Compost and/or composted manure. 2 parts organic potting soil (found at many nurseries) to part compost. If I have the money I also put in some worm castings. There's so much different crap you can mix together. Watch out for the soils that contain a lot of big wood chunks and/or bark. Goto a store like Home Depot and check out their selection. They always have decent soil mixes.
I grow in 10 gallon pots, not in the ground so... My last grow I used: 55% potting soil 15% perlite 10% building sand 20% worm castings When I mixed the final 10 gallon pot I added bat guano, decreased eachother % abit. This year I wanna try using peat moss too maybe no building sand.