Understand Carbon to Nitrogen Ratios before Buying Cover Crop Seed

By now you’ve heard of the “carbon penalty” some producers face with residue and cover crops—but what does that really mean? And should it deter you from planting cover crops?

Experts say it doesn’t mean you shouldn’t use cover crops—just know what you’re planting and its effect on soil. Nitrogen release—or tie up—is affected by many factors, according to Julia Gaskin, sustainable agriculture coordinator for the College of Agricultural and Environmental Sciences at the University of Georgia.

Factors impacting nitrogen release by cover crops:

• Soil temperature and moisture content
• Carbon to nitrogen ratio
• Cover crop quality and its effect on decomposition
  • More carbohydrates mean faster decomposition
  • Lignin, tannin and polyphenol slow it down

Each of these factors is related to species, variety, growth stage and environmental conditions, she adds.

The optimum diet for soil microorganisms is a 24:1 carbon to nitrogen (C:N) ratio, according to USDA Natural Resources Conservation Services (NRCS). They use 16 units of carbon for energy and the other eight parts for maintenance.

This means certain plants, such as mature alfalfa hay with a C:N ratio of 25:1, feed the microorganisms perfectly without creating a nitrogen surplus or deficit. That’s fine and dandy, but not what row crop farmers are looking for, you want to see a nitrogen surplus from cover crops or residue.

Take a look at common covers and residues from previous crops or other common plants and their C:N ratio. Use this information to help inform your decision regarding cover crops and nutrient application—note, the lower the ratio, the more nitrogen that is available to crops.

Source: USDA NRCS

Year after year of growing something with a high C:N ratio such as wheat straw could leave your crops starving for nitrogen. However, adding in a low C:N ratio crop such as hairy vetch will help microorganisms break down straw faster and make nitrogen available to the microorganisms, according to NRCS.

Multi-species mixes can help you strike the balance of C:N ratios—and is a practice many farmers are using. Since 2008, farmers have increased their use of cover crops by 13% each year, according to a survey from the Sustainable Agriculture Research Education program. This would mean 50 million acres of cover crops by 2025.

In 2017, farmers planted about 17 million acres of cover crops. Of those acres, nearly 50% were in cereal rye, with oats and radish lagging behind at 26% and 23%, respectively.