270 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The limit of concentration of irrigation water has been placed 

 by some authorities at 2,000 or 3,000 parts per million. With these 

 concentrations, however, injury will result if the alkali is allowed 

 to accumulate, and is not washed from the soil. Even compara- 

 tively small amounts of mineral matter may give rise to alkali in 

 time, if the soluble salts are allowed to accumulate. 



Thomas H. Means 1 found water containing as much as 8,000 

 parts per million of soluble salts used in the Desert of Sahara, 

 many of the crops grown being quite sensitive to alkali. The 

 Arab gardens are divided into plots about twenty-feet square, 

 with drainage ditches about three feet deep between them. A 

 large quantity of water is applied at least once a week ; more 

 often, water is applied twice, the check method of irrigation 

 being used. Thus a continuous downward movement of water is 

 maintained, and, since the soils are light and sandy, they are well 

 drained, and there is little opportunity for the soil water to be- 

 come more concentrated than the water applied. 



It is evident that the more salts contained in the water, the 

 better should be the under-drainage, and the more freely the 

 water should be used. On clay soils, the matter is more difficult. 

 Alkali is so hard to remove from some of these soils, even if 

 under-drained, that it is doubtful if any except water of high 

 purity should be used on such soils. 

 1 Circular No. 10, Bureau of Soils. 



