Japanese maples, known botanically as acer palmatum, are trees grown for their dramatically colored and spiky leaf foliage. While not heavy feeders, they appreciate acidic soils, so organic soil amendments are usually sufficient to feed them when grown in good quality soil. The nitrogen in synthetic fertilizers can alter the most attractive physical characteristics of Japanese maples such as changing the leaf color or patterning so they must be used sparingly, if at all.
Amend the soil around your Japanese maple with several pounds of sphagnum peat moss to boost the acidity of the surrounding soil at planting. Amend the soil after planting by scratching peat moss into the top inch or 2 of soil around the drip line with a hand cultivator. Do not dig any deeper to prevent damage to shallow roots. Replenish the peat moss every three to four years to top up the soil acidity.
Water the maple deeply after amending the soil with peat moss to help the nutrients percolate into and distribute through the soil.
Mulch around the base of your Japanese maple with an organic material that will feed the soil as it degrades. Use several inches of shredded bark, compost, leaf mold or cocoa bean hulls to create a level layer starting a few inches from the trunk to out past the drip line.