SOIL REACTION 41 1 



attributed by these authors to the colloidal matter in the 

 coverings of the hyaline sphagnum cells. The original sphagnum 

 was almost as " acid " as the peat, hence it is unnecessary to 

 assume that the " acid " is a decomposition product. Moreover, 

 if an acid is really present it is very insoluble, as an aqueous 

 extract of peat is practically neutral to litmus ; and this is 

 generally true of most acid soils. These views were based on 

 the purely chemical work of Linder and Picton, who showed 

 that when AsgSg is precipitated from colloidal solution by 

 BaClg a small amount of Ba is carried down with it and a 

 corresponding amount of HCl is set free. If Ca, Sr, or K. 

 chloride is used the same amount of HCl is left behind, showing 

 that equivalent amounts of the base are absorbed in each case. 

 The Ba thus carried down is held very firmly and cannot be 

 removed by washing, although it is easily replaced by other 

 bases by digesting with an appropriate salt in solution. The 

 work of Whitney and Ober concerning the electrical properties 

 of colloidal solutions gave some theoretical basis to Baumann 

 and Gully's views and further support was accorded by the fact 

 that acidity is developed on shaking an acid or a neutral soil 

 with neutral salt solutions. This phenomenon was first noticed 

 by Thompson in 1845, and was explained by Way about 1850 

 as due to an interchange of bases between the neutral salt and 

 a constituent of clay — a hydrated aluminosilicate of an alkali 

 or an alkahne earth. Later, when the general theory of surface 

 adsorption had been put forward and developed by Willard 

 Gibbs, J. J. Thomson, Freundhch, Wo. Ostwald and others it 

 was extended and applied in a somewhat modified form and 

 under the name of " selective adsorption " to soil interchanges 

 by Cameron and later by E. G. Parker and by Harris. Parker 

 found that when an acid soil is shaken with a solution of KCl 

 the K was displaced by Al and other bases in exactly equivalent 

 amounts, the CI remaining unchanged. When NaOH was also 

 present with the KCl just as much K was absorbed by the soil, 

 but no bases from the soil replaced it. Parker concluded that 

 the base is adsovhed by the soil and a real acidity developed, 

 which then dissolves from the soil the bases found in the solution. 

 This conclusion was supported by washing out the soluble bases 

 from the soil with HCl, washing the soil free from HCl, and treat- 

 ing with KNO3 solution : a considerable amount of free HNO3 

 was found in the solution besides the usual A1(N03)3, etc. 



(C) Basic Exchange Theory 



This adsorption view is, however, not universally held and 

 many still hold that a real interchange of bases occurs, not 

 however with the insoluble organic acids of the soil, but with 

 the complex aluminosihcates of the clay fraction. This modern 



