uced. Similarly the slow combustion or decay of nitrogenous
organic matter, which constantly takes place all over the earth's
surface, may be regarded as another source of this form of combined
nitrogen. Ammonia may be similarly formed by the combustion, either
quick or slow, of nitrogenous organic matter. It exists in the air as
nitrate or nitrite of ammonia, and also as carbonate of ammonia.[67]

_Amount of combined Nitrogen falling in the Rain._

The importance of the combined nitrogen in the air as a source of
soil-nitrogen is best gauged by the amount falling annually on the soil
dissolved in rain. This has been found to vary considerably. In the rain
falling in the vicinity of large towns the amount is greater than in
rain falling in the country. Thus at Rothamsted, in England, the average
amount for several years was only 3.37 lb. nitrogen per annum per acre,
of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln,
in New Zealand, 1.74 lb. fell annually per acre--as ammonia,.74, as
nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which
.93 was as ammonia, and 2.84 as nitric acid.[68] That the combined
nitrogen derived from the air by the soil may be considerably in excess
of this is highly probable. Soils, especially when damp, may absorb much
larger quantities from the air of the combined nitrogen it contains. We
must remember that the air in contact with the soil-surface is
constantly being changed, and that there is thus a constant renewal of
the air passed over the ground. The result is that the amount of air
from which combined nitrogen may be removed is very great.[69]

_Nitrogen in the Soil._

It has been remarked as a fact worthy of notice that nitrogen is
essentially a superficial element. By this is meant that it is only
found, as a rule, on the earth's immediate surface. This statement can
only be admitted to be true within certain limits. The chief source of
nitrogen, in addition to the atmosphere, is, of course, vegetable and
animal tissue.[70] As vegetable and animal tissue are only found to any
extent on the earth's surface, nitrogen is therefore chiefly found
there. The natural deposits of nitrogen salts, such as the
nitrate-fields of Chili and the saltpetre soils of India, &c., also
only occur superficially. Notwithstanding these facts, however, the
amount of nitrogen which exists at probably considerable depths from the
surface must be very great. There are