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