Tire rings = adjustable mini raised bed gardening!
Whatever works. I found a great article in a magazine the other day about growing in bags of top soil. It was pretty efficient and the author said she had beefed up the soil each season, but the bags lasted her for years and years. She was up to something like 150 bags, LOL!
This is a similar acrticle:
http://www.rodale.com/growing-vegetables-bags