Microplastics in compost: what 2025 studies found and what to do at home

Compost can carry microplastics, and some of them can end up in the vegetables grown in it, but this is an early finding, not a reason to stop composting. A 2025 Australian study counted 1,500 to 16,000 plastic particles in each kilogram of processed organic waste. A 2025 study by Thai researchers found particles in the edible parts of lettuce, radish and tomato grown with contaminated compost. The practical answer is cleaner inputs: keep plastic out of what you compost, and look at what you buy.

This is the fourth lesson of the path “New findings every home grower should know”. It covers what the two studies measured, what the numbers mean for one pot or bed, what is still unknown, and what you can do at home.

What microplastics are

Microplastics are plastic pieces smaller than 5 mm, the size FAO uses. Larger plastic breaks down into them over time, and they can break further into nanoplastics, which are smaller still. FAO notes that agricultural soils are thought to receive far greater amounts of microplastics than rivers and seas, yet most research so far has looked at water.

Study 1: how much is in compost

Griffith University’s Australian Rivers Institute tested processed organic waste, including compost, from 11 facilities in Victoria, Australia. The results, published in the Journal of Environmental Management in 2025:

• Every kilogram held between 1,500 and 16,000 microplastic particles.
• Most particles were 20 to 500 micrometres across, that is 0.02-0.5 mm, too small to pick out by hand.
• Most were microfragments, from larger plastic items, and microfibres, from fabrics such as clothing.
• Some fragments closely matched the physical and chemical make-up of bin bags sold as compostable or biodegradable, which the researchers suspect as one source.

The lead author noted that Australia has no national standard for microplastics in processed organics. The team called for monitoring and for stopping plastic at the source.

Study 2: does it reach the vegetables

A team from Sirindhorn International Institute of Technology at Thammasat University and Thailand’s National Center for Genetic Engineering and Biotechnology (BIOTEC) published the second study in Water, Air, & Soil Pollution in October 2025. They grew lettuce, radish and tomato using four composts whose plastic content had already been measured.

Crop	Part tested	Microplastics found (particles per gram, dry weight)
Lettuce	leaves	118 ± 136
Radish	storage root	140 ± 130
Tomato	ripe fruit	81 ± 90

They also found that the compost with the most microplastics slowed early growth and gave smaller plants, and that lettuce was hit hardest. Leaf pigments were unaffected. The authors’ own conclusion is careful: microplastics can build up in edible parts even over a short growing period, and long-term build-up and human health risk still need research.

These are real findings, but they are early: one survey of compost and one short growing trial. Neither study set a level that is safe or unsafe to eat.

Worked example: reading the numbers

How many particles go into one pot?

1. You mix 5 kg of compost into a container mix.
2. Low end: 5 × 1,500 = 7,500 particles.
3. High end: 5 × 16,000 = 80,000 particles.
4. The range is more than tenfold, and that spread comes from the source: different facilities, different inputs.

What does “118 ± 136” mean? The ± is the spread between samples. When the spread (136) is larger than the average (118), some samples held very few particles and others many times the average. One average figure hides that. It tells you contamination varies a lot with the compost, which is good news: the input is something you can change.

What you can do at home

• Keep plastic out of the bin. Only plant scraps, leaves, plain paper and cardboard. If you are not sure something is plastic-free, leave it out. The home compost lesson covers what goes in.
• Be wary of “compostable” bags in a home bin. The Australian study suspects them as a source. A small home pile rarely gets as hot as a large one, so anything slow to break down stays longer.
• Look at bought compost. Spread a handful on a tray. Visible bits of film, fibre or coloured plastic suggest more you cannot see. Ask the supplier what goes in.
• Pick out old plastic in beds. FAO lists non-biodegradable mulch film among the highest-concern farm plastics; old film left in soil breaks into smaller pieces.
• Keep composting. Compost still improves structure, water holding and nutrients. The problem is plastic in the inputs, not compost itself.

Common mistakes

• Taking one number as the whole story. “16,000 per kg” is the top of the range, not a typical value.
• Assuming washing fixes it. Washing removes surface soil and dust, which still matters for other reasons (washing vegetables), but not particles inside plant tissue.
• Treating “compostable” as “safe in any pile”. The label describes how a bag is meant to break down, not proof that it leaves nothing behind, and these bags are one of the suspected sources.
• Giving up compost for bagged mineral fertiliser alone. That throws away the soil benefits without removing plastic from your life or garden.

What is still unknown

How much of what reaches a vegetable reaches a person’s body, and what it does there, is not settled. The amount needed to cause harm has not been set. Expect more studies; the useful thing today is less plastic going into soil.

Sources

• Study finds one kilogram of compost contains up to 16,000 microplastic particles — Griffith University News
• Integrated Impacts of Microplastics Contaminated Compost on Soil–Plant Systems and Bioaccumulation in Lettuce, Tomato, and Radish — Water, Air, & Soil Pollution (Springer)
• Plastics in agrifood systems: The good, the bad and the ugly — FAO