I think trump's tweet was more about scaring Canada to be honest. From what I understand, the US gets about 90% of its potash from Canada, produces a small amount domestically and has a few small international sources. To replace what they get from Canada would require more than just a Belarusian supply. They would also need supplies from Russia and the middle east combined to fully replace it.
Potash is the single biggest lever Canada has in trade negotiations and trump is finally realizing it. Its essential, and there isn't another ready source big enough.