TRANSFORMATION OF ORGANIC MATTER 701 



"humus" cannot serve as a source of energy for nitrogen-fixing bacteria. 

 Even crude "humic acids" (extracted with NaOH and precipitated with 

 HC1) cannot be used as nutrients by the majority of microorganisms, 

 but may stimulate the activities of various organisms in a physico- 

 chemical way as accompanying mineral impur t'es wil do. 89 



It is true, however, that "humus" does decompose in well aerated 

 and limed soils and that it does not accumulate under these conditions. 

 The exact nature of the organisms concerned in these processes and the 

 processes themselves still await investigation. 



When the soil is heated, treated with antiseptics, or air dried, its 

 organic matter decomposes more readily than in the untreated soil. 

 As a matter of fact the amount of C0 2 formed from sterilized soil inocu- 

 lated with a suspension of bacteria or fungi may run parallel to the 

 availability of the organic matter in the soil. Konig and Hasenbaumer 90 

 determined the amount of C0 2 evolved from 500 grams of soil in seven 

 weeks, after the soil was sterilized and inoculated with pure cultures of 

 some microorganisms. 



co 2 



mgm. 



Sterile soil 60 .7 



Bac. ramosus 179.2 



Bac. vulgatus 273 .8 



Act. chromogenus 156 .0 



The evolution of C0 2 by a pure culture of an organism from the soil 

 itself, to which no fresh organic matter has been added, varies not only 

 with the organism but with the amount and availability of the organic 

 matter in the soil. Trichoderma was found 91 to produce in eight days, 

 from 100 grams of sterile sandy loam manured with cow manure each 

 year, 124.1 mgm. C0 2 , while only 37.4 mgm. were produced from the same 

 quantity of the corresponding unfertilized soil. These quantities are 

 much larger than those obtained by Konig and Hasenbaumer because of 

 the difference in composition of soil used and the greater activity of the 

 fungus over the bacteria. The fact that a soil kept at an optimum mois- 

 ture and temperature will give off a constant stream of C0 2 indicates 

 that the soil organic matter is constantly decomposing, truly at a low 



89 Ritter, G. A. Einige Versuche betrefend die physiologische Bedeutung 

 der Humusstoffe des Bodens. Intern. Mitt. Boden., 2: 301-311. 1912. Centrbl. 

 Bakt. II, 34: 577-666. 1912. 



90 Konig and Hasenbaumer, 1920 (p. 620). 

 •» Waksman and Starkey, 1924 (p. 723). 



