to zero pressure-head

to obtain the water-table elevation. Abrupt changes in

soil permeability with depth in the vicinity of the water

table complicate the use of this method and make it

necessary to install piezometers at or near the water

table.

(36) Ground-Water Graphical Methods

(36a) Water-Table Contour Maps

Equipment

Drafting instruments and supplies.

Procedure

On a scale map of the area being investigated, write

in the water-table elevations at locations at which watertable level measurements have been made. By standard mapping procedures used for ground-surface contour plotting, i. e., interpolation and extrapolation,

draw in lines of equal water-table elevations. Thé

principles that apply in surface contour mapping also

apply for water-table contours. Where slopes change

abruptly, more points are required to locate the contours accurately. Conversely, in areas of little change

in slope, measurement points may be farther apart. In

areas of rolling or varied topography where water

tables in general follow surface slopes, the number of

data required to construct water-table contours may be

prohibitive. A water-table contour line is the locus of

points on the water-table surface for which the hydraulic head is constant. In a three-dimensional flow system, such a line represents the intersection of an equal

hydraulic-head surface with the surface of the water

table.

Water-table contour maps provide direct visual information on the slope of the water table, and it is to

be expected that generally there is a horizontal movement of ground water in the direction of slope of the

water table. In the absence of subsurface artesian

conditions and if the areal application of water to the

soil surface is uniform, a region of steep slope of the

water table would be expected to occur where barriers

to the horizontal movement of ground water occur or

where the hydraulic conductance of the soil strata

below the water table is low. On the other hand, areas

of low slope in the ground-water table may indicate

the presence of aquifers that permit the ready transfer

of ground water in the horizontal direction. Such

information is pertinent to the analysis and solution of

drainage problems.

( 36b ) Water-Table Isobath Maps

Equipment

Drafting instruments and supplies.

Procedure

On a scale map of the area, write in depths to water

table from the ground surface at locations at which

water-table and ground-surface elevations have been

obtained. Construct isobath lines, i.e., lines of equal

depth to water table, by the standard mapping procedures that are used for ground-surface and watertable contour mapping, i. e., interpolation and extrapolation, and other procedures. Where either surface

topography or water-table slopes change abruptly, more

points of measurement are required for accurate construction of equal depth-to-water lines.

Depth to water table may also be shown by circumscribing areas within which depth to water table is in

a specified range. On a scale map, note depths to

water table as above. Select a convenient number of

•depth ranges, such as 0 to 2, 2 to 4, 4 to 10, 10 to

20, > 20, and delineate areas within which depth to

water table is in the designated ranges. Distinguish

between areas with a Crosshatch, color, or other convenient code system. Maps such as the foregoing provide graphic information on the adequacy of drainage

and, therefore, aid in showing areas in which artificial

drainage may be needed.

(36c) Profile Flow Patterns for Ground

Water

Equipment

Drafting instruments and supplies.

Procedure

On a profile section showing the soil surface and

available information on subsoil stratigraphy, write in

hydraulic-head values at points where hydraulic-head
SALINE AND ALKALI SOILS 119

120

no

LÜ

2

9100

<

>

UJ

-I

ÜJ

üj 90

o

z

LÜ

o:

LÜ

u.

LÜir 80

70

_GR0UND SURFACE ^^/// ^^v//// V> vy/z/V^AyV/' VH^V//' '\^vU-//.' Vv^\V/,» ;> \^\xy//v//

30 20

WEST

10 O 10

DISTANCE FROM DRAIN- FEET

FIGURE 30.—Equal hydraulic-head lines below a water table on a profile section in the vicinity of an open drain. Example from Delta

area, Utah. The direction of the hydraulic gradient is represented by arrows and indicates upward water movement from an

underlying source.

measurements have been made, i. e., points where

piezometers terminate in the soil (fig. 30). By standard methods, which are used for contour mapping,

interpolation, and extrapolation, draw lines to connect

points of equal hydraulic head. Convenient hydraulichead intervals may be selected, extending over the range

of measured values for hydraulic head. Usually an

interval is selected that allows a number of equal

hydraulic-head lines to be sketched on the same profile.

The component of flow in the plane of the profile is

normal to lines of equal hydraulic head, if the profile

section is plotted to a 1 : 1 scale. With the 1 : 1 scale,

flow lines can be sketched in at right angles to the

equal hydraulic-head lines, with arrows to show the

direction of flow. If the vertical scale is exaggerated

the relation between stream lines and equal hydraulic

head lines on the plotted profile is no longer orthogonal

Where the vertical and horizontal scales are not equal

therefore, the hydraulic-head distribution may be prop

erly plotted, but flow lines should not be indicated.

For cases where hydraulic head changes in a vertical

direction, indicating a vertical component of flow, the

elevation of the water table can be determined by piezometers that terminate at the water-table level, or by

extrapolation from a series of known points below the

water table, as outlined in Method 35c. Draw equal

hydraulic-head lines to intercept the water table at the

respective equal hydraulic-head elevations.

An equal hydraulic-head line may intercept the water

table at any angle, depending upon the flow direction.

The water table is not necessarily a flow line as is often

assumed, although it may be. A component of upward

flow that exists below the water table may continue

upward through the soil above the water table to the

soil surface by capillarity. Likewise, downward flow

may occur in the unsaturated soil above a water table.

Kejerences

Christiansen (1943), Reeves and Jensen (1949).

(36d) Water-Table Isopleths for Showing

Time Variations in the Elevation

of the Water Table

Equipment

Drafting instruments and supplies.

Procedure

A large seasonal variation in the water table often

occurs in irrigated areas. In such cases it may be useful to show graphically the variations, both in space

and time, by the use of water-table isopleths.^^ By this

*^ Private communication from M. Ram, Water Utilization

Division, Ministry of Agriculture, Tel Aviv, Israel.
120 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

method a series of observation wells is established on

a straight line across the area under investigation and

water-table elevations are recorded over a period of

time.

The graphical representation of the data is accomplished as follows: On a vertical scale at the left margin

of a sheet of tracing paper, make a time scale on which

the dates are shown for the various sets of readings of

the observation wells. Start the time scale near the

top of the sheet with the initial set of readings. Draw

horizontal constant-time lines across the sheet at the

time values for the various sets of readings. Across

the top of the sheet, draw a profile of the elevation of

the soil surface along the line of observation wells. Use

any convenient vertical scale and mark the location of

the observation wells on the horizontal scale. Draw in

horizontal lines representing convenient elevation intervals over the range of variation in the elevation of the

water table and plot the water-table profile for the initial

set of readings. Project the points of intersection of

this profile curve with the elevation scale lines downward to the horizontal constant-time line of the initial

set of readings. Place elevation numbers above these

points on the constant-time line. Repeat this process,

locating successive elevation points on successive constant-time lines for each set of well readings. Connect

constant water-table elevation points on successive constant-time lines with smooth curves. These curves are

called isopleths and show the variation with time of

the points of equal water-table elevation along the line

of water-table observation wells.

The sources of ground water as well as subsurface

stratigraphy must be taken into account in the interpretation of isopleths. The method provides a convenient graphical summary of water-table observations

and can be used to advantage in showing the rate of

subsidence of the water table following an irrigation

season. This information relates directly to the drainability of soils.

Physical Measurements

(37) Intrinsic Permeability

(37a) Permeability of Soil to Air

Apparatus

The apparatus for this measurement is shown in

figure 31. Compressed air is admitted through a calcium chloride drying tube to an airtight tank of constant volume. An outflow tube leads to a water

manometer and to the soil sample container. The soil

sample container consists of a tinned iron can with an

extension made from a 4-cm. section of brass tubing

counter-bored to give a snug fit on the top of the can.

Punch an outlet hole, approximately 5/32 in. in diameter in the bottom of the can. Use a disk of brass

screen, 20- or 40-mesh, with 2 layers of fiberglass sheet

CALCIUM CHLORIDE

DRYING TUBE

AIR STORAGE

TANK

WATER

MANOMETER

SOIL SAMPLE

CONTAINER

"W

FIGURE 31.—Apparatus using air flow to measure the intrinsic

permeability of soil by Method 37a. The manometer readings yi and ya represent successive heights of the free-water

surface above the rest position.

as a filter in. the bottom of each can. The soil-packing

machine, mentioned below, is useful for this measurement.

Remarks

This method for measuring air permeability can be

used for either disturbed or undisturbed samples. The

following procedure is for disturbed soils. In order

to get consistent results for comparing one soil with

another and to determine the effect of various treatments, a standard procedure for preparing and packing

the soil must be followed. Make determinations in

triplicate.

Procedure

F*ass air-dried soil through a wire-mesh sieve with

1-mm. openings. Obtain representative subsamples as

outlined in Method 1. Attach the brass cylinder to the

top of a can and fill the container about three-fourths

full of soil. Dump the soil, which has been well mixed,

into the container rather than by pouring or scooping.

With a spring-load of 3-kg. wt. on top of the soil, drop

the container 200 times on a solid block of wood from

a height of 2.5 cm. A cam-operated mechanical dropper has been used for this purpose. Remove the brass

cylinder from the can and use a spatula to strike the

soil off level with the top of the can. Cover the soil

with a disk of filter paper, place a lid on the can, and

complete the seal with a tight rubber band or beeswax.

Connection to the air source is made by means of a short

piece of copper tubing soldered to the lid of the can.

Pass compressed air through the drying tube to the

tank until a manometer displacement of 40 to 50 cm.

of water is attained. Record the air temperature in

the tank and measure the rate of drop of the manometer level as air is allowed to flow from the tank

through the soil sample. It is desirable to allow an

initial 10-cm. drop of the manometer level before

height and time readings are started.

Calculations

The intrinsic permeability of the soil using air is

given by the equation :
SALINE AND ALKALI SOILS 121

k\ = 2.30LVSr,

AP,

in which

A:'a'^ Intrinsic permeability with air, cm.^

L =Length of soil column, cm.

F = Volume of tank, cm.^

7; = Viscosity of air at the temperature at which the

determination was made, dyne sec. cm."^

(poises).

A = Cross-sectional area of sample, cm.^

Pa=Atmospheric pressure, dynes/cm.^

5 — Slope of log y vs. time curve ==

logioji-logioy^

At

j== Displacement of the water surface in one arm

of the manometer, cm.

Ai= Time interval in seconds for the water surface in the manometer to drop from ji to 72-

The time for a convenient and measurable drop in

manometer level can be controlled by the volume of the

tank used. For most soils prepared as outlined above, a

213-liter (55-gal.) drum gives a convenient time interval for a measurable manometer change. For soils of

lower permeability, a 24-liter tank is used.

While c. g. s. units are suggested above, any consistent set of units can be used.

References

Kirkham (1947), Soil Science Society of America

(1952).

{37b) Permeability of Soil to Water

Apparatus

Constant-level water-supply reservoir, glass siphon

tubes, and rack for supporting samples; soil containers

made from 3-oz. tinned iron soil cans with %2-inch hole

punched in bottom and brass cylinder extensions,

4-cm. high; 50- or 100-ml. graduates.

Procedure

The procedure for preparing and packing samples is

as outlined in Method 37a. Water-permeability determinations may be made on the same samples used for

air permeability. Care must be exercised to avoid disturbance of the sample in handling.

Following the air-permeability determination, place a

brass cylinder extension on top of the can containing

the soil sample and seal in place with an elastic band or

beeswax. Place the soil sample on the rack, cover the

soil surface with filter paper, and admit water to the

sample from the supply reservoir with the siphon. The

water level is adjusted so that the height of soil plus

water column is 2 times the soil column, giving a

hydraulic gradient of 2.

Record the water temperature, the time at which

water is admitted to the container, and the time at

which water first percolates through the sample. Measure the volume of percolate for a number of successive

time intervals. The amount of water passed through

the soil and the number of volume measurements made

will depend upon the purpose of the determination.

Usually 3 to 6 in. is used, a depth corresponding to an

irrigation. For comparing one soil with another and

for determining the effect of various treatments on a

given soil, use a value obtained after the hydraulic

conductivity has become more or less constant.

Calculations

Intrinsic permeability of the soil using water is given

by the equation :

h\- '^ k- '^ ^^

dwg dy,g A AhAt

in which

k\ = Intrinsic permeability with water, cm.^

A:=^Hydraulic conductivity, cm./sec.

F = Volume of percolate in time, Ai, cm.^

L = Length of soil column, cm.

A/i = Difference in hydraulic head between the inflow and outflow ends of the soil column,

cm.

^ = Cross-sectional area of the soil column, cm.^

Ai= Time interval for volume of percolate V to

pass through the soil, sec.

7; = Viscosity of water at the recorded temperature, dyne sec. cm."^ (poises).

c/w —Density of water, gm./cm.^

g = Acceleration of gravity, cm. sec."^

Intrinsic permeability is related to hydraulic conductivity as indicated by the above equation. If desired,

hydraulic conductivity may first be calculated and then

converted to intrinsic permeability by multiplying by

the ratio -q/dy.g. While c. g. s. units are suggested

above, any consistent set of units can be used.

Remarks

Soils may be compared with respect to permeability

on the basis of values obtained at a fixed time after

wetting or after a specified amount of water has passed

through the sample. For comparing changes in structure between different soils, the time basis has been

found to be preferable.

Reference

Soil Science Society of America (1952).

(38) Bulk Density

Apparatus

Balance, drying oven, moisture boxes, and core sampler. The latter can be anything from an elaborate

power-driven machine to a short section of thin-walled

brass tubing with an internal closely fitting ring of

clock spring soldered in place to form the cutting lip.

(See drawing of soil sampler in Appendix.)
122 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Procedure

Details of procedure will depend on the type of core

sampler and soil conditions. Usually a flat soil surface, either horizontal or vertical, is prepared at the

desired depth, and the core sampler is pressed or driven

into the soil. Care should be taken to see that no compaction occurs during the process, so that a known

volume of soil having field structure is obtained. The

oven-dry weight of the sample is then determined.

Calculations

Bulk density (¿b) = ( wt. of oven-dry soil core) /(field

volume of sample). Bulk density is expressed as

pounds per cubic foot or grams per cubic centimeter.

For practical purposes, the latter is equal numerically

to apparent specific gravity or volume weight.

(39) Particle Density

Apparatus

Balance, vacuum desiccator, and pycnometers.

Procedure

Weigh a pycnometer when filled with air (JFa),

when filled with water (íF^), when partially filled with

an oven-dried sample of soil [W^), and when completely filled with soil and water (Wsw)- To exclude

air, pycnometers containing the soil with enough water

to cover should be subjected to several pressure reductions in a vacuum desiccator and then allowed to stand

for a number of hours under reduced pressure before

completely filling with water for weighing W^^. The

particle density (dp) of the soil in gm. cm."^, is then

given by the formula:

d,=d^ (r«-ra)/(r,+r,-ra-rsw)

where rfw is the density of the water in gm. cm."^.

Slightly different and perhaps better values will be

obtained for dp if a nonpolar liquid such as kerosene,

xylene, or acetylene tetrachloride is used for the displacing liquid.

(40) Porosity

The porosity of soil is the fraction of the soil space

not occupied by soil particles. The porosity (n) may

be calculated from the formula:

n= {dp — db)/dp

if the bulk density (d^,) and the particle density (dp)

are known. Solutions of this equation may be found

graphically by use of the nomograms given at the right

of figures (ch. 2).

(41) Particle-Size Distribution

Remarks

The method as given is essentially that described by

Kilmer and Alexander (1949), except that the 0.005-

mrn. determination has been omitted. The names used

at present by the United States Department of Agriculture for the soil separates are as follows: The diameters

2.0 to 1.0,1.0 to 0.5, 0.5 to 0.25, 0.25 to 0.1, and 0.1 to

0.05 mm., respectively, separate very coarse, coarse,

medium, fine, and very fine sands; particles from 0.050

to 0.002 mm. are called silt, and particles with effective

diameters less than 0.002 mm. are designated as clay.

With the International System, the diameters 2.0, 0.2,

and 0.02, respectively, separate the classes represented

by the numerals I, II, and III, while particles of diameters less than 0.002 mm. are represented by IV.

Apparatus

Set of sieves; size openings, 2-, 1-, and 0.5-mm. round

hole ; 60-, 80-, 140-, and 300-mesh per in. Pyrex nursing bottles, 8-oz., with rubber stoppers; Lowy 25-ml.

automatic pipet ; haw pipet rack ; Pasteur-Chamberland

filters, short, "F" fineness. Analytical balance, drying

oven, steam chest, motor stirrer, reciprocating shaker,

desiccator, beakers, and evaporating dishes.

Reagents

A. Hydrogen peroxide, 30 percent solution.

B. Dispersing agent. Dissolve 35.7 gm. sodium

metaphosphate and 7.94 gm. sodium carbonate in water

and dilute to 1 liter. The sodium metaphosphate is

prepared as follows: 125 gm. of monosodium phosphate (NaH2P04-H20) is slowly heated in a platinum

dish to 650° C. This temperature is held for 11/^ hr.

The platinum dish and its contents are removed from

the furnace and the sodium metaphosphate is cooled

rapidly by pouring it out in narrow strips on a clean

marble slab. The sodium carbonate is used as an

alkaline buffer to prevent the hydrolysis of the metaphosphate back to the orthophosphate which occurs in

acidic solutions.

Procedure

GENERAL STATEMENT.—Samples are routinely run in

sets of eight; the necessary equipment is designed

accordingly. The sample is treated with hydrogen

peroxide, washed, filtered, and dispersed. The sand is

separated from the silt and clay by washing the dispersed sample through a 300-mesh sieve. The various

sand fractions are obtained by sieving, while the

20-/A and 2-/i fractions are obtained by pipeting.

Organic matter is determined on a separate sample by

the dichromate reduction method (Peech and coworkers, 1947).

