The Dirt on Soilless Growing
Soil is a growing medium, but it's not the only one. When planting seeds into trays, a soilless growing mix - traditionally containing some combination of vermiculite, perlite, sphagnum peat moss, and possibly bark or coir - is used to reduce soilborne disease pressures. In other words, the seeds are hydroponically grown.
Hydroponics refers to growing plants without soil. Hydroponic growing media can be liquid, air or an aggregate mix. In addition to the standard soilless potting medium typically used to start seeds, sand, pine bark, rice hulls, sawdust, clay-aggregate ...