TRANSFORMATION OF ORGANIC MATTER 683 



of oxygen. 29 Under anaerobic conditions, the organic matter is not 

 decomposed completely and a large part of the energy is left in the form 

 of intermediary products; the amount of C0 2 liberated is small and 

 cannot serve as good an index of decomposition as under aerobic con- 

 ditions. When the amount of oxygen absorbed is used as an index 

 of oxidation in soil, the rate of absorption is found to increase with the 

 temperature, the amount of water (up to a certain point), and the 

 amount of calcium carbonate; oxygen absorption is favored by con- 

 ditions obtaining in the surface soil as opposed to those in the subsoil. 30 



When a soil is air-dried and then moistened, or when it is partially 

 sterilized by means of heat or chemicals, there follows a decided increase 

 in microbiological activities, resulting in the liberation of greater amounts 

 of C0 2 . When undecomposecl organic matter is added to the soil, it is 

 decomposed with a rapidity depending not only on the nature of the sub- 

 stance added, but also upon the presence of available nitrogen, soil re- 

 action, moisture, aeration, etc. The amount of C0 2 produced from the 

 decomposition of a certain organic substance depends also upon the 

 nature of the organisms which are concerned in the process; different 

 organisms attack the same substance, but yield different products. 



The nature and composition of the organic material added to the 

 soil greatly influence the type of organism developing and the 

 mechanism of the process of decomposition. Glucose decomposes in 

 the soil very rapidly. The absence of available nitrogen does not be- 

 come a limiting factor, since nitrogen-fixing organisms can use glucose 

 as a source or energy and develop readily when large amounts of it are 

 added to the soil. One per cent of glucose is rapidly decomposed in 

 forty-eight hours. The decomposition of cellulose, however, is carried 

 out by certain specific bacteria and fungi, which require an available 

 source of nitrogen; the amount of the latter in the soil will, therefore, 

 become the limiting factor in the decomposition of cellulose. Natural 

 organic substances, which are poor in nitrogen, like cereal straws, corn 

 stover, wood products, contain various water soluble substances. They 

 decompose more rapidly than pure cellulose, but here also the nitrogen 

 soon becomes the limiting factor. In the decomposition of natural 

 organic substances, the monosaccharides and starches are decomposed 

 first, followed by the pentosans, celluloses, pectins, and proteins; the 

 strongly resistant carbonaceous residue (consisting largely of lignins and 



"Wollny, 1897 (p. 477). 



30 Russell, E. J. Oxidation in soils and its connection with fertility. Jour. 

 Agr. Sci., 1: 261-279. 1905. 



