SOIL CHANGES DUE TO IRRIGATION 79 



abnormally rich in soluble constituents. Normal soils, by 

 the power of absorption, retain most of the soluble mate- 

 rials, so that the concentration of the drainage water is 

 kept low, as above given. 



55. Concentration of soil moisture. Results of strik- 

 ing interest are obtained when the possible concentra- 

 tions of soil water are calculated. If it be 1 assumed that 

 a soil with .1 per cent of soluble matter under ordinary 

 laboratory methods contains an average of 20 per cent of 

 moisture to a depth of 10 feet, which is the approximate 

 condition of an irrigated clay loam immediately after a 

 5-inch irrigation, and if all the soluble matter goes into 

 solution in the water thus added, the soil solution will have 

 a concentration of about 5,000 parts of dissolved matter 

 for every 1,000,000 parts of water. This is far in excess of 

 the composition of any drainage water from such soils 

 under normal field conditions. Moreover, as evapora- 

 tion goes on, this concentration must increase consider- 

 ably. Arid soils usually contain more than .1 per cent of 

 soluble matter; if .5 per cent is held by the soil, for instance, 

 the concentration under the above assumptions will be 

 25,000 parts of dissolved substance for every 1,000,000 

 parts of water a concentration larger than that of the 

 drainage water from the above mentioned alkali reclama- 

 tion tract near Salt Lake City. Little is known, as yet, 

 about the exact concentration of soluble matter in the 

 film held about the soil grains; but it must be compara- 

 tively high. In such solutions the feeding roots of plants 

 are bathed. 



56. Loss by drainage. The repeated application of 

 water to soils, in quantities sufficient to drain through, 

 results disastrously, because, even though the quantity of 

 soluble matter taken out each time is small, in the end the 



