Rubidium is frequently used as a tracer of K + in plants and soils. However, uptake of Rb + from soils by plants, fungi, and even animals is greatly influenced by soil properties. Field biomass concentrations of Rb + usually vary by one-two orders of magnitude, largely according to soil acidity. A growth experiment was conducted using a sedge, Carex pilulifera, transplanted in 50 natural and semi-natural unfertilized soils representing a wide range of chemical properties. It is suggested that pH-dependent reactions regulating the Rb + solubility in soils and an increased plant uptake of Rb + , when K + is deficient, are major conditions accounting for differences in plant uptake of Rb + from contrasting soils. The pool size of exchangeable, indigenous soil Rb + , within the ranges usually encountered, seems to be of less importance.