Interactive Redox–pH Effects on Iron and Manganese Solubility in Soils of the Farafra Oasis, Egypt

Micronutrient deficiencies, particularly of iron (Fe) and manganese (Mn), are major constraints to crop productivity in calcareous soils of arid and semi-arid regions. The Farafra Oasis, Egypt, is characterized by coarse textures, moderate CaCO₃ content, low organic matter, and alkaline pH—conditions that limit Fe and Mn solubility despite large total reserves. This study aimed to elucidate the interactive effects of redox potential (Eh), pH, carbonate buffering, and soil physicochemical properties on the solubility and bioavailability of Fe and Mn under field and laboratory conditions. Seven representative soils, ranging from sand to clay, were characterized using XRD, XRF, and detailed chemical analyses. Seasonal field monitoring under sorghum cultivation assessed Eh, pH, moisture, O₂, CO₂, and DTPA-extractable Fe and Mn across irrigation cycles. Laboratory incubations of sandy and clay soils evaluated redox and pH buffering capacities under aerobic and anaerobic conditions. Results revealed that total Fe (14,800–20,200 mg kg-¹) and Mn (280–820 mg kg-¹) were poor predictors of plant availability, as DTPA-extractable fractions consistently remained below 0.03% of the total. Soil 1 (sand) exhibited the lowest DTPA-Fe (4.5 mg kg-¹) and Mn (3.0 mg kg-¹), whereas Soil 7 (clay, OM-rich) recorded the highest (6.5 and 6.2 mg kg-¹, respectively). Extractable Fe and Mn correlated negatively with pH, EC, and CaCO₃, and positively with OM, CEC, and moisture/ CO₂ flux (R² = 0.93 for Fe; R² = 0.91 for Mn). Laboratory incubations confirmed that redox regime exerted stronger control on Eh than pH. These findings demonstrate that Fe and Mn availability depends not on total content but on dynamic interactions among alkalinity, salinity, redox buffering, and organic matter. Integrated strategies—organic amendments, Fe-EDDHA fertilization, foliar Mn supplementation, and optimized irrigation—are recommended to sustain crop productivity in reclaimed desert soils.