Functional Partitioning of Cation Exchange Capacity in Highly Weathered Tropical Soils of Southwestern Nigeria

This study investigated the contributions of soil organic matter (SOM), clay, and various forms of Fe and Al oxides to the cation exchange capacity (CEC) of highly weathered tropical soils in central southwestern Nigeria. Soils representing major types in the region—Typic Paleustults, Rhodic Kandiustults, and Typic Haplustults—were sampled by genetic horizon. SOM was removed using hydrogen peroxide, and parallel subsamples underwent selective extractions targeting crystalline (dithionite–citrate–bicarbonate), poorly crystalline (acid ammonium oxalate), and organically complexed (sodium pyrophosphate) Fe and Al oxides. Changes in CEC following each treatment were used to assess the functional roles of the removed components. SOM removal resulted in substantial reductions in CEC across all soils, accounting for 38%–47% of total CEC in surface horizons, confirming SOM as the dominant source of exchangeable charge. Removal of oxides produced variable responses depending on the form and soil depth. In many subsurface horizons, CEC increased, indicating that these oxides often mask clay exchange sites. Conversely, decreases in CEC observed in some surface horizons indicate that oxides can also contribute pH‐dependent charge or stabilize organic functional groups. Clay fractions exhibited modest intrinsic CEC, consistent with kaolinitic mineral dominance. These findings demonstrate that CEC in these highly weathered tropical soils is controlled primarily by SOM and the degree to which Fe and Al oxides regulate access to clay exchange sites. Enhancing organic matter and managing oxide‐induced masking are therefore key strategies for improving nutrient retention and soil fertility in tropical agroecosystems.