PREPARATION OF THE SAMPLE.—The air-dried sample

is mixed and quartered. The quarter reserved for

analysis is rolled with a wooden rolling pin to break

up the clods. The sample is then passed through a

sieve with 2-mm. round holes. Rolling and sieving of

the coarse material are repeated until only pebbles are

retained on the sieve. The material not passing the

sieve is weighed and reported as a percentage of the

air-dry weight of the whole sample.
SALINE AND ALKALI SOILS 123

REMOVAL OF ORGANIC MATTER.—A 10-gm. sample

of the air-dry soil containing no particles larger than

2 mm. is weighed on a rough balance and placed in a

250-ml. electrolytic Pyrex beaker. About 50 ml. of

water is added, followed by a few milliliters of 30 percent hydrogen peroxide. The beaker is then covered

with a watch glass. If a violent reaction occurs, the

cold hydrogen peroxide treatment is repeated periodically until no*more frothing occurs. The beaker is

then heated to about 90° C. on an electric hot plate.

Hydrogen peroxide is added in 5-ml. quantities at

about 45-min. intervals until the organic matter is

essentially removed as determined by visual inspection.

Heating is then continued for about 30 min. to remove

any excess hydrogen peroxide.

REMOVAL OF DISSOLVED MINERAL MATTER.—Following the hydrogen peroxide treatment, the beaker is

placed in a rack and about 150 ml. of water is added

by means of a jet strong enough to stir the sample

well. The suspension is filtered by means of a short

Pasteur-Chamberland filter of "F" fineness. Five such

washings and filterings are usually sufficient except

for soils containing much coarse gypsum. Soil adhering to the filter is removed by applying a gentle backpressure and using the forefinger as a policeman. The

beaker is then dried on a steam bath, placed overnight

in an oven at 110° C, cooled in a desiccator, and then

weighed to the nearest milligram. After the sample is

transferred to a nursing bottle for dispersion, the ovendry weight of the beaker is obtained. Weight of

oven-dry organic-free sample is used as the base weight

for calculating percentages of the various fractions.

DISPERSION OF THE SAMPLE.—^To the oven-dry

sample is added 10 ml. of sodium hexametaphosphate

dispersing reagent B, and the sample is transferred to an

8-oz. Pyrex glass nursing bottle by means of a funnel,

a rubber policeman, and a jet of water. The volume

is made to 6 oz., and the bottle is stoppered and

shaken overnight on a horizontal reciprocating shaker

with 120 oscillations per minute. A similar volume of

dispersing agent is placed in a liter cylinder, the volume

made to 1,000 ml. and well mixed. A sample is taken

with the pipet, dried, and weighed to obtain the weight

correction referred to in the section on calculations.

This weight correction is obtained for each new solution of sodium metaphosphate.

SEPARATION OF THE SANDS FROM SILT AND CLAY.—

The dispersed sample is washed on a 300-mesh sieve,

the silt and clay passing through the sieve into a 1-liter

graduated cylinder. The sieve is held above the

cylinder by means of a clamp and a stand. Jets of

water should be avoided in washing the sample. The

sieve clamp is tapped gently with the side of the hand

to facilitate the washing procedure. Washing is continued until the volume in the cylinder totals about 800

ml. The sands and some coarse silt remain on the

sieve. It is necessary that all particles of less than

20;* diam. be washed through the sieve. The sieve

is removed from the holder, placed in an aluminum

pan, and dried at 110° to 120° C. While the sands are

drying, another sieve is used for the next sample. The

material on the sieve is then brushed into a platinum

dish and further dried for about 2 hr. The dish is then

placed in a desiccator, the contents to be sieved and

weighed when convenient. The silt and clay suspension in the cylinder is made up to 1 liter with distilled

water, covered with a watchglass, and set aside until the

pipetings are to be made.

PiPETiNG.—Pipetings are made for the 20/* and 2/x

particles in the order named. The 20/x particles are

pipeted at a 10-cm. depth, the sedimentation time varying according to the temperature. The 2/x fraction is

pipeted after a predetermined settling time (usually

6 to 61/^ hr.), the depth varying according to the time

and temperature. A Lowy 25-ml. automatic pipet with

a filling time of about 12 sec. is used. Prior to each

sedimentation process, the material in the sedimentation

cylinder is stirred for 6 min. with a motor-driven stirrer

(8 min. if the suspension has stood for more than 16

hr.). After removal from the stirrer, the sedimentation cylinder is surrounded with insulating material

and the suspension is stirred for 30 to 60 sec. with a

hand stirrer, an up-and-down motion being used. This

stirrer is made by fastening a circular piece of perforated brass sheeting to one end of a brass rod. A

wide rubber band is placed around the edge of the brass

sheeting to prevent abrasion. The time is noted at completion of the stirring. About 1 min. before the sedimentation is complete, the tip of the 25-ml. pipet is

lowered slowly into the suspension to the proper depth

by means of a Shaw pipet rack. The pipet is then filled

and emptied into a 60-ml. weighing bottle having an

outside cover. One rinse from the pipet is added. A

vacuum is used to dry the pipet for use on the next

sample. The weighing bottle is dried in an oven at 95°

to 98° C. and then further dried for about 4 hr. at

110°. The initial drying is done at a lower temperature to prevent spattering of the suspension. The

weighing bottle is then cooled in a desiccator containing phosphorus pentoxide as a desiccant and weighed.

SIEVING AND WEIGHING THE SAND FRACTIONS.—^The

dry sands, including some coarse silt, are weighed and

brushed into a nest of sieves. Sieves and specifications

are as follows:

Sieve Opening

(mm.)

1.0__.

0.5.__

0.25..

0.177.

0.105-

0.047_

Specifications

Perforated brass plate, round holes. No.

3 straight, 0.04-in. diam. holes, 240

holes per in.^

Perforated brass plate, round holes, No.

00 staggered, 0.02-in. diam. holes,

714 holes per in.^

60-mesh, Bureau of Standards (Phosphor Bronze wire cloth)

80-mesh, Bureau of Standards (Phosphor Bronze wire cloth)

140-mesh, Bureau of Standards (Phosphor Bronze wire cloth)

300-mesh (Phosphor Bronze wire

cloth), 0.0015-in. wire.
124 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

(An 80-mesh sieve is included in order to obtain

International Society of Soil Sciences fraction I.) The

sands are then shaken for 3 min. on a shaker having

vertical and lateral movements of l/^ in., making 500

oscillations per minute. For a different shaker, the

time of shaking would have to be determined by

microscopic study. The summation method of weighing is used. The first sand fraction is weighed, the

second fraction added to it, the total weight determined,

and so on. If the sum of the weights of the fractions

is equal to the total weight, it is assumed that no weighing error has been made.

Calculations

PiPETED FRACTIONS.— (^ — B) Ä^Z) =percent of pipeted fraction where y4 = weight in gm. of pipeted fraction, ß=-weight correction for dispersing agent in gm.),

K=

1,000

and

volume contained by pipet

100

organic-free oven-dry weight of total sample

(The 20jLt fraction) — (the 2ju. fraction) = International Society of Soil Sciences fraction III. United

States Department of Agriculture silt is obtained by

subtracting the sum of the percentages of sand and

clay from 100. International Society of Soil Sciences

fraction II is obtained by subtracting the sum of the

percentages of fractions I, III, and IV from 100.

SAND FRACTIONS.—

(Weight in grams of fraction on sieve X 100) -^

(organic-free oven-dry weight of total sample) =

percent of fraction

References

Kilmer and Alexander (1949), Peech and others

(1947), and Tyner (1940).

(42) Aggregate-Size Distribution

(42a) Wet Sieving

Remarks

This is a modification of the mimeographed tentative

method that was distributed in August 1951 by the

Committee on Physical Analyses of the Soil Science

Society of America. The method in brief consists of

placing a sample of soil on a nest of sieves that is

oscillated vertically under water. The amount of soil

remaining on the individual screens is determined, and

aggregation is expressed as the mean weight-diameter

of the aggregates and primary particles. After weighing, the aggregate separates are combined and dispersed and washed through the nest of sieves. The

resulting separates make it possible to correct the previous separates of aggregates for primary particles and

to calculate the aggregation index. This is a singlevalue index of the aggregation of a soil.

Apparatus

Yoder-type wet-sieving apparatus, sieve holders, 4

sets of 5-inch sieves with 2-, 1-, 0.5-, 0.25-, and 0.10-mm.

openings (corresponding to United States Screens Nos.

10, 18, 35, 60, and 140), drying oven, moisture cans,

balance Pyrex watchglasses, and 6-in. diameter porcelain funnel.

Procedure

Collect the soil sample with spade or garden trowel,

preferably when the soil is moist, avoiding excessive

compaction or fragmentation of soil. Dry the sample

slowly and, when sufficiently friable, pass it gently

through an 8-mm. sieve and air-dry. If the soil is

stony, pass the sample through a 4-mm. sieve and discard all primary material greater than 4 mm. in size.

Mix the soil and take subsamples in accordance with

Method 1. Make determinations in duplicate on 40- to

60-gm. subsamples. Weigh the subsamples to the

closest 0.1 gm. and determine the moisture content by

drying a separate subsample at 105° C.

Install the nests of sieves in the water slowly and at

a moderate angle to avoid entrapping air bubbles below

the sieves. Adjust the mechanism so that the top sieve

makes contact with the water surface when the oscillation mechanism is at the top of its stroke. Distribute

the sample on the top sieve so that wetting occurs by

capillarity and wait 5 to 10 min. after the soil surface

appears wet to insure saturation of the aggregates.

Oscillate the sieves for 30 min. with a stroke of 3.8 cm.

and a frequency of 30 cycles per minute, keeping soil

submerged at all times. Some attention may be required during the first few minutes of operation, in

order to prevent water from spilling over the top sieve,

and later, to prevent the top sieve from rising above

water level.

Remove the sieves from the water and drain for a few

minutes in an inclined position. Remove excess water

from the bottom of the screens with absorbent tissue

and place the sieves on watchglasses. Dry in a circulating oven at not higher than 75° C. because high

temperatures cause some soils to adhere. Then remove

the soil from the sieves, dry at 105°, and weigh.

In order to determine how much of the soil retained

on the individual sieves represents aggregates and how

much is gravel and sand, the oven-dried soil taken from

the five sieves is dispersed and washed through the

sieves with a stream of water. The oven-dry weight of

the primary particles remaining on each sieve is then

determined.

Calculations

The amount of soil remaining on each sieve is expressed as percentage of the total sample. Prepare a
SALINE AND ALKALI SOILS 125

graph, plotting the accumulated percentage of soil

remaining on each sieve as ordinate against the upper

limit of each fraction in millimeters as the abscissa,

and measure the area shown by the curve connecting

these points and by the ordinate and the abscissa. If

1 mm. (sieve size) represents 1 unit of the abscissa and

10 percent a unit on the ordinate, a square unit will

represent 0.1 mm. mean weight-diameter of the aggregates of the sample. Multiplying the number of square

units of the area by 0.1 gives the mean weight-diameter

of the entire sample, including the material that has

been washed through the smallest sieve.

The results from the wet sieving of the dispersed

sample are plotted and calculated in the same way.

The difference between the mean weight-diameters of

the original and the dispersed samples gives the aggregation index.

Remarks

The water container in which the sieve nest is oscillated can be of any desired size or shape, providing

its area is at least 1.6 times the area of the sieves. The

temperature of the water should be in the range 20°

to 24° C, and the water should not be excessively

saline. Fresh water should be used for each set of

determinations. Rubber bands cut from old inner

tubes are convenient for holding loosely fitting sieves

together.

References

Russell (1949), Van Bavel (1950).

(42b) Aggregation of Particles Less than

50 Microns

Remarks

This procedure measures the degree of aggregation

of the silt and clay (less than 50/x) fraction for those

soils that do not contain enough large aggregates to be

adequately characterized by wet-sieving. The method

involves measuring the concentration of two suspensions of the same soil, one of which is dispersed by

any standard dispersion procedure to give total silt

plus clay. The other suspension, prepared by mild

(end-over-end) agitation of the sample in water, gives

a measure of the unaggregated silt plus clay. The

difference in concentration between the two suspensions

provides a measure of the amount of silt plus clay

particles th2t is bound into water-stable aggregates

larger than SO/x in size.

This procedure may also be used as a rapid exploratory test to determine the effect of various soilaggregating chemicals in producing water-stable

aggregation.

Apparatus

Dispersion apparatus with high-speed stirring motor,

metal cup, 1-liter hydrometer jars, thermometer, and

Bouyoucos hydrometer or hydrometer-pipet. If the

pipet procedure is used, a Lowy automatic pipet, a

Shaw pipet rack, and tared moisture boxes are needed.

Procedure

Weigh two 50-gm. subsamples of air-dried soil

prepared as in Method 42a. Make a moisture determination on a separate subsample.

TOTAL SILT PLUS CLAY.—Disperse one of the subsamples in the dispersion apparatus. Transfer to a

hydrometer jar and dilute with distilled water to the

required volume. For procedure a (below) the final

volume is 1,130 ml., determined with the hydrometer

in the suspension; for procedures b and c, the volume

is 1,000 ml. (Note: If either procedure 6 or c is to be

used, stopper and invert the cylinder 2 or 3 times and

record the temperature. This is necessary in order to

determine in advance the settling time used.)

Stopper, invert, and shake the cylinder vigorously several times and determine the total silt plus clay in the

suspension as directed under procedures a, b, or c, given

below.

UNBOUND SILT PLUS CLAY.—Incline the hydrometer

jar containing the second subsample to a nearly horizontal position and shake lightly to spread the sample

over a distance of 10 or 12 cm. along the side of the jar.

Add distilled water slowly and in such manner as to

favor wetting by capillarity rather than by flooding.

When soil is completely wetted, dilute to the appropriate volume, as given above, but do not allow water to

fall directly on soil. Allow the soil to slake for at least

15 min. Record the temperature. Stopper the cylinder

and gently invert it 20 times (do not shake) within a

period of about 40 sec, requiring about 1 sec. for

inversion with a 1-sec. interval between inversions.

After the required settling period, determine the amount

of unbound silt plus clay in suspension by the same

procedure a, b, or c used for the total silt plus clay

measurement.

PROCEDURES FOR MEASURLNG CONCENTRATION OF SUSPENSIONS.—(a) Hydrometer. After final mixing, insert the hydrometer and take a reading after 40 seconds, as prescribed by Bouyoucos (1936). Immediately record the suspension temperature in degrees F.

and apply a temperature correction as follows: Add

0.2 to the hydrometer reading for each degree above

67° F.; subtract if below.

The corrected hydrometer reading gives the grams

per liter of silt plus clay in suspension.

(b) Hydrometer-pipet. The hydrometer-pipet (Hellman and McKelvey, 1941 ) measures the concentration

of soil particles in grams per liter of a suspension that

has been pipeted from a known depth and that is uniform throughout when measured by the hydrometer

contained within the pipet. The settling time and

depth of sampling are the same as for the pipet procedure c, and the hydrometer readings require the same

temperature corrections as for the hydrometer procedure a.
126 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

After final mixing and a few seconds prior to time self-alining to accommodate to any slight lack of paralof sampling, squeeze the bulb and insert the hydrometer- lelism in the line of bearing on the sample. The bars

pipet into the suspension to a depth of 12.5 cm. Start are coated with a strip of soft rubber with a cross secfilling the pipet after the prescribed time interval in tion 0.16 cm. square. The breaking machine is mounted

seconds, as indicated under procedure c. Care should on the platform of a beam balance. The briquet sample

be taken to keep the suspension well mixed by occa- is broken by upward motion of the two lower bars,

sionally allowing a bubble to rise through the suspen- which are supported by the platform of the balance,

sion in the pipet. Read the hydrometer at the top Upward motion of the upper bar is constrained by a

of the meniscus and apply the proper temperature cor- cross frame above the balance. The breaking force is

rection as prescribed for the Buoyoucos hydrometer. supplied by water accumulating in a vessel hung from

The corrected hydrometer-pipet reading gives the the end of the balance beam. The breaking force is

grams per liter of silt plus clay in suspension. applied at the rate of 2,000 gm.-wt./min., and the beam

(c) Pipet. After final mixing, insert the Lowy or motion that occurs when the sample breaks can be used

other suitable pipet into the suspension to a depth of to automatically stop the accumulation of water in the

12.5 cm. Start filling the pipet after the time-interval vessels.

in seconds indicated by the accompanying data. Other The briquet molds are precision made from %-m.

depth-time-temperature relationships may be obtained brass strip with inside dimensions of 31/2 cm. by 7 cm.

from the nomograms of Tanner and Jackson (1948). by 0.952 cm. high. Rectangles of hard white photorp. , , , fifraphic blotting paper are cut to the size 5 cm. by

12.5-cm. depth ^Vè ^"^- ^ screen-bottomed tray, 50 cm. square, is

Temperature: (seconds) made by pulling brass window screen taut and solder-

°C. or °F. ing it to a rigid l/^-in. galvanized pipe frame. A pan

22 (71V) S4 ^^ other water container slightly larger than 50 |cm.

24 (75)__I"III"IIIIIIIIIIIIIIII"IIIIIIII 51 square. Graduated cylinder. Trémie funnel with

26 (79) 49 straight cylindrical tube 2 cm. diam. and 10 cm. high.

28 (82.5) 46 (See drawing of apparatus in Appendix.)

30 (86) 44 ^ 5 I^I^ FF /

The weight in grams of oven-dry (105° C.) material Procedure

in the 25-ml. aliquot is multiplied by 40 to give grams i«*iii •• ¿i i-* 1 c

r^ r u 1 1 • • o 0 Make the determmation on 6 replicate samples ot

per liter of silt plus clay in suspension. .V 1 ^ "^ ^^ 10 J u i

^ r / r g^jj ^jjg^ ha\e been passed through a 2-mm. round-hole

Calculations sieve, using the subsampling procedure outlined in

Method 1. Samples should be just slightly larger than

Percent aggregation = (wt. of total sih plus clay in ^jn fin the briquet molds. Cover the inside of the

dispersed suspension, gm., minus wt. of silt plus clay in molds with a thin layer of vaseline so that the soil will

undispersed suspension, gm.) XlOO/(wt. of total silt not stick to the mold. Place the screen-bottomed tray

plus clay in dispersed suspension, gm.). in the pan. Place the molds on the blotting paper on

« , the screen. Rest the trémie on the blotting paper at one

ne;erences ^^^ ^^ ^^^ ^^j^ j^^^p ^jj ^^ ^ ^^.j g^bsample into the

Bouyoucos (1936), Reliman and McKelvey (1941), trémie. Move the funnel around inside the mold while

and Tanner and Jackson (1948). raising continuously so as to give a uniform smooth

filling of the mold.

