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A New Source of Nitrogen

December 11, 2020 |

By Kirsten Keleher


Organic tobacco growers spend a lot of time thinking about nitrogen. After all, nitrogen has a bigger impact on tobacco yield and quality than any other nutrient. According to a recent study led by Alex Woodley, assistant professor in the crop and soil sciences department at North Carolina State University (NCSU), nitrogen may come in more forms than previously recognized.

Under current popular methods, most in-season nitrogen fertilizers applied at planting time come from animal byproducts (bone, feather or blood meal), but there may be a more cost-effective solution: relying on cover crops in the off-season.


Legumes, Legumes, Good for Your Crops

Many people haven’t thought about legumes since their sixth grade biology class, so here’s a quick refresher: These small plants have nodules in their roots with nitrogen-fixing bacteria. What does that have to do with tobacco growers? They have the ability to make the nitrogen in the atmosphere usable to plants.

In Woodley’s two-year study, the researchers compared three different leguminous cover crops —crimson clover, hairy vetch and Austrian winter pea—to a control tobacco crop grown without cover crops. In addition to the cover crops, the researchers also added varying levels of feather meal to their tobacco crop during the second year.

Their finding: Using legumes as a cover crop allows you to use half the amount of fertilizer to get the same yield. This could mean an initial savings of 25 percent to 50 percent.


Choosing Your Legume

So how does this work? First, you’re going to want to decide which cover crop to use. There are many cover crop options, all of which are good alternatives to your basic fertilizer. Woodley’s team explored three options:

  • Crimson clover: It did a fine job of returning nitrogen to the system, had good biomass yields and was easy to terminate. However, the researchers observed a large difference in the productivity of the crop between two years. This can best be explained by the heavy (and rather unusual) rainfall experienced in one of the years, which caused significant leaching.
  • Hairy vetch: Overall, it put a lot of nitrogen into the system and was relatively easy to grow. The problem with hairy vetch is that if you terminate the crop too late, you may be faced with tough seeds that quickly go from money-saver to weed. With such a prolific plant, you also run the risk of it having just the right growing conditions to store nitrogen in the soil for longer, thus causing a greater, unfixable detriment in the form of greening.
  • Austrian winter pea: It too had a high biomass and was relatively easy to terminate, but because this is a newer crop to the Southeast, it could be more prone to disease, which has been an issue with commercial production of peas for grain.

In conclusion, crimson clover leads the pack. It doesn’t have the downfalls of Austrian winter pea or hairy vetch, and it still produces a quality yield. The runners up: Austrian winter pea followed by the ever-so-tricky hairy vetch.


Time to Plant

According to Matthew Vann, assistant professor in NCSU’s crop and soil sciences department, growers should plant their cover crop before the first frost, sometime prior to mid-October. “You can just leave it there until late March to mid-April,” he says. Most importantly, growers should remove their cover crop with enough time left before planting their tobacco to prevent greening. “About three weeks before you plant your tobacco, mow down the cover crop and incorporate it into the soil with multiple passes of a disc harrow.”

Over the next few weeks, the remains of the cover crops will rot, at which point you should add any other organic fertilizer, disc again, and then plant your tobacco. “Of course, there are certain conditions that are ideal if you are introducing an entirely new crop to your rotation,” Vann adds. “Most importantly, you have to consider the weather, which in any case is going to be your most variable factor. If conditions are too hot and dry or too cold and wet, crucial soil microbes that assist in the breakdown of the cover crop may fail.”

Of course, it’s not up to the grower for just-right Goldilocks conditions to magically present themselves. Still, Vann says that even with less-than-ideal conditions, the cover crop method is likely not any riskier than your typical nitrogen plan.

Nitrogen-fixing abilities are just one benefit of cover crops. “They also provide ecological benefits, primarily in the realm of soil conservation,” Vann says. “Cover crops decrease erosion and increase moisture retention as well as organic matter content of the soil. The plants also sequester carbon, where they take carbon dioxide out of the atmosphere and store it in the soil.” What does all of this mean? Ultimately, better growing conditions for you in the long run.



Category: Chemicals & Fertilizers, Soil

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