Vicky Carthew, of Dunedin, asks :-

Is soil watered from a fluoridated supply over the years likely to build up high levels of fluoride, thus affecting fluoride levels in fruit and vegetables grown in it?

Loga Loganathan, a soil scientist at Massey University, responded.

Fluoride can accumulate in soils watered from fluoridated water supplies. The rate of accumulation, however, depends on the fluoride concentration in the water and the rate of irrigation.

New Zealand market gardens and fruit orchards are commonly irrigated from ground water and surface water which have low fluoride concentration. Those home gardeners who are on fluoridated town water supplies use water which has fluoride concentrations of up to 0.5 ppm (mg/litre). Under a very frequent irrigation practice (200 to 400 mm/yr) this will add up to 1 to 2 kg fluoride/ha/year to the soils which is significantly lower than the rate of fluoride addition to soils from other sources such as phosphate fertilisers and other soil amendments (greater than 5 kg fluoride/ha/yr) (single superphosphate fertiliser contains approximately 1.5 per cent fluoride and 9 per cent phosphorus, common rate of fertiliser addition is 20 kg phosphorus/ha/yr).

The topsoil (top 10 cm soil depth, active rooting depth of most plants) fluoride concentration of most New Zealand soils range from 100 to 600 ppm (mg/kg), depending on the fertiliser history and type of soil in the land. The rate of topsoil fluoride concentration increase from irrigation with a fluoridated water supply is less than 1 to 2 ppm per year (calculated assuming that some of the added fluoride moves below the topsoil).

Although the fluoride addition from fluoridated water supply is slowly increasing the soil fluoride concentration, it is unlikely that it will affect the fluoride levels in vegetables and fruits because of two reasons. Firstly, most vegetable and fruit plants do not take up appreciable fluoride from soil because fluoride is very strongly adsorbed to soil particles and therefore plant-availability of soil fluoride is very low. Secondly, the fluoride taken up by the plant roots is not transported to fruits and vegetables to any significant degree. I have not come across any data to show that plants irrigated with fluoridated water in New Zealand caused fluoride accumulation in fruits and vegetables.