Fluoride contents in commonly used commercial phosphate fertilizers and their potential risks in China

The application of phosphate fertilizer is an important source of anthropogenic fluoride in agricultural soil. However, relatively few studies have examined the fluoride content of phosphate fertilizers, and that has limited our understanding of the phosphate fertilizer contribution to soil fluoride accumulation and distribution. To examine this problem, we first quantified the total fluoride (TF) and water-soluble fluoride (WF) contents of six of the most commonly used commercial phosphate fertilizers in China (potassium dihydrogen phosphate (MKP), calcium superphosphate (SSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), ternary compound fertilizer (NPK), and water-soluble macroelement fertilizer (WSF)). After calculating the [P₂O₅]/TF ratio for each of those fertilizers, we used those ratios and the average P₂O₅ application per crop of five typical crops grown in China (apples, greenhouse vegetables, wheat, corn, and rice) to estimate the annual fluoride accumulations in their soils after application of each type of phosphate fertilizer. Among the six fertilizer types, SSP, DAP, and NPK had much higher total fluoride and water-soluble fluoride contents than MKP, MAP, and WSF had. During crop production, the risk of fluoride accumulation was lower with MKP, MAP, and WSF (high [P₂O₅]/TF ratios) and higher with SSP, DAP, and NPK (low [P₂O₅]/TF ratios), especially in cash crops (fruit and greenhouse vegetables), which traditionally have unreasonably high P₂O₅ applications. Based on our findings, we proposed steps that should be taken to help effectively mitigate fluoride accumulation in China's agricultural soils.