h is most important, is _Temperature_.
According to Schloesing and Muentz the temperature at which maximum
development takes place is 37 deg. C.[117] (99 deg. F.), at which temperature it
is ten times as active as at 14 deg. C. (57 deg. F.) Below 5 deg. C. (40 deg. F.) the
action is extremely feeble. It is clearly appreciable at 12 deg. C. (54 deg.
F.), and from there up to 37 deg. C. (99 deg. F.) it rapidly increases. From 37 deg.
C. (99 deg. F.) to 55 deg. C. (131 deg. F.), at which temperature no nitrification
takes place, its activity decreases; at 45 deg. C. (113 deg. F.) it is less
active than at 15 deg. C. (59 deg. F.), and at 50 deg. C. (122 deg. F.) it is very
slight. These results by Schloesing and Muentz have not been exactly
confirmed by Warington. He has found that a considerable amount of
nitrification goes on at a temperature between 3 deg. and 4 deg. C. (37 deg. and 39 deg.
F.), while the highest temperature at which he has found it to take
place is considerably lower than 55 deg. C. (131 deg. F.) Thus he was unable to
start nitrification in a solution maintained at 40 deg. C. (104 deg. F.) It
would thus seem that the nitrifying ferments are able to develop at
lower temperatures than most organisms; and although nitrification
entirely ceases during frost, yet in a climate such as our own there
must be a considerable proportion of the winter during which
nitrification is moderately active.

_Presence of a sufficient quantity of Moisture._

The presence of moisture in a soil is another of the necessary
conditions of nitrification. It has been shown that it is at once
arrested, and indeed destroyed, by desiccation. Other conditions being
equal, and up to a certain extent, the more moisture a soil contains the
more rapid is the process. Too much water, however, is unfavourable, as
it is apt to exclude the free access of air, which, as we have just
shown, is so necessary, as well as to lower the temperature. During a
period of drought the rate at which nitrification takes place will,
therefore, be apt to be seriously diminished.

_Absence of strong Sunlight._

It has been found that the process goes on much more actively in
darkness; indeed Warington has found in his experiments that
nitrification could be arrested by simply exposing the vessel in which
it was going on to the action of sunshine.

_Nitrifying Organisms destroyed by Poisons._

It has already been pointed out that nitrification is arr