It's a quandary of food production: The same drive for efficiency that lowers the cost of eating also can damage our soil and water.
Take the case of one simple, essential chemical element: phosphorus.
Phosphorus is one of the nutrients that plants need to grow, and for most of human history, farmers always needed more of it. "There was this battle to have enough available phosphorus for optimum crop production," says , a scientist with the University of Maryland's Wye Research and Education Center, which sits between farm fields and the Chesapeake Bay on Maryland's Eastern Shore.
That's also the tension in this story: agriculture on one side, and water quality on the other.
Traditionally, farmers got phosphorus from animal manure. So if you grew crops like corn or wheat, it was good to have poultry or hogs nearby. Your grain fed the animals, and their manure fed your crops. Everything worked together.
Then came industrial fertilizer: big phosphorus mines; factories for making the other important nutrient, nitrogen; and railroads or highways to carry that fertilizer to any farmer who needed it.
"With the development of the inorganic fertilizer industry, it's possible to grow grain without having animals nearby. So you can decouple the animal agriculture from the grain agriculture," Staver says.
And decouple they did. Farmers concentrated on just one kind of production. So did entire . Georgia, Arkansas and Alabama, for instance, now produce the largest number of chickens — more than a billion of them every year. But they don't grow much chicken feed. They haul in grain from far away.
As that grain flows from fields to chicken houses, or hog farms, so do the nutrients in it, such as phosphorus and nitrogen.