Effect of Soil Redox and pH on Nutrient Availability in Paddy Soils.

A microcosm experiment was conducted to determine the influence of soil redox and pH on the potential metal and nutrient available in lowland rice soils.  Soil suspensions, under controlled and uncontrolled pH environment, were incubated alternately from oxidizing to reducing and from reducing to oxidizing conditions. The water-soluble and ammonium acetate (NH₄OAc)-extractable concentrations of Zn, Fe, Mn, P and Si were analyzed. In controlled pH condition, the highest concentrations of water-soluble and NH₄OAc-extractable Zn, Fe and Mn were observed in soil suspension at pH 4.0, whereas the highest concentrations of water-soluble P and Si were recorded at pH 7.0.  In uncontrolled pH experiment, the concentrations of water-soluble and NH₄OAc-extractable Fe and Si in both soils varied, whereas the water-soluble and NH₄OAc-extractable concentrations of Zn, Mn and P in both soils remained constant.  The concentrations of Fe and Mn were greatly influenced by soil redox potential. The results suggested that availability of plant nutrients in lowland rice soils was associated with the oxidation-reduction and reduction-oxidation cycles.