272 SOILS: PR0FEBTIE8 AND MANAGEMENT 



the crop yields are controlled more directly by lack than 

 by excess of water. It is a common observation that 

 soils which ordinarily give a low yield in seasons of nor- 

 mal or low rainfall give good fields in a wet season, 

 indicating how dominant is this influence of moisture on 

 soil fertility. 



189. Methods of checking evaporation losses. — All 

 methods for the reduction or elimination of evaporation 

 losses depend on one or both of two functions : (1) the 

 actual control of evaporation as it occurs at the surface ; 

 and (2) the prevention of the movement of capillary 

 water upward to take the place of the moisture already 

 lost. It has been shown that as water is lost at the sur- 

 face of a soil, movement is induced and capillarity is 

 set up. Such action, if allowed to continue, must ulti- 

 mately bring about great losses. The obstruction of 

 capillarity would obviously lower these losses to a marked 

 degree. As it is difficult and often impracticable to en- 

 tirely eliminate evaporation, the most successful methods 

 of water control usually m elude a change in the* structural 

 condition of the soil which tends toward a lower capil- 

 larity, especially at the surface. Of all the methods of 

 moisture conservation, the use of a mulch has been found 

 most satisfactory. The consideration of mulches is 

 therefore one of the most important phases in the study 

 of moisture control. 



190. Mulches for moisture control. — Any material 

 applied 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, such as 

 the keeping down of weeds and the maintaining of a 

 uniform soil temperature. By the conservation of the 

 moisture, more water remains in the soil for the solution 



