cornThe primary cause of nitrate loss into Iowa’s surface water is bare soils during periods when crops aren’t growing and not because of a misapplication of fertilizer by farmers, Iowa agronomists and soil scientists said.

“More than anything else, the amount of nitrate that ends up in surface water is controlled by temperature and precipitation during periods when crops aren’t growing,” said Michael Castellano, an Iowa State University (ISU) assistant professor of agronomy who specializes in nutrient movement through soils. “There is room for improvement on rate, placement, timing and source of nitrogen applications, but that improvement will be a relatively small contributor to reduction of nitrogen loss from our soils and crop systems.”

Because of that, regulations that attempt to reduce fertilizer rates below agronomic recommendations or force one-size-fits-all conservation regulations on 89,000 Iowa farms are a folly and likely to fail and could actually increase nutrient loss from fields, said Rick Robinson, Iowa Farm Bureau Federation environmental policy advisor.

 “This scientific aspect of the source of nitrate loss is critical to consider as Iowans work to reduce the amount of the nutrient in rivers that are the source of drinking water for Des Moines and other Iowa communities,” Robinson said. “That’s why it is so important to support the science-based Iowa Nutrient Reduction Strategy, which takes a broad look at reducing the factors of the loss of nutrients from farm fields and point sources. It offers farmers an array of practices based on science, and demonstration programs to find the ones that best fit their operation and reduce nutrient loss.”

Soil nitrogen naturally high

Nitrogen levels are naturally very high in Iowa’s famous deep, black soils. That’s especially true in the area known as the Des Moines lobe in north-central Iowa. Average Iowa soil contains 10,000 pounds of nitrogen per acre in organic matter, far more than the 150 to 200 pounds per acre that farmers typically apply to raise corn.

“It’s really a remarkable soil formation, and only a few other places in the world have it,” Castellano said. “It’s those rich soils that help make Iowa the global leader in crop production.”

Most of the time, that organic nitrogen is locked in as 3 to 5 percent soil org­­anic matter and is not susceptible to loss. However, warm and moist conditions prompt microbes to transform the organic nitrogen into nitrate, which is susceptible to loss and can end up in surface water.

That transformation into nitrate often occurs in the early spring when fields are either not yet planted or crops have barely emerged, Castellano said. “If there are no living roots there to absorb the nitrate, then it could be lost from the soil profile and wind up in waterways,” he said.

Cover crops can be a very effective tool to offset that loss, Castellano said. “They address that window of bare soil after harvest and before planting.”

More and more farmers are experimenting with cover crops. Acreage in 2014 jumped to an estimated 300,000 acres, up from less than 10,000 in 2009. A survey completed in late 2014 showed that nearly one-quarter of Iowa farmers had tried cover crops.

However, Castellano cautioned, cover crops aren’t the single answer for every farmer in Iowa to reduce nitrate loss because they don’t work everywhere and in every year, just like most practices. In some parts of the state, particularly northern counties, the growing season is typically not long enough to get a crop harvested and a cover crop established. Many farmers also find it difficult in the spring to terminate a cover crop and plant corn on time.

“Cover crops are a good tool. They won’t work in all cases, but they are good tool,” Castellano said.

Along with cover crops, farmers are using a number of other technologies to reduce nitrate loss, said Nate Pierce, western region agronomy business director for ag-supplier GROWMARK. Technologies, like nitrogen stabilizers and precision nitrogen programs, which were experimental a few years ago, are becoming common practices for a lot of farmers, he said.

 In addition, GROWMARK’s N-Watch program, which is designed as an educational tool to help farmers learn to monitor the nitrogen in the top two feet of their soil and consider factors such as weather that influence nitrogen movement, is also gaining popularity,” Pierce said. “Farmers are really learning more about the nitrogen system and how they can help reduce losses,” he said.

 Several edge-of-field technologies have also shown results have shown promise in reducing nitrate loss. Many farmers are adopting some of those, such as wetlands, bioreactors and saturated buffers, in combinational with in-field management techniques to reduce nitrate loss from fields.

While there are a lot of practices to reduce nitrate loss, lowering fertilizer application rates below the ISU recommendations is not recommended, Castellano said. A farmer who reduced fertilization levels below ISU recommendations risks a long-term decline in the soil organic matter, the ISU agronomist explained. That organic matter is needed to hold onto organic nitrogen, he said.

 “Soil is like a bank account,” Castellano said. “If you are taking more nitrogen out in grain than you a putting in with fertilizer, you will eventually deplete your balance and you would only exacerbate the water quality challenges.”

 It’s about retaining a nitrogen balance, Castellano said. “Nitrogen fertilizer inputs that are insufficient to maximize economic return for the farmer can cause a long-term soil organic matter loss,” he said. “However, nitrogen fertilizer inputs in excess of crop demand can increase nitrate loss and reduce profitability. That’s why it is important to fertilize at ISU recommended rates.”