They prefer ericaceous soil Keith

Assuming you're talking about japanese Acers, the palmatum types, they do prefer non alkaline soil. If its a different form of Acer, such as Acer griseum, they couldn't care less...

I think you should use just neat ericaceous compost, and not mix it with soil at all. They mostly like acid soil, as Donna said. Annie

Anneashby, I think Kmitch is planting into the ground - he just wanted to know whether to mix some ericaceous into the planting hole. Kmitch, if it is a japanese acer, soil doesn't have to be really acidic, just neutral to slightly acidic will be fine.