I/O FIXATION OR ASSIMILATION OF NITROGEN 



nitrogen compounds from the atmosphere. After seven 

 months 50 kilograms of soil showed an increase of 12.38 

 grams of nitrogen, the amount due to the ammonia and 

 nitric acid in the rain during this time being less than 

 half a gram. Indirect evidence of similar and larger 

 increments has been obtained also from analysis of the 

 soil and of the produce from it during several years, 

 when the crops were grown without nitrogenous manures. 



In the first 6 inches of soil of an arable field from 

 20 to 30 Ibs. or more of combined nitogen per acre is 

 added annually, the only source of which is the free 

 nitrogen of the air. From the fact that no gain of nitrogen 

 was found in soils which were first heated to 120" C. in 

 a current of superheated steam, Berthelot concluded that 

 the addition was in some way connected with the exist- 

 ence of minute living organisms in the unheated soil, 

 although he was not able to isolate them satisfactorily. 

 However, through the pioneer work of Hellriegel, Wil- 

 farth, Winogradsky, and Beijerinck, several organisms 

 capable of " fixing " or assimilating the free nitrogen 

 are now well known : the chief of them are species of 

 Clostridium, Azotobacter^ and Pseudomonas. There seems 

 little doubt that there are other forms of bacteria besides 

 these which are able to build up nitrogen compounds from 

 the free nitrogen of the air, and evidence exists which 

 appears to indicate that some of the lower fungi are 

 similarly endowed. 



The power to utilize gaseous nitrogen is not possessed 

 equally by all the organisms named. Pseudomonas rddi- 

 cicola in association with leguminous plants is able to 

 add indirectly large amounts of complex nitrogenous 

 compounds to the land, and these, as well as those 

 manufactured by the other bacteria mentioned, ulti- 



