Substrate Enrichment with Clinoptilolite: Limits and Possibilities for Container-Grown Seedling Propagation

Abstract This study examines the benefits of adding clinoptilolite (zeolite) to forest nursery substrates (growth medium) to optimise cultivation of European beech ( Fagus sylvatica L.) seedlings. Ten growth medium variants were tested, each consisting of a basic peat substrate (nutrient-enriched / non-enriched) with the addition of different quantities of zeolite in different forms (ammonium nitrate and potassium sulphate-enriched / non-enriched). Zeolite had a limiting effect on plant growth, with greater seedling growth in substrates with a low proportion of zeolite (5%), and lower root biomass and root: shoot ratios in substrates with higher proportions. In the latter case, exchangeably-bound divalent base cations (Ca2 + and Mg2+) were enriched in both the sorption complex and aqueous solution due to cation exchange. At the same time, antagonistic K + cations were depleted in leaves, despite high concentrations in the substrate. Higher doses also reduced above-ground height/root collar diameter, with substrate type as main cause, irrespective of zeolite content. These findings confirm a dose-dependent effect of zeolite, with low concentrations (5%) stimulating seedling growth and higher doses (especially 20%) leading to impaired rood development, primarily due to K + deficiency caused by Ca 2+ and Mg 2+ antagonism. By identifying physiological thresholds for zeolite use in silvicultural substrates, we provide important new information for ensuring the success of European beech seedling cultivation for climate-resilient regeneration projects.