CHAPTER XXI 

 THE NITROGEN CYCLE 



Of the various elements composing the nutrients used 

 by plants, nitrogen has the highest commercial value. 

 It is, moreover, absorbed in large quantities by agricul- 

 tural plants and the supply is constantly liable to loss in 

 drainage water and in the gaseous form. Its importance 

 to agriculture has led to much study of its occurrence, 

 combinations, reactions, and movements in the soil. 



When it is recalled that the nitrogen gas of the atmos- 

 phere is the one primitive source of the world's supply of 

 nitrogen, it becomes apparent that the agencies that have 

 been instrumental in its transfer from one condition to 

 another have been extremely active. The movement of 

 nitrogen from air to soil, from soil to plant, from plant 

 back to soil or to animal, and from animal back to soil, 

 with a return to air at various stages, involves many 

 forces, many factors, many organisms, and many re- 

 actions. 



365. Decay and putrefaction. — Decomposition of the 

 nitrogenous organic matter of the soil, consisting largely 

 of the proteins, begins with either one of two processes 

 — decay or putrefaction. Decay is produced by aerobic 

 'bacteria, and naturally occurs when the conditions are 

 most favorable for their development. When the condi- 

 tions are otherwise, the growth of these bacteria is checked, 

 and then further decomposition would be extremely slow 



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