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Moderately dry (slightly damp) substrate will be fine for a Grammosotola. Overflow the water dish once a month and otherwise keep it full with clean water, letting the substrate dry out. You and your T will be fine.
I keep water bowls full and moisten substrate weekly, letting it dry out to the touch - not bone dry.
I also mist weekly (to put moisture on sides, plants and webbing) and drop water into webbing of my avics.
It's a trial and error thing based on your relative and ambient conditions.
Good point. I should have said excess humidity is just as bad a bone dry environment; and both have been found to be detrimental to avics.
If you have an amazing cross breeze you can probably have wet substrate as the airflow will "drain off" excess humidity. If you have cross ventilation and...
I share Tom Moran's formula for substrate
4 parts Coco fibre
4 parts peat moss
1 part clean, dry sand
1 part shredded sphagnum moss, or vermiculite, or both!
You can add organic soil if you need more firm substrate, just make sure there are no pesticides, fertilizer or pine, redwood, cedar, etc...