real crop
such as wheat. Where turnips follow wheat, there is a period during
which the soil is left uncovered, and during which most serious loss of
nitrates is apt to ensue. The risk of loss is enhanced by the fact that
the assimilation of nitrates by cereals ceases before the season of
their maximum production in the soil. The soil is then left bare of
vegetation during the autumn, which is the most critical period of all,
and the result must be serious loss. In order to minimise this loss, the
practice of growing catch-crops has been had recourse to. As, however,
this practice will be dealt with elsewhere, nothing further need here be
said.

FOOTNOTES:

[97] As the formation of nitrites is a stage in the process, the term
_nitrification_ includes the formation of nitrites as well as nitrates.

[98] Nitre seems to have been known as early as the thirteenth century.

[99] Lawes and Gilbert, for example, have shown that in the Rothamsted
soils it only amounts to a few parts per million of soil.

[100] See Appendix, Note I., p. 196.

[101] The artificial production of nitre seems to have been first
effected by Glauber in the seventeenth century.

[102] The lime-rubbish from old buildings, especially those parts which
have come in contact with the earth, or plastering from the walls of
damp cellars, barns, stables, &c., have been found to be rich in nitrate
of lime, and, as has been long well known, constitute by themselves a
valuable manure. The formation of the nitrate of lime can be accounted
for by the contact of the lime with nitrogenous matter of different
kinds.

[103] As much of the nitric acid in this solution was present as nitrate
of lime, it was usually treated with a solution of potassium carbonate,
the result being the precipitation of the lime as carbonate, pure
saltpetre being left in solution, according to the following equation--

K_{2}CO_{3} + Ca(NO_{3})_{2} = 2 KNO_{3} + CaCO_{3}.

Under the French mode of manufacture, the process was considered to have
developed satisfactorily when 1000 lb. of earth, at the expiration of
two years, yielded 5 lb. of nitre.

[104] Pasteur had already in 1862 expressed the opinion that
nitrification might probably be in some way connected with ferments. A.
Mueller (see 'Journal of Chemical Society,' 1879, p. 249) was the first
to advance the opinion that nitrification was due to the action of a
ferment. This conclusion he was led to by the observation tha