ORGANIC-MATTER ROLE IN ILLITIC SOIL COLLOIDS FLOCCULATION .1. COUNTERIONS AND PH

Dispersed soil particles have a negative impact on soil structure and contribute to soil erosion and contaminant movement. The colloidal pro perties of soil clays have been found to be different from specimen cl ays. The purpose of this study was to investigate the effect of organi c matter on the flocculation of soil colloids ss a function of electro lyte concentration, pH, and exchangeable or soluble bivalent cation ty pe. The flocculation of suspensions of two illitic soils was studied u sing light transmission. Soil suspensions were prepared in Na-K-Ca and Na-K-Mg perchlorate solutions at a total electrolyte charge concentra tion of 5, 10, and 15 mol(c) m(-3) at pH 6 and 8. Flocculation at a gi ven soluble bivalent cation charge fraction was increased as the organ ic C content of the soil colloids decreased. Calcium was more effectiv e than Mg as a flocculant at the same soluble or exchangeable bivalent cation charge fraction. Increasing pH caused an increase in the flocc ulation of one of the soils, but this trend was reversed after the rem oval of organic matter by H2O2 treatment. The amount of electrolyte re quired for flocculation, the effect of pH, and the relative effectiven ess of Ca vs. Mg all depended on soil organic matter content.