1. Ammonium bicarbonate, with the chemical formula NH4HCO3, contains approximately 17% nitrogen. It is a white, chemically unstable crystalline substance that readily absorbs moisture and decomposes, especially in high humidity or temperature conditions. The decomposition process produces ammonia gas, which has a strong, pungent odor. When dissolved in water, it forms an alkaline solution (pH 8.2–8.4). At 70°C, ammonium bicarbonate completely decomposes under atmospheric pressure. Even at 20°C, when stored openly, it loses about 9% of its content after one day, 48% after five days, and 74% after ten days. In moist environments, it can absorb water, deliquesce, and form lumps, which is a sign of slow decomposition. To maintain its effectiveness, proper storage and application methods should be used to minimize volatilization loss.
Ammonium bicarbonate is suitable for all types of soil and crops. It works best as a base fertilizer and should be applied deep into the soil to reduce losses due to evaporation. This makes it ideal for use in various agricultural practices where quick nitrogen availability is not required.
2. Urea, with the molecular formula (NH2)2CO, contains around 46% nitrogen. It is typically a white, crystalline substance with strong hygroscopic properties. Modern urea products are often translucent and treated to reduce moisture absorption. Its hygroscopicity increases with higher temperatures and humidity. Urea is a neutral fertilizer, meaning it does not significantly alter soil pH over time. Once applied to the soil, it is converted into ammonium carbonate by urease enzymes produced by microorganisms. The hydrolysis reaction is: (NH2)2CO + 2H2O → (NH4)2CO3. The rate of hydrolysis depends on factors such as soil acidity, moisture, temperature, soil type, maturity, and depth of application. At 10°C, this process takes about 10 days, while at 20°C, it takes 4–5 days, and at 30°C, only about 2 days. As a result, urea's fertilizing effect is relatively slow, so it should be applied in advance to ensure nutrient availability during crop growth.
Urea is suitable for all soil types and crops. It can be used as a base fertilizer, top dressing, or even for foliar spraying at concentrations of 1–2%. Its versatility makes it a widely used nitrogen source in agriculture.
3. Ammonium chloride, with the formula NH4Cl, contains 24–25% nitrogen. It is a white, crystalline substance that is highly soluble in water and slightly hygroscopic. It does not tend to clump, making it easy to handle and store. However, ammonium chloride is acidic and classified as a physiological acid-forming fertilizer. Due to its acidic nature, it should not be used on acidic soils, saline-alkaline soils, or crops sensitive to chloride ions, such as fruit trees and tobacco.
The presence of chloride ions can inhibit nitrifying bacteria, reducing the leaching of nitrogen in the soil compared to other nitrogen fertilizers. Therefore, ammonium chloride is often preferred for paddy fields. To minimize its negative effects, it should be applied in conjunction with irrigation to help flush chloride ions deeper into the soil. It is not recommended for use as a seed fertilizer due to potential damage to germinating seeds.
4. Ammonium nitrate, with the formula NH4NO3, contains 33–35% nitrogen. It is available in both crystal and granular forms. Crystalline ammonium nitrate is highly hygroscopic, while granular forms are coated with a moisture-proof agent, making them less prone to moisture absorption. It dissolves easily in water and has a neutral pH, making it suitable for a wide range of crops.
Ammonium nitrate provides two forms of nitrogen: ammonium (NH4+-N), which is less mobile in soil, and nitrate (NO3--N), which is more mobile and readily absorbed by plants. This dual nitrogen supply makes it an effective and environmentally friendly fertilizer, as it leaves no residual material in the soil. It is considered a physiologically neutral fertilizer, making it ideal for many agricultural applications.
Ammonium nitrate is best suited for top-dressing in dry farming systems. It should not be used as a base or seed fertilizer, nor should it be applied to paddy fields, as it may lead to excessive nitrogen leaching or runoff. Its balanced nutrient release and low risk of soil acidification make it a popular choice among farmers.
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