Has the USA's potash supply been reduced due to strained relations with Canada? They are our top supplier, by far.

Fertilizer is pretty fungible and is a global market, so even if the US is primarily supplied by Canada, and overall global demand remained constant, prices would go up since there will be supply reduction due to the Hormuz strait being closed.

US and Canadian production is largely irrelevant to the price. These are world comodities. If worldwide production drops, prices rise. As with oil/gas producers, domestic potash producers are under no obligation to sell locally. If prices are higher in europe/asia/africa, that domestic potash will be loaded onto ships until domestic prices rise to match.

Are you forgetting the nitrogen? :)

A lot of crops need nitrogen. What has been impacted by Trump's Iran war is the supply of Urea through the Straight of Hormuz.

If the closure persists then no doubt other sources can ramp up to fill the void, but it's going to be too late for this season. Some Asian farmers have already chosen not to bother planting rice crops since the increase in fertilizer (urea) cost has meant they'd be losing money.

Fuel prices are also impacting imported produce prices.

Yes. Despite what others have said, yes. But, in general, because of the current global dynamics, fertilizer is more expensive wherever you're going to be getting it from. It just doesn't help that the US has picked a trade war with all allies at the same time, while also engaging in real wars that disrupt global supply chains of critical resources.

It's the nitrogen fertilizer almost all of which is manufactured from methane + air.