Air plants, also known as Tillandsia, do not need soil to grow. Instead, they absorb moisture and nutrients through their leaves from the air and other organic matter. However, they do need a substrate to anchor themselves on and provide some support. The best substrate for air plants would be a porous medium that allows air circulation around the plant. Some good examples of these are rocks, sand, bark, or a combination of these. Avoid using soil as it can retain too much moisture, leading to rot and other fungal diseases. When caring for air plants, ensure that they are not watered too frequently and that excess water is able to drain away. It is also important to provide adequate light, humidity and air circulation for their optimal growth.