Potting soil or growing media, also known as potting mix or potting compost (UK), is a substrate used to grow plants in containers. The first recorded use of the term is from an 1861 issue of the American Agriculturist. Despite its name, little or no soil is usually used in potting soil.

Materials used for growing mediums include: peat, coconut coir, wood products like bark and wood fiber, perlite, stone wool, soils/tufts, and recycled paper and cardboard. Other materials used include rice hulls, sand, vermiculite, and calcined clays.

Typical potting mixes include one or more materials which retain moisture, one or more materials which aid in aeration and drainage, and fertilizer. Moisture-retaining materials and aerating materials can be combined in any ratio, depending on the particular needs of the plant. Soils are minimally used as growing media because they compact and lose pore space after repeated watering and can be too heavy for growing potted plants. Mediums used for growing plants in pots typically are a mix of organic and inorganic ingredients.

Good growing mediums have a number of properties including moisture and nutrient retention capacity, quick water infiltration, pore space for aeration (plants roots need oxygen), drainage for excess water, decompose slowly, and provide support for the plants growing in them. They also have an optimal range of pH, cation exchange properties, and lack substance that are toxic to plants These are also dependent on the type of plant grown since there is wide variation in moisture and nutrient needs among different plants.

This part is usually made up of peat (usually with limestone to reduce acidity) or coconut coir. It serves to absorb water and nutrients. Tree bark, mainly of pine, may also be used.

The use of peat is controversial since the harvesting of peat moss from peatlands (which includes unique habitats such as bogs and fens) can degrade these peatlands. Peatlands are home to a diverse range of plant and animal species. Peat also has a very slow accumulation rate, as little as 1mm per year, so they take a long time to regenerate. Peatlands are also carbon sinks, constituting 3% of the world's surface but storing up to 30% of the carbon sequestered in the soil. The removal of the layer of CO₂ absorbing plants releases CO₂ into the atmosphere, contributing to climate change.

As such, alternatives such as coconut coir are promoted by some organisations.

Sand and grit may be used for drainage and aeration. Perlite and vermiculite improve both aeration and water retention.