Interaction between anions and soil colloids was governed by 2 antagonistic processes, anion exclusion and positive anion adsorption. The predominantly negative charge on the colloids caused anion repulsion; positively charged sites and chemisorption resulted in positive adsorption.Experimentally determined adsorption was the resultant of the 2 processes and yielded true net adsorption by correction for continuous anion exclusion.Assuming the Gouy Chapman theory of the electric double layer, de Haan calculated exclusion as the apparent distance from the colloid surface free from anions of different valency. The method was valid for systems containing monovalent and divalent cations and anions with an approximation for trivalent anions, and was extended to interacting double layers.The product of apparent distance of exclusion and of the colloid's specific surface was the volume of exclusion (V ex ), the same as the experimental adsorption value. Thus anion exclusion measurements yielded values for the colloid's specific surface. V ex was determined by a tracer method for Cl -, S0 42-, and P0 43-, and by potentiometric titration for Cl -. Adsorption of different anions could be determined simultaneously.Theoretical derivations were confirmed in experiments with fairly pure clays and in 12 Dutch soils, with special attention to phosphate adsorption. The correction for anion exclusion allowed refined measurements of anion adsorption and explained the bonding mechanisms between anions and soil colloids.
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