First, "dirt" is what you find under your fingernails or on the bottom of your shoes :-) Plants grow in soil.

For budget considerations, fill (like for sale on Craigslist) can work but you are still going to need to amend and for that you need compost or composted manure. For brand new raised beds, I recommend a ratio of about 3 parts fill/soil to 1 part compost or composted manure. The alternative is to import an already premixed planting or garden mix from your local nursery or bulk supplier.

You are going to have to invest some $$ any way you look at it and spending the effort on the soil is by far the best garden investment there is. Make sure your wife understands the savings you will reap over the coming years from growing your own fresh and tasty veggies and fruits!! (how much can a single raised bed cost??? - sheesh!)

btw, worm castings are great but do not go very far - you need a whole lotta worms to supplement a 32sf veggie garden. But the leachate makes a great fertilizer!!

Thank you for the advice. I actually called it "dirt" on purpose because I knew it was not fit to be called "soil" in the first place. I will check my local nursery to see what soil they've got, what composted manure they've got, and what the cost would be for either option. The recommendation on a 3:1 ratio really helps me put some numbers to this project.
The lumber to build the veggie box was just over a hundred bucks, and while I can see the long-term benefits (cost saving, nutritional, and culinary benefits) it's tough to justify the added expense in our current financial situation. I'm just so excited to grow my own food. And my 6-month old already loves tomatoes, imagine how much she's going to enjoy the ones out of my backyard. And squash and cucumbers and the list goes on.
Any other advice I should keep in mind? Do I need to let the soil sit before I start planting?

If money is an issue, then one thing I can tell you is that it is necessary to fill the box all the way up this year. You can break up your native soil and add amendments to it. Then, in the off season, you can add more to soil to it, worm castings and straw.

We built a large garden and money is our biggest limiting factor so we have just made it our goal to increase the fertility with time and as the budget allows. You do what you have to do.

The best thing to add is your own properly made compost. Be skeptical of any free "soil". You can try and test it before you buy large amounts, grow some beans or radish in a container filled with the soil in question. If the plants grow good and you are satisfied, then get the soil. I found decent free compost from my city, filled a little over 1,000 SQFT of garden 3-4".

You have stacked 2 by 12s? Even with nominal sizes and setting the boards 2 inches down, that's over 2 cubic yards of volume! That's going to be tough to fill with organic matter alone, so I can see your need for dirt. I have found in my ambitions a need to add lots of TIME to the workings of what I do in the ground, so whatever I do today (unless I want to throw money at the situation) is really just getting set for next season.

Twenty to twenty one inches of growing medium isn't needed to grow veggies. Nice? Yes. Necessary? No. You don't have to go whole hog all at once (that comes naturally later...).

If you have or know someone who has a pickup truck or trailer, you can get compost from a recycling landfill for cheap. Many even sell a compost/topsoil or a compost/sand mix for perhaps fifteen or so dollars. The price varies and you don't say where you live so I can't look it up for you.

Soil, what plants grow in, is made up of the mineral portion, the sand, silt, and clay in various proportions and organic matter. Loam is considered to be the best soil for growing in and it is generally considered to be about 45 percent sand, 25 percent silt, 25 percent clay, and 5 percent organic matter, so that should be about what you should shoot for. However, few of us have access to loam so we amend what we have so it contains about 6 to 8 percent organic matter. What I have is sand, no silt or clay, and that 6 to 8 percent OM works quite well for me. My sister has clay with very little sand or silt but amending that to contain about 6 to 8 percent OM works quite well for her just as it does for me.
Ma Nature takes whatever the soil is wherever it is and adds organic matter, every year and plants where she is in charge grow quite well.

Kimmsr rocks!

I am considering lining the bottom of the box with some cinder blocks (I've already got them on my side yard just taking up space) to take up some volume and help keep the amount of soil/dirt down. I'd put down the blocks, then a layer of garden liner, then start with the soil and dirt mixture. Any reason why I should not? Without blocks I'll have about 21 inches of depth, with blocks I'll have about 12. Is that deep enough to grow subterranean veggies like carrots and potatoes?
I built the vegetable box 2 feet tall mostly to keep me from breaking my back every time I worked in the garden.

As you can see, every person has different opinions. I don't dig, use organics heavily, and worship Ruth Stout. A friend happily uses 50-50 composted dairy manure and sand and I grow stuff in nearly 100% once-was-horse-manure, and it works well for me.

I can get 3 cubic yards of horse manure/bedding mixture onto my pickup truck and it's free, which is also important to me. The last load I got had a yard given to a neighbor, a yard put in a wire bin for later, and a yard dumped into a fresh empty bed that I'll probably be able to plant food in this Fall. No dirt. No clay. No digging-in. No composting.

It's all part of my being lazy, for sure.

A back can only take so much abuse in one lifetime and so "the higher the better" is a good way to plan. I've seen concrete block beds that are perhaps four feet high and my first "raised" bed here in the desert was about 3 inches high.

I can't grow carrots since every single one (ever!) has been donated shortly after birth to the local critter food supply. Potatoes, however, will tend to grow out of the surface, so leave room for a little extra mulch to shade the tubers from the sunlight.

If you mean putting the cinder blocks over the entire bottom as a base, I would not do that. While many things can grow with just 12 inches of soil, more is better. If they are in contact with your native soil, they have that opportunity to go further which will make them stronger.

Take one layer of 2x12 off and make another bed, then you'll have twice the space for not much more money. Check with your municipality to see if there is composted leaves you can pick up for free. Many do have that.

That 12 inch depth would be just barely enough for potatoes but about what full size carrots would need. You may want to look into potato towers.

Here is a link that might be useful: potato tower

Potatoes have to be hilled or progressively mulched in any case, so starting them out in a raised bed makes little sense unless one has pure clay.

You mentioned cinder blocks. I was thinking you could use those as the sides of your raised bed, but since you already built it, it's good. They could still be used as the sides for your potato hill.

Many people have built potato towers and have grown good, edible potatoes in them. Sometimes new concepts (less than 25 years old) take time to be accepted.

Here is a link that might be useful: potato tower

Kim, was anybody suggesting that potato towers typically produce inedible tubers? The del with the towers is that they do not increase yields much if at all, and therefore are a lot of un-necessary effort.

Mr. Brown. The name is KIMM not Kim. I think you have misread and misunderstood what I wrote, again.