Fertility – Nutrients
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Nitrogen (N). Nitrogen has a greater effect on tobacco yield and quality than any other nutrient. Too little nitrogen reduces yield and results in pale, slick cured leaf. Too much nitrogen may increase yield slightly but may also make mechanical harvesting and curing more difficult, delay maturity, extend curing time, and result in more unripe cured leaf. Excessive nitrogen also stimulates sucker growth, which can lead to excessive use of maleic hydrazide (MH) and increase problems with hornworms and aphids.
Nitrogen is also very leachable, and overapplication may contribute to groundwater contamination in deep, sandy soils.
Soil analysis is not used to estimate the N rate needed for tobacco because it is a leachable nutrient and the soil test is unreliable. Instead, recommendations for N on flue-cured tobacco have been developed based on soil texture, depth to clay, and previous crop. The range is 50-80 lb/acre with higher rates recommended for soils with more sand and lower rates for soils with more clay content. For depth to clay, the following table shows recommended N rates:
Adjustments to the final nitrogen rate also need to e made based on the previous crop. Use the lower end of the rate range following legumes such as soybeans or peanuts as well as heavily fertilized but poor corn crops.
Phosphorus is not very leachable, even in sandy soils, and a good tobacco crop only removes about 15 pounds per acre (as P205). However, many times this amount has been applied to tobacco fields over the years, resulting in at least “high” levels of available phosphorus in about 85 percent of the fields used for tobacco. Most fields in North Carolina used for flue-cured tobacco production do not need fertilizer P. Pay close attention to the soil test results for the P index and recommendations. Residual P can be utilized to satisfy crop demand during the growing season and, in most, cases supplemental P is not needed. Growers reluctant to not apply P can apply 5 pounds per acre in a transplant water fertilizer. Research indicates that 5 pounds in the transplant water is equivalent to 40 pounds per acre band applied after transplanting. If transplant water fertilizer is not an option, P can be supplied in a sidedress application post-transplanting. When using blended materials, producers are encouraged to utilize materials with a low P analysis such as 6-3-18.
Potassium is leachable, especially in deep, sandy soils, and a good crop removes about 90 pounds per acre (as K20). However, about 60 percent of our tobacco soils contain at least “high” levels of available potassium because of more abundant soil sources and excessive application. Also, subsoils in tobacco fields often contain substantial amounts of potassium and other leachable nutrients that are seldom measured by soil tests because only topsoils are usually sampled. When considering what an appropriate K rate is for a specific field consider the residual soil K content, soil texture, and depth to clay. Strong potential exists for the reduction of K application rates in many locations throughout North Carolina where medium to high K-indices, medium to fine soil texture, and shallow topsoil exist.
Calcium and Magnesium
If soil pH is kept within the desirable range of 5.8 to 6.2 with dolomitic limestone, the available levels of calcium and magnesium will usually be high enough to meet the needs of the crop. Otherwise, 40 to 50 pounds of calcium (Ca) and 15 to 20 pounds of magnesium (Mg) per acre are needed from the N-P-K fertilizer. Even with proper liming, some magnesium deficiency may occur on deep, sandy soils (more than 15 inches to clay) under severe leaching conditions. In these instances, supplying 15 to 20 pounds of magnesium per acre in the fertilizer may be desirable in the second and third seasons after lime application.
However, using N-P-K fertilizers containing calcium and magnesium will not substitute for using dolomitic lime if soil pH is too low. Be especially aware of low soil pH. The state’s latest soil test summaries show that about 30 percent of the tobacco fields tested in the last several years have had a pH lower than 5.5, and piedmont soils generally were more acid than those in the coastal plain.
Sulfur deficiencies are most likely on deep, sandy soils (over 15 inches to clay) that are low in humic matter (less than 0.5 percent). Because sulfur leaches, deficiencies are more likely in these soils following heavy rainfall in the winter and spring, especially if sulfur is omitted from the fertilizer of the next tobacco crop.
