Field experiments were conducted over 3 years at 21 sites of varying phosphorus (P) fertiliser histories (Colwell P range: 9–170 g/g) in the Manjimup–Pemberton region of Western Australia to examine the effects of freshly applied (current) and previously applied (residual or soil test ) P on the yield of potatoes (Solanum tuberosum L. cv. Delaware). Phosphorus was placed (banded) at planting, 5 cm either side of and below seed planted at 20 cm depth, at levels up to 800 kg P/ha. Exponential [y = a – b exp (–cx)] regressions were fitted to the relationship between tuber yield and level of applied P at all sites. Weighted (according to the variance) exponential regressions were fitted to the relationship between yield responsiveness (b/a, from the yield versus level of applied P relationship) and Colwell P, and two P sorption indices—phosphate adsorption (P-adsorb) and a modified phosphate retention index (PRI(100)). A weighted exponential regression was also fitted to the relationship between the level of applied P required for 95% of maximum yield (Popt; also from yield versus level of applied P) and P-adsorb and PRI(100). A weighted linear regression best described the relationship between Popt and Colwell P. Phosphorus application significantly (P<0.10; from the regression analysis) increased total tuber yield at all but 4 sites. Marketable tuber yield response paralleled total tuber yield response at all sites and averaged 85% of total yields (range 63–94%). Colwell P gave a good prediction of the likely yield response of potatoes across all sites. For example, the yield responsiveness (b/a) of potatoes in relation to Colwell P decreased exponentially from 1.07 at 0 g/g to 0, or no yield response, at 157 g/g Colwell P (R2 = 0.96) i.e. the critical Colwell P for 95% of maximum yield of potatoes on soils in the Manjimup–Pemberton region. Similarly, no yield response (b/a = 0) would be expected at a P-adsorb of 180 g/g (R2 = 0.69) or a PRI(100) of 46 (R2 = 0.61). The level of applied P required for 95% of maximum yield (Popt) decreased linearly from 124 kg/ha on infertile sites (<5 g/g Colwell P) to 0 kg P/ha at 160 g/g Colwell P (R2 = 0.66). However, a more accurate prediction of Popt was possible using either P-adsorb or PRI(100). For example, Popt increased exponentially from 0 kg/ha at <181 g/g P-adsorb (high P soils) to 153 kg/ha at a P-adsorb of 950 g/g (low P soils) (R2 = 0.75) and exponentially from 0 kg/ha at a PRI(100) of <48 (high P soils) to 147 kg/ha at a PRI(100) of 750 (low P soils) (R2 = 0.80). PRI(100) is preferred as a soil test to predict Popt for potatoes in the Manjimup–Pemberton region because of its superior accuracy to the Colwell test. It is also preferred to P-adsorb because of both superior accuracy and lower cost as it is a simpler and less time consuming procedure — features which are important for adoption by commercial soil testing services. A multiple regression including Colwell P, P-adsorb and PRI(100) only improved the prediction of Popt slightly (R2 = 0.89) over PRI(100) alone. When tubers were 10 mm long, the total P in petioles of youngest fully expanded leaves which corresponded with 95% of maximum yield was 0.41% (dry weight basis). These results show that, while the Colwell soil P test is a useful predictor of the responsiveness of potato yield to applied P across a range of soils in the Manjimup–Pemberton region, consideration of both the soil test P value and the P sorption capacity of the soil, as determined here by PRI(100), is required for accurate predictions of the level of P fertiliser required to achieve maximum yields on individual sites.
The relative effectiveness of broadcasting compared with band-placement of phosphorus (P) fertilisers (0–480 kg P/ha) was compared using potatoes grown on P-deficient sandy soils over 2 seasons in Western Australia (Karrakatta sand in 1993, experiment 1; and Spearwood sand in 1996, experiment 2). The maximum yield of potatoes when P fertiliser was broadcast and incorporated to 20–25 cm before planting (broadcast) was 17 t/ha higher than when P was placed in 2 bands 5 cm to the side of and below seed piece level (banded) in experiment 1, and 13 t/ha higher in experiment 2. However, higher rates of applied P were required to reach 99% of maximum yield on the broadcast compared with the banded plots in both years (i.e. 174 v. 134 kg/ha in experiment 1, and 279 v. 125 kg/ha in experiment 2). Despite the lower levels of applied P required to achieve maximum yield in the banding treatment, banding P fertiliser for potatoes grown on Karrakatta and Spearwood sands would result in significant economic loss. The higher yield in the broadcast treatment corresponded with significantly (P<0.001) higher P concentrations (about 2-fold) in petioles of youngest fully expanded leaves from 56 to 131 days after sowing. When tubers were 10 mm long, the petiole P concentrations corresponding with 95 and 99% of maximum yield were 1.13 and 1.28%, respectively, for the broadcast P treatments in experiment 1, and 0.95 and 1.11% in experiment 2. The reduced yield in the banded treatments was assumed to be due to P fertiliser toxicity in the soil and not P toxicity in the plant tissue. Phosphorus uptake by tubers was significantly (P<0.001) higher (about 2-fold) when P was broadcast rather than banded, especially at high levels of applied P. Phosphorus recovery efficiency by tubers (P uptake by tubers/P applied, both in kg/ha) was higher when P was broadcast rather than banded, particularly at high levels of applied P (e.g. at 480 kg applied P/ha, recovery efficiency was 0.07 in the broadcast treatment compared with 0.03 in the banded treatment). These results show that, for growers to avoid significant economic loss, broadcast applications of P fertilisers should continue to be recommended for potatoes grown on the low P-fixing, sandy soils of the Swan Coastal Plain.
Summary. The response of winter-grown potatoes (Solanum tuberosum L. cv. Delaware), as determined by yield, to applied (broadcast) phosphorus (P) (0–480 kg/ha) and to residual P was measured on an acutely P-deficient, newly 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 corresponded to maximum economic yield. For 95% of maximum yield 99 kg/ha was necessary. Phosphorus recovery efficiency by tubers (P uptake by tubers/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 the top 15 cm of soil was determined on residual P sites in each experiment to which P was applied (as superphosphate) 9 months earlier at levels from 0 to 800 kg/ha. These soil test P levels were related (R2 = 0.91) to total tuber yield. The soil test P level required for 95% of maximum total yield was 33 g/g and for 99% was 51 µg/g. When tubers were 10 mm long, the total P in petioles of youngest fully expanded leaves which corresponded to 95% of maximum yield was 0.7% (dry weight basis), and for 99% was 0.87%. These results, while based on responses measured at 2 sites only, provide strong evidence that maximum yield of winter-grown potatoes on Karrakatta sands can be achieved with lower levels of P fertiliser than are currently used in commercial practice (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 soils of the Swan coastal plain.
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