104 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



The same is true of the forms of nitrogen in the stable manures and 

 green manures that are added to the soil in varying quantities and, 

 in the case of some soils, at quite frequent intervals. Various 

 commercial organic fertilizers, such as guano, dried blood, cotton- 

 seed meal, and tankage, contain their nitrogen in the form of 

 organic compounds. Most of the nitrogen added to the soil in 

 artificial fertilizers is in the form of inorganic compounds, largely 

 nitrates, ammonium compounds, and amides. However, even 

 some of the synthetic fertilizers, such as cyanamid and urea, require 

 the previous interaction of microorganisms before the nitrogen is 

 made available to growing plants. 



Transformation of Nitrogen in Soil. — As long as the nitro- 

 gen is in an organic form it remains to a large extent unavailable 

 as far as the growth of higher green plants is concerned. The 

 organic complexes must be mineralized or decomposed, and, after 

 a series of transformations, the nitrogen finally becomes liberated 

 as ammonia. Many plants can absorb their nitrogen in this form. 

 Others require their nitrogen in the form of nitrite. The trans- 

 formation of the ammonia first to nitrous acid (nitrite), then to 

 nitric acid (nitrate), is carried out by specific bacteria, under 

 favorable conditions, in the soil. The nitrate may be used by the 

 plants; it may be assimilated by microorganisms in the presence 

 of available energy ; it may be reduced to nitrite, to gaseous oxides 

 of nitrogen, to ammonia, and to atmospheric nitrogen; finally, the 

 nitrate may be leached from the soil. 



The various transformations to which nitrogen is exposed in 

 soil are illustrated in Fig. 49. 



Fixation of Atmospheric Nitrogen. — While practically all 

 living organisms rely for their nitrogen upon organic compounds, 

 like proteins and their derivatives, or upon inorganic compounds, 

 such as nitrates or ammonium salts, there are certain microbes 

 which are able to make use of elementary (molecular) nitrogen to 

 supply the need of this element for their growth. Such an assimi- 

 lation of nitrogen by organisms from the molecular state is termed 

 biological nitrogen fixation. 



The ability of utilizing gaseous atmospheric nitrogen and fixing 

 it in their bodies in the form of complex organic and simple inor- 

 ganic compounds is limited largely to two groups of bacteria. 

 These include: (1) the non-symbiotic bacteria which lead a free 

 existence in the soil and obtain their energy from various organic 



