Researchers have revealed that plastic mulch used to boost strawberry growth leaves behind large amounts of material fragments in the soil. Findings from the study are likely to apply to worldwide plastic use in agricultural production.

A team of scientists from California Polytechnic State University, US, surveyed strawberry fields in the state after the seasonal film removal which covers the plants. The researchers discovered up to 213,500 pieces of microplastics – particles larger than 5mm across – per hectare on field surfaces alone. 

They also found that plastic pollution reduced soil moisture, microbial activity and plant-available nitrogen, essential for soil health and crop productivity.

The findings, presented at the Goldschmidt geochemistry conference in Lyon, France, raise concerns about the long-term sustainability of using plastic mulch in agriculture, especially as California is the largest shedder of agricultural plastic in the US and one of the major strawberry producers in the world.

“What we are seeing is a huge quantity of plastic material being shed wher the mulch is used to enhance strawberry production. These can remain in the soil for decades or longer,” says Dr. Ekta Tiwari, the study’s lead author.

“Plastic mulch provides benefits, but at the expense of long-term soil quality. It’s difficult and expensive to remove these particles from the soil, so once they are there they can stay indefinitely.”

Plastic even in the best managed fields
Polyethylene (PE) and other plastics are widely used in agriculture, for example, to make polytunnels (hoop greenhouses). Another common industry application is plastic mulch films, which have various benefits for crops. 

They cover the soil around the plant, which can help with weed and pathogen control, water conservation and soil protection from fruit splashing, especially for strawberries.

The mulch is laid in rows and taken away after the crop’s season. However, some plastic fragments may remain in the soil after careful removal by farmers as they stick to the ground and break off.

“After decades of annual plastic mulch application and removal, the researchers observed the accumulation of plastic fragments within farm soils, even in well-managed fields,” notes the study. 

“Researchers found that as the levels of macroplastic pollution increased, soil moisture content, microbial respiration and plant-available nitrogen declined.”

There are existing alternatives to PE mulches, such as biodegradable plastic mulches or natural mulches such as straw, which are expensive, according to the scientists.

Alternatives to PE mulch 
Researchers identified most of the particles as PE, widely used for making plastic bags and films. They also analyzed some soil samples for microplastics – smaller particles less than 5mm across – but have not yet reported their results.

“We tend to think that strawberries are simply things to be enjoyed, but this shows that even something as delicious as fresh strawberries can come with a cost to the environment. We are working with manufacturers to see if we can mitigate these costs,” Tiwari underscores.

She suggests that alternatives to using PE mulches, such as biodegradable plastic mulches or natural mulches like straw, could be considered by farmers and policymakers, but the lead author also acknowledges that material alternatives might be connected to higher economic costs or lower effectiveness.

“Plastics, and plastic mulches in particular, are vital to maintaining agricultural production. They are used for various purposes, including soil moisture retention, soil warming/cooling and weed or pest control. Agricultural plastic use is increasing worldwide, with California being the largest user of agricultural plastic in the US,” notes professor Sean Schaeffer from the department of biosystems engineering & soil science at the University of Tennessee, US.

“Research on the fate and transport of plastics in soil and water systems is relatively recent, so studies like this are vital to increasing our understanding of the plastic problem’s scope. We currently know relatively little about the distribution, size and types of plastic in soils in the largest states, in land area and in agricultural production,” he concludes.