I would just use straight potting soil with a bit of your own soil and well composted scraps like you have. Potting soil is somewhat like garden soil but sterile and probably less dense. Not a big deal as long as it will retain water and drains well.

Potting is going to be peat moss, coir, finely ground bark, or a combination of those, probably with either perlite or vermiculite mixed in, not a good base for a raised bed.
The soil you should want would be about 92 to 95 percent mineral (sand, silt, clay) and 5 to 8 percent organic matter. Contrary to what most people believe there is really nothing wrong with clay soil that organic matter will not fix.
You may want to look into the concept of waffle gardening.

Here is a link that might be useful: waffle gardening

I have found kimmsr's info about clay to be very true. We have very dense clay around here, so raising beds was my best option over the short term. I put some pine bark fines (very fine mulch) in my beds, then added our clumpy, sticky clay, then another layer of mulch. I let it sit for a few weeks until the clay was rendered "mixable" by the even moisture level it attained when mulched. I combined it well with a garden fork, then added in some compost and other good stuff. Clay and mulch made up the bulk of the mix, and plants have done very well in it. Now, two seasons later, earthworms and cultivation have turned it into very nice, friable soil that bears no resemblance to its humble beginnings as mid-Atlantic clay!

In Arizona heat, I would imagine the water-retentive qualities of clay would be a plus?

If money is less important (and the idea of filling that much volume with potting soil makes me think is may be), you could just hit up a local garden center and buy bagged or bulk mixes. I have found raised beds to be rather forgiving of different soil compositions due to the drainage you get above ground level.

This post was edited by jadie88 on Sun, Apr 13, 14 at 6:56

Sorry to disagree but potting soil is nothing like garden soil. First, it has no actual soil in it (hence the term "soil-less mix") and therefore no mineral content. It is designed expressly for fast drainage and would most likely drain way too fast for raised beds. And would be prohibitively expensive to fill raised beds that large - you're looking at nearly 4.5 cubic yards per bed and most bagged potting soils come in 1.5 or 2 cubic FOOT bags. That's 60-80 bags per bed!

I would purchase whatever bulk soil you needed and augment with compost. Clay and compost are pretty common components of a great many garden soils. If not enough of your own compost available, I'd look to whatever composted manures or composted recycled yard waste you can get your hands on - surely there are some farms or ranches close by that could provide.

A typical commercial potting soil is compost with peat moss and perlite. Peat moss for water retention, and perlite for drainage. The difference between this and garden soil is additional moisture retention and additional drainage.

If all you have available is soil-less potting 'mix', it's probably coconut coir, peat moss, and perlite.

There are a million ways to grow plants... I wouldn't stress out on it. Start small, have fun, experiment, learn from others in your neighborhood, etc. Herbs are a great place to start and some are year round in warm climates like rosemary, bay tree, lavender.

This post was edited by bardamu on Sun, Apr 13, 14 at 19:05

Clay is actually full of lots of good nutrients for your plants but they are "locked-up" and need to be broken down by mixing with lots of compost and organic matter. Its too bad I can't ship some of my clay soil to Arizona where they could turn it into top rate soil by mixing it with lots of composted materials.

The sand in the desert soil is good for allowing soil particles to loosen up and allow water to flow through it. Too much sand and clay make your soil like concrete. When mixed in the right proportion, it can make your soil better.

>> what to put into my new raised vegetable garden beds

ANYTHING organic, and lots of it. There is no one-size-fits-all label for what desert soil is so there is no one-size-fits-all remedy for it. My last place was heavy rocky clay with a four or six inch thick later of caliche about three feet down. Now I'm at the top of a hill five miles away and have sand sand sand.

Some say you have to have some ideal soil mix with exact percentages of silt, clay, sand, loam, lego blocks, perlite, vermiculite, peat and so on. Just add composted organics to what you have, mulch everything to death, and give yourself lots of time. All gardening is long term and starting EVERYTHING from scratch is way too much work. I think that's why people try to solve such problems with elaborate mixtures and disposable money. Home Depot (with its magic bagged goods) is not your friend.

I may be excessively biased about it, though. Just sayin'....

Most of us understand that loam is the best soil there is, and if one ever gardened in loam there would be agreement. However, loam is not readily available. Conversations with many people over the years tells me that many people think "topsoil" is loam but that is simply not true. "Topsoil" is, simply, the top 4 to 6 inches of soil from some place and may contain some organic matter. "Topsoil" is what ever the seller wants it to be, although usually the seller will try to make what is being sold as "topsoil" at least somewhat black since they realize most people would expect "topsoil" to be black.

A soil with adequate amounts of organic matter, whether the base mineral component is sand or clay, will work quite well for any garden planting bed.

Here is a link that might be useful: what is loam

My only issue is that buying soil and having it delivered is nearly impossible. I live 20 miles away from the nearest house. When I lived in Oregon and nebraska gettng soil was no problem but when my home was built all soil was raked away until they hit kalichi, which is a layer similar to concrete, last of rock little bit of sand and clay sticking it all together. That why I went for raised beds. Is there a company like Home Depot that might sell soil in the bags. I have around 200 bags of miracle grow thanks to a recent amazing sale at Home Depot but I have never had good luck growing in simply potting soil. I'm new to this area and don't know anyone to ask.

That Miracle Grow Potting Soil is going to be Peat Moss, perlite or vermiculite, and a fertilizer. There is no soil, as we commonly know it, in those mixes and it is meant to be used in relatively small containers, not planting beds.
Possibly that soilless mix could be mixed with some mineral soil, about 92 percent mineral soil to 8 percent of the potting mix, to arrive at something resembling garden soil.