Understanding Phosphorus Mobilisation in Surface Run-off from Soils

Managing phosphorus (P) import and export from pastures is a key issue in global agriculture. Knowledge of the amount and mobilisation of nutrients stored in soils can contribute to reducing nutrient runoff to water bodies, optimising soil fertility and improving fertiliser use efficiency. In particular, knowledge gained into the behavior of P can be crucial in developing strategies to prevent the export of nutrients from the soil to nearby water bodies which can lead to the degradation of aquatic environments. In this study, the degree to which P desorption and sorption reactions control the concentration of P in runoff was studied on a well characterised soil from the Camden region of NSW. Twelve runoff trays were constructed and rained upon with solutions of varying P concentrations. Runoff was collected at intervals throughout the runoff events and analysed for reactive P using the molybdenum blue method. Results showed that for the lower rain P concentrations desorption reactions controlled the concentration of P in runoff and vice versa, at the higher rain P levels. As the event continued the soil had less influence and runoff P levels tended to move towards the rain concentration. These results suggest that relatively large P concentrations can be removed from water as it flows over a low P soil. Such insights support the concept that P can be stripped from runoff to prevent its export from the paddock to adjacent water ways and that this can be achieved if farmers maintain a low P buffer zone between their productive pasture and nearby waterways