When we think of plastic pollution, we usually picture a bottle floating on ocean waves or a garbage patch drifting in the middle of the Pacific. Yet a growing body of research shows that an equally serious, far less visible problem is unfolding literally beneath our feet — in the soil of forests, parks, and gardens. Microplastics, plastic fragments smaller than 5 millimeters, are infiltrating terrestrial ecosystems at a pace that has surprised even the scientists studying them.
In this article, we look at where microplastics in soil actually come from, how they affect the organisms living underground, what this might mean for trees and entire forest ecosystems, and what each of us can do to reduce the scale of the problem. It is a topic that shows protecting nature today requires looking not only at what is visible to the naked eye, but also at what happens at the microscopic scale.
What Microplastics Are and Where They Come From on Land
Microplastic is a general term for plastic particles ranging from 1 micrometer to 5 millimeters in size. They fall into two main categories: primary microplastics, manufactured directly in this form (such as the pellets used in cosmetics), and secondary microplastics, created when larger objects — plastic bags, packaging, synthetic textiles, or car tires — break down under UV radiation, friction, and weathering.
For years, research and media attention focused almost exclusively on the oceans, since that is where microplastics were easiest to observe and photograph. Recent analyses, however, suggest that microplastic concentrations in some terrestrial soils can be several times higher than in seawater. Soil functions as a kind of reservoir — plastic particles that end up in it are not as easily washed away or broken down as one might expect, and can persist there for decades.
Where Microplastics in Forest and Urban Soils Actually Come From
There are surprisingly many sources of microplastic in soil, and most of them are not the first thing that comes to mind when thinking about plastic pollution:
- Car tires — tread wear during driving releases enormous quantities of rubber and plastic microparticles that settle along roadsides and eventually reach roadside tree plantings and drainage ditches.
- Sewage sludge used as fertilizer — wastewater treatment plants cannot filter out the finest textile fibers released when washing synthetic clothing, and sludge from such plants is sometimes applied to agricultural land as fertilizer.
- Agricultural and garden plastic films — plastic mulch, used to protect crops from weeds, gradually crumbles into tiny fragments that remain in the soil even after the main sheet of film has been removed.
- Atmospheric transport — studies show that microplastic can travel long distances on the wind, reaching even remote, seemingly untouched forest and mountain areas.
How Microplastics Affect Soil Organisms and the Microbiome
Soil is not an inert mineral mass but a living, remarkably complex ecosystem in which earthworms, mites, nematodes, fungi, and bacteria play crucial roles in decomposing organic matter and cycling nutrients. The presence of microplastic in this environment disrupts many of these processes in ways we are only beginning to understand.
Studies on earthworms — organisms considered among the most important “engineers” of soil — show that exposure to high concentrations of microplastic can slow their growth, impair digestive function, and reduce fertility. Because earthworms aerate the soil and mix its layers, any disruption to their functioning has cascading effects on soil structure, its ability to retain water, and the availability of nutrients for plants. Similarly troubling signals are emerging from research into the soil microbiome — the community of bacteria and fungi, including the mycorrhizal fungi that partner with tree roots — where microplastic exposure can alter the composition and activity of these microorganisms.
Microplastics as Carriers of Other Pollutants
Microplastic particles rarely occur in soil in a pure form. Their porous, rough surface makes it easy for heavy metals, pesticides, and other chemical pollutants present in the environment to bind to them. In this way, microplastic acts as a kind of “shuttle,” transporting harmful substances deeper into the soil profile and potentially into organisms that ingest these particles — making the problem considerably more complex than the presence of plastic alone.
What We Know About Microplastics’ Effects on Trees and Plants
Research into microplastics’ direct effects on trees is still in its early stages, but the findings so far give cause for concern. Experiments on seedlings of various tree species show that high concentrations of microplastic in the growing medium can limit seed germination, slow root growth, and reduce the efficiency of water and mineral uptake.
A particularly interesting — and worrying — line of research concerns the effect of microplastic on mycorrhizal symbiosis, the partnership between tree roots and soil fungi that is essential to the health of an entire forest. Choosing tree species suited to local conditions, much as we do in our own planting projects, is one factor that can help mitigate the effects of such disruptions. If microplastic interferes with the functioning of the fungal network, it could indirectly weaken trees’ resilience to drought, disease, and other environmental stressors, even when the trees themselves show no visible signs of harm.
