Here's a link to a paper which will help to get you started in the right direction. It's somewhat regionally oriented, but contains the basic tenets of soil amendment.
http://www.ces.ncsu.edu/depts/...
You could probably find some similar references which are a little more applicable to your area if you check out the ag universities in your region.
I would only take issue with one of their suggestions, which is the use of gravel to assist drainage. My personal feeling is that gravel is simply large sand, and I once spent a few days removing gravel from a planter bed which had become dank and waterlogged due to the gravel added lby the previous owner. Gravel has been shown to actually inhibit soil percolation.
I've used potting soil as a soil amendment, but some of them are very peat-heavy, and also tend to contain a lot of sand, (in addition to being very expensive), so now I use what is referred to as "planting mix".
Ground, composted bark is an excellent product, and used to be a staple of the soil amendment trade, however in recent years it has become either scarce or relatively expensive, at least here in my area.
Bark is designed to protect the tree, and breaks down more slowly, retaining its structure. Wood breaks down fairly quickly and seems to foster the growth of various fungi which can form a water repellent network of mycelium in your soil.
Today most bagged mixes seem to contain a high proportion of ground recycled wood from "regional recycling/composting" operations.(the local landfill). The label might read, "Composted forest products", but that's a clever, "green-sounding" euphemism for scrap wood. The problem with these landfill composting operations is that in many instances, the people doing the work are not properly trained or knowledgable in what constitutes a proper raw material for composting. I've seen things such as telephone poles, railroad ties, plywood, MDF, particle board, painted and pressure-treated wood in those composting areas. Before I was aware of this, I bought a few bags of this stuff, and when I opened them, they did not have the earthy, pleasant aroma of a healthy soil amendment, they smelled like the dump. I think maybe the loader might have taken too deep of a cut when scooping it up.
Mushroom compost used to be an excellent soil amendment, but again, economics has made its use tricky. Mushroom growers sterilize their beds prior to use, and some of the chemicals they use are pretty nasty. Years ago, I bought a cubic yard of "organic" container soil mix containing mushroom compost, and used it to pot up daylilies. They started out OK, but quickly took on the appearance of being stressed. By the end of the summer, every one of them had declined, and when I repotted them, their roots were rust-colored, and the rootball smelled like Clorox. I talked to the people at the soil yard, and they said that they had a lot of complaints about that batch, and that they had traced it to a mushroom grower who had used bleach to sterilize the beds prior to use. Mushrooms will grow on practically anything, it turns out, and you'll probably find a lot of raw, or nearly raw wood products in mushroom compost today.
The situation isn't hopeless, and good clean products can still be found, just be cautious.
