Effect of soil pH and mineralogy on the sorption and desorption of phosphite and phosphate in Ultisols of the Southeastern Coastal Plain

The development of transgenic crop varieties capable of utilizing phosphite (Phi) as a phosphorus (P) source is a promising strategy to increase P use efficiency while decreasing reliance on phosphate (Pi)-based fertilizers. However, little information is available on Phi sorption and desorption in soils. We conducted batch experiments to investigate the sorption of Phi and Pi by three Ultisols from the Coastal Plain of the southeastern United States. At the soils' acidic field pH (pH 4.7–6.2), Pi had a higher affinity than Phi for soil solids, where maximum sorbed concentrations of Pi were an average of 44% greater than those of Phi. The sorption of both P species decreased when experiments were repeated adjusting soil pH to 6.5. More Phi than Pi was recovered during desorption experiments, indicating that Phi was more reversibly sorbed and, therefore, may be more plant-available than Pi. Multiple linear regression between calculated linear distribution coefficients (K_D; 10 mg P L⁻¹ input solutions) and soil properties suggested that amorphous Al- and Fe-oxides controlled Pi sorption. Alternatively, amorphous Al-oxides and gibbsite controlled Phi sorption. Our results show that a lower affinity of Phi than Pi in Ultisols could improve P availability for plant uptake of Phi-based fertilizers but may increase the risk of soil P buildup over time.