Simulation of Cation Exchange Involving Hydrogen Ion in Soil

Abstract

AbstractA computer program for simulating cation‐exchange reactions in soil was developed and used for simulating the determination of exchangeable acidity with 1 M KCl and of hydrolytic acidity with 1 M NaOAc. The acidification process resulting from extraction of an initially base‐saturated soil with a dilute solution of an acid having various dissociation constant may also be simulated. From the calculated pH‐base saturation relationships for assumed K‐H exchange coefficent (KK‐H) varying from 1 × 101 to 1 × 10−6, it is noted that for KK‐H values of 1 × 10−n, the inflection point of the pH‐base saturation curve is at the pH value of n. Thus, the permanent charge sites of mineral soils appeared to have a KK‐H value of 1 × 10−1 and greater, while for the pH‐dependent charge sites the KK‐H values are smaller than 1 × 10−2. The acidity extracted by 1 M KCl solution corresponded to the H+ on the permanent charge sites, with KK‐H values of 1 × 10−2 and greater, but it may also include some portions of H+ bound on exchange sites with KK‐H values smaller than 1 × 10−2. Exchangeable H+ from sites with KK‐H smaller than 1 × 10−5 were practically not displaced by 1 M KCl solution. The use of 1 M NaOAc instead of Kcl could quantitatively displace the H+ held on the sites with KNa‐H values >1 × 10−3 by a single extraction, while those of 1 × 10−6 and greater may be displaced by repeated extractions. Calculations with KNa‐H values of 1 × 10−7 did not converge, but available soil data indicate that the pH‐dependent sites of soils have KNa‐H values in the order of 1 × 10−7. Use of a dilute acid solution in place of KCl or NaOAc solution simulates the acidification process of acid precipitations.