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The planet needs to be able to support life but initially ve devoid of it. Which I would think would make it's Soil like that of a desert. The regolith may be important for early agriculture. It can effect sky color.

How do I decide the contents of the planet's soil along with further details and it's consequences on things like sky color.

I hope it's alright to ask.

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    $\begingroup$ Do you want to have Earth-like grasses and plants? $\endgroup$ – Alexander Dec 13 '19 at 19:06
  • $\begingroup$ Yes. Life that exists on earth is what I mean't. Just not when the planet is first discovered and colonized. $\endgroup$ – YLong Dec 13 '19 at 19:12
  • $\begingroup$ "Soil can affect sky color"? How so? $\endgroup$ – F1Krazy Dec 13 '19 at 19:21
  • $\begingroup$ Dust is the specific term. Doing a basic google search on martian dust would be better for you. $\endgroup$ – YLong Dec 13 '19 at 19:32
  • $\begingroup$ Soil is a product of action of soil bacteria, earthworms and other living organisms. You can't scientifically have soil on sterile world. However, non-organic soil can be quite fertile, once those organisms are introduced. Volcanic basalts, for example, break down to make quite fertile soil. $\endgroup$ – Alexander Dec 13 '19 at 19:32
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The problem here is that the soil and atmosphere that is used by Earth life today is not naturally occuring (geologically speaking). It is made by other Earth life. The most important parts of it is a combination of organic molecules that were synthesized by organisms long ago that took raw naturally occurring molecules like CO2, H20, and N2 from the air and built various other molecules out of it to form their bodies. Then they died, and their husks decomposed into soil. Without this process, the regolith on an alien world will lack important nutrients. Most notably, any reactive form of nitrogen (NO3, NH4) which most Earth plants need to survive.

Another key factor of soil is the symbiotic fungi, bacteria, worms, etc that live it it that help plants create and absorb the various nutrients they may not be able to process on their own.

Between these two factors, you will not find organic soil on a planet that has never had life. The closest you can hope to find is a planet that used to support life, but does not anymore. Even in this you will still have to bring Earth cultures of soil microbes, and possibly nitrate fertilizers and some pioneer species of plants to help prepare the soil for full blown agriculture just depending on how Earth like the old form of life was.

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  • $\begingroup$ I did know that. I intended the planet to have nitrogen in it's atmosphere. Would that be implausible without a nitrogen cycle? Also on subject: could there not be some source of nitrates or nitrites that can be mined? $\endgroup$ – YLong Dec 13 '19 at 20:22
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    $\begingroup$ N2, NO2, and HNO3 are naturally occurring nitrogen sources that happen without life, but as far as I've been able to find, NO3 and NH4 require other life forms to enrich the soil 1st and most plants need these specific nitrates to survive. $\endgroup$ – Nosajimiki - Reinstate Monica Dec 13 '19 at 20:37
  • $\begingroup$ Thank you nosajimiki. $\endgroup$ – YLong Dec 13 '19 at 20:42
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Your planet was full of life but was sterilized by a gamma ray burst.

Your planet was lush, with forests, prairies, rich soil and all the other things you want for your story. However in an adjacent system a gamma ray burst took place. For four minutes your world was bathed in energetic rays.

Everything died. And did not rot, because everything died. The wind and rain continued, and gradually broke down the remains of life. Fires consumed some of the oxygen in the atmosphere, which was not replenished. CO2 levels rose and O2 levels fell and then reached a new equilibrium.

The situation when your story begins depends on how long it has been since this world died. Some of the rich soil might be lost to wind and erosion but you can have as much remain as you need to make your story work.

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Most of our atmospheric oxygen comes from photosynthesis, and most of our regolith is either sand or filled with biomass. So my guess is that you're looking at mostly sand, with the precise details depending on things like chemical abundances, the density, temperature, etc of your planet, and the presence or absence of water (the absence of water on the surfaces of Mars and the Moon seem to be part of why their regoliths are so much finer than Earth's sand).

It's hard to get specific without more information. Ex, is the planet lifeless because it's too hot / cold / dry / full of toxic chemicals / has too thin or thick an atmosphere? How big is it, both in terms of mass, and radius? Orbits and rotations will also affect the temperature. Is it geologically active? If so, you'd expect more basaltic and basalt-derived rocks and regolith.

I find this page useful. It's on the long side, but has clear subsections for surfaces, regolith, atmosphere, etc. It does, though, leave a great deal open-ended, since the precise surface composition can vary based on a variety of factors.

The planet you described sounds to me like it's somewhere between Earth and Mars. Given that, I'm guessing an Earth-like sky, with landmasses mostly covered in sand, with flood basalts and basaltic sediments optional depending on how geologically active this planet has been. However, you could easily go with something more Lunar (more vulcanism and less water) or Martian (lots of surface iron oxides). You could probably get away with copper compounds if you want blue sand, but you'd need to explain where the iron and silicon went (maybe blue sand wouldn't realistically cover more than a small area?).

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