VII.] 



FIXATION OF NITROGEN 



177 



nitrogen as long as that is below the maximum required 

 by the plant. Field experiments at Rothamsted with 

 leafy crops like mangolds, to which a very small amount 

 of nitrogen was supplied in order to give them a start, 

 showed that the increase thus produced was only pro- 

 portional to the nitrogen supplied, so that there is no 

 evidence that even a plant which has begun to grow 

 vigorously can then continue its development by taking 

 nitrogen from the atmosphere. 



From all these experiments the conclusion was 

 drawn that cultivated plants are unable to "fix" 

 atmospheric nitrogen, but obtain this indispensable 

 element in a combined state from the soil together 

 with the ash constituents; and such was the opinion 

 that prevailed for something like thirty years. 



Notwithstanding the conclusive nature of all the 

 laboratory experiments, there was still a residuum of 

 facts obtained under field conditions which were in- 

 explicable on the theory of the non-fixation of nitrogen, 

 and these facts were chiefly connected with the growth 

 of leguminous crops. 



Boussingault's crop statistics have already been 

 referred to ; the following table gives a short summary 

 of the kind of results he obtained : — 



The amount of nitrogen removed was equal to 



If 



