and render it unfit for respiration, did not Nature, with her
wondrous laws of compensation, provide for its purification. It has
already been stated, how the atmosphere returns to the hills, in clouds
and vapor, condensed at last to rain, all the water which the rivers
carry to the sea; and how the well-drained soil derives moisture, in
severest time of need, from its contact with the vapor-loaded air. But
the rain and dew return not their waters to the earth without treasures
of fertility. Ammonia, which is one of the most valuable substances
found in farm-yard manures, and which is a constant result of
decomposition, is absorbed in almost incredible quantities by water.
About 780 times its own bulk of ammonia is readily absorbed by water at
the common temperature and pressure of the atmosphere; and, freighted
thus with treasures for the fields, the moisture of the atmosphere
descends upon the earth. The rain cleanses the air of its impurities,
and conveys them to the plants. The vapors of the marshes, and of the
exposed manure heaps of the thriftless farmer, are gently wafted to the
well-drained fields of his neighbor, and there, amidst the roots of the
well-tilled crops, deposit, at the same time, their moisture and
fertilizing wealth.

Of the wonderful power of the soil to absorb moisture, both from the
heavens above and the earth beneath--by the deposition of dew, as well
as by attraction--we shall treat more fully in another chapter. It will
be found to be intimately connected with the present topic.

_Thorough drainage supplies air to the roots._ Plants, if they do not
breathe like animals, require for their life almost the same constant
supply of air. "All plants," says Liebig, "die in soils and water
destitute of oxygen; absence of air acts exactly in the same manner as
an excess of carbonic acid. Stagnant water on a marshy soil excludes
air, but a renewal of water has the same effect as a renewal of air,
because water contains it in solution. When the water is withdrawn from
a marsh, free access is given to the air, and the marsh is changed into
a fruitful meadow." Animal and vegetable matter do not decay, or
decompose, so as to furnish food for plants, unless freely supplied with
oxygen, which they must obtain from air. A slight quantity of air,
however, is sufficient for putrefaction, which is a powerful deoxydizing
process that extracts oxygen even from the roots of plants.

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