Comparison of Isotopic Dilution and Extraction Methods in Assessing Labile Phosphorus in Soils

Abstract

Abstract Soil samples representing field experiment plots of dissimilar P fertilization history were analyzed for inorganic P reserves by the Chang and Jackson fractionation procedure. The lability of these reserves was assessed by an isotopic dilution method (E t value) and by extraction with acid (pH 4.65) NH4OAc solution (PAAc), acid NH4F solution (Bray-P) and with water (Pw) at solution to soil ratios of 60:1, 400:1 and 800:1. The isotopic dilution recovered 33–38% of P assumed to be bound by hydrated Al and Fe oxides, and 17–30% of the total fractionated inorganic P. By the extraction tests 1–64% of the isotopically exchangeable P pool was recovered. The ranking of the soil samples by the extraction tests was different from that obtained by the isotopic dilution method. On the other hand, the accumulation of fertilizer P tended to be more sensitively reflected in PAAc, Bray-P and Pw at 60:1 than in the E t value of the extraction test indices, PAAc was most closely related to the P quantity parameter...