86 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



than pH 5.6 will favor the development of aerobic bacteria, with- 

 out, however, being unfavorable to fungi (see Table 20). 



Many of the aerobic bacteria use cellulose as the exclusive 

 source of energy and are unable to develop upon the simple sugars, 

 indicating that this process of decomposition may not proceed 

 through the sugar stage or at least does not lead to the accumula- 

 tion of the lower carbohydrates. Bacteria decompose cellulose, 

 leaving a mucilaginous mass consisting of bacterial cells imbedded 

 in certain gum-like compounds (hemicelluloses) synthesized by the 

 organisms. Small amounts of various acids and certain yellow 

 and orange pigments are also formed. The aerobic bacteria also 

 require considerable quantities of nitrogen and phosphorus for 

 the decomposition of the cellulose, for the same reasons that they 

 are required by fungi. 



Anaerobic bacteria (see Fig. 9) capable of decomposing cellu- 

 lose occur in arable soils only in limited numbers. They are 

 found abundantly in peat and marsh soils, in rivers, in manure 

 piles, and in other habitats where fresh organic matter is abundant 

 and free access of air is prevented. Some of the anaerobic bacteria 

 are thermophilic in nature, being capable of active growth and 

 decomposition of cellulose at 60° C. Both the organisms that 

 grow at normal temperatures and those that grow best at high 

 temperatures (55 to 65° C) can bring about very rapid decomposi- 

 tion of cellulose with the formation of various organic acids, alco- 

 hols, and gases, such as methane, hydrogen, and carbon dioxide. 

 Omeliansky found that out of 3.3 gm. of cellulose decomposed, 

 there were produced 2.2 gm. fatty acids, 1.0 gm. CO2, and 0.014 gm. 

 hydrogen. A thermophilic bacterium produced from the decom- 

 position of 42 gm. cellulose, 21.6 gm. acetic acid, 10.3 gm. ethyl 

 alcohol, and 11.9 gm. CO2, in addition to considerable quantities 

 of hydrogen. In addition to acetic acid, formic, lactic, and butyric 

 acids have been found as products of decomposition of cellulose by 

 anaerobic bacteria. The formation of several of these products 

 was explained by the following reactions: 



(C6Hio05)n + ^iHoO = 2n(CH3-CO-COOH) + 2nH2 



Cellulose Pyruvic acid 



CH3COCOOH + Ho = CH3CHOHCOOH 



Pyruvic acid Lactic acid 



CH3COCOOH = CH3CHO + CO2 



Pyruvic acid Acetaldehyde 



