Soil Zn Availability Decreases with Redox Potential in Anaerobic Stirred Slurries.

Zinc deficiency is worse at low redox potential, such as in flooded rice soils, due to interactions of Zn with redox-active soil components including carbonates, sulfides, and Fe or Mn oxides. Using soils varying in pH, organic matter content, and reactive Fe content, we tested the redox potential at which DTPA-extractable Zn dropped below the critical limit of 1 mg kg^-1 in an anaerobic sturred slurry under nitrogen flow in a 2-L beaker, and then also tested the time required for Zn availability to increase to its original level after the system was oxidized. Simultaneous measurements were made of sulfide in the slurry, of Zn, Mn and Fe in the DTPA extracts, and of methane emitted into the beaker headspace. The effect of added Zn fertilizer was also tested to show its availability under different redox conditons. Zn availability decreased with decreasing redox potential in all soil samples, related in various soils to increases in Fe and Mn in solution. In some soils, Zn availability decreased immediately after the critical level when the redox potential dropped below -200 mV, while in other soils, there was a time lag between redox change and the decrease in Zn availability. The information gathered in this experiment will allow better prediction of the likely response of flooded soils to Zn fertilization and can help develop recommendations for Zn fertilizer and water management for optimum plant nutrition.