Peak potash?
Minerals Industry - Energy and Resources - Government of Saskatchewan
Saskatchewan has the largest potash industry in the world, accounting for about 33% of the world’s trade in potash. The potash reserves in Saskatchewan are massive. The exceptional size and quality of ore reserves combine with the lowest production costs in the world. By conservative estimates, Saskatchewan could supply world demand at current levels for several hundred years.
The earth has potash (the K in NPK) reserves for several hundred years. After that, where does it come from? Generally the organic source for potash is from wood ash, but I doubt that burning large areas of forest would be looked upon as a positive by the public.