In tropical regions, excessive exchangeable acidity and aluminium (Al3+) influence the availability of ammonium (NH4+), nitrate (NO3-), and phosphorus (P) because of low soil pH. This study focuses on the effects of silico-manganese slag (SiMn slag) and chemical fertilisers (CF) on mitigating acidity, exchangeable Al3+, extractable manganese (Mn), and iron (Fe) for improving selected chemical properties [pH, NH4+, NO3-, P, and electrical conductivity (EC)] of a tropical acidic soil named Bekenu series (pH = 5.13). An incubation study was conducted by mixing one kilogramme of Bekenu series with different rates of SiMn slag and CF: T0 = soil only; T1 = SiMn slag only; T2 = soil + 100% CF; T3 = soil + 100% SiMn slag; T4 = soil + 75% SiMn slag + 25% CF; T5 = soil + 50% SiMn slag + 50% CF; and T6 = soil + 25% SiMn slag + 75% CF. After 30, 60, and 90 days of incubation, compared with T0, SiMn slag and CF significantly increased soil pH, exchangeable NH4+, available P, and EC, in addition to suppressing exchangeable acidity and Al3+, as well as extractable Fe and Mn. This indicates that SiMn slag could be used as a liming amendment, whereas chemical fertilisers remain necessary for supplying nutrient sources. In future studies, the justification for fixing the suitable application rate of SiMn slag should focus on acid-neutralising capacity, calcium carbonate (CaCO3) equivalent, target pH, exchangeable acidity neutralisation requirement, or silicon/calcium/magnesium (Si/Ca/Mg) supply. Future greenhouse and field trials are recommended to further validate the effects of SiMn slag on soil-plant interaction.