52 IRRIGATION PRACTICE 



loss is insignificant, amounting at most to only 1 inch of 

 rainfall in six years. 



The most effective method of checking evaporation 

 from the soil is to stir the top soil thoroughly with any 

 one of the many kinds of cultivators now found on the 

 market and built especially for the purpose. (Fig. 12.) 



37. Self -mulching soils. Under arid conditions, 

 some soils possess a self-mulching power. The abundant 

 sunshine, high temperature and low relative humidity 

 of arid sections, cause a very rapid evaporation. After 

 an irrigation on a very hot summer day the top soil may 

 be dried out so rapidly that the lower soil lasers cannot 

 send moisture upward in time to supply the loss. Under 

 such conditions the evaporation is automatically 

 decreased. The dry top soil, thus induced, is an effec- 

 tive check upon the upward movement of water. This 

 may be one explanation of the fact that in many virgin 

 arid lands much of the rainfall remains stored for months 

 at a time. Added to this is another condition of frequent 

 occurrence. Arid soils are, as a rule, rich in lime. In some 

 cases the calcareous substances of arid soils make up one- 

 fourth to one-half of the soil itself. Such soils, as they 

 dry out, become loose. It frequently happens, there- 

 fore, that when such a soil, after an irrigation, is dried 

 out by rapid evaporation, the surface layer falls into a 

 natural mulch which is fairly effective in stopping evapo- 

 ration. Buckingham reports an interesting experiment, 

 in which he found that the rapid evaporation due to arid 

 conditions so dried out the top soil that the loss of water 

 in a year was only 11.2 inches as against 51.6 inches from 

 a similar soil under humid conditions which permitted a 

 slow but. steady evaporation. 



The stirring of such self-mulching soils does not always 



