The Risk of Herbicide Contaminated Soil Amendments
 | Author:
Allison Kosto, MSU Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
The Risk of Herbicide-Contaminated Soil Amendments Allison Kosto, MSU Broadwater County Extension Agent Imagine this scenario… You worked hard all spring to prepare your garden. You did a soil test, amended the soil, planted the seeds and watered diligently. However, as the plants start to grow, you notice strange symptoms. The leaves are twisting and curling, and some are turning yellow. You do not see any signs of insects or disease and have no idea what is going on. What do you do now?! Isn’t this frustrating?!
The garden in this scenario is showing classic signs of herbicide injury. There are lots of possible explanations for herbicide injury. I have a good story about what Tordon looks like in the garden when the sprayer does not get cleaned properly…. But I will save my husband from embarrassment for another time. One possible cause of herbicide injury that is often overlooked is through contaminated soil amendments.
Some herbicides persist in the environment longer than others. These chemicals do not break down quickly in the environment and leave a residual. While this can be a desired characteristic, it also can cause issues in the wrong environment or setting. Some of these chemicals can also survive the animal digestive process, be present in manure and survive the high temperatures of composting.
To be clear, not all herbicides have these characteristics. The most common chemicals with this trait are growth regulator herbicides, known as pyridine carboxylic acid (PCA) herbicides. Examples of active ingredients are aminopyralid (Milestone), clopyralid (Transline) and picloram (Tordon). These are common rangeland or pasture herbicides.
Typical herbicide injury symptoms are leaf curling, chlorosis (yellowing), stunted growth and twisted growth. Some plants are more susceptible than others. Some of the more suspectable garden plants to PCA herbicides are peas, beans, tomatoes, potatoes, sunflowers, petunias, cucumbers and squash. Trees and shrubs can also be affected. However, it is common to see herbicide injury in trees the following spring after exposure.
Identifying herbicide injuries can be challenging. Testing for herbicide residues in plant material, soil and amendments is available through the Montana Analytical Laboratory. However, it is expensive, about $250-300, and often cost-prohibitive for a home garden. Another technique is to complete a bioassay.
A bioassay is an experiment that helps detect the presence of a substance, in this case, an herbicide. Collect samples from the soil, compost or amendment with the suspected issue, and place in pots. Blend with a potting mix if you are testing compost or manure. Plant seeds: peas or beans recommended. Place in a sunny area or beneath a grow light and water. Next, monitor for any symptoms as the plants grow. Specialists are available through MSU Extension who can look at the symptoms and help identify possible herbicide exposure.
If you determine that you have herbicide contamination, there is not much you can do about it unfortunately. It can take three years or more for the herbicide to completely break down. If you have a raised bed, you could replace the soil. Another option, although not always possible, is to move the garden to a new location. We do not recommend eating fruits and vegetables from herbicide-contaminated gardens, especially if plants are showing symptoms.
Ideally, it is best to avoid the situation whenever possible. Make sure that you are getting non-contaminated soil amendments. Ask questions: 1. Tell me about the process of making your compost or amendment. Do you know the history of herbicide use? If pesticides were used, what type? 2. Do you use plant growth testing or bioassays to look for herbicide contamination? 3. Have you had any issues with herbicide contamination in the past?
Another great option is to create your own compost. However, make sure that you fully understand the history of all your inputs of your home compost. For example, if you use manure, do you know that it is free of herbicides? If you use grass clippings, do you apply herbicides (including Weed n Feed) to your lawn? Always read the pesticide labels when applying pesticides. When in doubt, contact a bioassay before applying any compost, manure or soil amendments to your garden.
Stop by MSU Extension in Broadwater at 416 Broadway in Townsend or contact 406-266-9242 or allison.kosto@montana.edu for more information on the subject or any gardening questions.
