Searching for an Optimized Potassium Fertilization in Grapevine (Vitis vinifera L.) cv. Tempranillo with an Emphasis on Berry Quality and Nutrient Composition

Potassium (K) is an essential macronutrient that plays a central role in grapevine physiology and fruit quality. This study aimed to evaluate the effects of different K concentrations in the nutrient solution on berry composition, color development, and mineral concentration in leaves, petioles, and berries of fruit-bearing cuttings of Vitis vinifera L. cv. Tempranillo. Fruit-bearing cuttings were grown under controlled greenhouse conditions and irrigated with modified half-strength Hoagland solutions containing 0%, 25%, 50%, 75%, or 100% K and a control nutrient solution. Berry quality parameters, including total soluble solids, acidity, anthocyanins, and phenolic maturity, were significantly influenced by K availability. Moderate K treatments (50–75%) produced the most favorable outcomes in terms of sugar accumulation, anthocyanin concentration, and nutrient balance. The results suggest that optimized K fertilization is critical for improving grape quality while avoiding potential negative effects associated with deficiency or excess. These findings provide valuable insights for K management in viticulture.