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Fertility know-how

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Taking soil samples and following test recommendations can improve fertilizer efficiency and protect the environment.

Rocky Womack

Growers should strongly consider taking soil samples and following the recommended analysis rather than relying on the amount of fertilizer they applied last spring, especially if they have an increase in pounds and are planting on idle land.

“In my area with the transition in the tobacco industry without quota, we are seeing more tobacco planted on land that historically has not been planted to tobacco,” says Taylor Clarke, unit coordinator and Extension agent in Meck-
lenburg County, Va. “It’s very important to take soil samples to evaluate phosphorus and potassium, especially phosphorus.”

The importance of a soil test is imperative. “There is no way of knowing how much residual fertility there is in your soil without a soil test,” says Stanley Holloway, an Extension agent in Yancey County, N.C.

Flue-cured tobacco grower Billy Coffee of Kenbridge, Va., follows the recommendations from his annual soil analysis, and says each field may require a different nutrient ratio. Depending on what the soil test recommendations are, he usually starts with a 6-12-18 fertilizer mix and puts down between 750 and 850 pounds per acre.

Clarke says the phosphorus level of the soil will determine which ratio of tobacco fertilizer to use. “If the soil test shows a high to very high phosphorus level, use a 1-1-3 ratio tobacco fertilizer,” he says. “If lower phosphorus levels are found, then 1-2-3 or 1-3-3 fertilizer may be needed.”

In an on-farm test conducted in Brunswick County, Va., this year, Clarke says he noticed a big benefit in early-season growth and uniformity when using a transplant starter fertilizer applied at five pounds of P205 per acre.

For superior growth, Coffee bands his fertilizer in the row. “We used to broadcast and didn’t leave a skip row,” he says. “Now, we are going back to skip rows.”

He returned to banding in the row because the process uses less fertilizer, which is important to him with the increase in fertilizer prices in the past year. He also likes banding in the row because it applies fertilizer directly where he needs it instead of where he doesn’t.

Another good reason for soil testing is to measure soil pH and lime applications. Holloway says the optimum pH for burley tobacco production is 5.5 to 6 for maximum growth and fertilizer availability. For a pH below 5.5, manganese toxicity becomes an issue. Black shank can pose a concern at lower pH levels, he says, but the disease is cause for even more concern in soils at a pH above 6.

Holloway advises applying only the recommended rates of nutrients in order to receive the maximum amount of economic returns. Applying only the recommended rates also helps protect the environment.

“Most burley growers have traditionally used complete fertilizers, such as 5-10-15, 8-16-24 or 9-18-27, for several years and have built fertility levels in the soil,” Holloway says. “Soil-test summaries over the past 10 years for North Carolina show that 70 to 80 percent of the soils analyzed for burley tobacco contain high to very high levels of phosphorus and potassium and require little or no addition of these nutrients for maximum production. Growers that continue to use standard rates of these complete fertilizers are wasting money and potentially harming the environment. A soil test will also help to avoid potential problems with micro-nutrient deficiencies.”

When using a complete fertilizer, Holloway advises determining if it is tobacco grade. “General-use fertilizers typically contain potassium chloride as the potassium source and should be avoided, while tobacco-grade fertilizers typically contain potassium sulfate,” he says. “Excessive chlorine levels in the tobacco can result in curing problems for growers and smokeability problems for cigarette manufacturers. The same applies if only applying potash; make sure you’re using 0-0-50, sulfate of potash, and not 0-0-60, muriate of potash, due to issues related to chlorine content.”

For the most efficiency, Clarke encourages Virginia flue-cured growers to apply a complete fertilizer at planting, and he discourages broadcast applications. “Broadcast applications are easy to make; however, uniformity of the spread pattern is always a concern,” he says. “If a grower is leaving out a skip row, it can increase fertilizer purchased by 10 percent. On clay soils, broadcasting fertilizer increases the need for extra phosphorus to be applied to achieve similar crop response compared to band applications because of soil binding [tie up]. Also, even though it does not seem logical, the chances of fertilizer salt injury are greater with broadcast applications compared to properly placed band applications.”

Broadcasting can have its place. Holloway says whether to broadcast depends on the grower’s management style, his equipment and his available labor. From a labor standpoint, he says broadcasting all the fertilizer at preplant might work more efficiently for some growers.

Coffee sidedresses with about 200 pounds per acre of 15-0-14 or 14-0-0. Clarke advises using a sidedress material with potassium that meets the soil analysis recommendation for each particular field.

Holloway says sidedressing almost always provides the most efficient use of fertilizer. He says growers have options when best applying sidedressing. One option is to broadcast 100 percent of the recommended phosphorus and potassium and 50 percent of the recommended nitrogen preplant. They can sidedress the remaining nitrogen at the first or second cultivation.

