How many cubic feet of soil, at what depth, do I need in order to stop purchasing food plants from the store, year-round?
This needs to be clearly stated and addressed if people are going to be convinced to grow their own food. Yes, it's impossible to select a perfect answer for all climates, but it's absolutely possible to select an approximate answer for all estimates.
To make this more accessible, I made up four arbitrary tiers that approximately separate "basil in a pot" from "an apple tree":
Tier 1: Herbs — basil, thyme, etc.
Tier 2: Greens — lettuce, dandelions, strawberries^
Tier 3: Buried foods — potatoes, carrots, beets, etc.
Tier 4: Large plants — fruit trees, tea bushes, artichokes, squash, etc.
^ These are grouped by my mental approximation of difficulty level, lowest to highest — not by taxonomic distinction. Please don't derail with arguments about "fruit or vegetable". They're all food plants, Brant. One strawberry plant is about the size of a head of lettuce, and they both have easy root systems, so they're both in tier 2.