If
this fertilising mud owed this property to nitrogenised matters;
what enormous beds of animal and vegetable exuviae and remains ought
to exist in the mountains of Africa, in heights extending beyond the
limits of perpetual snow, where no bird, no animal finds food, from
the absence of all vegetation!

Abundant evidence in support of the important truth we are
discussing, may be derived from other well known facts. Thus, the
trade of Holland in cheese may be adduced in proof and illustration
thereof. We know that cheese is derived from the plants which serve
as food for cows. The meadow-lands of Holland derive the nitrogen of
cheese from the same source as with us; i.e. the atmosphere. The
milch cows of Holland remain day and night on the grazing-grounds,
and therefore, in their fluid and solid excrements return directly
to the soil all the salts and earthy elements of their food: a very
insignificant quantity only is exported in the cheese. The fertility
of these meadows can, therefore, be as little impaired as our own
fields, to which we restore all the elements of the soil, as manure,
which have been withdrawn in the crops. The only difference is, in
Holland they remain on the field, whilst we collect them at home and
carry them, from time to time, to the fields.

The nitrogen of the fluid and solid excrements of cows, is derived
from the meadow-plants, which receive it from the atmosphere; the
nitrogen of the cheese also must be drawn from the same source. The
meadows of Holland have, in the lapse of centuries, produced
millions of hundredweights of cheese. Thousands of hundredweights
are annually exported, and yet the productiveness of the meadows is
in no way diminished, although they never receive more nitrogen than
they originally contained.

Nothing then can be more certain than the fact, that an exportation
of nitrogenised products does not exhaust the fertility of a
country; inasmuch as it is not the soil, but the atmosphere, which
furnishes its vegetation with nitrogen. It follows, consequently,
that we cannot increase the fertility of our fields by a supply of
nitrogenised manure, or by salts of ammonia, but rather that their
produce increases or diminishes, in a direct ratio, with the supply
of mineral elements capable of assimilation. The formation of the
constituent elements of blood, that is, of the nitrogenised
principles in our cultivated plants, depends upon the presence of
inorganic matters i