CHAPTER XII 

 THE FIXATION OF NITROGEN 



As has been seen, the supply of nitrogen in the soil is the 

 result of processes that have been going on from time im- 

 memorial under natural conditions. When human activi- 

 ties enter into consideration, the equilibrium of natural 

 forces is upset. The decomposition of the organic matter 

 goes on more rapidly and more completely in cultivated 

 soil than in the virgin forest or prairie. The more aerobic 

 conditions favor the more rapid decomposition of the humus 

 that lias accumulated, and, unless care is taken to add in- 

 creased quantities of organic matter to the soil, it soon 

 becomes so depleted that profitable crops can no longer be 

 grown. 



The depletion of organic matter is of especial importance 

 from the standpoint of the nitrogen supply, because the 

 humus is the chief source of nitrogen for the soil organisms. 

 Some nitrogen is lost in the drainage water, and also in the 

 crop removed from the land. It is probable that some is 

 also lost in the decomposition of nitrogenous matter, and 

 certainly in the process of denitrification. The depletion of 

 the nitrogen content of the soil has been considered by some 

 observers to be a most serious problem. Some have main- 

 tained that the population of the world will be limited be- 

 cause of the constant loss of nitrogen from the soil. It is 

 probable that such fears are unfounded, for many factors 

 are at work tending to maintain the nitrogen content of 

 the soil. 



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