What the substrate needs to do
Use a clean soil-style substrate for Phormictopus auratus that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Phormictopus auratus, the substrate in a Cuban Bronze Tarantula enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Cuban Bronze Tarantulas are terrestrial, so give Phormictopus auratus enough substrate for normal digging while also keeping the fall distance from the lid low. A stable ground-level hide should sit securely on or in the substrate.
Match substrate depth to the current size of Phormictopus auratus. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Cuban Bronze Tarantulas, maintain moderate moisture gradient with a water dish and strong ventilation. Add water in a controlled area and judge how the enclosure changes over several days instead of trying to keep every centimetre equally wet.
Substrate moisture for Phormictopus auratus should work together with ventilation. Persistent condensation, a stale smell or widespread mould usually means the setup is staying too wet.
Compaction and stability
Press the lower substrate layers firmly enough that a retreat or burrow for Phormictopus auratus does not collapse immediately. The upper surface can remain looser where appropriate.
For Phormictopus auratus, keep rocks, heavy cork and ornaments in a Cuban Bronze Tarantula enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Phormictopus auratus enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Phormictopus auratus, replace all substrate for a Cuban Bronze Tarantula only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.