388 IRRIGATION PRACTICE 



purposes depends primarily upon the proportions of the 

 salts in the water. The poisonous action of irrigation 

 water is not the sum of the poisonous actions of its various 

 constituents; for, as observed, the effect of any compound 

 is qualified by the presence of other compounds. The whole 

 subject is in a confused state and needs extensive in- 

 vestigation. 



The real danger in the use of saline waters for irriga- 

 tion, whether they contain 1,000 or 8,000 parts of soluble 

 matter to 1,000,000 parts of water, is in the residue of 

 salts from the evaporated matter. For example, if an 

 irrigation water contains 1,000 parts of soluble matter 

 in each 1,000,000 parts of water, and if 18 inches of this 

 water are used over an acre each year, 4,000 pounds of 

 alkali an acre are added each year. As this is repeated 

 year after year, the accumulation of salts becomes so 

 great as to render the land unfit for farming. To over- 

 come this difficulty it is necessary, when saline waters are 

 employed in irrigation, to reverse the usual rule, and to 

 use quantities of water so large that drainage is assured. 

 The excess of salt is then washed into the country drain- 

 age, and alkali accumulations are prevented. Such hand- 

 ling of saline waters is, however, dangerous in that the 

 excess of water, heavily alkaline, may appear on the 

 lower lands, there to cause injury. Before saline waters 

 are used for irrigation, they should be investigated care- 

 fully as to their composition and their probable effect 

 on the land. 



233. Alkali deposits. The great deposits of alkali or 

 alkali impregnated soils and rocks common to arid 

 countries are another source of alkali. These deposits, 

 yet to be studied exhaustively, are associated with the 

 geological history of the country. In early geological 



