For decorative rather than structural purposes, how well does concrete made with soil (fine sandy loam) instead of sand hold up over the years? What ratio of soil to gravel to cement should I use for best aging results?

Firstly, you don’t want organic material in your concrete. It will weaken and discolor your concrete. There are mixes that support organics, but they are expensive along with requiring special equipment and knowledge.

That said, use about 60% gravel to 40% sand. This will get you close to a “well graded” mix. One part cement to 6 parts gravel/sand mix. Add just enough water to be workable. You can alter the amounts by a fair amount, but this is pretty close to traditional mixes.