82 IRRIGATION PRACTICE 



laws of absorption, they are held very near to the surfaces 

 of the soil grains and are not easily dislodged by the 

 gravitational water passing through the first foot. The 

 downward movement of water is comparatively rapid and 

 largely gravitational; the upward movement compara- 

 tively slow and capillary. Therefore, in irrigated soils, 

 fairly rich in soluble matters, the tendency is to concen- 

 trate the soluble materials in or near the top soil. 



Arid soils are frequently 50 to 70 feet deep and at times 

 that distance from the ground water. The irrigation 

 water in such soils, if wisely applied, moves downward 10 

 to 15 feet. It is only, then, within this limit that the 

 soluble matters are moved upward. If the soil is rich in 

 soluble matters, this concentration may result in injury 

 to the plants; if, as is the usual case, the percentage of 

 soluble matters is low, no injury results, but the plant- 

 foods from lower depths are made easily available to 

 plants. Even where the soil is rich in soluble materials, 

 the farmer can, by judicious irrigation, and by the proper 

 cultivation of the soil, keep the soluble substances so 

 well distributed that no damage can result to the growing 

 crop. 



58. Salinity of river waters. The natural waters 

 used in irrigation are never quite pure, for no natural 

 water is free from dissolved substances. Even rain-water 

 dissolves from the air considerable quantities of nitrates 

 and other substances. When the water that falls upon the 

 land as rain or snow moves toward the rivers by seeping 

 through the soil or by flowing over the ground, it succeeds 

 in dissolving, during its descent, relatively large quanti- 

 ties of soil materials. The more deeply such water soaks 

 into the soil before it finally reappears as a spring, or the 

 longer it flows over the soil, the higher will be its concen- 



