1 86 THE LIVING ORGANISMS OF THE SOIL [chap. 



ous plants made but little growth and developed no 

 nodules. Success followed when about 8 cwt. per acre of 

 soil from a field which had previously carried the crop in 

 question were sown broadcast over the land in April, 

 and harrowed in just before seeding. In one case, over 

 7 tons per acre of green serradella were grown where 

 the land had been treated with 8 cwt. of soil from an 

 old serradella field, whereas the crop failed after germi- 

 nation where no inoculation had been practised. 



Fixation of Free Nitrogen by the Soil. 



As already indicated, Berthelot attributed to the 

 soil itself the power of fixing a small quantity of atmo- 

 spheric nitrogen, a power which was lost when the soil 

 was sterilised and maintained under conditions prevent- 

 ing infection. This gain of nitrogen was independent 

 of the small amount of ammonia absorbed by soil from 

 ordinary air, which always contains a trace of ammonia ; 

 and at first it was attributed to the microscopic green 

 algae which clothe the surface of ordinary moist soil. 

 The experiments of Kossowitsch, and of Kriiger and 

 Schneidewind, have, however, shown that the growth of 

 pure cultures of these algae is dependent on a supply 

 of combined nitrogen, and that no fixation of free 

 nitrogen takes place whether the algae growth be small 

 or large. It is possible, however, that they may live 

 in symbiosis with nitrogen-fixing bacteria and supply 

 the carbohydrate, by the combustion of which the 

 energy needed for the fixation of nitrogen by the bacteria 

 is obtained. More recently, however, several organisms 

 have been isolated from the soil which are capable when 

 growing in a free state of fixing nitrogen drawn from the 

 atmosphere, and it is to these that the gains of nitrogen 

 observed by Berthelot must be attributed. Winogradsky 

 was the first to isolate an organism of this type, which, 



