I planted 15 crabapple trees on the family farm last year in hopes of having a nice orchard in a few years. I'm going to keep them well fertilized
I wouldn't suggest fertizing them much in the early years until they have fully adapted to your soil. Also in the early years, let them blossom and then prune off all of the apples. Nourishment will then go to the tree and not the apples which will make the tree healthier.