I have a city on a "sky island". I want to have farms around my city on all the rest of the island.

There is nothing around the island no matter the distance or direction. But the gravity works (meaning that all that is not the island feels a force of attraction directed downwards and "downwards" is an absolute direction). If you fall from the island you will die of hunger, thirst or boredom. The water is supplied by a fountain that fills ponds and canals. When the day comes a diffuse light appears everywhere. Therefore a cellar large enough even closed will be lightly illuminated. But the weather is monotonous - no rain, no wind and a constant temperature. There are trees on the island, but no forests and all the water bodies are artificial channels

With such a world, I am worried about potential difficulties for my culture and my farms. Then I maintain farms identical to that which can surround any city on earth under a temperate climate? The water supply is not a problem but is not due to the rain some cultures are better suited than others? What impact can the absence of weather and especially wind of the flora and fauna of the island?

  • $\begingroup$ When you say nothing do you mean atmosphere or vacuum? $\endgroup$
    – Joe Bloggs
    Aug 30, 2017 at 12:52
  • $\begingroup$ @JoeBloggs I mean atmosphere otherwise I know that it will be very calm $\endgroup$ Aug 30, 2017 at 13:00
  • $\begingroup$ How is your soil replenished? How is anything replenished that goes over the edge? $\endgroup$ Aug 30, 2017 at 13:43
  • $\begingroup$ @MichaelRichardson I have an infinite supply of water but I have not thought about the soil. Do you have any idea of ​​the speed at which this can go in the absence of rain and wind? $\endgroup$ Aug 30, 2017 at 13:59
  • $\begingroup$ If you are irrigating, the soil runoff is going to be considerable. Directed watering or light sprinkling with no run-off would have a greatly reduced effect. Where do your canals end up? Is excess water just dumped off the edge? How is the foliage on the island watered? $\endgroup$ Aug 30, 2017 at 15:11

3 Answers 3


Nutrient deficiency will eventually be a problem in a closed system with irreplaceable losses.

In addition to water, CO2 and sunlight, plants require macro and microelements. Humans do too. Macroelements include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Nitrogen can be fixed from atmospheric nitrogen by certain bacteria, but the others must be present in the soil. Microelements include zinc, copper and other things you will see on the label of vitamin / mineral supplements.

Soil can be depleted just by water coursing through it, or by growing crops and then taking them away and the minerals they contain with them. If the soil is depleted of these elements it must be resupplied either by organic matter coming in from other areas and decomposing (e.g. - putting seaweed or manure in the fields) or mineral bearing rocks breaking down and releasing their components in a form available to plants.

from http://www.californiaearthminerals.com/media/mineral-nutrient-depletion-in-us-farm-and-range-soils.pdf

In natural systems, living plants capture most of the elements released by decomposing primary minerals, and by decomposing organic matter.[6][6] The small amount that is lost is made up (at least partially) by the release of elements from primary minerals. This continues until the quantity of primary minerals is reduced to the point that it cannot supply enough nutrient elements to keep up with plant demands. In natural systems, this depletion can occur in a few thousand years in warm & very wet climates, or nearly forever in cool dry climates. Removal of nutrients by cropping and grazing accelerates this process.

As plants take up nutrients from the soil solution, the soil solution must be continually resupplied with nutrients from other reservoirs. Clays and organic matter can adsorb and hold mineral nutrient elements, and then release them to the soil solution as the soil solution is depleted. However the clays must get nutrient elements from the primary minerals in the soils. For the primary minerals to supply enough nutrient elements, the following conditions must exist: 1. The primary minerals must be rich in nutrient elements. 2. The primary minerals must be finely divided (very small particles). 3. Finely divided primary minerals must be present in sufficient quantities. 4. The primary minerals need to be relatively soluble for sufficiently rapid release to occur.

