I use half totally organic soil/eco-earth and half sand. The sand really helps with drainage. There should be no fertilizers or those little moisture ball things in the soil at all - just pure organic soil.
I mixed mine with about half play sand and half organic gardening soil. I'm not a plant person and learned afterwards that apparently I should have used coarser grain sand and POTTING soil not gardening soil but it's been a month and my plants are doing well.