From Steaming to Biosolarization, Growers Have Options to Replace Chemical Fumigants
By Thomas Skernivitz, Senior Editor, American Fruit Grower and American Vegetable Grower
There has long been a rally cry for alternatives to the chemical fumigation of soil. Some voices are louder than others.
“This is a philosophical thing, and it may be controversial, but I'm going to say it anyway,” Steve Fennimore, an Extension Specialist with the University of California, Davis (UC Davis), says. “People have gotten too passive about pest management — soil pest management, above-ground pest management. We basically have depended upon the agricultural chemical industry to come up with pesticides. We've gotten passive — or maybe we always were passive.”
Soil steaming has shown results in managing weeds in open field production and in the greenhouse. It also helps manage pests including as well, including Pythium, Sclerotinia minor, Verticillium wilt, Macrophomina, Phytophthora, and nematodes. Photo by Steve Fennimore
With a focus on soil health, Fennimore and other researchers in the U.S. and Canada are taking the offensive, in particular with three alternatives to chemical fumigants: soil steaming, anaerobic soil disinfestation (ASD), and biosolarization.
‘Cooking the Pests’ Having researched soil steaming the last 15 years in the Salinas Valley of California, Fennimore has become a major proponent of the technique, which uses steam to sterilize soil in open fields, high tunnels, and greenhouses. His hope is that growers, particularly on the vegetable side, become even bigger fans. He readily notes, as a disclaimer, that he is in the process of developing a steaming device; an endeavor he encourages growers to consider themselves.
“I work on crops like lettuce and broccoli and strawberries, and as far as weed control, which is one of the strengths of steaming, those are not markets [for machinery]. It's not [the growers’] fault. It's just a limitation of the system,” Fennimore says. “So, you ask about a take-home message for farmers: If you want to use this technique, you're going to have to develop the technology yourself.”
Fennimore encourages growers in the West to contact him. “I'm close enough. Let's try to work together,” he says. “We're trying to develop organizations to develop commercial-scale steamers in the expectation that there's going to be some type of commercial adoption of this.”
Why such optimism? “Because it works,” Fennimore says. “The weed control is quite good.”
The “perfect” steam treatment, according to Fennimore, revolves around an application of 158° F for 20 minutes.
“You hit the temperature. You maintain the dwell time. What happens? Basically, you're cooking the pests,” he says. “You're killing the seed embryos. They don't have to germinate. It's easy to kill them if they are germinating, but you kill the embryos right where they stand in the seed.”
His research, Fennimore says, has shown success against pests beyond weeds as well, including Pythium, Sclerotinia minor, Verticillium wilt, Macrophomina, Phytophthora, and nematodes.
“We can kill those pests pretty much like fumigants,” Fennimore says. “The difference between fumigants and these is that fumigants are highly regulated in California. You can only do it at certain times at certain places. You've got to leave plastic on for a long time to restrict the movement of the fumigant — the escape — so that neighbors aren't exposed.”
Growers should exceed 150° with steaming but never 160°, Fennimore warns. Anything higher could result in soil sterilization.
Anaerobic soil disinfestation involves the amendment of soil with a readily decomposing source of carbon to initiate rapid microbial growth, in this case with partially decomposed chopped alfalfa. Photo by Tom Forge
“Believe me, you do not want to sterilize the soil. We are not sterilizing soil. We are pasteurizing it. That is a very important point,” he says. “The reason steam works, and the reason we can get away with pasteurizing the soil, is that the bad guys are easier to kill than the good guys. The good guys — the beneficials — are more tolerant.”
Fennimore spent this summer using a continuous moving machine to apply steam in bands at a depth of 4 inches. This covered 20% of the total acreage of the field, which was then used to plant lettuce and carrots.
“It has to be aligned with where you're going to plant your crop. And then the next day — actually within a few hours — you can come and plant right in the center of that,” Fennimore says. “What you end up with is basically a clean band, with your crop coming up in the center of the band.”
The weeds outside those bands are considered the “cheap weeds” by Fennimore. “They are easy to cultivate,” he says. “Cultivation is, what, 12, 15 bucks an acre for a mechanical cultivation? It's a cheap activity; very labor efficient.”
To the contrary, hand weeding immediately next to the crop is very labor-intensive and costly, especially in an organic system in which herbicides are not used. But band steaming alleviates that problem.
“What we're doing is creating a partially clean zone. We're not really touching the soil outside of the band because those weeds are easy to control,” Fennimore says. “The root zone near the crop is the most critical to establishing the young seedling, and that's what we're trying to do. We're trying to protect the roots on the on the seedling crop so that we get a good start. Every farmer that might listen to this or read this will know that if you've got a problem in the seedling stage, you’ve got a problem that will be with you for the season.”
In the end, soil steaming allows the grower to take control of the situation, and the process does not have to be “horribly expensive,” Fennimore says.
“A small grower can get it set up with their own equipment if they have a small tractor. It can be reasonable, and if you share the equipment with a group, then you can greatly reduce the cost,” he says. “So, don't be intimidated by this technique. And don't assume that the chemical companies are the only option. There are other options.”
Pathogen ‘Cereal’ Killer
Another of those options is anaerobic soil disinfestation (ASD), which incorporates easily decomposable organic materials into the soil, followed by irrigation to saturation and finally soil cover with impermeable plastic.
