Phosphorus requirements of winter-planted lettuce (Lactuca sativa L.) on a Karrakatta sand and the residual value of phosphate as determined by soil test

Abstract

The phosphorus (P) requirements of crisphead lettuce (Lactuca sativa L. cv. Oxley) was measured over 2 consecutive winter plantings using superphosphate that was freshly applied and applied 9 months before planting, 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 relationships (R2 = 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 efficiency 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, R2 = 0.95), respectively at <10 ¦g/g soil test P (newly cleared sites). The marketable yield was 82 and 95% of total yield at 276 and 427 kg P/ha respectively. Bicarbonate-soluble P extracted from the top 15 cm of soil was determined on residual P sites over 2 years where P was applied at 0-600 kg/ha. These soil test levels were related to head yield in a Mitscherlich relationship (R2 = 0.88). The critical soil test P values required for either 95 or 99% of maximum relative yield, over the 2 years, were 80 and 115 ¦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 regression, R2 = 0.80), respectively. Soil and plant testing could be used to assist in reducing fertiliser costs, improving utilisation of freshly- and previously-applied fertiliser P by lettuce and reducing P losses to water systems on the Swan Coastal Plain in Western Australia.