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Simulation of Cation Exchange Involving Hydrogen Ion in Soil

Department of Agricultural Chemistry, National Taiwan Univ., Taipei, Taiwan, ROC

*Corresponding author (khhoung{at}ccms.ntu.edu.tw).

ABSTRACT

A 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 (K_K-H) varying from 1 x 10¹ to 1 x 10^–6, it is noted that for K_K-H values of 1 x 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 K_K-H value of 1 x 10^–1 and greater, while for the pH-dependent charge sites the K_K-H values are smaller than 1 x 10^–2. The acidity extracted by 1 M KCl solution corresponded to the H⁺ on the permanent charge sites, with K_K-H values of 1 x 10^–2 and greater, but it may also include some portions of H⁺ bound on exchange sites with K_K-H values smaller than 1 x 10^–2. Exchangeable H⁺ from sites with K_K-H smaller than 1 x 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 K_Na-H values >1 x 10^–3 by a single extraction, while those of 1 x 10^–6 and greater may be displaced by repeated extractions. Calculations with K_Na-H values of 1 x 10^–7 did not converge, but available soil data indicate that the pH-dependent sites of soils have K_Na-H values in the order of 1 x 10^–7. Use of a dilute acid solution in place of KCl or NaOAc solution simulates the acidification process of acid precipitations.

Received for publication June 4, 1993.