Effectiveness of Nitrogen in Soil
F
armers frequently ask about the relative effectiveness of fall versus spring nitrogen fertilizer applications, and they come from both irrigators and dryland producers. “Fall fertilization can range from very effective to disastrous depending on soil moisture conditions, the form of nitrogen fertilizer used, and how it is applied,” says Ross McKenzie, Agronomy Research Scientist, Alberta Agriculture and Rural Development (ARD), Lethbridge. Various soil processes and environmental conditions will affect the fate of fertilizer nitrogen in soil. Fertilizer nitrogen is applied to soil in the form of urea 46-00, anhydrous ammonia 82-0-0, or liquid fertilizer 280-0 which contains urea, ammonium, and nitrate. Urea and anhydrous ammonia quickly convert to ammonium, which won’t leach and normally won’t volatilize. It is the ammonium and nitrate forms that are taken up by plants, but generally most nitrogen is taken up in the nitrate form. If the soil is warm, moist and well aerated, ammonium is rapidly oxidized to nitrate through the nitrification process, which is a soil biological process performed by highly specialized soil bacteria. Banding nitrogen fertilizer slows the nitrification process by creating an environment near the band that inhibits the activity of the bacteria converting ammonium to nitrate. Therefore, if urea or anhydrous ammonia is banded in late fall, most of the nitrogen is retained in the ammonium form over winter until the soil warms up in the spring. If the fertilizer is broadcast or banded in early fall, likely most of the ammonium will be converted to nitrate prior to freezeup. Large nitrogen losses caused by anaerobic bacteria through denitrification can occur when soils are water saturated during the spring. Denitrification converts nitrate to gaseous nitrogen forms such as nitrous oxide. Once this process occurs the nitrogen fertilizer is lost from the soil. “Research has also shown that denitrification will occur in virtually all of our agricultural soils,” says McKenzie. This is not surprising since denitrification is not a particularly specialized function. Many different types of soil bacteria use denitrification as an alternative form of respiration when oxygen is in short supply. “In terms of fertilizer management, this means that no soil type or region of the province is completely safe when it comes to losses of fall-applied nitrogen. In most years, nitrogen losses through denitrification 14 • IRRIGATING ALBERTA - Spring 2012
in the southern Alberta are relatively small and fall banded nitrogen is usually 90 to 100 per cent equally effective to spring banded nitrogen.” In cases where spring banding causes a significant loss of seedbed moisture, fall banding can be superior to spring banding. Typically, denitrification can be significant on irrigated soils after extended saturating rain conditions in spring and summer. “Denitrifying bacteria are less than two millionths of a meter in size and respond to environmental conditions in the micro-pores of soil,” notes McKenzie. “This means that the microclimate in soil is important.” In fields with localized wet, depressional areas, denitrification can occur. Over winter nitrogen losses can vary greatly over a short distance in fields with rolling topography. Fall-applied nitrogen can be very effective on upland areas of a field and totally ineffective in depressional areas just a short distance away. It is important to remember fall-application always puts your fertilizer nitrogen at risk. The level of risk is generally assessed at the regional level, but whether or not losses occur is a function of very localized moisture and temperature conditions. General rules about nitrogen application methods and timing include: - Generally spring banded is the most effective method of application and fall broadcast the least effective. - Fall banded nitrogen can be as effective as spring banded if there is no extended period of saturation in the spring. - Fall banded nitrogen may be more effective than spring banded when lack of seedbed moisture is a concern and the spring banding operation dries the soil and reduces the quality of the seedbed for spring seeding. With all this information in mind here are a few tips to consider before fertilizing in the fall: - If your soils tend to be saturated with water for extended periods in the spring, then fall application is probably not a good option. However, if saturated soil conditions are normally not a problem, then you should get good results from fall banding. - Soil test to determine the optimum rates of fertilizer required. Producers should sample zero to six, six to 12, and 12 to 24 inches to determine the cumulative nitrogen to two feet. Based on soil analysis, then you can decide the ideal rates of nitrogen fertilizer to apply.