Switchgrass Lessens Soil Nitrate Loss Into Waterways

Nitrates that leach into the soil can affect Iowa communities that depend on the rivers for clean drinking water. (ISU/Bob Elbert)

By planting switchgrass and using certain agronomic practices, farmers can significantly reduce the amount of nitrogen and nitrates that leach into the soil, according to Iowa State University research.

Matt Helmers, associate professor of Agricultural and Biosystems Engineering, and Antonio Mallarino, professor of Agronomy, have been studying the amount of nitrates that pass through soil into tiling systems from several different types of crops and fertilizer treatments for the past three summers.

"One of the biggest things we found is that when alternative biomass sources like switchgrass are grown, even when they use fertilizer, we see dramatically lower nitrate concentrations (in the drainage water)," said Helmers.

The research compared fields that were planted with continuous corn while harvesting just the grain; continuous corn taking the grain and stover; and planting continuous corn taking all possible biomass from the fields. Half of those fields were treated with fertilizer and the other half with manure.

Other fields tested systems that rotate corn and soybeans, and others looked at switchgrass plots that received nitrogen fertilizer.

The results showed that fields planted in continuous corn and treated with fertilizer had the most amount of nitrates leach below the crop root zone into the tile system. The fields with the least amount of nitrates that leached through the soil were planted in switchgrass and treated with fertilizer or manure.

"If there is enough societal benefit and water quality benefit from growing switchgrass on these soils, there may be potential incentives for producers to grow (switchgrass)," Helmers said.

Helmers estimates that at least a third and possibly as much as half of all farmland in Iowa use tile systems to drain excess water from the fields. Nitrates that leach into the soil and are carried downstream are believed to contribute to a dead zone in the Gulf of Mexico where few plants or animals survive.

(Iowa State University)