I use a soil based compost, such as John Innes 3, in containers but I top dress the pots each year with leaf mould, well composted stuff, which is the most friendly fertiliser I know, and it smells great, like damp woodland floor. In the ground they are much more tolerant of soil type than often stated, although shallow chalk soils aren't preferred. Good cultivation of the planting site is the key to success. Phil J