﻿344 Prof, J. N. Mukherjee on 



not seem to be tenable. The preferential adsorption of an 

 ion by the soil does not mean hydrolytic decomposition of 

 the salt. It appears from the summary given by Russell 

 •that the equivalent exchange of bases lies in the way of 

 regarding the reaction as an adsorption process (loc. cit. 

 pp. 71, 75 ? 76). It would be apparent from the previous 

 discussion that this, in itself, does not contradict the adsorp- 

 tion hypothesis. 



Soil can be regarded as a complex colloidal system. It is 

 a complex gel consisting of aluminium and other silicates, 

 free silica, ferric hydroxide, etc. The gel is mixed with 

 insoluble crystalloids. It also contains small quantities of 

 adsorbed electrolytes and organic matter in indefinite and 

 varying proportions. The gel adsorbs anions by chemical 

 affinity. These anions may be : — 



(1) of organic acids, such as humus acid ; 



(2) of simple electrolytes like chlorides, sulphates, car- 



bonates, etc. ; 

 (3) hydroxyl ions from water. 

 . Owing to the complex chemical nature of the gel and the 

 enormous specific surface of gels, large quantities of anions 

 may be adsorbed. An equivalent number of cations remain 

 near the surface as the mobile second sheet or as electricalty 

 adsorbed. The exchange of bases is simply due to the dis- 

 placement of these ions. When the displacement is quantitative 

 equivalent amounts are exchanged. The anions primarily 

 adsorbed or the cations in the second sheet are not of one 

 kind. The relative numbers and chemical natures of these 

 ions will evidently vary with the different soils. 



An extract with pure water will be neutral unless the soil 

 contains free acids. An extract with a neutral salt can only 

 be acid when the cations displaced from the second sheet (or 

 electrically adsorbed) contain hydrogen ions or such ions as 

 aluminium, which hydrolyse in dilute aqueous solutions. 

 The role of the aluminium ions in determining the acidity of 

 the soil extract has been pointed out by Daikuhara (Bull. 

 Imp. Central Agri. Expt. Station, Tokio, ii. pp. 1-40 (1914)), 

 and has been fully confirmed by Rice (loc. cit.). The function 

 of organic acids lias constituted a great objection against the 

 adsorption hypothesis. The hydrogen ions in the second 

 sheet have probably, in most cases, their origin in these 

 acids. This view thus correlates the exchange of bases 

 observed with soil with such exchanges as have been observed 

 in the adsorption of electrolytes (cp. Michaelis). 



That sometimes considerable quantities of bases are ex- 

 changed should be referred to the enormous surface of these 





