activity the dormant
resources of the soil, they restore the proper balance between its
different constituents, and supply the excessive demand of some
particular elements. Thus, for instance, in a soil containing an
abundant supply of mineral matters, a salt of ammonia or nitric acid
increases the crop, by promoting the absorption of the substances
already present. So likewise a soil on which young cattle and milch cows
have been long pastured has its fertility restored by phosphate of lime,
because that substance is removed in the bones and milk in relatively
much larger proportion than any others.

The choice of a special manure is necessarily dependent on a great
variety of circumstances, and is governed partly by the nature of the
soil, and partly by that of the crop. It is obvious that cases may occur
in which any individual element of the plant may be deficient, and ought
to be supplied, but experience has shown that, as a rule, nitrogen and
phosphoric acid are the substances which it is most necessary to furnish
in this way, and which in all but exceptional cases produce a marked
effect on the crop. The other substances, such as potash, soda,
magnesia, etc., occasionally act beneficially, but the results obtained
from them are very uncertain, and frequently entirely negative.

It has been commonly asserted that phosphates are specially adapted to
root crops, and ammonia or nitrates to the cereals, and this statement
is so far true, that the former are used with advantage on the turnip,
while the latter act with great benefit on grain crops and more
especially on oats and barley. The effect of the latter, however, is
more or less apparent in all crops and on all soils, because it promotes
the assimilation of the mineral matters already present. But its
peculiar importance lies in the power which it has of promoting the
rapid development of the young plant, causing it to send its roots out
into the soil, and to spread its leaves into the air, thus enabling it
to take from those two sources, abundance of the useful substances
existing in them. But it ought to be distinctly understood, that the
statement that particular manures are specially suited to particular
crops must be assumed with some reservation, because everything depends
upon the nature of the food contained in the soil. It is well known that
there are many soils in which ammonia acts more favourably on the turnip
than phosphates, and _vice versa_, and