Extractability and plant availability of phosphate from p-goethite complexes

Abstract

Phosphate sorbed on soil iron (Fe) and aluminum (Al) oxides is only slowly and poorly desorbed. Factors such as percentage of phosphorus (P) saturation, time of incubation and temperature can affect P release from the mineral surfaces to the soil solution, and hence P availability to plants, but the kind of relationship involved is not yet clear. In this work the influence of the percentage of P sorption and ageing of goethite-P complexes on P extractability and bioavailability were evaluated. An Al-substituted goethite was brought at percentages of P saturation between 25 and 90% of its maximum P sorption capacity. The ageing of the goethite-P complex was simulated by subjecting P-complexes to series of wet-dry cycles at different temperature. Six extractants (0.01 M CaCl2, Olsen, 5×10− 3 M Na citrate and oxalate, anionic exchange resins and Fe-oxide impregnated filter papers) were used and pot trials with Lolium perenne L. were performed. Exponential correlations were found between extracted P and the percentage of P saturation. The effect of all chemical extractants was more dependent on P saturation than the plant. The influence of P saturation decreased in the order: oxalate>CaCl2>resins>Fe-ox papers>citrate>Olsen>Lolium. At low P saturation the plant resulted to be more efficient in P uptake than the chemical extractants, probably because various mechanisms of extraction are utilized at the same time. The ageing of the phosphated goethite caused a net decrease in the chemical extractability of P only after 8 wet–dry cycles at 70°C. Ageing had no evident effects on the availability to plants.