ProCrop

Nitrification Rate of Ammonia Fertilizers Effect on Fall Application Timing

Similar to all other soil-applied N fertilizers, once urea has been converted to ammonium, the ammonium converts to nitrate through a process called nitrification. 

NH₄⁺ NO₂^- NO₃^-
ammonium nitrite nitrate

Because this biological reaction is controlled by microorganisms, the rate of nitrification is dependent upon the level of biological activity. Cold temperatures (<50F) slow this reaction. Nitrification inhibitors, such as N-Serve�, and high ammonium concentrations are two other factors that slow the nitrification rate.

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.

Long term average 

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, soil temperatures at 4 inches 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. Broadcast applications of urea need to be incorporated to prevent losses from possible volitazation unless applied to cold soils in late October. Sandy soils and soils prone to flooding should have nitrogen applied in the spring.

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