Weather is an unpredictable wildcard for farmers in Iowa, but the drought poses bigger challenges than usual. Drought can affect nitrogen fertilization management by reducing the system’s yield – both in grain harvest and environmental degradation – which leaves more nitrogen in the soil and increases the likelihood of nitrate loss after a heavy rainfall.

Drought can have a surprisingly small impact on yield (as farmers reported this fall), but arid conditions still reduce nitrogen losses: lower rainfall moves less nitrate from the soil into rivers and streams, and dry soils limit microbial denitrification losses into the atmosphere. How is nitrogen fertilizer lost to the atmosphere? When the soils are wet and microbes don’t have oxygen to breathe, they instead breathe in nitrate and release nitrogen gases into the atmosphere.

Less nitrogen loss is good, but the nitrate that is not lost remains prone to loss. Farmers in drought-stricken regions of the state have the option to use this nitrate. Soil surveys can help farmers determine how much nitrate is left and catch crops can help preserve what remains in the soil.

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Iowa State University predictive models show that many soils in Iowa are currently much higher in nitrate than the average case, particularly in the high-yielding landforms Des Moines Lobe and Iowa Surface in north-central Iowa. Because of this, Iowa Corn and Iowa State University are encouraging Iowa farmers to take important steps this fall to better manage soil nitrogen so we can avoid potential water quality issues next spring.

Iowa State University said, courtesy of Dr.  Sotirios Archontoulis, Associate Professor of Agronomy, ISU, predicted the nitrate content in the soil from September 2021.

We experienced similar conditions in the drought of 2012, when lower yields and less environmental damage after harvest led to high residual nitrate levels in the soil. That fall, 2013 was followed by one of the wettest springs in Iowa when heavy rains caused massive nitrate floods in rivers and creeks. Unfortunately, this resulted in extremely high levels of nitrates in the Des Moines and Raccoon Rivers, which exceeded the Safe Drinking Water Act standard of 10 parts per million and forced the Des Moines Water Works to operate their denitrification facility.

Therefore, we encourage Iowa farmers to take additional steps to carefully manage future nitrogen fertilizer applications – especially this fall.

We encourage Iowa farmers to speak to their certified crop advisor or agronomist and consider the following:

  • Determine the nitrate content in the field soil: the yields can be surprisingly high despite the drought, but the soil nitrate content can also be surprisingly high due to the drought.
  • If there is a high nitrate content in the soil, reduce the fertilization in autumn, divide the fertilization between autumn and spring or apply 100% in spring.
  • Plant catch crops to use excess nitrate, reduce erosion and improve soil health.
  • Use nitrification inhibitor products. All nitrogen applications in autumn should be accompanied by a nitrification inhibitor, regardless of the drought and nitrate status of the soil.
  • Regardless of drought or soil nitrogen status, soil temperatures must be 4 inches below 50 degrees and trending downward before nitrogen fertilizers are applied in the fall.

Lance Lillibridge

We applaud farmers across Iowa and the Corn Belt for the increasing use of the four R’s of nutrient management to ensure the right source of fertilizer is applied in the right place, at the right rate, and at the right time. Given the current drought conditions and the potential for high levels of nitrates in our soils, we encourage Iowa farmers to take additional steps this fall to carefully manage nitrogen fertilizers to keep nitrate out of our waterways next spring.

Mike Castellano

Lance Lillibridge is the president of the Iowa Corn Growers Association; he farms in Benton County. Mike Castellano is Professor of Agronomy and William T. Frankenberger Professor of Soil Science at Iowa State University.