UC looks at 'biosolarization' as fumigant alternative

Using the power of the sun and agricultural waste may be a viable alternative to fumigants in controlling weeds and other pests in the field, according to the University of California.

Researchers at UC Davis say they are encouraged by early results from collaborative experiments with "biosolarization," a process that combines the sun's heat with soil amendments to manage weeds and other soil-borne pests.

"It looks promising," said food science and technology professor Christopher Simmons, who is testing biosolarization with various crops and working with farmers throughout the state. "We still have a lot of work to do, but biosolarization is showing real potential as a safe, sustainable way to control pests while improving crop quality and yield."

The power of solarization is already well-known, UC Davis reported. Laying a clear plastic tarp over moist soil can trap solar radiation and heat the soil enough to kill weeds and other soil-borne pests. But the process can take four to six weeks, which is often too long for commercial fields to lay fallow.

Biosolarization can accelerate and improve the process, Simmons said. He and his team are adding organic amendments such as grape and tomato skins or ground nut hulls to the soil before they tarp it, which promotes growth of beneficial bacteria. The helpful microorganisms compete with pests and temporarily make the soil more acidic—and therefore less hospitable to weeds and other pests. Together, the soil-heating and microbial activity can reduce the treatment time to days, not weeks, he said.

"By activating beneficial microbes in the soil, biosolarization has the potential to improve soil health over the long term," Simmons explained.

He further noted that though fumigants affect beneficial microorganisms along with unwanted pests, "biosolarization allows more innocuous and beneficial microorganisms to persist in the soil."

For farmers to adopt biosolarization as an alternative to fumigants, Simmons said the treatment must be effective, predictable and economical. For this reason, researchers are testing biosolarization with a wide variety of crops, amendments and soils against different pests in various locations at commercial scale throughout the state.

"We have field trials underway with lettuce, tomatoes, melons and various cover crops," he said, "and we have a long-term, 10-acre trial with almonds at a conventional orchard in Chico."

In Chico, researchers are collaborating with almond grower Rory Crowley at Nicolaus Nut Co., with funding support from the Almond Board of California and the Western Center for Agricultural Health and Safety. They are one year into a 25-year experiment to see if almond-processing residues and the sun can boost soil health and reduce weeds and other soil-borne pests.

"It's been great for the soil," Crowley said. "Using biosolarization and a mustard cover crop, we've increased organic matter by 1.25 to 1.75 percent, which is a huge jump. That's good for carbon sequestration and the overall health of the soil."

It's too soon to tell if the soil improvements will translate to greater crop yield, but Crowley said he thinks biosolarization could become a good pest-management tool and a valuable use for what would otherwise be agricultural waste.

"We need to find a home for the co-products of almond processing, so why not see if we can use them to improve soil health while controlling pests?" he said.

Researchers are testing biosolarization on several annual and cover crops in plots on the UC Davis campus, using agricultural waste streams from tomato and wine processing. Soon, they will begin tests with strawberries.

Simmons said he hopes to demonstrate to farmers that biosolarization can be effective and economical under a wide range of conditions against a broad number of pests.

"We're making significant ground," he said. "We're hopeful biosolarization can help farmers return food and agricultural waste back into the system to control pests and improve crop production."