188 THE LIVING ORGANISMS OF THE SOIL [chap. 



brown colour with a solution of iodine, a method which 

 is convenient for the observation of the organism. It is 

 aerobic, and is, in fact, a strong oxidising agent, the 

 dextrose or other carbohydrate which it requires being 

 converted by it into carbon dioxide and water, together 

 with small quantities of lactic and acetic acids, alcohol, 

 and sometimes butyric acid. 



A very characteristic bye-product is the dark brown 

 or black pigment from which the organism derives its 

 specific name, a pigment which may play its part in the 

 usual coloration of humus. 



As a rule, about 9 or 10 mg. of nitrogen are fixed 

 for each gram of carbohydrate oxidised, but the ratio 

 obtained varies considerably under different conditions ; 

 cultures which have been repeatedly transferred, being, 

 as a rule, less effective than the impure culture derived 

 directly from the soil. 



Azotobacter chroococcum and its kindred forms are 

 widely distributed in soils from all parts of the world ; 

 it has been found in most cultivated soils, and the author 

 has observed it in virgin soils from East Africa, India, 

 New Zealand, Egypt, Russia, Monte Video, Ohio, and 

 Sarawak. 



It is, however, not to be discovered in acid soils ; the 

 presence of calcium carbonate appears to be essential to 

 its development. Certain minor differences are to be 

 seen in the Azotobacter organisms present in the soil 

 from different parts of the world. From tropical and 

 semi-tropical soils in East Africa, for example, a form 

 has been isolated which is a very effective fixer of 

 nitrogen, but which differs from the normal in not giving 

 rise to the brown pigment ; another form, again, from 

 Monte Video gives rise to a green fluorescence in the 

 culture medium. 



The amount of nitrogen fixed by Azotobacter may 



