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