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CHAPTER 7

CONTROLLING SALINITY

Glenn J. Hoffman (University of Nebraska, Lincoln, Nebraska)

Joseph Shalhevet (Agricultural Research Organization, Bet Dagan, Israel)

Abstract. Strategies are presented to minimize the detrimental impacts of salinity, sodicity, and toxicity in irrigated agriculture. The tolerances of major agricultural crops to salinity are given along with the amount of leaching required to prevent crop yield loss. The differences among irrigation systems and the affect of the quantity and salt concentration of the applied water on the distribution and concentration of soil salinity are also presented. The chapter concludes with discussions on the application methods and the amount of applied water required to reclaim soils too high in salt concentration to produce economical crop yield. Keywords. Crop salt tolerance, Leaching, Reclamation, Salinity, Sodicity, Toxicity.

7.1 INTRODUCTION A major challenge to irrigated agriculture is the hazard of salt-affected soils and

waters. Saline irrigation and drainage waters and saline and sodic soils are threats to sustaining irrigated agriculture. Salinity is of more concern in arid than humid regions, where precipitation is a major source of water for crop production, but it will become more prevalent in humid regions as supplemental irrigation or drainage problems in-crease.

Except for extremely old soils, salts from weathering of rocks and minerals seldom accumulate in sufficient quantities to form a saline soil. Salt-affected soils usually develop in areas that receive and accumulate salts transported from other locations by water. Accumulations can also occur by atmospheric deposition of oceanic salts along coastal areas; by seawater intrusion from estuaries and coastal groundwater basins, inland saline lakes and playas; by upward movement of saline waters from groundwa-ter; and from leaching of saline lands. Sources of salt from agriculture include irriga-tion return flows, drainage waters, soil and irrigation amendments, animal manures and wastes, fertilizers, and sewage sludges and effluents.

Soluble salts are present in all natural waters, and it is their concentration and com-position that determine the suitability of soils and waters for crop production. Water quality is normally based on three criteria: (1) salinity, (2) sodicity, and (3) toxicity. Salinity is the osmotic stress caused by dissolved salts on crop growth. When the pro-portion of sodium compared to other cations becomes excessive, soil structure deterio-rates and the soil is said to be sodic. Toxicity encompasses the effects of specific sol-utes that damage plant tissue or cause an imbalance in plant nutrition.

Design and Operation of Farm Irrigation Systems 161

Soils and waters have no inherent quality independent of the specific conditions in question. Thus, soils and waters can only be evaluated fully in the context of a speci-fied set of conditions. A meaningful assessment of water quality for irrigation should consider such site-specific factors as rainfall, irrigation management practices, cli-mate, chemical and physical properties of the soil, and the chemical reactivity of con-stituents dissolved in the water.

This chapter describes the influence of soil salinity on crop production, introduces management techniques to cope with saline soils and waters, discusses the impact of the type, design, and movement of an irrigation system on salinity control, and pre-scribes methods to reclaim salt-affected soils. The chapter also addresses methods to assess and reclaim sodic soils and briefly discusses toxic constituents that may be pre-sent in soils or irrigation waters.

7.1.1 The Scope of the Salinity Hazard Land occupies about 134 million km2 of the earth’s surface. Of that total, not more

than 70 million km2 are arable and only 15 million km2 are cultivated (Massoud, 1981). Of the cultivated lands, about 3.4 million km2 (23%) are saline and another 5.6 million km2 (37%) are sodic. The amount of salt-affected land is summarized in Table 7.1. Salt-affected soils cover about 13% of the total arable lands and occur in over 100 countries. Figure 7.1 shows the distribution of saline and sodic soils worldwide. Note that lands north or south of about the 50° latitude are not salt-affected because precipi-tation is too high and/or irrigation is not practiced.

Table 7.1. Area of salt-affected soils worldwide (from Szabolcs, 1989).

Region Salt-Affected

Land (hectares) North America 15,800,000 Mexico and Central America 2,000,000 South America 129,200,000 Europe 50,800,000 Africa 80,500,000 South Asia 87,600,000 North and Central Asia 211,700,000 Southeast Asia 20,000,000 Australia and Pacific Islands 357,400,000 Worldwide 955,000,000

Figure 7.1. Global distribution of salt-affected soils (adapted from Szabolcs, 1989).

162 Chapter 7 Controlling Salinity

Figure 7.2. Potential water and soil salinity problems in the U.S.

(adapted from USDA, 1988).

The potential for salinity hazards in the conterminous United States is illustrated in Figure 7.2. The map is based upon indicators of salinity in surface soils and geologic formations on a scale of river basins or watersheds. About 30% of the land in the con-terminous United States has a moderate to high potential for salinity. Western portions of the United States, because of geologic formations and dry climates, are highly sus-ceptible to salinity problems.

7.1.2 Overview of Salinity Problems 7.1.2.1 Historical perspective. Excess salinity, which is frequently the result of in-

adequate drainage, has plagued irrigated agriculture for millennia. A well-documented case occurred in ancient Mesopotamia (now Iraq) about 4000 years ago. Cities in the river valleys of the Tigris and Euphrates flourished in the 4th millennium B.C. The development of these cities was based on the irrigation of wheat. Flooding, seepage, overirrigation, and siltation resulted in a rising water table, which led to excessive soil salinity (Gelburd, 1985). After 1000 years of successful irrigated agriculture, wheat was largely replaced by more salt-tolerant barley. Consequently, these cities lost popu-lation and power shifted to Babylon (Boyden, 1987). Numerous references are made to irrigation canals in historic records of Mesopotamia, but no record of drainage ca-nals being constructed has been found. The drainage of low-lying lands required pumping, a technology unavailable at the time.

Historical salt and drainage problems have also been documented in North and South America. The inhabitants of the Viru Valley on the coast of Peru developed an irrigation system between 400 and 0 B.C. (Willey, 1953). The population peaked about 800 A.D. before decreasing dramatically in the 13th century when people relo-cated from the valley floor to the upper narrows of the valley. Historians attribute this relocation to increasing soil salinity and rising water tables from inadequate drainage (Armillas, 1961). Another example is the Hohokan Indians (Willey, 1953). They lived in the Salt River region of what is now Arizona and practiced flood irrigation begin-ning about 300 B.C. No evidence of the Hohokan civilization exists after 1450 A.D. Historians surmise that waterlogging and salt accumulation caused crop failures.


Similar problems have occurred in Asia. In the Indus Plain, located in present-day India and Pakistan, irrigation began about 2000 years ago. Since the mid-1800s, seri-ous salinity and drainage problems have been experienced (Taylor, 1965).

7.1.2.2 Current outlook. Human-induced salination is not confined to antiquity. One recent example is the region of the Amu Darya and Syr Darya rivers in the Com-monwealth of Independent States. In the 1950s these river waters were diverted for the development of irrigation before reaching the Aral Sea. These irrigated lands produced 90% of the former USSR’s cotton and 40% of its rice (Trofimenko, 1985). Due to these major diversions, the Aral Sea began shrinking. Between 1960 and 1990 the level of the Sea dropped 13 m in elevation and its surface area decreased 40%. This shrinking has had devastating effects on the ecosystem and has caused health hazards (Micklin, 1991).

Another example of prominence is the San Joaquin Valley of California. Irrigation in the Valley began about 1850. Federal and state agencies envisioned a master drain to serve the west side of the Valley. By 1975, however, only the southern third of the drain, terminating at Kesterson Reservoir, had been completed. Budget restrictions and environmental concerns prevented completion of the drain to San Francisco Bay. Meanwhile, the drain conveyed saline drainage waters to Kesterson Reservoir, which served as an evaporation pond. In 1982, selenium toxicity of fish and, in 1983, de-formed and dead waterfowl were found in the Reservoir. The selenium content of the water averaged 329 µg /L (Lee, 1990), 15 times higher than the recommended concen-tration for irrigation water for long-term protection of plants and animals (Pratt and Suarez, 1990). The discovery of selenium toxicosis from irrigation drainage water emphasizes the need for concern about environmental impacts downstream from irri-gation projects (Letey et al., 1987). This was the first documented case of selenium toxicosis related to irrigation, but excess selenium is present in many basins in the western U.S. (and deaths have occurred in China from excess selenium in the drinking water). This problem has led to irrigation-drainage studies in seven western states in the U.S. where selenium toxicity is suspected (Sylvester et al., 1988).

As these two modern-day examples illustrate, salinity can still create significant water management problems. However, there are numerous examples of successful irrigation enterprises under saline conditions from around the world; a few examples are given here. In the Arkansas Valley of Colorado, irrigation water containing 1500 to 5000 mg/L of total dissolved salts has been used successfully to grow alfalfa, sor-ghum, and wheat (Miles, 1977). Water containing 2500 mg/L of total dissolved salts has been used for decades in the Pecos Valley of Texas (Moore and Hefner, 1977). In regions of India receiving monsoon rains, wheat has been irrigated with waters con-taining up to 10,000 mg/L (Dhir, 1977). Good cotton yields have been obtained in Uzbekistan irrigating with drainage water containing 5000 to 6000 mg/L (Bressler, 1979). In the semi-arid continental-monsoon climate of China, saline water (2000 to 5000 mg/L) has been applied to wheat, corn, and cotton since the 1970s with excellent yields (Fang et al., 1978).

7.1.2.3 Future concerns. History has shown that irrigated agriculture cannot sur-vive in perpetuity without appropriate salt management and drainage. How long irri-gated agriculture can survive without adequate salinity control depends on hydrogeol-ogy and water management. The rate of salt accumulation in the soil is determined by the salinity of the applied waters, the salinity of the soil profile, and the rate that salts


are leached out of the root zone. If layers restricting leaching are present near the soil surface, salt accumulation and waterlogging develops within a relatively short time, decades or less. If no restricting layers exist and the vadose region has a large water storage capacity, irrigation may be practiced for a long time, perhaps centuries, before salinity or waterlogging may become a threat.

7.2 QUANTIFYING SALINITY HAZARDS Salinity, sodicity, and toxicity normally must be quantified for proper diagnosis and

management. When sampling water, 200 to 500 mL are usually sufficient for a multi-tude of laboratory analyses. As for any sampling protocol, proper procedures must be followed to prevent contamination and ensure accurate results. One procedure, often ignored, is proper and complete labeling of samples. Samples should be refrigerated at about 4°C (never frozen) and analyzed as soon as possible. Samples from wells should be collected after pumping for at least half an hour. Typically, well-water quality will not change significantly during the irrigation season. In a few cases, particularly when the groundwater level changes substantially during the pumping season, periodic sam-pling may be required because of possible changes in water quality.

Soil salinity can be analyzed in the laboratory based on water either extracted di-rectly from the soil or taken from saturated soil samples. An alternative to laboratory analysis is measuring salinity directly in the field.

If a field is to be sampled for laboratory analysis, a sample of about 0.5 kg of soil is needed for each depth of interest. Samples should be air-dried, passed through a 2-mm sieve, thoroughly mixed, and placed in durable, labeled containers. Labels should in-clude sampling date, site, and soil depth.

Selecting a soil sampling strategy for salinity problems depends on the investiga-tor’s objectives and the potential sources of variance in the samples. In relatively small areas, sampling based upon judgment coupled with combining samples may be ade-quate. Judging which locations to sample can be based on crop growth, location of drains and irrigation appurtenances, and visual appearance of soils or plants. Fre-quently, soil samples are composited to reduce the cost of analysis. Samples are often composited from affected areas and compared with samples from unaffected areas. When monitoring salinity with time, when an unbiased or accurate evaluation is needed, or when salinity is assessed over a large area, sampling errors because of spa-tial variability must be minimized to detect differences in salinity. The reader is en-couraged to review Hanson and Grattan (1990) for detailed descriptions of systematic sampling procedures.

7.2.1 Salinity Knowing the amount of each individual solute in the soil water is ideal for diagno-

sis and management. Unfortunately, commercial field instruments are only capable of estimating the total solute concentration, typically by measuring electrical conductiv-ity (EC). Electrical conductivity measurements are very valuable because most plants respond to the osmotic potential of the soil solution in the plant root zone, which can be estimated by measuring electrical conductivity.

