Urea is a physiologically neutral fertilizer that does not leave harmful residues in the soil and is safe for long-term use. However, during the granulation process, if the temperature is too high, a small amount of biuret may form, which can negatively affect plant growth. In China, the biuret content in urea should be kept below 0.5%. When the biuret level exceeds 1%, it cannot be used as seed, seedling, or foliar fertilizer. During other application periods, urea should not be applied in excessive amounts or at high concentrations to avoid damage to crops.
As an organic nitrogen fertilizer, urea must first be converted into ammonium carbonate or ammonium bicarbonate by the enzyme urease in the soil before it can be absorbed and utilized by plants. Therefore, it is recommended to apply urea 4–8 days before the crop’s peak nutrient demand to ensure proper absorption.
Urea is suitable for use as a basal fertilizer, topdressing, and sometimes as a seed fertilizer. Before hydrolysis, urea exists in a molecular form and is not adsorbed by the soil. This makes it prone to leaching with water. Additionally, the ammonia formed after conversion is volatile, so deep placement is necessary to reduce losses and improve efficiency.
Naturally occurring in urine, urea is also widely produced industrially. In China, urea accounts for about 40% of total nitrogen fertilizer production and is one of the most commonly used nitrogen fertilizers, second only to ammonium bicarbonate. Its use began in the early 20th century, and since the 1950s, its high nitrogen content (45–46%) and improved industrial processes have led to widespread adoption globally.
China started building medium-sized urea plants in the 1960s, and from 1986 to 1992, annual production exceeded 9 million tons. Today, urea remains a key component of the country’s nitrogen fertilizer industry.
Beyond its agricultural use, urea has multiple applications. For example, in fruit trees like apples, applying a 0.5% urea solution 5–6 weeks after flowering helps regulate flower bud development by increasing leaf nitrogen levels. In peach and nectarine cultivation, urea is used for thinning fruits, with optimal concentrations between 8% and 12%.
In hybrid rice production, urea can replace gibberellin to promote outcrossing and synchronize flowering. It is also used in pest control; a mixture of urea, detergent, and water in a 4:1:400 ratio effectively controls aphids, mites, and caterpillars. Urea-iron complexes are also used to prevent iron deficiency in plants, improving chlorophyll production.
Proper storage is essential. Urea should be kept in dry, well-ventilated areas, away from moisture to prevent clumping. If stored in large quantities, it should be placed on wooden pallets with adequate space for air circulation. Sealed bags should be used if the product is not fully consumed to maintain quality.
Urea is not only a major fertilizer but also an important raw material in industries such as plastics, resins, and pharmaceuticals. It can also serve as a protein supplement in animal feed, though care must be taken to avoid overuse, as excessive urea can harm livestock kidneys.
In terms of production, natural gas, coal, and petroleum are the main raw materials. Due to fluctuating prices, natural gas-based production is often more cost-effective. For instance, companies using natural gas as a feedstock tend to have higher profit margins compared to those relying on coal or oil.
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