If you ask any farmer what their biggest limiting factors are on their farm, what do you think the biggest answers would be? I think the majority of farmers would say water, soil pH, or nitrogen. However, one of the biggest limiting factors often times gets overlooked; the carbon to nitrogen ratio. The C:N ratio plays a significant role in microbial activity and microbial diet, which, in turn, has major implications in residue breakdown and nutrient cycling.
What is the C:N ratio?
The carbon to nitrogen ratio in the soil is simply the ratio of carbon mass to nitrogen mass that is in the soil at that current time.The Haney test measures the C:N ratio by comparing the total water extractable organic carbon to the total water extractable organic nitrogen. These pools of carbon and nitrogen are significant in measuring the C:N ratio because water extractable organic carbon is what the microbes eat, and water extractable organic nitrogen is a stable form of plant available nitrogen. It’s also the most readily available nitrogen for building enzymes via microbial processes.
The C:N ratio of a field plays an important role in the available food for microbes. The ideal microbial diet would be a C:N ratio of 24:1. At this ratio, microbes will consume the ideal amount of carbon, and will leave little to no carbon or nitrogen behind.
C:N ratios in selecting cover crop species
The C:N ratio of a soil is key in determining which species of cover crops to plant. If you plant a cover crop with a high C:N ratio, like cereal rye, the microbes will have a harder time breaking all of that down. This would be similar to our diet consisting of only celery. It’s a healthy source of food, but it would take a long time for us to eat all of it, and it’s not an ideal diet. Since there is a larger proportion of carbon in comparison to nitrogen, the microbes have to find additional nitrogen to balance out the large amount of carbon that they have to consume. This leads to microbes immobilizing excess nitrogen in the field that the plant could use.
On the other hand, if you plant a legume cover crop with a low C:N ratio, the microbes will eat that up very quickly. This, to the microbes, is like feeding them candy. They break it down very quickly, and leave a lot of excess nitrogen in the process.
Planting soybeans into a high C:N ratio cover crop, like cereal rye, is the ideal scenario. Driving up the C:N ratio of the soil before planting soybeans leaves little excess nitrogen for the soybean to utilize. If there’s excess nitrogen for the soybeans, they use that nitrogen and don’t nodulate as much to fix atmospheric nitrogen (soybeans are lazy!). Leaving minimal excess nitrogen makes the beans work harder for the nitrogen that it needs, increasing nodulation and root mass. This makes the entire plant stronger and more resilient.
The ideal scenario for corn is planting into a low C:N cover crop, like hairy vetch or another legume. Since the microbes will break down the cover crop residue very quickly and return the nutrients in a crop available form, the corn will be able to utilize the legume delivered nitrogen. This is critical to reduce the need for synthetic nitrogen.
Measuring C:N ratios and making management decisions
We know that planting a legume cover crop with a low C:N ratio is ideal for quickly returning nutrients to your corn crop, but how do we know how much nutrients are out there, and how much we can cut back on synthetic nitrogen? At Continuum Ag, we look at the water extractable nitrogen number on the Haney test to make accurate recommendations on how much nitrogen you can cut back. We know that planting a legume cover crop leaves you with extra nitrogen in the field that the corn crop can use, but quantifying the amount that’s actually out there is key in reducing nitrogen rates, and we can help with that! We write recommendations to reduce nitrogen, and it’s all housed on our TopSoil® tool where you can view your water extractable nitrogen in the field, as well as the variable rate nitrogen recommendation based on what your corn needs.
Managing C:N ratios is also critical so that your microbes have the right food source. Planting cover crops with different C:N ratios is key for determining how much nitrogen is going to be in the field for the crop to utilize, but making sure that your fields have the ideal C:N ratio in the soil is also key for making sure the microbes have a well balanced diet. If your C:N ratio is higher than 20:1, it is crucial to lower that number by planting a cover crop with a lower C:N ratio. On the flip side, if the C:N ratio in your soil is lower than 10:1, plant a cover crop with a high C:N to raise it back to ideal levels.
The whole purpose of maintaining the C:N ratio of your soil is to give your soil microbes a well balanced diet. Just like humans, having a well balanced diet makes microbes more active, which leads to them breaking down residue more quickly, cycling more nutrients, and improving overall soil health.
“Carbon to Nitrogen Ratios in Cropping Systems” (2011) USDA Natural Resource Conservation Service. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcseprd331820.pdf
“Here’s Why the Carbon – Nitrogen Ratio Matters” (2015) Successful Farming, Gil Gullickson https://www.agriculture.com/crops/cover-crops/heres-why-carbonnitrogen-ratio-matters_568-ar48014