CHAPTER XL 



THE FIXATION OR ASSIMILATION OF 

 NITROGEN. 



The " Fixation " or Assimilation of the Free Nitrogen 



of the Air. In the processes of decay, putrefaction, and 

 denitrification some of the world's combined nitrogen is 

 destroyed, the nitrogen being set free in the gaseous 

 elementary form. Without the existence of an opposite 

 synthetic process, by means of which the free nitrogen of 

 the atmosphere is brought into combination again, it is 

 obvious that the stock of nitrogen compounds would 

 decrease. To some extent such synthesis or chemical 

 union of nitrogen and oxygen takes place in the air 

 through the agency of lightning and other electric dis- 

 charges, the nitrous and nitric acids so formed being 

 eventually brought down in the rain. The amount, 

 however, of the combined nitrogen added to the soil 

 annually in this country by showers and dew is very 

 small, being only 4 or 5 Ibs. per acre, and a large pro- 

 portion of it is not derived from free nitrogen, but 

 consists of compounds of ammonia, which have escaped 

 or evaporated into the air from the soil and decaying 

 organic matter. 



Berthelot, in some experiments carried out about 1885, 

 showed that bare uncropped soil exposed to the air in- 

 creased very considerably in combined nitrogen content, 



even when careful notice was taken of the addition of 



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