Contaminated compost showing up in Missouri

COLUMBIA, Mo. – Compost contaminated with herbicide is showing up in Missouri.

“The culprit can be one of any three herbicides which have been approved for use on pastures and forage crops,” said David Trinklein, University of Missouri Extension horticulture specialist.

Newer versions of herbicides with active ingredients such as clopyralid, picloram and aminopyralid can pass through the digestive systems of foraging animals and arrive, unchanged, in the manure. If that manure is composted, gardeners could unwittingly introduce these plant-killing compounds into their soil, Trinklein said.

“My first encounter with this was with a gentleman in northwestern Missouri who was using buffalo manure for composting,” Trinklein said. “We were able to demonstrate, on tomato, that there was something in the compost.”

So why not just test compost before selling it? Commercial testing for these chemicals is costly.

“The test is between $300 and $400 per chemical,” Trinklein said. “Say you test for picloram and it’s not found in the sample. You’ll have to spend another $400 to test for another chemical. You would have to keep testing until you find the culprit. Meanwhile, you’ve spent $400 a test.”

It’s not just compost. These herbicides can show up in mulch too.

“Let’s say a farmer put up hay but it got wet. Normally that would be great for mulch,” Trinklein said. “But if that hay had been treated with picloram, avoid using it since there is the potential for the herbicide to leach into the soil and harm plants.”

Once these herbicides have been added to the soil, through compost or mulch, there aren’t many options for correcting the problem. You can remove and replace the soil, but that is expensive, labor-intensive and time-consuming.

You also have the option to wait until soil microbes break down the herbicides naturally, but these new-generation herbicides have lengthy half –lives of months to years.

“These herbicides, ultimately, will break down, but it might take a couple of years, depending on the concentration,” Trinklein said. “We would hope that in most cases we could plant the following year, but one may want to avoid herbicide-sensitive plants, such as tomato.”

The good news is there’s a simple and inexpensive way to test for these compounds. Take green bean seeds and plant them into the compost.

“Green beans are very sensitive to herbicide,” Trinklein said. “If the seedlings come up okay, the compost is good. If they come up twisted and damaged, allow the compost to age until the herbicide is broken down.”

It is important not to panic. “We don’t want to dissuade people from using compost and organic matter,” Trinklein said. “They’re the gardener’s best friend when it comes to building healthy soil. Gardeners should find out whether the compost supplier has used animal manure, and if so, make sure the animals have not been fed forages sprayed with these herbicides.”

In this situation, being informed and proactive is your best defense. Homeowners and green-industry professionals should use the green bean test before they work compost into the soil. Also, talk with your garden supplier. Make sure they’re aware of the problem and discuss the steps they are taking to avoid it.