CAPILLARY WATER IN THE SOIL 107 
growth. The hygroscopic water and the gravita- 
tional water are of comparatively little value in 
plant growth. 
Field capacity of soils for capillary water 
The tremendously large number of soil grains 
found in even a small amount of soil makes it pos- 
sible for the soil to hold very large quantities of 
capillary water. To illustrate: In one cubic inch 
of sand soil the total surface exposed by the soil 
grains varies from 42 square inches to 27 square 
feet; in one cubic inch of silt soil, from 27 square 
feet to 72 square feet, and in one cubic inch of an 
ordinary soil the total surface exposed by the soil 
grains is about 25 square feet. This means that the 
total surface of the soil grains contained in a column 
of soil 1 square foot at the top and 10 feet deep is 
approximately 10 acres. When even a thin film 
of water is spread over such a large area, it is clear 
that the total amount of water involved must be 
large. It is to be noticed, therefore, that the fine- 
ness of the soil particles previously discussed has a 
direct bearing upon the amount of water that soils 
may retain for the use of plant growth. As the fine- 
ness of the soil grains increases, the total surface 
increases, and the water-holding capacity also 
increases. 
Naturally, the thickness of a water film held around 
