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I want to create a world were soil mining is a major industry. Imagine a vaguely Earth sized world where almost the entire surface of the planet is under, ocean, ice caps or is a hot or cold desert with nonexistent or very poor soils.

Soil or soil like organic matter does exist, but only quite deep underground and the inhabitants are forced to dig soil mines so that they can bring it to the surface for use in growing crops. How could such a strange state of affairs come about?

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    $\begingroup$ I assume no hydroponics. I also assume that sand, which must exist on any planet with wind and/or water, does not meet your expectations. Also, I assume you're looking for a planetary evolution answer, not a "here's what state the planet can be in" answer? Or am I wrong? $\endgroup$ – JBH Jun 17 at 16:46
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    $\begingroup$ Also, could you look at this chart and tell us which of those soil types exist on the surface and which must be mined underground? That way we know exactly what you're looking for. $\endgroup$ – JBH Jun 17 at 16:47
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    $\begingroup$ Also, wouldn't decomposing organic matter quickly provide a new surface layer of soil? And by "quickly" I mean "in a matter of decades". $\endgroup$ – Vilx- Jun 18 at 6:48
  • $\begingroup$ @JBH Yes sand does exist as does water, but the water is not that available for agriculture (hot deserts, cold deserts and oceans, maybe some underground and a little semi desert. Yes looking for a planetary evolution answer. $\endgroup$ – Slarty Jun 18 at 6:53
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    $\begingroup$ @alexgbelov well that was my question! But I imagine that was not always the situation. Either they arrived later from elsewhere and were faced with it or their world changed drastically somehow and they had to deal with it. There is a tiny bit of wriggle room in that word "almost" perhaps some people are hanging on in a small area of semi desert and want to improve it. $\endgroup$ – Slarty Jun 18 at 15:24
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Glaciers cover the old landscape.

Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.

In your world, glaciers cover what used to be fertile fields. Your people tunnel down through the glacier to access those fields and their soil. Under the ice, they may find other remnants of the world that once was.

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  • $\begingroup$ the edge of a glacier however produces some of the most fertile soils in existence. So the glaciers would have to run from pole to pole to achieve the desired effect. $\endgroup$ – John Jun 18 at 16:50
  • $\begingroup$ @John - is that true for an advancing glacier? I know receding glaciers leave ground up rock flour in their wake but I am less sure above advancing. $\endgroup$ – Willk Jun 18 at 22:36
  • $\begingroup$ It is true of all glaciers, the toe of a glacier is basically always melting. $\endgroup$ – John Jun 18 at 22:51
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Post apocalypse

The asteroid that killed dinosaurs did not kill all of them by direct kinetic strike. Most of the world was affected in two ways:

  • Temperature on half of the world rose too fast due to increased pressure (caused by the flying debris, which did cover almost a whole hemisphere) - this would have cooked bacteria in the upper layers of the soil, making many lands sterile for a long time;
  • The Earth was then covered by clouds of ash, which not only kept sunlight from reaching the ground but which also got deposited everywhere after some time.

If the world has been hit by an asteroid like that in recent eras, it may take a million years or more for land to recover naturally. If mankind was just getting off bunkers months after the asteroid hit and the dust has settled, they'll have to dig for Earth that has good nutrients, and will have to treat that soil in order to make it arable.

In lieu of an asteroid, a global nuclear winter would do. You have to dig through all that radioactive ash in order to get good soil. This may also happen due to global warning (not a typo).

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    $\begingroup$ It could also have been poisoned, Nausicaä of the Valley of the Wind style. $\endgroup$ – Sanford Bassett Jun 17 at 15:55
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The planet never evolved terrestrial life

Soil is primarily made up of the decomposed remains of other life, so a planet where life never made it to solid ground will have all of it's soil in water and around its bodies of water, but the vast majority of dry land will just be exposed bedrock, sand, and volcanic ash.

Is this possible?

Absolutely. Plant life only moved to solid ground on Earth about 475 million years ago, but Earth has had aquatic life for billions of years. So, if you were to just find an Earth like world that is 500 million years or so behind us on its evolutionary path, then you will not find usable soil on most of the dry land. Using pioneer plants, you could begin terraforming this world's dry land into soil, but this process takes a few thousand years, and most pioneer plant species are not agriculturally useful. So, in the mean time, your colonists would need to rely on dredging up soil from rivers and lakes to create artificial top soil.

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If your planet has only very little of natural land and the rest would be covered with water, then as soon as the inhabitants reach a certain number, the would have to get creative for food production, as there is only so and so much space for farming.

The first thing to do would probably be to move all housing and industry on platforms in the sea, but once that is done and all available natural land is filled up with farms, the only way to increase space for farming would be, to put soil on the platforms and farm there.

That of course means that the soil has to come from somewhere and as you don't want to destroy more fo the farming land you already have than is absolutely necessary, digging deep comes to mind.

Now bare in mind that soil is not dirt and it takes more than to mix the right amounts of nutrients and micronutrients with sand. The little critters living in the soil are just as important as all the nutrients, because they make them available for the plants.

That is why I would start the world with a bit of good soil, so the critters can be transplanted to the artificial soil.

A world like that would have to be very focussed on recycling all the organic material that is available. As it becomes valuable soil.

So this world would not actually be mining soil, there simply is no soil underground, just rocks, gravel and minerals. They would be mining the ingredients for soil and then make it.

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That is not too far from what we do now to make our crops grow, since soil depletes quickly as you grow crops. "Crops perform better when they have nutrient-rich soil. Healthy soils contain a balance of nitrogen (N), phosphorus (P) and potassium (K). These three nutrients help plants grow in different, yet each important, ways:

Nitrogen helps a plant’s leaves grow; Phosphorus supports a plant’s root growth and flower and fruit development; and Potassium is a nutrient that improves the plant’s overall health."

Google Potash mines images and you will get some great ideas for what an underground "soil" mine would look like. Potash is made of potassium.

Then you'd need Nitrogen--"Nitrogen fertilizers are made from ammonia (NH3), which is sometimes injected into the soil directly. The ammonia is produced by the Haber-Bosch process. In this energy-intensive process, natural gas (CH4) usually supplies the hydrogen, and the nitrogen (N2) is derived from the air." So rather than mining Nitrogen you would be manufacturing it. No doubt human waste could help in the processing...

Lastly you need phosphorus- "Most of the phosphorus used in fertilizer comes from phosphate rock, a finite resource formed over millions of years in the earth's crust. Ninety percent of the world's mined phosphate rock is used in agriculture and food production, mostly as fertilizer, less as animal feed and food additives."

Google phosphate mining and it appears that form of mining is usually strip mining on the surface. But... creative license..

So over all you need not come up with too fantastical of an idea which the world would have to mine soil. Since we already do. Any of your ideas, Global freeze, Global flood. Global disaster...

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  • $\begingroup$ You can't grow plants in straight fertilizer. So while mining for fertilizer demonstrates that you can take minerals from underground, it doesn't explain how to create anything but the worst growing mediums (sand, clay) from mining. $\endgroup$ – JBH Jun 17 at 16:43
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If you look at human evolution, and movement to cities, you could take the angle of megastructure civilizations. These massive societies all contained in a few buildings around the world would still need to grow plants to feed people and to develop things like medicines. The thing with moving everyone inside though is that the population will still increase, requiring the need for more and more soil to produce food.

To make the "outside world" uninhabitable, you could pick from almost any list, authoritarian government mandate, nuclear war, supervolcanic eruptions, the return of the glaciers, or even an atmosphere that is in heavy deterioration which leads to heavy radiation of organic topsoil.

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