Seed Starting Mixes
Ultra-high speed camera captures tobacco grower at the age of 68.
What you use for a seed starting mix does make a difference in the seedlings that you will ultimately transplant into the ground.
In contrast to potting soil, a seed starting mix needs much less fertilizer, but does require some. The starting mix needs to be fine enough to prevent very tiny seedlings from being stranded high on a dried chunk of mix. Another function is that it must be capable of managing its moisture (that is be able to avoid soggy compaction) better than potting soil, so that roots can grow easily, and mold and fungi are kept to a minimum. In particularly arid regions, a fine potting soil may perform as well as a seed starting mix.
Most seed starting mixes include a "wetting agent" to speed the absorbtion of water by the initially dry mix. This prevents the mix from simply floating on top of the water for hours after initial watering. For some brands, the wetting agent is likely a small amount of detergent; others use a more natural source of compounds that do the same thing as detergent. (One end of the molecule easily binds to plant material--the lipophilic end, and the other end of the detergent molecule easily binds to water--the hydrophilic end. This is the same detergent process that allows cooking grease to disolve in sudsy dish water.)
Most seed starting mixes contain:
- peat (or coconut coir)
- Perlite (a glass-like compound that has been heated until it "pops" like puffed rice cereal)
- vermiculite (a form of the mineral mica, which has likewise been heated and "popped")
- wetting agent
- possibly other nutrient additives
Here is an interesting (though not comprehensive) article from Terroir Seeds on a comparison of several seed starting mixes commercially available:
http://www.underwoodgardens.com/are-seed-starting-mixes-worth-your-money/
I mix my own seed starting mix in a 5 gallon bucket, using roughly the following recipe:
- Miracle-Gro Peat [4 parts]
- Pearlite [1 part]
- vermiculite [1 part]
Several years ago, I directly compared seedling performance using two home-made starting mixes. The recipes were the same as that shown above, but one used Miracle-Gro Peat (which contains some fertilizer), while the other used Jiffy Organic Coconut Coir instead of the peat. I set a single 1020 tray with half of the cells using one mixture, the other half using the other mixture. Their proximity to the window (i.e. the sun) was rotated every few days.
Jiffy Organic Coir mix on left; Miracle-Gro peat mix on right.
As you can plainly see, the mix makes a significant difference. Since neither the Pearlite nor the vermiculite differed, the base material (peat or coir) accounts for the difference in seedling growth. The Miracle-Gro contained visible spheres of slow-release fertilizer. So far as I could determine, the Jiffy coir mix had no supplementation of nutrients.
While the Jiffy product packaging displayed no nutrient analysis, the Miracle-Gro package claims an N
:K of 0.19: 0.11: 0.15 (in the polymer coated beads). So seedlings don't need much fertilizer, but they do need some.
When I do my mixing, I don't weigh or measure. I fill the 5 gallon bucket about half-way with Miracle-Gro peat, then pour in some Pearlite and vermiculite. Since the latter two ingredients serve only to minimize soil compaction and retain some water, the exact proportion is not important.
My 1020 tray setup is a latice tray on the bottom (for strength and stability when moving the trays), followed by a 1020 tray
without holes (so that it holds water), then a 1020 tray
with holes (to allow excess water to drain into the tray below) and finally a 1020 cell insert. I currently use a 48-cell insert. After initially filling the 1020 cell inserts with my mix, I pour 1-1/2 quarts of water into the underlying tray (by removing one of the cell packs), then mist the top of the mix with water, to greatly accelerate the uptake of water from below into the mix.
My germination occurs within a jar of moistened starting mix, resting on a seedling heat mat. Germinated seeds are then transfered individually, using forceps, into a dimple in the center of each cell of the tray, and misted again. The lidded jars of germinating seeds are maintained until all of the seedlings in the trays have successfully begun to grow.
Bob