9 8 



AGRICULTURAL BACTERIOLOGY. 



of these two different processes, the building-up and pulling- 

 down nitrates, in the soil ? That the nitrifying bacteria build 

 nitrates out of lower compounds is easy enough to understand, 



Fiu. 17. 



Nitrates (/nscil) 



'I'lie nitrogen cycle. 



and experiment shows that it actually occurs. But if both 

 nitrification and denitrification were going on at all times the 

 result would, in a short time, be a total disappearance of soil 

 nitrogen. If the nitrification were constantly building the soil 

 nitrogen compounds into the form of nitrates, and then the 

 denitrifying bacteria were as constantly seizing the nitrates thus 

 furnished and reducing them to ammonia or to free nitrogen, 

 it is clear that the continuation of the process would in the end 

 free the soil from all nitrogen bodies. Kvidently such a con- 

 dition does not exist, for no such constant dissipation of nitro- 

 gen occurs, and the soil nitrates increase rather than decrease 

 as the total result of the action of the soil bacteria. 



The denitrifying bacteria are very vigorous, while the nitri- 



