ver is raised, it obtains its carbon from the
atmosphere; and, if it be plowed under, and allowed to decay, a portion
of this carbon is deposited in the soil. Carbon constitutes nearly the
whole of the dry weight of the clover, aside from the constituents of
water; and, when we calculate the immense quantity of hay, and roots
grown on an acre of soil in a single season, we shall find that the
amount of carbon thus deposited is immense. If the clover had been
removed, and the roots only left to decay, the amount of carbon
deposited would still have been very great. The same is true in all
cases where the crop is removed, and the roots remain to form the
organic or vegetable part of the soil. While undergoing decomposition, a
portion of this matter escapes in the form of gas, and the remainder
chiefly assumes the form of carbon (or charcoal), in which form it will
always remain, without loss, unless driven out by fire. If a bushel of
charcoal be mixed with the soil now, it will be the same bushel of
charcoal, neither more nor less, a thousand years hence, unless some
influence is brought to bear on it aside from the growth of plants. It
is true that, in the case of the decomposition of organic matter in the
soil, certain compounds are formed, known under the general names of
_humus_ and _humic acid_, which may, in a slight degree, affect the
growth of plants, but their practical importance is of too doubtful a
character to justify us in considering them. The application of manures,
containing organic matter, such as peat, muck, animal manure, etc.,
supplies the soil with carbon on the same principle, and the decomposing
matters also generate[Q] carbonic acid gas while being decomposed. The
agricultural value of carbon in the soil depends (as we have stated),
not on the fact that it enters into the composition of plants, but on
certain other important offices which it performs, as follows:--

[On what does the agricultural value of the carbon in the soil
depend?

Why does it make the soil more retentive of manure?

What is the experiment with the barrels of sand?]

1. It makes the soil more retentive of manures.

2. It causes it to appropriate larger quantities of the fertilizing
gases of the atmosphere.

3. It gives it greater power to absorb moisture.

4. It renders it warmer.

1. Carbon (or charcoal) makes the soil retentive of manures, because it
has in itself a strong power to absorb, and retain[R] fertilizin