(43) Modulus of Rupture Strike off excess soil level with the upper surface of

the mold. Add water to the pan until free water surKemarks rounds every mold. Allow samples to stand for 1 hour

In the following procedure, the maximum force re- after all the soil samples become wet. Raise the screen

quired to break a small specially molded briquet of very carefully so as not to jar the samples and transfer

soil is measured, and from this breaking force the maxi- to a forced-draft oven at 50° C. After drying the

mum fiber stress in the standard sample is calculated. briquets to constant weight, remove from the molds and

Shrinkage of soil material on drying is a pertinent determine the breaking strength,

property and can be determined from the dimensions

before and after drying of the briquet samples used Calculations

in this test. , . .

Use the formula s=^ [ZFL) / {Ihd"), where s is the

Apparatus modulus of rupture (in dynes per sq. cm.), F is the

The machine for breaking the sample makes use of 2 breaking force in dynes (the breaking force in

parallel bars 5 cm. apart for supporting the sample. g"'-^t- X 980) ; ¿ is the distance between the lower two

The breaking force is supplied from a third overlying supporting bars, b is the width of the briquet, and d is

bar centrally located and parallel with respect to the the depth or thickness of the briquet, all expressed

supporting bars. The bar above and one bar below are in cm.
Chapter 7

Methods of Plant Culture and Plant Analysis

Plant-Culture Techniques Adapted to SaltTolerance Investigations

There are certain aspects of salt-tolerance investigations involving extensive plant breeding and selection

programs that are being conducted by crop specialists

at a number of agricultural experiment stations.

Furthermore, since salt tolerance of plants may be influenced profoundly by climatic conditions, investigators in a given region may find it desirable to undertake salt-tolerance studies under local climatic conditions in order to determine the crops best suited to

their region. In anticipation of a continued interest

in such breeding and testing programs, techniques used

in such studies at the Salinity Laboratory are included

in this handbook.

(50) Artificially Salinized Field Plots

Information is needed regarding the response of crop

plants to adverse saline conditions as related to climatic factors, planting techniques, and cultural practices used in the field. However, field observations

relating crop growth to the salinity of the soil are

difficult to evaluate, because of the wide range of salt

concentrations frequently encountered within a small

area in the field. In order to obtain information on

these aspects of salt tolerance, an artificially salinized

field-plot technique may be employed (Wadleigh and

Fireman, 1949). Small, 14-ft. square plots are used,

and each cro|) is managed according to practices generally followed in the principal regions where it is

grown under irrigation. Density of stand, spacing,

fertilizer practice, irrigation methods, and other conditions are all reproduced as closely as possible. The

salt tolerance of field, vegetables, forage, and tree crops

may be studied by this technique.

In the preparation of plots, careful leveling is necessary to insure uniform ¡distribution and penetration of

the salinizing water, and all parts of a basin-irrigated

plot should be brought to within ^,4 in. of the mean

plot level. Similar j)recautions should be used where

furrow irrigation is to be practiced so that all bed

surfaces and furrows will be at uniform levels. The

salinizing waters can be restricted to the plots by

borders; and, if the plots are closely spaced, the borders

can be supplemented with 6-in. boards around the plot.

Asphalt roofing paper 18 in. wide can be attached to

the boards and buried to a 12-in. depth. If the soil

is sufficiently permeable and adequate irrigations are

applied, salt concentration in the plot tends to reach a

steady value following the first several irrigations with

a minimum of variability within the plot.

In order to avoid the development of alkali conditions in the soil, it has been standard practice to add

salts to the irrigation water as equal parts of sodium

chloride and calcium chloride. This is readily accomplished for small plots by dissolving the desired amount

of salts in water in a galvanized tank and mixing with

the aid of a circulating pump. Salinity levels commonly employed in salt-tolerance tests are 0, 3,000,

6,000, and 9,000 p. p. m. of added salts, although a

more dilute series may be desirable for the more saltsensitive crops, such as beans and fruit trees; and a

more concentrated series for salt-tolerant crops, such

as barley and beets. Salinization of the plots is begun

after the seedlings or transplants are established because of the greater sensitivity of most crops to salinity

during germination and immediately following transplanting. To avoid the shock, which too abrupt a

change in soil salinity may induce in the plant, it is

advisable to increase salinity stepwise during the first

3 or 4 salinizing irrigations. By applying relatively

light irrigations at frequent intervals, the salinization

of a series of plots may be completed in 7 to 10 days.

However, owing to dilution of the salinizing water by

residual soil moisture, a few additional salinizing irrigations may be required to establish a relatively constant concentration of salt in the plot.

Subsequent irrigations are usually applied at conventional frequencies, as judged by the condition of

the control plot. The salt status of the salinized plots

is determined periodically during the growth of the

crop by taking soil samples at various depths in the

beds and furrows and determining the conductivity of

the saturation extract {ECe)- Yield data and observations on crop quality and composition can then be

correlated with the salt status of the several plots in the

series.

Plots treated in the above manner may be reused

after leaching the salt from the soil. Although the plots

remain nonalkali, recovery is facilitated by the addition of gypsum at the rate of 1 ton per acre. To insure

continued uniformity of the plots with regard to soil

structure (as measured by water penetration, aeration,

and other properties), it is desirable to grow some salt127
128 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

tolerant grass following leaching and prior to reusing

the plots.

The above procedure may be modified for certain

purposes. For crops that may be specifically sensitive

to high concentrations of soluble calcium or chloride,

the composition of the added salt may be changed to

vary the sodium: calcium ratio, or sulfate may be substituted for some or all of the chloride. The solubility

of calcium sulfate is a limiting factor in making such

substitutions, and the effect of high ratios of sodium

to calcium on the exchangeable-sodium-percentage must

be taken into account.

Inasmuch as this procedure does not include observations on the salt tolerance of the crop during germination, this must be determined separately, either by a

laboratory technique (Ayers and Hayward, 1949) or

by a modification of the artificially salinized field-plot

technique. The plots may be salinized prior to planting with a series of waters of graded concentrations.

Salinity levels are then determined by ECe measurements, and the seeds are planted according to standard

or experimental practice. Irrigating with water,

usually much less saline than that used in salinizing

the plots, will result in a redistribution of salt, especially where furrow irrigation is practiced. Germination counts will serve to delimit the critical level of

salinity which can be tolerated by the crop at the time

of planting under the specific cultural conditions

employed.

(51) Drum Cultures

Drum cultures have been used extensively for studies

of salt tolerance relating to the specific effects of various

added salts and to the frequency of irrigation as it

interacts with salinity in affecting the growth of plants

(Wadleigh and coworkers, 1951; Wadleigh and Ayers,

1945). Since the number of treatments may be greatly

increased with a minimum of space and effort by using

drum cultures, this technique has a very definite place

in salinity investigations.

Salinization of drum cultures requires a different

technique from that employed with artificially salinized

plots. Any salt added to the drums remains in the soil

and is not moved downward past the root zone by subsequent irrigations as in field plots. It is, therefore,

necessary to add the salt only in the initial irrigation

of the drums or by mixing salt with the dry soil in

amounts calculated to give the desired salinity levels.

Thereafter, nonsaline water must be used in irrigating

to avoid further increases in the salt concentration of

the soil. However, repeated irrigations with nonsaline

water will tend to leach the salt downward in the drum,

and a very steep gradient in salt concentration will

ensue.

In order to maintain a relatively uniform distribution of salt in the soil, it is necessary to irrigate alternately on the surface and by subirrigation. For subirrigation, provision is made for introducing the

irrigation water into a layer of fine gravel in the bottom

of the drum before filling with soil. With alternate

surface and subirrigation the distribution of the salt

in the soil can be maintained more nearly uniform.

Frequency of irrigation may be determined by daily

weighings of the drum cultures and calculation of mean

soil-moisture content. It is important to avoid overirrigation of the drums, since drainage, if permitted,

will result in loss of salt; or, if drainage is prevented,

the saturated soil condition will inhibit proper root

activity.

( 52 ) Sand and Water Cultures

Frequently, problems difficult to solve by soil-culture

methods can be studied more satisfactorily by sand or

water cultures, because the latter allow for a more precise control of the substrate. Salinity studies using sand

or water cultures involve the addition of various salts

to a base nutrient solution. The salts may be added in

isosmotic concentration to facilitate comparisons of

growth or in isoequivalent concentration to permit a

readier comparison of accumulation of the elements in

question. Occasionally, the two methods of adjusting

salt concentration are combined to permit both

isosmotic and isoequivalent comparisons, or a series of

concentrations of each salt may be used, and the effect

of osmotic pressure and equivalent concentration determined by graphical analysis of the data. By these

techniques the effects of various ions, such as sodium,

potassium, calcium, magnesium, chloride, sulfate, and

bicarbonate, on the growth and composition of plants

may be studied. In sand or water cultures the treatments are not altered as a result of cation exchange,

nor is fluctuating moisture content a disturbing factor

as it is in soil cultures.

For sand and water cultures, provision must be made

for adequate nutrition by use of a base nutrient solution, proper control of the pH of the nutrient solution,

and adequate aeration.

Methods of Plant Analysis

[See the introductory notes at the beginning of chapter 6]

(53) Sampling and Preparation of Plant

Samples

In collecting plant samples for chemical analysis in

connection with salinity studies, the usual care should

be practiced to obtain material representative of the

plant population. Since plant organs may differ

markedly in their selectivity in accumulating various

ions, it is frequently desirable to collect separate fractions of the various plant parts: leaves, stems, and

roots. In taking leaf samples it is the usual practice

to select mature, fully expanded leaves, avoiding

senescent ones, since salt accumulation may vary with

age of leaf. If some leaves are affected by leaf burn

or other visual symptoms of salt injury, separate

samples of affected and normal leaves may furnish data

of diagnostic importance.
SALINE AND ALKALI SOILS 129

move surface contamination of the plant material

ushing and brief rinsing in distilled or demineri water. If iron or other heavy metals are to be

ûdtermined, rub the entire surface gently in 0.3 N

hydrochloric acid and rinse. Dry the sample rapidly

in a forced-draft oven at 70° C. Grind the sample in a

Wiley mill, or by means of a mortar and pestle if

metallic contamination is to be avoided. Mix well and

store in tightly stoppered containers. Label each

sample, giving information on species, plant part

sampled, fresh weight, date, collector, and other

pertinent data.

If soluble sap constituents are to be determined,

freeze weighed amounts of fresh plant material in widemouthed jars and store at —10° C. To obtain sap,

thaw the plant material quickly by immersing the container in running water, press at 10,000 to 15,000 lb.

per sq. in. until the major portion of the sap has been

released. Centrifuge the liquid 15 min. at RCF= 1,500,

pour off the supernatant liquid, and save for analysis.

The expressed sap may be kept for several days in a

refrigerator if a few drops of toluene are added, but

analysis of the fresh sap is preferred. When the determination of insoluble constituents is desired, combine the residue from the centrifuge tube with the press

cake, determine moisture content, and analyze for desired constituents. Correct these results on the basis

of moisture content of the press cake and the concentration of soluble constituents of the sap.

Procedures for chemical analyses of plant materials

routinely employed in salt-tolerance studies are described under Methods 54 to 62. Additional determinations of various plant constituents are sometimes desirable, depending upon the specific problems under

investigation. Methods employed in studies of bicarbonate-induced chlorosis have been described by Wadleigh and Brown (1952).

(54) Ashing

( 54a) Wet Digestion

This procedure is preferable to dry ignition, because

of the possibility of loss of mineral constituents at high

temperatures during dry ignition.

Reagents

A. Nitric acid, cone.

B. Perchloric acid, 72 percent.

Procedure

Transfer 1.000 gm. of dried plant material or an

equivalent volume of sap (usually 10 ml.) to a 50-ml.

beaker and add 20 ml. of reagent A. Cover with a

watchglass and allow to stand until initial reactions

subside. Heat until solid particles have nearly disappeared, cool, add 10 ml. of B. Caution: Perchloric

acid is explosive in presence of easily oxidizable organic

matter. Heat gently at first, then heat more vigorously

until a clear, colorless solution results. Do not take to

dryness; discontinue heating when the volume is reduced to approximately 3 ml. Cool and transfer quantitatively to a 100-ml. volumetric flask, make to volume,

mix, allow to stand overnight and filter through a dry

filter paper without washing. Retain this solution and

use for analyses as described under Methods 55 to 58

and 61.

Reference

Toth and associates (1948).

( 54b ) Magnesium JSitrate Ignition

Since the wet-digestion procedure does not quantitatively retain sulfur, magnesium nitrate ignition should

be used in the determination of sulfur in plants. Total

phosphorus may be determined in the ignited material

as well, since phosphorus is quantitatively oxidized to

phosphate by this ignition.

Reagents

A. Magnesium nitrate solution. Add 113 gm. of

magnesium oxide to 300 ml. of water and stir to a

paste. Add nitric acid (1 + 1) until the magnesium

oxide is in solution. Add a little excess magnesium

oxide and boil; filter and dilute to 1 liter.

B. Hydrochloric acid (1 + 1).

Procedure

Weigh 2.000 gm. of the sample and transfer into a

large porcelain crucible or casserole. Add 10 ml. of

reagent A, taking care that all the material is brought

in contact with the solution, and heat gently on a hot

plate to 180° C. until the reaction is complete. Transfer the crucible while hot to an electric muffle and

allow to remain at low heat (muffle must not show any

red) until the charge is thoroughly oxidized. No black

particles should remain. (It may be necessary to break

up the charge and return to the muffle.) Remove from

the muffle and allow to cool. Moisten the ash with

water and add 10 ml. of B, which should provide an

excess of acid. Evaporate to dryness, moisten with B,

and again evaporate to dryness to dehydrate silica.

Add 5 ml. of B and sufficient water to bring the salts

into solution. Allow to stand on the steam bath until

solution is complete. Filter, M^ash, and make to 100

ml. with water. Retain this solution for analysis of

sulfur and phosphorus according to Methods 60 and 61.

Reference

Association of Official Agricultural Chemists (1950,

2.8 (e),p. 8, and 6.35, p. 104)

(55) Calcium

(55a) Calcium by Flame Photometer

Apparatus

Perkin-Elmer model 52 flame photometer with

acetylene or propane burner.
130 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Reagents

A. Lithium chloride, 0.05 N. Dissolve 2.12 gm. of

lithium chloride in water and make to 1 liter.

B. Sodium chloride, 5.00 meq./l. Dissolve 0.2922

gm. of dry sodium chloride in water and make to 1 liter.

C. Potassium chloride, 12.5 meq./l. Dissolve 0.9320

gm. of dry potassium chloride in water and make to 1

liter.

D. Calcium chloride, 50.0 meq./l. Dissolve 2.503

gm. of pure calcium carbonate (calcite crystals) in

50 ml. of 3 N hydrochloric acid and dilute to 1 liter.

E. Magnesium chloride, 50.0 meq./l. Dissolve 5.1

gm. of MgClg'óH^O in approximately 900 ml. of water.

Standardize by Method 78 and add the calculated

amount of water to bring the magnesium conce^ntration to 50.0 meq./l.

Remarks

The optimum concentration of lithium chloride for

use as an internal standard varies with individual flame

photometers but is usually 5 to 10 meq./l.

Interference by sodium, potassium, and magnesium

is not an appreciable factor unless the concentration of

the interfering element is at least 5 times the concentration of calcium. Interference may be compensated for

by determining the concentration of the interfering

element(s) in the unknown and by adding this concentration ( ±20 percent) of the interfering element(s)

to the standard solutions. In the determination of

calcium in plant tissue, interference is seldom encountered.

Procedure

Prepare a series of standards containing 0, 1, 2, 3,

and 4 meq./l. of calcium (0, 1, 2, 3, and 4 ml. of reagent D per 50 ml.) and the concentration of lithium

chloride found to be optimum for the instrument.

Transfer an aliquot (usually 10 ml.) of the acid

digest (Method 54a) containing 0.05 to 0.20 meq. of

calcium to a 50-ml. volumetric flask and add the same

concentration of lithium chloride used in the standard

series. Add compensating solutions B, C, and E when

necessary, and make to volume. Obtain instrument

readings for standard solutions and unknowns with

the wave-length indicator set at the point corresponding to the calcium emission maximum at 6,220 A.

Calculations

Milliequivalents of Ca per 100 gm. of dry material =

(meq./l. of Ca from calibration curve) X500/ml. in

aliquot.

(55b) Calcium by Oxalate Method

The acid digest (Method 54a) may be analyzed for

calcium by Method 77, if a flame photometer is not

available.

Calculations

Milliequivalents of Ca per 100 gm. of dry material =

normality of KMn04 X 10,000 X (ml. of KMn04 - ml. of

blank) /ml. in aliquot of acid digest.

(56) Magnesium

Remarks

Methods for determining magnesium by the use of

thiazole yellow or related dyes have not given acceptable

results at this laboratory. The following method, while

time-consuming, has proved to be reliable. The calcium is removed as the oxalate, and magnesium is precipitated as magnesium ammonium phosphate hexahydrate. The phosphate is determined colorimetrically,

and magnesium is calculated by reference to a calibration curve.

Apparatus

Photoelectric colorimeter or spectrophotometer.

Reagents

A. Oxalic acid, 1 N. Dissolve 63 gm. of oxalic acid

in water and make to 1 liter.

B. Methyl orange, 0.01 percent in 95 percent ethanol.

C. Ammonium hydroxide, cone.

D. Hydrochloric acid ( 14-1 ).

E. Magnesium chloride, 5.00 meq./l. Dissolve 0.51

gm. MgCla'óHgO in approximately 900 ml. of water.

Standardize by Method 78 and add the calculated

amount of water to bring the magnesium concentration

to 5.00 meq./l.

