I'm not an expert by any means and maybe A1 will have some insight on this but since the bottom of the container is cut out and the earth will be acting as a giant wick I myself would use regular garden soil mixed together with some of the native soil. JMO

Damon

Me too, I would do as Damon said and I would use what works for me and my plants according to my climate..

Living in Arizona, I would assume that you would want a soil that drains well, but holds moisture for a good amount of time...

For sure I would use a mix that drains well since you will be using a drip system..Plants can rot in a soil that stay wet too long even in Arizona, just ask my cousin..Especially in the winter cooler months..

Mike

Thanks guys, that was pretty much what I was thinking. This will be my first summer here so my mix is going to be fairly experimental regardless. I'll probably include some peat and some perlite, but maybe (cover your eyes, Al) vermiculite instead of the perlite?

In re: the drip, everything on the line (mostly roses, not desert plants) is new this year and will need more frequent watering this summer than well-established plants, but I do have decent shade. I'm expecting to water every other day in the worst of the heat. The container will be in a relatively shady location as well, and though the rose will be huge someday, it isn't yet. So I'm looking for something to hold water for 48 hours.

I would use about 60-70% native soil if it's decent; if it's not, I would use builders sand (same %) and make up the remaining 30-40% with good compost. This is another example of everyone recognizing that this would actually be a small raised bed, so there is little concern for the PWT that would assuredly accompany the same mix in conventional containers, and using vermiculite would pose no problems either. ;o)

Al