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 below 0.5%. When it exceeds 1%, urea cannot be used as seed or foliar fertilizer. During other application periods, urea should not be applied in excessive amounts or in highly concentrated forms.
As an organic nitrogen fertilizer, urea needs to be hydrolyzed into ammonium carbonate or bicarbonate by the enzyme urease in the soil before crops can absorb it. Therefore, it’s best to apply urea 4–8 days before the crop’s peak nutrient demand to ensure proper breakdown and uptake.
Urea is suitable for both base fertilization and top-dressing, and can sometimes be used as a seed fertilizer. Before hydrolysis, urea exists in a molecular form and is not easily retained by the soil. This makes it prone to leaching with water. Additionally, the ammonia formed during hydrolysis is volatile, so deep placement is recommended to minimize loss.
Naturally occurring in urine, urea is widely used in agriculture. In China, it accounts for about 40% of total nitrogen fertilizer production and is one of the most important nitrogen sources, alongside ammonium bicarbonate. Since the early 20th century, urea has been used as a fertilizer, and its popularity grew rapidly after the 1950s due to its high nitrogen content (45–46%) and improved production techniques. China began building medium-sized urea plants in the 1960s, and by the late 1980s, annual production exceeded 9 million tons, making it a key component of the country's agricultural industry.
Beyond its primary use as a fertilizer, urea has several other applications. For example, in apple orchards, spraying a 0.5% urea solution on leaves 5–6 weeks after flowering can help regulate flower bud development and manage fruiting levels. In fruit trees like peaches and nectarines, urea can be used to thin fruits when applied at concentrations between 8–12%. It also plays a role in hybrid rice seed production by helping synchronize flowering and increase outcrossing rates.
Additionally, urea can be used in pest control when mixed with detergent and water in a 4:1:400 ratio, effectively controlling aphids, mites, and caterpillars. Urea-iron complexes are also used to treat iron deficiency in plants, offering a cost-effective alternative to traditional iron supplements.
Proper storage is essential for maintaining urea quality. It should be kept in dry, well-ventilated areas, away from moisture. Improper storage can lead to clumping and reduced effectiveness. Large-scale storage requires wooden pallets and adequate space for ventilation. If not fully used, opened bags must be sealed properly to prevent degradation.
In terms of production, urea is made from natural gas, coal, or petroleum—often referred to as "gas head," "coal head," and "oil head" processes. Due to rising costs of oil and coal, the gas-based method is currently more cost-efficient. Companies using natural gas as a feedstock often enjoy higher profit margins compared to those relying on coal or oil.
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