PRESS BULLETIN No. 116.
Florida Agricullural Experiment Stallon.
THE AIR IN THE SOIL.
BY B. F. FLOYD.
AIR FOR THE ROOTS.
We may forget that roots breathe the same as do other parts of the
plant. They obtain oxygen from the air and throw off carbon dioxide gas.
The amount of air that is available for the use of the roots depends princi-
pally upon the condition of the soil. The spaces that occur between the soil
particles are filled with air from which the roots obtain their oxygen. These
spaces vary with the kind of soil and the treatment it has received. In clay
soils these spaces are very small; hence, under equal conditions, the roots in
clay soils are not likely to be as well supplied with air as are those in sandy
soils, where the spaces are much larger.
AERATION BY PLOWING OR DRAINING.
A most important result obtained from plowing or draining, is the admis-
sion of air into the soil. Soil that is water-logged has its air-spaces filled
with water. Nearly all of our farm crops, excepting rice, would die in such a
soil for lack of oxygen for the roots. As the water is removed by drainage,
air comes in to occupy the spaces that were filled with water, and the soil is
improved. Water, passing through the soil after a rain or after irrigation,
tends to ventilate the soil by putting the air in motion. This accounts for
some of the advantages obtained from rain and irrigation.
Clay soils should not be plowed while wet, as this presses out the air and
leaves the soil in a poorly aerated condition. Cultivation when the soil is in
the right condition, breaks the cakes or clods and allows the air to enter
freely. Plowing at the same depth, year after year, produces a hard pan
known as a plowsole. This interferes with the free passage of roots, mois-
ture, and air to the subsoil. Varying the plowing depth each year will pre-
vent this condition.
After rains or irrigation, soils are apt to have a crust formed. This pre-
vents the free passage of air into the soil. We should, therefore, use a culti-
vator to break this crust.
In sandy soils, the spaces between the soil particles being large, air en-
ters freely under ordinary conditions, and constant stirring of the soil may
allow an excessive amount of air to pass in.
The soil is inhabited by micro-organisms whose work is to break down
April 17, 1909.
humus and other organic compounds. The nitrogen of these substances is
finally changed into a nitrate, in which form it is available for the use of the
plant. If there is a lack of aeration, these organisms cannot do their work.
Other organisms that grow only in an atmosphere lacking in oxygen, develop,
causing harmful effects. But in the presence of an excess of air and plenty
of moisture the former organisms may multiply and act so rapidly that large
quantities of humus may be transformed into soluble forms which may be
washed away. This rapid transformation of humus is known as a "burning
out" of the soil. Since these soluble forms of nitrogen are likely to be
leached from the soil, it is better that they should become available slowly.
A rapid transformation of the humus is undesirable from another standpoint.
In addition to its importance as a chemical or food constituent, humus is very
necessary to the best physical condition of the soil. It not only enables the
soil to hold moisture, but it also acts as an absorbent for certain fertilizing
elements, thus keeping them from leaching away.
In order to conserve the humus in soils that are being "burned out," the
soil must be treated so as to reduce the amount of air which enters. If this
is due to constant cultivation, such as is sometimes practiced in orange
groves, the practice must be changed, or humus should be supplied to the soil
to make up the deficit.
State papers please copy.