No. 23 Article 6/October 3, 2008

Nitrogen for the 2009 Crop

In the fall, the uncertainty of weather conditions and how long it will be before it is too wet or too cold to till the soil or to apply nutrients for the next growing season encourages producers to get in the field as soon as possible after harvest. While the window of opportunity for doing all these fall field operations is not very large, it is important to exercise good judgment to realize their full potential. Of paramount importance is management of nitrogen (N). This nutrient is both one of the most expensive inputs in today's farming operations and a nutrient that can pose environmental concerns. Whether we think of cost, environmental implications, or both, we simply cannot afford poor management of nitrogen. This is a year in which being smart about N use will likely pay off. To achieve that goal, it is important to understand some key factors. The following are recommendations that will enhance the efficiency of N management this fall.

Time of Nitrogen Application

Some forms of N are more susceptible to loss than others. Chemically speaking, ammonium (NH4+) is a positively charged ion that behaves similarly to potassium K+ ions. Ammonia stays in the soil, held by the negative charges of clays and organic matter, and is not susceptible to leaching or denitrification. However, through the nitrification process, NH4+ can convert to nitrate NO3-, which is susceptible to leaching and denitrification (conversion to N2 or N2O gas). These conversions are all mediated by soil organisms. Since temperature has an important impact on the activity of these organisms, it is critically important to wait to apply N until soil temperature at the 4-inch depth is below 50F and is maintained at or below this value through the winter. In most years, the 50F temperature allows for N applications before soils become too wet or frozen.

Application of anhydrous ammonia with a nitrification inhibitor (see discussion below) can start after soil temperature at 4 inches is below 60F. Although the rate of nitrification is significantly reduced when soil temperature is below 50F, microbial activity continues until temperatures are below 32F. Air temperatures can fluctuate substantially during the early fall. For this reason, do not apply N before the 3rd week of October in central Illinois or the 2nd week of October in northern Illinois, even if air temperatures are getting cooler. Because of temperature considerations, fall N application should not be done south of a line roughly parallel to Illinois Route 16. In areas near this boundary, soil characteristics should be evaluated to determine whether fall application is appropriate. Soils that have high potential for NO3- leaching in the fall or early spring (i.e., sandy soils or those with excessive drainage) should not receive fall N applications. Daily maximum 4-inch bare-soil temperatures for Illinois this year at the end of September to beginning of October were in the upper 60s to mid-70s. Up-to-date soil temperatures can be accessed at However, it is strongly recommended that temperatures of soils in individual fields be monitored prior to N application.

Nitrogen Source

Anhydrous ammonia is a preferred nitrogen source for fall application because it has a slower nitrification rate than other N sources. Once applied in the soil, ammonia (NH3) reacts quickly with soil water and is converted to NH4+. Nitrification inhibitors (such as dicyandiamide [DCD] and Nitrapyrin, also known by the trade name N-serve) are chemicals that inhibit the activity of bacteria responsible for the first step in the process of nitrification (conversion of NH4+ to nitrite [NO2-]); this intermediate can then be quickly converted to NO3-. Proper use of these inhibitors will reduce the rate of nitrification, thus maintaining for a longer period a greater proportion of the applied N in the NH4+ form. Since nitrification rates increase under warm temperatures and moist conditions, nitrification inhibitors are especially useful when those soil conditions prevail. To avoid volatilization losses during application, make sure the soil is neither too wet nor too dry to secure adequate closure of the soil behind the applicator knife. Also, it is important to apply at the proper depth; for fine-textured soils, 6 to 8 inches is sufficient to keep NH3 gas from escaping the soil.

As mentioned, ammonium is a stable form of nitrogen that is readily adsorbed to exchange sites in the soil particles and organic matter. On the other hand, NO3- does not attach to exchange sites, but remains in the soil solution and can be lost through denitrification and leaching. Therefore, the source of N used for fall applications is an important consideration. Nitrogen sources containing nitrogen in the NO3- form (such as ammonium nitrate [NH4NO3] or urea ammonium nitrate [UAN]) should not be used in the fall to provide N for corn because part of the N is already in a form that can be easily leached or denitrified.

Urea (CO[NH2]2) converts to NH3 and then to NH4+ within a few days of application. However, the use of urea in the fall is discouraged because it has been shown to be less effective than fall-applied anhydrous ammonia. The lower efficiency of urea is mainly due to greater risk of NO3- losses before rapid nutrient uptake by the crop the following spring.

