82 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



gums and slimes, which are produced by a variety of bacteria 

 and fungi, either as excretion products of the cells, in the form of 

 capsules, or as cell constituents. 



Decomposition of Cellulose. — With certain few exceptions, 

 cellulose forms the largest single group of constituents of plant 

 materials added to the soil. Such fibrous materials as flax, hemp, 

 cotton, and jute are largely made up of pure cellulose. Cellulose 

 is not present in a free state in any great abundance in the 

 vegetative structure of the plant, but is combined with lignin, 

 cutin, and pectin, the compounds thus formed giving rise to 

 ligno-celluloses, cuto-celluloses, and pecto-celluloses. 



Cellulose is an amorphous polysaccharide, giving on hydrolysis 

 with strong mineral acids or with appropriate enzymes the di- 

 saccharide cellobiose, which in its turn is readily hydrolyzed to 

 glucose. When acted upon by soil microorganisms, sugars can 

 be demonstrated as intermediate products only in exceptional 

 cases. Cellulose can be readily decomposed by a large number of 

 organisms which are present in normal soils, including various 

 fungi, bacteria, and actinomyces, and possibly also some protozoa. 

 The nature of the microbes responsible for the decomposition of 

 cellulose varies with the conditions; it is different in soil than in 

 the manure pile or in the peat bog; the chemistry of the process of 

 cellulose decomposition depends upon the environmental condi- 

 tions and the organisms concerned in the process. 



Fungi Decomposing Cellulose.— The fungi capable of 

 decomposing cellulose include a large number of organisms found 

 among the Ascomycetes, Basidiomycetes, and Fungi Imperfect i. 

 The following genera have been shown to contain one or more 

 species capable of decomposing cellulose: Trichoderma, Asper- 

 gillus, Penicillium, Fusarium, Cephalosporium, Verticillium, 

 Sporotrichum, Monosporium, Alternaria, Hormodendrum, Humi- 

 cola, Chaetomium, various Polyporynae, and Agaricinae. These 

 fungi are especially active in the so-called raw-humus forest 

 soils, which are acid in reaction and are only seldom favorable for 

 the development of cellulose-decomposing bacteria. The forest 

 humus, comprising the upper few centimeters of forest soil, 

 consists largely of a mass of partly or completely disintegrated 

 leaves, needles and their residues; this is so much interwoven with 

 colorless and brown fungus mycelium that, on examining the mass 

 of partly disintegrated organic residues under the microscope, 



