Total and soluble fluorine concentrations in relation to properties of soils in New Zealand

Soil fluorine (F) concentrations continue to increase in agricultural soils receiving regular applications of phosphatic fertilizer. Continued accumulation of soil F poses a risk to grazing ruminants and may pose a future risk to groundwater quality. This paper examines the range of total F (F-t) concentrations and forms of soluble F species and their relationship to selected soil properties in New Zealand agricultural soils. The F-t and soluble F (soil F extracted with water (F-water) and 0.01 M KCl (F-KCl)) concentrations in 27 soil samples (0-75 mm depth) taken from predominantly pasture sites in the North and South Islands of New Zealand were much less than those reported in the literature for sites contaminated with F from industry. The F-t concentrations ranged from 212 to 617 mu g F g(-1) soil. The F-toxicity risk to grazing animals in farms at these sites through soil ingestion is small at present, but farms with very large F-t concentrations (i.e. > 500 mu g F g(-1)) need to adopt suitable grazing and fertilizer management practices to avoid future F-toxicity risk. The F-t concentration had very strong positive correlations with both total soil P and total soil Cd concentrations, reflecting the link between P fertilizer use and F accumulation in the soils. It also had significant positive correlations with organic matter and amorphous Al oxides contents, indicating that F is strongly bound to Al polymers adsorbed to organic matter and amorphous Al oxides. The F-water and F-KCl concentrations and free F- ion concentrations in water (F-water(-)) and 0.01 M KCl (F-KCl(-)) extracts were generally two and three orders of magnitude, respectively, less than the F-t concentrations and were much less than the concentrations considered phytotoxic. The F-water and F-KCl concentrations were positively related to soil organic matter content and negatively related to soil pH. Regression models relating F-water and F-KCl concentrations to soil organic matter content and soil pH suggest that F can be very soluble in extremely acidic soils (pH(water) < 4.9) with large organic matter contents and therefore F potentially may contaminate groundwater if these soils are also coarse-textured and the water table is shallow.