Nitrogen fertility management in organic tobacco greenhouse production

Nitrogen (N) is the most important nutrient for ensuring a healthy tobacco crop that produces high quality leaf. Accordingly, the majority of this research focuses on organic N fertilizer sources. North Carolina soils tend to have sufficient levels of legacy phosphorus (P) and potassium (K) for flue-cured tobacco; however, the producers must frequently test soils for essential nutrient concentrations and follow the technical recommendations to keep all the essential nutrients at adequate levels for tobacco.

Greenhouse management
A tobacco float system is a semi-hydroponic system in which seedling nutrients are applied to the float system solution. Seedlings are grown on a soilless media complex that contains trace amounts of agricultural lime and fertilizer. The limited quantity of fertilizer within the media blend is not enough to promote sufficient seedling growth and development; therefore, supplemental nutrients must be added to the underlying water solution seven to ten days after floating and again roughly three weeks later. It takes 50 to 60 days for seedlings to reach transplant size. Just as soil must be tested to determine if any nutrient or fertility issues must be addressed, float system water should be tested for pH and alkalinity/bicarbonate levels.

To improve nutrient recommendations for organic tobacco producers, research was designed to evaluate three organic N programs that might (1) provide sufficient N for seedling growth, (2) limit phosphorus exposure, and (3) reduce bicarbonate concentrations (prevent high float water pH).

Even with high differences in bicarbonate and nitrate concentrations, the team did not observe appreciable differences in transplant usability (Table 3). Starting water quality, in terms of pH and bicarbonate concentrations, was acceptable; baseline pH and bicarbonate concentrations were 5.97 and 0.32 meq L-1. The elevated bicarbonate concentrations observed in this study were a result of fertilizer interaction and degradation and not an inherent water quality issue (Figure 6). For float systems, farmers may use source water with naturally occurring elevated bicarbonate concentrations. The team did not investigate how well aeration can address inherently high bicarbonate concentrations and thus do not know how well aeration would work under this circumstance.

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For more information:
University of North Carolina 

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