The Influence of Fertilizers on the Behavior of Fluoride Fractions in the Alkaline Soil

Abstract

It has been established that fluoride bioavailability in the soil is defined by the form which it exists rather than the amount of total fluoride. In the soil, fluoride exist in different fractions such as water soluble (Ws-F), Exchangeable (Ex-F), bound to iron/manganese (Fe/Mn-F), organic matter bound (Or-F), and the residual (Res-F). All of these fractions are bioavailable however to different extents in an order Ws-F>Ex-F>Fe/Mn-F>Or-F>Res-F. Agricultural practices such as fertilizer application alters the behavior of these fractions which further affects fluoride bioavailability in the soil. This study investigated the influence of the three commonly used fertilizers (Di-ammonium Phosphate (DAP), Urea, and cow-manure) on the bioavailability of soil fluoride in an alkaline soil. The soil was mixed with either one of the fertilizers then incubated for a period of five months. All three fertilizers increased the amount of Ws-F by 7.8 ± 0.6, 4.1 ± 0.2, 9.6 ± 1.1 mg/kg and Fe/Mn-F by 2.24 ± 0.3, 0.44 ± 0.2, and 2.1 ± 0.2 mg/kg, for DAP, Urea, and manure amendments, respectively, but had no impact on the amount of Or-F. All three fertilizers were observed to enhance the bioavailability of fluoride in the soil by increasing the amount of Ws-F. The fertilizers could have increased the bioavailability of fluoride in the soil directly or indirectly through alteration of pH and the soil elemental composition. The three fertilizers might not be suitable for use in fluoride contaminated alkaline soils as they accelerate fluoride release and hence bioavailability in the soil.