Urea, containing up to 46% nitrogen, is often referred to as the "Hercules" of solid nitrogen fertilizers. Compared to other nitrogen-based fertilizers, it is easy to transport, store, and apply across a wide range of soils and crops. It can be used as a base fertilizer or for top-dressing, and is particularly effective when applied as a foliar spray. However, to maximize its effectiveness and economic benefits, it's essential to follow the "four wants" and "four don’ts" when applying urea.
The "Four Wants" are:
First, **deep application**. Although urea is chemically stable, it can be converted into ammonium carbonate in the presence of soil urease. This compound then breaks down into unstable ammonium bicarbonate and ammonium hydroxide, which quickly release ammonia gas. This makes urea highly volatile, especially in alkaline soils. Deep application—typically 15–20% more efficient than surface application—helps reduce ammonia loss and prevents seedling burn in drylands. In paddy fields, it also minimizes leaching and denitrification losses. Farmers often say, “Fertilizer goes deep into the soil, a mu of land and two acres,†emphasizing the importance of this practice.
Second, **apply it in advance**. Urea needs time to convert into forms that plants can absorb, and its effectiveness depends on temperature, moisture, and organic matter in the soil. At 10°C, it takes about 7–10 days to break down; at 30°C, only 2 days. Therefore, in warm seasons, it should be applied 2–3 days earlier than other nitrogen fertilizers. In colder months, like early spring, it should be applied 6–8 days ahead of time, especially for winter crops such as wheat and oilseed rape.
Third, **apply the right amount**. With its high nitrogen content, urea should be used more sparingly than other nitrogen sources. Overuse can lead to excessive growth, lodging, pests, and even seedling damage. For example, if 20 kg of ammonium bicarbonate is needed, only 7.5 kg of urea is required. Experienced farmers often say, “Urea isn't the same as ammonium bicarbonate.â€
Fourth, **apply it according to the field conditions**. Urea has limited adsorption in the soil and is prone to leaching, especially in sandy soils. In paddy fields, it should be incorporated into the soil 5–7 days before irrigation. If watered immediately after application, much of the nitrogen may be lost. For dryland top-dressing, watering after application helps move the fertilizer deeper into the soil, mimicking the effects of deep placement. However, avoid flooding to prevent leaching.
The "Four Don’ts" include:
First, **don’t mix with alkaline fertilizers**. Mixing urea with materials like lime, wood ash, or calcium magnesium phosphate can increase ammonia volatilization. It’s best to apply them separately, with a 3–4 day gap in summer and 7–8 days in winter.
Second, **don’t use as a seed dressing**. High concentrations of urea can damage seeds by disrupting proteins and harming germination. Biuret, a byproduct of urea, is also toxic. Seed dressing should not exceed 2.5 kg per mu, and dry mixing is recommended.
Third, **don’t water immediately after application**, especially in warm weather, as this can cause severe leaching. Wait 2–3 days in summer and 7–8 days in winter. In paddy fields, maintain a thin water layer and avoid applying before heavy rain.
Fourth, **don’t apply foliar sprays at too high a concentration**. While urea is one of the most effective foliar fertilizers, exceeding 1.5–2% can burn leaves. Concentrations vary by crop: 1.5–2% for grains and cotton, 0.5–1% for vegetables, and 0.2–0.3% for seedlings. Always check for biuret content, as solutions over 1% are unsuitable for foliar use.
By following these guidelines, farmers can ensure optimal nutrient uptake, reduce waste, and improve crop yields effectively.
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