Cereal brans, molasses, ethanol, and vegetable wastes are the main sources of carbon used in ASD. The soil remains covered from three to 10 weeks. Accumulation of toxic anaerobic decomposition products, antagonism by anaerobic organisms, lack of oxygen, and the combination of all these factors are the main mechanisms of action of the technique against plant pathogens.
Brazilian researchers, in a study published this year in Applied Soil Ecology, conclude that ASD demonstrates great potential to be used as an ecofriendly approach to control soilborne pathogens, insects, and weeds. “Organic and conventional farmers can benefit from ASD,” they write, “and this technique can be integrated with other sustainable control tactics, including crop rotation, biocontrols, and the use of organic amendments.”
Researchers with Agriculture and Agri-food Canada, the country’s department of agriculture, are currently studying the effects of ASD. Although data has yet to be released, the organization finds the method “exciting,” according to Agriculture and Agri-food Canada Research Assistant Paige Munro.
“You incorporate some easily decomposable organic material into the soil. Commonly used products are cereals and molasses, whatever's on hand,” Munro says. “We're using an alfalfa/Timothy grass hay mixture. Whatever is available. You can use orchard grass.”
After soil is watered to the point of saturation, it is covered with an impermeable plastic tarp that is left in place for several weeks.
“These steps create a low-oxygen environment, and that facilitates the accumulation of anaerobic decomposition products,” Munro says. “Those end products from that decomposition, in combination with the low-oxygen environment, help kill plant pathogens, such as fungi, bacteria, and nematodes.”
Biosolarization builds soil health by heating and killing pests and pathogens, while also providing a compatible environment for beneficial and benign microbes to “wake up” and start to metabolize soil amendments. Photo by Christopher Simmons
The process is thought to boast other positive effects, such as increasing nutrient availability in the soil, Munro says, but more research is needed to reach that conclusion.
“This is an interesting method in several ways,” Munro says. “There's lots to consider to kind of tweak what you're after. You can change the carbon source. You can change the application rate of that carbon source. You can change the incubation period for the tarp. Soil temperature is really important — the warmer the better, really, to get the best efficacy. Also, the plastic cover you use is important and also expensive, which is a consideration for this method.”
Finally, growers would need to determine what soil-borne pathogens are present.
Any Given Sun Day What does a grower get when aerobic soil disinfestation and solarization are combined? The answer — biosolarization — is near and dear to the heart of Christopher Simmons, a UC Davis professor who has researched the technology since 2011.
“I truly feel it’s one of those golden nuggets that you come across in terms of a solution that can offer so many different benefits,” Simmons says. “Oftentimes we talk about biosolarization in the context of pest control and as a technology to displace conventional fumigants, and the data show that it could be very good at that. But there is just a litany of benefits that also come with the technology that you would never get with conventional pesticides. And this feeds into the whole area of soil health and agricultural sustainability.”
Because a key element of biosolarization involves placing organic matter amendment in the soil to drive the biological processes that allow the technique to work, the digestion of these amendments also produces plant nutrients.
“You have a diverse, thriving microbial community to maintain nutrient turnover and help to exclude the recolonization of pests and pathogens,” Simmons says. “You have organic matter that's acting like a sponge to hold water in the root zone to promote soil aggregation. And humification is going to help prevent erosion and keep nutrients in that root zone, where growers want them. So, it speaks to many different dimensions of soil health and maintaining a healthy soil that's going to be productive for many years to come.”
Like ASD, setting up the biosolarization plot involves three key elements. First, growers need to broadcast and till in some type of compatible organic matter for microbes to eat.
“Typically, we're looking at residues that are abundant and low cost in California,” Simmons says. “This would be things like spent grain from brewing, skins and seeds from wine-making or tomato processing, and the hulls and shells from almond processing.”
Once those are incorporated into the soil, water is added via drip tape or drip line. Finally, those plots are mulched or covered with clear, transparent plastic tarp. “This could be the same tarp that you would use if you were fumigating,” Simmons says. “In this case we're just repurposing it.”
Once those three elements are in place, the soil is wetted with the irrigation system, which activates the process. The combination of the moisture and the food in the soil allows the bacteria and fungi to “wake up,” and start to degrade and metabolize, Simmons says. Meanwhile, the tarp acts like the windshield of a car parked in the sun on a hot day, he adds.
“It's allowing solar radiation in, trapping that heat and causing the temperature to elevate in the soil much like when your car gets very, very hot; hotter than the ambient environment once it's parked in the sun,” Simmons says.
As is the case with soil steaming, this “cooks” the pests and pathogens, Simmons says, while also providing a compatible environment for beneficial and benign microbes to “wake up” and start to metabolize those amendments.
“And what they're doing is producing fermentation products because the tarp is blocking oxygen from getting into the soil,” Simmons adds. “What that does is trigger the microbes to act in a certain way and to produce fermentation products that we call organic acids.”
Within the UC Davis program, biosolarized soils have led to improved yield for tomatoes grown in the treated soils, Simmons says. Meanwhile, the largest and longest-term study to date involves an almond orchard that, five years in, has been tracking the initial benefits to soil fertility, including the persistent elevation of potassium and nitrogen in the biosolarized soils. This would lead to long-term multi-year suppression of root lesion and ring nematode while eventually benefiting tree growth and health by way of larger trunk diameters and larger and greener canopies.
“Those data together really tell kind of the complete or almost complete narrative because we haven't, yet gotten to yield measurement, which is what we're doing currently,” Simmons says. “But they do show the multifaceted benefits that can come from biosolarization — from the test control to the soil chemistry and then eventually the positive interactions with the crops.”
