What Is Nitrification in Soil? How Ammonium Becomes Nitrate
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What is nitrification in soil? It is the process by which ammonium, a form of nitrogen, is converted to nitrate by soil bacteria. Both forms can be used by plants, but nitrate is the form that moves freely with soil water, and ammonium is the form that tends to stay attached to soil particles. Nitrification is one step in the soil nitrogen cycle, and its speed depends on temperature, moisture, and the supply of organic matter. This guide explains the process, the bacteria involved, and the conditions that shape it. It relies on two university extension sources.
Quick answer
- Nitrification converts ammonium (NH4+) to nitrate (NO3-), and it is carried out by soil bacteria.
- The University of Nebraska-Lincoln source says ammonium will nitrify to nitrate over time, usually in less than three weeks during the growing season.
- The Mississippi State extension source describes two groups of bacteria: one produces nitrite and the other converts nitrite to nitrate.
- Nitrification releases hydrogen ions, which can lower soil pH, according to the Mississippi State source.
Where nitrification fits in the nitrogen cycle
The University of Nebraska-Lincoln source says that all nitrogen in or added to the soil is subject to the processes of the nitrogen cycle. Some of those processes benefit plant nutrition, while others provide no benefit or are detrimental to plant growth. The source explains that soil bacteria can convert nitrogen from forms that are not available to plants into available forms, and the reverse.
Nitrification sits between two earlier and later steps. Mineralization comes first. The Nebraska source defines mineralization as the process by which organic nitrogen is converted to inorganic, or plant-available, nitrogen. Specifically, it is the conversion of organic nitrogen to ammonium. Nitrification then converts that ammonium to nitrate. Other processes can remove nitrogen from the soil entirely, including denitrification, which the same source describes as the process by which nitrate is converted to gaseous forms that escape into the atmosphere.
The Nebraska source notes that ammonium has several possible fates. It can be taken up by plants, be converted by nitrification, be fixed by clay minerals, or be converted to ammonia gas that can be lost to the atmosphere through volatilization. Nitrification is therefore one of several routes for ammonium, and the route a given molecule takes depends on conditions in the soil.
What the process does step by step
The Mississippi State source describes nitrification in two steps. It says that ammonium can exist in the soil only a short time in most cases, because it is converted to nitrate soon after its formation or after it is added as fertilizer. The process is called nitrification, and it involves two groups of soil bacteria. The first group, Nitrosomas bacteria, produces nitrite (NO2-). The second, Nitrobacter species, converts that nitrite to nitrate soon after it forms.
The Nebraska source gives a time scale. It says that ammonium will nitrify to nitrate over time, and that this usually takes less than three weeks during the growing season. It also states that as with mineralization, nitrification is carried out by soil bacteria. A gardener should therefore think of nitrification as a process that happens on the scale of weeks rather than days in a typical warm season.
The Mississippi State source also notes a chemical side effect. It says that hydrogen ions are produced, which increases soil acidity or lowers soil pH. The same source mentions that commercially available nitrification inhibitors can slow the process and keep nitrogen in the ammonium form for longer. The article does not recommend any product. It notes the mechanism only because the source describes it.
Why the nitrogen form matters for plants
Plants can use either ammonium or nitrate, and the sources describe them as behaving differently in soil. The Mississippi State source says that positively charged ammonium is attracted to negatively charged sites on soil particles, as are other cations. Ammonium is therefore available to plants, but it does not leach. Nitrate is negatively charged, so it remains in the soil solution and moves with soil water.
The Nebraska source says that both forms are inorganic and both are plant-available. It adds that both sources can be used by corn and other agronomic crops, and that nitrate is the preferred form. The source gives a reason for the preference in its wording: nitrate is preferred by these crops, while ammonium first has to be converted.
The difference in movement is important for the way nitrogen is lost. The Mississippi State source says that nitrate may leach out of the root zone when rainfall is excessive, or may accumulate at the soil surface in dry weather. Because nitrate moves with the water, the timing of rain and irrigation can influence how much of it stays where roots can reach it.
Temperature, moisture and organic matter
The rate of nitrification follows the same environmental controls as the earlier step of mineralization. The Nebraska source says that the rate at which bacteria work depends on soil temperature, soil moisture, and the amount of organic matter in the soil. It also says that as soils warm up in the spring, bacteria become increasingly active and use organic matter as an energy source.
Organic matter plays two roles in this picture. It supplies the nitrogen that is later mineralized to ammonium, and it supplies the carbon that microbes use as energy. The Nebraska source describes a related effect called immobilization. It says immobilization is the process by which plant-available inorganic nitrogen is converted to plant-unavailable organic forms, and that it usually occurs when high levels of residue with a high carbon to nitrogen ratio are added to the soil. The carbon residue is an energy source for microbes, which need nitrogen in the soil solution to grow.
The Nebraska source notes that bacteria can reduce ammonium and nitrate levels quite dramatically and can outcompete plants for the nitrogen source. In most production situations, it says, nitrogen needs to be applied in sufficient quantities early in the season to overcome this effect, or placed below the residue so that it is not near the carbon source. The point for home gardeners is that a large addition of fresh carbon-rich material can temporarily tie up nitrogen.
