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Let us assume that I built a dome on the moon's surface. That dome is pressurized and has nuclear-powered lights to deal with the long lunar night (or is in one of the poles). I also have water, either from the moon's lunar craters or from an icy asteroid gently landed on the surface and covered with something to protect it from evaporating under the sun.

How barren is the lunar soil? Why can't plants grow there and what can be done to fix its problems?

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    $\begingroup$ You're going to need electrolytes. Plants crave it. In all seriousness, you need to add carbon dioxide along with nutrients, water and a source of light. Also remember that plants can grow without traditional soil. $\endgroup$ – gwally Apr 26 at 10:50
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    $\begingroup$ You should consider not using the soil at all and going for hydroponics. en.m.wikipedia.org/wiki/Hydroponics They offer way better yields per area and can be done vertically, further improving yields and efficiency. Some solutions use a fish farm in combination with the plant farm, diversifying food production and cheaply producing nutriants for the plants. $\endgroup$ – TheDyingOfLight Apr 26 at 11:03
  • $\begingroup$ Is there a long lunar night? $\endgroup$ – Willk Apr 26 at 12:17
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    $\begingroup$ It takes 28 earth days to rotate. $\endgroup$ – Geronimo Apr 26 at 12:27
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    $\begingroup$ @gwally Also you will need to speak with it ;) $\endgroup$ – jean Apr 26 at 20:18
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Experiments have been done about this:

https://theunconventionalgardener.com/blog/growing-plants-in-lunar-soil/

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0103138

When humans will settle on the moon or Mars they will have to eat there. Food may be flown in. An alternative could be to cultivate plants at the site itself, preferably in native soils. We report on the first large-scale controlled experiment to investigate the possibility of growing plants in Mars and moon soil simulants. The results show that plants are able to germinate and grow on both Martian and moon soil simulant for a period of 50 days without any addition of nutrients. Growth and flowering on Mars regolith simulant was much better than on moon regolith simulant and even slightly better than on our control nutrient poor river soil. Reflexed stonecrop (a wild plant); the crops tomato, wheat, and cress; and the green manure species field mustard performed particularly well. The latter three flowered, and cress and field mustard also produced seeds. Our results show that in principle it is possible to grow crops and other plant species in Martian and Lunar soil simulants. However, many questions remain about the simulants' water carrying capacity and other physical characteristics and also whether the simulants are representative of the real soils.

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    $\begingroup$ Neat! Cress and mustard sandwiches here we come :) $\endgroup$ – Ynneadwraith Apr 26 at 11:14
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    $\begingroup$ I think I have read somewhere that actual Martian soil, when exposed to water, liberates hypochlorite, which not exactly plant friendly. Was it accounted in the study? $\endgroup$ – L.Dutch - Reinstate Monica Apr 26 at 14:17
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    $\begingroup$ @L.Dutch " it should be noted that none of the simulants include percholorates" $\endgroup$ – Ville Niemi Apr 26 at 14:50
  • $\begingroup$ @L.Dutch The simulant used in the study actually predates the discovery of perchlorates in Martian soil. $\endgroup$ – Ville Niemi Apr 26 at 14:51
  • $\begingroup$ @L.Dutch The reason given for newer simulants not including perchlorates is the health hazard to humans performing experiments. That to me implies that any Martian farmer would pre-process the soil they use to remove the perchlorates regardless of whether plants deal with it. $\endgroup$ – Ville Niemi Apr 26 at 14:54
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Lunar soil is made of regolith

Regolith covers almost the entire lunar surface, bedrock protruding only on very steep-sided crater walls and the occasional lava channel. This regolith has formed over the last 4.6 billion years from the impact of large and small meteoroids, from the steady bombardment of micrometeoroids and from solar and galactic charged particles breaking down surface rocks.

There are two profound differences in the chemistry of lunar regolith and soil from terrestrial materials. The first is that the Moon is very dry. As a result, those minerals with water as part of their structure such as clay, mica, and amphiboles are totally absent from the Moon. The second difference is that lunar regolith and crust are chemically reduced, rather than being significantly oxidized like the Earth's crust. In the case of the regolith, this is due in part to the constant bombardment of the lunar surface with protons (i.e. hydrogen (H) nuclei) from the solar wind. One consequence is that iron on the Moon is found in the metallic 0 and +2 oxidation states, whereas on Earth iron is found primarily in the +2 and +3 oxidation states.

