The world is clearly producing, using, and discarding too much plastic. But the problem is that plastic is so convenient and cheap that it’s hard to give it up. So as an alternative, consumers have recently turned to so-called compostable and biodegradable alternatives for things such as plastic cups, cutlery, bags, and food containers, hoping to mitigate any further plastic environmental harm.

But this compostable plastic may not be all that compostable. According to a new study, many of these products don’t actually break away and just stay in the soil instead.

In a UK-wide study, researchers found about 60% of home-compostable plastics don’t fully disintegrate in home compost bins, and end up in the soil. The study also found that most people are much confused about labels such as compostable and biodegradable plastics, which leads to people not disposing of plastic waste properly.

Plastic ain’t fantastic

Plastic waste is one of the leading environmental problems across the world. Plastic consumption has increased four times over the past 30 years, according to a recent OECD report. Globally, less than 9% of plastic waste is actually recycled, while 50% ends up in landfills, 22% evades waste management systems and 19% is incinerated.

Both biodegradable and compostable plastics can break down, but they aren’t exactly the same. Biodegradable plastic is defined by its ability to biodegrade into substances found in nature, and in a small timeframe. Compostable plastic also biodegrades, but it’s designed and tested to be processed at home or industrial composting facilities.

At least that’s the theory. But the practice doesn’t really align.

“The typical fate of landfill or incineration is not usually communicated to customers so the environmental claims made for compostable packaging can be misleading,” study author Danielle Purkiss said in a statement. “Our study was created in response to feedback that highlighted many systemic issues in compostable plastic packaging.”

The problems with compostable plastic

Purkiss and her colleagues at University College London created a citizen science study they called the Big Compost Experiment. They asked participants from the UK to fill out a survey about opinions and behavior on plastic waste. Then, they were invited to do a home composting experiment and finally send traces of their chosen compostable items.

The researchers collected the data over a period of 24 months. Results showed most people were willing to buy compostable plastics but were confused about their labelling. Out of a sample of 50 item images, they found over 45% had no clear home composting certification labelling and 15% had an industrial composting certification.

But what’s more concerning is that 60% of the plastic certified as home compostable didn’t fully disintegrate in home compost bins. Participants said they used their compost in their flower and vegetable gardens. The experiment showed that compost had plastic that hadn’t decomposed and then ended up polluting the garden’s soils.

The study also showed that compost bins are important places for biodiversity, as participants sent pictures that showed over 14 different categories of organisms – such as fungi, mites, and worms. It’s this ecosystem of organisms that is responsible for biodegrading the items put in the composter, including compostable plastics.

For Purkiss, the study has shown that home composting, when not properly controlled, is largely effective and not a good method to dispose of compostable packaging. She asked to review and improve overall home compostable plastics. A material by itself isn’t sustainable, she said, only its production, collection, and reprocessing can actually be sustainable.

The study was published in the journal Frontiers.