of determinations of
nitrates in cropped soils, receiving, however, no manure, and taken to a
depth of 9 feet, will be found in the Appendix.[77] The first 27 inches
only contain some 5 to 14 lb. per acre, and the most of that is found in
the first 9 inches. This shows how speedily nitrates are assimilated by
the growing crop. An interesting point shown by these analyses is that
nitrates almost entirely cease in cropped soils a certain depth down,
but that at a still lower depth they again occur in small quantities.

_Nitrates in manured Wheat-soils._

Lastly, we give in the Appendix[78] the amount of nitrates found in
wheat and barley soils, differently manured, at Rothamsted. From a
perusal of these tables, it will be seen that the amount (under various
conditions of manuring) of nitrates in the first 27 inches varies from
21.2 lb. per acre to 52.2 lb. for the wheat-soils, and 20.1 to 44.1 lb.
per acre for the barley-soils.

THE SOURCES OF SOIL-NITROGEN.

We shall now consider the sources of soil-nitrogen, the conditions which
determine its increase, and the amount of that increase, as well as the
sources of loss, and the conditions which determine this loss.

_That dissolved in Rain._

The natural sources of the soil-nitrogen are several. We have first of
all the atmospheric nitrogen. Of this let us first consider that present
as combined nitrogen. This, as we have already seen, consists chiefly of
nitrates, nitrites, and ammonia, and reaches the soil dissolved in rain
or in other meteoric forms of water, such as snow, hail, fog,
hoar-frost, &c.

_That absorbed by the Soil from the Air._

It is also absorbed by the soil from the air, especially when the soil
is in a damp condition, as has been proved by Schloesing's experiments,
already referred to. The total amount which falls dissolved in the rain,
per acre per annum, varies very considerably in different parts of the
world, but in any case only amounts yearly to a few pounds per acre.[79]
That absorbed by the soil from the air may be probably very much more
considerable. Schloesing in his experiments found that this latter might
amount to 38 lb. per acre per annum. These results, however, were
obtained under circumstances most favourable for absorption--viz., with
a damp soil and in the vicinity of Paris, where the air is presumably
richer in combined nitrogen than it is in the country. The nitrogen
absorbed, it may be mentioned, was almost entire