Carbon content in soil particle size and consequence on cation exchange capacity of alfisols

Many of the cultivated soils of sub-Saharan Africa typically have a surface horizon low in clay and with a low cation exchange capacity (CEC). Measurements made on long-term trials suggest that not all the soil organic matter (SOM) plays an equal role as regards the soil CEC. To study the influence of the different SOM size fractions on the CEC, soils with or without application of manure or compost coming from trials in Chad and Côte d'Ivoire were separated without destruction of the SOM into five organo-mineral fractions : coarse sand, fine sand, coarse silt, fine silt, and clay. The CEC of these fractions increases inversely with their size. The clay fraction which contains half of the SOM contributes about 80 p. cent of the CEC of the soils. The CEC of the fractions is largely a function of their carbon content, but the organic CEC per unit C of the clay fraction appears to be four times greater than that of the other fractions. Applications of manure or compost increase the CEC of the soils by increasing the soil C only when this C increase concerns the fine fractions of the SO