692 PRINCIPLES OF SOIL MICROBIOLOGY 



and probably also of bacteria can serve as a source of "humus" in the 

 soil; all organic substances utilized by microorganisms for nutrients can 

 thus become indirect sources of "humus." Typical black "humus" 

 was found to be formed only as a result of the participation of a group 

 of substances: lignin, protein, pigments and tannins; a mixture of 

 lignin and the latter alone, when acted upon for a considerable period of 

 time, may suffice. Celluloses are decomposed completely by soil fungi 

 without the direct formation of humus-like substances, except through 

 the synthesized microbial protoplasm. 59 Grave doubts were expressed 60 

 as to whether a specific humification of plant material takes place in the 

 soil, since the so-called humus extract of soils and peats is not a typical 

 soil product formed in the soil by the action of microorganisms, but 

 similar extracts can be obtained from unchanged vegetable material, 

 i.e., the "humus" found in the soil was believed to depend entirely 

 on the process of extraction employed. 



The formation of "humus" in the soil is thus found to be due either to 

 chemical or microbiological agencies. The different theories explaining 

 this phenomenon can be summarized as follows: 



1. "Humus" is formed from the interaction of carbohydrates with 

 amino acids or polypeptides, formed fr?>m the decomposition of plant 

 proteins. 61 Proteins were frequently considered 62 to be the most im- 

 portant sources of humus in the soil. Certain amino acids, like trypto- 

 phane, may be concerned in the reaction which produces black insoluble 

 humin, but this reaction cannot take place without the presence of 

 some as yet unidentified component of the protein molecule. 63 



2. "Humus" is a result of oxidation of benzene ring compounds. 64 

 The oxidation of quinone, hydroquinone and phenol results in the for- 



E9 Waksman and Heukelekian, 1924 (p. 443); 1925 (p. 443). 

 60 Gortner, 1916 (p. 690). 



61 Maillard, L. C. La formation de matiere humique sans l'oxygene d'atmo- 

 sphere. Compt. Rend. Acad. Sci., 154: 66. 1912; 155: 1554-1556. 1912; 156: 

 1159. 1913; Genese des matieres proteiques et des matieres humiques. Paris. 

 1913. 



62 Benni. Uber die Entstehung des Humus. Diss. Gieszen. 1896. Suz- 

 uki. Ref. Centrbl. Agr. Chemie., 37: 347. 1908. 



63 Gortner, R. I., and Norris, E. R. The origin of the humin formed by the 

 acid hydrolysis of proteins. Jour. Amer. Chem. Soc, 37: 1613. 1915; also 39: 

 24. 1917; 42: 821, 632, 2378. 1920; 45: 550. 1923; Jour. Biol. Chem., 26: 127. 

 1916. 



64 Eller, W., and Koch, K. Synthetische Darstellung von Huminsauren. 

 Ber. deut. Chem. Gesell., 53: 1469-1476. 1920; Ann. Chim. phys., 431: 133, 162, 

 177. 1923. 



