266 PRINCIPLES OF SOIL MICROBIOLOGY 



The Mucorales are unable to decompose celluloses and most hemi- 

 celluloses. 106 The ability of fungi to decompose different celluloses does 

 not depend on the solubility of the latter in acids, but on the chemical 

 composition of the substances in question. 107 Certain Mucorales, like 

 Rhizopus and Mucor stolonifer, are able to decompose pectins but not 

 celluloses, whereas Botrytis and other fungi decompose the fiber 

 itself. 108 



According to the earlier investigators fungi are the proper humus 

 builders in the soil. The fallen leaves, at the end of the vegetative 

 period in the fall, are found to be penetrated with fungus mycelium, which 

 decomposes the leaves readily, with the production of humic substances. 

 These accumulate, because they cannot serve as a source both of carbon 

 and of nitrogen 109 ' 110 but, in the presence of available sources of carbon, 

 they can be used as sources of nitrogen by fungi. More recent infor- 

 mation tends to show that, although fungi decompose most of plant 

 residues (with the exception of lignins) completely, they synthesize 

 extensive protoplasm, which is an important part of the soil organic 

 matter. 



Decomposition of nitrogenous substances by fungi (ammonia formation) . 

 Just as in the decomposition of celluloses and allied compounds, fungi 

 play an important role in the decomposition of organic nitrogenous 

 compounds. In the presence of available carbohydrates, the fungi 

 utilize the nitrogen compounds only as sources of nitrogen; in the com- 

 plete or relative absence of available carbohydrates, they utilize the 

 nitrogenous substances as sources of carbon and of nitrogen. In view of 

 the fact that the energy requirements of the fungi are greater than their 

 nitrogen requirements, a great deal more of the protein molecule will be 

 broken down to supply the necessary carbon. The excess of nitrogen 

 present in the protein molecule over that required by the fungus for the 

 building up of its own proteins will be left as a waste product, in the 



106 van Iterson, 1904 (p. 264); Hagem, 1910 (p. 237) ; Waksman and Heukelekian, 

 1924 (p. 443); Waksman and Skinner, 1926 (p. 190). 



107 Schellenberg, 1908 (p. 264). 



108 Behrens, J. Taurote von Flachs und Hanf. Centrbl. Bakt. II, 10: 524- 

 530. 1903. 



109 Reinitzer, F. Ueber die Eignung der Huminsubstanzen zur Erniihrung 

 von Pilzen. Bot. Ztg., 58: 59-73. 1900. 



110 Nikitinsky, J. Uber die Zersetzung der Huminsiiure durch physikalisch- 

 chemische Agentien und durch Mikroorganismen. Jahrb. Wiss. Bot., 37: 365- 

 420. 1902. 



