Disappearing Mulches

09/01/2010 02:00:00 AM

By Renee Stern
Contributing Editor

Growers rely on plastic mulches to control weeds, warm soil and improve crop quality, but disposal costs and other issues have produced keen interest in biodegradable alternatives.

Some products already on the market include paper and compostable cornstarch-and-biodegradable polymer blends. Cost and longevity remain grower concerns, however.

By reducing herbicide applications and trips through the field, “Compostable [mulch] is a huge step in the right direction,” says Tom Thornton, co-owner of Cloud Mountain Farm & Nursery in Everson, Wash.

But, he says, “If we have to put in the labor to pull it up at the end of the season, whether to take it to a landfill or a compost facility, that’s unacceptable.”

Thornton serves on the advisory board for a new multistate research project on biodegradable mulches and high-tunnel production systems. He’s eager to find cost-effective ways for his own operation to control weeds without herbicides.

Project researchers in Washington, Texas and Tennessee are comparing two cornstarch-based mulches and an experimental polylactic acid-based material designed to biodegrade. The trials use two control mulches: the standard black plastic and a biodegradable paper mulch.

Other researchers are pursuing a biodegradable plastic, including Michael Orzolek, professor of vegetable crops at Penn State University in University Park, and Mathieu Ngouajio, associate professor of horticulture at Michigan State University in East Lansing.

Economics and materials sciences, too

The combined mulch and high-tunnel research, now ending its first growing season, tackles not only horticultural aspects but also taps experts in economics and materials sciences. Project director Debra Inglis, a Washington State University professor and Extension plant pathologist at the university’s Mount Vernon Research Center, says growers want to adopt sustainable practices, such as replacing plastics with renewable materials.

The trials in Mount Vernon; Lubbock, Texas; and Knoxville, Tenn., will test differences in crop yield and quality and in mulch degradation in widely different growing conditions, Inglis says. As a plant pathologist, she’ll be checking for any signs of root or foliar disease variations.

The mulch tests use plots planted with tomatoes.

Doug Hayes, professor of chemical and biosystems engineering at the University of Tennessee in Knoxville, leads the project’s work on a new spun-bonded nonwoven film of polylactic acid. Tiny but strong fibers create a flexible material that should hold up through the growing season, Hayes says.

Ideally, after harvest, growers till a biodegradable mulch into their fields, where soil microorganisms convert it into water and carbon dioxide.

Researchers will bury samples in the soil and check them over time to determine how long that process takes.

Tweaking the formula

The first growing season showed the new material holding up “pretty well,” degrading at a rate somewhat less than anticipated, Hayes says.

Inglis says too much light penetration seems to be allowing more weed germination beneath the film.

Tweaking the formula for next year’s trials should address both issues.

“The rate of biodegradation has to be a happy medium between not too fast and not too slow,” says Randy Shogren, research chemist at the U.S. Agriculture Department’s National Center for Agricultural Utilization Research in Peoria, Ill.

Shogren developed a paper mulch coated with vegetable oil-based resins that performed well in the field; it also resists some weed species that poke through plastic. But to avoid tearing the paper during application, mulch-laying machines must run at slower speeds.

Thornton has tried paper mulch in the past. “It holds up on the surface, but where you have to bury it, it rots in six weeks,” he says.

Orzolek, at Penn State, is working with a bioplastic derived from plant sugars that should hit full commercial production next year. Mirel mulch, from Cambridge, Mass.-based Metabolix, degrades in two to three months after it’s plowed into the soil, he says.

Ngouajio’s research on a biodegradable plastic polymer is on hold pending additional funding. Two years of field tests showed promise, but he hopes to improve the material to speed conversion by soil microbes into water and carbon dioxide components.

Goal: No disposal

The goal for a biodegradable mulch is a complete breakdown in the soil before the next growing season begins. That step also is needed for acceptance by organic growers, Ngouajio says.

Most biodegradable mulches on the market now are based on starches, particularly from corn. The possibility of those products incorporating starch from genetically modified corn also makes organic growers leery, Ngouajio says.

Biodegradable mulches will cost more up front—Orzolek estimates $150 to $180 per acre, compared with $80 per acre for a product growers must remove from their fields—but should reduce overall mulching expenses.

“The growers’ main interest is not having to dispose of it,” he says. “Remove the drip tape and rototill, then drive away and you’re done.”

Despite interest in alternative mulches, George McManus III, owner of L.H. Piggott & Girls Inc., in Benton Harbor, Mich., has stuck with black plastic as the most cost-effective option to warm fields for a longer growing season.

For now, “It’s cheaper to remove the old (plastic) and start over,” McManus says. “But any time you can break something down it’s better than loading up the landfill.”

Contact The Grower at vlboyd@att.net or (209) 571-0414.

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