Use a Simple Compost Test to Avoid Contaminated Materials in Your Garden

I’ve read your reports about the problem of grass clippings, compost, manure and hay being contaminated with herbicide residues that can harm garden plants. Is there a way to screen these materials before I bring them into my garden?

Bonnie North
Asheville, North Carolina

You are right to be concerned about the risk of herbicides lurking in organic materials. The pesticides of concern are clopyralid and aminopyralid, both in the “pyralid” class. Here are some simple tests you can use to screen organic materials to be sure they don’t contain traces of these herbicides.

To test compost, set up at least six 4-inch seed pots, and fill half of them with potting soil. Fill the other half with a mixture of two parts of the compost you want to test and one part potting soil, and be sure to label the pots. Plant the containers with peas (in cool weather) or beans (in warmer conditions). If pyralid herbicide residues are present, germination will be poor, and seedlings that do grow will have curled leaf edges.

To test manure, plant at least six seedling pots with peas or beans, and let them grow for a couple of weeks. Mix a slurry of equal parts manure and water, and strain off 2 cups of liquid. Drench half of the seedling pots with the manure water; water the others as usual. If the manure is tainted, symptoms will appear within a few days.

To test mulch, use the same procedure as with manure, but soak the hay or other materials overnight before straining off the water.