How is Soil-biodegradable mulch treating soil?

There is no away

What happens to the plastics that we use? The most common answer: they get thrown away. But there is no AWAY! My advisor, Dr. Carol Miles, shared this in one of our meetings, and I realized it is true! The plastic waste we have generated every day for the past 70 years still exists on this earth either in its original form, recycled into another product, or gradually breaking down into microplastics. It takes hundreds of years for plastics to break down. Plastic revolutionized our daily lives and benefitted industries including agriculture by saving energy and creating lightweight and durable products with minimal material usage and low cost (Andrady and Neal, 2009). But now we must deal with the consequences of plastic pollution.

Use of plastic mulch in agriculture

Plasticulture is the use of plastics in agriculture in the form of mulch, fumigation tarps, irrigation tubes, greenhouse films, and many more products. The history of plasticulture began when plastic was first used as a greenhouse film by Professor E.M. Emmert at the University of Kentucky to replace more expensive glass (Kasirajan & Ngouajio, 2012). Plasticulture also brought the ‘white revolution’ to China when white polyethylene mulch substantially increased peanut production. Grain and cash crop yields in China also increased by more than 20% with the help of plastic mulch technology due to soil warming and moisture conservation effects. However, due to the large amount of mulch residues left in the field after harvest, plastic mulch technology evolved from the ‘white revolution’ to ‘white pollution’ (Liu et al., 2014). The plastic mulch used in China was very thin and could not easily be removed from the field. In other regions of the world, used plastic mulch cannot be recycled due to contamination with soil and crop debris and ends up in landfills, stockpiled, burned, or buried on the farm (Fig. 1). Despite the substantial benefits of plastic mulch, the environmental and agricultural sustainability challenges it has imposed are exceptionable. Plastic mulch itself makes up about 40% of the plastic films in agriculture. The sources of the other 60% include but are not limited to fumigation tarps, drip irrigation tape, row covers, plastic tunnels, silage bags, hay bale wraps, and nursery pots and trays (Kasirajan & Ngouajio, 2012). Meanwhile, the mulch film market size is growing at a compound annual growth rate of 7.8% during 2021-2026, so the problem is only getting worse unless science-based viable alternatives and strong policy interventions come into play (MarketsandMarkets, 2021).

Figure 1. Polyethylene mulch removed from the field.

A possible alternative

Researchers are evaluating soil-biodegradable plastic mulch (BDM) to determine its viability as an alternative to conventional plastic mulch. As the name suggests, it is designed to be incorporated into the soil to decompose into carbon dioxide and water by microbial activity but looks and acts the same as polyethylene plastic mulch (Fig. 2). Soil-biodegradable plastic mulches are composed of biobased feedstocks derived from fossil fuels or a mix of both. Several research studies have shown that BDMs provide similar crop production benefits as plastic mulch with the added advantage of agricultural plastic waste reduction. Recently, soil health has been the most important topic of discussion as there is limited information available on the impact of BDMs on soil health. The United States Department of Agriculture-Natural Resources Conservation Service defines soil health as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans.

Figure 2. Soil-biodegradable plastic mulch degradation process after tillage (Photo: Madrid et al., 2022)

Soil-biodegradable plastic mulch and soil health

Several short-term studies have shown no or minimal risk to soil physical, chemical, and biological properties due to BDM use (Bandopadhyay et al., 2020; Sintim et al., 2019). While the study did find pronounced effects on soil properties, soil health indicators, and soil functions, the changes were due to the study location and season (Sintim et al., 2019). Four years of continuous use of BDM in Knoxville, TN, and Mount Vernon, WA, showed overall positive effects on soil health and groundwater quality (Sintim et al., 2021) and another 6-year study in China evaluated soil properties with BDM in a garlic-maize rotation and did not find negative impacts on soil health (Zhang et al., 2022) (results summarized in figure 3).

Figure 3. Left, compared to the no-mulch treatment, BDMs, and conventional plastic mulch increased yields, water infiltration rate, and aggregate stability and reduced nitrate and nitrite post-harvest, nutrient leaching, and microbial activity. Right, BDMs increased available nutrients and soil microbial activity and reduced measured bulk density.

Soil health and BDM biodegradation are interrelated. In a seminar conducted by Washington State University Northwestern Washington Research and Extension Center (WSU NWREC), a grower from New England who has been using BDM for >20 years shared his experience of BDM degradation being weather-dependent and faster in healthier soil. He added, “BDM was almost completely gone by spring after tilling it in the previous fall. BDM fragments were quarter the size of initial fragments after tillage”. Soil type, temperature and moisture, soil microbial communities, and mulch fragment size affect site-specific, in-field BDM biodegradation. And enhanced biodegradation of BDM will likely not interfere with plant-soil interactions.

BDM has come a long way as research is resolving the concerns and issues of agricultural professionals and growers. Researchers are primarily focused on addressing biodegradation and soil health impact questions. It is yet to be demonstrated if the repeated use and incorporation of BDM into soil does not affect soil health in the long run.

Want to learn more about BDM’s? Head over to: https://smallfruits.wsu.edu/plastic-mulches/