nts, however, considerably modified this conclusion, and showed
that nitrification may take place to a depth of at least 6 feet.[118]
But although it may take place at this depth, it probably, as a general
rule, is limited to the surface-soil, as it is only there the conditions
for obtaining circulation of air are sufficiently favourable. A great
deal, of course, will depend on the nature of the soil--_i.e._, as to
its texture. In a clayey subsoil the principal hindrance to
nitrification will be the difficulty of obtaining sufficient aeration.
In clay soils it is probable, therefore, that nearly all the
nitrification goes on in the surface layer; in sandy soils it may take
place to a greater depth.[119]

_Action of Plant-roots in promoting Nitrification._

In this connection the action of plant-roots in permitting a more
abundant access of air to the lower layers of the soil, and thus
promoting nitrification, is worth noticing. This has been observed in
the case of different crops. Thus the action of nitrification has been
found to be more marked in the lower layers of a soil on which a
leguminous crop was growing than on that on which a gramineous. "The
conditions which would favour nitrification in the subsoil are such as
would enable air to penetrate it, as artificial drainage, a dry season,
the growth of a luxuriant crop causing much evaporation of the water in
the soil. Such conditions, by removing the water that fills the pores of
the subsoil, will cause the air to penetrate more or less deeply and
render nitrification possible. Subsoil nitrification will thus be most
active in the drier periods of the year" (Warington).

_Nature of Substances capable of Nitrification._

What kinds of nitrogenous substances are capable of undergoing this
process of nitrification are not yet well known. The question is, of
course, one of great importance, as the rapidity with which a
nitrogenous body nitrifies will be an important factor in determining
its value as a manure. Unfortunately, on this subject we know, as yet,
very little. We are well aware that the nitrogen present in the humic
matter of the soil is readily nitrifiable. In the experiments on
nitrification the nitrogenous bodies used have been chiefly ammonia
salts, so that it is difficult to say whether, in the case of other
nitrogenous substances, micro-organic life of a different sort has not
also been active and has converted the nitrogen into ammonia, and
t