What Happens to Nitrogen When Manure is Applied on Frozen Soil or Snow?
Applying manure on top of frozen soils or even snow is never a good option. In some areas like Minnesota, though, a wet and delayed harvest and early-arriving winter backed some into making that choice. So what will happen to that nitrogen in the manure? Melissa Wilson, University of Minnesota manure management and water quality Extension specialist, gives her take on the matter.
When manure is applied on the surface of frozen soils or on top of snow, we have two concerns. First, it cannot seep into the ground, so if there is any runoff in your fields, it can carry the manure to low spots or away from the field entirely which may cause environmental issues. We have already seen widespread rain in December across southern Minnesota and snowmelt in January in many parts of the state. Fields with higher amounts of residue are less likely to have as much runoff as fields with low residue, so this problem may be worse in some fields and not others.
The second problem we have to consider is the ammonia losses. Manure has two main forms of nitrogen: organic-nitrogen and ammonium-nitrogen. When ammonium-nitrogen is on the soil surface instead of being mixed in with the soil, it can volatilize and be lost as ammonia gas. This is mainly driven by chemical and physical factors. While the freezing temperatures slow the reaction down, research suggests it doesn’t stop it entirely. Plus, with the freeze thaw cycles we have seen this year, it is difficult to pinpoint how much will be lost as the manure sitting on the surface freezes and thaws, too. This problem is likely to impact all manure types, but especially swine manure since the total nitrogen content is roughly 60% to 80% ammonium-nitrogen when applied.
The good news is that with the cooler temperatures, the conversion of ammonium-nitrogen in manure to nitrate-nitrogen form is minimal. We do not expect nitrate leaching or denitrification to be increased because of the conditions in which manure was applied this year. This is because bacteria are responsible for the conversion, and the freezing temperatures minimize their activity in the winter. This could change depending on the kind of spring we have, however.
Unfortunately, we cannot predict exactly how much nitrogen was lost this year if it was applied on frozen soil or snow. Manure nutrient release can vary depending on specific circumstances. Use this link for a table that gives guidelines about nitrogen availability and loss by method of manure application and animal type. My best advice is to keep an eye on your crop this upcoming year and be prepared to sidedress additional nitrogen if the crop is looking deficient. The Minnesota Pollution Control Agency says you can apply an additional 20% of total crop N needs above UMN nitrogen guidelines (PDF) if soil conditions or cool weather warrants additional nitrogen application.