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Say a group of people travel to a different planet where they have to wear spacesuits to live (because there's no oxygen and because of the atmosphere's density). What would happen if they planted trees from earth in a greenhouse on this new planet, completely sealed, with all the conditions necessary for the trees to live, but on that planet's soil instead of earth's soil? Would the plants still grow? Would they still breathe out oxygen like on earth? And, if not, is there any way to grow a lot of earth's trees in outer space? (because there's a limit to how much dirt they could bring on a spaceship)

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    $\begingroup$ Very much depends on what the planetary surface actually is. A surface of caustic soda will react very differently to one of smashed up basalt. By the way: It’s not really ‘soil’ until it has organic material mixed in or organic material can readily grow there. Before that point it’s called ‘regolith’. $\endgroup$ – Joe Bloggs Aug 1 at 12:35
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    $\begingroup$ You don't need to bring dirt, you need microbes. The Martian (the book more than the movie) did an excellent job of explaining this, and showing the process for getting said microbes, a resource you will have too if you don't shove it out an airlock. $\endgroup$ – Cyn says make Monica whole Aug 1 at 16:43
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Maybe. It all depends.

Trees rely on soil for a lot of things, and the alien soil would need to supply it and also refrain from being harmful.

  • Not poison the tree: Some extraterrestrial soils are actively poisonous. (Mars' soil, for example, is highly oxidizing and would, untreated, almost certainly kill anything planted in it.)
  • Retain water: Soil holds water and the tree's roots absorb that water.
  • Provide fixed nitrogen, potassium and phosphorous: These three nutrients are essential for plant growth and needed in fairly high quantity. In an established soil community on Earth, they are supplied by mineral weathering and recycling of old plant material. Fixed nitrogen, in particular, would be unlikely to be usefully present on a lifeless planet that was Oxygen/Water based.
  • Provide micronutrients: There are many other elements which plants need in race amounts and rely on the soil to provide.
  • Provide support: The soil holds the tree's roots and the roots hold the tree erect. Without soil, trees fall over without external support.

Probably the best way to use alien soil is in hydroponics: The soil presumably lacks organic matter, so process it to remove nasties (oxidizers, over-supplies of specific minerals, etc) and use it as a growth medium that provides nothing more than support for nutrient-rich water. (Perlite, for example, is sort of a Quaker-puffed-mineral which is completely inert and used for just this purpose here on Earth.)

You still need to get CO2 in the air from somewhere, protect the tree's leaves from too much UV, and protect them from desiccation.

But it's not very likely the an extraterrestrial planet's soil would be useful right out of the box, so to speak.

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You are better off using lichens.

A lichen (/ˈlaɪkən/, LEYE-ken or (USA) /ˈlɪtʃən/, LICH-en) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. The combined lichen has properties different from those of its component organisms.

They are the first organism to colonize lava fields after eruptions, and grow in conditions as extreme as in the tundra.

Lichens grow on and in a wide range of substrates and habitats, including some of the most extreme conditions on earth. They are abundant growing on bark, leaves, and hanging from branches "living on thin air" (epiphytes) in rain forests and in temperate woodland. They grow on bare rock, walls, gravestones, roofs, and exposed soil surfaces. They can survive in some of the most extreme environments on Earth: arctic tundra, hot dry deserts, rocky coasts, and toxic slag heaps. They can live inside solid rock, growing between the grains, and in the soil as part of a biological soil crust in arid habitats such as deserts. Some lichens do not grow on anything, living out their lives blowing about the environment.

When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.

Last but not least, they can be used as food

Lichens are eaten by many different cultures across the world. Although some lichens are only eaten in times of famine, others are a staple food or even a delicacy. Two obstacles are often encountered when eating lichens: lichen polysaccharides are generally indigestible to humans, and lichens usually contain mildly toxic secondary compounds that should be removed before eating. Very few lichens are poisonous, but those high in vulpinic acid or usnic acid are toxic. Most poisonous lichens are yellow. In the past, Iceland moss (Cetraria islandica) was an important source of food for humans in northern Europe, and was cooked as a bread, porridge, pudding, soup, or salad. Wila (Bryoria fremontii) was an important food in parts of North America, where it was usually pitcooked. Northern peoples in North America and Siberia traditionally eat the partially digested reindeer lichen (Cladina spp.) after they remove it from the rumen of caribou or reindeer that have been killed. Rock tripe (Umbilicaria spp. and Lasalia spp.) is a lichen that has frequently been used as an emergency food in North America, and one species, Umbilicaria esculenta, is used in a variety of traditional Korean and Japanese foods.

