Zeolite has good potential for use in agriculture because its properties positively affect the physical and chemical characteristics of soil, which indirectly benefits plant growth. Among other things, its strong affinity for ammonium (NH4+) is supposed to reduce the possibility of nitrate leaching from soil, since its negative charge can temporarily bind the ammonium form of nitrogen and thus slow down mineralisation. In our pot experiment, designed from April to May 2024 in a greenhouse at the laboratory field of the Biotechnical faculty of the University of Ljubljana, we tested the effectiveness of added zeolite to organic fertilizer on the quantity and quality of lettuce (Lactuca sativa L.) yield. We were also interested in its effect on nitrification and nitrate leaching. The effect of three types of ready-made pelleted organic fertilizer with different proportions of zeolite (0 %, 6 %, 12 %) at three doses was tested. As expected, the addition of porous aluminosilicate did not reduce fertilizer effect, but it also did not improve it. The measured nitrate levels in lettuce were well below the limit value for nitrate in leafy vegetables (4000 mg NO3/kg for green lettuce grown in greenhouses and harvested between April 1 and September 30), while the levels in the substrates did not differ between treatments. The ammonium nitrogen content (NH4-N) of the substrate was higher when fertiliser with 12 % zeolite (BZ12) was used compared to that without (B). The results showed that zeolite, added in this proportions, had no effect on slowing nitrification. Furthermore, the type of fertilizer did not have a statistically significant effect on the SPAD value in lettuce.
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