Jennifer, i think your idea on the first post of adding 4" of compost every year and nothing else is an excellent idea. If you havent watched charlea dowding on youtube, you might enjoy it. I watched one today where he built a new bed by adding 4" of compost, then covered it for 3 months with a wool rug. No tilling, no carboard, no nothing. He flipped over the carpet and it was full of worms.
As stated, once that organic matter is added, worm castings will happen on their own. Everything will happen. Its good stuff.
James mentioned SEA-90. I can see a benefit from that and it is a worthwhile purchase every couple of years. It has every mineral known to man. I am able to get seawater from the coast so i dont have to purchase it.