Cation-exchange reactions between NH4+ saturated zeolites and solutions containing essential cations occurring as main cations in natural soil solutions (K+, Na+, Ca2+, Mg2+) were studied. Natural clinoptilolite and two synthetic zeolites (PC and phillipsite) prepared by hydrothermal alteration of power station fly ashes were used as zeolites and saturated with NH4+ in a NH4Cl solution (c = 5 mol.l-1). K+ and Ca2+ yielded the best cation-exchange in the case of all three zeolites. The equilibrium isotherms of ion-exchange reactions were measured in concentration limits usual for real soil solutions. The obtained results indicate that the NH4+ ions might be relatively quickly released to the soil solution after the application of NH4+ saturated zeolites to the soil. The exchange occurs preferably with K+ ions (and also with Ca2+ in the case of the PC zeolite) present in the soil solution.
