Effects of soil properties on phosphorus fractions in subtropical soils of Iran

The distribution of inorganic P fractions and their relationships with soil properties was studied in 17 slightly acidic to slightly alkaline soils (pH range 5.37 to 7.61). The soils were selected from agricultural fields of the north of Iran. Inorganic P fractionation included successive extraction with NaOH (NaOH-P), citrate-bicarbonate (CB-P), citrate two times (C1- and C2-P), Citrate-ascorbate (CAs-P), citrate-dithionite-bicarbonate (CBD-P), sodium-acetate buffer (NaOAc-P) and HCl (HCl-P). Results showed that the abundance of P fractions was in the order NaOH-P (35.67 mg kg-1) , NaOAc-P (39 mg kg-1), C2-P (49 mg kg-1), CBD-P (102 mg kg-1), CB-P (136 mg kg-1), CAs-P (156 mg kg-1), C1-P (197 mg kg-1), HCl-P (417 mg kg-1). Among soil properties, pH had almost a linear negative relationship with NaOH-P and a linear positive relationship with HCl-P; in addition, it significantly affected C1-P, CAs-P and NaOAc-P. Oxalate extractable Fe showed a significant positive correlation with NaOH-P and CAs-P, while calcium carbonate equivalent (CCE) had a significant positive correlation with NaOAc-P and HCl-P.