F. Ammonium chloride, 3 percent. Filter before use.

G. Ammonium dihydrogen phosphate, 5 percent.

Filter before use.

H. Phenolphthalein, 1 percent in 60 percent ethanol.

I. Ammonium hydroxide in ethanol and ether. Mix

20 ml. of concentrated ammonium hydroxide with 980

ml. of a mixture of equal volumes of ethanol, ether, and

water.

J. Sulfuric acid (1 + 6).

K. Ammonium vanadate, 0.25 percent. Dissolve 2.5

gm. of ammonium vanadate in 500 ml. of boiling water,

cool somewhat, and then add 60 ml. of reagent J. Cool

to room temperature and dilute to 1 liter. Store in a

brown bottle.

L. Ammonium molybdate, 5 percent. Store in a

brown bottle.

Procedure

Transfer an aliquot (usually 10 ml.) of the acid

digest from Method 54a containing 0.01 to 0.05 meq.

of magnesium to a 25-ml. volumetric flask. Prepare a

series of standards in 25-ml. volumetric flasks containing 0, 0.01, 0.02, 0.03, 0.04, and 0.05 meq. of

magnesium (0, 2, 4, 6, 8, and 10 ml. of reagent E).

To all standards and unknowns, add 3 drops of B,
SALINE AND ALKALI SOILS 131

acidify with D, if necessary, and add 1 ml. of D in

excess. Add 1 ml. of A, heat to boiling, and neutralize

with C. Cool and add more C if necessary to keep

basic (yellow). Make to volume and filter through a

dry filter paper ; do not wash. Transfer a 5-ml. aliquot

of the filtrate to a 15-ml. centrifuge tube and add 1 ml.

each of F and G and 1 drop of H. Heat to about 90° C.

in a water bath, and then add C until permanently pink.

After 15 min., add an additional 2 ml. of C, stopper,

mix, and let stand overnight.

Centrifuge at RCF==2ß00 for 10 min., decant carefully, drain on filter paper for 10 min., and wipe the

mouth of the tube with a clean towel or lintless filter

paper. Suspend the precipitate and rinse the sides of

the tube with a stream of 5 ml. of reagent I from a pipet

equipped with a rubber bulb. Centrifuge at RCF=

2,000, decant, drain for 5 min., and wipe the mouth

of the tube. Repeat this washing procedure once.

Pipet 10 ml. of reagent J into the tube and twirl for

a few seconds. After 5 min. transfer the contents

quantitatively into a 100-ml. volumetric flask with a

total of 50 ml. of water. Pipet 10 ml. each of K and L

into the flask while swirling the solution rapidly.

Make to volume and mix. After 10 min. measure the

percent transmission of the unknown and standard

solutions at 4,000 A. or by means of an appropriate

blue filter.

Calculaiions

Milliequivalents of Mg per 100 gm. of dry material=

(meq. of Mg from calibration curve) X 10,000/ml. in

aliquot of acid digest.

(57) Sodium

(57a) Sodium by Flame Photometer

Apparatus

Perkin-Elmer model 52 flame photometer with acetylene or propane burner.

Reagents

A. Lithium chloride, 0.05 N. Dissolve 2.12 gm. of

lithium chloride in water and make to 1 liter.

B. Sodium chloride, 5.00 meq./l. Dissolve 0.2922

gm. of dry sodium chloride in water and make to 1 liter.

C. Potassium chloride, 12.5 meq./l. Dissolve 0.9320

gm. of dry potassium chloride in water and make to 1

liter.

D. Calcium chloride, 50.0 meq./l. Dissolve 2.503

gm. of pure calcium carbonate (calcite crystals) in 50

ml. of 3 N hydrochloric acid and dilute to 1 liter.

E. Magnesium chloride, 50.0 meq./l. Dissolve 5.1

gm. of MgClü'óHaO in approximately 900 ml. of water.

Standardize by Method 78 and add the calculated

amount of water to bring the magnesium concentration

to 50.0 meq./l.

Remarks

The optimum concentration of lithium chloride for

use as an internal standard varies with individual

flame photometers, but is usually 5 to 10 meq./l.

Sodium is frequently found in relatively small

amounts in plant material, so that interference often

occurs. Potassium interferes if the potassium : sodium

ratio is 5 or greater; and calcium, if the calcium:

sodium ratio is 10 or greater. Since these ratios are

frequently exceeded in plant materials, it is advisable

to determine calcium and potassium first, so that the

approximate concentrations of interfering elements

may be added to the sodium standard solutions. Magnesium does not cause interference unless magnesium :

sodium ratios are in excess of 100. Such magnesium :

sodium ratios are very rarely encountered.

Procedure

Prepare a series of standards containing 0, 0.1, 0.2,

0.3, and 0.4 meq./l. of sodium (0, 1, 2, 3, and 4 ml.

of reagent B per 50 ml.) and the concentration of

lithium chloride found to be optimum for the instrument.

Transfer an aliquot (usually 10 ml.) of the acid

digest (Method 54a) containing 0.005 to 0.020 meq.

of sodium to a 50-ml. volumetric flask and add the

same concentration of lithium chloride used in the

standard series. Add compensating solutions C, D,

and E when necessary and make to volume. Obtain

instrument readings for standard solutions and unknowns with the wave-length indicator set at the point

corresponding to the sodium emission maximum at

5,890 A.

Calculations

Milliequivalents of Na per 100 gm. of dry material =

(meq./l. of Na from calibration curve) X500/ml. in

aliquot.

(576) Sodium by Uranyl Zinc Acetate

Reagents

A. Uranyl zinc acetate. Weigh 300 gm. of uranium

acetate dihydrate, 900 gm. of zinc acetate dihydrate,

and 10 mg. of sodium chloride into a large flask. Add

82 ml. of glacial acetic acid and 2,618 ml. of water.

Stir or shake until the salts are dissolved, leaving only

a small amount of sodium uranyl zinc acetate precipitate. This may require several days. Filter before use.

B. Ethanol, saturated with sodium uranyl zinc acetate. Filter before use. Sodium uranyl zinc acetate

may be prepared as follows: Add 125 ml. of reagent A

to 5 ml. of 2 percent sodium chloride solution, stir, and

after 15 min. collect the precipitate in a porous-bottomed porcelain crucible. Wash several times with

glacial acetic acid, then several times with ether. Dry

in a desiccator.
132 AGRICULTURE HANDBOOK 60, U. IS. DEPT. OF AGRICULTURE

C. Ether, anhydrous.

D. Phenolphthalein, 1 percent in 60 percent ethanol.

E. Calcium chloride dihydrate, 10 percent in water.

F. Ammonium hydroxide (1 + 1).

G. Acetic acid, 2 percent.

H. Acetic acid, glacial.

Procedure

Transfer an aliquot (usually 25 ml.) of the acid

digest (Method 54a) sufficient to give 50 to 200 mg. of

sodium uranyl zinc acetate to a 50-ml. sugar flask.

Add 1 drop of reagent D and 5 ml. of E and make the

solution basic (pink) with F to precipitate phosphate.

Make to 55 ml. and filter through a dry paper ; do not

wash. Transfer a 50-ml. aliquot to a 100-ml. beaker,

acidify with H, and evaporate to dryness on a steam

bath. Cool, dissolve the residue in 2 ml. of G, and add

75 ml. of filtered reagent A. Stir the solution and allow

to stand for 1 hr. Filter through a porous-bottomed

porcelain filtering crucible, taking care to transfer all

the sodium uranyl zinc acetate precipitate onto the

filter by means of a small wash bottle filled with A.

Wash the beaker 5 times with 2-ml. portions of A and

pass the washings through the filter. Allow the crucible

to drain completely, because it is important to have

the filter and the precipitate free of the reagent before

washing with the alcohol. Wash the crucible 5 timea

with 2-ml. portions of B and, after removing all the

alcohol by suction, wash once or twice with C. The

suction is continued until the precipitate is dry. Allow

the crucible to stand in a desiccator for 2 hr. and weigh.

Return the crucible to a suction apparatus and wash

with small portions of water until all the soluble material is dissolved and passes through the crucible.

Wash with alcohol and ether as above. Dry and weigh.

The difference between the first and last weight

represents the weight of sodium precipitate. The

precipitate is assumed to have the composition:

( UO2 ) sNaZn ( CH3COO ) 9-6H20 ; molecular weight,

1538.079; percent sodium, 1.4952.

Calculations

Milliequivalents of Na per 100 gm. of dry material =

(gm. of sodium uranyl zinc acetate precipitate) X

7,152/ml. in aliquot.

( 58) Potassium

(58a) Potassium by Flame Photometer

Apparatus

Perkin-Elmer model 52 flame photometer with

acetylene or propane burner.

Reagents

A. Lithium chloride, 0.05 N. Dissolve 2.12 gm. of

lithium chloride in water and make to 1 liter.

B. Sodium chloride, 5.00 meq./l. Dissolve 0.2922

gm. of dry sodium chloride in water and make to 1 liter.

C. Potassium chloride, 12.5 meq./l. Dissolve 0.9320

gm. of dry potassium chloride in water and make to 1

liter.

D. Calcium chloride, 50.0 meq./l. Dissolve 2.503

gm. of pure calcium carbonate (calcite crystals) in 50

ml. of 3 N hydrochloric acid and dilute to 1 liter.

E. Magnesium chloride, 50.0 meq./l. Dissolve 5.1

gm. of MgClg-óHaO in approximately 900 ml. of water.

Standardize by Method 78 and add the calculated

amount of water to bring the magnesium concentration

to 50.0 meq./l.

Remarks

The optimum concentration of lithium chloride for

use as an internal standard varies with individual flame

photometers but is usually 5 to 10 meq./l.

Interference by sodium occurs if the sodium : potassium ratio is 5 or greater, and by calcium if the calcium : potassium ratio is 10 or greater. Magnesium

does not cause interference until the magnesium : potassium ratio is in excess of 100. Interference in the

determination of potassium is very rarely encountered.

Procedure

Prepare a series of standards containing 0, 0.25,

0.50, 0.75, and 1.00 meq./l. of potassium (0, 1, 2, 3,

and 4 ml. of reagent C per 50 ml.) and the concentration of lithium chloride found to be optimum for the

instrument.

Transfer an aliquot (usually 10 ml.) of the acid

digest (Method 54a) containing 0.01 to 0.05 meq. of

potassium to a 50-ml. volumetric flask and add the

same concentration of lithium chloride used in the

standard series. Add compensating solutions B, D,

and E when necessary and make to volume. Obtain

instrument readings for standard solutions and unknowns with the wavelength indicator set at the point

corresponding to the potassium emission maximum at

7,(580 A.

Calculations

Milliequivalents of K per 100 gm. of dry material=

(meq./l. of K from calibration curve) X500/ml. in

aliquot.

(58b) Potassium, by Cobaltinitrite

Reagents

A. Nitric acid (1 + 15).

B. Trisodium cobaltinitrite, 20 percent. Store at

about 5° C. and filter before use. The solution is

stable for some time but should be prepared fresh at

about biweekly intervals.

C. Nitric acid ( 1 +1,500). Dilute 10 ml. of reagent

A to 1 liter.
SALINE AND ALKALI SOILS 133

D. Ethanol, 95 percent.

E. Potassium chloride, 0.0100 N. Dissolve 0.7456

gm. of dry potassium chloride in water and make to 1

liter.

Procedure

Evaporate to dryness in a 50-ml. beaker an aliquot

(usually 25 ml.) of the acid digest (Method 54a) containing 0.05 to 0.35 meq. of potassium. Add 10 ml.

of water, 1 ml. of reagent A, and stir to dissolve. Add

5 ml. of B, stir and allow to stand for 2 hr. at 15° to

20° C. Filter in a porous-bottomed porcelain filtering

crucible, the tare weight of which is known, using C

in a wash bottle to make the transfer. Wash 10 times

with C and 5 times with 2-ml. portions of D. Aspirate

until quite dry. Wipe the outside with a cloth, dry for

1 hr. at 110° C, cool in a desiccator, and weigh.

Prepare a series of standards containing 0, 0.05,

0.15, 0.25, and 0.35 meq. of potassium (0, 5, 15, 25,

and 35 ml. of reagent E) in 50-ml. beakers and proceed

as directed for the aliquots of the acid digests.

Calculations

Milliequivalents of K per 100 gm. of dry material =

(meq. of K from calibration curve) X 10,000/ml. in

aliquot.

(59) Chloride

Remarks

A modification of the method described by Clark

and others (1942) has been found to give results in

close agreement with those obtained by AOAC procedures with a considerable saving in time. The sample for chloride analysis together with a tube containing acid and a well containing base are placed in a

tightly closed weighing bottle (fig. 32). The acid

digests the filter-paper plug in the acid tube and reacts

with the sample, volatilizing chloride as hydrogen chloride, which is absorbed by the potassium hydroxide.

The absorbed chloride is then titrated with mercuric

nitrate.

Apparatus

Make the following items from ordinary glass

tubing:

1. Acid tube, approximately 1.0 cm. inside diameter,

with one end drawn out to a capillary tip. Capacity,

2 ml.

2. Outer well, approximately 1.1 cm. inside diameter,

4 to 5 cm. long, sealed at one end.

3. Inner well, approximately 1.0 cm. outside diameter, 1 to 2 cm. long, sealed at one end.

4. Support for inner well of such length that when

assembled, the top of inner well is at, or slightly above,

the top of the outer well.

Clamp. Any screw-type clamp, equipped with rubGROUNO GLASS

I JOINT ^

OUTER WELL -

INNER WELL

WITH KOH

SUPPORT FOR

INNER WELL

30 ML. WEIGHING

BOTTLE WITH

OUTSIDE CAP

ACID TUBE WITH

H2SO4

-FILTER PAPER

^.»<1\^^-?f^^^C0-rS?;:g| PLUG

FIGURE 32.—Apparatus for the chlortde determination

(Method 59).

her cushions which will hold the cap securely on the

weighing bottle.

CAUTION.—After cleaning, rinse all glassware shown

in figure 32 with nitric acid (1 +100) and dry in oven.

Use forceps, thereafter, in handling acid tubes, wells,

and supports to avoid chloride contamination. Avoid

contaminating the air with hydrogen chloride.

Reagents

A. Potassium hydroxide, 50 percent.

B. Sulfuric acid, cone.

C. Ethanol, 95 percent.

D. Diphenylcarbazone, 1 percent. Dissolve 1 gm.

of diphenylcarbazone (Eastman No. 4459) in 100 ml.

of 95 percent ethanol. Store at 5° C.

E. Mercuric nitrate, 0.01 N. Dissolve 1.7 gm. of

Hg(N03)2*H20 in water to which 10 ml. of nitric

acid (1 + 7) have been added and make to 1 liter.

Store in brown bottle in the dark.

F. Potassium chloride, 50.0 meq./l. Dissolve 3.728

gm. of dry potassium chloride in water and make to 1

liter.

G. Hydrogen peroxide, 30 percent.

H. Nitric acid (1 + 7).

I. Bromphenol blue, 0.05 percent in ethanol.

Procedure

Weigh accurately in a 30-ml. weighing bottle a

sample (usually 0.1 gm.) of dried plant material containing 0.02 to 0.25 meq. of chloride. To determine

chloride in expressed sap, evaporate a suitable aliquot

(usually 1 ml.) to dryness in the weighing bottle at

70° C. Prepare a series of standards containing 0,

0.05, 0.10, 0.15, 0.20, and 0.25 meq. of chloride (0, 1,

2, 3, 4, and 5 ml. of reagent F) and evaporate to dryness in the weighing bottles.
134 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Plug the acid tube with macerated filter paper and

assemble the apparatus as shown in figure 32. Add 3

drops of reagent A and 1 drop of C to the inner well.

Add 2 ml. of B to the acid tube and moisten the ground

surface of the cap with B. Seal the weighing bottle by

twisting the cap into position and clamp securely.

Place the assembly in an oven at 110° C. overnight.

Remove, cool, and open the weighing bottles, using

rubber gloves.

Transfer the inner well with forceps to a porcelain

casserole and add 5 ml. of water. Add 3 drops of

reagent I, neutralize with H, dissolving all of the residue

in the inner well, and add 1 drop of H in excess. Add

3 drops of G, 1 drop of D, and titrate with E to a

purple or pink color, depending on volume. Prepare

a standard curve; it is not linear over the entire range.

Calculations

Milliequivalents of Cl per 100 gm. dry material=

(meq. of Cl in sample) X 100/gm. of sample.

Milliequivalents of Cl per liter of sap= (meq. of Cl

in aliquot) X 1,000/ml. in aliquot.

References

Association of OflScial Agricultural Chemists (1950,

6.41-6,46, p. 105,106), Clark and others (1942).

(60) Sulfur

Reagents

A. Hydrochloric acid, cone.

B. Barium chloride, 10 percent. Filter before use.

C. Methyl orange, 0.05 percent in ethanol.

Procedure

Transfer an aliquot (usually 50 ml.) of the digest

(Method 54b) containing 0.1 to 2.0 meq. of sulfate to a

250-ml. beaker. Add 2 drops of reagent C, acidify

with A if necessary, and then add 1 ml. of A. Make to

approximately 100 ml. with water, heat to boiling, and

add an excess of B, drop by drop, with constant stirring.

Cover with a watchglass and allow to stand on a steam

bath for several hours, until the volume is reduced to

approximately 50 ml. After cooling, filter the precipitate of barium sulfate through an ignited and weighed

Gooch crucible and wash with water until free of

chloride. Dry, ignite in a muffle at low red heat, cool,

and weigh. Ashless filter paper may be used in place

of the Gooch crucible, but the muffle in which the

ignition is made must be well ventilated.

Calculations

Milliequivalents of total S (expressed as sulfate) per

100 gm. of dry material=gm. BaS04 X 42,841/ml. in

aliquot.

(61) Phosphorus

F'hosphorus is retained quantitatively by both the

magnesium nitrate ignition and wet digestion, provided

the perchloric acid digest is not allowed to go to

dryness.

Apparatus

A photoelectric colorimeter or spectrophotometer.

