ompounds. 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