“best. In consequence, the overall trajectory of farm yields was relentlessly downward. Corn harvests in upstate New York went from thirty bushels an acre in 1775 to barely a quarter of that half a century later. (A bushel is 35.2 liters, or 32 U.S. quarts.) A few eminent scientists, notably Nicholas Theodore de Saussure in Switzerland, Justus von Liebig in Germany, and Humphry Davy in England, established a relationship between nitrogen and minerals on the one hand and soil fertility on the other, but how you got the former into the latter was still a matter of debate, so farmers almost everywhere continued to cast desperate and often ineffective dressings onto their fields. Then, in the 1830s, there suddenly came the miracle product the world had been waiting for: guano. Guano—bird droppings—had been used in Peru since the time of the Incas, and its efficacy had been remarked on by explorers and travelers ever since, but it wasn’t until now that anyone thought to scoop it into bags and sell it to desperate farmers in the northern hemisphere. Once outsiders discovered guano, however, they couldn’t get enough of it. A dressing of guano reenergized fields and increased crop yields by up to 300 percent. The world was seized with what came to be known as “guano mania.” Guano worked because it was packed with nitrogen, phosphorous, and potassium nitrate— While guano was making life better for farmers, it had one very serious effect on city life: it killed the market in human waste. Previously, the workers who emptied city cesspits had sold the waste to farmers just outside the cities. That had helped keep costs down. But after 1847 the market for human waste collapsed, so disposal became a problem that was generally solved by tipping the collected waste into the nearest convenient river, with consequences that, as we shall see, would take decades to sort out. By 1850, the average farmer had the dispiriting choice of spending roughly half his income on guano or watching his yields wither. Clearly what was needed was a synthetic fertilizer—something that would feed crops reliably and economically. It was just at this point that a curious figure named John Bennet Lawes steps into the story. Bryson, Bill. At Home (p. 401). Knopf Doubleday Publishing Group. Kindle Edition. Bryson, Bill. At Home (p. 401). Knopf Doubleday Publishing Group. Kindle Edition. Bryson, Bill. At Home (p. 401). Knopf Doubleday Publishing Group. Kindle Edition. Bryson, Bill. At Home (p. 400). Knopf Doubleday Publishing Group. Kindle Edition. Bryson, Bill. At Home (pp. 399-400). Knopf Doubleday Publishing Group. Kindle Edition. ”