To grow plants you need to have pedolith

Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Earth's body of soil, called the pedosphere, has four important functions:

  • as a medium for plant growth
  • as a means of water storage, supply and purification
  • as a modifier of Earth's atmosphere
  • as a habitat for organisms

It's more or less the same situation one find immediately after a volcanic eruption: the solidified lava cannot host life as it is, it needs to be weathered and transformed to become pedolith..

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  • $\begingroup$ The reduced state will hurt the roots, won't it? $\endgroup$ – Geronimo Apr 26 at 12:03
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It is said by this popsci article that both lunar and martian soil are viable planting mediums. The test shows that martian soil is much better than lunar soil. The journal's report backs that up. Popsci does not give final conclusion: it is possible, they say, but many questions remain. The soil seems to dry-out quickly, they add. On the other hand, you must remember that eons of meteor bombardment makes ultra-fine dust. You must remove the dust, or your planter will turn the soil into a concrete block. (Don't know how coarse was their tested soil, if it dried so quickly). The soil may be processed to remove fine dust. As it was never exposed to water, you may see how it reacts with it. Once a reaction (if any) takes place, the chemical reactivity is neutralized. You may start adding nutrients and beneficial microorganisms and start planting.

Nasa's article states that the 4 elements necessary for growth are not available in the soil, with oxygen being bound. They are naturally absorbed through water and the atmosphere: carbon, oxygen, hydrogen and nitrogen. All other elements may be present in the soil.

Bbc article shows that marigolds can thrive on the minerals. So yes, that is possible. Don't forget that under lab conditions on earth they receive the 4 elements C, O, H, N from air, water and co2, so that the minerals in the soil make-up for what's missing. This allows us to extract water from polar ice, provide humans and poultry as a source of CO2 and maybe all what we need to bring to the moon is some nitrogen fertilizer to start the nitrogen cycle. With an adequate supply of minerals you can maintain a cycle.

DISCLAIMER: Part of my experience comes from a failed attempt to plant in a soil patch which was stripped or unpaved. The area was under concrete for many years and too inert for any planting. It took some time until plants began taking a roothold. This shows the importance of introducing oxygen and essential organisms into a soil which has always been sterile.

References

Popsci: https://www.popsci.com/article/technology/crops-grow-fake-moon-and-mars-soil

The journal: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0103138

Nasa: https://forum.nasaspaceflight.com/index.php?topic=32005.0

Bbc: http://news.bbc.co.uk/2/hi/science/nature/7351437.stm

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I think it's worth mentioning that even if you could use lunar soil to grow plants, it may not be a good idea for your Lunarians:

  1. https://www.livescience.com/62590-moon-dust-bad-lungs-brain.html
  2. https://science.nasa.gov/science-news/science-at-nasa/2005/22apr_dontinhale

"The real problem is the lungs," he explains. "In some ways, lunar dust resembles the silica dust on Earth that causes silicosis, a serious disease." Silicosis, which used to be called "stone-grinder's disease," first came to widespread public attention during the Great Depression when hundreds of miners drilling the Hawk's Nest Tunnel through Gauley Mountain in West Virginia died within half a decade of breathing fine quartz dust kicked into the air by dry drilling--even though they had been exposed for only a few months. "It was one of the biggest occupational-health disasters in U.S. history," Kerschmann says. This won't necessarily happen to astronauts, he assures, but it's a problem we need to be aware of--and to guard against.

Quartz, the main cause of silicosis, is not chemically poisonous: "You could eat it and not get sick," he continues. "But when quartz is freshly ground into dust particles smaller than 10 microns (for comparison, a human hair is 50+ microns wide) and breathed into the lungs, they can embed themselves deeply into the tiny alveolar sacs and ducts where oxygen and carbon dioxide gases are exchanged." There, the lungs cannot clear out the dust by mucous or coughing. Moreover, the immune system's white blood cells commit suicide when they try to engulf the sharp-edged particles to carry them away in the bloodstream. In the acute form of silicosis, the lungs can fill with proteins from the blood, "and it's as if the victim slowly suffocates" from a pneumonia-like condition.

Obviously we don't know for sure how it might happen, but before I start pulling in large quantities of moon (or martian) soil I'd like to make sure we've determined for sure that the soil won't kill me, even if the plants like it.

Personally, I'd just go for the hydroponics.

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