The pH of potting soil measures its acidity or alkalinity on a scale from 0 to 14, with 7.0 being neutral. Unlike natural garden soil, which is influenced by geography and bedrock, the pH of commercial potting mix is intentionally engineered by manufacturers. This chemical balance, controlled by organic components such as peat moss or coir, determines how successfully a plant can access the nutrients it needs to grow.
The Standard pH Range of Potting Mix
Standard commercial potting mixes are designed to maintain a pH that is slightly acidic to neutral, typically falling between 5.8 and 6.5. This narrow range offers the widest availability for the macro and micronutrients required by most container plants. The ingredients used in these mixes often start out highly acidic; for instance, sphagnum peat moss is naturally very acidic, sometimes measuring as low as 3.5 to 4.5. To counteract this acidity, manufacturers blend in a buffering agent, most commonly calcitic or dolomitic limestone. Dolomitic lime is often preferred because it supplies both calcium and magnesium, two minerals plants require for healthy growth.
Why pH is Critical for Nutrient Absorption
The pH level acts like a chemical gatekeeper, controlling the solubility of mineral nutrients and determining whether a plant’s roots can absorb them. When the pH is outside the optimal range of 6.0 to 6.5, nutrients become chemically locked up and inaccessible to the plant, a condition known as nutrient lockout.
In highly acidic potting mixes (pH below 5.5), several issues arise that restrict growth. Macronutrients like phosphorus, calcium, and magnesium become less soluble and are difficult for the plant to take up. Conversely, micronutrients such as iron, manganese, and aluminum become excessively soluble, which can lead to toxicity. Aluminum toxicity, in particular, hinders root development and limits the plant’s ability to absorb water.
When the pH becomes alkaline (above 7.5), a different set of nutrients is affected. Micronutrients like iron, zinc, and copper become insoluble and chemically precipitate out of the soil solution. The plant cannot absorb these elements, leading to deficiencies that often manifest as chlorosis, or yellowing leaves, especially in new growth.
Adjusting Potting Soil pH for Specific Needs
Home gardeners can use simple test kits or digital meters to monitor the pH of their potting mix, especially when growing specialized plants. Potting soil is less buffered than garden soil, making it more susceptible to pH changes from watering and fertilization over time. Adjustments must be made gradually to avoid shocking the plant.
Lowering pH
For plants that thrive in highly acidic conditions, such as azaleas or blueberries, the pH can be lowered by incorporating elemental sulfur or aluminum sulfate. Elemental sulfur is a slow-acting agent that relies on soil microbes to convert it into sulfuric acid. A less permanent method involves watering with a diluted solution of white vinegar.
Raising pH
To raise a potting mix’s pH for plants like lavender or cacti, gardeners can incorporate pulverized dolomitic limestone or calcitic lime. Wood ash is also a viable option, as it contains high levels of calcium and potassium, but its exact neutralizing strength is difficult to determine. Adjustments should be monitored with retesting to ensure the target range is achieved.