Characterization and evaluation of chabazite- and mordenite-rich tuffs, and their mixtures as soil amendments and slow release fertilizers

In the present study, zeolite tuffs of diverse chemistry, mineralogy and origin were characterized and tested for their efficiency as slow release fertilizers (SRF) through a series of leaching tests. Mixtures of zeolite tuffs were created and tested as loamy soil additives, in order to evaluate any synergistic effect on their cation exchange capacity (CEC) and SRF properties. The studied materials were a chabazite-rich tuff (SOR), a mordenite-rich tuff (KIM), 3 mixtures of them (SOR/KIM 30/70, 50/50 and 70/30) and a commercial clinoptilolite-rich tuff (BLG). CEC (sodium acetate method) was 95.7 for BLG, 100.9 for KIM and 92.2 cmol kg(-1) for SOR, while their mixtures displayed intermediate values. SRF values vary, with the SOR/KIM 30/70 exhibiting the slowest NH4+ release in 29 days (1.7%) compared to the control sample (19%). Zeolites promoted PO4-3 release, while their selectivity for Mg2+ and K+ was clearly affected by their different structure and chemistry. BLG exhibited the strongest selectivity for Mg2+, having the lowest release (39%). The results demonstrate that mixtures of zeolite tuffs could have a potential use in agricultural processes, by improving parameters like: chemistry, mineralogy, CEC and SRF efficiency, and resulting in the production of a competitive and functional soil additive.