Figure 4a–c shows the release of ammonium ions, sulfate ions and the molar ratio (SO4 2P:NH4 c ), respectively. Figure 4c shows that the release of ions occurs in two stages, both stages are characterized by different SO4 2P:NH4 c molar ratios. The first stage occurs during the first 8 days of the release experiment. It corresponds to the release of both ammonium and sulfate ions as (NH4)2SO4 that is related to the release of this salt is occluded and trapped in
the pore system of the tuff. The same phenomenon was observed by other researchers. Flanigan (1977) observed in zeolite fertilizers prepared from a mixture of phillipsite and high concentrations of KH2PO4 solution, that the salt occluded in the phillipsite was caused by the replacement of the zeolite water by the salt. Gracia Hernadez and others (1994) working on a phillipsite fertilizer preparation, using a mixture of phillipsite tuff and ammonium sulfate
solution, noted that some of the salt was occluded in the tuff material, and this fraction has a stoichiometric dissolution regime. Figure 4c demonstrates the relatively slow dissolution rate for a readily soluble ammonium sulfate salt. This slow dissolution is related to the zeolite pore system that constitutes a barrier for the release of the salt (Garcia Hernadez and others 1994).