684 PRINCIPLES OF SOIL MICROBIOLOGY 



waxes) is decomposed only very slowly. :u Organic matter containing 

 sufficient nitrogen, like ground leguminous plants, cotton seed meal, 

 dried blood, and fungus protoplasm, decompose even more rapidly; 

 the nitrogen, sooner or later, depending on its concentration, becomes 

 liberated as ammonia. Substances rich in proteins decompose at an 

 entirely different rate than those composed principally of carbohydrates. 

 Materials rich in oxygen and low in carbon decompose more quickly 

 than those rich in carbon and poor in oxygen. 32 The following amounts 

 of various organic substances, on a per cent basis, were decomposed in 

 21 days, as indicated by the carbon dioxide, using 10 grams of material 

 per 100 grams of soil: 



per cent 



Clover 59 .7 



Glucose 42 . 1 



Rice straw 29.0 



Oak leaves 17 .7 



Wheat straw 14.5 



Cellulose 11 .8 



Similar results were obtained 33 when the decomposition of 0.5 per cent 

 organic matter in the soil was studied. The addition of available nitro- 

 gen (NaN0 3 ) to a nitrogen poor substance, like straw, stimulates its 

 decomposition but not that of a substance containing sufficient nitrogen, 

 like alfalfa meal (fig. 53). The influence of available nitrogen upon the 

 composition of a nitrogen-poor substance, like cellulose, depends also 

 upon the nature of the soil, as shown in figure 54, where the stimulating 

 effect was greatest in a well manured, fertile soil (5A), least in an unma- 

 nured, poor soil (7A), and medium in an unfertilized but limed soil (7B). 

 These results confirm the earlier observations that the C0 2 content of 

 the soil rises and falls with the amount of organic matter present and 

 that the addition of manure to the soil stimulates the evolution of 

 carbon dioxide. Fresh manure stimulates this evolution more than 

 old decomposed manure, because of the introduction of both available 

 energy and available nitrogen. Grinding of peat increases its rate of 

 decomposition. 



31 Van Suchtelen, F. H. H. tJber die Messung der LebenstJitigkeit der aero- 

 biotischen Bakterien im Boden durch die Kohlensaureproduktion. Centrbl. 

 Bakt. II, 28: 45-89. 1910. 



32 Dvorak, 1912 (p. 428). 



" Starkey, R. L. Some observations on the decomposition of organic matter 

 in soils. Soil Sci., 17: 293-314. 1924. 



