142 Irrigation and Drainage 



the plant ; and if this amount would have been lost, either by 

 downward percolation or by evaporation from the surface, then the 

 subsoiling has been a gain. 



3. Earth Mulches 



When the damp surface of a soil is covered with a 

 dry layer of earth, the rate of evaporation from it is 

 very much decreased. It is because of this fact that 

 thorough surface tillage is able to so conserve the soil 

 moisture stored in the upper four to six feet of culti- 

 vated fields that fair crops may be grown with very 

 little rain ; and it is in the effective handling of these 

 mulches that the hope of farmers in sub -humid districts 

 must be laid. 



Conditions modifying the effectiveness of mulches. The laws 

 which govern the loss of water through mulches have not yet 

 been sufficiently worked out to permit a full discussion of this 

 important subject, but several important facts have been defi- 

 nitely settled, and may be here stated. 



In the first place, when other conditions are the same, the 

 thicker or deeper the layer of loose, dry soil is, the less rapidly 

 can the soil moisture pass upward through it, to be lost by 

 evaporation. 



It was found, for example, that when soil covered with no 

 mulch lost water in the still air of the laboratory at the rate of 

 4.375 acre-inches per 100 days, the same soil stirred to a depth 

 of .5 inches lost but 4.017 acre-inches, and when stirred to a 

 depth of .75 inches lost 3.169 acre-inches in the same time. In 

 another case, when the loss of water from the unmulched surface 

 was 6.2 acre-inches per 100 days, stirring this same soil to a 

 depth of 1 inch reduced the loss to 4 acre -inches, while stirring 

 it to a depth of 2 inches left the loss but 2.8 acre -inches per 

 100 days. 



So, too, when corn was cultivated to a depth of 1 to 1.5 



