FORMS OF SOIL WATER
Water which comes to the soil and is absorbed exists in the soil
principally in two forms: Free water and capillary water.
FREE WATER
Free water is that form of water which fills our wells, is found in
the bottoms of holes dug in the ground during wet seasons and is often
found standing on the surface of the soil after heavy or long
continued rains. It is sometimes called ground water or standing water
and flows under the influence of gravity.
Is free water good for the roots of farm plants? If we remember how
the root takes its food and moisture, namely through the delicate root
hairs; and also remember the experiment which showed us that roots
need air, we can readily see that free water would give the root hairs
enough moisture, but it would at the same time drown them by cutting
off the air. Therefore free water is not directly useful to the roots
of house plants or farm plants, excepting such as are naturally
swamp plants, like rice, which grows part of the time with its
roots covered with free water.
[Illustration: FIG. 26.--CAPILLARY TUBES.
To show how water rises in small tubes or is drawn into small spaces.]
[Illustration: FIG. 27.--CAPILLARY PLATES.
Water is drawn to the highest point where the glass plates are closest
together.]
[Illustration: FIG. 28.
A cone of soil to show capillarity. Water poured about the base of
this cone of soil has been drawn by capillary force half-way to the
top.]
[Illustration: FIG. 29.
To show the relative amounts of film-moisture held by coarse and fine
soils. The colored water in bottle _A_ represents the amount of water
required to cover the half pound of pebbles in the tumbler _B_ with a
film of moisture. The colored water in bottle _C_ shows the amount
required to cover the soil grains in the half pound of sand in tumbler
_D._]
CAPILLARY WATER
If you will take a number of glass tubes of different sizes, the
largest not more than one-fourth of an inch in diameter, and hold them
with one end of each in water or some colored liquid, you will notice
that the water rises in the tubes (Fig. 26), and that it rises highest
in the smallest tube. The force which causes the water to rise in
these tubes is called the capillary force, from the old Latin word
_capillum_ (a hair), because it is most marked in hair-like tubes, the
smaller the tube the higher the water will rise. The water which rises
in the tubes is called capillary water.
Another method of illustrating capillary water is to tie or hold
together two flat pieces of glass, keeping two of the edges close
together and separating the opposite two about one-eighth of an inch
with a sliver of wood. Then set them in a plate of water or colored
liquid and notice how the water rises between the pieces of glass,
rising higher the smaller the space (Fig. 27). It is the capillary
force which causes water to rise in a piece of cloth or paper dipped
in water.
Take a plate and pour onto it a cone-shaped pile of dry sand or fine
soil; then pour water around the base of the pile and note how the
water is drawn up into the soil by capillary force (Fig. 28).
Capillary water is the other important form of water in the soil. This
is moisture which is drawn by capillary force or soaks into the spaces
between the soil particles and covers each particle with a thin film
of moisture.
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
develop. If there is a tendency for free water to fill the soil a
large part of the time, the farmer can get rid of it by draining the
land. We get here a lesson for the grower of house plants also. It is
that we must be careful that the soil in the pots or boxes in which
our plants are growing is always supplied with film water and not wet
and soggy with free water. Water should not be left standing long in
the saucer under the pot of a growing plant. It is best to water the
pot from the top and let the surplus water drain into the saucer and
then empty it out.
Which soils have the greatest capacity for film water?
=Experiment.=--Place in a tumbler or bottle one-half pound of pebbles
about the size of a pea or bean; pour a few drops of water on them and
shake them; continue adding water and shaking them till every pebble
is covered with a film of water; let any surplus water drain off. Then
weigh again; the difference in the two weights will be approximately
the weight of the film water that the pebbles can carry. Repeat this
with sand and compare the two amounts of water. A striking
illustration can be made by taking two slender bottles and placing in
them amounts of colored water equal to the amounts of film water held
by the pebbles and sand respectively. In the accompanying illustration
(Fig. 29), _A_ represents the amount of water that was found necessary
to cover the pebbles in tumbler _B_ with a film of moisture. _C_ is
the amount that was necessary to cover with a film the particles of
sand in _D_. The finer soil has the greater area for film moisture. It
has been estimated that the particles of a cubic foot of clay loam
have a possible aggregate film surface of three-fourths of an acre.