Thanks for writing. It's best to grow Japanese maples in a spot shaded from afternoon sun and in moist, well-drained soil that's been enriched with lots of organic matter, such as compost or peat moss. If you do this, you shouldn't need to fertilize because the organic matter feeds the soil for you.
If you're not able to incorporate organic matter, then you can use a general-purpose garden fertilizer in spring or early summer. But I'd be wary of fertilizing much --- it can do more harm than good.
---Justin, Senior Garden Editor, BHG.com"