"Your #1 source for information about how to compost."
HOW TO COMPOST
What a Good Pile Needs
What You Need
Extras & Additives
What to Use - or Not!
Building the Pile
When is it Finished?
Just Do It
More Than Just Dirt
Water & Soil Remediation
A Few Concerns
What You Can Do
Are Pesticides in Compost?Compost feedstock contains numerous and diverse active microorganisms, all with their own characteristics and capabilities. This diversity means a greater chance that a pesticide will encounter a microbe that can degrade it. - Persistence and Degradation of Pesticides in Composting (PDF format), California Integrated Waste Management Board, 2002.
No organic gardener wants to introduce pesticides, and to have them arrive via compost would seem like the ultimate betrayal. Even if they do somehow get into your pile, many pesticides of all kinds -- herbicides, insecticides, fungicides -- break down into harmless chemicals during the composting process. But some, including several that have become increasingly common in recent years, do not.
How Compost Degrades PesticidesDuring the composting cycle, pesticide levels in the feedstock (the material that went into the pile) are reduced by a variety of processes. Some toxins decay into simpler molecules. Some form bonds with other compounds (adsorption). Some volatilize, or escape into the atmosphere. Some leach from the pile, draining away with liquid run-off. Some undergo humification, becoming part of humus molecules. And some undergo mineralization, which Ohio State University Extension calls the most "desirable fate" for pesticides.
Mineralization, the preferred end for pesticides, refers to the breakdown of organic compounds into their inorganic (or mineral) and organic constituents. The remaining organic constituents that contain carbon breakdown further into a variety of simple molecules that include carbon dioxide and water. The CO2 volatilizes, or evaporates, the water joins the soil solution, and the inorganic, or mineral part of the pesticide molecule takes its place in the soil environment. The result is that the pesticide has been permanently transformed into non-toxic molecules.
When the Ohio State Extension Office says that "The most desirable fate [for pesticides] is complete 'mineralization' or complete biodegradation to CO2," it does not mean that the pesticide turns into carbon dioxide. Instead, the carbon in the pesticide eventually bonds with oxygen to form carbon dioxide which then evaporates.
ClopyralidAfter the environmental disaster that was DDT, chemical engineers developed pesticides that did not persist in the environment as had DDT and other organophosphates. These efforts were so widely successful that it now comes as a shock to encounter a pesticide that, like the earlier generation of chemicals, resists degradation.
This is such a recent phenomenon that a literature review published in 1999 and 2000 concluded that composting degraded most pesticides "moderately well to very well." This review of scientific papers on the fate of pesticides in compost concluded that "Investigations of pesticide residues in composting feedstocks and finished compost detected few of the target pesticides. The compounds that were found occurred at low concentrations."
But that review, so massive that it was published in two parts, was completed in 2000 before the herbicide Clopyralid came into common use. Marketed under a wide variety of names including Reclaim, Stinger, Transline, Confront, Lontrel, Curtail and Millenium Ultra, clopyralid resists breakdown in compost, in soil, and in animal guts. Cattle that eat clopyralid-contaminated feed excrete it unchanged. It is also very mobile in both water and soil, meaning that it can end up in waterways and will follow them to reach areas far from where it was originally applied. In other words, it could end up in your garden and hence in your compost even though you never used it.
Fortunately, clopyralid does not accumulate in animal tissues, which means that it cannot be passed along the food chain as was DDT. In fact, according to the Ohio Extension factsheet, "clopyralid does not affect human or animal health or grasses, corn, berries, tree fruit, or the vast majority of woody and perennial ornamental plants."
However, it is extremely toxic at very low levels to members of the Leguminosa family (pea and bean), the Solonaceae family (potatoes, tomatoes, eggplant), and the Composita family (sunflowers, asters, dandelions, daisies). It became infamous after two instances of crop damage in Washington were traced to clopyralid-contaminated compost.
Several states have now severely restricted the use of clopyralid. Because it persists in the environment for so long, it will be with us for many years, though in increasingly diluted concentrations.
How Pesticides Get Into CompostThe most obvious route by which pesticides get into compost is through the composting of pesticide-treated plants. Over a hundred Canadian cities and towns, as well as several provinces, have passed laws that restrict the use of pesticides making their compost reasonably safe from pesticide contamination.
American composts, particularly municipal composts, are not pesticide-free. While many municipalities throughout North America no longer routinely apply pesticides to public land, private householders throughout the United States are still free to apply synthetic pesticides. Since most municipal composting programs accept material from all comers, this means that much municipal compost may be contaminated.
How much remains in the compost depends largely on how contaminated the feedstock (composted material) was, and by what. The composting process also makes a difference. Clearly, if clopyralid was used extensively in or near your town, municipal compost may contain significant residues. High temperatures, thorough turning, and long composting periods all help to give microbes their best shot at pesticides, thus reducing the amounts that remain in the finished compost.
Manures can also contain pesticide residues if the animals that produced them are fed treated feed.