CRITICAL POTASSIUM CONCENTRATION AND POTASSIUM CALCIUM PLUS MAGNESIUMRATIO IN POTATO PETIOLES ASSOCIATED WITH MAXIMUM TUBER YIELDS

In order to establish critical potassium (K) concentration levels and K/calcium (Ca) plus magnesium (Mg) [K/Ca+Mg] ratios in potato petioles associated to maximum total tuber yields, an experiment was conducted under Brazilian conditions. Six K levels (0, 60, 120, 240, 480, and 9 60 kg K2O/ha as potassium sulphate) were applied in a randomized compl ete block design experiment with four replications. Baraka potato tube rs were seeded, spaced 0.8 x 0.3 m, following the agronomic cultural p ractices recommended for the crop. After 48 days from plant emergence, plant tissue samples was collected from each plant, the youngest full y expanded leaves (YFEL) and the oldest but not senescent leaves (ONSL ). Petioles from these leaves were analysed to their K, Ca, and Mg con tents. At harvest, total, high grade, and weight tuber yields were inc reased to 733, 719, and 660 g/plant for the 353, 335, and 384 kg K2O/h a levels, respectively. These yields are double the Brazilian potato y ield average. Petiole Ca and Mg concentrations were decreased by the K fertilizer treatments, whereas the opposite occurred with the K/Ca+Mg ratio. Potassium concentrations in the petioles from the YFEL and ONS L increased up to 10.44 and 7.13 g K/100 g petiole dry matter at the 6 72 and 654 kg K2O/ha levels, respectively. The K petiole gradient conc entration was not affected by K fertilization. The K/Ca+Mg ratio for Y FEL associated with the maximum total tuber yield was 7.24, while the K concentrations were 8.91 and 6.16 g K/100 g dry matter in petioles f or the YFEL and ONSL, respectively.