Evaluation of Three- And Five-Step Inorganic Phosphorus Chemical Fractionation Procedures Along With Inductively Coupled Plasma Determination for Calcareous Soils

Quantitative phosphorus (P) fractionation is important in understanding soil P chemistry and mobility. Different P fractionation schemes have been used for calcareous soils, but little is documented as to how these procedures are compared. In this study, the redistribution of P due to the additions of NaHCO3 (for labile P) and citrate-bicarbonate (CB) and replicated extractions to a commonly used three-step P fractionation scheme along with the effect of salt washings on inductively coupled plasma (ICP) analysis of P fractionation extracts was investigated. The P chemical fractionation experiment was carried out with surface and subsurface samples of calcareous soils. Spike recovery of P by ICP analysis was near 100% for the NaHCO3, CB, and HCl fractions, whereas the recovery in the NaOH and citrate-bicarbonate-dithionite (CBD) fractions was 81% and 61%, respectively. The loss of P recovery in NaOH-P and CBD-P was due to the high NaCl concentrations in the final extracts and could be described by a highly significant linear equation (y = −12.5x + 99.1; R2 = 0.97, P < 0.001). The addition of the NaHCO3 increased P release in the subsequent NaOH extraction and the increase was dependent on the oxalate-extractable Fe and Al of the soil. The CB addition lowered the CBD-P fraction and the CB-P was modestly correlated with CB-Mn (R2 = 0.56). Replicated extractions caused a relatively large redistribution of P from the residual P fraction to the NaHCO3-P, NaOH-P, CB-P, and CBD-P fractions, suggesting that repeated extractions may be necessary to ensure a complete fractionation of inorganic P. The HCl-P fractions were generally less affected by additions of NaHCO3 and CB extractions or by replicating extractions. These results have important implications to soil P assessment, and caution should be taken to properly interpret P fractions from experiments using differing fractionation procedures.