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Salinity Management
Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman
Pages 131-145 (doi: 10.13031/ISM.2021.7) in Irrigation Systems Management. ,
Abstract. See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.
7.1 Introduction
Salinity is frequently a threat to irrigated agriculture. Have you ever wondered, What is the impact of salinity on crop production or the environment? How and where would you measure salinity? There are various types of salinity problems which can affect crops and soil in different ways. How can a producer best cope with the threats of excess salts? These questions are answered in the following sections.
All soils and irrigation waters contain salt. In humid areas—soils, surface waters, and groundwaters—are normally low in salinity. Salt concentrations are minimal because rainfall typically exceeds crop water requirements, which results in dilution of any salts in the soil. The excess water normally percolates through the soil flushing salts below the crop root zone. In dry climates potential evapotranspiration exceeds rainfall. Thus, small amounts of water percolate through the soil to remove salts. With time, the salt content of soils in arid regions may increase and crop yields decrease. When crop productivity is reduced by the presence of excess salt, the soil is said to be salt affected. Documented occurrences of salt-affected soils are illustrated in Figure 7.1. Estimates of the amount of irrigated land impacted by salination are given in Table 7.1 for the world and five selected countries. For detailed information on salinity management refer to Tanji (1990) and Wallender and Tanji (2012).
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| Figure 7.1. Salt-affected soils across the world (Reproduced from Wicke et al., 2011 with permission from the Royal Society of Chemistry.) |
| Table 7.1. Estimate of irrigated land damaged by salination during the mid-1980s for the top five irrigated countries and the world (adapted from Postel, 1990). | ||
| Country | Area Damaged(million ac) | Share of Irrigated Land Damaged (%) |
| India | 50 | 36 |
| China | 18 | 15 |
| United States | 13 | 27 |
| Pakistan | 8 | 20 |
| Former Soviet Union | 6 | 12 |
| World | 150 | 24 |
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| Figure 7.2. Two bands of salt precipitated out of solution along the top of furrow irrigation beds in California. |
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| Figure 7.3. Loss of cotton plants caused by excess salinity. |
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| Figure 7.4. Impact of increasing salinity (from left to right) on the size of a bunch of celery. |
The primary cause of increasing salt content in soils is evapotranspiration. As water is removed from the soil by plant roots or evaporates from the soil surface, salts are left behind. If salt concentrations become so high that they can no longer be held in solution, precipitation of salt occurs. Precipitation is the chemical process whereby dissolved salts change to their solid form. In the field, precipitated salts appear as a white to gray crust on the soil surface. Figure 7.2 shows salt precipitated on the soil surface in the Imperial Valley of California. Within the soil, nodules or layers of precipitated salts, called caliche, are found in some salt-affected soils. Within the crop root zone the salt concentration is controlled by the ability of the crop’s root system to extract water. This ability is associated with the salt tolerance of the crop.
The amount and types of salts in soils or waters determine the type of salt problem. The three types are salinity, sodicity, and toxicity. The most widespread problem, caused by the total concentration of dissolved salts, is referred to as salinity. The poor crop stand shown in Figure 7.3 is the result of excess salinity preventing cotton seeds from germinating or killing young seedlings in an Arizona cotton field. The bunches of celery in Figure 7.4 show the impact of salt on crop yield. Salinity, nearly zero on the left, increases progressively to high salt concentrations on the right that almost killed the plant. The impact of salinity on crop growth and yield is associated with osmotic stress, which is measured as osmotic potential (Chapter 2).
Sodium, present in excess, deteriorates the soil structure and inhibits movement of water into and through the soil. A soil affected by excess sodium is referred to as a sodic soil or, the outmoded term, alkaline soil. Figure 7.5 shows the effects of excess sodium in a corn field in Idaho. The white chunks are precipitated salts and the black areas are organic matter released when the excess sodium destroyed the soil structure. Some crops are sensitive to specific ions such as chloride, boron, sodium, and certain heavy metals in relatively low concentrations. Trees and other woody crops, in particular, are sensitive to specific ions. In these circumstances, excessive concentrations of specific ions are toxic. Figure 7.6 illustrates the toxic effects of three specific ions with considerable potential to damage or adversely affect plants: sodium (Na+), chloride (Cl-), and boron (B).
