Tobacco media typically consists of peat mixed with a combination of vermiculite (a natural mineral that expands with the application of heat) and perlite (naturally forming volcanic glass that also expands with the application of heat) in various proportions. The Kentucky and Tennessee guide points out that research has not yet found any specific combination of these elements that works the best or one brand that consistently outperforms any other.
More important than the type of media is how you use it. Bob Pearce, with the University of Kentucky’s College of Agriculture, reports that evenly filling trays is one of the most important things a grower can do to ensure healthy transplants. The seeder needs to apply one seed per cell, in the very center, down in the dibble mark (the indentation in the top of the media).
Dibbling, or making a half- to three-quarter-inch depression in the soil surface in each cell, has been shown to increase germination success. A rolling dibbler or dibble board should be matched to the tray style or brand so that the depression ends up as close to the center of each cell as possible.
Pearce also advises that, in order to make sure you have filled the cells consistently, you look periodically at the bottom of the cells. The media should be visible at the bottom of the hole. Make sure to check right before they go into the water. If sufficient media is not in the cell, water will not wick up and the seed will not germinate. Also, handle the trays gently when in transport, and add moisture if necessary during the fill: If the media is too dry, the seeds could fall out.
Pearce cautions that typically, media purchased this season would not need additional moisture as it is manufactured not to need additional water. If you are using media from last season, however, you will most likely need to add water to the bags, typically between a gallon to a gallon and a half per medium-sized bag.
You can check the moisture level of your medium with a simple test, according to Pearce. “Just take a handful of the mix and squeeze it in the palm of your hand. When you open your hand, the mix should retain its ball shape. If water comes out, then the medium is too wet. If it turns into a pile of loose mix, then it is too dry.”
Finally, the seed should be at the center of each cell at the bottom of the dibble. Uniform setup and planting will increase chances of uniform growth.