Laboratory analyses of soil water samples are required if the concentration of indi-vidual solutes is desired. The standard method for laboratory analysis utilizes water extracted from soil samples after they are brought to saturation by adding salt-free water. This procedure enables water to be extracted easily for analysis. The ratio of the


water content of the saturated soil paste to the field water content is frequently about 2. This ratio, however, ranges from 1.8 to 3.0 among various soils. In the following sections, measurements of electrical conductivity and concentrations of specific sol-utes are frequently reported on the basis of field water content. To convert from con-centrations in the soil water (Cw) to concentrations in a saturated soil extract (Ce), knowledge of the bulk density of the soil (Db), volumetric field water content (1w), and the gravimetric water content of the saturated soil paste on a percentage basis (satura-tion percentage, SP) are required. The relationship is: Ce = 100Cw1w/(Db S P) (7.1)

If the soil is relatively wet, soil water samples can be collected directly from the soil by displacement, absorption, compaction, suction, centrifugation, or pressure membrane extraction techniques. Of these, only suction is routinely used in the field. Soil water can be extracted by applying vacuum into a porous cup or tube buried in the soil. The porous sampler can be made of various ceramic or metallic materials. This technique normally works when the soil water potential is greater than –30 J/kg.

Devices that can be installed in the field for direct measurement of soil salinity in-clude porous-matrix salinity sensors, four-electrode units, inductive electromagnetic meters, and time domain reflectometry probes. These methods estimate soil salinity but not the concentrations of individual ions. Direct field measurements are particu-larly helpful for mapping large areas or monitoring changes in salinity with time. They also help determine the intensity and locations for detailed soil sampling.

Porous-matrix salinity sensors consist of a pair of small, corrosion-resistant elec-trodes separated by a housing of ceramic or glass that is porous to soil water. The elec-trodes connect to continual recording monitors or to manually operated, hand-held meters or recorders. A temperature sensor in the probe is used to correct the electrical conductivity readings for temperature. Response to changes in soil salinity can take a day or longer because of the slow movement of soil solution by diffusion into the po-rous housing. Calibration of the sensors can change, so periodic re-calibration is nec-essary. Accuracy of commercial units is about ±0.5 dS/m (Oster and Willardson, 1971).

Four-electrode units consist of four electrodes inserted into the soil in a straight line at a predetermined spacing. The electrical resistance to current flow between the inner pair of electrodes is measured while a constant, alternating electric current is passed through the soil between the outer electrodes (Rhoades and Ingvalson, 1971). The electrical conductivity of the bulk soil depends upon the salt and water contents of the soil, soil bulk density, and texture (Rhoades et al., 1990). A distinct advantage of the four-electrode unit is that the spacing between the electrodes can be altered to change the volume of soil measured. The depth of current penetration and thus the depth of measurement is approximately equal to one-third of the distance between the outer electrodes. Salinity within a small soil volume can be measured with portable probes (Rhoades and van Schilfgaarde, 1976) or permanently buried probes (Rhoades, 1979). These units consist of four annular rings molded in a plastic or rubber cylinder that is tapered slightly to maintain soil contact as it is inserted into an augured hole. The volume of soil measured can be varied by changing the spacing between the annu-lar rings; commercial units typically measure within a soil volume of 2 L.

An inductive electromagnetic meter can be hand-carried across the field to assess soil salinity rapidly (Rhoades and Corwin, 1981). A transmitter coil located at one end


of the meter induces current flow in the soil which creates a secondary magnetic field. The magnitude of the current flow in the soil is a function of the secondary field strength and the electrical conductivity of the soil. By taking meter readings at various heights above the soil surface, salinity by soil depth intervals can be estimated. The meter is especially valuable for surveying salt-affected soils and mapping saline seeps.

Time domain reflectometry is a method for measuring volumetric water content and electrical conductivity of the soil simultaneously. Soil water content is related to the transit time and dissipation of an electromagnetic pulse launched along a set of metal-lic parallel rods embedded in the soil. Salinity in the soil attenuates the pulse. This measuring device is particularly beneficial because it measures soil salinity and water content with the same probe (Dalton, 1992).

Salinity is reported using a variety of units. Various units and appropriate conver-sion factors are presented in Table 7.2 to facilitate comparison and conversion among units of measure. Concentration (C), the amount of substance per unit volume, is typi-cally reported in SI metric units as moles per cubic meter of solution (mol/m3). Alter-natively, C can be reported as grams per cubic meter (g/m3) or milligrams per liter (mg/L). The units of g/m3 or mg/L are numerically equivalent to parts per million (ppm). Traditionally, ionic concentration has been expressed as milliequivalents per liter of solution (meq/L). To convert ionic concentration in units of meq/L to mol/m3, divide by the valence of the ion. For example, divalent cations like calcium at a con-centration of 20 meq/L would be equivalent to a concentration of 10 mol/m3. To con-vert to g/m3, multiply the concentration in mol/m3 by the atomic weight of the ion. For example, sodium at a concentration of 20 mol/m3 would be equivalent to a concentra-tion of 460g/m3. Total salt concentration in units of g/m3 or mg/L is merely the sum of the concentrations of each ion present. In units of mol/m3, it is the sum of either the cations or the anions, but not both. The relationship between salt concentration (g/m3 or mg/L) and electrical conductivity (dS/m or mmho/cm) can be approximated by C = 640 EC.

Table 7.2. Units and conversion factors for various measurements of salinity.

Measure Symbol Unit Abbreviation

for Unit Conversion Factor Electrical

conductivity EC deciSiemens per meter millimhos per centimeter

dS/m mmho/cm

dS/m = mmho/cm

Concentration C

grams per cubic meter milligrams per liter parts per million moles per cubic meter milliequivalents per liter

g/m3 mg/L ppm

mol/m3 meq/L

ppm = mg/L = g/m3 mol/m3 = meq/L ÷ ion valenceg/m3 = mol/m3 × atomic weight

7.2.2 Sodicity

Another water quality concern is an excess concentration of sodium, which can cause a deterioration of soil structure. When the amount of sodium becomes excessive, soil mineral particles tend to disperse and water penetration decreases. This becomes a problem when the rate of infiltration is reduced to the extent that the crop is not ade-quately supplied with water or when the hydraulic conductivity of the soil profile is too low to provide adequate drainage. Sodium may also add to cropping difficulties because of crusting seed beds, temporary saturation of the surface soil, and the in-


creased potential for disease, weeds, soil erosion, lack of oxygen, and inadequate nu-trient availability. If calcium and magnesium, and not sodium, are the predominant cations adsorbed on the soil exchange complex, the soil tends to have a granular struc-ture that is easily tilled and readily permeable.

The sodium-adsorption ratio (SAR) of irrigation water is generally a good indicator of the exchangeable sodium status that will occur in the soil. SAR is defined as:

( ) 21 /

MgCa

Na

CC

CSAR+

= (7.2)

where all ion concentrations are in mol/m3. If the units are meq/L, the sum of CCa + CMg must be divided by 2. Most irrigation waters from surface sources in arid areas are supersaturated with calcite but precipitation is negligible. For such conditions, Equa-tion 7.2 is normally a suitable indicator of SAR for soil water near the surface under steady state conditions.

When groundwater, especially drainage water, is used for irrigation, Equation 7.2 does not accurately assess the hazard of excess sodium. A more accurate SAR (Suarez, 1981) adjusts the calcium concentration of the irrigation water to the expected equilib-rium value in the soil and includes the effects of carbon dioxide (CO2), bicarbonate (HCO3), and salinity upon the calcium originally present in the applied water but now part of the soil water. This adjusted SAR (adj SAR) can be written as:

( ) 21 /MgCa

Na

C *C

Cadj SAR+

= (7.3)

where C*Ca is a modified calcium concentration. Values for C*Ca are given in Table 7.3. The adjusted SAR of the irrigation water is generally a better indicator of the ex-changeable sodium status that will occur in the soil than SAR.

The permissible value of the SAR or adjusted SAR is a function of salinity. High levels of salinity reduce swelling and aggregate breakdown (dispersion) and promote water penetration, whereas high proportions of sodium produce the opposite effect

Table 7.3. The modified concentration of calcium (C*Ca) expected to remain in soil-water near the soil surface following an irrigation with a given salinity

and ratio of bicarbonate to calcium (adapted from Suarez, 1981). Electrical Conductivity of Applied Water (dS/m) [a] Ratio of Bicarbonate

to Calcium (both mol/m3) 0.1 0.5 1.0 2.0 4.0 0.1 6.1 7.2 7.6 8.2 9.0 0.2 4.2 4.5 4.8 5.2 5.7 0.5 2.3 2.5 2.6 2.8 3.1 1.0 1.4 1.6 1.6 1.8 1.9 2.0 0.90 0.98 1.0 1.1 1.2 4.0 0.57 0.62 0.66 0.70 0.77 8.0 0.36 0.39 0.41 0.44 0.48

20.0 0.20 0.21 0.22 0.24 0.26 60.0 0.09 0.10 0.10 0.12 0.12

[a] Table values are based upon assumptions that soil has a source of lime or silicates, no precipitation of magnesium, and a partial pressure of CO2 near the soil surface of 70 Pa.


Figure 7.3. Relative rate of water infiltration as affected by salinity and sodium-adsorption ratio (adapted from Rhoades, 1977, and Oster and Schroer, 1979).

Figure 7.3 represents the approximate boundaries where chemical conditions se-verely reduce infiltration of water into soil, where slight to moderate reductions occur, and where no reduction is expected in most soils. Regardless of the sodium content, waters with an electrical conductivity less than about 0.2 dS/m cause degradation of the soil structure, promote soil crusting, and reduce water penetration. Rainfall and snow melt would be prime examples of waters low in salinity that reduce water pene-tration into soils. As Figure 7.3 illustrates, both the salinity and the sodium-adsorption ratio of the applied water must be considered simultaneously when assessing the po-tential effects of water quality on soil water penetration.

7.3 CROP TOLERANCE 7.3.1 Crop Salt Tolerance

Concentrations of soluble salts in the soil beyond a threshold level depress the growth and yield of all crop plants. Growth depression is the most distinct salinity injury symptom, usually to the exclusion of other injury signs from stress such as wilt-ing, chlorosis, or necrosis. Crops can tolerate a certain level of salinity (the threshold) without a measurable yield loss; the more salt tolerant the crop, the higher is this threshold level. Beyond the threshold, yield is reduced linearly by further increases in salinity until yield becomes zero. This relationship is depicted in Figure 7.4.

Under field conditions, the soil solution is made up of a mixture of salts. The pre-dominant ones are chloride, sulfate, and bicarbonate salts of sodium, calcium, and magnesium. The yield response functions, as illustrated in Figure 7.4, pertain to a mixed salt solution as it normally occurs in nature. Some research reports in the litera-ture relate yield to the concentration of a single salt, usually sodium chloride. The use of single salts in experiments may create toxic effects and nutritional imbalances, which are not necessarily related to salinity as it occurs in nature. Under most field conditions crops respond to the total salt concentration. This response is termed the osmotic or solute effect. Some crops, notably tree and vine crops, are specifically sen-sitive to certain solutes, as will be discussed in Section 7.3.3. This toxicity is some-times termed the specific solute effect.


Figure 7.4. Relative classifications of crop salt tolerance

based upon threshold and slope values.

The relation of yield to salinity is normally expressed relative to the maximum yield that may be obtained under similar soil and climatic conditions at levels of salin-ity much less than the threshold value. Maas and Hoffman (1977) suggested the fol-lowing equation to express the relationship between crop yield and soil salinity:

( )⎪⎩

⎪⎨

⎧

≥

≤≤−−

≤≤

=

oe

oee

e

rCEC

CECttECstEC

Y0

1000100

(7.4)

where Yr = relative yield, % t = threshold salinity s = rate of yield reduction with increasing soil salinity beyond the threshold Co = level of soil salinity above which the yield is zero ECe = average root zone salinity measured as the electrical conductivity of the saturated soil extract.

Crops differ greatly in their response to salinity in both the threshold (t) and slope (s) values. A sensitive crop such as strawberry has a threshold value of 1 dS/m and a slope of 33% per dS/m, while a tolerant crop such as barley has a threshold of 8 dS/m and slope of 4.5% per dS/m. The threshold and slope values for the salinity response function are presented in Table 7.4 (Francois and Maas, 1994). The original tabulation of these factors was reported by Maas and Hoffman (1977).

