"You've heard of peak oil—the idea that the globe's easy-to-get-to petroleum reserves are largely cashed, and most of what's left is the hard stuff, buried in deep-sea deposits or tar sands. But what about peak phosphorus and potassium? ...These nutrients, which are essential for plants to grow, are extracted from soil every time we harvest crops, and have to be replaced if farmland is to remain productive."
"Are We Heading Toward Peak Fertilizer?"
Source: Mother Jones, 12/03/2012