Earlier studies have demonstrated close inverse relationships between Rb+ concentrations in plants and pH or base (including K+) saturation of soils. This study aims at elucidating conditions in soils influencing plant uptake of Rb+. Growth experiments with Carex pilulifera L. were performed, modifying the acidity and K+ supply of acid soils and solutions. We were unable to assess any reduction in Rb+ uptake by adding precipitated CaCO2 to acid soil unless pH was raised to near neutrality. Though not fully compensating the loss of soil solution K+and exchangeable K+ from uptake by the growing plants, soil treated with 0.5 mM K+ (as KCl) reduced the Rb+ concentration in the shoots by 40% without measurably changing soil pH. Experiments varying the pH and K+ concentration of a nutrient solution (20% Hoagland), spiked with 6 uM Rb+, clearly demonstrated that plant uptake of Rb+ and K+ was unaffected by acidity in the pH range 3.6–5.0 tested, whereas Rb+ uptake was reduced by ca. 50%, when K+ concentration was increased from 1.2 to 3.6 mM. The sensitivity of this reaction indicates that shortage or low availability of K+ controls Rb+ uptake from acid soils, being probably more important than soil acidity per se. Secondary effects of high soil acidity, such as leaching losses of K+, might also be of importance in accounting for the high uptake of Rb+ from such soils. It is suggested that leaf analysis of Rb+ may be used as a method to assess early stages of K+ deficiency in plants on acid soils.