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 drainage) under various rainfall intensities (5, 25, and 50 mm/h). Phosphorus mobility was high in soils with significant macropore structure. However, all soils exhibited some degree of preferential flow of P, including the heavy-textured soils with high P adsorption that were not P saturated. A phosphorus adsorption index based only on the chemical properties of the soil did 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 over 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 storages must aim to increase the residence time of P within soils and thereby allow mineral and organic fractions time to sorb P. Phosphorus loss through wet soils was significantly less than P loss through dry soils with high macroporosity. Application of P fertiliser to soils with high macroporosity may need to be delayed until later in the growing season than is currently practised.