There will then be no appreciable
movement of the soil-moisture. Such a condition is approximated in
late winter or early spring before planting begins. During the
greater part of the year, however, no such quiescent state can
occur, for there are numerous disturbing elements that normally are
active, among which the three most effective are (l) the addition of
water to the soil by rains; (2) the evaporation of water from the
topsoil, due to the more active meteorological factors during
spring, summer, and fall; and (3) the abstraction of water from the
soil by plant roots.
Water, entering the soil, moves downward under the influence of
gravity as gravitational water, until under the attractive influence
of the soil it has been converted into capillary water and adheres
to the soil particles as a film. If the soil were dry, and the film
therefore thin, the rain water would move downward only a short
distance as gravitational water; if the soil were wet, and the film
therefore thick, the water would move down to a greater distance
before being exhausted. If, as is often the case in humid districts,
the soil is saturated, that is, the film is as thick as the
particles can hold, the water would pass right through the soil and
connect with the standing water below.
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