Leaching behavior of steelmaking slag fertilizer under repeated wetting and drying conditions simulating upland soil

Abstract. To determine how steelmaking slag dissolves and modulates soil acidity and exchangeable cations under upland-like repeated wetting–drying conditions, we conducted a soil-column experiment. Specifically, we aimed to identify the Ca-supplying phases responsible for pH correction, evaluate their persistence during extended leaching, and define the layer-scale reach of the effect to inform application planning (rate, placement, and maintenance). Soil columns incorporating discrete slag-amended layers were prepared together with unamended controls. A repeated wetting–drying leaching test was run up to 24 weeks; after termination, each column was sampled by layer, and soil pH and exchangeable CaO were measured. Additionally, surfaces and cross-sections of slag particles embedded in the columns were observed to identify dissolving phases and secondary precipitates. In the control columns, soil pH remained in the acidic range (4.8–5.5), whereas slag-amended layers maintained pH 6.0–6.5 for 24 weeks in the test columns. Adjacent unamended layers in the test columns showed no detectable change, indicating that the effect was confined to the amended layers. Exchangeable CaO increased in soils mixed with slag. Microstructural observations revealed alteration and dissolution of free lime (f-CaO) and dicalcium silicate (2 CaO ⋅ SiO2), with calcium carbonate (CaCO3) precipitates on particle surfaces. These Ca-supplying phases persisted after 24 weeks of leaching. Sustained Ca release from f-CaO and 2 CaO ⋅ SiO2, together with CaCO3 precipitation, produced localized, durable pH correction in slag-amended layers while leaving adjacent layers unchanged. The defined reach and persistence provide a mechanistic basis for application planning in acidic upland soils – informing rate, placement within the profile, and maintenance intervals.