I'm guessing it's because (a province in?) Canada is threatening to withhold potash shipments to the US if Trump's tariffs go into effect. Potash is a fertilizer.
I see.
If I remember, it's more than that, we extract potassium from it for a lot of different usage. But yeah Potassium rich soil is a common fertilizer.
There's also a Larry Niven sci fi novel that sort of revolves around potassium in many points of the story. I really like it, I've re-read it a few times.