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Soil Health Minute: Nitrogen release (or not) with cover crops

There's this idea floating around that cover crops will take up nitrogen this growing season and then release that nitrogen for next year's crop. Add on top of that, an ability to anticipate when that nitrogen will be released by selecting specific cover crops in a mix (this is the C:N ratio stuff you hear about).

But, we have to ask, do we have data showing that for our region? And can we actually anticipate release based on C:N ratios in our region?

Luckily, Dave Franzen, North Dakota State University soil specialist, is working on this concept and getting us some answers ... and coming up with a few more questions in the process as well. It's interesting, and reassuring, that what he's finding in our region is similar to what other researchers are starting to find and publish as well.

Over the past three years, Dave has been working on research plots in a corn-soybean-wheat rotation in two different systems. The first system in Gardner, N.D., is a recent conversion to no-till (within three years) with no history of cover crop use. The second system in Rutland, N.D., is a long-term no-till (40 years) with a long history of cover crop use (20 years) and a four crop rotation. These are two very different systems, but showing similar results. Here's what we're finding:

Throw C:N ratios out the window — the books say that a cover crop mix C:N ratio of < 30:1 should result in a release of nitrogen to the following year's crop. Are we seeing this? Let's take a look at results from the Rutland site. It actually took more nitrogen to grow a corn crop after a wheat plus cover crop than it did to grow a corn crop after just wheat with no cover crop. We did everything right to get nitrogen to "catch and release" — we used a mix that had an 18:1 C:N ratio and included multiple species from different plant types (volunteer wheat, field pea, flax, radish, turnip), we seeded it in a timely manner after wheat harvest to get 2 ton per acre dry matter, we used this mix on the Rutland long-term no-till and cover crop history. This happened again in 2018 with about a 40 pounds per acre nitrogen drag for the cover crop treatment compared to the no cover crops. A similar situation happened in 2017 for wheat nitrogen requirements at the Gardner site — where an interseeded cover crop (oats and radish) established in soybean led to a 20 pound N per acre nitrogen drag for the next year's wheat crop. So, in this case, had we relied upon what the books say and anticipated a release of nitrogen from the cover crops, we would have lost yield.

Does this mean that cover crops are bad? Absolutely not, there are many benefits to using cover crops, we just don't want you to rely on nitrogen release the next year and hurt your corn or wheat crop yields. To protect yourself, you can always use a check strip where you apply sufficient nitrogen and compare to the rest of the field where you may have backed off on nitrogen, watch those strips, and side-dress nitrogen if/when needed.

Where is the nitrogen and when will we get it for the crops? We know from all of the soil and plant material testing being done with this project that nitrogen is being taken up by the cover crop, but where is it being held and when will it be released? Dave is now exploring how clay chemistry may be playing a role in nitrogen retention and release. This is the exciting part about researching a very complex system — there are always more questions, so stay tuned.

For full details from Dave, check out his article in the July 18 NDSU Crop and Pest Report (https://www.ag.ndsu.edu/cpr/soils/nitrogen-credit-from-cover-crops-07-18-19). While you're checking out the article, you might as well subscribe to this report and keep up to date with the latest work at NDSU.

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