Debatably, we're behind the curve here (the last frost date here was mid-April), but we just built our Square Foot Garden.

The biggest challenge, by far, was finding vermiculite. Mel (the SFG guy) says it should be easy to find, but to call around first. Sure enough, it should have been easy to find, but everyone was out of stock, with no accurate estimate of when it would come in. Since we were already late, waiting a couple of weeks was out of the question, so I persevered.

Wal-Mart, Home Depot, Lowe's, etc. only had it in small (expensive) bags. To get enough would have been about $80. Most nurseries I called usually had it in 4 ft³ bags, but were out of stock (they had the small bags in stock too). The price across the board was in the area of $30. Finally I called the local brickyard (after the weekend—they're only open during normal business hours). They had #3 grade (the kind Mel likes) in 4 ft³ bags for $15. They had 149 in stock. Yeah, if you're going to buy vermiculite (or perlite), be sure to call the brickyard.

Why does a brickyard have it? Vermiculite is a great insulator and is used around e.g. brick ovens (or around fireplaces, etc.). Its place in garden soil is that it holds a lot of water and air and makes the soil more friable (loose), all of which are great for roots. Some people have success using perlite, but I read that it holds less water, and in our hot dry summer I think I want all the water-holding capacity I can get, so I held out for vermiculite and ended up saving $15 in the end because of it.