Urea was first discovered in urine in 1773 by the French chemist Hilaire Rouelle. In 1828, the German chemist Friedrich Wöhler obtained urea by treating silver cyanate with ammonium chloride in a failed attempt to prepare ammonium cyanate.
This was the first time an organic compound was artificially synthesized from inorganic starting materials, without the involvement of living organisms.
More than 90% of world production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use (46.7%). Therefore, it has the lowest transportation costs per unit of nitrogen nutrient. Urea is the most concentrated nitrogen mineral fertilizer (contains 46% of N). It is a non-acid-forming fertilizer and can be used for all types of soil and for any crops as major fertilizer and in solution for additional foliar fertilization. Urea will be more effective then ammonium nitrate for irrigation and soil leaching. Besides, it is an essential nitrogen fertilizer by cultivation of rice and cucurbit crops.
Many soil bacteria possess the enzyme, urease, which catalyzes the conversion of the urea molecule to two ammonia molecules and one carbon dioxide molecule, thus urea fertilizers are very rapidly transformed to the ammonium form in soils. Among soil bacteria known to carry urease, some ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas are also able to assimilate the carbon dioxide released by the reaction to make biomass via the Calvin Cycle, and harvest energy by oxidizing ammonia (the other product of urease) to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially, Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils and is a major cause of water pollution from agriculture. Ammonia and nitrate are readily absorbed by plants, and are the dominant sources of nitrogen for plant growth. Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in 'foliar feed' fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution which is an advantage for mechanical application.
The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea is usually spread at rates of between 40 and 300 kg/ha but rates vary. Smaller applications incur lower losses due to leaching. During summer, urea is often spread just before, or during rain to minimize losses from volatilization (process wherein nitrogen is lost to the atmosphere as ammonia gas). Urea is not compatible with other fertilizers.
Because of the high nitrogen concentration in urea, it is very important to achieve an even spread. The application equipment must be correctly calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application as a spray or through irrigation systems.
In grain and cotton crops, urea is often applied at the time of the last cultivation before planting. In high rainfall areas and on sandy soils (where nitrogen can be lost through leaching) and where good in-season rainfall is expected, urea can be side- or top-dressed during the growing season. Top-dressing is also popular on pasture and forage crops. In cultivating sugarcane, urea is side-dressed after planting, and applied to each ratoon crop.
In irrigated crops, urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea will dissolve in its own weight in water, but it becomes increasingly difficult to dissolve as the concentration increases. Dissolving urea in water is endothermic, causing the temperature of the solution to fall when urea dissolves.
Urea is the most popular and economical of all nitrogenous fertilizers being used worldwide. Urea has a nitrogen content of 46%, a higher concentration than this is available in other solid sources of nitrogen. Unlike phosphate and potassium fertilizers, which normally require a single application, urea needs to be applied several times during the growing season. Urea is an ideal source of nitrogen for all kinds of crops (wheat, barley, rice, maize etc.) In addition, urea is a major raw material for melamine, and an important raw material for the manufacture of other industrial products.
Granular urea is chemically the same as Prilled urea. Granular urea, however, is slightly larger and harder. Today, the use of Prilled urea is more prevalent because it is more resistant to breaking down when being blended with the other components of fertilizer.
Hazards and safety: The product is relatively safe and chemically stable. When it decomposes it produces hydrogen cyanide, ammonia, oxides of nitrogen and carbon. As a dust, it may form an explosive mixture with air.
Applications: Broadcasting or in irrigation water (fertigation); best if split applied at least three times throughout the growing season; suitable for all crops across a wide range of environmental conditions.
Storage and handling: The product is normally stored in bulk and in bags.
Packaging: The product is mainly exported in bulk. Also, in 1000 and 50 kg bags.