SUMMARY 99 



combined at this stage we account for 125 pounds of carbon and 

 12.5 pounds of nitrogen. Continuing with the decomposition as 

 previously, there is a progressive increase in the amount of carbon 

 accounted for as carbon dioxide and less present in residual unde- 

 composed organic matter. After decomposition of the microbial 

 cell substance represented by (D) and (E) there are eliminated 8.3 

 pounds of nitrogen as ammonia, which are more than sufficient to 

 permit the organisms to decompose the 127 pounds of residual 

 undecomposed organic matter remaining at this time (F). Conse- 

 quently, the excess nitrogen remains as ammonia, which may now 

 be considered as a waste product since it will not be further used 

 by the organisms causing the decomposition. From this point on, 

 more and more ammonia will be added to the waste products, and 

 the remaining organic matter will be accounted for entirely as 

 microbial cells. It can be seen from this outline that eventually 

 ammonia will be fiberated from the decomposition of organic 

 materials having as wide an initial ratio of C to N as 80 to 1. 

 This entails the repeated minerahzation of the nitrogen of the 

 microbial cells, and eventually leads to a C : N ratio of 10 to 1, 

 which persists at this figure as long as there is any organic matter 

 remaining. 



In normal soils there is practically always a rather close rela- 

 tionship between the amounts of organic carbon and nitrogen 

 present. Even when organic materials of wide ratios of carbon to 

 nitrogen are added to soils, there is a tendency for these ratios to 

 become narrower with time, until they become about 10 to 1, at 

 which level the ratio remains practically indefinitely, although the 

 total amounts of organic carbon and nitrogen may be continually 

 decreasing. In the light of the diagrammatic conversion of organic 

 matter pictured above, it may be apparent why the ratio becomes 

 10 to 1 and does not become lower. Some variations from this 

 ratio frequently occur, as determined by differences in the organisms 

 causing the decomposition and by accumulations of resistant plant 

 residues of wide C : N ratios. 



SuMMAEY. Decomposition of Organic Matter in Soil and 

 Formation of Soil Humus, or Soil Organic Matter. — A brief 

 summary of the facts discussed in this chapter allows us to obtain 

 a clear idea of the general processes involved in the decomposition 

 of organic materials of plant and animal origin added to the soil 

 and the formation of the dark-colored substances which comprise 



