on, and becomes a most excellent
manure. It probably contains small quantities of nitrogen (obtained from
the leather), which adds to its value. Unless tan bark be composted with
lime, or some other alkali, it may produce injurious effects from the
_tannic acid_ which it is liable to contain. Alkaline substances will
neutralize this acid, and prevent it from being injurious.

One great benefit resulting from the use of spent tan bark, is due to
its power of absorbing moisture from the atmosphere. For this reason it
is very valuable for _mulching_[AD] young trees and plants when first
set out.

SAWDUST.

[Why is sawdust a good addition to the pig-stye?

What is the peculiarity of sawdust from the beech, etc.?

What is a peculiarity of soot?

Why may soot be used as a top dressing without losing its ammonia?]

_Sawdust_ in its natural state is of very little value to the land, but
when decomposed, as may be done by the same method as was described for
tan bark, it is of some importance, as it contains a large quantity of
carbon. Its ash, too, which becomes available, contains soluble
inorganic matter, and in this way it acts as a direct manure. So far as
concerns the value of the ash, however, the bark is superior to sawdust.
Sawdust may be partially rotted by mixing it with strong manure (as hog
manure), while it acts as a _divisor_, and prevents the too rapid action
of this when applied to the soil. Some kinds of sawdust, such as that
from beech wood, form acetic acid on their decomposition, and these
should be treated with, at least, a sufficient quantity of lime to
correct the acid.

_Soot_ is a good manure. It contains much carbon, and has, thus far, all
of the beneficial effects of charcoal dust. The sulphur, which is one of
its constituents, not only serves as food for plants, but, from its
odor, is a good protection against some insects. By throwing a handful
of soot on a melon vine, or young cabbage plant, it will keep away many
insects.

Soot contains some ammonia, and as this is in the form of a _sulphate_,
it is not volatile, and consequently does not evaporate when the soot is
applied as a top dressing, which is the almost universal custom.

GREEN CROPS.

[What plants are most used as green crops?

What office is performed by the roots of green crops?

How do such manures increase the organic matter of soils?]

_Green crops_, to plow under, are in many places largely raised, and are
alwa