Hi,
In ground, I just use 10/10/10 in bullets, a hand full every 2 weeks to every month.
In pots, the problem is elsewhere :
1. The trees get root-bound fast.
2. The dirt in the pot gets depleted but also its structure gets sandy with year. One must replace at least half of it every two or 3 years.
3. In the heat, in the pots, the roots get cooked and that's bad.
4. In the heat, the dirt dries out, the dirt gets dry and the tree gets a shock. That will stop the tree. The tree will loose time and then grow again if watered properly of course, if not, the tree may shed leaves or even die-back. In pot, in the heat, IMO, one must use a plate to keep some water available to the tree.
5. use straight pots, and not pots that are thinner at the base
If you don't fertilize, do not expect to get the best out of your trees ...