It's a bag of inexpensive topsoil from Lowe's. It's really pretty and fluffy, mostly decomposed pine bark and some sand. For my smaller containers I mix 5 parts of this stuff to 1 part perlite. Perlite breaks down eventually, so for large containers I add an additional 1 part expanded shale (this stuff never breaks down and it makes sure your roots have available oxygen, especially when the organic portion of your soil begins to break down.) It's an investment, so I try to buy a lot of this. The biggest drawback is the heaviness of the shale, and the fact that I have to go to one certain nursery to buy it. So now I"m on the lookout for another product that will do what expanded shale will do.
So far I'm having great success with this mix!