CH. Ill] 



The Soil Cycle 



47 



3. In absence of air some of the nitrates are reduced 

 to gaseous nitrogen which escapes. This process is called 

 denitrification. 



4. A loss of nitrogen also occurs when rich soils are 

 cultivated. It is possible that this is also denitrification, 

 but the evidence is not yet sufficiently definite to justify 

 the use of the term. 



5. The losses indicated in the two preceding para- 

 graphs are made good as follows. In presence of easily 

 oxidisable organic matter, especially of carbohydrates, 

 certain organisms are able to fix gaseous nitrogen and 

 build it up to the form of protein. This process is called 

 nitrogen fixation ; it must not be confused with nitrifi- 

 cation. 



The protein formed during nitrogen fixation, however, 

 can undergo nitrification in the usual way, being de- 

 composed to form ammonia, which is then oxidised to 

 nitrate. 



These processes form a double cycle in the soil which 

 may be thus expressed : 



PROTEIN 



Nitrogea 



by denitrifying 

 organisms 



