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Saline and Alkali Soils Are Soils That Saline Soil

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Saline and Alkali Soils Are Soils That Saline Soil 282 YEARBOOK OF AGRICULTURE 1957 gions of an arid or a scmiarid climate. Under humid conditions, the soluble salts originally present in soil materials and those formed by the weathering of Saline and minerals generally are carried down- ward into the ground water and are transported ultimately by streams to Alkali Soils the oceans. In arid regions, leaching and trans- C. A. Bower and Milton Fireman portation of salts to the oceans is not so complete as in humid regions. Leach- Saline and alkali conditions ing is usually local in nature, and solu- ble salts may not be transported far. lower the productivity and This occurs because there is less rain- value of large areas of agri- fall available to leach and transport the salts and because the high evapo- cultural land in the United ration and plant transpiration rates States—an estimated one- in arid climates tend further to con- centrate the salts in soils and surface fourth of our 29 million acres waters. of irrigated land and less ex- Weathering of primary minerals is the indirect source of nearly all soluble tensive acreages of nonirrigated salts, but there may be a few instances crop and pasture lands. in which enough salts have accumu- lated from this source alone to form a Saline and alkali soils are soils that saline soil. Saline soils usually occur in have been harmed by soluble salts, places that receive salts from other consisting mainly of sodium, calcium, locations; water is the main carrier. magnesium, chloride, and sulfate and secondarily of potassium, bicarbonate, RESTRICTED DRAINAGE usually con- carbonate, nitrate, and boron. tributes to the salinization of soils and Salt-aifccted soils are problem soils may involve low permeability of the that require special remedial measures soil or the presence of a high ground- and management practices. water table. Soluble salts may harm soils by in- High ground-water tables often are creasing the salt concentration of the related to topographic position. The soil solution and by increasing the per- drainage of waters from the higher centage saturation of the soil adsorp- lands of valleys and basins may raise tion complex with sodium. the ground-water level near to the soil The second effect occurs when so- surface on lower lands. Low perme- dium salts predominate. It is more per- ability of the soil causes poor drainage manent than the first because adsorbed by impeding the downward movement sodium usually persists after most of of water. The impedance may be the the soluble salts are removed. result of an unfavorable soil texture or Saline soils contain excessive amounts structure or the presence of hardened of soluble salts only. Alkali soils con- layers, called hardpan. tain excessive adsorbed sodium. Be- Salt-affected soils occur extensively cause leaching may have occurred pre- under natural conditions, but the salt viously, alkali soils do not always con- problem of greatest importance in agri- tain excess soluble salt. They are desig- culture arises when previously produc- nated as nonsaline-alkali or saline- tive soil becomes salt-affected as a re- alkali soils according to their content sult of irrigation. of salts. Irrigated lands are often located in Salt-afifected soils occur mostly in re- valleys near streams; because they can SALINE AND ALKALI SOILS 283 be irrigated easily, the lower and more can interchange freely with those in level soils usually are selected for culti- the soil solution, the proportions of the vation. Such soils may be adequately various adsorbed cations are related to drained and nonsaline under natural their concentrations in the soil solution. conditions, but the drainage facilities Calcium and magnesium are the prin- may not be adequate under irrigation. cipal cations in the soil solution and Irrigation waters may contain from 200 on the particles of normal, productive pounds to as much as 5 tons of salt per soils of arid regions. When normal soils acre-foot, and the annual application come in contact with irrigation or of water may amount to 5 acre-feet or drainage waters containing a high pro- more an acre. Considerable quantities portion of sodium, this cation becomes of soluble salts thus may be added to the dominant one in the soil solution irrigated soils in a short time. and replaces part of the original ad- Farmers who bring new lands under sorbed calcium and magnesium. As a irrigation often have failed to recog- consequence of the adsorption of sodi- nize the need for establishing artificial um, alkali soils are formed. drains to care for the additional water and the leaching required to prevent THE ACCUMULATION of soluble salts the accumulation of soluble salts. As a and adsorbed sodium by soils impairs result, the water table may rise from a their productivity in several