214 THE LIVING ORGANISMS OF THE SOIL [chap. 



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 

 easily be rendered evident by an increased yield of 

 crop. Koch treated soil in pots with large quantities of 

 sugar, 2 per cent, 4 per cent, and even more of dextrose, 

 and then sowed oats, buckwheat, etc. At first the sugar 

 was injurious, and the first crop suffered in consequence ; 

 but the proportion of nitrogen in the soil increased, and 

 the second and third crops were far greater than those 

 in the check plots of untreated soil. When the soil, 

 after the application of the sugar, was placed in an 



