NITROGEN-FIXING BACTERIA 97 



It is not known whether other butyric organisms are also able to fix 

 atmospheric nitrogen, but presumably other, perhaps prototrophic, bacteria 

 of the soil do so. It is highly probable that we shall find such in forests 

 where the earth is never ' manured ' and where, notwithstanding, enormous 

 quantities of nitrogen are fixed and worked up into the living substance of 

 plants. But all researches in this direction must be received very critically, 

 for there can be no doubt that the idea now very common in agricultural 

 circles that all soil bacteria can assimilate free nitrogen is quite incorrect. 

 Green and blue-green algae were formerly supposed to be able to do so, 

 but this opinion is now held to be incorrect (65), and although it is stated 

 that mould-fungi can fix free nitrogen the experiments on which the state- 

 ment is based did not take into consideration all the possible sources of 

 error. The time will perhaps come when ' Nitragin ' will be replaced by 

 pure cultures of free-living, nitrogen-fixing bacteria, which will be ' sown ' so 

 as to alternate with nitrogen- consuming crops. 



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