Factors affecting the change in extractable phosphorus following the application of phosphatic fertiliser on pasture soils in southern Victoria

Nine pasture soils from high rainfall zones of southern Victoria were analy sed for a range of chemical and physical properties before receiving a sing le application of P fertiliser in the form of triple superphosphate (TSP), single superphosphate (SSP), or TSP and lime (5 t/ha) at amounts ranging fr om 0 to 280 kg P/ha. Soils were analysed for bicarbonate-extractable P conc entration, using both the Olsen P and Colwell P methods, 6 and 12 months af ter fertiliser application. A strong positive linear relationship existed a t all sites between the amount of P applied and both the Olsen P and Colwel l P concentrations. The slopes of these relationships measured the change i n extractable P concentration (Delta EP) per unit of P applied, whilst the inverse of the Delta EP value indicated the amount of P fertiliser required above maintenance to increase the extractable P concentration by 1 mg/kg. These values ranged from 5 to 15 kg P/ha, depending on soil type. The Delta EP measured by the Olsen (Delta EPOlsen) method was closely related to sel ected soil properties and P sorption measures, whilst the Delta EPColwell v alues were also closely related to selected soil properties and P sorption measures, but only when one particular site, an acidic sand, with a high or ganic carbon content was excluded from the analysis. In general, simple, di rect measures of soil P sorption could allow the estimation of Delta EP val ues on different soil types. The application of P in the form of SSP resulted in a trend for higher Delt a EP values than occurred with TSP. This difference was significant on 3 si tes (P < 0.05), but depended on the method of extraction and the time after fertiliser application. The application of lime significantly (P < 0.001) increased soil pH (H2O and CaCl2) and decreased the concentration of exchan geable Al, 6 months after treatments were applied, but generally had little impact on Delta EP values.