Relation between Phosphorus and Iron in Florida Phosphatic Soils

Abstract

Abstract Iron phosphate minerals are uncommon in Florida phosphatic soils, even when quantities of extractable P and Fe are appreciable. This study was conducted to determine the forms, associations, and interactions of P and Fe in phosphatic soils (>7 g/kg total P) and incubated suspensions. Five phosphatic and four nonphosphatic soils (for comparison) were analyzed by x‐ray diffraction (XRD), selective dissolution, and differential scanning calorimetry (DSC). Nodules found in phosphatic soils contained no crystalline Fe or Fe‐P minerals detectible by XRD, despite high levels of dithionite‐citrate‐bicarbonate (DCB) extractable Fe and P. A broad XRD peak (21–23° 2θ), probably due to poorly crystalline Fe forms, disappeared after DCB treatment. The DSC analysis indicated that the Fe of the nodules was less heat stable than goethite. Goethite was detected in all nonphosphatic soils analyzed for comparison, but in none of the phosphatic soils studied. Oxalate‐extractable Fe was higher in phosphatic than nonphosphatic soils, though the reverse was true for DCB‐extractable Fe. The relationship between oxalate‐extractable Fe and P was linear ( R 2 = 0.98). Results indicated an association between P and poorly crystalline Fe in phosphatic soils, and suggested that P perturbed the formation of crystalline Fe phases. Incubation studies verified that P can reduce the crystallinity of synthetic Fe phases.