Depends whether you are planting in a container or in the garden. If planting in a container, then John Innes Ericaceous is the one to use. In the garden, then your soil needs to be acidic. If it is acid but heavy soil, then add sharp sand or grit to improve drainage. You must use horticultural grade as common sharp sand or grit will contain calcium in the form of shell and chalky residue.

The jury is out on the need for acidic soil for Acers. General consensus is they prefer slightly acidic soil but all prefer well draining and non compacted soil. I now just use compost from garden centres and leave the expensive ericaceuous soil to my Camellia and Blueberries and top up with leaf mulch and general feed and they are all doing well.

http://gardenerscorner.co.uk/forum/threads/soil-type-for-acer.21541/

http://www.japangarden.co.uk/Acers-c-347.html

In reality, its not that Japanese Acers demand acid soil, but more that they don't like limey, very alkaline conditions. Which means that something with a neutral ph is just fine. Other forms of Acer are not particularly fussy about soil ph.

There is the old addage with Acers (and Cornus and to a large extent Camelia too I believe), that says - It is not so much the prescence of Acid, but the Abscence of lime.

Which is just another way of saying the same as Bamboo.

If you do have alkaline soil, Acers will still grow, and can be helped with an occasional dose of 'sequestrine' feed. They do colour a bit better in neutral or acidic soil, but the main thing is to keep them sheltered from wind.

More or less agree with all here, but would add a moisture retentive soil is also beneficial, so not so suitable for dryer sandy soils, acidic or otherwise.