Do Pesticides Break Down In Compost?

The process of composting is an excellent way to turn your food scraps into rich, beautiful soil that benefits your garden. But what happens when pesticides end up in your compost pile? Does it break down, or will it remain in the compost and kill the plants you apply it to?

Generally, pesticides break down in compost and become harmless, but there are a few exceptions. While you might be confident in your compost pile’s organic nature, you might be surprised that pesticides can turn up in the pile, even when you don’t spray. Here’s what you should know.

How Do Pesticides End Up In Compost?

You might question how pesticides can end up in your compost, especially if you don’t use anything on your plants. Well, even though you’re working hard to ensure your compost pile stays organic, your efforts might be fruitless. 

The problem lies with the fresh produce available in U.S. grocery stores. While you could purchase the more expensive organic produce in an effort to avoid pesticides, there’s no guarantee there won’t be any residue on the produce you buy. In some cases, organic companies simply use pesticides derived from natural ingredients, allowing them to find the loophole to continue labeling as organic. 

If you opt for regular, non-organically labeled produce, there’s a good chance the product has pesticide residue on it. In fact, a nonprofit group called Environmental Working Group (EWG) conducted a study that found more than 70% of non-organic produce sold in the U.S. has pesticide remnants. 

Can I Wash Produce To Remove Pesticides?

With the high chance of pesticides on your produce, you might be wondering if you can wash your fruits and vegetables. However, this isn’t an effective way to remove the entirety of pesticides left behind on your produce. 

The FDA reports that washing fruits and vegetables with soap, detergent, or a commercial produce wash hasn’t been proven more effective than water at removing 100% of pesticide residue. While washing your produce is still a good idea, it’s unlikely you’ll be able to rid the product of pesticides. 

It doesn’t hurt to soak fruits and veggies in a vinegar water solution (10 percent vinegar, 90 percent water), as this will help cleanse the surface. That said, a quick rinse with clean water will remove the majority of bacteria (up to 98%), so vinegar isn’t necessary. 

Does Composting Destroy Pesticides?

The consensus from most research is that most pesticides break down in compost. Research in various studies and experts found that detectable pesticide levels in finished compost are usually at very low concentrations. 

Of course, there are a few exceptions to this, so if you want your compost to remain organic, you should avoid adding anything that has been sprayed with pesticides. 

The composting environment isn’t friendly to produce pesticides. The combination of heat and microbial action in a healthy compost pile breaks down most of these pesticides, leaving very low concentrations (if any) behind. 

Even if broken-down pesticides end up in your finished compost, the low concentration usually isn’t absorbed by plant roots. That said, plants may still absorb the low concentrations of pesticides, but it typically isn’t cause for concern. 

Exceptions To The Rule

As mentioned, most pesticides break down in compost and are rendered harmless. However, there are a few exceptions to this. Clopyralid, a weed killer for lawns, is one of the most recognized examples that fits in this category. This pesticide is incredibly mobile in water and soil and can even pass through cattle entirely unchanged. 

In other terms, this means Clopyralid could appear in your garden and compost, even if you haven’t used it yourself on your lawn. Some plants are more resilient to this pesticide, while others are sensitive to it (nightshades, legumes, Asteraceae, etc.). 

To be on the safe side, it’s best to steer clear of using cow manure or grass clippings that could be contaminated with Clopyralid for composting with these plants. While you don’t have to scrap compost containing these elements simply because of the pesticide potential, be careful of what plants you use it on. 

For example, if you’re growing legumes, peppers, or sunflowers, stick with a purely organic, pesticide-free compost.

Aside from Clopyralid, a new group of pesticides is growing in popularity. Research has shown these pesticides, known as Neonicotinoids, to be highly toxic to honeybees and other pollinators. However, other research concludes there aren’t enough complete studies to form a conclusive opinion. That said, honeybees and other pollinators are a helpful (often essential) part of a successful garden, so it’s best to steer clear of Neonicotinoids altogether. 

Try to avoid adding inorganic food scraps that may have been treated with potentially harmful pesticides. This way, you won’t have to worry about accidentally adding elements that will harm your plants later down the road. 

Here are a few additional toxins that could cause problems in your compost pile:

  • Bifenthrin: This insecticide is used to get rid of pests, such as ants and spiders. It’s a persistent insecticide that is found in certain large-scale composting facilities. Given its persistent nature, it can harm plants secondarily. 
  • Aminopyralid: This herbicide is often used on broadleaf weeds as a pre-emergent, broad-spectrum treatment. It’s banned for residential use in certain states, but it can appear in specific commercial-based compost feedstock. 

How Does Composting Break Down Chemicals?

So, we know that most pesticides break down in the composting process, but how does it happen? When the chemicals in pesticides end up in a healthy composting pile, some begin to break down, decaying into simpler molecules. 

Other chemicals bond with other compounds via absorption, while others break down by volatilizing (evaporating into the atmosphere). Or, these chemicals might break down by mineralizing. 

Mineralizing is ideal for certain pesticides, as it occurs when organic compounds break down into their organic and inorganic (or mineral) parts. The organic compounds left behind, including carbon, break down even further into other molecules, such as carbon dioxide or water. The carbon dioxide released enters the air, whereas the water remains in the compost pile. 

The non-toxic inorganic compounds left as a result of pesticide break down, turn into minerals, and eventually find their place in the soil. 

How Long Do Pesticides Stay In Soil?

The length of time pesticides and herbicides remain in soil depends on the type and level of concentration you’re dealing with. Persistent herbicides can remain in the soil for a few months to more than three years before completely breaking down into inert compounds. 

When measuring the amount of time pesticides, herbicides, and fungicides take to break down in the environment, researchers measure the length of time the pesticide persists in the environment by half-life. The half-life measurement indicates the amount of time it takes for the organic material to be reduced by 50%. 

In most cases, pesticides have a half-life between a few hours to 4-5 years. Microbes in the soil break down most pesticides, so microbial activity (and whether the environment is suitable for the microbes) heavily impacts the timeframe. 

Most modern pesticides don’t last nearly as long as pesticides used decades ago (such as DDT). However, in some parts of the world, folks use copper-based fungicides, which last forever in the soil. 

Frequently Asked Questions (FAQs)

Will Compost Containing Pesticides Kill Plants?

In some scenarios, compost that contains pesticides can kill plants. However, it depends on the particular pesticide and its concentration in the compost. Some plants are highly susceptible to certain pesticides but resistant to others. 

Other pesticides may effectively kill many plants in your garden, providing the concentration levels are high enough. However, the composting process breaks down most pesticides, rendering them ineffective and harmless to your garden. 

That said, it doesn’t hurt to take note of what you put in your compost pile.