Patricia Becker, center director for Common Ground Organic Garden Supply and Education Center in Palo Alto, California, explains that the most important thing to know when buying a bag of planting mix is whether or not it’s been sterilized—you want live microorganisms in your dirt to help your plants grow healthier roots. If the only option is sterilized “dead” dirt, you can augment it with some worm castings or liquid fish and kelp.