Research Looks at Methods of Getting Calcium into Blueberries

Speaking at the Oregon Blueberry Field Day, July 26 at the North Willamette Research and Extension Center in Aurora, David Bryla, a research horticulturist with the USDA Agricultural Research Service in Corvallis, provided data on two years of calcium research in blueberries.

USDA Agricultural Research Service horticulturist David Bryla provides data on calcium research in blueberries at the Oregon Blueberry Field Day.

Bryla is working with his research technician, Scott Orr, as well as with Lisa DeVetter, an associate professor in small fruits from Washington State University in Mount Vernon, and Wei Yang, OSU professor and district berry Extension agent for the North Willamette Valley on the project.

Typically, growers have applied calcium sulphate or gypsum on the soil in an attempt to raise the calcium level in the plant, Bryla said. More recently, growers have been trying various calcium sprays, and researchers have looked into sprays to see if there is any positive effect.

“Some of the work, including work done by Bernadine Strik and Amanda Davis, where they tried various calcium sprays at labeled rates, didn’t find any effect,” Bryla said. DeVetter also found no effect from calcium sprays, he said.

“However, there was some work done by Eric Gerbrandt up in B.C. looking at Draper fruit drop, which is thought to be a calcium disorder, and in that case, they found that using foliar sprays and calcium chloride actually reduced the amount of fruit drop they had in Draper,” Bryla said. Gerbrandt is research director of the British Columbia Blueberry Council.

“More recently, there’s been a study done in Chile where they looked at calcium chloride sprays on Liberty, and they did find increased calcium in the fruit, and they found better fruit quality at postharvest. So there does seem to be some promise. But the question is how do we get more calcium into the fruit?”

A review of calcium research on blueberries out of Georgia has shown that calcium in a plant moves only through xylem vessels, which transport water and nutrients from roots to tissue.

“So, one key to getting more calcium in the fruit is going to be having a nice healthy root system with new roots where the calcium could be taken up,” Bryla said.

Also, he said, if plants are actively transpiring, they are going to be more likely to move calcium into a plant, and typically into the leaves.

“Fruit, on the other hand, doesn’t transpire a lot,” Bryla said. “So, that is why it’s so hard to get calcium in the blueberries.”

As a nutrient, calcium has a critical function in maintaining a fruit’s membrane integrity and cell-wall strength, Bryla said.

“Calcium in the cell walls of fruit acts as a stabilizing agent binding with the pectin in the fruit,” he said. “So, if you don’t have enough calcium in the fruit, it leads to weakened cell walls and membrane break down. And that in turn is going to affect your fruit growth, it is going to affect the firmness of the fruit and it could increase susceptibility of the fruit to pathogens.

“The more calcium you have, the firmer the berries, which means less bruising at harvest and extended shelf life,” he said.

Bryla added that past research in various fruit crops has shown an inverse correlation between the amount of calcium in the fruit and the size of the fruit.

“Now, we haven’t done that in blueberries, but the idea is basically the more calcium in the fruit, the bigger the fruit,” he said.

In the current research project, researchers have applied calcium directly to the soil, through drip emitters as part of fertigation and with foliar sprays.

To date, the research is showing that foliar applications of calcium chloride delivered more calcium to the berry and on a more consistent basis than the other application methods and other calcium formulations.

“The treatment where we applied foliar calcium chloride always increased the amount of calcium that we’re getting in the berry,” Bryla said. In the trial, researchers measured the amount of calcium at harvest in the cultivars Duke, Bluecrop and Aurora.

Fertigation through a drip system generally did not have an effect on calcium in the fruit, Bryla said.

“The only time we did see any significant effect of fertigation is with the calcium thiosulfate and it was only during the first year and it was only during the first and third harvest of Elliott,” Bryla said. “So, just a small effect.”

Researchers also found little benefit in applying calcium directly to soil. As with fertigation, researchers did find more calcium in the soil solution than in the control plots, but the calcium didn’t make it to the berries.

“Fertigation did increase the levels of calcium in the soil solution, particularly around the drip emitters,” Bryla said. “Soil applications had less of an effect, but better than the control. So, there was more calcium in the soil solution with the soil applications, but not nearly as much as we got with the fertigation.”

Calcium applications in all cases, however, had no effect on yield or berry weights in Duke or Bluecrop, Bryla said. And applications on the Aurora cultivar actually decreased yield.

Bryla noted that the results in Aurora could be partly explained by the fact that the study was conducted in 2021, the year of the heat dome and drought. “What happened is the foliar applications of calcium chloride affected this cultivar in particular,” he said. “So, we actually had more heat damage with the foliar applications of calcium chloride in Aurora and it turns out that Aurora is the most sensitive cultivar that we know of to heat. So, it kind of makes sense.”

Researchers also noted an adverse effect in Bluecrop. “We actually got smaller berries in Bluecrop with the calcium chloride sprays,” Bryla said.

“We did find firmer berries when we used foliar applications of calcium chloride in Duke and in Bluecrop,” Bryla added. “And at postharvest, we saw firmer berries in Aurora.

“We also saw larger diameter with the foliar applications in Duke, as well as when we fertigated with gypsum,” he said. “And we saw a significant difference between that and the control treatment.

“So, there are some side effects going on, but just not as dramatic as we had hoped to see,” he said. “So, we still need to figure out how to get more calcium into the fruit.”

Researchers have also found that growers hoping to increase calcium in blueberries will need to apply it between petal fall and the late-green stage of fruit development.

“In the early green stage, the stomates, which are small pores on the fruit, are wide open,” Bryla said. “Once that fruit starts ripening, starts to turn pink and blue, those stomates get covered with wax. So, there is no way for water to move through it.

“If you try to add calcium after that, it’s not going to do anything,” Bryla added. “You might get it into the leaves, but you won’t get it into the fruit itself. And if it goes into the leaves, calcium tends to be immobile with plants, so we don’t think much of it, if any at all, gets remobilized the following year to the fruit.”