CONDITIONS CONTROLLING RELATIVE UPTAKE OF POTASSIUM AND RUBIDIUM BY PLANTS FROM SOILS

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 influenci ng 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 add ing precipitated CaCO3 to acid soil unless pH was raised to near neutr ality. 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% w ithout measurably changing soil pH. Experiments varying the pH and Kconcentration of a nutrient solution (20% Hoagland), spiked with 6 uM Rb+, clearly demonstrated that plant uptake of Rb+ and K+ was unaffect ed by acidity in the pH range 3.6-5.0 tested, whereas Rb+ uptake was r educed 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 a vailability of K+ controls Rb+ uptake from acid soils, being probably more important than soil acidity per se. Secondary effects of high soi l 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 sugges ted that leaf analysis of Rb+ may be used as a method to assess early stages of K+ deficiency in plants on acid soils.