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‐µm 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 × 10 −8 and +0.5 × 10 −8 m 2 s −1 V −1 , with isoelectric points (i.e.p.) ≤ 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 × 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.