Not to be entirely glib, because the issues around phosphorous (and oil, and everything else we’re running out of) are serious, but let’s no longer put “peak” in front. Instead, let’s assume, until told otherwise, that we’re running out of everything, and things that we’re not running out of are the things that should be prefixed.

But I digress. So, time to worry more about phosphorous:

From Kansas to China's Sichuan province, farmers treat their fields with phosphorus-rich fertilizer to increase the yield of their crops. What happens next, however, receives relatively little attention. Large amounts of this resource are lost from farm fields, through soil erosion and runoff, and down swirling toilets, through our urine and feces. Although seemingly mundane, this process cannot continue indefinitely. Our dwindling supply of phosphorus, a primary component underlying the growth of global agricultural production, threatens to disrupt food security across the planet during the coming century. This is the gravest natural resource shortage you've never heard of.

More here. And more on the same subject from 2008.

Source: http://paul.kedrosky.com/archives/2010/04/phosphorous_yes.html