ertain organic forms and as ammonia
salts, it is now a well-known fact that the chief, and by far the most
important, source of nitrogen is nitric acid. Probably more than 90 per
cent of the nitrogen absorbed by green-leaved plants from the soil is
absorbed as nitrates. The tendency of all nitrogen compounds in the soil
is towards conversion into nitric acid. It is the final form of nitrogen
in the soil. The precise method in which this conversion takes place is
a discovery of only a few years' standing. The great economic importance
of this discovery, made by the French chemists Schloesing and Muentz, and
associated in this country with the names of Warington, Munro, and P. F.
Frankland, is only gradually being appreciated. It is without doubt one
of the most interesting made in the domain of agricultural chemistry of
late years.

_Nitrification._

It was in the year 1877 that the two French chemists above referred to
published the results of some experiments they had carried out, which
proved that nitrification--the name given to the process by which
ammonia or other nitrogen salts are converted in the soil into nitric
acid--was due to the action of micro-organic life.

The basis of the theory rests upon the fact that dilute solutions of
ammonia salts or urine, containing all the necessary constituents of
plant-food, if previously sterilised, may be kept for an indefinitely
long period of time, provided the air supplied be filtered through
cotton wool,--so as to prevent the entrance of micro-organisms--without
any formation of nitrates. Introduce, however, into such a solution a
little fresh soil, and nitrification will soon follow.

The conditions under which the nitrification ferment acts, as well as
the nature of the ferment, or rather ferments, have subsequently been
carefully studied by Schloesing and Muentz, Winogradsy, Deherain,
Kellner, and other Continental observers, and especially by Warington,
Munro, and P. F. Frankland in this country. These conditions cannot be
gone into here. They will be fully discussed in the chapter on
Nitrification. Briefly stated, they are a certain range of temperature
(between slightly above freezing-point and 50 deg. C., the maximum activity
taking place, according to Schloesing and Muentz, at about 30 deg. C.); a
plentiful supply of atmosphere oxygen (hence the fact observed by
Warington, that nitrification is chiefly limited to the surface-soil); a
certain amount of mois