The health and productivity of nearly all plants are directly tied to the chemical environment of the soil they grow in. This environment is largely governed by a single measurement: pH. pH is a scientific measure of acidity or alkalinity, representing the concentration of hydrogen ions in the soil water. Soil pH is considered a master variable because it dramatically affects the chemical processes that occur around plant roots, specifically dictating what nutrients the plant can actually absorb. Even soil that is rich in nutrients will fail to nourish a plant if the pH level is not within a suitable range.
Defining Soil pH
The pH scale is a numbered range from 0 to 14, used to classify a substance as acidic, neutral, or alkaline (basic). A measurement of 7.0 is considered perfectly neutral, while any value below 7.0 is acidic, and any value above 7.0 is alkaline. The scale is logarithmic; for example, a drop of one full pH unit represents a tenfold increase in acidity, which explains why large changes in soil pH are difficult to achieve. Most cultivated plants thrive best in a slightly acidic to neutral range, typically between pH 5.5 and 7.5, though certain plants, like blueberries and azaleas, are adapted to much more acidic environments.
How pH Controls Nutrient Availability
The primary reason soil pH is important is its direct influence on the solubility of the chemical compounds that form plant nutrients. If a nutrient is not soluble, it remains chemically bound to soil particles and is physically inaccessible to the roots, a phenomenon known as “nutrient lock-up.” For example, in alkaline soils with a pH above 7.5, essential micronutrients like iron, zinc, and manganese tend to precipitate into insoluble forms. Plants growing in these conditions often exhibit symptoms of deficiency, such as yellowing leaves, despite the soil containing sufficient quantities of the elements.
Conversely, in highly acidic soils, typically with a pH below 5.5, some elements become excessively soluble, leading to potential toxicity. Aluminum, naturally present in most soils, becomes highly soluble and can inhibit root growth and function. Manganese also becomes more available and can reach toxic concentrations, causing leaf discoloration or cupping in susceptible plants. Low pH also reduces the availability of macronutrients like phosphorus, nitrogen, potassium, calcium, and magnesium, as their chemical forms are less easily taken up in acidic water.
Testing and Adjusting Soil pH
To ensure plants have access to nutrients, the first step is to accurately determine the current soil pH. Home testing kits, which use color-coded chemical reactions or digital probes, offer a quick estimate for the home gardener. For the most reliable measurement, a professional soil test conducted by a laboratory is recommended, as this provides a precise number and often includes recommendations for adjustment. Soil samples should be taken from multiple locations in the garden and mixed to get a representative reading.
Once the pH level is known, it can be adjusted through the addition of soil amendments. To lower the pH and make the soil more acidic, elemental sulfur or ammonium sulfate can be applied. Sulfur is converted by soil bacteria into sulfuric acid, gradually lowering the pH over time. To raise the pH, agricultural lime (calcium carbonate) is the most common amendment; dolomitic lime is used if magnesium is also needed. All changes to soil pH should be made gradually over months and worked into the top six inches of soil to avoid stressing the plants.