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 re tards 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 treat ment 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 im proves the drainage, and tends to fix in the soil certain of the valuable fertilizing constituents, espeeially alkaline phosphates and humus, which would otherwise be lost in the subsequent leaching; for it must be borne in mind that, al though the leaching process is effective in re moving the noxious salts, it is likely to carry away with them a. large part of those ingre dients 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 con vert black alkali into white.
Alkali lands are commonly either entirely de void 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 toler ance depending much upon the kind and propor tion of the salts present, as well as upon the na ture of the plant itself. Hilgard proposes to uti lize the natural vegetation as an index of the kind of salts predominating in a soil. Thus, un der California conditions, the Samphires cornia subterminalis and Allcnrolfea oeciden talis), Alkali-heath (Prankcnia gramlifolia cam pestris), and Cressa eretica' truxillensis are especially indicative of excessive amounts of salts of any kind; Tussock grass (Sporobolus airoidcs) and Greasewood (Sareobal 11R rermien lotus) of the presence of large amounts of black alkali; and Samphires and Saltworts (8ticrfla t orre ya n a and Sio•tht suffruteseens) of white al kali. The natural vegetation also furnishes, ac cording to Hilgard, a means of determining the reclaimability of alkali soils. Thus, when tus sock grass. greasewood, the Samphires. Saltworts. Alkali-heath, and Cressa occupy the ground as an abundant and luxuriant growth, such laud is considered irreclaimable for ordinary crops sin less 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 Aus tralian salt-bushes (,-Lt•iplex spp.), Alodiola rumben•, Tussock grass (Sporobolus airoides), Wild (Beckman/fin eurealormis), and Barnyard grass (Panieum, crus-yalli). Of ordi nary farm crops which show a marked tolerance of alkali may he mentioned rice, the millets, beets, English rape, sunflowers, asparagus, cel ery, spinach, onion, alfalfa, Bokhara, clover, grapes. The Australian salt-bushes, especially dtriplcx 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.