Soil pH measures the acidity or alkalinity of the soil or growing medium on a scale from 0 to 14. This chemical property determines the conditions under which plant roots function and, most importantly, the availability of nutrients. If the pH is incorrect, plants cannot absorb nutrients effectively, even if fertilizer elements are abundant. Managing this measurement is fundamental to successful cultivation because it governs the plant’s ability to access resources already present in the soil.
The pH Scale and Measurement Methods
The pH scale is logarithmic, meaning each whole number represents a tenfold change in acidity or alkalinity. A reading of 7.0 is neutral; values below 7.0 indicate increasing acidity, and values above 7.0 indicate increasing alkalinity. For example, adjusting from pH 6.0 to pH 5.0 signifies a tenfold increase in acidity, dramatically altering the soil’s chemistry.
Measurement Methods
Home growers can determine soil pH using several methods. Colorimetric chemical test kits are the simplest option, involving mixing a soil sample with a reagent solution and comparing the color to a chart. While fast, these kits offer only a general estimate. More accurate readings are obtained using a portable digital pH meter, which measures the electrical activity of hydrogen ions in a soil-water slurry. For the most reliable results, professional laboratory testing provides a highly accurate pH reading and often includes recommendations for adjusting the target pH level.
Mechanism of Nutrient Availability and Toxicity
The primary way pH affects plant growth is by controlling the solubility of mineral elements in the soil water solution. Nutrients must be dissolved to be taken up by the roots, and their solubility depends heavily on the hydrogen ion concentration. When the pH is outside the ideal range, nutrients can become chemically “locked up” and unavailable for absorption.
Availability Issues
At low, acidic pH levels (typically below 5.5), the availability of major macronutrients like phosphorus, calcium, and magnesium is significantly reduced. Phosphorus forms insoluble compounds with iron and aluminum in highly acidic conditions, preventing root uptake and causing deficiencies. Conversely, in alkaline conditions (usually above pH 7.5), the availability of micronutrients decreases sharply. Elements such as iron, manganese, and zinc precipitate out of the soil solution into solid forms that roots cannot absorb. This micronutrient deficiency often manifests as chlorosis, or yellowing of the leaves.
Elemental Toxicity
Soil pH also directly influences the risk of elemental toxicity. In highly acidic soils, particularly those below pH 5.0, elements like aluminum and manganese become excessively soluble. Soluble aluminum is especially detrimental, as it damages fine root hairs, inhibiting water and nutrient uptake and severely stunting overall growth.
Optimizing pH for Specific Plant Requirements
Most cultivated species thrive in a slightly acidic to neutral range, often called the “sweet spot,” typically between pH 6.0 and 7.0. Maintaining this range maximizes the efficiency of fertilization because the majority of essential plant nutrients are optimally soluble and available for absorption.
Certain plants require soil conditions that deviate significantly from this average. Acid-loving, or ericaceous, plants require a strongly acidic environment, often in the pH 4.5 to 5.5 range, for proper growth. Classic examples include blueberries, rhododendrons, and azaleas, which develop iron deficiencies if the pH is too high. Other plants prefer neutral or slightly alkaline conditions, tolerating a pH between 7.0 and 8.0. Vegetables like asparagus and ornamentals such as lilacs perform better when the soil is closer to neutral.
Adjusting Soil pH for Optimal Growth
When soil testing reveals a pH level that is too low, alkalinity must be increased. This is commonly achieved by adding liming materials, such as agricultural limestone (calcium carbonate). Dolomitic lime, which contains both calcium and magnesium carbonates, is used when both elements are needed to raise the pH. Adjusting the pH upward is a slow process, requiring the material to be mixed thoroughly into the soil. Retesting several months after application is advised to gauge the change.
To decrease the pH and make the soil more acidic, elemental sulfur or aluminum sulfate are the most common amendments. Elemental sulfur is slower-acting but long-lasting, as soil microbes must convert it into sulfuric acid. Aluminum sulfate works more quickly, but over-application risks aluminum toxicity. Incorporating acidic organic matter, such as sphagnum peat moss, can also help lower the pH in small areas. Regardless of the method, any adjustment should be gradual and based on current soil test recommendations to avoid stressing plants.