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.