PHOSPHORUS REQUIREMENTS OF WINTER-PLANTED LETTUCE (LACTUCA-SATIVA L) ON A KARRAKATTA SAND AND THE RESIDUAL VALUE OF PHOSPHATE AS DETERMINEDBY SOIL TEST

The phosphorus (P) requirements of crisphead lettuce (Lactuca sativa L . cv. Oxley) was measured over 2 consecutive winter plantings using su perphosphate that was freshly applied and applied 9 months before plan ting, at 0-600 kg/ha on a newly cleared Karrakatta sand of low natural P fertility. There was a significant (P < 0.001) head yield response to level of applied P in both years. Phosphorus uptake by whole plants and plant shoots was related to level of applied P in Mitscherlich re lationships (R(2) = 0.88). Phosphorus recovery efficiency (fertiliser P uptake by shoots/P applied, both in kg/ha) by shoots decreased from 0.16 at 50 to 0.04 at 600 kg applied P/ha. Phosphorus recovery efficie ncy by whole plants (shoots plus roots) decreased from 0.18 at 50 to 0 .05 at 600 kg P/ha. The level of freshly applied P required for either 95 or 99% of maximum relative yield over the 2 years (maximum yield, 86 t/ha) was 276 and 427 kg P/ha (Mitscherlich relationship, R(2) = 0. 95), respectively at less than or equal to 10 mu g/g soil test P (newl y cleared sites). The marketable yield was 82 and 95% of total yield a t 276 and 427 kg P/ha respectively. Bicarbonate-soluble P extracted fr om the top 15 cm of soil was determined on residual P sites over 2 yea rs where P was applied at 0-600 kg/ha. These soil test levels were rel ated to head yield in a Mitscherlich relationship (R(2) = 0.88). The c ritical soil test P values required for either 95 or 99% of maximum re lative yield, over the 2 years, were 80 and 115 mu g/g, respectively. Phosphorus in the wrapper leaf at early heading required for 95 or 99% of maximum yield was 0.59 +/- 0.03 and 0.61 +/- 0.03% (spline regress ion, R(2) = 0.80), respectively. Soil and plant testing could be used to assist in reducing fertiliser costs, improving utilisation of fresh ly- and previously-applied fertiliser P by lettuce and reducing P loss es to water systems on the Swan Coastal Plain in Western Australia.