hereby prepared the way for the process of nitrification.

That various manures, such as bones, horn, wool, and rape-cake are
readily nitrifiable, has been shown by experiment. Laboratory
experiments have also been carried out on such different nitrogenous
substances as ethylamine, thiocyanates, gelatin, urea, asparagin, and
albuminoids of milk. But in all these experiments, how far these bodies
have been directly acted upon by the nitrifying organisms, or how far
they have first undergone a preparatory change in which their nitrogen
has been first converted into ammonia, is impossible to say. It is at
least quite probable that all the organic forms of nitrogen have first
to be converted into ammonia ere they are nitrified.

_Rate at which Nitrification takes place_.

A question which is practically of no little importance is the rate at
which nitrification takes place. From what has been already said as to
the nature of the conditions favourable for the process, it will be at
once seen that this will depend on how far these conditions are present
in the soil. In point of fact the rate at which nitrification takes
place will vary very much in different soils. A greater difference,
however, in the rate at which it takes place, will be found even in the
same soils at different periods of the year. In this country, where the
most favourable temperature for its development is seldom reached, it
never goes on at the same rate as in tropical climates. One of the
causes of the greater fertility of tropical soils is due, doubtless, to
the very much longer duration of the period of nitrification, as well as
to its greater intensity. As, however, temperature is not the only
condition, and the presence of moisture is quite as necessary, it may be
that its development is seriously retarded in many tropical climates by
the extreme dryness of the soil during long periods.

_Takes place chiefly during the Summer Months_.

Although in this climate, as has already been pointed out, nitrification
probably goes on during most of the winter months, owing to the fact
that the temperature of our soils is only occasionally below the
minimum temperature at which the process takes place, yet there can be
little doubt that the great bulk of the soil-nitrates are produced
during a few months in summer. A fair conception of this amount is
afforded by the interesting experiments on the composition of
drainage-waters made at Rothamsted, whic