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In my story an area was formerly beneath an ocean or lake. Some form of life leeched the soil of a specific mineral or substance.

Technology level is that of ~1700 Spain.

Now that the water has retreated over a geological timescale and this lifeform is gone, the soil is still so deficient that crops cannot grow on the land, even with typical fertilisers. The only thing that does the trick is fertiliser made from the life form that originally depleted the soil, as it obviously has a high concentration of this mineral.

The soil actively supports large scale agriculture but only when continuously supplied with a specific fertilizer. This situation has persisted for decades and will continue for the near future at least. The fertilizer is a single point of failure without which agriculture would start to fail.

What form of life would be capable of leeching a lake/ocean like this? Plant, animal and otherwise are all acceptable. Extinct or current makes no difference.

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  • $\begingroup$ Water may do this all by itself. Land reclaimed from the sea is commonly unusable for agriculture until seriously ameliorated, by removing the salt, bringing in soil from elsewhere etc. And there is nothing which cannot be supplied by normal fertilizers; after all, we are able to grow commercial crops with no soil at all. $\endgroup$
    – AlexP
    Commented Apr 1, 2020 at 11:19
  • $\begingroup$ I'm looking for a situation where a specific fertiliser must be continuously supplied, for decades at least, and if stopped the crops will begin to fail. $\endgroup$
    – user72058
    Commented Apr 1, 2020 at 11:31
  • $\begingroup$ Isn't this the normal situation? Nitrogen and phosphate fertilizers must be continuously supplied, or else a smart scheme of crop rotation (including "green manure", i.e. growing legumes and plowing them back into the soil) must be employed (with the same effect), otherwise crops will begin to fail. And it does not take decades. Look up the illuminating and sad story of Soviet Union's Virgin Lands Campaign. $\endgroup$
    – AlexP
    Commented Apr 1, 2020 at 11:55
  • $\begingroup$ This depends a lot on the crops, different crops have different nutrient needs, corn for instance is notorious for depleting soil . $\endgroup$
    – John
    Commented Apr 1, 2020 at 12:07
  • $\begingroup$ Edited question to clarify. The fertiliser supply as a single point of failure is a key plot point. Salt is a candidate, except salt is unlikely to return to soil once the soil has been ameliorated (at least in my limited knowledge, I'll look into it more) $\endgroup$
    – user72058
    Commented Apr 1, 2020 at 12:24

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Your missing element is sulfur.

Here are purple sulfur bacteria.

purple sulfur bacteria https://askmicrobiology.com/purple-sulfur-bacteria/

A closeup: you can see the granules of sulfur these organisms accumulate within their bodies.

closeup sulfur bacteria

These bacteria belong to the purple photosynthetic sulfur bacteria that oxidize sulfide into sulfur which is deposited in intracellular granules in their cytoplasm.3

https://microbewiki.kenyon.edu/index.php/Chromatium

Sulfur deficiency is a real agricultural problem. It is not hard to deplete some soils of sulfur.

sulfur deficiency and non deficient plant

https://keys.lucidcentral.org/keys/sweetpotato/key/Sweetpotato%20Diagnotes/Media/Html/TheProblems/MineralDeficiencies/SulfurDeficiency/S%20deficiency.htm

Sulfur deficiency in soil is more problematic in the US over recent years because of the phase out of high sulfur coal. Burning that coal caused acid raid but the sulfuric acid in the rain helped replete soil sulfur.

This is what you wanted - an organism which collects a nutrient. I will assert here that the accumulated bodies of the purple sulfur bacteria dried up and blew away when the water dried. Maybe they had a burst of growth right at the end and the water turned red.

Now the soil that is there has no remaining sulfur and crops are stunted. If this red growth can be skimmed from other waterways, the sulfur in the bacteria can relieve the deficiency of the soil.

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  • $\begingroup$ There were some very good answers, but this one very closely fit the question. A well earned tick. $\endgroup$
    – user72058
    Commented Apr 5, 2020 at 22:30
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You may have the opposite problem.

Land "reclaimed" by nature from shallow, low- or non-flowing water by a natural process called eutrophication (filling in with vegetation that forms mats and eventually replaces the water with vegetable matter) may be poisoned with hydrogen sulfide or other sulfides. Hydrogen sulfide trapped in the soil would make the land stink, but it might also reach local concentrations that could be lethal to animals who graze there, or farmers who attempt to till the area. Hydrogen sulfide and other sulfides, in an oxidizing environment, can form sulfates when rehydrated, leading to acid soil that inhibits growth of many crops.

Acidic soil can self-renew acidity, especially if it's porous and has some circulation of ground water. In this case, it might be impossible to raise some crops without continual addition of a buffering agent like chalk or soda ash.

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The soil actively supports large scale agriculture but only when continuously supplied with a specific fertiliser. This situation has persisted for decades and will continue for the near future at least. The fertiliser is a single point of failure without which agriculture would start to fail.

Apart from calling it a fertilizer, you are basically talking about water. Since the beginning of agriculture, no crop can grow without water, and this situation will last probably as long as we human will be on this planet.

We can grow crop without soil using hydroponics, but as the name suggests we need water for it.

And plants/trees, with the humidity they release through their leaves, are an important part of the water cycle. And by doing it they deplete the soil from water.

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