Reagents

A. Sulfuric acid (Ifó).

B. Ammonium vanadate, 0.25 percent. Dissolve

2.5 gm. of ammonium vanadate in 500 ml. of boiling

water, cool somewhat, and add 60 ml. of reagent A.

Cool to room temperature and dilute to 1 liter. Store

in a brown bottle.

C. Ammonium molybdate, 5 percent.

D. Potassium dihydrogen phosphate, 2.50 millimoles/1. Dissolve 0,3404 gm. of potassium dihydrogen phosphate in water and make to 1 liter.

Procedure

Transfer an aliquot (usually 5 ml.) of the solution

prepared by Method 54a or 54b containing 0.002 to

0.020 millimoles of phosphate to a 50-ml. volumetric

flask. Prepare a series of standards containing 0,0.005,

0.010, 0.015, and 0.020 millimoles of phosphate (0, 2,

4, 6, and 8 ml. of reagent D). Add to each flask containing unknown or standard, 5 ml. each of A, B, and

C successively, shaking the flask during each addition.

Make to volume, allow to stand 15 to 30 min., and

determine transmittance at 4,000 A., or by means of a

suitable blue filter.

Calculations

Millimoles of phosphate per 100 gm. of dry material= (millimoles of phosphate from calibration

curve) X 10,000/(ml. in aliquotXgm. of sample digested).

1 millimole of phosphate=1 meq. of H2PO4".

(62) Boron

Remarks

Leaf samples, collected and prepared according to

Method 53, are usually the best index to the boron status

of the plant.

Apparatus

A spectrophotometer or photoelectric colorimeter.

Alkali-resistant (boron-free) glassware, porcelainware, platinum, or fused-quartz dishes. Avoid the use

of borosilicate glassware.
SALINE AND ALKALI SOILS 135

Reagents

A. Calcium oxide, powdered.

B. Hydrochloric acid (1 + 1).

C. Hydrochloric acid, cone.

D. Sulfuric acid, cone.

E. Carmine, 0.05 percent by weight in cone, sulfuric

acid (0.920 gm./l. ). Shake until completely dissolved.

F. Boric acid, 100 p. p. m. boron. Dissolve 0.5716

gm. of boric acid in water and make to 1 liter.

Procedure

Weigh a portion of the dry sample (usually 2.000

gm.) containing not more than 1.00 mg. of boron and

transfer to a porcelain casserole or platinum dish. Add

0.1 gm. of reagent A per gram of sample and mix well.

Ignite as completely as possible in a muffle at 500° to

550° C, cool, and moisten with water. Cover with a

watchglass and introduce 3 ml. of B per gram of sample, which should make the solution strongly acid.

Heat on a steam bath for 20 min. Transfer quantitatively to a 100-ml. volumetric flask, make to volume

with water, and filter through a dry filter paper.

Prepare a series of standard solutions containing

0 to 10 p. p. m. boron (0 to 1.00 mg./lOO ml.) by

diluting 0, 2, 4, 6, 8, and 10 ml. of reagent F to 100 ml.

Pipet 2 ml. of each of the standards and of the unknowns into Erlenmeyer flasks. Add 2 drops of C to

each standard and 2 drops of water to each unknown.

Add 10 ml. of D to each Erlenmeyer flask, mix, and

cool. Add 10 ml. of E, mix, and allow to stand at least

45 min. for color development. Determine the transmittance at 5,850 A., or by means of a suitable yellow

filter.

Calculations

Parts per miflion B in dry plant material= (p. p. m. B

from calibration curve) X 100/gm. of sample.

Reference

Hatcher and Wilcox (1950).
Chapter 8

Methods of Analysis of Irrigation Waters

[See the introductory notes at the beginning of chapter 6.]

(70) Collection of Irrigation Water

Samples

The minimum quantity of water required for the

ordinary chemical analysis is about one-half gallon

(1.9 liters). In special cases, larger quantities may be

necessary.

Care should be taken to obtain a representative

sample. Satisfactory samples of some waters can be

obtained only by mixing several portions collected at

different times, the details as to collection and mixing

depending on local conditions. Samples from wells

should be collected after the pump has been running

for some time, and samples from streams should be

taken from running water.

In general the shorter the elapsed time between collection and analysis of a sample, the more reliable will

be the analytical data. Changes resulting from chemical and biological activity may alter the composition

of the sample. No satisfactory method for sterilizing

a water sample to prevent bacterial action has been

proposed.

References

American Public Health Association and American

Water Works Association (1946, p. 1).

(71) Records, Reports, and Expression of

Results

At the time of collection, a label, bearing a short

identifying description, should be attached to the bottle.

Additional information can be recorded on a "Collector's Description of Water Sample" form as shown.

One item not specifically called for on this form, but

often of importance, is the elevation above sea level of

an appropriate reference point at the well. When this

is known, it is possible to refer the water level in the

well to sea level. This value may be useful in qualityof-water studies. The importance of an accurate and

complete description, especially as regards location,

cannot be overemphasized.

Two other blank forms used at the Laboratory in

connection with the analyses of water samples are

shown on pages 138 and 139. The laboratory work

136

sheet is used for recording the original data obtained

from the chemical analysis. One such sheet is used for

each sample. A laboratory number is assigned which,

with the description from the "Collector's Description of

Water Sample," is entered at the top of the page. Upon

completing and recording each separate determination,

the analyst enters his initials and the date.

For purposes of uniformity, the following rules for

reporting analytical lesults are used:

pH.—Report to the nearest 0.1 unit.

ELECTRICAL CONDUCTIVITY {ECXIO^ at 25° C).—

Report to the nearest 0.1 when less than 100, and to 3

significant figures for values above 100.

DISSOLVED SOLIDS.—Report in parts per million

(p. p. m.) to the nearest whole number, but not more

than 3 significant figures.

BORON.—Electrometric titration method; report to

the nearest 0.01 p. p. m. when less than 10 p. p. m.

boron, and to 3 significant figures above 10 p. p. m.

boron.

Colorimetric method; report to the nearest 0.1

p. p. m. boron but not more than 2 significant figures.

SILICA.—Report in p. p. m. SÍO2 to nearest whole

number, but not more than 3 significant figures.

FLUORIDE.—Report to the nearest 0.1 p. p. m.

fluoride or to the nearest 0.01 meq./l.

CATIONS AND ANIONS.—Report to the nearest 0.01

meq./l. up to 100, and to 4 significant figures if above

100 meq./l.

( 72 ) Electrical Conductivity

Remarks

Electrical conductivity is commonly used for indicating the total concentration of the ionized constituents

of a natural water. It is closely related to the sum of

the cations (or anions) as determined chemically, and

it usually correlates closely with the total dissolved

solids. It is a rapid and reasonably precise determination that does not alter or consume any of the sample.

Apparatus

Wheatstone bridge, alternating current, suitable for

conductivity measurements. This may be a 1,000-cycle

a. c. bridge with telephone receivers, a 60-cycle a. c.
SALINE AND ALKALI SOILS 137

Collector's No.

Name and/or owner

UNITED STATES DEPARTMENT OF AGRICULTURE

Agricultural Research Service

Soil and Water Conservation Research Branch

United States Salinity Laboratory

Riverside, California

COLLECTOR'S DESCRIPTION OF WATER SAMPLE

_; Lab. No. _; Date. _; Collector

Spring, Stream, Lake, Well? (circle one)

County

Location %, Sec.

Miles—distance nearest town

T ^ ; R

uses sheet

Distance and direction from section corner

or landmark

Other description

Depth ; Depth to upper perforation . _; Casing diameter.

Discharge .

Temp.

_; Static level . _; Draws down to .

-; Odor _; Gas . Color .

° C. or °F.

Use: Irrig., Municipal, Ind., Stock, Domestic

Approximate acreage served, crops

Condition or symptoms of land or crops .

Owner's opinion of water quality

Collector's remarks

Report to:

(Please draw a map on the reverse side, if necessary, to show the exact location of the sampling site).
138 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Water aample No#^ Description:

pH

Conductivity at260C.

Toc.

k .

R

ECxlO^

Dissolved Solids

D.S.

p.p.m.

Boron

B

p.p.m»

Sum of

Cations

Sum of

Anions

BCxl06

Anions

Percent

Na

Calcium Magnesium Sodium

Ca

meq./l.

Mg

meq«/l.

Na

meq./l.

Potassium

K

meq./l.

Carbonate and

Bicarbonate

Sulfate Chloride

CO,

meq»/l»

HCO,

meq#7l.

SO^

meq./l.

CI

meq./l«

Nitrate

NO,

meq./l.

Received laboratory:

Analysis completed:

Reported :

Reported to:
SALINE AND ALKALI SOILS 139

UNITED STATES DEPARTMENT OF AGRICULTURE

Agricultural Research Service

Division of Soil and

Plant Relationships

U» S. Salinity Laboratory

Rubidoux Unit

Riverside, California

REPORT OF WATER ANALYSIS

Description

Conductivityt ECxlOfe26<^C.

Percent Sodium

Boron (B) parts per million

Dissolved Solids:

tons per acre-foot

parts per million

Hydrogen-ion activity (pH)

Silica (SiOg) parts per million

1 1

Cations

Milligram

equivalents

per liter

Arts per

million Anions

Minima equivalents

per liter

Bwts per

million

Calcium (Ca)

Magnesium (Mg)

Sodium (Nft)

Potassium (K)

Carbonate (CO^)

Bicarbonate (HCOj)

Sulfate (SO^)

Chloride (Cl)

Fluoride (F)

Nitrate (NC^)

Sizm XXX Sum XXX

Analysed byt

Reported tos

Reported by:
140 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

bridge with an a. c. galvanometer, or one of the newer

bridges employing a cathode ray tube as the null

indicator.

Conductivity cell, either pipet or immersion type,

with platinized platinum electrodes. The cell constant

should be approximately 1.0 reciprocal centimeter.

New cells should be cleaned with chromic-sulfuric acid

cleaning solution, and the electrodes should be

platinized before use. Subsequently, they should be

cleaned and replatinized whenever the readings become

erratic or when an inspection shows that any of the

platinum black has flaked off. The platinizing solution

contains platinum chloride, 1 gm.; lead acetate, 0.012

gm.; in 100 ml. water. To platinize, immerse the electrodes in the above solution and pass a current from a

1.5-volt dry battery through the cell. The current

should be such that only a small quantity of gas is

evolved, and the direction of current flow should be

reversed occasionally.

Reagents

A. Standard potassium chloride solution, 0.01 N.

Dissolve 0.7456 gm. of potassium chloride in distilled

water and make to 1 liter at 25° C. This is the standard reference solution and at 25° C. has an electrical

conductivity of 1411.8X10« (0.0014118) mhos/cm.

Procedure

Place 4 tubes of reagent A in a water bath. ( For

subsequent sets of determinations, discard the first tube

of potassium chloride solution, shift the others one

place, and insert a tube of fresh solution.) Place 2

tubes of each sample in the bath, adjust the temperature

to approximately 25° C, and hold at this temperature

for 20 to 30 min. If the room temperature is not close

to 25° C, it is better to adjust the temperature of the

bath to approximately that of the room and hold it at

that temperature until equilibrium is attained. The

bath temperature is here represented by t. Rinse the

electrode in three of the tubes of potassium chloride

solution, transfer to the fourth, and measure the cell

resistance {R\). Rinse the electrode several times in

one tube of the water sample, transfer to the other

tube, and read the resistance (/?t). The electrical conductivity {EC at 25° C.) of the sample is calculated

from the equation :

0.0014118 X/?\

ECRt

This is multiplied by 1,000,000 (10«) and reported as

EC X 10« at 25° C, or as EC, micromhos/cm. at 25°.

The expression "electrical conductivity" is synonymous with "specific electrical conductance." The standard unit for conductivity is the mho/cm. It is so large

that most natural waters have a value of much less

than 1 unit. For purposes of convenience in recording

or expressing such results, the value in mhos/cm. is

muhiplied by 10'^ (decimal point moved 6 places to the

right ) and reported as EC X 10« at 25° C. The several

methods of reporting conductivity are shown below,

using as an example a western surface water with a

conductivity of 0.00117 mho/cm.:

EC =0.00117 mho/cm.

EC X 10^= 1.17 mmhos/cm.

£Cxiœ=ii7 (=Kxiœ)

EC X10«= 1,170 micromhos/cm.

References

Wilcox (1950), National Research Council, International Critical Tables (1929, v. 6, p. 234).

(73) Boron

(73a) Boron^ Electrometric Titration

Remarks

The addition of mannitol to a neutral, unbuffered

solution of mixed salts containing boron causes the

solution to become acid. The quantity of standard

alkali required to titrate the solution back to the initial

pH is an accurate measure of the boron present. Electrometric or direct methods of titration may be used.

The choice of apparatus for the electrometric titration of boron should be determined by the instruments

available, the number of analyses to be made, and the

frequency of use. Three sets of apparatus are described below, any one of which will give satisfactory

results. The first requires a minimum of equipment.

The operation depends on the fact that a 0.7 N calomel

electrode and a quinhydrone electrode come to a null

point (reversal of polarity) at approximately pH 7.0.

Apparatus

Galvanometer. An enclosed lamp and scale type

sensitive to 0.025 microampere per scale division.

Quinhydrone electrode. A piece of platinum wire

7.5 cm. (3 in.) in length, with suitable contact above

the surface of the solution. This type is preferable to

an electrode of platinum sealed through glass and connected with mercury, as minute cracks develop in the

glass and cause erratic results.

Calomel electrode, 0.7 N with respect to potassium

chloride. A silver-silver chloride electrode can be used

in place of the 0.7 N calomel electrode. For details see

Wilcox (1932).

Motor-driven stirrer.

Switch, single-pole single-throw.

The electrodes are connected through the switch to

the galvanometer. A shunt to protect the galvanometer

is desirable but not essential.

The second apparatus is a simple potentiometer

(fig. 33). In addition to the parts listed above, the

following are required: resistance wire, 1,500 ohms

tapped at 60 ohms; and a 1.5-volt dry cell. A calomel

electrode, 0.1 N with respect to potassium chloride, is

substituted for the 0.7 N electrode described above.

The third apparatus makes use of either a potentiometer or a pH meter as the indicating system. The
SALINE AND ALKALI SOILS 141

GALVANOMETER

no VOLTS-AG

T^

TRANSFORMER

S.RS.T.

SWITCH

D.RS.T A^

SWITCH 6

60 TI

OMUCl

DRY CELL 1.5 VOLTS

^F^

FIXED RESISTANCE

1440 OHMS

¡^ CALOMEL

BEAKEI

D.P.D.T.

SWITCH

f

S.RS.T.

SWITCH

FIGURE 33.—Diagram of electrometric titration apparatus,

showing electrical circuit. The 6 volt a. c. line from the

transformer supplies the light in the reflecting galvanometer.

instrument is set so that, at balance, the solution under

test will have a pH reading of 7.1. The following

electrode pairs have been found satisfactory: quinhydrone and 0.1 N calomel; quinhydrone and saturated

calomel ; glass and saturated calomel.

Reagents

A. Quinhydrone, reagent quality, free from heavy

metals.

B. Bromthymol blue indicator solution, 1 percent.

Methyl red may be substituted.

C. Sulfuric acid. Approximately 1 N.

D. Sulfuric acid. Approximately 0.02 N.

E. Sodium hydroxide. Approximately 0.5 N,

carbonate-free.

F. Sodium hydroxide. Standard 0.0231 N, carbonate-free (1 ml. is equivalent to 0.25 mg. boron).

G. Boric acid solution. Dissolve 0.5716 gm. dry

H3BO3 in distilled water and dilute to 1 liter. The

H3BO3 may be dried in a desiccator with calcium

chloride. One ml. contains 0.1 mg. boron. This solution is used in standardizing reagent F.

H. Mannitol, neutral. Synthetic mannitol is preferable to the natural product. The "blank" titration for

5 gm. of mannitol should not exceed 0.1 ml. of

reagent F.

Procedure

Transfer an aliquot of the sample, containing not

more than 1 mg. boron, to a 400-ml. beaker and dilute,

if necessary, to 250 ml. Add a few drops of reagent B

and acidify with C, adding 0.5 to 1 ml. in excess. Bring

to boil, stir, cautiously at first, then vigorously, to expel

carbon dioxide. Cool to room temperature, preferably

in a water bath. With the S. P. S. T. switch open and

the shunt, if used, set at 0.1, introduce the electrodes

and stirrer into the solution. Start the stirrer and

add E to approximate neutrality as shown by B. Add

about 0.2 gm. of A and close the switch in the electrode

circuit. The galvanometer should indicate approximate balance. If it swings to the right, excess alkali

is indicated, and if to the left, excess acid. Adjust

with either F or D until the galvanometer shows no

deflection.

If a shunt is used, reverse the switch, thus eliminating it from the circuit and permitting the galvanometer

to function at its greatest sensitivity. Again adjust to

balance with either dilute acid or alkali. The galvanometer should be steady, showing at most only a slow

drift. This is the initial point of the titration. Bring

the shunt into the circuit by reversing the D. P. D. T.

switch or open the S. P. S. T. switch, if the shunt is

omitted. Add 5 ±0.1 gm. of mannitol. If boron is

present, the indicator will change to the acid color and

the galvanometer will swing to the left. Add reagent F

until approximate balance is again attained; eliminate

the shunt, if used, and complete the titration, bringing

the galvanometer back to the original null point. This

is the end point.

Note the number of milliliters of reagent F required

after adding the mannitol at the initial point of the

titration. From this, subtract a blank determined by

substituting distilled water for the sample and proceed

as indicated above. The net volume of F multiplied

by the equivalency (mg. boron per ml. NaOH) gives

milligrams of boron in the aliquot titrated. Report as

parts per million boron. The equivalency of F is

established by titrating an aliquot of G. The buret

used should be of such accuracy that the volume of F

can be read to 0.01 ml. Borosilicate glassware (Pyrex)

can be used for this determination. New beakers

should be cleaned by filling with acid and heating on

the steam bath before use. If the concentration of

phosphate exceeds 10 p. p. m., it should be precipitated

with lead nitrate and the excess lead removed with

sodium bicarbonate.

