Madera Farmer Praises the Benefits of Soil Solarization

August 08, 2005 — By Statewide Integrated Pest Management Program, University of California-Davis

Tom Willey is spreading the word about solarization and how this inexpensive, chemical-free approach killed the weeds plaguing his 75-acre organic farm in the Central Joaquin Valley.

Willey recently talked to nearly 30 people at a workshop on solarization sponsored by University of California Cooperative Extension at his farm in Madera. Jim Stapleton, plant pathologist for the University of California Statewide Integrated Pest Management Program, and Richard Molinar, small farms advisor for Fresno County, joined him for the presentation.

Soil solarization works like a greenhouse to trap the sun's heat to temperatures that kill insects, plant diseases, weed seeds, nematodes and soil pathogens. Solarization has become a widespread and growing practice for organic growers, home gardeners and other users.

Soil solarization is a perfect fit for small-scale, specialty crop growers because specialty crops have few labeled pesticides due to the high development and support costs and low returns for the manufacturers. Also, many of the farms are located next to urban areas, and the use of pesticides and fumigants is further restricted.

Master gardeners, pesticide applicators, farm advisors and those with small farms rode a tractor-trailer out to Willey's fields to see for themselves how solarization works. Participants watched a tractor stretch the clear plastic across the field. After several days of sunshine, soil temperatures rise to as high as 165 degrees at the surface. It takes four to six weeks of sunny weather to pasteurize the soil. The larger the solarized area, the more heat is generated and maintained and the longer lasting the effects.

Willey has been farming in Madera since 1980 and earned his organic certification in 1987. He and his wife, Denesse, employ 55 staff to tend everything from Asian turnips to rutabagas.

Willey says cheeseweed, or little mallow, was threatening his crops, and hand weeding it was "like painting the Golden Gate Bridge—a never ending job." Cheeseweed is a winter annual that forms dense bushes and can grow four feet high. It has flowers that are held in clusters, and the fruit resembles a miniature wheel of cheese. The pervasive weed was choking his winter seedlings, and he had to act fast.

"We launched ourselves into solarization," says Willey. "There's a real learning curve to it. You can't buy it and pour it out of a can to make it work."

Simple steps are to level the soil and be sure it is free of weeds, debris or large clods. Water the soil thoroughly, and use clear polyethylene plastic that is 0.6 to 2 mil and is ultra violet (UV) treated. Cover the area, making sure it is airtight with no holes.

"Solarization is a knowledge-based, rather than product-based, soil disinfestation method," says Stapleton. "Users are largely missing the benefit of having trained consultants for their particular geographic locations. To address that problem, we collaborated with the Kearney Agricultural Center Geographic Information Systems unit to create statewide air-temperature maps to help users determine the suitability of their area for solarization."

Users may access the maps at the UC Kearney Research and Extension Center (REC) Web site at www.uckac.edu/iwgss/.

Stapleton recommends using solarization from June to mid-July when weeds are more easily controlled. He suggests using a wide soil bed that is at least 20 to 24 inches wide across the top with furrows on both sides of the bed. To reduce the number of furrows or trenches, Willey uses beds that are 52 inches wide. This minimizes the amount of land that is nonproductive.

Willey was spending $2,000 to $3,000 an acre hand weeding his acreage. Using solarization on one of his 4 fields every 4 years has reduced that amount to $400 an acre, including the cost of the plastic and labor.

During the last eight years, Stapleton and Molinar have been conducting weed research on soil solarization at the Kearney REC in Parlier and on farms in the surrounding San Joaquin Valley.

Two on-farm field experiments were conducted in the Fresno-Clovis area from 1997 to 1999. The first experiment was with strawberries in an urbanized area with residential housing less than 100 feet from the edge of the field, and it had been 3 years since the previous fumigation using methyl bromide.

The results of a field study with parsley, along with the two on-farm experiments with strawberries, showed that solarization effectively controls weeds in fall and spring specialty and organic crops in the San Joaquin Valley. The research has provided guidelines and technical support for growers in a wide variety of specialty crops.

"During solarization, helpful microorganisms living in the soil benefit," says Stapleton. "Soil that has been solarized allows plants to draw on the nutrients, especially nitrogen, calcium and magnesium, more readily. Seeds germinate more quickly. Plants grow faster and stronger, often maturing earlier with substantially higher yields than in soil that isn't solarized."

However, there are exceptions. Stapleton says that certain weeds, such as yellow and purple nutsedge, are not consistently controlled by solarization.

The advantages of solarization include ease of use by the grower, low treatment costs and no hazards to the grower, workers or public, which is important for farms close to urban areas. Solarization is acceptable for use in organic production, and no permits or pesticide reporting is required. Growers also have the option of leaving the plastic in place after treatment as a bed mulch to further justify the cost.

Solarization must be timed soon after the spring harvest, but before planting for the next crop, so one of the disadvantages for San Joaquin Valley growers is that the land will be idle for four to six weeks during the summer. To avoid losing a growing season, growers can rotate their crops to take advantage of the land before and after treatment. Maintenance and getting rid of the plastic are also important considerations. An easy way to collect the plastic is to roll it onto a telephone cable spool.

Learn more about solarization from the University of California Statewide Integrated Pest Management Web site at www.ipm.ucdavis.edu, or the UC Kearney Research and Extension Center at www.uckac.edu/iwgss/.

Statewide Integrated Pest Management Program
University of California-Davis
One Shields Avenue
Davis, CA 95616-8621
www.ipm.ucdavis.edu

Contact
Stephanie Klunk
(503) 754-6724