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.
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