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Applying Nitrogen Fertilizer in the Fall
Soil temperatures reach 50 F about October 1 in northern North Dakota and October 8th in southern North Dakota. While surface soil temperatures can vary considerably day to day, soil temperatures at 4 inch depth change more slowly. Conversion of banded ammonia products to nitrate will be slow due to cool soil temperatures. Periods of warmer than usual temperatures should not delay application of incorporated fertilizer.
Timing of fall ammonia application in North Dakota should be based on a combination of calendar and soil temperature. Fall N should not be applied before the last week of September and only then if soil temperatures at the 4 inch depth taken between 6 AM and 8 AM fall to 50 degrees F or lower. Application of N before this time can be conducted within 2 weeks if a nitrification inhibitor is applied with the ammonia. Banded urea application should be made after the first of October if soil temperatures are below 50 degrees. Broadcast applications of urea should be delayed until about the second week of October and incorporated within 2-3 days if surface temperatures are warm and within 3-5 days if soil temperatures are cool. Rainfall or snowmelt amounting to about 1/2 inch of moisture is as good as incorporation in most soils. No-till soils with a heavy mulch layer needs more precipitation than bare soils. Urea can be applied to soils late in the fall during the freeze and thaw cycles, but should not be applied to soils frozen so deeply that water cannot enter them. Research at Carrington has shown yields can decrease 50% in wheat when urea is applied to deeply frozen soils.
When soil temperatures and moisture are favorable for nitrification, urea broadcast and incorporated into the soil undergoes a much faster nitrification rate than does anhydrous ammonia (AA). This is because each small granule of urea is surrounded by a larger volume of soil. After quick hydrolysis (2-4 days), microorganisms in the soil rapidly begin to convert the ammonium to nitrate. Nitrification rate of broadcast UAN (28% solution N) is even more rapid because of more intimate contact between the large soil volume and each spray droplet. Nitrification is much slower with AA because ammonia is injected into the soil and remains in a 2 to 6-inch diameter band. (Drier soils result in a wider band while the ammonia remains in a narrower band when soils are wet.) This highly-concentrated band of ammonia stunts the microorganisms and in effect acts as its own nitrification inhibitor until the microorganisms recover. Surface, deep-band applications of urea with N-Serve may have nitrification rates similar to that of AA, but usually the rate of nitrification is slowest in AA, followed by banded urea, and fastest in broadcast urea.
UAN solutions are not recommended for use in the fall.
On coarse textured (sandy) soils, fall or early spring rainfall may be enough to leach the nitrogen. Fall application of nitrogen on sandy soils is not recommended.
The danger of N loss through leaching is not in the fall of the year, but in the early spring snowmelt and spring rains.
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