Symptoms of sulfur deficiency are very similar to and often mistaken for those of nitrogen deficiency. When a plant is low in nitrogen, the lower leaves are paler than the upper leaves and “burn up” prematurely. However, sulfur deficiency begins as yellowing in the buds; the leaves gradually pale from top to bottom, and the lower leaves do not “burn up” prematurely unless nitrogen is also deficient. Because sulfur is required for nitrogen use in the plant, adding high rates of nitrogen to sulfur-deficient crops will not turn the crops green, and can, in fact, reduce leaf quality. Therefore, accurate diagnosis of the deficiency is very important and often requires tissue analysis.
Soil tests for sulfur are sometimes unreliable. Therefore, to reduce the chance of sulfur deficiency on deep, sandy soils, add 20 to 30 pounds of sulfur (S) per acre from the N-P-K fertilizer every year. Sulfur deficiency occurring before lay-by can be corrected by banding 100 to 150 pounds of Sul-Po-Mag or potassium sulfate (0-0-50) as soon as possible after the deficiency is identified. However, sulfur deficiency on soils less than about 12 inches to clay is often temporary, even when no extra sulfur is applied, because adequate sulfur is usually contained in subsoils and will be absorbed as roots reach this depth.
Copper and Zinc
Known deficiencies of copper or zinc are extremely rare for tobacco. Rates suggested on the soil test report will be sufficient for several years, and future test results should be used to determine if and when copper and zinc should be reapplied.
Manganese deficiency begins to show on the lower leaves as flecks very similar to those caused by high ozone concentrations in the air (commonly called weather fleck). While weather fleck can occur anywhere in the state, manganese deficiency occurs primarily on low-manganese, overlimed soils in the coastal plain. Using too much lime causes soil pH to increase, which reduces manganese availability to plant roots. Tobacco plants that develop manganese deficiency are grown on soils with a pH of 6.2 or higher and low levels of soil manganese (availability index less than 26). Based on recent soil test results, 7 percent of the tobacco soils in the coastal plain were pH 6.5 or above. Therefore, tobacco planted in these soils is at risk for manganese deficiency, particularly on soil types such as Goldsboro, which have slightly higher organic matter than other coastal plains soils. Tobacco performs well when soil pH stays in the 5.8 to 6.2 range. Other major crops, such as soybeans, corn, and small grains, also perform well in this pH range if soil phosphorus is high. Therefore, when these crops are in rotation with tobacco, they usually should not be limed at rates higher than those suggested by the soil test for tobacco.
Manganese deficiency can be corrected by soil or foliar application of several manganese sources. Manganese sulfate is a relatively soluble, inexpensive source that can be used for soil or foliar treatment. The more expensive chelated sources generally perform satisfactorily as foliar sprays but are not superior to sulfates when applied to the soil. For soil applications, mixing the manganese source with acid-forming fertilizers increases its effectiveness, and banding is usually better than broadcasting. Do not broadcast manganese on soils with a pH greater than 6.1 because it will be converted to a less available form. For band application, special blends may be required because premium fertilizers usually do not contain enough manganese to correct a deficiency. When applying manganese, the general recommendation for actual Mn in North Carolina is to add about 3 pounds per acre banded, 10 pounds per acre broadcast, or 0.5 pound per acre as a foliar spray. Foliar application of manganese is an efficient way of correcting an unexpected deficiency because lower rates are often as effective as much higher rates of soil-applied manganese.
There is no suitable soil test for chloride, but this nutrient is included in most N-P-K tobacco fertilizers. You will apply sufficient chloride when you use N-P-K fertilizers guaranteeing chloride at suggested rates. Suggested rates of most fumigants also supply adequate amounts of chloride as chlorine; when Telone C-17 or Chlor-O-Pic is used, the N-P-K fertilizer does not need to contain chloride. Otherwise, the fertilizer should include enough chloride to provide a maximum of 20 to 30 pounds per acre. Muriate of potash is an inexpensive source of potassium; however, its potassium chloride and its use for supplying more than 20-30 lbs of K can result in chloride toxicity. Higher rates will not improve yield but can reduce quality. Chloride may not be included in some fertilizers, particularly blends or liquids, unless requested by the grower.