Iron availability, electrochemical changes and nutrient content of rice as influenced by green manuring in a submerged soil

Abstract

In a greenhouse experiment, the effect of Sesbania aculeata green manure (10 g kg^-1 soil) on soil solution Fe concentration, electrochemical changes and Fe, Mn, Zn, N, P, K and S content of rice was investigated under submerged conditions in a loamy sand soil (an Ustifluvent, pH 8.6). Soil solution samples collected anaerobically under gravity at 14 days interval up to 84 days of submergence were analysed for Eh (redox potential), pH, EC (electrical conductance), HCO₃ ^- and Fe²⁺ concentration. Green manuring under submerged conditions markedly decreased soil solution pH and pE, and increased EC, partial pressure of CO₂ and Fe concentration. Maximum Fe concentration, EC and partial pressure of CO₂ and minimum pE and pH were obtained at 14 days. Despite large variations in pH, pE and Fe concentration, the values of the expression: pE — pFe²⁺ + 3pH were nearly constant and close to the standard values of 17.9, indicating that the Fe(OH)₃-Fe²⁺ system controlled the solubility of Fe²⁺ throughout the period of submergence. However, after the peak of Fe²⁺ concentration in the soil solution, the Fe₃(OH)₈—Fe²⁺ and Fe(OH)₃—Fe₃(OH)₈ systems were predominant in untreated control and green manured soil, respectively. Dry matter yield and Fe, Mn, Zn, N and S content of rice significantly increased with incorporation of green manure.