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

Abstract

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.

Pasture Soils Contaminated with Fertilizer-Derived Cadmium and Fluorine: Livestock Effects

“…In soil solutions extracted by 0.01 M CaCl 2 from highly acidic (pH in 0.01 M CaCl 2 , 4.7-4.8) Australian pastoral topsoils (0-100 mm depth), 95%-98% of the F was estimated to be complexed with Al (McLaughlin et al 2001). However, in the subsurface soils (200-300 mm depth) that had pH (in 0.01 M CaCl 2 ), >5.2, F − was the dominant form of F in soil solutions.…”

“…In highly acidic soils with large inputs of F, however, the F concentration of pasture may be high. For example, in an Australian pasture soil of pH (CaCl2) of 4.6, it was reported that the mean pasture herbage F concentration (22 mg F/kg DM) in P-fertilized plots (annually topdressed with 125 kg SSP/ha for 36 yr) was signifi cantly higher than that (11 mg F/kg DM) in the unfertilized plots (McLaughlin et al 2001). Similarly, of 12 soils around aluminium smelters in Norway, in 11 soils having pH 4.1-5.8 and a water soluble F of 1-6 mg/kg, the F concentration in ryegrass shoots was 3-9 mg/kg, but in a highly F-contaminated soil having a pH of 4.9 and water-soluble F concentration of 33 mg/kg the F concentration in ryegrass shoots reached levels of 15-18 mg/kg (Arnesen 1997).…”

Fertilizer‐Borne Trace Element Contaminants in Soils

“…Fluorine is usually present in soils as the fluoride ion (F À ), usually retained on soil mineral surfaces by ligand exchange reactions, or incorporated into soil minerals, for example fluorite. Fluorine in soil solution is either present as the free F À ion or as complexes with Al or H [117,118]. The free F À ion is highly reactive with soil components and is retained strongly by soils [119], thus fertilizer-derived F accumulates at the soil surface [42,118].…”

“…Fluorine in soil solution is either present as the free F À ion or as complexes with Al or H [117,118]. The free F À ion is highly reactive with soil components and is retained strongly by soils [119], thus fertilizer-derived F accumulates at the soil surface [42,118]. The strong retention of F by soil also results in low soil-plant transfer coefficients [120], unless F is complexed by H [121] or Al [122].…”

Effect of low molecular weight organic acids on adsorption and desorption of fluoride on variable charge soils

“…Fluoride is also an impurity in phosphate fertilizers. Long-term application of phosphate fertilizer can lead to the risk that F concentrations in a soil may increase to phytotoxic levels (McLaughlin et al 2001). Fluoride can be adsorbed strongly by variable charge soils (Bower & Hatcher 1967;Shao & Chen 1986;Liu 1988;Yu 1997) and this process decreases the activity and availability of F ) to plants.…”