CHAPTER V 

 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



Sources of Nitrogen in Soil. — The atmosphere is the source 

 of supply of nitrogen in the soil; a continuous exchange is operating 

 between the nitrogen that now exists in various combinations in the 

 soil itself and what remains above the earth in the atmospheric 

 cloak. The gaseous covering of the atmosphere contains the rela- 

 tively inert molecular form of nitrogen. This becomes available 

 to green plants only as it is changed into combined forms by cer- 

 tain specific microbes or by strictly non-biological processes, such 

 as union of nitrogen and oxygen to form nitrate through the 

 agency of electrical discharges, through catalytic combination 

 of nitrogen and hydrogen to form ammonia, or by a reaction 

 between nitrogen and calcium carbide to form calcium cyanamid. 

 Natural electrical discharges add small amounts of inorganic 

 nitrogenous compounds to the earth; the commercial exploitation 

 of fixation processes has resulted in the further addition of vast 

 quantities of combined nitrogen to soils. However, the principal 

 natural mechanism by which plants obtain their combined nitrogen 

 and by which the lost quantities of nitrogen are restored to the soil 

 is that operated by some of the microbes which make the soil their 

 natural habitat. 



The mere presence of nitrogenous compounds in the soil does 

 not solve the problem of satisfying the requirements of higher 

 plants for this element. Green plants require certain very specific 

 nitrogen compounds, and the production of a continuous supply of 

 these substances is largely the result of the activities of micro- 

 organisms. The nitrogen compounds in the soil, from which the 

 supply of nitrogen made available for plant growth originates, are 

 varied and many are quite complex in composition. They com- 

 prise largely organic compounds produced as a result of the growth 

 of green plants and animals, as well as of the numerous microbes. 

 These compounds include proteins, proteoses, peptones, peptides, 



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