Farmers in the United States and China who grow strawberries, melons, and other fruits and vegetables often face the same arduous challenge—after harvesting they must gather up and dispose of the plastic mulch used to increase production. After months in the hot sun, the plastic sheeting starts to shred and break apart, leaving fragments behind in the soil. 

More than 350 million tons of plastic are produced and consumed globally every year, and the proportion of plastics that can be reused and disposed of safely and effectively is very low—currently only about 9 percent of plastic is recycled. Single-use plastics, like cups, utensils, and food packaging, get most of the airtime as plastic waste pollution, but agricultural plastic that is polluting soil is an overlooked ticking time bomb and an urgent, global problem. 

The march of plastic mulch 

In the United States alone,126 million pounds of plastic mulch covers the soil to reduce weed growth, conserve water, and warm the soil to extend the growing season, increasing yields by 30 percent. In an effort to identify land-based sources of plastic pollution in the ocean, Pam Krone of the Monterey Bay National Marine Sanctuary on assignment from NOAA and her team found that plastic is used extensively for multiple purposes. Plastic mulch, irrigation, fencing, erosion control and in-field packaging is used on California farmland that grows more than half of the U.S. lettuce and celery and more than a quarter of other fresh fruits and vegetables.

The benefits of plastic sheeting in agriculture were demonstrated in the 1950s by Professor Emmert at the University of Kentucky. Known as the “father of agricultural plastic development” he showed how plastic could be used for greenhouses, mulches, and row covers. Plastic mulch was also developed in Asia by Japanese scientists in the 1950s and was used for tobacco production in the 1960s. The technology was introduced in China in the 1970s. Plastic mulch use then spread beyond China and by 2000 was being used widely in the Philippines.

More plastic mulch is used in Asian agriculture than the rest of the world. China is now the largest user of plastic mulch film, which covers 10 percent of China’s cultivated farmland—18.4 million hectares, an area the size of the state of Nebraska. 

For over 40 years, Chinese farmers have used plastic mulch to help grow fruit, vegetables, grain, peanuts, cotton and tobacco. However, the plastic they used was as thin as plastic wrap, only 4 to 8 microns—a zip lock bag is 38 microns. The thin plastic easily tears making it almost impossible to pick up, so the shredded plastic stays in the soil. 

The 2011 Chinese plastic residue survey found an average of 34 kilograms of plastic residue was polluting each hectare of farmland, equivalent to over 6,000 plastic grocery bags. According to Professor Yan Changrong of the Chinese Academy of Agricultural Sciences the 2017 second national agricultural plastic survey found the average grew to 52 kg/hectare. The biggest impact of what has been called a “white revolution” was the use of plastic mulch with drip irrigation to grow cotton in the cold, dry areas of northwest China, nearly doubling the yield over traditional cotton areas in the Yangtze and Yellow River basins. Now the highest intensity plastic use is in Xinjiang Province in northwest China where the plastic pollution is 4 times more at 200 kg/hectare (over 36,000 plastic grocery bags). The cost of collecting and recycling the mulch film at the end of the season is about 450 yuan ($70) per hectare if it can be collected at all.

Agricultural plastic pollution cancels out the benefits

The direct economic benefit of plastic mulch is an increased rural income estimated at 120 to 140 billion yuan per year ($19 to $22 billion USD) according to Professor Yan. The global consumption of plastic mulch is almost 2 million tons per year, of which about 75 percent is used in China. Because of the extensive and continuous use of plastic mulch, agricultural plastic film residual pollution is uniquely Chinese. 

At first plastic fragments accumulating in the soil were a nuisance, getting caught up in equipment. For example, in northwest China, cotton growers must frequently stop their equipment and clean off the collected plastic fragments to apply the next sheet of plastic while planting the cotton seeds, increasing planting time and labor costs. The plastic fragments also present problems at harvest. Plastic trapped in the cotton bales reduces its quality and cannot meet the high-end textiles standards.

Research is now showing that plastic is damaging the soil’s structure, retarding plant development, and changing the soil carbon cycling that affects greenhouse gas emissions. While plastic sheeting is often used to help farmers adapt to a drier climate, at least 9 studies in China have uncovered that plastic mulch use is increasing greenhouse gas emissions from farming by 10 to 32 percent, exacerbating the climate crisis. The emissions come from the increased nitrous oxides emitted from the soil, the greenhouse gas emitted during plastic production, and the fuel to transport and apply it to the agricultural fields. 

Soil plastic residues also decrease crop yield by 13 percent and water use efficiency by 7.8 percent. The highest abundance of plastic in agricultural soil is in Xinjiang Province of western China—plastic mulch has been used continuously for over 20 years in this region and the plastic pollution is now reducing crop yield by 19 percent.

China takes the lead to end plastic soil pollution

While agricultural plastic pollution is significant, the use of agricultural plastic is largely unregulated. In the United States some state and local governments restrict plastic burning to control air pollution. Recycling programs for agricultural plastics have started and failed, mainly because dirt accounts for half of the plastic’s weight, and the soil damages the recycler’s shredding equipment.

In December 2019, the Chinese Ministry of Ecology and Environment and the National Development and Reform Commission (NDRC) proposed new regulations to reduce the agricultural plastic problem. The regulations require the use of higher quality, thicker mulch that can be reused, collected, and recycled. The new plastic order ban from the NDRC forbids the production and sale of the traditionally used thin mulch film of less than 10 microns. The order goes on to promote the use of biodegradable mulch and enhance recycling. NDRC is also supporting pilot demonstrations on the recycling and treatment of agricultural plastic waste.

Agricultural plastics have yielded so many benefits that use is likely to continue, in spite of pollution problems. In China, plastic mulch use is expected by 2025 to grow to 2.28 million tons and cover an area of 23.4 million hectares. Research support is needed to foster the development of biodegradable mulch films and technology to gather and remove plastic residues from soil and improve the reuse and recycling of plastic mulch. The impact of soil plastic residues on crop growth and soil health must be considered in policy-making related to agricultural sustainable development under global climate change.