154 AGEICULTUKAL AND INDUSTEIAL BACTERIOLOGY 



convert the nitrites into nitrates. The first of these 

 processes is termed nitrosation, the second nitratation, and 

 the two together nitrification. All of these terms are mis- 

 nomers. Instead of being nitrification the process is in 

 reality an oxidation. 



Denitrification. It has been previously noted that micro- 

 organisms living in the presence of organic matter and of 

 nitrates, in the absence of free atmospheric nitrogen, may 

 reduce the nitrates to nitrites making use of the oxygen thus 

 secured for oxidizing carbon compounds. Other microorgan- 

 isms carry the change still farther, breaking down the nitrites 

 with the formation of free nitrogen gas. This latter process, 

 like the former, takes place only in the absence of free 

 atmospheric nitrogen and is not very common in soil. How- 

 ever, it constitutes in a sense a leak in the nitrogen cycle, 

 the nitrogen passing off into the free gaseous nitrogen of 

 the air. 



Nitrogen Assimilation. The green plants as well as many 

 microorganisms living in the soil take up nitrogen either in 

 the form of ammonia or of nitrates, and build it up into 

 complex organic compounds. This process is the reverse of 

 that which we have just been considering. It is the build- 

 ing up of proteins from inorganic nitrogen. Animal pro- 

 teins are in all cases derived directly or indirectly from 

 plant proteins. This completes the cycle of nitrogen. 



Nitrogen Fixation. The fact that there is a leak in the 

 nitrogen cycle, and that nitrogen may also be lost from 

 nitrogenous compounds in the process of burning makes it 

 evident that there must be methods whereby nitrogen can 

 be returned from the atmosphere to the nitrogen cycle. 

 This is in large part accomplished by certain bacteria which, 

 as a result of the energy secured by the oxidation of carbon 

 and other compounds, are able to take up free atmospheric 

 nitrogen, combine it with other elements, and work it up 

 into their own protoplasm. Such bacteria in some cases live 



