As stated above, the nitrogen naturally in the soil is contained almost
entirely in the organic matter. Any process which tends to decompose
or destroy this organic matter, such as nitrification or other forms
of oxidation, will also tend to reduce the total stock of nitrogen in
the soil. Because of this fact the matter of restoring nitrogen to the
soil becomes of very great importance. Of course a part of the nitrogen
removed in crops may be returned in the manure produced on the farm;
and nitrogen may also be bought in the markets in such forms as sodium
nitrate (containing 15 to 16 percent of nitrogen), ammonium sulfate
(containing 20 to 21 percent of nitrogen), and dried blood (containing 12
to 15 percent nitrogen); but, when we bear in mind that such commercial
nitrogen costs about 15 cents a pound, and that one bushel of corn
contains about one pound of nitrogen, it will be seen at once that the
purchase of nitrogen cannot be considered practicable in general farming,
although in market gardening, and in some other kinds of intensive
agriculture, commercial nitrogen can often he used with very marked
profit.
Nitrogen is removed from the soil not only in the crops grown, but also,
and frequently in larger amounts per annum, in the drainage waters, and
in some other ways, as by denitrification and by the blowing and washing
of the surface soil. Professor Snyder, of the Minnesota Experiment
Station, has shown that during a series of years the total loss of
nitrogen from some Minnesota soils in some cases amounts to several times
the amount actually used in the crops produced.
Considering all of these facts, and the additional facts that there
are about seventy-five million pounds of atmospheric nitrogen resting
upon every acre of land, and that it is possible to obtain unlimited
quantities of nitrogen from the air for use of farm crops, and at very
small cost, the inevitable conclusion is that the inexhaustible supply
of nitrogen in the air is the store from which we must draw to maintain
a sufficient amount of this element in the soil for the most profitable
crop yields.
It is often stated that leguminous plants, such as clover, have power to
obtain free nitrogen from the air. This is not strictly true. Red clover,
for example, has no power in itself to get nitrogen from the air. It is
true, however, that the microscopic organisms[3] which commonly live in
tubercles upon the roots of the clover plant do have the power to take
free nitrogen from the air and cause it to unite with other elements to
form compounds suitable for plant food. The clover plant then draws upon
this combined nitrogen in the root tubercles, and makes use of it in its
own growth, both in the tops and in the roots of the plant.
These nitrogen-gathering bacteria live in tubercles upon the roots of
various leguminous plants,[4] such as red clover, white clover, alfalfa,
sweet clover, cowpeas, soy beans, vetch, field-peas, garden-peas, field
and garden beans, etc. These tubercles vary in size from a pinhead to a
pea, varying with the different kinds of plants, being especially small
upon some of the clovers, and very large upon cowpeas and soy beans. The
tubercles are, of course, easily seen with the eye, but the tubercle is
only the home of the bacteria, somewhat as the ball upon the willow twig
is the home of the insects within. The bacteria themselves are far too
small to be seen with the unaided eye, although they can be seen by means
of the most powerful microscope. Several million bacteria may inhabit a
single tubercle. It is not necessary to see the bacteria, because if we
find the tubercles upon the roots of the plant, we know that the bacteria
are present within, as otherwise the tubercle would not be formed.
Although the plant itself, as clover, for example, has no power to
feed upon the free or uncombined nitrogen in the air, yet these
nitrogen-gathering bacteria do have the power to absorb the free nitrogen
and cause it to combine with other elements, forming nitrates or other
compounds which are suitable forms of nitrogen for plant food.
It has also been demonstrated that, as a rule, there are different
species of nitrogen-gathering bacteria for markedly different species
of leguminous plants. Thus we have one kind of bacteria for red clover,
another kind for cowpeas, another kind for soy beans, and still a
different kind for alfalfa.[5]