How We Grow

2020 Jan/Feb How We Grow

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A L M O N D O R C H A R D 2 0 2 5 G O A L S ZERO WASTE 11 Trial Suggests Almond Shells Have a Forever Home in Strawberry Fields Research under way in the state's Central Coast region shows an exciting possibility for a collaboration between growers of almonds and strawberries – one that may improve the environmental and economic bottom line for both crops. Though still cautious, researchers hope their latest work will demonstrate a new way to increase strawberry yields, reduce nitrogen losses, protect water quality and further show that the value of almonds goes beyond those tasty kernels. Their study involves adding ground almond shells – a rich source of carbon – to the soil, specifically after broccoli or cauliflower is harvested in the fall and before a subsequent crop of strawberries is planted on that same soil. Carbon from the shells feeds microbes that help retain nitrogen in the soil longer, preventing it from being washed out by rain or irrigation before strawberry plants can use it. A trial held in Watsonville and completed in 2018 produced "a good surprise" for these researchers, according to Dr. Joji Muramoto, a University of California Cooperative Extension (UCCE) specialist and longtime researcher at UC Santa Cruz. "The best scenario we were anticipating before the experiment was to reduce the nitrate leaching potential without negatively affecting successive crop yield," Muramoto said. "To our good surprise, we also saw an increase in strawberry yield, especially when we used ground almond shells." Though a very limited trial, the yield increases were significant – about 25 percent more cartons of strawberries per acre were produced compared to the control plots, enough to increase grower returns by an estimated $7,800 per acre. According to Peter Navarra, an organic agronomist for berry marketer Driscoll's, that kind of return is enough for a strawberry grower to consider investing the approximately $400 per acre it costs to buy and apply the right amount of ground almond shells. Encouraged by those results, Muramoto and Navarra are now working to repeat the early success that was supported by the Almond Board of California (ABC), which donated 24 tons of ground almond shells for the study. Solving the crop rotation challenge In this study, researchers are trying to solve a nagging problem for Central Coast growers, who produce much of the nation's fresh produce. Many of these growers rotate a "cole" crop grown in the summer and harvested in the fall – such as broccoli, cauliflower or broccolini – with a crop of strawberries, planted in October or November and harvested the following spring. This crop rotation is popular among strawberry growers because incorporating broccoli residues into the soil is known to suppress Verticillium dahliae, a lethal soil- borne pathogen for strawberries. However, this rotation can greatly increase nitrogen loss potential during winters. Because growers only harvest about 30 percent of the vegetative mass of crops, like broccoli, the remaining 70 percent of the plant remains in the field to be mowed and tilled into the soil. This residue contains about 200 pounds per acre or more of nitrogen, which can release significant amounts of nitrate under the mild winters of coastal California. Young strawberry plants, however, barely take up any nitrogen the first few months, almost behaving as though the land were being fallowed. *Photos by Dr. Joji Muramoto, University of California Cooperative Extension In California's Central Coast region, strawberries are commonly planted in October or November, before the rainy season, then harvested the following spring. Ground almond shells are spread onto a recently harvested field of broccoli and then incorporated into the soil, where they help immobilize nitrogen until a subsequent strawberry crop can use it.

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