Big difference is that phosphate and potash deposits are not...

Big difference is that phosphate and potash deposits are not relatively widespread like iron ore and uranium deposits. We would've seen plenty of action already if they were.
Look for other ag inputs...chemical manufacturer and supplier NUF - glyphosate has nearly quadrupled in price since this time last year. Check their sp.
It is a relatively illiquid sector for share investors. Most gains this year will be to those in the futures market, the limited listed input suppliers, and to the family run grain farms throughout the areas which receive enough rainfall to have an average or above average year.