l to a depth of 9 inches, this would be equal to more
than 1 cwt. per acre--an amount of nitrogen equal to that contained in
6 cwt. of commercial nitrate of soda. These experiments are interesting
as showing what is probably the maximum rate of nitrification under the
most favourable circumstances, and where there is an abundant supply of
easily nitrifiable nitrogen. That nitrification ever takes place in our
soils to this extent is not to be for a moment supposed.

Warington, in his Rothamsted experiments, has found that the greatest
rate, working with ordinary arable soil (first 9 inches) from the
Rothamsted farm, was .588 parts per million of air-dried soil per
day--_i.e._, 1.3 lb. per acre (equal to about 8 lb. of nitrate of soda).
Similar soil, when supplied with ammonia salts, showed nearly double
this quantity. Higher results were obtained by Lawes and Gilbert with
rich Manitoba soils, the average rate being .7 parts per million per
day.

The last of these interesting laboratory experiments on the rate of
nitrification we shall refer to, are those by Deherain. He experimented
with soils containing different amounts of nitrogen and moisture. With a
soil containing .16 per cent of nitrogen he obtained, during a period of
90 days, rates of nitrification varying from .71 to 1.09 per million
parts of soil. The maximum quantity was formed when the soil contained
25 per cent of moisture. On a soil considerably richer--viz.,.261 per
cent of nitrogen--a higher rate of nitrification took place--1.48 parts
per million. The highest rate obtained in these experiments showed,
when calculated to pounds per acre, about 5-1/2, taking the soil to a
depth of 9 inches. When the soil was alternately dried and moistened the
process was most rapid.

_Portion of Soil-nitrogen more easily Nitrifiable than the rest._

Lastly, it may be noticed that in the above-cited experiments, and
others of a similar kind, the process goes on most rapidly at first, and
steadily diminishes thereafter. This is due to the fact, that there is
generally a certain quantity of nitrogen in most soils in a more easily
nitrifiable condition than the rest, so that when this becomes oxidised
nitrification proceeds more slowly. It would further seem that the
nitrogen of the subsoil is less easily nitrified than that of the
surface-soil.

_Rate of Nitrification deduced from Field Experiments._

While the above experiments throw much light on the question o