One reason many people try cover crops is to attempt to capture nitrogen left behind after the growing season. The theory is that growing cover crops capture nitrogen still in the soil and hold it over into the following season. However, now people are asking if the cover crop itself needs nitrogen applied in the fall to help it get rolling.

This information was prepared by the Indiana Conservation Partnership, led by Natural Resources Conservation Service personnel including Don Donovan and Clint Harrison, district conservationists; Kris Vance, public affairs specialist; Victor Shelton, state agronomist/grazing specialist; Tony Bailey, state conservation agronomist; and Shannon Zezula, state resource conservationist.

Many cover crop species are excellent at accumulating nitrogen and making it available for your next cash crop, Donovan says. Yet some are asking if it’s advantageous to apply supplemental nitrogen to a cover crop. Two goals of Indiana farmers using cover crops are reducing nutrient loss and reducing commercial nutrient inputs. Is there value in spending money to apply commercial fertilizer to a cover crop in times of narrow profit margins?

In many cases, cereal rye is either flown on in standing corn or seeded immediately after corn harvest. If the corn matured as it should and yields are near planned levels, much of the available nitrogen has been removed from the soil. Although cereal grains need nitrogen to thrive, there’s still likely enough nitrogen left. More will become available when the residues start to break down. Yet the decision depends on your goals and other factors.

Decision factors
If water quality is a concern, don’t apply additional nitrogen. If water quality isn’t a concern and you’re trying to improve your soil organic matter levels with high-carbon cover crops, it’s important that they get off to a good start in the spring coming out of dormancy. An application of 20 to 30 pounds of N per acre may improve early spring growth, and lead to more biomass improvements in soil organic matter levels. 

If available, it’s smart to put manure where you’ll have or already have cover crops. Cover crops can hold as much as 90% of the nutrients from manure applications, Donovan says. These nutrients will be available for future cash crops. Never apply manure to frozen, snow-covered or saturated ground, even on a cover crop.

Another option is to include a legume such as crimson clover in your cover crop mix, even if you’re not planting corn next, Donovan says. The legume will produce nitrogen that the grass cover crop can use. It also adds diversity in your cover crop mix for microbiology in the soil.

Other options
Should manure not be available or you don’t wish to include a legume, provide some nitrogen by applying phosphate for next year’s crop needs near cereal rye seeding time. Both MAP, 11-52-0, and DAP, 18-46-0, include nitrogen with phosphorus.  

Only apply supplemental N after the cover crop emerges to avoid excess fertility, in case conditions keep the cover crop from emerging in the fall. Never apply to frozen, snow-covered or saturated ground, Donovan emphasizes.

It’s important to remember to treat your cover crops like your cash crops. Things such as seed selection, proper planting methods, timely planting and even proper fertilization are critical for establishment and success, Donovan stresses. You may want to consider supplemental fertilizer application as part of your cover crop management plan.