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gard to their importance as nutriments; it only demands the
acknowledgment of their equal significance as matters indis-
pensable to vegetable life; and as the mineral constituents
can only be supplied by the soil itself, whilst the sources for
ammonia are various; it recommends such a system of ma-
nuring as is in best accordance with these facts. If we apply
ammonia to the one of the soils above referred to, and soluble
mineral salts to the other, we restore an equilibrium of the
nourishing substances in both soils, and by so doing render
them productive in the very spirit of the mineral theory.
But we also know that the same result can be effected by put-
ting both soils in the fallow until the one has appropriated
to itself the required quantity of ammonia from the atmos-
phere, the other that of soluble mineral salts, as produced by
the steadily advancing degradation of its mineral constit-
uents. A soil which has become deficient in ammonia may,
therefore, once regain its fertility without any artificial ap-
plication being made to it; as can also a soil which is de-
ficient in soluble mineral salts, but contains such constituents
as will produce them by their degradation; whilst a soil
which has no mineral constituents whatever, will remain un-
productive until it is artificially supplied with them by na-
ture.

In practice, where we cannot wait until the soil has re-
covered its nourishing ingredients by its own exertions, we
have to apply both ammoniacal and mineral manures to re-
store its fertility; and if we choose to apply only the latter,
with the expectation that the ammonia required would be
furnished by the atmosphere, we would be acting just in the
same way as if we had added ammoniacal salts alone to a soil
deficient in soluble mineral salts, with the expectation that
they would be furnished in time by the process of degrada-
tion. In neither case will our expectations be realized in
practice, nor are they in compliance with the spirit of the
mineral theory.

Lawes' and Gilbert's observations illustrate most remark-
ably the importance of ammonia as a nutriment for plants.
They found that the more of nitrogen we apply to a soil by
manure, the less is its quantity in the grains of the wheat
crop raised upon it; and as the absolute quantity of nitrogen
of the whole crop, in toto, is augmented in proportion to that
of the soil, it naturally follows that these extra additions
made, increased to such an extent the weight and number of
the grains, as to effect even a decrease in their per centage,
or relative quantity of nitrogen. Besides, we know by the
statements of these agriculturists, that the market price of