Response of onions (Allium cepa L.) to phosphate fertiliser placement and residual phosphorus on a Karrakatta sand

Abstract

The phosphorus (P) requirement of irrigated onions (Allium cepa L. cv. Creamgold) was measured over 2 consecutive spring plantings using superphosphate that was freshly-applied and applied 9 months before planting, at 0–800 kg P/ha on a newly cleared Karrakatta sand of low natural P fertility. The response of onions to placement of phosphate fertiliser (banded or broadcast) was also investigated. There was a significant (P<0.001) bulb yield response to level of applied P in all experiments. There was no significant effect of placement on yield although the concentrations of P in the youngest mature leaves and bulbs were on average 18% higher (i.e. 0.40 v. 0.34%) than in the broadcast treatment. A rectangular hyperbola described the relationship of P uptake by shoots or bulbs to level of applied P. Recovery efficiency (RE) of fertiliser P (P uptake by bulbs at rate i of applied P – uptake in absence of applied P/rate i of applied P) by bulbs after curing decreased from 0.43 at 50 kg P/ha to 0.06 at 600 kg P/ha. Recovery efficiency by bulbs at applied P required for 95 and 99% of maximum yield was 0.20 and 0.14 respectively.

The level of freshly-applied P required for 95 and 99% of maximum relative yield over the 2 years (maximum yield, 80–100 t/ha) was 122 and 203 kg P/ha (Mitscherlich relationship, R² = 0.82), respectively, at <10 g/g Colwell P soil test (newly cleared sites). The marketable (total – reject) yield was 94% and 92% of total yield at 122 and 203 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 0–800 kg/ha. These soil test levels were related to bulb 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 50 and 80 g/g respectively. Phosphorus in the youngest mature leaves required for 95 and 99% of maximum yield ranged from 0.22–0.28 to 0.26–0.32%, respectively, from the Mitscherlich regressions, depending on plant stage (i.e. leaf number or days after sowing) although there was no consistent trend with age. Soil testing can be used to reduce current applications of fertiliser P without reducing yield. Plant testing can be used to monitor the P status and associated fertiliser needs of onions on sands. Both these testing procedures need to be verified in commercial crops with a wide variation in soil test P levels and management practices. Soil and plant testing could therefore be used to reduce fertiliser application and cost, improve fertiliser RE by onions and reduce fertiliser P losses to water systems on the Swan Coastal Plain. Changing placement from broadcasting to banding does not appear to improve the efficiency of phosphate fertiliser use by irrigated onions on Karrakatta sands.