THE FORMS OF SOIL WATER 20l 



texture and the structure of the soil and the character 

 of the crop. Assuming for the time being, however, that 

 the pore space is entirely filled with water, or, in other 

 words, that the soil is saturated, three forms of water 

 are found to be present hygroscopic, capillary, and 

 free, or gravitational. These forms differ, not in their 

 composition, but in the position that they occupy in rela- 

 tion to the soil particles. 



The hygroscopic and capillary w r ater are both film 

 forms ; that is, they surround the soil particle, being held 

 partly by the attraction of the particle and partly by the 

 molecular attraction of the liquid for itself. The hygro- 

 scopic film is very thin, being water of condensation, or 

 adsorption. When this film is satisfied and moisture is 

 still present, the capillary water film begins to form. The 

 line of demarcation between hygroscopic and capillary 

 water is not sharp. The general difference between the 

 two forms may be considered as being not only one of 

 position, but also one of movement, this power being pos- 

 sessed only by the capillary film. With a change in any 

 controlling condition, such as temperature, hygroscopic 

 water may change to capillary, or capillary water to 

 hygroscopic, as the case may be. As the capillary water 

 continues to increase and the film becomes thicker and 

 thicker, a point is at last reached at which gravity over- 

 comes the surface tension of the liquid and drops of water 

 form which tend to move downward through the air 

 spaces, being now subject to movement by the attrac- 

 tion of gravity. Free, or gravitational, water then also 

 becomes present in the soil. If water is still added, the 

 gravitational water continues to increase until the air 

 is almost entirely displaced and a saturated condition 

 results. There may be a change of capillary to free water 



