Ensuring that the seedlings emerge and grow at the same rate is essential to getting a high percentage of usable transplants. Research conducted by NCSU tobacco specialists has shown that as little as a three-day difference in emergence in a quarter of the seedlings could reduce usability, and clipping could not reverse the negative impacts of uneven growth in these studies. Growers can promote consistent growth by filling trays evenly, maintaining ideal temperatures and fertilizing appropriately.
In addition, aim to avoid spiral root to keep your transplants on track. Spiral roots can result in significant reduction to usable plants. Also called aerial roots, they occur when a root does not grow down into the media but instead grows on the surface, sometimes wrapping around the plant. According to the 2011 Kentucky and Tennessee Tobacco Production Guide, spiral root results from a complex interaction between the variety, the characteristics of the seed, media properties and weather conditions. While occurrences of spiral root have decreased in recent years, you can minimize the chance of it happening to your seedlings by avoiding packing media too tightly into trays.
Another way to promote consistent seedling growth is through proper fertilization. Growers should be monitoring and managing fertilizer programs in particular during the transplant stage, as fertilizer salt injury is the most common nutritional problem in float systems.
Fertilizer works by distributing nutrients to seeds in the form of salts. The salts dissolve in the water after fertilizer is added to the waterbed. As the media absorbs the water, the nutrients move into it. Salt injury occurs if there is too little humidity, high temperatures or strong winds, all of which can accelerate moisture evaporation. This doesn’t give the salts, and ultimately the nutrients in the salts, enough time to dissolve. The transplant is left with accumulated salt instead of much-needed nutrients.
Using a fertilizer-free medium or a medium with low levels of fertilizer can help prevent salt accumulation during germination, according to the 2011 NCSU flue-cured guide. It also recommends testing salt levels with handheld conductivity meters every 24 to 48 hours from seedling emergence until the roots reach the waterbed. If the sample either reads above 1,000 microseimens, and the plants are pale or have stopped growing, or if it reads above 1,500 microseimens, regardless of plant appearance, apply overhead water to leach and dilute the salts.
Handheld conductivity meters can also be useful for checking fertility levels in waterbeds. The test can determine how well fertilizer and media have mixed. The most accurate readings are gathered at the start of the season. The meter measures total conductivity from all nutrients. As a plant grows, it absorbs nutrients at different rates, altering the relationship between conductivity and nutrient levels and making it harder to measure. The only way to get a completely accurate measure of the nutrient concentration later in the season is to send a water sample to a lab.