During the last couple of years, numerous gardeners and farmers have reported damage to vegetable crops – mainly tomatoes, potatoes, beans and peas – after applying manure, hay or straw to the soil.
Symptoms include curled, cupped or twisted leaves; misshapen or small fruit; poor germination; and even death of plants. While these symptoms could indicate biotic sources, such as insects or diseases, the presence of certain herbicides in animal-based material added to the soil should be considered.
Recently, the gardening and farming community has been active in trying to identify solutions to the issue of herbicide carryover. Certain classifications of herbicides used to kill broadleaf weeds in hay fields or pasture can pass through the animal’s digestive system and still be active in manure, even after it is composted.
The herbicides of greatest concern are aminopyralids (commonly referred to by the trade name Milestone), clopyralids (trade names Curtail, Transline and others) and picloram (trade name Picloram).
While it may be very difficult to determine if these herbicides are present in your compost, manure or soil, you can easily perform a bioassay before planting.
Using the bioassay: General procedures
This bioassay, developed by Washington State University, is intended for use by homeowners, gardeners and even farmers. In this protocol, peas (or beans) are planted and allowed to grow for two to three weeks until three sets of leaves have appeared. The plants are compared with control plants grown at the same time and evaluated for herbicide damage.
Test material (compost, manure or topsoil).
Potting mix (compost-free, peat-based commercial mix with fertilizer included).
Four-inch plastic pots (new, manufacturer not specified; volume of 0.75 a liter).
Garden pea seeds.
1. Evaluate test material (compost, manure, topsoil mix).
Record observations of odor and general condition of compost to be tested.
2. Set up control pots.
Fill three pots with potting mix, tapping several times on the countertop to settle mix uniformly.
3. Prepare test pots and label pots. The mix will vary depending on garden scenario.
If testing soil or topsoil mix where plants grown exhibited suspicious symptoms in the previous growing season, fill three pots with straight soil/topsoil mix.
If testing compost received for the current growing season, mix two parts compost to one part plain potting mix in a clean plastic bag. (The ratio is 2 to 1 by volume, compost to potting mix.) Fill three pots with the compost blend, tapping several times on the countertop to settle blend uniformly.
4. Plant three seeds in each prepared and labeled pot, pushing seeds into potting mix so they are just under the surface.
5. Grow plants
Water each pot carefully. Keep potting mix uniformly moist; minimize water leaching into saucer.
If excess water drains into saucer, allow it to be reabsorbed into the pot.
Maintain consistent growing conditions – could be done inside near a bright window or in a greenhouse – where temperatures should not drop below 50 degrees at night.
6. Evaluate plant growth
Record germination from each pot.
Grow plants until three sets of leaves appear – from 14 to 21 days, depending on growing conditions.
Compare plants from compost-blend pots to control.
If poor germination or distorted growth is seen, then you may not want to use that soil or manure. Typically, the herbicides in question take at least 18 months to break down. However, that is in ideal conditions: ample moisture, high humidity, good airflow and active microbial activity. Frequently, manure or compost piles are not adequately maintained, and the herbicide could persist for years.
If you have questions, samples or concerns, call the Colorado University Extension Office at 382-6463.
firstname.lastname@example.org or 382-6464. Darrin Parmenter is director and horticulture agent of the La Plata County Extension Office.