Soil acidity is measured by the pH scale, where 7.0 is neutral and values below 7.0 indicate increasing acidity. The pH scale is logarithmic, meaning a drop from pH 6.0 to pH 5.0 represents a ten-fold increase in acidity. A range of 6.0 to 7.0 is optimal for most plant species. When the soil pH drops below approximately 5.5, the chemical environment changes dramatically, restricting plant growth and compromising the soil’s ability to sustain a productive ecosystem.
Altered Nutrient Availability and Element Toxicity
As the pH drops below 5.5, the solubility of elements like Aluminum (Al) and Manganese (Mn) rises sharply, leading to toxicity in plants. Aluminum toxicity is the most significant chemical barrier to crop production in strongly acidic soils, typically occurring when the pH falls below 5.0 to 5.5.
Soluble aluminum ions damage the delicate tissues of plant roots, inhibiting their division and elongation. This root damage severely restricts the plant’s ability to absorb water and essential nutrients from the soil, leading to stunted growth and poor overall health. Manganese can also become excessively available in acidic conditions, resulting in toxicity symptoms like bronzing or yellowing of leaves and black spots on stems.
Low pH causes essential nutrients to become chemically unavailable to plants. Macronutrients such as Calcium (Ca) and Magnesium (Mg) are easily leached out by rainfall due to the high concentration of hydrogen ions. Furthermore, low pH causes Phosphorus (P) to chemically bind with soluble Aluminum and Iron (Fe). This reaction forms insoluble compounds that plants cannot take up, effectively locking the phosphorus away.
Suppression of Beneficial Soil Biology
The biological community within the soil is also highly sensitive to changes in pH, with acidic conditions significantly reducing the activity and diversity of beneficial microorganisms. Bacteria, in particular, are much more sensitive to low pH than fungi, causing the microbial community structure to shift toward a fungal dominance. This change disrupts the healthy balance needed for efficient soil processes.
The activity of bacteria responsible for critical functions, such as the Rhizobium species that form symbiotic relationships with legumes, is severely inhibited below a pH of 6.0. These bacteria are essential for converting atmospheric nitrogen into a usable form for plants (nitrogen fixation). When this process is suppressed, legumes fail to thrive, and the soil loses a natural source of nitrogen fertilizer.
Acidic conditions also slow the rate of organic matter decomposition. Microorganisms break down dead material, releasing stored nutrients back into the soil. When this decomposition slows, nutrient cycling is compromised, leading to a buildup of undecomposed material and a reduced supply of naturally derived nutrients.
Practical Steps for Soil Acidity Management
The first step in managing soil acidity is to accurately assess the current pH level of the soil. A professional soil test provides the active pH and the soil’s buffer capacity, which measures the soil’s ability to resist pH change. This information determines the precise amount of corrective material required for the specific soil type.
The most common and effective method for neutralizing soil acidity is the application of liming materials. Agricultural lime (primarily Calcium Carbonate) works by replacing hydrogen and aluminum ions on the soil particles, thereby raising the pH. If the soil test indicates a deficiency in magnesium, Dolomitic lime, which contains both calcium and magnesium carbonates, is the preferred choice.
Lime is a slow-acting material and must be applied well in advance of planting to allow sufficient time for it to react with the soil. For the fastest results, the lime should be thoroughly mixed into the top few inches of the soil rather than just spread on the surface. Where liming is impractical, an alternative strategy is to select plants that are naturally tolerant of low pH, often called acidophiles.