Last summer in California, some organic wheatgrass and the compost it was growing in underwent random pesticide testing. Test results showed residues of bifenthrin. A closer look revealed it wasn’t the wheatgrass that contained the residues, but the compost it was growing in. By August, the California Department of Food and Agriculture announced Nortech Gold Compost, produced by Nortech Waste LLC, was no longer approved for organic use. By October, Wonder Grow Compost, produced by Grover Landscape Services, and Clean City Compost, produced by Feather River Organics, had also lost the certified organic stamp of approval. All three composts contained notable bifenthrin residues, and all three composts were produced from municipal waste (i.e., residential yard trimmings and food waste).

Bifenthrin falls into a class of pesticides known as pyrethroids. Pyrethroids are essentially synthetic versions of pyrethrin, an insecticide naturally produced by some Chrysanthemum species. However, pyrethroids are petroleum-based products designed to be more effective and persistent than their natural counterpart. Bifenthrin is considered highly photostable and, when added to the soil, it forms a strong bond with soil particles. Listed as a restricted-use pesticide, it can only be sold to and used by certified pesticide applicators, largely because it is highly toxic to aquatic organisms, moderately toxic to mammals and a known groundwater and stream sediment pollutant. Although it is used to control pests on over 100 agricultural crops, urbanites in California use it to control structural pests by applying a band of it around foundations. They also use it to control lawn and landscape pests.

If residue levels on organic produce are found to be greater than 5 percent of the EPA’s tolerance for that residue, the produce cannot be sold, labeled or represented as organically produced (NOP Rule Section 205.671). But, what about residue levels in compost? Those measured in the three composts ranged from .1 to .4 ppm, and bifenthrin tolerance levels set by the EPA range from .05 ppm for peanuts and pistachios to .07 ppm for carrots, radishes, turnips and .5 ppm for tomatoes, peppers and eggplants. Does the limit on produce pesticide residues also apply to compost? Enter Section 205.203(c) of the NOP Rule, which states, “The producer must manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil or water by plant nutrients, pathogenic organisms, heavy metals or residues of prohibited substances.” Based on this standard, there does not seem to be any tolerance for pesticide residues in compost.

When the Organic Materials Review Institute caught wind of this issue, it initiated its own investigation and began testing various compost products to assess how widespread the problem was. As I write, the investigation is ongoing and focused on yard waste composts; the goal is to thoroughly assess all yard waste compost brands on the OMRI list. Meanwhile, OMRI turned to the NOP for guidance on tolerance thresholds for compost. The NOP did not foresee the issue of pesticide-laced compost when it drafted its production standards, and so could not answer OMRI’s questions. The NOP is now in the process of addressing it, though, and the proposed approach is to set an unavoidable residual environmental contamination (UREC) level of 5 percent of the lowest EPA tolerance level established for the pesticide detected.

We all know compost varies from farm to farm, although large-scale compost production companies have perfected the art of producing a consistent product. Does that mean, though, that pesticide residue levels are consistent for a particular brand? More likely, residue levels vary from batch to batch because the sources and proportions of ingredients vary from batch to batch. This begs the question: How often will a brand of compost undergo testing for pesticide residues? And, who will be responsible for it, the compost producer or the organic grower?

An alternative to expanded testing is to ban the use of municipal-waste compost on certified organic farms. Many gasp when they hear such a thing and for two good reasons. The first is many mid to large-scale organic farms simply can’t produce enough compost for their needs and must purchase it from elsewhere. A 2001 survey of commercial composters in California reported nearly 4.25 million tons of compost was being produced annually and farmers purchased 47 percent of it. The second reason is massive amounts of compostable materials are being tossed in the dumpster: of the 42 million tons of waste headed for California landfills every year, 30 percent, or 12.6 million tons, is compostable. The state of California recognizes this as unsustainable and wants to change it. The California Integrated Waste Management Board has set the goal of reducing the amount of compostables in the waste stream from 30 percent to 15 percent by 2020. How? Increase the production of and demand for compost.

Perhaps the reason NOP regulations did not address the issue of pesticide residues in compost is because many scientific studies in the 1980s and ’90s demonstrated how the composting process could significantly degrade and inactivate many pesticides. But, when a pesticide is characterized by strong bonds and designed to be persistent, study results are not so positive. In short, each pesticide is unique and its decomposition rate is affected by a number of variables in addition to bond strength, such as how long it has been composted for and type of microbes present. To complicate things further, plants may vary in the extent to which they absorb pesticides. Remember, the organic wheatgrass did not contain any bifenthrin residues, only the compost it was growing in had residues.

California has been singled out in this discussion, but this is not just California’s issue. All growers who purchase compost derived from municipal yard wastes should be aware of it.

The author is a biologist who lives and farms in Vermont’s Northeast Kingdom.