One of these I am familiar with is calcium deficiency in Hawaiian soils. The volcanic rock comprising the soil does not have much calcium to start with and it is easy to deplete it. lemons with end rot from calcium deficiency https://www.ctahr.hawaii.edu/nelsons/glossary/End_rot.htm

Nutrient deficiency in the soil can be prevented or remedied by enlightened (e.g. you know what minerals are missing and you supply them) or empirically effective (e.g. nightsoil makes crops grow better) farming techniques.

If your island is a closed system with nutrient losses over the edge into the void (either as water coursing down through the island and off the bottom, or wind/water erosion off the top, or kids throwing stuff off the edge) it will be hard to avoid gradual irreplaceable losses. But you can invent replacements and have make it cool too. Maybe you could have flocks of birds from off-island stop by periodically and do a lot of pooping? Occasional immense wind jellyfish flops down on the island to die?

Rains of spiders?

rain of spiders

from http://www.foilhatninja.com/its-raining-frogs-hallelujah/


What you describe is actually a greenhouse. Typically greenhouses are used for growing warm-season vegetables like tomato, pepper, cucumber and beans. Average temperature as for my experience may vary from 17 to 28C. Please note, that you may need some artificial light in addition to the natural one. It is also your responsibility to keep the reproducing in check since there may be no insects, wind and other pollinators. Hand-pollinating is a common practice, so it shouldn't be too much of a problem.

So, as I see it, it should be possible to grow warm-season plants in such climate. Please, do correct me if I'm wrong.


Size and makeup of the island

How large is your island? How many people are on it -- in the city and in the farmland surrounding? How is the land divided between urban spaces, farmland, canals/waterways, roads, etc., rural housing and so on? How deep is the soil? Is there bedrock below the soil layer or sand or air? What kind of soil makes up the farmland?

If 75% of the island is urban spaces + infrastructure spaces (roads, canals, etc.) then you won't have enough farmland to support the population. If the urban population is too high, you won't be able to grow sufficient crops to feed them.

If the soil is not good farming soil with proper water tables and sufficient depth, etc., then crops won't grow well. Soil is a complex combination of things that must maintain proper balances to function.

Farming techniques

I can't see what technology level your island has. Farming is different based on the technology level. Medieval farming required crop rotation, fallow fields, and so on to support the population. Modern farming has a much higher yield per acre/hectare, because we use modern pesticides, herbicides, and fertilizers. And our crops, whether GMOs or through centuries of selective breeding, are much more nutrient-rich than they used to be. Yields have risen over time.

Again, technology levels play into how much land a given number of people can farm effectively. Medieval farmers worked by hand with the help of oxen. It is the foundation for the word acre, which was defined as the amount of land a farmer could till with an ox in one day. Modern farmers use tractors and can, with sufficient crews, manage farms covering thousands of acres.

If your have no oxen or equivalent and no machinery, then farming becomes a highly labor-intensive job. It will mean many hands are required to grow food. Without livestock or machinery, you may not be able to grow enough food to support an urban population -- everyone must farm for subsistence. But livestock require food too...

You also have to have sufficient land to grow multiple kinds of crops. If your island only eats corn, then malnutrition will eventually set in. So you must supply a variety of crops to give a balanced diet.

With no seasons, your farmers can always have a crop growing. With proper, centralized, planning, you can even stagger crops so that there are always fresh crops ready for harvest at any given time -- provided you have enough land and farmers available.

Fauna matter, too

At the very least, your crops will require bees or some equivalent species to cross-pollinate. Your island either requires insects as well, or requires some other means to pollinate your crops. (Bees also provide honey, which allows for mead...)

Your soil also requires various living organisms to support plant life. This ranges from bacteria to worms and other creatures that help transform nutrients, aerate the soil, etc.

water, water, everywhere

Your crops require water. Your world provides no rain. Therefore, water must get to the plants via irrigation. Irrigation has been in use for thousands of years by different cultures using different techniques, but your people will have no alternative to using this.


Your crops require nutrients. In ancient times, soil depletion was countered with crop rotation. This allows different crops to extract different nutrients. You will need to either supply massive amounts of chemical fertilizer, or use some combination of natural fertilizer with crop rotation. Otherwise, your soil will become useless within a few generations.


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