Response of spring-planted lettuce (Lactuca sativa L.) to freshly-applied and residual phosphorus and to phosphate fertiliser placement on a Karrakatta sand

Abstract

Summary. The phosphorus (P) requirements of crisphead lettuce (Lactuca sativa L. cv. Salinas) were measured over 2 consecutive spring plantings using superphosphate that was freshly applied and applied 9 months before planting, at 0–600 kg P/ha on a newly-cleared Karrakatta sand of low natural P fertility. The response of lettuce to placement of phosphate fertiliser (banded, broadcast) was also investigated. There was a significant (P<0.001) head yield response to level of applied P in all experiments. There was no significant effect of placement on yield even though the concentrations of P in the wrapper leaves were on average 12% higher in the broadcast treatment. A Mitscherlich function described the relationship of P uptake by whole plants and plant shoots to level of applied P. Recovery efficiency of P fertiliser (fertiliser P uptake by shoots/P applied, both in kg/ha) by shoots decreased from 0.12 at 50 to 0.05 at 600 kg applied P/ha. Phosphorus recovery efficiency by whole plants (shoots plus roots) decreased from 0.13 at 50 to 0.05 at 600 kg P/ha.

The level of freshly-applied P required for 95 and 99% of maximum relative yield over the 2 years (maximum yield, 89–112 t/ha) was 185 and 286 kg P/ha (Mitscherlich relationship, R² = 0.93), respectively, at <10 µg/g soil test P (newly-cleared sites). The marketable yield was 82 and 92% of total yield at 185 and 286 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 = 0.90). The critical soil test P values required for 95 and 99% of maximum relative yield, over the 2 years, were 81 and 121 µg/g respectively. Phosphorus in the wrapper leaf at early heading required for 95 and 99% of maximum yield was 0.40 ± 0.02 and 0.42 ± 0.02% (spline regression, R² = 0.81), respectively. Soil and plant testing could be used to assist in reducing fertiliser costs, improving the efficiency of use of freshly- and previously-applied fertiliser P by lettuce and reducing P losses to water systems on the Swan Coastal Plain in Western Australia. Changing placement from broadcasting to banding will not improve efficiency of phosphate fertiliser use by lettuce on Karrakatta sands.