THE SOIL AS A CULTURE MEDIUM 631 



Bacteria are adsorbed by various colloids 29 as well as by sand. 30 

 Since soils contain an abundance of substances in a colloidal condition, 

 it is but natural to expect a marked influence upon the bacteria. The 

 following method can be used for the study of this phenomenon. 31 

 One cubic centimeter of a bacterial culture is added to 9 cc. of water 

 and the mixture placed in a flask containing 5 grams of soil. After 

 shaking for one minute, the soil is allowed to settle for ten minutes. 

 The number of bacteria is then determined in the suspension both by 

 plating and by direct microscopic examination. It was found that 

 pure sand has little adsorptive action. Some bacteria, like Bac. my- 

 coides, Bact. prodigiosum and Staph, pyogenes, are adsorbed rapidly 

 and completely (80 to 98 per cent) ; other bacteria, like Bad. coli, are 

 only weakly adsorbed (10 to 20 per cent). Adsorption of the bacteria 

 was found to lead to a diminution not only in numbers but also in their 

 chemical activities. Decomposition of organic matter in the soil 

 seems to be carried out largely by the unadsorbed bacteria, probably 

 due to the lower oxygen tension upon the soil colloidal particles. Ad- 

 sorption does not diminish the action of anaerobic bacteria upon or- 

 ganic matter in the soil. 



The absorption of inorganic materials by microorganisms is quite 

 marked, 32 some bacteria and fungi possessing a greater absorptive power 

 than higher plants per unit of cells. 33 



Soil solution. The water present in the soil and added through 

 rainfall dissolves some of the soil constituents. If the soil conditions 

 were stable the solution would soon become saturated. Constant 

 evaporation, rainfall, change in weather conditions, development of 



29 Eisenberg, P. Ueber spezifische Adsorption von Bakterien. Centrbl. 

 Bakt. I, Orig., 81: 72-104. 1918. 



30 Frey, W., and Erismann, H. Beitrage zur Theorie der Bakterienfiltration. 

 Centrbl. Bakt. I, 88: 306-336. 1922. 



31 Dianowa, E. W., and Woroshilowa, A. A. The adsorption of bacteria by 

 soils and its influence upon microbiological activities (Russian). Nautchno- 

 Agron. Zhur. No. 10. 1925; Chudiakow, N. N. Centrbl. Bakt. II, 68: 345-358. 

 1926. 



32 Beijerinck, M. W. tTber die Absorptionserscheinung bei den Mikroben. 

 Centrbl. Bakt. II, 29: 161-166. 1911; Stoklasa, J. tlber die biologische Absorp- 

 tion der Boden. Chem. Ztg., 35: 1425. 1911; Labes, R. tTber die fcrdernde 

 Wirkung von Kohlensuspensionen und anderen Korpern mit groszer Oberflach- 

 enentwicklung, etc. Biochem., Ztschr. 130: 1-13. 1922. 



33 Beard, E., and Cramer, W. Surface tension and fermentation action. 

 Proc. Roy. Soc. B., 88: 575. 1915; 98: 584. 1915. 



