Influence of oxalate and soil organic matter on sorption and desorption of phosphate onto a spodic horizon

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