Nitrogen Recommendations for Corn That Credit Cover Crops and Soil Organic Matter

This nitrogen recommendation tool allows a user to input site-specific soil and cover crop data and calculate a fertilizer recommendation for corn that accounts for N availability from soil organic matter and cover crop residues. The Quick Start Guide below explains the required inputs. Recommendations can be calculated using a graphical web-based application for a single field, or in a spreadsheet for multiple fields. The theory behind how the tool works, along with the equations used in the calculations, is described in the article "Soil Organic Matter and cover crop-based nitrogen recommendations for corn." 

Available formats

• Online web-based application
• Excel spreadsheet for multiple fields

Quick Start Guide

Inputs

Enter the following information into the graphical tool or the spreadsheet to calculate an N fertilizer recommendation for corn:

• Average Corn Yield Goal: This 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.
• Soil Clay and Sand Content (%): These values are best obtained from a soil sample of the field collected from 0-8" depth and analyzed for texture (sand, silt, and clay %) by a laboratory. The soil texture only needs to be measured once in the lifetime of a field, as this property does not change. Soil texture can also be estimated from an NRCS Soil Survey Map.
• Soil Carbon (%): This 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: This is most accurately determined with a soil sample analysis of % carbon and % nitrogen from a lab. The tool is calibrated for a 0-8 inch soil sample depth. Soil C:N is calculated as % carbon divided by % nitrogen. For estimation purposes, a default soil C:N of 10 can be used.
• Winterkilled Cover Crop Biomass N (lbs N/ac): This 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 (lbs N/ac): This should be measured or estimated in spring at peak biomass prior to termination.  It can be measured in the field using a handheld NDVI sensor, by a lab from a representative grab sample of biomass or estimated from a look-up table.
• Price of Corn ($/bu): The expected price of corn.
• Cost of Nitrogen ($/lb N): The expected cost of N fertilizer on a dollar per pound of N basis.

Outputs

The following numerical results are displayed in the graphical tool and the spreadsheet. The graphical tool also includes a simulated N fertilizer response curve for the site, indicating the predicted corn yield with no N fertilizer applied, and how the yield will respond to additions of N fertilizer, which is used to determine the economically optimum N rate recommendation.

• Critical Spring Cover Crop C:N Ratio for N Immobilization: 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. This value changes slightly depending on the soil clay and sand content.
• Cover Crop Yield Credit:  This value 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.
• Additional Recommended Nitrogen: 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.

The spreadsheet contains additional results based on intermediate calculations that are not described here.

This tool has been developed with grant funding from Northeast SARE, USDA NIFA, USDA NRCS and Penn State Extension.