FIXATION OF ATMOSPHERIC NITROGEN 339 



Chemical and Biological Relations. — The second and third 

 quarters of the nineteenth century saw the birth of a number of theories 

 dealing with this problem. It was suggested that nitrogen compounds 

 may be formed in the soil by the oxidation of nitrogen to nitric acid. 

 Compounds of iron, manganese and lime were supposed in some way 

 to make such oxidation changes possible. It was likewise suggested 

 that nascent hydrogen may be generated in the decomposition of organic 

 matter in the soil, and reacting with elementary nitrogen, may give 

 rise to ammonia. The various hypotheses were not supported by 

 experimental proof; moreover, the situation was complicated by the 

 knowledge, based on empirical observations, that crops of the legume 

 family seemed to be more or less independent of the supply of combined 

 nitrogen in the soil. Indeed, clovers and other legumes had, appar- 

 ently, the ability to increase the content of combined nitrogen in the 

 soil as was indicated by the experiments of Boussingault and of Lawes 

 and Gilbert. Finally, the mystery was solved by" the investigations 

 of Berthelot and Hellriegel and Wilfarth who furnished the proof that 

 elementary nitrogen may be utilized by plants when certain biological 

 relations are met. These relations involve the presence and activities 

 of microorganisms that by themselves, or in conjunction with higher 

 plants, make available to growing vegetation the great store of 

 atmospheric nitrogen. 



NoN-svMBiOTic Fixation of Nitrogen 



Historical. — Non-symbiotic nitrogen fixation, or Azofication, has 

 already been defined as the production of nitrogen compounds out of 

 atmospheric nitrogen by bacteria independently of higher plants. The 

 part played by bacteria in this process was not recognized until 1885, 

 when Berthelot published some of his data on the accumulation of com- 

 bined nitrogen in uncropped soils. His results seemed to explain a 

 number of scattered observations, made since the middle of the century, 

 on the apparent increase of the nitrogen content of cultivated soils. 



While Berthelot's experiments proved that the nitrogen gains 

 occurred only in unsterilized soils and were, therefore, due to micro- 

 organisms, it remained for Winogradski to' demonstrate, in 1893, that 

 the formation of nitrogen compounds by certain types of bacteria 

 may be accomplished in culture media nearly or quite devoid of com- 



