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.
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    $\begingroup$ Is your structure a sphere or a ring? if a ring is spinning around the central star, the centripetal force will create gravity on its surface. $\endgroup$ – Bald Bear Nov 5 '19 at 15:35
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    $\begingroup$ I think your problem is that over time, organic matter is going to fall away rather than compost back into the soil. When leaves fall off the trees, for example, they won't go back into the "ground". So this is not going to be self-sustaining unless as Bald Bear suggests, you have a spinning structure. $\endgroup$ – workerjoe Nov 5 '19 at 15:52
  • $\begingroup$ It's a sphere, and the mass in the center isn't technically a star. It's an artificial gravity well and power-source of the structure. There's not necessarily a large amount of movement between the two things. It's a stable 1G pull toward the center, at the surface level of the interior of the shell. $\endgroup$ – Nymn Nov 5 '19 at 15:54
  • $\begingroup$ The structure itself can sustain the process, that's one of its functions. I can recycle mass from the center to rebuild what it needs on the exterior. So even if there is erosion toward that point it can be sustained. Even so, we want a composition or method for holding soil at the surface as long as possible to reduce loss. $\endgroup$ – Nymn Nov 5 '19 at 15:57
  • $\begingroup$ "There is plant life to help hold things in place." How would these plants be growing without soil in the first place? $\endgroup$ – Snowshard Nov 5 '19 at 18:15

I'm going to run with the principle of this being an artificial structure with exactly this use in mind.

What that means is that we're not attaching to a smooth surface but rather a prepared texture specifically designed for plants to adhere to with relative ease.

We should also consider that water will need travel within this surface somehow. Since soil and water are intertwined the two factors need to be considered together.

The engineers will need to consider some sort of growing medium for their plants, it must hold water, it must hold the weight of trees and other large plants until they engage with the primary support structure. It doesn't strictly need to resemble soil, it merely needs to perform the fundamental functions as required by plants.

Plants will grow on just about any surface they can get their roots into, breaking anything that doesn't flex enough to allow their roots in. For these purposes I'm envisaging a structure that doesn't look unlike a tree or shrub in its own right, but has a relatively solid structure of porous ball leaves*. It has a main stem that is attached to the structure of the environment, branching massively down to tiny "twigs" that hold the porous ball on the end. The internal surface of the habitat consists of a solid forest of these trees creating a surface on which the organic plants grow.

The branching system allows the structures the required flexibility to allow the roots of plants to grow around and attach themselves to it. Through this structure and into it's 'leaves' will flow the water and nutrients the plants need.

Since all plants will have to reproduce via suckering (asexual) or tubers (sexual), this habitat also provides environment for creatures able to distribute those tubers further afield preventing large monocultures of plants from building up.

*or fruit if you prefer but by numbers and density they're closer to leaves

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  • $\begingroup$ So, if I understand right we're describing an artificial type of "tree" to host the necessities of plant life? Neat concept, could help prevent scabbing and give some extra strength to deal with weather conditions. Maybe even the idea of a biodegradable framework that could grow in regions that are having issues so that structure can keep stabilize problem areas. $\endgroup$ – Nymn Nov 6 '19 at 14:26
  • $\begingroup$ @Nymn, that's the right idea, in time it will have a full ecology of its own between root systems, fungi, fauna, and general biodegradation of roots and tubers. $\endgroup$ – Separatrix Nov 6 '19 at 14:39

The existence of epiphytes (such as the mosses you mentioned) shows that you don't necessarily need soil at all. Sticky seeds, and plants that spread via rhizomes and runners can spread over the inside surface of the structure, anchor growth in cracks and holes and extend branches and tendrils down towards the light. Everything else can just bind itself to existing vegetative structures.

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  • $\begingroup$ This does add a nice variable that doesn't force usage of potentially toxic compositions to sustain the ecosystem and allows for perhaps more varied environments. Thanks for the sources. $\endgroup$ – Nymn Nov 5 '19 at 16:23

I first posted this as a comment to another question of the OP, but it does belong here:

Since actual soil would have to be almost solid or somehow otherwise held in place with special technology, I think the most obvious solution is to not use normal soil at all:
Plants can be grown on/in hydroponics systems, which are much easier to maintain in artificial environments like buildings or space stations.

So if a super advanced civilization would build a giant space-greenhouse, why not take a power&gravity source and build a hydroponics system around it?
It's a pretty neat solution actually: a sphere shape means you can optimally use the light generated by the source as well as minimalise the required building materials of the structure (which is an issue for planet sized things).
The 'floor' contains the hydroponics system and supplies water and nutrients to the plants (any recycling system that collects matter 'lost' to the core can plug into this), while all plants are kept in the stable environment inside of the structure.

Such a system would mostly be suited for plants that can grow roots into the hydroponics system, but the system could also provide different surface patches optimised for shallow (i.e. grasses) or deep (i.e trees) roots.
For humans / animals this system would be almost sterile or inert. They may need to 'tap' certain kinds of plants as water sources, unless they are able to access the water pipes of the underlying structure.
If all the 'soil' is an artifical hydroponics system it also means that there are no natural (anorganic) building materials easily accessible to anyone living there. They would have to rely on wood or salvage parts of the structure (maybe from said damage / exposure to the outside of the structure?).

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  • $\begingroup$ Yeah, the society here would be very dependent on utilizing plant material for everything in their lives and tapping into the plants to extra-water and sap. Which is great for my big picture. Tapping plants for water is a totally possible situation when you think of how people can use plants like a cactus or maple trees. Since we'd obviously be lacking any stable bodies of water, other than maybe weird ponds forming in plants intertwining. $\endgroup$ – Nymn Nov 6 '19 at 15:40

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