Plant and Soil 227: 223–233, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 223 Effect of fluoride supply on fluoride concentrations in five pasture species: Levels required to reach phytotoxic or potentially zootoxic concentrations in plant tissue D. P. Stevens 1,4 , M. J. McLaughlin 1 , P. J. Randall 2 & G. Keerthisinghe 3 1 CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia. 2 CSIRO Plant Industry, PO Box 1600, Canberra, ACT 2601, Australia. 3 Present address: Joint FAO/IAEA Division, International Atomic Energy Agency, PO Box 100, A-1400, Vienna, Austria. 4 Corresponding author ∗ Received 7 October 1999. Accepted in revised form 25 July 2000 Key words: fluoride, plant uptake, pasture Abstract Recent findings have highlighted the possibility of increased fluoride (F) concentrations in herbage through F taken up from soil via the plant root. This paper aimed to assess the risk of F concentrations reaching phytotoxic or zootoxic concentrations in pasture plants. Five plant species commonly found in improved pastures in Australia, the sown species subterranean clover (Trifolium subterranean) and cocksfoot (Dactylis glomerata), and weeds barley grass (Hordeum leporinum), scotch thistle (Onopordum acanthium) and sorrel (Rumex acetosella) were grown in complete nutrient solutions with graded levels of added F to determine the effects of F − activity in solution on phytotoxicity and uptake of F by their roots. A model was developed using data from these solution culture experiments and data from the literature. The model assessed uptake of F by plants grown over a range of soil pH values and determined the risk of F taken up through the plant roots reaching phytotoxic concentrations, or concentrations potentially injurious to grazing animals, in the plant shoots. Modelling data suggested that the plants studied would not accumulate phytotoxic concentrations of F in shoots or concentrations of F deleterious to grazing animals through root uptake in neutral pH agricultural soils. The risks from F addition to soils in phosphatic fertilisers leading to reduction in pasture growth or animal health are therefore low. However, in highly F-polluted soil, as the soil becomes more acidic or alkaline, the risk of zootoxic concentrations of F in shoots of plants would increase. Abbreviations: S-UCE – shoot uptake coefficient (mol F kg −1 dried plant shoot/mol F kg −3 of growth solution; assuming 1 dm 3 of growth solution = 1 kg); R-UCE – root uptake coefficient (mol F kg −1 dried plant/mol F kg −3 of growth solution; assuming 1 dm 3 of growth solution = 1 kg) Introduction The effect of fluoride (F) pollution of soils on plants has generally been discounted, primarily because it is well known that F adsorbs strongly to most soils at neutral pH and is, therefore, assumed to be unavailable to plants (Farrah et al., 1987; Larsen and Widdowson, 1971; Wenzel and Blum, 1992). However, as soils become more acidic, F can be released into solution ∗ FAX No: 88303 8565. E-mail: Daryl.Stevens@adl.clw.csiro.au and form complexes with Al. Some AlF complexes may reside on the soil exchange phase and be involved in adsorption of F as this complex, but data suggest that the equilibrium favours increased solubility of F at lower pH (Anderson et al., 1991; Barrow and Ellis, 1986; Farrah et al., 1987; Larsen and Widdowson, 1971; Wenzel and Blum, 1992). In very acid soils (pH < 4.0), the formation of HF could also favour increased solubility of F. As soils become more al- kaline the increased negative charge on soil surfaces desorbs F and could increase the F ion concentration