ALKALI SOILS. A term applied to soils occurring in regions of deficient or irregular rainfall, which contain unusually large amounts of soluble salts concentrated in or near the sur face. tinder certain conditions of moisture these salts appear on the surface of the soil in the form of a crust or powder known as rch in India, sabach in Egypt, and alkali in America. The main constituents of this saline efflorescence are sodium sulphate, sodium carbonate, and sodium chloride, mixed in varying proportions. There are present besides, according to Hilgard, salts of calcium and magnesium and smaller amounts of potassium sulphate, sodium phosphate, and sodium nitrate, these salts furnishing the most important elements of plant food. Two main classes of alkali arc distinguished: "black" al kali, in which sodium carbonate predominates, and which is on this account highly corrosive and injurious to vegetation; and "white" alkali, the predominant constituent of which is sodium sulphate, and which is much less harmful to plant growth than is the black form. A saline form in which sodium chloride predominates is also frequently met with. Black alkali derives its name from the dark-colored spots and water puddles observed where it abounds, which owe their color to the organic matter dissolved from the soil by the sodium carbonate.
Practically the same soluble (alkali) salts oc cur in all soils, but in humid regions the abun dant rainfall prevents their accumulation on or near the surface, carrying away in the drainage those salts for which the soil has not a strong absorptive power. In regions of deficient rain fall, on the other hand, the scanty moisture which reaches the soil merely serves to dissolve the salts and carry them down a short distance into the ground, whence they are rapidly drawn up by the capillary rise of the water. The mois ture, evaporating at the surface, leaves the salts accumulated there. Such accumulations of alkali are also found in regions which have a rainy and a dry season (as in parts of India), and where the rains occur commonly in sudden and violent downpours, which quickly pass without wetting the soil to any considerable depth.
In irrigated regions alkali frequently appears at the surface of the soil as a result of excessive application of water combined with defective drainage. Irrigation water, carried by canals running through porous, sandy soils, or applied in excessive amounts on the higher lands, seeps through to the lower-lying lands, carrying with it the soluble salts. Conditions are sometimes aggravated by the use of irrigation water rich in soluble salts.
Alkali soils generally occur in circumscribed areas ("spots") , but sometimes as broad stretches of "alkali deserts." Such soils are common in arid regions, i.e., where the average annual rainfall is less than 20 inches (500 millimetres). According to Eilgard, "the arid region thus defined, includes, in North America, most of the country lying west of the one hun dredth meridian, up to the Cascade Mountains, and northward beyond the line of the United States; southward, it reaches far into Mexico, the Mexican plateau. In South America it includes nearly all the Pacific Slope (Peru and Chile) south to Araucania; and eastward of the Andes the greeter portion of the plains of western Brazil and Argentina. In Europe only a small portion of the Mediterra nean border is included; but the entire African coast belt opposite, with the Saharan and Lib yan deserts, Egypt, and Arabia are included therein, as well as a considerable portion of South Africa. In Asia. Asia Minor, Syria (with Palestine), Mesopotamia, Persia, and north western India up to the Ganges, and northward, the great plains or steppes of Central Asia east ward to :Mongolia and western China, fall into the same category, as does also a large portion of the Australian continent." There are exten sive regions, especially in European Russia. which are not strictly arid according to this definition, but in which alkali soils are of fre quent occurrence.