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Applied Nitrogen

December 11, 2020 |

Nitrogen can be a grower’s best friend or worst enemy. If plants don’t get enough of the nutrient, their growth is stunted. If they get too much, the excess leaf growth that results can attract pests and disease. That’s why savvy tobacco farmers apply nitrogen in a calculated manner, taking into account recommendations based on type of tobacco, soil type and composition, weather and their own experience.

Nitrogen Needs

Nitrogen is a necessary component of many plant processes, including photosynthesis. Tobacco plants that are deficient in nitrogen first exhibit decreased growth, with leaves that stop expanding. If nitrogen deprivation continues, the color of the plant’s lower leaves changes as nitrogen is channeled to the upper leaves. The leaves of the plant become a pale green, eventually taking on a light yellow hue. If no nitrogen is added, the plant becomes bleached and can turn brown as tissues die.

Burley tobacco can become susceptible to target spot and brown spot if plant nitrogen levels are low.

Too Much of a Good Thing

The effects of excess nitrogen vary by tobacco type and with environmental factors. Boosting nitrogen in flue-cured tobacco fields can increase the yield of the crop, but the increased leafy growth can create some problems for harvesting and curing. The plants take longer to mature, and increased leaf volume can result in longer curing times. Sucker growth can be promoted, which may result in overuse of maleic hydrazide (MH), a suckericide that can leave undesirable residue on the leaves. Big, leafy growth may attract hornworms and aphids.

Burley fields may also see more aphids in fields where topping is delayed or in later-set fields where excess nitrogen is applied.

When experimenting with dark tobacco, University of Kentucky researchers found that “excessive nitrogen application will not likely increase yield but may have detrimental effects on leaf quality and increase levels of harmful carcinogens such as TSNAs.” When tobacco plants absorb increased nitrogen, the alkaloid levels in the plant get ramped up, which in turn can increase tobacco-specific nitrosamine (TSNA) levels.

Recommended Amounts

Base nitrogen rates are guidelines set by researchers for a tobacco type and region. Rates vary depending on depth of topsoil and soil texture. Base rates may also be adjusted based on which crops were previously grown in the field. For example, a heavily fertilized, low-yielding corn crop or legumes may leave the soil with more nitrogen than usual.

Growers of flue-cured tobacco should use 50 pounds to 80 pounds of nitrogen per acre depending on their topsoil depth. Adjustments can be made based on soil tests and grower experience. Burley and dark tobacco growers should use between 150 and 275 pounds of nitrogen per acre. In areas where burley and dark tobacco are grown, nitrogen does not remain in the soil for very long, so soil tests are of limited use to growers.

Field history and drainage are better indicators of nitrogen needs for burley and dark tobacco. Well-drained soils require less nitrogen, particularly when tobacco follows a sod or a sod-legume crop. More nitrogen fertilizer is needed for tobacco that is grown in moderate or low-drainage fields or fields whose previous crop was tobacco.

Connecticut broadleaf tobacco has a shorter growing season than burley and dark tobacco and is therefore more sensitive to excess nitrogen. Additionally, thick leaves are undesirable in broadleaf markets. Growers should apply less than they would for burley or dark tobacco. Researchers at the University of Kentucky recommend 150 pounds to 175 pounds of nitrogen per acre.

Andy Bailey, extension tobacco specialist at the University of Kentucky, says timing isn’t crucial for Connecticut broadleaf. “It probably doesn’t matter if this nitrogen is applied all before transplanting, or half at transplanting and the other half sidedressed at two weeks after transplanting.”

Bobby Toon of Carlisle County, Kentucky, is growing broadleaf for his second year, and is still trying to find a “happy balance” of nitrogen. Toon says they will probably use 150 units to 175 units total, with 75 to 80 of those being used as sidedress.

Source and Methods

Granular and liquid solutions of nitrogen fertilizers are available. Liquid solutions (28–32 percent nitrogen) are half urea and half ammonium nitrate. They require more equipment for application.

Granular fertilizers include calcium ammonium nitrate (27-0-0) and urea (46-0-0). Burley and dark growers should be sure to maintain a pH range of 6.4 to 6.6 with a liming program. Flue-cured growers should keep their soil’s pH in the range of 5.8 to 6.2.

Soil pH affects nutrient availability, so it is important to use nonacidic forms of nitrogen like sodium nitrate, calcium nitrate or sodium-potassium nitrate in acidic soils. Excess acidity can result in the uptake of too much manganese or aluminum, reducing root growth and/or leading to toxicity.

Fertilizer can be applied via broadcast, which covers the whole field, or by sidedress, which focuses the chemical along the side of a row. Toon sidedresses his tobacco at shoe-top height (less than 16 inches) at the same time he applies Ridomil.

Here Comes the Rain Again

Excess rainfall or irrigation can wash away nitrogen and other nutrients, so growers must keep an eye on the weather. Determining when and how much nitrogen to add after a period of heavy rains is a tricky task and miscalculations can affect quality. During these times, most growers rely on experience.

North Carolina State University’s 2020 Flue-Cured Tobacco Guide cautions against anticipating leaching and urges growers to only apply additional nitrogen if absolutely necessary. Additions of between 50 and 80 pounds of nitrogen may be necessary, the guide states. Amounts added vary based on contributing factors like topsoil depth to clay, age of the crop and estimated amount of water moving through the root zone. Recommendations are made as a percentage of the suggested base rate that was applied before leaching.

Daniel Johnson of D.L. Johnson Farms in Georgia adjusts his basic formula based on the circumstances. “We apply 10 pounds at transplant and then 70 pounds afterwards in two plowings.” He looks to the weather to decide if more is needed. “We watch it and see what the rainfall has been.”

Leaching is less of an issue for heavier Kentucky and Tennessee soils. Toon says he doesn’t usually need to add additional nitrogen to his dark tobacco fields past 275 units even during wet seasons. “Even with the rainfall last year, our nitrogen was pretty high.”

Best Guess

Fertilizer application is yet another instance where farming combines science and experience. Obtaining the best yield for each tobacco field requires calculations, canniness and maybe a little bit of luck. As Johnson says, “in farming, there are no normal years.”

Category: Chemicals & Fertilizers, Soil

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