Table 7.4. Salt tolerance of agricultural crops[a] (adapted from Francois and Maas, 1994). Crop Salt Tolerance Parameters

Common Name

Botanical Name[b]

Growth Measure-

ment Threshold (t) ECe (dS/m)

Slope (s) (% per dS/m)

Relative Tolerance Rating[c]

Alfalfa Medicago sativa Shoot 2.0 7.3 MS Almond Prunus duclis Shoot 1.5 19 S Apricot Prunus armeniaca Shoot 1.6 24 S Asparagus Asparagus officinalis Spear 4.1 2.0 T

(continued)


Table 7.4 continued. Barley Hordeum vulgare Grain 8.0 5.0 T Barley (forage) Hordeum vulgare Shoot 6.0 7.1 MT Bean Phaseolus vulgaris Seed 1.0 19 S Bean, mung Vigna radiata Seed 1.8 20.7 S Beet, red Beta vulgaris Storage root 4.0 9.0 MT Bermuda grass Cynodon dactylon Shoot 6.9 6.4 T Blackberry Rubus macropetalus Fruit 1.5 22 S Boysenberry Rubus ursinus Fruit 1.5 22 S Broadbean Vicia faba Shoot 1.6 9.6 MS Broccoli Brassica oleracea

(botrytis) Shoot 2.8 9.2 MS

Cabbage Brassica oleracea (capitata)

Head 1.8 9.7 MS

Carrot Daucus carota Storage root 1.0 14 S Celery Apium graveolens(dulce) Petiole 1.8 6.2 Clover, alsike Trifolium hybridum Shoot 1.5 12 MS Clover, berseemTrifolium alexandrinum Shoot 1.5 5.7 MS Clover, ladino Trifolium repens Shoot 1.5 12 MS Clover, red Trifolium pratense Shoot 1.5 12 MS Clover, strawberry

Trifolium fragiferum Shoot 1.5 12 MS

Corn Zea mays Ear 1.7 12 MS Corn (forage) Zea mays Shoot 1.8 7.4 MS Cotton Gossypium hirsutum Lint or seed 7.7 5.2 T Cowpea Vigna unguiculata seed 4.9 12 MT Cowpea (forage)

Vigna unguiculata shoot 2.5 11 MS

Cucumber Cucumis sativus Fruit 2.5 13 MS Date palm Phoenix dactylifera Fruit 4.0 3.6 T Eggplant Solanum melongena fruit 1.1 6.9 MS Fescue, tall Festuca elatior Shoot 3.9 5.3 MT Flax Linum usitatissimum Seed 1.7 12 MS Foxtail, meadow

Alopecurus pratensis Shoot 1.5 9.6 MS

Garlic Allium sativum Bulb 1.7 10 MS Grape Vitis vinifera Shoot 1.5 9.6 MS Grapefruit Citrus × paradisi Fruit 1.2 13.5 S Guar Cyamopsis tetragonoloba Seed 8.8 17 T Guava Psidium guajava Shoot &

root 4.7 9.8 MT

Guayule Parthenium argentatum Rubber 7.8 10.8 T Kenaf Hibiscus cannabinus Stem 8.1 11.6 T Lemon Citrus limon fruit 1.5 12.8 S Lettuce Lactuca sativa Top 1.3 13 MS Love grass Eragrostis Shoot 2.0 8.4 MS Muskmelon Cucumis melo Fruit 1.0 8.4 MS Onion Allium cepa Bulb 1.2 16 S Orange Citrus sinensis Fruit 1.3 13.1 S Orchard grass Dactylis glomerata Shoot 1.5 6.2 MS Pea Pisum sativum Seed 3.4 10.6 MS

(continued)


Table 7.4 continued. Peach Prunus persica Shoot or

fruit 1.7 21 S

Peanut Arachis hypogaea Seed 3.2 29 MS Pepper Capsicum annuum Fruit 1.5 14 MS Plum, prune Prunus domestica Fruit 2.6 31 MS Potato Solanum tuberosum Tuber 1.7 12 MS Radish Raphanus sativus Storage root 1.2 13 MS Rice, paddy[d] Oryza sativa Grain 3.0 12 S Rye Secale cereale Grain 11.4 10.8 T Rye (forage) Secale cereale Shoot 7.6 4.9 T Ryegrass, perennial

Lolium perenne Shoot 5.6 7.6 MT

Sesbania Sesbania exaltata Shoot 2.3 7.0 MS Sorghum Sorghum bicolor Grain 6.8 16 MT Soybean Glycine max Seed 5.0 20 MT Spinach Spinacia oleracea Top 2.0 7.6 MS Squash, scallop Cucurbita pepomelopepo Fruit 3.2 16 MS Squash, zucchini

Cucurbita pepomelopepo Fruit 4.7 9.4 MT

Strawberry Fragaria × ananassa Fruit 1.0 33 S Sudan grass Sorghum sudanense Shoot 2.8 4.3 MT Sugar beet Beta vulgaris Storage root 7.0 5.9 T Sugarcane Saccharum officinarum Shoot 1.7 5.9 MS Sweet potato Ipomoea batatas Tuber 1.5 11 MS Tomato Lycopersicon

lycopersicum Fruit 2.5 9.9 MS

Tomato, cherry Lycopersicon lycopersicum

Fruit 1.7 9.1 MS

Trefoil, big Lotus pedunculatus Shoot 2.3 19 MS Trefoil, narrowleaf birdsfoot

Lotus corniculatus Shoot 5.0 10 MT

Triticale × triticosecale Grain 6.1 2.5 T Turnip Brassica rapa Storage

root 0.9 9.0 MS

Turnip (greens) Brassica rapa Top 3.3 4.3 MT Vetch, common Vicia angustifolia Shoot 3.0 11 MS Wheat Triticum aestivum Grain 6.0 7.1 MT Wheat (forage) Triticum aestivum Shoot 4.5 2.6 MT Wheat, durum Triticum turgidum durum Grain 5.9 3.8 T Wheat, durum (forage)

Triticum turgidum durum Shoot 2.1 2.5 MT

Wheat (semidwarf)

Triticum aestivum Grain 8.6 3.0 T

Wheatgrass, tall Agropyron elongatum Shoot 7.5 4.2 T [a] These data serve only as a guideline to relative tolerances among crops. Absolute tolerances vary, de-

pending upon climate, soil conditions, and cultural practices. S, sensitive; MS, moderately sensitive; MT, moderately tolerant; T, tolerant.

[b] Botanical and common names follow the convention of Hortus Third when possible. [c] Ratings are defined by the boundaries in Figure 7.4. [d] Because paddy rice is grown under flooded conditions, values refer to the electrical conductivity of the

soil water while the plants are submerged; less tolerant during seedling stage.


7.3.2 Factors Modifying Crop Salt Tolerance Crops are sometimes exposed to conditions that differ from those under which salt

tolerance data were obtained. A frequently asked question is whether the differences are significant with respect to the application of salt tolerance ratings to a variety of field conditions. Factors which may interact with salinity to cause a different yield response include soil, crop, and environmental conditions.

7.3.2.1 Soil. Soil factors which may influence salt tolerance are texture and struc-ture, fertility, and management schemes that alter spatial and temporal distribution of soil salinity. Soil structure and texture impact salt tolerance through their influence on infiltration, water holding capacity (WHC), aeration, and the ratio of saturation water content to field water content. For the same potential evaporation rate, a sandy soil with low WHC will lose proportionately more water than a fine-textured soil, resulting in a more rapid increase of salt concentration in the soil solution and potentially greater damage to the crop. This possible damage can be mitigated by irrigating a sandy soil more frequently than a clay soil. The impact of aeration on salt tolerance is not well understood. Some studies report yield reduction due to salinity may be less pronounced under poor soil aeration conditions (Aubertin et al., 1968; Drew et al., 1988); other investigations indicate about the same (John et al., 1976) or greater ef-fects (Kriedemann and Sands, 1984; West and Taylor, 1984).

The impact of soil fertility on the yield response to soil salinity has been studied ex-tensively. The most critical nutrient interacting with salinity is nitrogen (N), but phos-phorus (P) and potassium (K), in rare situations, may also show interactions. Three types of relationships between the level of fertilizer application and salinity might oc-cur: (1) the addition of fertilizer results in the same relative yield increase at all levels of salinity; (2) there is a larger response to fertilizer at low compared to high levels of salinity, and (3) there is a larger increase in relative yield due to fertilizer application at high salinity levels. Of 51 reports examined on the interaction of fertilizer applica-tion and salinity, only five showed the third type of response, a more positive response to fertilizer under high rather than low salinity. The remainder of the reports were di-vided equally between the first and second types of responses. The overwhelming conclusion is that the addition of nitrogen fertilizer can seldom overcome the deleteri-ous effect of salinity on crop yield. In nutrient solutions, high P concentration may be toxic at high salinity levels. Under most field conditions, the concentration of calcium in the soil solution is high enough to result in precipitation of P, resulting in an in-creased P fertilizer requirement. Thus, excess P is seldom found in salt-affected soils. Most crop plants possess high selectivity for K uptake, thereby reducing the danger of K deficiency.

The salinity distribution in the root zone must be known to calculate the average root zone salinity. Average root zone salinity is used to report crop salt tolerance. The bulk of the evidence indicates that the mean salinity with depth, integrated over time, is the most representative salinity. The results of Bower et al. (1969) and Shalhevet et al. (1969) are for soil salinity values varying widely with depth, with the maximum values occurring at either the bottom or top of the root zone. In a field experiment on organic soil (Hoffman et al., 1983b), sprinkler irrigation and subirrigation treatments created widely different soil salinity distributions, but corn yield was shown to re-spond to the average soil salinity. An exception to this conclusion is the result of Bernstein and Francois (1973) who suggested that the representative soil salinity is the


weighted salinity in proportion to the relative depth distribution of water uptake. For widely spaced plantings, such as orchards, the spatial salt distribution in three dimen-sions within the root zone of a single tree must be considered.

7.3.2.2 Crop. Crop factors that may modify the response to salinity are variety, rootstock, and stage of growth. The normal breeding and selection process of crop varieties typically emphasizes high productivity rather than tolerance to salinity. Con-sequently, varietal differences in salt tolerance are not common among field and gar-den crops. Nevertheless, some varietal differences do exist and may cause confusion with regard to the application of salt tolerance data. Some documented examples are: barley (Epstein et al., 1980), wheat (Torres and Bingham, 1973), tomato, soybean, lettuce (Shannon, 1980), and melon (Shannon and Francois, 1978). Breeding programs to improve crop salt tolerance have resulted in a limited number of tolerant varieties. Shannon and Noble (1990) list eleven such varieties. Large differences in salt toler-ance are known among rootstocks of subtropical crops and deciduous fruit trees. These crops are frequently salt sensitive and are known to be especially sensitive to chloride and sodium salts.

Determination of the relative sensitivity of crops to salinity at different stages of growth is difficult because the effect on growth during an early stage may influence the response during later growth stages. Some studies claiming to show stage of growth sensitivity, in fact, demonstrate a sensitivity to the duration the roots were ex-posed to salinity. Mass and coworkers (1986; 1989 a,b) applied salinity treatments for 30-day periods during the vegetative, reproductive, and maturity stages of growth for several crops. The major conclusion from their results is that crops are specifically sensitive during the early seedling period. Re-analysis of their results on a time-weighted basis, taking the number of days from treatment initiation to harvest as the weighing factor, shows very similar results for the vegetative and reproductive stages. Once salinity is applied for a significant period of time and then removed, there does not appear to be significant recovery—the damage is sustained to the end of the sea-son. Naturally, the later salinity is applied the less the damage will be. An important management option resulting from this analysis is that saline water may be applied late in the season with very little damage to crop production (Francois et al., 1994).

Salinity may delay seed germination and seedling emergence, but most crops are capable of germinating at higher salinity levels than they can tolerate at later stages of growth. For example, a salinity level of 4.7 dS/m at planting was shown to reduce pea-nut yield by 50%, while seedling growth after emergence was not reduced by half until ECe reached 7.5 d/Sm and 50% germination was achieved at an ECe of 13 dS/m (Shal-hevet et al., 1969). Similar results were obtained with corn (Maas et al., 1983) where germination was satisfactory at an electrical conductivity of the soil water, ECsw, of 10 dS/m while the threshold for grain yield was at an ECsw of 5.5 dS/m. Threshold salin-ity, ECsw, for seedling growth, however, was only 1.0 dS/m.

7.3.2.3 Environment. Six elements of the aerial environment have been shown to impact crop salt tolerance: temperature, humidity, rainfall, radiation, carbon dioxide, and air pollution. Temperature is a critical environmental factor for crop salt tolerance. High temperature increases the stress level to which a crop is exposed, either because of increased transpirational demand or because of the effect of temperature on the bio-chemical transformations in the leaf. The increase in the level of stress due to high tem-perature increases the sensitivity of the crop to salinity (Magistad et al., 1943, for al-


falfa, bean, beet, carrot, cotton, onion, and tomato; Lunt et al., 1960, for cherry, chrysan-themum, and kidney bean; Hoffman et al., 1978, for bean; and Ehlig, 1960, for grape).

