Plastic mulch is a product used, in a similar fashion to normal mulch, to suppress weeds and conserve water in crop production and landscaping. Certain plastic mulches also act as a barrier to keep methyl bromide, both a powerful fumigant and ozone depleter, in the soil. Crops grow through slits or holes in thin plastic sheeting.
Plastic mulch is often used in conjunction with drip irrigation. Some research has been done using different colors of mulch to affect crop growth. This method is predominant in large-scale vegetable growing, with millions of acres cultivated under plastic mulch worldwide each year.
In many areas of the country and for many crops, mulching fields with sheets of plastic has become de rigueur. Not only do these impermeable blankets help retain moisture, but they also warm the soil. As a result, mulched crops tend to ripen sooner, a boon to many farmers.
Disposal of plastic mulch is cited as an environmental problem. However, technologies exist to provide for the recycling of used/disposed plastic mulch into viable plastic resins for re-use in the plastics manufacturing industry.
Advantages
The use of plastic mulches along with the use of drip irrigation has many benefits such as:
Earlier planting dates: The use of plastic mulch alters soil temperature. Dark mulches and clear mulches applied to the soil intercept sunlight warming the soil, allowing earlier planting as well as encouraging faster growth early in the growing season. White mulch reflects heat from the sun effectively reducing soil temperature. This reduction in temperature my help establish plants in mid-summer when cooler soil might be required.
Soil moisture retention: Plastic mulches reduce the amount of water lost from the soil due to evaporation. This means less water will be needed for irrigation. Plastic mulches also aid in evenly distributing moisture to the soil which reduces plant stress.
Weed management: Plastic mulches prevent sunlight from reaching the soil which can inhibit most annual and perennial weeds. Clear plastics do not prevent weed growth. Holes in the mulch for plants tend to be the only pathway for weeds to grow.
Reduction in the leaching of fertilizer: The use of drip irrigation in conjunction with plastic mulch allows one to reduce leaching of fertilizers. Using drip irrigation eliminates the use of flood and furrow irrigation that applies large quantities of water to the soil which in turn tends to leach nitrogen and other nutrients to depths below the root zone.
Drip irrigation applies lower amounts of water with fertilizers injected and thus these fertilizers are applied to the root zone as needed. This also reduces the amount of fertilizer needed for adequate plant growth when compared to broadcast fertilization.
Improved crop quality: Plastic mulches keep ripening fruits off of the soil. This reduced contact with the soil decreases fruit rot as well as keeps the fruit and vegetables clean. This is beneficial for the production of strawberries, for example.
Reduction in soil compaction: The plastic mulch covering the soil decreases the crusting effect of rain and sunlight. The reduction in weed quantity means a decreased need for mechanical cultivation.
Weed control between beds of plastic can be done using directly applied herbicides and through mechanical means. The soil underneath the plastic mulch stays loose and well-aerated. This increases the amount of oxygen in the soil and aids in microbial activity.
Reduction in root damage: The use of plastic mulch creates a practically weed free-area around the plant, resulting in less need for cultivation except between the rows of plastic. The roots are not damaged because to of the lack of cultivation. Due to these factors, the use of plastic mulch leads to an improvement in the overall growth of the plant.
Disadvantages
These benefits, however, come at some expense to the environment, a new Agriculture Department study finds. The practice increases both erosion and runoff of toxic pesticides. Many farmers had noticed that more rain flows from plastic-mulched fields, via dirt furrows between the covered rows, than from fields covered with plant-stubble mulch.
However, "nobody had bothered to ask how much more (water runs off)," notes Cathleen J. Hapeman, an inorganic chemist at the Beltsville (Md.) Agricultural Research Center (BARC). So, during 2 years, she and her colleagues collected all the rainwater flowing from tomato fields and measured the amount, as well as any contaminants in it.
In 1998, the year with the more dramatic results, they found that an average of 63 liters of water ran off each square meter of the plastic-mulched soil. That’s four times the runoff from a field mulched with material from a plant known as hairy vetch.
Even "more disturbing," Hapeman reported at the American Chemical Society meeting last month in New Orleans, is that the plastic-covered field lost 4,950 kilograms of dirt per hectare that year—almost 15 times as much as the vetch-mulched field. Clearly, she observes, with such a slowly renewing resource as soil, "you cannot sustain such losses for very long."
Then, she looked at chemical runoff. Each of the two test fields had been sprayed with the same amount of the fungicide chlorthalonil and the insecticide endosulfan. Because the plastic-mulched field has less exposed soil that can bind the pesticides, rain washed away 19 times as much of the chemicals from it as from vetch-covered rows.
The researchers then added this runoff to containers holding local aquatic inhabitants, including hard clams and diatoms. The plastic mulch’s runoff was usually much more toxic than the vetch’s, says Hapeman.
Using plastic mulch enables farmers to harvest crops 3 or 4 weeks early. Such vegetables can command high market prices. "However, I’m having a hard time justifying that 3-week-earlier harvest in exchange for this loss of soil and pesticides," Hapeman says.