gary:

I use mostly the controlled release complete fertilizers like Osmocote. Mostly the same one I use for blueberries. One or two applications a year are all my plants need.

fruitnut-
what about in a soiless growing mix, still just osmocote twice a year? any specific timing to the applications (prior to leafing or during fruit growth, etc.)? Just spread around the top and water in? I was going to mix some into the soil but was thinking about using a low strength miracle grow or soluble fertilizer every other watering.

I haven't grown in soil less mix. Fertilizer hasn't been an issue and the trees have grown well. Fruit trees especially in pots just need enough fertilizer to grow adequate leaves.

I've had Zn deficiency on pluot trees grown in the ground but none on any kind of trees in pots. This year I do have some Fe deficiency on a few potted trees mostly apricot. But I think that's from cold wet soil and pH.

You don't want a potted tree growing too big and they won't just because of the small root volume. But I don't see any need to fertilize every other watering, once a month at most. If the tree doesn't develop adequate leaves, fertilize more. Perhaps in a mix without soil more fertilizer would be needed. But the trees growth is your best guide.