112 Decomposition of Plant and Animal Residues 



mals, especially the ruminants. The latter carry out the digestion 

 largely by means of an extensive population of bacteria and proto- 

 zoa that inhabit their digestive tracts. The breakdown products of 

 cellulose, the sugars, alcohols, and organic acids, are utilized by the 

 animals for their nutrition. Association or symbiosis is thus estab- 

 lished, the animal providing a habitat or shelter for the microbes, 

 and the latter digesting the food for the host. Whether this associa- 

 tion holds true also for shipworms and other mollusks capable of 

 digesting cellulose in wood still remains to be determined. 



Among the environmental factors that influence the nature of the 

 microorganisms concerned in the destruction of cellulose under a 

 particular set of conditions, the most important are moisture, reaction, 

 aeration, temperature, and a sufficient supply of nitrogen and other 

 nutrient elements. 



A high moisture content (80-95 per cent) favors the development 

 of anaerobic bacteria and is injurious to the growth of fungi and of 

 most actinomycetes. A medium moisture (50-75 per cent) is fa- 

 vorable to filamentous fungi and to aerobic cellulose-decomposing 

 bacteria; some of the fleshy fungi, like the wood-destroying forms, 

 develop at a lower moisture than the filamentous forms. A very low 

 moisture ( 10 per cent or less ) completely stops the activities of most 

 cellulose-decomposing organisms, although some, such as insects, 

 may still be able to make a certain amount of growth; destruction of 

 paper in books and in paper files takes place at a rather low moisture. 



The reaction of the medium also has a marked influence upon 

 the nature of the microbiological population responsible for the 

 process of cellulose decomposition. The aerobic bacteria belonging 

 to the Cytophaga group are able to grow at pH 6.1-9.1. Soils more 

 acid than pH 6.0 may be lacking in this group of organisms entirely, 

 although other cellulose-decomposing bacteria are able to develop 

 at pH 5.0-6.0. Actinomycetes grow at pH 5.5-9.5, whereas fungi 

 develop within much wider reaction ranges, at pH 3.0-9.5. Some 

 cellulose-decomposing fungi, like Trichoder)na, are able to grow even 

 at as high an acidity as pH 2.1-2.5. A slightly alkaline reaction (pH 

 7.5) favors, therefore, the growth of bacteria, whereas an acid reac- 

 tion is injurious to bacteria and is favorable to fungi. Addition to 

 the soil of acid-reacting fertilizers, which results in a low pH, favors 

 the development of fungi concerned in cellulose decomposition; addi- 

 tion of alkaline fertilizers, especially lime, reduces considerably the 

 numbers of fungi and leads to the development of bacteria which 

 are responsible for decomposition of the cellulose. 



