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Calcium needs to be available to apple trees early in the season during the cell-division phase, because that’s when the tree is drawing nutrients from its reserves and from the soil, and it’s possible to get nutrients into the fruit.

After fruit cell division stops, nutrient applications will only help keep the tree and canopy supplied so they don’t rob the fruit of calcium, Kraig Klicker of SoilTech Northwest, Inc., told growers at the annual Lake Chelan Horticultural Day last winter.

Growers can improve fruit calcium levels with soil-applied or foliar nutrients applied in the fall or early spring.

Klicker said there are two stages in the growing season when a nutrient analysis can be helpful. The first is in the spring when the fruit weighs between 30 and 50 grams—when cell division is taking place. An analysis at this time provides a report card on the nutrition program since last fall. All the calcium that’s going to be in the fruit is already there. From that point on, the cells expand and the calcium becomes diluted as the cells get bigger. If a nutrient analysis shows a low calcium level, that should be a red flag, he said.

“With the early fruit analysis, you basically want to be able to identify the trends,” he said. “Take a fruit analysis when it’s at golf-ball size and be ready to react to the information.”

Two weeks before harvest

A fruit nutrient analysis can also be done about two weeks before harvest to provide a report card on what’s been going on since the early fruit analysis and should indicate how well the fruit may
or may not store, based on its mineral composition.

At that point, the potassium level should be between 130 and 150 milligrams per 100 grams of fruit. Ideally, the ratio of the potassium level plus magnesium divided by the calcium level should be between 28:1 and 30:1. If it’s above 30, bitter pit is more likely to develop in the fruit. For Gala, the number should be below 20, ideally. When it’s higher than 20, stem splitting and other calcium-related disorders are more likely to develop.

At harvest, the nitrogen level should be between 50 and 70 milligrams per 100 grams, and the nitrogen-to-calcium ratio should be below 14:1. Klicker said he prefers it to be below 10.

“Don’t get hung up on the numbers, but watch the ratios,” he advised.

Calcium levels were unusually low during the 2005 season in Washington State. There were cool, overcast days during the early part of the growing season, when cell division was taking place, and soil temperatures were low. As a result, the tree canopies grew lush and large, to the detriment of the fruit, Klicker said. “We could tell there would be fruit quality issues in June and July.”

Early fruit analyses that SoilTech Northwest did industrywide last season indicated that calcium levels were 25 percent lower, on average, than in 2004. When levels are low, growers need to apply foliar nutrients throughout the growing season, he stressed. “One to two sprays are not enough.”

Calcium levels differ by variety as well as season, he said. For example, in analyses that SoilTech did at harvest last season, the calcium level for all varieties averaged 4.26 milligrams per 100 grams, but the average was 5.5 for Gala, a variety that is efficient at taking up calcium. The average was 5.3 for Pink Lady, 3.7 for Braeburn, 3.6 for Granny Smith, and only 2.9 for Honeycrisp.

A higher level of calcium means the fruit will store better, but when the calcium level is high and the potassium level is low, the fruit doesn’t taste good, Klicker said. Fruit with a higher potassium level tastes better. Honeycrisp tends to have high potassium levels and low calcium levels. “It tastes great, but it doesn’t hold up as long,” he said.

Klicker noted that the better the eating quality of the fruit, the harder it is to store, while good storing fruit tends not to taste as good.