Potassium nutrition of irrigated potatoes in South Australia. 3. Effect on specific gravity, size and internal bruising of tubers
The effects were studied of potassium sulfate and potassium chloride applied at rates up to 640 kg/ha K on tuber specific gravity, size and susceptibility to internal bruising. The field experiments from which these data were obtained were conducted during the 1982-83 and 1983-84 seasons throughout the main potato-growing areas of South Australia. For the deficient and marginal site groupings, there was a consistent positive trend in specific gravity as potassium sulfate rates increased up to 640 kg/ha K. There was a slight negative trend when potassium chloride was used. Potassium fertiliser had no consistent effect on specific gravity at sites in the non-responsive grouping. Petiole or tuber potassium concentrations were poorly correlated with specific gravity. In the deficient and marginal groups increasing rates of potassium increased the yield of 80-350, 350-450, 450-680 and greater than 680-g tubers. The percentage increase in yield was greatest for the larger size grades (350-450 g or larger). At sites in the non-responsive group potassium had no effect on tuber size. At 2 out of 9 sites the effect of potassium chloride on tuber size was significantly different from that of potassium sulfate. Potassium chloride at rates of 160 and 320 kg/ha K reduced the susceptibility of tubers to internal bruising at 3 out of 4 sites. Potassium sulfate was effective at 3 out of 6 sites but only at the highest rate tested (640 kg/ ha K). Susceptibility to internal bruising was significantly (P t 0.01) negatively correlated with both petiole and tuber potassium concentrations. Prognostic critical potassium concentrations above which internal bruising was minimised were 8.30% in petioles or 1.84% in tubers. These values were calculated using the Cate-Nelson separation.