Soil Mix for Raised Containers?

This is the first year that I'm trying my hand a bit of conventional (or not-so conventional gardening), and one of the challenges I must overcome is what could be considered a perpetual drought. Not that I don't get plenty of rain, living in VA, but 100% of my house water is fed by a rain-cistern. No well, and no city-water. This precludes me from planting in-ground, as I simply don't have the water resources to haul the hose out and soak the ground with 40-50 gallons of water.

With this in mind, I've decided to try container gardening in 30-gallon plastic bins (for this first year, at least. If all goes well, I'll build more appropriate, sturdier ones next year).

This is obviously going to require lots and lots and LOTS of soil. After drilling some drainage holes in the containers, I had planned on layering the bottom with a couple inches of river-rock, with the soil above composed of something like 4:1:1:1 topsoil / compost / sand / vermiculite, or just cut out the vermiculite entirely and make up the difference with sand (sharp edged, uniform pool-filter sand).

I was also planning on using Ollas in the center of each container, and covering the top of the soil with at least an inch of pine-bark mulch.

I'm wondering if I could also get away with mixing a good bit of mulch into the soil itself as "filler", to greatly reduce the amount of actual dirt I'll have to use to fill the containers (I plan on having at least 6 30-gallon tubs, and 2 or more 15-gallon tubs, along with a few 1.5-5 gallon pots here and there.

I also plan on reclaiming the runoff from at least the larger bins. Any other advice for either soil-mixtures or ways of conserving a little water?

Vegetables thrive on compost. Mulch and compost help fight bad nematodes as well. Keep in mind that raised beds soil changes as the plants grow and becomes depleted of what the plants are using, so the soil will need replaced in the future. Sand comes in the mulches as they deteriorate, it can also turn to concrete as it locks into place. even lasagna type beds keep it simple. Peat is also used in raised beds.

I had to look up Ollas-it sounds like a fabulous idea.
I would caution against reusing the water in the containers- unless there was lots of rain that summer it might end up with too much salts in the potting soil.
I do like the idea of the mulch-it will help keep the water loss minimal. I would tend to not mix the mulch in with the soil. If you put rocks or gravel or sand or something in the bottom of the bins, you might want to put some permeable material like landscape cloth between it and the potting soil, so it stays separate.
Vermiculite has two problems-the first is that it is bad to inhale the dust. The other is that over time is breaks down and compacts into a thick and poorly permeable substance. It is great for brief use like seed starting, but it might be a problem over time-I am guessing you would not want to replace the soil every year. You might instead want to use perlite or pumice (if cheap in your area which I doubt).

I'd skip the pine or any kind of wood. Problem with it is it uses nitrogen to decompose, so it robs the soil. Vermiculite would be good as it holds water, peat would also hold water, avoid perlite. Check out your local Farm supply for soil ammendments, it can be MUCH cheaper at the the Farm supply and the soil products can be better there than the local hardware store.

Lasagna beds might help too- and save on other expenses...

Thanks for all the responses. I'll check out some articles on lasagna gardening. I've heard of it before, but never really looked into it thoroughly.

As for things breaking down in the soil, it will all get replaced next year either way, since if I have success this year, I'll build proper beds out of untreated wood, at which point the soil from this year will be "refreshed" with new compost, soil, and amendments.

However, I will look into alternatives for the vermiculite. I just wanted to make sure that the soil drained well enough to prevent root-rot or mildew.

That may be why the mix I used (Square Foot Gardening) had Perlite in it. My soil drains well already so the Perlite additive just made it so no water stuck around for the plants. If you have clay or heavy soil Perlite is good to improve drainage and it can be bought pretty cheaper than vermiculite.

I had always heard Va farmers were called rock farmers. I know the soils are different depending on where you are around the state. Good luck.

I have found a product by Kellog (available @ HD & Lowes) made for raised beds and potting to be beneficial for all my raised beds. As my beds are deep, I augment it with my self made compost (animal & kitchen waste, aged over 6 mos). I do use the above product on it's own in the many pots I have outdoors. Our "soil" here in No AZ is like concrete AND filled with caliche clay. All above aside, check with a local-local nursery on soil for pots as I have very successfully used a product made specifically for our high desert mountains by the local nursery.

I use potting MIX and compost and occasionally add perlite if needed for drainage.