Potting Soil and Soilless Mixes Commercially available potting mixes and soilless mixes are usually made up of different organic ingredients including varying proportions of sphagnum peat moss, shredded bark, and sawdust, as well as mineral ingredients such as vermiculite, perlite, calcined clay, and sand. These are typically lightweight and weed and disease free. Many different brands and types of mixes are offered in garden stores, and quality varies widely. There is no regulation by state or federal agencies over the content of these mixes and there are no standards that define what constitutes a good potting mix. Each ingredient in a soilless mix is chosen to provide a certain characteristic.
Peat moss is acidic and holds water very well.
It is usually fibrous, brown, and sometimes chunky. Ground bark is cheaper than peat moss and is a by-product of the timber industry. It does not hold water and nutrients as well as peat moss, but may provide better aeration. It provides some nutrients. It also holds water and added nutrients, which are then readily available to the plant.