Effect of soil cultivation and winter pugging on fluorine distribution in soil profiles under pasture following long-term applications of phosphate fertilisers

Abstract

In situations where large amounts of soil are ingested, elevated fluorine (F) concentrations in topsoil under pasture may pose a potential F risk to grazing livestock. A study conducted on a Pallic Soil (Aeric Fragiaqualf in US Soil Taxonomy) under pasture in New Zealand showed that 21 years of annual applications of phosphate fertilisers (mostly single superphosphate; mean P rate of 27 kg/ha.year; 1985–2005) significantly increased total soil F concentrations down to 150 mm soil depth but had no effect at 180–210 mm depth. The topsoil (0–30 mm depth) F concentration increased from 140 to approximately 210 mg F/kg. For agricultural soils, these topsoil F concentrations are low and are unlikely to present any risk of chronic F toxicity to livestock at present, but continuous inputs of large amounts of F through P fertiliser applications will increase topsoil F concentrations to levels that may cause F toxicity in the future if accompanied by large amounts of soil ingestion. Cultivation of soil to 150 mm depth using a mouldboard plough once every 6–9 years during pasture renovation significantly increased total soil F concentration at 60–150 mm depth but had no effect on the surface and deeper soil horizons (0–60 and 150–210 mm depths). However, the amount of post-1985 fertiliser-derived F in the 0–60 mm depth as a percentage of the total amount of post-1985 fertiliser-derived F recovered in all depths within 210 mm was less for cultivated soil (31%) than for uncultivated soil (51%), indicating that cultivation can reduce the topsoil F concentration and hence the F risk to grazing animals. Moderate winter pugging of soil caused by dairy cattle had no effect on F distribution in soil profiles. The F distribution in the surface soil horizons (0–30 and 30–60 mm depths) predicted by an empirical computer-based F model corresponded reasonably well with the measured F values.