The idea of someone sprinkling urine on building rooftops sounds like a major health hazard, but that’s just what Amsterdam’s water utility, Waternet, recently proposed. Though it sounds like a cruel prank, the project is actually part of an effort to find cheap, natural alternatives to traditional fertilizer. In order to flourish, Amsterdam’s many green roof tops need lots of nitrogen, potassium, and phosphorus, substances that some say could become scarce by 2030. Knowing that human urine contains a lot of all three of these nutrients, Waternet recently set up a demonstration project to harvest pee from public urinals. It hopes to process what it gleans into an upcycled fertilizer for public gardens and vegetated rooftops around the city.