A commercial soil-less potting soil. We call it soil-less because it actually doesn't contain any soil. The best kinds of mixtures for using in a container are usually going to have peat moss, a little bit of fine bark chips in it, some pearlite, and as you can see in this mixture it's very light and very airy and it's a great place for the roots of the plants to grow. We would never recommend using top soil in a container because generally speaking it's too heavy and it doesn't allow for good drainage out of the container. As we talked about before, good drainage is essential to any container garden, and when we're looking at our soil mix we want to use something that'll drain really, really well, so you always want to use a commercial soil-less potting mixture, if anything that's labeled potting soil would be just fine.