Threshold Effects on Zinc Biofortification: A Cause of the Failure of Conventional Agronomic Interventions on Alkaline Soils

Zinc deficiency affects two billion people globally, with biofortification strategies demonstrating marked heterogeneity across alkaline soils that cover twenty-three percent of agricultural land, where restricted zinc availability severely limits intervention effectiveness. This field-based factorial experiment on maize (Zea mays L.) was conducted over two consecutive growing seasons (2022-2023 and 2023-2024) in Kelurahan Soe and Kuatae Village, Kota Soe District, South Central Timor Regency, East Nusa Tenggara Province, Indonesia, to identify critical pH thresholds governing zinc biofortification efficacy and characterize soil factors modulating bioavailability through comprehensive quantitative analysis. A split-plot factorial experimental design examined pH gradients from 6.5 to 9.5, comparing soil-applied inorganic sources (ZnSCu), chelated formulations (Zn-EDTA, Zn-DTPA), and foliar applications across multiple growing seasons. Segmented regression modeling integrated soil parameters, including available zinc, carbonate content, and cation exchange capacity, to identify threshold effects. Analysis revealed a critical threshold at pH 7.8 (95% CI: 7.6-8.0) beyond which biofortification effectiveness declined precipitously. Available soil zinc decreased 93.7% across the pH gradient, with zinc sulfate applications showing 89.3% reduced efficiency. Chelated sources (Zn-EDTA, Zn-DTPA) maintained moderate effectiveness through pH 8.0, while foliar applications achieved superior performance above pH 8.5, demonstrating 263.6% relative efficiency at pH 9.5 through enhanced remobilization mechanisms. The exponential relationship between alkalinity and intervention outcomes (R2 = 0.96) was quantified across representative field conditions. This pH-based framework enables targeted strategy selection, optimizing resource allocation for biofortification programs operating in alkaline soil regions. Future research should prioritize multi-location validation trials and investigate nanotechnology-enhanced formulations for severely alkaline environments.