212 THE LIVING ORGANISMS OF THE SOIL [chap. 



Fixation of Free Nitrogen by the Soil. 



As already indicated, Berthelot attributed to the 

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

 pheric 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 ot free 

 nitrogen takes place whether the algal 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, 

 when grown under anaerobic conditions and supplied 

 with soluble carbohydrate, breaks the latter down with 

 the formation of butyric and other acids, and at the same 

 time draws someof the gaseous nitrogen present into com- 

 bination. This particular organism Clostridium Pastori- 

 anum is very widely diffused and can readily be isolated 

 from pond mud and similar material, where organic matter 

 is decaying under comparatively anaerobic conditions. 

 The extent of the nitrogen fixation is, however, small ; 