A second option when sidedressing is to broadcast 50 percent of all recommended nutrients, sidedress the remaining nitrogen at first cultivation and the remaining fertilizer at second cultivation.

“A lot is dependent on the soil test results,” Holloway says. “If only phosphorus is needed, then it should all be broadcast preplant. Potassium may benefit from split applications, and typically the most efficient nitrogen use is by split applications.”

Nitrogen
In burley tobacco, Holloway says North Carolina’s standard nitrogen rate is from 150 to 200 pounds per acre. “There have been numerous studies conducted looking at nitrogen rate and yield,” he says. “In almost all instances, there has been no benefit to applying more than 200 units of nitrogen per acre.”

For some growers, nitrogen leaching can pose a concern. “In years when there is excessive early-season rainfall, an additional application of 25 to 50 pounds of nitrogen per acre prior to layby can be beneficial in optimizing yields,” Holloway says. “A faster-reacting, shorter-life nitrogen source such as calcium nitrate or nitrate of soda, if available, is recommended over ammonium nitrate due to excessive nitrogen concerns in the curing process.”

When thinking of a fertilizer management plan, every grower must ask himself what his objective is in raising a tobacco crop. “Does he want a high-yielding, high-quality crop or does he want an average crop in terms of yield and quality with minimal inputs?” asks Bill Easterwood, director of market support and regulatory compliance with Yara North America Inc. in Tampa, Fla.

Whatever his decision, nitrogen will play a key role, and he must determine what his nitrogen source will be. Easterwood remembers reading the book Principles of Flue-Cured Tobacco Production by W.K. Collins and S.N. Hawks Jr. of North Carolina State University. He says their advice and tobacco production principles still apply today. For instance, Easterwood says tobacco will absorb ammonium and nitrate nitrogen, but if the plant absorbs too much ammonium nitrogen, the quality and yield will be reduced. Too much ammonium also can prevent or reduce the amount of calcium, magnesium and potassium absorbed by the plant.

Because too much ammonium nitrogen may reduce quality and yield of flue-cured tobacco, how much should growers apply without losing their profits? Easterwood refers to the book by Collins and Hawks again. He says they recommended that at least 50 percent of the nitrogen in preplant fertilizer be in the nitrate form. He agrees.

Economics
Easterwood says David Smith, an Extension specialist with North Carolina State University, compared CN-9 (all nitrate) to UAN-30 (25 percent nitrate) as tobacco nitrogen sources during 2003-2005. He says Smith found that no fertilizer treatment differences were within a 95 percent level of confidence.

(The 95 percent confidence level is a statistical tool commonly used in scientific research. It means, essentially, that if a test were conducted 100 times, the resulting data would be within a certain number of percentage points in 95 of the 100 tests.)

Smith says that NCSU researchers have been unable to confirm results from earlier studies conducted in the 1960s that showed economically significant yield advantages by using nitrate. “The bottom line is that in numerous tests, we have yet to find an advantage of one form of nitrogen over the other,” Smith says. “Nitrate nitrogen is three times more expensive than 30 percent UAN, and we have collected no data to support a benefit of that higher cost when those data are interpreted by accepted scientific standards.”

“Since the buyout there has been an increased awareness of the need to reduce tobacco production costs,” Smith continues. “A significant number of growers have changed their fertilization practices and are now using higher percentage ammonium products such as 30 percent UAN. I am not aware of any problems, and as far as I know, growers are pleased with the results.”

Easterwood maintains, however, that using a nitrate fertilizer should provide economic benefits to the grower. “As a scientist, I understand that 95 percent confidence is required to determine treatment differences,” Easterwood says. “However, I also know that 95 percent confidence is very, very rarely achieved in production agriculture in this type of test due to field variability.

“During three years of testing, CN-9 treatments increased crop value $292 per acre compared with UAN-30, which should be economically valid to the grower,” Easterwood says. “All-nitrate fertilizer cost was $62 more per acre than UAN-30. Therefore, profitability was increased $230 per acre.”

Easterwood says a grower leaves money in the field if he does not use nitrate, even though the input cost increases (based on the $62 increase per acre by using nitrate fertilizer).

“Let’s look at a five-year potential of a grower’s loss if he does not use nitrate. Increased fertilizer cost will be $310 per acre for five years [$62 x 5],” he says. “If he makes the above crop in just one of the five years, he will lose $18 per acre [$310-$292]. Two good years out of five increase his profitability by $274 per acre [$292x2-$310]. Three good years out of five yield $566 per acre. Four good years out of five yield $858 per acre. Five good years out of five yield $1,150 per acre.”