Plastic (polyethylene) mulches
have been used in commercial vegetable production since the early
1960’s. 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 are 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 biodegradeable,
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 recyling.
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 has been estimated at over $100 per acre.
An alternative to polyethylene mulch is biodegradeable mulches. Organic
mulches such as hay, straw, sawdust, etc., provide some of the benefits
of plastic mulch, but may be more exensive, 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, biodegradeable polymers have been developed for agricultural
uses. Biodegradeable films are often thinner than traditional polyethylene,
but otherwise are quite similar. Biodegradeable films are available
in clear, black and a variety of colors. They may be made from renewable
resources such as starch, cellulose or degradeable polymers.
Film Testing
There are two types of biodegradeable 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 breakdown (from several months to several
years). These are not the same as the photdegradeable 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
though 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
4. Toxicity - that heay metals concentrations are less than 50 percent
recommended values.
Few products presently meet the compostable standards. However, products
may be biodegradeable, 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 used of biodegradable mulches. Biodegradeable 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 biodegradeable mulches and peppers
on black biodegradeable mulches at Agriculture and Agri-Food Canada,
Harrow and Ridgetown College, University of Guelph, Ridgetown, Ontario
has shown performance of the biodegradeable mulches can be similar
to polyethylene. Biodegradeable 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
weeds will grow through the biodegradeable films rather than being
smothered as often occurs under the stronger polyethylene films.
