Saline and Alkali Soils

C. A. Bower and Milton Fireman.

Saline and alkali conditions lower the productivity and value of large areas of agricultural land in the United States an estimated one-fourth of our 29 million acres of irrigated land and less extensive acreages of nonirrigated crop and pasture lands.

Saline and alkali soils are soils that have been harmed by soluble salts, consisting mainly of sodium, calcium, magnesium, chloride, and sulfate and secondarily of potassium, bicarbonate, carbonate, nitrate, and boron.

Salt-affected soils are problem soils that require special remedial measures and management practices.

Soluble salts may harm soils by increasing the salt concentration of the soil solution and by increasing the percentage saturation of the soil adsorption complex with sodium.

The second effect occurs when sodium salts predominate. It is more permanent than the first because adsorbed sodium usually persists after most of the soluble salts are removed.

Saline soils contain excessive amounts of soluble salts only. Alkali soils contain excessive adsorbed sodium. Because leaching may have occurred previously, alkali soils do not always contain excess soluble salt. They are designated as nonsaline-alkali or saline-alkali soils according to their content of salts.

Salt-affected soils occur mostly in regions of an and or a semiarid climate.

Under humid conditions, the soluble salts originally present in soil materials and those formed by the weathering of minerals generally are carried downward into the ground water and are transported ultimately by streams to the oceans.

In and regions, leaching and transportation of salts to the oceans is not so complete as in humid regions. Leaching is usually local in nature, and soluble salts may not be transported far. This occurs because there is less rainfall available to leach and transport the salts and because the high evaporation and plant transpiration rates in and climates tend further to concentrate the salts in soils and surface waters.

Weathering of primary minerals is the indirect source of nearly all soluble salts, but there may be a few instances in which enough salts have accumulated from this source alone to form a saline soil. Saline soils usually occur in places that receive salts from other locations; water is the main carrier.

RESTRICTED DRAINAGE usually contributes to the salinization of soils and may involve low permeability of the soil or the presence of a high groundwater table.

High ground-water tables often are related to topographic position. The drainage of waters from the higher lands of valleys and basins may raise the ground-water level near to the soil surface on lower lands. Low permeability of the soil causes poor drainage by impeding the downward movement of water. The impedance may be the result of an unfavorable soil texture or structure or the presence of hardened layers, called hardpan.

Salt-affected soils occur extensively under natural conditions, but the salt problem of greatest importance in agriculture arises when previously productive soil becomes salt-affected as a result of irrigation.

Irrigated lands are often located in valleys near streams; because they can be irrigated easily, the lower and more level soils usually are selected for cultivation. Such soils may be adequately drained and nonsaline under natural conditions, but the drainage facilities may not be adequate under irrigation. Irrigation waters may contain from 200 pounds to as much as 5 tons of salt per acre-foot, and the annual application of water may amount to 5 acre-feet or more an acre. Considerable quantities of soluble salts thus may be added to irrigated soils in a short time.

Farmers who bring new lands under irrigation often have failed to recognize the need for establishing artificial drains to care for the additional water and the leaching required to prevent the accumulation of soluble salts. As a result, the water table may rise from a considerable depth to within a few feet of the soil surface in a few years.

During the early development of irrigation projects, water is frequently plentiful, and there is a tendency to use it in excess. This hastens the rise of the water table. When the water table rises to within 5 or 6 feet of the surface, ground water containing more or less dissolved salt moves upward into the root zone and to the soil surface. Ground water, as well as irrigation water, then causes the soil to become saline.

ALKALI SOILS CONTAIN excessive amounts of adsorbed sodium.

Because of the presence of negative electrical charges at their surfaces, soil particles adsorb and retain cations, such as calcium, magnesium, and sodium. While the adsorbed cations are combined chemically with the soil particles, they may be replaced or exchanged by other cations that are added to the soil solution. Each soil has a reasonably definite capacity to adsorb and exchange cations, and the percentage of this capacity that is taken up by sodium is referred to as the exchangeable-sodium-percentage. The exchangeable-sodium-percentage of alkali soils is usually 15 or more.

