LOSS OF SOIL WATER
LOSS OF SOIL WATER AND MEANS OF CHECKING THE LOSS
We noticed in previous paragraphs that soil might at times have too
much water in it for proper ventilation and so check the growth of the
roots of the plant. Now is it possible that soil water may be lost or
wasted and if so can we check the loss?
In the experiment to find out how well the soils would take in the
rainfall (page 40) we noticed that the clay soil took in the water
very slowly and that on a field of clay soil part of the rain water
would be likely to run off over the surface and be lost. Free water
may be lost then, by surface wash.
We noticed methods of checking this loss, namely, pulverizing the soil
with the tillage tools and putting organic matter into it to make it
absorb the rain more readily.
We noticed that water poured on the sand ran through it very quickly
and was apt to be lost by leaching or percolation. This we found could
be checked by rolling the soil and by putting organic matter into it
to close the pores.
We learned that roots take water from the soil for the use of the
plant and send it up to the leaves, which in turn send it out into the
air, or transpire it, as this process is called. We learned also that
the amount transpired is very great. Now water that is pumped up and
transpired by the crops we are growing we consider properly used. But
when weeds grow with the crop and pump and transpire water we consider
this water as lost or wasted.
Water may be lost then by being pumped up and transpired by weeds. And
this is the way weeds do their greatest injury to crops during dry
weather. The remedy is easily pointed out. Kill the weeds or do not
let them get a start.
There is another way, which we are not apt to notice, by which water
may be lost from the soil. When the soil in the pans in a previous
experiment (page 26) had been wet and set aside a few days it became
very dry. How did the water get out of this soil? That at the surface
of the soil evaporated or was changed into vapor and passed into the
air. Then water from below the surface was pumped up by capillary
force to take its place just as the water was pumped up in the tubes
of soil. This in turn was evaporated and the process repeated till all
of the water in the soil had passed into the air. Now this process is
going on in the field whenever it is not raining or the ground is not
frozen very hard.
Water then may be lost by evaporation.
How can we check this loss?
Suppose we try the experiment of covering the soil with some material
that cannot pump water readily.
=Experiment.=--Take four glass fruit jars, two-quart size, with
straight sides. If you cannot get them with straight sides cut off the
tops with a hot iron just below the shoulder; tin pails will do if the
glass jars cannot be had. Fill these with moist soil from the field or
garden, packing it till it is as hard as the unplowed or unspaded
soil. Leave one of them in this condition; from two of them remove an
inch or two of soil and replace it in the case of one with clean, dry,
coarse sand, and in the case of the other with chaff or straw cut into
half-inch lengths. Stir the soil in the fourth one to a depth of one
inch, leaving it light and crumbly. Now weigh the jars and set them
aside. Weigh each day for several days. The four jars illustrated in
Fig. 30 were prepared in this way and allowed to stand seven days. In
that time they lost the following amounts of water:
Amounts of water lost from jars of prepared soil in seven days.
No. 1 packed soil--lost 5.5 oz. equal to about 75 tons per acre.
No. 2 covered with straw--lost 2 oz. equal to about 27 tons per acre.
No. 3 covered with dry sand--lost 0 oz. equal to about tons per acre.
No. 4 covered with crumbled soil--lost 2.5 oz., equal to about 34 tons
per acre.
Why did not 2, 3 and 4 lose as much water as No. 1?
The soil in jar No. 1 was packed and water was pumped to the surface
by capillary force and was evaporated as fast as it came to the
surface.
In No. 2 the water could rise rapidly until it reached the straw, then
it was stopped almost entirely. But the straw being coarse, the air
circulated in it more or less freely and there was a slow loss by
evaporation. In jar No. 3 the water could rise only to the sand, which
was so coarse that the water could not climb on it to the surface, and
the air circulated in the sand so slowly that there was not sufficient
evaporation to affect scales weighing to one-quarter ounce. No. 4 lost
less than No. 1 because, as in the case of the sand, the water could
not climb rapidly to the surface on the coarse crumbs of soil. The
loss that did take place from No. 4 was what the air took from the
loosely stirred soil on the surface with a very little from the lower
soil. Simply stirring the surface of the sod in No. 4 reduced the loss
of water to less than half the loss from the hard soil in No. 1.
This experiment gives us the clew to the method of checking loss of
water from the soil by evaporation. It is to keep the water from
climbing up to the surface, or check the power of the soil to pump the
water to the surface by making it loose on top. This loose soil is
called a soil mulch. Everything that we do to the soil that loosens
and crumbles the surface tends to check the loss of water by
evaporation from the soil below.
[Illustration: FIG. 30.--TO SHOW THE EFFECT OF A SOIL MULCH
1. Packed soil, lost in 7 days 5.5 ozs. water, equal to 75 tons per
acre.
2. Packed soil, covered with straw, lost in 7 days 2 ozs. water, equal
to 27 tons per acre.
3. Packed soil, covered with sand, lost in 7 days 0 ozs. water, equal to
tons per acre.
4. Packed soil, covered with soil mulch, lost in 7 days 2.5 ozs.
water, equal to 34 tons per acre.]