Desorption and plant availability of myo-inositol hexaphosphate adsorbed on goethite

The specific adsorption and high affinity of inorganic phosphate for Fe oxides limit its desorption and, hence, its availability to plants. Organic P compounds such as inositol phosphate show even greater affinity for soil oxides. This may result in low desorption and, hence, poor bioavailabilty, explaining the accumulation of these compounds in soils, although inositol phosphate desorption from Fe oxides has never been quantified. Desorption is controlled by several parameters such as soil pH, percentage of P saturation, ligand competition, and the characteristics of the P-containing compounds. In this work the desorption of myo-inositol hexaphosphate (IHP) and inorganic phosphate (Pi) from goethite (Gt) was studied while assessing the effect of pH, citrate and bicarbonate extractability and the number of desorption cycles. P availability to Lolium perenne L. was also studied in the presence of IHP or Pi added in solution or sorbed on goethite. The amount of desorbed P, as IHP, increased with pH and the number of desorption cycles, but in all cases it did not exceed one-fifth of the desorbed Pi, suggesting a limited availability of IHP when adsorbed on goethite, especially at low pH. Citrate and bicarbonate solutions extracted a small amount of IHP with respect to Pi, probably the result of the inability of their mechanisms to detach the four P groups involved in the IHP bonding with the goethite surface and of the high negative charge of the IHP-Gt complex that hampers the approach of ligands to the surface.The ryegrass that received P as IHP showed limited growth, signs of P deficiency, and a low shoot/root ratio as a consequence of the poor nutrient availability, particularly when IHP was supplied in solution and Gt was present in the pots. In contrast, significantly larger biomass productions were always obtained when P was supplied in inorganic form, and no significant differences among the tests were observed.