18 The Colorado Experiment Station. 
Nitrification. 
The change of ammonia and ammonia compounds in the soil 
into nitrates was looked upon for a long time as a purely chemical 
process. The transformation was explained as due to electrical dis¬ 
charges in the atmosphere, to natural combustion, or to the oxida¬ 
tion of organic matter in combination with calcium, magnesium, 
iron and manganese compounds. The reason for believing this was 
that nitrates are almost universally present in cultivated land com¬ 
bined with calcium or magnesium. In 1862, Pasteur expressed the 
belief that the formation of nitrates in the soil and in decaying 
organic matter might be due to biological activities. Muller, a 
few years later while studying the nitrites and nitrates in sewage 
and drinking water, came to look upon their presence as the work 
of micro-organisms. The exact nature of nitrification was not fully 
understood, however, until 1877 when Schlosing and Muntz dem¬ 
onstrated clearly that it was brought about by biological agents. 
Their classic experiments showed that when a dilute solution of 
ammonia is passed through a long tube filled with soil, nitrates 
appear in the resulting liquid in place of the ammonia contained in 
the original solution; in other words, contact with the soil had 
changed the ammonia to nitrate. Furthermore, if the soil in the 
tube is first sterilized by heat or chloroform or other germicides, 
and the ammonia solution is passed through subsequently, no change 
takes place. But if the sterilized soil is reactivated by inoculating 
it with a small quantity of fresh soil, its nitrifying power is again 
restored. It was quite clear from this that a close relation must 
exist between the life in the soil and the nitrifying process. Ac¬ 
cordingly, bacteriologists set to work at once to discover, if possi¬ 
ble, the specific micro-organisms which were responsible for the 
transformation. Years were spent in diligent search, but only 
negative results rewarded their labor. At last, in 1890, Winograd¬ 
sky succeeded in isolating certain bacteria which were capable of 
producing nitrates from ammonia solutions. His investigations 
showed, further, that nitrification or the oxidation of ammonia 
into nitrates takes place in two separate steps, and that two distinct 
classes of bacteria are involved in these changes. The first stage 
is the oxidation of ammonia into nitrous acid and nitrites; the 
second, the oxidation of the nitrites into nitric acid and nitrates. 
Winogradsky designated the organisms which are responsible for 
the first step as nitrous or nitrite bacteria and called the two species 
Nitrosomonas and Nitrosococcus. The former represents varieties 
obtained from soil in Europe, Asia and Africa, while the latter. 
