I think the difference between the "organic" and "inorganic" soil comes from what is added to the soil and how it's treated. The "organic" potting mix in plastic bags is expensive, I need about 4 cu yards (a 4 ft x 8 ft raised bed, 10 inch deep, needs 1 cu yard). A quick calc shows that I need 4 x 45 bags
Source: http://www.ext.colostate.edu/pubs/garden/07235.html
"There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was alive. Inorganic amendments, on the other hand, are either mined or man-made. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, manure, biosolids, sawdust and wood ash. Inorganic amendments include vermiculite, perlite, tire chunks, pea gravel and sand."