In January, NPR’s Dan Charles reported that the city of Des Moines, Iowa – concerned about human and environmental health impacts and rising water treatment costs – intended to sue upstream counties over fertilizer run-off in rivers. Rainfall and snow melt carry the water downstream to Des Moines. From there, it travels down to the Gulf of Mexico, harming wildlife, estuaries, and fisheries along the way. The issue is not isolated to Iowa.
“Bill Stowe, general manager of the Des Moines Water Works, told Iowa Public Radio in an interview last week that “we are seeing the public water supply directly risked by high nitrate concentrations.”
Stowe says the source of these nitrates is pretty clear. Farmers spread nitrogen fertilizer on their corn fields, it turns into nitrate and then it commonly runs into streams through networks of underground tile pipes that drain the soil.”
“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. The plants need it to grow. And as a result, during that period, there’s usually not much nitrate flowing into streams and rivers.
“Our problem is, we only grow plants for five months out of the year,” she says.
Most Midwestern farmers grow corn and soybeans, which are warm-season plants. And after they’re harvested, for seven long months, from fall until the following spring, nitrate continues to form naturally in the soil. It can be released from decaying plant roots or from microbes, “and if there’s nothing to suck it up, to scavenge it, then it’s going to move…”
What Carlson is describing is the use of “cover crops.” Their impact can be dramatic on reducing nitrate releases. Cover crops can also reduce erosion, a big problem in Iowa and elsewhere. For additional information about nutrient and fertilizer management, visit this University of California ANR webpage.