COLLOID CHEMISTRY OF KAOLINITIC TROPICAL SOILS

Highly weathered soils of the humid tropics may exhibit loss of aggreg ate 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 th e surface charge and flocculation of colloids extracted from four repr esentative kaolinitic tropical soils were examined by microelectrophor esis and light scattering methods. Electrophoretic mobilities (u) of c lay-sized particles (0.1-1.0-mu m diameter) were predominantly negativ e between pH 2 and 6 for all soils, but decreased in magnitude with in creasing 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 po ints (i.e.p.) less than or equal to 2.5. Photon correlation spectrosco py revealed an abrupt change from small to large particle size with in creasing proton concentration in the pH range 2.5 to 3.5. Light scatte ring by whole-soil suspensions indicated a proton-induced transition f rom 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 streng th dependence of colloidal stability observed for the soils is consist ent with an electrostatic stabilization mechanism among their clay-siz ed particles.