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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Daily soil temperatures at 4 inches under sod and bare soil at the Northwestern IL Ag R&D Ctr. in Monmouth in 2016 (Data Source: WARM).
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Fall Nitrogen: Strategies to Hang onto it Until the 2017 Crop Needs it

Progress: Harvest and other Fall Operations. For the week ending November 6, the USDA's National Ag Statistics Service estimates that 95 percent of the corn and 92 percent of the soybeans in the Western Illinois crop reporting district have been harvested (weblink). For many farmers, the focus now turns to other fall field work such as tillage,fall herbicide applications, applying dry fertilizers (DAP, potash, lime) or fall anhydrous ammonia applications. Particularly true for many acres of rented cropland, the low crop prices and lingering high costs of production have crop budgets projected to be running in the red for the foreseeable future. As a result, producers will be looking for any way to shave production costs, whether it be by forgoing fall tillage or planning to switch some corn acres to soybean.

Another important way to maximize production efficiency is by making sure that nitrogen (N) applied in 2016 both remains in the field and provides the maximum return on investment by translating into corn yield in 2017.

Factors affecting soil N retention. If everything goes as planned, anhydrous ammonia molecules injected into the soil react with water to form positively charged ammonium ions which adhere to soil particles. Enzymes in soil bacteria can convert ammonium ions (NH4+) to nitrite ions (NO2-) and then to nitrate ions (NO3-) which can easily leach out of the root zone with water movement. Research has shown that nitrification can occur down to freezing temperatures. In saturated soils, other bacterial enzymes convert nitrite and nitrate ions into gasses which can then move out of the soil profile and into the atmosphere. These chemical reactions are temperature dependent: higher soil temperatures speed up reactions and cooler soil temperatures slow them down.

Fall N applications and soil temperature. Fall anhydrous applications are a delicate balance between delaying applications until soil temperatures have fallen but not delaying so long that the soil surface has frozen. While warmers soils risk nitrogen loss through nitrification and leaching, frozen soils do not seal as well behind applicator knives, risking loss through volatilization. As a compromise, soil scientists recommended waiting to apply fall anhydrous until soils cool to below 50 °F (and falling) at a depth of 4 inches in bare soil.

The Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) outside of Monmouth in Warren County is one location in the Illinois State Water Survey's Climate Network and Water and Atmospheric Resources Monitoring (WARM) Program. As part of the program, sensors are positioned 4 inches below the soil surface under both sod and bare soil. These sensors collect both soil temperature and moisture measurements.

Soil temperatures vary field by field depending upon many factors including soil organic matter and color, drainage, and crop residue cover. While the Illinois State Water Survey's network of 19 WARM temperature sensors located throughout the state can serve as a reference, experts suggest collecting soil temperature measurements from each field before N application.

1989-2015: when have fall soil temperatures stayed below 50 °F? At the NWIARDC, in the 27 year period between 1989 and 2015 (at 4 inches under sod) the first date after which point each fall the maximum soil temperature remained lower than 50 °F occurred most often during the weeks of November 5 through 11 and November 19 through 25, but in 3 years has occurred as late as the week of December 2 through 9 (Figure).

For weeks into the fall, temperatures will fluctuate up and down as air temperatures rise and fall as the sun rises and sets. While 2016 soil temperatures have trended downward over time, one can see these temperature fluctuations exhibited in the data (Figure).

Practices that can both reduce off-farm nitrogen losses and improve water quality. The Illinois Nutrient Loss Reduction Strategy (NLRS) was developed in an effort to improve water quality both in-state and downstream from Illinois. The NLRS serves as a comprehensive research-based 'menu' of strategies that can be adopted to minimize both soil nitrogen and phosphorus losses. Two strategies that can be employed when applying anhydrous ammonia in the fall are using nitrification inhibitors and using 'MRTN' or 'maximum return to nitrogen' rates.

Nitrification inhibitors are products that are applied in-furrow along with anhydrous ammonia and work to slow the speed at which bacteria convert ammonium ions to nitrate thereby reducing the speed of potential N loss through leaching.

Maximum return to nitrogen is a range of recommended N rates. These rates are based upon data collected from hundreds of corn nitrogen rate experiments conducted throughout many production regions in the Corn Belt. In Illinois, rates can be customized by region, crop rotation, and a range of nitrogen and corn price scenarios. Without a crystal ball regarding the weather conditions that will prevail throughout the upcoming cropping season it is unlikely that the MRTN N rate will be 100 percent accurate. However, the range of N rates provided by MRTN calculators is based upon hundreds of site-years of data and is an unbiased source for a 'best guess' estimate.

In 2012 University of Illinois Commercial Agriculture Educator Dennis Bowman released free Android and Apple MRTN (maximum return to N) apps. The data that is used in the apps is continuously updated with the most current data taking the place of the oldest data points.


Bowman, D. 2012. Maximum Return to Nitrogen apps.

Fernández et al. 2009. Managing Nitrogen. Illinois Agronomy Handbook. Chapter 9.

Illinois EPA. 2015. Illinois Nutrient Loss Reduction Strategy.

USDA-NASS, Heartland Regional Field Office. Nov. 7, 2016. Illinois Crop Progress and Condition.

Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2016). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495.

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