You can make your own potting mix:
- 30% peat moss or coco coir
- 30% perlite or vermiculite
- 30% compost or soil
Vermiculite is an important soil conditioner composed of "millions of cells" which store air and water. It has no food value. Often a key ingredient in soilless potting mixes, vermiculite is prized for its ability to retain moisture. The coarse vermiculite is especially well-suited to creating soil and soil-less mixes.
In Chicago, the local retailer Brew and Grow has some of the best prices for large bag coarse horticultural vermiculite. Vermiculite Large 4 cu/ft Bag is sold for $24.95: http://bit.ly/PN6vPX
Brew & Grow - Irving Park - Chicago, IL http://bit.ly/Qtjvx2