1. Ammonium bicarbonate, with the chemical formula NH₄HCO₃, contains approximately 17% nitrogen. It is a white, chemically unstable crystalline substance that readily absorbs moisture, decomposes, and releases ammonia gas, giving off a strong, pungent odor. The decomposition rate increases with higher humidity and temperature. It dissolves easily in water and produces an alkaline solution (pH 8.2–8.4). At 70°C under atmospheric pressure, it completely decomposes. When stored at 20°C, the loss rates after 1, 5, and 10 days are 9%, 48%, and 74%, respectively. In humid conditions, it can absorb water, deliquesce, and clump together, which is a sign of slow decomposition. Proper storage and application methods are essential to minimize its volatilization. It is suitable for all types of soil and crops, and should be applied as a base fertilizer, buried beneath the soil for better efficiency.
2. Urea, with the molecular formula (NH₂)₂CO, contains about 46% nitrogen. It is typically a white crystal with strong hygroscopic properties. Modern urea products are often transparent and have been treated to reduce moisture absorption. At temperatures between 10–20°C, its moisture absorption is minimal, but as temperature and humidity increase, so does its hygroscopicity. Urea is a neutral fertilizer and does not harm the soil when used long-term. Once applied to the soil, it is hydrolyzed into ammonium carbonate by soil microorganisms' urease enzyme. The hydrolysis reaction is: (NH₂)₂CO + 2H₂O → (NH₄)₂CO₃. The rate of hydrolysis depends on soil pH, moisture, temperature, soil type, maturity, and depth of application. Under normal conditions, hydrolysis takes about 10 days at 10°C, 4–5 days at 20°C, and 2 days at 30°C. Therefore, urea’s effect is relatively slow, and it should be applied earlier than other fertilizers. Urea is suitable for all soil types and crops and can be used as a base fertilizer, top dressing, or foliar spray (with a concentration of 1–2%).
3. Ammonium chloride, with the formula NH₄Cl, contains 24–25% nitrogen. It is a white, crystalline solid that is soluble in water, hygroscopic, and does not tend to clump, making it easy to store. However, it is acidic and classified as a physiological acid fertilizer. It should not be used on acidic soils, saline-alkaline soils, or crops like fruit trees and tobacco, as chloride ions can inhibit nitrifying bacteria. This reduces the leaching of nitrogen in the soil compared to other nitrogenous fertilizers. Hence, it is a preferred choice for rice paddies. To minimize the negative effects of chloride ions, ammonium chloride should be applied with watering to move the ions deeper into the soil. It is not recommended for use as a seed fertilizer.
4. Ammonium nitrate, with the formula NHâ‚„NO₃, contains 33–35% nitrogen. It comes in two forms: crystals and granules. Crystalline ammonium nitrate is highly hygroscopic, while granular forms are slightly less so due to a moisture-proof coating. It dissolves easily in water and has a neutral pH. It contains both ammonium (NHâ‚„âº-N), which is less mobile in the soil, and nitrate (NO₃â»-N), which is more mobile. Both forms are efficiently absorbed by plants, making ammonium nitrate a high-quality nitrogen fertilizer that leaves no residue in the soil. It is physiologically neutral and ideal for top-dressing dryland crops. However, it should not be used in paddy fields or as a base or seed fertilizer due to its potential for runoff and instability.
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