What happens to toxins in compost and soil?

Question: What happens to toxins in soil? More specifically, when I compost my banana peel, I know there is a small amount of Thiabendazole, Imazalil, Azoxystrobin, Myclobutanil, and probably a half dozen other pesticide/fungicides on it. I have to imagine that these end up in the soil.

Photo by szczel (www.flickr.com)

Photo by szczel (www.flickr.com)

Do these things break down? What breaks them down (microbes, worms, sunlight, and/or time… or maybe something else completely? Do they build up in soil?

Have tests/studies been done to see if these toxins transfer to plants, or if they affect plants? Are there ways to test soil for a wide variety of toxins? Or, do you sort of have to know what you are looking for and test for that?

Answer: It is true that food scraps, particularly scraps from conventionally grown produce, can contain pesticide residues. We are at a point where we’re able to measure these residues in parts per billion concentrations. To put that into perspective, a part per billion is the same as $0.01 per $10,000,000.

It’s also true that many of these compounds are environmentally active at very low concentrations, and the impact on human health is not fully understood. It may never be understood. But it’s important to remember that these compounds are used for a reason. Take, Thiabendazole, for example. It’s used to control fungal disease in plants and roundworms in people.

When these compounds enter a compost pile, a number of things can happen. In many cases they are degraded by the microbes in the pile that also degrade the banana peel. They can also be absorbed onto the organic matter in the compost, undergoing structural changes as a result that make them less active. Or the heat in the compost pile can alter their chemical form.

In limited cases, the breakdown by microbes is incomplete and the byproduct or metabolite is more hazardous than the original compound. That was the case with DDT, where the breakdown product DDE was worse than the DDT. But it’s really important to remember that the formulations of these compounds have gotten more sophisticated and they are used at much lower quantities today. I am talking about a worst case here, so you will understand that I am not trying to dismiss your concerns.

There are also a select few compounds that fail to break down in compost at all, but rather concentrate. Clopyralid is one example of these. In Washington State, this herbicide persisted in compost, resulting in compost that effectively killed certain types of plants. You can read more about it here http://puyallup.wsu.edu/soilmgmt/Clopyralid.html.

The most likely case, though, is that whatever traces of herbicides there may be in your compost are at such low concentrations that any impact will be far outweighed by the benefits of the compost. For example, researchers studied the impacts of adding antimicrobial compounds in municipal biosolids to soil. They used a range of stress indices to measure the impacts of these compounds, which are specifically designed to kill bacteria, on soil bacteria. What they found was that the biosolids added so much organic matter and nutrients to the soil that the impact was a significant net positive.

In those studies, the researchers added antimicrobial compounds to the biosolids in quantities that would yield significant results. You certainly wonʼt do the same with your compost. In other words, the benefits from the compost outweigh any risks. What has also been seen for the vast majority of these compounds is that plants do not take them up from soils.

Bottom line: Making compost and growing food is a wonderful thing to do, and you are not doing harm to the soil, the plants, or yourself in the process.

–Answered by Sally Brown, University of Washington

Have a question for Soils Matter? Post it as a comment below, or email us at soils-matter@soils.org

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15 responses to “What happens to toxins in compost and soil?

  1. A question I have often wondered about. Thank you to the question writer and the answer writer. I have heard that fungi that can be used in water treatment. Would they be useful here?

    • You’re welcome! I’ll forward your question about fungi to our urban and garden soils expert and get you an answer as soon as I can.

    • Funghi are wonderful creatures. They are one member of the larger group of microorganisms that live in soils. The others are the bacteria, the actinomycetes, and the archaea. While my favorites are the ones that bear edible fruit (chanterelles and porcini to name a few), they are also highly specialized funghi that can degrade lignin. Lignin is a highly complex form of organic matter that is characterized by multiple and complex carbon-to-carbon bonds that are very difficult to degrade.

      Lignin is one thing that makes wood so strong, and white rot funghi are one type of fungus that can degrade wood. There has been extensive research on the ability of these funghi to break down toxic organics and there are cases where their degradation ability has been taken advantage of. However, most of the organic chemicals that we are concerned about these days are not anywhere near as toxic to soil microorganisms as those in the past, and do not require specialized species to decompose. Wastewater treatment is already typically a primarily microbial process. Aerobic secondary treatment is a microbial feast–with oxygen bubbled through the water to facilitate microbial decomposition of organic compounds.

