Collecting plastic fragments was a game he played while helping his parents farm when growing up in rural Shandong Province, says Dr. Zhao Kaiguang, who is now an associate professor of Environment and Natural Resources at The Ohio State University: “I wanted to collect the most, but did not realize the serious negative implications of leaving plastic in the soil.”

Ocean plastic pollution looms large as a growing threat to both marine ecosystems and seafood safety. Sadly, microplastics polluting the soil is an even bigger, yet less visible, threat to food security that demands more attention and action. As much as 23 times more microplastics are dumped on agricultural soils than are disposed of in the ocean.

For decades, farmers in the United States and China have used plastic mulch to increase yield, reduce herbicide use, and conserve water. In the United States alone, 126 million pounds of plastic mulch covers the soil. China is now the largest user of plastic mulch film, which accounts for 10 percent of China’s cultivated farmland—an area of the state of Nebraska. 

If not carefully collected for recycling or disposal, the plastic fragments left behind in farming can create a nuisance, be blown away or washed into ditches and waterways. The large pieces of plastic are an obvious source of pollution, but the bigger threat may be the very small, microplastic particles that also accumulate in the soil. Microplastics are considered to any plastic smaller than 5 millimeters, or about the size of a pencil eraser. In  a study conducted in Chinese vegetable fields, 78 microplastics were found in 1 kilogram of soil, meaning a 5-gallon bucket of soil would contain almost 2,000 pieces of microplastic.

Multiple pathways of microplastics into soil 

The plastic used on and in the soil as mulch, as well as drainage pipe and irrigation tubing, are a direct source of microplastics. These materials break apart over time through the 3Ds— being damaged, degraded and discarded. The disintegration of plastic into smaller and smaller fragments is considered diffuse pollution, making it hard to measure and control. And since the use of agricultural plastic exceeds the collection and degradation rates, microplastics have been accumulating in soils for decades. Microplastics in waterways and farmland can vary by eight orders of magnitude, from just a few fibers to over a trillion pieces of plastic per liter.

Some of the other pathways delivering microplastics into soil include the plastics fibers shed from washing clothes and microplastics in beauty products that are washed down the drain. Microplastics are removed in wastewater treatment plants, but then become part of the sewage sludge used as organic fertilizer that is spread on farm fields.  

In the United States, an estimated 21,000 tons of microplastic is applied to agricultural soils through sewage sludge application each year. In Europe, an average 2,900 microplastics were found per kilogram of soil that is fertilized with sewage sludge. More than 71,000 microplastics were found in Denmark on agricultural soils fertilized with sewage sludge. A five-gallon bucket of these soils could have over a million pieces of microplastic.

What is not filtered out in treatment plants flows into streams, lakes and even wells that farmers use to irrigate fruits and vegetables. Studies show up to 28 microplastics in every cubic meter of water in Vietnam’s rivers and lakes, which is in the low range when compared to waterways in Indonesia, Philippines and China. In the United States, up to 15 particles of microplastics per liter were even found in well water in central Illinois.

Microplastics even fall to earth and onto the soil from the air. About four percent of dust is now plastic. Burning plastic, often seen as a solution to the plastic waste problem, releases a range of contaminants including microplastics. Put simply, microplastics are now everywhere.

Macro impact of microplastics in soil

Microplastics become part of the food chain through plants and animals. These plastic particles have been found in carrots, lettuces, pears, and apples. The tiny plastic particles are easily ingested by soil animals and can penetrate tissues and cells. Scientists in China and New Zealand found that the higher the soil microplastic content, the greater the impact on soil animals like nematodes and earthworms. 

Not surprisingly, plastic enters and passes through the body when people and animals eat plastic in food. An average adult consumes between 40,000 and 50,000 microplastic particles a year when eating plastic contaminated food. In March 2022, a group of scientists from Amsterdam published the first sobering study that detected plastic particles in human blood. While the long-term impacts on health is unknown, the ubiquitous spread of microplastics is a growing concern of health professionals. 

The plastic particles not only release chemical additives, but with their large surface area, they are also ripe to absorb and concentrate chemicals in the soil. Dr. Mary Beth Kirkham, a soil scientist at Kansas State University studying soil heavy metal contamination, describes in an interview how she discovered the impact of microplastics on plant health. With the presence of microplastic, plants take up more toxic cadmium in the soil, which in turn results in greater cadmium in human and animal food. She described plastic as a vector for the plant uptake of toxic heavy metals.  

However, not all chemicals react the same way to microplastic. Chinese scientists studying both microplastics and arsenic uptake in rice found mixing microplastic in the soil leads to a decrease in arsenic uptake. Professor Yan Changrong at the Chinese Academy of Agricultural Sciences (CAAS) cautioned not to overestimate the effects of microplastics, as we still have limited knowledge of the impacts.

China is leading the way on soil plastic pollution research and its researchers are finding impacts of microplastic on plant growth. Researchers in Europe and South Korea also found both new and old plastic material in the soil, leaching chemicals that reduce plant germination and growth. 

Taking action

Few laws, rules or policies are in place to reduce the impact of microplastic in soil.  Managing plastic mulch is a good starting place, given it is a big contributor to soil plastic pollution.

The Circular Plastics Alliance identified five strategies to reduce soil plastic pollution when using agricultural plastic mulch. Extended Producer Responsibility (EPR) collection systems is one strategy. Ireland, for example, placed legal responsibility on the plastic producer to support recycling through a plastic levy in 2017. Banning open burning is another tactic, and some U.S. state and local governments already restrict plastic burning. A third way is setting a minimum thickness for mulch film. China has been a leader in establishing standards to improve plastic thickness and quality. The new plastic order ban from China’s National Development and Reform Commission forbids the production and sale of the traditional thin mulch film that is difficult to collect and recycle. Requirements for mulch collection is another strategy. The fifth alternative would be to offer farmer incentives to collect and properly handle used mulch film.

To reduce the number of microfibers washed down the drain and into the soil, the U.S. EPA recommends changing the way we wash clothes by ensuring full loads, shorter wash cycles, and using front-loading washers—all strategies to reduce fiber shedding. The agency also recommends installing a washing machine microfiber filter to capture the material.

California is the first state in the U.S. to adopt a plan to combat microplastic pollution. While not a regulatory body, the California Ocean Protection Council committed $3 million to work on reduction targets. They plan to start preventing plastic from entering the environment and monitor what is already there. 

Plastic pollution that has been accumulating for decades also is the focus of the February 2022 U.N. Environment Assembly global treaty. Unlike the previous administration, the Biden White House is embracing participation in global environmental treaties, and the U.S. now supports the development of a policy tool to combat ocean plastic pollution.

The United States and China must commit to take action now and join forces to help bring solid science and policy together to begin mitigating and cleaning up microplastics in the soil. International collaboration could be a model of support for much needed research on sources of microplastic and their impacts on plant, animal and human health. Agricultural research on non-plastic alternative materials to achieve high yields with minimal environmental impact also requires support. The Global Plastic Treaty is only the first step. Solving the problem of soil microplastic pollution requires sustained international support.