630 PRINCIPLES OF SOIL MICROBIOLOGY 



with the soil bases (Ca, Mg). When bases are absorbed by the soil, 

 they displace an equivalent amount of another base which is combined in 

 the soil either with the inorganic zeolites or with the organic compounds. 



The adsorption of dyes by soils, which depends upon the surface of 

 the soil, has been used for the estimation of colloids of the soil by as- 

 suming that only the colloids in the soil take part in this process. 25 



Colloids play an important part in making the soil a favorable med- 

 ium for the growth of microorganisms, by absorbing the soluble fer- 

 tilizing elements added to or produced in the soil and by their buffering 

 properties in preventing rapid changes of the soil reaction. The growth 

 of bacteria was found 26 to be a function of the soil surface; in culture 

 media colloidal silicic acid and its compounds, as well as colloidal 

 ferric and aluminum hydrates and humus stimulate nitrogen fixation 

 by Azotobacter, possibly by absorbing nitrogen gas. A colloid (like soil 

 extract, gelatin, etc.) quickly shortens the period necessary for germ- 

 ination of the spores of Bac. amylobacter A. M. and Bred, in a nutri- 

 ent solution. 27 In the presence of 0.25 to 1.0 per cent gelatin, the 

 period of incubation, from inoculation to beginning of fermentation, 

 was reduced from fifty-one days in a nutrient solution free from 

 colloids to three days. In the presence of a colloid, a clear zone is 

 found to surround the spores; this zone is absent in a suspension of 

 spores in a colloid-free solution. The shortening of the period neces- 

 sary for spore germination depends on the dispersion of the colloid and 

 is explained chiefly by adsorption phenomena. 28 



26 A detailed discussion of the subject of absorption is found in the following 

 papers. Whitney, M., and Cameron, F. The chemistry of the soil as related to 

 crop production. U. S. Dept. Agr. Bur. of Soils, Bui. 22. 1903; Bui. 23. 1904; 

 Cameron, 1911 (p. xiv), p. 61; Wiegner, G. Die Festlegung des Stickstoffs 

 durch sogenannte Zeolithe. Jour. Landw., 61: 11-58. 1913; Prescott, J. The 

 phenomenon of absorption in its relation to soils. Jour. Agr. Sci., 8: 110-130. 

 1916; Gedroiz, 1922 (p. 629); Fischer, E. A. The phenomena of absorption in 

 soils: a critical discussion of the hypotheses put forward. Trans. Farady Soc, 

 17: 305-316. 1922; Mattson, 1922 (p. 628). 



26 Sohngen, N. L. Einflusz von Kolloiden auf mikrobiologische Prozesse. 

 Centrbl. Bakt. II, 38: 621-647. 1913. 



"Lantzsch, 1921 (p. 620). 



28 Further information on the influence of colloids upon the activities of micro- 

 organisms is given by Plotho, O. Der Einflusz kolloidaler Metallosungen auf 

 niedere Organismen und seine Ursachen. Biochem. Ztschr., 110: 33. 1920; 

 Schade, H. Die Kolloide als Trager der Lebenserscheinungen. Die Naturw., 

 9: 89-92. 1921. 



