What the substrate needs to do
Use a clean soil-style substrate for Poecilotheria miranda that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Poecilotheria miranda, the substrate in a Bengal Ornamental enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Bengal Ornamentals are arboreal, so substrate is mainly there to manage moisture, catch waste and support the lower part of the enclosure. Poecilotheria miranda still needs enough depth for stable planting or cork placement, but the main retreat belongs above ground.
Match substrate depth to the current size of Poecilotheria miranda. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Bengal Ornamentals, maintain moderate moisture with strong ventilation 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 Poecilotheria miranda 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 Poecilotheria miranda does not collapse immediately. The upper surface can remain looser where appropriate.
For Poecilotheria miranda, keep rocks, heavy cork and ornaments in a Bengal Ornamental enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Poecilotheria miranda enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Poecilotheria miranda, replace all substrate for a Bengal Ornamental only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.