Over the last 20 years there have been a number of news reports about herbicide contaminated manure, compost, straw and organic fertilizer. This is a real problem for gardeners because such products will harm and even kill your plants. The danger is real!
- How can this happen?
- How common is this problem?
- Should you be concerned?
I’ll answer all of these questions in this post. Every gardener should read it
Contaminated Compost, Straw and Organic Fertilizer
Farmers use herbicides to grow crops such as straw or hay and some of which is then fed to animals. The herbicide is quite safe for animals and simply passes through their digestive system, ending up in the manure. Even when the manure is composted the herbicide persists in the compost.
Gardeners might buy the manure, or compost made from the manure. This compost may also be used to make a number of bagged commercial “organic fertilizers”. If any of these products contain certain herbicides and end up in your garden, they can damage and even kill your plants. This is especially a problem for seedlings and smaller vegetable plants.
Herbicides That Contaminate Compost
Not all herbicides are a problem. Some like glyphosate are short lived and don’t pose a problem. Others are digested in the animal’s stomach. The higher temperature during composting can degrade some to a safe level and still others will bind to soil, rendering them harmless to plants.
There are however a few herbicides that survive all of this and remain in soil for years. These so-called plant growth regulator (PGR) herbicides belong to the pyridine group of compounds and include clopyralid, aminopyralid, aminocyclopyrachlor and picloram. The following levels are known to cause harm to plants:
- clopyralid – 10 ppb
- aminopyralid – 1 ppb
- aminocyclopyrachlor – unknown
- picloram – 5 ppb
In the rest of this post, I will call these persistent herbicides.
As an example, aminopyralid, according to the manufacturer Dow Chemical, will affect legumes, carrots, lettuce, potatoes, some roses, tomatoes and dahlias. Assume it affects anything that is not a grass and it can take up to 3 years to break down in soil.
Symptoms of Herbicide Damage
In many cases the herbicide is not lethal, but it does affect new growth. New leaves and flowers are deformed. Seeds germinate poorly or are deformed after germination. At higher concentrations, whole plants can be killed.
Keep in mind that these symptoms can also be caused by nutrient deficiencies, diseases, insects and herbicide drift.
The dose makes the poison, but in this case quite a low dose can harm plants and that is why this is such a big problem.
Herbicide Sprayed Grass Clippings
This problem is not just a farming issue. Persistent herbicides are also available in lawn care products. In some cases they have been removed for home use, but are still used on commercial lawns and golf courses. If sprayed grass clippings are added to gardens or to a compost pile, the herbicide can harm plants. Some products say right on the label. “do not add grass clippings to compost”.
So what do gardeners do? A smart gardener will add a mulching blade to their mower and leave all grass clippings on the lawn, but others might put it in the green composting bin. It then goes to the municipal composting facility and ends up in the compost they produce. Unsuspecting gardeners take that home and watch their plants die.
Herbicide Contaminated Hay and Straw
Straw and hay are good mulches, especially in the vegetable garden. If that straw or hay has been sprayed with persistent herbicides, enough herbicide can leach off the mulch to harm plants and contaminate your soil for several years to come.
Herbicide Contaminated Compost
Contaminated compost (clopyralid) was first detected in 1999 at a compost facility in Washington state and has now been found in many states. In the past year, gardeners in the US have again reported problems using commercial soil to grow vegetables in containers – the soil was contaminated.
The U.S. Composting Council (USCC) and the EPA acknowledge there is a problem with clopyralid. Pyridine herbicides are up for registration review in 2021. “EPA is working to address compost contamination concerns for certain persistent herbicides during registration review”.
There are lots of composting facilities and few labs that can detect the persistent herbicides at a low enough level. Testing is expensive and there is not enough of it.
Herbicide Contaminated Organic Fertilizer
There are reports in the US about the B & Q Organic Living Tomato Food product being contaminated with persistent herbicide and being recalled after gardeners reported damage to their plants. I have not found the original recall notice.
This summer, gardeners in Norway and Sweden reported tomato and cucumber plants that were not growing well, and instead, looked limp with curly leaves. Further investigation found that they had used commercial organic fertilizer that was contaminated with persistent herbicides. “These tests detected 992 µg clopyralid per kg in one liquid fertilizer approved for organic farming, and 73 µg/kg in a pelleted fertilizer”. How toxic is this? Clopyralid can have a negative effect on plant growth at concentrations as low as 1 µg/kg in soil or growth medium. Aminopyralid is 10 times more toxic.
Much of this contamination in Europe is from the use of beet vinasse, a byproduct of the sugar beet industry. A few large manufacturers in France, Germany and Poland produce products under various labels resulting in several products all showing herbicide contamination.
The problem was reported in the UK as far back as 2005 and it is still a problem today. Some of the contamination has come from the use of local animal manure, but in other cases it has come from commercial bagged and branded potting compost (including veganic and peat free), mushroom compost and some liquid feeds.
How Common Are These Problems?
Considering the large number of gardeners who use these products and the small number of reported cases, it is a rare event. However, when it does happen, it is a serious problem. As long as persistent herbicides are on the market there is a chance that manure, compost, straw or commercial organic fertilizer could be contaminated.
What Organic Products are Safe?
Unfortunately, straw and compost are very popular but you can avoid persistent herbicides by selecting products that are not derived from pastures, such as:
- Blood meal
- Bone meal
- Cottonseed meal
- Fish meal
- Kelp meal
- Wood chips
- Pea straw
You also won’t find herbicide contamination in chemical fertilizers.
Testing for Persistent Herbicides
You can do a simple DIY bioassay yourself to test your soil, or an amendment to see if it is safe. Follow these steps:
- Take two pots and fill one with good soil and the other with a mixture of good soil and the amendment being tested.
- Plant some peas, beans or tomatoes in both pots.
- Water and provide some light for the seedlings to grow.
- Compare the plants after about 30 days. If both pots have healthy plants, the amendment is not contaminated. If the plants in the pot containing the amendment are smaller, or have unusual leaves, you have a contaminated amendment.
How to Fix Contaminated Soil
Once soil is contaminated it can take several years for microbes to decompose the persistent herbicides. Until that happens you can’t grow sensitive plants in that soil, so what do you do?
If it is a small area, you can remove the soil and replace it. Remove a few inches of soil, and do a bioassay on the remaining soil to see if it is contaminated. If it is, remove more soil.
If soil removal is not feasible consider these options.
- Grow plants that are not affected, such as corn (a type of grass), brassicas or other grasses.
- Grow in containers for a couple of years until the soil in the ground is suitable.
- Flush the soil with lots of water and see if you can wash the herbicides away. This works best in sandy soil.
- Aerate, water well and add fertilizer to increase microbial activity.
- Grow cover crops that are not sensitive and remove the plants at harvest time.
- Do nothing and wait for nature to resolve things. This could take 2-5 years depending on the herbicide and its level.