Many societies, ancient and contemporary, have innovated ways of supplying their fields with fixed nitrogen and phosphorus—two crucial ingredients for crop productivity. One is crop rotation, which alternates nitrogen-fixing and nitrogen-exhausting crops. Farmers around the world make use of chickens, ducks, and geese to add “fresh” guano to their fields. Cattle manure is another useful alternative—although it often lacks in phosphorus. Much more labor intensive than simply adding fossil-fuel derived synthetic fertilizer, these practices tend to build up soil, limit greenhouse gas emissions, and lead to less run-off into rivers, lakes, and oceans.
Persian pigeon towers are one of the more elegant solutions to the nitrogen-phosphorus problem. These are essentially castles built for thousands of wild pigeons, strategically placed in the middle of the fields. Their droppings were shoveled up once a year and sold to nearby farmers. While most pigeon towers existing today are in disrepair, the oldest still standing are dated to the 16th century (but they are assumed to have existed over 1,000 years ago) and helped fuel the cultivation of Persia’s legendary orchards, melons, and wheat production. [Read more…]