Descriptions of sand beds seem to indicate pure coarse sand maybe 12 inches over native soil. An old formula for cold climate cactus suggested 1 part soil to 9 parts sand. Sand beds in our area often seem to allow roots to reach our clay topsoil--which feels inconsistent somehow.
I've heard recently about using mostly crushed trap rock in a bed. I'm wondering now about crushed limestone.
We had a hot, humid, but dry summer in New England USA. Cacti and Yucca didn't flinch, and Viola pedata would have looked good except for the rabbits; but I lost several Penstemons. For misc. other plants, I'm not sure if it was dryness or humidity that was the problem. (and/or spider mites?)
Right now I'm thinking about 9 parts coarse sand, 1 part small crushed marble, 1 part crusted trap rock (basalt?) and 1 part sandy compost. Just thinking that coarse quartz sand seems to be an extremely sterile medium. I have been using some liquid fertilizer in my sand bed.
Guess there is no one size fits all. Then there is the question of watering and when. Guess I don't expect any answers.
Will try to post my results from time to time.
Charles Swanson NE New England USA
Afterthought:We have some beaches here with quartz sand with the consistency of table salt. No surprise that a beach pea grows; but there must be an explanation for the strong growth of a beach grass and Hudsonia.