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BACTERIOLOGY OF THE SOIL 49 



of fixing free nitrogen, has yet gained an enormous quantity of 

 nitrogen during the twenty years under review, a quantity which 

 at the lowest reckoning amounts to about twenty-five pounds per 

 acre per year ? The nitrogen brought down in the rain would 

 account for perhaps five pounds per acre per annum, a little 

 more will come in the form of dust, bird droppings, and other 

 casual increments, while some may be due to the fixation of 

 tmospheric nitrogen by bacteria in the soil not associated 

 with leguminous plants, like the Azotobacter chroococcum of 

 Beijerinck and Winogradski's Clostridium pasteurianum. The 

 Azotobacter has been found abundantly in the Rothamsted 

 soils, and in the case of grass land like the present the decay- 

 ing vegetation would supply the carbohydrate which the 

 bacterium must oxidize in order to fix nitrogen, it is quite pos- 

 sible that it may have effected considerable gains of nitrogen.' 

 Only recently has it become known that much of this 

 increased fertility is due to the fixation of free nitrogen by 

 bacteria present in the soil. These nitrogen-fixing organisms 

 may be conveniently divided into two primary groups. (1) 

 Free bacteria living in the soil; (2) Bacteria living in the 

 root-tubercles of leguminous and certain other of the higher 

 plants. It is only intended to briefly deal here with the 

 members of the first group. The second group will be con- 

 sidered under the heading 'Bacteria,' p. 508. 



Berthelot, a French scientist, was among the pioneers to 

 indicate that gain of nitrogen in the soil was due to bacteria. 

 Then followed the more exact investigations of Winogradski, 

 Beijerinck, and others, which resulted in absolute proof being 

 derived from pure laboratory cultures, that not only did certain 

 bacteria possess the power of fixing free nitrogen in a form in 

 which it could be directly utilised by the higher plants, but the 

 particular species were isolated and their general characteristics 

 accurately studied and described, so that they can be recognised 

 with certainty by other bacteriologists. Among such species 

 may be mentioned Azotobacter chroococcum, A. agilis, A. vine- 

 laudii, A. Beijerincki, Clostridium pasteurianum, Bacillus 

 mesentericus, etc. At a later period it was discovered that the 

 power of fixing atmospheric nitrogen was greatly increased 

 when two or more species of bacteria worked in unison. It 

 was further observed that the presence of certain substances 

 in the soil greatly favoured the work done by nitrogen-fixing 

 bacteria, while on the other hand their work is greatly- 

 retarded by the presence of other substances. Speaking 



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