They don't require much. If growing them in containers, I'd suggest just a slow release fert (like Osmocote) once a season, in spring. And you can always supplement with any liquid fertilizer you may be using during the summer months. I dilute to no more than half strength.

If in the ground in reasonably good soil, you don't really need to fertilize. The trees should be getting everything they require from the soil. Lightly mulching with any good compost won't hurt.