A COMPARISON OF THE CONTRIBUTIONS OF CLAY, SILT, AND ORGANIC-MATTER TO THE EFFECTIVE CEC OF SOILS OF SUB-SAHARAN AFRICA

The contributions of clay, silt, and organic matter (OM) to the effect ive cation exchange capacity (ECEC) of soils in subSaharan Africa were examined using data from more than 2131 soil samples collected from C ote d'Ivoire, Nigeria, Tanzania, and Uganda. The data were partitioned into highland humid (HLH), lowland humid (LLH), subhumid (SH), nonhum id (NH), low altitude (LA), mid altitude (MA), 0-20-cm depth (D-1), an d 20-40-cm depth (D-2) following preliminary analysis. The soils of ea ch zone are generally low in ECEC and OM, and low to medium in clay an d silt contents, and all exhibited wide variations both within and acr oss the zones. In most of the zones, ECEC correlated significantly wit h all three variables. In the NH zone, however, clay exhibited a nonsi gnificant correlation with ECEC, and silt showed similar characteristi cs with ECEC in the SH zone. The absolute CEC of clay was highest in t he HLH zone, where it made a relative contribution (RC) of 67% 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 a s high as or higher than the combined contributions of clay and silt t o ECEC, except in the SH and LA zones. For the entice subregion, OM al one could account for about 60% of the mean ECEC of the soils. This de monstrates the overall importance of OM in maintaining the nutrient re serve in tropical areas, The overall variations in ECEC attributable t o the three variables could range from 56% in the SH zone to 95% in th e UH zone. The distinct equations obtained for each zone show that int erzonal application of the equations is not advisable, in particular b ecause of the varying contributions of clay and silt. The study genera lly reveals that silt not only contributes significantly to ECEC but i s more important than clay in some zones.