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Issue No. 22, Article 4/September 24, 2010

Still Too Early for Nitrogen

Last fall was one that many farmers would not want to repeat: harvest was late, soils were wet, soil compaction was common, tillage operations were almost unheard of, and nutrient application was a rarity. This fall, harvest is progressing very well and much earlier than in 2010. Many fields have been not only harvested, but tilled as well. It seems that no one wants to chance not being able to get in the field this fall. While it is good to get the crop out of the field early, collect soil samples while the weather is still nice, and do some tillage while soil conditions are adequate, it is no time at all yet for anyone to be in the field applying nitrogen (N).

Every fall, those who need to apply N worry that if they wait too long for temperatures to drop sufficiently, soils might become too wet to make the application. While the window of opportunity is limited, it is important to exercise good judgment in order to realize the benefit of such application. Managing N well is important, because this nutrient is both one of the most expensive inputs in today's farming operations and one that poses environmental concerns. Whether the consideration is cost, environmental implications, or both, we simply cannot afford poor N management. Being smart about N can pay large dividends.

Achieving that goal requires understanding some key factors. The following guidance will enhance the efficiency of your N management this fall.

When in Fall to Apply Nitrogen

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 critical to wait to apply N until the soil temperature at the 4-inch depth is below 50°F and is maintained at or below this value through the winter. In most years, that threshold allows for applying N before soils become too wet or frozen. It is also critical to understand that although the rate of nitrification is significantly reduced when soil temperature is below 50°F, microbial activity continues until temperatures are below 32°F. So to minimize risk, instead of trying to push the application as close as you can to the 50-degree mark, it is wise instead to wait as long as possible after the temperature drops below it.

Air temperatures can fluctuate substantially during early fall, so even if they are getting cooler, do not apply N before the second week of October in northern Illinois and the third week in central Illinois. Because of temperature considerations, N should not be applied in fall 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 with high potential for NO3- leaching in the fall or early spring (sandy soils and those with excessive drainage) should not receive fall applications.

Up-to-date soil temperatures are available. However, I recommend strongly that you monitor temperatures of soils in individual fields before applying N. This week we had air temperatures in the upper 80s to lower 90s in many places in Illinois--it felt like summer! (And I guess it was, as a glance at my office calendar, with illustrations by Norman Rockwell, tells me that it is indeed summer until the 23rd of September this year.) Current daily maximum 4-inch bare-soil temperatures for Illinois are in the upper 70s to mid-80s. These temperatures, combined with the rain some places are getting, are making paradise-like conditions for bacteria to be actively nitrifying any ammonium they get ahold of. While harvest progress might make you feel like it is already time to apply N, you can be certain that if you drop any N in the soil this early, it will be transformed to nitrate in a hurry and likely won't be there to feed the crop next year.

What Nitrogen Sources to Use and to Avoid

Anhydrous ammonia is a preferred N source for fall application because it has a slower nitrification rate than other 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 more of the applied N in the NH4+ form. For this reason I strongly encourage using such inhibitors for fall applications of N. As I mentioned earlier, nitrification continues until soils are frozen; using an inhibitor will lower the rate of nitrification between the time of application and when soils freeze.

N losses with anhydrous ammonia can occur at the time of application. To avoid volatilization losses during application, make sure the soil is neither too wet nor too dry to secure adequate closure 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.

Another source that can be used for fall applications is ammonium sulfate ([NH4]2SO4). Just as with anhydrous ammonia, this source should be applied late in the fall, when soil temperatures are below 50°F. It is an excellent source for no-till fields, where broadcast applications are preferred. It can also be applied on frozen ground, so long as the slope of the field is less than 5% and the potential for surface water runoff is very low. Ammonium sulfate is more acidifying than any other N source. This is not a problem as long as the pH of the soil is maintained at adequate levels. As a general rule, 5 pounds of lime is needed to neutralize 1 pound of N from ammonium sulfate, compared to 2 pounds of lime per pound of N from ammonia or urea.

