Potting soils and potting mixes are used in container gardening and differ in content from garden or lawn soil. The mix is designed to support the growth of a single plant or small group of plants within a confined space. Containers do not allow plant roots access to nutrients or moisture outside the confines of the container, so the potting soil or potting mix must meet most of the plants' needs.
Potting mix does not contain soil. Each manufacturer has its own formula for potting mix regarding specific products and the ratio of elements. Commonly found elements, though, include peat moss or sorghum and either perlite or vermiculite. Many potting mixes also include pine bark or a similar product. The three elements provide a mix that is lightweight and less prone to compaction than garden soil.
Peat moss and sorhgum are made up of decomposed moss plants. Peat moss allows moisture to move through the potting mix, trapping enough for the plant to take up through the root system. Both perlite and vermiculite are minerals that have been "puffed up"; both serve to create air flow within the mix. The use of pine bark or other woody byproduct acts as interior mulch, conserving moisture and nutrients within the mix.
Not all potting mixes are soilless, in which case it is considered a potting soil. A potting soil may be comprised of sand and compost mixed with soil. Sand is used to give the soil a higher rate of drainage. Within a container, the excess water needs to drain quickly, more quickly than in the garden. If water pools within the container, the plant may suffer from root rot and die.
Compost is added to provide nutrients to the mix. The soil in a potting soil may contain nutrients as well, but is likely to become compacted or depleted before the plant completes its life cycle. The addition of compost extends the usability of the soil and reduces compaction.
Commercial potting mixes may contain a slow-release fertilizer. The fertilizer is integrated into the mix in a fashion similar to integrating decaying matter into a garden plot. As the matter breaks down, it fertilizes the soil. Within a potting mix, the fertilizer "breaks down" and feeds the soil over time.
Without the benefit of added slow-release fertilizers, potting mixes require fertilizers or plant food to be added at regular intervals. In potting soils containing compost, additional compost may need to be added as well as the plants reach maturity.
Both potting mixes and potting soils present risks. An inexpensive commercial potting mix may contain sand as a filler, with too little peat moss or perlite. Such potting mixes may not be able to retain nutrients or moisture in sufficient amounts.
Soil used in potting soils must be free of soil-borne disease and pests. Trapping disease or pests within a container results in the death of the plant within a short time of planting.