Species richness in vascular plants was related to the plants’ calcifuge or calcicole behaviour using documentation from forests and open-land vegetation at about one thousand sites in the southern parts of Sweden. It is concluded that vegetation of strongly acid soils (pH-KCl < 4.5) have fewer vascular plant species than comparable vegetation of other soils, whereas there are no consistent differences in species richness between slightly-moderately acid and calcareous sites. Low species richness is particularly related to high concentrations of Al³⁺ and H⁺ ions (either soil solution concentrations or concentrations of exchangeable ions), not to a lack of calcium carbonate. The majority of plant species are able to render the sparingly soluble phosphate, iron and manganese compounds of high-pH soils available, but they are unable to tolerate much Al³⁺ or H⁺. Acidicole (calcifuge) species have developed the power of tolerating Al³⁺ and H⁺, which may be considered a secondary property of plants, but they have lost the power of solubilizing critical mineral nutrients in high-pH soils. The reasons why these ecophysiological properties are inversely related in the current flora are obscure, difficult to account for experimentally and a main ecological problem. In areas with cool-temperate climates the flora was partly or mainly extinguished by the Pleistocene glaciations. Comparatively fewer calcifuge than calcicole species have, since then, had enough time to develop, and the number of calcifuges is lower, in spite of the fact that most natural and seminatural soils of these areas are nowadays acidic.