By David Nicolai
As soil temperatures cool down, now is the time to plan your nitrogen management for 2012.
The first important question is "How much nitrogen should I apply?" University of Minnesota Extension nitrogen guidelines should be a starting point.
A new Extension nutrient management website (www.extension.umn.edu/nutrient-management) houses current fertilizer suggestions and data from the University of Minnesota. This website was made possible by funding from the Minnesota Agricultural Fertilizer Research and Education Council.
Nutrient calculators are now available on the website. There is also a link to the regionally developed corn nitrogen calculator hosted by Iowa State University (direct link http://bit.ly/z3evN).
The nitrogen calculator is easy to use. The user enters his state (Minnesota), type of fertilizer, price per ton and expected corn price per bushel. The calculator will then calculate the Maximum Return to Nitrogen, the nitrogen rate where the economic net return to nitrogen application is maximized. Users can also enter multiple fertilizer and corn prices at the same time so that they can view and print a comparison of prices. There are also online nutrient calculators for soybean and wheat crops on the nutrient management website.
The amount of nitrogen to apply for corn depends on four factors, all of which are taken into account with the online nitrogen calculator:
•productivity of the growing environment
•price of fertilizer nitrogen to value-of-corn-crop ratio
•amount of risk a grower wants to assume.
The nitrogen application for corn should be adjusted based on the amount of residual nitrate nitrogen as indicated in the Extension corn nitrogen guidelines.
The best time to test soils for soil nitrate-nitrogen will depend on several factors. First, if the field is in western Minnesota (approximately west of Highway 71), a soil sample from the surface 2 feet can be taken in the fall or spring. For the fall soil sample for nitrate-nitrogen to be accurate, it must be taken after the soil temperatures in the surface 6 inches have stabilized at 50 degrees or lower.
Fall applications of nitrogen are not recommended in southeastern Minnesota, according to established best management practices for the state. In south-central Minnesota, late fall applications with a nitrification inhibitor are acceptable but with greater risk. In southwestern, west-central, and northwestern Minnesota, late fall applications without a nitrification inhibitor are acceptable with greater risk. In late fall applications, the nitrogen fertilizer must be an ammonium type.
What is meant by late fall? Late fall application should occur only after soil temperatures in the surface 6 inches are at or less than 50 degrees. Applications made at temperatures greater than this (earlier in the fall) may convert from ammonium to nitrate. This means that applying nitrogen when soils are still too warm can lead to loss of that nitrogen. This is not what farmers want to happen to their fertilizer and time investment.
Plan fall-applied nitrogen so that it stays in ammonium form throughout the fall, winter and early spring. Otherwise, when present in the nitrate form, there is a substantial risk for loss of nitrogen.David Nicolai, U-M crops educator, and Dan Kaiser, U-M soil nutrient management specialist