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I have a story in which a King, knowing his kingdom will fall, would like to have a scorched earth policy. The King would order all residents to begin salting the earth to ensure nothing there can grow. The city would have plenty of salt nearby as it is situated near a salt mine (I'm personally imagining Salzburg). There's some historic accounts of "salting" cities to starve people so it was thought about in the classical/medieval eras but all accounts seem to be very vague and improbable.

Let's say you were to salt a 5km radius from a city. That comes out to nearly 20,000 acres. And for a larger area, say 10km radius from a city that's an insane 77,000 acres to have to salt. And initially I wanted it a 20km radius which is 310,000 acres.

I'm not entirely sure there was enough salt throughout the entirety of pre-industrial earth to be able to salt anywhere near that much, even if only half was farmable land.

But the problem is any smaller and I can't see it being that large of an impact.

The question actually becomes three fold:

  1. How much land realistically would need to be salted around a city to starve residents?

  2. How much salt would this require?†

  3. Even with a literal salt mine would it be realistic for an average sized kingdom to already have/have to produce that much salt within a reasonable time frame (say in under 3 months before the invasion)?

†I know different soils and different plants react to salt differently, so for simplicity assume they only grew wheat (or any other grain) in whatever soil can support the grain.

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    $\begingroup$ Are you really, really sure you need hard-science on this? Wouldn't science-based suffice? It will be quite hard to find scientific papers on availability of salt around a city you just imagined... $\endgroup$ – Mołot May 11 '18 at 8:42
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    $\begingroup$ "The King would order all residents to begin salting the earth to ensure nothing there can grow", OK, but... Why would they obey him and destroy the value of their land? Kings come, kings rule, kings fall, nothing special. Especially in medieval times, when there was no such thing as "patriotism" or allegiance to a nation? Such an order seems the best way for the king to speed up his demise. After all, the farmers don't really care that much to what particular king they pay taxes. $\endgroup$ – AlexP May 11 '18 at 8:58
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    $\begingroup$ Have you read Wikipedia's page on "Salting the Earth" ? I don't believe it was ever done as you describe. The Roman's are said to have done this to Carthage, but given the extreme value of salt ( this answer on History SE equates it to gold ! ) it's extremely unlikely that anyone would use it as you describe. $\endgroup$ – StephenG May 11 '18 at 8:59
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    $\begingroup$ @Thatguypat If you destroy land, you are already dead next season, with death more painful than a sword: hunger. on the other hand, invaders need someone to grow food on invaded lands. So your assumption does not help, it is still better to overthrow king and surrender to invaders. $\endgroup$ – Mołot May 11 '18 at 9:14
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    $\begingroup$ "Invaders would kill everyone who remains" is highly unrealistic. Nobody killed farmers, not even the Mongols. Farmers are the basis of production, you want them alive and growing crops to feed your noble warriors. The Middle Ages were a time of low population; people were one of the most important assets of a kingdom. Most European countries had lots of uncultivated land simply because they did not have enough farmers. $\endgroup$ – AlexP May 11 '18 at 9:24
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Wow, I like counting. Soooooo: Sowing by hand usually takes one handful every two steps. The range would be approximately 1,5 metres wide. Assuming two steps are 1 meter one handful would be enough for 1,5 square metres. Handful of salt (the rock one, like Himalayan salt) is around 50 grams.

So you have 310,000 acres which is 1254527427 square metres. Divided by that 1.5 square metres give you 836351618 handfuls needed to cover the whole area.
$$836351618 * 50 grams = 41817.58 tons$$

Now some history, medieval salt mine Wieliczka excavated around 7000-8000(modern) tons a year source in polish. So 5 times less what would you need. But enough for that 5 km circle.

BUT
What you need to take into consideration is the place and reason for salting the earth. It actually make no sense in salting earth around city:

  1. enemy is at the gates, he have whole country behind him to pull resources from.
  2. Salting earth was a tactic during march of the army where resources were depleted, reserves were too far away and army couldn't pull food from earth. Imagine a game where your sorcerers use all his mana on spells, there is no mana potion around, he used everything he had stored and then he meet the boss.

