By Jeremy Klatt, Iowa Department of Natural Resources environmental specialist
The wet weather in Iowa this spring has many livestock and crop producers concerned about nitrogen availability from fall-applied manure. For animal producers with a Manure Management Plan (MMP), there are ways to address this concern.
First, producers should review their fall application rates and determine if the maximum allowable nitrogen application rate was applied. Additional nitrogen, up to the rate allowed by the MMP, can be applied without any field testing.
Secondly, state law allows for additional nitrogen application, even if the maximum allowable rate was applied in the fall, if recommended by the Late Spring Nitrate Test as described by the Iowa State University publication CROP 3140, Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production. This test involves taking soil samples when corn is 6-12” tall and provides recommendations based on the concentration of nitrate in the soil at that time.
Any additional nitrogen applied must be documented in the facility’s application records for five years. If the Late Spring Nitrate Test is the basis for additional nitrogen application, those soil samples also must be included in the records.
The MMP regulations also allow for a change in the crop planned for a field due to wet weather, even if it would result in application of nitrogen above the allowed nitrogen rate for the new crop. For example, if manure was applied in the fall for a planned corn crop, but due to wet conditions soybeans are planted instead.
Lastly, the law limiting application of manure for a soybean crop to 100 lbs. N per acre does not apply after June 1 of each year. Therefore, if a confinement producer needs to apply manure to a field that will be planted to soybeans after June 1, it can be based on the nitrogen uptake of the soybean crop (i.e. 3.8 lbs. N/bushel) and the optimum crop yield for the field.
Should a confinement owner have any questions about this information, contact your local DNR field office.