High atmospheric humidity tends to decrease the crop stress level, thus reducing sa-linity damage. High humidity generally increases the salt tolerance of salt-sensitive more than salt-tolerant crops (Hoffman et al., 1971; Hoffman and Jobes, 1978).

Rainfall has no direct effect on the salinity response function, but it may increase leaching and replace potentially saline sources for satisfying evapotranspiration. Rain-fall prior to and during the irrigation season makes it possible to use more saline irri-gation water because of the dilution of the soil solution. This impact is quantified in Section 7.4.1.

For some crops, high atmospheric carbon dioxide may reduce salinity damage. Schwartz and Gale (1984) found only an 18% yield reduction for beans at high carbon dioxide levels (2.5 mL CO2/L of air) compared with 64% reduction for the same in-crease in salinity at the ambient carbon dioxide concentration (0.3 mL CO2/L of air). No interaction with carbon dioxide was demonstrated for corn.

The apparent tolerance to salinity improves with an increase in the concentration of ozone, an important air pollutant. For example, at low levels of salinity, high levels of ozone caused a 95% yield reduction for bean while only a 44% reduction occurred for the same ozone level at high salinity levels (Hoffman et al., 1973). As with other fac-tors (e.g., soil fertility), when yield potential is suppressed by one limiting factor, the relative effect of another limiting factor is reduced.

Contrary to expectation, plants may be more sensitive to salinity as solar radiation is reduced. Despite half the yield potential in the shade as in the sun, salinity caused a 37% reduction in dry matter for Maranta leuconeura, while in the sun the reduction was only 15% (Nolan et al., 1982). Reduced radiation significantly decreased the salt tolerance threshold of cantaloupe grown in a greenhouse (Meiri et al., 1982).

7.3.3 Tolerance to Specific Solutes Specific solutes, such as boron, chloride, and sodium, are potentially toxic to crops

that take up these solutes through the root system and accumulate them in the leaves. Many trace elements are also toxic to plants at very low concentrations. Suggested maximum concentrations for a number of trace elements are given by Pratt (1973). Fortunately, most irrigation supplies contain insignificant concentrations of these po-tentially toxic trace elements and toxicity is generally not a problem.

7.3.3.1 Boron. Boron is a trace element found occasionally in saline soils. At con-centrations only slightly greater than the 0.2 to 0.5 g/m3 needed for optimum growth, boron may be phytotoxic. Boron toxicity symptoms typically appear at the tip and along the edges of older leaves as yellowing, spotting, and/or drying of leaf tissue. The damage gradually progresses between the veins toward midleaf. A gummosis or exu-date on limbs or trunks is sometimes noticeable on boron-affected trees, such as al-mond. Many sensitive crops show toxicity symptoms when boron concentrations in leaf blades exceed 250 mg/kg (dry-mass basis), but not all sensitive crops accumulate boron in their leaves. Stone fruits (e.g., peach, plum, almond) and pome fruits (pear, apple, and others) are examples of crops that do not accumulate boron in leaf tissue. Many crops have been tested for foliar damage caused by boron (i.e., Eaton, 1944). A much smaller selection of crops has been evaluated for yield loss.

The Maas-Hoffman model for salt tolerance has also been applied to boron toxicity. Both relative yield and relative shoot growth have been found to be described ade-


quately by this model (Francois, 1984, 1986; Bingham et al., 1985). The Maas-Hoffman model for boron tolerance is: Yr =100 – sb (Cb – Cbt) (7.5) where the terms are the concentrations for boron (b) rather than electrical conductivity for salinity as in Equation 7.4. The model coefficients for threshold and slope (when measured) and qualitative tolerance ratings for crops studied to date are given in Table 7.5. Although the threshold tolerance for boron varies widely among crops, the slopes Table 7.5. Coefficients of threshold (Cbt) and slope (sb) for boron tolerance using the Maas-

Hoffman model. Boron concentrations are for soil water (adapted from Maas, 1990). Boron Concentration of Soil Water

Crop Growth

Measurement Threshold, Cbt

(g/m3) Slope, sb

(% per g/m3) Qualitative

Rating[a] Alfalfa Shoot 4 - 6 T Barley Grain 3.4 4.4 MT Bean Shoot 0.7 - 1 S Beet, red Shoot 4 - 6 T Broccoli Head 1.0 1.8 MS Cabbage Shoot 2 - 4 MT Carrot Shoot 1 - 2 MS Cauliflower Curd 4.0 1.9 T Corn Shoot 2 - 4 MT Cotton Shoot 6 - 10 T Cowpea Shoot 2.5 12 MT Cucumber Shoot 1 - 2 MS Grape Shoot 0.5 - 0.7 S Grapefruit Shoot 0.5 - 0.7 S Lemon Shoot < 0.5 S Lettuce Shoot 1.3 1.7 MS Muskmelon Shoot 2 - 4 MT Onion Shoot 0.5 - 0.7 S Orange Shoot 0.5 - 0.7 S Peach Shoot 0.5 - 0.7 S Peanut Seed 0.7 - 1 S Pecan Shoot 0.5 - 0.7 S Pepper Shoot 1 - 2 MS Plum Shoot 0.5 - 0.7 S Potato Tuber 1 - 2 MS Radish Storage root 1.0 1.4 MS Sorghum Grain 7.4 4.7 T Squash Shoot 2 - 4 MT Strawberry Shoot 0.7 - 1 S Sugar beet Storage root 4.9 4.1 T Sunflower Seed 0.7 - 1 S Sweet potato Tuber 0.7 - 1 S Tomato Fruit 5.7 3.4 T Walnut Shoot 0.5 - 0.7 S Wheat Grain 0.7 - 1 3.3 S

[a] Rating is based upon the threshold value. Separations between sensitive (S), moderately sensitive (MS),moderately tolerant (MT), and tolerant (T) are concentrations of 1, 2, and 4 g of boron per m3 of soil so-lution.


Table 7.6. Chloride tolerance limits of some fruit-crop cultivars and rootstocks (adapted from Maas, 1990).

Crop Rootstock or Cultivar

Maximum Permissible Cl- in Soil Water without

Leaf Injury (mol/m3) Avocado West Indian

Guatemalan Mexican

15 12 10

Sunki mandarin, grapefruit, Cleopatra mandarin, Rangpur lime

50

Sampson tangelo, rough lemon, sour orange, Ponkan mandarin

30

Citrus

Citrumelo 4475, trifoliate orange, Cuban shaddock, Calamondin, sweet orange, Savage citrange, Rusk citrange, Troyer citrange

20

Grape Salt Creek, 1613-3 Dog ridge

80 60

Stone fruit Marianna Lovell, Shalil Yunnan

50 20 15

Berries Boysenberry, Olallie blackberry Indian Summer raspberry

20 10

Grape Thompson seedless, Perlette Cardinal, Black Rose

40 20

Strawberry Lassen Shasta

15 10

of the yield reductions only vary from 1.4% to 12% (Table 7.5). This indicates that although crop yields are reduced by relatively low concentrations of boron, yields do not decline rapidly as boron levels increase in excess of the threshold value.

7.3.3.2 Chloride. Most herbaceous crops are not particularly sensitive to chloride. Soybeans are an exception; salt-sensitive soybean cultivars accumulate excessive amounts of chloride. This problem has been avoided by breeding cultivars that restrict the transport of chloride to the shoots. Woody plant species, on the other hand, are all generally susceptible to chloride toxicity. Tolerances vary among species and even among varieties or rootstocks within a species. These differences usually reflect the prevention or retardation of chloride accumulation in the crop canopy. Table 7.6 lists chloride tolerance limits for some rootstocks and cultivars of fruit crops for which data are available. The data in Table 7.6 indicate the maximum chloride concentrations permissible in the soil water that do not cause leaf injury.

7.3.3.3 Sodium. Plant growth and yield may be reduced by the accumulation of toxic levels of sodium. Sodium injury is generally limited to woody species like avo-cado, citrus, and stone-fruit trees. Sodium concentrations as low as 5 mol/m3 in soil water or 0.25% to 0.5% on a dry weight basis in leaves can cause injury in some tree crops. Sodium is normally retained in the roots and the lower trunk of stone-fruit trees, but after several years, the conversion of sapwood to heartwood apparently releases the accumulated sodium, which is transported to the leaves causing tip and margin necrosis (Maas, 1990).


In sodic, nonsaline soils, total soluble salt concentrations are low and, conse-quently, calcium and/or magnesium concentrations may be nutritionally inadequate. These deficiencies rather than sodium toxicity are usually the primary cause of poor plant growth among non-woody species. Sodium uptake by plants is strongly regu-lated by calcium in the soil solution. Thus, the presence of sufficient calcium is essen-tial to prevent the accumulation of toxic levels of sodium. As a general guide, calcium and magnesium concentrations in the soil solution above 1 to 2 mol/m3 each, with the concentration of calcium being at least as great as magnesium, are nutritionally ade-quate in sodic, nonsaline soils.

7.3.4 Crop Water Production Functions The function relating yield (Y) to transpiration (T) is the water production function.

The slope of this function is the water use efficiency. The production function is lin-ear, crop-specific, and independent of ambient conditions, as long as T is normalized with respect to potential evaporation (Ep) (de Wit, 1958). In the field, it is seldom pos-sible to measure transpiration directly. The more commonly used relationship is yield as a function of applied water (AW). Water quantities applied in the field (irrigation plus rainfall) are equivalent to the sum of transpiration, evaporation, changes in soil water storage, leaching, and runoff. An example of the relationship between yield and water application is given in Figure 7.5 for tomato. In figures such as 7.5, the amount of water applied at zero yield is typically taken to be the amount of water applied that evaporates or may be transpired without yield production. This component may shift the intersect on the water quantity axis (see Figure 7.5), but will not change the slope. The leaching component may change the slope and the shape of the curve from linear to exponential.

Figure 7.5. Yield of marketable tomato fruit as a function of water application depths for three levels of salinity of the irrigation water (Vinten et al., 1986).


The relation of yield to water application is independent of salinity, as long as there is no leaching (Childs and Hanks, 1975). Shalhevet (1984) summarized experiments in which both salinity and water quantity were varied to show production functions for nine crops. Salinity may affect the production function in three ways. First, the maxi-mum yield possible (the yield plateau) will be lower as soil salinity increases (see Fig-ure 7.5). At the same time, transpiration will be reduced commensurate with the slope of the particular crop water production function. Second, the intercept on the water quantity axis may shift to the right because the evaporation component may be greater than under nonsaline conditions. For example, Shalhevet et al. (1969) found half of the total evapotranspiration to be lost in evaporation because the peanut canopy was slow to develop and did not reach full soil cover under saline conditions. Third, the need to satisfy the leaching requirement may result in a curvilinear function as demonstrated in Figure 7.6. A larger quantity of saline water must be applied to maintain a given yield level compared to nonsaline conditions. In Figure 7.6 the deviation from linear-ity reflects the leaching fraction, (AW2–AW1)/AW2). The distance Ymax – Y4 is the yield loss due to an irrigation salinity of 4 dS/m at maximum leaching.

Figure 7.6. Relative yield of tall fescue as related to the ratio of applied water

to potential evaporation (adapted from Letey et al., 1985).

7.4 LEACHING Water must drain through the crop root zone to prevent solutes from increasing to

concentrations detrimental to crop production. All soils have an inherent ability to transmit soil water provided a hydraulic gradient exists and the hydraulic conductivity is reasonable. This is usually denoted as natural drainage capacity. Drainage occurs when the hydraulic gradient is positive in the downward direction. Natural drainage in many instances is sufficient to leach salts from the root zone. Most soils of fine tex-ture, soils with compacted layers, and soils with layers of low hydraulic conductivity may be so restrictive to downward water movement that the natural drainage capacity is insufficient to provide adequate leaching. In addition, the hydrogeology may be


such that the hydraulic gradient is predominately upward, which leads to waterlogging and salination.

If natural drainage is not adequate, a man-made or artificial drainage system must be installed to sustain productivity. Before designing an artificial drainage system, the natural drainage capacity should be determined. If, after applying excess water, the hydraulic gradient and the soil’s hydraulic conductivity permit soil water to drain out of the crop root zone, the intensity of the artificial system can be reduced. Alterna-tively, upward flow into the root zone can intensify the need for artificial drainage.

With either natural or artificial drainage, water leaching below the crop root zone must go elsewhere. It may take decades or just a season for productivity to be reduced, depending on the hydrogeology of the area, but without drainage to an appropriate outlet agricultural productivity can not be sustained.

