If there were a foolproof way to predict the optimum nitrogen fertilizer rate for corn after a cover crop, most producers would likely be on board.

But calculating that is complicated, and many factors must be considered.

“There is a lot of uncertainty that you must think about,” said Hanna Poffenberger, a University of Kentucky soil science professor, at the recent Northeast Cover Crops Conference in Maine.

If you are growing a cover crop and have questions about how much nitrogen the cover crop provided and how much your corn may need, here’s some things to keep in mind:

Remember the N source. A good portion of nitrogen comes from mineralization of soil organic matter and crop residue.

In fact, based on a review of soil studies, Poffenbarger estimated that half or more of the nitrogen uptake of corn comes from the soil.

Cover crops can provide a good source of nitrogen to the following corn crop, but that depends on what the cover crop is. Different covers will take up varying amounts of nitrogen for their own growth, especially in spring.

Legumes, for example, can provide an additional amount of nitrogen — 50 to 150 pounds per acre — because of their ability to fix nitrogen from the air. But not all of it will be available to corn in the same growing season, Poffenbarger said.

“They can't fully replace fertilizer nitrogen, but they can definitely reduce the amount that would be needed," she said.

How much N can they provide? Again, this really depends on the cover crop being grown.

One thing that is clear, though, according to Poffenbarger, is that first-year cover croppers may not see an immediate nitrogen benefit to the following crop.

When ground is not covered in winter and early spring, nitrates can be lost — especially if it has been wet — and this can lead to water quality problems. Poffenbarger said cover crops can reduce nitrate leaching by 50% or more due to their ability to scavenge for nutrients.

“One of the major benefits of cover crops is that they can catch that excess nitrate and very nicely convert that into organic matter that can help to build up that nitrogen capacity and supply in the system over the long term," she said.

But there is a flip side. Depending on what cover crop is being grown, they could also be taking away potential nitrogen from the following corn crop.

Cover crops grow biomass, and this biomass needs nitrogen to get bigger. Some of that nitrogen will be released as the cover crop decomposes in spring, but the bottom line, Poffenbarger said, is that you will often see less inorganic nitrogen — the stuff available to plants — with a cover crop growing than if no cover crop were present, especially in situations when it is dry or there are no obvious ways for nitrogen to be lost.

Based on studies that measured soil inorganic nitrogen in fields following cover crops vs. fields growing no cover crops, Poffenbarger said the soil inorganic depletion rate is between 5 and 15 pounds of nitrogen per acre.

In the case of legumes, such as a hairy vetch, since they are getting a lot of nitrogen through fixation, they don't deplete the soil nitrogen as much as a non-legume, such as rye.

What will corn need? Poffenbarger has used a cover crop nitrogen calculator tool to do simulations on how much nitrogen a cover crop can release to the following corn crop.

She did simulations based on data from farm fields near State College, Pa. For hairy vetch, at 3,000 pounds per acre biomass, the tool predicted 113 pounds of nitrogen per acre in the cover crop, but only a portion was derived from the soil — 23 pounds. Most of it was fixed. The simulated six-week nitrogen release was around 43 pounds per acre.

A cereal rye-hairy vetch mix at 6,000 pounds per acre accumulated 108 pounds of nitrogen. The soil-derived nitrogen was 50 pounds, and the simulated six-week nitrogen release was 29 pounds per acre.

For cereal rye at 5,000 pounds per acre, 50 pounds of nitrogen, all soil derived, was accumulated in cover crop biomass, but the simulated nitrogen release after six weeks was only 3 pounds per acre.

"This really tells you that species selection plays a big role in how much nitrogen release you can expect, but also it impacts how much nitrogen depletion there could be” during cover crop growth, Poffenbarger said.

Poffenbarger participated in a Precision Sustainable Agriculture study that looked at the effect of cover crops on later corn yields.

The study was done at 15 locations across the U.S., including the Midwest and Northeast. It evaluated cereal rye, a legume (hairy vetch or crimson clover) and a rye-legume mix, and its later effect on corn yield.

Cover crops were planted the previous fall following corn silage or small grain. After termination in spring, five to six different nitrogen rates were applied — all split-applied at planting and sidedress. All fields were managed no-till and most had no history of cover crops.

The nitrogen rate ranged from 0 to 320 pounds of nitrogen per acre.

The 2021 results — 2022 results weren’t available — showed that without fertilizer, the plots with rye produced some of the lowest corn yields, while plots planted with legumes helped produce the best corn yields.

The covers produced between 2,000 and 6,000 pounds per acre of biomass.

"So that's already telling us a little about the rye impact on nitrogen supply to corn, having more of a deficit following the rye," Poffenbarger said.

In the Northeast locations — Maryland, Delaware, Pennsylvania and New York — corn required more nitrogen following the mix or rye than no cover crop.

The rye likely decreased nitrogen losses over winter and early spring, but because the rye took up nitrogen and released it slowly, it increased the fertilizer requirement for corn without increasing yield.

Poffenbarger said the nitrogen is likely going to build soil organic matter. "So, we're going to be building up organic matter from that initial biomass in the cover crop,” she said.

The average rate of soil carbon storage is 285 pounds per year. Assuming a carbon-nitrogen ratio of 10-to-1, Poffenbarger estimated about 28.5 pounds of nitrogen per acre is needed to build that up.

"And so, we have that building phase where the microbes are taking some of that nitrogen away from the crop and putting it into forming new organic matter," she said. “So, in time, the farmer will reap the benefits of greater soil organic matter, but there is this kind of investment phase first.”

Online tools

Below are some tools that you can use to help predict the amount of nitrogen your crop might need after a cover crop.

The tool at covercrop-ncalc.org was developed by the Precision Sustainable Agriculture program and was the same one used by Poffenbarger in her research:

aesl.ces.uga.edu/mineralization

soilhealth.spes.vt.edu/CoverCropCalculator.html

smallfarms.oregonstate.edu/calculator

extension.psu.edu/nitrogen-recommendations-for-corn