60 BIOLOGY AND TECHNIQUE 



first believed, depends essentially upon simple oxygen extraction 

 from the nitrates by the bacteria, and for this reason goes on most 

 actively when the supply of atmospheric oxygen is low. The first 

 bacteria described as possessing this power of denitrification were 

 the so-called B. denitrificans I and II, the first an obligatory 

 anaerobe, the other a facultative aerobe. Since then numerous other 

 bacteria, among them B. coli and B. pyocyaneus, have been shown 

 to exhibit similar activities. It is important agriculturally, there- 

 fore, to know that many species which are able to utilize atmospheric 

 oxygen when supplied with it, will get their oxygen by the reduction 

 of nitrates and nitrites when free oxygen is withheld. It is thus 

 clear that a loss of nitrogen is much more apt to proceed rapidly 

 in manure heaps which are piled high and poorly aerated. There 

 are other factors, however, in regard to the physiology of these 

 microorganisms, which must be considered for practical purposes. 



In order that these bacteria may develop their denitrifying 

 powers to the best advantage, it is necessary to supply them with 

 some carbon compound which is easily absorbed by them. This, in 

 decomposing material, is furnished by the products of the carbohy- 

 drate cleavage going on side by side with the proteolytic processes. 

 It is still more or less an open question whether the facilitation of 

 denitrification brought about in manure heaps by the presence of 

 hay and straw is due to the carbon furnished by these materials, 

 or whether it is due to the fact that bacilli of this group are apt 

 to adhere to the straw which acts in that case as a means of 

 inoculation. 



The actual danger of -nitrogen depletion of the soil by denitri- 

 fying processes is probably much less threatening than was formerly 

 supposed ; for, in the first place, the conditions for complete denitri- 

 fication are much more perfect in the experiment than they ever 

 can be in nature, and the nitrifying processes going on side by 

 side with denitrification make up for much of the loss sustained. 



ANABOLIC OR SYNTHETIC ACTIVITIES OP BACTERIA 



Nitrogen Fixation by Bacteria. The constant withdrawal of 

 nitrogenous substances from the soil by innumerable plants would 

 soon lead to total depletion were it not for certain forces continually 

 at work replenishing the supply out of the large store of free 

 nitrogen in the atmosphere. This important function of returning 