  3. Salting the earth was more of a cheap way to poison water reservoirs. With rain the salt was transferred to wells and creeks making them unusable for humans and animals. Usually war wagons were caring only food as armies relied on water sources on the way. Also it was cheaper to just scorch the earth. Just one torch and horse and in one day you could kill food resources on dozens of kilometres. Plus with good wind you could set fire trap against enemy.

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  • $\begingroup$ Also, it does not have as long of a negative impact as staking your fields would. Because once the war is over your people still have to eat $\endgroup$ – Garret Gang May 11 '18 at 14:03
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Don't use salt, use seawater. It's free, it's plentiful and it has 35g of salt (mainly sodium chloride) per litre. Only salt tolerant plants can live close to the sea, where the wind will blow a bit of spray now and then, so any crop which can't survive in a saltmarsh will die. Downside is you'll have to lug barrels and barrels of seawater around, so you'd better have lots of ox carts and horse drawn wagons on hand.

Don't use salt, set the crops on fire. Do an actual scorched earth. You don't have to spend months preparing for it - you just send your trusty soldiers galloping around with their flint and steel to set fire to fields and granaries. Of course it won't work if its been raining for the last fortnight, or if your crops grow in paddy fields.

People don't like doing things which will cause their children to starve to death. They'll be refusing to do it, holding a peasants revolt, pretending to obey but actually scattering sand instead of salt, etc. And having a secret tract of land which they didn't salt, in the hope of having enough food for their family to survive.

People can spot a way to become rich. Hey if all my neighbours' crops die and mine don't, my grain will be worth a fortune!

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    $\begingroup$ The Mongols at one point ran seawater through their conquered enemies' irrigation system, before destroying it. You'd still have to haul it uphill from sea level. $\endgroup$ – Davislor May 11 '18 at 18:11
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Wheat is relatively salt-tolerant; this source appears to suggest that 150 mM of salt per meter (I assume that's square meter) will reduce wheat biomass production by 50% even for resistant wheat strains.

That's almost 9 grams per meter^2. 9 metric tons per square kilometer.

Other things equal, clay soils are more affected than sandy soils, and they would already have reduced yield. There will be other salts already present which partly count so you don't have to add that much salt.

By comparison, modern corn farmers typically add 5 tons of ammonium nitrate per square kilometer. It isn't a tremendous amount by modern standards. But then, a modern nation that wanted to destroy cropland could seed it with radioactivity. The land doesn't have to be very radioactive before it will be considered valueless for farming.

The paper makes it look complicated. Less water means it takes less salt, but without enough water, yield would already be low. Some places, too much water means that saline groundwater gets high enough to hurt roots. Ignoring the complications, it's a ballpark figure of 9 tons/km^2.

(Does that make sense for Carthage, doing it with oxcarts and hand tools? I'm not sure. Wikipedia says there are no ancient sources for the story, but it was a legend as early as 1299 AD. Sowing salt over the city itself would be a cheaper symbolic gesture.)

For a hi-tech society, this is no big deal. If it's time to add fertilizer, adding salt instead is no harder. If you expect to be genocided before harvest time, then there's no particular reason not to.

http://plantstress.com/Articles/salinity_i/salinity_i.htm

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That depends on a myriad of other factors, how much excess food does the city produce? how much food is already stored in the city? What type of Salt mining is going on in the Kingdom? Shaft or Solution Mining? how many people live in the city. how is this mined salt stored?

Plants get their water through osmosis, this means the ratio of salt to water is higher in the root than the ground soil, so just by adding maybe about $100\space\text{grams per m}^2$ would be more than enough to change the ratio for most crops (Yes these are ball park numbers and based off logic not actual tests, and this is for most crops but potentially not all), however this also requires the salt to be dissolved by rain water, but if this number is accurate, then an acre of land would be $$4046\space\text{m}^2.$$ Call it $4000$ to make maths easier. So you would need $$4000\text{m}\times 100\text{g} = 400,000\text{g}$$ So about $400\text{kg}$ per acre...

Roughly $7\text{ billion}$ people on just under $4$ billion acres of arable and permanent cropland. You can more than triple this figure if you include land for cattle to graze to then be slaughtered for meat. This is about $0.5$ acres of farmland per person, or over $1.5$ acres per person if we include cattle land, and this also doesn't include the sea based food the world eats so one could comfortably extend that to 2 acres per person per year...

