I've spent a fair amount of time this winter reading about potting soils. My reading has included a review of many potting soil labels. I've come to two conclusions: (1) Many, if not most, potting commercial potting soils contain decomposed wood chips (and related byproducts of wood chipping, like leaves, twigs, and bark); (2) Many high-end potting soils proudly claim to contain "forest humus."
What the heck is "forest humus"? I couldn't find a good definition online, but the definition of "humus" is pretty much what I thought: "In soil science, humus (coined 1790–1800; < Latin: earth, ground) refers to any organic matter that has reached a point of stability, where it will break down no further and might, if conditions do not change, remain as it is for centuries, if not millennia. Humus significantly improves the structure of soil and contributes to moisture and nutrient retention. In agriculture, humus is sometimes also used to describe mature compost, or natural compost extracted from a forest or other spontaneous source for use to amend soil." Wikipedia.
So, based on that definition, the high-end potting soils contain decomposed material from the floor of a forest. What falls on the floor of a forest? Primarily leaves, twigs, and bark, I assume, with a smattering of animal droppings and other things.
If that's the case, then "forest humus" is the same thing as a pile of decomposed wood chips (fresh from the tree service). Therefore, it would seem a person could make a pretty good potting soil from decomposed wood chips, especially since a wood chip pile that comes straight from a tree service contains mostly carbon (wood, twigs, bark) but also a significant strain of nitrogen (from the leaves), and the ideal compost carbon-nitrogen ratio is about 25:1.
With that in mind, why would any person ever buy potting soil? If you go to your municipality's compost site, they'd probably love it if you took away a pickup full of decomposed wood chips. If your city doesn't have a compost site, I'm sure a tree service would welcome you as well. I would think a planter filled with fully (or nearly-fully) decomposed wood chips, a cup of slow-release fertilizer, and an agent to add air space (coarse sand, vermiculite, or perlite) would make a good potting soil.
Any thoughts? I'm not sure I'm correct about this, so I welcome correction. Barring none, I'm going to experiment with this approach.