Although mixing up a batch of potting soil is not likely to save much money, it will allow the gardener to tailor the mix to specific goals or specific plants. Potting mixes have gotten more sophisticated as horticulturalists have identified components that are beneficial for sowing seeds and growing plants. Potting soils vary in their exact composition, but several ingredients are commonly found in potting mix.
Loam is soil with 7 to 27 percent clay, 28 to 50 percent silt and less than 52 percent sand, according to the University of Connecticut Cooperative Extension. With the sterilization process, weed, disease and insect infestation should be eliminated from the loam. Even though it is free, using loam from the backyard for potting mixes is not recommended can lead to deformed, stunted or dead plants, which end up costing more in the long run.
Coarse sand, or builder sand, improves aeration and drainage. Excess sand in a potting mix will make planting containers too heavy. When creating a potting mix, sand should not be mixed with clay-based soil, according to the University of Pennsylvania Extension.
Soil-less potting mixes are often about half sphagnum moss, the partially decomposed organic material that accumulates in a peat bog's lower level. The coarse texture of sphagnum peat moss contributes to good aeration. Peat moss also improves the water-holding capacity of a potting mix, but too much may prevent proper soil drainage.
A clay in the mica family, vermiculite occurs in nature as laminated flakes. Vermiculite's accordion-shape folds are able to retain air, nutrients and water, though its air and water-holding capacity become reduced if it vermiculite is compacted. For potting mix, only horticultural-grade vermiculite is recommended.
Perlite may be used in a potting mix instead of sand or vermiculite. Perlite is expanded, fluoride-based volcanic rock that promotes good drainage. It is lighter than sand because it holds more air, but perlite does not retain water. Perlite needs to be moistened before mixing to prevent the inhalation of harmful perlite dust. And it has a tendency to float to the top of the pot when plants are watered.
A complete multipurpose slow-release fertilizer is an important addition to soil-less potting mix, which contains few nutrients. The clay or minerals in a soil-based mix may be sufficient if plants will only be kept in containers for a limited time. If, however, the plants will remain in containers for several years, adding fertilizer to a soil-based potting mix will improve plant growth.
Potting mixes made of the ingredients mentioned tend to have a pH below 6. According to the University of Pennsylvania Extension this is too low for the majority of plants. Agricultural lime or calcitic limestone will raise the pH level of the potting mix while also adding calcium, a nutrient for strengthening plant cell walls.