Ideal Rhododendron soil?

[redcoat]

Hi,

I am planting several rhodos, but have discovered I have very clay soil which holds water like a tank!

I was going to just amend the soil, but then realized that a hole filled with amended soil will still collect water (obvious now!). Consequently I want to plant the rhodos mostly above the ground, as suggested here: http://www.rhododendron.org/planting.htm.

Can anyone suggest the ideal soil mix to use for the raised portion please?

cheers

David