56 NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS 
use to the animal world. It is estimated that some 38,000 tons of 
urea are excreted daily by the human race. To this quantity 
must be added the far greater amount excreted by other animals, 
for all animals, large and small, secrete it or an allied substance, 
and the total is enormous. What becomes of it all? 
Thus the nitrogen of the nitrate absorbed by the plant has 
reached two quite different conditions. Part of it is still in the 
highly complex form of proteid, either in the dead body of the 
animal or the plant. A second part has been partly broken down 
in its passage through the animal’s body, and has reached the con- 
dition of urea or some allied body. But in neither condition is it 
within reach of another generation of green plants. It must be 
still further broken down before it is available for plants. 
ORGANIC NITROGEN—ITS DECOMPOSITION 
Decomposition in General—This means the breaking to 
pieces of complex compounds so as to form simpler ones. The 
term thus defined is a very broad one, and covers a long series of 
changes, of a purely chemical nature. But more commonly the 
term has a narrower meaning, and refers to the breaking down of 
organic products under the influence of microorganisms. This is 
one of the most important functions of soil bacteria. The destruc- 
tion of nitrogenous compounds, urea, proteids, gelatins, or other 
bodies, is brought about by several agencies, but the chief one is 
undoubtedly that of microérganisms. A small amount of the pro- 
teid appears to be decomposed in plant tissue without the aid of 
bacteria; another portion is broken down by yeasts; another by 
molds and other fungi. But decomposition is chiefly due to a class 
of bacteria called the decomposition bacteria. 
But even as thus limited, this term is still a broad one including 
different species of bacteria and various types of decomposition. 
Two types are generally recognized, under the names of decay 
and putrefaction. These two terms are frequently not very clearly 
distinguished, being used indiscriminately to refer to the decom- 
