Drainage layer

A drainage layer is a bottom layer, typically of expanded clay balls, gravel, or a similar coarse material, placed beneath the soil in a bioactive enclosure so that excess water from misting or a water feature drains down and collects there instead of sitting in the soil and waterlogging it. It functions similarly to the layer at the bottom of a planted aquarium or a potted plant's drainage holes.

A substrate barrier, usually a mesh layer, sits between the drainage layer and the soil above it to stop the soil from washing down and clogging the drainage space over time. The depth of the drainage layer generally scales with enclosure size and how much misting or watering the setup gets, since a lightly misted desert-style bioactive build needs far less drainage capacity than a heavily misted tropical one.

Skipping a drainage layer in a humid, heavily misted enclosure is one of the more common reasons a bioactive substrate turns sour or grows mold, since the soil has nowhere for excess water to go and stays saturated. Forgetting the substrate barrier between the drainage layer and the soil is a related mistake that lets soil settle into the drainage layer and defeats its purpose within a few months.