7.4.1 The Leaching Requirement The amount of irrigation water needed to satisfy the crop’s water requirement can

be estimated from water and salt balances within the crop root zone. The major flows of water into the root zone are irrigation, rainfall, and upward flow from the ground-water. Water flows out by evaporation, transpiration, and drainage. Under steady-state conditions, the change in depth of soil water and salt storage are essentially zero. If the total water inflow is less than evaporation plus transpiration, water is extracted from soil storage and drainage is reduced, with time, the difference between inflows and outflows becomes zero. In the absence of net downward flow, salt will accumulate, crop growth will be suppressed, and transpiration will be reduced by a value consistent with the crop water production function.

In the presence of a shallow water table, deficiencies in the irrigation and rainfall amounts may be offset by upward flow from the groundwater. Upward flow will carry salts into the root zone. If upward flow continues and sufficient leaching does not oc-cur, soil salinity will ultimately reduce crop growth and water consumption. Over the long term, a net downward flow of water is required to control salination and sustain crop productivity.

Rarely do conditions controlling the water that flows into and out of the root zone prevail long enough for a true steady state to exist. However, it is instructive to con-sider a simple form of the steady-state equation to understand the relationship between drainage and salinity. If it is assumed that the upward movement of salt is negligible, the quantities of salt dissolved from soil minerals plus salt added as fertilizer or amendments is essentially equal to the sum of precipitated salts plus salt removed in the harvested crop, and the change in salt storage is zero under steady-state conditions, the leaching fraction (L) can be written as:

d

a

d

a

a

d

ECEC

CC

DD

L ===

(7.6)

where EC is electrical conductivity and the subscripts d and a designate drainage and applied water (irrigation plus rainfall). Equation 7.6 applies only to salt constituents that remain dissolved. The importance of Equation 7.6 is greater than the accuracy of predicting the drainage amount or salt concentration because the relationship between the leaching fraction and root-zone salinity is extremely important in managing soil salinity.


The minimum leaching fraction that a crop can endure without yield reduction is termed the leaching requirement, Lr, which can be expressed as follows:

**

*

d

a

d

a

a

dr EC

ECCC

DD

L ===

(7.7)

The notation in Equation 7.7 is the same as in Equation 7.6 except the superscript (*) distinguishes required from actual values. Several models have been proposed to relate Cd

* in Equation 7.7 to some readily available value of soil salinity that indicates the crop’s leaching requirement. Hoffman and van Genuchten (1983) determined the line-arly averaged, mean root-zone salinity by solving the continuity equation for one-dimensional vertical flow of water through soil, assuming an exponential soil water uptake function (Raats, 1974). The linearly averaged salt concentration of the root zone given as the concentration of the saturated extract (C) is given by:

]e)1(ln[1 /δδ Z

aLL

ZLLCC −−++=

(7.8)

where Ca = salt concentration of the applied water L = leaching fraction Z = depth of the root zone δ = an empirical constant assumed equal to 0.2 Z.

Figure 7.7 illustrates the relationship given in Equation 7.8, taking into account the salinity of rainfall and irrigation water. Note salinity in Figure 7.7 is expressed in terms of electrical conductivity of the soil saturation extract (ECe or Ce) assuming C is twice Ce.

Plants adjust osmotically as soil salinity increases (Maas and Nieman, 1978). Be-cause salt-tolerance trials are usually designed to maintain leaching fractions of about 0.5, the osmotic adjustment consistent with no loss of yield can be estimated by the mean soil salinity at 50% leaching, as given in Figure 7.7. As a first approximation,

Figure 7.7. The mean root-zone soil salinity as a function of the salinity of the applied water and leaching fraction (L) (adapted from Hoffman and van Genuchten, 1983).


Figure 7.8. Leaching requirement (Lr) as a function of the

salinity of the applied water and the salt-tolerant threshold value for the crop (adapted from Hoffman and van Genuchten, 1983).

the leaching requirement can be expressed as a function of the crop salt tolerance threshold value by reducing C at any given L by C at 50% leaching. Figure 7.8 illus-trates this relationship among the salinity of the applied water, the salt-tolerance threshold of the crop, and the leaching requirement. Hoffman (1985) compared calcu-lated leaching requirements from this and other models with experimental results. Of the four models tested, the one presented here agrees well with the measured values throughout the range of Lr of agricultural interest.

The following illustrates the procedure for estimating the leaching requirement. As-sume tomatoes are to be grown in an arid region and irrigated by water with an ECi of 3 dS/m. The salt-tolerance threshold, t, of tomatoes is 2.5 dS/m (Table 7.4). If rainfall is insignificant compared to irrigation and there is no upflow from a shallow water table, the leaching requirement can be determined directly from Figure 7.8. In this case, the leaching requirement equals 0.2. The drainage water would have an EC of 15 dS/m from Equation 7.7. To calculate the salinity of the applied water when rainfall is significant, the equation used is:

CaDa = CrDr + CiDi (7.9)

The variable C is salt content and D is depth. The subscripts a, r, and i, indicate ap-plied, rain, and irrigation water, respectively. An iterative process must be used to determine Lr since Di is unknown until Lr is known. As an illustration, assume evapotranspiration, De + Dt, is 750 mm and rainfall, Dr, is 150 mm. Begin calculations by assuming that Di is 900 mm and Cr = 0. Because Da = Dr + Di, then Ca = CiDi/(Dr + Di) = 3(900)/(150 + 900) = 2.6 dS/m. With Ca = 2.6, Lr for tomato is 0.18 and Di = De + Dt + Dd = 750 + 0.18 Di. Thus Di is 915 mm. This is sufficiently close to the assumed value of 900 mm that further iterations are unnecessary.

Accounting for nonuniformity of irrigations in estimating Lr has not been addressed to date. If the leaching requirement is not met everywhere in the field, saline soil will develop wherever evapotranspiration plus the leaching requirement is not met. One must evaluate whether to apply copious amounts of water to assure that the leaching


requirement is met throughout the field or to accept some reduction in yield in parts of the field rather than overirrigate most of the field.

Extensive advancements in irrigation technology and management are needed to approach the goal of matching the leaching requirement. Present irrigation practices in many areas are inefficient and inadvertently provide excessive leaching. This is costly. It leads to a loss of water, energy, and nutrients, and increases the need for drainage facilities. Consequently, knowing the leaching requirements of crops and striving to attain them with properly designed and managed irrigation systems is vital. Neverthe-less, under the best conditions at present, some excess irrigation water is generally applied to achieve maximum yield.

7.4.2 Soil Salinity with Restricted Leaching An example of the effect of virtually no drainage on soil salinity is given by Peck et

al. (1981) for southwest Australia. Figure 7.9 illustrates the chloride concentration and downward velocity of the soil solution in a native eucalyptus forest with an annual rainfall of 800 mm. Chloride concentration was highest at a soil depth of 7 m, where the downward velocity of the soil solution equaled 0.04% of the annual rainfall, or 0.3 mm/yr. Below 7 m, the chloride concentration decreased linearly to less than 2000 mg/L just above the water table at a depth of 17 m.

The deeper the soil, the greater the capacity to store salt with minimal yield reduc-tion. High levels of salinity in the lower portion of a crop root zone have a minor in-fluence on yield if the upper portion is maintained at a relatively low level of salinity (Bingham and Garber, 1970; Bernstein and Francois, 1973). Plants compensate for reduced uptake of water from a zone of highly salinized soil by increasing uptake from a zone low in salinity (Wadleigh et al., 1947; Lunin and Gallantin, 1965; Shalhevet and Bernstein, 1968;). Although this minimizes yield loss, there is general concern about how much salt can be stored in the crop root zone before leaching is needed.

Chlor ide Concent rat ion (m g/L) Soil Solut ion Velocit y (mm /yr)

Figure 7.9. Soil chloride concentration and the downward rate of soil-water movement as a function of soil depth in poorly drained, non-irrigated soils of

southwest Australia (Peck et al., 1981).


Table 7.7. Change in the area and yield of cotton and tomato in Broadview Water District compared to Fresno County (from Wichelns et al., 1988).

1968 to 1972 1978 to 1982 Change (%)

Broad- view

Fresno County

Broad- view

Fresno County

Broad- view

Fresno County

Cotton Area (ha) Lint yield (kg/ha)

715 1,190

76,000 1,190

2,130 1,305

167,000 1,190

+200 +10

+140 0

Tomato Area (ha) Fruit yield (Mg/ha)

605 72

8,150 50

170 59

16,300 64

–0 –8

+100 +28

One of the first studies to address the question of how much salt can be stored in

the root zone without significant yield reduction involved alfalfa grown in a green-house and irrigated with water having an electrical conductivity (ECi) of 1 dS/m (Francois, 1981). The plants were grown without leaching in a sandy loam soil with water table depths of 0.6 m, 1.2 m, or 1.8 m for periods of 9, 14, and 20 months, re-spectively. In all cases, yield was reduced less than 25%, yet 14, 30, and 45 Mg/ha of salt were stored in the lower portions of the three different soil profile depths. Drastic reductions in yield occurred only when the salt began to accumulate in the upper por-tion of the root zone. This study demonstrates that regardless of soil depth, alfalfa can be grown for a considerable period of time without leaching, provided the upper part of the root zone, where most roots are concentrated, is maintained at a low level of salinity.

On a larger scale, the Broadview Irrigation District on the west side of California’s San Joaquin Valley is a documented example of the effect of accumulating soil salin-ity because of limited drainage (Wichelns et al., 1988). The district, consisting of 4000 ha of field crops, has had adequate imported surface water for irrigation since 1957. This imported water contained approximately 300 mg/L of salt (ECi of 0.5 dS/m). An-nual rainfall averages 150 mm. To facilitate leaching, subsurface drains were installed on more than 80% of the irrigated land. The district, however, had no drainage outlet until 1983, so it blended its surface runoff and subsurface drainage effluent with the irrigation water supply. The proportion of drainwater mixed with the low-salinity irri-gation water increased from near zero in the early 1960s to about half in the early 1980s, when the mean salt content of the drainage water was about 2800 mg/L. Thus, although the fields were leached, the salts were reapplied to the field and no salt dis-posal occurred. As illustrated in Table 7.7, crop selection switched to salt-tolerant crops, such as cotton, to maintain economic income. The amount grown and the yield of more salt-sensitive crops, such as tomato, dropped drastically as soil salinity in-creased over time. Irrigators in nearby Fresno County, in contrast, did not blend drain-age effluent with their irrigation water and they were able to maintain both tomato and cotton yields throughout the same period of time.

7.4.3 Impact of Shallow Groundwater The upward movement of shallow groundwater and its subsequent evaporation at

the soil surface leads to salination. To minimize the rate of salt accumulation, artificial drainage is normally installed to lower the water table. The depth of the water table, soil properties, and the rate of upward movement must be known to establish the ap-propriate depth to maintain the water table. This information is also needed to estimate the amount of groundwater available for consumption by plants.


7.4.3.1. Capillary flow. Starting from saturation, the drying rate at the soil surface is first limited by the atmospheric evaporative conditions. As the soil surface dries, the evaporation rate becomes limited by the rate of water movement to the soil surface in the liquid phase. As the soil dries further, liquid movement ceases but vapor move-ment upward continues, but is relatively unimportant. In arid and semiarid regions, evaporative demands are high so water-transmitting properties of the soil normally limit upward flow.

Gardner and Fireman (1958) studies the relationship between the rate of upward flow and the depth of the water table. Their study verified a relation, proposed by Gardner (1957), between hydraulic conductivity, K, and soil suction, S, of the form:

bSaK n +

=

(7.10)

where a, n, and b are constants. For many soils, values of n equal to 2 or 3 fit experi-mental data well. Figure 7.10 gives the theoretical maximum rate of upward flow from a stationary water table for two soils as a function of the depth to the water table.

The discussion above is valid when no drainage is discharged. When a net down-ward flux is maintained by an appropriately designed drainage system, leaching oc-curs. In this case groundwater salinity loses its significance as a criterion determining the desired depth of drainage. The main criterion becomes the need to maintain a suf-ficiently deep aerated root zone.

