THE BIOLOGY OF THE SOIL 99 
The Circulation of Nitrogen. 
After what we have said in this chapter, we can now 
complete the balance-sheet of the atmosphere, which we 
left unfinished on p. 79. 
1. The Sources of Nitrogen in the Soil.—(1) The nitro- 
genous compounds present in the soil are produced, as we 
have seen, by the agency of bacteria in the decomposition 
of humus. 
(2) During thunderstorms, in the path of the lightning, 
oxygen and nitrogen unite, and the resulting oxides of 
nitrogen are swept down by rain into the soil as nitrous 
acids ; these are readily oxidized and combined as nitrates. 
(3) Certain soil-bacteria, some living free in the soil 
and others located in root-nodules, are able to fix or 
assimilate the free nitrogen of the air. The latter is 
ultimately restored to the earth in the form of nitrates 
(p. 95). 
2. The Losses of Nitrogen in the Soil.—(1) The nitrates 
and ammonium-compounds taken up by plants are 
restored to the soil when the plants decay. If the plants 
are eaten by animals, the ingredients are returned to the 
soil in their excreta, or finally in their bodies after death. 
The loss to the soil in this case is only temporary. 
(2) Nitrates and ammonium-compounds are _ very 
soluble in water, and much of what is formed in the soil 
is carried away by water and lost in the sea. 
Everywhere, and at all times, there is leakage of nitrogen 
from the soil. The loss is made good from the air in the 
ways we have described. The air, in fact, is the ultimate 
source of all the fixed nitrogen in Nature, just as it is the 
source of all its carbon. 
