Chemical Fractionation and Sorption of Phosphorus in Greek Inceptisols

Phosphorus fractionation and sorption, both separately and jointly, were studied in two cultivated Inceptisols in Northern Greece. Hedley's extraction was used to separate soil phosphorus (P) into inorganic Pi (geochemical) and organic Po (biological) fractions. Direct extractable P by Olsen (Olsen-P), by Mehlich III (M3-P) and by ammonium oxalate (Pox) was also determined. Phosphorus sorption was carried out with 1:10 soil/solution ratio and sorption parameters were derived from the Langmuir model to the experimental data. Most of the total P occurred in inorganic P forms (74% of Pt), while organic P comprised only 26% of the Pt. Among the various inorganic P forms relatively large amount of residual P (111 mg kg -1 ) was observed, while occluded P in calcium phosphate minerals (d.HCl-Pi) and in Fe, Al-oxides (c.HCl-Pi) existed in equal amounts (83.1 and 83.7 mg kg -1 respectively). The phosphorus sorption parameters showed positive relationships with clay content, cation exchange capacity and the sum of exchangeable calcium plus magnesium. Overall, this study indicated that Ca and Mg compounds strongly influence the P chemistry in moderately weathered soils, with relatively high concentration of primary P minerals.