The water depth that will restrict upward flow to a minimum for these two soil types can be determined from Figure 7.10. Lowering the water table from the surface to a depth of about 1 m will be of little benefit in most soils. Upward flow at these shallow depths could exceed 2.5 mm per day for clay soils and be even greater for

Figure 7.10. Maximum theoretical rate of upward water flow for Chino clay and Pachap-pa sandy loam as a function of the depth of the water table (Gardner and Fireman, 1958).


coarser-textured soils. Lowering the water table from 1.2 m to 3 m in Pachappa sandy loam decreases upward flow by a factor of 10. When the water table is at 2.5 m, fur-ther lowering of the water table reduces upward flow only slightly. Keep in mind that upward movement and evaporation of water from the surface of the soil are possible even with a water table at a depth of 10 m. Harmful amounts of soluble salts could slowly accumulate if the groundwater is sufficiently saline and rainfall and irrigations amounts are sufficiently low. These results, verified by field observations, have led to the installation of most subsurface drainage systems at depths of at least 1.5 m wher-ever salinity poses a hazard.

7.4.3.2 Crop water use from groundwater. Water supplied to a crop by capillary rise from shallow groundwater can be an important resource. Benefits of using this water include reduced irrigation, lower production costs, the movement of less groundwater to deeper aquifers, and a decrease in the amount of groundwater that re-quires disposal through subsurface drainage systems. Although researchers have made progress in determining the individual effects of salinity and water table depth on crop water use, the combined effects of these two factors are not well understood. Some experiments that have been conducted are presented, but generalizations are difficult to make.

In a lysimeter study in Texas on Willacy fine sandy loam, Namken et al. (1969) studied the impact of two irrigation regimes and three depths to the water table on saline soil profiles (Figure 7.11). For the first year of the study, the salinity level of the groundwater, ECG, was 6 to 8 dS/m. During the last three years, the ECG ranged from

Figure 7.11. Soil salinity distribution for different salt contents and

depths of groundwater (Namken et al., 1969).


0.9 to 1.6 dS/m. The groundwater supplied 57%, 38%, and 28% of the total water used by a cotton crop when the water table was at depths of 0.9 m, 1.8 m, and 2.8 m, re-spectively. When the water table was 1.8 m deep or deeper, the upper part of the pro-file remained nonsaline, while the lower part became salinized. When the depth was 0.9 m, the salinity of the groundwater influenced the entire profile.

In a field study in the San Joaquin valley of California, cotton grown on a loam soil received at least 60% of its evapotranspiration from a water table located 2.0 m to 2.5 m below the surface with an ECG of 6 dS/m (Wallender et al., 1979). The fewer the number of irrigations, the more groundwater contributed to evapotranspiration. How-ever, the lint yield was reduced.

Some of the most consistent data showing the relationship between evapotranspira-tion provided from groundwater and water table depth have been obtained with cotton (Figure 7.12). The relationship between cotton’s use of water from the groundwater and the depth of the water table for soils ranging from clay to clay loam is based on results from three different field experiments on the west side of California’s San Joa-quin Valley (Grimes et al., 1984; Hanson and Kite, 1984; Ayars and Schoneman, 1986). The relationship for sandy loam is based on a lysimeter study in Texas (Nam-ken et al., 1969). The impact of salinity is not included in Figure 7.12 but results from California indicate that cotton’s uptake of groundwater is not reduced measurably until ECG exceeds 12 dS/m.

Use of groundwater by alfalfa and corn varies from 15% to 60% of the total sea-sonal use, but the data are too inconsistent to establish a relationship. In the Grand Valley of Colorado, alfalfa’s use of groundwater from a water table depth of 0.6 m varied from 46% to 94% of the total seasonal use in two different years, when ECG equaled 0.7 dS/m (Kruse et al., 1986). Groundwater use varied from 23% to 91% of

Figure 7.12. Contribution of shallow, saline groundwater to evapotranspiration

(ET) of cotton as a function of soil type and depth to water table (adapted from Hoffman et al., 1990).


the total seasonal use in other years, when ECG equaled 6 dS/m. In a joint study, Kruse et al. (1985) reported that corn obtained 52% to 68% of its seasonal water requirement when the water table was 0.6 m deep and obtained 25% to 32% of its seasonal water requirement when the water table was 1 m deep. The proportion of use remained unaf-fected when ECG varied from 0.7 dS/m to 6 dS/m.

7.5 SALINITY IMPACTS ON IRRIGATION DESIGN 7.5.1 Influence of Irrigation Method

The pattern of salt distribution within a given field depends on the variability in soil properties, differences in water management, and the design of the irrigation system. The soil salinity profile that develops as water is transpired or evaporated depends, in part, on the water distribution pattern inherent with the irrigation method. Distinctly different salinity profiles develop for different irrigation methods. Each irrigation method has specific advantages and disadvantages for salinity management.

The major methods of flood irrigation are borders and basins. Border methods commonly have excessive water penetration near the levees, at the upper end, and at the lower end of the strips if surface drainage is prevented, and inadequate water pene-tration midway down the strip, which may result in detrimental salt accumulation. If insufficient amounts of water are supplied to one end of the basins, the far ends may have excessive salt accumulations. The basin method of flooding has the potential for more uniform water applications than other flooding methods provided the basins are leveled, sized properly, and have uniform soils.

With furrow irrigation, salts tend to accumulate in the seed beds because leaching occurs primarily below the furrows. If the surface soil is mixed between crops and the irrigation water is not too saline, the increase in salt over several growing seasons may not be serious. If excess salt does accumulate, leaching by another irrigation method may be required or the application of special agronomic techniques (such as planting on the side slopes of the beds, planting in the furrows, etc.) can be helpful. In the fur-row and flood methods, the length of run, irrigation application rate, soil characteris-tics, slope of the land, and time of application are the factors that govern the depth and uniformity of application. Proper balance among these factors is beneficial in control-ling salinity.

Sprinkler irrigation is unavoidably accompanied by wetting of crop foliage unless below-canopy sprinkling is possible. Salts can be absorbed directly into the leaves, thus some crops experience foliar injury and yield reductions that may not occur when they are surface irrigated with the same water (Maas et al., 1982). Table 7.8 gives ap-proximate concentrations of chloride and sodium in sprinkled waters that cause foliar injury for some crops. Intermittent sprinkling with saline water frequently causes more foliar injury than sprinkling continuously. This is attributed to the increased salt con-

Table 7.8. Relative susceptibility of crops to foliar injury from saline sprinkling waters (adapted from Maas, 1990).

Sodium or Chloride Concentration (mol/m3) Causing Foliar Injury < 5 5 to 10 10 to 20 > 20

almond grape alfalfa cauliflower apricot pepper barley cotton citrus potato corn sugar beet plum tomato sorghum sunflower


centration each time water evaporates from the leaves. Foliar injury can be particularly acute if sprinkling is done during the day when high evaporative conditions exist.

Flooding and sprinkler irrigation systems that wet the entire soil surface create a profile that at steady state increases in salinity with soil depth to the bottom of the root zone, provided that moderate leaching is applied, application is uniform, and no shal-low, saline groundwater is present. If irrigations are infrequent, the salt concentration in the soil solution increases with time between irrigations, particularly near the soil surface. In the presence of a shallow, saline water table and, particularly, with inap-propriate irrigation management techniques, salts will accumulate near the soil sur-face. Frequently, this situation leads to a severe salinity problem.

Trickle or drip irrigation systems where water is applied from point sources have the advantage that high leaching is provided near the emitters and high soil water con-tents can be maintained in the root zone by frequent but small water applications. Plant roots tend to proliferate in the leached zone of high soil water content near the water sources. This allows water of relatively high salt content to be used successfully in many cases. Possible emitter clogging, the redistribution of water required to germi-nate seeds, and the accumulation of the salts at the soil surface between emitters are management concerns.

The salinity profile under line water sources, such as furrows and either porous or multi-emitter drip irrigation systems, has lateral and downward components. The typi-cal cross-sectional profile has an isolated pocket of accumulated salts at the soil sur-face midway between the line sources of water and a second, deep zone of accumula-tion, with the concentration depending on the amount of leaching. A leached zone occurs directly beneath the line source. Its size depends on the irrigation rate, the amount and frequency of irrigations, and the crop’s water extraction pattern.

Whereas the salt distribution from line sources increases laterally and downward, the distribution from point irrigation sources, such as micro-basins and drip systems with widely spaced emitters, increases radially from the water source in all directions below the soil surface. As the rate of water application increases, the shape of the sa-linity distribution changes. The mathematical model of Bresler (1975) predicts that the salinity distribution in uniform, isotropic sand changes from elliptical (with the maxi-mum movement vertical) to more circular as the rate of water movement decreases. In isotropic and layered soils, the horizontal rate of water movement is greater than the vertical, resulting in relatively shallow salt accumulations. For tree crops irrigated with several drip emitters per tree, the wetting patterns may overlap, thereby reducing the level of salt accumulation midway between the emitters under a tree.

The continuous upward water movement from a subsurface irrigation system re-sults in salt accumulation near the soil surface as water is lost by evapotranspiration. Subsurface systems provide no means of leaching these shallow accumulations unless the soil is leached periodically by rainfall or surface irrigations.

The salt distribution profiles shown in Figure 7.13 are typical of those just de-scribed. The lateral distribution of salts under sprinkler irrigation is relatively uniform, with evaporation accounting for the salt accumulation near the soil surface. The salin-ity distributions for both line source systems (furrow and drip) are similar, with salin-ity levels relatively low beneath the water sources and relatively high midway between the sources. Observed differences in salt accumulation below the side slopes of the


Figure 7.13. Influence of the irrigation system on the soil salinity pattern of bell pepper at two levels of irrigation water quality (adapted from Bernstein and Francois, 1973).

planting bed are caused by additional leaching during furrow irrigation. Of course, the more saline irrigation water accounts for higher salt concentrations for each case illus-trated. Because the salt distributions were determined after only one irrigation season for soil that had previously been well leached, salt had not yet accumulated at greater soil depths.

7.5.2 Conjunctive Use of Waters Historically, conservative salinity standards have been applied to assess the suit-

ability of water for irrigation. Consequently, saline waters, regardless of their origin, have normally been avoided even as supplemental irrigation supplies. Using rational water assessment procedures, however, saline waters, typically exported from irriga-tion districts, still have potential value for irrigation (Rhoades, 1977, 1987). One situa-tion for using saline irrigation water is in conjunction with nonsaline water. Where high-quality water costs are prohibitive, crops of moderate to high salt tolerance can be irrigated with saline water, especially at later growth stages, with economical ad-


vantages. This strategy may be economical even with some reduction in crop yields. Irrigating with saline drainage water would permit the expansion of irrigated agricul-ture while reducing drainage disposal and pollution problems (Rhoades, 1987).

Two strategies for conjunctive use of saline irrigation water are blending and sea-sonal cycling with a source of low-salinity water. Each strategy has it advantages and limitations.

7.5.2.1 Blending. The mixing of saline and nonsaline waters to obtain a composite water suitable for irrigation is usually referred to as blending. The goals of blending are to improve the suitability of the saline water and to increase the total water supply. However, the entire volume of any saline water cannot be consumed in crop produc-tion; the higher its salinity, the less that can be consumed. Blending is beneficial only when the resulting mixed water has a salinity lower than the threshold salinity of the crop and leaching is provided by rain or by irrigation. For example, the threshold sa-linity of zucchini squash is 4.7 dS/m (see Table 7.4). Blending saline water of 5 dS/m in equal volume with low-salinity water of 1 dS/m will yield water of 3 dS/m. The resultant soil salinity (ECe) will be about 4.5 dS/m (ECe =1.5 × ECi) which is lower than the threshold salinity. Blending doubles the supply of water for irrigation without any loss in yield. Using the saline water alone would result in a 26% yield reduction assuming the resultant soil salinity is 7.5 dS/m.

If this same blended water is used to irrigate strawberries, with a threshold salinity value of 1.0 dS/m, the resultant yield would be nil. Consequently, the portion of low-salinity water used in the blend, which could have resulted in only a 5% yield loss on half the area, is made useless. The low-salinity water can be used to irrigate corn with-out yield loss because its threshold salinity is 1.7 dS/m. Irrigating with saline water alone will result in 70% yield reduction while the reduction with the blended water will be 35%. The same 35% yield reduction would result from irrigating half of the area with nonsaline water and half with saline water without the need of blending. Blending in this case offers no advantage. Additionally, for some crops the blended water will require leaching, thereby expending more nonsaline water.

If the blending strategy is adopted, two mixing processes, network dilution or soil dilution, are possible (Shalhevet, 1984). With network dilution, water supplies are blended in the irrigation conveyance system. With soil dilution, the soil acts as the mixing medium. Meiri et al. (1986), conducting a three-year study in Israel on a rota-tion of potato and peanut under drip irrigation, concluded that crops responded to the weighted mean water salinity, regardless of the blending method. Other examples of blending techniques are given by Rains et al. (1987) and Rolston et al. (1988).

