a crop, and
fall into absolute infertility unless the substances removed from it are
restored from some other source in the form of manure. When this is
done, the fertility of the soil may not only be sustained but greatly
increased, and, in point of fact, all cultivated soils, by the use of
manure, are made to yield a much larger crop than they can do in their
natural condition.

The fundamental principle upon which a manure is employed is that of
adding to the soil an abundant supply of the elements removed from it by
plants in the condition best fitted for absorption by their roots; but
looked at in its broadest point of view, it acts not merely in this way,
but also by promoting the decomposition of the already partially
disintegrated rocks of which the soil is composed, setting free those
substances it already contains, and facilitating their absorption by the
plants.

In considering the practical applications of the broad general principle
just stated, it might be assumed that a manure ought invariably to
contain all the elements of plants in the quantities in which they are
removed by the crops, and that when this has been accurately ascertained
by analysis, it would only be necessary to use the various substances in
the proportions thus indicated. But this, though a very important, and
no doubt in many cases essential condition, is by no means the only
matter which requires to be taken into consideration in the economical
application of manures. And this becomes sufficiently obvious when the
circumstances attending the exhaustion of the soil are minutely
examined. When a soil is cropped during a succession of years with the
same plant, and at length becomes incapable of longer maintaining it,
the exhaustion is rarely, if ever, due to the simultaneous consumption
of all its different constituents, but generally depends upon that of
one individual substance, which, from its having originally existed in
the soil in comparatively small quantity, is removed in a shorter time
than the others. To restore the fertility of a soil in this condition,
it is by no means necessary to supply all the different substances
required by the plant, for it will suffice to add that which has been
entirely removed. On the other hand, if an ordinary soil be supplied
with a manure containing a very small quantity of one of the elements of
plant food, along with abundance of all the others, the amount of
increase which it yields must ob