Inorganic phosphorus fractionation of highly calcareous soils of Iran

Highly calcareous soils are abundant in Iran. The calcium carbonate equivalent (CCE) of these soils reach up to 650 g kg(-1). Although phosphorus (P) fertilizer is being widely used in these soils, little information, if any, is available about P status in such soils. The objectives of this study were to 1) determine inorganic P forms in 18 surface soils of southern Iran, 2) study P readsorption during different stages of fractionation schemes, 3) assess the ability of NaOH to extract aluminum (Al)-P, and 4) evaluate the relationships between P availability indices and inorganic P forms. Eighteen soil samples with a wide range of physicochemical properties were selected for this study. Inorganic P forms was determined by sequential extraction with NaHCO3, NH4OAc, NH4F, NaOH, citrate dithionite (CD), and H2SO4, which are referred to as Ca2-P, Ca8-P, Al-P, Fe-P, occluded P (O-P), and Ca10-P. Phosphorus readsorption in different stages was determined by 1 M MgCl2. Furthermore, a fractionation scheme without an NH4F step was used to evaluate the ability of NaOH to extract Al-P. NaHCO3 (Olsen-P) and MgCl2-extractable P (Exch-P) were regarded as P-availability indices. The abundance of different P forms was in the order Ca2-P < Fe-P < Al-P < O-P < Ca8-P < Ca10-P. Ca2-P was highly correlated with Olsen-P and Exch-P. Ca2-P, Olsen-P, and Exch-P showed a relationship with CCE, citrate-bicarbonate-dithionite extractable Fe (Fe(d)), and Al (Al(d)). Phosphorus readsorption appeared to be important only in the Ca8-P step, and the content of readsorbed P was related to Ca8-P, CCE, and clay content of the soils. In the present study, Al-P and Fe-P accounted for 10 and 5% of the sum of the inorganic P fractions, respectively, and Fe-P showed a strong relationship with Fe(o), whereas Al-P showed a significant relationship with oxalate-extractable Al (Al(o)) and Al(d). It was found that one extraction with NaOH is not a good indicator for Fe- and Al-P, and the ability of NaOH to extract Al-P was reduced with increase in Al-P content.