ways. considerable depth to within a few feet Because of the presence of consider- of the soil surface in a few years. able dissolved salt and the absence of During the early development of ir- significant amounts of adsorbed sodi- rigation projects, water is frequently um, saline soils generally are flocculat- plentiful, and there is a tendency to ed. Their tillage properties and per- use it in excess. This hastens the rise of meability to water therefore are equal the water table. When the water table to or higher than those of similar non- rises to within 5 or 6 feet of the surface, saline soils. The abnormally high salt ground water containing more or less concentration of the soil solution of dissolved salt moves upward into the saline soils, however, reduces the rate root zone and to the soil surface. at which plants absorb water; conse- Ground water, as well as irrigation quently growth is retarded. The retar- water, then causes the soil to become dation of growth is almost directly re- saline. lated to the total salt concentration of the soil solution and is largely inde- ALKALI SOILS CONTAIN excessive pendent of the kind of salts present. amounts of adsorbed sodium. The salinity status of soils is ap- Because of the presence of negative praised in terms of effects on crop electrical charges at their surfaces, soil growth by measuring the electrical particles adsorb and retain cations, conductivity of the solution extracted such as calcium, magnesium, and sodi- from saturated soil paste. The elec- um. While the adsorbed cations are trical conductivity of a solution is a combined chemically with the soil par- good measure of its total salt concen- ticles, they may be replaced or ex- tration, and the water content of satu- changed by other cations that are rated soil is related to the field-mois- added to the soil solution. Each soil has ture range. Thus the electrical con- a reasonably definite capacity to ad- ductivity of the saturation extract is sorb and exchange cations, and the directly related to the total salt concen- percentage of this capacity that is tration of the soil solution under field taken up by sodium is referred to as conditions. the exchangeable-sodium-percentage. The effects of salinity on growth are The exchangeable-sodium-percentage largely negligible when the electrical of alkali soils is usually 15 or more. conductivity reading (expressed in mil- As cations adsorbed on soil particles limhos per centimeter) is less than 2. 284 YEARBOOK OF AGRICULTURE 1957 At readings in excess of about 16, only leaching with irrigation water to re- a few very salt-tolerant crops yield sat- move excess soluble salts. The improve- isfactorily. The yields of very salt- ment of alkali soils involves (besides sensitive crops may be restricted at drainage and leaching) the replace- readings as low as 2; moderately salt- ment of adsorbed sodium by calcium tolerant crops grow satisfactorily below or magnesium and the use of practices readings of 8; only salt-tolerant crops that develop good soil structure. grow satisfactorily when readings range Adequate drainage is essential for between 8 and 16. the permanent improvement of saline While the primary effect of soil salin- and alkali soils. Leaching operations ity on crops is one of retarding growth and the application of amendments by limiting the uptake of water, certain for the replacement of adsorbed so- salt constituents are specifically toxic to dium will be largely ineffective unless some crops. Boron, for example, when the ground-water table remains deep present in the soil solution at concen- enough to prevent appreciable upward trations of only a few parts per million, movement of water. is highly toxic to many crops. The permissible depth to the water Alkali soils remain flocculated and table in various types of soils under their properties usually are similar to irrigation and drainage requirements those of saline soils as long as consider- and methods are discussed in the chap- able amounts of soluble salts are pres- ter on soil drainage, page 378. ent. If the excess salts are removed by leaching, however, saline-alkali soils SOILS CAN BE LEACHED by applying generally become nonsaline-alkali soils, water to the surface and allowing it to and their physical properties deterio- pass downward through the root zone. rate markedly. Leaching is most efficient when it is As the concentration of the salts in possible to pond water over the entire the soil solution is lowered by leaching, soil surface. the adsorbed sodium present causes Water can be ponded on nearly level undesirable characteristics to develop. land in shallow basins formed by the The soil may become strongly alkaline construction of earthen dikes or bor- (pH readings above 8.5), the particles ders 2 to 4 feet high. The dimensions of may disperse, and the soil may be- individual basins depend on the slope come unfavorable for the entry and of the land.
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