Nitrogen fertilizer management is highly impacted by changes and variability in climatic conditions. The specific aspects with the greatest effect include wet conditions in the fall, shortened winters, delayed spring fieldwork season, excessive precipitation early in the growing season and unusually heavy individual rainfall events.
The overarching strategy for dealing with all of these conditions is to have a plan for your ideal management, know what your available alternate options are and understand how and why to make changes based on conditions.
The N cycle
Understanding the nitrogen (N) cycle is key if you are to successfully evaluate what is happening in your fields. Most of the N, whether mineralized from organic sources or applied as fertilizer, will start out in the ammonium form.
Ammonium is a positively-charged cation that does not readily move in the soil. Ammonium will convert to nitrate through a natural microbial process that involves heat, moisture and time.
It is the nitrate form that is problematic, as this negatively-charged anion will both leach from the soil into groundwater and denitrify (moving into the atmosphere as a gas) when the soil is saturated.
Forms of N
Not all forms of N fertilizer are created equal when it comes to behavior in the environment. Anhydrous ammonia (AA) is both the most concentrated form of N fertilizer and the most stable in the environment. AA will kill microbes in the soil that cause the conversion to nitrate, delaying the process. In addition, a nitrification inhibitor can be added to further delay the process.
Conversely, urea is a neutral molecule that is water-soluble, so it can begin moving in the soil profile before it even converts to its basic ionic forms. Urea does not adversely impact soil microbial life, so it will convert to ammonium and then on to nitrate much more rapidly than AA. This is the reason fall urea is discouraged and should not be considered when the fall is wet.
In an extensive fertilizer source and timing study conducted by University of Minnesota researchers over several years, locations and crop rotations showed an overall average of 30 pounds less N required to achieve optimum yield when applying urea in the spring versus the fall, and yields increased an average of 13 bushels per acre with spring application.
It is important to realize that any application of N in the fall during wet conditions could be at risk.
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Several research projects conducted at the University of Minnesota’s Southern Research and Outreach Center in Waseca have shown that while there are similar losses of nitrate to tile water when comparing fall application of AA with a nitrification inhibitor versus spring N application, yields are often reduced. This is likely due to denitrification.
Management for shortened winters will be similar to that of wet falls, with a need to move fertilizer application to the spring to avoid N loss. The other management consideration for an extended fall and early spring is to use cover crops to try to scavenge any residual N in the soil profile.
Delayed spring fieldwork seasons have been a major problem in Minnesota the past two years. Research has shown the importance of planting early in order to take advantage of the entire growing season.
Farmers should place a higher priority on planting in a timely fashion than on applying fertilizer. In most areas, growers should be able to apply fertilizer in-season. An application of starter fertilizer at planting is sufficient under most circumstances until the full rate can be applied.
The exception to this is corn following corn, where it is a good idea to have at least 30 to 50 pounds of N applied at the time of planting.
Excessive moisture early in the growing season has caused issues with N loss in many locations over the past decade. As a rule of thumb, nitrate will move about 6 inches down in the profile for every excess inch of water that drains. This means about 5.5 to 6 inches of drainage is necessary to leach the majority of the N out of the profile.
Loss of all applied N is highly unlikely, but it is common for portions of nitrate to move deep into the profile, causing deficiency symptoms to manifest in the crop until root growth reaches it.
As previously mentioned, denitrification is a microbial process and therefore affected by temperature. When the soil is completely saturated and the soil temperatures are 70°F or higher, loss through this process can be significant.
It is difficult to know exactly how much N has been lost, but the University of Minnesota has a decision aid worksheet you can use to assist with the process. Other technologies such as soil testing, sensing and crop modeling can be of assistance too, but there has not been enough research under these circumstances to know how effective they are.
When making an in-season N application, one needs to be mindful of the growth stage of the crop. Corn enters its rapid growth period at about V8, and you should strive to have all of your fertilizer applied by then.
Y-drop applicators have become very popular over the last few years, and this is a fine method of side-dressing. If you choose to top-dress with urea, you should use a urease inhibitor to prevent volatilization loss.
In the absence of a crystal ball to forecast the exact weather conditions you will experience, it is not possible to know the best N management plan until after the growing season is over.
Nevertheless, as long as you have a plan, know your alternative options and understand how and when to modify your practices, you should feel confident you’re doing the best you can to maximize profit and minimize loss to the environment.