Good potting soil fulfills the needs of seeds and plants. Contrary to popular belief, fertile soil may be detrimental to germination, though it undoubtedly benefits growing plants. Seeds need light, water retentive soil or soilless mix of low fertility, while established plants--which also require moisture retention and do not like to be compacted--will thrive in a well-drained, slightly denser soil enriched with plenty of organic matter and nutrients. This recipe--based on a soil mix suggested by the Master Gardener Newsletter from the University of Illinois Extension Service--may be adapted for seeds or plants.
Sift the topsoil through a scrap piece of ¼ inch wire mesh to remove larger stones, insects or other debris.
Put one part each topsoil and peat moss in an appropriately sized container.
Add ½ part sand and ½ part either perlite or vermiculite to the mix.
Stir everything together and moisten well with water. The soil should be damp, but not soaked. If you should accidentally use too much water, merely spread the soil out on a pad of newspapers or a plastic sheet in the sun for a couple of hours to dry.
Allow the mix to set for a few hours (to allow water to soak into the peat moss and vermiculite or perlite) before using.