Calculations

Parts per million B=(ml. NaOH-blank) X mg. B

equivalent to 1 ml. NaOH X 1,000/ml. in aliquot.

References

American Public Health Association and American

Water Works Association (1946, p. 87-90), Association of Official Agricultural Chemists (1950, 31,52,

p. 547), Wilcox (1932).
142 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

( 73b ) Boron, Colorimetric, Using Carmine

Apparatus

A spectrophotometer with matched square cuvettes.^^

Centrifuge tubes, flasks, beakers, pipets, and burets

(boron-free glass). Alkali-resistant (boron-free)

glassware, porcelainware, platinum, or fused quartz

dishes are satisfactory. The use of borosilicate glassware is to be avoided. Convenient sizes are centrifuge

tubes, 15-ml.; flasks, 125-ml.; beakers, 100-ml.; pipets,

2-ml. ; burets, 10-ml. automatic.

Reagents

A. Sodium hydroxide, approximately 0.1 N, boronfree.

B. Hydrochloric acid, cone.

C. Hydrochloric acid, dilute (5 ml. cone.+ 95 ml.

water).

D. Sulfuric acid, cone.

E. Carmine solution. A 0.05 percent solution by

weight of carmine in cone, sulfuric acid (0.920 gm./l.).

Shake until completely dissolved.

F. Standard boric acid solution. Stock solution.

Dissolve 0.5716 gm. of recrystallized H3BO3 in distilled

water and dilute to 1 liter. One ml. of this solution

contains 0.100 mg. of boron.

Preparation of the Standard Curve

Dilute portions of reagent F to obtain standards over

the range of 0 to 10 p. p. m. boron. Treat 2 ml. of each

solution as described under Procedure and determine

percent transmittance. For a reference, 2 ml. of distilled water is carried through the entire procedure and

set at 100 percent transmittance.

Procedure

Pipet 2 ml. of the sample which should contain not

more than 0.02 mg. boron into an Erlenmeyer flask.

Add 2 drops of reagent B. Add 10 ml. of D. Mix

and cool. Add 10 ml. of E, mix, and allow to stand at

least 45 min. for color development. Determine the

percent transmittance at 585 m/A against a reference

solution of 2 ml. of distilled water carried through the

entire procedure. For colored samples such as certain

soil extracts, follow the procedure under paragraph

headed "Boron Concentration Too Low," except ignite

gently after evaporating the sample to dryness.

Calculations

Read the boron concentration from the concentration-transmittance calibration graph. Where the boron

concentration is such that the measured transmittance

value falls outside the recommended portion of the

transmittance range (this method suggests 20 to 95

^* A Coleman Model 14 Universal spectrophotometer with 13

by 13 by 105 mm. matched square cuvettes and filter PC-4 is

quite satisfactory. Any good photoelectric colorimeter should

be adequate, although perhaps somewhat less accurate.

percent), the sample is either diluted or concentrated

to meet these conditions.

BORON CONCENTRATION TOO GREAT.—Dilute the

sample with distilled water to a known volume, mix,

pipet 2 ml. into an Erlenmeyer flask, and proceed as

directed above.

BORON CONCENTRATION TOO LOW.—Pipet a suitable

aliquot of the sample into a beaker, a platinum dish,

or other suitable vessel. Make alkaline with reagent A

and add a slight excess. (The same amount should be

added to all samples, including a reference.) Evaporate to dryness on a steam bath or in an oven at 95° C.

Cool, add 5 ml. of C, and triturate with a rubber

policeman. Pour the solution into a conical centrifuge

tube and centrifuge at ÄCF= 1,000 to 1,500. Pipet 2

ml. of the clear solution into an Erlenmeyer flask and

follow the procedure shown above, correcting the reading from the standard curve to conform with the

aliquot taken.

Reference

Hatcher and Wilcox ( 1950).

(74) Dissolved Solids

Procedure

Filter the sample to obtain a perfectly clear liquid.

Evaporate a suitable aliquot containing not more than

1.0 gm. of residue to dryness in a weighed platinum

dish. Dry to constant weight at 105° C. Cool in a

desiccator and weigh. Reserve for the determination

of silica under Method 76.

Calculations

Parts per million DS=gm. residue X 1,000,000/ml.

in £iliquot.

Reference

Association of Official Agricultural Chemists (1950,

313, p, 535).

(75) pH of Waters

Procedure

See Method 21c.

(76) Silica

{76a) Silica, Gravimetric

Procedure

Acidify the sample or the residue from Method 74

with hydrochloric acid and evaporate to dryness on a

steam bath in a platinum dish. Continue the drying

for about an hour. Thoroughly moisten the residue
SALINE AND ALKALI SOILS 143

with 5 to 10 ml. hydrochloric acid. Allow to stand

10 to 15 min. and add sufficient water to bring the

soluble salts into solution. Heat on a steam bath until

solution of salts is effected. Filter to remove most of

the silica and wash thoroughly with hot water. Evaporate the filtrate to dryness and treat the residue with

5 ml. hydrochloric acid and sufficient water to effect

solution of soluble salts, as before. Heat, filter, and

wash with hot water. Transfer the 2 residues to a

platinum crucible, ignite in a muffle furnace, cool, and

weigh. Moisten the contents of the crucible with a

few drops of water. Add a few drops of sulfuric acid

and a few milliliters of hydrofluoric acid and evaporate on a steam bath under a hood. Repeat the treatment if all the silica is not volatilized. Dry carefully

on a hot plate, ignite, cool, and weigh. The difference

between the two weights is the weight of silica.

Calculations

Parts per million SiOg^gm. SÍO2X 1,000,000/ml. in

aliquot.

Reference

Association of Official Agricultural Chemists (1950,

31.19, p, 539),

i76b) Silica^ Colorimetric ^^

Apparatus

Spectrophotometer or photoelectric colorimeter.

Reagents

A. Ammonium molybdate solution, 20 percent, stock

solution. Dissolve 50 gm. (NH4) 6Mo7024-4H20 in 200

ml. water (do not heat), make to 250 ml. and filter.

B. Sulfuric acid, 10 N. Add 70.2 ml. cone, sulfuric

acid with stirring to 185 to 190 ml. water, cool, transfer

to 250-ml. volumetric flask, and dilute to the mark.

Solutions A and B may be stored in glass because of

the small amount used per determination.

C. Ammonium molybdate, sulfuric acid mixture.

Add 1 ml. of reagent B and 2 ml. of A to 200 ml. of

water. Use 10 ml. for each determination. A fresh

lot of this reagent should be prepared for each set of

samples.

D. Standard silica solution, 50 p. p. m. SiOg. To

prepare, dissolve more than the calculated amount of

crystalline Na2Si03-9H20 in water, filter, and analyze

gravimetrically, as described under Method 76a. Add

the calculated amount of water necessary to dilute the

solution to exactly 50 p. p. m. SÍO2. Store in a polyethylene bottle, not in glass.

^' This method was adapted for use with a spectrophotometer

from a method proposed by Scripps Institution of Oceanography

at La Jolla, California.

Preparation of the Standard Curve

Dilute portions of reagent D to obtain standards

over the range of 0 to 50 p. p. m. SÍO2. Treat 1 ml. of

each solution as described under "Procedure" and determine percent transmittance.

Procedure

To 1 ml. of the sample add 10 ml. of reagent C and

mix thoroughly. Determine the percent transmittance

after standing 10 min., but not more than 45 min., at

350 m/x against a reference solution of 1 ml. of distilled

water carried through the entire procedure. Read the

silica concentration from the standard curve and report

as p. p. m. SÍO2.

(77) Calcium

Reagents

A. Bromcresol green (sodium salt), 0.1 percent in

water.

B. Hydrochloric acid, 6 N.

C. Oxalic acid solution, 1 N. Dissolve 63 gm.

(COOH)2-2H20 in 1 liter of water.

D. Ammonium hydroxide solution (1 + 1).

E. Sulfuric acid, dilute solution (45 ml. water plus

5 ml. cone, sulfuric acid).

F. Standard potassium permanganate, 0.05 N.

Procedure

Take an aliquot of the sample containing between

0.20 and 2.0 meq. of calcium and concentrate, if necessary, to a volume of approximately 200 ml. Add 2 to 3

drops of reagent A, acidify with B, and then aid 0.5 ml.

of B and 0.5 ml. of C for each 100 ml. of solution.

Heat to boiling and neutralize with D. An excess of C

is added gradually (5 ml. for each 100 ml. of solution)

with constant stirring; the hot solution is made slightly

alkaline with D and allowed to boil gently for several

minutes. Cool and let stand until the precipitate of

calcium oxalate settles. During the cooling, further

additions of D may be necessary, in order to keep the

solution faintly alkaline.

Filter through a good grade filter paper designed

for fine precipitates, receiving the filtrate in a 400-ml.

beaker. Reserve the filtrate for the determination of

magnesium. Transfer the precipitate to the filter paper

and wash both beaker and precipitate with water until

free from soluble oxalates. Remove the beaker containing the filtrate and substitute the original beaker.

Puncture the tip of the filter paper and wash the precipitate down into the beaker. Pour 50 ml. of reagent

E through the funnel and rinse with water. The beaker

is heated nearly to boiling and the liberated oxalic acid

titrated with F until faintly pink. Add the filter paper

and continue the titration until a very slight permanent

pink color appears.
144 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

Calculations

Milliequivalents per liter of Ca= 1,000 X normality

of KMn04X (ml. KMn04-blank)/ml. in aliquot.

Reference

Blasdale (1909).

( 78) Magnesium

Reagents

A. Hydrochloric acid (141).

B. Diammonium-hydrogen phosphate solution. Make

up a 20 percent solution of diammonium-hydrogen

phosphate in water. Filter before use. A fresh lot of

this reagent should be prepared for each set of samples.

C. Ammonium hydroxide solution (1 + 1).

D. Ammonium hydroxide, cone.

Procedure

Acidify the filtrate from the calcium determination

(Method 77) with reagent A then add 2 ml. in excess.

Evaporate on a hot plate. If the weight of pyrophosphate is expected to be 0.0500 gm. or more, reduce the

volume to 100 ml.; otherwise, evaporate to 50 ml. and

allow to cool. Add 5 ml. of B for each 50 ml. volume,

then C drop by drop with stirring, until the solution is

strongly alkaline. After a few minutes add 10 ml. of D

for each 100 ml. final volume. On the following day,

filter on ashless paper and wash with dilute ammonium

hydroxide (5 + 95). Transfer the paper with the precipitate to a weighed silica or porcelain crucible, dry,

and ignite to whiteness in a muffle. Cool in a desiccator

and weigh.

Calculations

Milliequivalents per liter of Mg=gm. Mg2P207X

17,969/ml. in aliquot.

Reference

Association of Official Agricultural Chemists (1950,

3L26,p.541).

(79) Calcium and Magnesium by the Versenate Method

Procedure

See Method 7.

(80) Sodium

(80a) Sodium by Uranyl Zinc Acetate^

Gravimetric

The method of Barber and Kolthoff is the basis for

the one here described. It has been modified in only

minor details.

Reagents

A. Uranyl zinc acetate:

Uranyl acetate, dihydrate 300 gm.

Zinc acetate, dihydrate 900 gm.

Acetic acid, 30 percent 270 ml.

Distilled water__ 2,430 ml.

Weigh the salts and transfer to a large flask; add

acetic acid and water; shake or stir occasionally until

the salts are dissolved. This may take several days.

Filter before use.

B. Ethyl alcohol, saturated with 50í/¿u/n-uranyl-zincacetate precipitate. Filter before use.

C. Ether, anhydrous.

Procedure

Evaporate an aliquot of water sufficient to give 50

to 200 mg. of the triple salt (usually 10 to 20 ml.) in a

Pyrex beaker to a volume of 1 to 2 ml. Cool. Add 20

ml. of the filtered reagent A. Stir the solution and allow

to stand for 1 hr. Filter through a porous-bottomed

porcelain filtering crucible, taking care to transfer all

the triple salt onto the filter by means of a small wash

bottle filled with A. Wash the beaker 5 times with

2-ml. portions of A and pass the washings through the

filter. Allow the crucible to drain completely, because

it is important to have the filter and the precipitate

free from the reagent before washing with the alcohol.

Wash the crucible 5 times with 2-ml. portions of B and,

after removing all the alcohol by suction, wash once

or twice with C. The suction is continued until the

precipitate is dry. Allow the crucible to stand in a

desiccator 2 hr. and weigh.

Return the crucible to a suction apparatus and wash

with small portions of water until all the soluble ma

terial is dissolved and passes through the crucible

Wash with alcohol and ether as above. Dry and weigh

The difference between the first and last weight repre

sents the weight of sodium precipitate. The precipi

täte is assumed to have the composition (U02)3NaZn

(CH3C00)9-6H20; molecular weight, 1538.079; percent sodium, 1.4952.

Calculations

Milliequivalents per liter of Na= gm. sodium-uranylzinc-acetate precipitate X 650.16/ml. in aliquot.

Reference

Barber and Kolthoiï (1928).

(80b) Sodium by Flame Photometer

Procedure

See Method 10a.

(81) Potassium
SALINE AND ALKALI SOILS 145

(81a) Potassium by Cohaltinitrite^ Gravi'

metric

Reagents

A. Nitric acid, 1 N.

B. Trisodium cobaltinitrite solution. Prepare an

aqueous solution containing 1 gm. of the salt of reagent

quality in each 5 ml., allowing 5 ml. for each determination. Filter before use. The solution is stable for

some time, but it is preferable to make up a fresh lot

before each set of determinations.

C. Nitric acid, 0.01 N.

D. Ethyl alcohol, 95 percent.

Procedure

The aliquot for analysis should contain between 2

and 15 mg. of potassium in a neutral aqueous solution

of 10-ml. volume. (Ammonia interferes and if present

must be removed by evaporation with sodium hydroxide.) Add 1 ml. of reagent A and 5 ml. of B, mix, and

allow to stand for 2 hr. at 15° to 20° C. Filter in a

porous-bottomed porcelain filtering crucible, the tare

weight of which is known, using C in a wash bottle

to make the transfer. Wash 10 times with C and 5 times

with 2-ml. portions of D. Aspirate until quite dry.

Wipe the outside with a cloth, dry for 1 hr. at 105° C,

cool in a desiccator, and weigh.

Modified Procedure

For very small quantities of potassium (0.2 meq./l.

or less). Evaporate 200 ml. of the sample in a platinum dish and remove silica as under Method 76a. This

aliquot may be used for the determination of dissolved

solids as under Method 74 and silica as under Method

76a. After removal of silica, evaporate the filtrate to

dryness to remove hydrochloric acid, add 10 ml. of

water, 1 ml. of reagent A, and 5 ml. of B, and put the

sample in the refrigerator at 5° to 15° C. overnight.

When gypsum is high, add more of the reagents (10

ml. of water, 1 ml. of A, and 5 ml. of B for each 5

meq./l. of gypsum present). The following morning,

remove the samples from the refrigerator, filter

through a porous-bottomed porcelain crucible, and

proceed as directed above.

If the sample is high in organic matter, such as a

sewage effluent, evaporate the filtrate to dryness, take

up in aqua regia (3 parts cone, hydrochloric acid + 1

part cone, nitric acid), and evaporate again before

potassium is precipitated.

The composition of the precipitate can be represented

by the formula K2NaCo(N02)6-H20. K=17.216 percent.

Calculations

Milliequivalents per liter of K = gm. di-potassium

sodium cobaltinitrite precipitate X4,403.4/ml. in

aliquot.

Reference

Wilcox (1937).

(81b) Potassium, by Cobaltinitrite, Volum^etric

Reagents

In addition to the reagents listed under the gravimetric procedure, except 95 percent ethyl alcohol, the

following are required:

A. Sodium hydroxide, approximately 0.5 N.

B. Sulfuric acid, cone.

C. Potassium permanganate solution, standard

0.05 N.

D. Sodium oxalate solution, standard 0.05 N.

Procedure

Follow the gravimetric procedure of Method 81a

through the precipitation and washing with nitric acid.

Omit washing with alcohol. Wash the precipitate into

a 250-ml. beaker, place the crucible in the beaker, and

make to about 100 ml. with water. Add 20 ml. of

reagent A and boil for 3 min. Withdraw into another

beaker a slight excess of C, make to 50 ml. with water,

and add 5 ml. of B. Pour the hot potassium cobaltinitrite solution into the cold potassium permanganate

solution, transfer the crucible, and wash the beaker

with a small quantity of water. Add an excess of D,

heat to boiling, and complete the titration with potassium permanganate.

Calculations

Milliequivalents per liter of K = normality of KMnOi

X (ml. KMn04-blank) X 181.81/ml. in aliquot.

Reference

Wilcox (1937).

(81c) Potassium, by Flam,e Photom^eter

Procedure

See Method 11a.

( 82 ) Carbonate and Bicarbonate

Reagents

A. Phenolphthalein, 0.25 percent solution in 50

percent alcohol.

B. Sulfuric acid, standard 0.050 N.

C. Methyl orange, 0.1 percent in water.

Procedure

Take an aliquot of the sample containing not more

than 1.0 meq. of carbonate plus bicarbonate and dilute
146 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

to 50 ml., if less than that volume. Add a few drops of

reagent A, and, if a pink color is produced, titrate with

B, adding a drop every 2 or 3 seconds until the pink

color disappears. To the colorless solution from this

titration or to the original solution, if no color is produced with phenolphthalein, add 1 or 2 drops of C,

continue the titration (without refilling the buret) to

the methyl orange end point, and note the total reading.

(Reserve the solution for the determination of chloride.)

Blank determinations should be run with the reagents

and carbon dioxide-free distilled water and corrections

made, if necessary.

Remarks

To facilitate calculations a table similar to that

shown in APHA Standard Methods (1946) is included

(table 18).

TABLE 18—The titration of hydroxide, carbonate, and

bicarbonate ions in the presence of phenolphthalein

and methyl orange indicators.

Result of titration '

P=0...

