44 



special means, in special cases, the organic nitrogen of the soil may 

 contribute to the supply of nitrogen to chlorophyllous plants. 



I have now considered in some detail the sources of already 

 combined nitrogen available to our crops ; and the evidence points to 

 the conclusion that, independently of the small amount of combined 

 nitrogen annually coming from the atmosphere in rain, and the minor 

 aqueous deposits, the source of the nitrogen, at any rate of most of our 

 crops, is the stores already existing within the soil and sub-soil, or 

 those provided by manure. It has further been seen that the combined 

 nitrogen is largely taken up as nitric acid, or rather as nitrates. But 

 it is nevertheless obvious, that we have yet to seek for an explanation 

 of the source of the whole of the nitrogen of the Leguminosse in 

 some cases. 



We are brought to enquire, therefore, what is the evidence relating 

 to the question of the fixation of free nitrogen, by the plant, by the soil, 

 or otherwise ? 



EVIDENCE AS TO THE FIXATION OF FREE NITROGEN. 



Even nearly a century ago, it was a matter of discussion whether 

 plants took up, or evolved, free nitrogen ; and it is just about half a 

 century, since Boussingault commenced a series of vegetation experi- 

 ments to determine whether plants do assimilate the free nitrogen of 

 the atmosphere. From the results then and subsequently obtained, he 

 concluded that they did not ; and results obtained at Rothamsted 

 nearly 30 years ago confirmed those of Boussingault. But others 

 came to an opposite conclusion ; and a somewhat active controversy 

 was maintained on the subject for some time. Eventually it seemed to 

 be pretty generally admitted, that plants did not directly assimilate 

 the free nitrogen of the air. During the last few years, however, the 

 discussion has assumed a somewhat different aspect. 



The question still is, whether the free nitrogen of the air is an 

 important source of the nitrogen of vegetation ; but whilst few now 

 adhere to the view that chlorophyllous plants directly assimilate free 

 nitrogen ; it is nevertheless assumed to be brought under contribution 

 in various ways coming into combination within the soil, under the 

 influence of electricity, or of micro-organisms, or of other low forms, 

 and so indirectly serving as an important source of the nitrogen of 

 plants of a higher order. 



Several of the investigations in the lines here indicated, seem to 

 have been instigated by the assumption, that natural compensation 

 must be found, for the losses of combined nitrogen which the soil 

 sustains by the removal of crops ; and for the losses which result from 

 the liberation of nitrogen from its combinations, under various 

 circumstances. 



It was about 1876, that M. Berthelot called in question the 

 legitimacy of the conclusion that plants do not assimilate the free 



