ii] THE SOIL AND PLANT FOOD 23 



atmosphere through their association with the bac- 

 teria of the root nodules. Wlien their roots, leaves 

 or stems perish and mingle with the soil these newly- 

 furnished nitrogen compounds are added to the 

 general stock already there. 



A few years later another set of bacteria was 

 found able to take in and use the free nitrogen of 

 the air, differing from the preceding in that they 

 work on their own account and do not form associa- 

 tions with plants. The earliest to be discovered was 

 Clostridium, but interest has centred largely round a 

 later find, Azotobacter, because it works in presence 

 of air and not, as happens with certain other organ- 

 isms, in its absence. 



The more one studies these nitrogen-fixing or- 

 ganisms the more remarkable do they appear. The 

 absorption of gaseous oxygen by living organisms and 

 the changes it brings about can be paralleled more 

 or less closely by artificial processes in the laboratory. 

 But the absorption of gaseous nitrogen by these 

 particular organisms cannot be imitated in the 

 laboratory and is without parallel in our experience. 

 A source of energy is needed, and a considerable 

 number of substances are known to serve, including 

 sugar, starch, cellulose, or residues of plants. As the 

 organisms are very widely distributed they may be 

 expected to operate wherever supplies of easily de- 

 composable organic matter are present in the soil, 



