Patterns of nutrient status and fertiliser practice on soils of the south coast of Western Australia

Declining water quality on the south coast of Western Australia has been associated with increasing input of nutrients, particularly phosphorus (P), from diffuse agricultural sources. A survey of surface (0-10 cm) soil properties (ammonium oxalate extractable iron, bicarbonate extractable P and K, pH, organic carbon and salinity) from 7950 sites mostly on agricultural land, along with information on fertiliser practices, land use, soil type, rainfall and time since agricultural development reveals patterns of nutrient status and fertiliser practice, and improved understanding of potential nutrient export hazards. Significant increases in soil bicarbonate extractable P and K have been brought about by continued traditional application of P based fertilisers. Soil organic carbon decreased after clearing, but then increased as soil fertility increased. There has been little change in soil pH or salinity. A significant interaction between time since agricultural development and ammonium oxalate extractable Fe indicated that the rate of change and absolute value of soil bicarbonate extractable P increased as time and ammonium oxalate extractable Fe increased. Soils with low ammonium oxalate extractable Fe (400 mg kg(-1)) reach a maximum soil bicarbonate P more quickly (\u3c 30 yr) than soils with high ammonium oxalate extractable Fe (\u3e 30 yr). Optimum land management should be based around the reducing P leaching through sandy soils with low ammonium oxalate extractable Fe (low P retention), and reducing runoff and erosion from soils with high ammonium oxalate extractable Fe (high P retention) and a long fertiliser history. More than half of the soils tested contained sufficient bicarbonate extractable P (high P status) to forego P applications for at least one year; however, 90% received P applications independently of the soil test information. Two thirds of the high P status soils were deficient in S, whilst 25% were deficient in K. Correcting these nutrient deficiencies would reduce the rate of increase in soil P and reduce the P loss export hazard. (C) 1998 Elsevier Science B.V