794 bacteria: their work 



duction of anaerobic conditions is effected by aerobic species of 

 bacteria. When grown in solutions containing sugar it produces 

 butyric and other acids, and in the ground apparently derives the 

 carbon necessary for its nutrition from some carbohydrate. 



(ii) By species of Azotobacter. — From the soil a number of 

 organisms have been isolated more recently, which are capable 

 of assimilating or ' fixing ' the free nitrogen of the air in appreci- 

 able amounts. 



The first representatives of the group were obtained by 

 Beijerinck in 1901 from soil and canal water. These he in- 

 cluded in the genus Azotobacter, naming the two species which 

 he examined in detail, Azotobacter chroococcum and A. agilis 

 respectively. 



Since the date mentioned other closely alUed organisms with 

 similar powers have been found in the soil in all parts of the 

 world. 



The Azotobacteria vary considerably in size and shape accord- 

 ing to the stage of their growth and development, but are usually 

 met with as short, thick, oval rods. In young cultures they are 

 motile, and are strongly aerobic, needing an unrestricted supply 

 of fresh air for vigorous growth and the exercise of their 

 nitrogen-fixing functions. 



They are present in the upper layers of all soils except those 

 of a sour nature, and are also abundant in river and well water. 



A good supply of phosphates, lime, and potash, as well as an 

 adequate amount of easily oxidisable organic and carbon com- 

 pounds, are essential for nitrogen fixation by Azotobacteria, and 

 they thrive best where nitrogen compounds are absent or present 

 in small amounts only. 



The energy needed to bring the free nitrogen into chemical 

 combination is derived from the combustion of organic-carbon 

 compounds of the soil : the process is most efficiently carried on 

 at temperatures between 25° and 30° C. 



These organisms are able to add considerable amounts of 



