I make my own every year. Ingredients vary depending on at what is available.
http://www.durgan.org/URL/?DQJED 17 September 2014 Seedling and Potting soil
Home, Seeding and Potting Mix was made and stored in the greenhouse. Quantity was 332 liters. Ingredients are compost base, bedding wood chips, mason sand, garden soil, 202020 fertilizer, and coconut fiber. Quantities of each was subjective based on experience. The ingredients were well mixed using the small Honda rototiller.