226 THE LIVING ORGANISMS OF THE SOIL [chap. 



the production of nitrates ; in consequence it is their 

 numbers that should be determined. A better indica- 

 tion is obtained by determining the rate of formation 

 of nitrate within a mass of the soil ; the difficulty 

 attending- the operation is, however, that of securing 

 standard conditions of moisture and mechanical con- 

 dition of the soil. 



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 

 observers, particularly Deherain and Maquenne (1882), 

 with regard to soils, confirmed these results and showed 

 that they were due to bacterial action. In a paper 

 published in 1882, Warington described an experiment 

 in which sodium nitrate was applied to a soil saturated 

 with water ; after standing for a week, the nitrate was 

 washed out of the soil, part of it had disappeared, and 

 part had become nitrite. The total of both nitric and 

 nitrous nitrogen only amounted to 20-9 per cent, of that 

 which had been originally applied. That the nitrate 

 had been reduced to gaseous nitrogen was seen by the 

 development of transverse cracks filled with gas in the 

 soil, and it was concluded that some of the nitrogen 

 applied in manures and unaccounted for in crop and 



