Amount of Water for Single Irrigations 223 



soil during times of heavy or protracted rains.. This 

 leaching is usually looked upon as a necessary evil, 

 which results in a waste of fertility. Whether this 

 conviction is well founded, or whether a certain 

 amount of soil washing is indispensable to unim- 

 paired fertility, it appears to the writer is one of 

 the important soil problems awaiting positive demon- 

 stration. The accumulation of alkalies in the soils 

 of arid climates, where relatively small leaching is 

 associated with large evaporation, and the tendency 

 of alkalies to become intensified where irrigation has 

 been long practiced, are facts which suggest that 

 there may be such a thing as too great economy of 

 water in irrigation. 



But, waiving this possibility of demand for water, 

 and all of those cases w r here the water is applied for 

 other purposes than meeting the ordinary needs of 

 vegetation, the fundamental conditions which deter- 

 mine the amount of water which should be applied at 

 a single irrigation are : (1) the capacity of the soil 

 and subsoil to store capillary water; (2) the deptli 

 of the soil stratum penetrated by the roots of the 

 particular crop ; (3) the rate at which the soil below 

 the root zone may supply water by upward capillarity 

 to the roots ; and (4) the extent to which the soil 

 and subsoil have become dried out. 



On the other hand, the conditions which determine 

 the frequency of irrigation are : (1) the amount of 

 available moisture which may be stored in the soil ; 

 (2) the rate at which this moisture is lost through 

 the crop and through the soil ; and (3) the degree 



