There are times you want your nitrogen tied up to prevent loss and there are times when you want it available to your crops.
By managing your C:N ratio, you will have more control on nitrogen cycle. Grass/small grain cover crops are excellent at tying up ni-trogen and are best used in the fall and early spring when cash crops aren’t growing, cereal rye can reduce nitrogen leaching by 90%. Broadleaves release nitrogen much faster as they decompose. Legumes fix nitrogen bringing in more from the atmosphere and putting it into your soil but also release nitrogen very quickly as they decompose.
Depending on your rotation, you will want to utilize different species and mixes depending on the next years crop needs. After corn and prior to soybeans is a good time to scavenge potential leached nitrogen in a small grain/grass cover crop and provide a slow release of nitrogen to the soybeans. Prior to corn, a mix of broadleaves, legumes, and small grain/grass cover crop provides benefits of increasing nitrogen, tying up leached nitrogen, and providing a mix of fast to slow nitrogen release. Another management option is controlling the maturity of the small grain/grass cover crop-the more mature it is, the more nitrogen it will take up and release nitrogen at a slower rate.
Taking soil samples to understand where you are at with nitrogen and organic matter is valuable tool to help determine your C:N ratio management as well as observing the rate in which your surface residue disappears throughout the year. If you’re finding your residue isn’t breaking down and you’re seeing residue from the previous year in August, reduce your C:N ratio and utilize more legumes and broadleaves. If you can’t keep residue in your field past July, slowly increase your C:N ratio by increasing your small grains/grasses to your cover crop mix.
Stop by your local conservation office or visit the Minnesota Cover Crop Design Toll Nov. 2018 to see cover crops with their C:N ratios and several other attributes.