Nitrate (NO₃⁻) versus Ammonium (NH₄⁺)
Nitrogen can be supplied to plants in different chemical forms. Understanding the characteristics and behaviour of nitrate and ammonium nitrogen helps explain why nitrogen source matters in fertiliser selection.
Overview
Nitrogen, one of the most important plant nutrients, can be supplied in multiple chemical forms. The two inorganic forms most relevant to fertiliser use are nitrate (NO₃⁻) and ammonium (NH₄⁺). Plants can absorb and utilise both, but these forms behave differently in soil and affect plant physiology in distinct ways.
Understanding these differences helps inform decisions about nitrogen source selection for different crops, soils, and growing conditions.
Key differences
| Characteristic | Nitrate (NO₃⁻) | Ammonium (NH₄⁺) |
|---|---|---|
| Ionic charge | Negative (anion) | Positive (cation) |
| Soil binding | Not bound (mobile) | Bound to soil particles |
| Leaching risk | Higher | Lower (until nitrified) |
| Plant uptake | Immediate, no transformation needed | Can be used directly or after nitrification |
| pH effect | Tends to raise rhizosphere pH | Tends to lower rhizosphere pH |
| Stability in soil | Stable (unless lost through leaching or denitrification) | Converts to nitrate through nitrification |
Behaviour in soil
Nitrate nitrogen
Nitrate carries a negative charge and therefore does not bind to negatively charged soil particles. This makes nitrate highly mobile in soil solution. It moves readily with water, making it easily accessible to plant roots but also susceptible to leaching below the root zone with excessive water movement.
Under anaerobic conditions (waterlogged soils), nitrate can be lost through denitrification—a microbial process that converts nitrate to gaseous forms of nitrogen that escape to the atmosphere.
Ammonium nitrogen
Ammonium carries a positive charge and binds to negatively charged clay and organic matter particles in soil. This binding reduces ammonium mobility and makes it less prone to leaching. However, soil microorganisms rapidly convert ammonium to nitrate through nitrification in well-aerated, warm soils.
The rate of nitrification depends on soil temperature, moisture, pH, and microbial activity. In cool soils or under waterlogged conditions, nitrification slows and ammonium remains stable longer.
Plant uptake and physiology
Most plants can absorb both nitrate and ammonium, but the uptake mechanisms and physiological effects differ:
Nitrate uptake
Nitrate is absorbed through active transport mechanisms and can be stored in plant vacuoles. Once inside the plant, nitrate must be reduced to ammonium before incorporation into amino acids and proteins. This reduction requires energy but gives the plant flexibility in regulating nitrogen metabolism.
Ammonium uptake
Ammonium is also actively absorbed but must be quickly assimilated into organic compounds to prevent toxicity. Excessive ammonium can interfere with cation balance and disrupt various metabolic processes. Most plants prefer moderate ammonium concentrations rather than ammonium as the sole nitrogen source.
Rhizosphere pH effects
The form of nitrogen taken up affects pH in the immediate root zone (rhizosphere):
- Nitrate nutrition: Uptake is often accompanied by the release of hydroxide (OH⁻) or bicarbonate ions, which can raise rhizosphere pH. This can be beneficial in acidic soils.
- Ammonium nutrition: Uptake requires plants to release hydrogen ions (H⁺) to maintain charge balance, acidifying the rhizosphere. This can help in alkaline soils but may exacerbate acidity problems in already acidic conditions.
Practical implications
In most situations, supplying a mix of both nitrogen forms or relying on nitrate provides good results. Potassium nitrate, which supplies nitrogen exclusively as nitrate, offers certain advantages:
- Immediate plant availability without waiting for soil transformations
- No risk of ammonium toxicity
- Suitable for neutral to acidic soils where pH maintenance is important
- Lower salinity effect on sensitive crops
- Effective in hydroponics and fertigation where precise control is needed
The choice between nitrogen forms should consider crop type, soil conditions, application method, and specific agronomic goals.
Frequently asked questions
Can plants use both nitrate and ammonium nitrogen?
Yes, most plants can absorb and utilise both nitrogen forms. However, the efficiency and physiological effects differ between species and growing conditions. Some crops show preference for one form, while others perform best with a balanced supply of both.
Why does nitrogen form affect soil pH?
When plants take up ammonium (NH₄⁺), they release hydrogen ions (H⁺) to maintain electrical balance, which acidifies the rhizosphere. Conversely, nitrate (NO₃⁻) uptake is often accompanied by the release of hydroxide ions (OH⁻) or bicarbonate, which can raise pH in the immediate root zone.
Is nitrate nitrogen more susceptible to leaching?
Nitrate ions carry a negative charge and do not bind to soil particles (which are also predominantly negatively charged). This means nitrate moves freely with soil water and can be lost through leaching in high-rainfall situations or with over-irrigation. Ammonium binds more readily to soil particles but converts to nitrate over time through nitrification.
When might ammonium be preferred over nitrate?
Ammonium may be preferred in flooded rice systems (where it remains stable), in situations where soil pH is already high, or when trying to limit nitrate leaching in very sandy soils. Some acid-loving crops also show preference for ammonium nutrition.
How does temperature affect nitrogen form preference?
Cool soil temperatures slow both nitrification (conversion of ammonium to nitrate) and nitrate uptake. In early spring or cool conditions, some crops may benefit from having both nitrogen forms available. Warm soils favour rapid nitrification and efficient nitrate uptake.