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BIOLOGY FOR BEGINNERS 



shortage if it were not for other means of return. It remains for 

 certain bacteria, living in the nodules which they form on the 

 roots of clover, peas, beans, alfalfa, and all members of this large 

 family of plants, to aid in making good the loss. 



These bacteria take the free nitrogen from the air, combine it 

 into soluble compounds, and thus replace in the soil most of this 

 essential element, which decay and oxidation had set free in the air. 



CYCLE. 



FIG. 162. Diagram showing how nitrogen compounds, after being used 

 by plants and animals, are either returned to the soil by decay, or reclaimed 

 from the air. This completes the "nitrogen cycle." 



Although the atmosphere contains an enormous amount (80 per 

 cent) of nitrogen, it is not in the form of compounds, and these 

 plants of the pea family are the only ones that can use free nitrogen. 



Another means by which free nitrogen of the air is combined 

 into useful compounds is by the action of lightning, which con- 

 verts some into oxides. These are washed back to the soil by rain 

 and help in completing the circle. 



In addition to these natural steps in the nitrogen circle we must 



