RELATION OF SOILS TO WATER
IMPORTANCE OF WATER TO PLANTS
We learned in a previous paragraph that plant roots take moisture from
the soil. What becomes of this moisture? We will answer this question
with an experiment.
=Experiment.=--Take a pot or tumbler in which a young plant is
growing, also a piece of pasteboard large enough to cover the top of
the pot or tumbler; cut a slit from the edge to the centre of the
board, then place it on top of the pot, letting the stem of the plant
enter the slit. Now close the slit with wax or tallow, making it
perfectly tight about the stem. If the plant is not too large invert a
tumbler over it (Fig. 21), letting the edge of the tumbler rest on the
pasteboard; if a tumbler is not large enough use a glass jar. Place in
a sunny window. Moisture will be seen collecting on the inner surface
of the glass. Where does this come from? It is absorbed from the soil
by the roots of the plant and is sent with its load of dissolved plant
food up through the stem to the leaves. There most of the moisture is
passed from the leaves to the air and some of it is condensed on the
side of the glass.
By experiments at the Cornell University Agricultural Experiment
Station, Ithaca, N.Y., it has been found that during the growth of a
sixty bushel crop of corn the plants pump from the soil by means of
their roots, and send into the air through their leaves over nine
hundred tons of water. A twenty-five bushel crop of wheat uses over
five hundred tons of water in the same way. This gives us some idea of
the importance of water to the plant and the necessity of knowing
something of the power of the soil to absorb and hold moisture for the
use of the plant. Also the importance of knowing if we can in any way
control or influence the water-holding power of the soil for the good
of the plant.
SOURCES OF SOIL WATER
From what sources does the soil receive water? From the air above, in
the form of rain, dew, hail and snow, falling on the surface, and from
the lower soil. This water enters the soil more or less rapidly.
ATTITUDE OF THE SOILS TOWARDS WATER
Which soils have the greater power to take in the rain which falls on
their surface?
[Illustration: FIG. 21.
To show what becomes of the water taken from the soil by roots.
Moisture, sent up from the roots, has been given off by the leaves and
has condensed on the glass.]
[Illustration: FIG. 22.--PERCOLATION EXPERIMENT.
To show the relative powers of soils to take in water falling on the
surface. _A_, sand; _B_, clay; _C_, humus; _D_, garden soil.]
=Experiment.=--Take four student-lamp chimneys. (In case the chimneys
cannot be found get some slender bottles like salad oil bottles or
wine bottles and cut the bottoms off with a hot rod. While the rod
is heating make a shallow notch in the glass with the wet corner of a
file in the direction you wish to make the cut. When the rod is hot
lay the end of it lengthwise on the notch. Very soon a little crack
will be seen to start from the notch. Lead this crack around the
bottle with the hot rod and the bottom of the bottle will drop off.)
(Fig. 23.) Make a rack to hold them. Tie a piece of cheese cloth or
other thin cloth over the small ends of the chimneys. Then fill them
nearly full respectively, of dry, sifted, coarse sand, clay, humus
soil, and garden soil. Place them in the rack; place under them a pan
or dish. Pour water in the upper ends of the tubes until it soaks
through and drips from the lower end (Fig. 22). Ordinary sunburner
lamp chimneys may be used for the experiment by tying the cloth over
the tops; then invert them, fill them with soil and set in plates or
pans. The sand will take the water in and let it run through quickly;
the clay is very slow to take it in and let it run through; the humus
soil takes the water in quite readily. Repeat the experiment with one
of the soils, packing the soil tightly in one tube and leaving it
loose in another. The water will be found to penetrate the loose soil
more rapidly than the packed soil. We see then that the power of the
soil to take in rainfall depends on its texture or the size and
compactness of the particles.
If the soil of our farm is largely clay, what happens to the rain that
falls on it? The clay takes the water in so slowly that most of it
runs off and is lost. Very likely it carries with it some of the
surface soil which it has soaked and loosened, and thus leaves the
farm washed and gullied.
What can we do for our clay soils to help them to absorb the rain more
rapidly? For immediate results we can plow them and keep them loose
and open with the tillage tools. For more permanent results we may mix
sand with them, but sand is not always to be obtained and is expensive
to haul. The best method is to mix organic matter with them by plowing
in stable manures, or woods soil, or decayed leaves, or by growing
crops and turning them under. The organic matter not only loosens the
soil but also adds plant food to it, and during its decay produces
carbonic acid which helps to dissolve the mineral matter and make
available the plant food that is in it.
Clay soils can also be made loose and open by applying lime to them.
=Experiment.=--Take two bottles or jars, put therein a few spoonsful
of clay soil, fill with water, put a little lime in one of them, shake
both and set them on the table. It will be noticed that the clay in
the bottle containing lime settles in flakes or crumbs, and much
faster than in the other bottle. In the same manner, lime applied to a
field of clay has a tendency to collect the very fine particles of
soil into flakes or crumbs and give it somewhat the open texture of a
sandy soil. Lime is applied to soil for this purpose at the rate of
twenty bushels per acre once in four or five years.
Which soils have the greater power to absorb or pump moisture from
below?