This is aslo with some fairly advanced farming techniques that were definitely not available in the medieval times, but these days a lot of food is wasted and some of that cropland is used for growing crops specifically for cattle or other farm animals, so that number could possibly go even higher per person... but lets assume for a minute that it doesn't...

if an average medieval city housed, say, $50,000$ people before the war, that would go down to about $35,000$ after all the men were off fighting or dying for their mean old king... so... here we go again

$$35,000\text{ people} \times 2 \text{acres/person}\times 400\text{kg per acre} = 28,000,000\text{kg}$$ But thats so that everyone would starve... that sort of answer. Point 1

So the question remains... how many people does your Kingdom support therefore how many acres of land would be needed to support these people.

Excess Food

If the city normally has about $10\%$ excess of food, this excess is normally traded away, then by simply salting $20\%-30\%$ of the farmland is likely to cause starvation for much of the populace this lessens the amount needed albeit $20\%$ of $28,000$ tonnes is a meager $5,600$ tonnes still one hell of a lot to be stored unused... this sort of answer point 2

The Farmers

Remember very few kingdoms in the medieval times had permanent armies, most were men that had jobs and were conscripted in times of war. If the King is about to lose, then it suggest that many of the farmers would logically already be dead or encamped elsewhere fighting the war for their king, and therefore would not be home to tend to their crops, therefore the yield of those fields would be a lot less, before the salting even began

Stored Food

Most cities in medieval times couldn't last on their stored food for more than a year, so rather than salting the earth, simply burning the crops to the ground would cause starvation for the populace, as it would take another full season for those crops to be grow, providing there was even a stock of seeds to do so

Stored Salt

Salt is hydrophilic, this means it actively absorbs water and therefore is not stored correctly then it will absorb moisture from the air, and begin the process of dissolving away into a Salty Brine that would then leak away

Type of Mining

If Shaft mining is the type used, then no realistic amount from medieval times would be enough to salt all the land to the point where no crops can grow, this unfortunaetly answers point 3

If it is solution mining, then an alternative is fairly simple, have the pumps that bring the Salty Brine to the surface be pumped directly onto the farmland, or have the the king could organised basic pumps to flood the farmland with salt water, sea water wouldn't be as potent as raw salt, but the Brine would be, and both far easier to do, and dependent on the terrain of the kingdom, may be able to flood much larger areas then you are hoping for, however would it be enought? doubtful

Another option is to pump the brine solution or poor the salt mine stores into the "presumably Nearby???" freshwater lake or river, this would (again dependent upon how well it was performed) turn the kingdom's fresh drinking water into salt water. potentially far more damaging to the invading force then even salting the farmland. however it would not be a permanent thing, unless it was a truly massive amount into a lake, then the river would eventually clear the excess salt

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  • $\begingroup$ Why does "sea-based food" increase the number of acres of arable land per person? You can't salt the sea. $\endgroup$ – Pink Sweetener May 11 '18 at 10:28
  • $\begingroup$ @PinkSweetener.Bbecause the math on acres per person is based on 7 billion people being fed by (including cattle/animal land) 12 billion acres 1.5 acres per person, this is the current world situation (roughly), but a lot of the world is fed at least partially by the sea, if this kingdom does not have access to the sea then land farming will need to make up this difference, therefore more acres per person, hope that makes sense $\endgroup$ – Blade Wraith May 11 '18 at 11:33
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I’m not sure that actual salt was used to ‘salt’ the land. Certainly it’s unlikely to have been anywhere near pure, as for much of history, salt has been expensive - the Romans famously paid their soldiers with salt. So any ‘salting’ would likely use inedible rock salt at best.

More realistically, here’s a description of a ‘scorched earth’ policy from the Bible: “They destroyed the towns, and each man threw a stone on every good field until it was covered. They stopped up all the springs and cut down every good tree.”

(Woah, font size changed when I pasted and can’t undo it on mobile...)

The stones from the town walls were carried and thrown on the fields; I wouldn’t imagine them literally covered with no soil showing, rather that there’s enough stones to prevent ploughing. Clearing the land of stones would then be a large job, and rebuilding the walls also, as you’d need to wander the area to re-collect the stones. The springs would probably be filled with the stones also, and the ‘good’ trees - probably the fruit trees - cut down.

It’s entirely feasible to significantly damage the fields around the city to make Re-developing it very difficult, but doesn’t require salt.

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