Clay minerals and phosphate sorption in soils

Solubilization of soil phosphate accumulated during the last decades in fertilized soils can be profitably used to decrease phosphate (PO4) fertilization while maintaining crop production. In this respect, it is important to clearly identify and rank the different soil constituents regarding their importance for the control of PO4 sorption. However, the relative importance of clay minerals versus Fe oxide-hydroxides is still controversial. Indeed, some researchers only considered the influence of Fe oxide-hydroxides, while others also included clay minerals in their PO4 sorption models. Through this communication our main objective is to present a set of evidences issued from the literature and also based on original data which support the significant influence of clay minerals on PO4 sorption in soils. The first evidence is based on the adsorption capacity of minerals and on the mineralogical composition of soils. Second, we showed that the adsorption properties of clay minerals versus pH do not resemble to that of Fe oxide-hydroxides, thus precluding the consideration of Fe oxide-hydroxides as a surrogate for clay minerals. Last, preliminary results of synchrotron-based X-ray absorption spectroscopy at the phosphorus K-edge showed that PO4 binding to clay minerals substantially involves structural Al-O sites and not only Fe-O sites. To conclude, both Fe oxide-hydroxides and clay minerals should be considered in PO4 retention modelling in soils as they considerably influence sorption processes