MODELING THE SOLID-SOLUTION DISTRIBUTIONS OF PROTONS, ALUMINUM, BASE CATIONS AND HUMIC SUBSTANCES IN ACID SOILS

WHAM, an equilibrium chemical model for soils, waters and sediments, c entred on a discrete-site/electrostatic model of humic substances (HS) , has been used to analyse batch titration data for organic and minera l horizons of acid soils. In most cases, tolerable fits were obtained by optimizing the soil contents of HS and aluminium, while keeping the model parameters (site densities, equilibrium constants, electrostati c terms) fixed. The optimized contents agreed reasonably with those es timated by chemical extraction. For some mineral soil samples, low in HS and high in aluminium, fitting of the titration data was improved b y assuming the formation and dissolution of Al(OH)(3) and adjusting it s solubility product. Solid-solution distributions of base cations (Na +, Mg2+, K+, Ca2+, NH4+) could be explained by non-specific counterion accumulation, with a small degree of selectivity. The WHAM sub-model for fulvic acid sorption accounted approximately for observed aqueous- phase concentrations of organic carbon and organically-complexed alumi nium.