I use Foxfarm potting soil mixed with sand and compressed coconut for my BP's, and ever since I started using it, I can maintain the humidity in the cage at at least 60%, usually much higher. You can really tell the difference in their sheds now. No more ripped or partial sheds, just shiny and beautiful pythons.
As far as potting soils, read the ingredients before you put it in your BP's cage. Ammonia derived from chicken feces, sewer sludge, and all kinds of fun chemicals are added to soil. Do you really want your snakes on that? It took me forever to find something I felt comfortable with.


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Diego
SNAKES
4.4.0 Corn Snakes (Different morphs)
1.1.0 Everglades Rat Snakes
1.0.0 Boa Constrictor
0.1.0 Dumeril's Boa
1.1.0 Rosy Boas (Mexican & Mid Baja)
1.1.0 Kenyan Sand Boas
0.1.0 Indonesian Dwarf Pacific Boa
0.1.0 Tangerine Honduran Milksnake
1.0.0 Honduran Milksnake
1.2.0 Ball Pythons
1.0.0 Woma Python
1.1.0 Cape York Spotted Pythons
1.1.0 Macklot's Pythons
0.0.1 Ribbon Snake
1.0.0 Western Hognose
1.0.0 Albino San Diego Gopher Snake
0.0.1 Sonoran Gopher Snake
0.1.0 Mexican Black Kingsnake
1.1.0 Gray Banded Kingsnakes (River Road)
0.1.1 California Kingsnakes
1.0.0 Yellowtail Cribo
0.1.0 Blacktail Cribo
LIZARDS
1.0.0 Frilled Dragon
3.1.0 Bearded Dragons (2 Normal, 1 RedXGold, 1 Citrus)
1.1.0 Eastern Collared Lizard
0.1.0 Merauke Blue Tongue Skink
2.3.0 Leopard Geckos
1.0.1 Yellow Niger Uromastyx
FROGS
2.2.0 Southern Bell Frogs
1.0.1 Green Tree Frogs
1.1.0 Striped Walking Frogs
1.1.1 White's Tree Frogs