CLAY DISPERSION IN TYPICAL SOILS OF NORTH CAMEROON AS A FUNCTION OF PH AND ELECTROLYTE CONCENTRATION

摘要

Knowledge of clay dispersion behaviour [which is highly influenced by ion concentration in the aqueous phase and by related surface charge (SC) of colloids] is important for rating soil erosion risk (by water). It can also be useful for improving soilmanagement systems. Clay fractions separated from samples of the A-horizon of a Vertisol, Ultisol and Oxisol were collected, representing typical soils of North Cameroon. These soils were very different in physicochemical and mineral parameters. The effect of pH and the multivalent ions Ca~(2+), SO_4~(2-) and PO_4~(3-) on SC and dispersion characteristics were determined. The water dispersible clay was found to be higher in the Vertisol and Ultisol than in the Oxisol, indicating that the <2 mum fractions from the Vertisol and Ultisol are more dispersible than that from the Oxisol. The clay dispersion ratio together with the dispersion ratio were found to be in good agreement with water dispersible clay and are negatively correlated with the amount oforganic matter and dithionite citrate bicarbonate soluble Fe and Al. Generally, SC of the <2 mum fraction was found to be negative when the pH was in the region of 3 to 9; thus the absolute value is highly pH-dependent. At pH 6 and 8, CaCl_2, Na_2SO_4 and Na_2HPO_4 additions had antagonistic effects on SC: Ca~(2+) increased SC, whereas SO_4~(2-) and PO_4~(3-) decreased SC indicating the adsorption of positively as well as negatively charged multivalent ions by soil colloids. Along with the increase of SC, there was a fall in repulsive forces and formation of Ca-bridges, the addition of Ca~(2+) induced flocculation more rapidly than SO_4~(2-) and PO_4~(3-) amendments.