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Influence of Oxalate and Soil Organic Matter on Sorption and Desorption of Phosphate onto a Spodic Horizon

Bhatti, J. S., Comerford, N. B., Johnston, C. T.
Soil Science Society of America journal 1998 v.62 no.4 pp. 1089-1095
oxalates, clay fraction, spodic horizons, sorption, soil organic matter, nutrient availability, Spodosols, coastal plain soils, aluminum, sorption isotherms, soil fertility, kinetics, phosphates, phosphorus, desorption
Phosphate (PO₄) availability limits the productivity of pine plantations growing on Spodosols of the southeastern USA. Oxalate has been shown to interact with both the sorption and desorption PO₄ onto soil mineral surfaces. In addition, organic matter, a crucial component of many soil surfaces, affects the adsorption of PO₄. We studied the effects of oxalate and organic matter on PO₄ sorption and desorption onto the whole soil and clay-sized fraction of a spodic horizon from a poorly drained Spodosol of the flatwoods region of the lower Coastal Plain of the southeastern USA. Common batch studies and mass balance of OH⁻ production and consumption were used to interpret the processes. Maximum reduction in PO₄ sorption was observed in samples where organic matter and oxalate were present. The molar ratio of OH⁻ ions released to PO₄ sorbed supports the idea of a ligand-exchange mechanism dominating the PO₄ sorption process. Some of the sorption sites appear to be common sites for PO₄, oxalate, and organic matter. Phosphorus desorption from the spodic horizon by the action of oxalate was through ligand exchange of oxalate for PO₄. The presence of soil organic matter increased the amount of PO₄ desorbed by oxalate. Oxalate appeared to form stable soluble complexes with Al in solution, thus inhibiting its reprecipitation. Florida Exp. Stn. Journal Series no. 05371.