In recent growing seasons, boron (B) nutrition has become an increasing issue across the state. While B demand for flue-cured tobacco is relatively low ( <1 pound per acre) (1), compared to macro and secondary nutrients, its uptake is critical for tobacco plants to realize their full potential. The extreme weather events of recent growing seasons, coupled with alternative fertilizer programs now used in production have furthered the need for B nutrition to be addressed in greater detail.
Boron Resources in the Soil
Boron is found naturally in many soils whose parent materials contained the element; however, B levels in highly weathered soils as well as soils found in humid regions are often not sufficient to meet agricultural demands (2). Additionally, increases in soil organic matter are quite often associated with higher levels of B (2). These characteristics of B availability in the soil have potential to create management issues for flue-cured tobacco producers because the crop is most often produced on highly weathered soil types, in humid growing regions, with far less than one percent organic matter.
Boron availability in the soil profile increases when soil pH is in the range of 5.5-7.0 (2). When soil pH is below 5.5 B is likely to complex with Fe and Al; alternatively, when soil pH is above 7.0 B is less likely to be found as Boric Acid (H3BO3), which is the plant available form of the nutrient (2). Boron is also highly leachable in the coarse soil type’s that are native to the eastern Coastal Plain. Ultimately, B nutrition for tobacco production must be strongly considered when developing a fertilizer program due to the nature of the soils in major production areas.
Boron Fertilizer Application
Boron has long been included as part of a micronutrient package when blending fertilizers used in tobacco production. However, in current times, many producers have transitioned away from applying blended fertilizers in favor of applying prescription based fertilizers due to increased cost of materials the lacking need for many of the nutrients contained in blended fertilizers. The result of multiple seasons of fertilizer application without the addition of a micronutrient package containing B as well as growing seasons with excessive rainfall is B deficiencies in the various tobacco producing regions of North Carolina.
For producers utilizing blended fertilizer materials, continued use of a micronutrient package is advisable. Blended tobacco grade fertilizers, such as 6-6-18 and 8-8-24, with a micronutrient package will typically contain around 0.08% B and cannot exceed 0.1% based on North Carolina law (02 NCAC 48B .0108). Common application rates of these materials should provide between 0.3 and 0.5 lbs B per acre (3). It should be noted that some fertilizer dealers/blenders have reduced amounts of B in tobacco fertilizer to as low as 0.04%. Ultimately, it is advisable to check with your fertilizer dealer to determine mixing ratios for B as deficiency issues have been observed under specific environmental conditions. For producers utilizing a prescription based fertilizer program, adding a micronutrient package to dry fertilizer materials is worth consideration. Fertilizer dealers with a history of tobacco production experience will be aware of blending requirements, but 0.08% B is a recommended target in terms of nutritional requirements. Lastly, for producers not utilizing a micronutrient package or for producers utilizing a fertilizer program with liquid N and granular K only, an application of B fertilizer is only recommended when a deficiency is observed. Correctly identifying a B deficiency is critical as B can be toxic to tobacco plants at application rates greater than 1.0 pound of elemental B per acre.
Boron Deficiency and Toxicity
Boron deficiency is seldom observed under field conditions when recommended production practices are followed (3). However, when deficiencies are observed it is most typically on soils with a pH above 6.2, when plants are experiencing rapid growth, during periods of excessive rainfall, or combinations of any of these factors. In the early stages of B deficiency the bud leaves become abnormally brittle and the midrib may break at several points (3). In extreme cases, younger leaves may be malformed and older leaves may break off of the plant.
Correcting a B deficiency can be accomplished through the foliar application of a boron containing material such as Borax. Careful consideration must be made prior to application because excessive rates of elemental B are toxic to tobacco plants. It is recommended that no more than 0.5 lbs of B per acre be applied to a deficient tobacco crop; furthermore, deficiencies have been corrected at rates as low as 0.3 lbs of B per acre. Boron toxicity has been observed at application rates as low as 0.9 lbs B per acre; therefore, it should only be applied when a true B deficiency has been correctly diagnosed. When B deficiency is suspected, it is advisable to immediately collect tissue and soil samples and submit them to the appropriate laboratory for analysis. Collected tissue should come from leaves at or very near the bud of the plant, as B is not plant mobile and deficiencies will first be observed in younger leaves.
Boron nutrition for flue-cured tobacco has received increasing attention in recent years because of the emphasis that many producers have placed on alternative approaches to fertilizer programs and because of extreme weather conditions. Boron is critical for the production of quality flue-cured tobacco and, as a result, the nutrient must be considered regardless of fertility program.
1.) Tso, T.C. 1990. Nutrition-Minor Elements and Heavy Metals. In: Production, physiology, and biochemistry of tobacco plant(pp. 313-368). Beltsville, Maryland: IDEALS, Inc.
2.) Troeh, F. R., & Thompson, L. M. 2005. The Micronutrients. In: Soils and soil fertility(6th ed., pp. 283-304). Ames, Iowa: Blackwell Publishing.
3.) Collins, W.K., & Hawks, S.N. 1993. Fertilization. In: Principles of tobacco production 2nd ed., pp. 41-59). Raleigh, North Carolina: W.K. Collins.