Anything, they just don't care. However, the purists will direct you to the plant's origins (and mine), New Zealand, where the soil is rich and fertile, volcanic (few stones). Phil J

I would use a fairly fast, draining mix, myself, but they are subject to root rot in the summer here.

I have rescued one from the local tip, dug a very large hole and put blood fish and bone in some compost (ordinary) and put in bottom and it has survived, it is now showing its gratitued by producing lots of new shoots, its very happy.