Response of winter-sown carrots (Daucus carota L.) to rate and timing of phosphorus application on Joel sands

The response of winter-sown carrots to rate and timing of phosphorus application on Joel sands was investigated at 2 commercial vegetable property sites on the Swan Coastal Plain in 1996. Phosphorus was applied at rates from 0 to 160 kg/ha at 2 times of application either 100% broadcast to the soil surface and incorporated before sowing (BS) or 25% broadcast and incorporated before sowing with 75% broadcast to the soil surface (not incorporated) after sowing in 5 equal-sized applications of 15% every 3 weeks, commencing 3 weeks after sowing (BS + AS). There was a significant increase in total yield in response to rate and time of application of phosphorus at both sites. Total yield was 3-10% higher in the BA + AS than in the BS treatment at site 1 and 10-35% higher at site 2. Marketable yield was 21% higher at site 2 in the BS + AS compared with the BS treatment. Total and marketable yields were maximised at 40-80 kg/ha applied P at both sites. The concentration of phosphorus (%DW) in the youngest mature leaves at mid-growth was up to 30-35% higher in the BS + AS than in the BS treatment. Consequently, the percentage of phosphorus corresponding to the rate of applied phosphorus necessary for maximum yield was higher in the BS + AS than in the BS treatment. Thus, critical concentrations of phosphorus in the youngest mature leaves required for maximum yield of carrots should take account of timing of phosphorus application. Exponential regressions best described the relationship of phosphorus uptake (phosphorus concentration x yield) by roots or whole plants to rate and timing of applied phosphorus. Phosphorus uptake by roots and whole plants increased with rate of applied phosphorus and was higher in the BS + AS than in the BS treatment. Recovery efficiency of fertiliser phosphorus by whole plants was 2.0-3.0-fold higher in the BS + AS treatment than in the BS treatment at 40-80 kg P/ha.