The Mississippi State source adds that the rate of denitrification is greatly accelerated by higher temperatures. Denitrification is a separate loss process, but it shares the same warm, active soil conditions. Gardeners who see plants that look short of nitrogen after a very wet, warm period may be seeing the combined effect of several processes.
When nitrogen is lost instead
Not all ammonium becomes nitrate, and not all nitrate stays in the root zone. The Nebraska source says that nitrogen can be moved by leaching out of the reach of plant roots, or can escape into the atmosphere through gaseous loss known as denitrification. It defines nitrogen leaching as the passage of nitrogen through the soil profile and downward beyond where crop roots can access it.
The Nebraska source describes denitrification as the process by which nitrate in the soil is converted to gaseous forms of nitrogen that can escape into the atmosphere. It says that almost all denitrification takes place in saturated or compacted soils that have a limited oxygen supply, and that it is common during flooded conditions. The Mississippi State source gives a similar account. It says that a major mechanism of nitrogen loss in Mississippi is denitrification in waterlogged soils, and that the process requires waterlogged soil, a carbon source for the microbes, and nitrogen as nitrate or nitrite.
The Mississippi State source explains the leaching side. It says that nitrogen leaching is more likely when there is more rain than a crop can use. Nitrate that remains in the soil water and moves away from root uptake areas with downward water movement is lost from the root zone. These two loss routes show why good drainage and sensible watering matter for nitrogen management.
The Nebraska source also notes that nitrogen can be lost as ammonia gas. It defines volatilization as the process by which ammonia is lost into the atmosphere, and it says that volatilization occurs naturally in soils but losses are usually small. The source notes that the process is larger when fertilizers are left on the soil surface and not incorporated.
What nitrification means for a garden
For a home garden, nitrification is best understood as a timing process. Nitrogen added as ammonium-based material is not immediately in the nitrate form, and it may take a few weeks to convert in a warm growing season. Nitrogen in organic form takes longer still, because it must first be mineralized. The sources do not provide a schedule for any particular garden, but they describe the stages clearly enough to explain why nitrogen can seem to arrive slowly.
The Mississippi State source gives a practical reason to care about the form. It says that ammonium is held by soil particles, while nitrate moves with water. A gardener who waters heavily right after adding a nitrate-rich material may lose some of it below the root zone, while ammonium is more likely to stay near where it was placed. The article does not turn this into a fertilizer recommendation.
If you want to learn more about how organic matter feeds the soil, the page on how to keep garden soil healthy covers the role of organic matter. For the water side of nitrogen movement, the article on how to make garden soil better discusses drainage and soil structure, which both affect how long nitrogen stays where roots can use it.
How we chose these sources
This article uses two university extension sources, both opened on 2026-10-27. The University of Nebraska-Lincoln land application training page is a technical resource on the nitrogen cycle and its processes. The Mississippi State University Extension publication on nitrogen in Mississippi soils explains nitrification and nitrogen loss with reference to regional conditions. The two sources agree on the core steps, which is why the article states the mechanism with confidence.
The article names no products and gives no prices or retail links. Where a source mentions a commercial inhibitor or a fertilizer type, the article reports only the mechanism and does not recommend a product.
What we could not verify
The two sources describe the process in general terms and use Mississippi and agronomic examples. They do not give a single nitrification rate that applies to every soil, and they do not provide a method for measuring the rate in a home garden. The article therefore describes the stages and controls, and it does not invent numbers.
What the sources do not say
The sources do not give a fixed number of days for nitrification in a particular garden soil. They do not state how much nitrogen a given amount of compost will release through nitrification, and they do not give a soil pH threshold at which the process slows. Readers who need those figures should check a local extension publication or a soil test.
Frequently asked questions
What is nitrification in simple terms?
Nitrification is the conversion of ammonium to nitrate by soil bacteria. The University of Nebraska-Lincoln source describes it as the process by which ammonium is converted to nitrate, and both forms are plant-available.
How long does nitrification take?
The Nebraska source says that ammonium will nitrify to nitrate over time, usually in less than three weeks during the growing season. The time depends on conditions, so the source does not give a single fixed number.
Which bacteria are involved?
The Mississippi State source says that Nitrosomas bacteria produce nitrite, and that Nitrobacter species then convert nitrite to nitrate soon after it forms. The Nebraska source says nitrification is carried out by soil bacteria.
Does nitrification change soil pH?
The Mississippi State source says that hydrogen ions are produced during the process, which increases soil acidity or lowers soil pH. The source does not say how large the change will be in a particular soil.
Why does nitrate wash away but ammonium stay?
The Mississippi State source says positively charged ammonium is attracted to negatively charged sites on soil particles, so it does not leach. Negatively charged nitrate remains in soil water and moves with it, so it can leach out of the root zone when rainfall is excessive.
Sources
- University of Nebraska-Lincoln, “Nitrogen Dynamics,” Land Application Training Online, https://water.unl.edu/land-application-training-online/nitrogen-dynamics/ (HTTP 200, opened 2026-10-27)
- Mississippi State University Extension Service, “Nitrogen in Mississippi Soils,” https://extension.msstate.edu/publications/nitrogen-mississippi-soils (HTTP 200, opened 2026-10-27)
