Nitrogen management this fall
How much N? To determine the economically optimal N rate at various corn and N prices, use the Corn Nitrogen Rate Calculator. While the calculator is designed to help you make the most profitable decision for N management, it does not account for carryover N that might have been unused by crops because of the dry conditions in many places this year. This topic was discussed in issue 21 of the Bulletin (September 7). Also, if you applied manure or the soil has high potential for N mineralization (like in a field coming off of alfalfa), you need to adjust the values from the calculator to reflect what will be available next year.
Once you determine how much N you will need, remember that you need not make the entire application in the fall. If a fall application makes sense but you don't like taking big risks, consider applying some N in the fall and the rest in spring. Many fields will likely have high nitrate levels this fall because of the drought, and it is uncertain how much of that N will be present for the next crop. If a good portion is available, that should be all the plant needs to get started until sidedress time, which would reduce the need to supply additional N in the fall. If N is not present because of excessively wet conditions in the spring, chances are that a fall application of N could suffer similar losses.
Applying N in the spring, or splitting the application to supply N closer to when plants will need it, can increase use efficiency because there is less chance for leaching or denitrification. Research has also shown better efficiency of nitrification inhibitors when smaller N rates are used in the fall. So splitting the total application might result in benefits on several fronts.
An ongoing study over three years showed that fall applications reduced yield 17 percent relative to preplant applications done within three weeks of planting. The difference in yield, averaged across N rates, was 23 bushels per acre less with fall than with preplant applications. We are conducting the study this year, but I do not currently have yield information. I suspect, though, that differences might not be as large this year because there was very little N loss potential in the spring and because the drought was more limiting than any other factor.
Use caution. Be aware that anhydrous ammonia is under a lot of pressure inside the nurse tank, and when released it reacts quickly with water. If ammonia comes in contact with skin, eyes, or mucous membranes, it will cause dehydration and burns, so please use extreme caution when handling it.