64 NITRIFICATION AND DENITRIFICATION 
acid, and practically all the other commonly used explosives are 
produced from nitrates. Nitrate formation is, then, a matter of 
the greatest significance. While there are, perhaps, some other 
methods by which nitric acid can be formed, beyond doubt the 
nitrate store in the soil has been formed chiefly through the process 
of nitrification. 
Nitrification in the Soil.—It is very easy to demonstrate that 
such nitrification actually takes place in ordinary soil. Ifwe place 
a quantity of soil in a proper vessel and subject it at intervals to a 
chemical analysis, it will be found that there is an increase in the 
amount of nitrates present, after it has remained undisturbed for a 
few weeks. This fact has been known for nearly a century. The 
next step in the discoveries was made in 1877, when it was demon- 
strated that this nitrification is associated with the presence of 
living matter in the soil. This can be proved by placing two lots 
of the same soil under such conditions that in the one phenomena 
of life may go on, while in the other they are stopped. If, for 
example, one lot of soil is sterilized by heating it sufficiently to 
destroy the living germs present, and then this soil is compared 
with another lot treated in all respects the same, except that it is 
not sterilized, the latter will be found to increase its nitrates, while 
the former will show no such increase. The same results are ob- 
tained if the soil is mixed with antiseptics which prevent bacterial 
growth. In short, anything which prevents the occurrence of life 
phenomena in the soil, prevents the nitrification. 
Isolation of the Nitrifying Organisms.—Such experiments 
repeated many times and verified by numerous observers demon- 
strated that nitrification is the result of a living process. Inas- 
much as such soil contains no plants large enough to be seen, it 
follows that the living agent of nitrification must be some form of 
microorganism. It proved, however, to be a very difficult matter 
to find the organisms concerned in the process. The number of 
bacteria in the soil is large and many different species are there 
found. But although many of these bacteria were isolated and 
