DAVID GREENE, HOST:
You know, a few weeks ago, we told you about a conflict between the city of Des Moines, Iowa, and farmers who live nearby. The Des Moines Water Works says it will go to court to stop farmers from releasing so much nitrate pollution into the rivers that supply the city with its drinking water. We're going to dig a little deeper into this because Dan Charles, the reporter on that story, got some very interesting feedback from a listener.
DAN CHARLES, BYLINE: Sara Carlson was driving to work in Ames, Iowa, when she heard me talking about nitrates in the water.
SARA CARLSON: And I was like, oh, I really hope he nails this.
CHARLES: Because this topic is Carlson's specialty. She works with a group called Practical Farmers of Iowa. They try to farm in ways that protect the environment. As Carlson listened to the radio, she was thinking I hope he doesn't say that nitrate just comes from farmers using too much nitrogen fertilizer.
CARLSON: Or at least doesn't say fertilizer only.
CHARLES: Well, here's what she heard.
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CHARLES: Farmers spread nitrogen fertilizer on their cornfields. It turns into nitrate, and then it often runs into streams through...
CARLSON: And then when you said it, I was like, oh, no.
CHARLES: Carlson was frustrated because she hears that all the time from journalists, politicians, also farmers. And each part of that sentence is technically true. Nitrogen fertilizer does turn into nitrate. Nitrate does run into streams. But she - and lots of scientists actually - say that ignores something else, the thing really sends nitrate into streams. And it makes Carlson upset because she says this half-truth gets in the way of solving a massive continent-wide problem. Here's the bigger picture, Carlson says. During the summer, when crops are growing on those fields, they scarf up most of the soil's available nitrate. They need it to grow. So during that time, there's usually not much nitrate flowing into streams and rivers.
CARLSON: Our problem is that we only grow plants for five months out of the year. That's our problem.
CHARLES: Most of the Midwestern farmers grow corn and soybeans - warm-season plants. And after they're harvested, for seven long months, nitrate is still forming naturally in the soil. It comes from decaying plant roots or microbes.
CARLSON: And if there's nothing to suck it up, to scavenge it, then it's going to move.
CHARLES: Rainfall and melting snow will carry it downstream to Des Moines and beyond. It damages wildlife and fisheries all the way to the Gulf of Mexico. We won't fix this mess by using less fertilizer, Carlson says.
CARLSON: The way to fix this is we need to have something growing from October till May.
CHARLES: During the cold part of the year. Farmers can do this. The easiest way is they can plant cold-weather crops - a grain called rye, for instance - right after the corn or soybean harvest. Those crops will grow through the fall, go dormant when everything freezes and come back to life in the spring. The impact of these so-called cover crops - crops that just cover the soil - can be dramatic. Tim Smith, a farmer in Eagle Grove, Iowa, joined a government-funded effort to protect water in the Mississippi River Basin a few years ago. The program's experts suggested planting cold-weather crops. Smith was not that interested at first.
TIM SMITH: I thought, why would I want to do that? But as I've used those cover crops for the last four years, I've realized that they probably have the greatest impact on water quality.
CHARLES: Measurements showed they cut the amount of nitrate in water draining from his fields about in half. Scientists who have studied cover crops say they typically reduce nitrate releases by about one-third. And there are other benefits - they protect the soil from erosion. Yet very few farmers in the Midwestern corn and soybean belt plant these cover crops - only about 2 percent in Iowa, for instance. Tim Smith says it's still a new idea, that farmers want to be sure it works before they try it. Also it costs some money, and the payoff is long-term, not immediate.
SMITH: Some farmers aren't aware of the necessity for water quality. You know, that's part of the issue.
CHARLES: That may be changing, though, he says. Most farmers don't like this Des Moines lawsuit over water quality. They feel like it's a slap in the face. But it did get their attention. Dan Charles, NPR News.
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