Different Soils for Different Toils
The expression “dirt cheap” must have come about before the advent of commercial potting soils. A gardener can drop a bundle even before acquiring seeds or plants. There’s nothing wrong, of course, with paying good money for a good-quality planting medium if it works for you. You might also find that you can make your own blend for a fraction of the cost and customize it exactly to your needs.
Although there is no label requirement for potting soils, the components generally include some organic matter, such as peat moss or coconut coir, sawdust or ground bark, or loam, then some material for keeping the mixture from binding too much, such as sand, vermiculite, or perlite. Limestone is added to counter the acidity of the peat or wood products, and fertilizer may also be added.
Commercial potting soils have usually been sterilized to reduce the chance of diseases developing. You can use your own garden soil for potting and seed mix as long as your garden is free of significant insect, fungus, or bacterial problems. If you are concerned about problems, heat moist soil on cookie sheets in the oven at 185°F for half an hour. If you can’t trust your own and don’t want to hassle with sterilizing, check out local loam suppliers. Chances are the loam is not coming from a source that has problems that will bother your plants.
Basic Potting Soil: 1 part soil; 1 part peat or sawdust (or 2 parts if not using soil), 1 part sand, limestone (about 1 pound per 6 cubic feet of mix), fertilizer (optional, same ratio as limestone).
For acid-loving plants, omit limestone, add ½-ounce iron sulfate. For shade-loving plants (begonia, etc.) reduce limestone by half, add iron sulfate as above.
Seed Starter Mix: 1 part soil, 1 part peat or coconut coir, 1 part vermiculite or perlite. (Note: Peat moss should be sifted through a ¼-inch mesh if it is too chunky.)
This is a place to start. Experiment. There are two reasons not to re-use potting soil: The risk of spreading disease is greater, and some of the components will break down, making the mix too dense. However, used soil can be added to your garden, either directly or processed with your compost.