How would a group of floating medieval kingdoms sustain themselves

Question

I started creating a world where benders exist - people who can control the elements earth, air, fire and water. All benders control their element by using their "mana" to control it. They can't make their element only use preexisting sources of it.

Long story short, floating kingdoms exist. To be more specific, four of them which are located on four separate floating islands each about the size of a city like New York and I'm having trouble coming up with a realistic enough way for them to grow enough food for their subjects because I'm thinking that maybe their altitudes would be to high for normal plant growth to occur.

They cannot return to the ground because the reason they went to the sky in the first place was to escape a plague that was basically turning people into zombies and returning to the earth was strictly prohibited to prevent any outbreaks.

Key Things To Know:

1. Flying ships are a thing
2. Going to the ground is prohibited
3. Each kingdom has access to all four types of benders

Answer 1

The problem with really high altitude is going to be more than growing food - how do the people survive (too cold, not enough oxygen)? If altitude is lower, say in line with the Ethiopian Highlands, then you could grow grass (and raise livestock) on the roofs of all the buildings.

However, you may have relatively small human populations in these cities, as the area of land necessary to support them will otherwise probably be larger than the total area of roof-space (see this post for some relevant details). It might be possible to harvest flocks of birds attracted to any arable crops your citizens plant, but scale issues will likely remain - real medieval cities needed large areas of 'countryside' to support their populations.

If you're willing to 'bend' the rules about your benders, then you could potentially overcome the lack of oxygen and temperature issues associated with high altitude, and if you're willing to postulate flying cities, then why not postulate fictitious, nutritionally dense, food that can be grown in the space available?

(edit: I've ignored the water issue: I assume that your 'water benders' can extract some from the air - included exhaled water vapour - and otherwise recycle urine into drinkable water. If not - as the comments note, water will be THE issue)

Answer 2

Maybe the bending powers include generating artificial light. Then the cities can contain enclosed hydroponic and aeroponic farms to grow food in a much smaller area than dirt farming would. If the benders can generate or acquire enough energy to levitate floating cities, they should be able to provide the much lesser energy needed to illuminate crops.

Or maybe benders can synthysize food from various chemicals.

Do your benders have to work in shifts to keep the floating cities floating, for example? If so, how does the floating city society ensure that there will be enough benders at every time to keep the city from falling to destruction? And does the society use similar practices to ensure that enough benders will produce enough food?

Remember that magic in fantasy often takes the place of science and technology in science fiction, and vice versa.

I can image some science fiction fans singing "anything magic can do science can do better" or some fantasy fans singing "anything science can do magic can do better".

So you might want to read James Blish's science fiction "Okie" stories collected in Cities in Flight and perhaps get some ideas that can be modified for fantasy stories.

And I remember some recent stories discussing how Tolkien's Dwarves could grow enough food in their underground cities.

How can Dwarves produce honey underground?

Giving Tolkien Architecture a Reality Check: Dwarvish Kingdoms1

How many people can you feed per square-kilometer of farmland?2

Answer 3

Based on the description of the conditions that these people live in I would say that there are several ways of sustaining a population on a floating island.

1) Agriculture on a massive scale is not possible due to lack of space but what if your floating landmasses have cave systems which could be used to grow mushrooms? Citizens of these floating cities would need to dig the cave, maintain it and grow mushrooms in a system similar to hydroponics. Mushrooms can grow easily but my family used to grow them on sacks of wood and sawdust. People living in your floating island could just stack these things for a rather large farm.

2) Are hanging gardens a possibility? They could perhaps grow fruits on a massive floating garden which hangs from the sides or bellow the island. A complicated system for harvesting will most likely be required but plants could grow "upside down". They would have access to plenty of sun and water from the air. Nitrogen supplies could be a bit problematic for the plants, but your citizens would need to get them somehow and apply them to the plant roots.

3) Hunting is an option, depending on what kind of wildlife inhabits their world. Are flying whales or any other large flying creature a thing in your world? If yes those floating ships could be used for hunting expeditions. They could hunt large flocks of birds with massive nets.

4) Raids on the ground or the ocean. I know you said going down to the ground is forbidden, but a man's gotta eat. Perhaps a blessing to a special cast of hunters could be given to venture on the ground to forage for supplies? Sending flying ships to hunt for fish would not violate those rules. Expeditions to tree-top farms?

5) Insect farms. Maggots will grow on rotting corpses and you mentioned zombies being a problem in the world. Kidnapping them, putting them down and using them for maggot fuel could perhaps provide some meager sustenance to your citizens.

6) Your floating population can easily get water from air by gathering morning dew and rainwater. They would, however, not have enough water for luxury and would most likely have terrible hygiene.

Answer 4

Fish

That is a surprisingly easy thing to farm. Make it the primary source of food for your nations, and massive buildings breeding them in thousands, and you're free to go. You can feed the fish with many things that is available at high altitude.

Edit: You can also use insects as food, though I do not like the idea :p