It's not just the application of nitrogen fertilizer to farmlands that causes nitrate pollution of water -- it's the timing of that application and the runoff that results. One solution seems to be winter cover crops.

"Three weeks ago, Sara Carlson was driving to her job in Ames, Iowa, when she turned on the radio and heard me talking about nitrates in Iowa's water.

'And I was like, 'I really hope he nails this,' ' she says.

This topic is Carlson's specialty. She works with a group called Practical Farmers of Iowa. These farmers are devoted to farming in ways that protect the environment."

Dan Charles reports for NPR's Morning Edition February 2, 2015. [2]