It is not practicable, as many suppose, to wash the salts off the surface by a rush of water, as they instantly soak into the ground at the first touch. Nor is there any certain relief from allowing the water to stand on the land and then drawing it off; in this case also the salts soak down ahead of the water, and the water standing on the surface remains almost unchanged. In very pervious soils and in the case of white alkali, the washing-out can often be accomplished without special provision for underdrainage, by leaving the water on the land sufficiently long. But the laying of regular underdrains greatly accelerates the work, and renders success certain.
Leaching-Down.—In advance of underdrainage, it is quite generally feasible, where the land has been leveled and diked for irrigation by surface flooding, to leach the salts out of the first three or four feet by continued flooding, thus taking them out of reach of the crop roots, or at all events giving the seed an opportunity to escape injury from alkali. This plan is especially effective in the case of alfalfa, the young seedlings of which are very sensitive, while the grown plant is rather resistant. In order to obtain this relief so as to know what is being accomplished, the farmer should ascertain beforehand how fast water will soak down in his ground;[174] for in heavy clay soils, and especially in those containing black alkali, the soakage is sometimes so slow that the upward diffusion of the salts keeps pace with the downward soakage; in which case nothing is accomplished by flooding, and underdrainage is the only remedy. But in most soils of the arid region flooding from three days to a week will remove the alkali beyond reach of the roots of ordinary crops. If subsequently irrigation is done by means of deep furrows, the alkali salts may be either kept at a low level continuously, or if the land be at all pervious, the alkali may ultimately be permanently leached out into the subdrainage by farther flooding. When the alkali has not accumulated near the surface to any great extent, irrigation by deep furrows may, alone, afford all the relief needed.
Fig. 71.—Lemon Orchard Affected by Alkali; Before Deep Irrigation.
In the case illustrated by figures 71 and 72, irrigation by shallow furrows with water too strongly charged with salts had so far added to the natural alkali-content of the land that the lemon trees were being defoliated. Upon the advice of the California Station the deep-furrow system was adopted, and within two years the results were as shown in [figure 72], the salts having been carried down and diluted so as to become harmless.
Underdrainage the Final and Universal Remedy for Alkali.—When we underdrain an alkali soil, we adopt the very means by which the existence of alkali lands in the humid regions is wholly prevented; the leaching-out of the soluble salts formed in soil-weathering as fast as they are formed. The long and abundant experience had with underdrainage in reclaiming saline sea-coast lands, applies directly and cogently to alkali lands. It is the universal remedy for all the evils of alkali, and its only drawback is the first expense, and the necessity for obtaining an outlet for the drain waters, which cannot always be had on the owner’s land. Hence it requires co-operation or legislation to render the great improvement of underdrainage feasible. Such legislation is well established in the old world, and has been enacted in several states even of the humid region. Where irrigation is practiced as a matter of necessity, underdrainage is a correlative necessity, both to avoid the evils of over-irrigation and to relieve the land of noxious alkali salts.
Fig. 72.—The Above Orchard after Alkali was Driven Down
by Deep Irrigation, followed by Cultivation.
The drainage law now existing in California does not go farther than to authorize the formation of drainage districts, within which the necessary taxes may be levied; and there is some difficulty in securing popular action. But bitter experience will doubtless in time compel unanimity, such as now exists, e. g., in Illinois, where drainage is not nearly so urgently needed as it is in the irrigation states.
Possible Injury to Land by Excessive Leaching.—It should not be forgotten, however, that excessive leaching of underdrained land by flooding is liable to injure the soil in two ways: first, by the removal of valuable soluble plant-food; and further, by rendering the land less retentive of moisture, such retention being favored by the presence of small amounts of alkali salts, not sufficient to injure crops. After the salts have been carried down to a sufficient depth to prevent injury to annual crops, and with proper subsequent attention to the prevention of surface evaporation, the flooding will not need to be repeated for several years. Thus in many soils excellent crops may be grown even in strong alkali land, pending the establishment of permanent drainage systems.