Gertrude Stein observed that a rose is a rose is a rose. Confronted by a bag of peat moss, she would have been on solid ground asserting that the ingredients of peat moss are peat moss. What distinguishes peat moss from other soil amendments is the way in which it is formed. Peat moss is a natural product of certain wetlands, formed by time and specific ground conditions. While the highest quality of peat moss contains sphagnum moss, other leaves, branches and even tree trunks comprise this very popular aerator of compacted garden soils.
How Peat Moss Is Formed
The substance we call peat moss differs from other forest-refuse products, such as shredded bark mulch or leaf-compost, in that peat moss is formed underwater. Boggy (chronically wet) ground enables tree and plant refuse to decompose in the absence of air, or anaerobically. Decomposition proceeds at a slower rate than decay in the presence of air, and final products differ also. Peat moss is characterized by large-cell structures that, when combined with soil, provide more space for air in the soil than sand, gravel, shredded bark or composted leaves alone. Sold dry, it also soaks up great amounts of water and in some situations--although not all--helps soil retain water for the benefit of plants.
How Peat Moss Improves Clay Soil
Peat moss is stringy and shreddy when dry. Combined with soils that have a high clay content, peat moss lightens soil, making it easier for water to reach plants and making it easier for small roots to penetrate the soil. Peat moss is not particularly nourishing to plants but, by breaking up heavy soil, it enables nutrients to reach plants more efficiently. Peat moss can also reduce rain runoff in heavy clay soil, making more moisture available to plants.
How Peat Moss Improves Sandy Soil
Some soil drains too rapidly to sustain plant growth. Usually a high sand content is the cause. Adding peat moss to sandy soil improves water retention. Further, sandy soil tends toward alkalinity. Since peat is formed in forest wetlands, many of the plants comprising it tend toward acidity rather than alkalinity; adding peat moss to sandy soil may help restore a pH balance favorable to a wider range of plants.
Cautions about Peat Moss
Peat moss is not very nourishing. Do not substitute it for fertilizer if your soil is low in nutrients. Peat moss can lessen the existing nutritional value of soil if combined with it in excess. To lighten soil, combine at least 2 parts soil with 1 part peat moss. Mix thoroughly. Peat moss looks like, but does not substitute for, shredded bark or wood chip mulch. Placed on the top layer of soil, thirsty peat moss can soak up much of the water intended for plants, leaving roots dry. Using it properly enhances garden soil. Using it excessively puts demands on a natural resource that takes a long time to develop. As Gertrude might say, a bog is a bog is a bog.