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

Plants put down 90% of their root system in the top 12" of soil. I have found no benefit from raised beds deeper than 15" (of growing medium/soil mix, exclusive of any drainage underlayment). 12" to 14" of actual growing medium will be fine. You want it to be 12" deep AFTER settling - my 15" usually settles down to around 12" to 14" actual depth by planting time. If paint and other yard chemicals (gas, lawnmower, oil, etc) were stored in the now-demolished shed, there is always the chance that some of that was spilled on the concrete slab. I wouldn't feel comfortable growing on that slab or near it without some soil testing. A gentleman who was very active in the development of solar stills used to build self-watering raised beds for use in dry climates such as Arizona/Nevada. I have plans for his beds, which were published in Countryside Magazine in 1998. The text of that article is available online, but no pictures. If you can find a copy of the magazine in the library or get a copy of it via interlibrary loan, that would have the pictures in it. Or it has been published in Countryside's 1998 anthology, available online for around $20. I just bought a copy of that anthology because my copy of the magazine in which it was originally published was lost. He uses an impermeable barrier to contain the pea gravel and maintain the water level - commercial pond liner. You are unlikely to find the stuff at Lowe's or Home Depot. That would protect you from the lingering effects of any remaining chemical contaminants. What he uses for "sand" is unclear - he states builder's sand is not suitable. I'm not sure what the stuff that blows around in the Arizona high desert that he uses instead is, but I'm planning on using mason's sand - this is finer than builder's sand. It is not the same stuff used to make concrete. What you have here is essentially a large container. You will only be able to fit one 10'x4' wide bed on the space you have. I would suggest mulching as deeply as possible, especially if you have droughty summers, to keep the soil evenly moist. This is for a self-watering system, and it will work as well as you did when following the plans. You can build much simpler raised beds (with no self-watering) if you're planning on building them at ground level - but I wouldn't put ANYTHING directly on that slab unless I had evidence that it was clean of any contaminants. If you are very short (like me) or have impaired mobility/reach (like me) you may not be able to reach all the way across a 4' wide bed. If you build with concrete blocks, however, it would be fairly simple to sling a 4' long piece of 2x12 (or wider, if you can find it) across the top of the bed (being careful not to pinch or crush any plants). Like a walkway, to get you in a little deeper without stepping on the bed and compacting it. However, because of my mobility issues, I prefer a 3' wide row. I square-foot garden in spaces such as this. Since you need at least 3' between rows for access, you could still really only fit one 10' row on that 6x10 pad, regardless of whether it is 3' wide or 4' wide. Here is a link that might be useful: Horace McCracken's self-watering raised bed plans This post was edited by zensojourner on Wed, Dec 17, 14 at 20:12

Any idea where a cheap source of straw would be? If out in the country you can often find farmers with left over bales of either hay or straw they will sell cheap or even give away. Otherwise check any farm & feed stores but most of the time they only have it in the fall. We grow our own but many buy up bales in the fall to use the following spring. Same goes for shredded leaves - bag them in the fall to use the following year. Lost of other suggestions for good mulches over on the Soil, Compost & Mulch forum here. Dave

One reason for using those liners is that they keep the soil away from the wood frames of the raised beds and therefore help keep the wood from rotting. However, those liners also help keep the wood wet which is the primary reason for rotting, so nothing is really accomplished. Some will tell you to use the liners, especially if treated wood is used because the liners are supposed to keep the contaminants from that PT wood out of the garden. Even with CCA PT wood the contaminants did not pose that much of a problem because any contaminants leached out did not travel far and did not get taken up by the plants. The major way any contaminants might pose a problem o you is by your making contact with the wood and then not properly washing. As a general rule of thumb those liners are an unneeded material. kimmq is kimmsr