By Don Donovan

If it’s been a few years since college soils class, any discussion of the carbon-nitrogen ratio is probably hard to get your head around. You may even say, “Who cares and why should I worry about it as I plan my 2018 planting season?”

If you’re a farmer using conventional tillage and not using cover crops, you may not need to lose much sleep over it. If you’re a no-till farmer bringing cover crops into your management system, it’s time to brush up on the subject to head off potential issues this spring.

How cycle works
The C-N ratio is just what it says: It’s the relative amount of carbon, typically contained in crop residue and cover crop residue, to nitrogen, including N in manure, soil-available N and commercial N in the soil. This is important in no-till and cover crop systems because the microbiology in the soil uses available nitrogen to help break down available carbon in crop and cover crop residue. The optimum C-N ratio is around 24-to-1. 

If you planted a potentially high-carbon cover crop such as cereal rye, keep in mind that as the rye matures, it gains in carbon. When you terminate the rye, available nitrogen in the soil will be used by microbiology in the soil to start breaking down residue. That nitrogen will become unavailable for your cash crop, and can be an issue if you’re planting corn. 

This is one reason successful cover croppers across Indiana use starter fertilizer when planting corn into high amounts of high-carbon cover crops. The starter fertilizer should contain adequate amounts of nitrogen to supply what the young corn crop needs. Soybeans tend to respond positively to a nitrogen-starved environment.

Cover crop choice
When picking a cover crop, if your primary goal is soil erosion control and/or building soil organic matter, you’ll need to include high-carbon cover crops such as cereal rye in your mix. Brassicas such as daikon radish and legumes such as crimson clover are low in carbon and high in nitrogen. They will help moderate the C-N ratio in a mix with rye, but generally don’t build as much soil organic matter or provide as much erosion control when used without a high-carbon cover crop in the mix.

If you’re planning to use cover crops in your operation, it’s time to hit the books again to become familiar with the C-N ratio. Using cover crops with the proper C-N ratio will build soil organic matter faster, get nutrient cycling working to produce N for your corn crop, and help prevent possible issues such as lack of N at critical times as your corn crop develops.

Donovan is a district conservationist with the Natural Resources Conservation Service based in Parke County, Ind. He is a contributor to Salute Soil Health, which appears regularly in Indiana Prairie Farmer on behalf of the Indiana Conservation Partnership.