First, Japanese Maples do not require much fertilizer. Do not add fertilizer when planting a Japanese maple tree or during its first season. Feed young maples only once a year for the first 2 or three years. That being said, you can add an organic soil ammendment to the backfill mixture at planting time.
Evaluate Soil Conditions First...
Japanese Maples thrive in a soil pH between 6.0 and 7.0 on the pH scale. With this somewhat narrow range, it might be a good idea to test the soil. You can purchase a test kit at your local nursery and garden center or buy a soil test kit online here. Or check with your local extension service to see if they provide soil testing services.
Fertilizing Japanese Maples
Young Trees (Beginning in second year): In spring, after new growth has emerged and all danger of a late frost has passed, you can use a slow-release shrub and tree fertilizer, or an organic all-purpose slow-acting fertilizer, on the soil under the tree out to 1 or 1 1/2 feet beyond the tips of the branches (the dripline).
Older Trees: Fertilize older trees from the dripline (perimeter of the canopy) out 5 to 10 feet. Use about a cup of fertilizer for each inch of trunk diameter, measured at the base. After several years, healthy Japanese maples that are planted in good soil and are well mulched do not need regular annual fertilizing. Do not fertilize after mid-August because the nutrients encourage tender new growth which is vulnerable to damage by the first fall frost.