Jane Says: Don’t Worry About Pesticide Residues in Compost
“Is it OK to compost trimmings from produce that isn’t organic, or will the pesticide residues build up in the compost and then in the plants I’ll grow in it next spring?”
What a great question! For the uninitiated, compost is a rich, dark, crumbly mixture of decomposed leaves, food scraps, and other organic matter (i.e., carbon-based, or derived from a living organism). Compost is much, much more than the sum of its parts. Thanks to fungi, worms, various invertebrates such as centipedes and beetles, and billions of beneficial microorganisms, that mix of nitrogen-rich and carbon-rich bits and pieces you toss on the compost pile—what those in the know call “feedstock”—is transformed into something else entirely. (The University of Illinois Extension does a nice job of, ahem, breaking down the process.) Many gardeners like to call compost “black gold,” and for good reason.
In addition to improving the structure and health of soil and adding nutrients to plants, composting cuts down on food waste at home. If you’re concerned about pesticide residues (and who isn’t?), you can find out what fruits and vegetables contain the most and least from the annual Shopper’s Guide published by the Environmental Working Group. These compounds can be detected at very low doses, and these days, residues in parts per million, billion, and even trillion concentrations can be accurately measured. To help put that into perspective, one part per billion, for example, is the equivalent of one drop of impurity in 500 barrels of water, according to the Extension Toxicology Network.
It is true that trimmings and scraps from conventionally grown produce will contain pesticide residues, but it is still OK to compost them. A literature review of more than 100 studies on pesticide biodegradation published in the journal Compost Science & Utilization in 2000 concluded in part that none of the composts analyzed in the cited studies exceeded concentrations thought to affect human health or be phytotoxic to sensitive plants.
I reached out to John Reganold, regents professor of soil science and agroecology at Washington State University and founder of the country’s first organic agriculture major and the nation’s largest certified organic teaching farm. “The heat and microbial action in most compost piles break down many produce pesticides,” he wrote back in an email. “The pesticides remaining in the finished compost are often diluted to very low levels once mixed with soil. There can be absorption of some remaining pesticides by plants, but concentrations in the plant should be extremely low and not an issue for concern. Finally, getting the produce trimmings composted and back in the soil promotes healthy soil.”
It’s also worth remembering that the terms conventional and organic are not as cut and dried as they may seem. Many conventional family farms practice integrated pest management and use synthetic pesticides selectively and judiciously. A great deal of USDA-certified organic produce comes from large corporate farms, and whether large- or small-scale, there are some organic farmers who rely heavily on natural pesticides that are not necessarily what the average consumer may think of as “safe.” The botanical insecticide rotenone, for instance, can be lethal because it attacks the mitochondria in living cells; the good news is that it breaks down in one to three days in soil and water, and in two to six days in sunlight.
That’s unfortunately not the case with all pesticides. Sometimes, “the breakdown by microbes is incomplete, and the byproduct or metabolite is more hazardous than the original compound,” writes Sally Brown, a soil-contamination authority at the University of Washington, in Soils Matter, Get the Scoop! published by the Soil Science Society of America. “That was the case with DDT, where the breakdown product DDE was worse than the DDT,” she continues. “But it’s really important to remember that the formulations of these compounds have gotten more sophisticated, and they are used at much lower quantities today.”
There are a few select compounds that do not degrade in compost but instead become concentrated. Clopyralid—often used to control weeds on lawns and turf, and sold under the trade names Reclaim, Stinger, Transline, Confront, Lontrel, Curtail, and Millenium Ultra—is one example. You can find out more about the herbicide from sources such as the fact sheet published by Beyond Pesticides and that of Ohio State University, mentioned above. In short, because plants in the Leguminosae, Solanaceae, and Compositae families are very sensitive to clopyralid, you might not want to compost grass clippings from your neighbor’s suspiciously weed-free lawn if you’re planning to use the end result on potatoes, tomatoes, peas, beans, lettuce, peppers, or sunflowers. And if you’ll be buying compost for your garden in the spring, look for one certified by the Organic Materials Review Institute and the U.S. Composting Council.
But generally speaking, the most likely scenario “is that whatever traces of herbicides there may be in your compost are at such low concentrations that any impact will be far outweighed by the benefits of the compost,” writes Brown. “What has also been seen for the vast majority of these compounds is that plants do not take them up from soils.”
That may be understating things. Included among the environmental benefits of compost listed on the EPA website is its ability to clean up soil contaminated with “semivolatile and volatile organic compounds (VOCs), including heating fuels, polyaromatic hydrocarbons (PAHs), and explosives.” Compost, the website continues, “has also been shown to bind heavy metals and prevent them from migrating to water resources or being absorbed by plants. The compost process degrades and, in some cases, completely eliminates wood preservatives, pesticides, and both chlorinated and nonchlorinated hydrocarbons in contaminated soils.”
The take-away? Making compost is a smart thing to do on several different levels, and getting the overall balance of nitrogen-rich “greens” and carbon-rich “browns” right is more important than fretting about incorporating food waste that isn’t USDA certified organic. For more about the hows and whys of composting, check out this column from earlier this year.