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stituent of soils, without which a proper mechanical texture
cannot exist. Where there is no humus in the soil, no
proper mechanical texture exists, nor can there be any regular
source for the supply of carbonic acid to meet the wants of
the growing crop. The supply of carbonic acid is, in truth,
not altogether dependent on the condition of the soil itself,
as is the case with water, which enters the system of plants
exclusively through their roots, and therefore always by the
medium of the soil. Independently of this manner of assim-
ilation, carbonic acid is also taken up by the plants, to a
large extent, directly from the atmosphere by the medium of
their leaves. It has even been proven by experiment that
plants can be raised in a soil entirely destitute of humus, in
which case they are solely referred to this latter mode of ap-
propriating their carbon. This state of things, however,
merely shows to what extent plants can be forced to feed
themselves in a partial manner, but in no way prove that
under this condition their development is the most prosper-
ous. Experience teaches us, on the contrary, that the co-
operation of the humus in the soil is indispensable for se-
curing a regular supply of carbonic acid to plants, and their
production in the most perfect form. The gas arising from
the humus furnishes a constant source of carbonic acid for
that portion of the atmosphere which immediately rests upon
the soil and surrounds the upper part of the plant. It also
thoroughly penetrates the soil itself, in the very body of
which it is produced, becomes there dissolved in water, and
in this form most fit to enter the rootlets of plants and to
provide thus a nutriment which plants, in their first period
of life, cannot appropriate to themselves otherwise, being
without leaves at that time.

A regular supply of carbonic acid for the whole term of the
growth of the plant can therefore be secured only by the co-
operation of humus, or what is the same, by a proper me-
chanical texture of the soil, and experience teaches us, in
accordance with this, that the beneficial influence of the
humus on the production of crops is so great that two soils
of equal capacity will, mutatis mutandis, yield average crops
in proportion to the quantity of humus they contain.

Ammonia, finally, is the substance from which the plant
derives all its nitrogen, or about one per cent, of its whole
weight. It is, like carbonic acid, a product of the decompo-
sition of organic matter, and therefore most intimately con-
nected, with the occurrence of humus. Far the greatest mass
of the organic part of plants consists of carbon and the ele-
ments of water, (hydrogen and oxygen,) and becomes con-