THE FORMS OF SOIL-WATER 159 



centage would be considerably higher, in some cases reaching 

 50 or 60 per cent. It must always be kept in mind, however, 

 that the point designated as the hygroscopic coefficient is more 

 or less arbitrary and that there is no sharp line of demarca- 

 tion between the moisture designated as hygroscopic and that 

 which lies near it, but is called capillary. 



89. The capillary water. 1 — The moisture above the 

 hygroscopic coefficient but not free to respond to gravity is 

 generally spoken of as the capillary water. The portion of 

 this moisture lying in contact or in the immediate neighbor- 

 hood of the hygroscopic water is probably capable of only 

 sluggish diffusion movement if any. 2 This part of the capillary 

 moisture is held largely by the colloidal matter and may be 

 considered as transitional between the true hygroscopic and 

 the more active capillary portion. Although so closely related 

 to the hygroscopic water in general properties and character- 

 istics, the soil does not assume it by absorption from vapor- 

 laden air. This separates it at least analytically from the 

 hygroscopic form of moisture. Moreover, it is probably 

 largely in the liquid state, which is hardly true of all of the 

 hygroscopic water. 



The more active capillary water exists in the large inter- 

 stices and as a film over the particles and the colloidal com- 

 plexes. It is held rather loosely by the soil, yet strongly 

 enough to counteract gravitation. This part of the capillary 

 moisture, being more or less beyond colloidal influence, is 

 free to respond to the forces active in true solutions and, there- 

 fore, may move from place to place as equilibrium stresses 

 may demand. "While the inner portion of the capillary water 

 is held by the absorptive power of the colloidal surfaces, the 

 outer and freer portion is maintained by the surface tension 



1 The colloidal conceptions regarding soil-moisture has made it advis- 

 able to give the term capillary a broader significance than its root 

 meaning justifies. 



a Bouyoucos, G. J., A New Classification of the Soil Moisture; Soil 

 Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 



