opposite of those which favour nitrification. It is only
when oxygen is excluded, or, which practically means the same thing,
when large quantities of organic matter are in active putrefaction, and
the supply of oxygen is therefore deficient, that denitrification takes
place. Schloesing, as we have already seen, found that in the case of a
moist soil, kept in an atmosphere devoid of oxygen, a reduction of its
nitrates to free nitrogen took place.

_Takes place in water-logged Soils._

The exclusion of oxygen from a soil may be effected by saturating the
soil with water; and Warington has found in experiments carried out in
an arable soil, by no means rich in organic matter, that complete
reduction of nitrates may be effected in this way. It would thus seem
that the process of denitrification will take place in water-logged
soils, or in the putrefaction of sewage matter in the presence of large
quantities of water. Whether this reduction will result in the
production of nitrites, nitrous oxide, or free nitrogen, depends on
different conditions. This process is one of great importance from an
economic point of view, as it reveals to us a source of loss which may
take place in the fermentation of manures. In the rotting of our
farmyard manure it is possible that the denitrifying organisms may be
more active than we have hitherto suspected, and that a considerable
loss of nitrogen may in this way be effected.

_Distribution of the Nitrifying Organisms in the Soil._

The nitrifying organisms are probably chiefly confined to the soil, and
do not usually occur in rain or in the atmosphere. That, however, they
are found in spots which we might be inclined to think extremely
unlikely, is shown by some recent interesting researches carried out by
Muentz, who discovered that the bare surfaces of felspathic, calcareous,
schistose, and other rocks at the summit of mountains in the Pyrenees,
Alps, and Vosges, yielded large numbers of them, and that they occurred
to a considerable depth in the cracks and fissures of the rocks. The
nitrifying organisms are also found in river-water, in sewage, and
well-waters.

_Depth down at which they occur._

In Warington's earlier experiments, the conclusion he arrived at was
that the occurrence of the nitrifying organisms was almost entirely
limited to the superficial layers of the soil, and that they were seldom
to be met with much below a depth of 18 inches. His subsequent
experime