74 SOILS AND FERTILIZERS 



the ease with which evaporation may take place. Moisture 

 saved from evaporation is at the immediate disposal of the 

 crop. 



87. Mulches for moisture control. — Any material ap- 

 plied to the surface of a soil primarily to prevent loss by 

 evaporation may be designated as a mulch. It may at the 

 same time fulfill other useful functions, like keeping down 

 weeds and maintaining a uniform soil temperature. The 

 mulch ordinarily used for fallow land is produced by stirring 

 the surface soil. Mulches may be formed of straw, leaves, 

 flat stones, cloth, sawdust and various other materials, but 

 the most practical material is soil. 



88. The soil mulch. — The soil mulch is made by stirring 

 the surface of the soil with some one of the ordinary tillage 

 implements. For fallow land a disk harrow, straight, or 

 spring tooth harrow may be used. For intertilled crops 

 numerous forms of cultivators are made for the special pur- 

 pose of going between the rows of plants. For small grain 

 a weeder or spike-tooth harrow, with the teeth slanted back- 

 ward, is frequently used while the grain is young. This 

 practice has much to recommend it in an arid or semi-arid 

 region. 



In making a soil mulch the object is to destroy the capil- 

 larity near the surface soil and thus to prevent the move- 

 ment to the surface of water from the portion of the soil 

 below the mulch. Stirring may accomplish this by breaking 

 up the cohesion of particles to such an extent that moisture 

 cannot pass from one to the other. 



89. Frequency of stirring. — Some kinds of soil re- 

 quire more frequent stirring than others. For instance, 

 a sand will maintain a mulch longer than a loam or clay. 

 The latter becomes moist from below and will gradually 

 allow moisture to reach the surface. Rain will also compact 

 a mulch and unless it is soon restored there may be more 



