Nitrogen-based fertilisers have banished hunger in the rich world and ushered in an era of abundance. But they are a double-edged sword - the glut of food also comes with a glut of nitrogenous pollution that threatens to destroy our rivers and oceans. In our latest programme about the elements of the periodic table, Professor Andrea Sella of University College London tells presenter Justin Rowlatt why exactly our crops - and we humans - could not survive without nitrogen.The BBC's Washington correspondent Rajini Vaidyanathan sees - and smells - first-hand the denitrification of raw sewage, and hears from water scientist Dr Beth McGee of the Chesapeake Bay Foundation about the eutrophication of America's largest river estuary.And, Justin travels to Norwich to meet Giles Oldroyd of the John Innes Centre, who is seeking to genetically engineer cereal crops that can fix nitrogen from the air. He also meets farmer David Hill, who explains the hi-tech lengths he goes to in order to squeeze the maximum yield out of his fertiliser.