You don’t have to be involved with cover crops very long to hear the term “C:N ratio.”
It means carbon to nitrogen, a simple ratio of the mass of carbon to the mass of nitrogen in a substance. For example, if there are 10 units of carbon for each unit of nitrogen, the C:N ratio would be 10:1. This ratio can have an effect on the decomposition rates for both cash crop and cover crop residues and will have an impact on nitrogen cycling. These numbers can have a direct affect on our nitrogen use and can help us be better managers in the context of overall soil health.
The Microbe Component
There’s more to it than just understanding the C:N of a particular cash crop or cover crop residue. That’s where microbes come in to play. Their C:N ratio is around 8:1 and they need to eat residue that is near 24:1 in order maintain optimum health. Roughly two-thirds of the carbon they eat is used for energy and the balance for maintenance. In light of that, the closer we as farmers can manage to supply the ideal C:N ratio of 24:1, the better we are at maximizing the opportunity for those microbes to process and release nutrients our cash crops can use. And this opportunity can affect our bottom line.
How To Achieve This Ratio?
It’s not an exact science, but we can supply a smorgasbord of residues that provide at least a portion of ideal C:N ratios at any given time.
But first, some examples of crop C:N ratios — corn stalks, 57:1; wheat straw, 80:1; mature alfalfa hay, 25:1.
And cover crops ratios — cereal rye (headed) 37:1; cereal rye (pre-boot) 26:1; and hairy vetch, 11:1. A mix of hairy vetch and cereal rye has emerged as a popular trend that addresses this dynamic. This example gives the microbes an opportunity to balance their own diet, just like we try to do as humans. A balanced diet equals health and productivity.
Does Tillage Aid Residue Decomposition?
The answer is yes, but tillage also kills, or at the very least beats up the microbes, the very tiny critters that are trying to not only survive, but also do their part in the biological system. If undisturbed and provided food in a 24:1 C:N ratio diet, they will thrive and provide farmers with many nutrient cycling benefits.
What Are the Economic Benefits of Getting the Ratio Right?
Tough question, but just like the bigger aspect of cover crops in general, this question needs to be asked in the context the big picture — the real big picture, like 10-20 years from now.
Yes, there is immediate profitability that could be derived from a proper C:N ratio. As an example, I grow no-tilled pumpkins. They are planted into a thick rolled cover crop mix of triticale and hairy vetch. Aside from making the microbes happy with a balanced diet, I am benefiting from lower applied nitrogen due to the hairy vetch legume providing 50% of the nitrogen need. Soil erosion on my hills is slashed, and as a bonus my pumpkins are cleaner due to the rolled mulch keeping the soil from splashing on the fruit. And in the long term, my soil organic matter is slowing increasing. Even though I can’t put an exact dollar amount on that, we all agree that it does indeed have significant value.
Where Can I Dig Deeper?
As you can see, this topic can be somewhat complex. There is plenty of good information on the internet. The USDA/NRCS has some very good detailed information.
Don’t get too hung up on the technical aspects of targeting ideal C:N ratios. Simply manage for diversity of plant species, reduce tillage as much as possible, and you’ll certainly provide a better opportunity for long-term soil health benefits.