A civilization is living on an ancient super structure, they don't know much about it, they just live there and have lived there for as long as they know. For our current situation we can ignore most of this structure. The interior of it is hollow with the center hanging around an artificial gravity well/power source, for this case just imagine it as a star. Everything living on the inside is literally hanging over the void above this.
The outside of the structure is unsuitable for life, and the old working have a tendency to periodically get exposed to open space and the old machines can take a bit to fix things. Sure many people live here, but it's a dangerous life. So unless they're pretty bad off most live on the interior of the structure overhanging the void.
To sustain atmosphere and life on the interior we have a soil that can stick to the interior of the surface with minimal loss to the central mass.
Details that may matter
- Gravity at the surface (pull toward the central structure), is approximately 1 G. Important to remember that down, is toward the central gravity well.
- Water entering the soil will be from the structure, the ceiling, there isn't rainfall or any dramatic weather to worry about.
- There is plant life to help hold things in place.
- I've been mostly considering chemical make-up or structure of the soil, but animals or insects that could help would be interesting to consider also.
- The current civilization in question isn't at the same tech level as those who build the structure they just live there. The super civ that built it long ago is doing a lot of handwaving to make things sustainable, how they do most of it doesn't matter. My people are more worried about what exists and not the off the wall tech that may be needed to make it exist.
- The super structure has in place its own super hand-waving tech in place that we don't understand that can recycle lost matter going to the center. So don't be concerned about some matter loss. It IS sustainable as long as it's slow enough that things grow and people can actually make decently long term cities in places (200-300 years.) This includes allow air near the "top" to stay at a level that can sustain life.
- Central mass is far enough away things aren't going to randomly hit it. It's not going to cook everything, it's designed to sustain the life not kill everything.
Assuming I'd need a metallic/magnetic or sticky soil that would allow plant life. Obviously lots of things that can resist against 1G of gravity, we have stuff all over that people stick to ceilings, but this has to be very long term, not just mud. My inquiry is over how we'd add soil to an inverted surface that could support plant life? Everything from mosses to trees.
- Additional information that may help me is if it'll be toxic or not to people, or other risk factors I may have if it's the primary soil source for a civilization.