This may sound a bit off base, but the environmentally friendly(er) ice melt pellets are calcium chloride. It's "kinder" to the environment than the typical NaCl used for ice melting on sidewalks, etc. I sprinkle a teaspoon of it around my tomato plants to help prevent blossom end rot. Being that you are located in MN the stores may still have some in stock.

It's unusual to fertilize with calcium. Most clay-rich soils have too much calcium, in fact.

I'm going to question whether you need calcium. I don't know what the soils in that part of Minnesota are like. Is it common that plants there require supplemental calcium?

I do not know if calcium is a common supplement here. Soil testing prior to original planting showed good balance with adequate organic content; no adjustment was recommended. After 7-8 years of growth with annual minimal applications of nitrogen fertilizer which was indicated by soil testing my apples started having cork spot and bitter pit. Trees are not subject to other causes for the bitter pit like overcrowding, water stress or light crop. Soil treatment with lime is not an option as my soil is already about 7.4. I have trees in 2 areas separated by a couple of hundred feet with one area having much sandier soil. Problem is in both areas to varying degrees.

If the pH is already 7.4 , my guess is there's probably already plenty of calcium in the soil, and the tree's problem is not caused by deficiency of calcium.

(one thing though, are you fertilizing your trees with liquid ammonia? That could be causing high pH, and if so what I stated might not apply. unlikely but it was worth bringing up just in case)

Cork spot and bitter pit is a common problem where you are. Are you aware that one of the causes of this may be too much nitrogen fertilizer?

"High tree vigor. Trees that are invigorated by heavy pruning, light crops, or nitrogen applications beyond sufficient ranges have a higher incidence of bitter pit. Pruning is vegetatively invigorating and trees that are heavily pruned will also have light crops and substantial vegetative growth. High nitrogen fertilizer doses also lead to substantial vegetative growth. Vegetative growth competes with fruit for available calcium. If calcium is shunted to vegetative growth it is not available for fruit growth. Further, since calcium moves with water in the transpiration stream, greater leaf area will shunt more water (and calcium) to the leaves rather than fruit."

https://hort.extension.wisc.edu/articles/bitter-pit-and-cork-spot/

I'm not sure you are going to be able to solve the problem with calcium foliar sprays. The tree only wants to take in so much calcium. Your main underlying problem is that the tree is growing too fast because you gave it too much nitrogen fertilizer.

The last thing, is it possible you've been watering the tree too much, or not watering it enough? Both of which can contribute to this problem.

Thx for input. Not liquid ammonia but granulated urea. Trees are spaced out and are mulched 2-3' out from the trunk. I'm not sure what to think at this point as I have always been very sparing with the nitrogen fertilizer putting 1/2 or less than the suggested amount and didn't use it last year at all. Over pruning also not a likely cause as every year I end up at harvest time thinking I should've pruned a lot more aggressively at winters end as trees are typically heavily laden with fruit. I'll learn something this year as pruning this year was far more aggressive so theoretically problem could be worse. Watering not a likely cause as I've only watered during extended periods of little or no rain using my brown lawn as an indicator. I am studying the article your linked as well. Since I have a lot of trees and they are affected to varying degrees I can try to correlate the damage to other variables mentioned as causes. thx again for advice. mark in MN

Are the trees very young? (like only 3 or 4 years)

Yeah, maybe you should try calcium if there aren't any other obvious causes.

If the trees are already big and seem to be growing very vigorously, you might want to stop adding fertilizer.

Trees are 6-10 years old. This problem 1st appeared in 2019 and was worse last year. Agree no fertilizer. As noted previously no fert in 2020.

I've done some quick research, and one thing I've come across, one possibility, they suggest the magnesium levels in the soil may be higher than normal in your area, and that could be competing with the uptake of calcium. Maybe next time before you plant a tree you should dig a very big wide hole and fill it with lots of compost, only mixed with a smaller amount of the soil you dug out.

That is just one thought.

The for suggestion. I have several trees yet to plant this year and will do as you suggest. Holes are already prepped and are wide as you suggest as I use the tiller on the tractor to prep an area for tree planting. I will also do another soil test and make sure I get a magnesium level.