As mentioned, ammonium is a stable form of N 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. The source of N used for fall applications is thus an important consideration. N sources containing N 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, using 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 for 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, I am currently researching it; the data is limited for determining whether such products could be used in the fall or should not be. Some of the most important considerations regarding their effectiveness are the thickness of the coating, the time required for the coating to degrade, and the integrity of the coating after handling and application. Since research results are still considered preliminary, I am not presently recommending these products for fall application in Illinois. After testing ESN, a PCU product, researchers in Minnesota have indicated that fall application of this product is acceptable, but it is a high-risk operation.

In recent years there has been renewed 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-, so the soil temperature recommendations already discussed also apply here. 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, an overapplication of phosphorus will result. For this reason, most often the application rate should be based on meeting the crop's phosphorus requirements rather than N requirements. If soil phosphorus levels are already adequate, an alternative is to apply the N requirements for one year and reduce or eliminate phosphorus application for the next few years to allow phosphorus levels to draw down back to where they were before the manure application. Knowing the soil phosphorus level and nutrient contents of the fertilizer is a must to determine the appropriate application rate.

How Much Nitrogen to Apply

To determine the economically optimal N rate at various corn and N prices, use this rate calculator. Just remember that the calculator does not account for carryover N. 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. Once you determine how much N you will need, it is important to remember that it is not necessary to make the entire application in the fall. Some producers might find it beneficial for their production system to apply a portion of the total rate in the fall and reserve the rest for a later application in spring.

Applying Nitrogen to Increase Crop Residue Breakdown

With more acres planted to corn following corn, there is great interest in managing corn residue. 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 from fall application of N to increasing 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 level, is the limiting factor for microbial decomposition of residue.

Evaluating Your Situation

Overall, research on N timing has shown that application in the spring, close to the time of rapid uptake, maximizes yield because there is less chance for leaching or denitrification. This is especially true for years where early spring conditions are warm and wet and corn planting is delayed. However, late-fall application of some N sources (previously discussed) is adequate, especially for medium- to fine-textured soils where cold winter temperatures prevail and early springs are not excessively wet and warm.

If a full rate preplant application is not an option for you, a possibility would be to apply a portion of the total N needed in the fall and wait until spring to apply the rest as a preplant or sidedress application. In some years sidedress applications are more effective than preplant, but the results depend on weather. Most often, though, under normal spring conditions there is little or no difference between fall and spring times of application.

Fall applications have both economic and logistic advantages. Soil conditions are typically more conducive to application, there is more time available than during the busy planting season, equipment and labor are better distributed, and often there are price incentives to buy anhydrous ammonia. The spring typically is wet, and soil compaction, especially for manure application, is of greater concern. Also, waiting until 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 how 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 big risks but a fall application makes sense, it may be better to apply part of the N in the fall and wait until 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 apply N in the fall, heed the following guidelines:

  • Wait until soil temperatures at the 4-inch depth are below 50°F.
  • Do not apply before the third week of October in central Illinois and the second week in northern Illinois, even if air temperatures are getting cooler.
  • Do not apply N, or N with a nitrification inhibitor, south of Illinois Route 16 or if soils are prone to leaching.
  • Use a nitrification inhibitor with anhydrous ammonia applications. Ammonium sulfate is an acceptable source.
  • Do not apply urea or nitrate-containing fertilizers.
  • If you are using animal manure, make sure it is incorporated into the soil, and follow the time-of-application guidelines discussed for commercial N management.
  • Apply the appropriate rate, taking into account leftover N when applicable, and consider applying only a portion of the total N needed in the fall and the rest in the spring.
  • Do not apply N to increase residue breakdown.

Consider both the risks and benefits of fall N application. If fall application is appropriate, follow the recommendations here to help increase the efficiency.--Fabián G. Fernández

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