If you're going to pay for Miracle-Gro, you might as well pay a little more for Pro-Mix or Black Cow or other "professional" mixes that usually come in bales of 3.8 cubic feet. Say $25-35 for ~ 4 cubic feet.
If your big-box stores don;'t carry the good professional mixes, look through the yellow pages for "indoor hydroponic shops". They always have the good potting mixes, even if many of their customers wear Grateful Dead tee shirts and have reddened eyes.
The very few times I bought MG potting mix, it was really junky, POWDERED peat dust, held WAY too much water, and never let any air in.
People who know how to avoid over-watering seedlings seem able to make it work, but I hate it! I've heard that the "MG mix" depends on whatever is cheapest in the region it is manufactured in, but I don't know that.
I go to Lowe's instead of Home Depot, because where I live, HD sells logyard trash for around the same price that Lowe's charges for good clean bark products. Like "fine bark nuggets" or "medium pine bark mulch". Say around $2.50 - $3 for two cubic feet, maybe three times cheaper than Pro-Mix.
You can make half-decent potting soil from pure bark, but it seems better to use around half "professional" potting mix plus half bark, to make the pro mix cost 1/2 as much. The good mixes use some sphagnum peat moss, which is hard to beat. MG uses (I think) the plain "peat" which is different from "sphagnum peat moss", if I have the names right. One is cheap, brown and dusty. The other has bigger fibers and is what they use in hanging baskets that need to stay hydrated.
But you have to screen the bark, discard the dusty fines, and chop up and re-screen the big chunks to get grit-sized bark fibers (smallest dimension around 1-2 mm). Pine, fir and balsam bark are best, followed by any evergreen with lots of suberin in the bark. Hardwood bark, not so good.
If your pro mix has no fines, and ALREADY drains plenty fast, you might want to add SOME smaller bark fibers, to hold more water. But usually pro mixes will hold more water than I want. (I tend to over-water, so I need faster-draining mix.)
