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 



