Objective This study aimed to clarify the differences in ameliorative effects and mechanisms of calcareous purple mudstone with varying particle sizes on acidic purple soil, in order to facilitate the development of more precise mitigation measures for acidified purple soils.

Method An incubation experiment was conducted to investigate the ameliorative effects of Jurassic Suining Formation (J₃sn) calcareous purple mudstones at three particle sizes (< 0.25, 0.25 - 1.00, and 1.00 - 2.00 mm) and three application rates (0.1%, 0.5%, and 1.0%) on acidic purple soil, combined with batch experiments to reveal the mechanisms by which particle size influences the ameliorative effects of calcareous purple parent rock.

Result The addition of calcareous purple mudstone significantly improved soil acidity, with greater effectiveness observed at higher application rates and finer particle sizes. At 1.0% application rate, the addition of < 0.25 mm calcareous purple mudstone increased soil pH from 5.17 to 5.72, decreased exchangeable acidity from 3.62 to 0.63 cmol kg⁻¹, and reduced exchangeable Al³⁺ from 3.07 to 0.30 cmol kg⁻¹. With increasing application rates and decreasing particle sizes, the exchangeable and water-soluble base cations and base saturation of acidic purple soil showed a consistent upward trend. At 1.0% application rate, the addition of < 0.25 mm calcareous purple mudstone increased the total exchangeable base cations from 21.51 to 24.69 cmol kg⁻¹, exchangeable Ca²⁺ from 18.62 to 21.62 cmol kg⁻¹, total water-soluble base cations from 172 to 403 mg kg⁻¹, and water-soluble Ca²⁺ from 136 to 358 mg kg⁻¹. In aqueous solutions, finer calcareous purple mudstone particles released more base ions but generated fewer hydroxyl ions (OH⁻) in solution. The primary active components in the remediation process were Ca²⁺ and CO₃²⁻ derived from CaCO₃ dissolution. Hydrolysis of CO₃²⁻ produced OH⁻, which directly neutralized H⁺ and Al³⁺ in the soil solution. Simultaneously, Ca²⁺ exchanged adsorbed acid ions on soil colloids, increased base saturation, and enhanced the neutralization reaction between acid ions and OH⁻. Finer particle sizes released more Ca²⁺, facilitating acid ion desorption and subsequent neutralization.

Conclusion The effects of particle size of calcareous purple mudstone on the amelioration of acidic purple soil are primarily governed by the surface area-to-volume ratio. Smaller particle sizes increase the contact interface between calcareous purple mudstone and soil, accelerating the dissolution and release of base ions (Ca²⁺ and CO₃²⁻). This enhanced ion release promotes the hydrolysis of CO₃²⁻, generating OH⁻ that directly neutralizes H⁺ and Al³⁺, while Ca²⁺ exchanges with adsorbed acid ions on soil colloids. As a result, both active and potential soil acidity are reduced more effectively with finer particles. In agricultural applications, finely ground calcareous purple mudstone is therefore recommended for optimizing the amelioration of acidic purple soils.