SOIL CHANGES DUE TO IRRIGATION 81 



In view of this danger, the irrigation farmer must so con- 

 trol the application of water to the soil that no more is 

 added than is necessary to produce the maximum film 

 around the soil grains. Drainage must, as a rule, be 

 avoided. A knowledge of the depth and character of the 

 soil and devices for measuring water make this easily done. 



57. Upward leaching. In yet another manner is the 

 nature of the soil materially influenced by irrigation. If 

 water is applied in moderation, and according to the best 

 principles of irrigation, the soil-water film is simply thick- 

 ened to a distance greater or smaller, according to the 

 quantity applied. The water thus added is in part lost 

 by evaporation at the top soil, and in part is taken from 

 the soil through the plant roots. While the plant roots 

 often penetrate the soil to a depth of 8 to 10 feet or more, 

 yet the greatest abundance of plant roots is found in the 

 upper soil. Under heavy irrigation, especially, when 

 plants are not obliged to drive their roots deeply in search 

 of water, the greatest root-development is usually found 

 in the upper 3 feet or so of the soil. However, even these 

 surface roots draw water from much greater depths; for, 

 as has already been explained, the removal of water in an 

 upper soil results in a slow capillary flow of water from 

 below, to re-establish equilibrium. As the water moves 

 upward, to replace that removed by the roots, it carries 

 with it some of the materials dissolved from the lower 

 soil layers. 



Under wise irrigation, therefore, there is a gradual 

 movement of the soluble soil constituents toward the sur- 

 face, where the soil moisture often becomes so concen- 

 trated that the salts crystallize out and form layers of 

 alkali. When irrigation is again applied, these soluble 

 matters are in part washed downward; but, owing to the 

 F 



