Oct. 2, 2012
This year the crops have matured early and harvest is moving ahead of normal. With a large amount of the soybeans and corn coming out, thoughts are turning to getting fertilizer applied for next year's crop. For phosphorus (P) and potassium (K), there are very few problems with an early fall application. These nutrients are not mobile in most soils. The only big concern with a broadcast application of P and K is getting the fertilizer incorporated into the soil so it is in a place for the plant roots to utilize them next spring. Incorporation also reduces the chances of P and K being lost through erosion.
Nitrogen (N) is a mobile nutrient and therefore must be managed different to get the most nutrient value and the least amount of loss to the environment. If you are in the southeastern part of Minnesota or farm sandy ground, DO NOT apply N in the fall. The rainfall in southeastern Minnesota along with the Karst geology will result in large losses of N from fall application. If you farm sandy ground, N applied in the fall will not be in the soil when spring arrives. Fall N application on sandy soil, irrigated or not, is a total waste of time and money and presents large risks of groundwater pollution.
In the south-central part of Minnesota, application of fall N is not the most efficient management option. If your operation requires you to apply some N in the fall, there are some things you can do to get the most N out of the fertilizer application. First, DO NOT apply N fertilizer before the soil temperature at the 6-in. depth is consistently below 50° F. Second, use only an ammonium form of nitrogen. Anhydrous ammonia would be the preferred, followed by urea. If you have a field that is consistently wet, you may want to consider the use of a nitrification inhibitor to slow the conversion of ammonium to nitrate. The use of the inhibitor is NOT a way to allow for application before the soil temperatures are below 50°.
In southwest, west-central and northwestern parts of Minnesota, fall application of ammonia-based N sources is ok if the soil temperature is less than 50°.
At the time of writing this, Minnesota agricultural areas were experiencing drought at one degree or another. The ground is hard – maybe harder than last year. With this in mind, a late-fall application of N after we receive some rain maybe the best fall option. It will reduce the chances of loss by getting a better soil cover of the ammonia band and also save on the wear and tear of the tillage and application equipment. If you use urea, it must be incorporated to keep it from volatilizing. Dry soils are good candidates of urea volatilization to occur. Research with fall N applications, has shown that anhydrous ammonia will have a lower loss of nitrogen than urea.
Also with the dry summer, it is strongly suggested that you take a soil nitrate-N test. This is particularly true if the 2012 crop was corn. With the dry summer, the crop may not have used all of the N fertilizer applied for the previous crop and left a large amount of residual nitrate-N that could be used by the 2013 crop. To be useful, a soil sample for nitrate should be taken to a depth of 2 ft. for corn. The sample should be taken after the soil temperature is below 50°. A soil sample taken before the soil temperatures are below 50° is a waste of money and time. The nitrate-N soil test value will be erroneous.
If the weather conditions continue dry into winter, you should strongly consider spring application. Spring applications result in less chance of N loss and you will also have a better idea of the crop potential in 2013. A spring preplant soil nitrate-N test will also be helpful, similar to the fall soil test described in above.