VII.] BACTERIA-FIXING NITROGEN 141 



to make good their footing they have to compete with 

 the already overcrowded population of the soil, the 

 numbers of which are only maintained at a certain level 

 by the limited extent of the food supply. It has thus 

 been found necessary to make several inoculations and 

 renewed sowings before lucerne can be regularly 

 established in soils to which it was entirely new, though 

 when one successful crop has been grown there is never 

 any difficulty later in establishing a second. At present, 

 however, the practical applications of pure cultures of the 

 nodule bacteria to the inoculation of soil are distinctly 

 limited, and the enormous returns that have been 

 promised for them can only be realised in very 

 special cases. 



Since the discovery of the nitrogen-fixing organisms 

 associated with the nodules of leguminous plants several 

 other bacteria have come to light which can fix nitrogen 

 without any such partnership with a higher plant. For 

 example, among the typical organisms bringing about 

 the decay of organic matter in such places as the mud of 

 ponds and marshes, there occurs a bacterium possessing a 

 limited power of fixing nitrogen, but which we have reason 

 to suppose does play some part in keeping up the stock 

 of nitrogen in soils. But the most actively fixing 

 organism that has been isolated is a comparatively large 

 bacterium, called Asotobacter, which has been identified 

 in virgin soils from all parts of the world. Certain char- 

 acteristic features possessed by Azotobacter QXisblo. it to be 

 readily detected, and the following experiments may be 

 performed to illustrate its action. A non-nitrogenous 

 culture medium must be prepared as on p. 122, omitting 

 the ammonium sulphate, and portions of 100 c.c. are 

 placed in flasks, i gramme of glucose and half a gramme 

 of carbonate of lime are added to each, after which the 

 flasks are plugged and sterilised in the usual way. From 