As cations adsorbed on soil particles can interchange freely with those in the soil solution, the proportions of the various adsorbed cations are related to their concentrations in the soil solution. Calcium and magnesium are the principal cations in the soil solution and on the particles of normal, productive soils of and regions. When normal soils come in contact with irrigation or drainage waters containing a high proportion of sodium, this cation becomes the dominant one in the soil solution and replaces part of the original adsorbed calcium and magnesium. As a consequence of the adsorption of sodium, alkali soils are formed.

THE ACCUMULATION of soluble salts and adsorbed sodium by soils impairs their productivity in several ways.

Because of the presence of considerable dissolved salt and the absence of significant amounts of adsorbed sodium, saline soils generally are flocculated. Their tillage properties and permeability to water therefore are equal to or higher than those of similar non-saline soils. The abnormally high salt concentration of the soil solution of saline soils, however, reduces the rate at which plants absorb water; consequently growth is retarded. The retardation of growth is almost directly related to the total salt concentration of the soil solution and is largely independent of the kind of salts present.

The salinity status of soils is appraised in terms of effects on crop growth by measuring the electrical conductivity of the solution extracted from saturated soil paste. The electrical conductivity of a solution is a good measure of its total salt concentration, and the water content of saturated soil is related to the field-moisture range. Thus the electrical conductivity of the saturation extract is directly related to the total salt concentration of the soil solution under field conditions.

The effects of salinity on growth are largely negligible when the electrical conductivity reading (expressed in millimhos per centimeter) is less than 2. At readings in excess of about 16, only a few very salt-tolerant crops yield satisfactorily. The yields of very salt-sensitive crops may be restricted at readings as low as 2; moderately salt-tolerant crops grow satisfactorily below readings of 8; only salt-tolerant crops grow satisfactorily when readings range between 8 and 16.

While the primary effect of soil salinity on crops is one of retarding growth by limiting the uptake of water, certain salt constituents are specifically toxic to some crops. Boron, for example, when present in the soil solution at concentrations of only a few parts per million, is highly toxic to many crops.

Alkali soils remain flocculated and their properties usually are similar to those of saline soils as long as considerable amounts of soluble salts are present. If the excess salts are removed by leaching, however, saline-alkali soils generally become nonsaline-alkali soils, and their physical properties deteriorate markedly.

As the concentration of the salts in the soil solution is lowered by leaching, the adsorbed sodium present causes undesirable characteristics to develop. The soil may become strongly alkaline (pH readings above 8.5), the particles may disperse, and the soil may become unfavorable for the entry and movement of water and air and for tillage. Adsorbed sodium also may be toxic and cause various nutritional disturbances in plants.

There are two principal aspects of the salt problem in irrigation agriculture. One is the improvement (reclamation) of soils that are salt-affected under natural conditions or have become salt-affected because of mismanagement. The other aspect is the management of productive or slightly salt-affected soils so as to prevent increases in the soluble salt and adsorbed sodium contents and thus prevent reduction in crop yields.

SALINE SOILS are improved by establishing artificial drains if a high ground-water table exists and by subsequent leaching with irrigation water to remove excess soluble salts. The improvement of alkali soils involves (besides drainage and leaching) the replacement of adsorbed sodium by calcium or magnesium and the use of practices that develop good soil structure.

Adequate drainage is essential for the permanent improvement of saline and alkali soils. Leaching operations and the application of amendments for the replacement of adsorbed sodium will be largely ineffective unless the ground-water table remains deep enough to prevent appreciable upward movement of water.

The permissible depth to the water table in various types of soils under irrigation and drainage requirements and methods are discussed in the chapter on soil drainage.

SOILS CAN BE LEACHED by applying water to the surface and allowing it to pass downward through the root zone. Leaching is most efficient when it is possible to pond water over the entire soil surface.

Water can be ponded on nearly level land in shallow basins formed by the construction of earthen dikes or borders 2 to 4 feet high. The dimensions of individual basins depend on the slope of the land. Normally the difference in elevation at the high and low points of the basin should not exceed 6 inches. It is wise to construct dikes on the contour where possible, especially if the land slopes very much. Overflow gates or spillways placed in dikes between adjacent ponded areas facilitate the control of water and allow a number of basins to be kept full simultaneously.