Soil pH measures the concentration of hydrogen ions in the soil solution, quantifying its acidity or alkalinity on a logarithmic scale from 0 to 14. A pH below 7.0 indicates acidity, with lower numbers representing a higher concentration of hydrogen ions. Soil pH is a master variable because it governs the solubility and availability of most plant nutrients. Nutrient availability is greatest in a slightly acidic to neutral range, typically between pH 6.0 and 7.0. When the pH drops too low, elements like aluminum and manganese can become toxic to plant roots, and the availability of nutrients like phosphorus and calcium severely decreases.
Natural Weathering and Base Cation Leaching
The geology of a region and the movement of water are primary drivers of long-term soil acidification. The inherent mineral composition of the parent material—the bedrock from which the soil originates—determines its natural resistance to acidity. Soils derived from quartz-rich, acidic rocks like granite typically have a lower buffering capacity and are naturally more prone to low pH, often falling into the strongly acidic range (pH 4.0–4.9).
Conversely, soils formed from basic, calcium-rich materials like limestone are buffered by compounds such as calcium carbonate, which neutralize incoming acids and maintain a neutral to slightly alkaline pH (6.5–7.7). As water percolates through the soil profile, a process called base cation leaching occurs. This is the slow, constant removal of positively charged basic ions—primarily Calcium, Magnesium, and Potassium—that are loosely held on the soil’s exchange sites.
Rainwater is naturally slightly acidic due to dissolved atmospheric carbon dioxide, and as it moves downward, it washes away these buffering cations. The removal of these basic ions allows acidic ions, specifically Hydrogen and Aluminum, to occupy the exchange sites, lowering the soil’s pH. Areas with high annual rainfall experience an accelerated rate of leaching, making them susceptible to soil acidification over geological timescales. Sandy soils are particularly vulnerable because their coarse texture allows rapid water movement and their low clay and organic matter content provides minimal buffering agents.
Acid Production from Organic Matter and Root Activity
Biological processes carried out by plants and microbes contribute to the release of acidifying hydrogen ions. As dead organic matter decomposes, soil microorganisms produce various organic acids, including humic and fulvic acids. These compounds, along with carbonic acid formed when microbial carbon dioxide dissolves in soil water, release hydrogen ions into the surrounding soil solution.
Plant root activity further acidifies the immediate soil zone, known as the rhizosphere, through cation exchange. To absorb positively charged nutrients like potassium or calcium, plant roots actively release a hydrogen ion to maintain a neutral electrical charge balance. This constant exchange near the root surface results in a localized decrease in pH.
A powerful biological source of acidity comes from the oxidation of sulfur compounds. This process is pronounced in disturbed wetlands or where elemental sulfur is applied as a soil amendment. Sulfur-oxidizing bacteria convert elemental sulfur or sulfidic minerals into sulfate, which simultaneously generates sulfuric acid. Sulfuric acid is a strong acid, and its production can drastically lower the soil pH in the affected area.
The Role of Nitrogen Fertilizers in Accelerated Acidification
Modern agricultural practices, particularly the heavy reliance on nitrogen fertilizers, are the most significant human-driven cause of rapid soil acidification. Many common synthetic nitrogen fertilizers, such as anhydrous ammonia, urea, and ammonium sulfate, introduce nitrogen into the soil in the ammonium form. This ammonium is then converted into nitrate by soil bacteria in a two-step biological process called nitrification.
The chemical reaction of nitrification is highly acidifying because it releases hydrogen ions directly into the soil solution. The overall process of ammonium oxidation to nitrate results in the net release of hydrogen ions for every molecule of ammonium converted. This localized production of acidity quickly overwhelms the soil’s natural buffering capacity, leading to a measurable drop in pH over a much shorter period than natural processes.
The acidifying potential varies significantly between different fertilizer types. Ammonium sulfate is considered one of the most acidifying nitrogen sources. Additionally, acid rain, a form of atmospheric deposition, contributes external acidity as sulfuric and nitric acids. These acids are formed from the emission of sulfur dioxide and nitrogen oxides by the combustion of fossil fuels. Acid rain further accelerates the problem by promoting the leaching of base cations and directly introducing hydrogen ions into the soil.