Ca Saturation Determines Crop Growth in Acidic Ultisols Derived from Different Parent Materials

Soil acidity has become a major yield-limiting factor, but it is unclear which acidity indicator is the best to use for estimating crop yield changes. In this study, four pH-adjusted Ultisols derived from different parent materials were used for Chinese cabbage and wheat pot experiments. Structural equation modeling (SEM), Gompertz and linear-plateau models were used to examine main contribution of soil acidity indices and to determine their critical values. The results showed that Ca saturation had the strongest direct effect on crop biomass and thus acted as the most important factor. The critical values of Ca saturation varied slightly with crops and soils, where it was 84.6, 93.5, 95.2 and 82.9% for Ultisols derived from plate shale, Quaternary red clay, red sandstone and granite, respectively. The critical values of exchangeable Ca and Al, and Al saturation and exchangeable Al : Ca ratio (or Ca : Al ratio) were also determined as 8.21 and 0.44 cmolc kg–1, 6.37%, and 0.069 (or 14.5), respectively. In summary, our findings evidenced that critical Ca saturation has the potential to evaluate the implementation of quality improvement of acidic soils for good crop production.