The hilarity of the potash tariff underlines how little he understands about anything.
Okay...the current cost of potash is 318 USD per ton. I think that anyone can immediately see a potential problem. A ton is a lot. 318 dollars is not. So a massive driver in the potash market is going to be transportation cost. It costs about .16c CAD per ton mile to transport by rail, so a 500 mile journey from Sask to the US farming belt is going to cost roughly $57 USD for transportation on your 318 dollar ton of potash. The cost to bring in that same ton from Russia is going to be somewhere in the range of 380-450 USD per ton...just for the shipping, and ~700-800 USD per ton total.
A 2016 PAS study showed a 44% increase in food prices for every doubling of fertilizer cost and this would double the cost of fertilizer more or less by itself.
So I said that to say this, there's no alternative to Canadian potash. There is no domestic American potash industry waiting for the evil Canadians and their protectionist ways to be vanquished before they ride in with their guns, jesus and capitalism to save American's money at the dinner table. A tariff on potash will not impact demand, it will just raise prices. It's a farming tax on American farmers, so ultimately American consumers, and doesn't impact Canada at all.