viously be measured, not by those which
are abundant, but by that which is deficient; for the crop which grows
luxuriantly so long as it obtains a supply of all its constituents, is
arrested as effectually by the want of one as of all, as has been proved
by the experiments of Prince Salm Horstmar and others, referred to in a
previous chapter; and hence, in order to obtain a good crop, it would be
necessary to use the manure in such abundance as to supply a sufficiency
of the deficient element for that purpose. If this course were
persevered in for a succession of years, the other substances which
would have been used in much more than the quantity required by the
crops, must either have been entirely lost or have accumulated in the
soil. In the latter case it is sufficiently obvious that the soil must
have been gradually acquiring an amount of resources which must remain
dormant until the system of manuring is changed. To render them
available, it is only necessary to add to it a quantity of the
particular substance in which the manure hitherto employed has been
deficient, so as to restore the lost balance, and enable the plant to
make use of those which have been stored up within it. The substance so
used is called a _special_ manure; that containing all the constituents
of the crop is a _general_ manure.

The distinction of these two classes of manures is very important in a
practical point of view, because a special manure is not by itself
capable of maintaining the life of plants, but is only a means of
bringing into use the natural and acquired resources of the soil. In
place of preventing or retarding its exhaustion, it rather accelerates
it by causing the increased crops to consume more abundantly, and within
a shorter period of time, those substances which it contains. On the
other hand, a general manure prevents or diminishes the consumption of
the elements of plant-food contained in the soil, and if added in
sufficient abundance, may cause them to accumulate in it, and even
enable an almost absolutely barren soil to yield a tolerable crop.
General manures must therefore always be the most important and
essential, and no others would be used if it were possible to obtain
them of a composition exactly suited to the requirements of the crop to
be raised. Practically, however, this condition cannot be fulfilled,
because all the substances available for the purpose, and particularly
farm-yard manure, are refuse mat