If you use your own garden soil for pot or bench plants, it may be wise to sterilize it to get rid of harmful bacteria, insects, and weed seeds. You can do this in your kitchen oven if you can stand a pungent, earthy smell for a day or so. Here is the way it works: fill a large roaster pan or preserving kettle with garden soil, cover with aluminum foil pricked in many places to let steam escape, and bake for half to three-quarters of an hour at about 180 F. Repeat the process until the required amount has been sterilized. Let cool completely and stir it well before you use it.
My first greenhouse plantings were made in soil sterilized in this way, but I admit that subsequently I have used soil taken directly from the garden without sterilizing it, and results were just as good.
(However, I did live in an apparently nematode-free area.) Late in spring as plants were removed to make way for new plantings or early in summer at the end of the growing season, I turned over the soil in the benches, mixed in an all-purpose fertilizer, some clean sharp sand, and a little peatmoss or unmilled sphagnum moss, even osmunda fiber whatever I happened to have on hand. A few weeds may appear in this mixture, but they are easily pulled and hardly a problem.
Preparation of soil can be as simple and routine as mine, making your greenhouse gardening easier and more enjoyable, or you can purchase the soil you need.