Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two –NH2 groups joined by a carbonyl (C=O) functional group.
Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.
Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, which was an important conceptual milestone in chemistry in 1828. It showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that only living things could produce the chemicals of life.
More than 90% of world industrial 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. Therefore, it has a low transportation costs per unit of nitrogen nutrient. The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea breaks down in the soil to give ammonium. The ammonium is taken up by the plant. In some soils, the ammonium is oxidized by bacteria to give nitrate, which is also a plant nutrient. The loss of nitrogenous compounds to the atmosphere and runoff is both wasteful and environmentally damaging. For this reason, urea is sometimes pretreated or modified to enhance the efficiency of its agricultural use. One such technology is controlled-release fertilizers, which contain urea encapsulated in an inert sealant. Another technology is the conversion of urea into derivatives, such as with formaldehyde, which degrade into ammonia at a pace matching plants’ nutritional requirements.