Managing nitrogen fertilizer applications and especially fall applied nitrogen is important for optimal plant nutrient availability, crop growth, and ultimately yield. Nitrogen is one of the three primary nutrients needed for crop production and is a mobile nutrient in the soil. Losses of nitrogen can lead to less return on investment on fertilizer dollars spent and have environmental consequences. Applications of nitrogen in the fall may be favorable to some farm operations to spread out workload and reduce the potential for spring field compaction. Some factors to consider before applying nitrogen in the fall are soil temperatures, forecast air temperatures, and using a nitrogen stabilization product.
Four-inch depth soil temperature measured at 50 degrees Fahrenheit and trending lower is an important consideration for nutrient management in the fall. Applying nitrogen fertilizer in the fall at the 50-degree F mark slows the nitrification process, completely ceasing at 32 degrees F. It is also important to take into account the short- and long-term forecast air temperatures. Soil temperatures may dip below the 50-degree mark with cool nights, but may also warm back up with stretches of warm day time air temperatures. Taking a soil temperature reading mid-morning and mid-afternoon to find an average daytime soil temperature will help you understand how much variability may be present in soil temperatures.
When ammonia nitrogen is applied to soil it reacts with the positively charged soil converting NH3- to NH4+. Ammonium nitrogen (NH4+) is stable in the soil but can begin the nitrification process when soil temperatures are above 50 degrees F. If soil temperatures are above 50 degrees, Nitrosomonas bacteria in the soil convert ammonium nitrogen (NH4+) to nitrite (NO2-). The two-step process continues when Nitrobacter bacteria then convert Nitrite to Nitrate (NO3-). Nitrate is the dominant form of nitrogen available to plants, but must be carefully managed since it is water-soluble making it susceptible to nitrate leaching, runoff, or denitrification.
Generally, much of the nitrogen fertilizer needed for cereal plants such as corn and wheat isn’t until mid-season, so it’s important to manage nitrogen accordingly and keep it stable in the soil or protected throughout late fall and early spring. One strategy for keeping nitrogen stable in the soil longer into the growing season is to use a nitrogen stabilizer.
Nitrogen stabilizers can help prevent nitrogen loss by preventing parts of the nitrogen cycle that lead to losses. There are two broad categories of nitrogen stabilizer products, urease inhibitors and nitrification inhibitors. Urease inhibitors prevent the enzyme urease from converting urea into ammonium and are generally used with Urea and UAN forms of nitrogen. Nitrification inhibitors are a pesticide that kills a specific group of bacteria that are responsible for converting ammonium to nitrate and may be used with UAN products such as 28% or 32%, anhydrous ammonia (NH3), or manure. Different types of nitrogen stabilizers have specific active ingredients that you can find on a product label to verify what you are using.
WHEN TO USE FALL-APPLIED NITROGEN
Waiting to apply fertilizer or manure until late in the fall season when the soil and air temperatures are cold while also considering using a nitrification inhibitor are considered best management practices if you are deciding on when to make fall Nitrogen applications. Protecting your investment on dollars spent on fertilizer such as nitrogen requires you to think about using all the tools and resources available to you to maximize your yield efficiency and field profitability.
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