Using the power of the sun is organic way to fumigate

Biosolarization is an alternative technology to soil fumigation for agricultural crops to control nematodes, bacteria, fungi and other pests that damage crops.

Chris Simmons, associate professor and vice chair in the Department of Food Science and Technology and director of research and outreach for the Western Center for Agricultural Health and Safety at the University of California, Davis, has been doing biosolarization research since 2011.

Currently, biosolarization, along with a suite of other methods that utilize various aspects of the solar heating and soil amendment processes leveraged by biosolarization, is being used more in row crops, and initially in organic cropping systems.

Biosolarization offers solutions to organic growers, and Simmons said he also sees advantages for conventional agriculture, pointing to the conventional strawberry business as the best example of tapping into this process.

"They are one of the most reliant industries on soil fumigants, and as the predominate fumigants get more scrutiny and become more controlled, and in some cases outright banned, they have invested heavily into looking at alternative approaches for soil pest management," Simmons said.

In the past 12 years, the California Strawberry Commission has invested heavily in fumigant alternatives research, spanning anaerobic soil disinfestation—an approach similar to biosolarization—and other technologies. As is typical, organic strawberry production has been the earliest adopter, with roughly 1,000 acres treated. However, research sponsored by the Strawberry Commission intends to make biosolarization effective for widespread use in conventional production.

Though strawberries are grown mainly in coastal regions, where cooler weather may challenge the solar heating component of biosolarization, the hotter weather of the Central Valley can enhance the pest control efficacy of biosolarization, Simmons said.

In the Central Valley, there is also more access to locally sourced, compatible organic matter that may be used as soil amendments for biosolarization, such as:

• Tomato processing residues.
• Almond hulls and shells.
• Brewers' spent grain.

"Things like hulls and shells from almond processing, spent grain from brewing, skins and seeds left over from tomato processing," Simmons said, are economical for biosolarization use, and because California has numerous specialty crops and processing, it has a wide array of materials to draw from for biosolarization.

To establish biosolarization, an amendment that may or may not have been pretreated—dried or milled—is spread onto the field using a truck or tractor implement, then it is disked or tilled into the soil. Next, drip line and plastic film are laid down. Once the irrigation is started, the process is activated, Simmons said.

During the daytime, solar heating induced by the plastic film combines with microbial activity promoted by the soil amendments. They start breaking them down and deplete oxygen in the process, while producing compounds that act against pests. This establishes several stresses on pests in the soil.

"When done right, you can get high level pest control across many pest categories similar to fumigation, but unlike fumigation you've produced a very active microbial community in the soil as they degrade the amendments," Simmons said.

The goal is to have the plastic film on and initiate irrigation in early to mid-July in the Central Valley. This allows ample time to stimulate stresses in the soil, remove the film and allow conditions to restabilize before planting, Simmons said.

"If the temperatures are cooler, you're probably going to have to dial up the amount of time that the film is on the soil to get a consistent knockdown of the pest or pathogen," he said.

Just as with fumigation, biosolarization is generally done in fallow fields as a pre-plant treatment. For a number of Central Valley specialty crops that are harvested from late spring through summer, there would be a period before establishing the cover crop where biosolarization could be done.

When the temperatures are high, the process can be completed in about a week. Depending on environmental conditions and the type and amount of amendment used, several weeks may be needed for the soil to evacuate residual compounds produced during biosolarization that could stress plants. Once the process is complete and the soil is stabilized, it is safe to introduce the crop, Simmons said.

Farmers interested in biosolarization should begin with small test plots to determine the optimal conditions for their crops, Simmons advised.

Chemical fumigants achieve near complete control of the targeted pests, especially in the root zone, he said, and it is possible for biosolarization to match this pest control benchmark while also delivering other benefits to soil health. He said the process:

• Adds organic matter to the soil.
• Improves water holding capacity.
• Promotes microbial activity.
• May accelerate nutrient turnover.

Simmons went on to say that biosolarization should ultimately improve yields and productivity. Currently, there are trials that are tracking long- term crop performance as organic matter breaks down and makes more plant nutrients available.

"We've already seen it in one of our studies, cultivating tomatoes in the biosolarized soil," Simmons said, and a longer-term study is being conducted in an almond orchard in Butte County, which will have its first harvest next year and provide yield data.

Simmons is also looking at biodegradable plastic.

"Most of biosolarization today is done with plastic that is removed from the field and recycled. It's not degradable, and it is not respooled to use again in biosolarization," he said, "because there's no feasible way to respool it.

"There are films on the market that claim to be biodegradable, and we're starting to look more into using those along with the prospect of tilling them into the soil to accelerate degradation," Simmons said, "but this could be problematic for organic growers."

Most of the degradable tarps currently on the market have some additives that are not organically approved. If an organic grower used them, these tarps would have to be completely removed from the soil and composted elsewhere, he said.

"There is a need for new types of bioplastics that are optimized for biosolarization in both organic and conventional cropping systems," Simmons said.

(Kathy Coatney is a reporter in Bend, Ore. She may be contacted at kacoatney@gmail.com.)