Whenever there is a sufficient amount of water available, a thin
film of water is found around every soil grain; and where the soil
grains touch, or where they are very near together, water is held
pretty much as in capillary tubes. Not only are the soil particles
enveloped by such a film, but the plant roots foraging in the soil
are likewise covered; that is, the whole system of soil grains and
roots is covered, under favorable conditions, with a thin film of
capillary water. It is the water in this form upon which plants draw
during their periods of growth. The hygroscopic water and the
gravitational 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 possible 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.
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