P</2 T

P=V2T

P>V2 T

P=T..

Titration value related to

each ¡on—

Hydroxide

0

0

0

2P-T

T

Carbonate

0

2P

2P

2 (T-P)

0

Bicarbonate

T

T-2P

0

0

0

*P=Titration to the phenolphthalein end point; r=total

titration to the methyl end point.

Calculations

Ion sought, milliequivalents per liter of either OH,

CO3, or HC03= 1000 X normality of the acidX [titration value (from table 18) in ml. acid —blank]/ml. in

aliquot.

References

American Public Health Association and American

Water Works Association (1946, p. 9), Association of

Official Agricultural Chemists (1950, 31.18, p. 539),

(83) Sulfate

Reagents

A. Hydrochloric acid, cone.

B. Barium chloride solution, 10 percent. Dissolve

100 gm. BaCl>-2H,0 in 1 liter of water and filter.

C. Methyl orange, 0.1 percent in water.

Procedure

Take an aliquot of the sample containing between

0.2 and 5 meq. of sulfate and, if necessary, dilute to a

volume of approximately 200 ml. Add a few drops of

reagent C and 1 ml. of A. Heat to boiling and add an

excess of B drop by drop with constant stirring. Allow

to stand on the water bath until the volume is reduced

to about 50 ml. After cooling, the precipitate of barium

sulfate is filtered through an ashless filter paper and

washed with water until free from chloride. The filter

paper is then carefully folded, placed in a tared porcelain or silica crucible, ignited in a well-ventilated

muffle at low red heat, and weighed.

Calculations

Milliequivalents per liter of S04= gm. BaS04X

8568.2/ml. in aliquot.

Reference

Association of Official Agriculture Chemists (1950,

3L27,p.541).

(84) Chloride

Reagents

A. Potassium chromate indicator. Dissolve 5 gm.

of potassium chromate in water and add a saturated

solution of silver nitrate until a slight permanent red

precipitate is produced; filter and dilute to 100 ml.

B. Standard silver nitrate solution, 0.05 N. Dissolve

8.4944 gm. silver nitrate in water and dilute to 1 liter.

Check by titration against pure sodium chloride or

standard potassium chloride (reagent A, Method 72).

Procedure

To the solution from the carbonate and bicarbonate

determination (Method 82), add 1 ml. of reagent A

and titrate with B. Correct for the quantity of silver

nitrate solution necessary to give, in 50 ml. of chloridefree water with 1 ml. potassium chromate indicator,

the shade obtained at the end of the titration of the

sample.

If the size of aliquot that is suitable for the carbonatebicarbonate titration is too large for the chloride determination, a smaller aliquot must be taken and neutralized to methyl orange. The aliquot should contain

not more than 2 meq. of chloride.

Calculations

Milliequivalents per liter of Cl = 1000 X normality of

the AgNO,X(ml. AgNOa"blank)/ml. in aliquot.

Reference

Association of Official Agricultural Chemists (1950,

3L10,p.536),
SALINE AND ALKALI SOILS 147

(85) Fluoride

Remarks

Method suggested is given in Standard Methods,

APHA (1946), substituting sulfuric acid for perchloric

acid and silver sulfate for silver perchlorate.

It has been found reliable for potable waters of ordinary composition. Up to the following limits expressed as parts per

million it is not interfered with by: Chloride ion (Cl)—500

ppm., sulfate ion (SO4 )—200 ppm., alkalinity (expressed as

CaCOa)—200 ppm., acidity (expressed as CaCOa)—200 ppm.,

iron (Fe) —2 ppm., aluminum (Al)—0.5 ppm., phosphate ion

(PO4) —1 ppm., color 25, turbidity 25.

If limits are exceeded, separate fluoride by distillation.

Reagents

A. Acid zirconium alizarin reagent. Dissolve 0.3

gm. zirconium oxychloride (ZrOClg'SHgO) in 50 ml.

distilled water contained in a 1-liter glass-stoppered

flask. Dissolve 0.07 gm. alizarin sodium monosulfonate in 50 ml. distilled water and pour slowly into the

zirconium oxychloride solution, while swirling the flask.

This solution clears on standing for a few minutes.

Prepare a mixed acid solution as follows: Dilute 112

ml. cone, hydrochloric acid to 500 ml. with distilled

water. Dilute 37 ml. cone, sulfuric acid to 500 ml.

with distilled water. After cooling, mix the two acids.

To the zirconium alizarin solution in the 1-liter flask,

add the mixed acid solution to the mark and mix. The

reagent changes in color from red to yellow within an

hour and is then ready for use. If stored in a refrigerator, it may be used for 60 to 90 days.

B. Standard sodium fluoride solution. Dissolve

0.221 gm. sodium fluoride in distilled water and make

up to 1 liter. Dilute 100 ml. of the stock sodium

fluoride solution to 1 liter with distilled water. One

ml. is equivalent to 0.01 mg. of fluoride.

Procedure

To 100 ml. of sample containing not more than 0.14

mg. fluoride and to standards made up to 100 ml. with

distilled water, contained in 100-ml. matched Nessler

tubes, add 5 ml. of reagent A, accurately measured from

a 5-ml. volumetric pipet. Mix and compare sample

with standards after standing 1 hr. at room temperature. Recommended standards are 0, 0.01, 0.02, 0.03,

0.04, 0.05, 0.06, 0.08, 0.10, 0.12, and 0.14 mg. of

fluoride. Since the color of the zirconium-alizarin lake

varies with temperature, samples and standards should

have the same temperature within 1^ or 2° C, before

adding the reagent.

Calculations

Parts per million F=mg. FXl,000/ml. in aliquot.

Reference

American Public Health Association and American

Water Works Association (1946, 39A-2, p. 76, and

39B-2,p,77).

(86) Nitrate

(86a) Nitrate^, Phenoldisulfonic Acid

(For water of low chloride content.)

Procedure

See Method 15.

{86b) Nitrate^ Devarda

(For water of high chloride content.)

Apparatus

Nitrogen distilling apparatus with scrubber bulbs.

Reagents

A. Devarda alloy.

B. Sodium hydroxide, saturated solution.

C. Boric acid, 2 percent solution.

D. Standard sulfuric acid, 0.05 N.

E. Bromcresol green-methyl red (BCG-MR) indicator solution.^^ Prepare a 0.1 percent bromcresol

green solution, adding 2 ml. 0.1 N sodium hydroxide

per 0.1 gm. of indicator. Prepare a 0.1 percent methyl

red solution in 95 percent ethyl alcohol, adding 3 ml.

0.1 N sodium hydroxide per 0.1 gm. of indicator. Mix

75 ml. bromcresol green, 25 ml. methyl red, and 100 ml.

of 95 percent ethyl alcohol. The indicator should be

gray in a solution containing boric acid and ammonium

sulfate in concentrations equal to those encountered in

the Devarda procedure. It is often necessary to add a

little of one or the other of the indicators until the

proper shade is obtained. The color change is from

green in alkali through gray at the end point to red in

acid solution.

Procedure

Place 50 ml. of the sample, or such volume as will

contain not less than 0.2 meq. nitrate, in a Kjeldahl

flask and add 2 gm. Devarda alloy. Make up to 300

ml. with distilled water, then add 2 ml. of reagent B,

allowing it to run down the side of the flask so that it

does not mix with the contents at once. Connect with

the distilling apparatus and rotate the flask to mix.

Heat slowly at first and then at such a rate that the

200 ml. of distillate required will pass over in 1 hr.

Collect the distillate in 50 ml. of C. The ammonia is

titrated with D, using indicator E

Calculations

Milliequivalents per liter of N03= 1,000 X normality

of acidX (ml. acid —blank)/ml. in aliquot.

Reference

Association of Official Agricuhural Chemists (1950,

2.30, p, 14).

^" Chapman, H. D. Private communication.
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1952. SOIL WATER AND PLANT GROWTH. In Soil Physical

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1944. MOISTURE RETENTION BY SOME IRRIGATED SOILS AS RELATED TO SOIL-MOISTURE TENSION. Joui. Agr. RcS.

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ROBERTS, R. C.

1950. CHEMICAL EFFECTS OF SALT-TOLERANT SHRUBS ON

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RUSSELL, M. B.

1949. METHODS OF MEASURING SOIL STRUCTURE AND AERATION. Soil Sei. 68: 25-35.

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1947. A RATIO LAW GOVERNING THE EQUILIBRIUM OF CATIONS

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1948. THE PERMANENT WILTING PERCENTAGE AS A REFERENCE FOR THE MEASURING OF SOIL MOISTURE. Amer.

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SALINE AND ALKALI SOILS 153

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1945. GROWTH AND BIOCHEMICAL COMPOSITION OF BEAN

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1949. SALT DISTRIBUTION UNDER FURROW AND BASIN IRRIGATED COTTON AND ITS EFFECT ON WATER REMOVAL.

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1897. AN ELECTRICAL METHOD OF DETERMINING THE SOLUBLE

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1932. ELECTROMETRIC TITRATION OF BORIC ACID. Indus. and

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E.

A DIRECT METHOD OF AGGREGATE ANALYSIS OF SOILS

AND A STUDY OF THE PHYSICAL NATURE OF EROSION

LOSSES. AmeF. Soe. AgFon. JOUF. 28: 337-351, illus.

DRAINAGE AND RECLAMATION OF LAKES AND OF THE

ZUIDERZEE. Soil Sei. 74: 75-89, illus.
Glossary

ABSORPTION.—The process by which a substance is taken into

and included within another substance, i. e., intake of water

by soil, or intake of gases, water, nutrients, or other substances by plants.

ADSORPTION.—The increased concentration of molecules or ions

at a surface, including exchangeable cations and anions on

soil particles.

AGGREGATE.—A group of soil particles coherin/^ so as to behave

mechanically as a unit.

AGGREGATION.—The act or process of forming aggregates, or

the state of being aggregated.

ALKALI SOIL.—A soil that contains sufficient exchangeable

sodium to interfere with the growth of most crop plants, either

with or without appreciable quantities of soluble salts. See

Nonsaline-Alkali Soil and Saline-Alkali Soil.

ALKALINE.—A chemical term referring to basic reaction where

the pH reading is above 7, as distinguished from acidic reaction where the pH reading is below 7.

ALKALINE SOIL.—A soil that has an alkaline reaction, i. e., a

soil for which the pH reading of the saturated soil paste is

higher than 7.

ALKALIZATION.—The process whereby the exchangeable-sodium

content of a soil is increased.

ATMOSPHERE.—See Standard Atmosphere.

BASE-EXCHANGE CAPACITY.—See Cation-Exchange-Capacity.

BULK DENSITY.—The ratio of the mass of water-free soil to its

bulk volume. Bulk density is expressed in pounds per cubic

foot or grams per cubic centimeter and is sometimes referred

to as "apparent density." When expressed in grams per

cubic centimeter, bulk density is numerically equal to apparent specific gravity or volume weight.

CATION EXCHANGE.—The interchange of a cation in solution

with another cation on a surface-active material.

CATION-EXCHANGE-CAPACITY.—The total quantity of cations

which a soil can adsorb by cation exchange, usually expressed as milliequivalents per 100 grams. Measured values

of cation-exchange-capacity depend somewhat on the method

used for the determination.

CELL CONSTANT.—See Conductivity-Cell Constant.

COEFFICIENT OF CORRELATION.—A statistic used in linear correlation that provides a measure of the proportion of variation

in one variable that is associated with variation in another

variable.

COEFFICIENT OF DETERMINATION.—A statistic used in linear

correlation that gives the fraction of the variance in one variable which is associated with variance in another variable. It

is the square of the coefficient of correlation and is usually

expressed in percent.

COEFFICIENT OF VARIATION.—Standard deviation expressed as

percentage of the mean.

CONDUCTIVITY.—See Electrical Conductivity and Hydraulic

Conductivity.

CONDUCTIVITY-CELL CONSTANT (k).—The product of the known

electrical conductivity (EC) of a standard solution in a

conductivity cell and the corresponding measured resistance

(R) of the cell containing the standard solution. That is:

k=ECXR. The value of the cell constant is determined by

the geometry of the cell and so is nearly independent of the

temperature, but EC and Ä must be evaluated at the same

temperature. Rearranging the equation and indicating temperatures by a subscript gives: ECt=k/Rt. In this form, the

equation may be used for evaluating the conductivity ECt

of an unknown solution in the cell at temperature (t), where

Rt is the measured resistance of the cell containing the solution at the temperature t and k is the cell constant as evaluated from a previous measurement of a standard solution.

154

CONSUMPTIVE USE.—The water used by plants in transpiration

and growth, plus water vapor loss from adjacent soil or snow

or from intercepted precipitation in any specified time.

Usually expressed as equivalent depth of free water per

unit of time.

DARCY'S LAW.—1. Historical. The volume of water passing

downward through a sand filter bed in unit time is proportional to the area of the bed and to the difference in hydraulic

head and is inversely proportional to the thickness of the bed.

2. Generalization for three dimensions. The effective rate of

viscous flow of water in Isotropie porous media is proportional to, and in the direction of, the hydraulic gradient.

3» Generalization for other fluids. The effective rate of

viscous flow of homogeneous fluids through isotropic porous

media is proportional to, and in the direction of, the driving

force.

DISPERSED SOIL.—Soil in which the clay readily forms a colloidal

sol. Dispersed soils usually have low permeability. They

tend to shrink, crack, and become hard on drying and to slake

and become plastic on wetting.

DRAINAGE.—1. The processes of the discharge of water from an

area of soil by sheet or stream flow (surface drainage) and

the removal of excess water from within soil by downward

flow through the soil (internal drainage). 2. The means for

effecting the removal of water from the surface of soil and

from within the soil, i. e., sloping topography or stream channels (surface drainage) and open ditches, underground tile

lines, or pumped wells (artificial drainage).

DRAINAGE REQUIREMENTS.—Performance and capacity specifications for a drainage system, i. e., permissible depths and

miodes of variation of the water table with respect to the root

zone or soil surface, and the volume of water that the drains

must convey in a given time.

EFFICIENCY OF IRRIGATION.—The fraction of the water diverted

from a river or other source that is consumed by the crop,

expressed as percent. See Consumptive Use. Often applied

to whole irrigation systems and takes account of conveyance

losses.

EFFICIENCY OF WATER APPLICATION.—The. fraction of the water

delivered to the farm that is stored in the root zone for use

by the crop, expressed as percent.

ELECTRICAL CONDUCTIVITY.—The reciprocal of the electrical

re;sistivity. The resistivity is the resistance in ohms of a

conductor, metallic or electrolytic, which is 1 cm. long and

has a cross-sectional area of 1 cm.* Hence, electrical conductivity is expressed in reciprocal ohms per centimeter, or

raihos per centimeter. The terms "electrical conductivity"

and "specific electrical conductance" have identical meaning.

EQUIVALENT; EQUIVALENT WEIGHT.—The weight in grams of an

ion or compound that combines with or replaces 1 gm. of

hydrogen. The atomic weight or formula weight divided by

its valence.

EQUIVALENT PER MILLION.—An equivalent weight of an ion or

salt per 1 million gm. of solution or soil. For solutions,

equivalents per million (e. p. m.) and milliequivalents per

liter (meq./l.) are numerically identical if the specific gravity

of the solution is 1.0.

ETHYLENE GLYCOL RETENTIVITY.—Weight of ethylene glycol

adsorbed per unit weight of soil under specified equilibrium

or quasi-equilibrium conditions. See Method 25.

EXCHANGE CAPACITY.—See Cation-Exchange-Capacity.

EXCHANGE COMPLEX.—The surface-active constituents of soils

(both inorganic and organic) that are capable of cation

exchange.

EXCHANGEABLE CATION.—A cation that is adsorbed on the exchange complex and which is capable of exchange with other

cations.
SALINE AND ALKALI SOILS 155

EXCHANGEABLE-SODIUM-PERCENTACE.—The degree of saturation

of the soil exchange complex with sodium. It may be calculated by the formula :

rep— J^-^JL^Ig^J^l*^ sodium (meq./lOO gm. soil) ^ ^^^

^^ "Cation-exchange-capacity (meq./lOO gm. soil)

FIELD CAPACITY.—The moisture content of soil in the field 2 or 3

days after a thorough wetting of the soil profile by rain or

irrigation water. Field capacity is expressed as moisture

percentage, dry-weight basis.

FIFTEEN-ATMOSPHERE PERCENTAGE.—The moisture percentage,

dry-weight basis, of a soil sample which has been wetted and

brought to equilibrium in a pressure-membrane apparatus at

a pressure of 221 p. s. i. This characteristic moisture value

for soils approximates the lower limit of water available for

plant growth.

FIFTY PERCENT YIELD-DECREMENT VALUE.—The measured value

of the soil salinity or alkali that decreases crop yield 50

percent as compared with yields of the same crop on nonsaline and nonalkali soils under similar growing conditions.

GROUND WATER.—Water in soil beneath the soil surface, usually

under conditions where the pressure in the water is greater

than the atmospheric pressure, and the soil voids are substantially filled with the water.

HYDRAULIC CONDUCTIVITY.—The proportionality factor in the

Darcy flow law, which states that the effective flow velocity is

proportional to the hydraulic gradient. Hydraulic conductivity, therefore, is the effective flow velocity at unit

hydraulic gradient and has the dimensions of velocity

iLT-').

HYDRAULIC GRADIENT.—The decrease in hydraulic head per unit

distance in the soil in the direction of the greatest rate oí

decrease of hydraulic head.

HYDRAULIC HEAD.—The elevation with respect to a standard

datum at which water stands in a riser or manometer connected to the point in question in the soil. This will include

elevation head, pressure head, and also the velocity head, if

the terminal opening of the sensing element is pointed upstream. For nonturbulent flow of water in soil the velocity

head is negligible. In unsaturated soil a porous cup must be

used for establishing hydraulic contact between the soil

water and water in a manometer. Hydraulic head has the

dimensions of length (L).

INDICATOR PLANT.—A native plant that indicates, in general,

and often in a specific manner, the nature of soil conditions

with regard to moisture and salinity. Dominant species arc

the most important indicators of such conditions.

