What the substrate needs to do
Use a clean soil-style substrate for Ceratogyrus marshalli that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Ceratogyrus marshalli, the substrate in a Straight Horned Baboon enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Straight Horned Baboons are fossorial, so give Ceratogyrus marshalli deep, compactable substrate that can hold a burrow. Depth is part of the enclosure design, not an optional extra.
Match substrate depth to the current size of Ceratogyrus marshalli. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Straight Horned Baboons, keep the enclosure mostly dry, with upper substrate with some lower moisture and clean water. 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 Ceratogyrus marshalli 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 Ceratogyrus marshalli does not collapse immediately. The upper surface can remain looser where appropriate.
For Ceratogyrus marshalli, keep rocks, heavy cork and ornaments in a Straight Horned Baboon enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Ceratogyrus marshalli enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Ceratogyrus marshalli, replace all substrate for a Straight Horned Baboon only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.