DETERMINATION OF THE RESIDUAL VALUE OF PHOSPHATE AND SOIL TEST PHOSPHORUS CALIBRATION FOR CARROTS ON A KARRAKATTA SAND

The residual value of phosphorus (P) applied at 0 to 320 kg P/ha as su perphosphate 9 to 12 months earlier was measured using yields of carro ts (Daucus carota L. cv. Western Red). The effectiveness of the previo usly- applied P was determined relative to the effectiveness of freshl y-applied P on an acutely P-deficient, newly cleared Karrakatta sand i n experiments over three years. Yield response was significantly (P < 0.01) higher to freshly-applied P than residual P at all rates except 320 kg/ha. The relative effectiveness of residual P increased from 46% of freshly-applied P at 20 kg/ha to 95% at 320 kg/ha. Bicarbonate-sol uble P extracted from the soil was determined on the residual P site i n each experiment. These soil test P values were related to yield. The critical soil test values required for 95% or 99% of maximum yield we re 45 and 60 mug/g, respectively. The rate of freshly-applied P requir ed for 95% or 99% of maximum yield decreased from 118 and 183 kg P/ha at less-than-or-equal-to 5 mug/g soil test to 0 at the critical values . High levels of residual P (320 kg P/ha) resulted in high soil test v alues (greater-than-or-equal-to 60 mug/g) and were as effective as fre shly-applied P. Results of this work showed soil testing could be used as part of the P management of carrots on sandy soils. This has twin benefits of reducing fertiliser costs to the growers and reducing the leaching of fertiliser P into waterways thereby reducing eutrophicatio n.