Characteristics and application rules of ammonium nitrate in agronomy

Ammonium nitrate (NH₄NO₃) is a highly effective, versatile nitrogen fertilizer. It contains 34.6% nitrogen, with half present in the readily available nitrate form and the other half in the sustained-release ammonium form. This combination ensures an immediate start for the crop and subsequent stable nutrition.

• Nitrogen content — at least 34.6%
• Ratio of ammonium to nitrate nitrogen — 1:1
• Maximum granule humidity — 0.3%
• Granule diameter — 1–3 mm
• Bulk density — 0.85 t/m³

Physicochemical properties and storage requirements of the fertilizer

In production, ammonium nitrate is obtained by neutralizing 45–58% nitric acid with gaseous ammonia, releasing 145.8 kJ of heat. The resulting solution is evaporated to a concentration of 99.6–99.8% and sprayed at a temperature of 174–176 °C in a granulation tower, where the droplets cool to form durable granules. The solubility of the fertilizer in water is extremely high: at 20 °C, 192 g of ammonium nitrate dissolves in 100 cm³ of water, and at 100 °C, as much as 871 g.

The main drawback of ammonium nitrate is its high hygroscopicity. In the open air, the granules quickly become damp, and upon drying, they cake into dense blocks. To restore the fertilizer to a granular state before application, it must be crushed and sieved through 2–3 mm meshes. To prevent caking, manufacturers add magnesium nitrate or ammonium sulfate, or coat the granules with limestone dust and phosphate rock flour (the content of insoluble impurities in this process does not exceed 0.1%).

Behavior of ammonium nitrate in soil and impact on acidity

Upon reaching moist soil, the nitrate dissolves instantly. Plants absorb the NH₄⁺ cation more actively than the NO₃^- anion, which is why the fertilizer is physiologically acidic, although it acidifies the soil less than ammonium sulfate. In acidic podzolic soils with calcium deficiency, an exchange reaction occurs: [SEC]H + NH₄NO₃ ⇄ [SEC]NH₄ + HNO₃. The released nitric acid temporarily increases the acidity of the soil solution, but this effect disappears as the nitrates are assimilated by the roots.

Ammonium nitrogen in the soil is gradually converted into nitrate by bacteria (nitrification): 2NH₄NO₃ + 4O₂ = 4HNO₃ + 2H₂O. This process also produces a temporary acidic effect. During this time, part of the nitrogen is lost as gases due to denitrification. Biological denitrification proceeds through sequential stages: nitrate (HNO₃) → nitrite (HNO₂) → hyponitrite (H₂N₂O₂) → nitrous oxide (N₂O) → molecular nitrogen (N₂). Bacteria break down nitrogen compounds according to the following equations:

• C₆H₁₂O₆ + 6NO₂ = 6CO₂ + 6H₂O + 3N₂
• C₆H₁₂O₆ + 6HNO₃ = 6CO₂ + 9H₂O + 3N₂O
• C₆H₁₂O₆ + 8HNO₃ = 6CO₂ + 10H₂O + 8NO

Nitrogen losses in the form of oxide (NO) and dioxide (NO₂) also occur through chemical pathways under high acidity of the medium (pH ≤ 5.5). In such conditions, unstable nitrous acid decomposes according to the formula: 3HNO₂ = 2NO + HNO₃ + H₂O. Additionally, reactions occur with decomposition intermediates and organic matter:

• 3NH₂OH (hydroxylamine) + HNO₂ = 5H₂O + 2N₂
• CH₂NH₂COOH (glycine) + HNO₂ = CH₂OHCOOH (hydroxyacetic acid) + H₂O + N₂

When applying ammonium nitrate, nitrogen losses are inevitable: 15 to 30% of the active ingredient escapes into the air as gaseous products (via the reaction 2NO + O₂ = 2NO₂). In the first year of the growing season, crop plants assimilate about 40–50% of the nitrogen from the fertilizer. The remaining portion is partially bound by soil microorganisms: 10–20% of nitrate nitrogen and 20–40% of ammonium nitrogen are converted into organic form (immobilized). In the second year after application, plants receive no more than 10–15% of this volume, which accounts for only 2–3% of the initially applied dose.

The immobilization process is particularly intense if you plough cereal straw and other plant residues with high carbon content and nitrogen deficiency into the soil. However, the applied nitrogen activates the soil microflora, which mobilizes the soil's own nitrogen reserves and converts them into a form available to plants. This phenomenon allows for a significant increase in the overall utilization rate of nitrogen nutrition in the field.

Ammonium nitrate remains one of the most effective nitrogen fertilizers, suitable for all types of soil and crops. It is applied at all main timings: for winter crops in autumn, in spring before sowing, directly at sowing in rows, and as top dressing during the growing season. Split application helps to regulate the nitrogen plant nutrition at key stages of development.

• Nitrogen uptake in the first year — 40–50 %
• Gaseous nitrogen losses — 15–30 %
• Immobilization of nitrate nitrogen — 10–20 %
• Immobilization of ammonium nitrogen — 20–40 %
• Nitrogen release in the second year — 2–3 %

Properties of Calcium Ammonium Nitrate and Ammonium Sulfate Nitrate

Calcium ammonium nitrate (NH₄NO₃ · CaCO₃) is produced by melting or mixing ammonium nitrate with chalk, lime, or dolomite. The resulting fertilizer is physiologically neutral, practically does not cake, and has reduced hygroscopicity, which allows it to be stored in stacks without strict requirements for humidity. It is a valuable soil amendment and a source of nutrition for acidic soils, where plants suffer from calcium deficiency. However, on neutral or alkaline soils, calcium ammonium nitrate has no advantages over the standard form.

Ammonium sulfate nitrate [2NH₄NO₃ · (NH₄)₂SO₄] is a double salt of ammonium nitrate and ammonium sulfate, grayish in color. Depending on the production technology at plants, this fertilizer is called Leunasalpeter or Montansalpeter. In terms of nitrogen content, ease of transport, and acidifying effect, this double salt occupies an intermediate position between conventional ammonium nitrate and ammonium sulfate. In terms of efficiency in the field, all three fertilizers are approximately equivalent.

Ammonium sulfate nitrate is obtained by the following methods:

• by mixing dry ammonium sulfate and ammonium nitrate;
• by introducing dry ammonium sulfate into a concentrated solution of ammonium nitrate;
• by simultaneous production of the double salt in a single solution.

Montansalpeter is prepared directly from a saturated solution of ammonium sulfate and ammonium nitrate.