How We Grow

2020 July/Aug 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 9 associate professor of agroecology in UC Davis' Department of Plant Sciences and also coauthored the WOR research report. "Water is central to how we think about agriculture in California, and this research provided a clear example of how growers can capitalize on soil health to increase water use efficiency." Under regular irrigation conditions (100% evapotranspiration, or ET), the WOR treatment blocks yielded more pounds of kernels every year for seven consecutive years compared to the burn treatment blocks, with annual differences ranging from 82.3 lbs./acre (first harvest in 2011) to 498.4 lbs./acre (2014). UC researchers also compared the impact of deficit irrigation (80% ET) between the WOR and burn treatments, and while yield increases were not statistically significant, the WOR treatment plots still yielded more than the burn treatment plots. The midday stem water potential – a direct measurement of water tension within the tree – was on average 13% higher in the deficit-irrigated WOR treatment trees compared to the midday stem water potential of the deficit-irrigated burn treatment trees, indicating less tree stress occurred in the plots where WOR occurred. "Trees experienced less stress in soils that had significantly higher levels of organic matter – those where Whole Orchard Recycling took place," Ludwig said. "That is because increased organic matter allows the soil to be a little more forgiving due to its better water holding capacity, which means the trees are subject to fewer extremes in terms of water availability as more water is held in the upper layer of the soil. Whether stress is due to keeping up with water demand on a hot afternoon, breaks between irrigations (such as during harvest), or strategic deficit irrigation, the trees that were planted on Whole Orchard Recycling plots appeared to be more buffered from those stresses." Ludwig also notes the study's impressive findings of a 20% increase in water use efficiency due to WOR, an improvement attributable to the increased yields for the same amount of water applied. "It seems that whatever water is being applied to orchards where WOR took place is being retained in the topsoil longer and therefore helping to produce more yield rather than going someplace else in the environment. Also, with increased water holding capacity in the soil, trees have more access to nitrogen or other nutrients that move with water," Ludwig said. Pay attention to nitrogen in year one Often-used options for tree disposal in the past, such as burning them in-field or sending them to a cogeneration power plant, are today limited due to air quality regulations and clean energy generation pricing. "There are still permits available for burning, but they are getting expensive, and in 2015, about half the cogeneration energy plants closed," Holtz said. "All of a sudden, growers were having to spend about $1,000 per acre to have trees removed and they had to decide what to do with all the wood chips." Holtz designed the WOR research trial at the Kearney Research and Extension Center to study the effects of returning high volumes of carbon locked in old trees back into the ground for the next orchard to use. Initially, the industry was skeptical, but when cogeneration plants started to shut down, the industry turned to Holtz to see how the trial was working out. "Growers felt that if you added a lot of carbon to the ground, you might be tying up your nitrogen and stunting the young trees," Holtz explained. Previous UC research determined that applying 25-35 lbs. of nitrogen per acre in the first growing season resulted in ideal growth rates for young trees. When WOR is performed, 40-60 tons of wood chips that contain 15-30 tons of organic carbon are incorporated into the soil. Holtz and the research team led by Mae Culumber, Ph.D., a UC Cooperative Extension farm advisor in Fresno County, found that the high carbon-to-nitrogen ratio immobilized nitrogen in the first year – in line with growers' initial concerns – which led to their recommendation of doubling nitrogen applications for WOR orchards in the first year only. Following this doubling in the initial year, nitrogen applications should follow traditional recommended amounts, as demonstrated by continued UC WOR trials. Research continues to assess to what extent the amount of nitrogen applied can be reduced in later years, factoring in considerations such as the rate at which nitrogen is released from the wood chips and/or improved water holding capacity, which also improves nitrogen use efficiency. "Today, when new growers who have used Whole Orchard Recycling call me, their main question is around nitrogen applications in the first year," Holtz said. "Even though we have to add nitrogen in the first year, growers should see "Our soils in the San Joaquin Valley typically only average one percent organic material or less. Whole Orchard Recycling is a once-in-25-year opportunity to add organic material to your soil." — Brent Holtz Continued from page 8

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