Polyethylene mulches leave behind ‘negative effects for decades’

A study on strawberry crops in California reveals: the extensive use of traditional plastic mulches causes the dispersion of micro and macro fragments in the soil. A phenomenon that is more serious than expected as over 350 particles per kilo of soil are counted

by Matteo Cavallito

 

The use of plastic mulches can result in the dispersal of particles that are expected to remain in the soil for a long time, thus damaging its health, a research from California Polytechnic State University presented at the Goldschmidt Geochemical Conference in Lyon in recent weeks has found. The study focused on Californian strawberry fields.

“What we are seeing a huge quantity of macroplastic plastic material—particles bigger than 5mm across—being shed where the mulch is used to enhance strawberry production,” said US university researcher Ekta Tiwari in a statement released on the occasion. “These can remain in the soil for decades or longer.”

Mulches are useful but not without drawbacks

Typically polyethylene-based, plastic mulches are often applied around the base of the plant. Their use helps to control weeds and pathogens, reduce water evaporation and prevent soil build-up on the fruit. Once the harvest is complete, the sheets are removed. And this is where the problems begin.

Even if the removal is actually done (but there are still many cases in which this is not done, in order to save time and costs), some plastic fragments escape, adhering to the soil. So much so that many years later it is still possible to observe their accumulation. During the study, the researchers focused in particular on so-called micro- and macroplastics, i.e. those pieces with a diameter of less than and more than 5 millimetres respectively.

The Study

The researchers explored the abundance and composition of plastic parts on several strawberry farms that used low-density polyethylene (LDPE) mulch sheets in central California. Here, according the presentation of the research, “The concentration of macroplastic fragments ranged from 3909 ± 614 particles/ha to 213500 ± 33730 particles/ha in surface soil following normative mulch removal.”

The analysis of the microparticles is still ongoing. At the moment “preliminary data suggests the MP concentration reached 352 ± 36 particles/kg dry weight of soil.”

Polyethylene, the study recalls, was the dominant polymer type among the macro/microplastics extracted, confirming that mulches may be a major source of plastic pollution in agricultural soils. The researchers are still exploring the relationships between plastic concentration and various biogeochemical characteristics of the soil.

Global agriculture makes extensive use of single-use plastics

The study, the researchers emphasise, provides some data that help understand the extent of plastic pollution in the US agricultural system and beyond. At the same time, it stimulates efforts to improve land management practices and assesses the consequences of soil contamination, which is an increasingly important issue. “Application of single-use plastic in the agriculture sector has increased tremendously, covering millions of acres of land worldwide,” the presentation continues.

US farms, in particular, “are expected to generate nearly 860 million pounds of plastic waste annually, with single-use low-density polyethylene mulches used in row crops contributing a significant amount to total plastic waste.”

The fragments that are bound to accumulate will inevitably become “a serious threat to the environment and human health.” According to the FAO, global demand for plastic films for agricultural use (greenhouses, mulches and silage) will increase by 50 per cent, from 6.1 million tonnes in 2018 to 9.5 million tonnes in 2030.

The biodegradable and compostable alternative

“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,” Tiwari added. “We are working with the manufacturers to see if we can mitigate these costs.” Some alternatives, anyway, are currently available. Alongside polyethylene products, in fact, there are also natural covers, such as those made from straw, as well as biodegradable and compostable mulches.

Bio-sheets made of compostable polymers in particular are proving to be a highly effective and easy-to-manage solution. Their usefulness was officially recognised by the European Parliament as early as October 2017. Unlike their traditional plastic equivalents, in fact, biodegradable films do not have to be collected and disposed of at the end of the crop cycle. Instead, they are incorporated into the soil where they biodegrade, transforming into carbon dioxide, water and biomass and, above all, without generating any toxic effects.