SOIL CHANGES DUE TO IRRIGATION 



77 



tain definite thickness. Water added beyond this point 

 fills the capillary tubes and under the influence of gravity 

 moves downward into the country drainage. As this 

 gravitational water moves downward, the soil-water film 

 clinging closely around the soil grains is not materially 

 affected. A small part of the outer film may be carried 

 downward, but the inner part, near the surfaces of the 

 soil grains, where the dissolved soil constituents are held 

 in greatest concentration, probably does not move at all 

 with the gravitational water. Enough is carried along, 

 however, to affect materially the composition of the drain- 

 age water. In one of the Utah experiments, water was 

 applied to a very loose gravelly soil, scarcely 2 feet deep, 

 and underlaid with a cobble rock formation of unknown 

 depth. Underground collecting chambers were con- 

 structed to collect the drainage water. So gravelly was 

 the land that within half an hour after water had been 

 applied, it drained through into the lysimeters. As an 

 average of one season's test, the following results were 

 obtained : 



It will be noted from these figures that, even under 

 conditions of easy and rapid drainage, much valuable 

 material is washed out of the soil. Nevertheless, as will 

 be shown, the parts of total solids in 1,000,000 parts of 



