ly in the form of
ammonia. It is to be noted that the nitrogen the soil obtains in this
way from the combined nitrogen of the air is not all pure gain. With
regard to the nitrates and nitrites, no doubt most of these are formed
by electrical discharge, although a small portion of them may be formed
by the oxidation of ammonia by means of ozone and peroxide of hydrogen.
With regard to the ammonia and the combined nitrogen present in the
organic particles in the air, a not inconsiderable proportion is
probably derived from the soil. Schloesing considers the chief source of
the ammonia present in the air to be the tropical ocean; but we must
remember that the source of much of the nitrogen in the tropical ocean
is, after all, the soil.

Leaving aside for a moment the question of the availability of the free
nitrogen of the air, let us consider the other sources of soil-nitrogen.

_Accumulation of Soil-nitrogen under Natural Conditions._

The chief source is of course the remains of vegetable and animal
tissue.[80] Plants are the great conservers of soil-nitrogen. By
assimilating such available forms of it as nitrates, and converting them
into organic nitrogen, they prevent the loss of this most valuable of
all soil constituents that would otherwise take place.

They also serve to collect the nitrogen from the lower soil-layers and
concentrate it in the surface portion. In a state of nature, where the
soil is constantly covered with vegetation, the process going on,
therefore, will be one of steady accumulation of nitrogen in the
surface-soil. To what extent this accumulation goes on, and how far it
is limited by the conditions of loss, will be considered immediately.
That it may go on to a very great extent is amply proved by the
existence of the so-called _virgin_ soils of countries like America and
Australia. There are cases, also, where the accumulation of nitrogen is
practically unlimited, although the result in such cases is not
necessarily a fertile soil. Such cases are peat-bogs. But let us pass on
to the accumulation of soil-nitrogen under the ordinary conditions of
husbandry.

_Accumulation of Nitrogen in Pastures._

The case which, under the conditions of ordinary farming, most resembles
a state of nature, is that of permanent pasture. It will be best,
therefore, to study first the conditions under which gain of nitrogen
takes place in this case.

_Increase of Nitrogen in the soil of Pasture-land._