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Timing Fall Nitrogen

Harvest is back in full swing after a period of substantial rain that fell over central and northern Illinois between Oct. 5 and 15. With harvest, thoughts turn to application of fall ammonia.

“Almost everyone is on board with waiting until soil temperatures are at or below 50 degrees before applying ammonia,” says Emerson Nafziger, an agronomist in the Department of Crop Sciences at the University of Illinois. “Cool soil, along with use of a nitrification inhibitor, lowers the rate of nitrification, and helps preserve nitrogen in the ammonium form. Nitrogen present in the soil as ammonium is safe from loss.”

Nafziger says some producers would like to start before soil temperatures reach 50, assuming soil temperatures will go down as air temperatures start to decline. “But if we apply when soil is at 60 degrees and soil temperatures don’t drop quickly, or if they rise again after application, nitrification will continue and will persist as long as soils stay warmer. In fact, nitrification does not stop dead at 50 degrees; as a biological process, its rate drops off as temperature falls, but temperatures need to be near freezing for nitrification to stop completely.”

Therefore, he cautions farmers to wait to apply fall ammonia not only until soil temperatures are 50 degrees or less, but until they are likely to stay cool. In Illinois, it has typically been safe to apply ammonia on or after Nov.1. But that’s not a sure thing. In the past two years, soil temperatures have risen above 50 degrees at least once between November and February. “Even so, Nov. 1 is usually a reasonable starting date to balance keeping nitrogen safe with getting fall application done,” Nafziger says.

Minimum air temperatures have fallen into the 40’s this past week, and producers are wondering if it might be okay to start applying now. Minimum soil temperatures at the 4-inch depth under bare soil have dropped to the upper 40’s to low 50’s over much of the state each day between Oct. 16 and 18.

The problem with using only the minimum soil temperature is that it doesn’t represent the soil temperature in the ammonia application zone. Minimum soil temperatures on clear days are typically five degrees or so less than the average soil temperatures for the day. “Even though we may need a jacket on cool mornings this week, ammonia applied now is not going to be in soils with temperatures less than 50 degrees for some days or weeks,” Nafziger says.

Air temperatures are forecast to stay in the 70’s the rest of this week, fall into the 50’s (with lows in the mid to upper 30’s) next week, and rise again the following week.

“We’re already past the average first frost date for central and northern Illinois, and even with more seasonal temperatures coming the last week of October, it doesn’t look like ammonia applied now will be as safe from nitrification and possible loss as will ammonia applied in November,” Nafziger says. “Delaying application moves us closer to having soil temperatures low enough to be safe for nitrogen.”

Average fall temperatures in Illinois have been trending slowly upward for some time. As in other recent years, waiting until Nov. 1 to apply fall nitrogen does not assure low soil temperatures as consistently as it did in the past. So, if a stretch of warm weather is still in the forecast at the end of October, it might make sense to wait a little longer.

“Otherwise, patience in waiting another ten days will likely be rewarded, even if – as is often the case when doing the right thing – the reward isn’t very visible.”

Source: University of Illinois

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