NITROGEN AND ITS RELATION TO PLANTS
244. Nitrogen. A substance which plays an important part in animal
and plant life is nitrogen. Soil and the fertilizers which enrich it,
the plants which grow on it, and the animals which feed on these, all
contain nitrogen or nitrogenous compounds. The atmosphere, which we
ordinarily think of as a storehouse of oxygen, contains far more
nitrogen than oxygen, since four fifths of its whole weight is made up
of this element.
Nitrogen is colorless, odorless, and tasteless. Air is composed
chiefly of oxygen and nitrogen; if, therefore, the oxygen in a vessel
filled with air can be made to unite with some other substance or can
be removed, there will be a residue of nitrogen. This can be done by
floating on water a light dish containing phosphorus, then igniting
the phosphorus, and placing an inverted jar over the burning
substance. The phosphorus in burning unites with the oxygen of the air
and hence the gas that remains in the jar is chiefly nitrogen. It has
the characteristics mentioned above and, in addition, does not combine
readily with other substances.
245. Plant Food. Food is the course of energy in every living thing
and is essential to both animal and plant life. Plants get their food
from the lifeless matter which exists in the air and in the soil;
while animals get their food from plants. It is true that man and many
other animals eat fleshy foods and depend upon them for partial
sustenance, but the ultimate source of all animal food is plant life,
since meat-producing animals live upon plant growth.
Plants get their food from the air, the soil, and moisture. From the
air, the leaves take carbon dioxide and water and transform them into
starchy food; from the soil, the roots take water rich in mineral
matters dissolved from the soil. From the substances thus gathered,
the plant lives and builds up its structure.
A food substance necessary to plant life and growth is nitrogen. Since
a vast store of nitrogen exists in the air, it would seem that plants
should never lack for this food, but most plants are unable to make
use of the boundless store of atmospheric nitrogen, because they do
not possess the power of abstracting nitrogen from the air. For this
reason, they have to depend solely upon nitrogenous compounds which
are present in the soil and are soluble in water. The soluble
nitrogenous soil compounds are absorbed by roots and are utilized by
plants for food.
246. The Poverty of the Soil. Plant roots are constantly taking
nitrogen and its compounds from the soil. If crops which grow from the
soil are removed year after year, the soil becomes poorer in nitrogen,
and finally possesses too little of it to support vigorous and healthy
plant life. The nitrogen of the soil can be restored if we add to it a
fertilizer containing nitrogen compounds which are soluble in water.
Decayed vegetable matter contains large quantities of nitrogen
compounds, and hence if decayed vegetation is placed upon soil or is
plowed into soil, it acts as a fertilizer, returning to the soil what
was taken from it. Since man and all other animals subsist upon
plants, their bodies likewise contain nitrogenous substances, and
hence manure and waste animal matter is valuable as a fertilizer or
soil restorer.
247. Bacteria as Nitrogen Gatherers. Soil from which crops are removed
year after year usually becomes less fertile, but the soil from which
crops of clover, peas, beans, or alfalfa have been removed is richer in
nitrogen rather than poorer. This is because the roots of these plants
often have on them tiny swellings, or tubercles, in which millions of
certain bacteria live and multiply. These bacteria have the remarkable
power of taking free nitrogen from the air in the soil and of combining
it with other substances to form compounds which plants can use. The
bacteria-made compounds dissolve in the soil water and are absorbed into
the plant by the roots. So much nitrogen-containing material is made by
the root bacteria of plants of the pea family that the soil in which
they grow becomes somewhat richer in nitrogen, and if plants which
cannot make nitrogen are subsequently planted in such a soil, they find
there a store of nitrogen. A crop of peas, beans, or clover is
equivalent to nitrogenous fertilizer and helps to make ready the soil
for other crops.
[Illustration: FIG. 162.--Roots of soy bean having tubercle-bearing
bacteria.]
248. Artificial Fertilizers. Plants need other foods besides
nitrogen, and they exhaust the soil not only of nitrogen, but also of
phosphorus and potash, since large quantities of these are necessary
for plant life. There are many other substances absorbed from the soil
by the plant, namely, iron, sodium, calcium, magnesium, but these are
used in smaller quantities and the supply in the soil does not readily
become exhausted.
Commercial fertilizers generally contain nitrogen, phosphorus, and
potash in amounts varying with the requirements of the soil. Wheat
requires a large amount of phosphorus and quickly exhausts the ground
of that food stuff; a field which has supported a crop of wheat is
particularly poor in phosphorus, and a satisfactory fertilizer for
that land would necessarily contain a large percentage of phosphorus.
The fertilizer to be used in a soil depends upon the character of the
soil and upon the crops previously grown on it.
[Illustration: FIG. 163.--Water cultures of buckwheat: 1, with all the
food elements; 2, without potash; 3, without nitrates.]
The quantity of fertilizer needed by the farmers of the world is
enormous, and the problem of securing the necessary substances in
quantities sufficient to satisfy the demand bids fair to be serious.
But modern chemistry is at work on the problem, and already it is
possible to make some nitrogen compounds on a commercial scale. When
nitrogen gas is in contact with heated calcium carbide, a reaction
takes place which results in the formation of calcium nitride, a
compound suitable for enriching the soil. There are other commercial
methods for obtaining nitrogen compounds which are suitable for
absorption by plant roots.
Phosphorus is obtained from bone ash and from phosphate rock which is
widely distributed over the surface of the earth. Bone ash and
thousands of tons of phosphate rock are treated with sulphuric acid to
form a phosphorus compound which is soluble in soil water and which,
when added to soil, will be usable by the plants growing there.
The other important ingredient of most fertilizers is potash. Wood
ashes are rich in potash and are a valuable addition to the soil. But
the amount of potash thus obtained is far too limited to supply the
needs of agriculture; and to-day the main sources of potash are the
vast deposits of potassium salts found in Prussia.
Although Germany now furnishes the American farmer with the bulk of
his potash, she may not do so much longer. In 1911 an indirect potash
tax was levied by Germany on her best customer, the United States, to
whom 15 million dollars' worth of potash had been sold the preceding
year. This led Americans to inquire whether potash could not be
obtained at home.
Geologists say that long ages ago Germany was submerged, that the
waters slowly evaporated and that the various substances in the sea
water were deposited in thick layers. The deposits thus left by the
evaporation of the sea water gradually became hidden by sediment and
soil, and lost to sight. From such deposits, potash is obtained.
Geologists tell us that our own Western States were once submerged,
and that the waters evaporated and disappeared from our land very much
as they did from Germany. The Great Salt Lake of Utah is a relic of a
great body of water. If it be true that waters once covered our
Western States, there may be buried deposits of potash there, and
to-day the search for the hidden treasure is going on with the energy
and enthusiasm characteristic of America.
Another probable source of potash is seaweed. The sea is a vast
reservoir of potash, and seaweed, especially the giant kelp, absorbs
large quantities of this potash. A ton of dried kelp (dried by sun and
wind) contains about 500 pounds of pure potash. The kelps are
abundant, covering thousands of square miles in the Pacific Ocean,
from Mexico to the Arctic Ocean.