Preventing Nitrogen Loss in your Fall Applications
It is well known that plants use Nitrogen most efficiently when applied as close as possible to the time of plant uptake. However, we know that regardless of how you are applying your Nitrogen – it is subject to potential losses. The risk of this loss depends on the type of Nitrogen source you are putting down, your soil type, temperature, and management practices.
Dealing with Nitrogen loss is a challenge every grower faces, whether applying Urea, UAN, or Anhydrous – spring or fall. As you begin preparing your fall Nitrogen fertility plans, it is important to understand how to mitigate each of these losses.
Your ultimate goal is preventing Nitrogen conversion in your fall applied fertilizer – to protect against nitrogen loss between fall and spring.
So, what do you need to know? It begins by understanding how the conversion process works, factors that cause conversion and ways you can prevent conversion from taking place.
Understanding Nitrogen conversion
Going to the effort and expense of putting Nitrogen down without understanding the factors that can result in Nitrogen loss is like filling your pockets with loose change, only to discover you have a hole in the bottom.
Nitrogen, in the form of urea and UAN, goes through a complex “conversion” process that is required to transform the applied fertilizer into a form that is usable to the plant. These processes are commonly known as hydrolysis and nitrification. Both are necessary. Yet, if they take place prematurely, these processes can lead to costly Nitrogen loss over the fall and spring – before the plants are even in the ground. Let’s take a closer look.
Step 1: the conversion of urea or UAN to Ammonium
When the urea (or urea component of UAN fertilizer) is applied, it reacts with a common enzyme in the soil, called “urease enzyme”. This enzyme converts the urea to Ammonia (a process known as urea hydrolysis). Ammonia is a gas, which can dissolve (or “volatilize”) into the atmosphere.
Fortunately for growers and plants, hydrogen molecules in the soil helpfully convert Ammonia to Ammonium – which is a stable (and plant accessible) form of Nitrogen.
It is important that you know the pH level of your soils, as this has a direct impact on the conversion process. Acidic soil (0 – 7 pH) has more available free hydrogen to stabilize the ammonia. The more basic the soil (7 – 14 pH), the less available free hydrogen.
Step 2: The conversion from Ammonium to Nitrate
The form of Nitrogen that is most accessible to plants is Nitrate. So how do you get from Ammonium to Nitrate? It is achieved thanks to the hard work of two essential bacteria in the soil.
Nitrification is the process that happens when Nitrosomonas Bacteria converts the stable Ammonium to Nitrite, which is next converted to Nitrate by another bacteria (Nitrobacter Bacteria). While the conversion to Nitrate is important when the crop is in the ground, it is not so desirable in the context of a fall application. This is because Nitrate exists in a mobile state that can be lost to the atmosphere if there are no plants to utilize it. Denitrification is the process where Nitrate is stripped of oxygen and released into the atmosphere as N2 gas.
Nitrogen can also be lost through leaching. This occurs when the negatively charged Nitrate particles become unstable because they do not bond with the soil particles, which are also negatively charged. A rain fall can wash this Nitrate away in course textured soil situations.
Preventing loss of applied Nitrogen
The most important thing to consider when fall applying urea or UAN is how to prevent conversion from taking place before the crop is in the ground – and therefore – how to avoid volatilization, denitrification and leaching.
3 things you need to know about the conversion from urea and UAN to Ammonium
• Urease enzyme is active in the soil between -20°C and +30°C. So, it is still active when the temperature drops below zero.
• It’s more concentrated in fields with higher thatch or organic material.
• Moderately dry soils and soils with high pH levels tend to have lower hydrogen content, and are at higher risk for volatilization. With less free hydrogen, the conversion from Ammonia to Ammonium takes longer.
3 things you need to know about the conversation from Ammonium to Nitrate
• While you want the plant to access Nitrate, you don’t want the conversion to happen in the fall or early spring when there is no crop in the field.
• As the temperature drops, aerobic bacteria (including Nitrosomonas and Nitrobacter) experience a drop in activity. They cease to move at the freezing point.
• Denitrification is most common in damp, moist climates with temperatures about 5°C and clay soil structures.
So, based on these insights, what is the ideal time to apply urea or UAN?
Like an anhydrous Ammonia application, when applying fall urea or UAN it is advisable to wait for cooler soil temperatures of 8°C or lower (mid October on, in most years). The likelihood that your fertilizer will fully convert all the way through to Nitrate is greatly reduced because of the declining activity of the bacteria in this conversion process.
The bigger challenge, then, is to inhibit or slow the conversion of urea (or urea in UAN) to Ammonium.
Protecting your fall Nitrogen investment
The greatest risk around a fall urea or UAN application arises from the fact urease enzyme is active until the thermometer drops below -20°C.
Using a urease inhibitor (also known as a “Nitrogen stabilizer”) allows you to confidently apply fall urea or UAN in October, when the bacteria activity is in decline.
Nitrate conversion can be prevented through the use of dicyandiamide (DCD) or nitraprynin-based products if earlier fall application is desired when soil temperatures are still warm for the bacteria to be fully active.
By delaying urea/UAN application until soil temperature cools to 8°C, and by employing a urease inhibitor throughout the fall season, you can do your part in preventing Nitrogen conversion in fall urea or UAN from taking place, and reap the logistical benefits of a fall Nitrogen application. Most importantly, you’ll protect your investment from vanishing into thin air.
Related blog posts: The Two Minute Drill: How to maximize useable Nitrogen in your crop.