The whole gambit fails on things with inelastic demand and no alternative sources.

That said I think there are alternative sources for potash, Canada is just the largest supplier globally (around a third, I'm reading) and by far the largest supplier of the USA.

Then it probably starts to matter how the price balances when one supplier can deliver but with higher shipping costs, and the other has lower shipping cost but a high tariff. If the tariff is high enough it would logically still depress the imports from Canada a bit.

But it seems like a really stupid economic warfare move, because that for sure hurts the USA more than Canada. That said, this is a stupid administration in the USA, so who the fuck knows.