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What is the advantage of building a raised bed vs just heavily amending your base soil until it's almost as high quality as a raised bed and just using that to garden in?
I find that raised beds are best for:
And I have also found that I can grow more per plant in the ground beds, but they start a bit later.
If you have the time and energy to heavily amend a ground bed, I think you will like the reults better. That is just my experience. I have about 6000 sq. ft. of veggies, and have been working in ground beds since I was 3.
The advantages of raised bed vs. ground bed are pretty much the same regardless of your native ground soil, so you can take any of the comparison lists that pop up from your favourite search engine. This UGA extension page discusses some of them.
In particular with a clay soil, you'll want to keep in mind that compaction, slower warming and decreased drainage are typical difficulties you'll have to deal with. Also, clay soils are generally more alkaline, and many veggies prefer mildly acidic or neutral soil. As such, you could be looking at a fairly large amount of work to just amend your existing soil for your needs. Also, doing so would take quite a bit of time and high-clay soils are, generally, among the harder soils to work with.
One thing you might want to consider is that raised beds don't necessarily need to compete with ground beds. I've known some people to put in beds that were only raised about 6" and built with recycled wood, then tore the wood from the beds out a couple of years later and left themselves with just raised mounds. Basically, they used the raised beds to act as a heavy top-dressing for getting good organics going in their garden. They planted shallower rooting veg in the first few years, then after they determined that the underlying soil was ready to just be used, they took advantage of the higher nutrient retaining qualities of the clay underneath and loved it.
The only way one can amend any soil is by adding decomposed organic matter. I actually now have sandy soil and miss my clay. You can't use a rototiller on clay, you could but you'll be making things worse. Clay is made of tiny, tiny pieces of rock that are flat. These flat surfaces have electrostatic charges and these pieces are 'glued' to each other. By agitating the soil, mixing, you only increase these charges.
Think about how concrete is made. The ingredients are gravels, clay (Portland Cement), gypsum, lime and water. All thrown into a big drum and kept agitating until poured.
I've worked clay soils my whole life and it is a bit of work the first time you set up your garden. First, map out where your beds will be and walkways. You never want to walk on plant beds. For this answer, I'll be talking about vegetable beds.
My beds are 3' across or an arm's length from a walkway. If you have plenty of room, 3-4' wide beds bordered on both sides by 3' walkways is ideal. In my green house, I've reduced my walkways to 18" and my beds are 5' across with trenches all the way around so that water doesn't sit on walkways. Start digging by removing the soil from one end of the bed down 18" up to 2'. Dump this soil outside the bed temporarily. Then continue down your bed dumping the soil in the hole you've just dug. As you dig, have a wheelbarrow of decomposed organic compost and throw a bunch in on top of the soil you've just dug out. Don't worry about chunks. Chop a little but don't get carried away. You'll notice that clay is usually moist. I've tried drying it out and end up having to need a jack hammer.
Your 'fluffed' up soil will get a good 3-4' high, it will eventually settle down to 1 - 1 1/2 foot. I also throw in handfuls of 5-7-3 fertilizer that includes bacteria and mycorrhizae as I throw the compost on the virgin soil. The main thing is the bacteria and mycorrhizae which are essential for plant roots.
Rake and smooth this mound of soil and compost. Dig trenches or leave trenches 6"X6" all along the sides and ends, throwing the soil on top of your new bed. This will keep your beds draining and walkways dry. Don't try mixing any more, soil organisms will do that for you. Get a big piece of plywood lay it on top of the bed, get on top and jump up and down. This is to remove big air pockets. Your bed is now ready to plant.
I don't do 'rows' of anything. I have a salad bowl row where I put all my radishes seeds, all kinds of gourmet lettuce seed, carrot seed in a shaker (like the one you find at pizza joints that hold hot pepper flakes). I mix all the seeds together and very lightly shake seed on the top of my bed. Very, very thinly. I take a rake and standing on the opposite side of the bed I rake into the trench and sides to pull a thin blanket of soil on top of these seeds. It is a quick 'flip' that you'll get right away. Then I turn the rake (this one is the bow rake for raking rock) on it's head and lightly tap down on top of the soil and seed.
Otherwise I like to plant in triangles...I also make hills and squares for things like peas, pole beans, squash that need support.
After your plants get established, you can fertilize according to different plants'needs and always keep the soil covered with decomposed organic compost (DOC), not mulch that isn't decomposed. (Soil organisms will constantly be eating this stuff, taking back down into the soil profile, pooping it out, mixing this into your soil for you)! Make your nursery allow you to open bags to see, touch and smell the compost. Every batch is different. If you can see chunks of sticks, bark it isn't decomposed, don't buy it. Well, at least make sure that 3/4 or the material is dark, crumbly and smells like earth...not bark. The best way is to make your own compost so that you know what is in it...
In the fall as each of your beds are done producing, replant using a 'green compost' such as annual rye, buckwheat...they even have great mixes. They grow all fall and winter. This keeps weeds at bay and in the spring, all you have to do is turn the plants back into your bed. Clean your trenches and you are ready for the new season with great soil! You could make your walkways lawn, gravel, bark chips or leave as soil (clay is slippery when wet...). This even looks professional...don't forget to rotate your crops every year. Hope this helps.
Having done plenty of garden beds, both raised and amended rows and hills, here on our farm, I can say that my experience is that:
Raised beds:
Amending the ground:
The old farmers down the road always do traditional rows and their gardens are amazing. I've had good gardens both ways but I tire from wood raised beds rotting out over time.
I've just dismantled 4 old raised beds to relocate things. The wood was rotted out and had ants and termites throughout... I'm done putting dimensional lumber in the garden.
I'm also putting in a strawberry bed for next season, about 12 x 50 and will put about 6-8" of soil/compost atop the existing soil (solarized to kill the existing weeds/grass). It'll have beans on it first, then buckwheat and then winter rye. I considered plowing up the existing clay but there's really no point I think in doing that. I've got locust logs to line the bed.
Carrots have done better in the raised beds due to the 8-16" of soil I've had in them... it doesn't compact too much. In the ground they've not done as well for me.
In many parts of Michigan, we have about 6-12" of good quality soil and then BAM! pure clay. I've found that raised beds are great for growing plants with deep root systems, such as carrots. Even with aggressive tilling and soil amendment, without raised beds my carrots would always take a 90 degree turn and knot up into balls when they hit that depth.
