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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. Recent bare soil temperatures at a 4 inch depth at the NWIARDC.

Fall nitrogen considerations

Factors affecting soil N retention. Soil N is lost through the activities of soil-borne bacteria through chemical reactions called nitrification and denitrification.  With nitrification, enzymes in bacteria convert ammonium ions (NH4+) to nitrite ions (NO2-) and then to nitrate ions (NO3-).  Leaching is the movement of dissolved nutrients out of the soil with water movement. While ammonium in the soil cannot be lost through leaching, both nitrite and nitrate can.  Denitrification is a bacterial process that takes place in saturated soils in which nitrite and nitrate are converted into gasses which leave the soil.  The chemical reactions that take place in soil-borne bacteria are temperature dependent.  High soil temperatures increase the speed of chemical reactions while cooler temperatures slow chemical reactions.  Research has shown that nitrification can occur down to freezing temperatures.

Fall N applications and soil temperature. Producers have many decisions to make when it comes to fall nitrogen.  One is the matter of timing the application to wait until soil temperatures are low enough to slow the nitrification process and minimize fall N loss but not delaying so long that the soil surface is frozen and does not close well behind the knife applicator.  Aware of both issues, 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.

At the Northwestern Illinois Agricultural Research and Demonstration Center, Brian Mansfield and Marty Johnson collect daily minimum and maximum bare soil temperature data (Figure).  Bare soil temperatures dropped below 50 degrees on October 23rd, 24th and 25th.  Warm air temperatures and sunny fall days warmed the soil back up throughout the rest of October and have since hovered just above 50 degrees.


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

Illinois Agronomy Handbook - Chapter 9: Managing Nitrogen. The Illinois Agronomy Handbook is a good reference to help remind us about research based recommendations that can help in making nutrient management decisions about N rates, splitting N applications, and nitrification inhibitors.

Soil temperatures. The Illinois State Water Survey has a network of 19 temperature sensors located throughout the state that can serve as a reference.  Soil temperatures vary field by field depending upon many factors including soil organic matter and color, drainage, and crop residue cover.  It is suggested that soil temperature measurements be collected from each field before N application.

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