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II. Characteristics of Saline and Alkali Soils
Chemical Composition of Some River Waters Used for Irrigation in
Western United States a
Eleph. B., N. Mes.
Milliequivalents per liter
“These analyses were made by the U. S. Regional Salinity and Rubidoux Laboratories, Riverside, California.
ECxl06 = conductivity expressed in micromhos per centimeter.
H. E. HAYWARD AND C. H. WADLEIGH
conditions ground water may contribute to the salinization of the soil.
This is particularly true if the water applied carries appreciable amounts
of dissolved salts as is frequently the case in irrigated areas. Furthermore, loss of drainage water from irrigated areas upstream and the
pick-up of saline ground water result in more salt downstream. The
range of quality in irrigation waters is shown in Table I which gives t,he
parts per million, electrical conductivity, chemical composition and
sodium percentage for a number of river waters used for irrigation in
western United States.
Although many salt problems are man-made, it should be recognized
that the occurrence of saline and alkali areas is related fundamentally
to changes in climatic conditions, the chemical composition of soil-forming materials in the primary rocks, and to geologic changes that have
taken place with time due to deposition, erosion, weathering and other
processes (Harris, 1920; Hilgard, 1906; de Sigmond, 1938).
There are numerous publications dealing with various aspects of saline
and alkali soils, some of which go back before the turn of the century
(Burgess, 1928; Gardner, 1945; Goss and Griffin, 1897; Hibbard, 1937;
Hilgard, 1886, 1895-1898; Kelley, 1937; Powers, 1946; Tinsley, 1902).
Magistad (1945) has reviewed a number of the schemes of classification
for saline and alkali soils and has reported the terminology proposed for
them. I n view of the differences in the meanings of terms as used in the
literature, the U S . Salinity Laboratory (1947) has published a terminology and description of saline and alkali soils. The terms as defined
in that publication will be followed in this review and are given below:
Alkali Soil-A soil that contains sufficient exchangeable sodium to int.erfere with the growth of most crop plants, either with or without appreciable quantities of soluble salts. (See Saline-Alkali and NomalineAlkali Soil).
Nonsaline-Alkali S o i G A soil which contains sufficient exchangeable
sodium to interfere with the growth of most crop plants and does not
contain appreciable quantities of soluble salts. The exchangeablesodium-percentage is greater than 15, the conductivity of the saturation extract is less than 4 millimhos per centimeter (at 25°C.) and the
pH of the saturated soil usually ranges between 8.5 and 10.
Saline-Alkali Soil-A soil containing sufficient exchangeable sodium to
interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. The exchangeable-sodium-percentage
is greater than 15 and the conductivity of the saturation extract is
greater than 4 millimhos per centimeter (at 25°C.). The pH of the
saturated soil is usually less than 8.5.
PLANT GROWTH ON SALINE AND ALKALI SOILS
Saline Soil-A nonalkali soil containing soluble salts in such quantities
that they interefere with the growth of most crop plants. The conductivity of the saturation extract is greater than 4 millimhos per
centimeter (at 25"C.), the exchangeable-sodium-percentage is less than
15, and the pH of the saturated soil is usually less than 8.5.
Alkalization--A process whereby the exchangeable sodium content of
the soil is increased.
Salinization-The process of accumulation of salts in the soil.
term indicates the degree of
saturation of the soil exchange complex with sodium and is defined as
Exchangeable sodium (m.e. per 100 g. soil)
ESP = Cation exchange capacity (m.e. per 100 g. soil)
Soluble-sodium-percentage-The proportion of sodium ions in solution in
relation to the total cation concentration, defined as follows:
Soluble sodium concent.ration (m.e. per liter)
Total salt concentration (m.e. per liter)
This term is used in connection with irrigation waters and soil extracts.
Successful agriculture on saline and alkali soils requires the use of
crops capable of producing a sat.isfactory yield under moderate intensities of salt or alkali accumulation. The question arises immediately as
to what constitutes the physiological capacity of a plant to tolerate salt
or alkali. That is, what is salt tolerance and how may it be defined?
The salt tolerance of a variety or a species may be evaluated in three
ways. Firstly, salt tolerance may be looked upon as the capacity to
persist in the presence of increasing degrees of salinity. A given species
may make little or no growth a t the higher levels of salt accumulation,
but i t does survive. That is, power of survival in increasingly saline
soils regardless of growth would be the measure of salt tolerance. This
is largely the criterion of the ecologist in evaluating halophytic environments, since the species most capable of persisting in a saline area becomes the climax vegetation of that area.
Secondly, salt tolerance may be regarded from the standpoint of
productive capacity a t a given level of salinity. For example, a number
of varieties of a given crop may be tested in a soil having a certain degree
of salinization and the highest yielding variety may be designated as the
most salt tolerant. This method of interpretation may give a differen&
evaluation of salt tolerance from the previous one, since experience has
H. E. HAYWARD AND C. H. WADLEIGH
shown that the capacity to produce well a t moderate levels of salinity
does not necessarily imply the ability to persist a t higher levels of salt
accumulation. This second criterion is especially useful to the agronomist
in comparing the performance of strains and varieties of a given crop.
Thirdly, the relative performance of a crop a t a given level of soil
salinity as compared to its performance on a comparable nonsaline soil
may be used as a criterion of salt tolerance. This method has certain
advantages over the previously mentioned concepts in that comparisons
between species are more readily evaluated. For example, although preference as to salt tolerance should be given to that variety of alfalfa
having the highest production on saline soil regardless of performance in
the absence of salinity, one could hardly compare salt tolerance in alfalfa
with that in cotton without taking into account the yielding power of
these respective crops when growing on comparable nonsaline soils.
Evaluating salt tolerance on the basis of relative yield will not necessarily result in the same order of classification as power of survival a t
high levels of salinity, but it will provide a more useful basis of appraising agronomic crops to be grown on moderately saline soil. I n variety
and strain testing, tshe data on relative yield should be supplemented by
data on absolute yield; ie., a strain may have a comparably poor relative
yield because of unusual vigor of growth on the nonsaline soil, and yet
yield the best of any of the strains a t the given level of salinity. Everything considered, defining salt tolerance on the basis of relative yield to
that of the nonsaline condition is to be preferred for general agronomic
I n discussing the physiological basis for the various degrees of salt
tolerance which prevail among crop plants, it may be helpful to consider
the characteristics of the natural halophytes. I n a review of this group
of plants, Uphof (1941) discusses the physiological characteristics of
halophytes, but it is apparent that the specific physiology of these plants
is not well known. The early investigators concluded that halophytism
was essentially xerophytism, since both halophytes and xerophytes are
adapted physiologically or anatomically to a scarcity of water. Anatomical studies, such as those of Chermezon (1910), later revealed that
the two groups of plants must be regarded as distinct physiologically.
Halophytes tend to have relatively high values for the osmotic pressure
of the tissue fluids. Fitting (1911) used an indirect method to measure
the osmotic pressure of the cell contents of various species of plants on
the North African Desert. The highest osmotic pressures, 100 atmospheres or above, were found in plants growing on dry or highly saline
soils. Those growing on moist nonsaline soils had osmotic pressures of
10-20 atm. The osmotic pressure of the various species tended to vary