=Experiment.=--Use the same or a similar set of tubes as in the
experiment illustrated in Fig. 23. Fill the tubes with the same kinds
of dry sifted soils. Then pour water into the pan or dish beneath the
tubes until it rises a quarter of an inch above the lower end of the
tubes (Fig. 24). Watch the water rise in the soils. The water will be
found to rise rapidly in the sand about two or three inches and then
stop or continue very slowly a short distance further. In the clay it
starts very slowly, but after several hours is finally carried to the
top of the soil. The organic matter takes it up less rapidly than the
sand, faster than the clay, and finally carries it to the top. By this
and further experiments it will be found that the power of soils to
take moisture from below depends on their texture or the size and
closeness of their particles.
We found the sand pumped the water only a short distance and then
stopped.
What can we do for our sandy soils to give them greater power to take
moisture from below? For immediate results we can compact them by
rolling or packing. This brings the particles closer together, makes
the spaces between them smaller, and therefore allows the water to
climb higher. For more lasting results we can fill them with organic
matter in the shape of stable manures or crops turned under. Clay may
be used, but is expensive to haul.
Which soils have greatest power to hold the water which enters them?
=Experiment.=--Use the same or similar apparatus as for the last
experiment. After placing the cloth caps over the ends of the tubes
label and carefully weigh each one, keeping a record of each; then
fill them with the dry soils and weigh again. Now place the tubes in
the rack and pour water in the upper ends until the entire soil is
wet; cover the tops and allow the surplus water to drain out; when the
dripping stops, weigh the tubes again, and by subtraction find the
amount of water held by the soil in each tube; compute the percentage.
It will be found that the organic matter will hold a much larger
percentage of water than the other soils; and the clay more than the
sand. The tube of organic soil will actually hold a larger amount of
water than the other tubes. (See also Fig. 25.)
In the experiment on page 40 we noticed that the sand took in the
water poured on its surface and let it run through very quickly. This
is a fault of sandy soils.
What can we do for our sandy soils to help them to hold better the
moisture which falls on them and tends to leach through them? For
immediate effect we can close the pores somewhat by compacting the
soil with the roller. For more lasting effects, we can fill them with
organic matter.
Which soils will hold longest the water which they have absorbed? Or
which soils will keep moist longest in dry weather?
[Illustration: FIG. 23.
To show how bottles may be used in place of lamp chimneys shown in
Figs 22 and 24.]
[Illustration: FIG. 24.--CAPILLARITY OF SOILS
To show the relative powers of soils to take water from below.]
[Illustration: FIG. 25.--WATER-ABSORBING AND WATER-HOLDING POWERS OF
SOILS.]
=Experiment.=--Fill a pan or bucket with moist sand, another with
moist clay, and a third with moist organic matter; set them in the sun
to dry and notice which dries last. The organic matter will be found
to hold moisture much longer than the other soils. The power of the
other soils to hold moisture through dry weather can be improved by
mixing organic matter with them.
We find then that the power of soils to absorb and hold moisture
depends on the amount of sand, clay, or humus which they contain, and
the compactness of the particles. We see also how useful organic
matter is in improving sandy and clayey soils.
THE EFFECT OF WORKING SOILS WHEN WET
By this time the soils we left in the pans (see page 26), sand, clay,
humus and garden soil, must be dry. If so, examine them. We find that
the clay which was stirred when wet has dried into an almost bricklike
mass, while that which was not stirred is not so hard, though it has a
thick, hard crust. The sand is not much affected by stirring when wet.
The organic matter which was stirred when wet has perhaps stiffened a
little, but very easily crumbles; the unstirred part was not much
affected by the wetting and drying.
The garden soil after drying is not as stiff as the clay nor as loose
as the sand and humus. This is because it is very likely a mixture of
all three, the sand and the humus checking the baking. This teaches
us that it is not a good plan to work soils when they are wet if they
are stiff and sticky; and that our stiff clay soils can be kept from
drying hard or baking by the use of organic matter. "And that's a
witness" for organic matter.
The relation of the soil to moisture is very important, for moisture
is one of the greatest factors if not the greatest in the growth of
the crop.
The power to absorb or soak up moisture from any source is greatest in
those soils whose particles are smaller and fit closer together.
It is for this reason that strong loams and clay soils absorb and hold
three times as much water as sandy soils do, while peaty or humus
soils absorb a still larger proportion.
The reason why crops burn up so quickly on sandy soils during dry
seasons is because of their weak power to hold water.
The clay and humus soils carry crops through dry weather better
because of their power to hold moisture and to absorb or soak up
moisture from below. It is for this reason also that clay and peaty
soils more often need draining than sandy soils.
When rain falls on a sandy soil it enters readily, but it is apt to
pass rapidly down and be, to a great extent, lost in the subsoil, for
the sand has not sufficient power to hold much of it.
When rain falls on a clay soil it enters less readily because of the
closeness of the particles, and during long rains or heavy showers
some of the water may run off the surface. If the surface has been
recently broken and softened with the plow or cultivator the rain
enters more readily. What does enter is held and is not allowed to run
through as in the case of the sand.
Humus soil absorbs the rain as readily as the sand and holds it with a
firmer grip than clay.
This fact gives us a hint as to how we may improve the sand and clay.
Organic matter mixed with these soils by applying manures or plowing
under green crops will cause the sand to hold the rain better and the
clay to absorb it more readily.