According to NASS, Illinois producers harvested 36 percent of the corn crop and 52 percent of the soybean crop by October 20. That’s still behind the average pace of harvest, but harvest continues in many areas this week, and as it progresses, fields in many areas are becoming available for fall field work to begin.

 

Many producers in central and northern Illinois have fall anhydrous ammonia application high on their to-do list, especially after the fall of 2018 and the spring of 2019, when getting any nitrogen fertilizer applied was a challenge. As we have seen before, most people were able to work around the weather issues to get N applied, in some cases by changing to in-season applications, and sometimes changing the N form to one easier to apply in the narrow windows of opportunity last spring. While we hope not to see a repeat of such challenges very soon or very often, this past year reminds us that retailers and producers are up to the challenge of getting N applied even when the weather doesn’t cooperate.

 

Timing of anhydrous ammonia application in the fall is a major issue, and there is a considerable amount of anxiety related to having to wait until soil temperatures are low enough for safe application. Ammonia and ammonium-containing N fertilizers are the only fertilizer materials that are safer from loss when applied to cool soils than to warm soils. This is because the soil contains large populations of nitrifying bacteria that convert ammonium ions to nitrate ions—this process is called nitrification. As a cation, ammonium ions are attracted to the negative charges on clay and organic matter surfaces; this attraction means that ammonium ions don’t move with water as it moves downward through the soil. As an anion (negatively charged ion), nitrate is not attracted to negative charges in the soil, and so can move downward with water.

 

Nitrifying bacteria extract energy from ammonium while converting it to nitrite and then to nitrate; in the process, three oxygen atoms are added and hydrogen atoms and water are released. Because this is a biological process, nitrification is sensitive to temperature. The bacteria operate (and multiply) fastest at temperatures in the low to mid-80s, and the cooler it is the lower their activity; the rate of nitrification is close to zero at 40°, and is only about one-fourth of maximum at 50°. This means that waiting until soil temperatures are 50° or less will mean slow nitrification, and once soil temperatures reach 40°, almost all ammonia applied (which converts quickly to ammonium) will remain in the ammonium form—and so safe from loss—until soil temperatures rise.

 

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