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In my world, there are two distinct regions close to each other:

  1. fertile and very densely populated
  2. infertile marshlands and very sparsely populated

During the mid-1900s, the cities in the region #1 start to grow very large but this makes the country depleted of the high-quality farmland.

So they start investigating the possibility of massive relocating the fertile topsoil from building lands to the region #2. The goals are:

  1. enable the cities and the economy to grow without limitations
  2. save the precious fertile land

This could be done by trains and trucks, the distance is around 200 km.

There is limited information about this topic on the internet which makes me wonder whether this practice is being done somewhere in the world and whether it is possible.

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    $\begingroup$ Yes. Done on regular basis. Also look for soil conservation, not all links on your venue but there are some. Also one of the conservation of soil is classification of land for farming and which is less good for farming then it is for buildings. But again relocation was done and is done. $\endgroup$
    – MolbOrg
    Jul 17 at 16:18
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    $\begingroup$ A note about marshes: the soil is great but it is too wet to farm. If you put more soil in, it does not get less wet. If you drain the marshes they will be fertile right then. You might need something other than a marsh to be the target area for soil remediation. $\endgroup$
    – Willk
    Jul 17 at 16:20
  • $\begingroup$ Are synthetic fertilizers allowed? $\endgroup$
    – Alexander
    Jul 17 at 20:38
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    $\begingroup$ why not start building on the marshland like new Orleans the marshland probably has better travel access, also you can farm marshland, see rice and cranberries $\endgroup$
    – John
    Jul 18 at 0:20
  • $\begingroup$ The reasons why Region #2 is less fertile may be important. Maybe there's not enough water, or too much salt, or the ph level is extreme, or it's at the wrong altitude. Maybe another monumental project (or two) is needed before you begin hauling soil. $\endgroup$
    – user535733
    Jul 18 at 2:21
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Doable. It's a lot of work, but doable.

The best way to tell if something is possible is to look for laws regulating it. After some googling I found the UK government's rules on moving topsoil.

According to that document, when moving topsoil in the UK care needs to be taking if you're moving it to the garbage dump to sterilise it before dumping, which to me implies that it stays alive and viable during transport.

So - it'll work. This is also confirmed by people selling/trading their soil online. There's classified ads in my state for "I have a massive pile of soil - please come take. Free.", and people collect them. The presence of such a trade implies this works.

You'd need to extract the top ~30cm of ground. Casual googling shows some crops need a minimum of 24cm of good soil to grow, so 30cm seems like a decent minimum. If your city is going to occupy 100km * 100km of good farmable land, that's 3000GL of volume that needs to be shipped. That's way too much for trucks. A rail car can hold 100KL of soil, so you'll need 30 million railway car trips.

If your freight trains can carry 100 carriages per trip, and travel at 50km/hr, it's going to take 8 hours per round trip to move the 100 cars.

It'll take 273 years for a single train to relocate that much topsoil. To do it in a decade, you'd need 28 trains running 24/7 transporting soil. Plus all the loaders getting it from the ground into trucks, and from trucks into trains, and from the trains back into dump trucks, and then dumped on the ground.

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On a small scale, yes... On a large scale, no

One day a honking long time ago my wife (who has a degree in agriculture) and I were driving by Utah's Strawberry Reservoir, which is surrounded by nothing taller than sagebrush, a lot of sagebrush... and precious little else (including grasses). I asked here how a large body of water could be surrounded by such infertile land.

I believe the details of her answer aren't valuable here. But a summary is. In the context of your question, environmentally, there's a reason the marshland is infertile and a reason the land everybody wants to live on is fertile. It has to to with bacteria and the weather and altitude and mountains and the forest development cycle and insects and animals and a whompingly high level of complexity... but the point is, there are specific reasons one area is fertile and another is not.

And if you don't address those reasons, moving the dirt will mean absolutely nothing.

  • Humanity can address those reasons fairly easily on a small scale, like someone's yard or garden. We bring in good soil. We water it. We fertilize it. We keep things away that will harm it. Not that big of a deal on such a small scale — and the dirt is obviously very valuable in this case and economically practical to move.

  • Humanity can kinda address those reasons on a medium scale. We have massive irrigation projects and massive fertilization projects, but to a lesser degree we can't keep the unwanted insects out too well (some, yes, but not most). It's a lot harder to change the environment on a medium scale and that's ignoring the cost of moving in good dirt to replace the bad dirt — which we pretty much don't do, because it costs a lot and the economics simply don't justify it.

  • Humanity today still cannot — to any degree — address those reasons on a large scale (see what many will consider proof that this is wrong below, it's a mixed argument). We cannot change the environment of a desert to a great enough degree that it would make any sense at all to port in a bunch of fertile loam. What you'd have, no matter what we did, was a bunch of dried out bacteria-dead dirt in a year or two. Changing the climate and ecology so drastically to so great an area is beyond our technology.

Conclusion

Given today's tech (2021...), yes, we could move a lot of valuable top soil anywhere we want, but it would cost a boat-load of cash and you could only move it to a somewhat less fertile region or its value would be wasted because we can't control anything more than that.

Therefore, no, what you're proposing wouldn't have value (unless that marshland isn't as infertile as you're question implies).

BUT!!!

This doesn't mean that you couldn't use the idea to forward an important lesson. Let's take the lesson of The Aral Sea.

