96 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



would be 1.60 pounds of nitrogen in excess of the requirement. 

 This excess of nitrogen would appear as ammonia as the decompo- 

 sition progressed, and would represent a waste product of the 

 nutrition of the fungi. 



Dried blood contains about 40 per cent of carbon and 10 per 

 cent of nitrogen. There would be 40 pounds of carbon and 10 

 pounds of nitrogen in 100 pounds of the material. 



40 X 0.35 = 14.00 pounds of carbon assimilated. 

 14.00 X 0.10 = 1.40 pounds of nitrogen assimilated. 



1.40 — 10.00 = —8.60 pounds of nitrogen, meaning that there 

 would be 8.60 pounds of nitrogen in excess of the requirements. 

 Here there would be a great excess of nitrogen over that required 

 for the nutrition of the fungi. 



Similar calculations may be made to indicate the results of the 

 decomposition of the same organic materials by bacteria. Less of 

 the carbon is assimilated (only 7 per cent), and there is a greater 

 amount of nitrogen required per unit of carbon used — ratio of C to 

 N is 5.0 to 1, or one-fifth as much nitrogen as carbon is required. 



It is quite evident that, in the decomposition of organic mate- 

 rials, such as cellulose and mature straw, there is a marked deficit 

 in nitrogen which must be supplied from some source, such 

 as inorganic ammonium salts, nitrates, or available organic nitro- 

 genous compounds. With organic materials, as in the case of 

 leguminous plants, which contain from 1.5 to 3 per cent of nitrogen, 

 there will generally be more nitrogen than is necessary to satisfy 

 the requirements of the organisms, and consequently there would 

 soon be some ammonia formed. This may not always occur, since 

 the plant is not homogenous but a complex aggregate of com- 

 pounds which decompose with unequal rapidity. It has been 

 observed that when the plant materials contain less than 1.7 per 

 cent of nitrogen there will be a more or less extended deficiency of 

 nitrogen for decomposition to proceed rapidly. With materials 

 containing more than 1.7 per cent of nitrogen no deficiency appears; 

 in fact, there will be more nitrogen than is required by the 

 microorganisms; this excess will appear as ammonia (later as 

 nitrate), and the amount of the excess will be determined by the 

 excess of nitrogen over 1.7 per cent which the organic material 

 contains. 



The assimilation of nitrogen by microorganisms should not 



