Why are C:N ratios so important for farming?

Our new season of Carbon College has launched, including an entire course on carbon systems. Check out the video above as well as this article to dive into the importance of C:N ratios. And to learn more about carbon farming, explore Carbon College’s free video courses.

Understanding and managing the carbon to nitrogen ratio of your soil is key to optimizing soil productivity as well as crop yields. An optimal C:N ratio will ensure crops are getting exactly what they need from soil, while a C:N ratio that drifts too far in either direction can have serious impacts on crop growth and yield, as well as soil health.

The carbon to nitrogen ratio is the weight of carbon per weight of nitrogen. Both carbon and nitrogen are important elements for all living organisms, but their ratio in the soil must be in the proper balance. The carbon to nitrogen ratio in your soil can affect many aspects of farming, in particular two areas: 

1. Crop nutrient cycling, primarily with nitrogen
2. Crop residue decomposition.

When residue with a high C:N ratio (which means there’s a lot more carbon relative to nitrogen) is tilled into soil, that residue will immobilize nitrogen (both already in the soil and applied) and prevent plants from meeting their nutritional needs. Immobilization is when plant matter decomposing microbes utilize all the available nitrogen in the recently-harvested crop residue to satisfy their metabolic needs. This leaves no nitrogen in the decomposing matter for growing plants to use. This can severely impact crop growth. 

Understanding the importance of the C:N ratio and having a plan for managing it is especially important for planning cash crop rotations and adding cover crops to your fields.

The average C:N ratio of soil microbes is 8:1. Think of C:N ratios like the proportion of vegetables and starches to steak in a well-rounded meal. Getting out of balance for too long in either direction can cause health problems later on down the line. 

As a general rule of thumb, roughly 67% of all carbon entering the soil as crop residue is oxidized by microbes and used for energy for life functions such as decomposition. The remaining 33% makes up the microbes and becomes part of their tissue.

Generally, this means the ideal food for microbes would have a C:N of 24:1 in order for them to maintain their own body C:N of 8:1. 

If high C:N residue is turned into the soil near the time of planting, that can lead to immobilization as well as poor nitrogen nutrition for grass crops such as corn, wheat, and rice. The break point for immobilization is generally a ratio of 20 to 25:1. Immobilization is much less of a problem for legumes like soybeans and alfalfa because they can fix atmospheric nitrogen for their nutrition. 

Immobilization generally only occurs when the high C:N residue is physically turned in to the soil. Crop residue left on the soil surface is less likely to cause immobilization because the limited soil contact slows down the composition of the residue. This causes less immobilization to occur.

What’s the best C:N ratio for your crops? The C:N ratio will change as the season progresses. Generally speaking, young plants have a significantly lower C:N ratio than fully mature plants.

For corn, The C:N changes across the season, but the stover left at the end of the season is about 57:1 up to 60:1. This high ratio means any nitrogen in the corn residue and soil profile after harvest quickly becomes immobilized. This is because soil microbes eat first and are more competitive for nitrogen than plant roots. This immobilization period will last for 2 to 4 weeks. In severe cases, it can last up to 8 weeks depending on soil and climatic conditions. 

For soybeans, the C:N ratio also increases during the growing season, but post-harvest soybean residue generally has a C:N of 25:1 - 40:1. Microbes decomposing above ground soybean stubble will immobilize nitrogen for a short period of time. However, the lower total biomass and the lower C:N ratio of this residue cause soybean’s immobilization period to be much shorter than corn’s. Overall, as mentioned above, immobilization is less of a concern for soybeans or following a soybean crop.

For wheat, the C:N ratio also changes across the season. However, the stubble left at the end of the season generally has a C:N ratio of 80:1. Wheat residue has one of the highest C:N ratios of grain crops. It produces less biomass compared to corn, but it may immobilize soil nitrogen as long as corn residue or longer.

C:N ratios are important to track and manage throughout the growing season in order to maximize crop yields. There are many ways of managing C:N ratios so that they’re at an optimum level. One way is by planting cover crops. Try out our Nitrogen Generation Estimator to discover just how much different cover crops can contribute to your nitrogen needs.