If it is to be shown that nitrification will
occur in the absence of any ferment, it is clear that all ferments must be
rigidly excluded during the experiments; the solutions must be sterilized
by heat, the apparatus purified in a similar manner, and all subsequent
access of organisms carefully guarded against. It is only experiments made
in this way that can have any weight in deciding the question.
Leaving now the theory of nitrification, I will proceed to say a few
words, first, as to the distribution of the nitrifying organism in the
soil; secondly, as to the substances which are susceptible of
nitrification; thirdly, upon certain conditions having great influence on
the process.
_The Distribution of the Nitrifying Organism in the Soil._--Three series
of experiments have been made on the distribution of the nitrifying
organism in the clay soil and subsoil at Rothamsted. Advantage was taken
of the fact that deep pits had been dug in one of the experimental fields
for the purpose of obtaining samples of the soil and subsoil. Small
quantities of soil were taken from freshly-cut surfaces on the sides of
these pits at depths varying from 2 inches to 8 feet. The soil removed was
at once transferred to a sterilized solution of diluted urine, which was
afterward examined from time to time to ascertain if nitrification took
place. These experiments are hardly yet completed; the two earlier series
of solutions have, however, been examined for eight and seven months
respectively.
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