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

Abstract

Abstract Phosphate (PO 4 ) 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 PO 4 onto soil mineral surfaces. In addition, organic matter, a crucial component of many soil surfaces, affects the adsorption of PO 4 . We studied the effects of oxalate and organic matter on PO 4 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 PO 4 sorption was observed in samples where organic matter and oxalate were present. The molar ratio of OH ‐ ions released to PO 4 sorbed supports the idea of a ligand‐exchange mechanism dominating the PO 4 sorption process. Some of the sorption sites appear to be common sites for PO 4 , oxalate, and organic matter. Phosphorus desorption from the spodic horizon by the action of oxalate was through ligand exchange of oxalate for PO 4 . The presence of soil organic matter increased the amount of PO 4 desorbed by oxalate. Oxalate appeared to form stable soluble complexes with Al in solution, thus inhibiting its reprecipitation.