Phlox perform best in well-draining, nutrient-rich soil that receives consistent moisture. Adding compost to the soil before planting your phlox will help these plants in a number of ways. Loam or compost can improve drainage and create a consistent moisture level-these materials tend to hold water evenly, while rocky or hard soil has dry patches and doesn't drain well. Compost also makes the soil more fertile, giving your phlox the nutrients needed to grow.