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

Authors: Szogi, Ariel; Padilla, Joshua - Josh; Shumaker, Paul

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Phosphate (Pi) is typically applied in agricultural settings to satisfy plant phosphorus (P) requirements. However, a large portion of applied Pi is not available to plants because it is strongly bound by soils. Phosphite (Phi) is an alternative form of P that may be more plant available than Pi. Using Phi as a P fertilizer is especially promising due to the introduction of genetically modified plants that can metabolize Phi. However, information on the soil properties that control Phi binding is lacking. We conducted experiments to compare the binding of Pi and Phi by three soils from the Coastal Plain of the southeastern United States. At the soils’ natural acidic pH, we found that an average of 44% more Pi was bound by the soils than Phi. Less Pi and Phi were bound by the soils at a higher (more basic) pH, although soils still tended to bind more Pi than Phi. We then performed additional analyses to determine the most important soil properties for binding both forms of P. For both Pi and Phi, the most important soil properties were the content of aluminum oxides, clay, and kaolinite (a clay mineral). Iron oxides were also important for the soil binding of Phi. Our results demonstrate that Phi is less strongly bound by some Coastal Plain soils, and therefore more plant available than Pi.

Technical Abstract: 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. In this study, 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, 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. The most significant reductions in sorbed concentrations of Pi and Phi occurred for the initially most acidic soils. 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. Correlations between calculated linear distribution coefficients (KD; 10 mg P L-1 input solutions) and selected soil properties suggested that sorption of Pi and Phi was controlled by the soils' contents of amorphous Al oxides, clay percentage, and kaolinite while 'Phi's KD was also significantly correlated with the soils' content of amorphous Fe oxides. Our results demonstrate that Phi has a lower affinity than Pi in Ultisols of the Coastal Plain, indicating that Phi-based fertilizers could improve P availability for plant uptake. However, the lower affinity of Phi also increases its mobility in the soil environment when applying Phi-based fertilizer.