94 IRRIGATION PRACTICE 



acid and forty-eight pounds of nitrogen. By using the 

 smallest percentage, 22 per cent of potash, in the above 

 table, 2 acre-feet of water would yield a little less than six 

 pounds of potash, a quantity entirely insufficient for the 

 production of a crop. By using the averages of some of the 

 other waters in the table, the potash added by 2 acre- 

 feet is ample to supply the crop needs. Any of the waters 

 in the table, save No. 6, with only 17 per cent lime, would 

 supply amply the needs of the crop for lime. In most 

 waters, the nitric acid is present in natural waters in very 

 small quantities, but it is not likely that the quantity 

 of water ordinarily used in irrigation throughout a season 

 would be sufficient to supply the crop needs. Phosphoric 

 acid is also present in small quantities and seldom can 

 supply, thoroughly, the crop needs. While, therefore, the 

 total soluble material contained by ordinary water 

 appears to be quite sufficient in quantity to supply the 

 total needs of the plant, the specific substances required 

 for successful plant-growth are fully met only in a few 

 waters. With moderate irrigations and waters of aver- 

 age composition, plants must draw upon the soil for at 

 least some of the constituents needed in their growth 

 notably for phosphoric acid, nitrogen and potash. Waters 

 in which these substances are present in larger propor- 

 tions may supply all the needs of the crop for mineral 

 matters. 



The property of the soil to retain certain ingredients 

 of the water that may be passing through it is of impor- 

 tance in this connection. Lime, magnesia, potash (notably 

 in clay soils), chlorine, and practically all the ingredients 

 of irrigation water, are partly absorbed by the soil through 

 which the water passes. The substances that are absorbed 

 and the degree of absorption are determined by the com- 



