Cover Crop nitrogen scavenging calculator

Estimate how much nitrogen your cover crop has taken up

This tool estimates the aboveground biomass, tissue chemistry, and nitrogen uptake of a winter cereal cover crop from three things you already know: the variety, when you planted, and how tall the stand is now. It is built on field trials by Dr. Eric Brennan (USDA-ARS) and Richard Smith (UC Cooperative Extension) in California vegetable systems.

Use it to decide when to terminate, and to understand how quickly the nitrogen your cover crop scavenged will become available to your next crop.

Refer to the Cover Cropping for Soil Nitrogen Recycling document (Español) for detailed instructions.

  1. Measure stem length. Pull 10 random cover crop plants along a diagonal line across the field. For each plant, use the main stem and measure from the crown of the roots to the top of the head. Average the 10 measurements.
  2. Select your variety and planting date. Choose the planting date closest to when you actually planted.
  3. Enter the date you measured, or the number of days since planting if you would rather count days yourself.
  4. Look at your results and the curve. The results box tells you where the stand is now. The chart below it projects how the carbon-to-nitrogen ratio will keep changing if you leave the stand standing longer.

About these estimates

Nitrogen uptake is not the same as nitrogen your next crop will receive. A result of 150 lb/acre of nitrogen uptake does not mean 150 lb/acre will become available after termination. Release of nitrogen depends on the carbon-to-nitrogen ratio at termination, how the residue is incorporated, soil temperature, and moisture.

All results describe aboveground (shoot) tissue only; roots are not included. The estimates come from replicated trials in central coast California vegetable systems, and assume a stand planted in rows 6–8 inches apart, at a density of at least 10 plants per square foot. Results in other soils, climates, and cropping systems will differ.

This tool does not recommend which cover crop to plant.

Data and Research

Developed in collaboration with Dr. Eric Brennan (USDA Agricultural Research Service) and Richard Smith (UC Cooperative Extension) whose field trials and analysis are the data source of this calculator.

Source:
Brennan, E. B., & Smith, R. F. (2026). Can simple metrics predict carbon to nitrogen ratio changes in rye and triticale cover crops? Agronomy Journal, 118, e70375. https://doi.org/10.1002/agj2.70375