Manure minute: Avoiding N losses from high-ammonium manure
July 8, 2019 by Chryseis Modderman
Field-applied manure supplies two main forms of nitrogen: organic N, and ammonium N. The ammonium portion is immediately available for plants to use, and the organic portion is not and needs time to break down to become plant-available.
Manure from monogastric animals, such as swine, tends to be high-ammonium, containing more ammonium nitrogen than organic nitrogen. Ruminants, such as cattle, are the opposite with more organic N than ammonium.
While the organic nitrogen fraction of manure will stay put and sit tight until it is broken down, ammonium has a bit of wanderlust and is more easily lost – especially when it is converted to nitrate. Therefore, high-ammonium content manure presents unique challenges for minimizing nitrogen losses, as compared to high-organic N manure. Note that high-organic N manures have their own set of challenges, but today let’s just focus on high-ammonium manure.
When manure is left on the soil surface, ammonium rapidly converts to ammonia gas and is lost to the atmosphere in a process called volatilization. If you surface-apply a high-ammonium manure and don’t incorporate within four days, you can expect to lose half of the total nitrogen to volatilization. Of course, the organic N portion will remain, but a 50 percent reduction in overall nitrogen is significant. In this situation, manure with less ammonium would lose less total nitrogen to volatilization.
When ammonium is converted to nitrate in a process called nitrification, there are additional ways in which it can be lost. Nitrate is easily dissolved and will travel readily with water; whether that is runoff into ditches and waterways, or leaching downward into groundwater. Nitrate can also be lost as a gas through a process called denitrification. Ammonium is only one step away from nitrate; and if nitrification conditions are right, large amounts of nitrate may form with high-ammonium manure. In contrast, organic N is two steps away from nitrate as it must first be mineralized into ammonium, then turned into nitrate through nitrification. So, while high-organic N manure can still present nitrate problems, those challenges are more delayed with better odds that a plant will use that ammonium before it ever gets the chance to become nitrate.
Avoiding nitrogen loss
To retain nitrogen in high-ammonium manure, keep your ammonium in that form by avoiding conversion to ammonia gas or nitrate which are easily lost. Here are a few tips:
Whether you use tillage or injection, get the manure under the soil surface as soon as possible after application. The longer you wait to till in manure, the more ammonium will be lost to the atmosphere; incorporation within 12 hours is recommended. Injecting liquid manure will also minimize volatilization losses since the manure will spend no time on the surface.
Apply in the spring, or sidedress in the summer. Spring pre-plant or summer sidedress applications minimize the length of time manure sits idle before being used by plants. That means less time for ammonium to convert to nitrate and be lost.
For fall applications, wait until the soil temperature is below 50 degrees Fahrenheit. Since nitrification is a biological process, waiting for cool soils will slow down the conversion to nitrate.
Side note on nitrification inhibitors: Do nitrification inhibitors work with manure? Sometimes, and it depends. Research on this topic has given mixed results that are sometimes conflicting. But most studies agree that the same benefit from inhibitors can be achieved by simply waiting for cool soil temperatures before applying in the fall. If you do use these products, remember that they are not a silver bullet and will wear off over time.
Overall, nitrogen in high-ammonium manure has the advantage of being readily available for plant use, but at the cost of being more easily lost through volatilization, leaching, and denitrification than low-ammonium manure. However, there are management practices that can help retain that nitrogen such as prompt tillage, injection, spring application, sidedressing, and application to cool soils in the fall.