Organic fertilisers to combat clopyralid contamination

Organic-PLUS partner NORSØK has produced a report describing a pot experiment to study the effects of a liquid organic fertiliser containing clopyralid on plant growth. Tests crops were pea (Pisum sativum L.) and tomato (Solanum lycopersicum L.), and the study was conducted in spring 2020 by NORSØK at its research farm in Tingvoll.

The report describes the effects of using fertilisers (and other inputs such as growing media) containing undesirable compounds (e.g., herbicide residues) on plant growth. Initially, the report gives a brief overview of how clopyralid and aminopyralid affect plant growth, and at which concentration levels these compounds may be harmful to sensitive crops. The full report (in Norwegian only) is available here but the key points are included in this summary.

Liquid fertilisers made from the sugar industry by-product vinasse have been under discussion for several years, e.g., in the US and UK, and more recently also in Scandinavia. Several professional and hobby growers have reported negative effects on plant growth after application of commercial organic liquid fertilisers. Chemical analyses in different countries have detected residues of herbicides in several cases.

 The study presented here was developed in collaboration with the Norwegian Agricultural Extension Service (NLR) and the pesticide laboratory at the Norwegian Institute of Bioeconomy Research (NIBIO). Using peas and tomatoes as test crops, a pot experiment was conducted with a nutrient-poor seedling soil.  To this soil, various concentrations of an organic liquid fertiliser product made from vinasse and containing clopyralid were applied, with liquid mineral fertiliser as a control. The liquid organic fertiliser impacted plant growth significantly. The lowest applied concentration (1 % of liquid fertiliser) resulted in elongated pea plants with a low dry matter content. The tomato plants developed significantly fewer buds and flowers, and the fruits that developed did not contain normally developed seeds. At higher fertiliser concentrations, pea and tomato plants were strongly impacted or died off. The concentration of clopyralid in the fertiliser was 0,48 mg per litre. The study demonstrated a clear correlation between the application of fertiliser containing clopyralid and plant damage in pea and tomato plants.

Clopyralid is an active substance in several herbicide formulations, and is applied in, e.g., sugar beets. Recently, the EU Commission added additional risk assessment criteria to be used in the evaluation and approval of herbicides containing clopyralid. Utilising plant waste for new purposes, e.g., in fertilisers, is good bioeconomy practice. Hence, the decision of the EU Commission is an important step towards reducing problems related to the presence of clopyralid in soil, compost and organic fertilisers. This is crucial for certified organic growers, who are often dependent on commercial fertilisers.

Liquid organic fertilisers are challenging to analyse because they contain a multitude of organic compounds.  NIBIO has developed an analysis to detect aminopyralid and clopyralid with a limit of detection of 2 µg per kg soil or compost, and 7 µg per kg of organic fertilisers. This limit is above the threshold concentration for negative impact on plant growth for several sensitive crops. Analytical methods and bioassays must be developed to ensure that growers who depend on liquid fertiliser products can apply them without any risk of crop damage.

The study was supported by the Horizon 2020 project “Pathways to phase-out contentious inputs from organic agriculture in Europe (Organic-PLUS)”, GA 774340 (2018-2022).

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