Nitrogen application contributes to soil acidification, especially when using ammonium-based fertilizers. Soil pH measures the hydrogen ion (\(\text{H}^+\)) concentration in the soil solution; a lower number indicates greater acidity. Maintaining proper soil pH controls the availability of essential plant nutrients like phosphorus, calcium, and magnesium. Overly acidic conditions can also increase the solubility of elements like aluminum and manganese to toxic levels, which harms plant roots and impairs crop growth.
The Chemical Process of Acidification
The primary mechanism for nitrogen-induced soil acidification is nitrification, a natural biological process. Soil-dwelling bacteria, primarily Nitrosomonas and Nitrobacter, carry out this process, converting ammonium (\(\text{NH}_4^+\)) into plant-available nitrate (\(\text{NO}_3^-\)). This conversion is a two-step oxidation reaction that generates acidity. For every molecule of ammonium converted to nitrate, two hydrogen ions (\(\text{H}^+\)) are released into the soil solution.
The release of these hydrogen ions directly increases the active acidity of the soil, lowering its pH. Over time, the constant introduction of \(\text{H}^+\) ions from repeated nitrogen fertilization can significantly acidify the topsoil layer. This chemical change results directly from the microbial activity necessary to transform nitrogen into a plant-preferred form.
Nitrate leaching is a secondary factor contributing to long-term soil acidification. Nitrate (\(\text{NO}_3^-\)) is highly mobile and can be washed below the root zone by rainfall or irrigation. As the negatively charged nitrate leaches out, it takes positively charged basic cations with it, such as calcium (\(\text{Ca}^{2+}\)), magnesium (\(\text{Mg}^{2+}\)), and potassium (\(\text{K}^+\)), to maintain charge balance.
The removal of these basic cations reduces the soil’s buffering capacity, its ability to resist changes in pH. Losing these neutralizing agents means the soil has less capacity to absorb the \(\text{H}^+\) ions generated by nitrification. This makes the soil more susceptible to further pH decline from future fertilizer applications, driving the overall acidifying impact.
Comparing Nitrogen Sources and Their Acidifying Potential
Different nitrogen sources vary considerably in their potential to acidify the soil, mainly based on the percentage of ammonium they contain. Ammonium sulfate (\(\text{NH}_4)_2\text{SO}_4\)) is the most acidifying commercial product because all of its nitrogen is in the ammonium form, and it contains sulfur that further contributes to acidity. The amount of lime required to neutralize the acidity produced by ammonium sulfate can be 1.5 to 2.3 times greater than that needed for other common sources.
Urea and Ammonium Nitrate (\(\text{NH}_4\text{NO}_3\)) have a moderate acidifying potential. Urea initially causes a temporary, slight rise in pH as it breaks down, but the subsequent nitrification of its ammonium content results in a net acidifying effect over time. Ammonium nitrate is less acidifying than pure ammonium fertilizers because half of its nitrogen is already in the non-acidifying nitrate form.
Nitrate-based fertilizers, such as calcium nitrate or potassium nitrate, are minimally acidifying or can even have a temporary alkalizing effect. When plants absorb the nitrate ion (\(\text{NO}_3^-\)), they often release hydroxyl ions (\(\text{OH}^-\)) or take up \(\text{H}^+\) ions to balance the electrical charge within their roots. This action can slightly raise the pH in the immediate root zone, making them beneficial for already acidic soils.
Organic sources, such as composted animal manures, are often less acidifying than synthetic fertilizers. The nitrogen in these materials is released slowly through mineralization, which matches plant uptake more closely than quick-release synthetic products. This slower, controlled release, combined with basic cations in the organic matter, results in a lower overall acidifying impact compared to high-concentration ammonium fertilizers.
Counteracting Nitrogen-Induced Acidity
Managing nitrogen-induced acidity begins with regular soil testing to monitor the current pH level. A standard pH test measures active acidity, while a buffer pH test determines the soil’s buffering capacity. This buffer measurement dictates the total amount of lime required to achieve the desired pH change. Soils high in clay or organic matter have greater reserve acidity and require more neutralizing material to shift the pH.
The most common and effective strategy for remediation is the application of liming materials, typically calcium carbonate (\(\text{CaCO}_3\)). Liming works by introducing a base that neutralizes the acid in the soil solution. The carbonate component reacts with the hydrogen ions (\(\text{H}^+\)) to form water and carbon dioxide, effectively removing the acidity.
The calcium ions from the lime also replace the acidic \(\text{H}^+\) and aluminum (\(\text{Al}^{3+}\)) ions attached to the soil particles. Liming is not a quick fix; the material must dissolve and react, which can take several months to a few years depending on the product’s fineness. Therefore, lime should be applied proactively based on soil test recommendations rather than waiting until severe acidity is present.
Farmers and gardeners can also employ best management practices to slow the rate of acidification. This includes using less acidifying nitrogen sources when possible, such as nitrate-based fertilizers, especially on already acidic land. Applying the required nitrogen in smaller, more frequent doses also helps, as this encourages the plant to take up the nitrogen before significant nitrification and leaching can occur.