What the substrate needs to do
Use a clean soil-style substrate for Theraphosa stirmi that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Theraphosa stirmi, the substrate in a Burgundy Goliath Birdeater enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Burgundy Goliath Birdeaters are terrestrial, so give Theraphosa stirmi 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 Theraphosa stirmi. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Burgundy Goliath Birdeaters, use moisture-retentive substrate with strong ventilation; damp rather than waterlogged. 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 Theraphosa stirmi 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 Theraphosa stirmi does not collapse immediately. The upper surface can remain looser where appropriate.
For Theraphosa stirmi, keep rocks, heavy cork and ornaments in a Burgundy Goliath Birdeater enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Theraphosa stirmi enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Theraphosa stirmi, replace all substrate for a Burgundy Goliath Birdeater only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.