m which their
organs are formed; but other substances are likewise necessary for the
formation of certain organs destined for special functions, peculiar to
each family of plants. Plants obtain these substances from inorganic
nature."

While insisting on the importance of the mineral constituents, he did
so in a more or less general way not sufficiently distinguishing one
mineral constituent from another.

As all plants contained certain organic acids, and as these organic
acids were nearly always found in a neutral state--_i.e._, in
combination with bases, such as potash, soda, lime, and magnesia--the
plant must be in a position to take up sufficient of these alkaline
bases to neutralise these acids. Hence the necessity of these mineral
constituents in the soil. According to him, however, the exact nature of
the bases was a point of not so much importance. He assumed, in short,
as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual
replaceability amongst the bases than can be now admitted.

Passing on to a consideration of the difference of the mineral
composition of different soils, he attributes this to the difference in
the rocks forming the soils. "Weathering" is the great agent at work in
rendering available the otherwise locked-up stores of fertility. He
attributes the benefits of fallow exclusively to the increased supply of
these incombustible compounds which were thus rendered available to the
plant. Treating of this subject, he says: "From the preceding part of
this chapter" (in which he has been explaining weathering) "it will be
seen that fallow is that period of culture when the land is exposed to
progressive disintegration by the action of the weather, for the purpose
of liberating a certain quantity of alkalies and silica, to be absorbed
by future plants."

_His Theory of Manures._

Treating of manures, he showed how the most important constituents of
manures were _potash_ and _phosphates_. In the first edition of his work
he also insisted on the value of _nitrogen_ in manures, condemning the
want of precautions, in the treatment of animal manures, against loss of
nitrogen.

In the later editions of his work he seems to have receded from that
opinion, and considered that there was no necessity for supplying
nitrogen in manures, since the ammonia washed down in rain was a
sufficient source of all the nitrogen the plant required. It was here
that Liebig went astray, first in den