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| Figure 7.5. Effect of excess sodium in a corn field. Note the white colored salts and the black organic matter. |
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| Figure 7.6. Leaf burn on trees caused by specific ion toxicity; (a) indicates minimal leaf damage, (b) shows moderate damage, and (c) illustrates significant damage (adapted with permission from Tanji et al., 2007). |
The major impact of salinity is a reduction in the osmotic potential caused by the salt concentration of the soil water. As the osmotic potential of the soil water external to the plant decreases (becomes more negative), the difference between the water potential internal and external to the plant roots is reduced. The smaller the difference between internal and external water potentials the higher the degree of difficulty for the roots to extract water from the soil. This phenomenon is frequently noted as a reduction in the availability of water to the plant. As soil water becomes more saline, plants must use more energy to extract soil water. This utilization of energy means less energy for plant growth and the plant becomes smaller and yield is reduced.
7.2 Origin of Salt in Soils
Salt-affected soils are part of the geochemical processes that have continued since ancient geologic time. Soluble salts originate from the weathering of primary minerals in rocks forming the continents. The types of soluble salts depend on the composition of the weathered rock. Normally, salts move from sites of weathering into the groundwater system, eventually moving into streams and then into oceans. The present-day location of salts is dominated by the amount of water that has passed through each point of the hydrologic system. If rainfall is high, as in humid climates, most salts have been transported into oceans or to deep groundwater systems. In arid environments where rainfall is limited, salts are frequently still present in the soil.
Salts accumulate in landscapes having certain relief and geologic conditions. Salt moves with water; thus, saline conditions are linked to lowlands or depressions where water naturally drains and accumulates. Often this situation is associated with restricted internal drainage of the soil, which is conducive to high water table conditions. Salts frequently accumulate in these low areas. Low-lying lands may be relatively small areas in fields or they may be as large as the Great Basin of Utah and Nevada. Drainage water collects at some terminus in closed basins and evaporates. Water in these terminals increases in salt content and, eventually, may lose biological value and become less attractive for recreation.
In addition to weathering, secondary deposits are a major source of saline soils. Throughout geologic history, large portions of the continents have been covered by saline seas. Marine sediments deposited during extended periods of inundation serve as parent material for large areas now devoted to agriculture. These secondary deposits include shales, sandstones, mudstones, and conglomerates. Saline marine shales, for example, are notorious sources of salt. A prime example is the Mancos shale formation that occurs extensively in the upper Colorado River Basin of Colorado, Wyoming, and Utah.
When new lands are developed and brought under irrigation, soils that are prone to salt accumulation are frequently very saline. Before crop production is economically feasible, these salt-affected soils must be reclaimed. The reclamation process, whether it be for saline, sodic, or toxic soils, requires copious amounts of nonsaline water to flush the salts from the intended crop root zone. Frequently, man-made drainage systems are required to augment natural drainage to remove the extra water applied to flush salts from the soil.
Once irrigated lands are in production, the primary source of salt is the irrigation water. The salt introduced into the crop root zone by irrigation is additive to any salt already present in the soil.
7.3 Measurement of Salinity
Electrical conductivity (EC) is used frequently to estimate the salt concentration of solutions. This method is based upon the fact that salts dissociate into charged ions in water and can conduct an electric current. As the concentration of salts increases, the capacity of the solution to conduct an electrical current, called electrical conductivity, increases. Electrical conductivity is expressed in units of Siemens per meter (S/m). For most natural systems, the EC unit of dS/m (10-1 S/m) is convenient and is equal numerically to millimhos/cm, an outmoded unit. The approximate relationship between osmotic potential (?o, bars) and electrical conductivity (EC, dS/m) is:
?o = 0.36 EC (7.1)
The relationship between salt concentration (C) in units of mg/L and EC is approximated by:
C = 640 EC (7.2)
It is important to remember that electrical conductivity is sensitive to the temperature of the solution. Between the temperatures of 15° and 35°C, a one degree increase in temperature increases EC by about 2%. A solution at 35°C that measures an EC of 5 dS/m will have an EC of 4 dS/m if the temperature of the solution is decreased to 25°C. For consistency, ECs are normally reported at a temperature of 25°C.
