SOIL WATER 63 



coarse ones. Clay has a large absorptive capacity and 

 the presence of certain compounds increases immensely the 

 content of hygroscopic moisture. 



65. Capillary water. — The essential difference between 

 capillary water and hygroscopic water is that the former is 

 capable of motion and most of it may be used by plants. 

 The fact that the capillary film is 

 thicker causes it to be less firmly held 

 by the soil particles, in consequence of 

 which the water near the outer surface 

 of the film can move in response to 

 certain forces, and the absorptive ac- 

 tion of roots is sufficient to withdraw FlG 13 _ D i agram _ 



it, Until the film becomes SO thin that matic drawing of soil par- 



very little except hygroscopic water £^ ™^! 

 remains. The difference between hy- lary water, (s) soil parti- 

 groscopic water and capillary water is ^^^^ ^ 

 illustrated in Fig. 13. 



66. Capillary water capacity. — Comparatively large 

 quantities of water may be held in soils by capillarity. In 

 fact by far the major portion of water used by crops is ob- 

 tained from the capillary form. The quantity present 

 varies with different soils and at different times in the same 

 soil. The conditions that tend to increase the capillary 

 moisture content of soil are the following : 



LA fine-grained texture, or in other words a large pro- 

 portion of small particles. Thus in a test of a fine sand, 

 a sandy loam and a clay soil they were found to contain 

 respectively 10, 15 and 20 percent of capillary water in 

 addition to the hygroscopic water. 



2. A soil structure that gives a maximum effective sur- 

 face exposure within the soil. For this reason the granula- 

 tion of a clay soil, or the compacting of a coarse sand will 

 cause a rise in its capillary capacity. 



