Published 1:08 am Friday, December 18, 2015
SANTA CRUZ, Calif. — With fumigants on their way out, organics could be the future of strawberry production in California, but persistent troubles with plant yields may be holding the method back.
Organic production amounted to 3,400 acres of strawberries this year, or about 9 percent of the 38,100 total acres of strawberries in the Golden State in 2015, said Mark Gaskell, a University of California Cooperative Extension farm adviser.
Organic acreage has increased precipitously each year since 2000, but growers have never been able to match the yields of neighboring conventional farmers, Gaskell said.
“Really, organic strawberry production has continued to increase for the past 25 years in California,” he said during a webinar on Dec. 16. “There are growers and farming operations that have been growing organic strawberries for 25 years or more. There are growers with considerable experience in growing organic strawberries, but yields are 40 to 60 percent of regular strawberry production.”
Organic growers encounter two main challenges — soil-borne diseases, for which fumigants such as methyl bromide are used on conventional berries, and nutrient deficiency.
With methyl bromide expected to be totally phased out by the strawberry industry by next year and with other fumigants such as chloropicrin and Telone facing increasing scrutiny, the California Strawberry Commission has spent millions of dollars in recent years researching alternatives such as crop rotation, using natural sources of carbon to eliminate soil pathogens and sterilizing soil with steam.
Among the concepts that UC and other scientists have been testing to control berry-busting bugs and diseases are raised-bed troughs and “soilless” fields, according to a more than 200-page summary of projects the commission published last year.
But while much attention has been paid to finding natural ways to combat soil-borne diseases that can wreck plants, often the primary factor limiting plant growth is a lack of nitrogen available to the organic plants when they need it most, Gaskell said.
“Our challenge is we have this extended harvest season” and much of the plants’ demand for nitrogen occurs later in the season, meaning pre-plant cover crops or compost alone might be inadequate to fulfill the plants’ demand, he said.
While conventional growers can turn to controlled-release nitrogen fertilizers to bolster productivity as the season wears on, no such timed-release fertilizers are available to organic growers, researchers say.
Under the current system, mid-season fertigation and foliar nitrogen applications are the only practical means for supplementing nitrogen levels, but those methods’ effectiveness can vary because of a variety of factors, ranging from soil type to weather. Liquid organic fertilizers can supply sufficient nitrogen in the late stage, but they have a high risk of nitrogen leaching if big rains come.
Gaskell and Joji Muramoto, a researcher in UC-Santa Cruz’ Department of Environmental Studies, have been conducting field trials along the Central Coast in recent years to test different cover crops, soil treatments and in-season fertigation levels to find one or two methods that work the best.
Among methods used to prepare fields before planting, the researchers found that composts and a sudan grass cover crop had no appreciable effects on yield, and the use of blood meal — a high-nitrogen powder fertilizer made from blood — was only effective in dry and warm winters, they said. In-season nitrogen injections had no effect in three out of four seasons, partly because of low distribution efficiency, the researchers said.
However, based on analysis of six field trials with 53 different treatment combinations, the scientists did find that nitrogen loss is strongly dependent on the amount of residual soil nitrate from the previous crop, the amount of nitrogen added in crop residue just before planting and the amount of pre-plant organic fertilizer used, they said.
More studies are needed to understand how to improve fertigation efficiency, reduce nitrogen loss during rainy winters and recognize the interaction between plant root health and nitrogen demand, the researchers said.
Meanwhile, most organic strawberry operations in California remain small, averaging 20 to 25 acres, Gaskell said. And growers who produce both organic and conventional strawberries notice a difference.
“Growers observe that the (organic) plants are always smaller … and yields are consistently lower relative to their conventionally grown varieties,” Gaskell said.
