A bioassay is a technique for determining if herbicide (or other chemical) residues are present and bioavailable in soil or water at high enough concentrations to adversely affect plant growth. This is a simple, economical, and direct method to determine if it is safe to seed or plant into areas previously treated with herbicides or into soil with an unknown history of herbicide use.

In its simplest form, a bioassay uses susceptible plants to identify if the herbicide is present in concentrations high enough to inhibit germination and/or alter plant growth. However, scientists sometimes use sensitive bioassay species to estimate herbicide concentrations in soil and water, and to identify unknown herbicide residues from exhibited injury symptoms.

When newly seeded or established plants show seemingly unexplained symptoms of injury, stress, or decline. Also, when seeding or planting sensitive plant species into areas previously treated with residual herbicides. Topsoil from abandoned farm land can often contain herbicide residues that can injure many plants. Another too-common occurrence is the presence of herbicide residues in compost (both commercial and municipal). Additionally, if you suspect that another product may have been contaminated with an herbicide, both the product and treated soil can be tested using a bioassay.

Collect representative soil samples

1. Sample from areas suspected of having herbicide residues as well as areas which are known to be free of herbicides. You will use the herbicide-free soil for comparison.
2. Take separate samples from high spots, low spots, and different soils. Also sample areas where sprayer overlap may have resulted in an over-application.
3. Take soil cores. Remove the thatch (if present) and keep only the upper two inches of soil as most residual herbicides will be bound in this zone. On sandy soils sample to four inches.
4. Take several samples from an area and combine them. You need enough soil to fill several pots in which you will grow the bioassay plants (I suggest 3- to 4-inch pots).
5. For contaminated compost, collect samples from the center of the pile and from the edges. Run separate bioassays on these samples.

Select the bioassay species

In general, the best bioassay species is the one you intend to grow; however, crop plants and turfgrasses sometimes do not grow well indoors in pots, nor do they respond rapidly or decisively enough to be reliable bioassay species. Therefore, it is often advisable to select particular species known to perform well in bioassays such as ryegrass, oats, beans, peas, and tomato. Table 1 provides a list of recommended bioassay species for different herbicide families or modes of action.

Seed and grow for about three weeks

Seed or transplant (tomato) the bioassay species in "clean" and "contaminated" soil. Place the pots in a greenhouse or on a sunny window sill and keep them watered (do not waterlog). Add a small amount of fertilizer to ensure plants will grow. Watch the plants for about three weeks (after germination) then evaluate plant growth. Plants need to be large enough to observe symptoms. Oats or ryegrass grown in "clean" soil should be about four inches tall when you evaluate the plants. Broadleaf indicator plants should have three to four true leaves (not counting the seed leaves).

Examine the overall growth, leaves, and roots. Look for stunting, yellowing (or other discoloration), abnormal leaf or stem growth, and root swelling or stunting.

Additional information on herbicide mode of action and injury symptoms is available on the Weed Management Information portal.

Tomato bioassay for auxin herbicide residues in soil

Joe Neal  CC BY-NC 4.0

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Tomato bioassay for auxin herbicide residues in soil

Joe Neal  CC BY-NC 4.0

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There are basically three options.

1) Leave the soil fallow for one growing season before planting.

2) Plant another species which is tolerant of the herbicide, such as selecting a different turfgrass species or installing a woody ground cover bed.

3) Incorporate (rototill) activated carbon into the soil to a depth of four to six inches. The recommended amount to detoxify herbicide residues is 100 lb activated carbon per acre for every pound of herbicide active ingredient (AI) per acre suspected to be present. Be aware that activated carbon does not detoxify all herbicide residues. Therefore, it is advisable to determine if the activated carbon can effectively detoxified the herbicide residues. Mix ¹⁄₂ ounce (dry measure) of activated carbon in 1 quart of water. Add 1 fluid ounce of this to each 4 inch pot of soil. [This will approximate an application of 600 lb activated carbon per acre.] Dump the soil in a bag and mix well; then return the soil to the pot and run the bioassay. If the plants grow well, proceed with the application of activated carbon to the field. If the plants are still stunted, contact your local Cooperative Extension office for addtional assistance.

A bioassay is a simple, inexpensive, and accurate way to determine if herbicide residues are present at high enough concentrations to affect seedling emergence or plant growth. By conducting a bioassay on new top soil or in new seedlings previously treated with an herbicide, you may avoid wasted time and seed, thus saving you time and money in the long run.

This information was originally published as Cornell Cooperative Extension WeedFacts #3 in 1991. Revised and edited by the author 2016 and 2021.