Acidic soils can lock up nutrients and release harmful metals, reducing crop growth. In a laboratory soil-column experiment, we tested steelmaking slag, an alkaline by-product, under repeated wetting and drying. The slag-mixed layer showed lasting improvement in acidity for twenty-four weeks, while nearby layers changed little. Slag released calcium slowly and formed surface coatings, suggesting durable, place-specific treatment and guidance for rate, depth, and reapplication.