Solubilization of Phosphate from Poorly Crystalline Fe and Al Hydroxides By AVAIL® Copolymer.

Less than 40% of fertilizer phosphate applied to soils is taken up by crops because of strong retention of P by soil solids. Our objective was to determine mechanisms by which AVAIL®, a maleic-itaconic copolymer used as a fertilizer additive, potentially affects retention of applied phosphate, and consequently plant availability. We measured competitive sorption of AVAIL® and orthophosphate in aqueous suspensions of ferrihydrite and poorly crystalline Al hydroxide (pxl-Al(OH)₃) at pH 6.2, and characterized phosphate bonding between Fe(III) and Al(III) in 1:1 (w:w) mixtures of these solids using P K-edge XANES spectroscopy. With increasing co-additions of AVAIL or P at the levels evaluated, sorption results showed dissolved P increasing up to 0.45 and 1.25 mmol L^-1 for ferrihydrite and pxl-Al(OH)₃, respectively, which represented 18 and 34% of added P. Negative relationships between sorbed P and sorbed AVAIL® implied a competitive adsorption mechanism between these two ligands, and solubilization of Fe by AVAIL® indicated complexometric dissolution of ferrihydrite. XANES results showed that 72-86% of sorbed P was bonded with Al(III) in the ferrihydrite/ pxl-Al(OH)₃ mixtures, with only a minor (<15%) effect of AVAIL® apparent when P was applied at the two levels tested in this study. Our results can be used to estimate banded application rates of AVAIL® and P fertilizers on soils with different P sorption capacities.