90 IRRIGATION PRACTICE 



64. Need of water surveys. The substances con- 

 tained by the water may in themselves be harmless ; but, 

 since they are applied to the soil from year to year in 

 such large quantities, they undoubtedly often fill many of 

 the capillary soil spaces or are deposited on the surfaces 

 of the soil grains, and thus affect the chemical composi- 

 tion and the granular condition of the soil. This subject 

 has as yet been poorly investigated, but is worthy of 

 careful investigation, so that irrigation practices may be 

 rationalized from the point of view of the varying com- 

 position of irrigation water. Systematic chemical sur- 

 veys of irrigation waters should be made in connection 

 with the study of the soils to which the waters are to be 

 applied. Only when such data are abundantly at hand 

 will it be possible to formulate for each section irrigation 

 practices that will be permanently satisfactory. In the 

 present stage of irrigation progress, it has become very 

 important to know the composition of irrigation waters. 

 As irrigation becomes older more problems will arise, 

 many of which can be solved only by a more thorough 

 knowledge of the waters used on irrigated soils. Water 

 surveys are as legitimate in irrigated districts as are soil 

 surveys. 



65. Composition of natural waters. While the total 

 quantity of soluble matter found in a given volume of 

 irrigation water is of great importance, the composition 

 of such soluble matter is of equal importance. In soils 

 are found the great majority of the chemical elements 

 and particularly those that are essential in plant-growth. 

 In the following table may be found the composition of a 

 number of natural waters selected from the data given 

 by Clarke. 



