High rainfalls and humid temperatures in tropical regions resulted in acidic soils because of excessive leaching of base cations and occupation of iron (Fe2+) and aluminium (Al3+) ions. Silico-manganese slag (SiMn slag) is an alkaline material which could neutralise soil acidity and regulate nutrient leaching. A 30-day leaching study was conducted to determine the effects of SiMn slag and chemical fertilisers (CF) on nutrient leaching in Bekenu series (Typic Paleudults). One kilogramme of Bekenu series was mixed 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. The co-application of SiMn slag and CF consistently increased pH and EC of the leachates, which resulted in significantly higher amounts of ammonium (NH4+), nitrate (NO3-), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) leached. After 30 days of leaching, co-applying SiMn slag and CF increased soil pH, electrical conductivity (EC), exchangeable NH₄⁺, Ca²⁺, and Mg²⁺ by reducing exchangeable acidity and Al³⁺. We concluded that SiMn slag did not mitigate nutrient leaching, but rather it may have ameliorated acidity to improve nutrient availability. The most suitable combinations are T5 (soil + 50% SiMn slag + 50% CF) and T6 (soil + 25% SiMn slag + 75% CF). Further pot and field studies are recommended to evaluate soil–plant interactions using SiMn slag.