♦o* . 



Nitrogen Fixation — Nonsynibiotic 



Nitrogen Fixation in Nature 



The supply of fixed nitrogen in the soil is very limited, ranging 

 normally from less than 0.1 to 0.2 per eent, and higlier in e.xeeptional 

 cases. Rainfall brings down small quantities of nitrogen that have 

 been fixed by electric discharges in the atmosphere. These are 

 chiefly in the form of nitrogenous oxides. The chemical and physico- 

 chemical fixation of nitrogen, through the agency of sunlight, for 

 example, may also be considered of very limited importance. The 

 major part of the elementary nitrogen that finds its way into the 

 soil and that is used for synthesis of plant and animal life is due 

 entirely to its fixation by certain groups of microorganisms. 



Two major groups of bacteria, usually designated as nonsymbiotic 

 and symbiotic forms, are primarily concerned in this process. Aside 

 from these, there is also a limited fixation of nitrogen by a variety of 

 different bacteria and fungi, and especially by blue-green algae. The 

 capacity of nonsymbiotic bacteria to fix atmospheric nitrogen and 

 the amount of nitrogen fixed depend largely upon the nature and 

 concentration of the available energy. Since soil humus cannot be 

 used as a source of energy and since nitrogen fixation is inhibited by 

 the presence of available forms of nitrogen in the soil, the significance 

 of the nonsymbiotic organisms in normal soil is still a matter of 

 speculation. On the other hand, the fixation of nitrogen through 

 the symbiotic action of leguminous plants and bacteria that grow 

 in the plant roots and produce nodules is of great economic im- 

 portance in agriculture. 



The root-nodule associations were the first to be recognized for 

 their ability to fix atmospheric nitrogen. The ability of leguminous 

 plants to improve the soil by increasing its supply of available nitro- 

 gen has been known for more than two thousand years. The role of 

 the bacteria in the process was established more than six decades 



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