CHAPTER IV 

 Bacteria Fixing Atmospheric Nitrogen 



Nitrogen fixation in nature. All higher plants, all animals and the 

 great majority of microorganisms depend for their nutrition on combined 

 nitrogen, whether organic or inorganic in nature, and can make no 

 use whatsoever of the great store of gaseous nitrogen in the atmosphere. 

 Large quantities of nitrogen, therefore, are removed every year from the 

 soil by the growing crops. In addition to that, several groups of soil 

 microorganisms are even capable of reducing nitrates and liberate 

 atmospheric nitrogen. The quantities of manure returned to the soil 

 are far from sufficient to replace the losses from the soil; the attempt to 

 replace this loss by artificial fertilizers may be sufficient to supply the 

 need of the growing plant but not to replenish the losses from the soil. 

 This is accomplished through the agency of nitrogen-fixing bacteria, 

 working alone or in symbiosis with higher plants. A small amount of 

 combined nitrogen is formed by chemical agencies, such as electrical 

 discharges, and is brought down with the yearly rainfalls, but this hardly 

 amounts to more than one or two pounds of nitrogen per acre per year, 

 while ordinary forest trees may remove in the wood and leaves over 50 

 pounds of nitrogen per acre per year. The rest of the nitrogen is 

 presumably fixed in the soil by the agency of microorganisms. 



The first organisms to be studied in connection with the fixation of 

 atmospheric nitrogen were the bacteria forming nodules on the roots of 

 leguminous plants; it was then believed that only those organisms that 

 live symbiotically on the roots of the plants are able, during this process 

 of symbiosis, to transform the gaseous nitrogen of the atmosphere 

 into combined forms. It was later found 1-3 that these bacteria are 

 also capable of fixing nitrogen in the absence of the host plant, when 



1 Beijerinck, M. W. Die Bakterien der Papilionaceenknollchen. Bot. Ztg. 

 46: 725-735, 741-750, 758-771, 782-790, 797-803. 1888. 



2 Maze, M. Fixation de l'azote par le bacille des node-sites des Ldgumineuses. 

 Ann. Inst. Past. 11: 44-54. 1898. 



3 Chester, F. D. Oligonitrophile Bodenbakterien. 4th Ann. Meet. Soc. 

 Amer. Bact. Washington, D. C. 1902. (Centrbl. Bakt. II, 10: 382. 1903). 



103 



