Fall N application: What, when, where and how
Where to Apply
Because temperatures do not stay below 50°F long enough during the winter in all of Illinois, fall nitrogen application should not be done south of a line roughly parallel to Route 16. In areas near this boundary, evaluate soil characteristics to determine whether fall application is appropriate. Soils with high potential for nitrate leaching in the fall or early spring (sandy soils or those with excessive drainage) should not receive fall nitrogen applications. Also, regardless of location in the state, do not apply nitrogen in the fall to soils with high potential for nitrate leaching or soils that are very poorly drained.
Given the length of time between application and utilization by the crop, application of manure and other organic nitrogen sources 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.
How to Apply, and How Much
When applying anhydrous ammonia, make sure soil conditions are fit for the application. Soils that are too dry or too wet can result in ammonia loss to the atmosphere because the application knife tracks may not seal properly. When soils are dry, increasing depth of application or reducing application rates typically can help minimize volatilization loss. When soil is wet, little can be done to minimize loss through volatilization. If you use manure, poultry litter, or other animal-derived fertilizers, incorporate them into the soil to avoid volatilization.
To determine the economically optimal nitrogen rate at various corn and nitrogen prices, use the corn nitrogen rate calculator at extension.agron.iastate.edu/soilfertility/nrate.aspx. While the calculator is designed to help you make the most profitable decision for nitrogen management, it does not account for carryover nitrogen that might not have been used by a crop if conditions were dry. Also, if you applied manure or the soil has high potential for nitrogen mineralization (as in the case of a field coming off of alfalfa), you will need to adjust the values derived from the calculator to reflect what will be available next year.
Once you determine how much nitrogen you will need, remember that you don't have to apply the entire amount in the fall. If you don't like taking big risks, but a fall application makes sense, it may be better to apply some nitrogen in the fall and the rest in spring. A portion of the total nitrogen requirement applied in the fall can provide all of what the corn crop will need to get started in spring. Applying the remainder closer to when plant will need the most nitrogen can increase use efficiency because there is less chance for leaching or denitrification. Also, research has shown better efficiency of nitrification inhibitors when lower nitrogen rates are used in the fall. Splitting the total application thus might produce benefits on several fronts.