CHAPTER XXI 

 SOIL ORGANISMS— THE NITROGEN CYCLE 



Of the various nutrient materials applied to the soil for 

 the use of plants nitrogen has the highest commercial value 

 and is absorbed in very large quantities. Moreover, nitro- 

 gen is lost from the soil in considerable amounts in drainage 

 water and possibly to some extent in gaseous form. The 

 great importance of this element and of its compounds in 

 agriculture and the possibility of it becoming a limiting factor 

 in crop production has le^d to much study regarding its re- 

 actions and movements in the soil. 



The original source of the world 's supply of combined nitro- 

 gen has been the atmosphere and, as the free gas is exceed- 

 ingly inert, 1 the natural forces which facilitate its combina- 

 tion must be extremely powerful. The movement of nitrogen 

 from air to soil, from soil to plant, from plant back to soil or 

 to animal, and from animal to soil, with a return to air at 

 various stages, involves many forces, many factors, many or- 

 ganisms, and many reactions. These complicated changes 

 are spoken of as the nitrogen cycle. 



226. The nitrogen cycle. — In tracing the various trans- 

 formations through which the nitrogen passes, the conspicu- 

 ous feature is the great complexity of the cycle. Apparently 

 the nitrogen cycle is much more extended and intricate than 

 either the carbon or sulfur cycles. This complexity, however, 



1 Because nitrogen is such an inert gas, it must not be inferred that it 

 forms inactive compounds with other materials. In combination it is 

 extremely active, seemingly being the basis of all plant and animal life 

 processes. 



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