It is one of the many paradoxes in gardening that the best soils for starting seedlings indoors contain no real soil at all. Ordinarily, weed seedlings sprout and grow in profusion in any speck of open ground. Why can't we just dig up a little good earth and set it aside for our winter work. As simple and as natural as that answer might seem, the truth is that using real topsoil brings several problems to container culture and especially to seed starting.
Outdoor soil is too heavy and too compact for good air circulation.
It also holds too little air and carries too many disease organisms. So instead the world has turned to soilless mixes. On this point the largest commercial growers and most of the best gardeners agree. Today's soilless mixes are mostly sphagnum peat moss, plus vermiculite and often a little perlite. The sphagnum peat used in good seed-starting and growing mixes is a very stable organic material that holds a great deal of water and air and does not decompose quickly.