Taylor Williams, Extension agent with the Cooperative Extension Service, says one of the most difficult decisions facing tobacco growers is how (and how much) to replace nitrogen, and possibly sulfur and potash after that excessive rain.
More than seven inches of rain fell on tobacco already too tall to side dress last week, he says.
Williams cites guidelines prepared by Dr. David Smith of the N.C. State University faculty in the 2006 edition of the Flue-Cured Tobacco Information booklet. Smith says three variables -- topsoil depth, water percolation and weeks after transplanting -- determine how much nitrogen to apply.
"To apply this as a real world example, let's assume I have tobacco in the Carthage area on a Dothan soil (our best tobacco land) that was planted May 8. I received six inches of rain on June 12 and 13. The soil has clay within 10 inches of the surface, and I assume that about four inches of rain percolated (six inches rain less one inch to recharge soil, one-inch runoff) five weeks after planting. Therefore, I would try to put out 20 percent of my total nitrogen for leaching losses, or 16 units (20% of 80 units)," Williams said.
Another example is a tobacco field in Eagle Springs on Candor sand that was planted May 1. It rained four inches on June 12 and 13, about six weeks after planting. The soil has clay about 17 feet (that's right, not inches) below the surface. Because rain is not likely to run off this soil and because not more than one inch of rain is needed to recharge soil moisture, the farmer can assume that about three inches percolated.
By consulting Smith's chart, the farmer estimates that 25 percent of the nitrogen, or 20 units, should be added.
Williams said Candor sand is "some of our droughtiest land, but our best growers grow fine irrigated tobacco here."
As for potassium and sulfur, Smith recommends that where topsoil is greater than 10 inches, one unit of potassium should be added for every unit of nitrogen applied as leaching loss adjustment.
Sulfur, and even magnesium, can be leached from soils, Williams says. If the potassium source is sul-po-mag, sulfur and magnesium are supplied, and if the nitrogen source is urea, sulfur is probably present, he adds.
"This is complicated," he said. "Fortunately we have good tools to test our ciphers. NCDA's plant tissue analysis service is tailor-made for checking your assumptions."
Farmers can call Williams (947-3188) or David Dycus (919-770-0635) if they would like to submit a sample.
Florence Gilkeson can be reached at 947-4962 or by e-mail at florence@thepilot.com.
