DENITRIFI CATION. 67 
are comprised under this head: i. The reduction of nitrates into 
nitrites. 2. The reduction of nitrates to give off free nitrogen. 3. 
The reduction of nitrites into free nitrogen. The term denitrifica- 
tion 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 nitrogen. 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 
may be built up again into nitrates by the nitrifying bacteria. But 
those that are reduced to a condition of free nitrogen pass off 
into the air and out of the reach of plant life. This nitrogen, 
therefore, represents an actual loss to the soil. Denitrification 
is a process very different from the general type of decomposition 
which we have described. Decomposition begins with proteids and 
reduces them to ammonia compounds. Denitrification begins with 
nitrates and nitrites, and liberates free nitrogen. 
The Denitrifying Bacteria. Denitrification is the result 
of the action of a class of bacteria known as the denitrifiers. Very 
many bacteria have the power of extracting the oxygen from nitrates, 
reducing them to nitrites, but the list of those that can liberate free 
nitrogen is shorter. Some of them act in aerobic conditions, and 
others in anaerobic conditions. The names B. denitrificans 
I and II have been given to two of them, but others have been found 
with similar properties. They are very widely distributed; they 
are found not only in soil and water, but in the air and all organic 
decomposing refuse. They are very abundant in the manure heap, 
especially if it contains much hay and straw, and they are likely 
to cause a considerable loss of nitrogenous matter by liberating 
the nitrogen as free nitrogen gas. Excrement always contains 
them, but they are more abundant in the excrement of herbivorous 
animals than of carnivorous animals. These bacteria, in order to 
grow vigorously, require some carbon-holding food, and they cause 
the largest amount of denitrification when abundantly supplied 
with carbohydrates. Sugars, starches, glycerin, or organic acids 
