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

An understanding of the rate at which fluorine (F) accumulates in soil from phosphorus fertiliser and how much is lost via leaching are important because of the challenge elevated F may pose to soil, plant and animal health. This study measured F accumulation in a soil under pasture (Inceptisol) that received single superphosphate (SSP) fertiliser over 63 years in a long-term fertiliser trial. It also assessed the downward movement of F in soil and effect of irrigation in a long-term irrigation trial. Results showed total F concentrations in topsoil (0-7.5 cm) that received 188 kg SSP ha−1 and 376 kg SSP ha−1 fertiliser increased from 251 mg kg−1 to 349 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, low compared to previous studies. The F concentration in the 376 kg SSP ha−1 treatment decreased with depth, but was significantly higher than the control treatment to 50 cm, suggesting movement of F down the soil. Fluorine concentrations also decreased with depth to 50 cm in soils that received either no irrigation or up to 770 mm yr−1 from irrigation. 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 the period immediately after P fertiliser application in mid-winter and wasn't enhanced by irrigation. This study should be repeated for other soil types to help our understanding of the potential for ongoing F accumulation in soil and implications this may have on future land use.