7.5.2.2 Cycling. The cycling strategy utilizes low-salinity water for pre-plant and early irrigation of salt-tolerant crops in the rotation and for all irrigations of salt-sensitive crops. The saline water is used to irrigate salt-tolerant crops at later stages of growth. The timing and amount of substitution will vary with salt tolerance of the crops grown, the quality of the two waters, the climate, and the irrigation system. Whatever salt buildup occurs in the soil from irrigating with saline water is alleviated in a subsequent cropping period when nonsaline water is applied. Furthermore, the yield of the sensitive crop is not reduced if proper pre-plant irrigations and appropriate water management are used during germination and seedling establishment to leach salts out of the seed area and shallow soil depths. Subsequent irrigations with non-saline water leach these salts farther down in the profile ahead of the advancing root


system and reclaim the soil. The cyclic strategy prevents the soil from becoming ex-cessively saline while permitting, over the long term, substitution of saline water for a significant fraction of the irrigation water requirements.

This strategy has been tested for two cropping patterns on a farm in the Imperial Valley of California (Rhoades et al., 1988). One test consisted of two successive rota-tions of wheat, sugar beet and cantaloupe. Wheat and sugar beet are relatively salt tolerant and cantaloupe is relatively salt sensitive. Results for this successive rotation are given in Table 7.9. In this rotation, Colorado River water (1.2 dS/m, relatively nonsaline water) was used throughout for the control treatment (C) and for the pre-plant and early irrigations of wheat and sugar beet in the other two treatments (Ca and Ac) and for all irrigations during the cantaloupe crop. The saline water (A) was from the Alamo River which had an EC of 4 dS/m. During the two rotations, 35% of the irrigation water was saline for treatment Ca while 53% was saline for treatment cA. No significant yield losses of wheat or sugar beet occurred in either cycle of the rota-tion from substituting saline water for Colorado River water after seedling establish-ment. Furthermore, the commercial yield of fresh-market cantaloupe in the fields that had previously been irrigated with saline water was not reduced significantly.

The second cropping pattern tested was a four-year block rotation consisting of two years of cotton followed by wheat (both relatively salt-tolerant crops) and then by al-falfa (a relatively salt-sensitive crop). For the two cotton crops, the three treatments were Colorado River water only (C), nonsaline water for seedling establishment fol-lowed by saline water (cA), and saline water from the Alamo River only (A). Colo-rado River water was used exclusively for wheat and alfalfa in all three treatments. During this four-year rotation, 25% of the irrigation water was saline for treatment cA and 45% was saline for treatment A. Lint cotton yield in 1982 was not reduced from use of saline water (see Table 7.9) even when it was applied during pre-plant and seedling establishment periods. In the second cotton crop (1983) there was no yield loss from use of saline water following seedling establishment. A significant loss of

Table 7.9. Yields of crops in two rotations cycling saline and nonsaline irrigation waters in the Imperial Valley of California (adapted from Rhoades et al., 1988).

Successive Rotation

Wheat 1982

(Mg grain/ha)

Sugar beet 1982-83

(Mg sugar/ha)

Cantaloupe 1983

(kg seed/ha)

Wheat 1984

(Mg grain/ha)

Sugar beet 1984-85

(Mg sugar/ha)

Cantaloupe 1985

(cartons/ha) Irrigation Water Quality

C 8.1 9.7 440 7.9 9.2 435 Ca 8.1 9.7 430 7.8 9.2 535 cA 8.3 9.2 400 8.0 8.8 525

Block Rotation

Cotton 1982

(Mg lint/ha)

Cotton 1983

(Mg lint/ha)

Wheat 1984

(Mg grain/ha)

Alfalfa 1985

(Mg forage/ha)

Irrigation Water Quality C 1.4 1.1 7.7 17.5

cA[a] 1.4 1.1 7.7 15.7 A[a] 1.5 0.7 7.7 16.6

[a] All treatments of wheat and alfalfa irrigated with Colorado River water only.


lint yield did occur in 1983 from using saline water for all irrigations (treatment A). This yield reduction was caused by excess salinity in the seed bed during the estab-lishment period, which reduced plant population. No loss in yield of wheat grain or alfalfa forage occurred from the previous use of saline water on cotton when irrigated with Colorado River water. Studies by Ayars et al. (1986) and Shennon et al. (1987) showed similar results.

There may be difficulty in adopting the cyclic strategy if the availability of saline water does not coincide with peak crop water use. In the San Joaquin Valley of Cali-fornia, drain effluent occurs primarily from January to June when most crops require high-quality water. Using drainage water late in the season may not be feasible if the flow rate needed to irrigate a field effectively exceeds the flow rate from the drains. To avoid this lack of drainage water, a storage reservoir could be constructed. Another option is to close the drains and allow the soil to act as the reservoir and then pump as necessary. The latter option is more desirable since it does not take land out of produc-tion. A third option is to develop a reuse system on a regional basis with separate dedicated areas of drainage water collection and use. However, regardless of where the drainage water is stored, a drainage water collection and distribution system must be constructed to implement this strategy (Grattan and Rhoades, 1990).

7.5.3 Environmental Consequences Irrigating with saline water to alleviate an environmental concern is possible, par-

ticularly where saline water disposal is impractical due to physical, environmental, or social constraints. Because many crops are sensitive to shallow or fluctuating water tables, a means of lowering the shallow water table is essential to sustain crop produc-tion. Drainage water, collected to lower the water table, can frequently be used for irrigation. This reduces the amount of nonsaline water required for salt-tolerant crops, and decreases the volume of drainage water requiring disposal or treatment. Many growers in the San Joaquin Valley of California, for example, are considering this option as a solution to reducing drainage volume in response to an environmental con-cern.

Another potential use of saline water is the irrigation of specific crops that have the ability to accumulate large quantities of undesirable constituents (e.g., selenium, ni-trate, boron, molybdenum) before the subsequent discharge of drainage effluent. A number of plants can be used for biofiltration, the term used to describe this process (Cervinka et al., 1987; Wu et al., 1988). For example, they found that certain grasses and native species in California are effective in accumulating substantial amounts of selenium in their shoots. Potentially toxic constituents could therefore be removed by harvesting the shoots. This alternative management practice is most attractive where drainage disposal is an environmental concern, the bioaccumulator has economic value, or other treatment processes are unavailable or too expensive.

Saline waters may contain specific solutes, such as boron and chloride, that can ac-cumulate in plants to levels that cause foliar injury and a yield reduction. In portions of the San Joaquin Valley of California, shallow groundwaters often contain boron concentrations in excess of 5 mg/L. The removal of boron from the soil requires far more water for leaching than does removal of salts. Thus, the environmental conse-quences of excess boron can be more detrimental than salinity.


7.6 SALINITY MANAGEMENT PRACTICES 7.6.1 Irrigation Scheduling

Irrigation scheduling involves timing as well as the quantity of water to be applied for each irrigation and for the entire season. Scheduling of irrigations may require modification for saline waters.

7.6.1.1 Irrigation water requirement. As discussed in Section 7.3.4, the slope of the crop-water production function for saline water is the same as for nonsaline water, provided leaching water is not applied during the irrigation season. Leaching by irriga-tion is frequently postponed and accomplished following the crop growing season or from winter rainfall.

If the irrigation water is moderately saline and maximum yields can be maintained by leaching, the seasonal water requirement can be estimated from the crop-water pro-duction function and the leaching requirement. When salinity is expected to reduce crop productivity, the yield decrement can be estimated from the salinity response function (see Equation 7.4). The water requirement can be estimated from the water production function. A simplified approach combines the water production function as defined by Stewart et al. (1976) with the salinity response function to obtain: ETr = 1– (s/c) (ECe – t) (7.11) where ETr = relative evapotranspiration

t = threshold of the salinity response function s = slope of the salinity response function c = slope of the water production function.

7.6.1.2 Irrigation interval. As a soil dries between irrigations, both the osmotic and matric potentials decrease. The rate at which these processes occur depends on ET and on the water desorption properties of the soil. Shortening the interval between irrigations causes a higher pre-irrigation soil water content and a lower pre-irrigation salt concentration; both features enhance plant growth.

Intuitively, one expects this favorable effect to be greater for saline than for non-saline irrigation water. However, several processes occur which result in a smaller response to irrigation interval under saline than under nonsaline conditions. More fre-quent irrigations result in an upward shift in the depth of the peak salt concentration into regions of greater root concentration (Bernstein and Francois, 1973). In addition to salt loading from irrigating to satisfy crop water requirements, the salt load in the soil also increases because of the need to apply additional water to compensate for leaching. Under sprinkler irrigation, reducing the irrigation interval may result in more severe leaf damage because of the more frequent wetting of the foliage. More signifi-cant, however, is the fact that salinity reduces ET, resulting in a slower rate of soil drying. Thus, for the same irrigation interval the total pre-irrigation soil water poten-tial (matric plus osmotic) may be lower, causing a greater yield reduction under non-saline than under saline conditions (Figure 7.14). In this case, reducing the irrigation interval will be more beneficial under irrigation with nonsaline than saline water.

Field experiments in three different soils in two ecological regions of Israel tested the impact of irrigation interval and irrigation water salinities on eggplant (Shalhevet et al., 1983) and corn (Shalhevet et al., 1986). A unified function of relative yield ver-sus mean root zone salinity could be used for both crops for all irrigation intervals tested. The results for the corn experiment are presented in Figure 7.15 for irrigation intervals from 3.5 to 21 days.


Figure 7.14. Soil matric potential as a function of time following an irrigation for

waters having an electrical conductivity (adapted from Shalhevet et al., 1986).

Figure 7.15. Field experiment showing the lack of impact from irrigation interval on corn yields as a function of soil salinity created by different irrigation water qualities

(adapted from Shalhevet et al., 1986).


Most experimental results show either no difference in relative yield response or a lower response to more frequent irrigations with saline than with nonsaline water. No differences in response were shown by Ayers et al. (1943) with kidney beans, Bern-stein and Francois (1975) with beans, Greenway (1965) with citrus, and Hoffman et al. (1983a) with tall fescue. Lower response was shown by Ayoub (1977) with senna, Bernstein and Francois (1975) and Goldberg and Shimueli (1971) with pepper, and Wagenet et al. (1980) with barley. Only guayule (Wadleigh et al., 1946) showed an advantage of increasing irrigation frequency with saline compared with nonsaline wa-ter. The conclusion from this analysis is that recommendations given regarding irriga-tion frequency with nonsaline water are suitable for saline water irrigation as well. Most studies show no benefit of decreasing the irrigation interval under saline conditions.

7.6.2 Crop Considerations A few crop management options are available to counteract a possible reduction in

yield from salinity. These options include crop selection, seed placement, and planting density.

7.6.2.1 Crop selection. As described in Section 7.3.1, crops vary widely in their tolerance to saline conditions. Crops can be selected from Table 7.4 to withstand the restrictions imposed by the expected soil salinity. Some leaching is frequently pro-vided by irrigation inefficiency or excess expectations of crop water use. When sig-nificant leaching is required, especially when drainage or infiltration is a problem, it might be preferable to select more tolerant crops than to depend on leaching to control soil salinity. When the irrigation water is high in a potentially toxic constituent, such as chloride or boron, crops selected must be tolerant to these specific solutes.

When nonsaline water is available, along with saline water, more tolerant crops, which are generally of lower economic value, may be selected to be irrigated with the saline source while the nonsaline water is applied to more valuable salt-sensitive crops. Leaching of accumulated salts can be provided during the periods of nonsaline irrigation provided the leaching requirement is met (see Section 7.4.1).

7.6.2.2 Seed placement. The pattern of salt accumulation within the seed bed for furrow irrigation is critical to seed germination (Bernstein et al., 1955) Placing a single or double seed row on the shoulder of the bed close to the water line, rather than in the center of the bed, places the seed in an area of the lowest salt concentration (see Figure 7.16). When alternate-furrow irrigation is practiced, seeds should be placed in the bed center or on the side of the bed away from the dry furrow. A sloping bed configuration with the seeds planted on the sloping side is an alternative solution to the salt accumu-lation problem in furrow irrigation (Bernstein and Fireman, 1957).

Under drip irrigation, care must be exercised not to place seeds between last sea-son’s points of irrigation where salt accumulation may prevent germination. When precision seeding is not feasible, sprinkler irrigation should be used to leach salts out of the surface soil if off-season rainfall has been inadequate to provide the required leaching.

