Peat moss is an important component of most potting soils and seed starting mediums. It holds several times its weight in moisture, and releases the moisture to the plants roots as needed. It also holds onto nutrients so that they aren’t rinsed out of the soil when you water the plant. Peat moss alone does not make a good potting medium. It must be mixed with other ingredients to make up between one-third to two-thirds of the total volume of the mix.