Potential phosphorus losses in overland flow from pastoral soils receiving long‐term applications of either superphosphate or reactive phosphate rock

Abstract

The forms and concentrations of P in overland flow were measured from intact pastoral soils obtained from the Winchmore long‐term P fertiliser trial. Treatments under evaluation were soils that received either 0, 188, 250 or 376 kg superphosphate ha⁻¹ yr⁻¹, or 175 kg reactive phosphate rock (RPR) ha⁻¹ yr⁻¹. The objective was to determine the magnitude of potential P transfers from soil to water following P fertilisation, and to determine if losses were different following RPR fertilisation compared with superphosphate. Overland flow was induced by the application of artificial rainfall at 15 mm h⁻¹, maintained for 1 h after flow commenced. Concentrations of dissolved reactive P (DRP) and total P (TP) mirrored the long‐term application rates, although prior to a fresh application of P, soils with P applied in RPR form lost more P during an event than soils with the same rate of P applied as superphosphate. After a fresh application of RPR and superphosphate treatments, up to 5.4 mg TP litre⁻¹ was lost in flow from the 376 kg superphosphate ha⁻¹ yr⁻¹ treatment, while P in flow from soils fertilised with RPR were commonly c. 0.11 mg litre⁻¹, but still greater than from the unfertilised control soils (0.02 mg litre⁻¹). Regression analysis indicated that DRP concentrations in flow from the fertilised soils were elevated above that lost before fertiliser application for a period of approximately 60 days. These results support earlier studies that demonstrate the greater risks of incidental P losses from soluble P fertilisers such as superphosphate (up to 60 days), and conversely the potential environmental benefits from RPR fertilisation of soils “at‐risk” of P loss (e.g., where much overland flow occurs such as in very wet soils and near stream channels). However, if good management practice is followed then the difference in P loss between superphosphate and RPR treated soils should be minimal over a period of a year.