Japanese Maples are light feeders.
When newly planted you can mix in compost with the soil.
In the spring you can use a light fertilizer in early spring.
Older trees can be fed a 13-13-13 fertilizer approx 3 weeks before new growth in early spring.
A large tree should take 7 tablespoons of fertilizer, a medium size tree 3 tablespoons and a small tree 1 tablespoon.