Soil reaction is one of the most important factors that affects solubility and availability of nutrients to plants. The aim of the present research was to determine the effect of soil reaction on the K/(Ca+Mg) ratio in the grapevine (Vitis vinifera L.) leaves of the cultivar Sauvignon Blanc (rootstock SO4) in the Plešivica wine growing region (Northwest Croatia). The research was conducted in the period 2009-2011 in the Plešivica wine growing region on very acid to acid soil (location Rečki gaj: pHKCl 3,54-5,01) and on alkaline soil (location Borička: pHKCl 7,01-7,41). Leaves samples for monitoring the dynamics of potassium, calcium and magnesium and their mutual relationship, have been taken each year three times during vegetation (in the following phenological stages: flowering, veraison and harvest). A significant effect on the K/(Ca+Mg) ratio in the grapevine leaves was determined in respect to soil reaction and the phenological stage in all three years of research. The wider ratio of K/(Ca+Mg) in the leaves in all three years was determined on the acidic soil (0.29, 0.58, 0.59) in respect to alkaline soil (0.20, 0.40, 0.38). The widest cation ratio was determined in the stage of flowering (0.31-1.02) while the most limited ratio was determined in the stage of harvest (0.15-0.34) in all research years regardless soil reaction.