7.6.2.3 Planting density. As pointed out previously, the most distinct symptom of salinity damage to crops is stunted growth. This reduction in growth may be exploited to overcome yield reduction in some crops by increasing the number of plants per unit area (Francois, 1982). Increasing planting density should be done by decreasing inter-row spacing rather than increasing intra-row spacing of plants (Keren et al., 1983). To date, this idea has only been tested on cotton.


Figure 7.16. Typical salt accumulation patterns in ridge and bed cross-sections under furrow irrigation (Bernstein et al., 1955; Bernstein and Fireman, 1957).

7.6.3 Infiltration Infiltration refers to the entry of water into the soil. An infiltration rate of 3 mm/h

or less is considered low for many situations while a rate of more than 12 mm/h is relatively high. Infiltration can be affected by many factors other than water quality, including soil texture, the type of clay minerals, and exchangeable cations. Only infil-tration problems caused by chemical constituents are discussed here. Figure 7.3 illus-trates that both salinity and the sodium-adsorption ratio of the applied water influence infiltration.

Water with an electrical conductivity less than 0.5 dS/m and especially below 0.2 dS/m tends to leach surface soils free of soluble minerals and salts, especially calcium and magnesium. This process reduces the stabilizing influence of salts on soil aggre-gates and soil structure. Without salts, and in particular without calcium and magne-sium, the soil disperses, and the displaced finer soil particles fill the smaller soil pore spaces, sealing the surface and reducing infiltration. Soil crusting and seedling emer-gence problems often result. Rainfall is a very low-salinity water and irrigated areas frequently experience exceptionally low rates of infiltration from rainwater and sig-nificant amounts of surface ponding, runoff, and erosion can occur.

Management options to improve infiltration can be based on either chemical or physical changes. Chemical practices involve altering the soil or water chemistry that influences infiltration. This is normally accomplished by adding a chemical amend-ment, such as gypsum, or blending sources of water with different chemical composi-


tions to reduce the potential hazard. Physical practices include cultivation, deep till-age, and preserving crop residues.

Chemical amendments added to soil or water to improve infiltration increase the soluble calcium content and/or increase the salinity of the applied water significantly. Amendments will not help if the cause of inadequate infiltration is soil texture, soil compaction, restrictive soil layers, or a high water table. Most amendments supply calcium directly (i.e., gypsum) or indirectly by applying an acid or acid-forming sub-stance (i.e., sulphuric acid or sulphur) that reacts with lime (CaCO3) in the soil to re-lease calcium into the soil solution. If lime is not present, acids or acid-forming mate-rials will not be beneficial. Comparative data for the more common amendments are given in Table 7.10. Amendments are expensive, and they are only justified if their use, substantiated by field trials, results in a significant improvement. Water amend-ments are most effective if the infiltration problem is caused by a low-salinity water (ECi < 0.2 dS/m) or by high SAR in a water of low to moderate salinity (ECi < 1 dS/m). If water salinity is moderate to high (ECi

> 1 dS/m) and SAR is high, soil-applied amendments are often more effective.

Gypsum is the most commonly used and widely available amendment. In practice it is unusual to get more than 0.5 to 2 mol/m3 of calcium dissolved from gypsum into an irrigation water supply. These relatively small amounts of calcium in a low-salinity water, however, may increase infiltration by as much as 100% to 300%. If the water is relatively saline, these small amounts of calcium are far less effective and increase infiltration to a much smaller degree. The rate at which gypsum goes into solution is dependent upon its particle size. Finely ground gypsum (less than 0.25 mm in diame-ter) dissolves much more rapidly than coarse material. The coarsely ground, low-purity forms of gypsum are more satisfactory for application to the soil; given time for dissolution, low grades of gypsum have been applied successfully in the irrigation water.

Tillage to create a rough, yet thoroughly disturbed, soil surface is frequently an ef-fective practice for improving infiltration. Cultivation prior to an irrigation roughens the soil and opens cracks and air spaces that increase the surface area exposed for in-filtration. Deep tillage (chiseling, subsoiling) for many soils, particularly fine-textured soils, improves deep water penetration for only a few irrigations because the soil sur-

Table 7.10. Chemical properties of various amendments for reclaiming sodic soil.

Amendment Chemical

Composition Physical

Description

Solubility in Cold Water

(kg/m3)

Amount Equivalent to 1 kg of 100% Gypsum (kg)

Gypsum CaSO4.2H2O white mineral 2.4 1.0 Sulfur S8 yellow element 0 0.2 Sulfuric acid H2SO4 corrosive liquid Very High 0.6 Lime sulfur 9% Ca + 24% S yellow-brown

solution Very high 0.8

Calcium carbonate CaCO3 white mineral 0.014 0.6 Calcium chloride CaCl2.2H2O white salt 977 0.9 Ferrous sulfate FeSO4.7H2O blue-green salt 156 1.6 Pyrite FeS2 yellow-black mineral 0.005 0.5 Ferric sulfate Fe2(SO4).9H2O yellow-brown salt 4400 0.6 Aluminum sulfate Al2(SO4)3.18H2O corrosive granules 869 1.3


face quickly returns to its original condition. Although improvement is not permanent, deep tillage may temporarily allow sufficient water to enter to make an appreciable improvement in stored soil water and in crop yield. Deep tillage, done only when the soil is relatively dry to prevent compaction, physically tears, shatters, and rips the soil. It is done prior to planting or during dormancy of permanent crops.

The preservation or application of crop residue, livestock manure, and other or-ganic matter on the soil surface will improve infiltration and is a widely accepted prac-tice. The fibrous and less easily decomposed crop residues, such as from barley, rice, wheat, corn, and sorghum, have improved water penetration markedly, whereas resi-dues from legumes and vegetable crops have been less effective. Relatively large quantities of residues are needed; for instance, manure has been added at rates of 40 to 400 Mg/ha. An application of organic residues in the range of 10% to 30% by soil volume in the upper soil layer may be needed to be effective.

7.6.4 Reclamation of Salt-Affected Soils Soils may be naturally saline or suffer from salination because of irrigation mis-

management or inadequate drainage. Such soils require reclamation before irrigated agriculture can be profitable. The only proven method of reclaiming salt-affected soils is by leaching. Sodic soils normally require the addition of an amendment or tillage to promote the leaching process. Soils high in boron are particularly difficult to reclaim because of the tenacity by which boron is held in the soil. Leaching can be enhanced by growing very salt-tolerant plants during the reclamation process. Reclaiming saline soils by harvesting plants with the salts they have taken up is not practical. Likewise, flushing water over the soil surface to remove visible crusts of salt is not effective. Adequate drainage and suitable disposal of the leaching water are absolute prerequi-sites for reclamation.

7.6.4.1 Saline soils. The amount of water required for the removal of salts from a saline soil depends on the initial level of salinity, soil physical characteristics, tech-nique of applying water, and soil water content. Leaching by continuous flooding is the fastest method but it requires larger quantities of water than leaching by sprinkler or by intermittent flooding. Intermittent flooding or sprinkling, though slower, is more efficient and will require less water, particularly on finer-textured soils, than continu-ous ponding.

The relationship between the fraction of the initial salt concentration remaining in the soil profile, c/co, and the amount of water leaching though the profile per unit depth of soil, dL/dS, when water is ponded continuously on the soil surface can be de-scribed by (c/co) (dL/dS) = K (7.12)

where K is a constant that differs with soil type. Equation 7.12 defines the curves in Figure 7.17 for organic (peat) soils when K = 0.45, for fine-textured (clay loam) soils when K = 0.3, and for coarse-textured (sandy loam) soils when K = 0.1. Equation 7.12 is valid when dL/dS exceeds K. Figure 7.17 summarizes the results of nine field ex-periments (Hoffman, 1986).

The amount of water required for leaching soluble salts, particularly for fine-textured soils, can be reduced by intermittent applications of ponded water or by sprin-kling. The results of three leaching trials by intermittent ponding where no water table was present are summarized in Figure 7.18. Agreement among experiments is excel-


Figure 7.17. Depth of leaching water per unit depth of soil required to reclaim

a saline soil by continuous ponding (adapted from Hoffman, 1986).

Figure 7.18. Depth of leaching water per unit depth of soil required to reclaim a

saline soil by ponding water intermittently (adapted from Hoffman, 1986).


lent, considering the variety of soil textures and the depth of water applied each cycle (50 to 150 mm) with corresponding ponding intervals varying from weekly to monthly. The relationship between c/co and dL/dS for intermittent ponding can be ap-proximated by (c/ co) (dL/dS) = 0.1 (7.13) For fine-textured soils, intermittent ponding requires only about one-third as much water to remove about 70% of the soluble salts initially present, compared to continu-ous ponding.

Reclamation by flooding may be relatively inefficient when tile or open drains are present as most of the water flow will take place through the soil near the drains, while midway between the drains there will be far less leaching. The regions between drains should be leached by basins separated from the regions above the drains, or leaching should be done by sprinklers to improve efficiency (Luthin et al., 1969).

When a soil is both saline and sodic, the initial removal of salts may create sodic conditions and reduce permeability. The equivalent dilution method, which is based on the principle that divalent cations tend to replace monovalent cations on the exchange complex as a result of the dilutions of the soil solution, was proposed to overcome this problem (Reeve and Doering, 1966). By successive dilution of initially applied high-salt water, sodic soils can be reclaimed without a reduction in infiltration capacity.

7.6.4.2 Sodic soils. The reclamation of sodic soils is very difficult, if not impossi-ble, without the use of chemical amendments to replace exchangeable sodium by cal-cium. In some cases, the equivalent dilution method can be used to reclaim sodic soils. Properties of typical amendments are given in Table 7.10. In a calcarious soil, the ad-dition of acids or acid-forming materials may dissolve sufficient lime to provide suffi-cient exchangeable calcium. Gypsum, however, is the most common additive because of its low cost, good solubility, and availability.

The gypsum requirement to reclaim a sodic soil depends on the amount of ex-changeable sodium (Na) to be replaced by calcium. It can be calculated from (Keren and Miyamoto, 1990) GR = 0.86 dsDb (CEC) (ENai – ENaf) (7.14) where GR = gypsum requirement

dS = depth of soil to be reclaimed, m Db = soil bulk density, Mg/m3 CEC = cation exchange capacity of the soil, mol/kg ENai and ENaf = initial and desired final exchangeable sodium fractions.

According to this equation about 10 Mg/ha of gypsum will replace 30 mol/kg of Na to a depth of 0.3 m for a soil having a bulk density of 1.32 Mg/m3. Typically, a 1-cm depth of water per hectare will dissolve about 0.25 Mg of gypsum. Thus, a 40-cm depth of water will be required to reclaim the soil in the above example to a depth of 30 cm. It is difficult to reclaim a deep sodic soil profile in a single leaching operation. The usual procedure is to partially reclaim the soil during the first year, then plant a shallow-rooted crop and continue the reclamation process in the following years until the entire profile is reclaimed (Keren and Miyamoto, 1990).

7.6.4.3 Boron leaching. Like other salts, excess boron must be removed. More wa-ter is needed to leach boron than other salts because it is tightly adsorbed on soil parti-cles. Similar to leaching soluble salts, the relationship between c/co and dL/dS can be approximated by


(c/co) (dL/dS) = 0.6 (7.15)

In addition, periodic leachings may be required to remove additional boron released from the soil particles over time (Oster et al., 1984).

7.7 SUMMARY AND CONCLUSIONS The design and management of irrigation systems must be adequate to prevent

harmful accumulations of salt in the crop root zone. Timely irrigations must be of suf-ficient quantity and uniformity to meet the crop’s needs and leach salts adequately, without excessive surface runoff or deep percolation. All irrigation water contains salt; as evapotranspiration proceeds, the salt concentration increases in the remaining soil water. Without proper irrigation management, the land can become waterlogged and salinized. Regardless of management, drainage water from irrigated lands carries salt that requires appropriate disposal.

The response of crops to salinity, sodicity, and toxicity varies widely among plant species. The relationship between crop yield and soil salinity has been quantified for many crops under typical growing conditions. This relationship, however, depends on a number of soil, crop, and environmental factors. Sodicity typically reduces infiltra-tion, which leads to reduced crop yields. Crops can also be sensitive to specific sol-utes, such as chloride, sodium, and boron. With proper crop selection and appropriate irrigation management, economic yields are usually possible under low to moderate saline conditions.

Salt-affected soils can be reclaimed through leaching. Sodic soils normally require a chemical amendment that supplies calcium or magnesium to replace sodium. Copi-ous amounts of water are frequently required to reclaim a salt-affected soil.

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