SOIL CHANGES DUE TO IRRIGATION 89 



Under more modern and improved methods of irriga- 

 tion, first-class crops are frequently raised with 1 or 1J^ 

 acre-feet of water. In such cases, the crop more nearly 

 takes up the substances added to the soil by irrigation 

 water. On the other hand, the water used for irrigation 

 ordinarily contains more than 250 parts of dissolved sub- 

 stances hi 1,000,000 parts of water. If the salinity is 500, 

 2 acre-feet of water would add to one acre, 2,600 pounds 

 of solid substances, and waters richer hi mineral matters 

 would leave hi the soil tremendous quantities of solid 

 matters. It is readily seen, therefore, how profoundly 

 irrigation water may affect soils under irrigation. Should 

 the irrigation water be heavily charged with substances 

 deleterious to soil or crop, immediate and irreparable 

 damage may be done. Little definite information con- 

 cerning the whole subject is as yet available. It is quite 

 evident, however, that the methods of irrigation must be 

 varied with regard to the composition of the water used. 



63. Use of concentrated waters. An irrigation water 

 of medium concentration may be used safely in modera- 

 tion, although it should be so used as to leave as little as 

 possible of its constituents in the soil. When concentrated 

 waters are used, excessive quantities are applied to force 

 drainage, so that the concentration of the free water in 

 the soil after irrigation is never above that of the water 

 used. This is the good reason behind the practice of 

 farmers, in districts where the soils or waters are heavy 

 in alkali, to use more water throughout the season than 

 in districts where the soils are freer from alkali and the 

 water of low concentration. This principle is frequently 

 the essential one in building up a district which of neces- 

 ity must depend upon highly mineralized water for its 

 supply of irrigation water. 



