On the other hand, if we reach a point where stockpiling human urine to supply phosphorous for agriculture (as opposed to merely conserving it locally) is economically viable, that implies some pretty scary things about the general availability of food and knock-on effects for general social stability. I'm not sure how much of it we're (literally) pissing into the sewers and whatnot, but I'd be surprised if agricultural runoff weren't a much greater percentage of the total.