Plastic mulch used in strawberry production leads to long-term pollution

Currently, plastic mulch films are extensively used in agricultural practices. When tucked in around the base of plants, they can help control weeds and pathogens, reduce water evaporation, and prevent soil splashing on fruit. 

However, according to a new study led by the California Polytechnic State University (Cal Poly), the plastic mulch used to support the growth of Californian strawberries sheds large quantities of plastic fragments that negatively impact soil quality. Such a situation is likely to occur in the case of other crops too, suggesting that this method’s risks may outweigh its benefits.

“What we are seeing is a huge quantity of macroplastic material – particles bigger than 5mm across – being shed where the mulch is used to enhance strawberry production. These can remain in the soil for decades or longer,” said study lead author Ekta Tiwari, a postdoctoral fellow in Geochemistry at Cal Poly.

Plastic fragments accumulate for decades

Although the mulch is removed after the crop’s seasonal production is complete, even careful land stewardship by farmers fails to remove all the plastic, since fragments usually adhere to the soil after removal. 

After decades of annual plastic mulch application and removal, even in extremely well-managed fields, there is a significant accumulation of plastic fragments.

“We carried out a systematic survey of strawberry fields after the seasonal removal of these plastic film. We found that the distribution was fairly uniform. On field surfaces alone, we found up to 213,500 macroplastic particles per hectare. That doesn’t include subsurface particles, which we did not survey,” explained Tiwari.

“In addition, we are currently analyzing the same soil samples for microplastics, which are smaller particles, less than 5mm across; these are not yet included in our findings.”

Plastic mulch degrades soil quality

By using a groundbreaking method called Fourier transform infrared spectroscopy, the experts found that most of the particles were polyethylene. Moreover, the analysis revealed that, as the levels of microplastic pollution increased, soil moisture content, microbial respiration, and plant-available nitrogen decreased.

“The 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 there indefinitely. 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 the manufacturers to see if we can mitigate these costs,” Tiwari said.

Further research is needed 

Since the current study has only focused on macroplastic pollution, the researchers are now evaluating the level of microplastic pollution too. Providing reliable data on the extent of plastic pollution in the U.S. agricultural system could help improve land management practices and assess the biogeochemical consequences of plastic accumulation in agricultural soils.

Although there are alternatives to using polyethylene mulches – such as biodegradable plastic mulches or natural mulches like straw – such methods come with additional costs, which makes them less popular among farmers. Further research is needed to identify cost-effective techniques that could protect crops, while avoiding the negative impacts of plastic mulch.

The study was presented the Goldschmidt Geochemistry Conference in Lyon, France (9-14 July).

More about plastic mulch

Plastic mulch is a product used in farming and gardening to suppress weeds and conserve water. It’s made from a thin layer of plastic film that is placed over the soil with the plants growing through slits or holes in the plastic. Here are some key points about plastic mulch:

Weed control

Plastic mulch forms a physical barrier between the soil and the atmosphere, thus preventing weed seeds from germinating and growing. This is because most weed seeds need light to germinate, and the plastic mulch blocks this light.

Moisture conservation

Plastic mulch acts as a barrier to evaporation, reducing the amount of water that is lost from the soil. This helps to conserve water and can be especially useful in regions where water is scarce or in times of drought.

Soil temperature regulation

Depending on its color, plastic mulch can either absorb or reflect sunlight, thus affecting the temperature of the soil underneath. 

Black plastic mulch absorbs sunlight and warms the soil, which can be beneficial for warm-season crops like tomatoes and peppers. Conversely, white or silver plastic mulch reflects sunlight and can help to keep the soil cooler, which can be beneficial for cool-season crops.

Increased crop yield

By suppressing weeds, conserving water, and helping to regulate soil temperature, plastic mulch can contribute to increased crop yield.

Despite these benefits, there are also several potential drawbacks to using plastic mulch. One is that it is a form of non-biodegradable waste, which can contribute to environmental pollution. Additionally, the use of plastic mulch can lead to soil degradation over time due to reduced organic matter decomposition and soil biodiversity. 

To address these issues, some farmers and gardeners use biodegradable mulches, which provide many of the same benefits but break down over time to reduce waste and soil degradation.

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By Andrei Ionescu, Earth.com Staff Writer

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