INFILTRATION.—The downward entry of water into soil.

INFILTRATION RATE; INFILTRATION CAPACITY.—The maximum

rate at which a soil, in a given condition at a given time, can

absorb rain. Also, the rate at which a soil will absorb water

ponded on the surface at a shallow depth when the ponded

area is infinitely large or when adequate precautions are taken

to minimize the effect of divergent flow at the borders. It is

the volume of water passing into the soil per unit of area

per unit of time, and has the dimensions of velocity {LT'^).

INTAKE RATE; INFILTRATION VELOCITY.—The rate of water

entry into the soil expressed as a depth of water per unit of

time. This term involves no restrictions on area of application or divergence of flow in the soil; therefore, the measuring procedure should be specified. It has the dimensions of

velocity (LT^).

INTRINSIC PERMEABILITY.—The factor k' in the equation,

k'dgi k'.,„ „ ,

v= — = — {dtg—Vy)

•n 1

where i;=flow velocity, d=density, g=scalar value for acceleration of gravity, ¿=hydraulic gradient, i7=yiscosity,

Fg=gravitational force per unit of mass, and V p=pressure

gradient. Intrinsic permeability has the dimensions of

length squared {V). See Permeability (Quantitative).

ISOBATH.—1. Having constant depth. 2. A line connecting

points of equal depth to water table.

IsoPLETH.—1. A graph showing the occurrence or frequency of

any phenomenon as a function of two variables. 2. A line

showing the variation in time and position along a field profile

of the point of intersection of a water-table contour line and

the profile.

LEACHING.—The process of removal of soluble material by the

passage of water through soil.

LEACHING REQUIREMENT.—The fraction of the water entering

the soil that must pass through the root zone in order to

prevent soil salinity from exceeding a specified value. Leaching requirement is used primarily under steady-state or longtime average conditions.

LIME.—Strictly, calcium oxide (CaO), but as commonly used

in agriculture terminology calcium carbonate (CaCO») and

calcium hydroxide (Ca(0H)2) are included. Agricultural

lime refers to any of these compounds, with or without magnesia, used as an amendment for acid soils.

MILLIEQUIVALENT.—One thousandth of an equivalent.

MiLLiEQUiVALENT PER LITER.—A millicquivalent of an ion or a

compound in 1 liter of solution.

MOISTURE PERCENTAGE.—1. Dry-weight basis. The weight of

water per 100 units of weight of material dried to constant

weight at a standard temperature. 2. Depth basis. The

equivalent depth of free water per 100 units of depth of soil.

Numerically this value approximates the volume of water per

100 units of volume of soil.

NONSALINE-ALKALI SOIL.—A soil that 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 and the electrical conductivity of the saturation extract is

less than 4 millimhos per centimeter (at 25" C). The pH

reading of the saturated soil paste is usually greater than 8.5.

ONE-THIRD-ATMOSPHERE PERCENTAGE.—The moisture percentage, dry-weight basis, of a soil sample that has been air-dried,

screened, wetted, and brought to hydraulic equilibrium with

a permeable membrane at a soil-moisture tension of 345 cm.

of water. This retentivity value closely approximates the

moisture equivalent value of many soils.

OSMOTIC PRESSURE.—The equivalent negative pressure that influences the rate of diffusion of water through a semipermeable membrane. Its direct experimental value for a solution is the pressure difference required to equalize the diffusion rates between the solution and pure water across a

semipermeable membrane. Osmotic pressure in atmospheres

may be calculated from the freezing-point depression AT

in °C. by the formula OP--12.06Ar-0.021Ar.

PARTICLE DENSITY.—The average density of the soil particles.

Particle density is usually expressed in grams per cubic centimeter and is sometimes referred to as "real density" or "grain

density."

PERCOLATION.—A qualitative term applying to the downward

movement of water through soil. Especially, the downward

flow of water in saturated or nearly saturated soil at hydraulic

gradients of one or less.

PERMANENT WILTING PERCENTAGE.—The moisture percentage

of soil at which plants wilt and fail to recover turgidity. It

is usually determined with dwarf sunflowers. The expression

has significance only for nonsaline soils.

PERMEABILITY.—1. Qualitative. The quality or state of a porous medium relating to the readiness with which such a

medium conducts or transmits fluids. 2. Quantitative. The

specific property governing the rate or readiness with which

a porous medium transmits fluids under standard conditions.

The equation used for expressing the flow should take into

account the properties of the fluid so that proper measurements on a given medium give the same permeability value

for all fluids that do not alter the medium. The physical

dimensions of the permeability unit are determined by the

equation used to express the flow. 5ee Intrinsic Permeability.

PLANT COMMUNITY.—An assemblage of plants living together

under the same environmental conditions.
156 AGRICULTURE HANDBOOK 6 0, U. S. DEPT. OF AGRICULTURE

POROSITY.—The fraction of the soil volume not occupied ísy

soil particles, i. e., the ratio of the sum of the volumes of the

liquid and gas phases to the sum of the volumes of the solid,

liquid, and gas phases of the soil.

POTASSIUM-ADSORPTION-RATIO.—A ratio for soil extracts and

irrigation waters used to express the relative activity of

potassium ions in exchange reactions with soil.

PAR=

V(Ca^^-fMg-)/2

where the ionic concentrations are expressed in milliequivalents per liter.

RECLAMATION.—The process of removing excess soluble salts or

excess exchangeable sodium from soils.

REGRESSION COEFFICIENT.—A statistic used in linear correlation

that gives the change in one variable that is associated with

unit change in another variable.

SALINE-ALKALI SOIL.—A soil containing sufficient exchangeable

sodium to interfere with the growth of most crop plants and

containing appreciable quantities of soluble salts. The exchangeable-sodium-percentage is greater than 15, and the

electrical conductivity of the saturation extract is greater

than 4 mmhos per centimeter (at 25" C). The pH reading

of the saturated soil is usually less than 8.5.

SALINE SOIL.—A nonalkali soil containing soluble salts in such

quantities that they interfere with the growth of most crop

plants. The electrical conductivity of the saturation extract

is greater than 4 mmhos per centimeter (at 25° C), and the

exchangeable-sodium-percentage is less than 15. The pH

reading of the saturated soil is usually less than 8.5.

SALINIZATION.—The process of accumulation of soluble salts

in soil.

SATURATED SOIL PASTE.—A particular mixture of soil and water.

At saturation the soil paste glistens as it reflects light, flows

slightly when the container is tipped, and the paste slides

freely and cleanly from a spatula for all soils except those

with high clay content.

SATURATION EXTRACT.—The solution extracted from a soil at its

saturation percentage.

SATURATION PERCENTAGE.—The moisture percentage of a

saturated soil paste, expressed on.%a dry-weight basis.

SEMIPERMEABLE MEMBRANE.—A membrane that permits the diffusion of one component of a solution but not the other. In

biology, a septum which permits the diffusion of water but

not of the solute.

SODHJM-ADSORPTION-RATIO.—A ratio for soil extracts and irrigation waters used to express the relative activity of sodium

ions in exchange reactions with soil.

SAR =

Na+

V(Ca^" +Mg*^)/2

where the ionic concentrations are expressed in milliequivalents per liter.

SOIL EXTRACT.—The solution separated from a soil suspension

or a soil at a particular moisture content.

SOIL-MOISTURE STRESS.—The sum of the soil-moisture tension

a_nd the osmotic pressure of the soil solution. It is the suction or negative pressure to which water must be subjected

to be at equilibrium through a semipermeable membrane

with the solution in soil.

SOIL-MOISTURE TENSION.—The equivalent negative pressure or

suction of water in soil. Experimentally, the suction of water

in soil is the pressure difference required across a permeable

membrane to produce hydraulic equilibrium between the soil

water and free water..

SOLUBLE-SODIUM PERCENTAGE.—A term used in connection with

irrigation waters and soil extracts to indicate the proportion

of sodium ions in solution in relation to the total cation concentration. It may be calculated by the formula:

Soluble sodium concentration (meq./l.) SSP=' xioo Total cation concentration (meq./l.)

SPECIFIC ION EFFECT.—Any effect of a salt constituent in the

substrate on plant growth that is not caused by the osmotic

properties of the substrate.

SPECIFIC SURFACE.—The surface area, per unit weight of soil,

commonly expressed as square meters per gram of soil

(m.Vgm.).

STANDARD ATMOSPHERE.—A unit of pressure defined as follows:

1 atmosphere^ 1.013X10' dynes per sq. cm.=14.71 pounds

per sq. in. -76.39 cm. of mercury column=I,036 cm. of water

column=34.01 ft. of water column. (Water and mercurv

at 20° C).

STANDARD DEVIATION.—A statistic used to measure the dispersion of a set of values around their mean.

SUCTION.—See Soil-Moisture Tension.

WATER TABLE.—The upper boundary for ground water. The

upper surface of the locus of points at which the pressure in

the ground water is equal to atmospheric pressure.
Appendix

EC

ECxlO'

ECxlO'

fCe

EC\, ECf,, ECho^

£Ciw

EC^yy

ECs

Rs

mho

mmho

)uraho

ESP

SAR

EPP

PAR

CEC

meq

mg./l

p.p. m

pHs; pHe; pHi; pHs--

Symbols and Abbreviations

Electrical conductivity in mhos/cm. LR Leaching requirement.

unless otherwise specified. HC Hydraulic conductivity.

Electrical conductivity in millimhos/ PWP Permanent-wilting percentage.

cm. (value in mhos/cm.X10'). FAP Fifteen-atmosphere percentage.

Electrical conductivity in micromhos/ SP Saturation percentage.

cm.(value in mhos/cm.X10*). SMT Soil-moisture tension.

Electrical conductivity of saturation OP Osmotic pressure.

extract. OPe Osmotic pressure of saturation extract.

Electrical conductivity of extract from ¿s,T Freezing-point depression, °C.

a suspension having the proportions Pw Percentage water, dry-weight basis.

of 1 gm. of dry soil to 1, 5, or 50 Pa Percentage water, depth basis.

gm. of water. rfw Density of water.

Electrical conductivity of irrigation ¿b Bulk density of soil.

water. dv Particle density of soil.

Electrical conductivity of drainage k Hydraulic conductivity; conductivitywater or soil solution at the bottom cell constant.

of the root zone. k' Intrinsic permeability.

Electrical conductivity of saturated soil Z)iw Depth of irrigation water applied to

paste. soil.

Resistance of soil paste in Bureau of Z)dw Depth (equivalent free depth) of

Soils cup. drainage water.

Reciprocal ohm; (ohm spelled back- Dew Consumptive use expressed as equivaward). lent free depth of water in a

Millimho. specified time.

Micromho. E Efficiency of irrigation water applicaExchangeable-sodium-percentage. tion.

Sodium-adsorption-ratio. n Porosity.

Exchangeable-potassium-percentage. C2-53 Example of classification of irrigation

Potassium-adsorption-ratio. water; C denotes conductivity (elecCation-exchange-capacity. trical) ; S denotes sodium {SAR) \

Milliequivalent. numbers denote respective numeriMilligrams of solute per liter of solu- cal quality classes. {See ch. 5.)

tion. m Meter.

Parts per million. As commonly meas- cm Centimeter.

ured and used parts per million is mm Millimeter.

numerically equivalent to milligrams m/i Millimicron.

per liter. fi Micron (10"* meter); also prefix

pH reading of saturated soil paste; micro.

saturation extract; 1 : 1 or 1 : 5 soil- A Angstrom (10"'" meter).

water suspension. Measured with RCF Relative centrifugal force.

glass electrode unless otherwise r. p. m Revolutions per minute.

specified. T? Viscosity.

Conversion Formulas and Factors

Conductivity to milliequivalent per liter:

meq./l.=10 XECXW, for irrigation waters and soil extracts

in the range from 0.1 to 5.0 millimhos per cm. See figures

4 and 20.

Conductivity to osmotic pressure in atmospheres :

OP=0.36XECX W for soil extracts in the range from 3 to 30

millimhos/cm.

Conductivity to parts per million :

p. p. m.=0.64X£CX10" for irrigation waters \n the range

100 to 5,000 micromhos/cm.

Parts of salt per million parts of irrigation water to tons of

salt per acre-foot of water:

Tons per acre-foot (t. a. f. ) =0.00136 X p. p. m.

Grains per gallon to parts per million :

p. p. m.=17.1 X grains per gallon.

Milliequivalents per liter (from chemical analyses) to parts

per million:

Multiply meq./l. for eaéh ion by its equivalent weight and

obtain the sum.

Gypsum (CaSOi^HzO) equivalent weight=86.09 gm.

Saturated gypsum solution at 25" C. contains:

30.5 meq./l.; 2.63 gm./l.; 2,630 p. p. m.; 3.5 tons of gypsum

per acre-foot of water. One milliequivalent of gypsum per

100 gm. of soil corresponds to 1.72 tons of gypsum per

acre-foot of soil (4,000,000 lb.). In other words, tons of

gypsum per acre-foot of soil=1.72X (milliequivalents gypsum per 100 gm. of soi' \ £CX 10^=2.205 at 25*» C.

1 standard atmosphere =^a. 13x10' dynes cm."'; 14.71 lb.

in. ^ 76.39 cm. of mercury column; 1,036 cm. of water

column; 34.01 ft. of water column. (Mercury and water

at 20° C.)

1 bar =10* dynes cm.'''= 1,023 cm. of water column.

1 millibar=one thousandth of a bar.

] mile=5,280 feet.

1 inch=2.54 cm.

1 foot=30.48 cm.

1 pound=453.59 gm.

1 acre=43,560 sq. ft.

157
158 AGRICULTURAL HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

1 acre-foot soil weighs 4,000,000 pounds (approximate).

I acre-foot water weighs 2,720,000 pounds (approximate).

Gallons per minute to cubic feet per second:

c. f.s.=0.002228Xg. p.m.

1 cubic foot per second (c. f. s.) =

50 miner's inches in: Idaho, Kansas, Nebraska, Nevada, INew

Mexico, North Dakota, South Dakota, Utah, and southern

California.

40 miner's inches in: Arizona, California (statute), Montana,

and Oregon.

38.4 miner's inches in: Colorado.

1 c. f. s. for 24 hours=1.98 acre-feet.

1 U. S. gallon=231 cubic inches,

8.345 pounds of water.

0.1337 cubic foot.

58,417 grains of water.

1 cubic foot=7.48 gallons.

1 cubic foot of water weighs 62.43 lbs.

1 cubic foot of soil in place weighs 68 to 112 pounds. Bulk

density 1.1 to 1.8 gm. cm.~^

Average particle density=2.65 gm. cm.-\ approximately.

(For soils which are low in organic matter.)

Chemical symbols^ equivalent weights, and

common names

Chemical symbol

or formula—

Equivalent

weight Common name

Ions:

Ca++

Mg++

Na+

K+

ciSO4—

CO3--

HCO3-

Salts:

CaCU

CaS04

CaS04.2H20 ..

CaCOa

M«Cl2

M¿S04

MgCOs

NaCl

Na2S04

NazCO,

NaHCOa

KCl

K,S04

K.CO3

KHCO3

Chemical amend

ments:

S

H2SO4

Al2(S04)3-18H20

FeS04-7H20. ...

Grams

20.04

12.16

23.00

39.10

35.46

48.03

30.00

61.01

55.50

68.07

86.09

50.04

47.62

60.19

42.16

58.45

71.03

53.00

84.01

74.56

87.13

69.10

100.11

16.03

49.04

111.07

139. 01

Calcium ion.

Magnesium ion.

Sodium ¡on.

Potassium ion.

Chloride ¡on.

Sulfate ion.

Carbonate ion.

Bicarbonate ¡on.

Calc¡um chloride.

Calc¡um sulfate.

Gypsum.

Calc¡um carbonate.

Magnes¡um chlor¡de.

Magnes¡um sulfate.

Magnes¡um carbonate.

Sodium chlor¡de.

Sod¡um sulfate.

Sod¡um carbonate.

Sod¡um b¡carbonate.

Potassium chlor¡de.

Potassium sulfate.

Potassium carbonate.

Potass¡um b¡carbonate.

Sulfur.

Sulfuric acid.

Aluminum sulfate.

Iron sulfate (ferrous).
^

SALINE AND ALKALI SOILS 159

2Y OD.

16 Th'ds/in.

MACHINE TO ACCOMOOATE

HCTAINER CYLINDERS - FREE

SLIDINO FIT.

RETAINER CYLINDERS

I ,f 1

SOIL RETAINER ASSEMBLY

Note;

INSIDE DIAMETER OF CUTTINO

EDGE MUST BE 0.009" SMALLER

THAN INSIDE DIAMETER OF

RETAINER CYLINDERS.

CORE EXTRACTOR

No. NAME SIZE No. MATERIAL

1 Hondles i" 2 Pipe, golvonized iron

2 Handle, fitting ¥ Cross, « «

3 Hondle, stem i' Pipe,

4 Cap, core sampler i" Plate, mild steel

5 Barrel, core sampler 2f-à""" Seamless tubing, steel

6 Cutting edge ^•K.OIO" Blued clock spring steel

7 Retainer disk f

Plastic

e « 1." le Ceramic

9 Rubber bond ^30 2

10 Wire, retainer disk 19 go. Wire, nichrome

II Block, core extractor nominal

2x2" 1 Wood, pine

12 Fingers, core extractor i" 2 Rod, tobin bronze

13 Finger depressor ring 2" 1 Thinwall conduit

14 Head, hommer 2 ¿dio 1 Mild steel

15 Guide, hammer fa... 1 - .

16 Retainer cylinder 2r,,9ga. 1 Seamless tubing, brass

17 .. .. 2 «

18 .. - 2 ..

19 " -1 ..

Soil sampler and core retainer.
160 AGRICULTURE HANDBOOK 60, U. S. DEPT. OF AGRICULTURE

^ M CM - <0

Modulus of rupture apparatus.

•tf U. S. GOVERNMENT PRINTING OFFICE : 1 O - 360-381

NATIONAL AGRICULTURAL LIBRARY

1022728352