NITROGEN FIXATION 177 



either by leaving the soil fallow for a year or, alternatively, 

 by growing acrop of clover, vetches, alfalfa, or other leguminous 

 plant. Either of these measures results in a replenishment of 

 the soil-nitrogen, and it is possible to grow further successful 

 crops of grain. Both of these natural fertilisation measures 

 owe their ef&cacy to the action of bacteria in " fixing " 

 atmospheric nitrogen and so rendering it available in the soil 

 in a form which can be utilised by plants. 



In the case of the field left fallow, the organism mainly 

 concerned is the strict aerobe, Azotobacter. Berthelot showed 

 in 1885 that if soil is left exposed to the air, then its nitrogen 

 content slowly increases and that this increase takes place 

 at the expense of atmospheric nitrogen. He further showed 

 that the responsible agent is biological, since the process 

 can be stopped by heat or by treating the soil with caustic 

 chemicals. It was some years before any organism was 

 isolated from soil which has the property of fixing nitrogen 

 and the first such organism isolated was CI. pastorianum, 

 a strict anaerobe which is of less importance than Azotobacter. 

 Azotobacter is able to grow rapidly in the presence of gaseous 

 nitrogen as sole source of nitrogen and of carbohydrate as 

 carbon and energy source. Growth is such that there is a 

 constant ratio between nitrogen fixed and carbohydrate 

 utilised. In soil the limiting factor is often the amount of 

 carbohydrate available, and this explains the practice of 

 some Indian farmers of enriching their soil by ploughing waste 

 molasses into it. Despite many studies on Azotobacter, we 

 are still without any definite knowledge of the chemistry of 

 the fixation process (see p. 90). 



In the days before artificial fertilisers were available, the 

 alternative open to the farmer, instead of leaving his fields 

 fallow, was to cultivate on his fields a crop of one of the 

 leguminous plants, i.e. those plants having nodules on their 

 roots. The function of the root-nodules was first made 

 apparent in 1888 by Hellriegel and Wilfarth, who studied the 

 growth of peas and the formation of root-nodules on their 



CHEM. A. B. 12 



