Understanding Carbon to Nitrogen Ratios Pays Yield Dividends

Cover crops aren’t a new concept in agriculture production; they are only being revisited in modern practices as producers look at ways to reduce input costs and increase productivity through healthier soils.

For decades, legumes were implemented into a crop rotation to fix nitrogen for the upcoming crop. As an additional benefit, legumes also helped maintain soil structure, and mitigated at least a fraction of the damage done by implemented tillage practices.

Today, forward thinking producers are looking back, using new knowledge combined with conventional practices to create a cocktail of minimal input, natural amendments to increase yields and improve soil health—enter the interest in a soil’s carbon to nitrogen ratio.

Soil microorganisms have an 8:1 carbon to nitrogen ratio, but because of the perpetual intake and expelling of waste, they need a 24:1 ratio to survive; of this ratio, the microorganisms will use 16 for energy and 8 for maintenance.

The higher the C:N ration the more time it takes microorganisms to break down and the more N is needed from another source…the soil. And the ratio can have a significant effect on the rate at which residue breaks down and soil and crop nutrient cycling occurs. The higher the ratio climbs from the optimal 24:1, the more nitrogen microbes must pull from the soil; conversely, the lower a ratio falls from 24:1, the more rapidly microorganisms will decompose the residue and the greater the available nitrogen release will be for the current or following cash crop.

However, keep in mind, that while a lower C:N ratio will speed the breakdown of residue and increase available nitrogen…the breakdown of residue will likely leave your soil exposed to the elements.

Knowing this information puts a producer at an advantage when looking at which cover crops to plant in a cash crop rotation – and how much nitrogen to apply for the cash crop.

To help producers in their cover crop and organic matter amendment decision making, the National Resources Conservation Service has put together a list of the C:N ratios of crop residues and other organic materials.