DENITRIFICATION 133 



ganic matter of the soil on coming in contact with nitrates was 

 oxidized and the nitrogen liberated. It was well-known that un- 

 der certain conditions nitrites and ammonia would react and both 

 lose their nitrogen. Could this be what was happening in the 

 soil? This question was answered in 1886 by Gayon and De- 

 petit who isolated from the soil two organisms— B. demtrifici- 

 cans a and |3 — capable of acting on nitrates with the evolution 

 of gaseous nitrogen. They also found a number of organisms 

 capable of reducing nitrates to nitrites. 



Organic substances, such as straw, carry the denitrifying bac- 

 teria. They are carried with the litter to the manure and later 

 with the manure to the soil. The true denitriiiers are less pre- 

 valent in cultivated soil, but the ones which yield nitrites or pro- 

 tein are more numerous as has been demonstrated by the work 

 of Putnam. He examined 201 species and found 139 capable of 

 reducing nitrates to nitrites, whereas none liberated nitrogen. 

 Even in manure those which afiFect partial denitriiication are most 

 numerous. Severin studied twenty-nine different microorganisms 

 from horse manure. Of this number eight were capable of com- 

 plete reduction of nitrates, provided the nitrate concentration be 

 not too great. Nine of the remaining were able to reduce nitrates 

 to nitrites. 



Loss of Nitrates from Manure and SoiL — The finding of 

 the denitrifying bacteria on straw and \n manure, together with 

 the established fact that they can under appropriate conditions 

 decompose nitrates with the evolution of gaseous nitrogen, led 

 Wagner in 1895 to emphatically declare that the application of 

 cow or horse manure to a soil is often not only unprofitable but 

 harmful, that when applied together with nitrates they cause, by 

 virtue of the microorganisms contained in them, the destruction 

 of the nitrates. More than that, the baneful effects do not stop 

 here, for the nitrates as they are gradually formed from the or- 

 ganic matter of the soil are also attacked by the denitrifying bac- 

 teria and their nitrogen set free. In reality, then, the animal 

 manures applied are not only useless in themselves but are harm- 

 ful because of their destructive effects on the oxidized nitrogen 

 derived from other sources. 



