Fertilization Practices in Greenhouse Tobacco Seedling Production: A Survey

Production of tobacco (Nicotiana tabacum L.) seedlings in greenhouses has become increasingly popular in the southeastern USA. In the most common system, styrofoam flats float on a bath of nutrient solution. A survey was conducted in 1993 and 1994 in South Carolina to determine the nutrient concentration of the solution after the plants were removed. Extension agents collected 183 samples from their growers on an anonymous basis. The samples were analyzed for NO£, NH~, PO~-, SO4 ~-, K+, Caz+, and Mg z÷ by ion chromatography. The analytical results show that most growers are following extension fertility recommendations; however, a few growers are heavily overfertilizing. The overfertilization may be an incorrect grower response to growth-limiting factors other than nutrient supply. The extension agents can use this information to better work with growers on their fertility practices. Residual PO~ concentrations uggest that P recommendations were too high, but new fertility programs have successfully lowered the residual P concentrations. The results also show that current methods of waste nutrient disposal should be adequate. S TOBACCO (Nicotiana tabacurn L.) producers are increasingly adopting greenhouse t chnology for production of transplants. For the most part, these greenhouses use a technology referred to as the float system (Rideout et al., 1994). Briefly, this system involves constructing wooden frames on the floor of the greenhouse. At the beginning of each season, the frames are lined with plastic sheeting and filled with nutrient solution formulated with water-soluble horticultural fertilizers such as 20-4.4-16.6, 16-1.7-13.3, or 15-2.2-12.5 (N-P-K). Solution and plastic are both only used for one growing season. Styrofoam fiats filled with peat and vermiculite-based soilless medium float on this nutrient solution. All irrigation is by capillary movement of nutrient solution up into the media. The nutrient level of the soilless medium is generally low enough that the majority of the nutrients needed for seedling growth must be provided by the nutrient solution. The composition of the nutrient solution when the seedlings are removed for transplanting is significant for several reasons. First, the solution may still contain appreciable quantities of nutrients, which must be disposed of in an environmentally sound manner. Given the seasonal nature of tobacco seedling production, the solution is not Department of Agronomy, Clemson Univ., Pee Dee Research and Education Center, Rt. 1, Box 531, Florence, SC 29501-9603. Technical contribution no. 4169 of the South Carolina Agile. Exp. Stn. Received 26 Feb. ! 996. *Corresponding author (jrdt@clemson.edu). Published in J. Nat. Resour. Life Sci. Educ. 26:111-115 ( 997). reused. The most common disposal method is to allow the solution to evaporate in the off season. Evaluation of this method and recommendation f a sound disposal method is critical, since much of the U.S. tobacco production is localized in the environmentally sensitive southeastern coastal plain. Second, residual nutrient concentration is an indicator of the efficiency of fertilization practices in use. High residual concentrations may indicate overapplication, while lack of a particular nutrient may indicate underapplication. Widespread overor underapplication may indicate an incorrect extension recommendation. Lastly, examining residual nutrient levels in a large number of greenhouses may give insight into grower adoption of, and adherence to, extension recommendations. The period (1993-1994) represented by this study was one of great change in recommended fertilization practices. For instance, in 1992, the recommended fertilizer was water soluble 20-8.7-16.6 (N-P-K) at a rate sufficient to provide I00 L-l N (Gooden et al., 1991). Based on P nutrition research results, the fertility recommendation was changed to 15-2.2-12.5 or 16-1.7-13.3 (N-P-K) at a rate sufficient provide 250 mg L-~ N in 1994 (Rideout et al., 1994). addition, the number of producers using greenhouses for seedling production greatly increased uring this time. The objective of this study was to determine the residual nutrient concentration in as many greenhouses a practical. From the residual concentration, we planned to make inferences concerning grower adherence to recommended practices, and to determine what additional educational efforts were needed. In addition, measuring residual concentrations over a wide area aided in identification of any potential environmental problems from waste nutrient disposal. MATERIALS AND METHODS