Surface charge characteristics of highly-weathered soils from tropical Queensland

Abstract

An examination of six highly-weathered soils from tropical north Queensland showed that they were dominated by colloids of the constant surface potential type. Their surface charge is controlled by the pH and ionic strength of the ambient medium, and this was demonstrated by potentiometric titration as well as by the direct adsorption of counterions.In order to ascertain the pH and ionic strength conditions under which these soils occur in the field, solutions obtained after incubation at a moisture tension of 0.1 bar were analysed to obtain data on their ionic strengths. Soil extracts, at soil:solution ratios of 1:1, 1:2.5, 1:5, and 1:10 were also examined. Ionic concentrations of the soil solutions were found to be low when compared with many values reported in the literature. The upper limit for the ionic strength was about 0.005.Ionic strength was well correlated with the electrolytic conductivity of the soil solution itself, and also of the soil extracts, thus obviating the tedium involved in the analysis of solutions to allow calculation of the ionic strength.A simple method for determining the cation-exchange and anion-exchange capacities of these soils was devised, in the light of knowledge gained from the soil solution study. The soil is equilibrated with unbuffered BaC12 solution (ionic strength = 0.006) and then Mg++ compulsively replaces Ba++ when the former is added as MgSO4. Cation-exchange capacity is determined by measuring the reduction in Mg concentration in the final MgSO4 solution, while anionexchange capacity is found from the C1- concentration in the same solution, after correcting for entrained chloride. If desired,'the exchangeable basic cations can be determined when they are extracted during the BaC12 equilibration process.