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Issue No. 23, Article 8/October 5, 2007

Fall Nitrogen Applications

Earlier planting and above-normal temperatures during portions of the growing season have induced corn to mature earlier this year. With the early harvest there is a tendency to take advantage of the good weather and soil conditions to start field operations as soon as possible in preparation for next year's crop. While a head start in planning and actual field operations could be beneficial, there are practices related to nitrogen-containing fertilizers that need to wait. Nitrogen (N) is both one of the most expensive inputs in today's farming operations and a nutrient that can pose environmental concerns. Judicious fall N management to protect the environment and to improve N effectiveness and profitability is ever so important.

Fall Nitrogen Application Timing

Since N transformations in the soil are largely mediated by soil organisms, and temperature has an important impact on their activity, it is critical to wait until soil temperature at 4 inches is below 50°F and is maintained at or below this through the winter. In most years, the 50°F temperature allows for N applications before soils become too wet or frozen. Application of anhydrous ammonia with a nitrification inhibitor (see the discussion below) can start after the soil temperature at 4 inches is below 60°F.

It is important to note that the rate of nitrification (the conversion of ammonium [NH4+] to nitrate [NO3-]) is significantly reduced below the recommended 50°F soil temperature, but microbial activity continues until temperatures are below 32F. Also, soils with high potential for NO3- leaching in the fall or early spring due to soil characteristics (i.e., sandy soils) or excessive drainage should not receive fall N applications. Temperature considerations mean that 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 N application is appropriate. Daily maximum 4-inch bare soil temperatures can be obtained on the web. However, it is strongly recommended that the temperature of soils in individual fields be monitored prior to applying N.

Nitrogen Source

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. Thus the source of N used for fall applications is an important consideration. 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 nitrogen is already in a form that can easily be leached or denitrified.

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+.

Since urea [CO(NH2)2] is converted to NH3 and then to NH4+ within a few days of application, this is an appropriate nitrogen source for fall 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 due mainly to greater risk of NO3- losses before rapid nutrient uptake by the crop the following spring. If surface applied, urea should always be incorporated within 2 to 3 days from the time of application, either by tillage or rain. The use of urease enzyme inhibitors (such as Agrotain: [N-(-n butyl) thiophosphoric triamide, or NBPT]) to slow the rate of conversion of urea to NH3 is recommended if rapid incorporation of urea is not possible. If urea is left on the soil surface, NH3 can be volatized to the atmosphere. Volatilization can occur especially when temperatures are above 55F, the soil surface is moist and drying quickly, or the soil surface is under heavy crop residue.

Nitrification inhibitors (such as dicyandiamide, or DCD) or Nitrapyrin (also known by the trade name N-serve) are chemicals that inhibit the activity of nitrosomonas bacteria. These bacteria are responsible for the first step in the process of nitrification; 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 of time a greater proportion of the applied N in the NH4+ form. Since nitrification rates increase under warm temperatures and moist soil conditions, the use of nitrification inhibitors is especially useful when those conditions prevail in the fall.

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 about it. Some of the most important considerations regarding 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 of these products. Since research data on the products is still considered preliminary, they are not presently being recommended for fall application.

Applying Nitrogen to Help Break Down Residue

Finally, with the expected increase in the number of acres with corn following corn, there is great interest in corn residue management. One common question has been whether applying N 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. This topic was covered recently in the article "Notes Heading into Harvest 2007" published September 7, 2007, in issue No. 22 of the Bulletin.--Fabián Fernández

Fabián Fernández

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