Damage in organically produced potted herbs: effect of compost amendment and nitrogen fertilization

M. Olivera, E. Meinken, D. Lohr
In organic potted herb production, issues such as germination inhibition, growth reduction, and leaf abnormalities like chlorosis and necrosis occur regularly. Although the presence of mineral nitrogen as ammonium in growing media is a consistent factor, direct ammonium toxicity and ammonia formation due to elevated pH levels cannot solely explain observed damage. Previous studies indicated that even extremely high ammonium concentrations did not necessarily cause harm, and damages also occurred at pH levels below 6.5, making ammonia formation unlikely. Furthermore, interactions between composts and organic fertilizers influenced plant damage, with certain combinations exacerbating damages, while others mitigating it. This study aimed to evaluate the effect of compost amendment alongside with different N fertilizers on damage occurrence in a plant response test with Chinese cabbage. The growing media consisted of peat mixed with 30% compost by volume, using two composts – one preventing damage (GWC A) and the other promoting it (GWC B) – either alone or in various mixing ratios. To assess the triggering effect of ammonium on the occurrence of damage, mineral N was added either as calcium nitrate or ammonium sulfate, each alone as well as in various ratios. Additionally, different types of organic fertilizers were included in the study. Weekly plant evaluations over one month revealed neither plant damage nor growth reduction when at least 25% of N fertilization was applied as nitrate, irrespectively of compost mixing ratio. When ammonium-N or organic fertilizers were used, at least 75% GWC A by volume were necessary to effectively prevent damage and to promote nitrification and plant growth; in mixtures with 50% or less GWC A, severity of damage and growth reduction increased sharply. These results propose approaches to understand underlying mechanisms of damage in organic herb production, and thus help to prevent them.
Olivera, M., Meinken, E. and Lohr, D. (2025). Damage in organically produced potted herbs: effect of compost amendment and nitrogen fertilization. Acta Hortic. 1416, 129-136
DOI: 10.17660/ActaHortic.2025.1416.17
https://doi.org/10.17660/ActaHortic.2025.1416.17
compost quality, plant response test, ammonium toxicity, nitrification
English

Acta Horticulturae