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The setting is a city enclosed within a safe boundary that protects it from a magical blight. This blight kills all animals outside the boundary when the sun is down. Farmers live inside this boundary, and travel outside to their farms during the day, returning before the sun sets.

While I imagine some parts of the ecosystem can adapt to this, such as birds, it poses problems for localised parts of the ecosystem that do not have the capability for large daily migration patterns, such as worms and other creatures that improve soil quality, which prompted this question.

Restraints:

  • The demand for a safe place to sleep means that land inside the boundary is at a premium. Simply farming inside the boundary is not practical when there are large swathes of unused land outside the boundary.
  • 'Animals' does not include plants, or micro-organisms. The plants live on at night, as does bacteria and the like.
  • Technology level is the standard medieval-ish level.
  • Magic exists, but for all intents and purposes is unaccessible to the farmers.
  • Climate is undetermined at the moment, if required it can be whatever suits your answer.

With that in mind I'm wondering:

  • Is it even possible for this to be viable farmland?
  • If it is possible, what crops are best suited to these conditions? (ie. ones that don't need to be pollinated by insects, ones that are best suited to low soil quality)
  • If it isn't possible, are there adaptions the farmers could make to make it viable? (extra fertiliser? Portable insect farms? Does leaving the fields to fallow have no effect if there is no life in the soil refreshing it?)
  • As a bonus, how many hours a day would be needed to work the land to produce crops under these conditions? (This restricts the amount of farmland is available, since a farmer has to be able to walk out to the field, work, and return to safety during daylight hours)
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  • $\begingroup$ how can you farm plants that die every night? $\endgroup$ – John Nov 29 '19 at 2:19
  • $\begingroup$ I understand 'living creatures' to exclude plants, compared to 'living things'. I've updated the question to clarify that the blight does not effect plants. $\endgroup$ – Asher Nov 29 '19 at 2:23
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    $\begingroup$ @SZCZERZOKŁY to provide accomodation for people who'd like to live to see tomorrow morning? Seems a fairly useful purpose. $\endgroup$ – Starfish Prime Nov 29 '19 at 10:51
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    $\begingroup$ @SZCZERZOKŁY it very obviously does not boil down to that, as it is about farming in the absense of any but temporary and tightly controlled animal life. That's quite a different issue, and quite a complex one. $\endgroup$ – Starfish Prime Nov 29 '19 at 11:04
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    $\begingroup$ @SZCZERZOKŁY if you take the trouble to read the question, you will see that it does not ask "how much food does the city need", and as a result the size of the city is supremely irrelevant. It asks if farming under these circumstances is possible, and those circumstances are totally unlike farming in space. You have read the question, right? $\endgroup$ – Starfish Prime Nov 29 '19 at 12:34
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When this magical blight first appears, it will be incredibly disruptive to the biosphere, and if it happens suddenly (in evolutionary time, ie less than hundreds or thousands of years) there will be mass extinctions both of animal species which don't reside in the safe zone, and plant species which rely on such animals for pollination.

Assuming that this construct has been in existence for enough time to stabilise (or the humans did some very speedy ecological engineering!), there will be a hierarchy of land surrounding the safe zone. The land closest to the boundary will be by far the most productive, because it can be fertilised by animals grazed out during the day and herded in at night; and pollinated by bee hives maintained within the safe zone. It will also be the land which can be most intensively farmed because it requires less commuting time for farm workers. There will be intensive competition between arable and pastoral farming in these areas, but since the greatest range that pollinators like bees will travel seems to be about 5km it would make most sense to grow arable crops in this immediate vicinity, with wind-fertilised grass pastures beyond for animals that are herded in each night, out to the limit of what land can be accessed in time.

A human can walk at about 5 km/h, so the absolute maximum radius for this 'tillable zone' is about 41km; but fields this far out would only be accessible for an hour a day for one month of the year, hardly intensive farming. The furthest a field could be from the boundary to still be reachable throughout the year would be 9km.

Farming, especially using medieval technology, was a very time-intensive occupation, with farmers labouring from before sunrise until after sunset every day. This would be inevitably curtailed by the 'curfew', but the intensity of farming would be reduced as a result.

