Sadly, plastic mulch has a dark side as fragments of plastic are accumulating and now polluting the soil. At first plastic fragments accumulating in the soil were a nuisance, getting caught up in equipment. 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 bringing it below high-end textiles standards.
Globally, 1.95 million tons of plastic mulch are applied each year, and 74 percent of it is used in China. Research now shows that plastic fragments left behind are damaging the soil’s structure, stifling plant development, and changing the soil carbon cycle that affects greenhouse gas emissions.
The problems with plastic mulch are emerging first in China because it is the largest user. The Chinese farmers also use ultra-thin mulch, less than 10 microns, which is much thinner than the 20-30 micron plastic used in the EU and the U.S. For comparison a plastic zip lock bag is 38 microns. The thin plastic easily tears, making it difficult and costly to remove from the field. The cost of collecting plastic mulch from cotton fields is more than 450 RMB/ha (nearly $70 USD/ha). An average of about 60 kg/ha of plastic is now contaminating some of China’s agricultural soils. That is equivalent to over 10,000 plastic grocery bags per hectare.
Chinese takes action to end agricultural plastic pollution
Chinese policies on agricultural plastic mulch were first advanced in 2003 with the Promotion of Clean Production Law. This was followed by the 2009 Circular Economy Promotion Law. However, the plastic pollution of China’s soil continued to worsen, so more comprehensive action was needed.
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 ban 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. This action was followed up in 2020 with spot checks on film quality and punishment for sale or use of illegal ultra-thin plastic film.
China’s National Green Agriculture Development Plan is now part of the 2021 14th Five Year Plan. This includes agricultural plastic film pollution control. The goal is to increase the collection rate of agricultural plastic film to 85 percent by 2025, with no additional plastic mulch residue left in the soil. To accomplish this, China will need to build up a market driven, sustainable collection and recycling system. The respective responsibilities of government officials, film producers, retailers, farmers, and recyclers are still not clear.
Research still needed
China is leading the way on agricultural plastic research. Research ranges from plastic use, application, recycling, pollution impacts and alternatives, like biodegradable plastic. Even after collection, however, agricultural plastic is difficult to recycle. About half of its weight is dirt, which damages the recycling equipment. Using water to wash the plastic may not be an option in dry areas of China as well.
If not carefully collected for recycling or disposal, the plastic fragments left behind in farming can create a nuisance, blown away into the air 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 be 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.
The impacts of microplastics in soil are still unclear. Some studies show that microplastics increase plant uptake of toxic chemicals while other studies show just the opposite. Professor Yan Changrong cautioned not to overestimate the effects of microplastics, as we still have limited knowledge of the impacts.
Biodegradable plastic mulches are a promising alternative to the currently used polyethylene-based mulches, but rigorous testing throughout all life stages is needed to ensure their use and disposal are environmentally safe. In-field testing of biodegradation under different soil and climatic conditions for a variety of cropping systems, with particular attention to release of microplastics, will show their degradation in soils and their effects on soil quality. U.S. researchers Markus Flury, Washington State University, and Douglas Hayes, University of Tennessee, have already conducted field studies in Tennessee, Texas, Washington, and several locations in China.
While providing for alternatives, in 2017 China developed a standard “Biodegradable Mulching Film for Agricultural Uses (GB/T35795-2017)”. The European Committee for Standardization (CEN) followed in 2018, adopting EN 17033: Plastic-Biodegradable Mulch Films for use in Agriculture and Horticulture, as the first international standard for biodegradable plastic in soil. However, without strict and recognized labeling requirements, the biodegradable claim can be misused.
Agricultural plastics have transformed agriculture and the lives of farmers all over the world, especially in China. It is unlikely that the use of plastics will be abandoned. The use of plastic mulch is now increasing throughout Asia and in the United States. Through careful use, collection, recycling, and hopefully in the future, affordable biodegradable plastic mulches the benefits can be retained while minimizing the environmental impact.