Ideally, soil salinity should be measured at the soil water content found in the field. This is not easily done although several methods are now available that operate at field water contents. The most common method of determining soil salinity is by extracting a solution from a soil sample that has been saturated. The procedure begins by taking a soil sample in the field. The sample is brought to saturation in a laboratory by adding distilled water and then a sample of the saturated soil solution is extracted by vacuum filtration. The electrical conductivity of this saturated extract (ECe) is then measured.
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| Figure 7.7. Relative grain yield of corn grown in the Sacramento-San Joaquin Delta of California as a function of soil salinity (adapted from Hoffman et al., 1983). |
7.4 Crop Salt Tolerance
The salt tolerance of a plant is defined as the plant’s capacity to endure the effects of salt. Crop salt tolerance is not an exact value because it depends on many factors. Although salt tolerance cannot be stated in absolute terms, relative crop response to known salt concentrations under typical conditions can be predicted. For a more complete reference on crop salt tolerance see Maas and Hoffman (1977).
The salt tolerance of a crop can be described by plotting relative crop yield as a continuous function of soil salinity (Figure 7.7). For most crops, this response function follows a sigmoidal relationship where crop yield is not reduced significantly as salinity initially begins to increase but, as salinity increases further, yield is reduced rather rapidly. Then, as salinity reaches high levels, crop yields, although low, do not decrease as rapidly as at moderate concentrations. For practical purposes this sigmoidal relationship for crop salt tolerance can be represented by two straight lines, one line is a tolerance plateau with a slope of zero and the other line is concentration dependent and its slope indicates the yield reduction per unit increase in salinity.
Figure 7.7 shows the “two straight lines” model fitted to actual field data for corn grain yield. The point at which the two straight lines intersect designates the salt tolerance threshold which is the maximum soil salinity that does not reduce yield appreciably below that achieved under nonsaline conditions. For soil salinities exceeding the threshold, relative yield (Yr) in percent can be estimated from:
Yr = 100 – S(ECe – T) for ECe > T (7.3)
where: T = salt tolerance threshold expressed in ECe units of dS/m,
S = slope expressed in % per dS/m, and
ECe = the mean salt concentration in units of electrical conductivity of saturated soil extracts taken from the crop root zone.
The threshold and slope values provide general guidelines about salt tolerance for crop management decisions. Irrigators need to know the level of soil salinity that initiates yield reduction (T, threshold) and the rate at which yield is reduced at salt levels greater than the threshold (S, slope).
Typical ears of corn from the experimental results plotted in Figure 7.7 are shown in Figure 7.8. The top row of ears were grown using nonsaline irrigation water; the bottom row with irrigation water having an EC of 8 dS/m.
| Table 7.2. Salt tolerance of major crops (adapted from Maas and Hoffman, 1977). | |||
| Crop | Salt Tolerance Threshold, T (dS/m) | Percent Yield Decline, S%/(dS/m) | Qualitative Salt Tolerance Rating[a] |
| Grain Crops | |||
| Barley | 8.0 | 5.0 | t |
| Corn | 1.7 | 12 | ms |
| Cowpea | 4.9 | 12 | mt |
| Rice | 3.0 | 12 | s |
| Sorghum | 6.8 | 16 | mt |
| Soybean | 5.0 | 20 | mt |
| Wheat | 6.0 | 7.1 | mt |
| Fiber, Sugar and Oil Crops | |||
| Cotton | 7.7 | 5.2 | t |
| Flax | 1.7 | 12 | ms |
| Peanut | 3.2 | 29 | ms |
| Sugar beet | 7.0 | 5.9 | t |
| Sugar cane | 1.7 | 5.9 | ms |
| Grasses and Forage Crops | |||
| Alfalfa | 2.0 | 7.3 | ms |
| Bermuda grass | 6.9 | 6.4 | t |
| Clover | 1.5 | 12 | ms |
| Fescue | 3.9 | 5.3 | mt |
| Orchard grass | 1.5 | 6.2 | ms |
| Ryegrass | 5.6 | 7.6 | mt |
| Trefoil, birdsfoot | 5.0 | 10 | mt |
| Vegetables and Fruit Crops | |||
| Asparagus | 4.1 | 2.0 | t |
| Bean | 1.0 | 19 | s |
| Cabbage | 1.8 | 9.7 | ms |
| Carrot | 1.0 | 14 | s |
| Celery | 1.8 | 6.2 | ms |
| Corn, sweet | 1.7 | 12 | ms |
