In most gardening magazines, there are numerous articles about refurbishing and enriching garden soils. However, all of them talk about tilling, digging, and then adding compost. Our garden is an older established garden with flowers that reseed themselves each year plus established perennials. How should the soil in this type of garden be enriched? There is a lot of clay about 8 inches down, and the soil above the clay is not the best. We add dry fertilizer in early spring, but that's all.
I've heard of people who dig up their existing plants and set them off to the side in pots while they add compost and till the entire garden. That kind of overhaul is extreme and only necessary if, one, you love labor, or two, your garden is in such atrocious shape that a complete overhaul is necessary. The simpler method, which is perfectly appropriate for 99 percent of established but "tired" gardens, is to add a thick layer of compost on top.
If you don't have your own compost, try to find some that is lightweight and crumbly; avoid the heavy, mucky stuff. Spread a 3-inch layer everywhere you can. If you're worried that self-seeding plants won't come up through a layer that thick, you can skimp in some places, but plan to add another layer of compost in the fall and again next year. Beneficial soil organisms actually feed on the partially-decomposed plant material in the compost. Their activity will slowly but surely improve the soil below.