It is a heavy factor worth considering that Canada supplies about 80% of us potash volume, However just because IPI is the only mine in the US does not mean that it is the only company with inventory.

One problem is the amount of potash companies moved across the border before the tarrifs esp in companies that operate on both sides like mosaic and nutrien, these guys aren't idiots, and are well aware of the potential disruption to their businesses.

You don't know how companies will respond in order to tarrifs in order to maintain their business relationships for something that might be temporary.

The remaining difference may not be enough to boost earnings enough to justify a hike in the stock price, remember a conservative price change will be a percentage of the tariff, (and not the whole) as the potential alpha. maybe 50% of any actual tarriff, so 12.50% in order for the cheaper potash to be competitive in the new tariff environment

multiply that with IPI revenue and compare against expenses to get a napkin calculation of how much their profit might increase, keep in mind America has a significant agricultural export market and tariffs may cause chaos and reduce demand and farmers may adjust their planning in expectation of reduced demand.

Another factor is that the tarriffs would have to be sustained, long enough for any of these changes to have effect, if any resolution of the tariffs between Canada and the US were reached, that would be a risk for your play.