tash salts and phosphoric acid.[111]

As we have seen above, the presence of organic matter is not necessary
for the process. In this respect these organisms are differentiated from
all other ferments hitherto discovered.

_Presence of a Salifiable Base._

The presence of a sufficient quantity of a base in the soil with which
the nitric acid may combine, when it is formed, is another necessary
condition.[112] The process only goes on in a slightly alkaline
solution. The substance which acts as this salifiable base is _lime_.
The presence of a sufficient quantity of carbonate of lime in the soil
will thus be seen to be of first-rate importance. This furnishes an
explanation of one of the many benefits conferred by lime on soils. The
activity of nitrification in many soils may be hindered by the absence
of a sufficiency of lime salts, and in such cases most striking results
may follow the application of moderate dressings of chalk. The absence
of the nitrifying organisms in certain soils, such as peaty and forest
soils, may be thus accounted for. In such soils humic acids are present
and the requisite alkalinity is thus awanting.

_Only takes place in slightly Alkaline Solutions._

But while a certain slight amount of alkalinity is necessary, this must
not exceed a certain strength, otherwise the process is retarded. This
is the reason why strong urine solutions do not nitrify. The amount of
carbonate of ammonia generated in them by putrefaction renders the
development of nitrification impossible by rendering the alkalinity of
the solution too great.[113] The practical importance of this fact is
considerable, as it shows the importance of diluting urine very
considerably before applying it as a manure. Similarly, when large
quantities of lime, especially burnt lime, are applied to soils, the
result will be to arrest the action of nitrification for the time. The
presence of alkaline carbonates in the soil, unless in minute
quantities, is apt, therefore, to seriously interfere with the
process.[114]

_Action of Gypsum on Nitrification._

It has been found by Pichard that the action of certain mineral
sulphates is extremely favourable to the process, and among these
_gypsum_. Warington has carried out some experiments on the action of
gypsum in promoting nitrification. The reason of its favourable action
is probably because it neutralises the alkalinity of nitrifying
solutions. It thus permits the process to go