      So you are correct in saying that microbes could be used for treatment–they are. Here I mean a broad spectrum of organisms. And in specialized cases, funghi have been used to degrade particularly toxic compounds. A quick search on Google Scholar will give you some examples. Luckily, our composts and wastewater have extremely low to no concentrations of those highly toxic compounds, as a result of them being banned from general use.

      –Answered by Sally Brown, University of Washington

  2. This is what I was hoping to hear. I volunteer for a community garden around the corner from my house. UCSD has offered to do some free soil and plant testing to make sure there are no lingering contaminants on the lot we use. They also did testing at my house to get a bigger picture. Everything was normal except for my strawberries which had a high levels of arsenic in them. We need more test samples to create a average for the final model, but I’m wondering if perhaps I should be avoiding the strawberries until that happens. They taste so damned good though.

    yours,
    James
    http://www.mindyourdirt.com

    • Eat the strawberries! But read this first. In order to truly enjoy how delicious your strawberries are and not worry that you are going to hurt yourself by eating them, it is important to understand a little bit about arsenic. As with most soil contaminants, the risk is a question of several factors. How much arsenic is in the strawberries? How much of the arsenic in the strawberries is actually adsorbed by your body when you eat them? And how much of your diet is made up of the strawberries?

      It is very likely that the concentration of arsenic in the strawberries, while it may be elevated, is not a concern because strawberries make up such a small portion of your diet. The total arsenic is reported on a dry weight basis and strawberries are primarily water. If you eat a pint of strawberries that weighs 6 ounces, you are likely eating well under an ounce of dry strawberries. This is a lot of strawberries, but a very low amount of food compared to your normal diet.

      Also when you eat a well balanced diet, typically your uptake efficiency–the amount of the contaminant that is absorbed into your system–is very low. This has been shown extensively for lead. Your body does not want the lead, and only takes it up when it is looking for the nutrients that you need. Here I am assuming that the same is true for arsenic. Arsenic is chemically similar to phosphorus. If you are eating well, your diet will be rich in phosphorus and so you will likely absorb very little arsenic.

      So enjoy your strawberries. And if you want to further reduce risk, eat them on top of ice cream or with cereal and milk.

      –Answered by Sally Brown, University of Washington

      • I’m glad you’ve said this whereas I’ve been eating them anyways. I’m aware that almost everything has some unwanted or dangerous trace elements in them. It and we are all stardust anyways right. The scientist that did the testing also alluded to a low risk strawberry scenario. Thanks as well for officially giving me permission to eat more ice cream!

  3. I have heard of some plants being used to target and remove specific toxins from soil. Is there a list somewhere of the plants can be used for these purposes? Or which toxins persist in soil and can’t be removed? If my strawberries or apples had a lot of arsenic I might want to remove the arsenic from the soil before I continued consuming these fruits. The presence of lead might be another toxin of concern since it is so common in urban soils. I don’t understand how toxins that accumulate in our bodies could be considered safe if they can be detected and described as being at a ‘high’ level.

      • In answer to your question about using plants to clean toxins from soil:

        There are highly specialized and exceedingly rare plants that are able to grow on metal-contaminated sites by taking the contaminants up into plant tissues. They are referred to as hyperaccumulator plants. One hyperaccumulating tree species, identified in the South Pacific, takes up so much nickel that the sap runs blue. Other hyperaccumulator plants have been identified that take up zinc, copper and nickel. No hyperaccumulators have been found that can take up lead. A fern that takes up arsenic, the Brake fern, has also been identified.

        The thought was that these plants could be grown on metal-contaminated sites as a way to clean up the sites over time. There was also a potential to grow crops on these sites as a new form of mining–a green ‘ore’. The challenges to making this idea a practical reality have been significant. If a soil contains 30 parts per million arsenic and the top 15 cm of soil weighs 2000 tons, that means that the arsenic in that soil weighs 60 kgs. If the plant had 1000 parts per million arsenic, that means that every kg of dry plant tissue would have 1 g of arsenic. So in order to get those 60 kgs of arsenic out of the soil, you would need to grow 60 dry tons of plant matter. Yields of ferns are low, so you are talking about growing annual crops of ferns for well over 20 years to clean the soil. And that’s assuming the plants can continue to take up the arsenic at really high rates even as the soil arsenic goes down. Also, unlike crops like corn and wheat, these are wild species. So it isn’t clear if it would even be possible to grow them like a conventional crop.

        In other words, hyperaccumulators are amazing plants. Can we use them now to clean our soils? No.

        –Answered by Sally Brown, University of Washington

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