| Lettuce | 1.3 | 13 | ms |
| Potato | 1.7 | 12 | ms |
| Strawberry | 1.0 | 33 | s |
| Sweet potato | 1.5 | 11 | ms |
| Tomato | 2.5 | 9.9 | ms |
| Woody Crops | |||
| Almond | 1.5 | 19 | s |
| Apricot | 1.6 | 24 | s |
| Blackberry | 1.5 | 22 | s |
| Date palm | 4.0 | 3.6 | t |
| Grape | 1.5 | 9.6 | ms |
| Grapefruit | 1.8 | 16 | s |
| Guayule | 15 | 13 | t |
| Orange | 1.7 | 16 | s |
| Peach | 1.7 | 21 | s |
| Plum | 2.6 | 31 | s |
| [a] s = sensitive ms = moderately sensitive t = tolerant mt = moderately tolerant | |||
Crops differ significantly in tolerance to soil salinity. The relative salt tolerances of major crops are given in Table 7.2. The table gives the salt tolerance threshold (T) and the percent yield decline (S). These two values can be inserted into the salt tolerance equation (Equation 7.3) to predict relative crop yield (Yr). Qualitative ratings for ease in comparisons among crops are also given in Table 7.2. The qualitative salt tolerance ratings are sensitive (s), moderately sensitive (ms), moderately tolerant (mt), and tolerant (t). These qualitative ratings can be seen in Figure 7.9.
| Table 7.3. Classification guide for saline irrigation water. | |||
| Irrigation Water | Salt Concentration (ppm or mg/L) | Electrical Conductivity (dS/m) | Crop Problems |
| Fresh | < 300 | < 0.5 | none |
| Slightly saline | 300–600 | 0.5–1 | rare |
| Moderately saline | 600–1,200 | 1–2 | occasional |
| Saline | 1,200–2,400 | 2–4 | common |
| Highly saline | 2,400–4,800 | 4–7 | severe |
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| Figure 7.8. Example ears of corn produced with irrigation water having no salt (top) and with salt concentrations equal to one-fourth the salt concentrations of sea water (bottom). |
A handy guide to classify potential crop damage from increasing salt levels in irrigation waters is given in Table 7.3. The reader is cautioned, however, that the
use of saline water depends upon the crop, soil, climate, geology, and management practices. Thus, this classification is only a rough guide.
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| Figure 7.9. Division boundaries for qualitative salt tolerance ratings of crops (adapted from Maas and Hoffman, 1977). |
7.5 Sodicity
If sodium is the predominate cation adsorbed in the soil, the clay particles in the soil swell and soil aggregates disperse. This deterioration leads to reduced penetration of water into and through the soil. When calcium and magnesium are the predominate cations, the soil tends to have a granular structure that is easily tilled and readily permeable. Excess sodium becomes a concern when the rate of infiltration is reduced to the point that the crop cannot be adequately 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 increased potential for disease, weeds, soil erosion, lack of oxygen, and inadequate nutrient availability (Hoffman and Shalhevet, 2007).
To assess the sodium hazard of irrigation water, the sodium absorption ratio (SAR) is normally calculated. SAR is defined as:
(7.4)
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| Figure 7.10. Division of waters that cause inadequate water penetration because of chemical conditions (adapted from Rhoades, 1982). |
where ion concentrations (C) are in units of moles of charge per m3 (molc/m3) for sodium (Na), calcium (Ca), and magnesium (Mg). Equation 7.4 is valid for soil water under steady-state conditions where the SAR of the irrigation water approximates the SAR of the soil water. The SAR for the soil water under nonsteady-state conditions needs to be adjusted. Figure 7.10 can be used to determine whether an irrigation water will lead to a sodicity problem. If the relationship between the SAR of the irrigation water and its salinity results in a point to the left in Figure 7.10, a sodicity hazard is likely to occur. If the point is between the two lines, a slight to moderate sodicity hazard is likely.
Ionic concentrations are sometimes reported in units of milliequivalents per liter (meq/L). The relationship between the two units frequently used to report ionic concentrations is:
(7.5)
where the valence of the ion can be one or more. Recall from chemistry that the valence of sodium is positive one and the valence of calcium and magnesium is positive two.
