THE SOIL AS A CULTURE MEDIUM 629 



crease in the decomposition of the soil organic matter, greater libera- 

 tion of C0 2 , mineralization of the organic matter, absorption of bases 

 and increase in colloidal matter. 21 



Van Bemmelen 32 was the first to show that soil organic matter 

 (humus) plays an important part in the absorption of both basic and 

 acid radicals from the soil solution; this process of absorption was found 

 to be similar in nature to that of an artificial calcium-aluminum silicate. 

 However, different forms of absorption in the soil are often recognized: 23 

 (1) biological absorption or the assimilation of the anions or cations 

 by microorganisms; (2) mechanical absorption, or the mere mechanical 

 retention of particles suspended in water; (3) physical, or surface ad- 

 sorption, which may be positive or negative, depending on the fact as 

 to whether the substance decreases or increases the surface tension of 

 the dispersion medium; (4) physico-chemical, or adsorption in the 

 narrow sense, which consists in the exchange of bases between the 

 added salt and the zeolitic or alumino-silicate (and humic) complex of 

 the soil ; and finally (5) chemical absorption, or the chemical interaction 

 between two substances giving difficultly soluble compounds, as in the 

 formation of calcium phosphate from the carbonate and soluble phos- 

 phate. Certain investigators, however, do not take the view that any 

 sharp differentiation exists between chemical and physical reactions, 

 which may all be due to electrical forces differing only in degree; this 

 applies especially to the exchange of bases in the soil. 



In proportion to their total mass, colloids exhibit a remarkable power 

 of adsorption because of the large surface that they possess. Adsorp- 

 tion increases with the concentration of the solute. The absorption 

 of ammonia by soils or by calcium silicates (permutite) from a solution 

 of an ammonium salt follows the laws of adsorption. 24 The same was 

 found to hold true for the adsorption of other bases. Of the acids, the 

 phosphates alone are absorbed, or rather precipitated on interaction 



21 Thaer, W. Der Einflusz von Kalk unci Humus auf die mechanische, physik- 

 alische und chemische Beschaffenheit von Ton-, Lehm- und Sandboden. GJt- 

 tingen. 1910 (Centrbl. Bakt. II, 32: 271-274. 1912). 



22 van Bemmelen, J. M. Die Absorptionsverbindungen und das Absorptions- 

 vermdgen der Ackererde. Landw. Vers. Sta., 35: 69-136. 1888; Die Absorp- 

 tion. Dresen. Steinkopff. 1910. 



23 Gedroiz, K. K. On the absorptive power of soils. Zhur. Opit. Agron., 

 19: 269-322. 1918; Leningrad. 1922. 



24 Wiegner, G. Zum Basenaustaush in der Ackererde. Jour. Landw., 60; 

 111-150, 197-222. 1912. 



