EFFECTS OF NITROGEN, PHOSPHORUS, AND POTASSIUM ON YIELD, SPECIFIC-GRAVITY, CRISP COLOR, AND TUBER CHEMICAL-COMPOSITION OF POTATO (SOLANUM-TUBEROSUM L) CV KENNEBEC

Five field experiments were conducted over 3 years in the Mt Lofty Ranges (4 sites) and the Upper South East (1 site) to examine the significance of main effects and interactions between nitrogen (N, up to 360 kg N/ha), phosphorus (P, up to 320 kg P/ha), and potassium (K, up to 480 kg K/ha) on total yield, yield of 80-350 g size grade, specific gravity, crisp colour, and chemical composition of tubers of potato cv. Kennebec. There were significant (P<0.05) main effects of increasing rates of applied N on total tuber yield and yield of 80-350 g tubers at 2 of the 5 experimental sites; the effect of applied P was significant at 3 sites, and applied K also at 3 sites. Application of N, P, and K increased total tuber yield by 13-432%, 19-145%, and 19-89%, respectively, at responsive sites. Increases in the yield of 80-350 g tubers were similar to those for total yield. There were significant first-order interactions (N x K, N x P, K x P) at 2 sites. The interactions were (i) response to the application of 1 nutrient limited by suboptimal rates of another; and (ii) yield depression, due to high rates of 1 or more nutrients. Nitrogen and P increased specific gravity at only 1 of the sites where they were applied. Potassium increased specific gravity at 1 site but decreased it at another. There were significant N x K, N x P, and K x P interactions at 2 sites. Vascular ring colour index (CI) and medulla CI were significantly (P<0.05) affected by rates of applied N, P, and K at 1, 3, and 4 experimental sites, respectively. Application of K (up to 480 kg K/ha) decreased vascular ring CI and medulla CI. Application of P increased the colour indices; for example, increasing the rate of P from nil to 160 kg/ha increased mean (+/- s.e.) vascular ring CI from 51 +/- 18 to 117 +/- 17. No interactions between N, P, and K in their effects on medulla CI were significant. For vascular ring CI, significant first-order interactions (K x N, K x P) were observed at 2 sites, at which application of N and P at nil K significantly increased vascular ring CI. At 240 or 480 kg K/ha, there was no change. Significant (P<0.001) curvilinear relationships were found between specific gravity and the concentrations of N, P, and K in the stem-end region of tubers; the coefficients of determination (r(2)) were 0.27, 0.62, and 0.76, respectively. For the combined medulla CI and vascular ring CI data, the corresponding values were 0.19 (P<0.001), 0.29 (P<0.001), and 0.05 (n.s.), respectively.