50 FIELD AND LABORATORY STUDIES OF SOILS 



Discussion: Moist soil as we find it under ordinary 

 conditions in the field holds the water as a thin film around 

 the soil grains. This form of moisture is given the name 

 film moisture, because of the fact that it forms a film over 

 the surface of the tiny soil particles. This moisture is 

 available for the use of the plants that may be growing in 

 the soil. 



If a handful of this moist soil is spread on a board for 

 several days and allowed to dry, it will have the appear- 

 ance of containing no moisture at all. The film moisture 

 will be gone, but the hygroscopic moisture will still be 

 present. This form of moisture, which is not available to 

 plants, varies with the moisture in the air and cannot be 

 driven out without heating the soil. 



The third form in which moisture occurs in the soil is 

 called free or gravitational water. This form is seen seep- 

 ing out of the banks of rivers and smaller streams. 



