442 SOILS: PBOPBBTIJSS AND MANAGEMENT 



complished by a series of changes from one compound to 

 another, as was seen to be the case with the non-nitroge- 

 nous materials. The final products are carbon dioxide, 

 water, usually some hydrocarbon gases resulting from 

 the carbon and hydrogen of the organic matter, and also 

 some hydrogen sulfide or other gas containing sulfur 

 or a final oxidation of the sulfur of the proteids into sul- 

 fates; while the nitrogen is ultimately converted into 

 nitrates, or into free nitrogen, although a portion of the 

 original nitrogen sometimes escapes into the air in the 

 intermediate stage, ammonia. 



The processes will be discussed under the following 

 heads, which represent certain more or less definite stages 

 in the decomposition: 1, decay and putrefaction; 2, am- 

 monification; 3, nitrification; 4, denitrification ; 5, fixa- 

 tion of atmospheric nitrogen. These various processes 

 form what has been termed the nitrogen cycle. 



