You have the choice of tilling up the ground under the raised bed and than adding great soil on top of this, and working in the two types of soil, or just adding compost right on top of the old soil and over time the soil beneath the compost will soften up. The advantage of tilling or digging the bottom layer of soil is that deep rooted crops like potatoes, parsnips and carrots will thrive. If you decide to just layer on the compost over the ground, put down a layer of newspaper to smother any weeds that may creep up from below and add at least 8 inches of new soil for seeding your vegetables. Shallow rooted crops like lettuce, kale, cabbage and other leafy greens do best in this shallow soil. Keep growing, Marianne Binetti