Peat moss is a common seeding medium and soil augmentation that is used primarily in indoor pot gardening. Because it is an organic material, it offers some benefits over vermiculite or pearlite as a sprouting medium. Peat moss accumulates under sphagnum moss in bogs that can be thousands of years old. Peat moss is old dead sphagnum moss. The term sphagnum moss refers to the living moss that can become peat once it dies and is covered by upper layers of living moss.
Peat moss adds a loose, airy texture to many harder, more dense soils like clay-based soils. By decreasing the density of the soil, peat moss allows more air in the soil and improves root health and function. The lightness and looseness of peat moss on its own makes it an ideal medium for starting seeds whose roots may not be strong enough to penetrate more dense soils.
The airy nature of peat moss means that it is able to hold more water than more dense soils. By holding more water around the roots, it dries out more slowly than other more dense soils. When growing plants from seed, peat moss reduces the risk of the newly germinated seeds and young roots drying out. As a soil augmentation, peat moss increases the capacity of other soils to hold and retain water, thus reducing the risk that the roots will dry out.
Although peat moss initially adds little nutritional value to the soil, it is an organic material that will release nutrients into the soil as it breaks down. It is a more acidic material than many soils and, as such, is an ideal soil augmentation for plants requiring a more acidic soil. For some plants, like orchids, peat moss provides an ideal rate of decomposition and acidity and produces good, slow release nutrition via decomposition and good water-holding traits.