7.6 Toxicity
Toxicity occurs as the result of the uptake and accumulation of certain ions within plant tissue. The toxicity of any ion is highly dependent upon the crop. Specific ions that may be toxic include boron, chloride, and sodium. Some ions, like chloride, can be absorbed directly into the leaves when moistened during sprinkler irrigation. Foliar damage from sprinkling is particularly acute during periods of high temperature and low humidity. Many trace elements, such as cadmium and lithium, are also toxic to plants at very low concentrations. Suggested maximum concentrations for many trace elements are given by Pratt (1973). Fortunately, most irrigation supplies contain insignificant concentrations of these potentially toxic trace elements and are generally not a problem.
7.7 Leaching
Salinity in the crop root zone can be controlled if the quality of the irrigation water is satisfactory and the flow of water through the soil is sufficient. Leaching, the net downward movement of soil water and solutes, is the key to successful irrigation where salts are a hazard. As the salinity of the irrigation water increases or if more salt sensitive crops are grown, leaching must be increased to maintain high crop yields. This chapter presents general guidelines for leaching that can be applied to various types of irrigation systems; guidelines specific to drip irrigation are presented in Hanson and May (2011).
The simplest general expression describing the actual amount of leaching is:
(7.6a)
dt= dz + dr (7.6b)
where: Lf = actual leaching fraction,
dp= depth of water draining below the crop root zone (deep percolation),
dt= total depth of infiltrated water,
Ca = weighted mean salt concentration of the applied water,
Cd = salt concentration of the draining water,
dz = mean depth of infiltrated irriga- tion, and
dr = depth of infiltrated rainfall.
The weighted mean salt concentration of the applied water can be calculated from:
(7.7)
where: Ci = concentration of irrigation water
Cr = concentration of rain water
The salt concentration of rainfall is so low that it is considered to be zero. Thus, the term Cr dr in Equation 7.7 is zero.
The leaching requirement (Lr) is the minimum leaching fraction that will prevent a reduction in crop yield. The Lr can be derived from Equation 7.6 as:
(7.8)
in which the superscript * distinguishes required from actual values. Because electrical conductivity (EC) is easily measured and is almost linearly related to the salt concentration of a relatively dilute salt solution, it is customary to substitute EC for C in these relationships.
Several mathematical models have been proposed to relate Lr to some readily available value of soil salinity that is indicative of the crop’s leaching requirement. One such model is represented graphically in Figure 7.11. This graphical solution relates the salinity of the applied water, crop salt tolerance threshold, and leaching requirement.
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| Figure 7.11. Graphical solution for the 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 salt tolerance of many annual crops increases as the growing season progresses. This suggests that if soil salinity levels are low enough at the beginning of the season and adequate amounts of low salt water are applied, soil salinity can be permitted to increase gradually during the irrigation season. For the next crop, rainfall, either singly or in combination with dormant season or pre-plant irrigations, can replenish soil water and leach accumulated salts to permit irrigation the next season without the need for further leaching. An important exception to this procedure is perennial crops, like trees, that form their buds for the next year during the latter half of the irrigation season. High salinity levels during bud formation will be detrimental to fruit production the following season.
If irrigation waters are saline, rainfall and out of season leaching may not be sufficient and leaching during the irrigation season will be required to prevent yield reduction. The key factor to remember is that leaching is not required until accumulated soil salinity surpasses the salt tolerance threshold for the crop. Leaching can be done each irrigation or less frequently, such as seasonally or at even longer periods, provided soil salinity is maintained below the salt tolerance threshold if yield losses are to be avoided.
Some irrigation systems are managed to apply copious amounts of water. Thus, in many cases, this excess amount of irrigation supplies water for leaching without a conscious effort by the irrigator. In some situations, the nonuniform applications of the irrigation system result in some areas of the field receiving water in excess of the crop water and leaching requirements, while underirrigated areas cause water and salt stress. This problem is best solved by an irrigation system that is more uniform in water application rather than applying more water to compensate for nonuniformity.
The leaching requirement model presented here assumes steady state conditions. In reality, steady state never occurs in the field. Several complex computer models have been developed which account for transient conditions that more closely represent field conditions (Minhas et al., 2020). These transient models predict that steady state models overestimate the leaching requirement (Letey et al., 2011; Corwin and Grattan, 2018). Unfortunately, these models require huge data sets, are not readily available to irrigators, and do not directly predict the leaching requirement. Nevertheless, the irrigator should be aware that the leaching requirements given in Figure 7.11 overestimate the amount of saline drainage water entering the environment.
