SOIL CHANGES DUE TO IRRIGATION 87 



from extreme purity to a concentration comparable with 

 saturated waters of inland lakes. 



61. Soil moisture and natural waters compared. As 

 shown above, the soil solution of a clayey loam contain- 

 ing about .1 per cent of soluble matter will contain in the 

 neighborhood of 5,000 parts of dissolved matter for 1,000,- 

 000 parts of water. This is considerably higher than the 

 concentration of the larger number of river waters, or even 

 of mineral springs. In the arid regions, the soluble matter 

 of soils often exceeds .1 per cent, and the concentration 

 of the soil solution, after irrigation, is probably higher 

 than 5,000. Moreover, if the top soil is not thoroughly 

 stirred, evaporation from the soil surface goes on very 

 rapidly and the soil solution becomes so concentrated 

 that, before the next irrigation, the concentration must be 

 nearly twice what it is immediately after an irrigation. 

 The effect of varying quantities of dissolved substances 

 in irrigation water on the growth of plants will be dis- 

 cussed in the chapter on alkali. It is of very great impor- 

 tance to the irrigation farmer. 



62. Ash constituents added by irrigation water. 

 When the quantities of water used in irrigation are so 

 large that there is a constant drainage through the soil, 

 the only probable effect of the water on the soil is the wash- 

 ing out of certain soil constitutents. When water is 

 added in moderation, so that the soil is filled to a certain 

 depth, but not in sufficient quantity to drain through, the 

 soluble matters contained by the water must of necessity 

 remain in the soil, except as they may be utilized by the 

 plant. Under existing practices 2 acre-feet of water 

 represent a very moderate annual irrigation. On suffi- 

 ciently deep soils, if the single applications are not too 

 large, this quantity of water does not cause material 



