As to mulching, it is already well recognized in the alkali regions of California as an effective remedy in light cases. Fruit trees are frequently thus protected, particularly while young, after which their shade alone may (as in the case of low-trained orange trees) suffice to prevent injury. The same often happens in the case of low-trained vines, small-fruit, and vegetables. Sanding of the surface to the depth of several inches was among the first attempts in this direction; but the necessity of cultivation, involving the renewal of the sand each season, renders this a costly method. Straw, leaves, and manure have been more successfully used; but even these, unless employed for the purpose of fertilization, involve more expense and trouble than the simple maintenance of very loose tilth of the surface soil throughout the dry season; a remedy which, of course, is equally applicable to hoed field crops, and is in the case of some of these—e. g., cotton—a necessary condition of cultural success everywhere. The wide prevalence of “light” soils in the arid regions, from causes inherent in the climate itself, renders this condition relatively easy of fulfilment.
Turning-under of Surface Alkali.—Aside, however, from the mere prevention of surface evaporation, another favorable condition is realized by this procedure, namely the commingling of the heavily salt-charged surface-layers with the relatively non-alkaline subsoil. Since in the arid regions the roots of all plants retire farther from the surface because of the deadly drought and heat of summer, it is usually possible to cultivate deeper than could safely be done with growing crops in humid climates. Yet even there, the maxim of “deep preparation and shallow cultivation” is put into practice with advantage, only changing the measurements of depth to correspond with the altered climatic conditions. Thus while in the humid States, three to four inches is the accepted standard of depth for summer cultivation to preserve moisture without injury to the roots, that depth must in the arid region frequently be doubled in order to be effective; and will even then scarcely touch a living root in orchards and vineyards, particularly in unmanured and unirrigated land.
A glance at [fig. 63, (chap. 22, p. 431)], will show the great advantage of extra-deep preparation in commingling the alkali salts accumulated near the surface with the lower soil-layers, diffusing the salts, say through twelve instead of six inches of soil mass. This will in very many cases suffice to render the growth of ordinary crops possible if, by subsequent frequent and thorough cultivation, surface evaporation, and with it the re-ascent of the salts to the surface, is prevented.
A striking example of the efficiency of this mode of procedure was observed at the Tulare substation, California, where a portion of a very bad alkali spot was trenched to the depth of two feet, throwing the surface soil to the bottom. The spot thus treated produced excellent wheat crops for two years—the time it took the alkali salts to reascend to the surface.
It should therefore be kept in mind that whatever else is done toward reclamation, deep preparation and thorough cultivation must be regarded as prime factors for the maintenance of production on alkali lands.
The Efficacy of Shading, already referred to, is strikingly illustrated in the case of some field crops which, when once established, will thrive on fairly strong alkali soil, provided that a good thick “stand” has once been obtained. This is notably true of the great forage crop of the arid region, alfalfa or lucern. Its seed is extremely sensitive to “black” alkali, and will decay in the ground unless protected against it by the use of gypsum in sowing. But when once a full stand has been obtained, the field may endure for many years without a sign of injury. Here two effects combine, viz., the shading, and the evaporation through the deep roots and abundant foliage, which alone prevents, in a large measure, the ascent of the moisture and salts to the surface. The case is then precisely parallel to that of the natural soil ([see p. 432, chapter 22]), except that, as irrigation is practiced in order to stimulate production, the sheet of alkali hardpan will be dissolved and its salts spread through the soil more evenly. The result is that so soon as the alfalfa is taken off the ground and the cultivation of other crops is attempted, an altogether unexpectedly large amount of alkali comes to the surface and greatly impedes, if it does not altogether prevent, the immediate planting of other crops. Shallow-rooted annual crops that give but little shade, like the cereals, while measurably impeding the rise of the salts during their growth ([see fig. 70, page 452]) frequently allow of enough rise after harvest to prevent reseeding the following season.
“Neutralising” Black Alkali.—Since so little carbonate of soda as one-tenth of one per cent may suffice to render some soils uncultivable, it frequently happens that its mere transformation into the sulfate is sufficient to remove all stress from alkali. Gypsum (land plaster) is the cheap and effective agent to bring about this transformation, provided water be also present. The amount required per acre will, of course, vary with the amount of salts in the soil, all the way from a few hundred pounds to several tons in the case of strong alkali spots; but it is not usually necessary to add the entire quantity at once, provided that sufficient be used to neutralize the sodic carbonate near the surface, and enough time be allowed for the action to take place. In very wet soil, and when much gypsum is used, this may occur within a few days; in merely damp soils in the course of months; but usually the effect increases for years, as the salts rise from below.
The effect of gypsum on black-alkali land is often very striking, even to the eye. The blackish puddles and spots disappear, because the gypsum renders the dissolved humus insoluble and thus restores it to the soil. The latter soon loses its hard, puddled condition and crumbles and bulges into a loose mass, into which water now soaks freely, bringing up the previously depressed spots to the general level of the land. On the surface thus changed, seeds now germinate and grow without hindrance; and as the injury from alkali occurs at or near the surface, it is usually best to simply harrow in the plaster, leaving the water to carry it down in solution. Soluble phosphates present are decomposed so as to retain finely divided, but less soluble earth phosphates in the soil.
It must not be forgotten that this beneficial change may go backward if the land thus treated is permitted to be swamped by irrigation water or otherwise. Under the same conditions naturally white alkali may turn black ([see above, chapter 22, p. 451]). Of course, gypsum is of no benefit whatever on soils containing no “black” alkali, but only (“white”) Glauber’s and common salt.
Removing the Salts from the Soil.—In case the amount of salts in the soil should be so great that even the change worked by gypsum is insufficient to render it available for useful crops, the only remedy left is to remove the salts, partially or wholly, at least from the surface of the land. Three chief methods are available for this purpose. One is to remove the salts, with more or less earth, from the surface at the end of the dry season, either by sweeping or by means of a horse scraper set so as to carry off a certain depth of soil. Thus sometimes in a single season one-third or one-half of the total salts may be got rid of, the loss of a few inches of surface soil being of little moment in the deep soils of the arid region. Another method affording partial relief is to flood the land for a sufficient length of time to carry the alkali three or more feet below the surface, then carefully preventing its re-ascent by suppressing evaporation ([see this chapter, p. 455]) as much as possible. The best of all, the final and universally efficient remedy, is to leach the alkali salt out of the soil into the country drainage; supplementing by irrigation water what is left undone by the deficient rainfall.