Soil \(\text{pH}\) refers to the measure of acidity or alkalinity in the soil, which is a logarithmic scale ranging from 0 to 14. This measurement directly controls the availability of essential plant nutrients. If the soil becomes too acidic (low \(\text{pH}\)) or too alkaline (high \(\text{pH}\)), nutrients like phosphorus, iron, or nitrogen can become chemically bound and inaccessible to plant roots. Certain fertilizers, specifically those containing nitrogen in the ammonium form, can significantly lower soil \(\text{pH}\), making the soil more acidic.
The Chemical Process Behind \(\text{pH}\) Shifts
The primary mechanism by which fertilizer application lowers soil \(\text{pH}\) is nitrification. Many common fertilizers introduce nitrogen into the soil in the form of ammonium (\(\text{NH}_4^+\)).
Soil-dwelling bacteria, primarily Nitrosomonas and Nitrobacter, facilitate a two-step conversion of this applied ammonium. Bacteria first convert ammonium into nitrite (\(\text{NO}_2^-\)), and then rapidly into nitrate (\(\text{NO}_3^-\)). This overall conversion makes the nitrogen available to plants.
The chemical reaction for this conversion is highly acidifying because it releases hydrogen ions (\(\text{H}^+\)) into the surrounding soil solution. For every molecule of ammonium converted to nitrate, two hydrogen ions are produced. The presence of these extra \(\text{H}^+\) ions is the chemical definition of increased acidity, which registers as a lower \(\text{pH}\) reading.
This process is continuous as long as ammonium is present. The extent of acidification depends on how much nitrate is absorbed by the plant versus how much is leached away. Plant uptake of nitrate can slightly offset the initial acidity.
Specific Fertilizers That Lower Soil \(\text{pH}\)
The potential for a fertilizer to lower soil \(\text{pH}\) is directly related to the amount of ammonium nitrogen it contains. The most common commercial fertilizer known for its high acidifying potential is Ammonium Sulfate (\((\text{NH}_4)_2\text{SO}_4\)). It is particularly acidifying because it contains both acid-forming ammonium and sulfur, which microbes convert into sulfuric acid.
Urea is another widely used nitrogen fertilizer that causes acidification. When added to the soil, urea rapidly converts into ammonium, which then proceeds through the nitrification process. The total acidifying effect of urea is substantial over time due to its high nitrogen concentration.
Ammonium Nitrate (\(\text{NH}_4\text{NO}_3\)) also contributes to soil acidity because half of its nitrogen is in the ammonium form. The other half is already nitrate, which is generally not acidifying. This makes Ammonium Nitrate a moderately acidifying choice compared to ammonium sulfate.
Non-Acidifying Options
Nitrogen-based fertilizers are the main drivers of fertilizer-induced soil acidification. Fertilizers that primarily supply phosphorus (e.g., Triple Superphosphate) or potassium (e.g., Potassium Chloride) are virtually \(\text{pH}\) neutral. Growers seeking to avoid acidification might choose nitrate-based nitrogen sources, such as Calcium Nitrate, which bypasses the nitrification process and has a neutral or slightly alkaline effect.
Testing and Adjusting Soil Acidity
Regular \(\text{pH}\) testing is an important practice for soil management. Simple home testing kits provide a quick estimate of soil \(\text{pH}\) using chemical reagents or probes. For precise and actionable results, a professional soil test conducted by a laboratory is highly recommended.
Laboratory analysis provides the current \(\text{pH}\) level and measures the soil’s “buffer \(\text{pH}\),” which indicates the soil’s resistance to \(\text{pH}\) change. This buffer measurement is used to calculate the exact amount of material needed to raise the \(\text{pH}\). The amount of amendment required is much higher in clay or high organic matter soils than in sandy soils due to this buffering capacity.
If fertilizer use has lowered the soil \(\text{pH}\) into an undesirable range, liming materials are used for remediation. Agricultural lime (calcium carbonate) or dolomitic lime (calcium and magnesium carbonates) are the most common amendments. These materials work to neutralize the excess hydrogen ions in the soil solution.
Lime should be applied and thoroughly mixed into the topsoil to work most effectively, although surface application is necessary for established lawns and no-till systems. The effect of liming is not immediate, often taking several months to a year to fully change the soil \(\text{pH}\). Growers can maintain balance by rotating between acidifying and non-acidifying fertilizers or by reducing the use of high-ammonium products.