Peat moss is partially decomposed organic matter found at the bottom levels of a peat bog, as described by Sherry Rindall on the Iowa State University Department of Horticulture's website. Once harvested, it is packaged and sold as a soil amendment in much of the U.S. The primary source of peat moss for the U.S. are large Canadian peat bogs, where it is commercially harvested for the horticultural market.
Peat moss improves the texture and water holding capacity of soil. When added to heavy clay soils, it loosens the soil and improves aeration. This allows roots to access oxygen needed for healthy growth. It also provides loose soil conducive to good root formation to provide plants with moisture and nutrients, and support for growing plants. In sandy soil, peat moss is instrumental in improving its water-holding capacity. Its absorbent nature takes in moisture when it rains and releases it to the surround soil as the soil dries.
The Cornell University Extension recommends adding one-third volume of peat moss when amending soil. Layering 2 to 6 inches, depending on the amount of organic matter already present in the soil, before tilling the soil provides adequate amounts of peat moss to the soil. Working peat moss into the garden soil with a garden tiller incorporates organic matter to the soil.
For home application, peat moss is relatively expensive, particularly when added to entire gardens. Other less expensive options include composted yard or garden wastes or well-rotted manure to amend soil.
Peat moss is acidic and lowers the pH of soil. Depending on the original pH of the soil and the amount of peat moss added, adjusting pH to 6.0 to 6.8 for gardening may be necessary. Typically, a soil test reveals the pH of the soil. Raising pH requires an application of limestone.
Harvesting peat moss may pose a threat to the environment. According to Linda Chalker-Scott on the Washington State University website, restoration projects after harvesting peat moss do result in new growth, but release high levels of CO2, contributing to greenhouse-gas production for several years. However, according to Cornell University Extension, the harvesting of peat moss is regulated and requires restoration and reclamation to develop a sustainable resource. It recommends conservative use of peat moss for horticultural needs.