Written by Sarah Jones
Compost is well-known as an organic fertiliser - but what if compost is adding more to our soils than just nutrients?
Research has shown that some compost may contain microplastics, making it a potential pathway for microplastics to reach the environment. But does it matter?
Prepare yourself for the excessive use of size-themed juxtapositions, and the inclusion of superfluous words purely because they rhyme with plastic, as we put these mini-polluters under the microscope.
What are microplastics?
Microplastics are simply any plastic that’s less than 5mm in length, i.e. as small, or smaller, than an ant and can be separated into two groups.
Microbeads are intentionally produced for use in cosmetics and household cleaning products (the manufacture of which has now banned in the UK). The second group are those that are accidentally produced by the breakdown of larger plastics.
These plastics can break down even further, into what are known as nanoplastics, which are less than 0.1 micrometre in size (10,000th of a mm).
A plastic microbead from a facewash, taken via scanning electron microscopy. Image: Andrew Watts (CC-BY-2.0)
Small plastics, big problem
A lot of news reports have focussed on microplastics in oceans and the negative impacts they might be having on aquatic ecosystems. But they might also be much more pervasive in the wider environment than previously thought.
Recent studies have shown that microplastics are turning up in a range of other environments, notably including soil, and it could be that compost and other organic fertilisers are to blame.
Microplastics aren't just found in the ocean
Food waste is now routinely collected separately from other rubbish, as an essential way to reduce the volume of household waste reaching landfill. From there, it is taken to industrial composting facilities, where one of several techniques can be used to process it into compost.
However, the general public may not be especially thorough at sorting the veg from the wrap.
A study conducted by researchers at the University of Bayreuth found that household organic wastes reaching composting facilities contained significant numbers of plastic fragments.
The types of plastic present were those originating from food packaging and containers.
Be careful when food composting that you've removed all plastic packaging!
Unless carefully screened, a lot of these plastic particles will remain throughout composting, sometimes breaking down themselves into micro and nano-plastics.
These particles then remain in the finished compost to be applied to soils where, crucially, they have the potential to enter the food chain.
Microplastics could also be reaching agricultural soils via another biodegradable waste, sewage sludge. Sewage sludge is the organic material that is left over after wastewater treatment, which can be applied as agricultural fertiliser under certain conditions.
Researchers at the Galway-Mayo Institute of Technology found that sewage sludge could contain up to 15,000 pieces of microplastic per kg.
It’s not just industrial organic wastes that have a plastic problem though. Your home-made compost could be harbouring a similar problem on a smaller scale.
Plastic can be hidden in surprising places, such as teabags, which can make its way into your finished compost.
Plastic not so fantastic
Does it matter if soils are full of microplastics? The short answer is yes.
But to elaborate: research has shown that microplastics can have potentially negative impacts on both the environment and human health.
A study conducted at Anglia Ruskin University, released earlier this year, showed that the presence of microplastics stunted the growth of earthworms.
Microplastics could be harming earthworms
What’s more, plastics can release additives into the soil that can negatively affect the hormonal systems of insects. Any harm to insect populations could have a significant impact on overall soil health.
Microplastic particles in soil may also end up in the human food chain. A recent study published in the journal Environmental Science & Technology looked at the number of microplastics in everyday foods.
The authors suggest that people could be ingesting between 39,000 and 52,000 particles of microplastic annually.
You could be injesting more microplastics than you might think
The exact effects of microplastics on human health are not yet known, but it is thought that once they’re in the gut, there is potential for them to release chemicals and additives.
Perhaps more worryingly, it has been suggested that the smaller microplastics and nanoplastics may have the ability to cross out of the gut and penetrate cells, making whole-body exposure to them a possibility.
An additional concern is that microplastics could also serve as a 'vessel' for other pollutants, such as toxic metals, pesticides, and even pathogens.
Get drastic on plastic - top tips for preventing it in your compost
The good news is that if you have your own compost heap, it’s easy to reduce the amount of microplastic that reaches your end product.
The most important thing to remember is to know your source material – plastic can only end up in compost if it’s in the stuff you put on your compost heap. Even teabags that are labelled as 'plastic-free' can contain small amounts of bio-plastics that may not break down on a home compost heap.
Be careful to check that teabags are definitely plastic free before putting them in your compost
Disposable paper cups, plates and bowls can be coated in plastic – make sure anything of this nature that you add to your heap is plastic-free and stated to be biodegradable. Plastic coated cartons and Tetra-pak should be recycled rather than added to your compost heap.