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Bits of plastic in compost and soil - what's the issue?

Tony Callaghan 08/08/2022

Bits of plastic in compost and soil - what's the issue?

Compost made from recycled garden waste (aka green waste) can contain bits of plastic. Gardeners are put off using green waste, peat-free, compost because of this.  In this blog, we explore the issues of plastics (see definition)* in compost and soil:

  • How do they get there?
  • Can they be removed?
  • Are they a problem?

This is a topic that is likely to generate a lot of strong feelings.  We welcome feedback and different views but please ensure it is given in a calm and reasoned manner.

Compost manufacture in the UK

In the UK, around 300 companies process about 15 million tonnes of green waste every year into about 5 million tonnes of compost. Most of this is made and sold under the regulated BSI “PAS100” certification scheme.  It’s hard to grasp the scale! If you think about a big artic lorry on the road, it will hold and transfer about 20 tonnes of green waste. That’s 750,000 arctic lorry trips every year, on average 50/week/site.

Compost sites receive green waste collected on behalf of local councils from our homes, parks and various landscape contractors.  All this garden “green waste” should all have no plastic in it. Regrettably, in practice, green waste always has items like plastic bags, plant string, ties, bits of plant labels, pots etc. Most of the contamination is accidental, it just gets scooped into the bin when clearing up the garden.  However, sometimes it appears that the "green waste bin" is being used as a garden trash bin!  We need to remove as much plastic as possible, but achieving plastic-free green waste collections is unlikely to be achieved.

At the compost site, visible larger items (e.g. plastic containers) can be hand-picked out, but it is not practical to hand-pick through 15m tonnes of waste. The waste is shredded and piled into what is termed ‘windrows’. Shredding converts 1 bit of plastic into 10  to 100 bits of plastic.  After 12-16 weeks the composting process is complete and the compost is screened (sieved) to remove ‘oversize’. Typically two screens are used (40mm and 25mm). Screening removes a lot of plastic, but smaller bits can and do fall through the 25mm screener into the compost.

Is there a technical solution to remove plastic from compost?

Visible plastic is reduced to almost zero when the compost is sieved at 10mm. The nature of sticky, moist compost and small screens makes this a slow and expensive process. Few compost companies do it and only a small percentage of compost gardeners will pay the extra - e.g. double the price for a bag of compost.

There are advanced IR-scanning solutions that might help remove plastics before the green waste gets shredded.  These systems are expensive to deploy but need investigation. However even deploying the very best technology, some plastic bits will end up in the compost.

We need to establish what level is acceptable and safe. The PAS100 certification scheme tests composts and set limits for plastic.

There is no excuse for compost ‘riddled’ with plastic – it does not meet the specification and should be returned to the supplier for a refund. However, anecdotal evidence suggests that customers get concerned and complain when they spot any plastic in a bag of compost.

Why do we use green waste (i.e. garden pruning and grass) to make compost?

If we do not recycle the 15 million tonnes of green waste created in the UK, four things will happen:

  • More precious peat will be dug up and used to make ‘compost’
  • Even more coir (coconut husk) will be imported from Asia to make ‘compost’
  • 15m tonnes of green waste would go to landfill (it can't there is no more space - but even if there was space, in landfill it would decay aerobically to create methane which is x24 times more potent greenhouse gas. Rough calc: 15mt waste would generate 220 million equivalent tonnes of CO2e.
  • We lose all the nitrogen and other minerals in the green waste into landfill.  Gardeners would have to buy an estimated 150,000 tonnes more man-made fertilisers!

Composting is the right thing to do - we just have to do it better in terms of reducing plastic contamination.

Do plastics in your compost and soil cause a problem?

There are strong online debates about plastics and micro-plastics and how they might or might not affect your health and soil health.  Hopefully, the following will offer some useful insight

Do they look unsightly

– yes, but if they are safe, this seems overridden by the recycling peat-free arguments

Are the bits of plastic in compost dangerous to wildlife?

