COLLOID CHEMISTRY OF KAOLINITIC TROPICAL SOILS

Highly weathered soils of the humid tropics may exhibit loss of aggregate stability and dispersion of hydroxyl-bearing soil constituents as a result of management-induced changes in soil solution chemistry. The effects of pH and 1:1 ''indifferent'' electrolyte concentration on the surface charge and flocculation of colloids extracted from four representative kaolinitic tropical soils were examined by microelectrophoresis and light scattering methods. Electrophoretic mobilities (u) of clay-sized particles (0.1-1.0-mu m diameter) were predominantly negative between pH 2 and 6 for all soils, but decreased in magnitude with increasing solution proton concentration. Measured u values were between - 3.0 x 10(-8) and + 0.5 x 10(-8) m(2) s(-1) V-1, with isoelectric points (i.e.p.) less than or equal to 2.5. Photon correlation spectroscopy revealed an abrupt change from small to large particle size with increasing proton concentration in the pH range 2.5 to 3.5. Light scattering by whole-soil suspensions indicated a proton-induced transition from kinetically stable to kinetically unstable suspensions in the same pH range. Rapid flocculation occurred in the electrophoretic mobility range - 1.2 to - 1.8 x 10(-8) m(2) s(-1) V-1. The pH and ionic strength dependence of colloidal stability observed for the soils is consistent with an electrostatic stabilization mechanism among their clay-sized particles.