Answer: Sounds kind of odd, doesn't it? It refers to mixes of materials that are sterile, and usually used for starting seeds. Soil has lots of decomposed organic matter in it, and a soilless mix doesn't. It's usually made of perlite, vermiculite and sometimes peat moss.
Perlite is a mineral that is expanded by heat. It's that little white puffy stuff, like bits of styrofoam, you've probably seen in mixes. It helps retain air and moisture. Mica is another mineral that is heated to form vermiculite. It's kind of shiny. It also retains air and moisture.
A soilless mix is particularly useful for starting seeds in a greenhouse or indoors, because it's so lightweight that seedlings can easily emerge and grow roots, and it's sterile, so no pathogens can attack the seedlings.
A soil mix should list its ingredients on the package. If it contains such things as potting soil, compost, tree bark, etc. it's not soilless. Unfortunately, the soilless mixes are usually more expensive than regular potting soil.
Q&A Library Searching Tips