232 SOIL PHYSICS AND MANAGEMENT 



percolation, by transpiration from leaves of plants and by evapora- 

 tion from the surface of the soil. 



(a) Decreasing Percolation. The amount of percolation de- 

 pends very largely on the texture of the soil itself. As a general 

 rule, the coarser the texture or the larger the air spaces the greater 

 the amount of percolation. This, of course, may be modified by 

 the amount of compaction and also by the organic-matter content. 

 The amount of percolation depends, too, on the openness of the 

 soil produced by tillage as given above. Excessive percolation 

 where it is due to coarseness of soil texture is very difficult to 

 prevent. 



The incorporation of some water-retaining material such as 

 clay or any of the finer soil constituents or organic matter with 

 the sand or gravel will aid in accomplishing the results desired. 

 The former is an expensive process, but has been done on a small 

 scale with excellent results. The use of organic matter is a 

 more practical but somewhat slower process unless under condi- 

 tions where abundant supplies of farm manure are at hand. Com- 

 pacting is very beneficial in case of sandy soils, but must be care- 

 fully done in the case of heavy soils. 



(b) Decreasing Transpiration from Plants. All plants in 

 their growth require enormous amounts of water, practically all 

 of which must be secured from the soil. We have seen that from 

 300 to 500 pounds of water are required for each pound of dry 

 matter produced. This means that crops remove large quantities 

 of water from the soil. 



The relative amount of water required may be reduced by an 

 abundance of plant food provided through cultivation, rotation 

 and fertilization. Weeds and other plants foreign to the crop 

 should be destroyed to prevent them from depriving it of the 

 moisture necessary for its growth. 



(c) Preventing Evaporation by Mulches. A mulch is any 

 material placed on or produced from the surface soil by tillage. 

 Its object is to prevent evaporation. To be effective a mulch must 

 be dry. Since moisture films pass very slowly into dry, loose soil, 

 practically all of the moisture that is lost is by interstitial evapo- 

 ration and diffusion through the mulch air to the atmosphere above. 

 This diffusion takes place very slowly. 



The following table gives the results obtained by Buckingham 



