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Manure Minute: The nitrogen cycle and manure


March 4, 2021
By Chryseis Modderman | University of Minnesota Extension

Nitrogen is a vital nutrient for plant growth, and it’s often the nutrient that gets the most attention when it comes to fertilizers. Fortunately, manure happens to be an excellent source of nitrogen. Understanding how manure nitrogen behaves after application is important in order to know how best to manage manure.

Nitrogen forms in manure
There are two forms of nitrogen in manure: inorganic and organic. The inorganic form is immediately plant available, while the organic form is not. Broadly, inorganic N refers to all forms of nitrogen that are immediately plant available, including ammonium and nitrate. For the most part, however, manure does not contain nitrate. So, the plant-available part of manure N is nearly all ammonium. 

Organic N is tied up and needs to be broken down by a process called mineralization in order to become plant available. This breakdown of organic N is what makes estimated available nitrogen in year two tricky. Mineralization is driven by soil microbes and the speed at which they work is heavily influenced by the environment; and we all know how fickle the weather and environment can be!

Besides mineralization, the other major N transformation that happens in the soil is nitrification. This is when ammonium is converted to nitrate. Remember that manure supplies ammonium and not nitrate; but ammonium can convert to nitrate once in the soil. This is also a microbially driven process that’s dependent on environmental factors.

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Nitrogen loss pathways
Of all the nutrients in manure, nitrogen is most easily lost to the environment. It’s important to keep N where you put it so that you don’t lose valuable nutrients and cause nutrient pollution. When manure is applied, volatilization is the first type of loss you need to worry about. That’s where ammonium is lost to the atmosphere as a gas when manure is left sitting on the soil surface. If you’re working with a high-ammonium manure, such as swine manure, you could lose up to 50 percent of your total nitrogen to volatilization in just four days. Luckily, there’s a pretty easy way to minimize volatilization loss: incorporate manure into the soil. Getting the manure below the soil surface through tillage or injection will greatly reduce volatilization loss.

Once in the soil, ammonium is going to pretty much stay put. However, if ammonium converts to nitrate (through nitrification), it can be lost more easily. Nitrate readily dissolves in water and will travel wherever water will. That means it can be lost downward through the soil profile (leaching), through tile drainage, or across the landscape as runoff. In addition, nitrate can be lost to the atmosphere as a gas (denitrification). As you can see, to prevent N loss, it’s worthwhile to keep it in the ammonium form and not let it transform into nitrate. The nitrification process can be slowed down by applying when soils are cool – below 50 F is generally recommended. But keep in mind that even in cold soils, nitrification still happens – just much more slowly. You can also avoid nitrification by applying manure to an actively growing crop or to soil that will soon have crops planted. If plants take up and use the ammonium, it won’t have the time to become nitrate.

Testing manure for nitrogen
While there are nutrient tables out there for nearly every type of manure, those are just estimates. We recommend sampling your manure for nitrogen as well as other nutrients. Actual N content of manure can vary based on a whole host of management factors including feed, housing type and storage system. Remember that not all of the N is immediately usable by the plant. So, instead of using total nitrogen to calculate application rates, you’ll want to multiply by the availability factor. Each state or region has its own recommended availability factor for each manure type, so be sure to use your local recommendations. •