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.
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
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