Test compost before adding to garden soil

COLUMBIA, Mo. – Many Missouri home gardeners made a horrible discovery this year. The compost they worked into their garden soil is contaminated with persistent herbicides.

David Trinklein, horticulture specialist for University of Missouri Extension, says the contamination is coming from the composted manure and bedding of animals that have grazed on forage sprayed with new-generation herbicides. These new herbicides, called pyralids, are designed to control broadleaf weeds in pastures and to last a long time. They are not broken down in a grazing animal’s digestive tract or in the composting process.

Contaminated compost worked into your garden will contain enough active herbicide to damage sensitive plants. These include many plants prized by home gardeners, such as tomatoes, beans, strawberries, marigolds and some varieties of roses, Trinklein says.

Fixing contaminated soil isn’t easy. Trinklein says it’s better to check for contamination before you buy or apply compost. This includes compost you made yourself using animal manure.

“I think the safest thing to do would be ask for a small sample of compost and do the green bean seed test,” Trinklein said. “That way you’ll know before you buy, and certainly before you apply, whether there’s any chance of contamination.”

The seed test is simple, he says. Grow green bean seeds using the compost. Since green beans are very sensitive to these herbicides, the sprouts will grow twisted and gnarled if the compost is contaminated.

Another option is to avoid compost made with horse manure or bedding, unless it can be verified that horses were fed hay that was not treated with pyralids. Horse manure is more frequently a source of contamination than manure from other common livestock animals.

“This might be due to the fact that most horse owners are particular about what they feed their animals, therefore they insist on hay that is weed-free,” Trinklein said. “It’s easier to obtain weed-free hay if it has been treated with these herbicides.”

What can you do if you unknowingly add contaminated compost to your garden? There aren’t a lot of options, Trinklein says. You can try to remove the contaminated soil and replace it with new topsoil. That’s neither easy nor inexpensive.

“The other thing that you can do would be to apply activated charcoal,” Trinklein said. “The finer the grind of the charcoal, the more effective it’s going to be.”

A moderate amount would be about 7 pounds of activated charcoal per 1,000 square feet, equivalent to a 20-by-50-foot garden, he says. He warns to be ready for sticker shock.

“I found it online in bulk for about $5 per pound plus shipping,” Trinklein said.

Another option is to wait for the herbicides to break down in the soil naturally. This usually occurs over time because of microbial activity. But how long would you need to wait?

“That’s the scary part. We don’t know,” Trinklein said. “We cannot tell people if you only wait weeks, months, years, your soil will be safe. Our learning curve about the problem still is steep.”

Sidebar: Green bean seed bioassay for contaminated compost

1. Use six 6-inch plant pots.

2. In three of the pots, put a 50-50 mixture of compost and potting soil.

3. In the other three pots, put potting soil only.

4. Plant green bean seeds in all six pots and wait.

5. If the green beans in all six pots come up looking normal, then the compost is probably safe.

6. If the beans in the 50-50 mixture come up twisted and gnarled, then the compost is likely contaminated with herbicides.