94 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



ammonia, nitrate, or in simple organic combination; the extent of 

 the assimilation of inorganic nitrogen depends upon the amount 

 of available energy material and its relative content of organic 

 nitrogen. 



Metabolism of Microbes and Decomposition of Organic 

 Matter. — Knowing the nature of the organisms effecting the 

 decomposition of an organic material, the composition of the 

 microbial cells, the energy balance of these cells, the composition 

 of the organic matter undergoing decomposition and the environ- 

 mental conditions, it is possible to interpret in a very definite 

 manner the end products which will be formed and the speeds with 

 which these will accumulate. Any one of these five variables 

 may show differences over a considerable range, but sufficient is 

 already known to interpret in general terms what may be expected 

 to take place. 



Fungi use the organic substance as a source of energy and as a 

 nutrient (a source of carbon); they assimilate, in the mycelium 

 and spores, from 20 to 50 per cent of the carbon contained in the 

 organic compound decomposed. The rest of the carbon goes off 

 as carbon dioxide or is left as incompletely decomposed material. 

 For the sake of simplicity let it be assumed that 35 per cent of the 

 carbon of the organic matter decomposed is assimilated by the 

 fungi. Bacteria assimilate much less of the carbon, utilizing 

 the available energy less economically. They may assimilate 

 from 1 to 30 per cent of the carbon contained in the organic mate- 

 rial decomposed. An average of 7 per cent may typify conditions 

 for many bacteria. Actinomyces are intermediate between the bac- 

 teria and fungi, assimilating from 15 to 30 per cent of the carbon. 



Accompanying the carbon assimilation, there is a consumption 

 of considerable amounts of nitrogen, which is largely synthesized 

 in the form of protein complexes. The carbon content of the cells 

 may be between 45 and 54 per cent. For convenience it may be 

 assumed that dry microbial cell substance contains 50 per cent of 

 carbon. The nitrogen content and the ratios of carbon to nitrogen 

 in the cells may be assumed to be as follows : 



Nitrogen content, 

 per cent 



Ratio of carbon 

 to nitrogen 



Fungi 



Bacteria 



Actinomyces . 



3- 8, av. 5 

 8-12, av. 10 

 7-10, av. 8.5 



10.0 to 1 

 5.0 to 1 

 6.0 to 1 



