Soil pH is a measure of the acidity or alkalinity of the soil solution, expressed on a scale from 0 to 14. A pH below 7.0 indicates acidity, while a measurement above 7.0 indicates alkalinity; a value of 7.0 is considered neutral. This chemical measurement is one of the most significant factors governing plant health.
The pH level directly influences the chemical environment in the soil, particularly the water surrounding the plant’s root system. This environment dictates what resources the plant can access. An unfavorable pH can severely limit a plant’s ability to thrive.
The Primary Mechanism: Nutrient Availability
The most direct way soil pH impacts plant growth is by controlling the chemical form and solubility of essential nutrients. When elements dissolve in the soil water, their ability to be absorbed by root hairs is highly dependent on the surrounding hydrogen ion concentration. This explains why nutrients may be physically present but chemically unavailable to the plant, a phenomenon often called nutrient lockout.
Macronutrients, such as nitrogen, phosphorus, and potassium, generally achieve their highest availability in the mildly acidic to neutral range, typically between pH 6.0 and 7.0. Within this range, these elements are structured in soluble forms that plant roots can efficiently take up.
Micronutrients, however, behave differently, often becoming less available as the soil becomes more alkaline (higher pH). Iron and Manganese, for example, tend to form insoluble compounds above pH 7.5, leading to deficiencies in plants. This lack of available iron often results in interveinal chlorosis, where the leaves turn yellow while the veins remain green.
Conversely, highly acidic soils, those below pH 5.5, can lead to the toxicity of certain elements. Aluminum and Manganese become excessively soluble in these low-pH conditions, resulting in concentrations high enough to impede root growth and damage plant tissue.
Impact on Soil Biology and Structure
Soil pH profoundly affects the living components of the soil ecosystem. Soil microorganisms, including bacteria, fungi, and actinomycetes, are responsible for breaking down organic matter and cycling nutrients back into plant-available forms. Their activity levels are strongly tied to the hydrogen ion concentration.
Most beneficial bacteria, including those involved in nitrogen fixation, thrive best in neutral or slightly alkaline conditions. A pH below 5.5 can significantly slow their metabolic rates, impeding the conversion of organic nitrogen into forms usable by plants.
Certain fungi, particularly those that form symbiotic mycorrhizal relationships with plant roots, are generally more tolerant of acidic conditions. They can often function effectively in a wider pH range than bacteria. Extreme pH levels, whether high or low, can also inhibit the activity of earthworms, which are important for creating soil aggregates and improving aeration and drainage.
Testing and Adjustment Methods
Determining the current soil pH is the necessary first step before any adjustment. While inexpensive home testing kits offer a quick preliminary reading, they often lack the precision required for large-scale amendments. Professional laboratory analysis provides the most accurate measurement, often including recommendations for the amount of material needed based on soil type.
To raise the soil pH, making it less acidic, the most common practice involves the application of liming materials. Agricultural lime, primarily calcium carbonate, works by neutralizing the hydrogen ions in the soil solution. The amount of lime required is heavily dependent on the soil’s buffering capacity; clay soils require significantly more lime than sandy soils to achieve the same change.
To lower the soil pH and increase acidity, growers typically incorporate elemental sulfur into the soil. Sulfur is oxidized by soil bacteria into sulfuric acid, a process that slowly releases hydrogen ions into the soil solution. This biological action provides a controlled, gradual reduction in pH over several months.
Acidifying with Fertilizers
Another method for acidifying soil involves using certain fertilizers, such as ammonium sulfate. When the ammonium ion is converted to nitrate by soil microbes, hydrogen ions are released, contributing to a moderate drop in pH. pH adjustments are not immediate; it can often take several months to a full growing season for the materials to fully react and for the desired change to stabilize.
Plant Preferences and Specific Ranges
Although the pH 6.0 to 7.0 range is optimal for the widest variety of plants, many species have evolved to thrive outside this neutral band. Understanding a plant’s specific requirements is important for successful cultivation.
Classic examples of acid-loving (calcifuge) plants require a highly acidic environment, typically preferring a pH below 5.5, to efficiently absorb necessary micronutrients like iron. Growing these plants in neutral or alkaline soil invariably leads to nutrient deficiencies and poor growth. These species include:
- Blueberries
- Azaleas
- Rhododendrons
- Camellias
Other plants are highly tolerant of or even prefer slightly alkaline conditions. Examples include lavender, clematis, and many members of the Brassica family, such as cabbage and broccoli. For these plants, a higher pH helps prevent certain diseases and ensures adequate calcium availability.