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Current and Future Status of Biodegradable Plastics

The Tomato Magazine
Feburary 2005

By John Warner
Agriculture and Agri-Food Canada
Greenhouse and Processing Crops Research Center
Harrow, Ontario, Canada N0R 1G0

Plastic (polyethylene) mulches have been used in commercial vegetable production since the early 1960s. Benefits include soil temperature modification, earlier crop production, higher yields, enhanced produce quality, improved soil water retention by reduced evaporation, reduced fertilizer leaching, reduced soil erosion, better management of certain insect pests and fewer weed problems.

Plastic mulches are relatively inexpensive to purchase, easy to use and readily available in a variety of colors and forms. Furthermore, crop management systems are well established for plasticulture. It is estimated that over 4,000 acres of vegetables re grown on plastic mulches in Ontario, Canada.

Disposal Problems
Plastic mulches, however, have a negative impact on the environment. They are produced using non-renewable resources, are not biodegradable, are often disposed of after a single season and disposal of used mulch is becoming more difficult.

Disposal may involve incineration, burying in a landfill site or recycling. Open burning of plastic is not allowed in most jurisdictions. Incineration to recover the fuel value is an alternative; however, incineration facilities that will accept waste plastic are not readily available. Land filling plastic mulches is expensive; the plastics persist for many years after disposal and restrictions are placed on the disposal of agricultural plastics in landfills. Some landfills refuse to accept used plastic mulches. Recycling of plastic mulches is difficult because of plant material, soil, moisture and possible chemical residues associated with them after their use in the field.

Some growers are stockpiling used plastic mulch because no other suitable alternatives exist. The pick up and disposal cost in U.S. dollars of plastic mulch has been estimated at over $100 per acre.

An alternative to polyethylene mulch is biodegradable mulches. Organic mulches such as hay, straw, sawdust, etc., provide some of the benefits of plastic mulch but may be more expensive, harder to handle and do not provide the soil warming effects encountered with plastic. Paper mulches have also been used, however, paper disintegrates at the shoulder where the mulch is buried and paper pieces tend to blow around.

Recently, biodegradable polymers have been developed for agricultural uses. Biodegradable films are often thinner than traditional polyethylene, but otherwise are quite similar. Biodegradable films are available in clear, black and a variety of colors. They may be made from renewable resources such as starch, cellulose or degradable polymers.

Film Testing
There are two types of biodegradable polymer films presently being investigated. One is a polyethylene which has a proprietary additive which allows oxidative degradation of the polyethylene film. The breakdown is a two-step process. The oxidative degradation is triggered by sunlight, heat and mechanical stress. The film becomes brittle, and the molecular fragments are then further broken down by microbial action to CO2, water and natural substances. The second step is the microbial action. Depending on how the film is formulated will affect the length of time required for it to break down (from several months to several years). These are not the same as the photodegradable mulches that were previously available that left plastic residues in the fields.

The other type of film under test is starch-based polyester. Starch is added to the formulation to break up the long polymer (carbon) chains to shorter lengths which are then subject to further breakdown through microbial activity to CO2 and water. Polyester is also biodegradable. There are also polyethylene films with starch added to enhance breakdown, but the problem is that molecular fragments of polyethylene may remain in the soil.

There are compostable standards in the U.S. (ASTM) and Europe (EN) which state that in order for a product to be compostable the following criteria must be met:
1. Disintegration-the ability to fragment into non-distinguishable pieces and safely support bioassimilation and microbial growth
2. Biodegradation-conversion of carbon to CO2 to the level of 60 percent over a period of 180 days (ASTM) standard) and 90 percent in 180 days (EN standard)
3. Safety-that there is no evidence of eco-toxicity in the finished compost and soils can support plant growth, and
4. Toxicity-that heavy metals concentrations are less than 50 percent recommended values.

Few products presently meet the compostable standards. However, products may be biodegradable, but in a slower time frame, or may not be adequately tested to meet the U.S. or European standards. Companies may also be promoting products which are not truly biodegradable. In Ontario, there is limited use of biodegradable mulches. Biodegradable mulches are just starting to become available and the cost tends to be higher than for polyethylene.

Testing needs to be done to determine the length of time that the products will last in the field, how consistent they perform from year to year, to determine their soil warming effects and effects on crop maturity, yield and quality.

Testing of sweet corn on clear biodegradable mulches and peppers on black biodegradable mulches at Agriculture and Agri-Food Canada, Harrow and at Ridgetown College, University of Guelph, Ridgetown, Ontario, has shown that performance of the biodegradable mulches can be similar to polyethylene. Biodegradable films generally are thinner and therefore, extra care must be exercised when laying the films to prevent tears and punctures. Weed control must be good under the mulch as weed will grow through the biodegradable films rather than being smothered as often occurs under the stronger polyethylene films.

Editor's Note: This research was presented Dec. 8, 2004, during the Plasticulture Session of the 2004 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Mich.

© 2005 Columbia Publishing

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