Microplastics in the Food Chain — From Soil to Larger Animals
Microplastic present in soil does not stay put forever — it enters the food chain at multiple levels. Earthworms and other soil invertebrates ingest plastic particles along with organic matter, sometimes mistaking them for food, and then become prey themselves for birds, hedgehogs, or small mammals living in the same ecosystem. In this way, a pollutant that began as an invisible fragment in the soil can, after a few links in the food chain, end up inside the bodies of much larger animals, including species we typically associate with a symbol of healthy nature.
Scientists describe this process in several stages that help illustrate the scale of the phenomenon:
- Breakdown of larger plastic waste into tiny fragments in and on the soil, driven by sunlight, frost, and microorganisms.
- Uptake of microplastic by soil organisms such as earthworms, mites, and nematodes during normal foraging.
- Transfer to predators — insectivorous birds, amphibians, and small mammals that feed on soil organisms.
- Potential bioaccumulation at higher levels of the food chain, though the exact scale of this phenomenon in terrestrial ecosystems is still the subject of intensive research.
While we still don’t know everything about the long-term consequences of this process for wildlife health, the mechanism itself — the gradual movement of pollution up the food chain — is already well documented in numerous field studies. This shows that reducing microplastic at the source, before it ever reaches the soil, is a far more effective strategy than trying to remove it from the ecosystem at a later stage.
What We Can Do to Reduce the Microplastic Problem in Soil
Although the scale of microplastic pollution can feel overwhelming, and its sources scattered and hard to control, there are many concrete actions that genuinely reduce the amount of plastic entering the soil — both at the individual and institutional level. Changing everyday habits at home and in the garden may seem minor, but multiplied across billions of daily decisions, it has a real effect on the amount of plastic circulating in the environment.
- Cut back on plastic garden films and choose natural alternatives, such as straw, bark, or biodegradable fabric, for mulching flower beds and vegetable patches.
- Air-dry laundry instead of using a tumble dryer, and if possible, use a washing machine filter that captures microfibers released when washing synthetic clothing.
- Support local green-space cleanup initiatives, since plastic litter left behind eventually breaks down into microplastic directly in the soil we are trying to protect.
- Choose clothing and textiles made from natural fibers where possible, reducing the amount of synthetic microfibers released into the environment during washing and use.
Equally important as individual consumer choices is supporting regulations and standards that limit the use of primary microplastic in everyday products and require textile manufacturers to implement microfiber-filtering solutions at the production stage. You can read more about how we care for the cleanliness of green spaces during our tree-planting and forest-cleanup campaigns on the One More Tree website.
Why Healthy Soil Is the Foundation of Successful Tree Planting
The microplastic problem makes it clear why planting trees alone is not enough if we do not also take care of the quality of the soil those trees are meant to grow in. A seedling planted in soil with a disrupted microbiome and reduced water-holding capacity will have a harder start and a lower chance of surviving the first, most critical years.
That is why, in our own projects, we pay close attention not only to the number of trees planted but also to the condition of the land where we carry out plantings — we assess soil structure, choose species suited to local conditions, and try to minimize disruption to natural soil processes. This approach, though less visible than raw planting numbers, is what determines in the long run whether new plantings actually survive and continue performing their ecological functions for decades. We encourage you to explore our educational articles, where we regularly cover similarly overlooked aspects of environmental protection.
An Invisible Problem That Demands Visible Action
Microplastic in forest and urban soils is a reminder that environmental pollution is rarely limited to what is visible to the naked eye. Real ecological challenges often play out at a microscopic scale, in places we will never see with our own eyes — yet they have a genuine impact on the health of forests, soils, and ultimately the entire planet.
Awareness of this problem is the first step toward changing everyday habits — from the textiles we choose, to how we care for our gardens, to supporting organizations dedicated to protecting soil and forests. Every decision that reduces the amount of plastic entering the environment is an investment in the health of ecosystems we all depend on — even if we won’t see the results of that investment with our own eyes right away.
