84 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



both as a source of energy and as a source of carbon for the building 

 up of the microbial cell substance, the ratio between the carbon 

 used for energy and the carbon used for cell synthesis is constant 

 for a given organism under a given set of conditions. There is also a 

 ratio between the amounts of cellulose decomposed and the nitro- 

 gen assimilated. For every 100 units of cellulose decomposed, 20 

 to 30 units of cell substance are synthesized, which contain about 

 1.5 to 2.5 units of nitrogen. Since cellulose is free from nitrogen, 

 this element has to be obtained from some other compounds to 

 enable the microbes to bring about the decomposition of the cellu- 

 lose. The amount of available nitrogen in the soil very frequently 

 becomes the limiting factor in the decomposition of cellulose. 

 The effect of the presence of available nitrogen on development of 

 fungi is apparent from Table 19. The influence which soil reaction 

 and available nitrogen may have is shown in Table 20. 



TABLE 20 



Influence of Reaction and Available Nitrogen upon Development 

 OF Cellulose-decomposing Bacteria and Fungi (from Dubos) 



The beneficial effects which additions of animal manures and 

 many different fertilizer materials exert on the decomposition of 

 cellulose may frequently be ascribed to the greater amounts of 

 available inorganic nitrogen which these substances put to the 

 disposal of the microbes in the soil. Since phosphorus, and to a 

 less extent potassium, sulfur, and other nutritive elements, are 

 also required for the synthesis of microbial cell substances, these 

 elements must be present in the soil in appropriate amounts in 



