Online Calculator for Corn Nitrogen Fertilizer Recommendations that Credit Cover Crops and Soil Organic Matter

Enter the inputs below to calculate a nitrogen fertilizer recommendation for corn based on site-specific cover crop and soil organic matter measurements. The equations in this tool have been calibrated based on field trials conducted across Pennsylvania over multiple years and are sensitive to regional climatic conditions. Because of the regionally-specific calibration, the tool should not be used to develop N fertilizer recommendations outside of Pennsylvania.

Revised with new calibration in February 2023.

Inputs

   The average corn yield goal should be based on the true average corn yield produced in the particular field over several recent years of production, including good, bad, and normal years. Inflating the average corn yield goal will lead to higher N recommendations that do not have positive economic returns.

  Clay % is most accurately determined with a soil sample analysis from a lab. For estimation purposes, clay % from a soil survey map can also be used, available at https://casoilresource.lawr.ucdavis.edu/gmap/

   Sand % is most accurately determined with a soil sample analysis from a lab. For estimation purposes, sand % from a soil survey map can also be used, available at https://casoilresource.lawr.ucdavis.edu/gmap/

   Soil Carbon % is most accurately determined with a soil sample analysis from a lab. The tool is calibrated for a 0-8 inch soil sample depth. For estimation purposes, organic matter % from a soil analysis can be converted to carbon % by multiplying organic matter by 0.59. If the organic matter is reported as %LOI (loss on ignition), multiply by 0.44.

   Soil C:N ratio is most accurately determined with a soil sample analysis of % carbon and % nitrogen from a lab. Soil C:N is calculated as % carbon / % nitrogen. For estimation purposes, a default soil C:N of 10 can be used.

   Winterkilled Cover Crop Biomass N should be measured or estimated for the winterkilled species of the cover crop in fall at peak biomass prior to frost killing. The credit from winterkilled cover crops is generally small and it is sufficient to use an estimate from a look-up table.

   Spring Cover Crop Biomass N should be measured or estimated in spring at peak biomass prior to termination. Cover crop biomass N can be estimated from a Greenseeker sensor NDVI reading (see conversion tool below) or a visual estimate.
   Spring Cover Crop C:N ratio should be measured or estimated in spring at peak biomass prior to termination. Spring cover crop C:N ratio can be measured by a lab from a representative grab sample of biomass or estimated from a look-up table.

Diagnostics and Results

   If the spring cover crop C:N ratio is greater than the critical C:N ratio for N immobilization, then the cover crop residue will immobilize (tie-up) N during the growing season and increase the N fertilizer requirement.
   The cover crop yield credit represents how the cover crop residues affect the predicted yield of an unfertilized corn crop. A positive credit indicates N supply from the cover crop and will reduce the N fertilizer recommendation. A negative credit indicates N tie-up by the cover crop and will increase the N fertilizer recommendation.
   The N fertilizer recommendation is the economically optimum rate of fertilizer to apply to reach the average yield goal. It is affected by the slope of the response curve, the price of corn and cost of N fertilizer. It represents the total amount of N that should be applied as a combination of at-planting N fertilizer, sidedressed N fertilizer, and available manure N.

This calibration of the Greenseeker sensor to predict cover crop nitrogen is based on research published by White et al. (2019) in an article that is freely available to the public here.