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Potassium is a critical nutrient for tobacco, promoting good root growth, stimulating the production of proteins and helping the plant fight off disease and drought. Recently, though, the high price of potash, the major source of potassium in tobacco fertilizers, has forced growers to think long and hard when planning their fertilization budgets.
 
“Over the past few years, potash prices have really skyrocketed,” says Bob Pearce, an Extension tobacco specialist at the University of Kentucky. He says current world potash prices may be falling back a bit, but declining world market prices don’t always translate directly to lower fertilizer prices for growers. Regardless of what the coming year brings in terms of price trends, potassium applications will be expensive, and it pays to be as efficient as possible when planning a fertilization program.

Tips for burley growers
If you’re aiming to be more efficient in your potassium use, Pearce says his first recommendation is to conduct a soil test. The test results will indicate the amount of potassium to apply—if any is needed in the first place. “We have good reliable soil tests, and we’re confident in our ability to predict the likelihood of the response to a potash application,” Pearce says. “Almost 30 percent of the fields that we’ve tested in the last few years didn’t need any potash at all.”
 
Pearce says some growers apply 200 to 300 pounds of potassium per acre as a matter of routine. “If they don’t need all that, there could be some big savings there,” he says. “At last year’s prices of about $1,200 per ton of potassium sulfate, you’re talking roughly $1.20 per pound [of potassium oxide (K2O)]. You could be saving up to $300 per acre.”

For burley growers, the type of potash used plays a big part in costs. So, for his second tip, Pearce says, if possible, to conduct a soil test in the fall to see if you can apply a less expensive form of potash. Kentucky tobacco specialists recommend growers use sulfate of potash if they need to apply potassium in the spring, but sulfate of potash is more expensive (and a less concentrated source of potassium) than the alternative, muriate of potash.

In much of Kentucky, growers can apply the less expensive muriate of potash, but only if they apply it in the fall. Springtime applications of a chloride-containing fertilizer such as muriate of potash at rates greater than 50 pounds of chloride per acre can cause increased chloride concentrations in the leaf, which can lead to fat stems and mold and rot during curing and storage. High chloride concentrations may also affect smoking characteristics, including a reduced burn rate and unpleasant flavors.

With a fall application of muriate, precipitation over the winter will eventually cause the excessive chloride to leach out of the soil, but the potassium will remain in the root zone. Pearce cautions that this tip only applies to fine- and medium-textured soils. It doesn’t apply in flue-cured growing areas with sandy soils, where the potassium will leach out along with the chloride. Nor should a grower attempt a muriate application in wet, boggy soil, which tends to retain nutrients and won’t allow the chloride to leach out.

The savings between these two applications could add up. According to Pearce’s research, growers could save up to $114 per acre with a fall application of muriate instead of a spring application of sulfate, assuming the two applications would produce similar yields and quality and potash prices remained the same as last year.

For many growers, however, knowing in the fall how much burley they might plant in the coming year is becoming increasingly challenging, making early applications more of a risk.
“Most growers in Kentucky could do this; it’s just a matter of knowing where they are going to put the tobacco and collecting a soil sample in the fall,” says Pearce. “But this is one of the biggest challenges since many growers don’t know in the fall if they’ll have a contract and how much they should spend to prepare fields.”

In cases like these, sulfate of potash is then the best option as the major potassium fertilizer used on tobacco fields after Jan. 1. This leads to Pearce’s third tip: if you do apply in the spring, do a banded rather than broadcast application.

“There’s good evidence that in our soils, if we apply fertilizer in a band near the row, we can reduce potash applications by 30 percent,” he says. “So, for example, if the soil test says you need 100 pounds per acre you can use 70 pounds. That may save you up to $100 per acre.”

Pearce has an additional caution. Regardless of the type of fertilizer you use, carefully evaluate the product before you buy. Recently some potassium fertilizer products have been marketed as having higher efficiency, and growers have been encouraged to apply much lower rates, which in some cases has led to yield losses. When choosing among different products and formulations, growers should always look for good scientific evidence to back up the product’s claims.

Research in flue-cured country
Flue-cured growers typically use potassium sulfate or potassium magnesium as their potassium source, and, as with burley, determining the best rate to use is more science than art.
 Matthew Vann, a graduate student specializing in tobacco production in North Carolina State University’s crop science program, is currently working on two research projects looking at maximizing potassium use efficiency. Vann says the research was driven not only because of the high cost of potash, but also because farmers are increasingly transitioning away from using fertilizer blends and need more information on how to apply each nutrient separately as it’s needed. “This is more efficient from a cost standpoint, especially if there is already enough potassium in the soil,” Vann says. “Why apply more than needed?”
 
In one project, he’s looking at potassium application at eight rates that range from 0 to 225 pounds per acre to see what rate amounts are better for growers on a long-term basis. In the second project, he’s looking at potassium rates with four different application methods and times: broadcast one month before transplant, broadcast one week before transplant, banded at transplant and a split application banded at transplant and banded at lay-by. “Different farmers want to put it out at different times of the year,” Vann says. “We’re trying to determine what’s the best rate at the best time.”

Vann says the data analysis won’t be finished until later in the year, but like all Extension research, the results should translate directly to recommendations for the grower and will help them save money where they can. This is especially important as prices of potassium magnesium, along with other potassium sources, have soared, rising from approximately $0.55 per pound in April 2008 to approximately $1.30 per pound in November 2008. “They now may be falling again,” Vann says. “But it still adds up, especially when you look at what some growers apply to the crop. I’ve heard of growers who apply up to 150 pounds per acre.”

As for general recommendations, like burley growers, flue-cured growers need to conduct a soil test as the first step in a fertilization program. According to NCSU’s Flue Cured Tobacco Guide 2010, about 60 percent of North Carolina soils contain at least “high” levels of potassium because of abundant soil sources and past applications, so growers should be careful to follow soil test recommendations. According to the guide, over-application probably won’t improve tobacco yield and quality, but it will increase production costs—and this is something every grower wants to avoid.