286 



SOIL PHYSICS AND MANAGEMENT 



deep to be most effective (Fig. 130). This mulch if maintained 

 will prevent excessive capillary movement until the crop is suffi- 

 ciently large to shade the ground. The maintenance of the mulch 

 then becomes of less importance. Alfalfa during three-fourths of 

 the time of its growth furnishes a very effective shade. Evaporation 

 from soil of orchards is prevented very materially by the shading of 

 the trees. Artificial mulches, as straw, leaves, sawdust, and manure, 

 may be used, but are too expensive for large areas and only possible 

 for high-priced crops under a very intensive system of agriculture. 

 3. Deep Plowing and Turning Under Alkali. The practice 

 of encouraging evaporation is sometimes resorted to for bringing 

 the alkali to the surface and then turning under so deeply that it 



Fia. 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 passed 

 through its most sensitive stage. By this means alfalfa and other 

 crops may be started. When the crop attains such size that it shades 

 the soil and the roots take up the water from beneath, comparatively 

 little moisture evaporates from the surface and the alkali is not 

 carried up to any extent. 



4. Neutralizing Black Alkali. The black alkali when present 

 in amounts of one-tenth of a per cent prevents the growing of most 

 crops. Sodium sulfate may be present in amounts five times as 

 great before it becomes injurious. By treating the black alkali 

 spots with gypsum (land plaster) a chemical reaction takes place 

 when moisture is present, producing sodium sulfate and calcium 

 carbonate. The former is not sufficiently soluble to be injurious, 



