FILM WATER

Take a marble or a pebble, dip it into water and notice the thin layer
or film of water that clings to it. This is a form of capillary water
and is sometimes called film water or film moisture. Take a handful of
soil that is moist but not wet, notice that it does not wet the hand,
and yet there is moisture all through it; each particle is covered
with a very thin film of water.

Now this film water is just the form of water that can supply the very
slender root hairs without drowning them, that is, without keeping the
air from them. And the plant grower should see to it that the roots of
his plants are well supplied with film water and are not drowned by
the presence of free water. Capillary water may sometimes completely
fill the spaces between the soil particles; when this occurs the roots
are drowned just as in the case of free water as we saw when cuttings
were placed in the puddled clay (see Fig. 18). Free water is
indirectly of use to the plant because it serves as a supply for
capillary and film moisture.

Now I think we can answer the question which was asked when we were
studying the habit of growth of roots but was left unanswered at the
time (see page 14). The question was this: Of what value is it to the
farmer to know that roots enter the soil to a depth of three to six
feet? We know that roots will not grow without air. We also know that
if the soil is full of free water there is no air in it, and,
therefore, roots of most plants will not grow in it. It is, therefore,
of interest to the farmer to see that free water does not come within
at least three or four feet of the surface of the soil so that the
roots of his crops may have plenty of well ventilated soil in which to