h we shall have occasion to
refer to immediately. It may be pointed out, however, that it is not
always safe to take the amount of nitrates found in drainage-waters as
an infallible indication of this rate, for this amount will depend to a
certain extent on the amount of rainfall, and would be misleading in the
case of a long period of drought. On the whole, however, it furnishes us
with extremely useful data for the elucidation of this important
problem.

_Process goes on most quickly in Fallow Fields._

It has been shown in the Rothamsted experiments that the process goes on
best in fields lying in bare fallow; and in this fact lies the
explanation of one of the many reasons why the practice of leaving
fields in bare fallow, so common in past times, and still practised in
the case of clay soils in some parts of the country, was so beneficial
to the land thus treated. But despite this fact, the practice of leaving
soils in bare fallow can scarcely be justified from this point of view,
as the loss of nitrates through the action of rain is very great in our
moist climate.

_Laboratory Experiments on Rate of Nitrification._

Several interesting experiments have been carried out with the object of
affording data for estimating the rate at which the process may go on in
our soils under certain conditions. An old experiment, carried out by
Boussingault, illustrates, in a general way, how rapid the process is
under favourable circumstances. A small portion of rich soil was placed
on a slab protected by a glass roof, and was moistened from time to time
with water. The amount of nitrate of potash formed under these
circumstances was estimated from time to time during a period of two
months. During the first month (August) the percentage was increased
from .01 to .18 (equal to about 5 cwt. of nitrate of potash per acre).
The increase during the second month (September) was very much
less,--indeed only about a seventh of the amount.[120] The soil
experimented with was an extremely rich garden soil, and all the
conditions for nitrification were most favourable.

Of recent experiments on the rate of nitrification, the most striking,
perhaps, are those by Schloesing. He mixed sulphate of ammonia with a
quantity of soil fairly rich in organic matter, and containing 19 per
cent of water. During the twelve days of active nitrification no less
than 56 parts of nitrogen per million of soil were nitrified per day.
Taking the soi