Conserving fall applied N: When have daily soil temperatures fallen to and stayed below 50 degrees?

As farmers wind up harvest in Western Illinois, focus turns to other fall field work such as tillage, applying herbicides and/or anhydrous ammonia applications. With crop budgets projected to be running in the red for the foreseeable future, 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 applied in 2014 both remains in the field and provides the maximum return on investment in 2015.

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 then 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. 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 them up and cooler soil temperatures slow them down.  However, research has shown that nitrification can occur down to freezing temperatures.

Chapter 9: “Managing Nitrogen” in the Illinois Agronomy Handbook contains useful information about nitrification inhibitors that can also slow the conversion of ammonium to nitrate.

Fall N applications and soil temperature. One way to maximize efficiency is to time anhydrous applications to occur when soil temperatures are low enough to slow the nitrification process and minimize fall N loss but to not delay until the soil surface freezes and does not close well behind the knife applicator.  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. Sensors are placed 4 and 8 inches below the soil surface under sod. These sensors collect  soil temperature and soil moisture measurements.

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

1989-2013: when have soil temperatures stayed below 50 °F? At the NWIARDC, in the 25 year period between 1989 and 2013, at both the 4 and 8 inch depths, the first day that the maximum soil temperature was less than or equal to 50 °F occured most often during the week of October 29 through November 4.

For weeks into the fall, temperatures will fluctuate up and down as air temperatures rise and fall and the sun rises and sets.

The maximum daily WARM soil temperature data collected at the NWIARDC each fall day between 1989 and 2013 shows us that 4 and 8 inch soil temperatures have most frequently been and stayed below 50 °F during the week of November 5 through 11 (Figure). This has also frequently occurred during the week of November 12 through 18 and as late as December 17 through 23. Luckily, the internet has revolutionized the speed at which we can monitor the current and forecasted weather conditions which will influence soil temperatures.

Maximizing N returns. Dennis Bowman, University of Illinois Commercial Agriculture Educator, released free Android and Apple MRTN (maximum return to N) apps in 2012.  MRTN is the nitrogen rate at which one will receive the maximum returns according to expected crop yields and grain and nitrogen prices.