The Aral Sea (once the fourth largest lake on the planet) was once a famously fertile lake in Russia — until the Soviets got the idea that they could change all those climatic and ecological conditions I was talking about earlier. The soviets wanted (among other things) cotton, which was (and is...) incredibly valuable. So they diverted the two largest rivers feeding the lake to irrigate a desert to grow cotton.

That should have rung a bell (and it is the source of my example earlier in my answer). The Soviets weren't moving trainloads of dirt... all they were doing, all they were doing, was diverting water.

Now, a lot of things went wrong with that particular stunt. The canals leaked. No one cared about what would happen to the bazzillions of people who depended on the Aral Sea for everything from food to jobs. Etc. But the moral of the story is this: Today the Aral Sea is something like 10% of its original size and so salty that it's functionally a dead sea. Lives were destroyed to make way for "progress" somewhere else.

But it should be noted (and in support of your question...) that the project didn't fail so much as it had massive unintended consequences that have persisted for the better part of a century. Cotton is still grown today and still a valuable crop.

So... (let's call this a conclusion to my conclusion)...

Could you do it? Sure, at great cost. Would it work? Well, maybe... kinda... yes, in a way. Would it be believable if you didn't bring the negatives into it?

And that's my point. The answer to that last question is no. People live and thrive on fertile farm country for a reason (yup, another ecological reason) and comparatively don't live in deserts for a reason (let's leave Arizona out of this!). And if you try to make one into the other, it's going to have consequences equal to or greater than the benefits.

And that's the lesson of the Aral Sea.

For every action there is an equal and opposite reaction... Mess with the balance of nature at your own risk.

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Drain the marsh instead

The closest thing Britain has to prairies is the Fens of Lincolnshire, Cambridgeshire, Suffolk and Norfolk.. This was all marshes until an enterprising Dutch engineer cut a sweet deal with the government to drain it. At the time of the English Civil War (1600s, Oliver Cromwell) the city of Ely was an island, and so hard to get to through the marshes that the Royalists retreated there as a last stronghold. It's still called "the Isle of Ely" today, except it's actually just a slightly raised area surrounded by farmland.

Drained marshland makes for brilliant farmland, because of centuries of build-up of fertile peat. There's absolutely no need to bring extra topsoil. However you do have the problem that peat dries out much more easily and then just blows away, so your farmers need to be aware that they must not carry out the same kind of ploughing regime that they'd use on other soils. In England it's taken a long time to get this message across.

By the way, for your timescales, this started in the 1700s and was basically finished by the mid-1800s. By 1900 this was the most significant arable farming area in Britain.

Topsoil isn't very deep

That's why we call it "top" soil. At most you're usually looking at a metre or so before you hit the less-fertile (or completely infertile) subsoil. Usually it's less than that. Sure you can scrape it off and move it elsewhere - but then you've created a patch of infertile ground which needs to build up humus again to form more topsoil.

Use compost and sand instead

If you really need to build up your soil, you have other options. The Dutch used sand (which is not fertile) to bring up the level of polders, and added compost from nightsoil and animal manure. You need plenty of fibrous matter in this, but the result is perfectly acceptable sandy soil. By doing this, you are creating more topsoil without destroying the fertile areas you already have.

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    $\begingroup$ "then you've created a patch of infertile ground which needs to build up humus again to form more topsoil." Which isn't a problem if you're constructing buildings on that patch of ground rather than trying to grow crops. $\endgroup$
    – nick012000
    Jul 18 at 9:41
  • $\begingroup$ mcculloughexcavating.com/… Dredged mud is good soil! $\endgroup$
    – Willk
    Jul 18 at 17:20
  • $\begingroup$ @nick012000 True enough. Or you could construct on the reclaimed land instead. :) $\endgroup$
    – Graham
    Jul 18 at 20:49
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When my uncle bought the land where he wanted to buy his house, he was lucky to find soil for all the 6 meters of depth he had to dig for the foundation, instead of the usual meter or soil and rest of rock. He quickly sold that soil and it helped repay the expense for buying the land.

The only limiting factor might be the transport expenses: shipping soil from Oregon to Japan might be a tad outrageous expense.

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This is totally possible if you want to do it on an industrial scale and you have the finances to do it.

In 2018 Australia mined 899 Mt of iron ore, that's 899 million metric tons in one year. That's drilled, blasted, dug up, screened, transported by rail approximately 600 km to the coast, stockpiled and then shipped thousands of kilometers to China, Japan and South Korea.

Prior to the Covid-19 pandemic and issues relating to mine safety, Brazil mined 450 Mt of iron ore in 2018 and likewise exported it to China, Japan and South Korea - half way across the world.

Between the two countries that's a total production of 1349 Mt or 1.349 Gt (1.349 billion metric tonnes) just during 2018.

Hematite is one iron ore mineral. It has a density of 5.15 g/cm3, which is the same as 5.15 t/m3. Magnetite is another important iron ore mineral. It's density is 5.17 to 5.18 t/m3.

For simplicity, assume an average density of 5.15 t/m3. The volume that 1.349 Gt of iron ore would occupy is,

(1.349 × 109)/5.15 = 0.262 × 109 m3 = 262 × 106 m3

Which is 262 million cubic meters.

From a technical perspective, moving top soil the way you want to is already possible, the question is how much will it cost and are your people prepared to pay that cost. In 2018 the price of iron ore was USD 65 per tonne. For Australia alone, it got 58.435 billion US dollars for its iron ore exports, hence why it was able to move that much broken rock to east Asia.

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