The pH Level of New Orchid Bark
Orchid bark, typically composed of processed fir or pine bark, is a naturally acidic growing medium. For most commercial mixes, the pH of fresh bark ranges from approximately 4.0 to 6.5, with fir bark often being slightly less acidic than pine bark. This means the medium is generally below the neutral pH of 7.0. The acidic nature of the bark mimics the epiphytic orchid’s natural habitat, where roots cling to tree branches and receive nutrients from rain and decaying forest debris.
Why Acidity Impacts Orchid Health
The acidity of the potting medium directly controls the availability of dissolved nutrients to the orchid’s roots. Most epiphytic orchids thrive in a slightly acidic environment, generally preferring a pH range of 5.5 to 6.5 for optimal function.
When the pH deviates too far from this preferred range, the plant can suffer from “nutrient lockout,” where essential elements are present but chemically unavailable for absorption. For instance, a pH that is too high, or alkaline, can cause micronutrients like iron and manganese to precipitate out of the solution, leading to deficiencies despite regular fertilization.
Conversely, if the medium becomes excessively acidic, the absorption of macronutrients, specifically calcium and magnesium, can become limited. Calcium and magnesium are important for cell wall development and overall leaf health, and their reduced uptake due to improper pH can make roots more susceptible to rot.
How Bark Acidity Changes Over Time
The pH of an orchid’s potting medium is not static but changes predictably as the bark ages inside the pot. Organic materials like fir and pine bark are naturally susceptible to decomposition by water, heat, and microorganisms.
As this process occurs, the bark structurally breaks down, and it simultaneously releases organic acids, causing the pH of the medium to drop. This means that over the course of one to three years, the initially slightly acidic medium becomes progressively more acidic, often referred to as turning “sour.”
The significantly lower pH level created by this decomposition can lead to a toxic environment for the roots, inhibiting nutrient uptake and root function. This chemical change, coupled with the loss of air space as the bark turns to sludge, is the primary reason repotting is a necessary part of orchid care.
Managing Potting Medium pH
The most effective strategy for managing medium pH is to prevent the organic material from breaking down excessively. This involves repotting the orchid with fresh bark every one to three years, depending on the bark quality and the watering frequency.
Regularly flushing the pot with clean, neutral water is also helpful to prevent the accumulation of fertilizer salts, which can otherwise alter the pH balance. For growers dealing with overly acidic media, or for certain orchid species that prefer slightly higher pH, small amounts of dolomitic lime can be incorporated into the mix. Dolomitic lime contains calcium and magnesium and works to neutralize excess acidity, raising the pH gradually over several months.