I stir in a cup of Pro-Gro or other organic fertilizer and a cup of Azomite or other rock powder, then moisten the mixture enough so that the dryness of the commercial potting mix, which contains peat moss or coir, is overcome. Then I let it sit for a day or more to warm up indoors.
Azomite is a rock powder mined in Utah that contains 70 naturally occurring minerals and trace elements harvested from a layer of volcanic ash that was later inundated with sea water. Although I do not have scientific proof of its ability to improve growth and vigor of plants, I have done some informal experiments with rock powders, and believe that they help.
If you use the same plot every year for decades, as I have, trace minerals of the soil may well get used up, so adding a wide variety of minerals makes sense to me. I add Azomite or finely ground granite powder to increase mineral diversity in my planting mix and also in my soil.
I re-use plastic pots each year, and believe it is a good practice to clean them before re-using. I wash them in the sink with soapy water, or sometimes fill the top shelf of the dishwasher to clean them. This helps to eliminate any bacterial residues that might not be good for my seedlings.
Also key to success with indoor seedlings is a good light source. I use ordinary fluorescent lights that I hang over my seedlings. I keep them about 6 inches above the seedlings, Light intensity diminishes exponentially with distance. My lights hang on chains, and I raise the lights as the seedlings grow. I have the lights on a timer so they are on just 14 hours a day. Little plants need rest, too.
Don't keep your seedlings too warm. Sixty-five degrees is good for the day time, but leave a window ajar at night to let temperatures drop to 55 degrees -- though I admit that don't do that most nights.
Growing seedlings from seed requires some work, but I do it every year because I love tending my seedlings. I save money doing it, and can grow plants I would never find for sale at a garden center, too.
Henry Homeyer can be reached at P.O. Box 364, Cornish Flat, NH 03746. Please include a self-addressed, stamped envelope if you want a question answered by mail. Henry's e-mail address is email@example.com. His website is www.Gardening-Guy.com.