Chemical fractionation of fluorine in soils with a long-term phosphate fertiliser history

Abstract

Elevated concentrations of fluorine (F) in pasture topsoils resulting from phosphorus (P) fertiliser (1–4% F) use are potentially toxic to grazing livestock if F absorption from ingested soil is high. Fluorine absorption by animals and the potential F toxicity is expected to vary depending on the chemical nature of the F in the ingested soil. In this paper we assess the solubility of F in 6 acidic topsoils (0–75 mm depth) under grazed pastures in New Zealand with widely different chemical characteristics, using a sequential soil P fractionation technique involving a cation/anion exchange resin, 0.1 m NaOH, 0.5 m H2SO4, and a residual fraction (17 m NaOH fusion). The results showed that F accumulates mainly in the NaOH-Fi fraction (inorganic F (Fi) adsorbed to Fe and Al oxides) and residual fractions (occluded F and recalcitrant organic F). In contrast, P accumulates mainly in the NaOH-Po (labile organic P fraction) and H2SO4-P (calcium phosphate compounds) fractions, with minor amounts in the NaOH-Pi fraction. The NaOH-Fo (labile organic F) fraction was not determined but the total F concentration calculated by summing all the extracted F fractions, except NaOH-Fo, was approximately equal to the total F concentration determined by NaOH fusion, indicating negligible F was in the NaOH-Fo fraction. Therefore, we suggest that the mechanisms of release of P and F in soils are different. Some of the F found in the residual fraction may have been derived from the previously extracted H2SO4-F fraction which was precipitated by the Al dissolved during H2SO4 extraction. It may therefore have been an artefact of the fractionation method.