Organic Answers Column – January 31, 2024 – Contamination

Herbicide Contamination Testing

Planning ahead for the spring garden doesn’t just include buying and starting seeds – if you’re concerned about the contents of organic products such as commercial compost or mulch, or if you’re putting in a garden in a new property where the previous tenant might have used contaminated hay, there’s an easy home test to find out if the soil or your organic products are contaminated with persistent herbicides. Recalcitrant herbicides linger and are capable of damaging tomatoes, peas, beans, lettuce and several other crops and the contamination of the soil can last for years. The home screening method is called a bioassay which is a proven scientific method.

Squash showing contamination.
Photo by Sally Cunningham.

Prepare for testing compost – set up at least six 4-inch seedling pots or two flats and fill half of them with a sterile peat moss-based potting soil (this is the only time I recommend a peat moss potting soil). Fill the other half of the containers with a mixture of two parts of the compost you want to test and one part potting soil. Be sure to label for clear identification. Plant the containers with peanuts. They grow fast and are most sensitive to the contamination. If residues of pyralid herbicides are present, germination will be poor, and seedlings that do grow will show curled leaf edges.

Testing manure that will be used in the garden – plant seedling pots or flats of potting soil with peanuts (the best choice) or peas and beans and let them grow for a couple of weeks. Mix a slurry of equal parts manure and water in a 5 gallon bucket or other container and let it sit for about an hour. Water half of the seedlings with the manure water and use just water on the other seedlings. If the manure is contaminated, symptoms will appear within a few days.

Compost can be contaminated with persistent herbicides; peanut plants grow fast & respond quickly to testing.

Mulch or Hay testing – use the same procedure as with manure testing, but soak the mulch, hay or other materials overnight before draining off the water.

In summer, you can pot up volunteer tomato seedlings from your garden to use as test subjects. If you know their source or product name, materials that turn out to be killer compost or murderous mulch should be reported to your local extension service.

Volunteer tomato plants can be used for testing for pesticide contamination

In tests in Washington, Pennsylvania, and in New Zealand, Dow’s clopyralid (still sold as Confront) was found to be the contaminant in compost that killed home garden and nursery plants. Aminopyralid, the active ingredient in common herbicides Milestone and Forefront, belongs to the same class of chemicals that includes clopyralid. Picloram is a similarly damaging product.

Tomatoes are highly sensitive to contamination, and symptoms including curled, cupped leaves and wilting new growth. These symptoms are often misdiagnosed as a virus or disease problem. Here are good web sites to check out for additional information:

Answers for addressing soil contamination:
Contaminated soil detox
Zeolite