SOIL MOISTURE AND DRY FARMING 99 



cerned in evaporation. Draughts from the first source 

 can only be partially prevented while grain crops oc- 

 cupy the soil. But it may be regulated by regulating the 

 number of the crops to be grown and also the kind of 

 the crops. Those from the second source (see p. 165) 

 may be greatly lessened but not entirely prevented by 

 the maintenance of the soil mulch even as persistently 

 as this may be practicable. Under no circumstances can 

 it be maintained so continuously as to entirely prevent 

 loss from evaporation. Even on the carefully managed 

 summer-fallow there will be loss. When rain falls, water 

 is taken from and near the surface before the mulch can 

 be renewed by using the harrow. Small showers and 

 frequent, aid in such escape. Vapor comes up from be- 

 low in hot weather and cracks are formed through which 

 moisture escapes. There are periods when a dust mulch 

 cannot be maintained, as when grain crops are in the ad- 

 vanced stages of growth. When the land is plowed in 

 the autumn some moisture is lost from a damp surface, 

 and the same is true in the early spring. From all these 

 causes moisture will escape, hence in many soils it has 

 been thought that not more than half the precipitation 

 that falls is retained. But it is very evident that the loss 

 of moisture will decrease as the dust mulch is main- 

 tained. The more, therefore, that the processes of culti- 

 vation are followed that will admit of maintenance of 

 the dust mulch, the less will be the loss of moisture from 

 the soil and subsoil. 



The utilization of subsoil moisture. The stored wa- 

 ter in the soil and subsoil is much more valuable than an 

 equal amount of rain water falling during the period of 

 crop growth. It contains nitrates formed the previous 

 season. These are not washed out as in humid regions. 

 It also increases the supply of potash and phosphoric 

 acid in the soil. It is in a considerable degree secure 

 from evaporation, and it enables the roots to penetrate 



