Calcium & potassium is for root-growth. My lawn-fertilizer in late fall has potassium. Calcium is to prevent leaves from falling off with tons of acidic rain. Calcium is best slow-released, as in oyster shell lime to neutralize acidic rain. Potassium is best as sulfate of potash (lowest in salt) or potassium-mag. (for non-clay soil), see below thread for more info.

https://www.houzz.com/discussions/5993773/random-qs-observations-about-growing-healthy-roses#n=96

There are differing points of view about fall fertilizing. If you've already stopped for fall watch your roses and you will see that they continue to grow anyway as long as the weather permits. You can't stop them! I prefer to feed them so they will go into winter well fed and have lots of stored energy to come back with next spring.

In any case, you can always add organic materials to the soil any time. By adding it in the fall it has all winter to break down and the nutrients to become available to the rose come spring.

@strawchicago z5 - Do yo think egg shells have a similar % of calcium to oyster shells? Maybe they would also be good, slow release for calcium over the fall/winter?

B cs_zone5B Egg shells is very slow to break down. I have them in my compost pile and see them a year later, NOT decomposed. Calcium do harden soil. It's always good to ask any horse manure source as to what type of calcium they use. Oyster shell is slow-released and more expensive and it doesn't harden soil fast like quick lime which shoots up the pH (bad for plants' roots). So any manure that clumps together & can't be broken down by rain ... most likely have quick lime. From Wikipedia: Quick lime or Calcium oxide is a key ingredient for the process of making cement. Quick lime in manure makes it hard to any fertilizer to decompose & usable for plants.

@strawchicago z5 Straw, thanks for this explanation on what type of Calcium is good for clay soil. Appreciate it!