I've just built a 6x6 ft by 14 inches deep raised bed for my dahlias. What kind of soil and amendments should I be using and how deep should the soil be? I'll likely be buying the soil in bulk and having it delivered. I've read of people cutting the lawn beneath the bed and placing the sod grass side down inside the bed to reduce the amount of soil necessary to purchase.
Starting a raised bed
We had organic garden soil delivered for our beds. The first ones we built, the place we ordered soil from suggested that for dahlias their landscape mix would be less expensive then their garden mix and would work as well...how ever, it contains soil scooped out of the Columbia River with all the mysterious pollutants that have joined the river over the years. I wish we had not been persueded to go this route because I can never grow food in those raised beds. The other beds, we have used a nice organic gardening soil,made locally but suitable for growing anything. So now, when we test the beds for fertility we have to run tests on the beds separately, which does add to the complexity of fertilizing the dahlias. And then we have to come up with two sepreate formulas of fertilizers and run two different soil tests. So I say, go with a good organic soil mix that is made locally and can be delivered by the truck load, And keep in mind that the level in the beds will sink every year and they will need topping up.
Also, I LOVE my raised beds. It is so much easier to work on the beds having them a foot taller then ground level. I can move my little garden stool along the rows and it is much easier for old knees to get up and down from the stool. Also, I don't have to bend down to the ground level to work in the beds. Do cover the bottom of your beds with rodent proof wire. 
. This photo was taken after "Cut down" last fall to show the structure of the beds.
dahlia1972 I wouldn't bother cutting the sod inside the beds and flipping it upside down. Whatever volume you gain is going to be pretty temporary I would think. I only did this when the lawn sod was removed from the paths/elsewhere and added to the beds. I would personally just put cardboard over the lawn within the beds, if that. My recommendation would be thick branches, small logs. Some people might add some organic nitrogen-rich fertilizer on them to make up for nitrogen getting locked up a bit as they decompose. It would possibly be a good use of coffee grounds if you have a source from a local coffee shop. Anyway, I'm not any expert, but that's what I would do.
dahlia1972 @blown_dry has great suggestions here! When I started raised beds in my vegetable garden I did the cardboard thing too and covered that with a mix of approximately 30% compost and 70% screened topsoil. If you go the cardboard route, make sure to peel off any labels or packing tape, as those will stick around and annoy you later. Cardboard is actually pretty awesome not only for killing the grass, but also the glue that is used in the cardboard pressing process attracts a lot of earthworms which do their thing and improve your soil. Now my veg raised beds are only 4" tall, so it wasn't too expensive to fill them with ordered soil and compost. If you have really tall ones, her suggestion of filling the bottom portion with branches is a good one because it's healthy for the soil plus it takes up some of the space. If you want to go a step further, read up on the hugelkultur method! 
My husband designed and built my lovely vegetable garden fence and gate with a little help from me. His health isn't so great these days so I'm very thankful and grateful for that gift. Anyway, you can see the raised beds here. 
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dahlia1972 This is a great discussion topic, I'm so glad you created this - there's such good advice here! I have raised beds on a small, urban lot and I'm here to share that I crammed all of my winter, organic straw mulch from the previous winter into the bottom of the new 15' raised bed I built last spring. I was hoping to take up space like you, and I can confirm that my soil dropped a dramatic 6" by halfway through the growing season. While this is zero surprise to me now, it was not something that I had anticipated for who-knows-where-my-brain-was reasons. So I'm a plus-one on blown_dry's advice to use not-floofy-stuff like thick branches and small logs! Also, we are intense coffee drinkers and egg eaters and we have a separate bin for composting grounds and shells only. I added a healthy amount of this to the raised bed and the soil was pillowy and delicious for lifting dahlia clumps in November!
Thanks for all this advice!! How deep should my soil be? The bed itself is 14 in., was hoping I could get away with 10 in of soil. I'll be putting chicken wire at the bottom of the bed and lining it in plastic.
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I have been happy with 12" of soil in mine . Not sure how it would do with less. I would recommend Rat wire rather then chicken wire. SOme creatures can easily chew through chicken wire. It isn't even strong enough to really protect chickens though it will keep them confined, but wild creatures can chew right through it. Rat Wire, aka "Hardware cloth" is so tightly constructed the little varmints can't get their jaws open to chew. It will last for a long time without rusting out. Ask me how I know these things....no don't....
Hardware cloth is good stuff. Chicken wire will rust out more quickly than hardware cloth also.
Hardware cloth is well worth the extra $$. Also, I wouldn't line the bottom with plastic, it would like;y impede the drainage and with a heavy rain you could end up with a raised swimming pool.
dahlia1972 I didn't use rat wire because we don't have an issue with tunneling where I am, but I did line my beds with a couple layers of Dewitt Pro-5 Weed Barrier to help keep the overzealous plant *friends out!
Seconding the not bothering with chicken wire. It will rust and is not strong enough to keep out gnawing critters. Not sure what the plastic will accomplish. It's one thing when it's used for paths and gets hosed off, etc., but as soon as you put soil or mulch on it it's not long before the weeds that colonize on top make the barrier meaningless and it needs to be dug out by someone later after it's got a bunch of dirts and roots on top. "Ask me how I know these things....no don't...." 
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I've decided not to use plastic on the bottom- my tubers have done really well in my mom's raised bed and she said it was lined with plastic, but apparently only the sides were lined to protect the wood, so I'll follow suit. I guess I'll scratch the chicken wire idea too. My friend's tubers were destroyed by voles so I'll try hardware cloth to protect them, as a vole rampage would be disastrous.
There is also a "gopher wire" made specifically for raised beds which might be worth looking into, especially if it is available locally.. It is similar to chicken wire but smaller mesh and heavier galvanization.It is probably cheaper than hardware cloth. I have never tried it but am considering it.
This is an excellent description of how to grow dahlias in a raised bed:
[https://mygardenbee.com/blog/f/growing-dahlias-in-raised-beds?blogcategory=Raised+Beds](https://)
I’ve followed Paula’s advice since I started growing dahlias, which is admittedly only a few years. But I return to it again and again because she’s very clear and she illustrates things well.
I’ve added a couple of raised beds for this season and I was just fiddling with the soil. We reconfigured one large (8’x16’X 6”) bed as well. Last year, I used an existing 10’x10’ bed and the shape just wasn’t ideal for cut flowers. The new bed will allow for long rows with an aisle in the middle for access. My smaller beds are tall like yours. They’re similar to the metal ones in this blog post. We used a bulk garden soil mix from a very good local company that includes compost. I’m amending it with compost and trying out some of the ideas in this post. I’d start with a soil test, of course. In fact, I am waiting on results from a new test that I sent in for this year. Right now, I’m going for the right texture, if that makes sense. Now that I’ve done this for a few seasons in a row, I have a better idea what I want the soil to feel like. The raised bed is great for that because you can experiment. I had very good luck with the tall raised bed I did last year. I used sticks as suggested above, and it drained beautifully. It was also easier to tend than the lower bed. My plants did well and the tubers were so easy to lift! Good luck!
JessieC Thanks for this information. What soil should are you testing? They won't be coming into contact with my ground soil, and I'll be ordering soil in bulk to be delivered, which I can't test beforehand.
dahlia1972 I tested the soil in the raised bed. It's not strictly necessary, but does help you dial in on what you need and what you don't.