218 NATURE AND PROPERTIES OP SOILS 



sted work, is roughly 14 inches. Rim-off and percolation may 

 be considered as about 50 per cent. The water requirement of 

 an ordinary crop is about 7 inches. This leaves a loss of 7 

 inches to be credited to evaporation. In other words, in a 

 clay loam soil in a climate like that of England, one-half 

 the rainfall goes as run-off and percolation, while the other 

 half is divided about equally between the plant and loss by 

 evaporation. While run-off and percolation may be checked 

 to some extent, not enough conservation can occur in this di- 

 rection to tide a crop over a period of drought. Some con- 

 sideration should, therefore, be directed towards the check- 

 ing of loss by evaporation, since moisture thus saved means 

 just that amount added to the water available for the use of 

 the crop growing on the soil. 



113. Evaporation control. — ^Any material applied to the 

 surface of a soil primarily to prevent loss by evaporation or to 

 keep down weeds may be designated as a mulch. Mulches are 

 of two general sorts, artificial and natural. In the former case, 

 foreign material is merely spread over the soil surface. Man- 

 ure, straw, leaves, and the like may be used successfully. 

 Such mulches while effective, especially in preventing weed 

 growth, are not generally applicable to field crops where in- 

 ter-tillage is practiced, since they would make cultivation im- 

 possible. Their use is, therefore, limited to such crops as 

 strawberries, blackberries, and the like. 



The second type of mulch is called a soil-mulch since it is 

 formed from soil itself. With proper tillage, a loose dry layer 

 of soil may be formed on the surface. Such a layer is designed 

 to obstruct capillary movement to such an extent as to reduce 

 evaporation loss to a minimum. In theory a soil-mulch should 

 be formed as quickly as possible so that the only moisture 

 sacrificed will be that which is present in the soil forming 

 the mulch. Moreover, the mulch should be renewed after 

 every rain and should, except in special cases, be not more 



