That's an excellent question! The success of your self-watering planter box largely depends on the soil you use. While you don't need a single "special" brand, you absolutely require a specific *type* of soil for the system to function correctly. Regular garden soil or heavy, dense potting mixes are not suitable, as they will clog the wicking system and can lead to root rot.
The ideal soil for a self-watering planter is a lightweight, well-draining, and high-quality potting mix. Look for mixes that contain ingredients like peat moss, coco coir, perlite, and vermiculite. These components create a structure that is loose enough to allow for excellent aeration but has the capillary action needed to effectively pull water up from the reservoir below to the plant's root zone.
You can often find pre-mixed "container" or "potting" soils that work well. A great DIY recipe is to use two parts high-quality potting soil, one part coco coir or peat moss, and one part perlite or vermiculite. This blend ensures optimal moisture control, providing consistent hydration without waterlogging your plants. Avoid adding sand or gravel to the bottom, as this can actually impede the wicking process. By starting with the right soil, you set the stage for a thriving, low-maintenance urban garden.