Have you heard the phrase “manure is the gift that keeps on giving”? It refers to many different aspects of manure, but the gift of nitrogen is one of them. The phrase refers to how quickly, or not so quickly, the nitrogen releases over time. This depends on several factors but there are still some things we are looking into through our research.
Animal Species and Storage
Manure nitrogen can come in two broad forms: organic and inorganic. The inorganic form is primarily ammonium, which is immediately plant-available once land-applied, similar to a chemical fertilizer. The organic form is nitrogen that is tied up in organic matter and carbon. It has to be decomposed and released by microorganisms in the soil before it transforms into inorganic, plant-available nitrogen. Thus, organic nitrogen is more “slow-release” – the gift that keeps on giving!
When we test manure for nitrogen content, we find different ratios of organic and inorganic nitrogen depending on the animal species and how the manure is stored. Hog manure and poultry litter tend to have higher inorganic nitrogen content than cattle manure. This is likely due to differences in feed and how different animals digest their food. On average, the nitrogen content of liquid hog manure is 75-80% inorganic nitrogen. This means that the majority of it is plant-available the first year after it’s applied!
How manure is handled and stored plays a role, too, especially whether bedding is used. Liquid and slurry manures (0-10% solids) tend to have much higher amounts of inorganic nitrogen than semi-solid and solid manure (10% solids and higher), which often have bedding in them. For example, liquid dairy and beef manures tend to have just over 50% inorganic nitrogen when they come out of storage and are ready for land application. Solid dairy and beef manure, however, only have about 10-15% inorganic nitrogen available! This means that solid manures will release nitrogen much more slowly over time than liquid manures.
Distribution of nitrogen in livestock manure by species and storage type. Inorganic nitrogen is shown in the red portion of each bar (with the percentage listed) and organic nitrogen is shown in the yellow portion of each bar. Liquid manure is defined as manure with 0-10% solids while solid manure has more than 10% solids. Photo by: University of Minnesota Extension
Application Method
Land application also plays a role in manure nitrogen availability. If manure is left on the surface, the inorganic nitrogen (primarily ammonium in the case of raw manure) can turn into ammonia gas (through a process called volatilization) and be lost. This is especially true in warm, windy conditions; though, volatilization can happen even in cold temperatures. Incorporating the manure quickly or injecting it below the surface helps to minimize these losses.
We try to take these losses into account when considering how to calculate manure nitrogen availability. However, the type of application equipment will affect things. For example, is a portion of the manure left on the surface after application? How much? Is the manure liquid enough to soak into the soil quickly or does it sit on the surface? As new equipment becomes available, one of our on-going research areas will be trying to determine how application method affects manure nitrogen availability.
Weather
Since soil microorganisms play a role in the nitrogen cycle and weather impacts microbes in the soil, you can guess that the weather plays an extremely important role in manure nitrogen availability. Soil microbes are like goldilocks. They want conditions that are warm (but not too hot or cold) and moist (but not too wet or dry). They can still work under poor conditions, it just may be slower and less efficient than in optimal conditions. In some years, the organic portion of manure nitrogen may release really well during a growing season, while in other years it’s too fast or too slow for the crops. As described above, the weather also affects ammonia volatilization if the manure is left sitting on the surface.
Cropping Systems
One topic of research evaluating manure nitrogen revolves around cropping systems and their effect on availability. If a farmer applies manure into a living crop (like a cover crop or hay ground) and wants to use no-till, does the crop canopy reduce ammonia volatilization if it’s tall enough? Another question: Does the previous crop affect nitrogen release? Anecdotally, manure seems to be more variable when used in continuous corn systems compared with corn-soybean rotations. Is that due to differences in nitrogen availability or is it something else?
What’s Next?
Most of our current recommendations regarding manure nitrogen availability come from research a few decades ago. With the unpredictable and changing weather patterns that we are experiencing in Minnesota, changes to livestock diets and housing, plus new manure application technologies, our team has been working on field trials to see if those calculations still work or if we need updates. As new information comes available, you will find updates to our website and our team will be giving presentations around the state. Don’t forget to follow us on social media!
More Resources:
- Do the “4Rs” apply to manure? You betcha!
- Tips for fall manure application and how to avoid nutrient loss
- Nitrogen Smart’s online course has advanced sessions on manure management, the 4Rs
