Does plastic affect the microbes in soil?

Q: I have questions on the breakdown of plastic in the soil, and its effect on soil microbe life and function. I’ve read that plastic is non-biodegradable, lasts forever unless broken down by sunlight, and that it inhibits microbial life. Many county landfills are giving away mulch made of chopped yard waste. This mulch also contains small pieces of chopped plastic, which comes mainly from the plastic bags that residents use to transport leaves, twigs, etc.

As soil is in need of organic material enrichment, I would like to use this mulch, but not at the expense of killing microbes: the life of the soil. Does plastic affect microbes in soil? Also do plants absorb plastic chemicals, which means I should not use this on vegetables, fruits?

plastic-in-compost

Compost from city sources can contain plastic bits from the bags people use to transport their yard waste. Do these plastic pieces hurt soil microbes? Photo: Montgomery City Division of Solid Waste Services (www.flickr.com)

A: To my knowledge, there has not been any specific research to measure the impact of the plastic contaminants in compost on soil microbes.

What I do know is that scientists have studied the impact of antimicrobial compounds in municipal biosolids on soil microbes. These antimicrobial compounds–namely the chemicals triclosan and triclocarban–were formulated to kill any and all bacteria. You can find these chemicals in many household products that advertise that they are “antimicrobial.” Examples include a range of soaps and certain brands of toothpaste. Because we use these products in our homes, they enter the municipal wastewater system.  Biosolids are the nutrient-rich byproduct of wastewater treatment and are often used to fertilize crops like wheat and corn. They are present in biosolids as small chemicals that are often attached to the organic matter in the biosolids.

Scientists tested to see if these antimicrobial compounds would harm soil bacteria. They found that the benefits of biosolids–which are very high in food for microbes–strengthened the soil microbes much more than the antimicrobial compounds hurt them. Total populations of microbes, respiration, and nutrient cycling were all measured.

Compost will be very similar to biosolids in that it provides food for soil microorganisms, which as you say are the life of the soil. It’s almost certain that the benefits of adding compost will far outweigh any negative effects from the plastics. There is one other thing to consider: Plastic contaminants will be enormous compared to the microbes. That  means the microbes won’t try to eat the plastic–it’s like a mouse trying to eat an elephant. This is another reason plastic pieces are unlikely to hurt soil bacteria.

To find research papers on this, search for O’Connor at University of Florida. He has done a lot of research on triclosan and triclocarban with other authors.

–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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8 responses to “Does plastic affect the microbes in soil?

  1. I appreciate your response/research.
    I was also wondering if plastic contaminants would then cause any farm that was certified organic (by USDA) to lose its certification. Sewage sludge, which also contains heavy metals like mercury from dental waste water, hospitals, etc., and all kinds of contaminant/metals/chemicals that enter municipal water treatment plants, makes its application not fit for organic certification.
    A sewage sludge product from far away Milwaukee, WI, wastewater treatment plants, called Milorganite, is sold throughout Florida. Farms that use this product cannot be certified organic, or can lose certification.
    Plastic is a toxic material, not a soil nutrient. So it appears that plastic materials are not organic. So I wonder what is plastic considered to be, when added to soil?
    I am concerned as landfills around the U.S. are providing compost/mulch with plastic pieces in efforts to reduce their fill.
    –Posted on behalf of Susan Z.

  2. For better or worse, our world is filled with a range of chemicals, created to meet a broad range of needs, which are now often present in detectible concentrations in soil, air, water, people, and wildlife.

    You speak of contaminants in biosolids, but you have to understand that the primary source of metals and other contaminants in biosolids is from our homes. In fact, the concentrations of many of these contaminants in biosolids products, such as Milorganite, are about the same as the concentrations in food and yard waste composts.

    To my knowledge, plastic contaminants are not justification for excluding a compost product from organic certification. For example, our fruits and vegetables typically come with small plastic stickers that have the code numbers to facilitate check out at supermarkets. It would be nearly impossible to remove these stickers from composts. Feedstocks like food scraps are a terrific feedstock for compost as they are high in nutrients and help to balance the high carbon content of woody materials.

    While it is not a good thing to have plastic contaminants in compost, it is important to remember that all things are a mix of good and bad. For composts that are carefully made and screened, the small quantity of plastics that remain is bad. However, the benefits associated with using the compost far outweigh any negative impacts from these plastic residuals. The same is true for biosolids.

    –Posted on behalf of Sally Brown, University of Washington

  3. Plastic is like sewage sludge in that it is not suitable for organic certification by USDA. I know exactly what is in my septic tank, but I am not allowed to spread it. But Milwaukee sells its sewage sludge in Milorganite, which can contain anything and everything from industrial wastes, chemicals, dental/hospital waste water (Mercury, et al.), all that enters the sewerage system. There is more to sewage sludge than just human fecal matter; there are chemicals, solvents, paints, heavy metals, chemicals from all kinds of industries, biocidal agents dumped down drains, sinks….. Its the hazardous, toxic chemicals, heavy metals that keeps sewage sludge off the USDa organic list.

  4. Hello, I want to carry out a research on the effect of plastics on crop productivity, my initial plan was to have a controlled environment agriculture in that I wanted to create 10 quadrants with different number of plastic. In each quadrant I will plant maize and at end I will measure the crop productivity of each quadrant. Please I need your suggestion?

    • We are sorry, but this question is outside the scope of our public-oriented blog. We wish you the best in your agronomy research!

  5. I don’t understand why anyone would use city compost, when it is possible to make your own compost and control what goes into it. It is not enough to say, you have to accept the good with the bad. The question is not will plastic harm soil microbes, but what cancer causing chemicals are in city compost made with pieces of plastic. I inherited a patch of soil, that someone mulched with black plastic and it stays intact. I am still picking out pieces of plastic and I can’t grow anything there at all. It is going to take years to pick out each little piece of plastic as the plastic is all ripped up and incorporated within the plot.

    • Hi Macy, thank you for your concern. We certainly do promote all households that can to make their own compost. It may not be possible for everyone to do that based on regulations and size of their home. We have an upcoming post on vermicomposting and on our soils.org/discover-soils site, we have information about compost and other amendments. Thanks for reading. SF

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