7.8 Reclamation
Reclamation of salt-affected soils is frequently required when semiarid or arid lands are first brought into agricultural production; when saline groundwater persists near the soil surface; or when irrigation and rainfall have failed to meet the leaching requirement. The only proven method of reclaiming salt-affected soils is the leaching of accumulated salts down below the crop root zone. For reclaiming sodic soils, an amendment or deep tillage may be required before leaching is effective. Soils excessively high in boron are particularly difficult to reclaim because of the tenacity by which boron is held in the soil.
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| Figure 7.12. Depth of leaching water per unit depth of soil required to reclaim a saline soil by continuous ponding (adapted from Hoffman, 1986). |
Adequate drainage is essential for reclamation. Natural internal drainage alone may be adequate, provided there is storage capacity in the profile for salt below the root zone or a permeable subsurface layer is present that drains to a suitable outlet. Where such natural drainage is lacking, an artificial system must be provided or reclamation will not be feasible.
7.8.1 Saline Soils
The amount of water that must leach through the soil profile to remove soluble salts depends primarily on the initial soil salinity level, the technique of applying water, and the soil type. Water suitable for irrigation is normally suitable for reclamation. The relationship between the fraction of the initial salt concentration (Co) remaining in the soil profile (Cf/Co) and the amount of water leached through the profile (dL) per depth of soil (dS) to be leached (dL/dS) when water is ponded continuously on the soil surface can be described by:
(7.9)
where K is a constant that differs with soil type. Equation 7.9 defines the curves in Figure 7.12 for organic (peat) soils where K = 0.45, for fine-textured (clay loam) soils where K = 0.3, and for coarse-textured (sandy loam) soils where K = 0.1. The initial offset at the top of each curve in Figure 7.12 is indicative of the amount of water that must be added to the profile before leaching commences.
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 sprinkling. The differences in leaching efficiency among the leaching methods (continuous ponding versus intermittent ponding or sprinkling to prevent water ponding on the soil surface) are caused by differences in dispersion and diffusion. The concept of soil pores is useful in visualizing these differences. The amount of solution retained in small soil pores is considerable for saturated soils, as for continuous ponding, and decreases with decreasing soil water content. Consequently, the drier the soil, as with intermittent ponding or sprinkling, the larger the fraction of water flowing through fine pores and the more efficiently the leaching water displaces the saline solution. The reclamation equation for intermittent ponding and sprinkling can be written as:
(7.10)
By intermittent ponding or sprinkling, the effect of soil type is minimal. One disadvantage of intermittent ponding or sprinkling is that the period of time required for reclamation may be extended beyond that required by continuous ponding.
7.8.2 Sodic Soils
The reclamation of sodic soil usually requires that water penetration into and through the soil be improved by either exchanging excess sodium in the soil with calcium, so that leaching can proceed or by initially leaching with saline water, and then by progressively decreasing the salinity of the applied water. If the choice is to replace sodium with calcium, then an amendment must be applied that either contains soluble calcium or dissolves calcium already present in the soil. Examples of amendments that contain calcium are gypsum, lime, and calcium chloride. Sulfur, sulfuric acid, and pyrite are examples of amendments that will react and dissolve calcium present in the soil. Occasionally, calcium present in the subsoil can be mixed with a shallow sodic layer by deep tillage, thus, eliminating or reducing the need for an amendment.
Successive dilutions of a high salt water containing calcium can be an effective method of reclaiming sodic soil. The basic requirement is an adequate supply of a high saline water and a low salinity water. After initially applying the highly saline water, this water is diluted in steps with the low salinity water until only the low salinity water is applied and the reclamation process is complete.
Tillage to create a rough, yet thoroughly disturbed, soil surface is a common practice for improving water infiltration. Typically, a sodic soil is tilled prior to each intermittent water application during reclamation.
7.9 Salinity and the Environment
Irrigation always degrades water quality and can cause a salinity hazard. Without proper management, the land can become waterlogged and salinized. Regardless of management, drainage water from irrigated lands carries salt that requires disposal. Questions arise as to whether salination is inevitable and if the environment is jeopardized.
