Potting soil is different from garden soil in that it is generally extremely light and fluffy. This consistency is usually achieved by adding nonsoil amendments. The same is true of African violet potting soil, except to a greater degree, since African violets need excellent drainage to survive at all.
Common African Violet Mix Additives
Perlite is a pelletized form of volcanic glass and is an extremely light substance that does not absorb water. Vermiculite is mica, heated to popping, and is also an extremely lightweight medium that retains water and nutrients very well. These two are added to African violet mix to keep the soil very, very light. Peat or sphagnum moss is another additive and is prized for the acidity it adds to the medium. Some mixes contain humus, which is an organically rich, lightweight substance and a boon to plants. Despite the name, soil is very rarely, if ever, added to African violet potting soil.
Growing Other Plants
Most potted plants need a very light potting soil mixture to make up for the fact that they are not in the ground, where the surrounding soil would normally even out the soil moisture quickly and keep it from becoming soggy. The exceptions to this rule are few, including cacti and sedums. Seeds and transplants do very well in a light soil mixture, and African violet potting soil may be perfect for them.
African violet potting soil may also contain fertilizers. Read the package carefully to determine what other additives may be in it before using it on a plant that does not need that kind of fertilization. The light acidity in African violet potting soil is fine for most plants, but alkaline or high pH-loving plants may suffer in acidic soils. In these cases, the soil can often be brought to more neutral pH by adding compost, and the extra organic matter will certainly help most any other plant thrive.
African violet potting soil is generally not used for other plants because of its expense. African violets need great care in the type of soil they grow in, and thus high-quality additives are often used, or sometimes the product is simply marked up because it is marketed toward a target audience. While it is usually fine to use African violet potting soil for other plants, other fine potting soils work just as well with much less expense.