Soilless potting mix is any growing medium that does not include dirt. These mixes offer several advantages over their classic counterparts, especially for container gardening. Most notably, soilless mediums tend to be less welcoming to insects, bacteria, fungi, and diseases, improving the chances of injury to the plants they support. They can also offer superior aeration and drainage, reducing the danger of rotting roots, and they can be lighter than typical soils, allowing for easy container relocation. Finally, they're less likely than traditional soils to contain spores or seeds for other plants, like weeds.
Some gardeners enjoy working with soilless mix because they like the control it gives them over a plant's environment. You can make your own dirt-free growing medium by combining things like peat moss, bark, sand, vermiculite, coconut coir, and perlite. Most gardeners include some kind of fertilizer to the mix, too.
Because they're highly flexible and relatively safe, these kinds of mix are especially valuable for starting seeds. They're also preferable for growing plants indoors, since they reduce the incidence of plant-eating predators who might be controlled in outdoor conditions by other factors like birds and weather.
A basic homemade soilless medium might include a one-to-one mixture of perlite and vermiculite and four to six parts peat moss. To add natural fertilizers, you can include a few parts compost, and/or half a cup of bone meal, blood meal, soybean meal, dolomitic limestone, or dried kelp powder for each eight gallons of mix.