Debbie, I mix OrganiMax compost with used potting soil at a rate of 2/3 old soil to 1/3 OrganiMax. If cost is an issue you can use the city compost, but I would also add a handful of the OrganiMax when planting. The Super Thrive is a plant enhancement and is added to the water when watering. You can layer the compost on the surface of the soil in the existing beds. Best And Happy Yardening, Nancy.