AbstractRetention of K, Ca and PbyPseudoscleropodium purumgrowing under oak in Windsor Forest was examined following treatment, over 9 days, with 10 mol m−3CaCl2or KH2PO4Concentrations of the added elements and Mg were determined in the intercellular, exchangeable (cell wall) and intracellular fractions of the green tissues before nutrient addition and at intervals (0–205 days) afterwards.Increases of cations in the intercellular fraction of treated shoots were transient. Addition of K and Ca markedly increased levels of these cations in the exchangeable fraction and significantly reduced the concentration of exchangeable Mg. The changes in exchangeable cations were progressively reversed under field conditions indicating that their levels are in dynamic equilibrium with element concentrations in precipitation and/or throughfall rather than the result of a continuousaccumulation process.Net uptake of K and Ca into the protoplasts of treatedP. purumwas slight but substantial net gain (+178%) of P occurred in KH2PO4treated plants. One-third of the additional P was lost within 15 days due, either, to redistribution within the shoot, or, to leakage. Intracellular Mg fell significantly below controls 2 weeks after treatment with CaCl2suggesting that Mg uptake involves prior adsorption onto the cell wall exchange sites. Fluctuations in K, Ca and Mg levels of untreatedP. purumindicated that appreciable net uptake of natural inputs occurred, particularly during autumn when leachates from the senescing tree canopy may be received.Element concentrations, including Mg, were highest at a shaded site where the moss remained moist for long periods suggesting that either ion absorption is favoured by protracted contact times or that metabolic nutrient requirement increases with increasingly mesic conditions.