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Cover Crops Complicate Nitrogen Management

March 15, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
Corn College 3

Plan when to kill a cover crop and manage the potential carbon penalty to prevent mistakes that could reduce yield

Cover crops can do wonders for soil health. But sound management is just as critical with cover crops as it is with cash crops such as corn, wheat and soybeans. With cover crops, the most common mistakes farmers make are failing to kill the cover crop at the right time and failing to manage the carbon penalty resulting from increased residue, says Farm Journal Field Agronomist Ken Ferrie. To learn more about how to maximize cover crop benefits, Ferrie conducted a study in 2012 with seed arranged by conservation farming consultant Dan Towery of Ag Conservation Solutions in West Lafayette, Ind. Ferrie shared his findings at Farm Journal Corn College this past July.

FJ Cover Crops pC28

Following a wheat crop (and after the straw was baled and removed), Farm Journal Field Agronomist Ken Ferrie planted a variety of cover crops. In some of the plots, the wheat residue was tilled and incorporated before planting corn. In other plots, the wheat residue was left untouched and corn was no-tilled into the field.

While there was moisture to germinate the cover crops and the cash crops that followed, the weather got gradually drier after planting. Ferrie and Towery agree the results might be different in a wetter season.

In the study, a variety of cover crops were planted following a wheat crop, in which the straw was baled and removed. In some plots, the wheat stubble was tilled in the fall and leveled before planting the cover crop. The following spring, the cover crop was incorporated with a vertical harrow before planting corn. In other plots, the wheat stubble was left untilled; in the spring, corn was no-tilled directly into the cover crop and wheat stubble residue.

The need for management arises because some cover crops impact the carbon penalty. The carbon penalty results when corn is planted after a crop that produced large amounts of residue with a carbon/nitrogen (C/N) ratio higher than 30/1, such as corn or wheat, or some cover crops, such as cereal rye.

Cover crop residue with a high C/N ratio adds a fresh helping of carbon in the spring, in addition to what’s already present from the previous crop. With that carbon-rich food source, the soil microorganisms that decompose residue increase rapidly. They use nitrogen from the soil, immobilizing it. If the timing is wrong, and if the nitrogen is not managed correctly, this can create a temporary nutrient shortage for young corn plants.

Nitrogen tie-up. The nitrogen taken up by microbes becomes available later, through mineralization, which can be affected by moisture and temperature. Immobilization and mineralization of nitrogen occur simultaneously in the soil, Ferrie explains. But, at various times, there is either net immobilization or net mineralization

Net mineralization late in the growing season, as nitrogen from cover crops becomes available, is beneficial to the corn crop. The amount and type of residue produced by the cover crop determines how soon it becomes available. In the 2012 study, the date the cover crop was killed was a big factor determining the onset of net nitrogen mineralization.

Cover Crops pC29

The effect of the carbon penalty was more pronounced in no-till corn following a cover crop planted into wheat stubble than it was in fields that had been tilled.

Ferrie tracked how long it took for the nitrogen in the various cover crops to reach a stage of net mineralization. Some of the cover crops winterkilled and were already dead at planting time.

Others were killed with a herbicide on three separate dates.

"We used soil nitrate tests to track net mineralization," Ferrie says. "In a block where we killed the grass cover crops on March 19, net mineralization did not show up for 60 days. The cover crops that had winterkilled showed net mineralization by March 29."

"That’s pretty typical for cover crops that winterkill," Towery adds. "It’s important to understand that this mineralization occurred before the drought set in."

With annual ryegrass covers, which were killed in the spring, Towery believes the lack of rainfall and soil moisture had a much more significant impact on nitrogen availability. "The extremely dry weather resulted in much less mineralization following annual ryegrass than would have occurred in a normal year," he says.

Where Ferrie killed annual ryegrass on March 15, net mineralization showed up in about 40 days. Where he killed it on March 26, net mineralization didn’t occur for approximately 125 days.

"That’s because the carbon content increased as plants continued to grow," Ferrie says. "One week made a huge difference because of the increase in tonnage and C/N ratio."

Dry weather damper. Towery believes the delayed nitrogen mineralization following an annual ryegrass cover crop was a one-year phenomenon, caused by extremely dry weather.

"Annual ryegrass is a nitrogen scavenger, and has a relatively low C/N ratio—as low as 17/1 if you burn it down early," Towery says. "Because the annual ryegrass was planted into wheat stubble, it may not have taken up much nitrogen because there wasn’t much in the soil to begin with—possibly less than 20 lb. per acre. I’ve seen annual ryegrass take up much more nitrogen than that—90 lb. to 120 lb. per acre, depending on field history.

"The low C/N ratio of annual ryegrass means that typically 50% to 75% of that nitrogen becomes available to corn plants six to 10 weeks after it is killed in the spring—in late June or early July," Towery continues. "Often, corn fields following an annual ryegrass cover will be dark green, or even almost black in July. That has to be from the nitrogen mineralized from the annual ryegrass, and the absence of a carbon penalty, resulting from the crop’s low C/N ratio.

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FEATURED IN: Farm Journal - March 2013



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