Mobility of phosphorus through intact soil cores collected from the Adelaide Hills, South Australia

Intact cores were collected from a variety of soils in the Adelaide Hills, South Australia, and tested for phosphorus retention and mobility (P in dra inage) under various rainfall intensities (5, 25, and 50 mm/h). Phosphorus mobility was high in soils with significant macropore structure. However, a ll soils exhibited some degree of preferential flow of P, including the hea vy-textured soils with high P adsorption that were not P saturated. A phosp horus adsorption index based only on the chemical properties of the soil di d not accurately predict the mobility of P through soils with macroporosity . A phosphorus mobility index was developed encompassing both soil chemical and physical parameters. Results showed the sandy soils, and the loams ove r clays with high macroporosity that are located in the more elevated parts of the Adelaide hills, are most susceptible to P leaching. Management to reduce P loss to groundwater, streams, or surface water stora ges must aim to increase the residence time of P within soils and thereby a llow mineral and organic fractions time to sorb P. Phosphorus loss through wet soils was significantly less than P loss through dry soils with high ma croporosity. Application of P fertiliser to soils with high macroporosity m ay need to be delayed until later in the growing season than is currently p ractised.