Dynamics of copper fractions and solubility in a savanna soil under continuous cultivation

Abstract

An understanding of the influence of soil management practices on the distribution, form and solubility of trace metals in soils is required to formulate appropriate guidelines for the applications of organic manures and inorganic fertilizers for sustainable soil fertility and environmental protection. The distribution, forms and solubility of Cu were investigated in a savanna soil under long-term cultivation and varying management practices which included fertilization with NPK, farmyard manure (FYM), NPK+FYM and a control which received neither NPK nor FYM for 50 years. Total Cu varied between 5 and 13 mg kg−1. Long-term application of FYM and FYM+NPK significantly increased DTPA extractable Cu in the surface horizon, and in the subsurface horizon in FYM fertilized fields. Averaged across the sampling depths, cultivation and management history did not affect the concentration of DTPA extractable Cu, water-soluble Cu, organically complexed Cu and residual Cu. But fertilization with FYM and FYM+NPK significantly increased organically complexed Cu in the surface layer as compared to NPK fertilization. Solubility equilibria studies indicated that Cu2+ activities in soil solution approached an apparent equilibrium with cupric ferrite (CuFe2O4), suggesting that the control on Cu2+ solubility was either a CuFe2O4 or a Cu solid with solubility similar to cupric ferrite. The soil management practices involving combined application of inorganic fertilizers and farmyard manure did not seem to pose any risk of inducing Cu deficiency, but sole application of inorganic NPK poses such a risk.