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Trials conducted on potato fertilization at different rates of nitrogen, phosphorus and potassium have shown that the elements able to influence the marketable tuber yield are nitrogen and phosphorus. The potato dry matter, which reflects other quality aspects such as the specific gravity and the starch content, increases with nitrogen fertilization till 150-200 kg ha-1 of nitrogen; beyond those rates values remain nearly unchanged. Dry matter increases also with the application of phosphorus and at low potassium rates. The objective of the research was to test the effect of different rates of N P K fertilizer on yield and some quality traits of potato. The test was conducted at the Campus of the Agricultural Faculty, Bari University, Italy. It involved the comparison of 6 fertilizing formulae N1 P1 K1, N1 P2 K1, N2 P1 K1, N2 P2 K1, N3 P1 K1, N3 P2 K1, obtained from the factorial combination of three nitrogen levels (N = 100-200-300 kg ha-1) and two phosphorus rates (P2O5 = 50-100 kg ha-1) against an unfertilized control N0P0K0. The dose of potassium was constant for all fertilizing formulae (K2O = 300 kg ha-1). The highest total and marketable yields of tubers per plant have been observed at the two highest fertilizing levels (N3 P1 K1, N3 P2 K1), which are not statistically different so that the best treatments is shown to be N3 P1 K1; the trend was similar for the mean weight of tubers. With the various treatments, no difference was observed in terms of yield of tubers belonging to the two first size classes (< 35mm and 35-55mm); what has increased with the fertilizing levels is the yield of tubers greater than 55 mm. Tuber specific gravity show, as expected, a positive correlation with the dry matter percentage. Both parameters increased shifting from the control to the N2 P2 K1 and decreased at the highest N level, without any difference being observed with the change in the P rate. The highest starch percentage (20.5%) was also observed in the crop fertilized by N2 P2 K1. Yield and yield components increase with the application of nitrogen fertilizer and do not change when applying high rates of phosphorus. The tubers with the highest specific gravity and starch content values are obtained when applying respectively 200 and 50 kg ha-1 of nitrogen and phosphorus.
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