SOIL MOISTURE 19 



tube, when the air spaces in the soil are completely filled 

 with water. This constant has no great agricultural value. 

 It represents a condition that should be avoided so far 

 as is possible in farming under irrigation, except in the 

 top foot of the soil while water is actually being applied 

 to the land. 



20. Summary. The principles upon which rational 

 irrigation practices are built, rest upon the facts pre- 

 sented in this chapter. The attraction existing between 

 soils and water causes water to cling as a film around the 

 soil grains. The hygroscopic coefficient represents the 

 water which is condensed from the water vapor of sat- 

 urated air; it is of no practical value to plants. The wilt- 

 ing coefficient, about one and one-half times the hygro- 

 scopic coefficient, represents the point below which plants 

 can not secure sufficient water from the soil for their needs. 

 The lento-capillary point is the point above which the 

 soil moisture is readily available to plants. Above this 

 point, also, film water moves freely in obedience to the 

 laws of capillarity. The maximum capillary capacity 

 represents the point at which the attraction between 

 the soil surface and water ceases to be active; it is satu- 

 rated. From the first hygroscopic coating to the maximum 

 capillary water capacity, the water is said to be in a capil- 

 lary state. Any water added above this point is free 

 water moving in obedience to the pull of gravity. (Fig. 5.) 



Much excellent work has been done on soil moisture 

 by investigators, both in Europe and America. F. H. 

 King, E. W. Hilgard, Milton Whitney and his associates, 

 have done much of the American work. Unfortunately, 

 for the arid regions, most of the work on soil moisture 

 has been done under humid conditions. For instance, 

 capillarity has been studied almost entirely by placing 



