54 NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS. 
(Fig. 1 6). Most of these bacteria produce this action through an 
enzyme that they secrete, named urase. Under proper conditions, 
as much as 97 per cent, of the nitrogen in the urea is converted into 
ammonia in a space of four days. The ammonia is a volatile 
product and has, consequently, a tendency to pass off into the air, 
as may readily be recognized from the odor of ammonia that is 
frequently perceived around a manure pile. This represents a per- 
manent loss of nitrogen, and should be avoided as much as possible. 
FIG. 16. Various bacteria causing the ammoniacal fermentation of urea (Beijerinck}. 
The loss is greater when the liquid is concentrated, and consequently 
less if the urine can be poured upon the soil at once, than if stored 
in vats or even mixed with solid manure. 
Although urea shows the ammoniacal fermentation most readily, 
other nitrogenous bodies, like proteids, etc., also may give rise to 
ammonia (Fig. 17), which is, indeed, one of the common end-products 
of proteid decomposition. The chemical changes that occur in 
proteid decomposition are complex and not wholly understood. 
The first step seems to be quite like that taken when they are 
digested in the digestive tract of animals, for, under the action of the 
