I grow an herb garden on my deck in a 2' x 4' raised planter (see photo). I usually fill the planter each year with new potting soil. Lowes had a sale on MiracleGro Moisture Control Potting Mix, so I just bought a couple of bags to fill the planter. I've had fairly good luck in past years with my herbs, but I'm wondering if I ought to add anything else to enhance the soil before putting my plants in. Advice? Thanks!