Soil acidity or alkalinity is measured by its pH. A pH value below 7.0 is considered acidic, while a value above 7.0 is alkaline. Maintaining a specific pH range is crucial because it governs the solubility and availability of nutrients for plants. For instance, in highly alkaline soils, micronutrients like iron and zinc can become chemically locked, leading to plant deficiencies. Many common garden plants thrive in a slightly acidic to neutral range of pH 6.0 to 7.0, but specialized plants, such as blueberries, azaleas, and rhododendrons, are acid-loving and require a pH much lower, often between 4.5 and 5.5.
Identifying Acidifying Nitrogen Sources
The most significant contributors to increasing soil acidity are fertilizers that contain nitrogen in the ammonium form (\(\text{NH}_4^+\)). The intensity of this acidifying effect is measured by a product’s Acid-Forming Potential (AFP).
Among the most common nitrogen sources, Ammonium Sulfate (\(\text{NH}_4)_2\text{SO}_4\)) has the highest Acid-Forming Potential per unit of nitrogen applied. This product is highly favored for intentional pH reduction, especially in alkaline soils, because it supplies readily available nitrogen and sulfur simultaneously. Ammonium Sulfate is estimated to be approximately 1.5 to 2 times more acidifying than other common nitrogen fertilizers.
Other common fertilizers that lower soil pH include Urea and Ammonium Nitrate (\(\text{NH}_4\text{NO}_3\)). While these are also acid-forming, their effect is less pronounced than Ammonium Sulfate.
The Chemical Process Behind Soil Acidification
The fundamental mechanism by which these fertilizers increase soil acidity is a biological process called nitrification. Nitrification is a two-step process carried out by specialized soil bacteria, primarily in the genera Nitrosomonas and Nitrobacter.
The process begins when soil bacteria convert the ammonium ion (\(\text{NH}_4^+\)) into nitrite (\(\text{NO}_2^-\)). The nitrite is then rapidly converted into nitrate (\(\text{NO}_3^-\)) by a second group of bacteria. The release of two hydrogen ions (\(\text{H}^+\)) for every one ammonium ion converted is the direct cause of the increased soil acidity.
If the resulting nitrate is absorbed by plants, some of this acidity is neutralized, but if the nitrate leaches out of the root zone, the hydrogen ions are left behind, contributing to what is known as “residual acidity”. This residual acidity remains in the soil, accumulating over time with repeated application of ammonium-based fertilizers.
Measuring and Applying Soil Acidifying Amendments
Before any acidifying fertilizer or amendment is applied, obtaining an accurate soil test is essential to determine the current pH level and the soil’s buffering capacity. Soil testing should be performed by a laboratory, and the frequency should typically be every three to five years, or more frequently in sandy soils or when actively trying to adjust pH.
The most effective materials for lowering pH are elemental sulfur and ammonium-based fertilizers, but their action differs significantly. Ammonium Sulfate is considered a fast-acting acidifier because its conversion to acid through nitrification occurs relatively quickly, often within weeks to a few months, depending on soil temperature and moisture. However, its use is limited by the plant’s nitrogen requirement, as over-application can damage the plants.
Elemental sulfur is the preferred option for making large, structural changes to soil pH because it contains no nitrogen, allowing for higher application rates. Sulfur must be biologically oxidized by soil bacteria to form sulfuric acid. Ammonium Sulfate will produce a faster, more immediate pH shift, while elemental sulfur provides a sustained and more profound long-term change.