74 NITRIFICATION AND DENITRIFICATION 
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 fiom ni- 
trates, 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. 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 may furnish this needed carbon, and the 
cellulose present in hay or straw will also furnish it. Any form 
of decaying matter that contains great amounts of hay or stubble 
is especially subject to denitrification. Horse manure, containing 
as it does large amounts of hay, shows greater losses of nitrogen 
than the manure of cattle, which contains less carbonaceous 
material. 
The extent of the actual losses caused by these denitrifiers in 
ordinary farm processes is not fully known. It is certain that in 
concentrated decomposing solutions the action is vigorous, but 
