Charge characteristics of soil organic matter fractions in a Ferric Lixisol under some multipurpose trees

oil organic matter (SOM) has a key role in maintaining soil fertility in we athered soils in the tropics. This study was conducted to determine the con tribution of different SOM fractions to the cation exchange capacity (CEC) of a tropical soil as influenced by organic matter inputs of different bioc hemical composition. Soil samples were collected from a 16-yr old arboretum established on a Ferric Lixisol, under five multipurpose tree species: Leu caena leucocephala, Dactyladenia barteri, Afzelia africana, Pterocarpus san talinoides, and Treculia africana. Fractions were obtained by wet sieving a nd sedimentation after dispersion with Na2CO3. Fractions larger than 0.053 mm were separated into mineral and organic components by flotation on water . Relationships between CEC and pH were determined using the silverthioureu m-method. For all treatments the organic fractions had the highest CEC, exp ressed on a dry matter basis, and the CEC of the fractions smaller than 0.0 53 mm was inversely related to their particle size: clay (< 0.002 mm) > fin e silt (0.002-0.02 mm) > coarse silt (0.02-0.053 mm). A positive correlatio n (significant at the 0.01 probability level) existed between the slope of the fitted CEC-pH relationships and the organic C concentrations of the who le soil and both silt fractions. The clay and fine silt fractions were resp onsible for 85 to 90% of the CEC of the soil. Organic inputs with a high C/ N and lignin/N ratio produced fine and coarse silt sized SOM fractions with the highest charge density. Therefore, inputs of slowly decomposing organi c residues seem to be promising for increasing the CEC of highly weathered soils.