SOIL WATER, DRAINAGE, AND IRRIGATION 295 



soil particles (fig. 142), 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, measuring their 

 surfaces, and comparing the last measure- 

 ment with the first. A marble exactly 

 1 inch in diameter will just slip inside 

 a cube 1 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 fill the cubic 

 inch completely, and their aggregate sur- 

 face area would be 3141.59 square inches. 

 In coarse silt the surface area of the 

 particles reaches an astonishing figure, 

 since in 1 cubic foot of such soil the 

 particles would have a surface area of 

 37,700 square feet. So large a surface 

 area makes possible a large water content. In some kinds of 

 clay 100 pounds of the wet soil may contain 40 pounds of 

 water, or 40 per cent of the whole weight. Since 1 cubic 

 foot of clay ordinarily weighs about 80 pounds, 40 per cent 

 of it, or 32 pounds, might be water. These facts make clear 

 the tremendous differences in the water-holding power of soils. 

 298. Soil water and soil temperature. Anyone who has 

 walked back and forth along the lake or ocean shore while 

 in bathing has noticed differences in the temperature of the 

 soil, and any well-trained gardener who desires to grow early 

 spring vegetables selects what he calls a " warm soil." What 



Fie. 142. Distribution 

 of soil moisture 



Diagram to represent 

 il particles, each cov- 

 d by a film of water. 

 wet weather or imme- 

 itely after a rain all 

 e spaces might be filled 

 with water 



