CATION SELECTIVITY COEFFICIENT VARIATIONS IN ACIDIC FOREST SOILS FROMSUTTON, QUEBEC

Single values of the Vanselow, Gaines-Thomas or Gapon selectivity coef ficients have been used to model cation exchange in terrestrial ecosys tems; however, selectivity coefficients that do not vary as a function of pH and ionic strength have questionable validity. We examined cati on exchange in two Humo-Ferric Podzols (Haplorthods) equilibrated with artificial soil solutions with two different ionic strengths in the p H range 3-6. The resulting selectivity coefficients varied by several orders of magnitude. These variations could not be fitted to regressio ns using the pH and ionic strength of the solutions. Estimating the ca tion concentrations using fixed values for selectivity coefficients wi thin the chemical composition range of the soils studied would generat e modelling errors of up to four orders of magnitude. An equation prop osed by McBride and Bloom, relating the amount of an ion in solution t o its relative abundance on the exchange complex was more easily fitte d. This adsorption equation can be modelled using a regression of pH a nd ionic strength. Single regression equations are presented for Ca, M g, K and Al that are capable of estimating the coefficients for two si tes and two different soil horizons.