Nitrate (NO₃⁻) vs Ammonium (NH₄⁺) in Plant Nutrition

Key differences between nitrate and ammonium

The table below summarises the main characteristics that distinguish nitrate and ammonium nitrogen in agricultural contexts. These differences have practical implications for fertiliser timing, placement, and crop response.

Uptake and metabolism

When roots absorb nitrate, the ion is often transported to the shoot where it is reduced to ammonium and then incorporated into amino acids. This reduction process requires energy (from photosynthesis) and occurs primarily in the leaves. Some nitrate reduction also takes place in roots, depending on plant species and nitrogen supply levels.

Ammonium, by contrast, is typically assimilated in the roots soon after uptake. This is partly because free ammonium can be toxic to plant cells at elevated concentrations. The rapid incorporation of ammonium into organic compounds (glutamine, glutamate) in root tissues prevents toxicity and allows safe transport of organic nitrogen to other plant parts.

In practical terms, plants supplied predominantly with nitrate often show somewhat higher shoot-to-root ratios, as shoot growth is favoured when nitrogen assimilation occurs in leaves. High ammonium supply can promote root development relative to shoot growth in some species.

Effect on soil and rhizosphere pH

One of the most important practical differences between the two nitrogen forms relates to their effect on soil pH. When plants take up ammonium cations, they release hydrogen ions (H⁺) to maintain electrical neutrality across root cell membranes. Over time, this acidifies the soil immediately surrounding the roots (the rhizosphere) and can contribute to broader soil acidification with repeated ammonium-based fertilisation.

Nitrate uptake has the opposite tendency. To balance the negative charge entering the root, plants release hydroxide (OH⁻) or bicarbonate ions, which can raise rhizosphere pH. This effect is generally less pronounced than ammonium-induced acidification, but it can be beneficial in already-acidic soils or when growing crops that struggle in low-pH conditions.

For growers managing soil pH, the nitrogen form in fertilisers is an important consideration. Soils that are already acidic may benefit from nitrate-based nutrition, while alkaline soils or those with pH-induced nutrient availability issues (such as iron chlorosis) may require careful attention to the balance of nitrogen forms.

Mobility and leaching considerations

Because nitrate carries a negative charge, it does not bind to negatively charged soil particles and moves freely with the soil solution. This mobility means nitrate is immediately available throughout the root zone, but it also means that excess nitrate can be leached below the rooting depth during heavy rainfall or over-irrigation—potentially contaminating groundwater and representing a loss of applied fertiliser.

Ammonium binds to cation exchange sites on soil particles, making it more resistant to leaching. However, under aerobic conditions, soil microorganisms convert ammonium to nitrate (nitrification), usually within days to a few weeks depending on temperature and soil conditions. This means that even ammonium-based fertilisers eventually contribute to the soil nitrate pool.

Best management practices for minimising nitrogen loss include matching application rates to crop demand, using split applications rather than single large doses, and timing applications to coincide with active crop uptake periods. In high-leaching-risk situations (sandy soils, high rainfall), smaller and more frequent nitrogen applications may be warranted regardless of the nitrogen form applied.

When nitrate nutrition is advantageous

Nitrate-based fertilisers like potassium nitrate offer several advantages in specific situations:

• Fertigation systems: The high solubility of nitrate fertilisers and their compatibility with irrigation water make them well-suited for fertigation through drip or sprinkler systems.
• Foliar applications: Nitrate nitrogen is readily absorbed through leaf surfaces and efficiently utilised, making it an effective choice for foliar feeding.
• Cool conditions: Nitrate is immediately available and does not require microbial conversion, which can be advantageous in cool soils where nitrification is slow.
• Alkaline soils: In high-pH soils where ammonium volatilisation losses may occur, nitrate provides a stable nitrogen source.
• Chloride-sensitive crops: When using potassium nitrate as a potassium source, the accompanying nitrate nitrogen avoids the chloride that would come with potassium chloride.

Balanced nitrogen nutrition

Research suggests that many crops perform optimally with a mixture of nitrate and ammonium nitrogen rather than either form exclusively. A balanced supply can provide the benefits of both forms: the immediate availability and mobility of nitrate, combined with the lower leaching risk and different metabolic effects of ammonium.

The ideal ratio varies by crop species, growth stage, and environmental conditions. Growers managing intensive production systems often use both nitrogen forms strategically—for example, applying ammonium-based fertilisers as a base dressing and supplementing with nitrate through fertigation during periods of high demand.

Understanding the characteristics of each nitrogen form allows informed decision-making about fertiliser selection and application methods tailored to specific growing situations.

Frequently asked questions