ALKALI SOILS. The term "alkali" is applied to those soils in which soluble salts have accumulated in amounts sufficient to cause injury to plant life. The toxic limit varies with the chemical composition of the soluble salts and also depends upon the variety of crop plants grown in the region. Soils of this character are usually found in regic ris of low rainfall, their origin being traced to the insufficiency of moisture to wash out and carry away the excess of soluble salts resulting from the weathering of the parent soil material. Under certain conditions the soluble salts accumu late at the surface, while under other conditions the greatest accumulation of soluble salts is found in a soil layer at some distance below the surface and varying with the depth to which the light rainfall of the region penetrates the soil or the depth to the water table. Extensive studies of alkali soils have been made in America, Egypt, Hungary and other countries where arid or semi-arid conditions have resulted in the accumulation in the soil of harmful quantities of soluble salts. The salts that most frequently predominate are the carbonates, chlorides and sul phates of sodium, magnesium and calcium, or a mixture of two or more of these salts, but other forms may be encountered. Under the broad definition of alkali soil the reaction is not necessarily alkaline, but under certain conditions the predom inating salt may be neutral, as in the vicinity of Great Salt Lake, Utah, where sodium chloride is present in abundance.
Alkali is commonly designated as white alkali or as black alkali, depending upon the appearance of the incrustations caused by excessive accumulations at the surface of the soil. Usually when large quantities of sodium chloride or sodium sulphate accumulate at the surface their presence is marked by white incrustations, hence the term white alkali. Since sodium car bonate, when present in large quantities attacks the vegetable matter of the soil, causing a dark brown stain, it is frequently called black alkali. This classification is not entirely satisfactory since a much less harmful salt, such as sodium chloride, may likewise impart a dark stain to the soil surface. Since the salts of which the alkali is composed are readily soluble in water, it is evident that, while they will remain inactive in a dry soil, they will dissolve when water is added and will move with the moisture. Farmers living in arid regions are familiar with the fact that parts of their fields heavily encrusted with alkali show no such crusts after heavy rains or after the application of irrigation water. The salts dissolve in the water and move down into the soil but again make their appearance at the surface as the water rises and evaporates leaving the salts behind. Many farmers in a new irrigation district have found to their dismay that after a few years of irrigation practice, alkali has appeared at unexpected places. If sufficient water is added not only to dissolve and start a downward movement of the salts, but also to leach them away into the country drainage, no harmful accumulation will occur. In many irrigated areas the greater part of the alkali accumula tions in cultivated fields is the result of seepage, by which the soluble salts are moved from higher to the lower areas where there is a tendency for the drainage water to collect. Such accumula tions can be prevented if precautions are taken to provide suitable drainage at the time the irrigation project is undertaken. Exten sive studies of methods for the reclamation of alkali soils in dif ferent areas have been made and it has been demonstrated that where adequate drainage is provided and irrigation water of good quality is available, reclamation can be accomplished.
BIBLIOGRAPHY.-C. W. Dorsey, "Alkali Soils of the United States," Bibliography.-C. W. Dorsey, "Alkali Soils of the United States," in U.S. Dept. Agr. Bur. Soils Bul. 3S (1906) ; F. S. Harris, Soil Alkali; its Origin, Nature and Treatment (192o) • W. P. Kelley, "The present status of alkali," and P. L. Hibbard, "Alkali Soils, origin, examination and management" in Cal. Agr. Exp. Sta. Circ. 219 (192o) and 292 (1915) respectively ; W. P. Kelley, "Variability of Alkali soil" and A. A. F. von Sigmond, "Contribution to the theory of the origin of alkali soils," in Soil. Sci. 14 (1922) and 21 (1926) respectively ; F. T. Shutt, "Alkali soils ; their Nature and Reclamation" in Canada Dept. Agr. Bul. 21 (19.23) ; "Hungarian Alkali Soils and Methods of Their Reclamation" (Cal. Agr. Exp. Sta., 1927) . A. G. McCall, Physical Properties of the Soil (19o8) and Studies of Soils (1915). (A. G. McC.)