Mapping the Invisible Plastic Reaching Our Farmland

We tend to picture plastic pollution as something that happens to the ocean: bottles on beaches, a gyre of debris in the Pacific. But some of it is going the other way, into the ground under our food. Every time compost, treated sewage sludge or manure is spread on a field to enrich the soil, it can carry a hidden cargo of tiny plastic fragments along with the nutrients. Until now, no one had drawn a global picture of how much, or where.

A team led by Federico Maggi of the University of Sydney set out to build that picture. Working with Fiona Tang of Monash University, Francesco Tubiello of the UN Food and Agriculture Organization, and Budiman Minasny of Sydney's Institute of Agriculture, they assembled a dataset they call MPGRIDS, described in Environmental Research Letters as the first global geospatial map of microplastic inputs to agricultural soils through organic amendments. The work was accepted for publication on 1 July.

Their central number is a range, not a single figure, and the width of that range is part of the story. Across the world's farmland, they estimate, somewhere between 33,000 and 2.3 million tonnes of microplastics reach the soil each year through organic fertilizers, with a median of about 910,000 tonnes. That is a spread of roughly seventy-fold between the low and high ends, a sign of how much is still unknown rather than a hedge. Locally, the input rates vary just as dramatically, from less than 0.01 to more than 100 kilograms of microplastic per hectare a year, depending on what gets spread where.

To reach those figures, the team pulled amendment-application data from 17 countries and microplastic-concentration measurements from 26, then extrapolated across the globe's cropland using a detailed map of where fruit, vegetables and pulses are grown. That extrapolation is exactly why the uncertainty is so large: for most of the planet's farmland, nobody has actually measured the plastic in the fertilizer going onto it. The model fills those gaps with the patchy evidence available.

Which is why the authors are unusually blunt about their own numbers. Rather than present the estimate as a firm accounting, they write that "these numbers underestimate reality," pointing to the "paucity of data and uncertainty" in existing measurements as a fundamental limit. In other words, the real figure is probably higher than even the top of their range, not lower. For a first global estimate, that candour is the appropriate posture: the map is a starting point, not a verdict.

What the map does offer is geography. Because it is spatially explicit, it shows where microplastics are likely piling up fastest: the regions and crop systems where heavy use of organic amendments meets high plastic contamination. That matters because microplastics in soil are not an inert nuisance. Research has linked them to changes in soil structure, water movement and the microbial life that farmland depends on, and there are open questions about whether some of that plastic makes its way into the crops themselves. Knowing where the burden concentrates is the first step toward measuring what it does.

None of this makes organic fertilizers the villain. Returning compost, manure and treated sludge to the land recycles nutrients and cuts reliance on synthetic inputs; the problem is the plastic that rides along, much of it traceable to contamination upstream in waste streams. The value of a map like MPGRIDS is that it turns a diffuse worry into something locatable: a way to ask where the plastic is entering, and to start closing the enormous gap between what has been measured and what is actually in the ground.

The dataset and methods are described in Environmental Research Letters.