NITROGEN FIXATION 455 



out of the nodules into the surrounding soil and some remain bound up 

 in the proteins of the bacterial cells themselves. The amount of nitrogen 

 fixation is influenced by a number of factors among the most important of 

 which are the quantity of available nitrogen and carbon compounds present 

 in the soil. When available nitrogen is high nitrogen fixation is retarded. 

 Apart from the depressing effects of high soil nitrogen content, nitrogen 

 fixation by the symbiotic bacteria is, in general, favored by the same factors 

 that promote good vegetative growth of the host plants. Surprising quan- 

 tities of atmospheric nitrogen may be fixed into organic compounds by these 

 bacteria. Under very favorable conditions a good crop of alfalfa may add 

 as much as 400 pounds of nitrogen per acre to the soil. The average, how- 

 ever, is much lower being estimated by Giobel (1926) as ranging between 

 100 and 200 pounds per acre when the common legumes are used as host 

 plants. 



Most of the nitrogen fixation by saprophytic bacteria is brought about 

 by two groups of organisms: ( i ) The Azotobacter group composed of coccus- 

 like aerobic organisms, and (2) the Clostridium group which are rod-shaped 

 anaerobic bacteria. Both types are common in well aerated soils. The aerobic 

 forms occur around the surface of the soil particles while the anaerobic forms 

 are found within aggregations of soil particles or in regions of the soil in 

 which the oxygen content has been depleted by respiration. These bac- 

 teria combine the gaseous nitrogen of the air with carbohydrate compounds 

 obtained from the soil. Azotobacter is usually absent from soils more acid 

 than pH 6 but Clostridium can tolerate soil acidities as great as pH 5. Both 

 groups can operate effectively in relatively dry soils. The quantity of atmos- 

 pheric nitrogen fixed by the saprophytic bacteria is much less than that com- 

 bined by the symbiotic organisms and it is probable that these organisms do 

 not add appreciable quantities of combined nitrogen to the soil. 



2. Ammonification. — In the process of decay the complex nitrogenous 

 compounds present in dead plant and animal tissues are broken down into a 

 number of simpler compounds, most of the nitrogen being released in the 

 form of ammonia. This process is termed ammotiification and the bacteria 

 involved are called ammonifying bacteria. Ammonification is not the result 

 of the activities of a single group of bacteria but may be brought about by 

 a large number of different micro-organisms, including the actinomyces and 

 filamentous fungi in addition to numerous groups of bacteria. Probably most 

 of the bacteria commonly present in soils are concerned in the formation of 

 ammonia from one kind of nitrogenous material or another. The amount of 

 ammonia produced in the decay of nitrogenous materials is influenced by ( i ) 

 the available carbohydrate supply, (2) the chemical composition of the nitrog- 



