Carbon to nitrogen ratio: The key to unlocking your soil potential

January 6, 2025 | Matt Swanson

As today’s farmers continue to implement cover crops on their farms, the effect of the carbon to nitrogen (C:N) ratio must be understood. In this article, we will discuss what the C:N ratio is, how the C:N ratio affects nutrients already in your soil, how proper C:N ratio is a key indicator of healthy soils, and in what way cover crops can affect your soil’s C:N ratio.

The C:N ratio measures the mass of carbon to nitrogen. Knowing your soil’s ratio when planning your crop rotations and implementing cover crops is essential. For example, the optimum ratio for soil microbes is 24:1. The 24:1 ratio stimulates microbes, allowing for timely plant residue decomposition, which helps release nutrients to your cash crop.

Unlocking your soil’s potential

Microbes must be fed a balanced diet to keep working for you in your soil. Corn residue has a C:N ratio of 57:1, which indicates a high carbon content and a poor environment for microbes to rapidly consume and break down plant residue. This means when corn residue is left in soil on a corn-on-corn rotation, microbes use nitrogen to break down the carbon-rich residue, making it unavailable to the crop.

Some producers will add more nitrogen in the spring to compensate for the “tied up” nitrogen. However, introducing a cover crop with a low C:N ratio, such as hairy vetch, can be a better solution to unlocking nitrogen in your soil.

Nitrogen is not the only nutrient that can be tied up in your soil due to an improper C:N ratio. Microbes can also use phosphorus for their own nutritional needs, and they may compete with plants for phosphorus in high C:N soils. Soil tests that historically have high phosphorus values could have a C:N ratio that needs to be adjusted.

Healthy soil maintenance

Maintaining a proper C:N ratio is key to having healthy soils. A C:N score of 24:1 promotes higher microbial activity and can lead to higher organic matter. Organic matter is plant, animal, and microbial residue in various stages of decomposition, and new research has shown that as much as 80% of it is microbial residue.

The Venn diagram below the crossover that forms soil organic matter (SOM). C:N scores of 24:1 promotes higher microbial activity and leads to a higher SOM.

High organic matter scores lead to more nutrient release, improved water-holding capacity, and reduced compaction. This further supports the importance of proper C:N ratio to soil health. A C:N ratio that is too high can lead to more fungi than bacteria in the soil, leading to slow cycling of organic matter and immobilized inorganic nitrogen. When your soil’s C:N ratio is too low, the residue breaks down faster and increases the chance of nitrogen leaching.

Cover crops can naturally help raise or lower your soil’s C:N ratio. Planting cereal rye as a cover crop before corn often receives bad publicity because of the thought of cereal rye having an allelopathic effect on corn.

Some believe that allelopathy, where cereal rye produces biochemicals that impact corn growth, survival and reproduction, is to blame for yield reductions in corn following cover crops. In other words, this is saying the roots of the cereal rye plant are releasing allelochemicals that harm the corn plant. However, according to Iowa State University Extension and Outreach, the allelopathic chemicals released by cereal rye are related to the seed size of the subsequent plant or crop.

The smaller the seed, the more susceptible the plant is. The large seed of corn and its planting depth will minimize the impact of any allelopathic chemicals from cereal rye. The yield drag producers see when planting corn into cereal rye is directly related to the cereal rye’s high C:N ratio and microbes in the soil are tying up the N. Integrating cover crops such as hairy vetch, radishes and crimson clover, all with low C:N ratios, can significantly aid in adjusting your soil’s C:N ratio closer to the ideal 24:1 to minimize the early season concerns of corn following a cover crop.

Iowa Soybean Association’s Research Center for Farming Innovation is driven to deliver more ways to increase soybean production while keeping conservation at the forefront. Maintaining a proper C:N ratio is crucial to soil health and unlocking nutrients. For more information on research in cover crops, please reference The Advances in Cover Crop Management Guide on the ISA website.

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