Written by Lauren Lehrer

“Do you want some strawberries?” asks research assistant Lucy Toyama.

She points to two one-gallon buckets sitting askew in the dirt alongside test rows of strawberry plants at the UC Santa Cruz Farm. We walk to the buckets and peek inside. The intoxicating sweetness wafts toward me; the deep red of the piles beckons me to taste. Yes, I want some strawberries; I want a whole bucket of them

ASD
Anaerobic**
Soil
Disinfestation

Toyama hands me a green plastic mesh basket, which I fill with the berries’ red decadence. This fruit grows from the research of UCSC professor Carol Shennan and lead horticulturist Joji Muramato. Their team is testing and developing a farming method called Anaerobic**Soil Disinfestation** or ASD, to grow strawberries and other crops using biological methods instead of harsh fumigants.** The lab works with farms in Santa Cruz, Monterey, and Santa Barbara counties.

Shennan and her team have worked for 12 years to develop ASD and are seeing it grow.

About 2.5 percent of California’s strawberry acres implemented ASD in the 2014-2015 growing season, a large increase from the previous year. The lab’s work now focuses on optimizing ASD for established organic farmers, which make up 80 percent of the ASD-treated acres in California.

As the team develops the method in the organic sector, they also seek to find a modified version of ASD that is viable for more conventional farms. If the researchers succeed, they could alter conventional farming’s use of harsh fumigants** by providing a biological alternative in a changing agricultural world.

“Making a commercially feasible alternative for fumigants and observing growers’ positive reaction to our research is very exciting,”--Joji Muramoto

Shennan and Muramato started testing ASD, a method pioneered in Japan and the Netherlands, in 2003 at UCSC’s farm. The team explored the possibility of expanding the method past organics and into conventional farming. In 2007, the team received funding from the U.S. Department of Agriculture’s methyl bromide transition program, and in 2008 the California Strawberry Commission funded the project again. This research seeks to provide conventional farmers with an alternative to the harsh fumigant** methyl bromide.

Methyl bromide kills nematodes,**Field workers must inject this colorless and odorless gas into the soil of conventional farms before planting the strawberry crop. This chemically intensive method has provided us with plenty of cheap strawberries for more than 40 years.

But methyl bromide has a nasty side.

The fumigant can corrode the skin and eyes of laborers, poisoning them and possibly causing cancer. Excess methyl bromide molecules ascend from the soil into the atmosphere and eat away at the ozone layer that shields our planet from the sun’s ultraviolet rays.

Because of these effects, the U.S. Environmental Protection Agency put the fumigant under critical use limits in 2005.

This fumigant, which strawberry farmers have used for decades, has had no effective alternative.

Because the immediate elimination of the fumigant would devastate the strawberry market, the EPA has granted critical use exemptions for conventional strawberry growers since 2005. Each year fewer exceptions are granted in an attempt to phase out the fumigant completely. Regulators expect in 2017 these exemptions will no longer be available for California growers. Conventional farmers desperately need to find other tools to control pests and diseases.

Fresh Berries, Photo Credit:  Judith Klinger
Fresh Berries, Photo Credit: Judith Klinger

These tools could be different harsh fumigants—or large-scale farming could turn somewhere else. The lab hopes ASD can become the alternative for conventional growers and a preferred option for organic growers.

ASD is a new recipe for the soil.

First, field workers prepare the soil with rice bran or grape pumice,****And since “anaerobic” is the A in ASD, we can bet it’s important.

Once the soil is anaerobic,**As the decomposers “breathe” using carbon, they also create byproducts. The cellular waste and lack of oxygen kills diseases in the soil. Once the ASD preparation is over, the farmer can plant the strawberries. As the workers dig holes through the tarp and into the earth, oxygen returns to the soil. The disease-free soil welcomes the planting of the young strawberry crop.

But it is not always that easy. In the last few years, a major challenge has arisen: drought. Water acts as an important supplement to ensure soil reaches an anaerobic state. When the tarps are still pulled over the rows, workers switch on the irrigation drips to flood the soil with water. The extra H2O fills in any tiny spaces between the soil particles so that oxygen has no space to sneak in, thus facilitating anaerobic conditions. During the ongoing drought, the need for extra water “is definitely a concern,” says Toyama.

“Due to the drought, some growers are doing ASD without adding water,” says Muramato. “It seems to work in some cases when the disease pressure is not too high.”

But this may not work with every field. This year, the lab is conducting water-free trials on test fields with high disease presence to see if ASD can still be effective in this new regime of limited water.

ASD is not perfect.

“It is important to remember ASD is not a silver bullet replacement for fumigants.”--Muramoto

“It is a biology-based method,” so things like temperature or soil type affect how well the method works. These variables change from field to field. For ASD to move to a large scale, the team believes, the technique must integrate other methods—such as crop rotation or a low-level fumigant.**

With increased labor, material costs, and the occasional challenge in turning the soil anaerobic, ASD is much more involved than the old method of spraying a fumigant.**

To perform ASD “takes time,” says Muramato. The farmer must create strawberry beds, line those beds with irrigation lines and drip tapes, and then cover the fields with plastic tarp. And ASD requires water application within 48 hours after the rice bran or grape pumice carbon source is incorporated. Though most farmers solve this by working one at a time with smaller sections, five acres or below, many practicing farmers feel there are “a lot of materials and a lot of work to do ASD,” notes Muramato.

ASD yields approach conventional farming yields.
ASD yields approach conventional farming yields. Photo Credit: Robin E.H. Ove

Positively, ASD provides comparable yield from organic farms to those from conventional farms using fumigants. Usually, organic yields are 50 to 80 percent of those from conventional farms for the same acreage; ASD organic growers now see much higher crop production than before. “With the price premium, [ASD] makes economic sense to many organic berry growers,” says Muramato.

There are hurdles for ASD, but recent results are encouraging. Working with groups like Farm Fuel, Inc., a Watsonville-based company that helps farmers implement ASD, and Agriculture and Land-Based Training Association (ALBA), an organization focused on programs for immigrant farmworkers, the UCSC team works to share its findings on ASD and teach improvements on the method.

ASD is used in other crops besides strawberries. The Shennan lab also works in brussel sprouts and broccoli fields, testing whether ASD can control for pests as well as the usual soil-borne diseases. Other research groups work with ASD in apple orchards and may begin on almond and walnut farms, where there have been promising trials.

Back at the UC Santa Cruz Farm, I pick a strawberry from the small basket Toyama handed me.

I bring the red fruit to my mouth. The skin was warmed by the sun, and as I bite in the flavors overwhelm my taste buds. ASD has some challenges before it can be implemented on a large scale with strawberries. But as methyl bromide is phased out, conventional strawberry farmers face the decision of new chemicals or new biological alternatives. Whether that method will be ASD, another biological method, or a different fumigant is yet to be seen. Regardless of the outcome, our strawberry prices may be going up. I just hope our future strawberries at the grocery store will taste as good the ones I received in my green basket at the farm.

Lauren LehrerLauren Lehrer is majoring in ecology and evolutionary biology at UCSC. She wrote this story for SCIC 160: Introduction to Science Writing.