Where salinity is a hazard, irrigation must have drainage. A net downward movement must occur through the soil profile to prevent the accumulation of soluble salts to a level detrimental to crops. Whether drainage is natural or man-made, the rate of movement of soil water must be sufficient to prevent salination. This drainage water must go somewhere. Depending on the geologic and hydrologic conditions, the need for drainage may become evident after only a few irrigations or after many decades.
Permanent irrigated agriculture frequently requires the sacrifice of some value elsewhere. An example is the Colorado River in the southwest corner of the United States. Lohman et.al. (1988) estimated damages from salinity for the period of 1976 to 1985 to be $311 million per year when based on a reference salinity of 500 mg/L, the Public Health Service standard for drinking water. Damages occurred to agriculture, households, water utilities, and industry. Of the figure quoted, $113 million reflect damages to agriculture.
Ultimately, saline drainage water must be transported out of the region, disposed of locally, or treated. It is technically feasible to treat saline water. Several desalination studies have evaluated reverse osmosis. The world’s largest desalination plant was constructed near Yuma, Arizona, to remove salt from irrigation drainage water before it returns to the Colorado River. However, it’s difficult to justify such an approach economically (van Schilfgaarde, 1982). An alternative to treating the water is to convey it to evaporation ponds. Experience in California indicates that 10 to 14% of the land area must be devoted to evaporation ponds. Loss of land, construction costs, and avoidance of leakage makes this alternative unattractive. Transporting saline water out of the region remains the primary means of disposal using natural or man-made water courses.
7.10 Summary
In regions where rainfall is not adequate to leach salts from the soil, water must be managed to avoid crop losses from excess salinity. Crops differ by nearly a factor of 10 in their sensitivity to salinity. With appropriate management to provide drainage and ensure downward movement of soil water through the crop root zone, crop productivity can be maintained even if salinity is a hazard.
The amount of water that must leach below the root zone to prevent yield loss depends on the salt content of the irrigation water and the salt tolerance of the crop. If both the soil and the irrigation water are low in salt concentration, no leaching may be required for several years, particularly if rainfall is significant.
Soils high in salinity can be made productive by applying copious amounts of water to leach the salt and reclaim the soil. The amount of water required to reclaim a saline soil depends on the soil type, the depth of soil to be reclaimed, and the method of applying the water.
Where salinity is a problem, salts must be flushed from the soil. The disposal of this salt can be detrimental to the receiving body of water, whether surface water or groundwater.
Questions
1. Describe saline and sodic soils.
2. Why do large imbalances occur in the distribution of salts in soils?
3. A saturated soil extract has an electrical conductivity of 5 dS/m at 20°C; what value of electrical conductivity should be reported and used in calculations?
4. What is the specific meaning of ECe?
6. Calculate the yield reduction expected for sorghum produced on a soil having a salt concentration of 2,500 mg/L.
6. At a salt concentration of applied water of 4,000 mg/L, what would the Lr be for sorghum?
7. In Example 7.5, what would the leaching requirement for tomato be if rainfall were 10 inches rather than 4 inches?
8. Define sodium absorption ratio.
9. Distinguish between leaching fraction and leaching requirement. Discuss the field conditions that would exist when Lr is less than Lf, and when Lr is greater than Lf.
10. Explain the concept shown in Figure 7.11.
11. What benefits are derived from intermittent soil drying in a salt reclamation project?
12. For a clay loam soil how much water must be applied before any significant reclamation will occur if a soil depth of 3 ft is to be reclaimed by continuously ponding water on the soil surface?
13. How much water would be needed to reclaim the field in Example 7.6 if the soil was a sandy loam and the water was applied by sprinkling to prevent surface ponding?
14. Under what conditions would deep plowing aid in the reclamation of sodic soils?
15. If you had access to a large quantity of lime sulfur (9% Ca + 24% S), would it be useful to reclaim sodic soil? If useful, why?
References
Corwin, D. L., & Grattan, S. R. (2018). Are existing irrigation salinity leaching requirement guidelines overly conservative or obsolete? J. Irrig. Drain. Eng., 144(8), 02518001.
Hanson, B., & May, D. (2011). Drip irrigation salinity management for row crops. Agriculture and Natural Resources (ANR) Publ. 8447. University of California, Davis.
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