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Background Even though extensive work has been done on P interactions with free Al-and Fe-hydroxide minerals, limited or no information exists on sorption/desorption processes of organic and inorganic phosphate forms with soil hydroxy-interlayered minerals.Purpose The purpose of this study was to assess the potential of soil clays rich in hydroxy-interlayered vermiculite (HIV) and smectite (HIS) minerals to adsorb and release inorganic and organic P forms as compared to geological reference gibbsite and goethite minerals. Methods Seven-day P adsorption/desorption experiments were conducted involving six soil clays containing >40% HIV, two soil clays containing >55% HIS, and two reference goethite and gibbsite clays equilibrated with P pools consisting of 10 mg P g -1 clay from KH 2 PO 4 or inositol hexakisphosphate. Results The results indicated a strong tendency for HIV and HIS minerals to sorb P at amounts equivalent to or even higher than those sorbed by gibbsite or goethite minerals. Organic P was sorbed at higher quantities and was retained with greater energy than inorganic P forms. Ammonium oxalate-treated samples produced a clearer picture of the net contribution of hydroxy-interlayered minerals to P sorption capacity compared to free Al/Fe hydroxide components, but the degree of Al-hydroxy-interlayering somewhat obscured P-sorption mineral quantity correlations. Increasing Psorption equilibration times generally increased total inorganic and organic-sorbed P loads only of the ammonium oxalate treated samples, suggesting increased P interaction with less accessible interlayer reactive sites. Reaction times had no significant effect on P desorption characteristics, although some samples released less P at longer equilibration times probably due to readsorption processes. Conclusions The findings suggest that soils containing high amounts of hydroxy-interlayered minerals may act as good P sinks and tolerate higher organic P loads than free Al or Fe hydroxides without increasing risks of P contamination.

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