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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. October soil temperatures at the NWIARDC; temperatures are recorded (in F) at a depth of 4 inches in bare soil.
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Fall N fertilization: soil temperature and nitrogen retention

Now that harvest is complete for most people, field activities turn to fall tillage and fertilization.

Why the Nitrogen Cycle matters. The nitrogen cycle is a complex set of processes in which nitrogen (N) is converted among gas, inorganic and organic forms (Figure). Many of these processes are mediated by living organisms (bacteria and plants), and so factors such as soil temperature and moisture can influence the conversion among the different forms of N. This has implications for both the amount of fall-applied N available to corn plants in the spring and summer and grain yields.

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. Fall-applied N is at risk for losses due to leaching and/or denitrification.

Fall N applications and soil temperature. High soil temperatures increase the speed of chemical reactions that occur within soil microbes. In cooler temperatures, these chemical reactions slow. Research has shown that nitrification can occur down to freezing temperatures. Farmers can encounter difficulties such as fall rains and freezing soils by waiting to apply fall N until soils cool down. With these difficulties in mind, soil scientists recommended timing fall N applications to wait until soils cool to below 50 °F (and falling) at a depth of 4 inches in bare soil.

Soil temperatures vary field by field depending upon many factors including soil color, water drainage, and crop residue cover. It is suggested that soil temperature measurements be collected from each field before N application. At the NWIARDC, bare soil, 4 inch depth maximum daily soil temperatures remained high for most of October (Figure). It has only been in the last 3 days of the month, that we have seen maximum soil temperatures below 50 °F.


Maximizing N returns. The University of Illinois and the Illinois Council on Best Management Practices have created an online N-rate calculator. This tool is also available as a mobile app and calculates the MRTN (maximum return to N). MRTN is the nitrogen rate at which you will receive the maximum returns according to crop yields and grain and nitrogen prices.

Fall N sources and nitrification inhibitors. Dr. Fabián Fernandez recently wrote an article for the Department of Crop Sciences Bulletin in which he reviewed properties of anhydrous ammonia and ammonium sulfate, the two fertilizer formulations recommended for fall applications in areas North of route 16, and in soils that do not have a high risk of leaching or saturation. He also discussed the use of nitrification inhibitors, which can be applied along with fall N in order to retard the nitrification process.

Projected fertilizer prices. Economist Dr. Gary Schnitkey, of the University of Illinois, recently wrote about trends in pricing for potash, DAP and anhydrous ammonia fertilizers in a farmdocDAILY article and speculated about the longer-term anhydrous price outlook.

Illinois Agronomy Handbook - Chapter 9: Managing Nitrogen

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