Black plastic mulch, often referred to simply as “black plastic,” is a thin sheet of petroleum-based plastic that farmers lay over the surface of their rows, usually installing a drip line under the plastic for irrigation. While it is allowed within organic agriculture, it is inherently unsustainable as it is a petroleum based product and difficult to recycle
Weed control is one of the primary challenges that farmers have faced around the world and through the ages. Before the introduction of herbicides into agriculture in the 1940s, cultivation, hand weeding, and, in some parts of the world, controlled flooding were the primary techniques used to suppress weed growth. The use of herbicides grew in popularity very quickly, and by the 1970s it was the dominant mode of weed control used by farmers in the United States.
These days, most conventional farmers deal with weeds using a combination of cultivation and herbicide. While these techniques are very effective at keeping weed populations low, they have many detrimental side-effects on the health of the soil, the environment, and people.
Soil disturbance and applications of herbicides damage the ecology of the soil, reducing its ability to absorb and retain water, store and cycle nutrients, and maintain good soil structure. As a result, erosion and leaching of nutrients are more likely to occur, removing valuable material from the field and damaging the water systems into which these materials flow. In addition to its effects on soil biology, plowing is fuel intensive and creates a hardpan, which can impede the growth of roots and the flow of water.
Some herbicides also have the potential to do harm to the environment and to human health when they enter streams, rivers, and lakes as runoff or when they get into the groundwater as leachate. In the 1950s, black plastic mulch was introduced to the market as another tool to help with weed suppression.
What is Black Plastic?
Black plastic mulch, often referred to simply as “black plastic,” is a thin sheet of petroleum-based plastic that farmers lay over the surface of their rows, usually installing a drip line under the plastic for irrigation. Plants are transplanted through holes in the plastic by hand or with machinery. At the end of the growing season, the material is removed from the field and disposed of.
Black plastic very effectively prevents plants from growing in the areas it covers. Compared to growing vegetables in bare soil, using black plastic greatly reduces the need for other forms of weed control, be it herbicide application, cultivation, or labour-intensive hand weeding. Another perk of using black plastic is that it warms the soil, in some cases allowing for earlier planting. For these reasons, black plastic has grown in popularity over the last 50 years.
However, black plastic has its downsides. While it is allowed within organic agriculture, it is inherently unsustainable as it is a petroleum based product and difficult to recycle. Every acre of land farmed using black plastic produces 100-120 lb of waste that typically goes to landfills. What’s more, when black plastic is used, 50-70% of a field is transformed into an impervious surface, increasing the volume of runoff by 40% and erosion by 80%.
And when herbicides and pesticides are used on fields covered in black plastic, the concentration of these chemicals in the fields’ runoff increases, making environmental and human health impacts even more of a concern. 6 Finally, the increase in soil temperatures during hot summer days under black plastic mulch has been found to shift the soil organisms community towards bacterial rather than fungal and increase microbial stress. Black plastic is also a substantial annual cost to the farmer at $250-$300 per acre for the material and about $20 per acre for disposal.
The solution? Cover crop mulch
For these reasons, researchers have been exploring cover crop mulch systems as an alternative to black plastic mulch. Several cover crop-based vegetable production systems have been developed and discussed in scientific literature, including the use of flail mowers, rollercrimpers, or undercutters to terminate the cover crop and transform it into mulch.
Whereas black plastic damages soil quality, cover crop mulches improve it by adding organic matter to the soil and increasing soil microbial life. Researchers have found that leaving cover crop residue on the soil surface resulted in crops with “increased disease tolerance…high vigor, higher marketable yield, and delayed senescence.”
These systems are also less expensive and faster to execute than black plastic, and they require no cost or labour for the removal and disposal of material at the end of the season. While researchers have made great headway in developing and demonstrating the efficacy of cover crop mulch systems, most of the systems that have been developed rely to some degree on synthetic herbicides to supplement the weed control provided by the cover crops.
For this reason, researchers at Rodale Institute have been working to develop a cover crop mulch system in which herbicides are not necessary for weed suppression, furthering the work of making cover crop mulch a viable option for organic as well as conventional vegetable producers.
The Northeast Sustainable Agriculture Research and Education (NE SARE) Program provided Rodale Institute with funding in 2010 to launch a three-year vegetable trial focusing on cover crop alternatives to petroleum-derived disposable black plastic mulch.
The work of John Teasdale and Aref Abdul-Baki
John Teasdale and Aref Abdul-Baki, both plant physiologists with the U.S. Department of Agriculture, began exploring cover crop mulch as an alternative to black plastic in the 1980s. They developed a mowed hairy vetch mulch system for tomatoes, in which the vetch is flailmowed just prior to tomato planting and one or two applications of herbicide are used to control re-growth of the vetch and other weeds that emerge throughout the season.
Through their research, they found that tomatoes grown in this system generally produced better yields, had less foliar disease, and required fewer commercial fertilizers than those grown in black plastic. In addition, the vetch mulch system produced profits that were two-thirds greater than profits in the black plastic system. Not only did Teasdale and Abdul-Baki establish that cover crop mulch systems can be a viable alternative to black plastic mulch, they demonstrated that these systems are beneficial to the soil, the plants, and the environment.
At the four collaborating farms in Pennsylvania and New Jersey, each farmer tested one cover crop system with the standard system. The cover crop systems provided serviceable weed control, added more biomass to the soil, contributed nutrients, and increased soil moisture and percent total soil carbon. Although marketable yields were lower, on average, some of the cover crop systems achieved higher profits across the three years. The cover crop mulch systems eliminated 91.5 lb of plastic waste per acre.
Challenges related to extreme weather conditions and late blight meant results were more variable than anticipated, but all of the partner farmers continue to use what they’ve learned from the project to reduce their reliance on black plastic.
Benefits the farmers experienced included discovering a method for more effective cover crop kill, substantial cost savings and new ways to use cover crops between rows for ecosystem benefits. One partner has already cut his black plastic use in half and hopes to expand even further.
The importance of healthy Soil Biota
Healthy soil contains a diverse set of microorganisms that provide many benefits to crops and to the farmer. These bacteria, protozoa, nematodes, fungi, and microarthropods decompose plant residues, improve soil aggregation and porosity, cycle nutrients from the soil organic matter and minerals into plant available forms, and can even protect plants against pathogens.
As a result, plants growing in fields with healthy soil biota will be more resistant to disease and will do better in times of stress, such as during drought or extreme heat. The fields themselves will be more able to absorb and retain moisture, and will be less likely to erode.
Source: Rodale Institute
