NITRIFICATION 6x 
carefully tested, for a long time none proved to have any power of 
nitrification. Most of them, indeed, produced the reverse effect, 
that of deoxidizing nitrates, but none of them raised the nitrites 
into a state of nitric acid. None of them could oxidize ammonia 
so as to form nitric or even nitrous acid. Ifa small quantity of 
soil is added to a solution of nitrite or of ammonia salts the nitrite 
or ammonia soon becomes converted into nitrate, under the influ- 
ence of the fermentation started by the presence of the soil. 
This shows that the soil must contain the nitrifying organisms. 
But the bacteria which are isolated from such soil by ordinary 
methods showed no power of nitrification. Evidently the nitrify- 
ing bacteria cannot be found by the ordinary bacteriological 
methods. 
The cause of the trouble as well as the secret of successful study 
was soon learned. In bacteriological studies the common method 
of isolating bacteria is to get them to grow in culture media made 
by the bacteriologist. The media commonly used contain a cer- 
tain amount of organic compounds which serve as food for the 
bacteria. But experiment soon showed that the presence of the 
smallest amount of organic matter is directly injurious to the nitri- 
fying bacteria, so that they will not grow at all in ordinary culture 
media. It was necessary to devise some culture media that con- 
tained no organic matter, and as soon as this was done it was pos- 
sible to isolate from the soil bacteria having the power, under 
proper conditions, of oxidizing ammonium and nitrite compounds 
into nitrates. For a while the results of experiments were in 
some confusion, since in some cases nitrates appeared to be 
formed, ‘while in others they did not. It became evident that 
nitrification was not a simple phenomenon, and further study 
showed, that the nitrification, as occiirring in ordinary soil, is a 
two-fold process. The first step in the process oxidizes the am- 
monia into nitrites. In most of the experiments the nitrogen was 
put into the culture fluids in the form of sulphate or carbonate 
of ammonia and this was readily oxidized into nitrite. The second 
5 
