We were asked why MINAGRIS doesn’t look at the impact of rubber in agricultural settings and soils.

A simple answer is that we’re examining how microplastics affect soil, and you can’t look at everything.

But it’s not surprising that this would be a question. Rubber is a multi-level pollutant. And it is often seen on farms. A small hill of worn-out tyres in field corners and rubber clamps or tyres on top of silage pits in farmyards is not an uncommon sight.

So let’s look more closely at rubber – the different types, how it differs from plastic, the way it acts as a pollutant, and why we’re not including it in our programme.

Rubber is complex

Rubber and plastic seem quite similar. Both can be flexible and they often look similar. However, they are different products. Rubber is a polymer, like plastic, but it is classified as an elastomer, which means it’ll return to its original shape after being stretched. Plastic is a polymer that is stronger and more suitable for moulding because it holds its shape rigidly.

Rubber is also not just one product. You can get natural and synthetic rubber. Synthetic rubber is the result of many different production methods and types of chemical additives. That matters when it comes to scientific method, as we will explain later.

But let’s first dig into the differences between natural rubber, synthetic rubber and plastic.

Picture credit: Deborah Talbot

In agricultural settings, natural and synthetic rubber are likely to be found in tyres, silage clamps, rubber flooring in animal stalls, hoses, pipes, and belts. Seventy per cent of the world’s natural rubber is used to make tyres.

Conversely, plastics have a wide variety of land uses, such as plastic mulch, sheeting, tarps, and packaging of produce.

All of these products are likely to be found in soil as they are discarded and degraded.

The environmental cost of rubber

Like plastic, rubber can cause environmental problems. These include its production, the level of waste once it has served its purpose, and what happens when rubber is absorbed by the soil and wildlife as it decomposes.

Rubber’s environmental impact is widespread, as most products are made with a combination of natural and synthetic rubber or are simply synthetic.

Take the example of tyres, not all of which are composed purely of natural rubber.

Studies have shown that used tyres have an environmental burden – the distribution of tyre particles as they wear and the leeching of chemical compounds and heavy metals that go into their production into soil and water.

Synthetic rubber production includes volatile organic compounds (VOCs) and other hazardous chemicals, which harm the environment and the health of humans and other animals.

Even natural rubber, which technically should be pollutant-free and will biodegrade without causing environmental problems, is not without cost. The fact that natural rubber is primarily cultivated as a monoculture means that trees are vulnerable to pests and diseases, particularly as climate change accelerates. Fertiliser and pesticide use are common, leading to soil degradation and aquatic pollution.

Environmentalists advocate for the greater use of natural rubber but with different cultivation methods – for example, relying on a more diverse product and introducing more vegetation biodiversity into commercial rubber production. Scientists are exploring the viability of species such as Taraxacum kok-saghyz and guayule as alternatives.

Innovators claim that synthetic rubber production does not need to rely on petroleum-based materials, and more sustainable bio-based alternatives are being developed. However, the environmental cost of these alternatives needs to be understood.

Why not rubber?

Synthetic rubber arguably has an equally detrimental impact on soil and wildlife. So why haven’t we included it in the Minagris study?  

We focus on plastics partly because you can’t look at everything in a research study. Policymakers need answers to questions that only an in-depth scientific study can answer. Spread your research too thinly, and it is likely you won’t have the time to dig deep.

To Minagris, plastic pollution is a global environmental harm that needs urgent action, as the graphic below illustrates.

Photo credit: Machado et al (2018)

However, the exclusion of rubbers is also about scientific testing.

To assess a substance’s impact on soil, we need reference spectra. A reference spectra is a simulated spectrum or classifier that can be used as a benchmark for comparison. It ensures that results are consistent and replicable.

We can only detect polymers when we have reference spectra. So, we have included types of plastic polymers for which we have spectra, or, in the case of PBAT, where we will soon have one.

Rubber is more complicated because, as we have already discussed, it can describe various products, making the interpretation of spectra difficult and unreliable.

On top of this, there’s the IR absorption problem. IR absorption is where infrared light is used to study the atoms in materials. Rubber particles usually have high IR absorption, so whatever signal is transmitted is too weak to be reliable.

In short, if we included rubber, much of our work would involve describing the diversity of spectra when we would have a low chance of getting successful results due to the weakness of the IR signal.

The nature of science

Do we need to look at rubber as part of MINAGRIS? We’d argue not.

In fact, it would be similar to asking why we aren’t looking at any chemical substance that might find its way into agricultural soil (for example, pesticides, which is the remit of another research project, SPRINT). Just because plastic and rubber look similar doesn’t mean they are when considering their composition, production and decay.

Each research programme must have a point of focus. For MINAGRIS, that focus is the impact of plastics on agricultural soil and how farmers can begin to assess the viability of alternatives.

We hope that there will be more studies to build on examples of existing research on how rubber behaves in agricultural settings.