70 VETERINARY BACTERIOLOGY 



development of these nitrifying organisms than by any other 

 single factor. Their importance in nature as essentials for the 

 growth of the higher plants and, therefore, of animals can scarcely 

 be overestimated. 



Nitrogen Fixation. The free nitrogen of the air is so inert 

 that very few living plants are capable of making use of it in 

 the building up of their bodies. None of the green plants can 

 bring this about of themselves. Certain molds and bacteria 

 are able to make use of this source of nitrogen, however, and 

 are, therefore, of the greatest economic importance. The fer- 

 tility of the soil is largely dependent upon the fixed nitrogen that 

 it contains, and the taking up of the free nitrogen by these 

 organisms ultimately renders it available to other forms of plants. 

 This does not mean that the bacteria take up the nitrogen from the 



A. 



Fig. 37. Free living or non-symbiotic nitrogen-fixing bacteria: A, Bacillus 

 (Clostridium} pastorianum; B, Azotobacter; a, A. chroococcum; b, A. agilis. 

 (A after Winogradsky, B after Beyerinck.) 



air and immediately transform it into nitrates for the use of the 

 higher plants, but that it is built up into their protoplasm and 

 ultimately is set free by the death and decomposition of the organ- 

 isms. Microorganisms which make use of free nitrogen commonly 

 utilize carbonaceous materials as a source of energy. 



Organisms capable of fixing nitrogen are subdivided into two 

 general groups, those which live free in soils and those which live 

 in or on the roots of certain plants in a kind of symbiosis. The 

 free living organisms which fix nitrogen belong to three general 

 groups: first, certain anaerobic types belonging to the general 

 group of butyric acid bacteria; second, certain aerobic species; 

 third, a few molds. The anaerobic organism known to fix the 

 nitrogen is Bacillus (Clostridium) pastorianum. Probably this 

 organism is not of the greatest importance, as the conditions for 



