Potting mix, not garden soil

A pot is not a small piece of garden. In the ground, water can drain away deep into the earth and worms and roots keep the soil open. In a pot, the mix has a floor, and whatever it is made of has to hold both water and air on its own.

Garden soil does not do that job well. Packed into a pot it tends to become dense, drains poorly and can carry weed seeds, pests and disease organisms. Colorado State University Extension advises against using it for houseplants.

Potting mixes, often called soilless mixes, are made for containers. They are light and open, hold moisture without staying waterlogged, and are sold free of pests and pathogens. Most are based on peat, coconut coir or fine bark, with perlite or similar materials for air. Many include a starter dose of fertiliser. For special plants there are special mixes: chunky bark mixes for orchids, and gritty, fast-draining mixes for cacti and succulents.

Potting mix does not last forever. The RHS notes that old mix breaks down and compacts, leaving less air for roots, which is one reason plants need fresh mix every few years.