54. NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS 
for the world. But these beds do not solve the question of con- 
tinued soil fertility, for they will soon be exhausted and some other 
source of nitrates must be discovered. Fortunately, we have 
learned that there are processes in nature by which the soil ni- 
trates are replaced, quite independently of the store of nitrates in 
Chili or elsewhere, and this replacement is a phenomenon of the 
life activities of the bacteria and other microdrganisms of the soil. 
If the nitrates absorbed from the soil are replaced, it must be 
from one of two sources or from both. 
1. From the Nitrogen in Organic Bodies—After plants absorb 
nitrates from the soil, they build them up into organic compounds, 
chiefly proteids, which subsequently may or may not be utilized as 
food by animals. Whatever be its history, whether in animals or 
plants, this proteid material always contains nitrogen; and even- 
tually, by processes which we shall study, this nitrogen may again 
assume the form of nitrates and restock the soil. 
2. The Nitrogen of the Atmosphere-—This is an inexhaustible 
source of supply, if it can be utilized. We shall learn that there 
are means by which it becomes available for plants. 
There seem to be no other possible sources for replacing soil 
nitrates, and we will therefore consider these two in detail. 
ORGANIC NITROGEN. ITS NATURE 
The nitrates are built up by the plants into a variety of com- 
pounds, mostly of the nature of proteids, like gluten of wheat, 
legumen of peas, and other similar bodies. All plants contain some 
such compounds which serve a purpose in the life of the plant. 
While the plant is still alive they remain as proteids without much 
change. After the death of the plant that produced them the pro- 
teids are at the disposal of nature’s forces of destruction. Some of 
the proteids are seized by animals and utilized for their life proc- 
esses. ‘They are slightly changed inside the animal’s body, but are 
not built into bodies more complex than proteids. The animal 
forms animal proteids out of them, producing myosin, gelatin, 
