Speaking from my experiences of temperate soil building...
Add organic material on top. Let roots die below.
You need quantity and quality of organic material on top, and you want big fat and/or deep roots below.
So you look for how to get organic material on top of your beds. Then it becomes a struggle to find resources to feed your beds. As for the roots, the research turns to finding the best roots for your situation, and maybe ones that can provide other benefits (food, seed, etc).
If you have an incline, it would be good to position your beds on the low end of a swale, but that might be going a little too far for you.
You could introduce worms. But worms come in the long run with added organic material.
We top dress with loads of goat pen muckings. We spread straw in the goat pens, then after a few weeks once the straw is pretty broken down, we muck it into plastic trash bins. I let it cook there in the sun for a week or two, then pile it onto everything. It doesn't burn the plants, the worms and plants love it, and it's pretty much free. Everything I add this to greens up and doubles its production.
'Science is the father of knowledge, but opinion breeds ignorance.' - Hippocrates
Shawn...what is under the beds...concrete, plywood, soil? One option that you could do in addition to all the top dressing/mulching suggested in the other replies would be to remove all the soil from the raised beds and incorporate some hugelkulture. How high is the raised bed above ground surface? You could either remove to ground surface if you have 3-4 foot high raised beds or dig 1 ft or so below ground surface if they are shorter. Then you could fill the hole with any and all organic material you have on hand using the following bottom to top stratification: firewood sized logs, branches/twigs/smaller woody, leaves, weedy green debris/grass cuttings, kitchen scraps and then put the materials referenced in other posts. You could mix in the removed soil into each layer to minimize open space, i.e., mix leaves and grass cuttings into soil in a wheelbarrow with a hoe/shovel. The woody material would hold water and also add nutrients as it decomposes. All the other organic matter would continue to breakdown and make the soil better. This is a lot more work upfront but it could pay off with great soil for years to come.
totally agree with what is written above, esp ..bury wood and pile on organics
Bloom where you are planted.
posted 7 years ago
I want to try hugelkulture ,but were still renting. As soon as were able to buy, im gonna put one in.
Location: Lexington, Kentucky Zone 6
posted 7 years ago
Even if just renting making the garden soil/ raised bed better for yourself and future renters is a pretty cool thing to do...and it is good practice for you before you have your own property...I know every time I do a new bed I learn something or see a way to do the next bed different and/or better! Plus I love and take any chance to dig a hole and play in the dirt!!
My favorite helpers are the worms. When I am working in an older area of the garden I'll put any worms I find in a bucket. After I get a good handful of them I'll transfer them to whatever new areas need some worm activity, add a little fresh kitchen trash under the mulch to get them going. Even after 8 years here I still have 'dead' areas. One day, eventually, this whole 1 1/4 acre will be living earth - again.
Edited to add, you should be able to find worms easily by digging just under the surface in any wooded area that is shady and cool. After my grandma moved to the city she often went to the woods to get four things: worms, polk, mushrooms (she was a pro), and soil for her ferns.