The odor of ammonia wafting from soil is a common sign for gardeners and farmers. This distinct smell indicates a temporary chemical imbalance within the soil’s structure. The odor is caused by gaseous ammonia (\(NH_3\)), a volatile compound. This gas is a natural byproduct created during the complex process of nitrogen cycling in the soil environment.
The Nitrogen Cycle and Ammonia Production
The process that generates the ammonia smell begins with the natural breakdown of organic matter, called ammonification. Soil microorganisms consume nitrogen-containing compounds found in decaying plants, animal waste, or organic fertilizers. These microbes excrete nitrogen in the form of ammonium (\(NH_4^+\)), a positively charged ion that typically binds to clay or organic particles in the soil.
Ammonium is an important plant nutrient, but its stability depends on the surrounding soil conditions. In well-aerated soil, other microbes convert ammonium into less volatile compounds, like nitrates, through nitrification. However, when the soil environment shifts, ammonium can be chemically converted into ammonia gas (\(NH_3\)). This transformation is accelerated when the soil \(\text{pH}\) rises above a neutral level, typically around \(\text{pH}\) 7.5 or higher.
At higher \(\text{pH}\) levels, the positive hydrogen ions required to keep nitrogen in the stable ammonium form are less available. This chemical shift causes the ammonium to release a hydrogen ion, resulting in the formation of gaseous ammonia. Since ammonia is highly volatile, it easily escapes from the soil and enters the atmosphere, a process known as volatilization. It is this escaping gas that creates the odor detected above the soil surface.
Practical Triggers for Ammonia Buildup
The conditions that encourage the conversion of ammonium to volatile ammonia are often related to soil management practices. A frequent trigger is the excessive input of nitrogen, especially from sources like fresh manure or synthetic fertilizers containing urea. When the microbial community is overwhelmed by a large dose of nitrogen, they produce ammonium faster than other microbes can convert it into nitrates.
Another factor is poor soil structure, which leads to inadequate aeration and drainage. Waterlogged or compacted soil creates anaerobic conditions, meaning oxygen levels are too low to support the microbes responsible for nitrification. The lack of oxygen halts the conversion of ammonium to nitrate, causing \(NH_4^+\) to accumulate in the soil profile.
When water remains stagnant, oxygen-dependent processes slow down, allowing \(NH_4^+\) concentration to build up. High soil \(\text{pH}\) exacerbates this issue by facilitating the chemical release of \(NH_3\) from the accumulated ammonium. For instance, applying high-nitrogen fertilizer to soil with a \(\text{pH}\) of 8.0 will cause more ammonium to convert to volatile ammonia gas than if applied to a \(\text{pH}\) 6.0 soil.
Remediation and Future Soil Management
Addressing the current ammonia odor requires immediate action to restore chemical balance and gas exchange. Begin by gently aerating the affected area using a garden fork to lightly turn the top few inches of soil without deep tilling. This introduces oxygen back into the soil, supporting the microbes that convert excess ammonium into stable nitrate.
Following aeration, the soil should be thoroughly leached with plain water to dilute and wash away excess nitrogen compounds. Applying several inches of water helps move soluble ammonium deeper into the soil profile, mitigating the concentration near the surface. This step quickly reduces the odor and lowers the phytotoxicity risk to plants.
Long-Term Management Strategies
For long-term management, preventative measures focus on controlling \(\text{pH}\) and nitrogen input:
- Perform a soil test to determine exact \(\text{pH}\) and nutrient levels.
- If high alkalinity is confirmed, incorporate amendments like elemental sulfur to gradually lower the \(\text{pH}\).
- Adjust the timing and amount of nitrogen fertilizer application, using slow-release or smaller, more frequent doses.
- Improve drainage by incorporating organic material like compost to prevent waterlogging and compaction.