Can two distinctive environmental phenomena like microplastic contamination and desertification interact? We don’t have definitive answers. But MINAGRIS research highlights a potential problem with high plastics use in arid soil conditions.  

MINAGRIS partner Charlotte Chivers recently visited the Cartagena region of Spain, an area characterised by arid conditions, water scarcity and increasing desertification.

In an area characterised by mostly clay loam soils, she observed extensive use of agricultural plastic as mulch and tree protection, alongside stockpiles of agricultural plastic waste awaiting delivery to recycling facilities in several locations.  

Photos taken at the Cartagena site visit. Photo credits: Charlotte Chivers.

Farmers are heavily reliant on plastic mulch as a cost-effective way to help water retention and weed suppression, reducing the need for pesticides. Often due to the lack of effective farm plastic recycling schemes, end-of-life plastics are often stockpiled in areas of farmland.

It is also difficult to remove all plastic from fields. Inevitably, some of it will be left to decompose into microplastics, which can linger in the soil over generations.

Recent research by MINAGRIS reveals a serious problem that could potentially compound desertification – that microplastics can make soils repel water. This means that soils not only become dryer, but they are also less able to grow crops.

The water repellency problem

Research by Špela Železnikar and colleagues for MINAGRIS showed that, as plastics degrade into microplastics, their high ‘hydrophobicity’ – the tendency of a substance to repel water – can induce soil water repellency.

Soil water repellency (SWR) is a major problem for agriculture and may be increasingly so as the impacts of climate change accelerate. When soil repels water instead of absorbing it, plants struggle to get the moisture they need, leading to lower crop quality and yield.

Our research conducted experiments on two types of soil – silty clay loam and sandy loam. They mixed soils with three different types of plastic film – a conventional plastic, a biodegradable plastic and a bio-based alternative (starch). They then tested how long it took for water to penetrate each soil.

The MINAGRIS research revealed that:

• Microplastics can alter how repellent soil is to water by coating soil particles. The microplastic polluted soil repels water, which will affect soil health and plant growth.
• Differences were observed between bio-based and non-biobased plastics – both conventional and biodegradable plastic significantly increased water repellency while the bio-based starch needed much higher concentrations before an impact was shown.

So while plastic film is used by farmers to hold water in soil in response to dry conditions, over time and with microplastic accumulation, it may make the problem of desertification worse. We would like more research to find out if this is in fact the case.

While the site Charlotte visited was clay loam soil, soil types vary across the region. Many desertification‐prone areas feature sandy soils that are far more susceptible to this effect.

Widespread plastic usage and poor recycling systems leading to plastics being left to decompose in agricultural fields may impact long‐term soil health and resilience in these fragile ecosystems, and in the longer term, food security.

It is vital that we innovate new, better and cheaper products to replace plastics in farming along with effective recycling schemes to support farmers to dispose of plastic safely.

Check out our simple guide about how to test soil water repellency on your land.

Read our accompanying factsheet.

Full paper about how microplastics can increase soil water repellency.