Myths often told and retold in the ongoing discussions over water quality issues in Iowa typically do not hold up to the scrutiny of science, Michael Castellano, an Iowa State University (ISU) soil scientist, said during a seminar last week at the 2015 Iowa Farm Bureau annual meeting in Des Moines.
For example, Castellano said, there is a myth that nitrate problems in Iowa waterways are primarily caused by farmers’ mismanagement of fertilizer. And that myth, he said, needs to be busted.
"Natural loss of nitrate is really the main thing that is causing us to lose nutrients into our waterways," Castellano said. "Fertilizer applications really have very little to do with it."
Iowa’s deep, black soils have on average some 10,000 pounds of nitrogen per acre stored in organic matter, the ISU associate professor said. "This nitrogen is a tremendously important resource for the state," Castellano said. "Along with favorable weather, it’s the reason we have such high crop production in the state."
Mismatch of timing
Nutrient losses typically occur in the early spring and late fall, when microbes unlock a portion of that natural nitrogen but there are no crops on the fields to take it up, Castellano said.
"It’s a mismatch of timing of when the nutrient is naturally produced and when the crop roots can take it up that leads to the nitrogen loss into the water," he said.
Another myth that needs busting is that water quality problems can be solved if Iowa farmers did all of their fertilizing in the spring, instead of in the fall, Castellano said. "I can’t tell you how many times I’ve heard if everyone applied fertilizer in the spring, we wouldn’t have this problem. But that is not the case, because fertilizer application has very little to do with nitrate loss," he said. Fertilizer applications are about 1.5 to 2 percent of the total nitrogen available.
There is also a myth out there that cover crops are the only viable solution to water quality problems, the ISU researcher said.
No magic formula
While cover crops have shown to be very effective, they won’t work for every farm in the state because of weather and soil conditions, Castellano said. Other practices, such as bioreactors, wetlands and saturated buffers, may work better in some situations, he said.
Some farmers may even find it’s better to idle some pieces of land that are not very productive and tend to lose nutrients more readily than higher quality land, Castellano said.
"There’s is not just one approach that works for water quality," Castellano said. "There is no silver bullet. It’s going to take time for farmers to realize which practices are best for their operations and find what fits in their part of the state."