Soil acidity, cation balance, exchangeable potassium status and cation retention capacity under K-bearing rock dust–organic amendment systems in a tropical Alfisol of southwestern Nigeria

Abstract Soil acidity and weak cation-holding conditions reduce nutrient-use efficiency in highly weathered tropical soils and constrain progress towards Sustainable Development Goals (SDGs) 2, 12 and 15. This study evaluated the 14-day soil chemical response to locally sourced potassium-bearing rock dust (RD), field-stabilized poultry manure (PM) and fresh Tithonia diversifolia biomass (TD) in a tropical Alfisol of southwestern Nigeria. A 3 × 3 split-plot field experiment with three replications comprised three randomized main-plot fertilizer designations—no mineral input (NA), RD and plots designated for subsequent NPK 15-15-15 application; and three organic-amendment subplots—NA, PM and TD. Baseline sampling preceded treatment application; RD and the organic materials were incorporated, and okra was sown at 0 weeks after amendment application (WAA), while the 2-WAA soil sampling was completed before NPK application. Accordingly, the NPK-designated plots are reported as NPK-pre and are not interpreted as NPK effects. At 2 WAA, RD plots recorded exchangeable K and operational cation-exchange capacity (CEC) means of 1.67 and 8.54 cmolc kg −1 , respectively, compared with 0.49 and 4.63 cmolc kg −1 under NA. PM reduced the baseline-referenced H + ratio from 1.07 under no organic amendment to 0.45 and increased operational CEC to 7.48 cmolc kg −1 . The FD × OA interaction was significant for exchangeable K ( P = 0.027); within RD plots, PM produced 1.90 cmolc kg −1 , exceeding RD × NA (1.54 cmolc kg −1 ) and RD × TD (1.57 cmolc kg −1 ). The interactions were not significant for operational CEC ( P = 0.145) or ECEC ( P = 0.085), whereas exchangeable acidity responded interactively ( P < 0.001). Electrical conductivity remained 0.03–0.04 dS m −1 . The findings indicate short-term modification of acidity and exchangeable-cation status without evidence of salinity build-up, but they do not establish long-term K retention, crop uptake or yield benefits.