on in unfavourable
conditions. Where, therefore, too great alkalinity exists for the
maximum development of nitrification, the best specific will be found to
be gypsum.[115] The practical value of gypsum as an adjunct to certain
manurial substances, where nitrification is desired to be promoted as
rapidly as possible, such as sewage and farmyard manure, will thus at
once become apparent. So far as there is a proper degree of alkalinity
maintained, the presence of large quantities of saline matter does not
seem to interfere with the process.

_Presence of Oxygen._

The nitrification bacteria belong, it would seem, to the aerobic[116]
class of ferment--_i.e._, they cannot develop without a free supply of
oxygen. Exclusion of the air is sufficient to kill them, and in those
portions of the soil where access of air is not freely permitted,
nitrification will be found to be correspondingly feeble. Thus it has
been found in experiments with different portions of soils, that but
little signs of nitrification occur in the lower soil layers. According
to experiments by Schloesing on a moist soil, in atmospheres
respectively containing no oxygen and varying quantities of it, the
action of oxygen in promoting nitrification was strikingly demonstrated.
In an atmosphere of pure nitrogen, entirely devoid of oxygen, the
process no longer took place, but the nitrates already present in the
soil were reduced and free nitrogen was evolved. In an atmosphere, on
the other hand, containing 1.5 per cent of oxygen, a considerable amount
of nitrification took place; while in the presence of 6 per cent,
nitrification took place to double the extent. An addition of 10 to 15
per cent again doubled the quantity. When the amount of moisture added
was increased, the effect of larger percentages of oxygen was found to
be less marked. The reason of this is that the oxygen probably acts as
dissolved oxygen; the addition of water meaning at the same time an
addition of available oxygen. This condition exemplifies the value of
tillage operations. The more thoroughly a soil is tilled the more
thoroughly will the aeration of its particles take place, and
consequently the more favourable will this necessary condition of
nitrification be rendered. The benefits conferred on clayey soils by
tillage will in this respect be especially great.

_Temperature._

Another of the conditions determining the rate at which nitrification
takes place, and one whic