Changes in bicarbonate extractable phosphorus over time when P fertiliser was withheld or reapplied to pasture soils

Field studies were established on 9 different soil types used for pasture production in the high rainfall zones of southern Victoria. Sites were selected to represent a range of phosphorus (P) buffering capacities (PBC) and were analysed for a series of chemical and physical properties before receiving P fertiliser treatments. A single application of P fertiliser in the form of triple superphosphate (TSP), single superphosphate (SSP), or TSP and lime (5 t/ha) was applied at amounts ranging from 0 to 280 kg P/ha at the start of the experiment, whilst treatments of 35 and 70 kg P/ha were reapplied at 6-monthly intervals. Soils were analysed for bicarbonate-extractable P concentration, using both the Olsen P and Colwell P methods, 6, 12, 18, 24, and 30 months after P fertiliser was applied. A strong positive linear relationship existed at all sites and time periods between the amounts of P applied as a single application and both the Olsen P and Colwell P concentrations. The slopes of these relationships measured the change in extractable P concentration per unit of P applied (ΔEP) and the rate of decline in ΔEP values represented the decline in the effectiveness of the P fertiliser with increasing time from application. The decline in these ΔEP values varied with soil type. The ΔEP values of some low to moderate P buffered soils remained 2-3 times higher compared with the most highly buffered soils, after 30 months. Despite this, the decline in ΔEP values between 6 and 30 months was difficult to predict using a single soil property. Multiple linear regressions involving a measure of PBC and either organic carbon or exchangeable hydrogen were useful methods of predicting the decline in ΔEP values across the 9 field sites examined in this study. In general, the reapplication of P fertiliser every 6 months resulted in significantly higher extractable P concentrations compared with the same rates applied in a single application, across all sites and rates of P applied. The application of SSP, TSP, and TSP and lime had little impact on ΔEP values 18-30 months after treatments were applied.