Fall Applied Manure - Will the Nitrogen Still be There?

Manure applications can be a valuable component of a nutrient management program but timing those applications to maximize the utilization of nitrogen can be challenging. Manures generally contain two forms of nitrogen, ammonium and organically bound nitrogen. Ammonium is immediately plant available and relatively immobile in the soil because it is a positively charged ion that held by the cation exchange capacity, similar to potassium. Organically bound nitrogen is also immobile in the soil, but it is not plant available until the organic matter is microbially decomposed, mineralizing the nitrogen.

In an ideal situation, manure applications would occur either into an actively growing crop or shortly before a crop is planted to gain the most benefit from the nitrogen. Unfortunately, our cold wet soil conditions in the spring and the types of application equipment we have available often do not allow for spring or early summer application. As a result, much of the manure in our region is land applied following harvest in the fall and early winter leaving several months for potential nitrogen loss.

The first step to maximizing the benefits of manure is to keep it on the field and in the soil profile. The most obvious potential loss of nitrogen when manure is fall applied is surface runoff. Runoff can be minimized by avoiding applications on saturated, frozen, or snow-covered ground; incorporation with tillage; or subsurface injection.

Once the manure has been incorporated into the soil profile, there is still the potential for substantial nitrogen loss to occur before the next crop can utilize it. The two potential mechanisms for loss are leaching and denitrification. Leaching has the greatest potential to occur on well drained soils during periods of heavy precipitation. However, for significant losses of nitrogen to occur due to leaching, the nitrogen needs to have been converted to nitrate prior to the precipitation. The conversion of ammonium to nitrate is a microbial process that will only occur when the microbes have adequate temperature and aeration in the soil to be active. Denitrification is the conversion of nitrate to gaseous forms that can be lost to the atmosphere. This is a microbial process that only occurs when soils are saturated, and the microbes are in an anerobic environment. Microbial activity is minimal when soil temperatures are below 50 degrees. If soil temperatures remain cold after application, the chances of significant nitrogen loss are minimized.

The winters and springs in this region are generally cold enough to keep most of the nitrogen in the soil profile. However, over the last few years this has not always been the case. The spring of 2017 was unusually warm with February and March temperatures reaching 70+ degrees. In one situation, using a soil nitrate and ammonium test, we were able to confirm the loss of nearly all the nitrogen following a fall hog manure application that was subsurface injected and treated with a nitrogen stabilizer. The grower in this situation had to supply a full rate of nitrogen fertilizer though sidedress to sustain his corn crop. The next season, following the very cold spring of 2018, the same grower in the same exact situation was able to confirm that no additional nitrogen was needed to produce his crop.

Following a relatively mild winter, the approaching spring of 2020 looks to be warmer than average leading to greater potential losses of nitrogen. If you are planning to plant corn into a fall manured field, a pre-sidedress soil nitrate test is the best tool to assure that you have adequate nitrogen. For more information regarding sampling procedure and data interpretation, please see our fact sheet.