SOILS 



297 



FIG. 256. 



plant food which the crop needs. It conserves the soil 

 moisture by putting the soil in a condition to absorb the 

 rainfall and also by preventing evaporation by forming a 

 layer of dust mulch on the surface and by breaking up 

 capillary connections. 



Since in most parts of the United States the rainfall is 

 insufficient during the growing months for the production 

 of maximum crops, it is quite essential that the ground be 

 cultivated as soon as possible after rains. The first effect of 

 cultivation is to increase evaporation, but soon the loose 

 dry soil on the surface acts as a mulch to retard evaporation. 



Dry Farming. A large part of the United States has 

 insufficient rainfall to raise good crops without irrigation. 

 In some of the western states it has been possible to conserve 

 the greater part of the rainfall for one or more years for 

 the use of crops during the growing season. This is called 

 dry farming and is accomplished by first plowing the 

 ground to a depth of eight or ten inches and then cultivat- 

 ing continuously, so that a dry dust mulch is formed on 

 the top of the ground. The water that falls as rain is 

 quickly absorbed and held as capillary water by the soil. 

 Capillary water moves very slowly through the dry dust 

 mulch, and thus the loss by evaporation is reduced to a 

 minimum. When sufficient water has collected in the soil, 



