Objective The acidification of purple soils has become increasingly severe, necessitating a clear understanding of how varying degrees of acidification affect crop growth, in order to provide scientific support for the precise management and sustainable utilization of acidic purple soils.

Method This study focused on acidic purple soils derived from Jurassic Shaximiao Formation (J₂s) mudstone, widely distributed across the Sichuan Basin. By externally adding different amounts of Al₂(SO₄)₃ and Ca(OH)₂, the soil pH was adjusted to a range of 3.88 - 7.60. Through pot experiments, we systematically analyzed the chemical properties of these purple soils under varying pH conditions and investigated the effects soil acidification on maize uptake of Ca and Al, as well as on maize growth characteristics. A two-segment piecewise linear regression model was applied to determine critical soil pH values.

Result Soil acidification increased exchangeable acidity, with exchangeable Al³⁺ reaching up to 9.36 cmol kg⁻¹. Concurrently, the levels of exchangeable Ca²⁺, exchangeable base cations, effective cation exchange capacity (ECEC), and base saturation decreased with increasing acidity. Notably, these purple soils retained relatively high levels of base cations and base saturation despite acidification. Acidification reduced Ca^{2 +} uptake by maize while significantly enhancing Al^{3 +} accumulation, thereby impairing plant growth. Critical pH values for maize growth parameters were identified as follows: plant height reached thresholds at pH 4.97 (seedling stage, 30 days after transplanting) and pH 4.17 (maturity, 100 days after transplanting); stem diameter at maturity showed a threshold at pH 4.94. The critical pH values for total biomass, ear weight, and shoot weight were 4.32, 4.26, and 4.95, respectively, while grain weight declined sharply below pH 4.21. Given that grain yield is the primary goal in maize cultivation, we define pH 4.21 as the acid damage threshold for J₂s-derived purple soils, corresponding to a critical exchangeable Al^{3 +} concentration of 6.18 cmol kg⁻¹. Compared to red soils, J₂s-derived purple soils exhibited a lower critical pH and greater tolerance to exchangeable Al^{3 +} , which could be attributed to the buffering capacity of exchangeable Ca^{2 +} against acidification and its antagonistic effect on Al^{3 +} toxicity.

Conclusion Although J₂s-derived purple soils possess a relatively low critical pH and inherent buffering capacity, the ongoing trend of soil acidification-coupled with significant variations in acidification behavior among purple soils formed from different parent materials-underscores needs to be heightened for awareness of crop toxicity risks.