Purchased potting mixes usually include soil as one of the components. Soil-less mixtures have no soil and consist of other organic materials. They tend to carry fewer pathogens and fungi. Soil-less mediums are better for plants that are prone to damping off or rotting at the soil level. This type of medium is used for orchids, bonsai and rooting cuttings. The blends of materials vary depending on the purpose of the medium, but they may include peat moss, compost, bark, coir, perlite, sand and vermiculite.
Make a basic soilless mix by combining 6 parts of sphagnum peat moss, 1 part perlite and one part vermiculite. Sand may also be used to replace perlite, but you will need to add 1/2 tbsp of limestone to balance the pH as the peat is acidic. Limestone will raise the pH and make the mixture more alkaline.
Use compost as the main component of a soilless mixture when possible. Peat moss takes a very long time to reproduce, which makes it a finite resource in the short term. Compost recycles important organic compounds into useful nutrients and is an important part of sustainable gardening.
Prepare a peat-less mixture with 4 parts coconut coir, 2 parts compost, 1 part perlite and 1 part vermiculite. Compost may be used as the main ingredient, but you will have to add sand for better drainage. A recipe for this would be 4 parts compost, 3 parts sand and 1 part perlite.
Switch ingredients around according to each type of plant's specific needs. Compost and peat are interchangeable as long as you add sand to the compost mix. Bark adds increased drainage just like sand. Coir is a good replacement for peat. The goal is to make a loose, well-drained and nutrient-rich medium for plant growth.
Add fertiliser and amendments as needed. Bone meal and dried kelp will add nitrogen to a soilless mixture. Limestone adds calcium and magnesium to increase the pH of the medium. Phosphorus is found in bone meal. Add any of these items to the mix at a rate of 1/4 cup per 4 gallons of mix.