SOIL OEGANISMS 



417 



power of different soils. Highly productive soils have gen- 

 erally been found to maintain a greater nitrifying efficiency 

 than less productive soils, but this is not always the case, 

 as factors other than available nitrogen may limit the pro- 

 ductiveness of a soil. 



With the formation of nitrate nitrogen, the main portion 

 of the nitrogen cycle is completed, since plants absorb most 

 of their nitrogen as the nitrate ion. Of this cycle, from 



NITROOEM 

 OF AIR 



MANURE 



DECAY 



PARTIALLY DECAYED 

 COMP0UMD5 



\ 



/ '-^.AWMONIFICATIOK 



J ""— AMMONIA 



REDUCTION 



FREE M 



, NITRIFICATION . 



HlTRATE5-#-N\TRlTES 



o^"^ ' -^ 



Fig. 59. — ^Diugram representing the movements of nitrogen between 

 soil, plants, animals and the atmosphere. These transformations 

 are termed the ** nitrogen cycle." 



plant to soil, and from soil to plant again, the nitrification re- 

 action is the weakest point, since the other biological changes 

 proceed to a certain extent in spite of unfavorable soil con- 

 ditions. Nitrification is easily retarded and may even be 

 brought to a standstill. As a consequence, the factors affect- 

 ing this particular portion of the nitrogen cycle are of special 

 interest. A soil favorable to nitrification is generally wholly 

 favorable to the other desirable processes involving nitrogen 

 xransxormaxions* 



