74 SOILS AND FERTILIZERS 



of water needed to produce a pound of dry matter 

 of the different grains, the figures showing that oats 

 require about five hundred pounds and corn about 

 two hundred seventy-seven pounds. The other grains 

 use amounts somewhere between these two. This 

 water, having accomplished its work of bringing plant 

 food to the leaves, is by them passed off into the air. 

 The soil also loses moisture by direct evaporation from 

 its surface, especially during dry, hot, windy weather; 

 hence the supply of water to the soil must replace the 

 losses through the plants and through evaporation if 

 vegetation is to thrive. It is estimated that each ton 

 of dry crop material on an acre requires four inches of 

 water. If, then, five tons of hay are to be produced 

 from an acre, a rainfall of twenty inches, or its equiva- 

 lent, must be provided. 



Conservation of Soil Water. To reduce the loss 

 of soil water by evaporation, or, in other words, to 

 conserve the moisture already in the soil, is a necessity 

 in regions where at least twenty inches of water does 

 not fall annually, and it is also of great benefit to the 

 growing crop, even where the rainfall is greater than 

 twenty inches. The most effective agent in this 

 conservation is cultivation. This breaks up the soil 

 at the surface and hastens evaporation there, but the 

 dry soil above, through which capillarity acts very 

 slowly, serves to keep the soil moist below and thus the 

 roots are kept supplied with moisture. When the next 

 rain comes, the loose surface soil will allow the water 

 to percolate freely through it to the soil below, for 

 gravity tends to draw free water downward. Breaking 

 up the surface of the soil for the purpose of conserving 

 the moisture is called forming a dirt mulch, or a dust 



