Bioassay for Auxinic Herbicides
in Soil, Manure or Compost
The methodology described below is designed to test samples of soil, manure or compost for the presence of auxinic (growth regulator) herbicides that may be detrimental to sensitive crops planted in the sampled soil or planted where the sampled manure or compost is used as a soil amendment.
This bioassay is designed to test only for auxinic herbicide residues and not other substances.
Materials needed:
- 4- or 5-inch flowerpots
- Plastic saucers
- Noncontaminated soil (loam texture) or potting mix
- Plastic bags
- Labels for pots
- Disposable gloves
- Crop seed — garden pea seeds or beans (variety not specified) or the intended garden plants
- Photographs of plants showing herbicide damage
Method of testing:
- Insect larvae found in fresh manure, compost or soil may feed on planted crop seeds or seedlings, so try to remove larvae from samples to be tested.
- Manure or compost samples should be placed in a clean bucket and mixed with an equal volume of loam soil or potting mix that does not contain herbicides.
- Use clean plastic bags to mix each manure or compost sample with soil or potting mix to avoid cross-contamination.
- If testing garden soil, collect soil samples from several representative spots throughout the garden and then combine and thoroughly mix the samples.
- Fill 4 pots with the soil to be tested or the manure- or compost-potting medium mixture, tapping the bottom of the pots several times on solid surface (countertop) to settle mix.
- Label pots.
- Each pot should be placed in a separate tray or on a saucer to eliminate cross-contamination when pots are watered.
- Additional pots containing only soil or potting mix not contaminated with herbicide with no manure or compost should be prepared to serve as a noncontaminated control.
- Position pots in random order on the bench.
- Space pots far enough apart to avoid splashing media from one pot to the next during watering.
- Once pots are prepared and in place, they should be watered and left to stand for 24 hours before test crop is planted.
- Plant 4 seeds in each pot by pushing seeds into the mix so they are just under the surface.
- Water each pot carefully to avoid splashing and washing pot contents onto bench.
- Keep pot contents uniformly moist, but not excessively wet.
- Minimize water leaching into tray or saucer. If excess water drains into tray or saucer, allow it to be re-absorbed back into the pot.
- If possible, maintain consistent growing conditions with 12 hours light and supplement with fluorescent grow lights as needed.
- Temperature in the area where plants are growing should not drop below 50 F at night.
Plant Injury Assessments
- Compare plants grown in pots that contain soil or the manure- or compost-potting medium mixture suspected of containing herbicide with plants grown in potting mix or soil that is not contaminated with herbicides.
- Observe the planted crop for symptoms of herbicidal activity, such as poor stand (effect on seed germination), chlorosis (yellowing), necrosis (dead leaves or shoots), or cupped or curled leaves.
- If herbicidal symptoms do not occur, the test crop can be grown.
- If there is apparent herbicidal activity, do not plant the intended crop. If there is evidence of an auxinic herbicide injury, then one option is to plant a grass crop.
- Plants should be assessed at emergence and then at least at weekly intervals until they have three true leaves or more.
If plants show damage, mixing the soil, such as by rototilling, can help accelerate aminopyralid herbicide breakdown by increasing the activity of the soil microbes that naturally break it down. (Note, however, that plant damage can indicate the presence of several other herbicides or non-herbicide contaminants.) Herbicide-damaged garden plants can be disposed of by spreading them out evenly in the garden and then rototilling them into the soil where soil microbes will begin to naturally break down the herbicide.
 |
 |
| Soybean - Auxinic Herbicide Injury Symptoms | Soybean - No herbicide |