Slow-release, controlled-release, and polymer-coated urea (PCU) are all common names of products that have been designed to control or reduce the conversion of urea to NH4+ and thus limit the potential transformation to NO3-. While the concept makes sense, at this time limited research results are available to make a definite statement. Some of the most important considerations on the effectiveness of these products are the thickness of the coating, the time required for degradation of the coating, and the integrity of the coating after handling and application. Since research data are still considered preliminary for these products, they are not presently being recommended for fall application.

With the high cost of fertilizer, there is interest in using manure, poultry litter, and other organic fertilizer forms to supply not only N but also phosphorus and potassium. These animal products are excellent nutrient sources, but they should be incorporated to avoid N loss by volatilization. Most of the N is in uric acid and NH4+ forms that can rapidly transform to NO3-. Thus, the soil temperature recommendations already discussed also apply to these fertilizers. Due to the length of time between application and utilization by the crop, applications should be done as far as possible from environmentally sensitive areas, such as on steep slopes and near bodies of water. If the application cannot be accomplished in late fall, do not apply on frozen soils in the winter; it is better to wait until spring. Before application, these fertilizers should be analyzed for nutrient content. Typically, if these sources are applied to meet the N needs of the crop, it will cause an overapplication of phosphorus. For this reason, most often the rate of application should be set to meet the crop's phosphorus requirements rather than N requirements. Knowing the soil phosphorus level and nutrient contents of the fertilizer are a must to determine the appropriate application rate.

How Much to Apply

To determine how much N to apply, use the N rate calculator at, or use Table 5 with information generated from the N rate calculator. These calculations do not account for carryover nitrogen. This year, since it was so wet in the spring, it is unlikely that much N will be carried over even if yields were lower than expected. However, if you applied manure, you will need to adjust the values from the calculator to reflect what will be available next year. If you are planning to plant corn into a field coming off of alfalfa, chances are high that there is enough N in the soil to produce a crop without any addition of N.

Table 5 gives suggested N rate ranges for corn following corn and corn following soybean for different regions in Illinois, at three N prices and five corn prices. The N prices of 70, 76, and 82 cents per lb are equivalent to anhydrous ammonia prices of $1,150, $1,250, and $1,350 per ton, respectively. Ranges for prices between the ones in the table can be estimated. The value near the center of the range is the N rate expected to provide the greatest return to N.

Applying N to Help Break Down Residue

With more acres planted to corn following corn, there is great interest in corn residue management. One common question has been whether application of N, such as UAN, on the residue this fall would help with the breakdown of corn stalks. Research has shown no benefit in fall application of N to increase microbial decomposition of corn residue in order to improve corn planting operations and N for the next corn crop. Typically, low temperature, and not N levels, is the limiting factor for microbial decomposition of residue.

Consider Your Options

At current N fertilizer prices, it is critical to have every pound of fertilizer going to corn yield. Overall, research on N application timing has shown that application in the spring, close to the time of rapid N uptake, maximizes yield because there is less chance for leaching or denitrification. In some years side-dress applications are more effective than preplant, but the results are dependent on weather. Most often, though, under normal spring conditions there is little or no difference between the two times of application. However, late-fall application of some N sources (previously discussed) is also adequate, especially for medium to fine-textured soils where cold winter temperatures prevail.

Fall applications have both economic and logistic advantages. Soil conditions are typically more conducive for application, there is more time available than during the busy planting season and equipment and labor are better distributed, and often there are price incentives to buy anhydrous ammonia in the fall. The spring typically is wet, and soil compaction, especially for manure application, is of greater concern. Also, waiting until the spring to apply fertilizer can delay planting, damage crops, and delay application of fertilizer to meet early nutrient uptake needs of the crop. Unfortunately, since spring weather conditions have a large influence on N efficiency, it is impossible to know for any given year how safe, or risky, it is to apply N in the fall. If the spring is dry, there is little risk of N loss from fall application (assuming N was applied correctly). On the other hand, if the spring is wet, the chance of N loss increases. All these points should be considered carefully to make the best possible decision. If you don't like taking high risks, but a fall application makes sense, it may be better to apply part of the N in the fall and wait until the spring to apply the rest. This approach is like buying an insurance policy--it gives peace of mind but costs money, and you can never be certain whether the investment will pay off.

In Summary

If you are preparing to apply N in the fall, heed the following guidelines:

--Fabián G. Fernández

Close this window