vil] LOSSES OF SOIL NITROGEN 197 



whether these nitrates, formed after harvest, are retained 

 for the succeeding crop or are washed out of the soil. 

 To sum up, increased nitrification, together with the 

 conservation of soil moisture and the warming of the 

 surface soil, are among the chief benefits derived from 

 all forms of surface cultivation. 



As so much of the fertility of a soil must depend on 

 the number of nitrifying organisms it contains, attempts 

 have been made to compare soils in this respect, by 

 seeding small quantities of them into a standard solu- 

 tion capable of nitrification and determining the amount 

 of nitric acid formed after a given time. Although con- 

 siderable differences are seen in the action of different 

 soils, satisfactory quantitative results have not yet 

 been obtained, because of difficulties in the way of 

 drawing strictly comparable samples of the soils, 

 and the uncertainty still attaching to the amount 

 which should be used for inoculation or the best period 

 of incubation. 



Denitrification. 



The term denitrification is most properly applied 

 to the reduction of nitrates to nitrites, ammonia, or 

 particularly to gaseous nitrogen, which is brought 

 about by bacterial action under certain conditions. 

 Of late, however, the term has been more loosely 

 used to denote any bacterial change which results 

 in the formation of gaseous nitrogen, whether de- 

 rived from nitrates, ammonia, or organic compounds 

 of nitrogen. 



Angus Smith was the first to observe the evolution 

 of gas from a decomposing organic solution containing 

 nitrates, which were destroyed in the process. Other 



