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 Hilgard, "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, including especially 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 greater 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 frequent occurrence.
Alkali injures plants by its corrosive action (in the case of black alkali) on the root crown, and by interference with osmotic action, by which seeds and plants take up the moisture and soil solutions, and thus prevents or seriously retards germination and growth. The latter effect results only when the soluble salts are present in considerable amount: on the other hand, a small amount of alkali appears to have a beneficial effect. Alkali, especially the black variety, also renders soils pasty and difficult to till and drain, and tends to form a tough hardpan impervious to water. Alkali soils are, as a rule, more moist than those free from a localized excess of soluble salts. This is due to the strong absorptive power of the salts for water and their retarding effect on evaporation.
Alkali soils are generally so fertile when freed from excess of noxious salts and their area is so rapidly increasing under careless methods of irrigation that the reclamation of alkali lands is a matter of the greatest agricultural im- portance. Alkali soils may be improved by (1) reducing surface evaporation, which may be effected by maintaining a loose tilth in the surface soil, by mulching, and by the growth of plants which root deeply and shade the soil, or which take up large amounts of soluble salts in their growth; (2) deep and thorough tillage; (3) the use of chemical correctives, such as gypsum, which in case of black alkali converts the corrosive carbonate into the comparatively
harmless sulphate; and (4) leaching out the excess of salts by irrigation in connection with underdraining. The first two methods of treatment are merely temporary expedients, and are of value only when the amount of alkali is small. The third also affords only temporary relief, and is of value mainly when the amount of alkali is small and of the black variety. It is, however, very effective when employed in connection with the fourth method, for it improves the drainage, and tends to fix in the soil certain of the valuable fertilizing constituents, especially alkaline phosphates and humus, which would otherwise be lost in the subsequent leaching; for it must be borne in mind that, although the leaching process is effective in removing the noxious salts, it is likely to carry away with them a large part of those ingredients upon which the productiveness of the soil depends. The California experiment station has found that from two and a half to three times as much gypsum as there is sodium carbonate present in the soil is required in order to convert black alkali into white.
Alkali lands are commonly either entirely devoid of vegetation, or else produce plants of little or no value to man. Plants differ widely as regards tolerance of alkali in the soil, the tolerance depending much upon the kind and proportion of the salts present, as well as upon the nature of the plant itself. Hilgard proposes to utilize the natural vegetation as an index of the kind of salts predominating in a soil. Thus, under California conditions, the Samphires (Salicornia subterminalis and Allenrolfea occidentalis) , Alkali-heath (Frankenia grandifolia campestris), and Cressa cretica truxillensis are especially indicative of excessive amounts of salts of any kind; Tussock grass (Sporobolus airoides) and Greasewood (Sarcobatus vermiculatus) of the presence of large amounts of black alkali; and Samphires and Saltworts (Suæda torreyana and Suæda suffrutescens) of white alkali. The natural vegetation also furnishes, according to Hilgard, a means of determining the reclaimability of alkali soils. Thus, when tussock grass, greasewood, the Samphires, Saltworts, Alkali-heath, and Cressa occupy the ground as an abundant and luxuriant growth, such land is considered irreclaimable for ordinary crops unless under-drained for the purpose of washing out surplus salts, as explained above. The more important and valuable of the plants which can withstand large amounts of alkali are the Australian salt-bushes (Atriplex spp.), Modiola decumbens, Tussock grass (Sporobolus airoides), Wild Millet (Beckmannia cureæformis) and Barnyard grass (Panicum crus-galli) . Of ordinary farm crops which show a marked tolerance of alkali may be mentioned rice, the millets, beets, English rape, sunflowers, asparagus, celery, spinach, onion, alfalfa, Bokhara, clover, grapes. The Australian salt-bushes, especially Atriplex semibaccata, have recently come into considerable prominence as a useful crop for alkali soils. They are highly tolerant of alkali, taking up large amounts of the soluble salts in their growth (nearly twenty per cent. of the dry matter of salt-bushes is ash), and they produce a forage of considerable value.
Bibliography. E. W. Hilgard, "Alkali Lands, Irrigation and Drainage," Appendix to Report of California Experiment Station, 1890 (Sacramento, 1892); "The Relations of Soil to Cli-
