. Soil physics and management. s may be done in two ways—by mulching and efficiency of a layer of soil in fine tilth to prevent evaporationhas already been discussed. This should be three or four inches 286 SOIL PHYSICS AND MANAGEMENT deep to be most effective (Fig. 130). This mulch if maintainedwill prevent excessive capillary movement until the crop is suffi-ciently large to shade the ground. The maintenance of the mulchthen becomes of less importance. Alfalfa during three-fourths ofthe time of its growth furnishes a very effective shade. Evaporationfrom soil of orchards is preve
. Soil physics and management. s may be done in two ways—by mulching and efficiency of a layer of soil in fine tilth to prevent evaporationhas already been discussed. This should be three or four inches 286 SOIL PHYSICS AND MANAGEMENT deep to be most effective (Fig. 130). This mulch if maintainedwill prevent excessive capillary movement until the crop is suffi-ciently large to shade the ground. The maintenance of the mulchthen becomes of less importance. Alfalfa during three-fourths ofthe time of its growth furnishes a very effective shade. Evaporationfrom soil of orchards is prevented very materially by the shading ofthe trees. Artificial mulches, as straw, leaves, sawdust, and manure,may be used, but are too expensive for large areas and only possiblefor high-priced crops under a very intensive system of Deep Plowing and Turning Under Alkali.—The practiceof encouraging evaporation is sometimes resorted to for bringingthe alkali to the surface and then turning under so deeply that it. Fig. 130.—An orchard well cultivated prevents the rise of alkali. (U. S. Reclamation Service.) will not rise to the surface until after the young crop has passedthrough its most sensitive stage. By this means alfalfa and othercrops may be started. When the crop attains such size that it shadesthe soil and the roots take up the water from beneath, comparativelylittle moisture evaporates from the surface and the alkali is notcarried up to any extent. 4. Neutralizing Black Alkali.—The black alkali when presentin amounts of one-tenth of a per cent prevents the growing of mostcrops. Sodium sulfate may be present in amounts five times asgreat before it becomes injurious. By treating the black alkalispots with gypsum (land plaster) a chemical reaction takes placewhen moisture is present, producing sodium sulfate and calciumcarbonate. The former is not sufficiently soluble to be injurious, ALKALI LANDS AND THEIR RECLAMATION 287 while the latter is very benef
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