Inorganic phosphorus forms in some entisols and aridisols of Egypt

Effective soil phosphorus (P) management from both environmental and agronomic point of view requires the knowledge of P forms determining its availability. This study examined the inorganic P forms of representative Egyptian soils and investigated their relationship to other soil properties. In twenty-two samples of Entisols and Aridisols collected from different geological deposits (fluvial, lacustrine, marine, calcareous, sandy and fluvio-sandy) seven P availability indices were determined (Olsen-P, Chang and Jackson-P, Bray1-P, Mehlich1-P, Saunder-P, Soltanpour-P and ammonium oxalate-P). Furthermore, inorganic P forms were sequentially fractionated by sodium bicarbonate (NaHCO3–P), sodium hydroxide plus sodium chloride (NaOH–P), citrate bicarbonate (CB–P), citrate bicarbonate dithionate (CBD–P), HCl (HCl–P) and HNO3–HCl–H2O2 (residual-P).Among the P availability indices determined, Olsen-P concentration was highest in fluvial soils, Chang and Jackson-P in lacustrine and calcareous soils, while Mehlich1-P in marine, fluvio-sandy and sandy soils. Ammonium oxalate-P was not detected in all soils. The dominant P fraction was HCl–P making up 68, 66, 58, 52, and 30% of the fractionated P in fluvial, marine, lacustrine, calcareous, and sandy & fluvio-sandy soils, respectively. Citrate bicarbonate dithionate-P (CBD–P) was not found in all the studied samples. The relative abundance of P fractions was in the order: HCl–P > residual-P > NaOH–P > NaHCO3–P > CB–P > CBD–P. The extracted fractions, except NaHCO3–P, correlated with CBD, ammonium oxalate and total Fe, Al, Mn, and Si as well as with clay content, cation exchange capacity, organic matter content, and active calcium carbonate ratio. Mehlich1-P and Soltanpour-P were negatively correlated to these soil properties.Olsen-P correlated with all soil inorganic P fractions. Bray1-P, Chang & Jackson-P and Saounder-P were only correlated to the NaHCO3–P fraction. Multiple regression analysis indicated that most of the examined availability indices were best predicted by NaHCO3–P. Therefore, the Olsen method was more suitable for extracting available P in these soils.