Peat moss, which is the partially decomposed remains of Sphagnum moss, is a highly popular soil amendment valued for its unique physical properties that enhance water retention and soil aeration. Peat moss is naturally acidic, typically possessing a pH range between 3.5 and 4.5. This inherent acidity makes it uniquely suited for specific horticultural applications but requires modification for general use.
The Chemical Basis for Peat Moss Acidity
The low pH of peat moss originates from the unique environment in which it forms: waterlogged peat bogs where decomposition occurs slowly under anaerobic conditions (without oxygen). This slow decay process inhibits the complete breakdown of organic matter, preventing the release of basic minerals that would otherwise neutralize acidity. As a result, the accumulating organic material contains high concentrations of organic compounds, notably humic and fulvic acids.
The acidity is further driven by a process called cation exchange, which is a characteristic of Sphagnum moss itself. The cell walls of the moss contain negatively charged sites that have a high capacity to bind with positive ions, or cations, such as calcium and magnesium. When these sites bind to other cations, they simultaneously release hydrogen ions (\(H^+\)) into the surrounding solution, lowering the pH. This high Cation Exchange Capacity (CEC) is a primary reason why peat moss is an effective acidifier.
Using Acidic Peat Moss in Gardening
The highly acidic nature of peat moss is a beneficial feature for plants that thrive in low-pH environments. Incorporating peat moss into the soil or a potting mix provides a direct means of lowering the soil pH, which unlocks nutrients otherwise unavailable to acid-loving plants.
Gardeners often use peat moss to create custom soil blends for these sensitive species, particularly in regions with naturally neutral or alkaline soil. Plants that flourish when peat moss is included in their growing medium include:
- Blueberries
- Azaleas
- Rhododendrons
- Camellias
For plants like blueberries, which require a highly acidic environment (pH 4.5–5.2), peat moss is a preferred amendment. Beyond its chemical benefits, its fibrous structure also ensures the good drainage and moisture retention that many acid-loving plants need.
Adjusting the pH of Peat Moss Mixes
Gardeners who wish to use peat moss for its excellent physical properties—such as improving soil structure and holding moisture—but need a neutral or slightly alkaline medium must neutralize its acidity. This is a common practice when using peat moss as a component in a general-purpose potting mix or for growing vegetables and flowers that are sensitive to low pH. Neutralizing the acidity involves adding a basic material to the mix.
The most common and effective material used to raise the pH is horticultural lime, either calcitic or dolomitic limestone. Commercial soilless substrates often have lime added during manufacturing to bring the pH up to a more optimal range of 5.4 to 6.4 for general use. This process requires thoroughly mixing the lime into the peat moss or potting mix before planting, as lime is not water-soluble and must be physically incorporated to react with the acidic material.
The specific amount of lime needed is variable, depending on the starting pH of the peat moss and the desired final pH, making soil testing an important step. Dolomitic lime is often preferred because it supplies both calcium and magnesium, two nutrients that can be deficient in peat-based mixes. While lime is the standard amendment, wood ash can also be used as an alternative, though it is less predictable in its effect on pH.