A comparison of the contributions of clay, silt, and organic matter to the effective CEC of soils of subSaharan Africa

The contributions of clay, silt, and organic matter (OM) to the effective cation exchange capacity (ECEC) of soils in subSaharan Africa were examined using data from more than 2131 soil samples collected from Cote d'Ivoire, Nigeria, Tanzania, and Uganda. The data were partitioned into highland humid (HLH), lowland humid (LLH), subhumid (SH), nonhumid (NH), low altitude (LA), mid altitude (MA), 0-20-cm depth (D1), and 20-40-cm depth (D2) following preliminary analysis. The soils of each zone are generally low in ECEC and OM, and low to medium in clay and silt contents, and all exhibited wide variations both within and across the zones. In most ofthe zones, ECEC correlated significantly with all three variables. In the NH zone, however, clay exhibited a nonsignificant correlation with ECEC, and silt showed similar characteristics with ECEC in the SH zone. The absolute CEC of clay was highest in the HLH zone, where it made a relative contribution (RC) of67% to the ECEC. The highest RC of silt (37%) was obtained in the NH zone, where the CEC of silt was also highest. Generally, the RC of OM was either as high as or higher than the combined contributions of clay and silt to ECEC, except in the SH and LA zones. For the entire subregion, OM alone could account for about 60% of the mean ECEC of the soils. This demonstrates the overall importance of OM in maintaining the nutrient reserve in tropical areas. The overall variations in ECEC attributable to the three variables could range from 56% in the SH zone to 95% in the LLH zone. The distinct equations obtained for each zone show that interzonal application of the equations is not advisable, in particular because ofthe varying contributions of clay and silt. The study generally reveals that silt not only contributes significantly to ECEC but is more important than clay in some zones.