First thing, how old are the deck boards? Older pressure treated lumber (10+ years?) may pose a risk to use for vegetable gardening. They used to use different chemicals int he process than they do today. You'd have to do the research on it, but it's definitely something to consider.

12 inches is fine for most things you'll grow. There may be a few exceptions I'm not aware of who need deeper root room and you may need to address that before filling. What you should fill it with would probably depend on how big you're planning on making the beds. If you build small builds you may need to use a soilless potting mix.. I have 4 by 20 and 4 by 12 foot beds and had a dump truck load delivered of soil. Make sure you get it tested though, I had mine tested and it showed almost no nitrogen content.

I've got pretty hard clay also. What I did was to use a garden fork to break up the soil (you know there is only about a 15 minute window to dig here! LOL).
Next put about 6" of composted manure down,worked that into the broken up soil, built the boxes, about 10" high (with hardware cloth for gophers), then filled with a nice mix from a landscape yard that had organic soil/compost.We now use a mix from the landfill (we are fortunate to have organic mixes at out landfill!) to top off every couple of years.
Happy Gardening! Nancy

>First thing, how old are the deck boards? Older pressure >treated lumber (10+ years?) may pose a risk to use for >vegetable gardening.

I don't know how old they are and I did not even think of this! I was trying to reuse the wood to avoid it ending up in a landfill but wow! Thanks for the tip -- I'll just go buy some 12" boards from the hardware store.

I'm thinking 12foot x 1.5foot beds.

Thanks for the info Supernova and NancyJane! I will put it to use and follow up some time!

Yeah, the old treated lumber (prior to I think it was 2003, but don't hold me to that) had some pretty toxic stuff in it. The biggest one being arsenic.

It kept from rotting and having mildew grow on it by pretty much being as poisonous as possible.

...and I don't think you can throw treated lumber in a landfill anymore.. need to check you local regs though...it may be considered hazardous material.

Do yourself a favor and make your beds out of something that won't rot. Stone if money is no object, or cinder blocks if money is tight. You'll be glad you did a few years down the road.

Good idea, barnhardt9999 -- what do you think about composite decking material?

Something is wrong here. If your soil doesn't drain well like you said then you should not go down but up with the beds. Build raised beds not deep beds, especially for clay soil. For the first year or two you could go buy some good compost to mix with the top few inches of your beds but you should start making your own compost asap .

I have 9 raised beds built on extremely tight clay. I used stacking concrete blocks that do not require mortar. This will be my fourth year with them and I LOVE them. I began with two rows of blocks, which made the beds about a foot deep, and then two years later, the blocks went on sale, so I added two more rows.

When you are dealing with clay, the answer is work and patience. My first year garden was pretty good, but each and every year, it has gotten better and better. My first year, I did like nancy jane and loosened and amended the soil under the beds to a spade's depth. Then added what I had. Some beds were filled with my homemade compost. Some were filled with horse manure I got from a friend. And some were filled with the bagged composted manure from a big box store. Each time I pulled out a crop and replanted, I dug, amended, and mixed again that first year. I have continued that routine ever since, but now it is truly a very simple process. The soil is pitch black and loose and friable. It is a pleasure to work.

Last year, I double dug the tomato beds for the first time, working in horse manure and blood meal as I went. It made a big difference in my tomato crop. I double dug the rest of the beds this year. I figure this took me back into that foot under the beds that I loosened the first year, but I added alot more organic material to it. It was not extremely difficult, and made a big difference in the condition of the soil.

Last fall, I planted red clover in my empty beds. Let it grow all winter and turned it under in late February. When I started planting in April I could hardly believe what a difference it made. This will become a yearly routine. I wish I had done it the first year too. I would have saved alot of hard digging....

You will love raised beds. I think they truly are the answer for clay soil. I can even raise beautiful LONG carrots in them! Just keep working at it and have patience.

ceth_k -- Yes, I am going up, not down. By "how deep," I'm essentially asking -- how high up I should go ?

donnabaskets -- thank you!!! I've been told this by a couple of friends... that it will take years of nurturing the soil to get the best results. That's fine by me! I'm going to get started this year! Thanks for all of your information -- I really do appreciate it!

Here is my two cents:
-- for the beds/boxes another alternative is cedar boards, which will last several years. I bought these 6" x 5ft x 3/4" boards from HD, for about $1.40 a piece. So with ten box I made a 5ft by 21/2ft by 10" boxes. had to get 2 by 2 s for corners and of course, some 1 1/2" screws. I made 7 0f these. Then , if you want bigger boxes, you can do it by the same lumber, say make them, 5ft by 5ft or 10ft by 5ft....
these cedar boards are mostly used for fences..

about the soil: If you are going to fill the boxes partly with your backyard soil, you have to mix with compost 50 /50. manures can be added as fertilizer too. but enough compost will do. Then you have to add chemical or organic fertilizers.... Soil is just a medium. It has to accomodate easy root growth, enough moisture retention and adequate drainage. Food can come later.

How deep is deep enough? Here's the only technical article I could find on root depth for vegetable plants. I found it a little shocking.

Here is a link that might be useful: Vegetable root depths

Eric: I've read some of that about root spread/penetration in the past and was shocked. Now I've seen some of the Square Foot Gardening literature that claims 6" is plenty (and 4 potato plants to the square foot!) and was shocked again. What to believe? I know my crowded asparagus, in an 18" high raised bed, has roots that don't go down a foot. Some is very healthy; on the other hand some plants seem to be dying - ???

What experience do others have at both ends of the spectrum (excluding those plants - carrot, parsnip etc. - that obviously need deep soil anyway) ?

Just as a point of reference, I made my raised bed 18 inches I believe it was, but I also put potatoes in mine and those tend to plant deep and grow deep.

General rule of thumb when working with containers (and a raised bed is not dissimilar from a very large container) is that the container should be big enough to put on top of whatever you are growing. Or in other words, ideally the root systems of many plants are as large beneath the soil line as the stems and leaves are above it.

There are exceptions of course, but always better to have a little too much space than not enough.