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Urea – Introduction
Urea was first identified in 1773 when it was isolated by crystallization from urine. It was first prepared synthetically in 1828 by Wohler from ammonia and cyanuric acid (Honti, 1976).
NH3 + HCNO → CO(NH2)2
The modern-day general method of synthesizing urea from ammonia and carbon dioxide at elevated temperatures and pressure has been known in principle since 1868, but commercial production by this method started not before 1922 in Germany, in 1932 in the United States, and in 1935 in England. However, there was some commercial production in Canada (by DuPont) starting in 1920 using calcium cyanamide as a raw material.
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Use of urea as a fertilizer developed rather slowly at first, partly because the early technical processes were cumbersome and expensive and partly because of some doubt about its agronomic suitability. Urea has been considered a slow-release fertilizer in Europe since it must undergo two transformations in the soil before it becomes available to most crops.
The first transformation is hydrolysis:
CO(NH2)2 + H2O → 2NH3 + CO2
The second transformation is nitrification in which ammonia is oxidized in the soil by microbiological means first to nitrite and then to nitrate. These reactions proceed rapidly in warm, moist soil, but they are quite slow in cool soils, which are characteristic of temperate climates (northern Europe) in the spring season.
Under some circumstances urea can be phytotoxic. The phytotoxicity may be caused by locally high concentrations of ammonia during the hydrolysis stage, or it may be caused by accumulation of nitrite during the nitrification stage. A possible third cause is the presence of excessive amounts of biuret as an impurity in urea.
Aside from toxicity, poor agronomic results may be caused by loss of ammonia to the atmosphere when urea is applied on the surface of the soil or as a top dressing to growing crops. This loss is caused by the formation of ammonia by hydrolysis under such conditions that part of it escapes to the atmosphere rather than being adsorbed by the soil. A primary use for straight nitrogen fertilizer in the United Kingdom and many European countries is for topdressing cereals and grass. Studies in England and the Netherlands have shown that on the average urea is only 80%-85% as effective as ammonium nitrate for this use.
This conclusion was confirmed by research conducted in the United Kingdom in 2006. The research commissioned by the Department for Environment, Food and Rural Affairs (DEFRA) found that the only realistic alternative to ammonium nitrate (AN) in the UK market was urea. However, it also showed that a switch to urea would lead to an increase in emissions of ammonia to the atmosphere which could hamper the UK’s commitment to reducing these emissions. The research therefore concludes that AN remains the most sustainable option for farmers.
In Germany, the revision of the fertilizer control order in 2017 provides as of 01.02.2020 for the agronomic application of urea fertilizer only in combination with urease inhibitors (unless urea can be incorporated into the soil immediately but not later than within four hours of application), with the intention to reduce emissions to the atmosphere and to increase nitrogen use efficiency.
Urea is generally satisfactory for rice and preferable to nitrates for flooded rice because nitrates are reduced to N2O or N2 in the anaerobic zone of the rice paddy and hence lost to the atmosphere. Furthermore, the rice plant, unlike most other crops, can utilize the ammonium form of nitrogen efficiently. In tropical, sub-tropical, and warm temperate zones, hydrolysis and nitrification (in aerobic soil conditions) are rapidly completed; thus, there is no delayed-release effect. For these reasons and because of its high concentration and favourable production cost, it has become the most popular nitrogen fertilizer in Asia and in many countries on other continents. An excellent discussion of the agronomic effects of urea has been presented by Tomlinson (1970), who concludes that urea can be used efficiently, but its use requires a higher degree of understanding than is the case with simple inorganic salts.
Urea is used as a cattle feed supplement where it is a cheap source of protein. It is used both in prill form (microprill at 0.2/0.4-mm diameter) and as a liquid in a mixture of urea (±75% solution), molasses, and phosphoric acid. It is the feed material for melamine plastic and for various glues. The glues are either urea-formaldehyde or urea-melamine-formaldehyde. The latter is waterproof and is used for marine plywood. It is also used as a de-icing agent on airfields and as an anti-NOx additive (AddBlue).
Best option for the environment and farming remains ammonium nitrate, says fertiliser research, press release, reference 429/06, Department for Environment, Food and Rural Affairs (DEFRA), United Kingdom, 04.10.2006.
Verordnung über die Anwendung von Düngemitteln, Bodenhilfsstoffen, Kultursubstraten und Pflanzenhilfsmitteln nach den Grundsätzen der guten fachlichen Praxis beim Düngen (Düngeverordnung), 26.05.2017
Tomlinson, T. E. (1970) Urea-Agronomic Implications, Proceedings International Fertiliser Society, 113.
Watson, C. J. (2000) Urease Activity and Inhibition – Principles and Practice, Proceedings International Fertiliser Society, 454.
Related IFS Proceedings
131, (1972), Some Fundamental Aspects of Urea Technology, S M Lemkowitz, M G R T de Cooker, P J van den Berg
166, (1977), Urea Stripping Process – Stripping Technology, Phase Equilibria and Thermodynamics, P J C Kaasenbrood, H A G Chermin, Cooker, P J van den Berg
257, (1987), Rationale for Mixed Ammonium Nitrate – Urea Fertilisers and Assessment of Granular Products, M K Garrett
725, (2013), Urea-based NPK Granulation – Examination of Constraints and Potential Solutions, S R Doshi
770, (2015), World-Wide Trends in Urea Process Technologies, J M G Eijkenboom, M J Brouwer
805, (2019), The Carbon Footprint of Fertiliser Production: Regional Reference Values, A Hoxha, B Christensen
830, (2019), Principles and Applications of a Directory of Urea Safety Incidents, with Case Studies, M J Brouwer
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