In the air, nitrogen consists of two atoms locked tightly together. Plants need those atoms “fixed,” or compounded with some other element: ammonium oxalate, for example, as found in guano, also known as bird poo, or potassium nitrate, also known as saltpeter and a main ingredient of gunpowder. Reserves of both guano and saltpeter were found in South America, mined, shipped around the world, and dug into soil. But by the century’s end, experts were fretting about what would happen when these reserves ran out.
