THE CHEMICAL CYCLE OF LIFE 



6i 



Nitrogen in 

 Atmosphere 



,^Bacteria 

 ^- and 



Fig. 20. The nitrogen cycle 



Nitrogen compounds are withdrawn from the soil by plants, represented by the tree in 

 the diagram. The nitrogen material feeds animals (as the squirrel) and also other plants 

 (as bacteria and fungi). Some of the waste products of life activity are returned to the 

 soil ; other nitrogen compounds are scattered and lost in the air. The only way by which 

 nitrogen from the atmosphere is regularly returned to the soil is through the action of 

 bacteria found on the roots of plants of the bean family, represented by a bean plant in 

 the diagram. Bacteria of decay bring about a return of the nitrogen in the bodies of dead 



plants and animals 



90. The nitrogen problem. In comparatively recent years 

 two solutions of the "nitrogen problem" have been presented. 

 One of them rests upon a better understanding of living things ; 

 the other is an application of chemical knowledge. It has been 

 found that there are present in many soils tiny one-celled plants 

 called bacteria. They are related to the germs that are known 



