Effect of nitrogen and potassium fertilization on the magnesium content in vineyard soil, and in the leaves and berries of Bianca and Sibera grapevine cultivars

During a three-year study, an impact of different levels of nitrogen and potassium fertilization was investigated on the magnesium content in vineyard soil and in grapevine plants grown in a cool climate condition. Three levels of K: 0, 50 and 100 kg K ha(-1) and three levels of N: 0, 50, and 100 kg N ha(-1) were applied. Two white grape cultivars were used as model plants: cv. Bianca and cv. Sibera (Vitis vinifera). Apart from soil analyses, plant tissues (leaf blades, leaf petioles and grapes) were assessed each year for Mg using the ICP-OES technique. Potassium concentrations in the soil solution as well as EC and the concentration of sulphates increased proportionally to the K fertilizer doses in every year of the study. Reverse relationships appeared for Mg and Ca. The soil Mg levels, despite significant reserves of Mg in the topsoils, were slowly depleted due to the absence of fertilizer Mg inputs and progressive soil acidification. A high level of K fertilization decreased the Mg concentration in soil, especially in the 3rd year of the trial. Seasonal variety in the Mg content of petioles and blades was found. The leaf Mg concentrations ranged from 0.23% to 0.27% in blades and from 0.29% to 0.39% in petioles for cv. Sibera and cv. Bianca, respectively. The cultivar Bianca had higher Mg concentrations than cv. Sibera in both analyzed leaf parts. Furthermore, petioles were characterized by a higher Mg concentration than blades. The mean K: Mg ratio was 6.7 in cv. Bianca leaves and 9.6 in cv. Sibera leaves. The cultivar Bianca was characterized by a low concentration of magnesium in berries compared to berries of the cv. Sibera plants. The Mg concentration in grape must was significantly correlated with the Mg level in leaf blades (r = 0.50) and leaf petioles (r = 0.40) as well as with the available Mg concentration in soil (r = 0.37).