—story and photos by Ross Courtney
Researchers in Prosser, Washington, are poking electrical sensors in the ground, burying a root camera and uprooting entire cherry trees to study how and when nutrients move within them.
The goal is precision nutrient management, delivering only what the tree needs when the tree needs it, said Bernardita Sallato, a Washington State University tree fruit extension specialist at the Irrigated Agriculture Research and Extension Center in Prosser.
Sallato is the principal investigator for a three-year project funded with $111,000 from the Washington Tree Fruit Research Commission.
Traditionally, Pacific Northwest cherry growers have fertilized following soil analyses and tissue tests, but they’re using tests based on standards developed for freestanding trees with older, vigorous rootstocks, Sallato said. That helps growers determine if their trees are sufficient or deficient in nutrients at a point in time, but it doesn’t reflect how demand changes.
Growers need to know specifically when their trees need certain nutrients and how different rootstocks and training systems may affect those needs, Sallato said. Otherwise, they waste fertilizers and increase the risk of nitrates leaching below roots and into groundwater, an issue that will become more important with the tree fruit industry beginning to pursue carbon credits and other sustainability certifications.
Research on apples has shown how different rootstocks affect root growth timing and nutrient uptake, but those questions have not been explored as much for sweet cherries, she said.
Knowing more about when trees most efficiently take up nutrients would help growers apply fertilizer at the right time. Sallato’s work may help answer why nutrients can be plentiful in the soil but not in the tree or, more importantly, the fruit.
“We’re still kind of shooting in the dark,” said grower Denny Hayden, a member of the research commission’s cherry committee and a collaborator on Sallato’s project.
Apple and cherry growers often fertilize with foliar sprays of certain nutrients, especially calcium, at prescribed times of the year. Sallato has long questioned how much those help, but her research may shed some light on the practice, Hayden said. Dialing in the precision of ground application might help reduce the need for more expensive foliar sprays.
Sallato and her team have set up several trials to help answer these questions.
In one trial, they are cutting, uprooting and grinding entire cherry trees to study biomass and nutrient partitioning — where nutrients are stored within the tissue of the tree — across three different rootstocks, training systems and tree ages. Those blocks are: a fifth-leaf Benton on MxM14, Gisela 12 and Gisela 5 rootstocks in a steep-leader training; 12th-leaf Skeena and Selah on Gi.12 with UFO training; and first-leaf Black Pearl on Gi.6, Gi.12 and Mazzard in a two-dimensional system.
Also, Sallato is comparing three techniques to monitor root growth from bloom until leaf fall. One is with root windows, or pits, dug in the ground and lined with clear plexiglass to allow visual root inspection. Another is a mini rhizotron, a specialized camera — a CI-600 In-Situ Root Imager from CID Bio-Science, to be exact — inserted into a plexiglass tube. The third involves placing electrical resistivity tomography sensors on the ground beneath the trees and using electrical current to map subsurface variations and visualize plant root structures and soil root interactions. Collaborators at the Pacific Northwest National Laboratory in Richland will monitor the sensors. The technology is new to orchards but has been used in archaeology, soil contaminant tracking and mining.
To measure nutrient availability and leaching, they track nutrient levels in the soil at three depths and at five times during the season.
After one year of trials, Sallato has determined the following:
—Vigorous Mazzard rootstocks led to a greater share of biomass in the roots, both fine and big, than precocious rootstocks Gi.6 and Gi.12.
—Root growth for all rootstocks began about a month after full bloom, once soil temperature reached 70 degrees.
—Trees on Gisela rootstocks had a second, small flush of root growth in mid-September, giving growers an extra, monthlong opportunity for efficient nutrient uptake.
—Nitrate levels in the sites with mature trees were within normal ranges, with highest levels in the first 8 inches of soil and levels declining with depth. That suggests little to no leaching between June and September.
California researchers have done similar work, also hoping to develop precision nutrient applications and stave off regulators concerned about nutrient pollution.
In 2019, Doug Amaral, a University of California Cooperative Extension water and soil specialist, led a three-year project to measure nutrient levels at various sites within the tree. His team also excavated entire trees — nine Bing, nine Coral Champagne and nine Rainier — and cut them up with chain saws to measure biomass and pinpoint where nutrients traveled. Among his findings: Trees accumulated 90 percent of their annual nitrogen by September.
Amaral’s research, funded by the California Cherry Board and the California Department of Food and Agriculture, yielded nitrogen “budget curves” that will help growers optimize their fertilizer applications by synchronizing them with tree demand.
The results are posted as industry standards on the cherry board’s website. For one example, Amaral determined that cherry trees accumulate between 80 and 90 percent of their nitrogen between April and September.
Coming later this year, Amaral said, will be a web-based model at the UC Davis website for tree fruit and nuts. •
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