Making muck

Hello, can anyone advise on the best way to make successful muck for rock plantings. I have potters clay but I am not sure what would be the best organic ingredient to ad to this.

Perhaps sphagmun moss ? can't get hold of sedge peat.

Thanks to all

Any kind of normal clay without any added chemicals (maybe not potters clay?) and depending on how heavy it is you can mix it with sphagnum peat moss or composted soil to a workable structure. Be sure it can still drain off water if no holes in the slap if that is the planting piece.

Regards
Morten