FORMS OF SOIL MOISTURE 



141 



68. Forms and availability. There are three forms 

 in which water may exist in soils: (1) Gravitational 

 water, or that which is free to move through the soil 

 under the influence of gravity. (2) Capillary or film 

 water, or that which is held against gravity by the 

 surface tension of the films of water surrounding the 

 soil particles. (3) Hygroscopic moisture, or that which 

 condenses from the atmosphere on the surface of the 



HYGROSCOPIC 



FORMS OF SOIL WATER 



CAPILLARY 



GRAVITATIONAL 



UNAVAILABLE AVAILABLE INJURIOUS 



AVAILABILITY OF' SOIL WATER TO PLANTS 



Fig. 43. Diagram illustrating the forms, proportions and 



availibility of soil water. 



soil particles, when the soil is allowed to become air 

 dry. 



There is no sharp change in the moisture condition 

 of the soil in passing from one form to the other. Still, 

 it is true that there are certain marked changes in some 

 of the physical properties of the soil, such as volume, 

 weight and resistance to penetration, which are in a 

 general way associated with these transition points. 



Not all of the water in the soil is available to use of 

 plants. It is a matter of general experience that for 

 most farm crops the saturated condition of the soil 

 is unfavorable to the best development. There are, 

 of course, many plants which are adapted to such con- 



