INFLUENCE OF ORGANIC-MATTER ON AGGREGATION IN OXISOLS RICH IN GIBBSITE OR IN GOETHITE .2. CLAY DISPERSION, AGGREGATE STRENGTH AND WATER-STABILITY

The structural implications of the influence of organic matter on Oxisol surface charges were further explored in this study. In Oxisol aggregates rich in gibbsite, amounts of dispersed clay were only 20-40% with moderate mechanical input energy and 30-70% with ultrasonic disruption. The stability was mainly attributed to the importance of non-electrostatic intra-aggregate bonds. Organic aggregates were also much more stable than mineral aggregates attributable to lower porosities and to greater hydrophobicity. Tensile strength of air-dried aggregates was relatively low in comparison with temperate soil counterparts and increased with necessary organic matter content. Consequently, organic aggregates had much more complex populations than mineral aggregates. The influences of mineralogy, structure (pore volume and heterogeneity) and intra-aggregate bonds on both aggregate tensile strength and water-stability are discussed.