53 



to sterilize intact. Nothing was found superior to peptone 

 although this medium in the absence of mineral salts and car- 

 bonaceous material is far from the optimum requirement of 

 most bacteria. 



DENITRIFICATION 



It must be evident that of all the organisms which have 

 been studied, comparatively few reduce nitrite to free nitrogen 

 in pure culture. Of the seventy soils under consideration, all 

 of those which would be considered as suitable for crop pro- 

 duction reduced nitrite to free nitrogen in a very short period 

 of time. It must therefore be concluded that either the very 

 few species of bacteria which effect such reduction are univer- 

 sally distributed, or that those organisms which will not per- 

 form this function in pure culture will work in symbiosis to 

 effect this end. The latter phenomenon has been established 

 with reference to a few organisms and will doubtless be extended 

 to include a great variety. The operation of this function 

 is not confined to nitrogenous media but is performed with 

 equal vigor in Ashby's medium in which either glucose or man- 

 nite are employed. The reduction seemingly taking place a 

 little slower in the case of glucose. In simple soil infusion of 

 considerable concentration I have been unable to detect the 

 slightest evidence of reduction of nitrates or nitrites on addi- 

 tion of these salts. That these reductions are effected wholly 

 according to the nascent hydrogen theory seems improbable. 

 By soil inoculation of the media I have been unable to reduce 

 sulphates to hydrogen sulphide except in a very few instances, 

 while with sewage sludge no difficulty is experienced. The 

 phosphates also seem refractory. A selective action involving 

 energy, nutrition, etc., may be concerned. 



But eight species of bacteria are commonly cited as re- 

 ducing nitrites to free nitrogen in pure culture: Bacterium 

 centropunctatum, (H. Jensen), Bacterium filifaciens (H. 

 Jensen), Bacterium Hartlebi (H. Jensen), Bacterium nitro- 

 vorum (H. Jensen), Pseudomonas pyocyanea (Migula), Bacil- 

 lus denitrificans (H. Jensen) and probably two members of 

 the Fluorescens group: Bacillus fluorescens and Bacillus 

 fluorescens liquefaciens. Maassen observed that Bacillus 

 praepollens broke down nitrates to free nitrogen only in sym- 