On balance, in compost, probably not.  Blue planet and scores of other researchers have made it abundantly clear that plastics floating in the sea and blowing around on land are a major threat to wildlife.  But small bits in the soil do not tangle and cause the same issue.

Will bits in the soil get eaten and cause a problem for soil insects?

It seems unlikely that 2-10mm bits of plastics will be eaten - they have no nutritional benefit. But there is growing evidence that microplastics are being consumed in various parts of the ecosystem. If the (micro) plastics are ingested will they cause issues? Probably but how so is as yet known. There are not digested by any known enzyme, so the most likely outcome is they will only pass via predators eating. At each stage of the transfer, the microplastic is most likely to stay in the digestion tract or be excreted as faeces.

Do plastics “leach” toxic chemicals (e.g. BHPs, PAHs, BTEX, EDCs etc).

The main group of plastic polymers (polyethylene, polypropylene, PET, PS) are non-toxic and contain few if any additives. Some plastics contain tiny amounts of additives to help with UV protection, fire resistance etc. You will find many scary reports on the internet that plastics contain hundreds of toxic chemicals that leach out and cause cancers etc.  I have no reason to doubt this is possible when we look across the many thousands of different plastics made. However, it is also true that in nature, in our food, and in our soil, there are thousands of naturally occurring toxic chemicals that are equally dangerous.  It comes down to how much is present and just how toxic they are. No one wants unnecessary risk, but we have to live and manage many risk factors.

It is worth noting that in soil & composting, microbes cannot decompose the core plastic polymers. However, they can and do decompose almost all other known organic (carbon-based) chemicals.  Many of the toxic leach chemicals noted are organic (carbon-based) and with the exception of PFOS, will be digested and removed by soil microbes within 12-18 months.

Plastics have been around for a hundred years and now exist in quantities measured in billions of tonnes. We should never let our guard down, and always have external independent organisations looking at H&S and controlling what is and is not used by the industry, but at this stage, I remain unconvinced that 'leaching' from common bulk plastics is a cause for concern.

Do plastics in soil biodegrade? Will they be there forever?

The plastic polymer molecules do not biodegrade –  because there are no microbes with enzymes to break down plastics*.

Although plastics do not biodegrade, we know that over time, plastics degrade through a number of physical and chemical processes.

As the polymer surface is attacked by UV light and other oxidising agents, molecules are broken off. These molecules can and are biodegraded. This combined degradation happens very slowly. But it is worth noting that figures stating plastics will be around for 1,000 years are often quoted, but every test is based on a specific set of test conditions. All those who read about hot composting will know reactions can be 32 times faster at 60C than at 10C, If the test was at 10C and was repeated at 60C, then the new estimate could easily be 30 years. There are many research programmes looking at how we can speed up plastic decay.

As the polymer is broken into ever smaller pieces we end up with microplastics so small they cannot be seen by the naked eye. These small particles can be ingested by animals. It is still not clear just how damaging this will be.

What is the issue with plastics?

Let us put our cards on the table: plastics per se are not the issue. The movement to imply “plastics are evil and the plastics industry is evil” is a human reaction to evidence (via Blue Planet) of an environmental disaster they can cause. Humans seek to blame one party rather than stand up and accept that human behaviour across the globe is the cause of the problem.

There would be no plastics in the ocean nor plastic in compost and soil if we did not throw them away. We need to fix this behaviour at personal, local, country and global levels.

Our SF40 and SF60 biochar super composts are premium products. We will keep working hard on the green waste composting processing side to ensure the absolute minimum levels of plastic is in the end product. For those that want an absolute guarantee, then the option to make your own biochar super compost by adding the compost humification agent to your own compost heap is an option.


*Define Plastics

The term plastic* is used in this compost and soil-related blog means the common group of plastics, i.e. PE, PP, PET, PS.  This group accounts for >80% of the total production of plastics.  It excludes the many hundreds of other specialist plastics and the cellulose/starch/PLA compostable plastics now on the market replacing single-use plastics).