Urea fertiliser, also known as carbamide, is the most important nitrogenous fertiliser. It is a white crystalline organic chemical compound containing about 46 per cent nitrogen. It is a waste product formed naturally by metabolising protein in humans as well as other mammals, amphibians and some fish. Synthetic urea is produced commercially from ammonia and carbon dioxide. Urea is widely used in the agriculture sector both as a fertiliser and animal feed additive, which makes the production of urea considerably high in comparison to other fertilisers. In the United States alone, approximately one million pounds of urea is produced every year.
Urea was first discovered by a French scientist, named Hillaire Rouelle in 1773. But, synthetic urea was started to produce in 1828, about 55 years after its discovery. Currently, urea is manufactured industrially by the dehydration of ammonium carbamate in a process involving elevated temperature and pressure. Normally, a high pressure reactor is used within which all these reactions are carried out.
Advantages of Urea Fertilizer
Urea has the highest nitrogen content, equal to 46 per cent. This percentage is much higher than other nitrogenous fertilisers available in the market.
The cost of production of urea is relatively low since carbon dioxide required for its manufacture is obtained from crude naphtha.
Urea in not subject to fire or explosion hazards, and hence there is no risk in the storage of urea.
Urea can be used for all types of crops and soils. After its assimilation by plants, urea leaves behind only carbon dioxide in the soil through the interaction of nitrifying bacteria. This carbon dioxide does not harm the soil.
Disadvantages of Urea Fertilizer
Urea is very soluble in water, and hygroscopic water (hygroscopic water creates a thin layer surrounding individual soil particles, which makes water unavailable to plants), and hence requires better packaging quality.
It is not as stable as other solid nitrogenous fertilisers. Urea decomposes even at room temperatures, particularly in a humid atmosphere releasing ammonia and carbon dioxide. The formation of ammonia and carbon dioxide results in serious loss.
If urea contains impurities more than 2 per cent, it cannot be used as a fertiliser, since the impurities are toxic to certain crops, particularly citrus.
Soil Application and Placement
Urea should be applied at the time of sowing. It should not come in contact with the seeds. It also can be applied as a top dressing. Since urea is highly concentrated, it should be used in combination with earth or sand before its application. It should not be applied when the soil contains free water or is likely to remain wet for three or four days after application.
Blending Urea with Other Fertilizers
Urea is readily blended with monoammonium phosphate or diammonium phosphate. But, urea must not be mixed with any superphosphate unless applied immediately after blending, because urea reacts with superphosphate liberating water molecules. This will produce a damp material that is hard to store and apply.