CHAPTER X 

 THE CYCLE OF NITROGEN 



While it is perhaps impossible to characterize any one 

 process as more essential than some other, where all are 

 necessary steps in a complex relationship, it is undoubt- 

 edly true that the cycle which nitrogen undergoes in nature 

 is invested with the greatest interest of any of the ele- 

 ments, because of the intricacy of the changes involved, 

 their dependence on one another, the completeness with 

 which the various steps have been traced, and the possi- 

 bility of controlling by scientific knowledge the progress 

 of these changes. 



Nitrogen is constantly being removed from the soil by 

 growing plants, and it is essential that in some manner the 

 nitrogen be returned to the soil and again made available 

 to the plant. In the discussion of the cycle of carbon it 

 was shown that not only were microorganisms instrumen- 

 tal in the return of the carbon of organic matter to a form 

 in which the plant can again make use of it, but that both 

 animals and plants are giving off carbon-dioxide as a pro- 

 duct of their respiration. In the case of nitrogen it will 

 be seen that microorganisms are the only agents by which 

 the nitrogen in plant and animal matter can be made 

 available to the green plant. 



The amount of free nitrogen in the world is enormous. 

 The air contains 80 per cent, of this element. Over every 

 acre there are 35,000 tons of this gas. Nitrogen is an inert 

 element anil does not readily enter into combination with 

 other elements, such as the oxygen of the air ; but there are 



94 



