few sedimentary rocks which do not
contain it. At Rothamsted a sample of calcareous clay, taken from a
depth of 500 feet, contained .04 per cent--that is, as much as is found,
on an average, in the Rothamsted clay subsoils.

_Nitrogen in the Subsoil._

On the whole, however, as we have said, nitrogen is chiefly found in the
surface-soil. The amount found in the subsoil at Rothamsted seems to
vary very slightly at different depths, the percentage amounting to from
.06 to .03.[71] Unlike the nitrogen of the surface-soil, that in the
subsoil seems to be of very ancient origin, being probably derived from
the remains of animal and vegetable life in the mud deposited at the
bottom of the ocean. It is more abundant in the case of a clay subsoil
than in a sandy subsoil.

_Nitrogen of Surface-Soil._

Nitrogen has a tendency to collect on the top layers of the
surface-soil, the first 9 inches or foot containing by far the largest
proportion of it. In the table given in the Appendix,[72] the rate at
which it decreases in amount the further down we go is clearly shown.
Determinations of the respective amounts of nitrogen in every 3 inches
of the soil, taken to a depth of one foot of the experimental
wheat-field at Rothamsted, showed that the percentage between the first
3 inches and the second 3 inches varied very slightly. A more marked
difference, however, was shown to exist between the nitrogen in the
second and third 3 inches; while the fourth 3 inches were distinctly
poorer--differing very little in their percentage of nitrogen from the
subsoil. This was the case in unmanured soil. In the case of heavily
manured soil, the increase in the soil's percentage, due to manure, was
shown to be felt to the depth of a foot, but not much below it.[73]

A careful perusal of the tables in the Appendix will show that the
quantity of nitrogen in the case of both arable and pasture soils
steadily decreases for the first 3 feet, but that below this depth
little decrease is seen, the percentage evidently becoming fairly
constant.

_The amount of Nitrogen in the Soil._

Very considerable difference exists in the amount of nitrogen present in
different soils. The majority of analyses refer only to the amount found
in the surface-soil--generally in the first 9 or 12 inches. As the soil,
further, is not a body exactly homogeneous in its character, very
considerable difficulty exists in obtaining reliable results. A great
deal depe