230 PLANT PHYSIOLOGY 



of oxygen and therefore always exists in soil, together with the 

 ordinary saprophytic bacteria, which live closely associated with 

 its colonies and consume oxygen with great avidity. 



The fixation of nitrogen is an endothermic process requiring 

 an external supply of energy. Clostridium bacteria use as a 

 source of energy the process of butyric fermentation. Vino- 

 gradsky has found a definite quantitative relationship between 

 the two processes. He found that 2 to 3 mg. of nitrogen are 

 assimilated for each gram of carbohydrates fermented. 



Somewhat later, the Dutch bacteriologist Beijerinck (1901) 

 discovered another widely distributed bacterium, which he calls 

 *'Azotobacter" (Fig. 67). This organism has the capacity of 

 very intensive respiration, which supplies it with sufficient energy 

 for the fixation of free nitrogen. Since a much greater amount 

 of energy may be obtained from the same quantity of carbo- 

 hydrates by aerobic than by anaerobic respiration it is but 

 natural that Azotobacter works with greater efficiency than 

 Clostridium, being able to fix as much as 15 mg. of nitrogen per 

 gram of sugar consumed. Besides these two most important 

 nitrogen-fixing bacteria, there are found in the soil large numbers 

 of others that assimilate atmospheric nitrogen. Certain authors 

 attribute the same capacity even to some of the molds and blue- 

 green algae, though this has yet to be confirmed. The detailed 

 chemistry of nitrogen fixation by these organisms is not clearly 

 understood. There are many reasons to suppose, however, that 

 nitrogen is combined with the hydrogen of the water molecule 

 and that ammonia is the primary product of this synthesis. 

 Ammonia is utilized for the formation of the more complex sub- 

 stances, such as the amino acids and proteins. 



Since the nitrogen-fixing bacteria require nonnitrogenous 

 organic substances in order to obtain the necessary energy for 

 their vital activities, the incorporation of plant material into 

 soil as, for instance green manure, even if.it is poor in nitrogen, 

 considerably increases the activity of these bacteria and thus 

 contributes to the accumulation of fixed nitrogen. A particu- 

 larly intense activity is displayed by these bacteria in tropical 

 countries, where good harvests are often secured from soils low 

 in nitrogen and not enriched by nitrogenous fertiUzers. In 

 colder climates, however, through the activity of microorganisms 

 there accumulates only about 10 to 20 kg. of nitrogen per hectare 



