After we built it, we lined the bottom with cardboard. We layered unfinished compost on top of the cardboard followed by a layer of leaves, then a layer of substandard dirt we got for free, then a layer of finished compost. We'll top it off with topsoil from a home center before planting.
We plan to grow lettuces, arugula, green onions, carrots, chives, parsley, thyme, oregano, cherry tomatoes, nasturtiums, and maybe some cilantro in this bed. I'm so excited!