ested by the
action of antiseptics, such as chloroform, bisulphide of carbon, and
carbolic acid. Another substance which has been found to have an
injurious action is ferrous sulphate or "copperas," a substance which is
apt to be present in badly drained soils, or soils in which there is
much actively putrefying organic matter. Maercker has found that in moor
soils containing ferrous sulphate, no nitrates, or mere traces of
nitrates, could be found. A substance such as gas-lime, unless submitted
to the action of the atmosphere for some time, would also have a bad
effect in checking nitrification, owing to the poisonous sulphur
compounds it contains. Common salt, it would seem, also arrests the
process; and this antiseptic property which salt exercises on
nitrification throws a certain amount of light on the nature of its
action when applied, as it is often done, along with artificial
nitrogenous manures.

_Denitrification._

In connection with the process of nitrification, it is of interest to
notice that a process of an opposite nature may also take place in
soils--viz., _denitrification_--a process which consists in reducing the
nitrates to nitrites, nitrous oxide, or free nitrogen. That a reduction
of nitrates takes place in the decomposition of sewage with the
evolution of free nitrogen, was a fact first observed by the late Dr
Angus Smith in 1867; and the reduction of nitrates to nitrites, and
nitric and nitrous oxides in putrefactive changes has been subsequently
noticed by different experimenters, who have further observed that such
reduction takes place in the case of putrefaction going on in the
presence of large quantities of water or where there is much organic
matter.

_Denitrification also effected by Bacteria._

This change was supposed to be of a purely chemical nature, and it has
only been recently discovered that it is effected, like nitrification,
by means of bacteria. It has been surmised by some that the action of
denitrification may be effected by the same organisms that effect
nitrification, and that it depends on merely external conditions which
process goes on. There is no reason, however, to suppose that this is
so, and several of the denitrifying organisms have been identified.

_Conditions favourable for Denitrification._

That it is a process that goes on to any extent in properly cultivated
soils is not to be supposed. The conditions which favour denitrification
are exactly the