Soil Bacteria and the Nitrogen Cycle 



399 



ceding section do. The nitrogen compounds thus formed are 

 used by the host plant, and when the latter is plowed under and 

 decays, the nitrogen compounds are made available for a succeed- 

 ing crop of wheat or corn. 



The nitrogen cycle. It may be well at this point to call to 

 mind all the facts we have learned concerning the uses of nitro- 

 gen and the transformations that it and its compounds undergo in 

 nature, involving the nitrogen of the air and the nitrogen com- 

 pounds of soils, of plants, and of animals (Fig. 246). 



If we start with green plants, carbohydrates (see diagram) com- 

 bine with (i) nitrates and form (2) amino acids. These are built 

 up into (3) proteins of plants. Plant proteins are the sources of 

 amino acids used by animals in building (4) animal proteins. 



Fig. 246. Diagram of the nitrogen cycle in nati 



