Reduction of nitrogen leaching from nursery potting mixes by Chabazite

Due to coarse textured, fast-draining growing media with poor buffer capacity, nitrogen leaching after heavy rainfall is a major concern in container nurseries. The amendment of growing media with Chabazite – a natural zeolite with high ammonium (NH₄⁺) exchange capacity – in combination with NH₄⁺ fertilization might reduce nitrogen losses significantly. This was tested in a pot trial with Photinia fraseri. Growing media amended with Chabazite were compared with a pumice containing growing medium as control. Chabazite was used on the one hand untreated and on the other hand it was charged with ammonium sulfate ((NH₄)₂SO₄) with loads of 1 and 5 g ammonium nitrogen (NH₄-N) pot^‑1_. During the first part of the trial, pots with untreated Chabazite were repeatedly fertilized with NH₄⁺ and leached after each fertilizer application. Pots with charged Chabazite were also leached on the same dates. In the second part, pots with untreated Chabazite received an excessive NH₄⁺ supply and were leached several times afterwards. In both parts of the trial, NH₄⁺ was adsorbed by Chabazite after fertilization. Higher amounts of NH₄⁺ were leached from growing media with pumice than from those with untreated Chabazite. In leachates from growing media with NH₄⁺-charged Chabazite, higher amounts of nitrate (NO₃^-) were only found in the case of charging with 5 g N pot^‑1. Whereas after excessive NH₄⁺ fertilization leaching of NH₄⁺ remained low in presence of Chabazite, summing up to 5% of the applied N, in the control leached NH₄⁺ was 28% of fertilized NH₄⁺. Thus, it can be concluded that fertilized NH₄⁺ was adsorbed to Chabazite and thus protected from direct leaching as well as from nitrification and subsequent leaching as NO₃^-. This protection decreased with increasing NH₄⁺ charging of Chabazite. Hence, the combination of Chabazite with repeated NH₄⁺ fertilization is a promising approach to reduce nitrogen leaching from container nurseries.