In the UK we get around 4,380 hours of daylight in total per year, distributed cyclically which we can naively model as:

$$Y_0 = \int\limits_{0}^{365}{4.5 \ sin(\frac{2 \pi x}{365} + 12}) dx = 4380$$

If we equally naively assume that the yield of a piece of farm land is directly proportional to the number of hours spent working it, and that medieval farmers will work every daylight hour they can, then the yield of a piece of land at distance $R$ from the boundary is:

$$Y(R) = \int\limits_{0}^{365} \max \left( 0, 4.5 \ sin \left( \frac{2 \pi x}{365} + 12 - 2 \frac{R}{5} \right) \right) dx $$

Which you can see here plotted from the spring equinox. The flat red area is the zone unreachable at that time of the year.

Assuming that the boundary is circular with radius $r_0$, then the total yield of the annulus of reachable land around the boundary is:

$$Y = \int\limits_{r=r_0}^{\infty} \int\limits_{\theta=0}^{2\pi} r \int\limits_{0}^{365} \max \left( 0, 4.5 \ sin \left( \frac{2 \pi x}{365} + 12 - \frac{r}{2.5} \right) \right) dx\ d\theta\ dr $$

Wolfram Alpha will helpfully solve this crazy integral, and gives me that $Y$ out to a distance of 20km (comfortably the 'zone of influence' of a medieval village) is about 55% of the 'normal' level of working (where farmers are distributed across the land such that they don't have to commute). Out to 40km (the point where the infected land becomes basically unreachable) the overall workability falls to less than 23%; but within the 5km 'pollination zone' the workability is about 88%.

In short, the farming yield of a village in this situation would be reduced by (at least) somewhere between 15% and 50%, with some alterations needed in distributions of farm types. On a purely logistical level, this isn't complete deal-breaker for the survival of the settlement.

Of course there are lots of other reasons why an isolated medieval settlement like this is not viable, of whatever size; external resources like metals and fuel will be quickly exhausted: timber in particular will be an extremely dangerous commodity to harvest, as lumberjacks will need to travel far to the retreating forests and then return with heavy loads as the sun sinks ominously. But you asked specifically about farming, and from a logistical standpoint at least, it's not impossible.

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    $\begingroup$ Yeah, fuel seems likely to be a bit of a killer, doesn't it? And that in turn puts some significant pressure on growing crops that can be eaten and provide reasonable nutrition without cooking. +1 for that observation alone, though the triple-integral for a medieval farming problem is worth it, too ;-) $\endgroup$ – Starfish Prime Nov 29 '19 at 17:23
  • $\begingroup$ fruit bearing plants are screwed as well. Although they would be a good choice for outlying areas since they don't require much upkeep without animals to eat the fruit, so people only need to travel out to them for harvest. $\endgroup$ – John Dec 1 '19 at 2:30
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Some ideas:

  • Only animals bigger than a certain size or mass die. Small animals like earthworms or insects survive because they are too small for the magical blight. Plants may survive because their cell structure is different to the cell structure of animals. (The magical blight is some kind of radiation)
  • Only warm blooded animals die. Very small animals are ectotherm, so they don't attract the magical blight. Plants are obviously cold, too. (The magical blight is some kind of bacteria)
  • Only animals with lungs die. Insects, earthworms and plants don't have lungs (The magical blight leads to a massive reduction of oxygen in the atmosphere)
  • Animals which sleep under the surface of earth survive. (The magical blight is some kind of radiation)
  • Only animals with a certain brain size die. The magical blight might be some visuals or sounds that drive people and animals crazy, so they die from a heart-attack.

Most of those ideas would lead to the same farming conditions that existed in the real medieval.

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    $\begingroup$ Could the blight be a widespread nocturnally activated fungi which extreme adaptation, besides the nocturnal part, would be to rapidly expell vast quantities of carbon dioxide? If the fungi is in symbiosis with the root system of the dominant crop, then that could be why there is a limit to its area of effect. $\endgroup$ – Dale Gusta Nov 29 '19 at 13:18
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    $\begingroup$ That's a good idea. CO₂ is heavier than normal air, so it will stay on ground level. You need a concentration of at least 10% CO₂ to kill humans in a few hours. Of course, all the CO₂ needs to be soaked up during the morning hours, so it will be safe to go out during the day. During the night, the funghi lose the ability to hold he CO₂, so the blow it out. That would even help all other plants to grow faster, so it is a good explanation on why the fields are not located inside the safe area. $\endgroup$ – Dorian Nov 29 '19 at 16:07
  • $\begingroup$ Insects and earthworms might not have lungs, but they still need oxygen. Plants too if I remember correctly. $\endgroup$ – Ryan_L Nov 29 '19 at 23:11
  • $\begingroup$ Yes, the insects and earthworms need a bit of handwaving. Maybe they can survive a higher CO₂ concentration than other animals or they hibernate at night. Plants produce oxygen from CO₂ by photosynthesis. At night, they also need a little bit of oxygen, but they need far less then animals and they're happy with a high CO₂ concentration. $\endgroup$ – Dorian Dec 2 '19 at 8:12
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All the obvious answers have been provided so I'll throw in one from left-field:

Burrowing animals caught above-ground fertilise the plants.

