How to test your organic compost and mulch for herbicide contamination (2024)

Tomatoes and other vegetables are highly sensitive to herbicide contamination, and the symptoms can fool you. They can include curled or cupped leaves, wilting or distorted new growth, all of which are often misdiagnosed as disease problems or even spider mites.

There's an easy home test to find out if your organic products are contaminated with persistent herbicides.

For compost testing

Set up seedling pots or 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 any legume, but peanuts are best. If certain persistent herbicides are present, germination will be poor and seedlings that do grow will show curled leaf edges. If a peanut plant is used, the leaves will almost instantly fold up.

For manure testing

Plant seedling pots or flats with peas, beans or peanuts (the best choice) 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.

For 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 for applying it to the seedlings.

About the contaminants

In summer, you can use tomato seedlings as test subjects. Materials that turn out to be contaminated should be reported to your local extension service, to the manufacturer of the product, and to the EPA's Special Review and Reregistration Division.

Dow's clopyralid (still sold as Confront) was found to be the contaminant in compost that killed home garden and nursery plants in Washington, Pennsylvania and New Zealand. 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. Unfortunately, these and other similar products are commonly recommended and used in agriculture where the compost, manure and mulch products come from.

Resources

Online: dirtdoctor.com or facebook.com/thedirtdoctor

Radio: "The Answer" KSKY-AM (660), 8-11 a.m. Sunday. ksky.com. The call-in number is 1-866-444-3478.

Mail: P.O. Box 140650, Dallas, TX 75214