Potting soil, also known as potting mix or potting compost, is a medium in which to grow plants, herbs and vegetables in a pot or other durable container. The first recorded use of the term is from an 1861 issue of the American Agriculturist.
Some common ingredients used in potting soil are peat, composted bark, sand, perlite and recycled mushroom compost, although many others are used and the proportions vary hugely. Most commercially available brands have their pH fine-tuned with ground limestone; some contain small amounts of fertilizer and slow-release nutrients. Despite its name, little or no soil is used in potting soil because it is considered too heavy for growing houseplants.
Some plants require potting soil that is specific for their environment. For example, an African violet would grow better in potting soil containing extra peat moss, while a cactus requires sharp (i.e. plenty of) drainage, most commonly perlite or sand. But potting soil is not ideal for all contained gardening. Insectivorous plants, such as the Venus flytrap and the pitcher plant, prefer nutrient-poor soils common to bogs and fens, while water-based plants thrive in a heavier topsoil mix.
Commercially available potting soil is sterilized, in order to avoid the spread of weeds and plant-borne diseases. It is possible to reuse commercial potting soil, provided that the remnants of plant roots, fungus, weeds and insects are removed from the mixture through heating before new planting can take place. Packaged potting soil is sold in bags ranging from 5 to 50 pounds (2.3–22.7 kg).
As with garden soil, potting soil can attract insects. For example, the fungus gnat is often found around houseplants because it lays eggs in moist potting soil.
Infections due to potting mix have been reported in Australia, New Zealand, the Netherlands and the United States.
On June 13, 2000, the U.S. Centers for Disease Control and Prevention (CDC) reported that a woman in Washington was hospitalized with pneumonia that was triggered by Legionella longbeachae, the bacterium associated with Legionnaires' Disease. The CDC also confirmed the presence of Legionella longbeachae in soil in Australia and Japan.