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. 



