METABOLISM 223 



stock but from the straw, particles of earth, etc., that stick to the 

 manure. These bacteria increase in number until at the end of 

 four weeks there may be 1000 per gram of substance associated 

 with these. Hence, we may conclude that the absence of nitri- 

 fication which has been noted by various workers when organic 

 matter is present may be due to some of the following factors: 

 (1) Excessive quantities of soluble organic matter. This has been 

 repeatedly found to be the case where excessive quantities of 

 carbohydrates have been added to the media. (2) A low per- 

 centage of potassium as suggested by Renault. (3) The physical 

 and chemical properties of the medium, as noted by Stevens and 

 Withers. (4) The presence of organic acid, as is the case in peats 

 and forest soils. In this condition it is the acid reaction which 

 interferes with the process and not the organic matter present. 

 (5) A substance may be toxic when tested by the solution method, 

 whereas in the soil it may be inert or actually beneficial. 



Energy. The nitrifying organisms are devoid of chlorophyll 

 and function best in the dark, yet they synthesize from the carbon 

 dioxid complex organic compounds. The energy necessary for 

 this synthesis is obtained by the nitrosomonas from the oxidation 

 of ammonia : 



2NH 3 + 3O 2 = 2HNO 2 + 2H 2 O + 157.6Cal. 



and by the nitrobacter from the oxidation of nitrous acid: 



2HNO 2 + O 2 = 2HNO 3 + 36.6cal. 



Lafar points out that if the quantity of nitrogen oxidized per 

 unit of time be taken as the standard for measuring the chemical 

 energy of these organisms, the nitrosomonas will be found the 

 mest active of the two. From this fact he concludes that the 

 conversion of the trivalent nitrogen of nitrous acid into pentav- 

 alent nitric nitrogen requires the expenditure of a greater amount 

 of internal force than is needed for the first step in the oxidation. 



Metabolism. The metabolism of these organisms has, therefore, 

 been the subject of considerable study. Winogradsky early sug- 

 gested that the ammonium carbonate in the first place probably 

 gives rise to an amid, somewhat similar to the transforming of 

 ammonium carbonate into urea: 



NH 4 NH 4 NH 2 



^ \ 



\co -> Nco 



NH 4 0/ NH 2 



ammonium ammonium 



carbonate carbamate 



