Fluorine in soils under pasture following long-term application of phosphate

An understanding of the relationship between the inputs and losses of fluorine (F) from pastoral soils that have received phosphate (P) fertiliser is important because of the potential threat elevated F may pose to soil, plant and animal health. This study measured F accumulation in a stony soil that has received regular addition of single superphosphate (SSP) fertiliser over a period of 63 years, along with assessing the vertical movement of F in soil and the effect of irrigation. Results showed total F concentrations in topsoil (0 – 7.5 cm) that received 188 kg SSP ha-1 (188PA) and 376 kg SSP ha-1 (376PA) fertiliser increased from 251 mg kg-1 to 349 mg kg-1 and 430 mg kg-1, respectively. The rates of F accumulation were estimated at 1.1 and 2.1 kg ha-1 yr-1 respectively, which are low compared to previous studies. The soil F concentration in the 376PA treatment decreased with depth and was significantly higher than the control treatment to 50 cm, suggesting movement of F down the soil profile. Fluorine concentrations also decreased with depth to 50 cm in soils that received either no irrigation or up to 770 mm per annum. However there was no significant difference in the amount of F with depth between irrigation treatments. We postulate that F was moving predominately in drainage water in this soil in the period after P fertiliser application in mid-winter rather than enhanced by irrigation. This study should be repeated for other soil types and factored into any analysis for the long-term implications of ongoing F accumulation in soil on future landuse.