People in the know about organic farming and food already know that some pesticides used by neighboring conventional farms can contaminate organic fields by traveling through the air, water or soil. But now scientists have discovered another threat to the purity of organic food from organic farms: the ghosts of pesticides past. In other words, chemicals applied to land decades ago can continue to influence the soil’s health even after switching to organic management. Researchers have recently identified pesticide residues at 100 farms, including all the organic fields studied, with beneficial soil microbes’ abundance negatively impacted by their occurrence.
Researchers examined pesticide levels and their impact on soil health on farms managed with conventional versus organic practices, as well as on farms converted to organic methods. The researchers measured surface soil characteristics and the concentrations of 46 regularly used pesticides and their breakdown products in samples taken from 100 fields that were managed with either conventional or organic practices.
Study findings overview
Surprisingly, the researchers found pesticide residues at all of the sites, including organic farms converted more than 20 years prior. Multiple herbicides and one fungicide remained in the surface soil after the conversion to organic practices; though the total number of synthetic chemicals and their concentrations decreased significantly the longer the fields were in organic management.
Additionally, the team observed lower microbial abundance and decreased levels of a beneficial microbe when fields had higher numbers of pesticides in the fields, suggesting that the presence of these substances can decrease soil health.
Journal Reference: 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. Widespread Occurrence of Pesticides in Organically Managed Agricultural Soils—the Ghost of a Conventional Agricultural Past? Environmental Science & Technology, 2021; 55 (5): 2919 DOI: 10.1021/acs.est.0c06405