The flora on this planet have adapted to the huge abundance of nutrients provided by animals dying in their vicinity. In this way, they have evolved barbs for stopping burrowing animals from escaping beneath the surface to escape the magical blight. This has examples in real life in the proto-carnivorous plant "Puya Chilensis" (sheep-eater plant) which gets sheep tangled in its barbs, resulting in them starving to death and rotting next to the plant, providing biomass to the soil. These plants are cultivated by humans as well, as they are useful for making fishing nets.

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Dead animals are processed to make fertilizers for the plants. Smaller ones are left to decay. Larger animals are either harvested for meat when freshly dead or processed to make fertilizer or fodder ingredients. Leaving large dead animals is not sanitary and may spread diseases.

Beehives may be relocated to the safe areas so that the bees return to their hives before sunset. The path of bees is fairly predictable as they return to their hives before sunset. Other pollinators are not.

Size limit was not specified, but saying that microorganisms can survive means there is a size limit somewhere. Are earthworms and pollinators safe? The boundary limit is not specified: How deep the blight effect goes under the soil level? I assume there is a limit here. Is it effective to build animal shelters underground, or under a mound? So, set your own size and depth limit.

In order to be able to meet work deadlines, the farmers will divide their field into two plots: the one nearer to safety will be dedicated to the labor-intensive crops: those that need lots of attention, mainly vegetables. Further out, those that are in a less need for intensive care, mainly fruit trees.

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  • $\begingroup$ Fertilize? The breakdown of amino acids, by removal of nitrogenous group - NH3, from the decomposition of animal matter to provide nitrogen by a myriad of microorganisms involved in the nitrogen cycle. Symbiosis between plant and microorganism is what helps the plants grow. $\endgroup$ – user70311 Nov 30 '19 at 7:36
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Hydroponic farms

The basics of hydroponics are very simple: instead of soil your plants grow with their roots hanging in nutrient-rich water.

With some effort this is achievable with medieval technology. No animals required.

Some questions:

But how much food can you produce this way
Enough to moderately feed a small population, the Netherlands is one of the largest producers of food in the world mainly due to this method.
But there won't be pollination!
true, no pears then, but there are plenty of self-pollinating plants or plants not grown for their fruit Where do you get your nutrients?
Humans and their livestock do this thing called "pooping". turning this into hydroponics-suitable nutrient slurry would be part of the "some effort" part of my answer.

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  • $\begingroup$ You seem to be assuming that a) all plants can be grown hydroponically, b) no plants need to be pollinated by animals and c) nutrients can be handwaved into existence... $\endgroup$ – Starfish Prime Nov 29 '19 at 13:07
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ones that are best suited to low soil quality.

Firstly, the fact that the soil will be accumulating dead animals already makes it rich in amino acids. Deamination of these amino acids that leads to ammonification provides a nitrogen rich environment in the soil. After nitrifying bacteria have converted ammonia into nitrogen that is useable by the plants. Rice paddies (depending on the location), lettuce, cabbage, rhubarb, brussel sprouts, kale, mustard greens, beans, etc. all require a sufficient amount of nitrogen for prolific growth.

As a bonus, how many hours a day would be needed to work the land to produce crops under these conditions?

The presence of organic matter (cellulose, lignin and pectin from dead plants; proteins from dead animals; chitin from skeleton of insects; polysaccharides from cell walls of fungi and other microorganisms; and finally urine and faeces) in soil leads to the phenomenon of humus. Insoluble humus to be exact - consists of unrecognized parent material of organic matter, residues of microbial decomposition of plants (that will eventually die) and animals.

Insoluble humus:

  1. Improves soil structure
  2. Facilitates slow release of nutrients
  3. Increases water holding capacity of soil

Humid material makes up 10% of soil weight, thus, tilling might not be required for insoluble humus.

Soluble humus provides the nutrients the insoluble will facilitate slow release of.

Work for farmers - providing urea (urine or faeces (hey, recycle that waste)).

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