SAN LUIS OBISPO, Calif. – The practice of plastic mulching in agricultural fields is causing negative impacts on soil health that could compound over time.

Plasticulture, the use of plastic materials in agricultural applications, is said to have begun in 1948 when a horticulturist at the University of Kentucky started using plastic film as an alternative to glass siding for greenhouses. Now, plastic is used in all aspects of farming, especially plastic mulch, which became commercially available in the 1960s. U.S. farmers are estimated to use 126 million pounds of plastic mulch annually.

Seeta Sistla, associate professor and soil ecologist at California Polytechnic State University, became interested in how plastic mulch impacts soil health when her lab received state and federal funding to investigate biodegradable plastics as an alternative to plastic mulch.

“When we were out setting up our trials for biodegradable plastics, we realized there was a tremendous amount of visible plastic left behind in the soil,” she said. “The research idea came from observation.”

To understand the presence of macro- and microplastics in agricultural fields and how they impact soil health, Sistla and her team sampled 12 farm fields on California’s Central Coast. The fields were surveyed after farmers had carefully removed the plastic mulch for the season, using best practices to avoid soil contamination. Despite best practices, Sistla and her team found about 25 kilograms or about 55 pounds of macroplastic debris per hectare. This debris covered up to 3.4 percent of a field’s surface area.

Additionally, the research team found microplastic contamination across all fields surveyed, which correlated with macroplastics such that the more macroplastics recorded, the more microplastics were recorded.

“It’s just impossible to remove all this plastic,” Sistla said. “Once you’ve laid it in the field, it rips and shreds. It’s really difficult to get rid of.”

Plastics left in the field negatively affect soil health, Sistla found. Macroplastic accumulation led to less soil moisture and plant-available phosphate, lower soil respiration, and reduced carbon pool sizes. These effects were observed at low plastic contamination levels, less than 10 percent of the contamination levels reported to degrade field soils.

Interestingly, Sistla noted that the relationships between macroplastic accumulation in the field and negative soil health effects weren’t very dramatic. The reduction in soil fertility was subtle, and the differences are likely offset when farmers add back fertilizer or increase irrigation. Still, Sistla cautions against disregarding the negative impacts of plastic mulch.

“If we’re moving toward more extreme plastic buildup over time, the directionality (of the research results) suggest we ought to reduce our plastic inputs,” she said. “If we continue to intensify our plastic mulch application, year in and year out, we are going to see a negative trajectory for soil health.”

In some parts of the world, like China, plastic accumulation in agricultural fields is so severe that the layers of plastic resemble baklava, Sistla said. Research studies conducted in China have indicated that the buildup of plastics can reduce crop yield, plant height, and root weight and negatively impact soil properties, including soil water evaporation capacity, soil water infiltration rate, and soil organic matter. One study estimated that polluted fields could reduce cotton yield by 6-10 percent.

Understanding the impact of plastic mulch on our environment extends beyond the field. Sistla and her team are investigating the connections between plastic mulch cover and plastic pollution in nearby waterways by working with the California Marine Sanctuary Foundation.

“There are hot spots of plastic accumulation, like large fragments along fence lines in farm fields, if the plastic doesn’t break down into smaller pieces and become incorporated into the soil as microplastics,” she said. “Depending on the efficacy of the fence line, we expect that we may be seeing hot spots of plastic accumulation that can bleed into waterways.”

Transitioning from plastic mulch to alternatives is an ongoing problem that researchers are approaching from several avenues, Sistla added, although several solutions offer new challenges. For example, one idea is to create durable plastic mulch to be used more than once, reducing the amount of plastic introduced in fields. However, these mulches would still be plastic and could negatively impact soil health. Another idea is to use biodegradable plastic mulch, but in fields with quick turnover, the mulch might not have enough time to biodegrade. Other groups are working to recycle plastic mulches and pelletize them for use in different industries. It’s difficult to recycle plastic mulch, though, because it’s often contaminated with organic matter.

Ultimately, the solution to plastic mulches will be multifaceted and stem from the work of many individuals.

“I think the more quickly we acknowledge the use of single-use plastics in agriculture and find viable alternatives, the better it will be for the long-term costs to our agricultural systems,” Sistla said.