Ferralsols in the Cameroon plateaus, with a focus on the mineralogical control on their cation exchange capacities

Land use intensity in the Cameroon plateaus, where Ferralsols and the associated highly weathered soils are dominant, has been steadily increasing, so that soil fertility status and its controlling factors on these plateaus should be elucidated to establish long-term land management strategies. Although it is known that Ferralsols have small, but variable CEC values, it has not been fully determined which clay components are the main source of CECclay in Ferralsols. Here, soils in three Cameroon plateau regions of different altitude (the lower South Cameroon (SC) Plateau with an altitude of 300-600 m, SCL; the upper SC Plateau with an altitude of 600-900 m, SCH; and the Adamawa Plateau with an altitude above 900 m, ADM) were collected, and their physicochemical and mineralogical properties were investigated. The relative abundance of illitic minerals, which are a minor but the most promising source of CECclay in Ferralsols, was determined by using three indicators: the relative abundances of 1.0 nm-thick structures as represented by XRD peak intensity (XRD1.0 nm), structural K in illitic minerals as represented by clay-K content, and the quantity of intermediate interlayer sites between illite and vermiculite structures (i.e., frayed edge sites) as represented by the radiocesium interception potential (RIP). The relative abundances of two low-activity clay components, kaolinite and gibbsite, were determined by differential thermal analysis, and the degree of Al-substitution in goethite was determined by differential x-ray diffraction analysis. All three quantitative indicators for illitic minerals decreased along the altitude gradient, SCL > SCH > ADM, and were positively correlated with CECclay. While kaolinite also showed a decreasing trend at higher altitude, it did not show any significant correlation with CECclay. In contrast to the decrease in these Si-bearing minerals, gibbsite and Al-substituted goethite increased along the same altitude gradient. These associative changes strongly indicate that illitic minerals were the clay component largely controlling CECclay and that de-silication or aluminization may proceed at higher altitude or on stable surfaces with older formation ages in the Cameroon plateau regions. Thus, mineralogical properties well explained the distribution of Ferralsols; they were marginal in the SCL region, while those with relatively high CECclay were dominant in the SCH region and those with lower CECclay were dominant in the ADM region.