Fungicides, herbicides, and insecticides are often used to protect crops in conventional agriculture. In contrast, organic farming avoids adding synthetic substances and relies on a healthy soil ecosystem. However, some organic farms are operating on land that has been treated with pesticides in the past. It is unclear whether pesticides have a long-lasting presence in organically managed fields and what effects they have on microbes and beneficial soil fungi in the long term.

Thomas D. Bucheli, Florian Walder, Agroscope, Zurich, Switzerland, Marcel G. A. van der Heijden, Agroscope and University of Zurich, Switzerland, and colleagues have examined pesticide levels and their impact on soil health on farms using either conventional or organic agriculture, including farms that had been converted to organic methods. The researchers measured the concentrations of 46 regularly used pesticides and their breakdown products in samples taken from 100 fields that were part of either conventional or organic farms. They extracted the pesticide residues from the soil samples and analyzed the extracts using high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS).

Surprisingly, the team found pesticide residues at all of the sites, including at farms that had been converted to organic agriculture more than 20 years ago. Thus, either pesticides persist in soil far longer than expected, or the organic fields have been contaminated via an indirect pathway through air, water, or soil particles from nearby conventional fields. Still, the number of different pesticides and their concentrations decreased significantly the longer the fields were used for organic farming. The researchers also observed lower microbial abundance and decreased levels of beneficial plant symbionts in fields with higher pesticide levels, suggesting that pesticide residues can decrease soil health.

• Widespread Occurrence of Pesticides in Organically Managed Agricultural Soils—the Ghost of a Conventional Agricultural Past?,
  Judith Riedo, Felix E. Wettstein, Andrea Rösch, Chantal Herzog, Samiran Banerjee, Lucie Büchi, Raphaël Charles, Daniel Wächter, Fabrice Martin-Laurent, Thomas D. Bucheli, Florian Walder, Marcel G. A. van der Heijden,
  Environ. Sci. Technol. 2021.
  https://doi.org/10.1021/acs.est.0c06405