DENITRIFICATION 73 
an increased vegetation may continue for a long time under the 
influence of these nitrifying bacteria which are constantly con- 
verting the soil nitrogen into nitrates. That this is the whole ex- 
planation in these cases is by no means sure, but it is certain that 
the nitrifiers do unlock much nitrogen previously not in an avail- 
able condition 
Number of Nitrifiers in Soil—Strange to say, these nitrifiers, 
for all their importance, have never been found in soil in great 
abundance. A few thousands per gram are the highest figures 
reported, while other kinds of bacteria, of which the functions are 
still unknown, occur in the hundreds of thousands or even millions 
per gram. It has never been fully explained how they can be of 
such importance while occurring in such small numbers. 
DENITRIFICATION 
There is another group of microorganisms in soil and other 
decaying masses acting in exactly the reverse direction from the 
nitrifiers. Whereas nitrification oxidizes ammonium compounds 
and nitrites, to form nitrates, denitrification takes the oxygen out 
of nitrates, reducing them to nitrites and ammonia, and may even 
reduce these to free nitrogen. Nitrification prepares plant foods, 
but denitrification destroys them. The one process is useful, the 
other detrimental, to soils. 
Three different types of reduction of nitrogen compounds 
are comprised under this head: (1) The reduction of mztrates into 
nitrites. (2) The reduction of nitrates to give off free nitrogen. 
(3) The reduction of nzirites into free nitrogen. The term denitrifi- 
cation is sometimes used to cover all of these types of reduction, 
and sometimes more particularly to refer only to the reduction of 
the nitrates and nitrites, so as to liberate free mstrogen. In its 
strict use it should be confined to the latter process. 
It is evident that these different types of reduction will have 
different effects upon soil fertility. Those portions of the nitrogen 
that are reduced to nitrates or ammonia, under proper conditions, 
