698 



PRINCIPLES OF SOIL MICROBIOLOGY 



nature of combination of the bases with the soil "humus" is insuffi- 

 ciently understood, some considering them to form definite chemical 

 compounds and some as forming absorption complexes. 77 



In general the fractionating of the soil organic matter can be repre- 

 ented by the following scheme : 78 



Soil 



Treating with NH 4 OH (or NaOH) 



Black solution 



Adding acid 



I 

 Insoluble humin 

 (Identity of components unknown) 



Oden 79 submitted some evidence to indicate that "humic acid" 

 is a definite chemical compound, with three or four replaceable hydro- 

 gens, forming salts with alkalies and being electrically conductive. He 

 believes with earlier investigators that the nitrogen is present in this 

 "humic acid" as an impurity, since the content of nitrogen does not 

 fit into the particular chemical formula. However, other investigators 80 



77 Gedroiz, K. K. The absorbing capacity of the soil and the zeolitic bases of 

 the soil. Zhur. Opit. Agron., 17: 472-527. 1916; Contributions to our knowl- 

 edge of the absorptive capacity of soils. Ibid., 19: 269-322. 1918; 20: 31-58. 

 1919. 



78 Beckley, 1921 (p. 693). 

 "Oden, 1919 (p. 671). 



80 Schmuck, A. The organic substances of the soil. Trans. Kuban Agr. 

 Inst. (Russian), I, No. 2- 1-92. 1923. 



