What is biosolarization? Simply put, it’s a way for growers to use the sun and put microbes to work for them, killing nematodes and some weeds and getting soil ready to plant a new orchard.
It’s a substitute for fumigation during preplant and replant that UC Davis researcher Chris Simmons says provides richer soil along with other benefits. His team has been studying the benefits of the process summed up in the image above.
“If you can match the performance of fumigation and you’re doing that with biosolarization, that’s going to yield benefits that you would never have with fumigation alone,” Simmons explained by phone. “With biosolarization you’re more selectively targeting the disease-causing organisms and what’s left behind is a very robust and diverse microbial community that is capable of rapid breakdown of organic matter in the soil, and that’s important for nitrogen cycling, and humification of soil, and other processes that improve fertility and nutrient retention in the soil.”
Simmons’ team tested the process at a site called Kittyhawk Ranch owned by the Nicolaus Nut Company.
A walnut orchard had just been removed, and Nicolaus Nut offered the UC team just under 10 acres out of 50 in total, to try the process as almonds were planted.
The team used hulls and shells to amend the soil, and combined that treatment with biosolarization, in which plastic sheeting was used to heat the soil. Anaerobic fermentation under the plastic kills unwanted pests in the soil.
The soil temperature can reach 140 or 150 degrees fahrenheit, and that alone can be enough to kill pest, Simmons said. At that temperature microbes that don’t need oxygen can produce acid that kills pests while remaining safe for humans.
“Kittyhawk has the distinction of being our first foray into fieldwork for biosolarization in the almond industry,” Simmons said, thanking the Almond Board for support for the research.
The project will hopefully yield “an appealing opportunity to provide a new outlet for the solid waste that the almond industry generates every season.”
Researchers estimate the process is already used on 20,000 acres in the Imperial Valley to control pests, largely in the leafy green industry.
“The priorities for an almond grower would be very different than a spring mix grower in the central valley. It would be more to control nematodes and fungal pathogens,” Simmons said. “We’ve been focused on nematodes.”
As you can see in the chart above, significant reduction in nematodes was achieved, with the best results for the top six inches and foot of soil when hulls were used with biosolarization.
Simmons’ chart below shows how irrigation water can be used to spread the organic acids that kill nematodes even further into the soil.
“The health of that orchard in the first year can have ramifications for the next two decades,” Simmons said, describing the benefits of added organic matter and fewer nematodes with a healthy population of beneficial microbes.
Drip or subsurface irrigation is costly, but where it is already in place, the cost of biosolarization is much reduced, Simmons said.
“Right now we’re focused on making it work as good as possible and giving it the best shot to control pests at the level fumigants would,” he said. “Once we find out what those conditions are, we’ll dig deeper and try to find out how to create those conditions at the best economics possible.”
Even if growers are replanting an area with no irrigation, Simmons said biosolarization may make sense as a reason to make the irrigation switch--aside from an ongoing benefit of greater water use efficiency, the process has many benefits to offer growers.
“We don’t think it’s exactly fair to only compare the price of biosolarization against the cost of fumigation because the benefits are not only pest control,” Simmons said. “What price do you put on healthy topsoil? What would be the value for those dimensions on top of the pest control? What is the value of enhanced nutrient retention and water retention?”