Cover crops make N available

In research plots at the Center for Environmental Farming Systems in Goldsboro, N.C., George Naderman, a retired Extension soil specialist and an associate professor emeritus at North Carolina State University, measured from 100 to 200 pounds more of nitrogen in the top 5 inches of soil where conservation tillage had been used for six crop years, compared to conventional tillage. Researchers had put on the same amount of N on both fields during operations.

Why would there be more N on the top 5 inches of no-till fields? Naderman says that whenever a farmer grows a crop in the winter (assuming the farmer doesn’t put more fertilizer on to plant it), it is going to take up some nutrients. And when the farmer later kills that cover crop and leaves it on the ground, those nutrients are likely to be near the surface.

“We were using wheat or oats as a cover crop,” Naderman says, speaking of his CEFS experience recently. “But anything growing takes up N.” Comparing the CEFS experience to Ray Styer’s cover-crop experience, Naderman notes the similarities.

“The thing is the legumes — in this case, the hairy vetch and crimson clover [that Styer planted as a cover] — grow during the winter, and since they are not killed by winterkill, are fixing N right along. That is N beyond what the farmer puts on in fertilizer during the summer for a crop like corn.”

On the other hand, the N that Naderman measured in the top 5 inches at CEFS is not necessarily available to be used by the crop; in fact, he says most of it was probably not immediately available. That is because it takes some time for that N in the soil to become conditioned so it can be used by a crop.

“Ray has been doing this since 1995 or longer,” Naderman notes. “That is 15 years. If we measure more N in there, it is tied up in the form of organic matter, but it begins to cycle into available N after some time — we don’t know exactly how long. But Ray is undoubtedly taking advantage of N that has been fixed or put in years ago as fertilizer that is still in the soil, and now it is becoming available. That is why his corn appears to grow without N. Actually, it must get N from somewhere, but this recycled nutrient is what his corn is using.”

Use it or lose it

Naderman makes the point that after years of production agriculture, there is typically some usable N residing in the soil. On top of that, wherever Ray has a legume mixed in with his cover crop, that legume is also producing N that can be used by crops.

“Nitrogen is a very wily and essential element in every single cell of life,” Naderman says. “Every cell has got to have N in it. So there are microbes in the soil, and Styer is feeding the microbes with all this organic matter, which makes them thrive. They take up the N, they use it for a while, then they die and it keeps recycling in the soil. But if it is not constantly cycling in biology, it is leachable, and it goes further down into the soil.”

Of course, if a farmer doesn’t capture the nutrients and make them available to the crop, they will continue to filter down through the soil — most likely, Naderman says, into the groundwater. At that point the nutrients become a waste at best, and perhaps worse, a pollutant.

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MEASURABLE RESULTS: George Naderman, a retired Extension soil specialist and an associate professor emeritus at North Carolina State University, wanted to know how much cover crops preserve nutrients. He found cover crops can tie up 100 to 200 pounds more nitrogen in the top 5 inches of soil.

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SLICE OF PERFECTION: A slice of topsoil can look as good as a slice of peach pie to a farmer who has taken steps over many years to enhance its organic matter and nutrients.

This article published in the March, 2012 edition of CAROLINA-VIRGINIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.