ters, the exact composition of which is
not under our control, and they do not necessarily contain their
constituents either in the most suitable proportions, or the most
available forms, and consequently when they are used during a succession
of years, certain of their constituents may accumulate in the soil, and
it is under such circumstances that special manures are both necessary
and advantageous.

Several different substances, but more especially farm-yard manure,
fulfil in a very remarkable manner the conditions of a general manure,
and supply abundantly, not merely the mineral, but also the carbonaceous
and nitrogenous matters necessary for building up the organic part of
the plant; and hence its use is governed by principles of comparative
simplicity, and really resolves itself into determining the best mode of
managing it so as effectually to preserve its useful constituents, and,
at the same time, to bring them into those forms of combination in which
they are most available to the plant. But the employment of a special
manure opens up nice questions as to the relative importance of the
different elements of plants which have given rise to much controversy
and difference of opinion.

In treating of the food of plants, it has been already observed that the
fixed or mineral constituents which are contained in their ash, are
necessarily derived exclusively from the soil, but that the carbon,
hydrogen, nitrogen, and oxygen, of which their organic part is composed,
may be obtained either from that source or from the air. The important
distinction which thus exists between these two classes of substances,
has given rise to two different views regarding the theory of manures.
Basing his views on the presence of the organic elements in the air,
Liebig has maintained that it is unnecessary to supply them in the
manure, while others, among whom Messrs. Lawes and Gilbert have taken a
prominent position, hold that, as a rule, fertile soils, cultivated in
the ordinary manner, contain a sufficient supply of mineral matters for
the production of the largest possible crops, but that the quantity of
ammonia and nitric acid which the plants are capable of extracting from
the air is insufficient, and must be supplemented by manures containing
them. A large number of experiments have been made in support of these
views, but the inferences which can be drawn from them are not
absolutely conclusive on either side, and it is necess