Phosphate Solubilization from Metal Hydroxide and Phosphate Materials in the Presence of Avail Polymers.

Due to its strong retention in soils, <40% of applied phosphate fertilizers is taken up by crops in one growing season. AVAIL® systems are polymer-based fertilizer additives developed to enhance availability of added phosphate. However, the mechanisms by which these chemicals maintain dissolved phosphate in soils are unclear. This study aimed to evaluate the mechanisms by which different types and levels of AVAIL® polymers solubilize phosphate in adsorbed or solid forms. Five types of polymers with different branching or functional groups were reacted at different levels with aqueous ferrihydrite along with co-added phosphate in a competitive sorption experiment. Separately, the polymers were reacted with Fe or Ca phosphates in dissolution experiments. Dissolved phosphate increased with increasing levels of added polymers, with the original AVAIL dissolving up to 34 to 49% more phosphate compared to the new polymers, except for a new formulation designated as VLS 9101. Decreasing trends of sorbed phosphate with increasing concentrations of sorbed polymers suggest that competition between the phosphate and the polymers for ferrihydrite surfaces are likely controlling the polymer responses. Similarly, dissolved phosphate increased with increasing polymer concentrations reacted with Fe and Ca phosphates. In the Fe phosphate system, the original AVAIL® formulation solubilized up to 36% more phosphate than the control, with less dissolution induced by any of various newer AVAIL® polymers. In the Ca-phosphate system, the newer polymer formulations designated as VLS 9101-01 and VLS 9102-01 dissolved up to 4% and 11% more phosphate than the original AVAIL®. Our results indicate that the new polymers are only more effective than the original AVAIL® for Ca phosphate, while all polymers prevent sorption of phosphate on ferrihydrite mainly via competitive sorption.