704 PRINCIPLES OF SOIL MICROBIOLOGY 



about 5 to 10 per cent of nitrogen and about 45 per cent of carbon. This 

 necessitates the assimilation of considerable quantities of available 

 nitrogen by microorganisms, with the result that the nitrogen is con- 

 verted from an available into an unavailable form. In other words, 

 for every 100 units of organic matter decomposed, from 0.5 to 4.0 units 

 of nitrogen may be assimilated. If the organic matter contains 0.5 per 

 cent nitrogen, as in the case of straw, additional available nitrogen has 

 to be provided, 1 to 2 parts of nitrogen for every 100 parts of organic 

 matter, before the latter can be completely decomposed. If the organic 

 matter, like clover or alfalfa, contains 1.5 to 2.5 per cent nitrogen, no 

 additional nitrogen is required for its complete decomposition by fungi, 

 but over a period of a few months no nitrogen will be liberated as NH 3 . 

 However, if the organic matter contains more than 2| per cent nitrogen, 

 some of it will be liberated as ammonia and the amount will depend on 

 the actual nitrogen content. Very little will be formed in the early 

 period from alfalfa which contains about 3 per cent nitrogen; more will 

 be formed from linseed meal, cottonseed meal and similar substances 

 which contain 4 to 8 per cent of nitrogen. Large amounts of nitrogen 

 will be liberated from dried blood which contains 11 to 13 per cent 

 nitrogen. 



The bacteria and actinomyces are much less economical in energy utili- 

 zation than the fungi and, although their nitrogen content is higher 

 (9-13 per cent), the total amount of nitrogen assimilated and stored away 

 is less than in the case of the fungi. An organic substance, like alfalfa, 

 can, therefore, liberate more available nitrogen when it is decomposed 

 by bacteria or actinomyces than when it is decomposed by fungi. Con- 

 ditions favoring the activities of bacteria and destructive to fungi, such 

 as liming an acid soil or treating a soil with volatile antiseptics, tend 

 toward a liberation of available nitrogen, as will be shown in detail 

 elsewhere. 



The equilibrium between the carbon and nitrogen, which becomes 

 established in the soil is, therefore, a result of three processes resulting 

 from the activities of microorganisms: (1) the complete decomposition 

 of most of the organic material, including most of the carbohydrates, 

 proteins, etc.; (2) the resistance of some materials as lignins to decom- 

 position; (3) the synthesizing activities of microorganisms. 



If fresh organic matter is added to a soil with a wide carbon-nitrogen 

 ratio, the microorganisms which attack it reduce the ratio, by liberating 

 part of the carbon as C0 2 and assimilating part of it into the microbial 

 cells. The products formed depend largely on the amount of available 



