Digging Into Plastic Pollution: How We Measure Microplastics in Soil
Microplastics have emerged as an invisible but growing environmental threat. These tiny fragments – ranging from millimeters to micrometers in size – can persist in the environment for decades, interacting with soil, water, and living organisms in ways we are only beginning to understand. While public concern often focuses on plastic pollution in the oceans, soils may actually hold far larger reservoirs of plastics, with direct consequences for food security and ecosystem health.
Plastics enter soils from many sources, including industrial emissions, litter, road runoff, landfills, and even particles carried by the air. On top of these inputs, agriculture plays a major role: mulch films, greenhouse covers, irrigation systems, and fertilizer coatings introduce plastics directly into farmland. The problem doesn’t stop there, sewage sludge and compost used as fertilizers bring additional plastic fragments and synthetic fibers. Over time, these particles accumulate in soils, disrupt soil structure and microbial communities, and may even transfer into crops.
Given the essential role soils play in sustaining terrestrial ecosystems and food production, understanding the extent and behavior of microplastics in soils is critical. Accurate analysis not only helps us assess the risks to soil health and food systems, but also provides the data needed to design effective mitigation strategies and inform environmental policy. However, measuring microplastics in soils is exceptionally challenging due to their diversity, small size, and the complexity of soil itself.
Why microplastics are so complex
Microplastics exhibit diverse characteristics across five key dimensions:
- Size range: Microplastics span from 1 µm up to 5 mm. This wide spectrum influences detectability and ecological behavior.
- Polymer types: Microplastics are made from a wide range of conventional and biodegradable polymers, each with distinct physicochemical properties.
- Shape: Plastics fragment into irregular particles, spheres, fibers, foams, and thin films, complicating recognition and classification.
- Chemical diversity: Beyond polymers themselves, microplastics can contain additives such as antioxidants, plasticizers, pigments, or sorbed contaminants like persistent organic pollutants, antibiotics, and heavy metals.
- Aging states: Plastics change once they enter the environment. Sunlight, weather, and microbes gradually break them down, altering their surfaces and making them more fragile. These changes affect how long they persist in soil and make them more difficult for scientists to detect.
This multi-dimensional variability means no single method can capture the full picture, creating a clear need for multi-analytical strategies.
How to separate microplastics from soil?
Because soil is full of natural particles that can mask or mimic plastics, separation is crucial. The most common approach includes density separation, where plastic particles float in a salt solution while heavier minerals sink. Low-density solutions like sodium chloride can recover lighter plastics such as polyethylene (PE) or polypropylene (PP). For denser plastics like polyethylene terephthalate (PET) or polyvinyl chloride (PVC), high-density solutions such as zinc chloride or sodium iodide are used.
Still, separation alone is not enough, since soil organic matter overlaps in density with many plastics. To address this, researchers use digestion methods to break down SOM while trying to preserve the plastics as much as possible. Oxidizing agents such as hydrogen peroxide or Fenton’s reagent are widely used because they effectively remove organic material without major damage to most plastics. However, harsher chemicals like strong acids or alkalis can degrade certain polymers, including biodegradable plastics. Enzymes provide a gentler alternative, though they are slower and more expensive.
Seeing the unseen: Tools to identify microplastics
Once Microplastics are separated from soil, the next challenge is figuring out what they are and how much is there. Scientists rely on a toolbox of particle-based and mass-based techniques, each with its own strengths and limitations.
- Microscopy (stereomicroscopy and fluorescence) is often the first step, giving researchers a direct look at the particles. It helps visualize shapes, colors, and sizes, but it comes with a catch – visual identification is prone to error, especially when natural materials look similar to plastics.
- Infrared-based and Raman micro-spectroscopies take the analysis further. These methods provide a molecular “fingerprint” for each particle, allowing precise identification of polymer types. Raman can resolve smaller particles than FTIR, but it is more sensitive to impurities and background noise in complex soil samples.
- Mass spectrometry methods, most often pyrolysis gas chromatography mass spectrometry, break plastics into chemical fragments for analysis. It shows which polymers (and additives) are present and how much of them there is (in mass), but because the sample is destroyed, details about individual particles are lost.
Moving Forward
Measuring microplastics in soils is still a developing science. Researchers are working to improve analytical methods, establish standardized protocols so results can be compared across studies. These steps are key to building a clearer picture of how much plastic is in our soils and what it means for the environment. But while the science advances, action can’t wait. We need to reduce plastic inputs now to protect soils, the foundation of healthy ecosystems, productive farms, and food security. Stronger science, paired with immediate action, is the path to safeguarding the ground beneath our feet.
At InPlasTwin, collaboration among partners is key to driving innovation and sharing knowledge across disciplines. This blog post reflects that spirit of cooperation, featuring insights from one of our experts.
This article was written by Dr. Nhu Phan, an InPlasTwin expert from VITO. Dr. Phan is a postdoctoral researcher specializing in the development of methods to analyze micro- and nanoplastics in complex environmental samples. She obtained her PhD as a Marie Curie ITN Fellow at Lancaster University (UK), where her research focused on “Micro- and Nanoplastics in Soil: Analytical Methods and Environmental Fate.” Currently, she contributes her expertise to several Horizon Europe projects on micro- and nanoplastics, including InPlastics and Inspire.
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