POT's biggest problem (Mosaic, too) is the increased competition that is entering the fertilizer field.
With prices for nitrogenous fertilizer at such stratospheric levels, one can reasonably expect that trend to continue. The current players may enjoy phenomenal spot pricing (and even forward pricing) for a while, but don't expect that trend to continue indefinitely as others seek to enter such a lucrative field.
Another big downside risk is that these stocks are parabolic and priced for perfection when no one can really know what will happen with public policy on shifting sources of biomatter for ethanol (corn to sugar cane? sugar can to cellulosic?), and whether recent drought patterns in many parts of the world will hold up or not.
Also, input costs to make potash based fertilizers are rising dramatically.
And China and other, more centrally-planning aligned economies, will probably seek to seed domestic-based fertilizer producers to alleviate dependence on foreign sources of the 'stuff' needed to support something as vital as food production.
Crop rotation, weather patterns, massive U.S. subsidies for corn-based ethanol, other government subsidies for rice - these are all variables that can kneecap these high fliers in a heartbeat if they change suddenly.
Not every aspect of the outlook for POT and MOS is coming up roses.