Phosphorus. It's in your teeth. It's in your pee. It's in mineral deposits in the ground. It also happens to be a necessary nutrient for plants. Back in 2013, I think that the fears of Peak Phosphorus peaked, with multiple stories in the news about an impending shortage of the valuable element (just google "Peak Phosphorus"!). No P would mean problems for agriculture and, of course, food supplies.
However, people are endlessly inventive when it comes to tackling tough problems, and here's a story I love in Earth magazine about finding ways to recycle pee in order to get P:
'P' is for phosphate: Could urine solve a fertilizer shortage?
http://www.earthmagazine.org/article/p-phosphate-could-urine-solve-fertilizer-shortage
