If it's going to live long-term in the container, it's probably best to give it something with a bit more structure. I'd suggest using a mix loam/topsoil and an ericaceous (or lime-free) compost. You could incorporate home-made leafmould or some composted shredded bark. Make sure the pot has good drainage by first putting in a layer crocks, gravel or broken up polystyrene (if you have some to hand).