Just read a very interesting article on plant nutrition and fruit size and firmness. The article is “Managing Nutrition for Fruit Size and Firmness” by John Kempf in the May 2019 issue of Acres Magazine (p. 10).
Hard to summarize it all, but he is postulating that to maximize fruit size and eventual firmness, one needs to do several things with your fertilizer regime. Get adequate Ca on the ground (with boron if needed) so it is available just after blossoming. Make sure K levels are not high until fruit cells have finished their dividing stage (2-4 weeks after fertilization), and then have adequate K and other nutrients. A very simplified version of what his article says, but it points to a different approach than I had typically been using.
I will give his method a go this year (as best I can). Starting with getting some gypsum (and a touch of boron) down asap, as it is still a few weeks before apple bloom here. Add chelated Mn to my foliars, and reduce or eliminate K from the ferts until a few weeks after fruit set. It will be interesting what if any effects this has.
Obviously there is quite a bit more to what Kempf says than what I’ve mentioned in this post. But I thought folks on this forum might be interested in what he had to say and maybe even giving it a try.