he ratio of nitrogen to carbon, which points to the fact that, just as
we should naturally suppose, the origin of the latter is very much more
ancient than the origin of the former. Thus in the first 9 inches of old
pasture-soil at Rothamsted, the ratio was 1:13; while in the subsoil, 3
feet from the surface, it was only 1:6. In the surface-soil it thus
approaches more nearly in composition ordinary vegetable matter.

_Nitrogen as Ammonia in Soils._

The second form in which nitrogen is present in soil is as ammonia. A
very considerable misapprehension has existed in the past as to the
amount of nitrogen in this form in soils. This mistake was due to the
method adopted in estimating it, which consisted in treating the soil
with boiling caustic alkalies and counting as ammonia what was given off
as such. It is now known that certain forms of organic nitrogen--as, for
example, amides--if treated in this way are slowly converted into
ammonia. Statements, therefore, which are found in the older text-books,
representing the amount of ammonia in soils as at over a tenth per cent,
must be regarded as utterly unreliable. Indeed it is highly probable
that ammonia only occurs in most soils in very minute traces. From what
we know of the process of nitrification, we see how it is wellnigh
impossible that ammonia should exist to any extent in the soil except
under very exceptional circumstances.

_Amount of Ammonia present in the Soil._

In ordinary soils it probably does not amount to more than from .0002
per cent to .0008 per cent, or an average of .0006 per cent.[75] In
rich soils, or in garden-soils, the amount may be considerably more.
Thus Boussingault found in a garden-soil .002 per cent. In peat and in
peat-mould even a higher percentage has been found--viz.,.018 for the
former and .05 for the latter.

_Nitrogen present as Nitrates in the Soil._

The third form of nitrogen in the soil is nitric acid. It is more
abundant in this form than as ammonia; but still, compared with the
organic nitrogen, its amount is trifling. Probably not more than 5 per
cent of the total nitrogen of a soil is ever present as nitrates. The
reason of this is twofold. First, as we have already remarked, the soil
has very little power to retain nitrogen in this form; and secondly,
where the soil is covered with growing vegetation the nitrates are
quickly assimilated by the plant as they are formed. It is for this
reason that we find the q