ary to consider
the matter in a general point of view.

Setting out from the proposition already so frequently referred to, that
the plant cannot grow unless it receives a supply of all its elements,
it must be obvious that if, to a soil containing a sufficiency of
mineral matters to raise a given number of crops, a supply of ammonia be
added, its total productive capacity cannot be thus increased; and
though it may yield larger crops than it would have done without that
substance, this can only be accomplished by a proportionate diminution
of their number. In either case, the same quantity of vegetable matter
will be produced, but the time within which it is obtained will be
regulated by the supply of ammonia. That substance differs in no respect
from any other element of plant-food, and used in this way is to all
intents and purposes a special manure, and acts merely by bringing into
play those substances which the soil already contains. Its effect may
not be apparent until after the lapse of a very long period of time, but
it ultimately leads to the exhaustion of the soil. If, on the other
hand, a soil be continuously cropped until it ceases to yield any
produce, it is manifest that the exhaustion must in this instance be
entirely due to the removal of its available mineral nutriment, because
the superincumbent air constantly changed by the winds must continue to
afford the same unvarying supply of the organic elements, and the power
of supporting vegetation would be restored to it, by adding the
necessary inorganic matters. Hence when a soil, which in its natural
condition is capable of yielding a certain amount of vegetable matter,
is rendered barren by the removal of the crop, it may be laid down as an
incontrovertible position, that its infertility is due to the loss of
mineral matters, and that it may be restored to its pristine condition
by the use of them, and of them only.

But the case is materially altered when we come to consider the course
of events in a cultivated soil. The object of agriculture is to cause
the soil, by appropriate treatment, to yield much more than its normal
produce, and the question is, how this can be best and most economically
effected in practice. According to Liebig, it is attained by adding to
the soil a liberal supply of those mineral substances required by the
plant, and that it is unnecessary to use any of the organic elements,
because they are supplied by the air in sufficie