DENITRIFICATION 



135 



in general farming and under average field conditions. We have 

 no hesitation in emphasizing again the view expressed above — 

 that under the wide range of field conditions, denitriiication is 

 not a phenomenon of economic significance to the general 

 farmer." 



Moreover, at Rothamsted a plot of ground one- thousandth of 

 an acre in extent has been kept from vegetation by hoeing for 

 thirty-five years. During this time it has lost one-third of its 

 original stock of nitrogen, but all except no pounds of this is 

 accounted for by the nitrates in the drainage water, as may be 

 seen from the following: 



Nitrogen in 

 Soil 



Nitrogen per Acre 

 {lbs.) 



Nitrogen 



Recovered 



as Nitrates 



Nitrogen 



Unaccounted 



for 



1870 



igos 



1870 



1905 



Loss from 

 1870-igos 



i87o~i8gs 





,146 



,101 



3500 



2450 



1050 



940 



no 



Russell, commenting upon the results, states: "The experiment 

 is not fine enough to justify any discussion of the missing no 

 pounds, but it shows that the loss of nitrogen is mainly due to 

 leaching out of the nitrates." 



Factors Influencing Denitrification. — Denitrification can 

 take place in a soil only when a source of carbon is present, nor 

 is the form in which the carbon exists without significance. Deni- 

 trifiers need large quantities of organic food and develop best in 

 the presence of the fresh plant and animal debris. Hence, it 

 would be unwise to use heavy dressings of nitrate and fresh ma- 

 nure or to plow under with the nitrates green-manuring crops. 

 Partly decayed barnyard or green manures do not appear to favor 

 denitrification. 



In very wet soils oxygen is excluded. Here the bacteria turn 

 to the soil nitrates for oxygen and hence liberate the nitrogen. 



