CHAPTER IV 



THE FIXATION OF NITROGEN 



For several centuries it has been a matter of common 

 observation and agricultural practice that soil impoverished 

 by the growth of cereals can be revitalized by allowing the 

 land to lie fallow or by growing leguminous plants, yet not 

 until the end of the nineteenth century were adequate 

 explanations forthcoming as to why these procedures caused 

 such beneficial effects. They are in fact due to micro- 

 organisms which, either themselves or when in association 

 with leguminous plants, possess the ability to use the 

 atmosphere as a source of nitrogen. The conversion of mole- 

 cular nitrogen (N2) into nitrogenous compounds which can 

 be assimilated by the organisms concerned is termed nitrogen 

 fixation. The ability to fix Ng appears to be restricted to 

 micro-organisms and even amongst them it occurs in but 

 a few genera. 



Priestley's claim that green plants absorbed N2 as well as 

 CO 2 and the suggestion of Sir Humphrey Davy that their 

 nitrogen might be derived from the atmosphere by the 

 agency of 'mushrooms and funguses' were the cause of 

 much controversy and stimulated several investigations 

 designed to test these statements by experiment [41]. 

 Boussingault in 1838 was the first to show that, when they 

 were grown in sand, the nitrogen content of clover plants 

 increased yet that of wheat did not. Although in the years 

 that followed, the swellings or nodules invariably found in 

 the root systems of leguminous plants were frequently com- 

 mented upon, half a century was to elapse before the 

 classical experiments of Hellriegel and Wilfarth established 

 that they were the site of the agency which enabled legumin- 

 ous plants to fix atmospheric Ng . They found that nodules 

 were formed only in non-sterile environments and that in 

 contrast to cereals the growth of leguminous plants was 

 normal even in the absence of fixed nitrogen (i.e. nitrate, 



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