432 NATURE AND PROPERTIES OP SOILS 



bacteria, B, mesentericiiSy B. pneumonice^ B. radiodacter^ B. 

 amylohacterj B, prodigiosiis, B. asterosporus, and B. lactis 

 viscusus have certain capacities in this direction. Duggar 

 and Davis ^ have shown that certain filamentous f nngi, such 

 as Phoma hetoe, Aspergillus niger, Pencillium digit atum, 

 and others have the ability of utilizing atmospheric nitrogen. 

 The power of fixing nitrogen is, therefore, possessed by a 

 large number of different organisms, yet from the data now 

 at hand the Azotobacter group seems to be of the greatest 

 economic importance. The nitrogen fixed enters the nitrogen 

 cycle when the organisms die, undergoing decay, ammonifica- 

 tion and nitrification, thus becoming available to higher plants. 



238. Conditions for azofication and the amount of nitro- 

 gen flxed.^ — The term azofication relates to the fixation of 

 nitrogen by the Azotobacter group, although it may be used 

 loosely in reference to all free-fixing activities. The soil con- 

 ditions favorable to this phenomenon are those which are opti- 

 mum for higher plants. This is especially true regarding 

 aeration, temperature, and moisture relations. The process 

 is encouraged by the application of lime when soils are acid 

 and seem to require considerable phosphorus. This element is 

 probably utilized in building up proteins within the bodies 

 of the organisms. Potassium, sulfur, iron, and magnesium 

 seem also to be essential to the phenomenon. The Azotobac- 

 ter themselves are influenced by catalytic agents such as 

 manganese. 



Since considerable energy is required for nitrogen fixation 

 the presence of organic matter in the soil becomes very im- 

 portant in this regard. Almost any non-toxic organic ma- 

 terial may serve as a source of energy, even cellulose being 

 very effective. Farm manure seems especially to encourage 



^^Buggar, B. M., and Davis, A. R., Studies in the Fhijsiology of the 

 Fungi; Ann. Mo. Bot. Garden, Vol. 33, j^^p. 413-437, 1916. 



^A very excellent review of literature and discussion of Azofication: 

 Greaves, J. E., Azofication; Soil Sei., Vol. VI, No. 3, pp. 163-217, 

 1918. 



