Contamination of soil with fluoride by long-term application of superphosphates to pastures and risk to grazing animals

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Fluoride (F) is an impurity in phosphatic fertilisers and significant amounts of F will be added to soils with long-term application of phosphatic fertiliser to soil. There is a risk that F concentrations in soil may increase to levels toxic to plants or grazing animals. If F added to soil remains in the upper soil profile, due to high reactivity with soil, grazing animals could increase their intake of F through inadvertent ingestion of F-enriched soil. This study determined the distribution of F in the soil profile of medium (>8 years) and long-term permanent pastures (>50 years) where F had been applied as an impurity in phosphatic fertiliser, and assessed the potential bioavailability of fertiliser F in soil. Long-term application of superphosphate was linked to increased concentrations of the more soluble fraction of F in soil. The more soluble F fraction in the soil surface led to increased F concentrations in herbage (from 11 to 22 mg F/kg). Fluoride concentrations in herbage were not high enough to be toxic to plants or grazing animals. Fluoride applied as an impurity in phosphatic fertiliser was generally found to accumulate in the upper soil profile (0–6 cm depth). Such increases could also increase F ingestion by ruminants through incidental soil ingestion while grazing. The risk of fluorosis to grazing animals is low at present, but with continued application of fertiliser to soil containing high concentrations of F, and depending on the bioavailability of fertiliser-derived F in soil, we estimate that fluorosis may become an issue in the next 25–50 years in highly fertilised pasture soils.