Similar to what happens on Earth, after lichens have enriched the substrate forming a soil, you can start growing grass-like plants and only much later you might think of trees.

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  • $\begingroup$ You're suggesting that I use lichens instead of trees on my planet. I researched them a bit and it sounds like a great suggestion. How would they react to being planted on the above alien planet? $\endgroup$ – user613 Aug 4 at 9:37
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Your plants will most likely die as they don't have the supporting ecosystem in the soil. If you look at the evolutionary timescales, land based (read as soil inhabiting) plants are first thought to have evolved on Earth around 700 million years ago, but life first formed on the planet a lot earlier; around 3.5 billion years old, probably around 1 billion years after the Earth first came into existence. Prior to that life was actually oxygenating the atmosphere; it's estimated to have taken around 2 billion years.

The point of all this is that if there is no oxygen in your atmosphere, then there's probably little to no biological activity going on there to start with, and certainly none that would provide an analogue to the ecosystem that exists in soil that trees and other plants rely on for their survival. For a start, dirt isn't just dirt; trees grow in a mix of biological material and rely on bacteria breaking down old plant matter to provide nutrients for them. Potting mix, if you're a gardener, doesn't often contain soil at all, but rather a whole bunch of composting biological matter with the accompanying bacteria, worms and the like that help turn it into the nutrients that the roots of trees pull out of healthy soil.

When you get right down to it, plants are nowhere near as hardy as we would like. Granted for the most part they're hardier than us, but put them on an alien planet and you're going to have soil Ph, foreign biological processes (if you're lucky), salinity, water capture and a range of other issues to deal with. On earth in most places all we have to add is water because the soil is already ideal for our trees. But, if the amount of fertiliser bought by the agricultural sector is anything to go by, even our soil is often not ideal.

Of course, this doesn't even cover the question of toxins in the atmosphere like sulphuric acid, what the temperature variations are, the relative humidity and ability of the soil to even hold water... It's a complicated exercise.

Bottom line is that you can't just bring your trees; you need to bring their entire ecosystem, probably building suitable soil out of the dirt and regolith already present from the ground up (no pun intended) before you think about planting your trees.

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Not trees, trees are delicate sensitive things, though they may not seem it at first glance.

You want weeds, dandelions, horsetail, bindweed, knotweed, grass.

Plants that will grow out of a crack in concrete and call it a good home. Plants that your every effort to annihilate has knocked 6 weeks growth out of and look who's back. It'll take you a while to work out what will grow, what won't, and what's straight up invasive in the new environment, then you can start planting properly.

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  • $\begingroup$ Wut? Plant a black locust in your yard, then tell me about how delicate and sensitive trees are. $\endgroup$ – kingledion Aug 1 at 15:58
  • $\begingroup$ @kingledion, we don't get them here, it's mostly sycamore and ash for the weed trees, but they're still comparatively sensitive even if you have to pull up a dozen every year even in a small garden. $\endgroup$ – Separatrix Aug 1 at 20:17
  • $\begingroup$ Those weeds are optimised for growth on earth. You may think they can just grow out of solid rock, but that rock is porous and home to microorganisms and natural rainfall or just condensation provides water. There are also plenty of rocky areas devoid of obvious life due to lack of water. Otherwise "weeds" would be growing on the top of Mt Everest and all our deserts would be covered by them. They're not as hardy as you think. $\endgroup$ – Daniel Aug 1 at 22:57
  • $\begingroup$ @Daniel, did you miss the bit in the question about all the necessary conditions? $\endgroup$ – Separatrix Aug 2 at 14:09
  • $\begingroup$ @Separatrix Apparently I did but my point was that trees and weeds need the same things to grow. Even on Earth those things are not always available which is why we have deserts and other barren areas. If the soil is capable of providing the nutrients for plant growth then the greenhouse system can be modified to grow anything from dandelions to redwoods. $\endgroup$ – Daniel Aug 3 at 22:35

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