By Karen Mancl
Plastic has been a revolutionary material that is light-weight, flexible, durable, and inexpensive to produce. China has been using plastic mulch films in agriculture since the 1970s. According to Professor Yan Changrong of the Chinese Academy of Agricultural Sciences, the direct economic benefits have been great at an estimated RMB 120 – 150 billion per year (USD 19 – 24 billion) of rural income by increasing water use efficiency and yield by 30%, raising poor farmers out of poverty. Unfortunately, continuous use of plastic mulch for decades has resulted in soil plastic pollution that is beginning to cancel out the benefits.
Biodegradable mulch films must be 100% degradable by microbes in nature, breaking down to carbon dioxide, water, and minerals without damaging the soil. Sadly, the perfect biodegradable plastic does not yet exist, one that balances its physical features with environmental protection. Right now, the plastic PBAT comes the closest. A big market for PBAT is plastic mulch films used in agriculture. In the United States alone, 126 million pounds of plastic mulch covers the soil. China is the world leader in the use of plastic mulch in agriculture, covering 10% of its farmland. Rather than assuming the expense and labor to gather up and haul plastic mulch to a landfill, biodegradable mulch can be tilled safely back into the soil.
In 2017, China developed a standard “Biodegradable Mulching Film for Agricultural Uses.” The European Committee for Standardization followed in 2018, adopting Plastic-Biodegradable Mulch Films for use in Agriculture and Horticulture as the first international standard for biodegradable plastic in soil. The standard requires lab testing for content, biodegradation in soil, ecotoxicity, and physical characteristics like strength. The international standard requires at least 90% degradation in the soil over two years.
Unfortunately, the biodegradable claim is being misused. Early degradable plastic mulches, known as oxodegradable and photodegradable, broke apart but did not completely degrade, leaving behind microplastic that polluted the soil.
Moreover, biodegradable plastic is having a hard time meeting the organic standard. For certified organic farming, biodegradable plastic must be 100% bio-based to meet the requirement of the National Organic Program (NOP). Current biodegradable plastic materials only include 20% plant material, so they do not meet the NOP requirement. Further, GMO (genetically modified organism) materials must be excluded from organic farming, and the corn used to make biodegradable plastic is genetically modified.
The Chinese started testing biodegradable plastic in 2010. By 2015, the Chinese Ministry of Agriculture and Rural Affairs organized demonstration tests on degradable mulch film. The tests revealed that biodegradable plastic behaves differently than traditional plastic mulch. Biodegradable plastics are not as strong as the PE (polyethylene) plastic used by farmers for decades. The mechanical strength of biodegradable mulch film is significantly weaker, making it difficult to use machines to apply the mulch. Biodegradable plastics can easily tear during application and planting. “Laying down biodegradable plastic is an art” notes Dr. Douglas Hayes, University of Tennessee. The biodegradable plastic can also start to break apart in the field, causing pieces of plastic to stick to the crop, contaminating and lowering the value of the harvest.
Also, biodegradable mulch film is not as effective in soil warming and moisture retention. Water vapor escapes biodegradable mulch film at 10 times that of the traditional mulch. Water loss leads to heat loss, resulting in poor soil warming.
We also need to consider what happens when plastics are moved off site. Soil degradable plastics do not degrade in the atmosphere or in water, so if they are blown or washed away, soil biodegradable plastics can be as polluting as conventional plastics. Dr. Hayes, working with Dr. Marcus Flury, Washington State University have studied the degradation of mulches over a range of soil types and climates in Tennessee, Washington State, Texas, and several locations in China. They found that soil degradable plastic does degrade in the soil but over 2 years results can vary widely. However, the same cannot be said when biodegradable mulch is moved off-site.
Another big obstacle to biodegradable plastic adoption is cost. The cost of purchasing and applying biodegradable mulch is 1.5 to 2 times higher than traditional plastic mulch, even accounting for the collection and disposal costs of traditional plastic. A survey of U.S. strawberry growers in California, Pacific northwest, Pennsylvania, and New York, found that half of the growers would try biodegradable plastic mulches if the price dropped.
What is needed
More research is needed to improve the performance of biodegradable plastic mulch. Professor Yan shares that the quality of biodegradable mulch is improving. The initial mechanical strength of biodegradable mulch film can now reach about 85% of the traditional PE mulch film. The water vapor loss rate has also improved by 75%. Currently few norms and standards for biodegradable plastic exist at the international level, but the European Standard (EN 17033) was a good start. Biodegradable plastic needs to be tested under real, variable soil and climate conditions to confirm the long-term biodegradation of plastic mulch films.
Karen Mancl is a Professor in the Ohio State University Department of Food, Agricultural & Biological Engineering. This column is provided by the OSU Department of Food, Agricultural & Biological Engineering, OSU Extension, Ohio Agricultural Research and Development Center, and the College of Food, Agricultural and Environmental Sciences, For more information, Mancl can be reached at 614-292-4505 or email@example.com. You can also check her website at setll.osu.edu.