700 PRINCIPLES OF SOIL MICROBIOLOGY 



However, more careful studies have established the fact that soil 

 "humus" cannot serve as an available source of energy for the great 

 majority of soil microorganisms. Hoppe-Seyler 85 believed that "humic" 

 substances afford a habitat and substrate for the soil bacteria, fungi, 

 algae and lower animals, but that "humus" itself cannot offer any food 

 to plants, or animals and cannot be decomposed by bacteria. The very 

 fact that "humus" may accumulate in the soil indicates the low avail- 

 ability of these compounds as sources of energy for microorganisms. 

 One could hardly expect that sources of available energy should exist in 

 the soil in great abundance in the presence of the various microorganisms 

 and not be attacked. The low availability of the soil organic matter 

 as a source of carbon is further confirmed by the fact that the addition of 

 a small amount of available nitrogen, such as NaN0 3 or (NH.O2SO4, 

 does not greatly stimulate decomposition of the organic matter, as indi- 

 cated by the evolution of carbon dioxide, except in soils with a wide 

 carbon-nitrogen ratio. The nitrogen part of the soil organic matter 

 seems to be more available for the activities of microorganisms than the 

 carbon part. On adding available energy, even in the form of cellu- 

 loses, various organisms are enabled to use some of the soil nitrogen. 

 This has been also clearly demonstrated by various investigators, 86 

 who cultivated fungi using purified "humus" as the only source of nitro- 

 gen; the "humus" could not be utilized, however, as a source of carbon. 

 The various claims put forth that "humus" can be used as a source of 

 energy for nitrogen fixing bacteria, as well as for fungi and urea bacteria 

 still need confirmation. 87 Warmbold 88 demonstrated previously that 



85 Hoppe-Seyler, F. Uber Huminsubstanzen, ihre Entstehung und ihre 

 Eigenschaften. Ztschr. physiol. Chem., 13: 66-121. 1889. 



86 Reinitzer, F. Uber die Eignung der Huminsubstanzen zur Ernahrung von 

 Pilzen. Bot. Ztg., 58: 59-73. 1900; Nikitinsky, J. Uber die Zersetzung der 

 Huminsaure durch physikalisch chemische Agentien und durch Mikroorganismen. 

 Jahrb. wiss. Bot., 37: 365-420. 1902. 



87 It is sufficient to cite the work of Robertson, R. A., Irvine, J. C, and Dob- 

 son, M. E. A contribution to chemistry and physiological action of the humic 

 acids. Biochem. Jour., 2: 458-480. 1907; Christensen, H. Uber Ureumspalt- 

 ung. Centrbl. Bakt. II, 24: 130. 1909; Krzemieniewski, 1908 (p. 579); Prings- 

 heim, 1908-1912 (p. 564). The same is true of the results of Lipman and Teakle, 

 on the use of displaced soil solution and residual soil as sources of energy by 

 Azotobacter; Lipman, C. B., and Teakle, L. J. H. The fixation of nitrogen by 

 Azotobacter in a displaced solution and in soil residue therefrom. Soil Sci., 

 19: 99-103. 1925. 



88 Warmbold, H. Untersuchungen liber die Biologie stickstoffbindender 

 Bakterien. Landw. Jahrb., 35: 1-123. 1906. 



