NOTE: This post is relevant for both farmers as well as home gardeners. There are 3 sections to this post:
1. Article from MU on contaminated compost; 2. Green bean test for contaminated compost; 3. US Composting Council response and action on your part if you have purchased contaminated compost.
Contaminated Compost
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, said
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
said.
Fixing
contaminated soil isn’t easy. Trinklein said 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 said. 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.”
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.
US Composting Council Response
The US Composting Council is continuing to pressure EPA to
force the chemical companies to reduce the persistence and take steps to reduce
nontarget impacts, and articles like this help.
(by Debbie Johnson, MU Senior Writer)
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