CHAPTER XX 



SOIL WATER, DRAINAGE, AND IRRIGATION 



226. Amount of water in the soil. It is commonly observed 

 that coarse, gravelly soils do not hold water for a long time 

 and that fine soils do. If a series of glass tubes are filled with 

 soils of different degrees of coarseness, and the same amount 

 of water is poured into each, it will be noted that less flows 

 out from the finer soil. Why is this ? 

 Water may be held in the soil in either 

 of two ways : it may adhere closely to 

 the surfaces of the soil particles, in which 

 case it is known as water of adhesion, 

 or it may be free between soil particles 

 (fig. 100), when it is known as free 

 water. Most of the free water may flow 

 away if there is adequate outlet for it, 

 but adhesion water is not easily removed. 



Obviously the more surface exposed, 

 the more water may be held by adhesion. 

 In the finer soils much more surface is 

 exposed than in coarse soils. This may 

 be illustrated readily by measuring the 

 exposed surface of a cube, then cutting 

 the cube into many small cubes and measuring their surfaces 

 and comparing the last measurement with the first. A special 

 student of soils, F. H. King, says : " Suppose we take a marble 

 exactly one inch in diameter. It will just slip inside a cube 

 one inch on each side, and will hold on its surface a film of 

 water 3.1416 square inches in area." If, however, we reduce the 

 diameter of the sphere " to one thousandth of an inch, it will 

 require 1,000,000,000 of them to completely fill the cubic 



205 



FIG. 100. Distribution 

 of soil moisture 



Diagram to represent 

 soil particles, each cov- 

 ered by a film of water. 

 In wet weather or imme- 

 diately after a rain all 

 the spaces might be filled 

 with water 



