RESPONSE OF WINTER-GROWN POTATOES (SOLANUM-TUBEROSUM L) TO APPLIED AND RESIDUAL PHOSPHORUS ON A KARRAKATTA SAND

The response of winter-grown potatoes (Solanum tuberosum L. cv. Delawa re), as determined by yield, to applied (broadcast) phosphorus (P) (0- 480 kg/ha) and to residual P was measured on an acutely P-deficient, n ewly cleared Karrakatta sand in experiments over 2 years. Tuber yield responded significantly (P<0.001) to level of applied P. Phosphorus at 162 kg/ha was necessary for 99% of maximum total yield, which corresp onded to maximum economic yield. For 95% of maximum yield 99 kg/ha was necessary. Phosphorus recovery efficiency by tubers (P uptake by tube rs/P applied, both in kg/ha) decreased from 0.14 at 30 kg P/ha to 0.04 at 480 kg P/ha. Bicarbonate-soluble P (soil test P) extracted from th e top 15 cm of soil was determined on residual P sites in each experim ent to which P was applied (as superphosphate) 9 months earlier at lev els from 0 to 800 kg/ha. These soil test P levels were related (R(2) = 0.91) to total tuber yield. The soil test P level required for 95% of maximum total yield was 33 mu g/g and for 99% was 51 mu g/g. When tub ers were 10 mm long, the total P in petioles of youngest fully expande d leaves which corresponded to 95% of maximum yield was 0.7% (dry weig ht basis), and for 99% was 0.87%. These results, while based on respon ses measured at 2 sites only, provide strong evidence that maximum yie ld of winter-grown potatoes on Karrakatta sands can be achieved with l ower levels of P fertiliser than are currently used in commercial prac tice (125-300 kg P/ha). The results also show that soil testing can be used to improve the P management of potato crops grown on the sandy s oils of the Swan coastal plain.