Herbicide Contamination in Compost: Risks, Research, Solutions

15th March, 2025

If you’ve made compost from materials outside your garden, and nothing has grown in it, the problem could well be due to a hidden herbicide. 

While most traditional herbicides do break down in compost, there’s a new breed of herbicide which is far slower to do so. 

Contaminated manure has caused headaches across the world, while recently the Stockbridge Technology Centre warned us that low-level contamination in store-bought compost is affecting sensitive plants such as tomatoes and dahlias.

So today I’m covering a new study around these herbicides. Expect to learn: 

• How herbicides are a particular problem with compost
• How moisture levels affect breakdown
• One thing that accelerates the breakdown of these herbicides
• 6 key tips to protect your garden and plants.

The study looked at two specific problem herbicides: aminopyralid and iodosulfuron-methyl-sodium, both of which are phytotoxic – i.e. they are poisonous to plants.

The researchers highlighted the problems with these herbicides 

• They can damage plants and reduce yield.
• They can contaminate soil and water. 
• They have the potential to enter the food chain. (While the EPA considers aminopyralid to be fairly safe for mammals, the University of Hertfordshire states that iodosulfuron-methyl-sodium is highly toxic to mammals.)

Using a controlled laboratory study, the team studied the impact of four factors on the breakdown of these herbicides: 

1. Application of green manure (made from grass, leaves and branches) to compost.
2. Herbicide dose.
3. Soil moisture
4. Microbial activity

Compost is essential for improving soil health – but the researchers also found that when compost is added to soil, it slows down the breakdown of the problem herbicides by up to 4.6 times. 

The scientists believed this to be down to the high level of organic matter in compost, which increased the adsorption of the herbicide. 

Composters such as Charles Dowding have noted in the past that herbicides don’t break down in compost until they are applied to the soil – this increased adsorption may explain why. 

Water levels affect the breakdown of the herbicides – but just how depends on the herbicide. 

Aminopyralid: Breaks down twice as fast in soil with water holding capacity of 50% as it does in soil with water holding capacity of 25%.
Iodosulfuron-methyl-sodium: Degrades slightly faster in drier soils, possibly because it binds to soil particles and no longer acts as a herbicide, despite still being present in the soil. 

The study also found that microbes play a key role in the breakdown of the herbicides. 

The irony is that when herbicides aren’t present, adding compost is one of the best ways to stimulate microbial activity in soil. 

However, as we saw earlier, when herbicides are present, compost also increases organic matter, which leads to greater adsorption and a slower breakdown of these chemicals.

Perhaps not surprisingly, more herbicide meant a longer problem. 

Aminopyralid:  The half-life increased by up to 2.6 times.
Iodosulfuron-methyl-sodium: The half-life increased by up to 1.9 times.

As I’ve personally experienced, herbicides are a real problem in manure, and can have devastating effects on plant growth. 

And that’s just plants – we still don’t know what the impact is on our health.

Herbicides have also been found in store-bought compost, and even at low level can impact sensitive plants such as tomatoes. 

It’s immensely frustrating, as manure in particular is a valuable, widely available and cheap (or free) resource!

I’ve now found a farmer who doesn’t use it, but many people won’t have that option. 

1. If you do have a problem with soil affected by herbicide, avoid adding compost until the soil has recovered. You can test whether your soil is ready by planting a crop sensitive to herbicides such as beans, peas, lettuce or spinach.

2. Avoid using manure unless you can confirm that aminopyralid and iodosulfuron-methyl-sodium are not used.

3. Where possible, make your own compost from sources you trust – i.e. your own garden.

4. Avoid using lawncare or other products which may contain aminopyralid, iodosulfuron-methyl-sodium or clopyralid (another problem herbicide).

5. Only buy compost from reputable companies. While there are no guarantees, these companies are more likely to have strict controls on materials and perform testing on finished compost.

6. If unsure, perform a germination test before using compost. Again, ensure you use plants that are particularly sensitive to these herbicides (e.g. potatoes, beans). 

With careful sourcing and testing, you can make great compost while avoiding herbicide contamination.

Read more

• Manure in Compost: A Cautionary Tale
• The Problem With Peat-Free Compost (And How to Fix It)

Alligare, LLC. (2020). Alligare Aminopyralid 2SL Herbicide (EPA Reg. No. 81927-82). U.S. Environmental Protection Agency. 

U.S. Environmental Protection Agency. (2005). Pesticide fact sheet: Aminopyralid. https://www3.epa.gov/pesticides/chem_search/reg_actions/registration/fs_PC-005100_10-Aug-05.pdf

University of Hertfordshire. (2025). Aminopyralid (Ref: XDE 750). Pesticide Properties DataBase. https://sitem.herts.ac.uk/aeru/ppdb/en/Reports/29.htm

University of Hertfordshire. (2024). Iodosulfuron-methyl-sodium (Ref: AE F115008). Pesticide Properties DataBase. https://sitem.herts.ac.uk/aeru/ppdb/en/Reports/401.htm

Zanardini, E., Ravagnani, A., Rossini, M., & Vischetti, C. (2024). Dissipation of two acidic herbicides in agricultural soil: Impact of green compost application, herbicide rate, and soil moisture. Agriculture, 15(5), 552. https://doi.org/10.3390/agriculture15050552